Biomaterials and bioactive natural products from marine invertebrates: from basic research to innovative applications

  Citation: Romano, G.; Almeida, M.; Varela Coelho, A.; Cutignano, A.; Gonçalves, L.G.; Hansen, E.; Khnykin, D.; Mass, T.; Ramšak, A.; Rocha, M.S.; et al. Biomaterials and Bioactive Natural Products from Marine Invertebrates: From Basic Research to Innovative Applications. Mar. Drugs 2022,20, 219. https:// doi.org/10.3390/md20040219 Academic Editor: Hermann Ehrlich Received: 14 February 2022 Accepted: 16 March 2022 Published: 22 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). marine drugs Review Biomaterials and Bioactive Natural Products from Marine Invertebrates: From Basic Research to Innovative Applications Giovanna Romano 1,* , Mariana Almeida 2,3 , Ana Varela Coelho 4, Adele Cutignano 1,5 , Luis G Gonçalves 4, Espen Hansen 6, Denis Khnykin 7, Tali Mass 8, Andreja Ramšak 9, Miguel S. Rocha 2,3 , Tiago H. Silva 2,3 , Michela Sugni 10 , Loriano Ballarin 11,* and Anne-Marie Genevière 12 1Marine Biotechnology Department, Stazione Zoologica Anton Dohrn, Villa Comunale, 80121 Naples, Italy; [email protected].it 23B’s Research Group, I3B’s—Research Institute on Biomaterials, Biodegradables and Biomimetics of University of Minho, Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark—Parque de Ciência e Tecnologia, Barco, 4805-017 Guimarães, Portugal; [email protected] (M.A.); [email protected] (M.S.R.); [email protected] (T.H.S.) 3ICVS/3B´s—PT Government Associate Laboratory, 4710-057 Braga, Portugal 4ITQB NOVA, Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal; V[email protected] (A.V.C.); [email protected] (L.G.G.) 5CNR-Institute of Biomolecular Chemistry, Via Campi Flegrei 34, 80078 Pozzuoli, Italy 6Marbio, UiT-The Arctic University of Norway, 9037 Tromso, Norway; [email protected] 7Laboratory for Immunohistochemistry and Immunopathology (LIIPAT), Department of Pathology, Oslo University Hospital-Rikshospitalet, 0450 Oslo, Norway; [email protected] 8Faculty of Natural Science, Department of Marine Biology, Charney School of Marine Sciences, University of Haifa, Haifa 3498838, Israel; [email protected] 9National Institute of Biology, Marine Biology Station, Fornaˇce 41, SI-6330 Piran, Slovenia; [email protected] 10 Department of Environmental Science and Policy, University of Milan, Via Celoria, 2, 20133 Milan, Italy; [email protected] 11 Department of Biology, University of Padova, Via U. Bassi 58/B, 35100 Padova, Italy 12 Biologie Intégrative des Organismes Marins (BIOM), Observatoire Océanologique de Banyuls-sur-Mer, Sorbonne Université, CNRS, 1 Avenue Pierre Fabre, 66650 Banyuls-sur-Mer, France; [email protected] *Correspondence: [email protected] (G.R.); [email protected] (L.B.) Abstract: Aquatic invertebrates are a major source of biomaterials and bioactive natural products that can find applications as pharmaceutics, nutraceutics, cosmetics, antibiotics, antifouling products and biomaterials. Symbiotic microorganisms are often the real producers of many secondary metabolites initially isolated from marine invertebrates; however, a certain number of them are actually synthesized by the macro-organisms. In this review, we analysed the literature of the years 2010–2019 on natural products (bioactive molecules and biomaterials) from the main phyla of marine invertebrates explored so far, including sponges, cnidarians, molluscs, echinoderms and ascidians, and present relevant examples of natural products of interest to public and private stakeholders. We also describe omics tools that have been more relevant in identifying and understanding mechanisms and processes underlying the biosynthesis of secondary metabolites in marine invertebrates. Since there is increasing attention on finding new solutions for a sustainable large-scale supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of aquatic invertebrate stem cells. Keywords: marine natural products; marine biomaterials; marine invertebrates; bioactivity; stem cells Mar. Drugs 2022,20, 219. https://doi.org/10.3390/md20040219 https://www.mdpi.com/journal/marinedrugs Mar. Drugs 2022,20, 219 2 of 45 1. Introduction Historically, natural products have played a key role in drug discovery, especially for cancer and infectious diseases [ 1 – 3 ]. Although most drugs still derive from terrestrial sources, the marine environment represents a unique resource of natural bioactive products as many marine compounds have chemical characteristics not found in natural terrestrial products. Aquatic invertebrates, due to their high genetic richness, have been a major source of marine natural products (MNPs) of social value, as they produce molecules (enzymes, biopolymers, bioactive compounds, secondary metabolites) that can find applications in various fields as pharmaceutics, nutraceutics, cosmetics, antibiotics, antifouling products, biomaterials and more [4]. The relevance of bioactive compounds of marine origin in drug development is demonstrated by the fact that currently thirteen sea-derived drugs have been approved in the EU and/or USA, four of which received approval in the last three years [ 5 ]. These drugs were developed for the treatment of different diseases including carcinoma, chronic pain, and Alzheimer’s disease. In addition, the clinical pipeline in 2020 contained more than twenty drug candidates in different clinical trials in phase III, II, or I [ 6 ]. The number of natural products isolated from marine organisms has indeed grown rapidly and continues to provide significant chemical biodiversity that contributes to the development of new therapeutic agents. The most studied marine invertebrates as sources of bioactive compounds include sponges, cnidarians, molluscs, echinoderms and ascidians [ 4 ], as also testified by the drugs currently available for therapeutic applications developed from marine natural products isolated from species belonging to these groups of animals [ 7 , 8 ]. The first compound developed for clinical use is a synthetic analogue of the C-nucleoside cytarabine (or Ara-C) isolated from the Caribbean sponge Tethya crypta. It was approved in 1969 and is still used to treat acute myelocytic leukaemia and non-Hodgkin’s lymphoma [ 9 ]. Another analogue of a nucleoside isolated from the same species is viradabine (Vira-A ® ), approved in 1976 as an antiviral against Herpes simplex. Thirty-two years later, Trabectedin (Yondelis ® ), an alkaloid isolated from the tunicate Ecteinascidia turbinata [ 10 ], was approved as an anticancer agent. Since then, other marine products were approved as anticancer agents, including eribulin mesylate (Halaven ® ), an analogue of halichondrin B from the sponge Halichondria okadai, plitidepsin (Aplidine ® ), a cyclic peptide from the ascidian Aplidium albicans, two derivatives of dolastatin 10 (brentuximab vedotin, Adcetris ® ), and polatuzumab vedotin (POLIVY ® ) from the mollusc Dolabella auricularia [ 7 ]. In addition, the potent analgesic ziconotide (Prialt ® ) produced by the gastropod mollusc Conus magus [ 11 ] is in clinical use for treating chronic pain. Although it has been demonstrated that symbiotic microorganisms produce many of the secondary metabolites initially isolated from macro-organisms, there is convincing evidence that a certain number of them are directly synthesized and released by cells of various tissues of the macro-organism, including immune cells deriving from the differentiation of stem cells [ 12 ]. Ideally, the possibility to produce bioactive compounds using aquatic invertebrate-derived immortalized cell lines would be of great importance in developing societal improvements and advances [ 12 ]. This could remove one of the major bottlenecks represented by a sustainable supply of sufficient amounts of bioactive compounds to support preclinical development and all phases of clinical trials required for a new drug to reach the market. Unfortunately, up to now, all the efforts to establish long-term cultures of aquatic invertebrate cells have failed to achieve significant results [ 13 ]. Therefore, the knowledge of the gene networks involved in biological processes underlying stem cell differentiation as well as in the biosynthetic pathways of useful bioactive metabolites is of great interest not only in medicine but also for industrial enterprises. In this sense, the continuous development of genomic, transcriptomic, proteomic and metabolomic tools is providing new opportunities to identify and understand those mechanisms and processes. There is also an increasing amount of scientific information available on marine biomaterials from marine invertebrates, among which the most promising are marine biominerals, Mar. Drugs 2022,20, 219 3 of 45 collagen, chitin and adhesive proteins [ 14 ]. These biomaterials show high biocompatibility and may find several applications in the biomedical field for tissue engineering and regenerative medicine [15], drug delivery [16] and sutureless wound closure [17]. In this review, we analysed the literature of the years 2010–2019 on natural products (bioactive molecules and biomaterials) from the main phyla of marine invertebrates and present relevant examples with the aim of identifying natural products of interest to public and private stakeholders. We also discuss the potentialities of the main phyla of marine invertebrates in terms of the production of useful natural compounds and in light of advanced approaches and technologies that can provide a sustainable exploitation of valuable products from this source. 2. Sponges Sponges (phylum Porifera) include nearly 9000 species of filter-feeding, benthic organisms living (mainly) in seawater and (about 220 species) freshwater environments. They are considered basal metazoan and include the clades Calcarea, Hexactinellidae, Demospongiae and Homoscleromorpha. 2.1. Biomaterials In recent years, marine organisms have been emerging as an alternative and sustainable source of biomaterials and other compounds of biomedical interest, prompted by their safety regarding zoonosis and the lack of ethical constraints. Marine sponges present an interesting and promising biotechnological potential which is still vastly unexplored. Indeed, the number of Porifera species known to date, which is around 8500 species [ 18 ], is far higher compared to species studied for biomaterials development, that would include a few dozens of species. These ancient animals are a source of various compounds that have been proven to have applicability in biomedical research, such as biosilica, collagen/spongin and chitin (Table 1). These biomaterials have common specific features that make them ideal for tissue engineering approaches, such as low immunogenicity and cytotoxicity, and biodegradability. Additionally, the skeleton of some sponges is a suitable material for tissue engineering templates as it possesses a highly interconnected porous architecture, similar to bone structures, that allow culturing cells and guiding cell growth and differentiation, stimulating the regeneration of human tissues to recover lost functions [ 15 ]. Given the unique and particular characteristics of sponges’ components, most developed applications aim at bone tissue engineering, although they are currently being employed in innovative biomedical applications, helping to disclose the full potential of blue biotechnology. Table 1. Biomaterials from sponges. (Sorted alphabetically according to Class of Biomaterial.) Class Producer Species Class of Biomaterial Biomaterial Origin/Structural Component Possible Applications References Demospongiae Petrosia ficidormis, Bioceramics Silicate, Calcium carbonates Silicate/calcium salts whole body Support for tissue regeneration [19] Demospongiae Petrosia ficidormis, Chondrosia reniformis and Agelas oroides Bioceramics Calcium phosphate (hydroxyapatite) skeleton Substitutes for synthetic Bioglass®[20] Demospongiae Petrosia ficidormis Inorganic polymer Biosilica whole body 3D support for osteoblast adhesion and growth [21] Demospongiae Spongia agaricina, Spongia officinalis, Spongia zimocca Inorganic polymer Hydroxyapatite whole body Bone substitute material [22] Demospongiae Suberites domuncula Inorganic polymer Biosilica skeleton Stimulates mineralizing activity [23] n.a. n.a. Inorganic polymer Silicate skeleton Stimulates osteogenesis in vivo [24] n.a. n.a. Inorganic polymer Silica/silicatein skeleton Regeneration of bone tissue [25] Mar. Drugs 2022,20, 219 4 of 45 Table 1. Cont. Class Producer Species Class of Biomaterial Biomaterial Origin/Structural Component Possible Applications References n.a. n.a. Inorganic polymer Biosilica/polyphosphate skeleton Promotes growth and differentiation of hMSCs *; 3D tissue printing of hMSCs *; Delivery of hMSCs * in fractures [25] Demospongiae Spongia agaricina Inorganic polymer Hydroxyapatite whole body Bone tissue engineering [26] Demospongiae Spongia agaricina Inorganic polymer Hydroxyapatite whole body Tissue engineering (bone scaffolds) [22] Demospongiae Ianthella labyrinthus Polysaccharides Chitin skeleton Scaffolds to culture cardiomyocytes differentiated from human-induced pluripotent stem cells (ipsc-cms); 3D tissue engineering [27] Demospongiae Ianthella flabelliformis Polysaccharides Chitin skeleton Drug delivery biomaterial [28] Demospongiae Ianthella basta Polysaccharides Chitin skeleton Tissue engineering and regenerative medicine [29] Demospongiae Pseudoceratina purpurea Polysaccharides Chitin skeleton Biomedicine [30] Demospongiae Mycale euplectellioides Polysaccharides Chitin skeleton Biomedicine [31] Demospongiae Acarnus wolffgangi and Echinoclathria gibbosa Polysaccharides Chitin skeleton Biomedicine [32] Demospongiae Pseudoceratina arabica Polysaccharides Chitin skeleton Biomedicine [33] Demospongiae Aplysina aerophoba Polysaccharides Chitin whole body 3D microporous chitinous scaffolds for hMSCs * in vitro [34] Demospongiae Ianthella basta Polysaccharides Chitin whole body Scaffolds for human mesenchymal stromal cells [35] Demospongiae Aplysina cavernicola, Aplysina cauliformis, Aplysina fulva, Aiolochroia crassa, Plysina aerophoba Polysaccharides Chitin whole body Chitin scaffolds for chondrocytes attachment [34] Demospongiae Aplysina aerophoba Polysaccharides Chitin whole body Ready-to-use scaffolds for cultivation of cardiomyocytes [36] Demospongiae Spongia lamella, Spongia officinalis Hippospongia communis Sarcotragus spinosulus Polysaccharides/Proteins Collagen/ proteoglycan skeletons Bio-based dressing for topical drug delivery [16] Demospongiae Aplysina archeri Polysaccharides Chitin skeletal fibres Ready-to-use 3D chitin scaffolds [37] Demospongiae Aplysina fulva Aplysina aerophoba Ianthella basta Polysaccharides Chitin skeletons Directed differentiation of human adipose tissue-derived hMSCs * within chitin-based skeletons [38] Demospongiae Chondrosia reniformis Proteins Collagen whole body Support and promote the migration, adhesion, and growth of epithelial cells [39] Demospongiae Biemna fortis Proteins Collagen whole body Bone repair and bone augmentation [40] Demospongiae Chondrosia reniformis Proteins Collagen whole body Sponge collagenous membranes [41] Demospongiae Callyspongiidae Proteins Collagen skeletons Scaffold for use in bone tissue engineering [42] Demospongiae Ircinia fusca Proteins Collagen whole body Composite scaffolds (marine collagen + chitosan + hydroxyapatite) for matrix-based bone repair and bone augmentation [43] Demospongiae Aplysina fulva Proteins Spongin whole body Spongin-enriched biosilicate scaffolds to support bone formation [44] * Human mesenchymal stem cells (hMSCs). Mar. Drugs 2022,20, 219 5 of 45 Biosilica is an inorganic polymer produced by poriferan from monomeric silicate substrates to build up their inorganic skeleton. The main players in poriferan silicification are low molecular weight proteins, such as silicateins and cathepsins in demosponges, while glassin, collagen, and chitin are involved in the formation of glass sponge (Hexactinellida) exoskeleton [ 45 ]. Interestingly, the patterning of silica architecture is based, in both hexactinellids and demosponges, on axial filaments composed primarily of actin [ 45 ]. Biosilica is able to stimulate mineralizing activity and induce osteogenesis in vitro , supporting tissue mineralization [ 46 ]. Therefore, it has been used in various biomedical applications, mainly in hard tissue engineering. For example, biosilica extracted from the demosponge Suberites domuncula enhanced mineralizing activity of osteoblasts by favouring the formation of hydroxyapatite, making this material promising for studies of bone replacement [ 23 ]. Wang and colleagues have employed biosilica and polyphosphate to supplement biologically inert alginate [ 47 ]. The supplemented alginate promoted the growth and differentiation of human mesenchymal stromal cells (hMSCs), which could be advantageous for application in 3D tissue printing of hMSCs and for the delivery of hMSCs in fractures [ 47 ]. In another example, surface modification of biosilica obtained from the calcination of the Petrosia ficidormis body allowed osteoblasts to grow and colonize the highly porous and interconnected bioceramic structure, inducing hydroxyapatite formation [ 21 ]. In addition, microspheres prepared by encapsulation of β -tricalcium phosphate ( β -TCP) supplemented with silica have presented morphogenetic activity on bone-forming cells in vivo , demonstrating that these biosilica-based scaffolds are promising biomaterials for bone repair/regeneration [25]. Silicate, a silica compound, has been shown to stimulate in vivo osteogenesis, demonstrating potential to ameliorate osteoporotic disorders [ 24 ]. Bioceramics obtained from P. ficidormis,Chondrosia reniformis and Agelas oroides have been considered suitable for biomedical applications, namely in tissue engineering, due to the coating of the surface resembling hydroxyapatite and its biocompatibility [ 19 ]. Furthermore, it has been shown that, due to their lack of cytotoxicity, these bioceramics can be used as substitutes for synthetic Bioglass®[20]. Collagen is the most abundant structural protein present in animal extracellular matrices, and it is strikingly similar in both invertebrates and vertebrates. This allowed the development of various applications using collagen from invertebrates as a biomaterial. Spongin is a protein of collagenous nature identified in the exoskeleton of some demosponges. Although the spongins’ exact molecular composition is not elucidated yet due to its abundant diversity within this clade of marine sponges, the presence of a collagenous domain and of short non-fibrillar type-IV-related collagens were demonstrated [ 48 , 49 ]. Many authors consider spongin to be collagen, but only a detailed proteomic analysis could definitely clarify this issue. This review refers to spongin as sponge collagen. Various structures to support the regeneration of distinct tissues or drug delivery applications can be developed using sponge collagen as scaffolds, hydrogels, particles or membranes. Collagen from Ircinia fusca resulted in a promising biomaterial, as it successfully promoted osseous tissue formation [ 40 ]. In combination with chitosan and hydroxyapatite, it has been employed in the development of scaffolds mimicking the required properties of bone extracellular matrix (ECM) aiming at bone tissue engineering with encouraging results [ 43 , 44 ]. Marine sponge collagen has application also in skin care and regeneration. C. reniformis collagen has been isolated and incorporated in formulations for topical administration, assessing the effect on skin biophysical parameters [ 50 ]. Moreover, this marine sponge collagen has been used on the development of microand nanoparticles envisaging the dermal delivery of drugs [ 51 , 52 ]. It has been also used as biomaterial for the development of membranes capable of mimicking human basal lamina, thus supporting and promoting the migration, adhesion, and growth of epithelial cells for epithelial repair, regeneration or replacement [ 39 ]. Other membranes for tissue engineering and regenerative medicine approaches have been produced using this collagen, with the possibility of alternatively improving the mechanical properties or the antioxidant performances of the Mar. Drugs 2022,20, 219 6 of 45 derived biomaterial by adapting the extraction procedure [ 41 ]. This versatility allows the tailoring of a membrane’s properties to the requirements of a specific application. Horny sponges (Order Dictyoceratida) are also a source of collagen with biomedical applicability as a bio-based dressing for topical drug delivery [ 16 ]. Due to its glycosaminoglycans content, this natural sponge skeletal scaffold is a bioactive and biocompatible carrier able to regulate the wound healing processes by releasing the drug while absorbing the excess of the wound exudate [16]. Chitin is a polysaccharide functionally comparable to keratin with many proven biotechnological and industrial applications. Chitin has been isolated from various demosponges, mostly from Verongiida, and in the biomedical field it has been mainly used as a scaffold for supporting various cell types, promoting their proliferation and differentiation due to its biocompatibility and mechanical properties [ 34 , 36 – 38 ]. Most sponges possessing chitin in their skeletal networks, mineralized with silica and calcium carbonates, are also partially brominated [ 53 , 54 ]. Aplysina aerophoba is a renewable source of unique 3D microporous chitinous scaffolds which are cyto-compatible and support attachment, growth and proliferation of hMSCs in vitro , being suitable for tissue engineering strategies [ 34 ]. The chitinous structure of this animal has also been studied as ready-to-use scaffolds for cultivation of cardiomyocytes, confirming the biocompatibility of the sponge biomaterial with this cell type [ 36 ]. Chitin-based scaffolds generated from verongid sponges promoted adhesion, proliferation and differentiation of adipose tissue-derived hMSCs into osteogenic and adipogenic lineages [ 38 ]. This finding enables the development of new biocompatible and functionally-active bioengineered structures. Naturally prefabricated 3D chitin scaffolds preserving their fibrous interconnected structure were also obtained from Aplysina archeri, raising interest for diverse technological and biomedical fields [ 37 ]. Chitin scaffolds isolated from Ianthella basta have also been suggested for stem cell-based tissue engineering applications due to their biocompatibility and capacity to maintain the cells’ ability to differentiate [ 35 ]. Furthermore, carriers of human mesenchymal stem cells (hMSCs) based on the chitinous skeleton of this species were developed as a stem cell cryopreservation method within 3D tissue engineered scaffolds, supporting cell adhesion, migration and proliferation, and maintaining cell viability after cryopreservation [ 29 ]. Ianthella labyrinthus is also a species with biotechnological potential, as it is a source of bandage-like 3D chitin scaffolds [ 27 ]. These scaffolds are naturally obtained by cleaning the sponge chitinous skeleton and are suitable for culture of cardiomyocytes differentiated from human induced pluripotent stem cells (iPSC-CMs), showing promise for the development of sponge chitin-based absorbable haemostats [ 27 ]. The biomedical potential of sponge-derived 3D chitinous scaffolds is extensive, as chondrocytes cultured in structures originated from various marine sponges are able to synthesize an ECM similar to that found in other cartilage tissue engineering constructs both in vitro and in vivo [ 55 ]. In a different approach, the natural 3D chitinous scaffold obtained from the skeleton of Ianthella flabelliformis was used as drug delivery biomaterial [ 28 ]. The prospective for the development of sponge chitin-based biomaterials for biomedical applications is auspicious, as recently many new and renewable sources of this polysaccharide have been discovered [30–33]. 2.2. Bioactive Molecules The interest in bioactive secondary metabolites from marine invertebrates started in the 1950s with the identification of modified nucleotides in extracts of the Caribbean sponge Cryptotethya crypta [ 56 ]. This discovery led to the development of cytarabine (Cytosar-U ® ) [ 57 ], the first drug on the market derived from a marine natural product. Since then, marine sponges (Figure 1) have remained a rich source of structurally diverse natural products, and 2639 new compounds were reported between 2010 and 2019 as reviewed in the ‘Marine Natural Products’ review articles published annually in Natural Products Reports, e.g., [ 58 – 60 ]. The majority of these compounds, more than 1700 or 65%, were initially described as bioactive. The dominating structural classes were terpenes (35%), alkaloids (29%) and lipids (22%), whereas peptides, polyketides and other Mar. Drugs 2022,20, 219 7 of 45 molecules constituted 14% of the compounds (Table S1). The majority (about 50%) of the bioactive compounds showed cytotoxicity towards tumour cell lines (Figure 2). Compounds with antimicrobial/antibacterial bioactivity are the second most represented (14%). Interestingly, several MNPs from sponges were identified as possible inhibitors of protein phosphatase [ 61 ], proteasome [ 62 ], topoisomerase [ 63 ] and other key proteins and enzymes involved in cell cycle and apoptosis induction (Table S1). Mar. Drugs 2022, 20, x 7 of 48 structures originated from various marine sponges are able to synthesize an ECM similar to that found in other cartilage tissue engineering constructs both in vitro and in vivo [55]. In a different approach, the natural 3D chitinous scaffold obtained from the skeleton of Ianthella flabelliformis was used as drug delivery biomaterial [28]. The prospective for the development of sponge chitin-based biomaterials for biomedical applications is auspicious, as recently many new and renewable sources of this polysaccharide have been discovered [30–33]. 2.2. Bioactive Molecules The interest in bioactive secondary metabolites from marine invertebrates started in the 1950s with the identification of modified nucleotides in extracts of the Caribbean sponge Cryptotethya crypta [56]. This discovery led to the development of cytarabine (Cytosar-U®) [57], the first drug on the market derived from a marine natural product. Since then, marine sponges (Figure 1) have remained a rich source of structurally diverse natural products, and 2639 new compounds were reported between 2010 and 2019 as reviewed in the ‘Marine Natural Products’ review articles published annually in Natural Products Reports, e.g., [58–60]. The majority of these compounds, more than 1700 or 65%, were initially described as bioactive. The dominating structural classes were terpenes (35%), alkaloids (29%) and lipids (22%), whereas peptides, polyketides and other molecules constituted 14% of the compounds (Table S1). The majority (about 50%) of the bioactive compounds showed cytotoxicity towards tumour cell lines (Figure 2). Compounds with antimicrobial/antibacterial bioactivity are the second most represented (14%). Interestingly, several MNPs from sponges were identified as possible inhibitors of protein phosphatase [61], proteasome [62], topoisomerase [63] and other key proteins and enzymes involved in cell cycle and apoptosis induction (Table S1). Figure 1. The demosponge Smenospongia aurea (left) and Aplysina fistularis (right). Photo by Joseph Pawlik (https://spongeguide.uncw.edu/, accessed on 13 February 2022). Sponges are well known for containing substantial amounts of symbiotic microorganisms, including bacteria, fungi and microalgae. It has thus long been assumed that many of the structurally diverse and bioactive secondary metabolites originally isolated from sponge extracts are produced by such microorganisms. It is therefore fair to say that the sponge holobiome is the source of the rich chemical diversity. Figure 1. The demosponge Smenospongia aurea ( left ) and Aplysina fistularis ( right ). Photo by Joseph Pawlik (https://spongeguide.uncw.edu/, accessed on 13 February 2022). Mar. Drugs 2022, 20, x 8 of 48 Figure 2. Proportion of different bioactivity associated to sponge-derived MNP, according to data in Table S1. Technical developments in molecular biology and analytical chemistry, especially within the field of omics, have made it possible to shed some light on the underlying mechanisms of production of sponge secondary metabolites. Renieramycins are a group of about 30 cytotoxic and antimicrobial tetrahydroisoquinoline quinones isolated from sponges belonging to the genera Haliclona, Xestospongia and Neopetrosia. Tianero et al. [64] demonstrated that renieramycin E (Figure 3) extracted from Haliclona sponges collected in Papua New Guinea was actually produced by the symbiont Endohaliclona renieramycinifaciens. Due to genome reduction, the symbiont has lost its capability for free living, and lives encapsulated in dedicated bacteriocytes. The term ‘chemobacteriocytes’ was introduced to describe such specialized sponge cells where symbiotic bacteria are living within the sponge, receive nutrients and are sheltered from bacterial competitors while they produce defence molecules that benefit the host sponge. Indeed, E. renieramycinifaciens live in the chemobacteriocytes where they produce renieramycins, which protects the Haliclona sponge from predators and pathogenic microorganisms. It is interesting to note that the distantly related bacteria Endoecteinascidia frumentensis is producing the chemically related defensive compounds ecteinascidins in a similar symbiosis with ascidians [65]. Thus, the renieramycins have the same chemical scaffold as ecteinascidin-743 (Figure 3 (2)), which is an approved drug for the treatment of ovarian cancer and advanced soft tissue carcinoma first isolated from the colonial ascidian Ecteinascidia turbinata, marketed under the commercial name Yondelis. This underscores the potential use of renieramycins as potential lead compounds for developing new anticancer drugs. 1 2 Figure 3. Renieramycin E (1) and ecteinascidin-743 (2). Figure 2. Proportion of different bioactivity associated to sponge-derived MNP, according to data in Table S1. Sponges are well known for containing substantial amounts of symbiotic microorganisms, including bacteria, fungi and microalgae. It has thus long been assumed that many of the structurally diverse and bioactive secondary metabolites originally isolated from sponge extracts are produced by such microorganisms. It is therefore fair to say that the sponge holobiome is the source of the rich chemical diversity. Technical developments in molecular biology and analytical chemistry, especially within the field of omics, have made it possible to shed some light on the underlying mechanisms of production of sponge secondary metabolites. Renieramycins are a group of about 30 cytotoxic and antimicrobial tetrahydroisoquinoline quinones isolated from sponges belonging to the genera Haliclona,Xestospongia and Neopetrosia. Tianero et al. [64] demonstrated that renieramycin E (Figure 3) extracted from Haliclona sponges collected in Papua New Guinea was actually produced by the symbiont Endohaliclona renieramycinifaciens. Due to genome reduction, the symbiont has lost its capability for free living, and lives encapsulated in dedicated bacteriocytes. The term ‘chemobacteriocytes’ was introduced Mar. Drugs 2022,20, 219 8 of 45 to describe such specialized sponge cells where symbiotic bacteria are living within the sponge, receive nutrients and are sheltered from bacterial competitors while they produce defence molecules that benefit the host sponge. Indeed, E. renieramycinifaciens live in the chemobacteriocytes where they produce renieramycins, which protects the Haliclona sponge from predators and pathogenic microorganisms. It is interesting to note that the distantly related bacteria Endoecteinascidia frumentensis is producing the chemically related defensive compounds ecteinascidins in a similar symbiosis with ascidians [ 65 ]. Thus, the renieramycins have the same chemical scaffold as ecteinascidin-743 (Figure 3( 2 )), which is an approved drug for the treatment of ovarian cancer and advanced soft tissue carcinoma first isolated from the colonial ascidian Ecteinascidia turbinata, marketed under the commercial name Yondelis. This underscores the potential use of renieramycins as potential lead compounds for developing new anticancer drugs. Mar. Drugs 2022, 20, x 8 of 48 Figure 2. Proportion of different bioactivity associated to sponge-derived MNP, according to data in Table S1. Technical developments in molecular biology and analytical chemistry, especially within the field of omics, have made it possible to shed some light on the underlying mechanisms of production of sponge secondary metabolites. Renieramycins are a group of about 30 cytotoxic and antimicrobial tetrahydroisoquinoline quinones isolated from sponges belonging to the genera Haliclona, Xestospongia and Neopetrosia. Tianero et al. [64] demonstrated that renieramycin E (Figure 3) extracted from Haliclona sponges collected in Papua New Guinea was actually produced by the symbiont Endohaliclona renieramycinifaciens. Due to genome reduction, the symbiont has lost its capability for free living, and lives encapsulated in dedicated bacteriocytes. The term ‘chemobacteriocytes’ was introduced to describe such specialized sponge cells where symbiotic bacteria are living within the sponge, receive nutrients and are sheltered from bacterial competitors while they produce defence molecules that benefit the host sponge. Indeed, E. renieramycinifaciens live in the chemobacteriocytes where they produce renieramycins, which protects the Haliclona sponge from predators and pathogenic microorganisms. It is interesting to note that the distantly related bacteria Endoecteinascidia frumentensis is producing the chemically related defensive compounds ecteinascidins in a similar symbiosis with ascidians [65]. Thus, the renieramycins have the same chemical scaffold as ecteinascidin-743 (Figure 3 (2)), which is an approved drug for the treatment of ovarian cancer and advanced soft tissue carcinoma first isolated from the colonial ascidian Ecteinascidia turbinata, marketed under the commercial name Yondelis. This underscores the potential use of renieramycins as potential lead compounds for developing new anticancer drugs. 1 2 Figure 3. Renieramycin E (1) and ecteinascidin-743 (2). Figure 3. Renieramycin E (1) and ecteinascidin-743 (2). Another investigation used molecular networking to study the chemical diversity of extracts from Smenospongia aurea and the bloom-forming cyanobacterium Trichodesmium sp., revealing substantial overlap in the metabolomes of the two samples including the cytotoxic smenamides, smenothiazoles and conulothiazoles [ 66 ]. The presence of typical cyanobacterial structural motives in these bioactive secondary metabolites found in the sponge also lends support to their hypothesized cyanobacterial origin. Traditionally, chemical investigations of sponges have been based on the identification of major secondary metabolites in crude extracts, either based on bioassay-guided fractionation or chemical analysis as exemplified above. However, more information can be obtained by combining mass spectrometry imaging (MSI) with morphological studies. Using a combination of molecular networking based on HR-MS/MS data, MALDI-MSI and fluorescence microscopy, Cantrell et al. were able to demonstrate that smenamide A and B, (Figure 4, 3 and 4, respectively) co-localized with cyanobacterial cells in the ectosome of a specimen of S. aurea. This observation lends further support to the hypothesis that smenamides are indeed produced by a cyanobacteria [ 67 ]. A number of other unrelated secondary metabolites were identified and investigated in the same study. Mar. Drugs 2022, 20, x 9 of 48 Another investigation used molecular networking to study the chemical diversity of extracts from Smenospongia aurea and the bloom-forming cyanobacterium Trichodesmium sp., revealing substantial overlap in the metabolomes of the two samples including the cytotoxic smenamides, smenothiazoles and conulothiazoles [66]. The presence of typical cyanobacterial structural motives in these bioactive secondary metabolites found in the sponge also lends support to their hypothesized cyanobacterial origin. Traditionally, chemical investigations of sponges have been based on the identification of major secondary metabolites in crude extracts, either based on bioassay-guided fractionation or chemical analysis as exemplified above. However, more information can be obtained by combining mass spectrometry imaging (MSI) with morphological studies. Using a combination of molecular networking based on HR-MS/MS data, MALDI-MSI and fluorescence microscopy, Cantrell et al. were able to demonstrate that smenamide A and B, (Figure 4, 3 and 4, respectively) co-localized with cyanobacterial cells in the ectosome of a specimen of S. aurea. This observation lends further support to the hypothesis that smenamides are indeed produced by a cyanobacteria [67]. A number of other unrelated secondary metabolites were identified and investigated in the same study. 3 4 Figure 4. Smenamide A; (3) smenamide B (4). The tryptophan derivative aplysinopsin 5 (Figure 5) [68] was dereplicated through molecular networking, and MSI analysis revealed that this compound was more or less evenly distributed throughout the sponge, giving no clues about its biosynthetic origin. Aplysinopsin is interesting for its diverse biological activities, including neuro-modulating activities [69]; it would therefore be of interest to identify more analogues in order to investigate the structure–activity relationship between this molecular framework and relevant biological targets. Figure 5. Aplysinopsin. Considering that the true producers of bioactive natural products isolated from extracts of marine sponges, in many cases, seem to be uncultivable microorganisms, and the fact that is it difficult to activate biosynthetic gene clusters (BGC) for secondary metabolite production when microorganisms are grown in cultures, there is still a great interest in using sponges, collected at various different geographical sites in different seasons, for identifying novel natural products. 3. Cnidarians Figure 4. Smenamide A; (3) smenamide B (4). Mar. Drugs 2022,20, 219 9 of 45 The tryptophan derivative aplysinopsin 5 (Figure 5) [ 68 ] was dereplicated through molecular networking, and MSI analysis revealed that this compound was more or less evenly distributed throughout the sponge, giving no clues about its biosynthetic origin. Aplysinopsin is interesting for its diverse biological activities, including neuro-modulating activities [ 69 ]; it would therefore be of interest to identify more analogues in order to investigate the structure–activity relationship between this molecular framework and relevant biological targets. Mar. Drugs 2022, 20, x 9 of 48 Another investigation used molecular networking to study the chemical diversity of extracts from Smenospongia aurea and the bloom-forming cyanobacterium Trichodesmium sp., revealing substantial overlap in the metabolomes of the two samples including the cytotoxic smenamides, smenothiazoles and conulothiazoles [66]. The presence of typical cyanobacterial structural motives in these bioactive secondary metabolites found in the sponge also lends support to their hypothesized cyanobacterial origin. Traditionally, chemical investigations of sponges have been based on the identification of major secondary metabolites in crude extracts, either based on bioassay-guided fractionation or chemical analysis as exemplified above. However, more information can be obtained by combining mass spectrometry imaging (MSI) with morphological studies. Using a combination of molecular networking based on HR-MS/MS data, MALDI-MSI and fluorescence microscopy, Cantrell et al. were able to demonstrate that smenamide A and B, (Figure 4, 3 and 4, respectively) co-localized with cyanobacterial cells in the ectosome of a specimen of S. aurea. This observation lends further support to the hypothesis that smenamides are indeed produced by a cyanobacteria [67]. A number of other unrelated secondary metabolites were identified and investigated in the same study. 3 4 Figure 4. Smenamide A; (3) smenamide B (4). The tryptophan derivative aplysinopsin 5 (Figure 5) [68] was dereplicated through molecular networking, and MSI analysis revealed that this compound was more or less evenly distributed throughout the sponge, giving no clues about its biosynthetic origin. Aplysinopsin is interesting for its diverse biological activities, including neuro-modulating activities [69]; it would therefore be of interest to identify more analogues in order to investigate the structure–activity relationship between this molecular framework and relevant biological targets. Figure 5. Aplysinopsin. Considering that the true producers of bioactive natural products isolated from extracts of marine sponges, in many cases, seem to be uncultivable microorganisms, and the fact that is it difficult to activate biosynthetic gene clusters (BGC) for secondary metabolite production when microorganisms are grown in cultures, there is still a great interest in using sponges, collected at various different geographical sites in different seasons, for identifying novel natural products. 3. Cnidarians Figure 5. Aplysinopsin. Considering that the true producers of bioactive natural products isolated from extracts of marine sponges, in many cases, seem to be uncultivable microorganisms, and the fact that is it difficult to activate biosynthetic gene clusters (BGC) for secondary metabolite production when microorganisms are grown in cultures, there is still a great interest in using sponges, collected at various different geographical sites in different seasons, for identifying novel natural products. 3. Cnidarians The phylum Cnidaria includes over 11,000 species distributed among Anthozoa (sea anemones, corals, sea pens), Scyphozoa (jellyfish), Cubozoa (box jellies), Hydrozoa (hydroids) and Staurozoa. The phylum is named after cnidae—the stinging organ which contains toxins used for prey capture and defence. 3.1. Biomaterials Corals are the main sources for biomaterials among cnidarians [ 15 ]. Their skeleton is composed of calcium carbonate mainly in the form of aragonite, which is a natural bioceramic. Stony coral skeleton can be used for biomedical applications as its mineral is stable with highly organized porous structure. One of the coral-derived materials is hydroxyapatite [ 70 ], which is structurally similar to human bone [ 71 ], biocompatible, nontoxic, biodegradable and of low immunogenicity [ 72 ]. Coral skeleton has been used both as hard scaffold for bone repair [ 73 , 74 ] as well as in collagen-based scaffold [ 75 ]. In addition, soft corals (Octocorallia), for which the mechanical properties of the skeleton depend on environmental conditions, such as the increased stiffness of deep-sea coral [ 72 ], also have high potential for biomimetic and biomedical applications [ 76 ]. The skeleton structure of black corals (Antipatharia) contains chitin that is also biocompatible and serves as a template for cell adhesion and differentiation [77]. Collagen from scyphomedusae is also an interesting resource [ 78 ]. Collagen extracted from Rhizostoma pulmo found application in tissue engineering, and several assays confirm that jelly-derived scaffolds have optimal adsorption and biocompatibility properties [ 79 ]. The Welsh company Jellagen is exploring not only the collagen derived from this jellyfish species but also derived biomaterials, as hydrogels and scaffolds, targeting regenerative medicine and biomedical research. In this regard, jellyfish collagen scaffolds have shown suitability for bone regeneration [ 80 ], and hydrogels produced by combination with other marine biopolymers are being proposed for tissue engineering using green processing approaches [ 81 , 82 ]. Other species of jellyfish have been also explored as sources of collagen for biomedicine, namely the Nomura’s jellyfish (formerly named Stomolophus nomurai meleagris Mar. Drugs 2022,20, 219 16 of 45 cancers, solid tumours and some leukaemias, and are therefore under clinical evaluation. Synthetic analogues of dolastatins, auristatins (POLIVY ® ), in a form conjugated to antibodies, have been recently approved against several tumours, including relapsed/refractory multiple myeloma, metastatic urothelial cancer and non-Hodgkin’s lymphoma (https:// www.marinepharmacology.org/approved, accessed on 13 February 2022). Another interesting group of gastropod molluscs producing bioactive compounds is represented by the Conidae. This group evolved a highly diversified family of neurotoxic peptides, named conotoxins, used as chemical weapons to paralyse their prey. Unlike most bioactive secondary metabolites isolated so far, conotoxins are direct gene products, deriving from post-translational modification of large protein precursors. Conopeptide biosynthesis occurs in the highly specialized venom gland. The up-regulation, in this structure, of peptidylprolyl cis-trans isomerase, suggests a potential and specialized role for this enzyme in the in vivo folding of conopeptides. The research on conotoxins began in the early 1970s and the first venom component characterized was alpha-conotoxin GI, a small peptide that proved to be a competitive nicotinic antagonist [ 140 ]. Since then, the research has been growing exponentially, and currently it is estimated that more than 80,000 natural conotoxins exist in various cone snails (Figure 12) around the world and each Conus species may possess an average of 100–200 conotoxins [141,142]. Mar. Drugs 2022, 20, x 17 of 48 bodies, have been recently approved against several tumours, including relapsed/refractory multiple myeloma, metastatic urothelial cancer and non-Hodgkin’s lymphoma (https://www.marinepharmacology.org/approved, 13/02/2022). Another interesting group of gastropod molluscs producing bioactive compounds is represented by the Conidae. This group evolved a highly diversified family of neurotoxic peptides, named conotoxins, used as chemical weapons to paralyse their prey. Unlike most bioactive secondary metabolites isolated so far, conotoxins are direct gene products, deriving from post-translational modification of large protein precursors. Conopeptide biosynthesis occurs in the highly specialized venom gland. The up-regulation, in this structure, of peptidylprolyl cis-trans isomerase, suggests a potential and specialized role for this enzyme in the in vivo folding of conopeptides. The research on conotoxins began in the early 1970s and the first venom component characterized was alpha-conotoxin GI, a small peptide that proved to be a competitive nicotinic antagonist [140]. Since then, the research has been growing exponentially, and currently it is estimated that more than 80,000 natural conotoxins exist in various cone snails (Figure 12) around the world and each Conus species may possess an average of 100–200 conotoxins [141,142]. Figure 12. The gastropod Conus textile. (Courtesy of Dr. Ernesto Mollo.) Conotoxins generally consist of 10 to 40 amino acid residues derived from RNA-encoded precursor proteins. Their structure is consolidated by the presence of two to four or more disulphide bonds, which provides an efficient protection from proteases. The enormous variety of conotoxins is due to hypermutation of conotoxin-encoding sequences, fragment insertion/deletion, and mutation-induced premature termination [143]. Research on conotoxins has provided numerous scientific and societal benefits, including their use as diagnostic agents, drug leads, as well as research tools in neuroscience, pharmacology, biochemistry, structural biology, and molecular evolution. In the last few years, a number of reviews focusing on conotoxins have been published [144–146], where additional information can be found. The growing interest towards conotoxins for pharmaceutical applications is based on their capability to bind various types of ion channels, thus interfering with the transmission of the neuronal impulse. The synthetic analogue of omega-conotoxin MVIIA, known as ziconotide (Prialt ® ), is a potent analgesic, approved since 2004 for the management of severe chronic pain in humans. 5. Echinoderms The phylum Echinodermata includes about 7000 species, making it the second-largest grouping of deuterostomes and the largest phylum that has no freshwater or terrestrial members. Echinoderms in the adult form live on ocean floors where they play an important role in benthic ecosystems. The echinoderms fall into five clades with overlapping, yet diverse, characteristics: Echinoidea (sea urchins) (Figure 13, left); Holothuroidea (sea cucumbers); Crinoidea (sea lilies, feather stars); Asteroidea (sea stars, starfish) (Figure 13, right); and Ophiuroidea (brittle stars). Echinoderms are known producers of bioactive glycosylated metabolites, dominated by steroidal and sulphated metabolites, saponins and glycolipids [147]. Figure 12. The gastropod Conus textile. (Courtesy of Dr. Ernesto Mollo.) Conotoxins generally consist of 10 to 40 amino acid residues derived from RNAencoded precursor proteins. Their structure is consolidated by the presence of two to four or more disulphide bonds, which provides an efficient protection from proteases. The enormous variety of conotoxins is due to hypermutation of conotoxin-encoding sequences, fragment insertion/deletion, and mutation-induced premature termination [ 143 ]. Research on conotoxins has provided numerous scientific and societal benefits, including their use as diagnostic agents, drug leads, as well as research tools in neuroscience, pharmacology, biochemistry, structural biology, and molecular evolution. In the last few years, a number of reviews focusing on conotoxins have been published [ 144 – 146 ], where additional information can be found. The growing interest towards conotoxins for pharmaceutical applications is based on their capability to bind various types of ion channels, thus interfering with the transmission of the neuronal impulse. The synthetic analogue of omega-conotoxin MVIIA, known as ziconotide (Prialt ® ), is a potent analgesic, approved since 2004 for the management of severe chronic pain in humans. 5. Echinoderms The phylum Echinodermata includes about 7000 species, making it the second-largest grouping of deuterostomes and the largest phylum that has no freshwater or terrestrial members. Echinoderms in the adult form live on ocean floors where they play an important role in benthic ecosystems. The echinoderms fall into five clades with overlapping, yet diverse, characteristics: Echinoidea (sea urchins) (Figure 13, left); Holothuroidea (sea cucumbers); Crinoidea (sea lilies, feather stars); Asteroidea (sea stars, starfish) (Figure 13, right); and Ophiuroidea (brittle stars). Echinoderms are known produc- Mar. Drugs 2022,20, 219 17 of 45 ers of bioactive glycosylated metabolites, dominated by steroidal and sulphated metabolites, saponins and glycolipids [147]. Mar. Drugs 2022, 20, x 18 of 48 Figure 13. The sea urchin Paracentrotus lividus (left) and the sea star Echinaster sepositus (right) (photo by Federico Betti). 5.1. Biomaterials Echinoderms are a valid source of biomaterials that can be used for various biomedical applications and can be grouped in two main categories: (1) the ‘structural’ materials, mainly used to develop scaffolds for regenerative medicine and tissue engineering; and (2) bioadhesives for different applied fields (Table 2). Among the former, exploitation of echinoderm ECM components, and particularly of collagen, are currently the most active research field. Indeed, marine collagens are a hot topic in biomaterial development [148] as they can represent a valid alternative to mammalian collagen in tissue engineering applications. Collagen has been successfully extracted from various echinoderm taxa, including sea urchins, sea cucumbers and starfish [149]. The collagenous ECM of echinoderms possesses structural features and mechanical properties that are similar to mammalian ECM. However, even more, these animals possess mutable collagenous tissues (MCTs) [150]. MCTs are peculiar echinoderm connective tissues with unique properties in the animal kingdom, being able to undergo drastic, rapid and reversible changes in their mechanical properties [151]. Echinoderm collagen is easily obtained in its native fibrillar form, still superficially decorated by glycosaminoglycans [149,152] and thus fully preserving its structural and functional features. This allows its use for the production of highly biomimetic and mechanically-resistant devices which can include thin membranes or 3D scaffolds for tissue regeneration [153]. Of note is that these devices improved skin regeneration when applied in an in vivo (sheep) model [154]. Recently, MCT-derived collagen has reached the market and is currently one of the top products of a high-tech company devoted to the discovery and development of marine natural compounds (https://mebioscience.com, accessed on 13 December 2022). Similarly, starfish body wall extracts, which mostly consist of ECM components, particularly collagen, are one of the main ingredients of a newly branded anti-age cream produced by a Korean company (https://www.koreanqueens.com/en/home/returning-starfish-cream.html, accessed on 13 December 2022). Therefore, echinoderms, and sea urchins particularly, are innovative and alternative collagen sources to produce efficient guided tissue regeneration membranes [149]. In addition to being the direct source of raw material, MCT has also been the source of inspiration for biomaterial development, particularly stimuli-responsive synthetic nanocomposites mimicking the unique mechanical properties of MCTs [155,156]. These echinoderminspired biomimetic materials (which are composed of cellulose whiskers and synthetic polymers) were developed to produce mechanically compliant brain microelectrodes [157]. Among the structural materials, echinoderm skeleton was used to obtain bioceramics. The skeletal elements of these animals are made of calcite (CaCO3) with high magnesium content and are arranged in stereomes displaying a highly porous and trabecular structure [158]. This provides a high resistance of the skeletal pieces combined with a light weight. This particular architecture originally stimulated the development of biomaterials Figure 13. The sea urchin Paracentrotus lividus ( left ) and the sea star Echinaster sepositus ( right ) (photo by Federico Betti). 5.1. Biomaterials Echinoderms are a valid source of biomaterials that can be used for various biomedical applications and can be grouped in two main categories: (1) the ‘structural’ materials, mainly used to develop scaffolds for regenerative medicine and tissue engineering; and (2) bioadhesives for different applied fields (Table 2). Among the former, exploitation of echinoderm ECM components, and particularly of collagen, are currently the most active research field. Indeed, marine collagens are a hot topic in biomaterial development [ 148 ] as they can represent a valid alternative to mammalian collagen in tissue engineering applications. Collagen has been successfully extracted from various echinoderm taxa, including sea urchins, sea cucumbers and starfish [ 149 ]. The collagenous ECM of echinoderms possesses structural features and mechanical properties that are similar to mammalian ECM. However, even more, these animals possess mutable collagenous tissues (MCTs) [ 150 ]. MCTs are peculiar echinoderm connective tissues with unique properties in the animal kingdom, being able to undergo drastic, rapid and reversible changes in their mechanical properties [ 151 ]. Echinoderm collagen is easily obtained in its native fibrillar form, still superficially decorated by glycosaminoglycans [ 149 , 152 ] and thus fully preserving its structural and functional features. This allows its use for the production of highly biomimetic and mechanically-resistant devices which can include thin membranes or 3D scaffolds for tissue regeneration [ 153 ]. Of note is that these devices improved skin regeneration when applied in an in vivo (sheep) model [ 154 ]. Recently, MCT-derived collagen has reached the market and is currently one of the top products of a high-tech company devoted to the discovery and development of marine natural compounds (https://mebioscience.com, accessed on 13 February 2022). Similarly, starfish body wall extracts, which mostly consist of ECM components, particularly collagen, are one of the main ingredients of a newly branded anti-age cream produced by a Korean company (https://www.koreanqueens.com/en/home/returning-starfish-cream.html, accessed on 13 February 2022). Therefore, echinoderms, and sea urchins particularly, are innovative and alternative collagen sources to produce efficient guided tissue regeneration membranes [ 149 ]. In addition to being the direct source of raw material, MCT has also been the source of inspiration for biomaterial development, particularly stimuli-responsive synthetic nanocomposites mimicking the unique mechanical properties of MCTs [ 155 , 156 ]. These echinoderm-inspired biomimetic materials (which are composed of cellulose whiskers and synthetic polymers) were developed to produce mechanically compliant brain microelectrodes [157]. Among the structural materials, echinoderm skeleton was used to obtain bioceramics. The skeletal elements of these animals are made of calcite (CaCO 3 ) with high magnesium content and are arranged in stereomes displaying a highly porous and trabecular structure [ 158 ]. This provides a high resistance of the skeletal pieces combined with a light Mar. Drugs 2022,20, 219 18 of 45 weight. This particular architecture originally stimulated the development of biomaterials designed by replication of the skeletal microstructure [ 159 ]. In an attempt to find new solutions for bone regeneration, some research was addressed to directly use the porous calcitic architecture as a three-dimensional scaffold for seeding mammalian cells (e.g., osteoblasts), thus providing preliminary evidence of the biocompatibility of the calcite stereome [ 160 , 161 ]. Alternatively, the calcitic skeleton of some sea urchin species was converted to calcium phosphate salts (e.g., hydroxyapatite) via hydrothermal/chemical reaction in order to obtain novel materials for bone regeneration [ 162 , 163 ]. Similar to the ECM components, echinoderm skeleton was also used as source of inspiration for material design rather than directly used for its development. Particularly, the microstructural organization of sea urchin spines (highly ordered nanoparticles in a biomineral mesocrystal matrix) inspired the design of an elastic concrete material that might be used in future construction processes [164]. Lastly, bioadhesives from echinoderms are a recent field of investigation in material science. It is well known that starfish and sea urchins temporarily but firmly attach to the substrate thanks to a duo-gland adhesive system relying on both adhesive and deadhesive secretions [165]. Table 2. Biomaterials from echinoderms. (Sorted alphabetically according to Class of Biomaterial.) Class Producer Species Family/Class of Biomaterial Biomaterial Origin/Structural Component Possible Applications References Holoturoidea Sea cucumbers (and other echinoderms) Proteins/neutral carbohydrates Mutable collagenous tissue (MCT) components Mutable collagenous tissue/ECM components design of an MCT-inspired synthetic material [166] Holoturoidea Sea cucumbers (and other echinoderms) Proteins/neutral carbohydrates Mutable collagenous tissue (MCT) components dermis/ECM components design of an MCT-inspired stimuli-responsive synthetic nanocomposite [156,157] Holoturoidea Sea cucumbers Proteins/neutral carbohydrates Mutable collagenous tissue (MCT) components dermis/ECM components design of mechanically tunable synthetic biomaterials [167] Holoturoidea Sea cucumbers Proteins/neutral carbohydrates Mutable collagenous tissue (MCT) components dermis/ECM components biomimetic design of artificial polymer nanocomposites [168] Holoturoidea Holothuria forskal, H. leucospilota, B. subrubra, P. graeffei Proteins/neutral carbohydrates Proteins rich in small side amino acid Cuvier tubule bioadhesives [165] Holoturoidea Holothuria tubulosa Proteins Collagen dermis/ECM components membranes for guided tissue regeneration [149,152] Asteroidea Pisaster giganteous Bioceramics High-magnesium calcite ossicles (skeletal microstructure) scaffold for mammalian cell culture [161] Asteroidea Asterias rubens Proteins/glycosylated proteins Glycosylated proteins tube feet bioadhesives [169,170] Asteroidea Echinaster sepositus Proteins Collagen dermis/ECM components membranes for guided tissue regeneration [149] Asteroidea Asterias rubens Proteins Sea star footprint protein 1 (Sfp1) tube feet bioadhesives [165,171] Echinoidea Heart urchins Bioceramics High-magnesium calcite ossicles production of bioceramic nanopowder [172] Echinoidea Sea urchins Bioceramics High-magnesium calcite ossicles (skeletal microstructure) production of structured hydroxyapatite material [173] Echinoidea Sea urchin Bioceramics High-magnesium calcite spine bio-inspired design of super-resistant concrete materials [164] Echinoidea Tripneustes gratilla Bioceramics High-magnesium calcite ossicles (skeletal microstructure) production of magnesium substituted β-tricalcium phosphate for bone graft materials [162] Echinoidea Paracentrotus lividus ECM components Collagen peristomial membrane/ECM components membranes/scaffolds for tissue regeneration [149,152] Mar. Drugs 2022,20, 219 19 of 45 Table 2. Cont. Class Producer Species Family/Class of Biomaterial Biomaterial Origin/Structural Component Possible Applications References Echinoidea Paracentrotus lividus ECM components Mutable collagenous tissue (MCT) components peristomial membrane/ECM components decellularized membranes for invertebrate cell culture [174] Echinoidea Paracentrotus lividus Proteins tube feet bioadhesives [175] Ophiuroidea Brittle stars Bioceramics High-magnesium calcite dorsal arm plates (microstructure) brittle-star-inspired micro-lens [176] Some of the key proteins of the attachment glue have been identified, and their molecular structure was characterized in an attempt to produce recombinant protein for the development of synthetic adhesive. Indeed, understanding the mechanisms and the key actors behind this very fast adhesion–de-adhesion process can be instrumental for the design of water-resistant adhesives. Recently, fractions of the adhesive protein Sfp1 identified in the adhesive of Asterias rubens were obtained by recombinant production and proposed as a material to develop coatings for various biomedical applications [ 177 ] and development of new antifouling strategies [165]. 5.2. Bioactive Molecules Echinoidea include 950 species distributed across all the oceans, from tropical to polar climates, and inhabiting marine benthic zones from the intertidal to 5000 m. Sea urchins contain several edible species for which effective culture methods have been developed. However, sea urchins have also generated interest for their peculiar defence system which protects them against microbial infections and fouling [178]. Accordingly, antimicrobial activities were identified in sea urchin coelomocytes or coelomic fluid [ 179 ], and several antimicrobial compounds were isolated. Among them, antimicrobial peptides, short cationic peptides containing positively-charged amino acid residues, were identified in various sea urchin species. Among them were strongylocins [ 180 , 181 ], centrocin 1 and 2 [ 182 , 183 ] or their analogues [ 184 , 185 ], and paracentrin 1 [ 186 , 187 ]. Sea urchin pigments, which are found in test spines but also in coelomocytes or gonads, gave rise to powerful antioxidant compounds [188]. Sea cucumbers belong to the clade Holothuroidea, globally found in deep seas and benthic areas. The body wall of these marine invertebrates contains most of their active constituents, mainly polysaccharides and collagen, which exhibit numerous biological activities, including anticancer, anti-hypertensive, anti-angiogenic, anti-inflammatory, antidiabetic, anticoagulation, antimicrobial, antioxidant, and anti-osteoclastogenic properties [ 189 ]. It also contains sea cucumber saponins, cerebrosides and gangliosides [ 190 , 191 ]. Starfish (class Asteroidea, around 1800 species) are benthic animals that inhabit diversified ocean ecosystems from rocky beaches to the deep sea. Most bioactive compounds, isolated from whole body extracts, are steroids and their glycoside derivatives, including asterosaponins and polyhydroxy steroid glycosides. They are frequently sulphated in the steroid and/or glycoside portions. Starfish-derived compounds have been shown to be promising anticancer and anti-inflammatory agents, but also present neuritogenic, antimicrobial and antifouling properties. Their in vitro cytotoxic effects against various cancer cell lines have been studied through the evaluation of IC50 data, inhibition rates and colony formation [147]. Overall, this phylum displays a great variety of bioactive compounds, most of them (38%) with cytotoxic or anti-proliferative activities (Figure 14, Table S4). Products displaying anticoagulant activity (16%), i.e., fucoidans and other sulphated polysaccharides, seem characteristics of echinoderms, compared to other invertebrates. Another characteristic bioactivity associated to MNP from echinoderms is the neuritogenic effect displayed by gangliosides isolated, for example, from the starfish Asterias amurensis [ 192 ]. Antimicrobial and antioxidant activities represent a lower proportion, accounting for 9% and 7% of the total bioactive compounds, respectively. Mar. Drugs 2022,20, 219 20 of 45 Mar. Drugs 2022, 20, x 21 of 48 seem characteristics of echinoderms, compared to other invertebrates. Another characteristic bioactivity associated to MNP from echinoderms is the neuritogenic effect displayed by gangliosides isolated, for example, from the starfish Asterias amurensis [192] . Antimicrobial and antioxidant activities represent a lower proportion, accounting for 9% and 7% of the total bioactive compounds, respectively. Figure 14. Proportion of different bioactivity associated to echinoderm-derived MNPs, according to data in Table S4. Echinoids are an important source of natural polyhydroxynaphthoquinones (PHNQ), of the 1,4-naphthoquinones group [193]. Investigation of the red-dark pigments derived from the spines and shells of sea urchins led to the identification of a sub-family of PHNQ called spinochromes and echinochrome A 15 (Figure 15); spinochromes A–E (e.g., spinochromes E 16, Figure 15) are the best known and the more accessible molecule of this class [194]. Found in many sea urchin species, spinochromes are also expressed in coelomocytes, eggs, ovaries and larvae [195]. Their chemical structures were progressively uncovered so that the molecules and various derivatives were synthesized in the laboratory [196–199]. A distinctive feature of these compounds is their ability to effectively intercept free radicals and bind the Fe 2+ ions responsible for the formation of reactive oxygen species (ROS) [200–202]. The spinochromes A–E show antioxidant activity but with distinct efficiencies and cytotoxicities on human cells [194]. Spinochrome D attenuates doxorubicin-induced mitochondrial damage in human cardiomyocyte cell line AC16 and human breast cancer cell line MCF-7 [203]. Spinochrome E and echinochrome A (see below) were produced by cultured coelomocytes [204], paving the way to the generation of sea urchin cell cultures producing complex bioactive compounds with therapeutic potential. Echinochrome A, first isolated from the spines of sea urchin Stomopneustes variolaris [205], acts as a cardioprotective agent and was further used, in a water-soluble form, as the active substance in the drug Histochrome introduced in Russia for preventing ischemia/reperfusion injury and ophthalmopathic complications [206–208]. The antioxidant and anti-inflammatory capabilities of echinochrome A are responsible for the cardioprotective effect [201,209]. Indeed, echinochrome A attenuates the oxidative stress caused by ROS. More recent reports suggest that it could be a candidate molecule to promote cardiac regeneration [210–212]. Echinochrome A was also recently investigated in the framework of stem cell therapy, as a potential drug for enhancing in vitro cardiomyocyte differentiation from mouse embryonic stem cells (mESCs) [213] and for promoting ex vivo expansion and stemness maintenances of hematopoietic stem and progenitor cells [214,215]. Additionally, echinochrome A could exert a wide range of biological effects, including antiFigure 14. Proportion of different bioactivity associated to echinoderm-derived MNPs, according to data in Table S4. Echinoids are an important source of natural polyhydroxynaphthoquinones (PHNQ), of the 1,4-naphthoquinones group [ 193 ]. Investigation of the red-dark pigments derived from the spines and shells of sea urchins led to the identification of a sub-family of PHNQ called spinochromes and echinochrome A 15 (Figure 15); spinochromes A–E (e.g., spinochromes E 16 , Figure 15) are the best known and the more accessible molecule of this class [ 194 ]. Found in many sea urchin species, spinochromes are also expressed in coelomocytes, eggs, ovaries and larvae [ 195 ]. Their chemical structures were progressively uncovered so that the molecules and various derivatives were synthesized in the laboratory [ 196 – 199 ]. A distinctive feature of these compounds is their ability to effectively intercept free radicals and bind the Fe 2+ ions responsible for the formation of reactive oxygen species (ROS) [200–202]. Mar. Drugs 2022, 20, x 22 of 48 fibrosis, anti-diabetic, anti-allergic, anti-acetylcholinesterase, mitochondria-protective and gastro-protective effects, as occurred in experimental models [216–224]. A di-glutathionyl functional analogue was synthesized with higher solubility and stability in aqueous solutions, and reduced toxicity while maintaining the anti-ischemic effect [225]. Echinochrome A also showed bactericidal activity [194,226,227] and antiviral ability in particular on herpes simplex virus type 1 [228,229]. 15 16 Figure 15. Echinochrome A (15); spinochrome E (16). Many polysaccharides of various biological origins have been isolated and used as a source of therapeutic agents. The most promising activities of these biopolymers are their immunomodulatory and anticancer effects [230]. A neutral, water-soluble polysaccharide, named SEP, was isolated from the eggs of the sea urchin Strongylocentrotus nudus [231]. SEP was found to be an α-(1→4)-d-glucan and was reported to display antitumour activity by stimulating immune cells, including NK and T cells, via TLR2 and TLR4 receptors [232]. Combining SEP with cytotoxic drugs [233,234] or other immunoregulatory agents [235] resulted in a potent synergistic antitumour effect in mice. Two types of acid polysaccharides, a prominent class of glycans, are among the most important components extracted from the body wall of sea cucumber: sulphated fucans (also named fucoidans) and fucosylated chondroitin sulphates (FCS) [236–238]. These polysaccharides are often endowed with high bioactivity related to their sulphate functional groups, which can interact with many positively-charged biological macromolecules, enzymes included. Sulphated fucans are complex fucose-rich polysaccharides, often extracted from brown seaweeds in which they were first discovered [239] and, to a lesser extent, from the body wall of sea cucumbers or the egg jelly coat of sea urchins (for review see [240–242]). Unlike marine algae, which express sulphated polysaccharides with complex, heterogeneous structures, marine invertebrates synthesize sulphated fucans and sulphated galactans with regular repetitive structures but with variation of the pattern of sulfation and the position of glycosidic linkage [243,244]. This simpler structure can help to elucidate structure–biofunction relationships. Recently, nanomedicine began to use fucoidans especially in the fields of cancer, regenerative medicine, and cardiovascular diseases [240]. In this framework, the specific biotechnological potential of the regular structure of echinoid fucoidans remains poorly investigated. However, it was shown that sulphated polysaccharides isolated from three sea urchin species have different anticoagulant and anti-selectin activities, which give them a potential activity in the attenuation of metastasis progression [245,246]. Fucosylated chondroitin sulphates (FCS) are structurally unique glycosaminoglycans found exclusively in marine invertebrates such as the body wall of sea cucumbers [247], crabs [248] and octopuses [249]. FCS are composed of the same chondroitin sulphate structure as vertebrates, although with additional branches of sulphated fucopyranose units. The fine structure of FCS is species-specific, and their biological activity is supposed to depend mainly on the degree of sulfation and position of sulphate groups, as well as on the distribution of branches along the backbone. Among the multiple potential biomedical applications of FCS, their potential anticoagulant and antithrombotic properties are the most interesting for their possible use as alternative drugs with low bleeding risk with respect to heparin anticoagulants. Therefore, FCS have received extensive attention and OH O H OH OH O O O Figure 15. Echinochrome A (15); spinochrome E (16). The spinochromes A–E show antioxidant activity but with distinct efficiencies and cytotoxicities on human cells [194]. Spinochrome D attenuates doxorubicin-induced mitochondrial damage in human cardiomyocyte cell line AC16 and human breast cancer cell line MCF-7 [203]. Spinochrome E and echinochrome A (see below) were produced by cultured coelomocytes [ 204 ], paving the way to the generation of sea urchin cell cultures producing complex bioactive compounds with therapeutic potential. Echinochrome A, first isolated from the spines of sea urchin Stomopneustes variolaris [ 205 ], acts as a cardioprotective agent and was further used, in a water-soluble form, as the active substance in the drug Histochrome introduced in Russia for preventing ischemia/reperfusion injury and ophthalmopathic complications [ 206 – 208 ]. The antioxidant and anti-inflammatory capabilities of echinochrome A are responsible for the cardioprotective effect [ 201 , 209 ]. Indeed, echinochrome A attenuates the oxidative stress caused by ROS. More recent reports suggest that it could be a candidate molecule to promote cardiac regeneration [ 210 – 212 ]. Echinochrome A was also recently investigated in the framework of stem cell therapy, as a potential drug for enhancing in vitro cardiomyocyte differentiation from mouse embryonic Mar. Drugs 2022,20, 219 21 of 45 stem cells (mESCs) [ 213 ] and for promoting ex vivo expansion and stemness maintenances of hematopoietic stem and progenitor cells [ 214 , 215 ]. Additionally, echinochrome A could exert a wide range of biological effects, including anti-fibrosis, anti-diabetic, antiallergic, anti-acetylcholinesterase, mitochondria-protective and gastro-protective effects, as occurred in experimental models [ 216 – 224 ]. A di-glutathionyl functional analogue was synthesized with higher solubility and stability in aqueous solutions, and reduced toxicity while maintaining the anti-ischemic effect [ 225 ]. Echinochrome A also showed bactericidal activity [ 194 , 226 , 227 ] and antiviral ability in particular on herpes simplex virus type 1 [228,229]. Many polysaccharides of various biological origins have been isolated and used as a source of therapeutic agents. The most promising activities of these biopolymers are their immunomodulatory and anticancer effects [ 230 ]. A neutral, water-soluble polysaccharide, named SEP, was isolated from the eggs of the sea urchin Strongylocentrotus nudus [ 231 ]. SEP was found to be an α-(1→4)-d-glucan and was reported to display antitumour activity by stimulating immune cells, including NK and T cells, via TLR2 and TLR4 receptors [ 232 ]. Combining SEP with cytotoxic drugs [ 233 , 234 ] or other immunoregulatory agents [ 235 ] resulted in a potent synergistic antitumour effect in mice. Two types of acid polysaccharides, a prominent class of glycans, are among the most important components extracted from the body wall of sea cucumber: sulphated fucans (also named fucoidans) and fucosylated chondroitin sulphates (FCS) [ 236 – 238 ]. These polysaccharides are often endowed with high bioactivity related to their sulphate functional groups, which can interact with many positively-charged biological macromolecules, enzymes included. Sulphated fucans are complex fucose-rich polysaccharides, often extracted from brown seaweeds in which they were first discovered [ 239 ] and, to a lesser extent, from the body wall of sea cucumbers or the egg jelly coat of sea urchins (for review see [ 240 – 242 ]). Unlike marine algae, which express sulphated polysaccharides with complex, heterogeneous structures, marine invertebrates synthesize sulphated fucans and sulphated galactans with regular repetitive structures but with variation of the pattern of sulfation and the position of glycosidic linkage [ 243 , 244 ]. This simpler structure can help to elucidate structure–biofunction relationships. Recently, nanomedicine began to use fucoidans especially in the fields of cancer, regenerative medicine, and cardiovascular diseases [ 240 ]. In this framework, the specific biotechnological potential of the regular structure of echinoid fucoidans remains poorly investigated. However, it was shown that sulphated polysaccharides isolated from three sea urchin species have different anticoagulant and anti-selectin activities, which give them a potential activity in the attenuation of metastasis progression [245,246]. Fucosylated chondroitin sulphates (FCS) are structurally unique glycosaminoglycans found exclusively in marine invertebrates such as the body wall of sea cucumbers [ 247 ], crabs [ 248 ] and octopuses [ 249 ]. FCS are composed of the same chondroitin sulphate structure as vertebrates, although with additional branches of sulphated fucopyranose units. The fine structure of FCS is species-specific, and their biological activity is supposed to depend mainly on the degree of sulfation and position of sulphate groups, as well as on the distribution of branches along the backbone. Among the multiple potential biomedical applications of FCS, their potential anticoagulant and antithrombotic properties are the most interesting for their possible use as alternative drugs with low bleeding risk with respect to heparin anticoagulants. Therefore, FCS have received extensive attention and several recent reviews have been published [ 250 – 254 ]. More recently, synthetic low molecular weight FCS have been examined as an alternative to natural FCS for their limited side effects [255]. Saponins, classified as steroid or triterpenic glycosides, with their amphipathic properties, are good surfactants with the ability to increase the permeability of cell membranes [ 256 ]. In Animalia, saponins are biosynthesized only by sponges, starfish and sea cucumbers [ 257 ]. Those isolated from Holothuroidea and sponges are triterpenic, although asterosaponins, the characteristic compounds of Asteroidea, are glycosides of Mar. Drugs 2022,20, 219 22 of 45 sulphated steroids. A review listing asterosaponin structures and their biological functions and bioactivities was recently published [ 258 ], so details will not be presented here. Extracts containing asterosaponins are highly ichthyotoxic and haemolytic; additionally, their high abundances in the mucus layer, body wall and stomach suggest their protective action against pathogens and predators [ 259 , 260 ]. Co-ARIS, an asterosaponin isolated from the starfish Asterias amurensis egg jelly, has a crucial role in the acrosome reaction (AR) required for oocyte penetration by spermatozoa. It was shown that Co-ARIS induces conformational changes in membrane microdomains, affecting the status of AR-signalling proteins [ 261 ]. Asterosaponins isolated from several starfish organs or the whole body have demonstrated anticancer, anti-inflammatory and antimicrobial reported bioactivities [ 259 ]. Interestingly, a synergism of anticancer action was identified when 2D and 3D cultures of human melanoma cells were pre-treated with thornasteroside A or asteropsiside A from starfish Asteropsis carinifera and then by fucoidan from the brown algae Fucus evanescens. This process relies on the regulation of cell cycle protein expression and of a chain of signalling proteins such as MEK1/2, ERK1/2, and MSK1. Produced signals regulate the cell cycle, cell proliferation and cell development, opening up the prospects for the development of effective combined chemotherapeutic methods for melanoma treatment [ 262 ]. Other encouraging results come from the enhanced efficacy of chemoradiation therapy by asterosaponin P1, isolated from Patiria pectinifera, in reducing the number and size of the colonies of colorectal cancer cells. The radiosensitizing activity of asterosaponin P1 occurred by apoptosis induction through the regulation of antiand pro-apoptotic protein expression followed by caspase activation and DNA degradation [ 263 ]. Nevertheless, these promising bioactivities of asterosaponins are not yet explored, envisaging their commercialization. 6. Tunicates Tunicates or urochordates are marine invertebrate chordates considered the sister group of vertebrates. They owe their name to the tunic, the external layer that embeds the body. It is secreted by the epidermis and is composed by an ECM rich in collagen and tunicin (a form of cellulose) fibres, and hosts cells deriving from the delamination of the epidermis and from the circulation. Tunicates include sessile species, collectively grouped in the clade Ascidiacea (ascidians) (Figure 16), and pelagic species forming the clades Thaliacea and Larvacea or Appendicularia. About 3000 species of tunicates live in the seas and oceans of the world and 2300 of them are represented by ascidians, the largest and most studied tunicate group. Within tunicates, Ascidiacea is the most diverse clade and comprises benthic and sessile forms. Mar. Drugs 2022, 20, x 24 of 48 Figure 16. The ascidians Ciona intestinalis (left) and Botryllus schlosseri (right). 6.1. Biomaterials Tunicates are also receiving attention from the biomedical field owing to their adhesiveness, regeneration capacity and tunic tissue features. The tunic is mainly composed of tunicin, a highly crystalline cellulose nanofiber (tunicates are the only known animal source of cellulose), a unique composition among animals, which is found associated with proteins, lipids, sulphated glycans and mucopolysaccharides [264]. Polysaccharides are extensively used in regenerative medicine approaches due to their high biocompatibility and functional properties, among which chitosan and alginate are probably the most studied members. In addition, cellulose is a polysaccharide with low cytotoxicity and, compared to other polysaccharides, has excellent mechanical properties, which makes it also attractive for the development of medical applications, such as wound dressings, bone tissue replacements, drug delivery, vascular grafts and scaffolds for tissue engineering [265,266]. Cellulose nanocrystals (also often referred to as nanowhiskers) which consist of the nanoscale crystalline region of the cellulose polymer, have been isolated from plants and bacteria, and to a less extent from tunicates, and investigated for biomedical applications. Tunicate nanocellulose is exploited for commercial purposes, and significant steps towards the scalable isolation of cellulose from invasive tunicates are being developed, thus offering a potential solution to the numerous challenges which invasive tunicates pose to global aquaculture communities [264,267] and offering a valuable sustainable source to the biomedical field. Membranes and liquid bandages manufactured from Styela clava tunics were prepared for healing skin wounds and showed positive results on skin regeneration [268,269]. Furthermore, a selenium-loaded cellulose film originated from this species accelerated cutaneous wounds during diabetic conditions [124]. Cellulose membranes were also prepared from Styela clava, demonstrating an osteoconductive effect in perforated rat frontal bone, which may be applied in injured bones [270]. In addition, aiming at bone tissue regeneration, cellulose nanocrystals isolated from tunicates demonstrated suitability as a scaffold for bone tissue engineering by promoting osteoblast growth and differentiation, and the recovery of damaged tissue [271]. This biomaterial isolated from Ascidiella aspersa induced guidance in skeletal muscle myoblasts, which could be used for skeletal muscle tissue engineering [272]. More recently, cellulose nanocrystals isolated from Halocynthia roretzi were employed in the development of hydrogels demonstrating highly ordered architectures and outstanding mechanical performances, having potential biomedical applications as artificial biomaterials, such as ligaments and tendons [273]. In addition to cellulose, the tunic also contains various glycosaminoglycans that can be extracted and find a potential use in the sanitary field [274,275]. In addition to containing tunicin, the tunic also contains proteins with 3,4-dihydroxyphenylalanine (DOPA) with a catechol moiety, and 3,4,5-trihydroxyphenylalanine Figure 16. The ascidians Ciona intestinalis (left) and Botryllus schlosseri (right). 6.1. Biomaterials Tunicates are also receiving attention from the biomedical field owing to their adhesiveness, regeneration capacity and tunic tissue features. Mar. Drugs 2022,20, 219 23 of 45 The tunic is mainly composed of tunicin, a highly crystalline cellulose nanofiber (tunicates are the only known animal source of cellulose), a unique composition among animals, which is found associated with proteins, lipids, sulphated glycans and mucopolysaccharides [264]. Polysaccharides are extensively used in regenerative medicine approaches due to their high biocompatibility and functional properties, among which chitosan and alginate are probably the most studied members. In addition, cellulose is a polysaccharide with low cytotoxicity and, compared to other polysaccharides, has excellent mechanical properties, which makes it also attractive for the development of medical applications, such as wound dressings, bone tissue replacements, drug delivery, vascular grafts and scaffolds for tissue engineering [ 265 , 266 ]. Cellulose nanocrystals (also often referred to as nanowhiskers) which consist of the nanoscale crystalline region of the cellulose polymer, have been isolated from plants and bacteria, and to a less extent from tunicates, and investigated for biomedical applications. Tunicate nanocellulose is exploited for commercial purposes, and significant steps towards the scalable isolation of cellulose from invasive tunicates are being developed, thus offering a potential solution to the numerous challenges which invasive tunicates pose to global aquaculture communities [ 264 , 267 ] and offering a valuable sustainable source to the biomedical field. Membranes and liquid bandages manufactured from Styela clava tunics were prepared for healing skin wounds and showed positive results on skin regeneration [ 268 , 269 ]. Furthermore, a selenium-loaded cellulose film originated from this species accelerated cutaneous wounds during diabetic conditions [ 124 ]. Cellulose membranes were also prepared from Styela clava, demonstrating an osteoconductive effect in perforated rat frontal bone, which may be applied in injured bones [ 270 ]. In addition, aiming at bone tissue regeneration, cellulose nanocrystals isolated from tunicates demonstrated suitability as a scaffold for bone tissue engineering by promoting osteoblast growth and differentiation, and the recovery of damaged tissue [ 271 ]. This biomaterial isolated from Ascidiella aspersa induced guidance in skeletal muscle myoblasts, which could be used for skeletal muscle tissue engineering [ 272 ]. More recently, cellulose nanocrystals isolated from Halocynthia roretzi were employed in the development of hydrogels demonstrating highly ordered architectures and outstanding mechanical performances, having potential biomedical applications as artificial biomaterials, such as ligaments and tendons [273]. In addition to cellulose, the tunic also contains various glycosaminoglycans that can be extracted and find a potential use in the sanitary field [274,275]. In addition to containing tunicin, the tunic also contains proteins with 3,4-dihydroxyphenylalanine (DOPA) with a catechol moiety, and 3,4,5-trihydroxyphenylalanine (TOPA) with a pyrogallol moiety [ 276 ]. These compounds are associated with wound healing and are also present in the adhesive components of tunicates [ 277 , 278 ]. Though the exact mechanism of the adhesion in tunicates remains unclear, TOPA compounds were recently proposed in tunicate-inspired adhesives, envisaging biomedical applications. Gallolfunctionalized copolymers exhibited stronger adhesive performances (typically seven times stronger in water) than the widely-used catechol groups, which could bring a new route to the development of new tunicate-inspired gallol polymers with potential as bioadhesives for the medical arena and for other applications [ 279 ]. In this regard, natural polymers such as chitin and its derivate chitosan are being conjugated with gallic acid, which has pyrogallol moieties, to create tunicate-mimetic hydrogels. These adhesives showed higher tissue adhesive properties than fibrin glue and mussel-mimetic adhesives [ 280 ] and improved haemostatic functions with potential application as sealants of internal tissues [ 281 ]. Similarly, a bio-inspired hydrogel conjugating pyrogallol to hyaluronic acid with two routes of pyrogallol oxidation revealed minimal cytotoxicity and immunogenicity in vitro and in vivo , demonstrating versatile applicability for tissue engineering (cell grafting) and drug delivery (therapeutic angiogenesis) [282]. In tunic proteins, the pyrogallol groups can form coordinative complexes with metal ions in the tunicate haemolymph to heal the tunic tissue. Based on metal–pyrogallol coordination of tunicate tissue, a gallic acid/metal ion complex-mediated coating was created to Mar. Drugs 2022,20, 219 24 of 45 accelerate hydroxyapatite remineralization on human teeth, showing potential applicability in the treatment of dentin hypersensitivity [ 283 ]. In another example, inspired by the hemocompatibility of heparin, magnetic nanoparticles with heparin-mimetic coating were developed using TOPA as a biological adhesive, and showed great application potential in haemodialysis as recycling anticoagulants [284]. A list of biomaterials from tunicates is reported in Table 3. Table 3. Biomaterials from tunicates. Class Producer Species Family/Class of Biomaterial Biomaterial Origin/Structural Component Possible Applications References not identified Polysaccharides Cellulose tunic Scaffold for bone tissue engineering [271] Ascidiacea Styela clava Polysaccharides Cellulose tunic Biomaterial for treatment of bone defect [270] Ascidiella aspersa Polysaccharides Cellulose tunic Biomaterial for skeletal muscle tissue engineering [272] Styela clava Polysaccharides Cellulose tunic Membrane for wound healing [124] Styela clava Polysaccharides Cellulose tunic Film for wound healing [269] Halocynthia roretzi Polysaccharides Cellulose tunic Hydrogel for biomedical applications [273] Styela clava Broussonetia kazinoki Polysaccharides Cellulose tunic Liquid bandage for wound healing [268] not identified Proteins TOPA 1 proteins tunic Adhesive hydrogel for biomedical applications [280] 1TOPA proteins: DNA topoisomerase 1. 6.2. Bioactive Molecules Ascidians are the source of a great variety of bioactive molecules of potential sanitary applications, including cytotoxic, antimitotic, antiviral and antimicrobial compounds [ 285 – 289 ], as well as are molecules that are of interest to the biomedical field owing to their adhesiveness [ 290 ]. Ascidian bioactive compounds belong to a high variety of chemical categories [ 287 , 291 ]. Most of the metabolites synthesized by ascidians contribute to create the physicochemical barrier preventing the entrance of foreign organisms in the internal fluids or the tunic colonization by encrusting organisms. In addition, natural endosymbionts contribute to producing part of these compounds to prevent other microorganisms from entering the host [287,292,293]. The most represented chemical class among the bioactive secondary metabolites isolated from tunicates in the selected period was that of alkaloids, including 50% of the isolated compounds, followed by polyketides (37%) and peptides (13%) ([ 289 ]; Table S5). Cytotoxicity against mammalian cell lines and anti-proliferative activity were the most frequently assigned bioactivities, accounting for 58% of the total number of bioactive molecules (Figure 17). Compounds with cytotoxic and antineoplastic properties isolated from ascidians belong to disparate chemical classes, and three of them have been entered into clinical trials. Mar. Drugs 2022,20, 219 25 of 45 Mar. Drugs 2022, 20, x 26 of 48 their adhesiveness [290]. Ascidian bioactive compounds belong to a high variety of chemical categories [287,291]. Most of the metabolites synthesized by ascidians contribute to create the physicochemical barrier preventing the entrance of foreign organisms in the internal fluids or the tunic colonization by encrusting organisms. In addition, natural endosymbionts contribute to producing part of these compounds to prevent other microorganisms from entering the host [287,292,293]. The most represented chemical class among the bioactive secondary metabolites isolated from tunicates in the selected period was that of alkaloids, including 50% of the isolated compounds, followed by polyketides (37%) and peptides (13%) ([289]; Table S5). Cytotoxicity against mammalian cell lines and anti-proliferative activity were the most frequently assigned bioactivities, accounting for 58% of the total number of bioactive molecules (Figure 17). Compounds with cytotoxic and antineoplastic properties isolated from ascidians belong to disparate chemical classes, and three of them have been entered into clinical trials. Figure 17. Proportion of different bioactivity associated to tunicate-derived MNP, according to data in Table S5. Didemnin B 17 (Figure 18, left), a cyclic dispeptide from the colonial ascidian Trididemnum solidum [294], was the first marine natural product to enter a clinical trial [295]. It displays strong antimitotic effects. Its collateral effects and the observation that dehydrodidemnin B, also known as plitidepsin 18 (Figure 18, right) (Aplidine ® ), was more potent than didemnin B [296] led to the end of the clinical development of didemnin B [297]. 17 18) Figure 18. Didemnin B (17) and plitidepsin (18). Plitidepsin, better known by the commercial name of Aplidine ® (marketed by PharmaMar, S.A), isolated from the Mediterranean colonial ascidian Aplidium albicans [298], exerts anticancer activity via the cell cycle arrest at G1-S, likely through the inhibition of N OO O O N H O N NH O OHO O O NHO N O O N O O Figure 17. Proportion of different bioactivity associated to tunicate-derived MNP, according to data in Table S5. Didemnin B 17 (Figure 18, left), a cyclic dispeptide from the colonial ascidian Trididemnum solidum [ 294 ], was the first marine natural product to enter a clinical trial [ 295 ]. It displays strong antimitotic effects. Its collateral effects and the observation that dehydrodidemnin B, also known as plitidepsin 18 (Figure 18, right) (Aplidine ® ), was more potent than didemnin B [296] led to the end of the clinical development of didemnin B [297]. Mar. Drugs 2022, 20, x 27 of 48 Ascidians are the source of a great variety of bioactive molecules of potential sanitary applications, including cytotoxic, antimitotic, antiviral and antimicrobial compounds [285–289], as well as are molecules that are of interest to the biomedical field owing to their adhesiveness [290]. Ascidian bioactive compounds belong to a high variety of chemical categories [287,291]. Most of the metabolites synthesized by ascidians contribute to create the physicochemical barrier preventing the entrance of foreign organisms in the internal fluids or the tunic colonization by encrusting organisms. In addition, natural endosymbionts contribute to producing part of these compounds to prevent other microorganisms from entering the host [287,292,293]. The most represented chemical class among the bioactive secondary metabolites isolated from tunicates in the selected period was that of alkaloids, including 50% of the isolated compounds, followed by polyketides (37%) and peptides (13%) ([289]; Table S5). Cytotoxicity against mammalian cell lines and anti-proliferative activity were the most frequently assigned bioactivities, accounting for 58% of the total number of bioactive molecules (Figure 17). Compounds with cytotoxic and antineoplastic properties isolated from ascidians belong to disparate chemical classes, and three of them have been entered into clinical trials. Figure 17. Proportion of different bioactivity associated to tunicate-derived MNP, according to data in Table S5. Didemnin B 17 (Figure 18, left), a cyclic dispeptide from the colonial ascidian Trididemnum solidum [294], was the first marine natural product to enter a clinical trial [295]. It displays strong antimitotic effects. Its collateral effects and the observation that dehydrodidemnin B, also known as plitidepsin 18 (Figure 18, right) (Aplidine ® ), was more potent than didemnin B [296] led to the end of the clinical development of didemnin B [297]. 17 18 Figure 18. Didemnin B (17) and plitidepsin (18). N OO O O N H O N NH O OHO O O NHO N O O N O O Figure 18. Didemnin B (17) and plitidepsin (18). Plitidepsin, better known by the commercial name of Aplidine ® (marketed by PharmaMar, S.A), isolated from the Mediterranean colonial ascidian Aplidium albicans [ 298 ], exerts anticancer activity via the cell cycle arrest at G1-S, likely through the inhibition of protein tyrosine phosphatases, a class of enzymes frequently associated with tumour progression [ 299 ]. It can also inhibit protein synthesis by interacting with the elongation factor 1 α and induce apoptosis in cultured cells through the overactivation of c-Jun N-terminal kinases [ 297 ]. Didemnin B (and, likely, also the closely-related Aplidine ® ) is synthesized by the symbiotic α-proteobacteria Tistrella mobilis and Tristella bauzanensis [295,300]. Trabectedin, formerly ecteinascidin-743 (ET-743) (Figure 2), is an alkaloid first isolated from the Caribbean colonial ascidian Ecteinascidia turbinata in 1990. It is the first anticancer drug from a marine source introduced to the market after encompassing all the clinical trials, and is particularly effective against solid tumours, especially soft tissue sarcomas and relapsed ovarian cancer [ 286 , 301 ]. Today, it is marketed by PharmaMar with the commercial name Yondelis ® . It has a unique mechanism of action based on the interaction with the minor groove of deoxyribonucleic acid (DNA), which leads to strand breaks and consequent inhibition of DNA synthesis and gene transcription, as well as interruption of the cell cycle and induction of apoptosis [ 287 , 297 ]. When used at a micromolar concentration, trabectedin can inhibit a number of transcription factors [ 302 ]. In nature, trabectedin is produced by the bacteria endosymbiont Candidatus Endoecteinascidia frumentensis [ 302 – 304 ], but, due to its scarcity in the ascidian tissues, today it is obtained by chemical synthesis. Mar. Drugs 2022,20, 219 32 of 45 19. 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