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Primer

Glia: much morethan the neuron’sside-kick

Beth Stevens

Neuroscientists should have knownbetter. Ramon y Cajal, a visionarywhose contributions on neuronalmorphology and circuitry havedominated modern neuroscience,also made significant observationsabout glia, the non-neuronal cellsin the brain. Yet his pioneeringwork on glial cells was all butignored. Over a century ago, herecognized the great numbers ofglia in the brain and their intimateassociation with neurons. Thoughtheir function was at that time amystery, he predicted that gliamust be doing more than simplyfilling the spaces between neurons.

Not surprisingly, Cajal wascorrect. Neuroscientists are nowcatching up and discovering thatglia not only support a number ofessential neuronal functions, butalso actively communicate withneurons and with one another. Bydoing so, glia influence nervoussystem functions that have longbeen thought to be strictly underneuronal control.

Types of Glial CellIn the human brain, glia outnumberneurons by a factor of ten, andtoday we can readily identifynumerous glial cell types in thevertebrate nervous system basedon their unique morphological andbiochemical features (Figure 1).The myelinating glia —oligodendrocytes in the centralnervous system (CNS) andSchwann cells in the periphery —provide the insulating layers ofmyelin membrane around axons,which allow neural impulses topropagate rapidly over longdistances (Figure 1A,B). The othermajor category of glia areastrocytes, which have numerousforms and functions in the CNS.Classical neurohistologists dividedastrocytes into two main classesthat are distinguished by

morphology and location, andperhaps by function as well.Protoplasmic astrocytes found ingrey matter are closely associatedwith synapses, while fibrillary (orfibrous) astrocytes in white mattercontact nodes of Ranvier. Althoughmost roles of astrocytes remain agreat mystery, there is little doubtthat they buffer ions andneurotransmitters in theextracellular space. Astrocyticprocesses also ensheath bloodvessels, where they may help toregulate the development andmaintenance of the blood–brainbarrier by inducing tight junctionsbetween endothelial cells.Moreover, astrocytes are, like mostglia, major sources of extracellularmatrix proteins, adhesionmolecules and neurotrophicfactors. The role of these signals isstill unclear, but they may help topromote neuronal growth,migration and survival. As we shallsee below, it is likely thatastrocytes have a variety of moreactive roles as well.

Microglia comprise about 10%of CNS glia, and exist in multiplemorphological states in the healthyand damaged brain. Like other glialcell types, much mystery stillsurrounds their function. They mayact as internal ‘barometers’ of theextracellular milieu, rapidly sensingand communicating changes in thelevels of extracellular ions andsignaling molecules to neurons andglia. Much like immune cells,microglia elicit inflammatoryresponses, and phagocytosecellular debris in response to injury.

The list of glial functions in thenervous system is extensive andcontinues to grow as we learnmore about their biology,physiology and the mechanisms bywhich glia communicate with eachother and with neurons.

Neuron–Glia and Glia–GliaCommunicationUnlike neurons, glial cells areelectrically inexcitable, one reasonwhy they have been largely ignoredby neuroscientists for much of thelast century. But, despite beingincapable of firing actionpotentials, glia are in fact highlyactive cells, communicatingprimarily through chemical signals.Like their neuronal partners, glia

express an array of voltage-gatedion channels, membranetransporters, neurotransmitter andneurotrophic receptors. Thus, theyare well equipped to transmit andreceive signals to and fromneurons.

Glial calcium signaling is ameasurable indicator of excitability,providing a powerful tool to studyglial cells in action. Time-lapsecalcium imaging in several modelsystems has recentlydemonstrated that glia located atsynapses, and in extrasynapticregions along axons, respond toneuronal activity with an increasein intracellular calcium. Likewise,chemical or mechanical stimulationof glia can elicit calcium responsesin nearby neurons. Moreover, gliaalso actively communicate withone another. Stimulation of oneastrocyte can trigger a localelevation in astrocytic calciumwhich can subsequently spread toneighbouring astrocytes as acalcium wave, which is propagatedat a rate of 100 m m/sec. Themechanisms underlying astrocyticcalcium waves include localdiffusion of second messengersthrough gap junctions andextensive chemical signaling alongastrocytic networks. ExtracellularATP released from stimulatedastrocytes activates purinergicreceptors on neighboringastrocytes, which can propagatecalcium signaling over significantdistances. The function ofastrocytic calcium signaling isunknown. The realization thatneurons and glia activelycommunicate with one anotherthrough reciprocal chemical andtrophic signals suggests that manyimportant roles for glia in nervoussystem development, plasticity anddisease remain to be identified.

Glia Regulate Nervous SystemDevelopmentAs glia are a major source oftrophic factors, it is not surprisingthat they are proposed to becritical regulators of neuronalmigration, growth and survivalduring development — consistentwith their well-accepted supportrole. A classic example is the roleof radial glia in neuronal migrationearly during development. Thesespecialized glia provide a

temporary scaffold for themigration of newborn cortical andcerebellar neurons. Their longradial fibers extend from theventricular to the pial surface andserve as permissive ‘guidancecables’ for neurons en route totheir final target area in the brain.In addition to serving as astructural framework, radial gliaprovide important trophic supportfor migrating neurons.

Interestingly, some radial glia cellsare neural stem cells that generatepyramidal neurons in the cerebralcortex. These recent findings raisethe question whether radial glia areglia cells after all.

Like most aspects of neuraldevelopment, neuronal pathfindingand axon guidance have beenlargely viewed from a neurocentricperspective. New research usingmolecular and transgenic

approaches is revealing a muchmore dynamic and instructive rolefor glia in the development of braintopography. Glia are now believedto play a pivotal role in directingaxonal growth — either as‘guideposts’ for pioneering axonsalong a pathway or at key‘decision/choice points’, such asthe midline of the nervous system.It turns out that midline gliaprovide a number of positive andnegative navigational cues thatdirect commissural axons to eithercross or not cross the midline. InDrosophila, for example, midlineglia secrete the chemoattractantNetrin, as well as chemorepellentSlit — two molecules previouslythought to be secreted byneuronal cells at the midline. In themouse, a parallel situation isexemplified by glia at the opticchiasm directing retinal ganglionaxons from the left and the righteye to either cross the chiasmtoward the contralateral visualcortex or to continue alongipsilateral tracts.

Nervous system development isdependent on reciprocal andinteractive trophic signalingbetween neurons and glia. Forinstance, there are several majorways by which neural cells signaleach other to survive. First,neurons can signal to each otheraccording to the classic‘neurotrophic hypothesis’, whichstates that neuronal survival isdependent on competition ofgrowing axons for limiting amountsof target-derived factors. Geneticstudies in the mouse stronglysuggest that neuron survival is alsohighly dependent on ability ofneurons to provide trophic supportto developing glia, which in returnsecrete a vast array ofneurotrophic factors to neurons. Aprime example is neuregulin-dependent signaling betweendeveloping Schwann cells andperipheral axons. Axon-derivedneuregulin-1 is required forSchwann cell survival, proliferationand lineage progression.Importantly, however, axonalneuregulin also regulates theproduction of trophic factors bySchwann cells, which in turnfeedback onto sensory neurons topromote their survival anddifferentiation.

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Figure 1. Major categories of glia in the nervous system.(A) Non-myelinating Schwann cells appear spindle shaped when cultured without axons(left). Myelinating Schwann cells extend multiple wraps of myelin membrane around asingle axonal segment. Myelinated segments are interrupted by regularly spaced Nodesof Ranvier, which serve to propagate action potentials. (B) A cultured oligodendrocyte dis-plays several elaborate extensions of myelin membrane. In contrast to Schwann cells, asingle oligodendrocyte can myelinate multiple axons (right). (C) Astrocytes do not formmyelin, but they communicate with neurons, glia, and blood vessels to regulate diversefunctions in the CNS. Cultured rodent astrocytes are pictured on the left. (Drawingsadapted from Figure 2,3, Kandel, Schwartz, and Jessell.)

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New research points toward acritical role for glia also in synapseformation. In vitro studies in thevisual system have demonstratedthat retinal ganglion cells formfewer synapses without continuoussignaling from astrocytes. Synapsenumber and efficacy aresignificantly compromised in aglia-less environment. However,this reduction can be rescued byaddition of astrocytes or mediumconditioned by astrocyte cultures,suggesting the involvement of asoluble glial signal. In addition,cholesterol, secreted fromastrocytes and bound to ApoE, isinternalized by neurons andsignificantly increases synapsenumber and efficacy. While thespecific molecular mechanismsand signaling pathways, by whichastrocytes and/or cholesterolregulate synapse formation, arenot known, these studies havebroad implications for theinvolvement of glia insynaptogenesis in other areas ofthe brain.

Glia as Regulators of SynapticActivity and PlasticityThere is little question that gliaperform several regulatory rolesessential for normalneurotransmission. Astrocyticprocesses surround pre- andpostsynaptic neurons, where theyrapidly clear and recycle ions andneurotransmitters from thesynaptic cleft. Duringneurotransmission, glutamate istaken up by astrocytes, convertedinto glutamine, and then releasedinto the extracellular space.Subsequently, neurons take upglutamine to synthesize theexcitatory and inhibitoryneurotransmitters glutamate andGABA, respectively. Importantly,astrocytes also buffer K+ and otherions released duringneurotransmission and protectneurons from excessive glutamateand other forms of toxicity.

While these classic glialfunctions are critical in regulatingneuronal activity, emergingevidence is revealing a far moredynamic role for glial cells at thesynapse. The realization that gliasignal back to neurons to directlymodulate synaptic activity has ledto a re-definition of the synapse.

The ‘tripartite synapse’ has beenproposed to incorporate synapticglia as the third participant,actively communicating with pre-and post-synaptic neurons (Figure2, inset). We now know thatperisynaptic astrocytes in the CNSand terminal Schwann cells,surrounding neuromuscularjunctions in the peripheral nervoussystem (PNS), respond to synapticrelease of neurotransmitters, suchas glutamate and ATP, withincreases in intracellular calcium.This activity-dependent calciumflux can regulate glial intracellularsignaling pathways, geneexpression and trigger the release

of neuromodulators back to thepresynaptic neuron. Whether thisglial transmission is excitatory orinhibitory appears to depend onwhich types of signaling pathwaysare activated in glia duringsynaptic transmission. Elegantelectrophysiogical and imagingstudies performed at the intactfrog neuromuscular junction havedemonstrated that activity-dependent activation of specificsignaling cascades in terminalSchwann cells can integratesynaptic activity and directlyinfluence neuronal activity. Forexample, sustained IP3-dependentcalcium flux in perisynaptic

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Figure 2. Dynamic reciprocal communication between neurons and glia in the brain.Multiple types of glia actively communicate with neurons and with each another to reg-ulate diverse brain functions. The end-feet of astrocytic processes actively modulatesynaptic activity (inset). Synaptic release of neurotransmitters is detected by post-synaptic neurons and synaptic glia, which express receptors for ATP (P2R), glutamate(GluR) and other activity-dependent signals. Synaptic activity triggers calcium flux andactivation of signal transduction cascades in surrounding astrocytes. This regulatestransmitter release from astrocytes, which can feedback to synaptic terminals and acti-vate neighbouring astrocytes via a calcium wave (illustrated in red). Microglia alsoactively respond to synaptic signals, especially following damage or excessive activity.Bi-directional and activity-dependent communication also occurs between myelinatingglia and axons during development.

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Schwann cells can regulatesynaptic strength, whereassynaptic activation of G-proteinsignaling cascades in Schwanncells can affect synapticdepression.

Active communication betweenastrocytes contributes anadditional layer of complexity tosynaptic signaling in the CNS. Asillustrated in Figure 2,neurotransmitters released bypresynaptic neurons do not onlysignal to post-synaptic neuronsand terminal astrocytes, but canalso trigger massive calciumwaves through astrocyticnetworks, which could transmitsynaptic signals over extensivecortical areas. Taken together,these new findings suggest broadimplications for the involvement ofglia in activity-dependent synapticplasticity during developmentallycritical periods, learning andmemory, as well as informationprocessing within neural circuits. Infact, the primary function ofprotoplasmic astrocytes nowappears to be to regulate synapsedevelopment, function, andplasticity, all of which are the resultof active dialogues with nearbyneurons.

Injury, Regeneration andMyelinationGlia are major determinants of howthe nervous system responds todamage or traumatic injury. Whydo peripheral nerves regrow andreinnervate targets after injury,while central axons fail toregenerate? The answer lies in thefundamental differences betweenthe types of glial signaling in thecentral and peripheral nervoussystems.

In the adult PNS, the glialinfluence on nerve repair isgenerally positive. If matureSchwann cells detach fromdamaged axons, they rapidly de-differentiate into a population ofhighly plastic cells that resembleimmature Schwann cells. Despitethe loss of axon contact, such cellssurvive, re-proliferate, and providean instructive and permissiveenvironment for axonal regrowth.Once the appropriate number ofSchwann cells is generated,ensheathment and re-myelinationare initiated along regenerated

axons. Schwann cells are alsoinstrumental in the maintenanceand repair of synaptic connectionsin denervated peripheral targets.Terminal Schwann cells areexceptionally sensitive to damagesignals from muscle cells anddirectly promote axon sprouting atdenervated neuromuscularjunctions. Immediately after nervetransection or injury, terminalSchwann cells become ‘reactive’and extend long lamellipodialextensions that form a ‘bridge’ thatfacilitate pioneer axons toreinnervate endplates.

In the CNS, the environmentfollowing neurotrauma isdramatically different to the PNS.Damaged central axons do notgenerally regenerate or become re-myelinated. Astrocytes andmicroglia, which in the healthybrain promote and protect nervegrowth and activity, undergosignificant changes in morphologyand function. Microglia rapidlytransform into phagocytic cells,initiate inflammatory responsesand release an array of cytokines,ions and cytotoxic substances,which promote neurodegeneration,astrocyte swelling, and impairsynaptic transmission. Astrocytesalso become ‘reactive’ within a fewhours of brain injury and undergochanges in morphology andproliferation, thereby releasingmany signals that damage neuronsand other glia.

There are several reasons for thefailure of CNS regeneration — allinvolving glia. The inflammatoryresponse mediated by microgliaand astrocytes certainly hindersaxon repair, but astrocytes canalso act as a physical andchemical barrier to axon growththrough formation of a gliotic ‘scar’around the injury site. In addition,myelinating oligodendrocytesproduce several inhibitors toaxonal growth, such as myelin-associated glycoprotein (MAG) andNogo surface molecules, which areexpressed in CNS, but not PNSmyelin. These mechanisms clearlyillustrate that neuron–gliacommunication is not alwaysbeneficial, but understanding thespecific mechanism underlying thedifferences in CNS and PNS injuryrepair, will probably aid in thedevelopment of therapeutic

strategies to treat injury anddegenerative diseases. Forexample, use of neutralizingantibodies against Nogo, as well astransplantation of Schwann cellsinto CNS injury sites have yieldedpromising results. Also,development and regenerationshow several parallels; therefore,understanding the mechanismsregulating neurite outgrowth andmyelination during developmentwill be likely to shed light on themechanisms underlying diseaseand injury.

A Bright Future for Glial CellsCollectively, these examplessuggest that any brain function,traditionally considered to beregulated solely by neuron–neuroncommunication, could involve glialcells. The inclusion of this newdimension to neural circuitry isgradually shifting long-standingneurocentric perspectives on howthe brain works. With the growingrealization that glial cells are activeparticipants in nervous systemfunction, a new area ofneuroscience has emerged, inwhich there is much to explore andre-explore.

Further readingAraque, A., Parpura, V., Sanzgiri, R.P.

and Haydon, P.G. (1999). Tripartitesynapses: glia, the unacknowledgedpartner. Trends Neurosci. 22,208–215.

Fields, R.D. and Stevens-Graham, B.(2002). New insights into neuron-gliacommunication. Science 298,556–562.

Kandel, E.R., Schwartz, J. and Jessell,T.S. (1991). Principles of NeuralScience, 3rd edn. (Elsevier, NewYork.)

Lemke, G. (2001). Glial control ofneuronal development. Annu. Rev.Neurosci. 24, 87–105.

Qiu, J., Cai, D. and Filbin, M.T. (2000).Glial inhibition of nerve regenerationin the mature mammalian CNS. Glia29, 166–174.

Stevens, B. and Fields, R.D. (2000).Response of Schwann cells toaction potentials in development.Science 287, 2267–2271.

Ullian, E.M., Sapperstein, S.K.,Christopherson, K.S. and Barres,B.A. (2001). Control of synapsenumber by glia. Science 291,657–661.

Section on Nervous SystemDevelopment & Plasticity, NICHD,National Institutes of Health, Bldg. 49,Room 5A78, MSC 4495, 49 ConventDrive, Bethesda, Maryland 20892, USA.E-mail: stevensb@mail.nih.gov

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