Role of Glia In Reproduction and Consequent Human Therapeutic Potentials … · 2013. 5. 11. · 1. Neural cell adhesion molecule(PSA-NCAM)-with GnRH –glial adhesiveness mediated

Article ID: WMC004222 ISSN 2046-1690

Role of Glia In Reproduction and ConsequentHuman Therapeutic Potentials-A Systematic ReviewCorresponding Author:Prof. Kulvinder K Kaur,Scientific Director, Obstetrics & Gynaecolgy (Reproductive Neuroendocrinology & Infertility Specialist, 721,GuruTeg Bahadur Nagar, 144003 - India

Submitting Author:Prof. Kulvinder K Kaur,Scientific Director, Obstetrics & Gynaecolgy (Reproductive Neuroendocrinology & Infertility Specialist, 721,GuruTeg Bahadur Nagar, 144003 - India

Article ID: WMC004222

Article Type: Review articles

Submitted on:11-May-2013, 06:20:59 AM GMT Published on: 11-May-2013, 12:05:31 PM GMT

Article URL: http://www.webmedcentral.com/article_view/4222

Subject Categories:REPRODUCTION

Keywords:Astrocytes, erbB, Tanycytes, PGE2, SynCAM , Neuregulins, Puberty, GnRH secretion.

How to cite the article:Kaur KK. Role of Glia In Reproduction and Consequent Human Therapeutic Potentials-ASystematic Review . WebmedCentral REPRODUCTION 2013;4(5):WMC004222

Copyright: This is an open-access article distributed under the terms of the Creative Commons AttributionLicense(CC-BY), which permits unrestricted use, distribution, and reproduction in any medium, provided theoriginal author and source are credited.

Source(s) of Funding:

None

Competing Interests:

None

WebmedCentral > Review articles Page 1 of 21

http://www.webmedcentral.com/article_view/4222http://creativecommons.org/licenses/by/3.0/http://creativecommons.org/licenses/by/3.0/

WMC004222 Downloaded from http://www.webmedcentral.com on 11-May-2013, 12:05:32 PM

Role of Glia In Reproduction and ConsequentHuman Therapeutic Potentials-A Systematic ReviewAuthor(s): Kaur KK

Abstract

Background: Recent evidence suggests thatastrocytes have important neuroregulatory functionsbesides classic functions of support and segregationof neurons which includes regulation of neuroncommunication,neurosecretion and synapticplasticity.The aim of this review was to focus onastrocyte-neuron interactions in the hypothalamus,specially with respect to their potential contributions tothe regulation of gonadotropin hormone(GnRH)secretion,and their role in initiation of puberty.

Methods: A systemic review of international literature by a search of PUBMED and authors files was donefor glia in sexual maturation/puberty, reproduction/incontrol of GnRH secretion with reference to mostlyanimal studies and occasional human MRI studies andcases of precocious puberty which are currentlygetting available with advent of MRI. .

Results: Both from animal studies as well asoccasional human MRI data it is clear that tanycyteplasticity varies with the state of ovarian steroids andthat how precocious puberty in humans occurs inhamartomas lacking GnRH neurons and that role ofg l i a l g r o w t h f a c t o r s ( T G F α/erbB1,neuregulins/erbB4)signaling is importantbesides prostaglandin E2 (PGE2) production forGnRH secretion.

Conclusions: Glial-neuronal interactions areimportant for neuroendocrine control of femalepuberty,regulation of GnRH secretion.Growth factorsof the epidermal growth factor(EGF) family activatingvia erbB receptors(with tyrosine kinase activity) play amajor role in glia-neuron communication.Inturn,neurons facilitate astrocytic erbB signaling viaglutamate dependent cleavage of erbB ligandprecursors.Genetic disruption of erbB receptors delays sexual development due to impaired erbBligand –induced glial PGE2 release.Besides theproduction of growth factors,glial cells contribute toinitiation of puberty by plastic rearrangement ofglia-GnRH neuron adhesiveness.

Introduction

It is increasingly clear that astrocytes play animportant role in maintaining central nervoussystem(CNS)function(1-3)and controlling key bodilyprocesses such as breathing(4),sleep(5)andreproduction(6).Because of their perivascular andinterneuronal localization astrocytes are wellpositioned to sense afferent neuronal and blood bornesignals and ideally suited for the temporal and spatialpropagation of these signals(7-9).The activation ofa s t r o c y t e s l e a d s t o t h e r e l e a s e o fgliotransmitters(8,10)that trigger rapid responses inneighbouring cells and thus contribute to the regionspecific homeostatic regulation of neuronal function.

As the most abundant cell type in the brain ,glial cellsoutnumber neurons by a 9:1ratio.The majority of thembeing astrocytes which are so named because of theirstarlike shape. Traditionally astrocytes have beenrelegated primarily to a supportive or structural role inbrain.However ,there is a growing literature thatsuggests astrocytes are also an important source ofneuroact ive substances,such as growthfactors,eicosanoids,and neurosteroids which maysubsequently affect neuronal development,survival,and neurosecretion. In the hypothalamus astrocytesregulate the secretory activity of neuroendocrineneurons(11-14).Asubset of such neurons secretes thed e c a p e p t i d e g o n a d o t r o p i n r e l e a s i n ghormone(GnRH),which controls both the initiation ofpuberty and adult reproductive function.In rodentsGnRH neurons are mostly located in the preopticregion of the ventral forebrain.The importance ofstudying the rodent model has been highlightedearlier(15).These GnRH neurons project to the medianeminence of the hypothalamus where GnRH isreleased into the pituitary portal blood for delivery tothe anterior pituitary.As the projection field ofneuroendocrine GnRH neurons ,the medianeminence (ME) of the hypothalamus is poised to playa crucial role in the precise regulation of GnRHrelease and therefore central to the control of thereproductive axis

The Role of Glial cells in the maturation ofneuronal circuits regulating GnRH neurons



The glial cells are important mediators of the sexualdifferentiation of neuronal connectivity induced bygonadal hormones.This is substantiated by findings ofseveral laboratories indicating that the morphology,immunoreactivity,enzymatic activity,and geneexpression of astroglia are sexually dimorphic inseveral brain areas and can be modified by thepostnatal actions of sexhormones.Furthermore ,thegl ia l cel ls express receptors for gonadalhormonesii)metabolize gonadal steroids,iii)andparticipate in the synthesis of endogenous steroids bythe nervous system(16).Sex differences in thedifferentiation of astroglia may impact on theorganization of neuronal network that regulates theactivity and secretion of GnRH neurons Exposure ofG n R H n e u r o n a l n e t w o r k i n f e m a l eanimals(guineapigs,mice,sheep,rhesus monkeys )totestosterone results in modification in number andfunction of synaptic inputs to GnRH neurons andanimals that have been exposed in utero totestosterone have an impaired ,male like response tothe E2 –stimulated surge.

Neuroglia plasticity in the arcuate nucleus mayintegrate the action of different hormonal signals,including estradiol (E2) and leptin,which maycoordinate GnRH release with other physiologicalchanges at the onset of puberty.Consequently, therole of glial cells in maturation of neuronal circuitsregulating GnRH neurons has been studied in detail inthe arcuate nucleus of the rat hypothalamus.In thisnucleus ,paral le l maturat ion of neuronalmembranes,glial cells and synaptic inputs during thej u v e n i l e a n d p r e p u b e r t a l m a t u r a t i o nperiod(17)generates a sexually dimorphic organizationof synapses and c glia such that ,after puberty females,but not males respond to neuroplastic actions ofE2(18).These sex differences are induced by theperinatal secretion of testosterone(Tn) in malerats.Perinatal Tn increases in astrocytes theexpression of a cytoskeletal protein that regulatesastroglia cells morphology,glia fibrillary acidicprotein(GFAP),increasing the growth of astrocyticprocesses and the extent of neuronal membranescovered by these processes .Coincident with thesechanges in astrocytic morphology there is a strongreduction in the density of dendritic spines andaxo-somatic synapses on arcuate neurons inmale(19).

Role of Glial-Neuronal Interactions in Initiation ofPuberty

Pubertal activation of GnRH secretion requires

information from glial cells,besides transsynapticinputs(20-reviewed in ojeda2010).Both astrocytes andependomyoglial cells lining the ventral surface of thethird ventricle(tanycytes)produce cell to cell signalingmolecules that stimulate GnRH release ,and that arenecessary for timing of puberty(21)Glial cellscontribute to the pubertal activation of GnRH secretionvia two complementary mechanisms.i)One involvesgrowth factors of at least four different familiesa)Transforming growth factorbeta(TGFβ) ,of the TGFβsuperfamily,is recognized by the cellmembranereceptors endowed with serine threonine kinaseactivityand that are located within Gn RH neurons(22).Upon binding TGFβ enhances GnRH geneexpression and Gn RH secretion(23).Growth factors ofthe other three families ,including theb)epidermalgrowth factor(EGF),family,c)basic fibroblast growthfactor (bFGF) and d)insulin like growth factor1(IGF1)are recognized by receptors with tyrosinekinase act iv i ty .Some of these receptors(FGFR,IGF-1R)are expressed in GnRH neurons,buterbB receptors (which recognize EGF and EGF –likepeptides)are mostly expressed on glial cellsthemselves.Genetic disruption of erbB receptorsdelays female sexual development due, at least inpart, to impaired erbB l igand induced gl ialprostaglandin E2(PGE2) release(21)While growthfactors of glial origin set in motion glia-to neuronsignaling pathways ,atleast one neuron-to gliaregulatory pathway initiated by glutamatergic neurons,has been shown to facilitate astrocytic signalingmediated by erbB receptors(24).

The second mechanism invo lves p last icrearrangement of cell adhesiveness. MoleculesMediating glial-GnRH neuron adhesiveness Although reviewed in detail by ojeda et al the main moleculespostulated to mediate glia-GnRH neuron adhesivecommunications in the hypothalamus are

1. Neural cell adhesion molecule(PSA-NCAM)-withGnRH –glial adhesiveness mediated by hemophilicinteraction with NCAM 140(25)2. GnRH neurons adhere to astrocytes by hemophilicinteractions mediated by synaptic cell adhesionmolecule 1(SynCAM 1) and 3)by heterophilicinteractions mediated by the binding of contactinpresent in GnRH neurons to the receptor Receptor–like Protein Tyrosine Phosphataseβ(RPTPβ)-thatuses its carbonic anhydrase(CAH) extracellularsubdomain to interact with contactin.Immunoreactivecontactin found to be abundant in OVLT andME,suggesting GnRH axons are an important site ofcontactin dependent cell adhesiveness.



Addi t iona l ly th ree mul t igene fami les o fadhesion/signaling molecules with complementaryfunctions has been reported in both the prepubertalfemale monkey hypothalamus and the GnRHsecreting cell linesGT1-7.One of these families iscomposed of a large number of synaptic specifierstermed i)neurexins ;another is formed byii)protocadherins,a group of membrane-anchoredproteins that function as a synaptic adhesionmolecules.iii)The third family consists of members ofthe contactin associated protein(Caspr)genefamily(26)Despite the unmistakable presence of thesemolecules in the neuroendocrine brain,thecontributions they have to adhesiveness of GnRHn e u r o n s a n d g l i a l c e l l s r e m a i n t o b eestablished,however contactin has been shown tointeract in cis with Caspr 1 whose cytoplasmic domaincontains a proline –rich sequence with a canonicalSH3 domain that associates with atleast four SHdomains –containing proteins C,includingSrc,Fyn,p85and PLCγ.(27)Altogether these resultsindicate that GnRH neurons adhere to astrocytesusing both heterophilic(contactin/RPTPβandhomophilic (SynCAM/SynCAM)interactions .Becauseboth systems have signaling capabilities it wouldappear that in addition to providing an adhesiveinteraction ,these molecules can activate intracellularsignaling cascades in both Gn RH neurons andastrocytes(21).(fig1)

The pubertal process can be set in motion prematurelyby the pathological activation of discrete subsets ofastrocytes fuctionally connected to the GnRHnetwork.For instance ,puberty inducing lesions of ofthe anterior hypothalamic area in rats ,results inactivation of TGFα and erbB1 receptor expression inastrocytes surrounding the lesion site(28,29).ii)Somehypothalamic hamartomas associated with sexualprecocity in humans are endowed with a rich networkof astrocytes containing TGFα and erbB1 receptors(30),suggesting that foci of glial activation in theproximity of GnRH neurons such as these ,may be acause of idopathic sexual precocity of central origin inhuman females.

Role of Glia in Control of GnRH secretion

The ME ,which is located ventral to the third ventriclein the tuberal region of the hypothalamus ,is one of theseven circumventricular organs and primarily containsneurosecretory axon terminals(31).It constitutes awindow of exchanges between the hypothalamus andperiphery that is facilitated by the presence ofpermeable brain capillaries feauturing fenestrated

endothelium(32,33).Thus it appears that the mostimportant function associated with the lack of bloodbrain barrier in this region is that it permits the releaseof neurohormones produced by neuroendocrine cellsfrom terminals into the pituitary portal circulation.It isalso important to acknowledge that the cellularprocesses through which neuroendocrine terminals release their neuropeptides into the circulation couldbe subjected to the direct modulatory influence of theblood borne factors acting on this region.

As the projection field of neuroendocrine GnRH neurons ,the ME of the hypothalamus is poised toplay a crucial role in the precise regulation of GnRHreClease ,and is therefore central to the control ofreproductive axis .The ME ,which is located ventral tothe third ventricle in the tuberal region of thehypothalamus ,is one of the seven circumventricularorgans and primarily contains neurosecretory axonterminals(31).It constitutes awindow of exchangesbetween the hypothalacmus and the peripherythat isfacilitated by the presence of permeable brainc a p i l l a r i e s f e a u t u r i n g f e n e s t r a t e dendothelium(32,33).Thus it appears that the mostimportant function associated with the lack of a bloodbrain barrier in this region is that it permits the releaseof neurohormones produced by neuroendocrine cells from terminals into the pituitary portal circulation.It isalso important to know that the cellular processesthrough which neuroendocrine terminals release theirneuropeptides into the circulation could be subjectedto the direct modulatory influences of b lood bornefactors act ing on this region.The pecul iarcytoarchitecture of the ME is mainly conferred bytanycytes ,which are specialized ependymoglial cellsthat form a belt l ining the floor of the thirdventricle(31).One dominant feature of tanycytes istheir marked polarization ;although tanycyte cellbodiesline the border of the third ventricle,they also sendprocesses to the vascular walls,where they makecontact through endfeet specialization.In additiontanycytes were recently shown to express efficienttight junction complexes at their apex that bestowthem wi th proper t ies o f the b lood bra inbarrier(BBB)(33). Although tanycytes are the dominantcelltype,astrocytes also reside within the internal zoneof the ME.

While in primates,including humans GnRH neuronalcell bodies are diffusely distributed in the forebrain andare particularly abundant in the preoptic region and inthe tuberal region of the hypothalamus;in rats they arenot present in the latter region.Deafferentation studiesin rodents together with work showing that release of



G n R H f r o m h y p o t h a l a m i c e x p l a n t s i spulsatile(34,35),thus led to the concept that atleastpart of the mechanisms synchronizing GnRH secretionmay reside within the tuberal region of thehypothalamus.These synchronizing events could evenoccur directly within the ME as ME explants were alsoshown to release GnRH in a pulsatile mode invitro(36,37).Intriguingly plastic events taking place within the ME modulate the direct access of GnRHneurons to the pituitary pcortal blood vessels and thatthese structural changes are directly correlated to theendocrine status of the individual,e.g.in rats directneurohaemal junctions are visualized at the onset ofthe preovulatory surge of GnRH when E2levels arehighest(38).

Ovarian cycle-related morphological plasticity atthe neurohaemal interface for GnRH neurons.

ME dynamics involve coordination of neuroendocrineaxons,tanycytes,and the parenchymatous basallamina ,the last structure secreted neurohormonesmust cross to enter the blood((39-41).Over the pastdecade ,it has been established that fluctuatingphysiological conditions during the ovarian cycle havethe power to reversibly alter structural relationshipsamong the various cell types of the ME thatspecifically interact with nerve terminals containingGnRH terminals(42-45).During the ovarian cycle,under conditions of low gonadotropin output ,GnRHneuroendocrine terminals are completely enwrappedby tanycyte endfeet,which prevent direct access to thepericapillary space and thus create a diffusion barrierhampering GnRH entry into the pituitary portalcirculation(38).Astructural rearrangement of tanycytesoccurs during the preovulatory surge resulting in therelease of the engulfed neuroendocrine terminals andthe establishment of direct neurohaemal contactsbetween GnRH neurons and the pituitary portalblood(38).In parallel to tanycytic endfeet retraction,GnRH axon terminals are frequently seen to sproutnew terminals towards the pericapillary space andthus appear to be attracted by the endothelial wallwhich they eventually contact(38).Similarly ,electronmicroscopy studies performed in gonadectomized rats,an experimental condition that results in increasedGnRH release ,showed that the distance of GnRHaxon terminal from the pericapillary space waspositively correlated to plasma LH levels(46).

Human Studies

With the advancement of magnetic resonanceimaging(MRI)techniques such as diffusionMRI(measurement of water diffusion coefficient that

provides information about the cellular structure oftissue)and proton MR spectroscopy(measurement of arange of cerebral metabolites including N-acetyla s p a r t a t e , c h o l i n e a n d c r e a t i n e t h a tprovidesinformation about tissue metabolism) tissuestructure can now be probed and imaged onmicroscopic scale in vivo(47,48). Recently Baroncini etal showed that Gn RH axon fibers were abundantlyapposed to tanycytic processes in the human MEraising the possibility that as in rodents,putativephysiological condition-induced plastic changesinvolving morphological interaction could play a role in the neuroendocrine control of GnRH secretion inhumans(49). A noninvasive longitudinal studymonitoring sexsteroid hormone controlled plasticity inwomen recently evidenced that structural changesactually occur within the hypothalamus during anartificial menstrual cycle(50).In this study ,femalevolunteers were subjected to diffusion and spectroscopy MRI at two stages of their artificialmenstrual cycle. Thirteen days after initiating oralc o n t r a c e p t i o n i . e . , w h e n t h ehypothalamic-pituituitary-gonadal(HPG)axis is fullyinhibited(51)and at the end of pill free interval,i.e whenmost of the steroidogenic negative effects wear off andnormal early follicular phase LH pulse pattern isfound(51).Results showed that removal of oralcontraceptive-mediated gonadal steroid negativefeedback on the reproductive axis dramatically andselectively favours diffusion in the hypothalamus andis associated with variations in the release ofcholine(the precursor of phosphadityl choline,thecorephospholipid in the cell membrane ),which is ametabolite mainly released by glial cells(52,53)whenchanges in cell membrane turnover occur(54).ii)Similarto studies conducted in brain slices showing thatchanges in the astrocytic coverage of neurons modifyextracellular space geometry and diffusion parameters(55),these human data raise the possibilitythat the microstructural changes monitored during thepillfree period (increased diffusivity of watermolecules)in the female hypothalamus could be due tothe retraction of the glial cell processes(50).

Role for Glia in the release of GnRH and MEfunctional plasticity

Initial studies showed that transforming growth factor α(TGFα),an epidermalgrowth factor (EGF)relatedpeptide expressed by tanycytes and astrocytes of theME(56)was able to stimulate GnRH release from MEexplants(57).TGFα does not stimulate GnRH releasedirectly ;instead it does so via a paracrine mechanismthat involves PGE2 release which subsequently acts



o n G n R H n e u r o n s t o i n d u c e G n R Hsecretion(58,59)but also triggers acute tanycytesretraction both in cultured tanycytes and inhypothalamic explants(60).Both invitro and in situstudies showed that the TGFα receptor erbB1,wasexpressed in tanycytes(61-63)Blockade of erbB1receptor tyrosine kinase activity in the ME delayspuberty(56)whereas TGFα overexpression induced viaeither transgenic approach(64) or by grafting cellsgenetically engineered to secrete TGFα into the MEaccelerates the onset of puberty in female rats(59).Injection of E2 and progesterone(Pg)was shown to increase TGFα Mrna expression in premature ratsand blockade of TGFαaction with tyrphostins ,erbB1inhibitors,delayed the occurrence of the first GnRH/LHpreovulatory surge at puberty(56).Because tanycytesof the ME express E2 receptors (60,65)E2 may actdirectly on these cells to promote both TGFαexpression and release on the day of prooestrus.Invitro studies conducted in primary culture of tanycytesshowed that12hr TGFα treatment promotes therelease of PGE2 ,and a PGE2 dependent release ofTGFβ1(63),a growth factor also known to be involvedi n t h e g l i a l c o n t r o l o f G n R Hsecretion(66-68).Morphometric studies in vitro showedthat both TGFα and TGFβ1 had dramatic but oppositeeffects on tanycyte morphology(63).When tanycytesmonolayers are treated with TGFα,during the first16hrs of treatment ,TGFα-erbB1 signaling acts ontanycytes to first promote outgrowthof their processesand then to elicit a PGE2 dependent production ofTGFβ1(63).Subsequently TGFα induced TGFβ1 releaseinduces retraction of the tanycytic processes duringthe following 6-8h(63). This sequence of eventsappears to recapitulate the E2 dependent changes ingrowth factor expression and morphology displayed bytanycytes during the preovulatory surge of GnRH.TGFβ1 mediated cellretraction in tanycytes ,which wereshown to express TGFβ receptors in vivo(22,68)r e q u i r e s t h e a c t i v i t y o f m a t r i xmatalloproteinases(63)that were alsoshown to beexpressed in the ME(69).In contrast to theaforementioned effect of PGE2 that promotes tanycyteendfeet retractionby promoting actin cytoskeletonremodeling (within30min) (60)TGFβ1-mediatedtanycyte retraction involves digestion of extracellularmatrix that causes substrate adhesion loss fort a n y c y t e s a s s h o w n b y t i m e l a p s e experiments(63)Thus these two mechanismsmediating tanycyte retraction appear highlycomplementary.

Role of Glial Neuregulin-erb2/4 signaling complexin modulating GnRH Release/tanycyte plasticity

Cultured hypothalamic astrocytes expressNRG1,NRG3 as wel l as erbB2 and erbB4receptors.When the cells are exposed to NRG1β orTGFα there is phosphorylation of erbB4 and erbB1receptors ,respectively,in addition to erbB2 receptorcrossphosphorylation.As a result ,production of PGE2increases(61)erbB2 receptors plays an important rolein amplifying intracellular signals initiated by TGFαand NRGs ;in vitro blockade of astrocytic erbB2synthesis prevents both the stimulatory effects ofNRG1 on PGE2 release and the increase in GnRHsecretion elicited by NRG1–conditioned astrocyteculture medium(61).Consistent with the presence oferbB2 and erbB4 receptors in cultured astrocytesimmunohistochemistry and in situ hybridization studiesdemonstrated the presence of erbB2 Mrna and proteinin hypothalamic asrocytes and tanycytes of the thirdventricle/ME,cand erbB4 in astrocytes,but not intanycytes(61).The key involvement of ME astrocytes inthe control of GnRH release was demonstrated usingtransgenic mice in which a dominant negative form ofthe erbB4 receptor ,lacking the intracellular domain,was specifically targeted to astrocytes(70,71).Themutant astrocytes exhibited a blunted PGE2 responseto neuregulin stmulation,ii)a reduced GnRH responseto neuregulin treatment in ME explants ,iii)diminishedplasma gonadotrophin levels andiv)delayed onset ofthe first preovulatory surge at puberty,all of these onthe face of normal erbB1 dependent function(70)PGE2) PGE2 originating from ME astrocytes following erbBreceptor activation could,in addition to stimulatingGnRH neurons themselves ,also modulate MEplasticity either by promoting actin cytoskeletonremodeling(60)and/orTGFβ1 expression(63)intanycytes. erbB4 expression within the hypothalamusis regulated by E2 and its expression levels aremaximal at the time of proestrus(61). In addition theexpression of SynCAM1,a synaptic adhesion moleculewhich is expressed in astrocytes as well as GnRHneurons and is a mediator of adhesion betweenhypothalamic astrocytes and GnRH neurons(72);isregulated by erbB4 signaling(73).

Role of vascular endothelial celss-NO

PGE2 synthesis in tanycytes of the ME could beprompted by two indepen dent but complimentary cell-based mechanisms,one involving glial-glialinteractions set in motion by the paracrine activationof TGFβ/erbB1 signaling pathway in tanycytes(fig2a)and another involving endothelial-tanycyte interactionand the release of nitric oxide(NO)by vascularendothelial cells ,which in turn directly modulatesCOX activity in tanycytes( 60).Both pathways could



be subject to the modulatory influence of Gonadalsteroids ,as estrogen are known to upregulate bothTGFα expression in astroglial cells (56) and COXexpression in tanycytes.( 60) (fig2b)

Other Glial Derived Factors

Although glial PGE2 is a major mediator of thestimulatory actions that TGFα and NRG’s exert onGnRH release astrocytes release additionalsubstances capable of stimulating GnRH release(74).Among these substances ,calcium,glutamate and ATPare the most conspicuous(75). Calcium reachesadjacent astrocytes via gap junctions(76)andstimulates the release of ATP and glutamate ,whichthen affect neuronal function upon binding to specificreceptors(75).In the primate hypothalamus ,GnRHneurons respond to extracellular ATP ,via P2X2 andP2X4 receptors with an immediate increase inintracellular calcium and release of GnRH(77,78).ATPand glutamate can also activate Ca mobilization inastrocytes (75);Ca release more glutamate,which inturn stimulates PGE2 formation (79),PGE2 elicitsfurther release of arachidonic acid .In turn arachidonica c i d i n h i b i t s g l u t a m a t e u p t a k e i n t oastrocytes(80)thereby increasing the halflife of theneurotransmitter in the synapses.

Neuron to Glia and Glia To Glia interactions incontrol of PGE2 release

In vitro experiments suggest that erbB signaling inhypothalamic astrocytes is functionally connected tothe neuronal glutamatergic system,the primary modeof excitatory transsynaptic communication used byhypothalamic neuons(81).and one that is known toincrease GnRH secretion.Neuronally releasedglutamate (Glu)coact ivates metabotrobicglutamatergic(mGlur) and AMPA glutamatergic(Glut)receptors in astrocytes, stimulating the activity of zincdependent matrix metalloproteinases(MMPs)of theADAM(a disintegrin and metalloproteinase )family.TheMMPs catalyze ectodomain shedding of thep r o E G F l i g a n d s i ) p r o T G F α a n dii)proNRG(neuregulin).In particular the processing ofproTGFα has been shown to involve themetalloproteinase ADAM17 also known as tumournecrosis alpha converting enzyme(TACE)(82).Thesubsequent release of matrix TGFαand NRG activateerbB1/erbB2 and erbB4/erbB2 heterodimersrespectively(24).The coactivation of glutamatergicreceptors,caused extracellular Ca2+ influx ,aCa2+/protein kinase C dependent increase in TACElike activityand enhanced release of TGFα(83) and

induces recruitment of erbB1 and erbB4 and theirproligandcs to the cell membranes ,where MMPcomplexes form as demonstrated by the directphysical association of erbB and glutamatergiucreceptors.TACE is abundant in astrocytes of the MEand becomes more active in this region at the time oft h e f i r s t p r e o v u l a t o r y s u r g e o fgonadotrophins.Inhibition of TACE activity in the MEdecreases GnRH secretion and delays pubertyindicating that an increased TACE activity in thisregion is necessary for the pubertal activation ofGnRH secretion to take place(83). The activation oferbB receptors in hypothalamic astrocytes inducesprofound morphological changes including theretraction of cytoplasmii)stellation of cellsandiii)theelongation of processes. 2)The activation of erbB alsopromotes release of PGE2(24),stimulates a cyclicAMP /PKA pathway in GnRH neurons through themobilization of EP2 receptors(84).Activation of thissignaling pathway induces a reversible membranedepolarization ,initiation of spike firing via a postsynaptic effect involving the activation of anonselective cation current (84)(fig3).

Role for astroglial PGE2 in dendritic plasticity inGnRH neurons

GnRH neurons exhibit a simple bipolar morphologywith one or two very long dendritic processes that canextend upto 1mm(85,86)Intriguingly ,recent studieshave demonstrated that the density of spines alongthese dendrites is subject to robust increases not onlyduring sexual development in immature animals ,butalso at the onset of the GnRH/LH surge induced bygonadal steroids in ovar iectomized adul tmice(87).Although sexual maturation and the surgemechanisms have been shown to require the neuronalexpression of sex steroid receptors(88),studiessuggesting that astrocytic mechanisms might controlthe stabilization of individual dendritic processes andtheir subsequent maturation into spines( 89),togetherwith the demonstration that specific juxtacrinesignaling pathways are involved in sculptingastrocyte-dendritic interactions(90),raises thepossibility that astrocytes play a role in thephysiological changes of synaptic structure underlyingGnRH neuronal maturation and function.PGE2 has infact been shown to mediate the dramatic neuronalspastic plasticity induced by estrogens in thedeveloping preoptic region( 91).This effect involves theactivation of AMPA and metabotrobic Glut receptors(92), as well as the EP2/PKA signaling pathway(91),recently found to be functional in neuronal spineplasticity in the adult hippocampus, have also been



shown to promote comparable changes in theimmature hippocampus(91).However ,in thehippocampus ,the underlying mechanisms do notappear to require PGE2 synthesis (91),suggesting thatincreases in PGE2 synthesis are selectively used byE2 to promote dendritic spine plasticity in thedevelopincg preoptic region .Further studies arerequired to determine whether estrogenic effects onthe plasticity of hypothalamic neurons such as thoseseen in newborn rodents can also occur later inpostnatal life and/or in adulthood.

Glia as the Metabolic sensing Unit

Astrocytes are essential for both neuronal metabolismand metabolism sensing and are a critical componentof the so called tripartite synapse,which also includesthe pre and post synapt ic processes o fneurons(93).Astrocytes take up glutamate released byneurons,use it for thir own cellular metabolism ,andrecycle the resultant glutamine for neuronalmetabolism(94 ).In addition to producing glutamate,astrocytes also regulate glutamate synthesis.Thisfunction changes according to the reproductive stageof the animals.For instance ,glutamate metabolismchanges in the adult mouse hypothalamus in responseto preovulatory levels of E2(95) and in female rat sduring the normal onset of puberty(96).They also takeup glucose,metabolize,and release it as lactate orstore it as glycogen (94 ).Although the exact degree towhich neuronal metabolism is dependent on astrocytederived lactate is controversial,it is clear that neuronscan take up lactate v ia monocarboxylatetransporters(97) and convert it to pyruvate foroxidative production of ATP(94). Because lactatebypasses most glucosensing pathways alteration inastrocyte lactate production by neurotransmitterss u c h a snorepinephrine,dopamine,serotoinin,glutamine,and γ-amino butyric acid(98) can override the effects ofglucose on neuronal glucosensing((99).Perhaps themost important function of astrocyte lactate productionfrom glycogen is to provide an energy reserve tocsupport neuronal function during hypoglycemia.Finally,the majority of fatty acid oxidation in the brain occursin astrocytes.Astrocytes can also produce ketonebodies,which are exported and traken up by MCTs toserve as an alternate source for neuronalmetabolism.AMPK is a major regulator of astrocyteketone production(100)so that studies in which AMPKactivity or other fatty acid metabolic pathways arealtered may produce physiological effects primarily byaltering astrocyte rather than neuronal metabolism.

Role of Tanycytes

Little is known about the function of tanycytes insupporting neuronal metabolism. However, processesof tanycytes(vimentin expressing)lining the thirdventricle divide theARC and VMN into compartmentsand effectively prevent diffusion of substances such asglucose and larger molecules from ME,which lacks aBBB,into theARC(33).Presumptive GK expressingglucosensing neurons line up along these processessuggesting asupportive role of tanycytes in metabolicsensing.Also tanycytes express both Glut2 and GK,which make them potentially glucosensingthemselves(101). Transient destruction of thirdvent r icu la r tanycy tes marked ly impa i rscounterregulatory responses to glucoprivation ,andthis is reversed when they regenerate.However muchmore work is required to further elucidate the role ofthese intriguing cellsas members of a metabolicsensing unit.

Other Molecular Mechanisms involved inneuron-glia interaction associated with GnRHregulation

Genomic and proteomic analysis carried out inlaboratories of ojeda et al have identified severalgenes that may potentially be involved in the initiationof the structural and functional neuro-glia remodelingof the ME at puberty and duting estrus cyclicityalthough its functional signif icance is st i l luncertain.Some of these genes may act as mastergenes or upper echelon genes ,which coordinate theexpression of a network of other regulatory genes andmaintain the hierarchical structure of thenetwork.Among these three are of particular interesti)Oct2(octamer binding protein 2)ii)TTF1(thyroidtranscription factor1)i i i)EAP1(enhanced atpuberty1)i)Oct2 is a transcriptional regulator of thePOU domain family of homeobox –containing genes,which may regulate TGFα and SynCAMtranscription.The expression of Oct2 increases in thehypothalamus during juvenile development and theblockade of the Oct 2 synthesis delays the age at firstovulation.In contrast sexual precocity is associatedwith increased expression of Oct2(102).ii)TTF1 likeOct 2,a homeobox gene enhances the transcriptionalactivity of genes that facilitate puberty,such asGnRH,erbB2(erythroblastic leukemia viral oncogenehomolog 2) and KISS1(human melanoma metastasissuppressor),and suppresses the expression of genesinhibitory to the pubertal process,such as thepreproenkephalin gene ,TTF1 is expressed by GnRHneurons and tanycytes and its expression increases



at puberty in the hypothalamus .TTF1 disruption isassociated with delayed puberty ,disruption of initialc y c l i c i t y a n d d e c r e a s e d r e p r o d u c t i v ecapacity(102),and recently TTF1 has also been foundto be a component of the molecular machinery whichcontrols circadian oscillations in GnRH genetranscription.(103).iii)The third candidate EAP1 encodes a nuclear protein expressed in GnRHneurons and in neuronal subpopulations involved inthe control of GnRH neurons ,such as glutamatergic,GABAergic ,proenkephalinergic ,and KiSS1neurons.Hypothalamic EAP1 Mrna levels increase in bothmonkeys and rats during female puberty .Similar toTTF1 ,EAP 1 enhances the transcriptional activity ofgenes that inhibit the pubertal process and itsknocking down in the hypothalamus delays pubertyand disrupts estrus cyclicity(102).Recent studies innonhuman primates confirms the requirement of EAP1for menstrual cyclicity (104)and single nucleotidepolymorphisms in the EAP1 gene has been associatedwith amenorrhea/oligomenorhea in non humanprimates(105) which raises the possibility thatpolymorphisms in EAP1 may increase the risk offunctional amenorrhea in humans.Also recentlyLomniczi et al showed that epigenetic mechanisms of transcriptional repression time the initiation of pubertyin rats.Silencers of the Polycombgroup(PcG)wereprincipal contributors with hypothalamic expression of2 key PcG genes ,EeD and Cbx7 decreased andmethylation of their promoters increased beforepuberty ,Pubertal increase in Kiss1 expression wasaccompanied by EED loos from Kiss1 promoter andenrichment of histone 3 modificatio associated withgene activation.Pulsatile GnRH release was disruptedby preventing evictin of EED from the Kiss1promoter.Hence epigenetic silencingis a mechanismunderlying neuroendocrine control of female puberty(106).

Conclusions

Since GnRH lacks estrogen receptors (categoricallyestrogen receptor α),it is generally believed thatestrogenic control of GnRH release occurs in anindirect manner.In addition to a role for inhibitory andexcitatory transmitter containing neurons ,thehypothalamic astrocytes have been documented torelease a variety of neuroactive factors ,including TGF α &TGFβ,IGF1,PGE2,and the neurosteroid 3-α-5-pregnane-20-one,each of which have been shownto stimulate GnRH release ,and receptors for eachfactor have been documented on Gn RH neurons.

Astrocytes have also been implicated in the regulationof synaptic plasticity in key areas of the hypothalamusthat control GnRH release,an effect achieved byextension and retraction of glial processes(i.e glialensheathment).Through this mechanism ,the numberof synapses on GnRH neurons and GnRH regulatoryneurons can be potentially modulated ,thereby influencing the activation state of GnRH neurons .Thesteroid hormone 17-β-estradiol ,which triggers theGnRH sand LH surge has been shown to induce theastrocyte regulated changes in hypothalamicsynapticplasticity as well as enhance formation andrelease of the astrocytic neuroactive factors ,therebyproviding another potential mechanistic layer forastrocytic regulation of Gn RH release.

With advent of MRI one can correlate in humans theplasticity changes in tanycytes during the menstrualcycle and try to find the aetiology of precociouspuberty with further studies on upper echelonregulatory genes such as Oct2 TTF1,and EAP1 wouldbe helpful in determining their role in coordination ofhormonal signals with the plasticity of tanycytes andGnRH axons in the median eminence ,the uptake ofIGF1 by tanycytes ,the neuroglial plasticity of thehypothalamic neuronal circuits regulating GnRHn e u r o n s , a n d t h e p l a s t i c i t y a n d t h eneurosecretoryactivity of GnRH neurons at pubertyand during reproductive cycles in adult brain.Finallyboth astrocytes and tanycytes(33,107) may act asmetabolic sensors and may contribute to metabolicregulation of reproduction besides dire ct glucosensingby GnRH neurons(108)through AMPK ,and astrocytescan protect GnRH neurons besides other neurons bythe release of TGFβ and activation of a c-Jun /AP1protective pathway(23).

References

1. Gordon GR,et al Astrocyte –mediated distributedplasticity at hypothalamic glutamate synapses.Neuron2009;64:391-403.2. Hennenberger C,Papouin T,Oliet SH,RusakovDA.Long term potentiation depends on release ofD-serine from astrocytes.Nature 2010;463:232-236.3. Panatier A,et al.Glia derived D-serine controlsN M D A r e c e p t o r a c t i v i t y a n d s y n a p t i cmemory.Cell2006;125:775-7844. Gourine AVet al.Astrocytes control breathingthrough PH depedent release of ATP.Science2010;329:571-575.5. Halassa MM et al Astrocytic modulation of sleephomeostasis and cognitive consequences of sleep



loss.Neuron 2009;61:213-2196. Prevot V,RioC,ChoGJ,Lomniczi A,Heger S,NevilleCM,Rosenthal NA,Ojeda SR,Corfas G.Normal femalesexual development requires neuregulin-erbBrecedptor signaling in hypothalamic astrocytes .JNeurosci.2003;23:230-239.7. Barres BA The mystery and magic of glia :Aperspec t ive on the i r ro le in hea l th anddisease.Neuron2008;60:430-440.8. Haydon PG,Carmignoto G.Astrocyte control ofsynapt ic t ransmission and neurovascularcoupling.Physiol Rev 2006;86:1009-1031.9. Iadecola C,Nedergaard M.Glial regulation of thecerebra l microvasculature.Nat Neurosc i2007;10:1369-1376.10. Classadonte J,Sharif A,Prevot V.Gliotransmissionbyprostaglandin E2:A prerequisite for GnRH neuronfunction.Front Endocrinol(Laussanne) 2011;2:91-10311. Hatton GI.Function –related plasticity inhypothalamus.Annu Rev Neurosci 1997;20:375-397.12. Ojeda SR,Prevot V,Heger S,Lomniczi A,DziedzicB,Mungenast A.Glia –to neuron signaling and theneuroendocrine control of female puberty.Ann Med.2003;35:244-255.13. Prevot V,Hanchate NK,Bellefontaine N,SharifA,Parkash J,Estrella C,Allet C,deSerannoS,CmpagneC,de Tassigny X,Baroncini M.Function relatedstructural plasticity of the GnRH system:A role forneuronal-glial-endothelial interactions,FrontNeuroendocrinol 2010;31:241-258.14. Theodosis DT,Poulain DA,Oliet SH.Activitydependent structural and functional plasticity ofastrocyte-neuroninteraction.Physiol Rev 2008;88:983-1008.15. Kaur KK,Allahbadia G,Singh M.Kisspeptins inhuman reproduction-future therapeutic potential.JAssist Reprod Genet 2012;Epub ahead of print.16. Garcia-Segura LM and Melcangi RC.Steroids andglial cell function .Glia 2006;54:485-498.17. Mong JA and Blutstein T.Estradiol modulations ofastrocytic form and function:implications for hormonacontrol of synaptic communication.2006;138:967-975.18. Csakvari E,Hoyk Z,Gyenes A,Garcia –OvejeroD,Garcia-Segura LM,and Parducz A.Fluctuation ofsynapse density in the arcuate nucleus during theestrus cycle.Neuroscience 2007;144:1288-1292.19. Mong JA and McCarthy MM. Steroid –induceddevelopmental plasticity in hypothalamic astrocytes:Implications for synaptic patterning.Journal ofNeurobiology.1999;40:602-609:2003-2010.20. OjedaSR,Lomniczi A,Sandau U.Contribution ofglial –Neuronal Interactions to the neuroendocrinecontrol of female puberty.Eur J Neurosci2010;32:2003-2010.

21. Lomniczi A ,OjedaSR,.Arole for glial cells of theneuroendocrine brain in the central control of femalesexual development.In Parpura V,HaydenP,editors.Astrocytes in (Patho)Physiology of thenervous System.Springer;NY:2009.P487-511.22. Prevot V,Bouret S,Croix T,Tnkumi T,JennesL,Mitchell V,Beavillain JC.Evidence that members ofthe TGFβ Superfamily play a role in the regulation ofof the GnRH neuroendocrine axis ;expression ofhypothalamic areas in female rat.J Neuroendocrinol.2000;12:665-670.23. Mahesh VB,Dhandapani KM,B rann DW.Role ofastrocytes in reproduction and neuroprotection.MolCell Endocrinol.2006;246:1-9.24. Dziedzic B,PrevotV,Lomniczi A,Jung H,CorneaA,Ojeda SR.Neuron to glia signaling mediated byexcitatory aminoacid receptors regulates erbBreceptor function in astrogl ial cel ls of theneuroendocrine brain.J Neurosci. 2003;23:915-926.25. Povlsen GK,Berezin V,Bock E.Neural celladhesion molecule -180 –mediated homophilic bindingi n d u c e s e p i d e r m a l g r o w t h f a c t o rreceptor(EGFR)downregulation and uncouples theinhibitory function of EGFR in neurite outgrowth.JNeurochem 2008;104:624-639.26. Mungenast A,Ojeda SR.Expression of three genefamilies encodind cell-cell communication molecules inthe prepubertal nonhuman primate hypothalamus .JNeuroendocrinol 2005;17:208-219.27. Peles E,Nativ M,Campbell PL,Sakurai T,MartinezT,Lev S,Clary DO,Schilling J,Barnea G,PlowmanGD,Grumet M,Schlessinger J.The carbonic anhydrasedomain of recptor tyrosine phosphatase β is afunctional ligand for the axonal cell recognitionmolecule contactin.Cell.1995;82:251-260.:28. Junier MP,Ma YJ,Costa ME,Hoffman G,HillDF,Ojeda SR.Ttansforming growth factor α contributesto the mechanism by which hypothalamic injuryinduces precocious puberty.Proct Natl Acad SciUSA.1991;88:9743-9747.29. Junier MP,Hill DF,CostaME,Felder S,OjedaSR.Hypothalamic lesions that induce femaleprecocious puberty activate glial expression of theepidermal growth factor receptor gene:Differentialregulation of alternatively spliced transcripts.JNeurosci 1993;13:703-713.30. Jung H,Carmel P,Schwartz MS,Witkin JW,BenteleKHP,Westphal M,Piatt JH,Costa ME,Cornea A,MaYJ,Ojeda SR.Some hypothalamic hamartomascontain transforming growth factor α ,a puberty–inducing growth factor,but not luteinizing hormone–releasing hormone neurons.J Clin Endocrinol Metab.1999;84:4695-4701.3 1 . P a g e R B . T h e a n a t o m y o f t h e



hypothalamo-hypophyseal complex.In :Knobil E;NeilJD,editors.The Physiology of Reproduction.Vol.1.NewYork:Raven Press;1994.p.1527-1619.32. Ciofi P,Garret M,Lapirot O,Lafon P,LoyensA,Prevo t V ,Lev ine JE .Bra in -endocr ineinteractions:amicrovascular route in the mediobasalhypothalamus.Endocrinology 2009;150:5509-5519. 33. Mull ier A,Bouret SG,Prevot V,DehouckB,,Differential distribution of tight junction proteinss u g g e s t a r o l e f o r t a n y c y t e s i n b l o o d–hypothalamusbarrier regulation in the adult mousebrain.JComp Neurol.2010;518:943-962.34. Bourguignon JP,Gerard A,Mathieu A,FrachimontP.Maturation of the hypothalamic control of pulsatilegonadotropin releasing hormone secretion at onset ofpuberty.I.Increased activation of N-methyl-D-aspartatereceptors.Endocrinology 1990;127:873-881.35. Bourguignon JP,Gerard A,varez GonzalezML,Freanchimont P.Control of pulsatile secretion ofgonadotropin releasing hormone from hypothalamicexplants.Hum Reprod.1993;8(Suppl2):18-22.36. RasmussenDD.Episodic gonadotropin-releasinghormone release from the rat isolated medianeminence in v i t ro .Neuroendocr ino logy1993;58:511-518.37. Urban JH,Das I,Levin JE.Steroid modulation ofneuropeptideY –induced luteinizing hormone releasinghormone release from median eminence fragmentsfrom male rats.Neuroendocrinology 1996;63:112-119.38. Prevot V,Croix D,Bouret S,Dutoit S,TramuG,Stefano GB,Beauvillain JC.Definitive evidence forthe existence of morphological plasticity in the externalzone of the median eminence during the rat estrouscycle:implications of neuro-glial-endothelialinteractions in gonadotropin –releasing hormonerelease.Neuroscience.1999;94:809-819.39. Kozlowski GP,Coates PW.Ependymoneuronalspecializations between LHRH fibers and cells of thecerebrovascular system.Cel l T issue Res1985;242:301-311.40. Meister B,Hokfelt T,Tsuroro Y,HemmingsH,Ouimet C,Greengard P,Goldstein M.DARPP 32 adopamine –and cyclic AMP-regulated phosphoproteini n t a n y c y t e s o f t h e m e d i o b a s a lhypothalamus:distribution and relation to dopamineand luteining hormone releasing hormone neuronsa n d o t h e r g l i a lelements.Neuroscience.1988;27:607-622.41. King JC,Rubin BS,.Dynamic changes in LHRHneurovascular terminals with various conditions inadults.Horm Behav. 1994;28:349-356.42. Garcia-SeguraLM,,Lorenz B,DonCarlos LL.Therole of glia in the hypothalamus:implocations forGonadal steroid feedback and reproductive

n e u r o e n d o c r i n e o u t p u t . R e p r o d u c t i o n2008;135:419-429.4 3 . O j e d a S R , L o m n i c z i A , S a n d a uUS.Glial-gonadotrophinhormone(GnRH)neuroninteractions in the median eminence and the control ofGnRH secretion.J Neuroendocrinol 2008;20:732-742.44. Prevot V.Glial –endothelial interactions areinvolved in the control of GnRH secretion .JNeuroendocrinol 2002;14:247-255.45. Prevot V,Dehouck B,Poulain P,BeauvillainJC,Buee-Scherrer V,Bouret S,Neuronal-glialendothelial interactions and cell plasticity in thepostnatal hypothalamus:implications for then e u r o e n d o c r i n e c o n t r o l o f r e p r o d u c t i o n . P s v c h o n e y r o e n d o c r i n o l o g y2007;32(Suppl1 ):S46-51.46. King JC,Letourneay RJ.Luteinizing hormone–releasing hormone terminals in the median eminenceof rats undergo dynamic changes after gonadectomy,as revealed by electron microscopic imageanalysis.Endocrinology 1994;134:1340-1351.47. Le Bhan D,.Looking into the functional architectureof the brain with diffusion MRI.Nat Rev Neurosci.2003;4:469-480.48. Ross B,Bluml S.Magnetic resonance spectroscopyof the human brain.Anat Rec. 2001;265:54-84.49. Baroncini M,Allet C,Leroy D,BeauvillainJC,Francke JP,Prevot V.Morphological evidence fordirect interaction between gonadotropin-releasinghormone neurons and astroglial cells in the humanhypothalamus.J Neuroendocrinol 2007;19:691-702.50. Baroncini M,Jissendi P,Catteau-Jonard S,DewaillyD,Pruvo JP,Francke JP,Prevot V.Sex steroidhormones related structural plasticity in the humanhypothalamus.Neuroimage 2010;50:428-433.51. Hemrika DJ,Slaats EJ,Kennedy JC,deVriesRobles-Korsen TJ,Schoemaker J.Pulsatile luteinizinghormone patterns in long term oral contraceptiveusers.J Clin Endocrinol Metab 1993;77:420-426.52. Manganas LN,Zhang X,Li Y,Hazel RD.SmithSD,Wagshul ME,Henn F,Benveniste H,DjuricPM,Enikopolov G,Maletic-Savatic M.Magneticresonance spectroscopy identifiesneural prigenitorce l l s in the l i ve human bra in Sc ience.2007;318:980-98553. Urenjak J,Williams SR,Gadian DG,NoblePM.Proton nuclear magnetic resinance spectroscopyunambiguously identifies different neural cell types.JNeurosci.1993;13:981-989.54. Herholz K,Coope D,Jackson A.Metabolic andmolecular imaging in neuro-oncology.Lancet Neurol2007 ;6:711-724. 55. Piet R,Vargova L,Sykova E,Poulain DA,OlietSH.Physiological contribution of the astrocytic



environment of neurons to intersynapt iccrossta lk .Proc.Nat l Acad Sct USA 2004;101:2151-21555 6 . M a Y J , J u n i e r M P , C o s t a M E , O j e d aSR.Transforming growth factor –α gene expression inthe hypothalamus is developmentally regulated andlinked to sexual maturation.Neuron 1992;9:657-670.57. Ojeda SR,Urbanski HF,Costa ME,Hi l lDF,Moholt-Siebert-M.Involvement of transforminggrowth factor α in the release of luteinizing hormonereleasing hormone from the developing femalehypothalamus.Proc.Natl.Acad.Sci.USA.1990;87:969809702.58. Ma YJ,Ojeda SR.Neuroendocrine control of femalepuberty:glial and neuronal interactions.J InvestigDermatol Symp Proc. 1997;2:19-22.59. Rage F,Lee BJ,Ma YJ,Ojeda SR.Estradiolenhances prostaglandin E2 receptor gene expressinlu te in iz ing-hormone re leas ing hormone(LHRH)neurons and facilitates the LHRH response toPGE2 by activating a glia-to-neuron signalingpathway.J Neurosci 1997;17:9145-9156. 60. deSeranno S,d’Anglemont deTassigny X,EstrellaC,Loyens A,Kasparonv S,Leroy D,OjedaSR,Beauvillain JC,Prevot V.Role of estradiol in thedynamic control of tanycyte plasticity mediated byvascular endothel ia l ce l ls in the medianeminence.Endocrinology 2010;151:1760-1772.61. Ma YJ,Hill DF,Creswi ck KE,Costa ME,CorneaA,Lioubin MA,Plowman GD,Ojeda SR.Neuregulinssignaling via a glial erbB2-erbB4 recetor complexcontribute to the neuroendocrine control of mammaliansexual development.J Neurosci. 1999;19:9913-9927.62. Ma YJ,Hill DF,Junier MP,Costa ME,FelderSE,Ojeda SR.Expression of epidermal growth factorreceptor changes in the hypothalamus during theonset of female puberty.Mol Cell Neurosci1994a;5:246-262.63. Prevot V,Cornea A,Mungenast A,Smiley G,OjedaSR.Activation of erbB-1 signaling in tanycytes of themedian eminence stimulates transforming growthfacor β 1 release via prostaglandin E2 production andinducescell plasticity.j Neurosci. 2003;23:10622-10632. 64. Ma YJ,Dissens GA,Merlino G,Coquelin A,OjedaSR.Overexpression of a human transforming growthfactor alpha(TGFα) transgeniereveals a dualantagonistic role of TGFα in female sexualdevelopment .Endocrinology 1994b;135:1392-1400.65. Langub MC Jr,Watson RE Jr.Estrogenreceptoe-immunoreactive glia-endothelia,andependyma in guineapig preoptic area and mediane m i n e n c e : e l e c t r o nmicroscopy.Endocrinology1992;130:364-372.

6 6 . B u c h a n a n C D , M a h e s h V B , B r a n nDW.Estrogen-astrocyte-luteinizing hormone-releasinghormone signaling:a role for transforming growthfactor –β 1.Biol Reprod 2000 ;62:1710-1721.67. Melcangi RC,Martini L,Galbiati M.Growth factorsand steroid hormones:a complex interplay in thehypothalamic control of reproductive functions.ProgNeurobiol. 2002;67:421-449.68. Bouret S,deSeranno S,Beauvillain JC,PrevotV.Transforming growth factor β 1 May directlyinfluence gonadotropin –releasing hormone genee x p r e s s i o n i n t h e r a thypothalamus.Endocrinology.2004;145:1794-1801.69. Esrella C,de Serranno S,Carin E,d’Anglemont deTassigny X,Mitchell V,Beauvillain JC,Prevot V.Matrixmetalloproteinases are expressed in the medianeminence of the hypothalamus and their activities varyduring the rat estrous cycle.Prog 86th Ann MrgEndocrine Soc;New Orleans ,LO,USA.2004.P3-266.70. Prevot V,Rio C,Cho GJ,Lomniczi A,HegerS,Neville CM,Risenthal NA,Ojeda SR,CorfasG.Normal female sexual development requiresneuregulin –erbB recepror signaling in hypothalamicastrocytes.J Neurosci.2003;23:230-239.71. Prevot V,Lomniczi A,Corfos g,Ojeda SR.erbB1 anderbB4 receptors act in concert to facilitate femalesexual development and mature reproductivefunction.Endocrinology. 2005;146:1465-1472.72. Sandau US,Mungenast AE,McCarthy J,BiedererT,Corfas G,Ojeda SR.The synaptic cell adhesionmolecule ,SynCAM1,mediates astrocyte-to astrocyteand astrocyte to GnRH neuron adhesiveness inm o u s ehypothalamus.Endocrinology;2011a;152:2353-2363.73. Sandau US,Mungenast AE,Alderman Z,SardiSP,Fogel ALI,Taylor B,Parent AS,Biederer T,CorfasG,Ojeda SR.SynCAM 1 a synaptic adhesion molecule,is expressed in astrocytes and contributes to erbB4receptor –mediated control of female sexualdevelopment.Endocrinology.2011b;152:2364-2376.74. Ma YJ,Berg-vonder Emde K,Rage F,WetselWC,Ojeda SR.Hypothalamic astrocytes respond totransforming growrh factor alpha with secretion ofneuroactive subsatances that stimulate the release ofl u t e i n i z i n g h o r m o n e – r e l e a s i n ghormone.Endocrinology. 1997;138:19-2575. Fields RD,Burnstock G.Purinergic signaling inn e u r o n - g l i a i n t e r a c t i o n s . N a t R e vNeurosci.2006;7:423-436.76. Haydon PG.Glia:Listening and talking to thesynapse.Nat Rev Neurosci .2001;2:185-193.77. Terasawa E.Role of ATP in synchronization ofCa2+oscillations in LHRH neurons in vitro.5th IntlC o n g r e s s o f



Neuroendocrinology(S20);Aug30-Sep6,2002;BristolUK-Ref type-abstract.78. Terasawa E,Keen KL,Grendell RL,GolosTG.Possible role of 5’-adenosine triphosphate insynchronization of Ca2+oscillations in primateluteinizing hormone releasing hormone neurons..MolEndocrinol 2005;19:2736-2747. 79. Zonta M,Sebelin A,Gobbo S,Fellin T,PozzanT,Carmignoto G.Glutamate –mediated cytosoliccalcium oscillations regulate a pulsatile prostaglandinr e l e a s e f r o m c u l t u r e d r a tastrocytes.LPhysiol.2003;552:407-414.80. Barbour B,Szatkowski M,Ingledew N,AttwellD.Arachidonic acid onduces a prolonged inhibition ofg l u t a m a t e u p t a k e i n g l i a lcells.Nature.1989;342:918-920.81. Van den Pol AN,Trombley PQ.Glutamate neuronsin hypothalamus regulate excitatory transmission.JNeurosci.1993;13:2829-2836.82. Peschon JJ,Slack JL,Reddy P,StockingKL,Sunnaborg SW,Lee DC,Russell WE,CastnerBJ,Johnson RS,Fitzner jn,Boyce RW,NelsonN,Kozlosky CJ,Wolfson MF,Rauch CT,CerettiDP,Paxton RJ,March CJ,Black RA.An essential rolefor ec todomain shedd ing in mammal iandevelopment.Science.1998;282:1281-1284.83. Lomniczi A,Cornea A,Costa ME,OjedaSR,Hypothalamic tumour necrosis factor-alphaconverting enzyme mediates excitatory amino acid–dependent neuron-gl ia s ignal ing in theneuroendocrine brain.J Neurosci.2006;26:51-62.84. Clasadonte J,Poulain P,Hanchate NK,Corfas G,Ojeda SR,Prevot V.Prostaglandin E2 release fromastrocytes triggers gonadotropin releasinghormone(GnRH)neuron firing via EP2 receptora c t i v a t i o n . P r o c . N a t l . A c a dSci.USA.2011;108:16104-16109..85. Campbell RE,Han SK,Herbison AE.Biocytin fillingof adult gonadotropin –releasing hormone neurons ins i tu revea ls ex tens ive ,sp iny ,dendr i t i cprocesses.Endocrinology 2005;146:1163-1169.86. Campbell RE,Gaidamaka G,Han SK,HerbisonAE.Dendro-dendritic bucling and shared synapsesbetween gonadotropin releasing hormoneneurons.Proc.Natl.Acad.Sci.USA.2009;106:10835-10840.87. Chan H,Prescott M,Ong Z,Herde MK,HerbisonAE,Campbell RE.Dendritic spine plasticity ingonadotropin-releasing hormone (GnRH) neuronsact ivated at the t ime of the preovulatorysurge.Endocrinology.2011;152:4906-4914.88. Mayer C,Acosta-Martinez M,Dubois SL,WolfeA,Radovick S,Boehm U,Levine JE.Timing andcompletion of puberty in female mice depend on

estrogen receptoralpha-signaling in kisspeptinneurons.Proc.Natl.Acad.Sci.USA2010;107:22693-22698.89. Nishida H,Okabe S.Direct astrocytic contactsregulate local maturation of dendritic spines.JNeurosci .2007;27:331-340.90. Murai KK,Nguyen LN,Irie F,YamaguchiY,Pasquale EB.Control of hippocampal dendritic spinem o r p h o l o g y t h r o u g h e p r i n A 3 / E p h A 4signaling.Nat.Neurosci.2003;6:153-160.91. Amateau SK,McCarthy MM.A novel mechanism ofdendritic spine plasticity involving estradiol induction ofprostaglandin E2.J Neurosci.2002;22:8586-8596.92. Amateau SK,M cCarthy MM.Induction of PGE2 byestradiol mediates developmental masculinization ofsex behavior.Nat Neurosci.2004;7:643-650.93. Araque A,Parpura V,Sanzgiri RP,HydonPG.Tripartite synapses:glia,the unacknowledgedpartner.Trends Neurosci 1999;22:208-215.94. Pellerin L,Bouzier-Sore AK,Aubert A,SerresS,Merle M,Costalat R,Mgistretti PJ.Activity dependentregulation of energy metabolism by astrocytes:anupdate.Glia2007;55:1251-1262.95. Blutstein T,Devi dze N,Choleris E,Jasnow AM,PfaffDW,Mong JA.Oestradiol upregulates glutaminesynthetase Mrna and protein expression in thehypothalamus and hippocampus :implications for arole of hormonally responsive glia in aminoacidtransmission.J Neuroendocrinol .2006;18:692-702.96. Roth CL,McCormack AL,Lomnoczi A,MungewnastAE,Ojeda SR.Quantitative proteomics identifies amajor change in glial glutamate metabolism at the timeo f fema le puber ty .Mo l Ce l l Endocr ino l2006;254-255:51-59.97. Pierre K,PellerinL,Debernardi R,RiedererBM,Magistretti PJ.Cell specific localization ofmonocarboxylate transporters,MCT1 and MCT2,in theadu l t mouse b ra in revea led by doub leimmunohistochemical labeling and confocal microscopy.Neuroscience.2000;100;617-627.98. Uchara T,Sumiyoshi T,Itoh H,Kurata K,.Lactateproduction and neurotransmitters;evidence frommicrodialysis studies.Pharmacol Biochem Behav.2008;90:273-281.99. Parsons MP,Hirasawa M.ATP-sensitive potassiumchannel –mediated lactate effect on orexinneurons:implications for brain energeticsduringarousal.JNeurosci. 2010;30:8061-8070100. Blazquez C,Woods A,de Ceballos ML,CarlingD,Gzman M.The AMP activated protein kinase isinvolved in the regulation of ketone body production byastrocytes.J Neurochem 1999;73:1674-1682.101. Garcia MA,Millain C,Balmaceda –AguileraC,Castro T,Pasor P,Montecinos H,Reinicke K,Zuniga



F,Vera JC,Onate SA,Nualart F.Hypothalamicependymal-glial cells express the glucose transporterGLUT2 ,a protein involved in glucosesensing.JNeurochem.2003;86:709-724.102. Ojeda SR,Lomniczi A,Mastronardi C,HegerS,Roth C,Prent AS,Matagne AE.Minireview:theneutoendocrine regulation of puberty:is the time ripefor a systems biology approach?Endocrinology2006;147:1166-1174.103. Matagne V,Kim JG,Ryu BJ,Hur MK,Kim MS,KimK,Park BS,Damante G,Smiley G,Lee BJ,OjedaSR.Thyroid transcription factor 1 ,a homeodomaincontaining trancdiptopn facror ,contributes toregulating periodic oscillations in GnRH geneexpression.JNeuroendocrinol. 2012;24:916-929.104. Dissen GA,Lomnoczi A,Heger S,NeffTL,OjedaSR.Hypothalamic EAP1(enhanced at puberty1 i s requ i red fo r menst rua l cyc l i c i t y innonhumanprimates.Endocrinology.2012;153:250-361.105. Lomniczi A,Garcia-Rudaz C,RamakrishnanR,Wilmot B,Khouangsathiene S,Ferguson B,DissenGA,Ojeda SR.Asingle –nucleotide polymorphism in theE A P 1 g e n e i s a s s o c i a t e d w i t hamenorrhea/ol igomenorrhea in nonhumanprimates.Endocrinology.2012;153:339-349.106. Lominiczi A,Loche A,Castellano JM,RonnekleivOK,Bosch M,Kaidar G,Knoll JG,Wright H,PfeiferGP,Ojeda SR.Epigenetic control of female puberty.NatNeurosci 2013;16(3):281-289.107. Fraying C,Britton R,Dale N.ATP-mediatedglucosensing by hypothalamic tanacytes.J Physiol2011;589:2275-2286. 108. Roland AV,M oenter SM.Regulation of GnRHneurons by glucose.Trends Endocrinol Metabol2011;22:443-449.



Molecules postulated to mediate glia-GnRH neuron adhesive communication in the hypothalamus.GnRH neurons adhere to astrocytesvia artleast three mechanisms:1)Haemophiliac interactionsinvolving NCAM140,2)Homophilic interactions mediated by SynCAM,and3)heterophilic interactionsmediated by the binding of contactin present in GnRH neurons to the receptor RPTβ expressed in glialcells.Each of these systems has signaling capabilities suggesting that ,in addition to providing anadhesive interaction,they can regulate astrocyte and GnRH neuron intracellular processes.Thisregulation is likely to occur via a variety of intracellular signaling molecules.-courtesy ref 20

Illustrations

Illustration 1

Fig 1



SCHEMATIC REPRESENTATION OF NEURAL-GLIAL-ENDOTHELIAL INTERACTIONS INVOLVED IN THECONTROL OF GnRH neurosecretion in the ME

A)Glial-neuronal interactions in the MEinvolve the production of epidermal growth factor(EGF)-relatedpeptides by glial cells .Activation of erbB1/erbB2 and erbB4/erbB2 heterodimers by TGFα andNRG,respectively,promotes the release of PGE2 from astrocytes .The binding of TGFα to tanycytic erbB1receptors results in the recruitment of erbB2 coreceptors and signal transduction.The ligand dependentactivation of erbB1 receptors in tanycytes results in biphasic plastic changes characterized by an initialphase of tanycyte outgrowth(1)and secondary phase of retraction(5).Although the initial outgrowth(1)is independent of TGFβ1 system ,the subsequent retraction requires PGE2 synthesis (2),a PGE2–dependent increase in the production of TGFβ1(3’)and matrix metalloproteinase(MMP)activity(4).Inaddition to promoting TGFβ1 synthesis by tanycytes(3’)PGE2 released by tanycytes(2)and astrocytes isable to directly stimulate GnRH release at nerve endings through the EP1 receptor(EP1)-mediatedmobilization of intracellular calcium stores(3).

B)Endothelial-neuronal interactionsat the level of ME involve the production of nitric oxide(NO)by theendothelial cells of fenestrated capillaries of the portal blood vessels.Upon its secretion ,NO diffusesfrom its source and production of PGE2 from tanycytes.PGE2 promotes the release of GnRH into theblood stream by the direct stimulation of nerve endings (3)and by promoting their access to thepericapillary spaceby inducing cytoarchitectural changes in tanycyte end feet(1-3’).Estrogens(Og) arelikely tobe the key humoral factors involved in the orchestration of endothelia-glia communication thatthat allows GnRH neurons to directly contact the piruitary portal blood vessels on the day ofprooestrus.Og treatment upregulates COX expression in tanycytes and stimulates endothelial nitricoxide synthase(Enos)expression in ME endothelial cells.-courtesy ref10 –with permission

Illustration 2

Fig 2



PGE2 acts as a gliotransmitter to stimulate GnRH neurn electrical activity.-Neuronally releasedglutamate(Glu)(1)co-activates metabotropic glutamatergic(mGlur)and AMPA glutamatergicreceptors(GluR)in astrcytes(2),stimulating the activity of zinc-dependent-matrixmetalloproteinases(MMP)of the ADAM(adisintegrin and metalloproteinase)family(3).The MMP’scatalyze ectodomain shedding of the pro-EGF ligands pro-TGFα and pro NRG(pr-neuregulin).In particular,the processing of peo-TGFα has been shown to involve the metalloproteinase ADAM 17,also known astumour necrosis factorα converting enzyme(TACE).The subsequently released mature TGFα and NRGactivate erbB1/erbB2 and erbB4/erbB2heterodimers respectively.The coactivation of glutamatergicreceptors induce the recruitment of erbB1 ,erbB4,and their pro-ligands to the cell membrane,wheremultiprotein complexes form,as demonstrated by direct the direct physical association of glutamatergicand erbB receptors.The activation of erbB receptors in hypothalamic astrocytes promotes profoundmorphological changes ,including thev retraction of cytoplasm,stellation of cells and elongatin ofprocesses.The activation of erbB receptors also prpmotes release of PGE2(4)which stimulates Camp/proteinkinase A(PKA)pathway in GnRH neurons through the mobilization of EP2 receptors(5).Activationf this signaling pathway induces a reversible membrane depolarization of GnRH neurons leading t theinitiation of spike firing via a postsynaptic effect involving the activation of non selective cationcurrent.-courtesy ref 10 with permission.

Illustration 3

Fig 3



DisclaimerThis article has been downloaded from WebmedCentral. With our unique author driven post publication peerreview, contents posted on this web portal do not undergo any prepublication peer or editorial review. It iscompletely the responsibility of the authors to ensure not only scientific and ethical standards of the manuscriptbut also its grammatical accuracy. Authors must ensure that they obtain all the necessary permissions beforesubmitting any information that requires obtaining a consent or approval from a third party. Authors should alsoensure not to submit any information which they do not have the copyright of or of which they have transferredthe copyrights to a third party.

Contents on WebmedCentral are purely for biomedical researchers and scientists. They are not meant to cater tothe needs of an individual patient. The web portal or any content(s) therein is neither designed to support, norreplace, the relationship that exists between a patient/site visitor and his/her physician. Your use of theWebmedCentral site and its contents is entirely at your own risk. We do not take any responsibility for any harmthat you may suffer or inflict on a third person by following the contents of this website.


IntroductionArticleIllustrationsIllustration 1Illustration 2Illustration 3

Documents

Role of Glia In Reproduction and Consequent Human Therapeutic Potentials … · 2013. 5. 11. · 1. Neural cell adhesion molecule(PSA-NCAM)-with GnRH –glial adhesiveness mediated