P og ess in Neu obiology 205 (2021) 102124
A ailable online 24 July 2021
0301-0082/© 2021 The Au ho (s). Published by Else ie L d. This is an open access a icle unde he CC BY license (h p://c ea i ecommons.o g/licenses/by/4.0/).
O iginal Resea ch A icle
The ole o Ex acellula Vesicles du ing CNS de elopmen
Nasim Bah am Sangani
a
,
b
, Ana Ri a Gomes
c
,
d
, Leopold M.G. Cu s
b
,
Ch is P. Reu elingspe ge
a
,
b
,
*
a
Depa men o Biochemis y, Maas ich Uni e si y, Ca dio ascula Resea ch Ins i u e Maas ich , Maas ich , he Ne he lands
b
GKC-Re Expe ise Cen e, Maas ich Uni e si y Medical Cen e, Maas ich , he Ne he lands
c
Depa men o Bioenginee ing and IBB – Ins i u e o Bioenginee ing and Biosciences, Ins i u o Supe io T´
ecnico, Uni e sidade de Lisboa, A . Ro isco Pais, 1049-001,
Lisboa, Po ugal
d
Ins i u o de Medicina Molecula - Jo˜
ao Lobo An unes, Faculdade de Medicina da Uni e sidade de Lisboa, Po ugal
ARTICLE INFO
Keywo ds:
Ex acellula esicles
Exosomes
In acellula communica ion
neu on
glia
CNS de elopmen
ABSTRACT
Wi h a di e se se o neu onal and glial cell popula ions, Cen al Ne ous Sys em (CNS) has one o he mos
complex s uc u es in he body. In e cellula communica ion is he e o e highly impo an o coo dina e cell- o-
cell in e ac ions. Besides elec ical and chemical messenge s, CNS cells also bene i om ano he communica ion
ou e, wha is known as ex acellula esicles, o ha monize hei in e ac ions. Ex acellula Vesicles (EVs) and
hei sub ype exosomes a e memb anous pa icles sec e ed by cells and con ain in o ma ion packaged in he o m
o biomolecules such as small agmen s o DNA, lipids, miRNAs, mRNAs, and p o eins. They a e able o e i-
cien ly d i e changes upon hei a i al o ecipien cells. EVs ac i ely pa icipa e in all s ages o CNS de el-
opmen by s imula ing neu al cell p oli e a ion, di e en ia ion, synap ic o ma ion, and media ing ecip ocal
in e ac ions be ween neu ons and oligodend ocy e o myelina ion p ocess. The aim o he p esen e iew is o
enligh en he p esence and con ibu ion o EVs a each CNS de elopmen al miles one.
1. In oduc ion
Ex acellula Vesicles (EVs) we e ini ially hough o unc ion as a
disposal mechanism ca ying cellula was e in o ex acellula space.
They a e nowadays es ablished as one o he lines o communica ion
be ween cells.
The name “ex acellula esicle” is a gene al e m used o desc ibe
h ee sub ypes o esicles: (i) Mic o esicles (ii) Exosomes and (iii)
Apop o ic bodies. I is impo an o men ion ha he e ms EVs and
exosomes a e used in e changeably in li e a u e. The e is indeed a high
inconsis ency wi hin he ield ega ding he nomencla u e used o
desc ibe ex acellula esicles. A wide ange o e minologies a e used in
li e a u e while no unambiguous de ini ion based on sub ype speci ic
ma ke s o sub ype speci ic isola ion me hods is a ailable o he
Abb e ia ions: As gI1, Aspa aginase-like p o ein 1; BMPs, Bone Mo phogene ic P o eins; CAM kinase II, Calmodulin-dependen p o ein kinase II; CT-1, Ca dio-
ophin-1; CNS, Cen al Ne ous Sys em; CME, Cla h in-Media ed Endocy osis; C3, Complemen componen 3; CFB, Complemen Fac o B; ESCRT, Endosomal
So ing Complexes Requi ed o T anspo ; EE, En i onmen al En ichmen ; EGFR, Epide mal G ow h Fac o Recep o ; EAAT2, Exci a o y Amino Acid T anspo e 2;
EVs, Ex acellula Vesicles; FBS, Fe al Bo ine Se um; FGF, Fib oblas G ow h Fac o ; FGF2, Fib oblas G ow h Fac o 2; GABA, Gamma Aminobu y ic Acid; GFAP,
Glial Fib illa y Acidic P o ein; GLT1, Glu ama e Syn hase (NADH); IGFBP6, Insulin-like G ow h Fac o Binding P o ein 6; ITGB4, In eg in Subuni Be a 4; IPCs,
In e media e P ogeni o Cells; ICAM, In acellula Adhesion Molecule; ILVs, In aluminal Vesicles; LAMP1, Lysosomal-Associa ed Memb ane P o ein 1; miRNA,
mic oRNA; MVBs, Mul i esicula Bodies; MAG, Myelin Assoccia ed Glycop o ein; MBP, Myelin Basic P o ein; MOG, Myelin Oligodend ocy e Glycop o ein; PLP,
Myelin P o eolipid P o ein; NGF, Ne e G ow h Fac o ; NPCs, Neu al P ogeni o Cells; NSCs, Neu al S em Cells; NECs, Neu oepi helial Cells; NSMAF, Neu al
Sphingomyelinase Ac i a ion Associa ed Fac o ; NMDA, N-Me hyl-D-Aspa a e; IFN-ү, In e e on-gamma; OPCs, Oligodend ocy e P ogeni o Cells; OSVZ, Ou e
Sub en icula Zone; PEDF, Pigmen Endo helium-De i ed Fac o ; RGCs, Radial Glial cells; oRGsou e , Radial Glia cells; RA, Re inoic Acid; SNPs, Sho Neu onal
P ecu so s; SNAREs, Soluble N-e hylmaleimide-sensi i e Fac o A achmen P o ein Recep o s; SHH, Sonic Hedgehog; SVZ, Sub en icula Zone; TGFB1, T ans-
o ming G ow h Fac o -be a 1; VEGF, Vascula Endo helial G ow h Fac o ; VEGFR2, Vascula Endo helial G ow h Fac o Recep o 2; VZ, Ven icula Zone; Wn ,
Wingless- ype MMTV in eg a ion si e amily; AMPA,
α
-Amino-3-hyd oxy-5-Me hyl-4-isoxazolep opionic Acid.
* Co esponding au ho a : PO Box 616, Depa men o Biochemis y, Ca dio ascula Resea ch Ins i u e Maas ich , Maas ich Uni e si y, 6200 MD Maas ich ,
he Ne he lands.
E-mail add esses: [email p o ec ed] (N. Bah am Sangani), [email p o ec ed] (A.R. Gomes), leopold.cu s@
maas ich uni e si y.nl (L.M.G. Cu s), [email p o ec ed] (C.P. Reu elingspe ge ).
Con en s lis s a ailable a ScienceDi ec
P og ess in Neu obiology
jou nal homepage: www.else ie .com/loca e/pneu obio
h ps://doi.o g/10.1016/j.pneu obio.2021.102124
Recei ed 12 Decembe 2020; Recei ed in e ised o m 16 Ap il 2021; Accep ed 20 July 2021
P og ess in Neu obiology 205 (2021) 102124
2
sub ypes. (Mo a e al., 2016; Yanez-Mo e al., 2015). Fo example, he
wo e ms EVs and exosomes a e used in e changeably, and a selec ion
o a e m seems o be based on au ho s’ p e e ence (Wi we & The y,
2019). The In e na ional Socie y o Ex acellula Vesicles (ISEV) has
p oposed a consensus nomencla u e in which EVs is a p e e ed gene ic
e m o desc ibe lipid bilaye pa icles ha a e eleased by cells and
unable o eplica e due o he lack o unc ional nucleus (The y e al.,
2018). In he p esen e iew we ollow hese guidelines, howe e , when
discussing indi idual s udies we adop he e ms used by he au ho s o
he s udy.
The sub ypes o EVs a e dis inguished based on hei size, o igin o
o ma ion, con en , unc ion, and mechanism o elease (Doyle & Wang,
2019). Wi h a size anging om 30nm- o 100 nm, exosomes a e he
smalles esicles ollowed by mic o esicles (100 nm-1000 nm) and
apop o ic bodies (1000-5000 nm)(Maia e al., 2018). I is impo an o
men ion ha di e en me hods ha e been p oposed o de e mine di-
ame e s and hus he e is an o e lap be ween he subclasses o EVs wi h
espec o size (Bo ges e al., 2013; Cocucci & Meldolesi, 2015; EL
Andaloussi e al., 2013). The e o e, o u he cha ac e ize EV sub-
popula ions, esea che s usually use me hods such as Nano T acking
Analysis (NTA) in combina ion wi h elec on mic oscopy, low cy om-
e y o Wes e n blo ing o he known EVs ma ke s.
The di e en EV subpopula ions a ise om di e en biogene ic
pa hways. Apop o ic bodies esul om a p ocess ha o ches a es he
demise o he cell and ha p oduces memb ane-encapsula ed cellula
agmen s ha can con ain o ganelles, p o eins, DNA and RNA.
Apop o ic bodies a e no homogenous and ha e a b oad size dis ibu ion
(Ba is elli & Falcie i, 2020; Xu e al., 2019) Mic o esicles a e o med by
di ec ou wa d budding o he plasma memb ane and can be eleased
in o he ex acellula space by “pinching o ” om he cell memb ane.
This p ocess equi es cy oskele al p o eins such as ac in, mic o ubules,
and molecula mo o s such as dynein, kinesis and myosin as well as a
collabo a ion be ween SNARES, Rab GTPases and e he ing ac o s (Cai
e al., 2007; Doyle & Wang, 2019; T ica ico e al., 2017).
As opposed o mic o esicles and apop o ic bodies, which ha e ela-
i ely simple biogene ic pa hways, exosomes a e gene a ed by a com-
plex mul is ep p ocess ha s a s wi h he o ma ion o ea ly endosomes
ha e ol e in o la e endosomes and mul i esicula bodies (MVBs)
con aining in aluminal esicles (ILVs). Ea ly endosomes a e o med by
he in agina ion o he plasma memb ane and g adually ma u e in o la e
endosomes (Doyle & Wang, 2019). Ma u a ion om ea ly o la e en-
dosome is mainly media ed by he small GTPases Rab5 and Rab7. In his
p ocess, which is known as Rab con e sion, he deple ion o Rab 5, he
ma ke o ea ly endosome, is accompanied by he ec ui men o Rab7
on la e endosomes (Po e yae e al., 2010; Rink e al., 2005). ILVs a e
o med by inwa d budding o he limi ing memb anes o la e endosomes
(Pipe & Ka zmann, 2007). Fusion o he limi ing memb ane o MVBs
wi h he plasma memb ane leads o he elease o ILVs in o he ex a-
cellula space. ILVs in he ex acellula space a e e med exosomes
(Ca uso Ba iso o e al., 2019; Minciacchi e al., 2015; an Niel e al.,
2006; Zhang e al., 2019a). The p ecise mechanisms o ca go so ing in o
ILVs ha e no been esol ed ye bu a e dependen on he biogene ic
pa hway o ILVs.
ILVs can be p oduced by he well-s udied endosomal so ing com-
plexes equi ed o anspo (ESCRT) dependen pa hway. This p o ein
machine y consis s o ou complexes (ESCRT-0,-I,-II,-III) which a e
sequen ially ec ui ed on he la e endosome memb ane (Colombo e al.,
2013; Henne e al., 2011). The wo p o ein subuni s o ESCRT-0, H s and
STAM1/2 (Vps27 and Hse1 in yeas ) a ange he ec ui men o ubiq-
ui ina ed ca go o he limi ing memb ane o he endosome whe e he
inwa d budding s a s (Henne e al., 2011; Pipe & Ka zmann, 2007).
ESCRT-0 hen ac i a es he TSG101 con aining ESCRT-I complex
(Colombo e al., 2013). Inhibi ion o he ESCRT-0 and I p o eins H s,
STAM1 and TSG101 esul s in he educ ion o exosome sec e ion
(Colombo e al., 2013). The ESCRT-I complex binds and ec ui s
ESCRT-II subuni s o he endosome memb ane, he eby ini ia ing
inwa d budding (And eu & Yanez-Mo, 2014; Colombo e al., 2013).
Subsequen ly, ESCRT-II nuclea es ESCRT-III complex assembly and
polyme o ma ion. Finally, The ATPase Vps4 joins he ESCRT-III poly-
me and induces i s disassembly om he memb ane acili a ing esicle
scission o o m he ILVs (Wolle e al., 2009). In addi ion o o ches-
a ing ILV o ma ion he ESCRT complexes also de e mine ca go so ing
in o he ILVs (Henne e al., 2011; Minciacchi e al., 2015).
The indings ha deple ion o key ESCRT subuni s did no esul in
ull inhibi ion o MVB o ma ion and exosome sec e ion poin ed o he
exis ence o ESCRT-independen mechanisms o ILV o ma ion (S u e s
e al., 2009; Zhang e al., 2019a). I was shown ha membe s o he
e aspanin p o ein amily such as CD9, CD63, CD81, and CD82, and
lipid a s play pi o al oles (And eu & Yanez-Mo, 2014; an Niel e al.,
2011). ILV o ma ion o he ESCRT-independen pa hway equi es he
p oduc ion o ce amide by neu al sphingomyelinase 2 (nSMase2)
(T ajko ic e al., 2008). Ce amide can induce coalescence o lipid- a
mic odomains o he endosomal memb anes ha p omo es
domain-induced inwa d budding gi ing ise o ca go-loaded ILVs
(T ajko ic e al., 2008).
nSMase2 also con ols he loading o he ILVs wi h compounds such
as RNAs by i s downs eam ac o NSMAF/FAN (neu al sphingomyeli-
nase associa ed ac o ) ha is in ol ed in he ec ui men o RNA
binding p o eins o ILV ca go selec ion (Leidal e al., 2020).
Once o med, MVBs can ei he use wi h lysosomes o lysosomal
deg ada ion o wi h he plasma memb ane o elease hei con en in he
o m o exosomes in o he ex acellula space (Pipe & Ka zmann, 2007).
Fac o s ha de ine he a e o MVBs emain o be esol ed. Membe s o
he Rab amily including Rab2b, Rab9a, Rab27a, Rab 27b, Rab35, ha e
been shown o be in ol ed in a icking and docking o MVBs o he
plasma memb ane ( o mo e in o ma ion on he ole o Rab amily in
EVs please see e e ences (Blanc & Vidal, 2018; Ei an e al., 2016). I has
been sugges ed ha choles e ol en ichmen o he MVB memb anes
s imula es usion o MVBs wi h he plasma memb ane (Doyle & Wang,
2019; Mobius e al., 2002; Mobius e al., 2003). Pos ansla ional
modi ica ions may also de e mine he a e o MVBs. A ecen s udy by
Villa oya-Bel i e al. showed ha ISGyla ion o TSG101 s imula es
lysosomal deg ada ion o MVBs and educes exosome sec e ion (Villa -
oya-Bel i e al., 2016).
The di e en biogene ic ou es also esul in di e en ca go
composi ion o exosomes. Some p o eins a e only ound in exosomes
p oduced by he ESCRT-dependen pa hway while o he s equi e he
ESCRT-independen ou e o hei sec e ion ia exosomes. Fo example
loading o ILVs wi h he epide mal g ow h ac o (EGFR) equi es H s
and STAM1 o ESCRT complexes (Bache e al., 2003; Raibo g e al.,
2002; U be e al., 2003). This was u he con i med by S u e s e al
who showed ha deple ion o ESCRT componen s inhibi EGFR so ing
in o ILVs (S u e s e al., 2009). Inhibi ion o nSMase2, which is a key
playe in he ESCRT-independen ou e o ILV o ma ion (see abo e)
signi ican ly educes CD82 media ed exosome elease o β-ca enin sug-
ges ing ha β-ca enin sec e ion equi es he ESCRT-independen
pa hway (Chai oungdua e al., 2010). Ano he example was gi en by
Theos e al. indica ing ha he so ing o melanosomal p o ein Pmel17
(Pmel17) in o ILVs is insensi i e o H s deple ion and he e o e Pmel17
so ing is a anged independen ly o he ESCRT machine y (Theos e al.,
2006).
Once eleased, EVs can a ge cells in hei icini y as well as
eaching hose loca ed a a dis ances. In gene al, he e a e h ee s a-
egies desc ibed o EVs a ge ing and up ake. One s a egy is o simply
use wi h he plasma memb ane o he a ge cells a e which he EV’s
con en is eleased in o he cy oplasm. Memb ane usion is hough o be
media ed by se e al p o eins amongs which a e soluble N-e hyl-
maleimide-sensi i e ac o a achmen p o ein ecep o s (SNAREs). The
assembly o he ou complemen a y SNARE mo i s media es a igh
connec ion be ween he wo lipid memb anes o use (Jahn & Schelle ,
2006; Kwok e al., 2021). The o he ou e in ol es ligand- ecep o
in e ac ion in which EVs dock wi h memb ane exposed ligands o
N. Bah am Sangani e al.
P og ess in Neu obiology 205 (2021) 102124
3
ecep o s on he plasma memb ane o he ecipien cells and igge
cascades o in acellula signaling e en s (Fu e al., 2020). In he ma-
jo i y o cases, howe e , in e naliza ion o EVs by endocy osis seems o
be he mos impo an ou e. The unde lying mechanisms a anging EV
endocy osis a e no ye clea bu se e al mechanisms ha e been sug-
ges ed ( o mo e in dep h in o ma ion he eade is e e ed o he
ecen ly published e iew on EVs anspo a ion and up ake by (Kwok
e al., 2021). These include cla h in-media ed endocy osis (CME). In his
model cla h in p o ein oge he wi h o he componen s such as
cla h in-adap o p o eins and sca old p o eins o m cla h in coa ed
endocy o ic esicles (Kaksonen & Roux, 2018). I is no ewo hy o
men ion he e ha CME is he majo pa hway in synap ic esicle p o ein
in e naliza ion (Saheki & De Camilli, 2012). Addi ionallly, CME ac i ely
pa icipa es in synap ic ca go e ie al om he plasma memb ane a e
physiological s imuli (G anse h e al., 2006; Nicholson-Fish e al., 2015).
O he mechanisms include phagocy osis, mic opinocy osis, and
ca eolin-dependen endocy osis, a p ocess h ough which ca e-like
s uc u es known as ca eola esicles a e o med and pinched o om
he plasma memb ane in o he cy osol (Kwok e al., 2021). Se e al
p o eins ha e been iden i ied o play a ole in EV up ake by ecipien
cells. These include lec ins, in eg ins, in acellula adhesion molecules
(ICAMs), and p o eoglycans (Mu phy e al., 2019; an Niel e al., 2018;
Zhang e al., 2019a; Zolle , 2009). Te aspanins, a amily o ans-
memb ane p o eins, also pa icipa e in he in e ac ion be ween EVs and
ecipien cells. In addi ion o hei oles in ESCRT-independen ILV
biogenesis and ca go so ing, Te aspanons also ha e a ole in de e -
mining he ecipien cell o exosome up ake (Janko ico a e al., 2020;
an den Boo n e al., 2013). No ably, he dis inc issue-homing
beha io o EVs is due o e aspanin-en iched-mic odomains (TEM) in
which e aspanins in e ac wi h in eg ins and de e mine a ge
selec ion ( an den Boo n e al., 2013).
The con en loaded in o EVs is gene ally a collec ion o biomolecules
ob ained om he pa en al cell. Howe e , he e a e a ew common
p o ein ma ke s de ec able in almos all EVs/exosomes. These include
classic exosome ma ke s such as e aspanin p o eins CD9, CD63, and
CD81,CD82 as well as HSP70, HSP90β, ALIX, TSG101 and Flo illin
p o eins (Doyle & Wang, 2019; Kowal e al., 2016). A la ge amoun o
s udies has been pe o med o iden i y and cha ac e ize EV con en by
Mul i-omics app oaches such as nex gene a ion sequencing and mass
spec ome y. The ou come is collec ed in h ee online da a eposi o ies:
Vesiclepedia, EVpedia, and ExoCa a (Kal a e al., 2012; Kee hikuma
e al., 2016; Kim e al., 2013). I is hough ha in ega ds o CNS,
miRNAs a e he s andou elemen s among he o he componen s o EVs
(Ba iz e al., 2016; Lua e e al., 2016b). They a e in ol ed in egula ing
synap ic plas ici y as well as neu op o ec ion (Micci e al., 2019; P op-
e zi e al., 2015). Mo eo e , each miRNA is able o a ge and ep ess he
ansla ion o as many as hund eds o mRNAs (En igh e al., 2003; K ek
e al., 2005; S e ana o & Sinden, 2014).
Nume ous s udies ha e in es iga ed he unc ion o EVs pa icula ly
in neu odegene a i e diso de s. Howe e , less is known abou hei
pa hological impac on neu ode elopmen al diso de s ( o a ecen e-
iew see Gomes e al. 2020) (Gomes e al., 2020). The pu pose o he
p esen s udy is o shed ligh on he ole o EVs by p esen ing a
comp ehensi e e iew o li e a u e on hei ole du ing CNS de elop-
men . This e iew sugges s new insigh s in o he po en ial link o exo-
somes and some o he unde lying pa hologies in diso de s o
neu ode elopmen .
CNS has he mos complex s uc u e in he body consis ing o a ious
cell ypes gene a ed in a spa io- empo al manne and each cell ype has a
dis inc mo phology and unc ion. Desc ibing he CNS s uc u e and
Fig. 1. Summa y o majo de elopmen al miles ones. Neu oepi helial cells (NECs) inside he neu al ube popula e he en icula zone (VZ) h ough symme ic
di ision. No ch signaling, he de ining ac o in cell a e decision, main ains he balance be ween p oli e a ion and di e en ia ion. NECs wi h inac i e No ch signaling
unde go asymme ic di ision o p oduce he i s neu ons, known as sho p ecu so neu ons (SNPs), as well as Radial Glial Cells (RGCs). Besides hei p oli e a ion,
RGCs in sub en icula zone (SVZ) di ide asymme ically o gene a e in e media e p ogeni o cells (IPCs) and neu ons. The ou e adial glial cells (oRGs) eside in
he ou e sub en icula zone (OSVZ) and gi e ise o he majo i y o neu ons (mos no ably in human). IPCs di ide symme ically o p oduce wo neu ons. Neu ons
mig a e away om VZ and SVZ o each he co ical pla e (CP) and co ical ma ginal zone (MZ) whe e hey e en ually become ma u e neu ons. Newbo n neu ons
s a hei mig a ion in he in e media e zone (IZ) by using he long basal adial p ocesses o he RGCs as he sca old (Coope , 2014; Hi o a & Nakajima, 2017; Tan &
Shi, 2013). Towa ds he end o neu ogenesis and by ecei ing signals om neu ons, RGCs swi ch o gliogenesis o p oduce as ocy es and oligodend ocy e p ogeni o
cell (OPC). Du ing synap ogenesis, axons s a o in e ac wi h nea by dend i es, a p ocess ha unde goes ma u a ion o es ablish and s abilize synapses. The excess in
synapse o ma ion is elimina ed du ing p uning o emo e he weak synapses. Ma u e oligodend ocy es acili a e he as anspo o signals along neu ons by
w apping he neu onal axons wi h myelin shea hs. The app oxima e imelines p o ided o each s ep a e om he ollowing e e ences: Neu ula ion (Mulle &
O’Rahilly, 1987; Mulle & O ahilly, 1988), Neu ogenesis (Bys on e al., 2008; Rakic, 1988), Gliogenesis (Choi & Lapham, 1978; Jako ce ski e al., 2009), Syn-
ap ogenesis and synap ic p uning (Hu enloche , 1979; Pe anjek e al., 2011), and Myelina ion (Jako ce ski e al., 2009; D. J. Mille e al., 2012).
N. Bah am Sangani e al.
P og ess in Neu obiology 205 (2021) 102124
4
unc ion is well beyond he scope o he p esen e iew and he e o e we
only speci y and b ie ly desc ibe he pa s highligh ed in EVs s udies o
he de eloping CNS. We s a om neu ogenesis ollowed by gliogenesis
and con inue o synap ogenesis, synap ic p uning, and e en ually
myelina ion. Addi ionally, since neu ogenesis is an ongoing p ocess we
will also ha e a look a he ole o EVs in adul neu ogenesis. Fig. 1
ep esen s a schema ic isualiza ion o he CNS de elopmen al s ages.
2. Emb yonic neu ogenesis
CNS de elopmen begins by he neu ula ion p ocess when he neu al
pla e olds inwa dly o e en ually o m he neu al ube. Sho ly a e
being o med, he neu al ube unde goes pa e ning by which h ee axes
a e de i ed: an e io -pos e io , medial-la e al, and do sal- en al
(Hemma i-B i anlou & Mel on, 1997). Neu al ube pa e ning is egu-
la ed by mo phogens such as Sonic Hedgehog (SHH), Bone mo phoge-
ne ic p o eins (BMPs), Re inoic Acid (RA), Fib oblas G ow h Fac o
(FGF), and Wn ha e en ually lead o o ma ion o ou di e en e-
gions in CNS: o eb ain, midb ain, hindb ain, and spinal co d (Gue ou
e al., 2014; Hemma i-B i anlou & Mel on, 1997).
Be o e he ini ia ion o co ical neu ogenesis, Neu oepi helial cells
(NECs) loca ed in he neu al ube unde go p oli e a ion (a an expo-
nen ial a e) h ough symme ic di ision. P oli e a ion occu s in he
p ima y p oli e a i e zone, he en icula zone (VZ), o he neu al ube
(Fie z & Hu ne , 2011; Ma inez-Ce deno & Noc o , 2018; Pin o & Go z,
2007). Changes in he p oli e a ion mode om symme ic di ision o
asymme ic di ision is he s a ing poin o neu ogenesis (Ca iness e al.,
2003; Go z & Hu ne , 2005). Asymme ic di ision p oduces one NEC
while he o he daugh e cell becomes a p ogeni o cell, also known as
adial glial cell (RGC). NECs a e also capable o di ec ly p oducing sho
neu onal p ecu so s (SNPs) (Gal e al., 2006; S ancik e al., 2010) which
a e known o be he i s popula ion o neu ons gene a ed in CNS. Cell
ansi ion om p oli e a ion o di e en ia ion is in luenced by No ch
signaling ha supp esses di e en ia ion and is mo e in a o o p oli -
e a ion (Imayoshi e al., 2010; Lu ol e al., 2002). Thus, newly gene -
a ed NECs wi h ac i e No ch signaling a e p og ammed o p ese e he
cell pool by sel - enewing di isions while hose wi h inac i e No ch
signaling ake a di e en pa h owa ds di e en ia ion. Symme ic and
asymme ic cell di isions a e also p esen in neu onal p ogeni o cells
hough a a mo e es ic ed manne compa ed o NECs (Go z & Hu ne ,
2005; Holgue a & Desplan, 2018). While symme ic p oli e a i e di i-
sion main ain he neu al p ogeni o cell pool, asymme ic cell di ision
ei he di ec ly p oduces neu ons o gene a es in e media e neu onal
p ogeni o cells, also known as ampli ying p ogeni o cells. They
mig a e adially o he second p oli e a i e zone, known as he sub-
en icula zone (SVZ) (Noc o e al., 2001; Noc o e al., 2004).
In e media e p ogeni o cells (IPCs) a e subjec ed o symme ic
neu ogenic di ision o p oduce wo pai s o pos mi o ic neu ons (Ma -
inez-Ce deno e al., 2006; Noc o e al., 2004; Tan & Shi, 2013).
Addi ionally, he e is ano he sub ype o p ogeni o cell, adial glia-like
p ogeni o cells, also known as ou e adial glial cells (oRGs). They
eside in he ou e sub en icula zone (OSVZ) in human. They a e
mo phologically di e en om RGCs and IPCs (Hansen e al., 2010; X.
Wang e al., 2011) and able o p oduce mo e IPCs h ough asymme ic
di ision (Hansen e al., 2010; LaMonica e al., 2013).The expansion and
complexi y o human neoco ex is la gely due o he OSVZ and mos
no ably due o oRGs. The OSVZ is popula ed wi h IPCs and oRGs
(Hansen e al., 2010). Recen s udies e ealed ha oRGs a e also p esen
in mice (Shi amukai e al., 2011; X. Wang e al., 2011) hough he ex en
o which hey con ibu e o co ical expansion is es ic ed. In mice, IPC
p oli e a ion does no exceed mo e han one o wo cell cycles while in
highe mammals such as p ima es, he IPCs ep esen highe
sel - enewing capaci y and hus inc ease he co ical size and complexi y
(Hansen e al., 2010) (X. Wang e al., 2011) (Ma inez-Ma inez e al.,
2016) (Tibe i e al., 2012).
Neu onal mig a ion o he co ical pla e, which ends he neoco ex
de elopmen , is a g adual p ocess. Pos -mi o ic neu ons p oduced in VZ
and SVZ mig a e h ough he in e media e zone o e en ually each he
op o he co ical pla e, whe e hey u he di e en ia e and u n in o
hei dis inc pheno ype.
3. Gliogenesis
Gene a ion o neu ons and gene a ion o glial cells ake place in a
sequen ial o de such ha neu ons a e he i s o be gene a ed ollowed
by as ocy es and, la e on du ing he pos na al pe iod, oligodend ocy e
p ecu so cells and oligodend ocy es (Adnani e al., 2018; J. Liu &
Casaccia, 2010). T ansi ion o gliogenesis occu s owa ds he end o
neu ogenesis when RGCs ecei e signals om newly o med neu ons,
which igge hei gliogenic a e o ake ac ion. The in insic and
ex insic ac o s a e he key playe s in swi ching om neu ogenesis o
gliogenesis (Adnani e al., 2018; F. D. Mille & Gau hie , 2007; Za -
ei-Khei abadi e al., 2020). Fo ins ance, cy okines p oduced by newly
gene a ed neu ons in he co ical pla e a e demons a ed o p omo e
gliogenesis h ough a nega i e eedback mechanism (Ba nabe-Heide
e al., 2005). Newly gene a ed co ical neu ons sec e e ca dio ophin-1
(CT-1) which ac s as an ex insic gliogenic signal o igge he con-
e sion om neu ogenesis o gliogenesis. Thus, CT-1 ins uc s mul i-
po en co ical p ecu so cells o p oduce as ocy es.
All he a o emen ioned s udies clea ly indica e ha RGCs a e he
common p ecu so cell pool ha c ea e he e ogenei y in he CNS by
gene a ing di e en coho s o cell ypes a di e en ime poin s and
places. This plu ipo en p ope y o RGCs he e o e makes hem a good
candida e in cell-based he apy o diseases wi h neu ological
pheno ypes.
EVs du ing neu ogenesis and gliogenesis
A la ge numbe o s udies ha e ocused on he po en ial he apeu ic
e ec o he neu al s em cell sec e ome in neu ological diso de s. Neu al
s em/p ogeni o cells sec e e EVs ha a e ins umen al in in e cellula
communica ion. In spi e o hei small size, hei con en co e s a wide
a ie y o biomolecules such as small agmen s o DNA, p o eins,
mRNAs, miRNAs as well as me aboli es and lipids. The con en o EVs is
de e mined by he pa en al cell and al hough i s quan i y is conside ed
low, i s e icacy o induce changes in he ecipien cell is ine i able. The
s udy by S e ana o e al. demons a ed ha he exosomal miRNA
eleased by human neu al s em cells (hNSCs) e lec s he miRNA con en
o he p oducing cells. Fu he mo e, he exosomal miRNA can be
ans e ed unc ionally o ecipien cells (S e ana o e al., 2016). They
i s pe o med nex gene a ion sequencing on cellula and exosomal
con en o hNSCs ha e ealed a di e en ially en iched subse o miR-
NAs in exosomes including Hsa-miR-1246, hsa-miR-4488,
hsa-miR-4508, hsa-miR-4492 and hsa-miR-4516. S oichiome y indings
by eal ime PCR on highly en iched Hsa-miR-1246 indica ed ha he e
we e a leas 10 copies o his miRNA pe exosomes. Mo eo e , unc-
ional analysis by 3’ un ansla ed egion dual luci e ase epo e assay
indica ed he educ ion in luci e ase ac i i y in Hela cells, used as a ge
cells, indica ing ha he amoun o ans e ed miRNA is su icien o
elici changes in ecipien cells.
EVs e icacy is also de e mined by hei dosage. In a s udy by S o na i
e al, EVs ex ac ed om emb yonic mouse neu al p ogeni o cells
(NPCs) om spinal co d we e shown o di e en ia e ecipien NPCs o
as ocy es (S ona i e al., 2019). To assess he dosage e icacy, NPCs
we e exposed o wo di e en concen a ion o EVs. Resul s indica ed
ha cells ea ed wi h highe concen a ion o EVs had a highe pe -
cen age o Glial Fib illa y Acidic P o ein (GFAP)-posi i e as ocy es.
EVs can di ec ly pa icipa e in neu ogenesis. S o na i e al. demon-
s a ed ha NPCs de i ed om emb yonic mouse spinal co d can p o-
duce and elease exosomes bo h a he p oli e a ion and di e en ia ion
phases (S ona i e al., 2019). Cul u ed in expansion medium, NPCs
eleased exosomes which we e con i med by he p esence o exosomal
ma ke s TSG-101 and e aspanin ma ke s CD63 and CD81. Fu he -
mo e, when s imula ed by di e en ia ion mediums con aining ei he
N. Bah am Sangani e al.
P og ess in Neu obiology 205 (2021) 102124
5
Fe al Bo ine Se um (FBS) o BMP, NPCs showed a highly en iched
as ocy ic pheno ype wi h less p e e ence o neu onal pheno ype.
Simila ly, he ex ac ed EVs, om bo h di e en ia ed as ocy es and
neu onal cells, also s imula e he p oli e a ing NPCs mo e owa ds he
as ocy ic lineage. This s udy indeed indica es he impo an ole o EVs
in ansi ion o NPCs om a neu ogenic o a gliogenic lineage.
Neu ogenesis is an ongoing p ocess ha con inues in o adul hood
hough a conside ably lowe a es esul ing in he c ea ion and main-
enance o only a small size pool o NPCs. The pauci y o NPCs in
adul hood is a majo obs acle o he apeu ic s a egies employing NPCs
o NPC-de i ed EVs. Re-p og amming o soma ic cells in o NPCs has
p o ided a new oppo uni y. Using ansc ip ion ac o s B n2, Sox2, and
Foxg1 Ma e al. ep og ammed mouse ib oblas and as ocy es in o
induced NPCs (iNPCs) (Ma e al., 2019b). In e es ingly, hey demon-
s a ed ha iNPCs we e able o elease EVs a highe le els when
compa ed o wild ype (WT)-NPCs as de e mined by Nanopa icle
T acking Analysis and Wes e n Blo ing o EV ma ke s Flo illin-1, Flo-
illin-2 and HSP70. In addi ion, iNPC-EVs p omo ed p oli e a ion o
WT-NPCs signi ican ly s onge as compa ed o EVs omWT-NPC. P o-
eomics analysis e ealed ha , in ac , he exp ession le els o g ow h
ac o ela ed domains such as g ow h ac o ecep o cys eine ich
domain, EGF-like domain, and EGF-like calcium-binding domain a e
highe bo h in iNPC and in hei EVs. Fu he analysis by pe u ba ion o
unc ion assay sugges ed ha he a o emen ioned g ow h ac o s in-
c ease he p oli e a ion a e in NPCs h ough hei downs eam MER-
K/ERK pa hways. In a ollow up s udy, i was demons a ed ha
NPC-o igina ed exosomes (EXO) can induce he di e en ia ion o
co ical NPCs in o neu onal cells in di e en ia ion medium whe eas
iNPC-de i ed exosomes (iEXO) appea ed o possess a much lowe po-
ency o di e en ia e co ical NPCs in o neu onal cells (Ma e al.,
2019a). Nei he exosomes popula ion induced signi ican di e en ia ion
owa ds glial di e en ia ion. Mic oa ay analysis e ealed ha EXOs
ca y miRNA-21a a a much highe le el han iEXOs. Using a
miRNA-21a mimic and a miRNA-21a inhibi o i was demons a ed ha
he mimic supp essed he p opo ion o GFAP
+
glial cells and inc eased
he p opo ion o uj1
+
neu onal cells, whe eas he inhibi o caused he
opposi e e ec . Collec i ely, hese esul s highligh he ole o
miRNA-21a in de e mining he NPC cell a e mo e owa ds neu ogenesis
a he han gliogenesis. This is in con as wi h he esul s ob ained by
S o na i e al. (S ona i e al., 2019). Possible explana ions a e: Fi s ly,
S o na i e al. collec ed NPCs om mouse spinal co d while he NPCs in
he Ma e al.s udy we e de i ed om co ex. Bo h collec ed NPCs a day
13.5 o mouse emb yonic de elopmen . Secondly, S o na i e al. in es-
iga ed he di e en ia ion o NPCs in FBS and BMP4 di e en ia ion
medium.
Cell- o-cell communica ion is a i al pa o CNS s uc u e, unc ion,
and homeos asis bo h du ing de elopmen and a e wa ds in adul hood.
Besides elec ical and chemical signals, exosome-based cell-cell in e -
ac ion is also impo an no only be ween neu ons bu also in neu o-glial
in e ac ions. A wide ange o examples is p o ided h oughou his e-
iew. Fo ins ance, Mo on and colleagues iden i ied a bidi ec ional
in e ac ion be ween NCSs and mic oglia, which is media ed by EVs
(Mo on e al., 2018). They i s e ealed ha mu ine neona al sub-
en icula NSCs elease EVs. This was con i med by de ec ing he EVs
ma ke s CD63 and CD9 as well as he exosome ca go p o ein ALIX. The
des iny o eleased EVs was also de e mined by acking CD9-GFP pos-
i i e pa icles. The educ ion o hese pa icles o e he ime o e lapped
wi h in lux o mic oglia sugges ing an ac i e clea ance o EVs by
mic oglia. They epo ed ha mic oglia-iba1 posi i e cells a e
co-localized wi h CD9-GFP posi i e pa icles eleased om elec o-
po a ed SVZ. Fo u he assessmen , NSC-de i ed EVs o SVZ we e
labeled wi h he lipophilic dye DiI and ansplan ed o P0 mu ine pups.
The labeled EVs we e de ec ed in mic oglia-iba1 posi i e cells as well as
cells ha we e double posi i e o CD68 and CD11b (neona al sub-
en icula zone ma ke s). These esul s con i med ha SVZ
NSC-de i ed EVs a ge mic oglia. Fu he analysis e ealed ha hese
EVs we e en iched wi h miRNAs including miR-9, Le -7, and miR-26,
which belong o miRNA amilies wi h a ole in egula ing mic oglia
mo phology and physiology (Kuma e al., 2015; Lehmann e al., 2012;
Yao e al., 2014; L. Zhang e al., 2015). T ans ec ing exosomes wi h
syn he ic Le -7 miRNA enabeled hem o change mic oglia mo phology
and induced cy okine elease ha supp essed p oli e a ion o SVZ NSCs.
I was p oposed ha NSCs and mic oglia es ablish an in ica e
communica ion ne wo k based on EV elease ha egula es SVZ NSC
p oli e a ion. NPC-de i ed EVs a e also ins umen al in con eying
unc ional immune esponses. The s udy by Cosse i e al. e ealed ha
he con en o NPC-de i ed EVs mi o s he ac i a ion s a us o he
pa en al cell (Cosse i e al., 2014). T ea men o mu ine SVZ NPCs wi h
p oin lamma o y cy okines caused selec i e so ing o componen s o
he in e e on-gamma (IFN-ү) pa hway in o he EVs. Amongs hese
componen s was he in e e on gamma ecep o 1 (I ng 1), which was
exp essed a he EV’s su ace. These EVs igge ed IFN-ү-dependen s a 1
signaling in NIH 3T3 cells. Ad anced-imaging echniques demons a ed
a apid adhesion and inco po a ion o hese EVs in he a ge cells. As
opposed o hei as inco po a ion, hei deg ada ion was shown o be
a he slow due o he lack o lysosomal-associa ed memb ane p o ein 1
(LAMP1) sugges ing a sus ained and obus impac o hese EVs on he
ecipien cell. Toge he hese expe imen s illus a e he ole o
NPC-de i ed EVs in immune esponses and ha his ole is modula ed by
he mic oen i onmen o he EV-p oducing NPC.
In addi ion o being con eyo s o immune esponses, EVs can also
p o ide p o ec ion agains in lamma ion. I was p e iously epo ed ha
NPCs in conce wi h endo helial p ogeni o cells a e able o educe
hypoxia-induced ROS o e p oduc ion in b ain endo helial cells (ECs)
(Wang e al., 2016). The s udy by Liu and colleagues in es iga ed he
unde lying mechanism h ough which NPCs p o ide p o ec ion agains
oxida i e s ess in b ain ECs and highligh ed he ole o NPC-de i ed
exosomes (NPC-EXs) (H. Liu e al., 2017). They demons a ed ha
miR-210 in NPC-EXs media es he an ioxidan e ec on ECs by using
miR-210 mimic, miR-210 inhibi o and sc amble con ol miRNA.
NPC-EXs ca ying miR-210 educed Nox2 le els and apop osis o ECs
exposed o he oxida i e s ess induce angio ensin II (Ang-II). In addi-
ion, NPC-EXs ca ying miRNA-210 diminished Ang-II induced up egu-
la ion o eph in A3 and p e en ed Ang-II induced loss o ECs abili y o
ube o ma ion h ough no maliza ion o he
phospho yla ed-VEGFR2/VEGFR2 a io.
I is wo h men ioning he e ha he EV-media ed c oss alk be ween
neu al and b ain endo helial cells co e s a wide a ay o p ocesses. Fo
ins ance, neu on-de i ed exosomes a ge endo helial cells o egula e
he b ain ascula in eg i y (Xu e al., 2017). These exosomes ha bo a
neu on-en iched miR-132 ha up egula es ascula endo helial cad-
he in p o ein, also known as Cadhe in 5 (Cdh5), by di ec ly a ge ing
euka yo ic elonga ion ac o 2 kinase (ee 2k) in endo helial cells.
Dys unc ion o miR-132 educes Cdh5 exp ession (and i s pa ne
β-ca enin) and is associa ed wi h se e e in ac anial hemo hage and
dys egula ion o b ain ascula in eg i y in zeb a ish la ae.
EVs seem o ha e a dynamic in e ac ion wi h hei mic oen i on-
men . No only is hei composi ion de e mined by he mic oen i on-
men (Cosse i e al., 2014), hey can also condi ion he
mic oen i onmen . The s udy by I aci e al. demons a ed ha
NPC-de i ed EVs ca y me abolic enzymes and ac as independen
me abolic uni s capable o in luencing he composi ion o hei mic o-
en i onmen (I aci e al., 2017). Me abolic and unc ional analyses
showed ha NPC-de i ed EVs ca y unc ional Aspa aginase-like p o ein
1 (As gl1) ha con e s Aspa gine (Asn) in o Aspa a e (Asp), which is
eleased in o he mic oen i onmen .
4. Neu ogenesis o be con inued (Adul Neu ogenesis)
The exci ing disco e y in 1960’s, which e ealed ha neu ogenesis
pe sis s o adul hood, e olu ionized he dogma a ound emb yonic-bo n
neu ons as he solo inhabi an s o CNS. Neu ogenesis in adul hood akes
N. Bah am Sangani e al.

P og ess in Neu obiology 205 (2021) 102124
6
place a a limi ed a e and es ic ed o only wo main egions in he
b ain also known as neu ogenic niches. The subg anula zone o he
den a e gy us in hippocampus (E iksson e al., 1998) and he sub-
en icula zone in he la e al en icula wall o he ce eb al co ex
(Johansson e al., 1999). The NPCs in den a e gy us ep esen simila
ea u es o adial glial cells and a e he e o e e e ed as RG-like cells
(Ma inez-Ce deno & Noc o , 2018), hei unc ion is shown o be
impo an in cogni i e ac i i y o hippocampus such as pa e n sepa a-
ion (Aimone e al., 2011), spa ial lea ning and memo y (Dup e e al.,
2008). The newly p oduced neu ons in he adul SVZ mig a e o ol ac-
o y bulb and a e di e en ia ed o in e neu ons (Lim & Al a ez-Buylla,
2016). Sel - enewal and mul ipo ency o neu al s em /p ogeni o cells o
emb yonic o igin, and o some ex en adul hood NSC, ha e made hem
po en ial candida es o he apeu ic pu poses.
In egene a i e medicine, neu al s em cell ansplan a ion has ound
i s place as a ela i ely p omising ea men . The e has been a as
g ow h in clinical indica ions o such he apeu ic app oach including
Pa kinson disease, Hun ing on disease, Alzheime disease, Amyo ophic
la e al scle osis, spinal co d inju y and s oke (Takagi, 2016; Tang e al.,
2017). Howe e , he e a e s ill many challenges o be add essed. The
e hical issues owa ds NSC ansplan a ion a e nowadays a he pa ed
by ep og amming o soma ic cells in o induced plu ipo en s em cells
(iPSCs). Ne e heless, one should keep in mind he ad e se ou come
e en s associa ed wi h using such cell lines including he isk o
umo igenesis and immune esponse (Li e al., 2008; Nam e al., 2015).
In addi ion, he adminis e ed s em cells need o mig a e o he a ge si e
and di e en ia e in o he desi ed neu onal cell ype. They should also be
able o in eg a e wi h he neu al ci cui and o m p ope synap ic con-
nec i i y wi h he hos cells in o de o exe hei he apeu ic e ec
(Ma sh & Blu on-Jones, 2017). Mo eo e , ascula obs uc ion is
ano he challenge on he way o achie ing desi ed e icacy om NSC
g a ing (Lua e e al., 2016b; Xin e al., 2014). Recen ly, EVs ha e
become mo e and mo e app ecia ed as he apeu ic al e na i e o he
cell-based he apy. Inc easing lines o e idence suppo hei he a-
peu ic unc ionali y in he b ain in i o (Spellicy & S ice, 2020; Zhang
e al., 2019b) In addi ion, due o hei size, blood-b ain ba ie (BBB) is
ac ually no a ba ie on hei way o a ge b ain cells (Saeedi e al.,
2019).
EVs in adul neu ogenesis
In addi ion o NPCs, adul neu ogenic niches also hos a a ie y o
o he cell ypes such as as ocy es, mic oglia, neu oblas s, endo helial
cells, pe icy es as well as ma u e and imma u e neu ons (Ba iz e al.,
2016; Lua e e al., 2017). P ope communica ion be ween hese cellula
componen s is i al bo h o neu ogenesis and o he in eg a ion o
adul newbo n neu ons wi h synap ic ci cui y in he hippocampus
(Ca len e al., 2002; J. T. Goncal es e al., 2016). A combina ion o
ex insic ac o s including mo phogens, g ow h ac o s, and neu o-
ansmi e s oge he wi h in insic signals such as ansc ip ion ac o s
and epigene ic egula o s guide neu ogenesis h ough i s mul iple
s ages. Al hough he ole o EVs in adul neu ogenesis has ye o be
i mly es ablished, he e a e se e al lines o e idence suppo ing hei
in ol emen . Thei po en ial pa icipa ion in adul neu ogenesis has
been e iewed by mul iple s udies (Ba iz e al., 2016; Lua e e al.,
2016a; Lua e e al., 2017). Fo ins ance, miR-34a, which has a p o ound
ole in adul neu ogenesis and di e en ia ion o de eloping neu ons, has
also been de ec ed in exosomes eleased in he medium o neu onal
cul u es (Mollina i e al., 2015). In addi ion, as men ioned by B´
a iz e al,
some o he p o ein modula o s o adul neu ogenesis a e also ound in
exosomes hough om di e en cell ypes (Ba iz e al., 2016). Among
hem a e T ans o ming G ow h Fac o -be a 1 (TGFB1) (C. S. Hong e al.,
2014; Raimondo e al., 2015; Sole e al., 2015; Szajnik e al., 2013;
To eggiani e al., 2014), Eph in-B2 (Ma hi anan e al., 2010), Vascula
Endo helial G ow h Fac o (VEGF) (Eks om e al., 2014; Thompson
e al., 2013; To eggiani e al., 2014) as well as p o eins ha a e
in ol ed in cell a e decision in neu ogenic niches such as Pigmen
Endo helium-De i ed Fac o (PEDF) (Rami ez-Cas illejo e al., 2006)
(Haj asouliha e al., 2013), Insulin-like G ow h Fac o Binding P o ein 6
(IGFBP6) (Ba kho e al., 2006), Epide mal G ow h Fac o Recep o
(EGFR) (G ane e al., 2009), and Fib oblas G ow h Fac o 2 (FGF2)
(Haj asouliha e al., 2013). The same is also epo ed o some o he
p o eins o signaling pa hways such as Wn , No ch, and SHH signaling
pa hways (Ba iz e al., 2016; Wendle e al., 2013).
Fu he s udies a e needed o in es iga e hei di ec ole in adul
neu ogenesis o wo impo an easons. Fi s ly, EVs can nega i ely
a ec adul neu ogenesis. The injec ion o blood exosomes collec ed
om majo dep essi e diso de pa ien s in o mice is shown o cause
dep essi e-like beha io (Wei e al., 2020). Da a om miRNA
sequencing e ealed highe le els o hsa-miR-139-5p in blood exosomes
which is a nega i e egula o o neu ogenesis and leads o dys unc ion o
adul hippocampal neu ogenesis (Wei e al., 2020). Secondly, EVs can
ha e he apeu ic po en ial and can imp o e neu ogenesis. Sys ema ic
adminis a ion o exosomes loaded wi h miR-124 can p omo e neu o-
genesis a e ischemia inju y (Yang e al., 2017).
I has been sugges ed ha e-ac i a ing some o he de elopmen al
signalling pa hways in he adul migh be bene icial o amelio a e he
b ain inju ies (Goncal es e al., 2018). Fo ins ance, e inoic acid (RA),
as a guidance molecule, is c ucial o axon/neu i e ou g ow h du ing
de elopmen (Dme ichuk e al., 2006). The RA signaling pa hway has
been shown o egula e emyelina ion and axonal/neu i e ou g ow h
a e spinal co d inju y (Goncal es e al., 2019;Goncal es e al., 2018).
In e es ingly, exosomes we e shown as he impo an in e cellula
anspo e o RA ha enable c oss alk be ween oligodend ocy e p e-
cu so cells (NG2
+
cells) and neu ons o media e emyelina ion and
axonal/neu i e ou g ow h. The impo ance o EVs in media ing he
neu on-glia c oss alk was u he highligh ed by F uhbeis e al. The
au ho s demons a ed ha he small ex acellula esicles eleased by
oligodend ocy es a e i al o axonal main enance and suppo axonal
as anspo . This s udy sugges ed ha since oligodend oglial exo-
somes a e c i ical o neu onal in eg i y, hey could be a causa i e link
be ween glial dys unc ion and axonal degene a ion (F uhbeis e al.,
2020).
5. Synap ogenesis
Once neu onal mig a ion is accomplished, neu ons unde go
mo phological changes, also known as neu omo phogenesis, o app e-
cia e synap ic connec i i y (Co nell & Toyo-Oka, 2017). These changes
include he g ow h o axonal and dend i ic cones, which e en ually gi e
ise o o ma ion o p e-synap ic e minal and pos -synap ic si es. Axons
elonga e o each he app op ia e pos -synap ic a ge s o o m synapses
and acili a e neu o ansmission. The choice o pos synap ic si e is
di e en in glu ama e gic and GABAe gic neu ons. Glu ama e gic syn-
apses a e o med on dend i ic spines while GABAe gic synapses ake
place on somas and p oximal dend i es (Sanes e al., 2019). Synap o-
genesis is a long- e m de elopmen al p ocess ha ini ia es in he
de eloping b ain a a ound he week 28 o ges a ion and con inues o he
pos na al pe iod (Hu enloche , 1990). Synap ic densi y inc eases
exponen ially du ing in ancy and eaches i s peak, which is 50% highe
han in adul , a a ound 1-2 yea s. This end begins o decline be ween
ages 2 - 16 yea s (Hu enloche , 1979) by synap ic p uning.
EVs du ing synap ogenesis
As wi h he o he s ages o CNS de elopmen , synap ic di e en ia-
ion and ma u a ion p ocesses a e egula ed by a dis inc se o p o eins
and miRNAs. In e es ingly, he e is e idence ha synap ic componen s
a e inco po a ed in o EVs, suppo ing hei ole a synapses. Resul s
ob ained by elec on mic oscopy e ealed ha soma o-dend i ic com-
pa men s o di e en ia ed co ical and hippocampal neu ons can
elease exosomes in i o (Lachenal e al., 2011). Fu he mo e, he
exosome elease was shown o be modula ed by glu ama e gic synap ic
ac i i y and calcium in lux. I was i s demons a ed ha ma u e a
co ical and hippocampal neu ons sec e e exosomes. Using wo di e en
concen a ion o KCl, exosome elease was shown o be media ed by
N. Bah am Sangani e al.
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7
KCl-induced depola iza ion since an ibodies agains exosomal ma ke s
Flo illin-1 and ALIX showed s onge immuno eac i i y in he cell cul-
u e medium ea ed wi h he high KCl concen a ion. The e o e, exo-
some elease om ma u e neu ons is closely associa ed wi h
depola iza ion. In addi ion, incuba ion wi h he calcium ionopho e
ionomycin, which causes a ise in cy osolic calcium, inc eased exosome
elease. Fu he mo e, an agonis s o GABA ecep o signi ican ly
inc eased elease o exosomes, due o enhancemen o glu ama e gic
spon aneous ac i i y. This was u he con i med wi h an agonis s o
NMDA and APMA ecep o s. These an agonis s bo h inhibi ed he in-
c ease in exosome elease con i ming ha glu ama e gic synap ic ac-
i i y modula es he exosome elease om ma u e neu ons. In addi ion,
exosomes we e ound o con ain GluR2/3 subuni s o AMPA ecep o s,
which led he au ho s o sugges ha exosomal elease o AMPA e-
cep o s, which is ollowed by glu ama e gic synap ic ac i i y, is a
mechanism o elimina e synap ic ecep o s as a esponse o al e a ions o
synap ic plas ici y.
The p esence o AMPA ecep o subuni GluR2/3 was p e iously
epo ed in exosomes de i ed om p ima y cul u es o a co ical
neu ons. Fau ´
e e al. demons a ed ha a p ima y co ical neu ons a e
able o sec e e exosomes (Fau e e al., 2006). Cha ac e iza ion o hei
con en by immunoblo ing e ealed ha hey ca y GPI ancho ed p ion
p o ein and neu onal cell adhesion molecule L1 as well as AMPA e-
cep o subuni GluR2/3. Au ho s sugges ed ha exosomes migh ac as a
disposal mechanism in synapses -whe e lysosomes a e no p esen - o
disca d AMPA ecep o s in e en s such as synap ic dep ession. Howe e ,
hese exosomes we e shown o lack NMDA ecep o subuni NR1 indi-
ca ing ha such disposal s a egy is no ele an o he o he glu ama e
ecep o s. I was also demons a ed ha exosomal elease by p ima y
co ical neu ons can be egula ed by depola iza ion and ha GluR2
sec e ion upon depola iza ion is associa ed wi h exosomes. In a e iew
by Smalheise , mul iple scena ios we e discussed abou he ole o
exosomes de i ed om pos synap ic memb anes in ans e ing synap ic
p o eins e.g. calmodulin-dependen p o ein kinase II (CAM kinase II)
alpha and mRNAs and miRNAs o p esynap ic e minals, an ac ion ha
highligh s hei in ol emen in synap ic plas ici y (Smalheise , 2007). In
pa icula , i was p oposed ha exosomal loading and in acellula
anspo o synap ic signaling molecules occu a he pos synap ic lipid
a s.
In addi ion o ans e o synap ic ela ed p o eins, EVs a e con i med
o ca y miRNAs wi h unc ionali ies a synapses. The s udy by Mo el
e al. indica ed a ole o exosomes in modula ing synap ic ac i i y
h ough as ocy e-neu onal communica ion (Mo el e al., 2013). In hei
s udy, miR-124a was highligh ed as he key componen ca ied by
exosomes om neu ons o as ocy es whe e i a ge s Glu ama e Syn-
hase NADH (GLT1). GLT1 is a oden analog o human exci a o y amino
acid anspo e 2 (EAAT2), an as oglial synap ic p o ein wi h a key ole
in glu ama e up ake and clea ance a synapses (Kim e al., 2011). I was
demons a ed ha GLT1 physiological unc ion depends on he signals
ecei ed by neu ons. P o ein and mRNA le els o GLT1/EAAT2 in as-
ocy es we e signi ican ly inc eased when co-cul u ed wi h neu ons
(Gegelash ili e al., 1997; Schlag e al., 1998). Mo eo e , due o ha ing
long 3´un ansla ed egion (UTR), au ho s p oposed ha GLT/EAAT2
could be a a ge o miRNA-media ed egula ions. I was i s demon-
s a ed ha p ima y cul u es om mouse ce eb al co ex neu ons and
as ocy es sec e e exosomes. In e es ingly, miR-124a was highly
en iched in neu ons and hei ha es ed exosomes while cul u ed as-
ocy es p esen ed only a minimum amoun o miR-124a. Neu onal
exosomes we e hen shown o be in e nalized in as ocy es as de ec ed
by ime-lapse imaging o he luo escen ly labeled exosome memb anes.
Fu he mo e, hese exosomes we e able o inc ease GLT1 p o ein le els
in cul u ed as ocy es. Di ec ans ec ion o as ocy es wi h miR-124a
inc eased GLT1 p o ein exp ession wi hou any change on mRNA le el
indica ing ha miR-124a egula o y e ec is a he ansla ional le el. In
i o expe imen s in which a speci ic an isense miR-124a was injec ed
in o he s ia um pa o he mouse b ain esul ed in educ ion o GLT1
and glu ama e up ake. Mo eo e , i was indica ed ha miR-124a egu-
la o y e ec on GLT1 is independen o ligand eph in3, a pu a i e sup-
p esso o GLT1, con i ming ha miR-124a associa ion wi h GLT1
exp ession is indi ec . Mo eo e , exogenous deli e y o miR-124a in o
he SOD1 mouse model o amyo ophic la e al scle osis e icien ly
educed pa hological loss o GLT1 in his model. Collec i ely, hese e-
sul s unde sco e he impo ance o EVs in egula ing synap ic unc ion
by a ge ing as ocy es ha ac alongside o neu ons o ensu e no mal
synap ic unc ion.
A ecen s udy by Sha ma e al. highligh ed he po en ial ole o EVs
in imp o ing neu ogenesis, synap ogenesis, and ci cui assembly in Re
synd ome (Sha ma e al., 2019). Re synd ome is a neu ode elopmen al
diso de and is caused by mu a ions in he me hyl-CpG-binding p o ein
2 (MECP2) gene (Ami e al., 1999). MECP2 gene exp ession is co e-
la ed wi h neu onal ma u a ion and synap ogenesis (Fukuda e al.,
2005; Shahbazian e al., 2002). P o eomics analysis conduc ed by
Sha ma e al. demons a ed ha exosomes om hiPSC-de i ed neu al
cul u es con ibu e o neu al ci cui de elopmen owing o he signaling
p o eins hey ca y. Au ho s showed ha hese p o eins a e absen om
exosomes collec ed om MECP2 loss-o - unc ion (MECP2LOF) cul u es.
Fu he mo e, ea men wi h con ol exosomes we e e ec i e, bo h in
i o and in i o, in imp o ing neu ogenesis. Exosome ea men also
imp o ed synap ogenesis and ci cui connec i i y in MECP2LOF cul-
u es. Based on he esul s, au ho s sugges ed ha MECP2 mu a ion
esul s in he al e a ion o p o ein ca gos and signaling bioac i i y o
exosomes.
EVs a e also in ol ed in synap ic g ow h and unc ion. Fo ins ance,
exosomes a e shown o media e communica ion be ween p e-and pos -
synap ic cells by ans e ing a e og ade signaling componen , Syn-
ap o agmin 4 (Sy 4), which is essen ial du ing de elopmen and o
main enance o synap ic plas ici y and g ow h (Ko ku e al., 2013). I
was demons a ed ha p esynap ic neu ons a neu omuscula junc ions
in D osophila elease Sy 4 ia exosomes. Sy 4 is ecei ed by pos -
synap ic muscles and egula e he ac i i y-dependen synap ic g ow h
and po en ia ion o spon aneous elease. In ac , exosomes coo dina e
he p esynap ic unc ion wi h pos synap ic ou pu by means o Sy 4
ans e . The EVs in ol emen in synap ic plas ici y goes beyond
ans e ing e og ade signaling. They also ans e he neu onal A c
mRNA. A c p o ein ac i i y is i al o long- e m memo y and consoli-
da ion o in o ma ion as well as synapse elimina on (Pas uzyn e al.,
2018) and i has been implica ed in se e al neu ode elopmen al diso -
de s such as Angelman synd ome (G ee e al., 2010; Pas uzyn &
Shephe d, 2017), F agile X synd ome (Pa k e al., 2008), and Schizo-
ph enia (F ome e al., 2014; Manago e al., 2016; Pu cell e al., 2014). I
has been demons aed ha A c EVs can ans e highly abundan A c
mRNAs o dend i es e.g. in esponse o neu onal ac i i y (Pas uzyn e al.,
2018). Simila ly, he A c homolog in D osophila, dA c1, uses EVs o i s
own mRNA ans e a neu omuscula junc ions (Ashley e al., 2018).
6. Synap ic p uning
The excess in synap ic con ac s c ea ed du ing ea ly in ancy will be
elimina ed in a p ocess called synap ic p uning. Sculp ing he synapses is
c ucial o p ope neu al ci cui o ma ion and plas ici y. I is es ima ed
ha nea ly hal o he synapses and neu ons will be emo ed by p uning
and apop osis (Jiang & Na delli, 2016; S iles & Je nigan, 2010).
Mic oglia a e he co e playe s in his e inemen p ocess (Ke enmann
e al., 2013; Scha e e al., 2012). Ha ing close associa ion wi h p e-
synap ic and synap ic elemen s, mic oglia a e hough o a ge weak
synapses o phagocy osis (S. Hong e al., 2016; Scha e e al., 2012;
T emblay e al., 2010). Dys unc ion o ecip ocal in e ac ion be ween
mic oglia and neu ons a e obse ed in neu ode elopmen al and
neu opsychia ic diso de s (Zhan e al., 2014).
EVs ole in synap ic p uning
EVs a e shown o ac as endogenous ac o s able o elimina e syn-
apses. Lee and collogues e ealed an in e play be ween Wn and P oline-
N. Bah am Sangani e al.
P og ess in Neu obiology 205 (2021) 102124
8
Rich 7 (PRR7) ia exosomes ha esul s in egula ing he numbe o
exci a o y synapses (Lee e al., 2018). Wn signaling is conside ed as one
o he i al synap ogenic ac o s du ing bo h synapse o ma ion and
main enance (Dickins & Salinas, 2013). PPR7 was i s iden i ied by
p o eomic analysis in pos synap ic densi y ac ion o a o eb ain.
Al hough i s unc ion was no clea a he ime, he in e ac ion wi h
NMDA ecep o and Pos synap ic Densi y P o ein 95 (PSD-95) sugges s a
ole o PRR7 in egula ing neu al ac i i ies (Mu a a e al., 2005). The
ecip ocal in e ac ion be ween PPR7 and Wn epo ed by Lee e al.
un a eled a ole o PRR7 a synapses. Bo h PPR7 and Wn we e shown
o be eleased in exosomes by a hippocampal neu ons. The opposi e
unc ions o Wn and PRR7 main ain he balance be ween synap o-
genesis and synapse emo al and in e es ingly his ac ion is acili a ed
by exosomes as signal ca ie s. Au ho s p o ided a se o molecula
mechanisms h ough which exosomal PRR7 elimina e synapses. These
include inhibi ing exosomal sec e ion o Wn , p o ein deg ada ion o
PSD-95, and ac i a ion o GSK3β as he downs eam componen o Wn
signaling. Fi s , i was shown ha PRR7, Wn 5a, and Wn 7a a e highly
en iched in exosomes de i ed om mouse hippocampal neu ons.
Fu he mo e, using NMDA ecep o an agonis , exosomal elease o
PRR7 was shown o be educed while AMPA ecep o an agonis did no
achie e he same esul indica ing ha neu ons elease PRR7 in a
NMDAR-dependen manne . One o he mechanisms o synap ic elim-
ina ion is Ubiqui in–P o easome Sys em (UPS), a s a egy ha is also
applied by PRR7 o educe synap ic sca olding p o eins including
PSD-95, Memb ane Associa ed Guanyla e Kinases (MAGUKs), and
SAP90/PSD-95-associa ed p o eins (SAPAPs). In addi ion, PRR7 o e -
exp ession inc eased he o al numbe o poly-ubiquina ed p o eins
con i med by an ibodies agains K48-speci ic poly-ubiqui ina ion.
In e es ingly, PRR7-con aning exosomes we e shown o a ge exci -
a o y synapses o emo al. This was e iden when incuba ion o naï e
neu ons wi h hese exosomes o 24 h esul ed in educ ion o exci a o y
synapse numbe s, de e mined by PSD-95 co-localiza ion wi h esicula
Glu ama e T anspo e 1 ( GLUT1), wi h no al e a ions in he numbe o
inhibi o y synapses.
In e es ingly, he e has been ecen ly a ole iden i ied o exosomes
as a egula o o synap ic p uning h ough hei phagocy ic abili y.
Cul u ing a pheoch omocy oma PC12 cells in a se um- ee medium
con aining Ne e G ow h Fac o (NGF), Bah ini and colleagues we e
able o induce he o ma ion o neu i e ou g ow h and synap ic-like
s uc u es (Bah ini e al., 2015). Au ho s also sough o assess he ole
o mic oglia in he clea ance o degene a ing neu i es. I was obse ed
ha neu i es become degene a ed wi hin wo days in he absence o
NGF, howe e in DMEM con aining 10% FBS he degene a ion a e was
lowe wi h mos o he neu i es s ill ese ed. In iguingly, when PC12
cells we e co-cul u ed wi h MG6, a mic oglial cell line de i ed om
mouse, in DMEM/F10 lacking NGF, neu i e elimina ion a e ema kably
inc eased indica ing he p omo ion o p uning by mic oglia. Conside ing
he p e ious indings ha depola ized neu ons sec e e exosomes (Fau e
e al., 2006) and ha neu onal de i ed exosomes a ge mic oglia
(Cosse i e al., 2012), Bah ini e al. also in es iga ed he associa ion
be ween PC12 cells-de i ed exosomes and mic oglial unc ion. PC12
cells we e p e-incuba ed wi h MG6 cells o 16 h. Compa ison o syn-
ap ic p uning in p e-incuba ed MG16 wi h con ol MG6 e ealed
s onge abili y o p e-incuba ed MG6 cells in neu i e p uning and i
was he e o e p oposed ha exosomes enhance mic oglia p uning ac-
i i y. Mic oa ay analysis iden i ied 183 di e en ially exp essed genes
in p e-incuba ed MG6 cells wi h “Phagosome’’ and “Complemen and
coagula ion cascades’’ being among he en iched e ms. Fu he quan-
a i e PCR analysis indica ed he up- egula ion o complemen ac o B
(C b) and complemen componen 3 (C3) genes. Since C3 mRNA le el
emained unchanged, au ho s sugges ed ha exosome egula o y e ec
on C3 is a he ansc ip ional le el as opposed o di ec ly ans e ing
he mRNA. Thus, exosomes de i ed om PC12 cells can be engul ed by
mic oglia whe e hey enhance phagocy osis by up egula ing he
exp ession o complemen ac o s. Howe e , he ac o s in exosomes
ha induce such changes in mic oglia ha e ye o be iden i ied.
7. Myelina ion
Fas anspo o ac ion po en ials is only possible when axons a e
enw apped by myelin shea hs. Myelina ion is a long-las ing e en
s a ing ea ly du ing pos na al pe iod and ex ending in o adul hood
(Semple e al., 2013). Oligodend ocy es and Schwann cells o ganize
myelina ion in cen al and pe iphe al ne ous sys ems espec i ely.
Oligodend ocy e P ogeni o Cells (OPC) a e p oduced by NECs in en-
icula zones (Be gles & Richa dson, 2015; Jiang & Na delli, 2016).
A e mig a ion h oughou he CNS, OPC will be dis ibu ed in g ay and
whi e ma e s whe e hey unde go di e en ia ion o become
p e-oligodend ocy es and e en ually ma u e myelina ing oligodend o-
cy es (Be gles & Richa dson, 2015; Jiang & Na delli, 2016). Once
di e en ia ed, hey ex end hei plasma memb ane owa ds axons and
w ap hem wi h mul ilaye ed myelin shea hs. The e is also an inc ease in
he gene exp ession o myelin ela ed genes such as Myelin Basic P o ein
(MBP), myelin P o eolipid P o ein (PLP), Myelin-Associa ed Glycop o-
ein (MAG), and Myelin Oligodend ocy e Glycop o ein (MOG) (Be cu y
& Macklin, 2015; an Tilbo g e al., 2018) in OPC p io o di e en ia-
ion. I is ob ious ha neu on-oligodend ocy e ecip ocal in e ac ion
and he exchanged biomolecules be ween hem a e he key ac o s o
he myelina ion p ocess.
EVs ole in Myelina ion
Accumula ing e idence poin s o he in ol emen o EVs in o ches-
a ing myelina ion in CNS. We only b ie ly discuss he e idence in his
sec ion and e e he eade o a ecen excellen e iew on his opic by
S. Domingues e al. (Domingues e al., 2020).
Exosomes sec e ed by oligodend ocy es a e ca ie s o p o eins ha
ha e been ecognized o play a ole in myelina ion. The s udy by
K ame -Albe s e al. e ealed ha exosomes sec e ed by oligodend o-
cy es con ain se e al myelin p o eins including PLP, 2’3’ -Cyclic-
Nucleo ide-Phosphodies e ase (CNP), MBP, and MOG (K ame -Albe s
e al., 2007). Mo eo e , i was demons a ed ha exosome sec e ion and
he e ogenei y o sec e ed exosomes a e egula ed by in acellula Ca
2+
le els sugges ing ha exosome elease by oligodend ocy es is coupled o
neu onal ac i i y. Besides myelin p o eins, he au ho s also iden i ied
nume ous o he p o eins including chape ones and enzymes wi h p o-
posed unc ions in he elie o cell s ess. Thus, hey sugges ed ha
oligodend ocy e-de i ed exosomes also p o ide ophic suppo o he
axons. Fu he cha ac e iza ion o oligodend oglial exosomes e ealed
he p esence o he classic myelin lipids galac oce eb oside and sul a ide
indica ing ha oligodend oglial exosomes a e uniquely equipped o
suppo myelina ion o axons.
As men ioned, he e is a bidi ec ional in e ac ion be ween oligo-
dend ocy es and neu ons and EVs a e one o he media o s. We no iced a
eedback p inciple men ioned by mos o he pape s. Exosomes a e
sec e ed om oligodend ocy es unde he in luence ecei ed by neu ons
o a ange a se o al e a ions in neu ons. Fo ins ance, in hei s udy
F uhbeis e al. demons a ed ha neu onal elec ical ac i i y (depola -
iza ion) igge s glu ama e elease, which induces Ca
2+
in lux in o oli-
godend ocy es h ough AMPA and NMDA ecep o s and subsequen ly
s imula es exosome elease (F uhbeis e al., 2013b). These exosomes a e
hen in e nalized by neu ons h ough endocy ic pa hway. U ilizing
mic o luidic chambe s i was shown ha exosomes we e aken up by
neu ons a axonal and soma odend i ic si es (F uhbeis e al., 2013a). To
un a el he bioac i i y o exosomes, a Boyden chambe co-cul u e o
neu ons and oligodend ocy es was p epa ed. Neu ons we e subjec ed o
oxida i e s ess and s a a ion ia hyd ogen pe oxide and absence o
B27 supplemen espec i ely. These changes inc eased me abolic ac-
i i y in he p esence o oligodend ocy e-de i ed exosomes sugges ing
ha exosomes con e p o ec ion o neu ons. The suppo i e ole o
oligodend oglial exosomes on neu ons is no limi ed o neu op o ec ion.
N. Bah am Sangani e al.
P og ess in Neu obiology 205 (2021) 102124
9
F ¨
ohlich e al. epo ed a b oad spec um o oles o
oligodend ocy e-de i ed exosomes, which include ac i a ion o
signaling pa hways such as MEK/E k and PI3K/Ak , neu onal gene
exp ession, and enhancemen o ac ion po en ial, as well as esis ance o
oxida i e s ess and p omo ion o neu onal su i al (F ohlich e al.,
2014).
In addi ion o se ing as ini ia ing ac o in myelina ion, neu ons can
also nega i ely egula e myelina ion. Bakh i e al. epo ed ha oligo-
dend ocy es sec e e exosome-like esicles ha has au oinhibi o y e ec
on cell di e en ia ion by educing he cell su ace expansion and sub-
sequen ly inhibi ing myelina ion (Bakh i e al., 2011). Rega ding he
egula o y ole o Rho-associa ed kinase (RhoA-ROCK) pa hway in
oligodend ocy e b anching and cell su ace expansion (Kippe e al.,
2009; Kippe e al., 2007; Liang e al., 2004), au ho s es ed he e ec o
Rock inhibi o and Myosin II inhibi o on cell su ace size. I was
obse ed ha such inhibi o s p e en ed he nega i e egula o y e ec o
exosome-like esicles on cell expansion. Thus, exosome-like esicles
inhibi cell su ace size h ough he ac i a ion o Rho-ROCK-myosin
signaling axis. Fu he mo e, incuba ion o oligodend ocy es wi h
neu onal condi ioned medium obus ly educed exosome elease and
he e o e au ho s p oposed he likelihood o one o mo e neu onal
ac o s in neu onal condi ioned medium ha p e en he exosome
sec e ion. The e o e, neu onal signals egula e myelin memb ane
biogenesis h ough con olling he exosome elease.
Oligodend ocy es ha e been shown o ecei e ins uc i e signals
-inco po a ed in exosomes- also om o he cell ypes such as as ocy es
and dend i ic cells. Fo ins ance, i was obse ed ha di ec con ac
cul u e o OPC wi h as ocy es signi ican ly enhanced exosome sec e ion
and OPC p oli e a ion (Zhang e al., 2020) when compa ed o OPC
co-cul u ed wi h AST supe na an g oup. T ansc ip ome sequencing
e ealed a se o di e en ially exp essed genes be ween he wo g oups
among which was he up egula ion o In eg in subuni Be a 4 (ITGB4), a
p o ein which media es cell adhesion sugges ing an impo an ole o
ITGB4 in OPC p oli e a ion. This was con i med by ITGB4 gene knock-
down, which esul ed in educ ion o exosome elease and p oli e a ion
a e. When exosomes we e added o he ITGB4 de icien OPC/as ocy e
co-cul u e p oli e a ion a e was es o ed. I was he e o e concluded
ha OPC p oli e a ion is egula ed by as ocy es h ough ITGB4 medi-
a ed exosomal sec e ion.
A conside able numbe o pape s ha e highligh ed he he apeu ic
e ec s o ex acellula esicles and exosomes in p omo ing oligoden-
d ocy es p oli e a ion and/o di e en ia ion as well as emyelina ion
and axon egene a ion. Fo example, in p ena al b ain inju y, which
a ec s whi e and g ay ma e and causes se e e neu ode elopmen al
pheno ypes, Mesenchymal s omal cell-de i ed exosomes we e e ec i e
in escuing myelina ion and educing inju ies o g ay and whi e ma e
(Thomi e al., 2019). In ano he s udy, ea men wi h Mesenchymal
s em cell-de i ed EVs in a a model o p e e m b ain inju ies e icien ly
amelio a ed in lamma ion induced hypomyelina ion, neu onal cell
degene a ion and long- e m whi e ma e mic os uc u al abno mali ies
(D ommelschmid e al., 2017). Wi h p omising ou comes, en i on-
men al en ichmen (EE) also seems an e ec i e ea men in imp o ing
b ain unc ion. EE is desc ibed as enhancemen o physical, social and
in ellec ual ac i i y (Pusic e al., 2016). Exosomes ha e been ound as
one o he e ec i e neu op o ec ion elemen s in EE (Pusic & K aig,
2014). Applica ion o bo h young and EE-se um exosomes on
Fig. 2. Schema ic o exosome sec e ion, s uc u e, and in ol emen du ing CNS de elopmen . The exosome ca gos a e so ed in o ea ly endosomes h ough ei he
endocy osis, inwa d olding o plasma memb ane, o di ec ly om Golgi ne wo k (Palmulli & an Niel, 2018), a p ocess ha is media ed by he endosomal so ing
complexes equi ed o anspo (ESCRT) machine y. In agina ion o he la e endosomes, also e e ed as Mul i esicula bodies (MVBs), o ms in aluminal esicles
(ILVs). These ILVs a e e en ually eleased as exosomes in o ex acellula space by usion o MVBs wi h plasma memb ane. The p ecise mechanism o elease is no
clea howe e , in acellula calcium and depola iza ion, media ed by glu ama e gic synap ic ac i i y, a e indica ed as igge s o exosome sec e ion. Some o he
exosome con en s a e highligh ed a each s ages o CNS de elopmen . As i is shown, miRNAs a e he mos s udied elemen o exosomes du ing neu o-gliogenesis.
CAM kinase II alpha is sugges ed as one o he candida e ca gos o synap ic exosome, a scena io which needs o be in es iga ed (Smalheise , 2007). In e es ingly,
exosomes ac i ely in ol e in myelina ion mainly by anspo ing myelin ela ed p o eins be ween neu ons and oligodend ocy es.
N. Bah am Sangani e al.