GLIAThe Muller cell: a functional element of theretinaEric Newman and AndreasReichenbach

Mullercellsarethe principalglialcellsof the retin~ assumingmanyof the functionscarriedout

by astrocytes,oligodendrocytesand ependymalcellsin other CNS regions.Miillercellsexpress

numerousvoltage-gatedchannelsand neurotransmitterreceptors,whichrecognizea varietyof

neuronalsignalsandtriggercelldepolarizationand intracellularCa2+waves.In turn, Mullercells

modulateneuronalactivitybyregulatingtheextracellularconcentrationofneuroactivesubstances,

including:(1) K+,whichis transportedvia Mulle*cellspatial-bufferingcurrents;(2) glutamate

andGABA, whicharetakenup by Mulle~cellhigh-affinitycarriers;and(3) H+,whichiscontrolled

by the action of MulleecellNa+-HCOJ- co-transportand carbonicanhydrase.The two-way

communicationbetweenMullercellsandretinal

role in retinalfunction.

TrendsNeurosci. (1996) 19, 307-312

0 F THE TWO principal types of macroglial cellsin the brain, oligodendrocytes are completely

absent in the retinae of most speciesand astrocytesarepresent only in mammalian species,and then only inthe nerve fiber layer. In their stead, the Muller cellserves as the principal glial cell of the retina. It per-forms many of the functions subservedby astrocytes,oligodendrocytes and ependymal cells in otherregions of the CNS.

Just as the retina has proveda valuableand access-ible tissue for elucidating the cellular and networkpropertiesof neurons, the Mullercell has servedas animportant model system for investigations of glialcells. Duringthe past quarter century, the physiologi-cal and morphological propertiesof Muller cells havebeen studied extensively. (See Refs 1-3 for recentreviews.)The amphibian retina, with its large, easilydissociatedMuller cells, has been a particularlyusefulpreparationin studiesof cell function.

Muller-cellmorphology

Miiller cells are radial glial cells which span theentire depth of the neural retina&6(Fig. 1). They arepresent in the retinae of all vertebrate species.Radiatingfrom the soma (in the inner nuclearlayer)isan inwardly directed process that terminates in anexpanded endfoot at the inner border of the retina,adjacent to the vitreous humor. Also projecting fromthe soma is an outwardlydirectedprocessthat ends inthe photoreceptor layer. Microvilli project from thisapical process into the subretinal space surroundingthe photoreceptors. In mammalian species withvascularizedretinae, en passant endfeet contact andsurround blood vessels within the retina while theendfeet of some cells next to the vitreous humorterminate on surface blood vessels. Secondaryprocesses branching from the main trunk of Mullercells form extensive sheaths that surround neuronalcell bodies, dendrites,and, in the optic-fiberlayer, theaxons of ganglion cells.

neuronsindicatesthatMullercellsplayanactive

Muller-cellphysiology

Muller cells, like other glial cells, express a widevariety of voltage-gatedion channels&lz.Their mem-brane conductance is dominated by inward-rectifierK+channels913,which give these cells an extremelylow membrane resistance, ranging from 10 to 21 Mflin different species14. The inwar&rectifier K+channelsare distributedin a highly non-uniform manner overthe cell surface. In amphibian species, 80-90?40of allchannels are localized to the endfoot at the retinalsurface,whilein mammalianspecieswith vascularizedretinae, channels are localizedto the surfaceendfoot,soma, and, in the cat, apical microvilli14.Muller cellsof various species also possess delayed-rectifier,fast-inactivating and Ca2+-activatedK+ channels, Na+channels, and Ca2+channels8,11,12.These channelsprobably do not modulate cell membrane potentialsignificantly, however, as their cellular conduc-tance are much smaller than that of the K+inward-rectifier channel. AmphibianMuller cells are coupledtogether by gap junctions, while mammalian cellsare coupled to astrocytes lying at the surface of theretina*5.

Muller cells also expressmany types of neurotrans-mitter receptors, including a GABA~receptor16,17andseveraltypes of glutamatereceptors18-20.They possesshigh-affinity uptake carriers for glutamate21-23andGABA(Refs21,24).

Mtillercells of the salamanderexpressa number ofacid–basetransport systems,includingan electrogenicNa+–HC03-co-transporter,an anion exchanger and aNa+–H+exchangerz5,Z6.They also have high levels ofcarbonic anhydrase,an enzyme that plays an impor-tant role in pH regulationby catalyzingthe hydrationof COZto H+and HC03- (Refs27,28).

How do these physiological properties contributeto glio-neuronal communication within the retina?Doesneuronal activity resultin changesin Muller-cellfunction? Can Muller cells modulate neuronal activ-ity? These questionswill be exploredin this review.

EricNewmanis atthe DeptofPhysiology,UniversityofMinnesota,Minneapolis,MN55455, USA,and AndreasReichenbach is atthe PaulFlechsigInstitutefor BrainResearch,DeptofNeurophysiology,LeipzigUniversity,D-04109 Leipzi&Federa[RepublicofGermany.

Copyright 01996, Elsevier Science Ltd. All rights reserved. 0166- 2236/96/$15.00 PII: S0166-2236(96)10040-0 71M Vol. 19, No. 8, 1996 307

GLIA E. Newman and A. Reichenbach – Miiller cells

CAP

Fig. 1. Drawing of Muller cell in the mammalian retina. Neuronalsomata and processesare ensheathed by the processesof a Miller cell(shaded blue). Abbreviations: A, amacrine cel~ B, bipolar cell; C, conephotoreceptor cel~’CAP, capillary; E~ Mii//er-ce// endfoot; G, ganglioncell; H, horizontal cel~ M, Mti//er cel~ MV, Mti//er-ce// microvilli; 1?,rodphotoreceptor cell. Modifiedfrom Ref.7.

Recognitionof neuronalsignalsby Mullercells

PotassiumMany substances released from active neurons,

including K+,neurotransmittersand metabolizes(suchas COZ),can potentially modulate Muller-cellbehav-ior. Important signalingfunctions have been ascribedto increases in the extracellular K+ concentration([K+]O)Z’.Increasesin IK+]Oresult in rapid cell depolar-ization in Miillercells30and in sloweractivation of the(Na+,K+)-ATPase31.Localizedincreasesin IK+]OgenerateK+spatial-bufferingcurrentswithin Mullercells, lead-ing to the redistribution of excess extracellular K+and to the generation of field potentials within extra-cellularspace32.Enhanced ~+]0has alsobeen showntostimulate glycogenolysisin mammalian Miiller cells7,and can triggerincreasesin the intracellularCa2+con-centration ([Caz+]i)1933.

NeurotransmittersNeurotransmittersreleasedfrom neurons constitute

a second signaling mechanism by which neuronalactivitycan modulateMullercells. Mullercells expressa varietyof receptors,includingthose for amino acids,catecholamines, neuroactive peptides, hormones andgrowth factors16-Z0,J*S6.Generally, these receptors dis-play high binding affinities and pharmacologicalproperties similar to those described in neurons. Inmost cases where electrophysiological studies havebeen performed,ligand binding elicits cell depolariz-ation, causedby direct opening of ion channels, as inthe case of the GABA~receptor in the skate andbaboon1617, or by second-messenger systems. Forexample, increases in glutamate, dopamine18’36andthrombin10are all believed to depolarizeMuller cellsby a secondmessenger-linkedreduction in K+conduc-tance in the cell.Carban dioxide

Active retinal neurons (particularlyphotoreceptorsin the dark) release C02, leading to substantialincreases in extracellular Pco . Such increases canresult in rapid intracellular a~idification in Mullercells. This acidification is generated largely by theaction of the enzymecarbonic anhydrase28,and mightmodulate several pH-dependent functions of theMuller cell, including carrier-mediated glutamateuptake37, acid–base transport, and gap-junctionalcoupling.

SignalingwithinMiillercells

CalciumNeurotransmittersand ions releasedby neurons can

activate second-messenger systems within Mullercells. One prominent example is the elevation of[Ca2+]iwithin Muller cells, which can be triggeredbyK+-induceddepolarizationas well as by activation ofligand-gatedreceptors1933.In dissociated salamanderMuller cells, raising IK+]Oresults in an influx of Ca2+(Ref. 33), presumably through voltage-gated Ca2+channels, while in cultured rabbit cells, glutamateelicits a Ca2+influx through non-NMDAreceptors19.Inthe absence of external Ca2+,stimulation of salaman-der Mullercells leadsto the releaseof Ca2+from inter-nal storesand to increasesin [Ca2+]iwhich begin in theapicalend of the cell and travel in a wave-likemannertowardsthe cell endfoot33(Fig. 2). These intracellularCa2+waves can be stimulated by elevated K+,gluta-mate and ATP,as well as by caffeine and ryanodine.Itis interesting to speculate that these Ca2+waves pro-vide a second pathway, independent of the neuronalnetwork,for signalsto be relayedfrom the outer to theinner retina.

In addition to the release of Ca2+from internalstores, several other second-messenger systems canbe activated by signals derived from neurons. Theseinclude the inositol phosphate and adenylatecyclase-cAMPsystems34r35.

Control of neuronalmicroenvironmentby MuHercells

Glial cells can modulate neuronal activity by con-trolling the concentration of neuroactive substancesin the extracellularfluid bathing CNS cells. The con-centrations of neurotransmittersand K+,for example,are regulated by glial-cell homeostatic mechanisms.Studies on Miiller cells have provided some of the

308 71NSVoL 19, ~0, 8,1996

E. Newman and A. Reichenbach – Mtiller cells GLIA

clearest examples of glial-cell control of the neuronalmicroenvironment.Potassium

One of the most thoroughlycharacterizedfunctionsof Muller cells is their regulation of IK+]Oin theretina3f29.Stimulation of the retina with light resultsinneuronal activation and increases in IK+]Oin the tworetinal synaptic layers (the inner and outer plexiformlayers)so.These light-elicited increases in IK+]Omustbe cleared rapidly in order to limit fluctuations inneuronal excitability. Muller cells remove excess K+from extracellularspaceby severalmechanisms. Asinother glial cells, K+is taken up and temporarilystoredin Muller cells, with influx occurring by both passive(K+ and Cl- uptake) and active [(Na+,K+)-ATPase]processes31.Potassium is also removed from extra-cellular space by a spatial-buffering mechanism32:increases in IK+]Owithin the two plexiform layersdepolarizeMullercells and leadto K+effluxfrom otherMi.iller-cellregions (Fig. 3). The resulting K+spatial-bufferingcurrent effectivelyredistributesextracellularK+from regions where IK+]Ois initially high to regionswhere it is low.

Spatial-buffering currents pass through inward-rectifiing K+channels, the predominant channel inMiiller cells913.Unlike other voltage-gatedchannels,these channels are open at the resting membranepotential. The voltage-and K+-dependentpropertiesofthese channels serve to augment the spatial-bufferingcurrents; in regionswhere IK+]Ois raised,channel con-ductance is increased. Neurotransmittersand otherfactors can modulate the conductance of Muller-cellK+channelslo’18’36,suggestingthat IK+]Oregulation byMiillercells is under neuronal control. Even a modestdecrease in K+ inward-rectifier conductance wouldreduceK+spatial-bufferingcurrentsand diminish ~+]0regulationby Muller cells.

In amphibian species,a largefraction of all inward-rectifyingK+channels in Mullercells is localizedto theendfoot at the retinal surface14’38.Thus, K+ spatial-bufferingcurrent preferentiallyexits from Mullercellsat the endfoot. The result of this specializedform ofspatialbuffering,termed ‘K+siphoning’32,is that mostexcess K+releasedby active neurons is transferredtothe vitreous humor, which acts as a large K+sink3940.The pattern of the spatial-bufferingcurrent is morecomplex in mammalian species, with excess K+directed to both the vitreous humor and the fluidspace surroundingthe photoreceptors41(the subreti-nal space; Fig. 3).

Modeling studies of amphibian and mammalianretinae indicate that K+siphoning is 1.6-3.7 timesmore effective in clearing excess K+from the retinathan is K+diffision through extracellular space31,3z,42.Experimental studies demonstrate directly that K+siphoning is a key mechanism for regulating retinalIK+]O.The light-elicited increases in IK+]Omore thandouble in the frog40,and more than triple in the cat41when K+-siphoning currents are blocked by Ba2+(Fig. 4).FJeurotransrnitters

Glial cells play an important role in removingneurotransmitters from extracellular space followingtheir release from synaptic terminals. This uptake isessentialfor terminating synaptictransmissionas wellas for preventingthe spreadof transmittersawayfromthe synaptic cleft. Muller cells possess high-affinity

0 33 82 139 205283 376485

ntdCa’+

Fig. 2. Calcium wave in a dissociated salamander Mtiller cell. TheC&+wove,elicitedbyadditionof 100 rw rpnodinein theabsenceof extrace//u/arCd+, begins ot the opico/ end ofthe cc// and truvek towards the cc// endfoot. The intrucelkdar Cc/+ concentration is imagedusing the Cd+-indicator dye fura-2. Images were obtained at 7s intervak. FromRef.33.

uptakecarriersfor many transmittersand are believedto regulate extracellular transmitter levels in theretina. Muller cells processes ramify extensively inthe two synaptic layers of the retina, suggestingtheimportance of these cells for removing neurotrans-mitters from extracellularspace.Glutamate

The high-affinity glutamate carrier of Miiller cellshas been studied extensively21-23.Expression of onesuch carrier, the L-glUtamate–L-aSPaflate transporter(GLAST), has been demonstrated in rat Muller cells43.

v vitreous humor

Fig. 3. Potassium spatial-buffering in MiWer cells. Potassium,released from active neurons into the inner p/exiform layer, is removedfrom extracellular space by a fC-@honing current flow through theMtiller cell. The excess /C entering the cell generates a depolarizationand drives out an equo/ amount of ~ from other cc// regions. In vascu-/arized mammalion retinae, P efflux occursfrom several cell regionshaving a high density of F channek: (1) the endfaot at the inner sur-face of the retina; (2) the apical endaf the cel~ and (3) the endfeet ter-minating on blood vessels.Heavy arrows indicate K+fluxes inta and outof the Miller cell. Abbreviations: IPL, inner plexiform laye~ SRI, sub-retinal space; V~, cellmembrane potential. A second IK+]Oincrease, inthe outer plexifarm laye~ has been amitted for clarity. The apicol endof the cell is shown at the tap. FromRef.3.

TTNSVO1.19, No. 8,1996 309

GLIA E. Newman and A. Reichenbach – Miiller cells

A Control Ba2+

B

[ 0.5mV

J I v

The Mtiller-cell glutamate carrier has a complexstoichiometry and questions concerning its detailsremain. In one scheme, the inward transport of oneglutamate molecule and two Na+is coupled to theoutward transport of one K+and one OH- (Ref. 44).Because the transporter is electrogenic, glutamateuptakeis voltage-dependent;cell depolarizationslowsdown or even reversesuptakeof the excitatory aminoacid4s(Fig. 5). In addition, becauseOH-is transportedalong with glutamate, uptake is pH dependentand isaccompariiedby an extracellularalkalinization44.GABA

Muller cells, including those of rabbit and mouse,also possess a high-affinity uptake system for GABA(Refs 21,46). Expressionof the GABAcarrier, GAT-3,has been demonstratedin Mullercells of the rat47.Thetransporter is electrogenic, and is believed to have astoichiometry of two Na+plus one Cl- plus one GABAmolecule, all transported inwardly24’48.GABAis theprimary inhibitory transmitter of the retina, and isreleasedby horizontalcellsand amacrinecells. In somemammalian species, including the rabbit, horizontalcells lack a GABAtransporter49,and the responsibilityfor removing GABAfrom the extracellularspace pre-sumablyfalls largelyupon Muller cells.pHandCO,

Neuronalactivity, triggeredby light, generatesa sig-nificant extracellular alkalinization in the retinaso.

w ~loPA0.5 s

Fig.5. Depolarization-elicited release of glutamate generated by reversal of the MiJller-cellglutamate uptake carrier. (A) Release of glutamate from a AWer cell (right) is monitored byrecording glutamate-elicited currents from an adjacent Purkinje cell (/efl). (B) Depolarizing aMiller cell from -60 to +20mV (top trace)elicits an inward current in the Purkinje cell(middle trace). The Purkinje-ce// current is generated by activation of its glutamate-containingreceptors. When extracelkslar IC is omitted (bottom trace) reversed glutamate transport byIvhWer cells is blocked and no glutamate current is recorded in the Purkinje cell. Photographin A, courtesyof BrianBillupsand DavidAttwell;B,modifiedfrom Ref.37.

Fig. 4. Mii//er cells regulate[fC]@in the retina. (A) Light-elicitedincreasesin the extrace//u/ar it concentration ([K’]O) are recorded withinthe innerplexiform/oyerofthe cat retina.WhenMti//er-ce//~ @annekare blocked by the addition of 3 m~ B&+, thus reducing spatia/-buffering currents, the light-elicited increase in [fC]O more than triples.(B) The /ight-e/icited decrease in[~]0 in the subretinal space more thanquadruples when B&+ is added, indicating that spatial-buffering cur-rents norma//y transfer excessf6 from the retina to the subretinal spaceas well as to the vitreous humor. The timecourse of the light stimulus,4s in duration, is indicated in the bottom trace. Modified fromRef.41.

This pH shift is thought to be regulated by Mullercells3’2b.SalamanderMuller cells possess a Na+–HCO~-co-transport system which has a stoichiometry ofapproximately three HCOJ- transported along withone Na+(Ref. 25). Because the transporter is electro-genic, cell depolarizationresults in an efflux of acid

26 Thu5, during periods of neuronal activ-equivalents .ity, when Muller cells are depolarizedby increasedIK+]O,the activity of the Na+-HCO~co-transport sys-tem will acidifyextracellularspace (Fig. 6). This acidi-fication helps to muffle extracellularpH variationsbypartially neutralizingthe neuronally generated extra-cellularalkalinization.

Muller cells might also regulateextracellularpH inthe retina by facilitating the removalof COZproducedby neuronal activity. Excessretinal CO, willbe rapidlyconverted to HC03- and H+by the enzyme carbonicanhydrase, which is found both within Muller cellsand on the cell surface27’28.The HCOj- might then betransportedto. the vitreous humor by Na+-HCOq-co-transporters,which, in the salamander,are localizedpreferentiallyat the cell endfoot252G.This homeostaticmechanism, termed ‘COZsiphoning’28for its resem-blance to K+siphoning, has yet to be demonstratedexperimentally.Such a scheme is consistent, however,with the observation that in the frog inhibitors ofcarbonic anhydraseresultin an enhancement of light-elicited pHvariationswithin the retinaso.

Muller-celimodulationof neuronalactivity

Muller cells, by controlling the concentration ofneuroactive substancesin extracellularspace, can sig-nificantly modulate neuronal activity. A reduction inthe uptakeof glutamateor GABAwill leadto enhance-ment of synaptic transmission. The accumulation ofK+in extracellularspace, resultingfrom a reduction inK+removalby Mullercells, will also lead to changes inneuronal excitability.Variations in pH can modulateneuronalactivityaswell.Eventhe smalldepolarization-inducedextracellularacidificationgeneratedby Mullercells can have a dramaticinhibitory effect on synaptictransmission.For example, an acidification of 0.05 pHunits in the salamanderretina producesa 24°h reduc-tion in synaptictransmissionbetween photoreceptorsand second-orderneurons51.Neuratransmitterrelease

Muller cells, in addition to influencing neuronalactivity by regulating the levels of substances in theneuronal microenvironment, might control neuronalactivity more directly. When depolarizedsufficiently,glutamate uptake by salamander Muller cells isreversedand glutamateis actually releasedinto extra-cellular space45(Fig. 5). This release might contributeto excitotoxic damageto neurons under pathological

310 TLVSVO1.19, No. 8, 1996

E. Newman and A. Reichenbach - Miller cells GLIA

Fig. 6. Depolarization-induced acid efflux generated by the Miiller-cell Na+–ffCOj- co-transport system. (A) Micrograph of a dissociatedsalamander Mti//erce/l. Scale bar, 20pm. (B) Image of extracel/u/arpHfor the cellshownin ~ measured by imaging the pH-indicator dyeBCECF(2~7’-biscarboxyethyl-5 (6)carboxyfluorescein) fixed to a cover-slip. The cell Na+–HC03- co-transporter is activated by depolarization([~]0 raised from 2.5 to 50m@. The resulting acid efflux is largest atthe cell endfoot, indicating that co-transporters are preferentially loca/-izedto thiscc//region.Thepseudocolor image is calibrated inpi-lunits(bar at right). FromRef.25.

conditions. Depolarizationcan also leadto the releaseof GABAfrom rat Miiller cells’z. It should be noted,however,that the releaseof glutamateand GABAfromMiiller cells has been demonstratedonly in cells thathave been pre-loadedwith the transmitters.It remainsto be shown that transmitter release occurs underin vivo conditions.

Nitric oxide (NO) synthase is expressed in Mullercells of salamanderand fish53,although releaseof NOfrom Muller cells has yet to be demonstrated. It isinteresting to speculate that the Ca2+wavesobservedin Mtillercells might triggerthe releaseof neurotrans-mitters, as is apparently the case for the release ofglutamate from cultured astrocytess’. Such ‘glio-transmitters’ could potentially modulate neuronalactivity.Metabolic sufi~oti

Retinal neurons are nourished by Muller cells.Glycogen stores in the retina are restricted to Mullercells”. Glycogenolysisin Muller cells is stimulatedbyneuronal activity’, and the direct transfer of lactatefrom Mi.illercells to neurons has been observedin the

56 Muller cells might also regulate bloodguinea pig .flow in retinal vessels in response to changes inneuronal activity. Retinal blood vessels are almostcompletely surroundedby Muller-cell endfeet whichcan release K+,acid equivalents, and perhaps NO (allvasodilators)when Muller cells depolarize2c’53’57.

Moreover, glial uptake of glutamate and GABAareimportant initial steps in the process of transmitterrecycling. Glutamine synthetase, an enzyme thattransamidates glutamate to glutamine, is localizedexclusively to Muller cells in the retinaz’. The gluta-mine synthesizedby Mi.illercells is recycled back toretinal neurons, where it servesas a precursorfor thesynthesisof additionalneurotransmitter.Inhibition ofthe glial enzyme in the rabbit causes a complete lossof neuronal finctionsB, demonstratingthe crucial rolethat Muller cells play in neurotransmission in theretina.

Concludingremarks

To date, research on Muller cells has been con-cerned largelywith their cellular propertiesand theircontributions to the regulation of the retinal micro-environment. These distinctive glial cells recognize avariety of neuronal signalsand actively control levelsof K+,H+and neurotransmittersin extracellularspace.In the coming years, research will provideadditionalinsights into the role of Mi.illercells in modulatingneuronal activity and retinal function.

Selectedreferences1 Newman, E.A. (1994) in The Principles and Practice of

Ophthab?dogy, Basic Sciences (Vol. 1) (Albert, D.M. andJakobiec,F.A.,eds),pp.398-419, Saunders

2 Reichenbach, A. and Robinson, S.Il. (1995) in Neuroglia(Kettenmann, H. and Ransom, B.R., eds), pp. 58-84, OxfordUniversityPress

3 Newman, E.A. (1996) TheNeuroscientist2, 110-1194 Uga, S. and Smelser, G.K. (1973) invest. Ophthalmol. 12,

434-448S Reichenbach, A. et al. (1988) Z. Mikroskop. Anat. Forsch. 102,

721-7556 Reichenbach, A. et al. (1989) Anat. Embryol. 180, 71-797 Reichenbach, A. et al. (1993) J. Chenr. Neuroanat. 6, 201-2138 Newman, E.A. (1985) Nature 317, 809-8119 Newman, E.A. (1993) J Neurosci. 13, 3333-3345

10 Pure, D.G. and Stuenkel, E.L. (1995)/. PhysioL 485, 337-34811 Chao, T.L et al. (1994) PfliigersArch. 426, 51-6012 Chao, T.I. et al. (1994) G2ia10, 173-18513 Brew, H. et aL (1986) Nature 324, 466-46814 Newman, E.A. (1987) J. Neurosci. 7, 2423-243215 Robinson, S.R. et al. (1993) Science262, 1072-107416 Malchow, R.P., Qian, H. and Ripps,H. (1989) Proc.NatL Acad.

Sci. U. S. A. 86, 4326-433017 Reichelt, W. et al. (1996) Neurosci..Lett.203, 159-16218 Schwartz, E.A. (1993) Neuron 10, 1141-114919 Wakakura, M. and Yamamoto, N. (1994) Vis. Res. 34,

1105-110920 Pure, D.G. (1995) Prog.Retinal Res. 15, 89-10121 Ehinger, B. (1977) Exp. Eye Res. 25, 221-23422 Brew, H. and Attwell, D. (1987) Nature327, 707-70923 Schwartz, E.A. and Tachibana, M. (1990) J. Physiol. 426,43-8024 Biedermann, B., Eberhardt, W. and Reichelt, W. (1994)

NeuroReport5, 438-44025 Newman, E.A. (1991) J. Neurosci. 11, 3972-398326 Newman, E.A. (1996) J. iVeurosci.16, 159-16827 Linser, P.J., Sorrentino, M. and Moscona, A.A. (1984)

Dev. Brain Res. 13, 65-7128 Newman, E.A. (1994) G2ia 11, 291-29929 Newman, E.A. (1995) in Neuroglia (Kettenmann, H. and

Ransom, B.R., eds), pp. 717–731, Oxford University Press30 Karwosld, C.J. and Proenza, L.M. (1977) J. Neurophysiol, 40,

24425931 Reichenbach, A. et al. (1992) Can. J. Physiol. Pharrnacol. 70,

S239-S24732 Newman, E.A., Frambach, D.A. and Odette, L.L. (1984)

Science225, 1174-117533 Keirstead,S.A.and Miller, R.F. (1995) GZia 14, 142234 Koh, S-W.M., Kyritsis,A. and Chader, G.J. (1984) J. Neurochem.

43, 199-20335 Osborne, N.N. and Ghazi, H. (1990) Prog. Retinal Res. 9,

101-13436 Biedermann, B. et al. (1995) NeuroReport6, 609-61237 Billups,B. and AtWell, D. (1996) Nature 379, 171-17438 Newman, E.A. (1985) J. Neurosci. 5, 2225-223939 Karwoski,C.J., Lu, H-K.and Newman, E.A. (1989) Science244,

578-58040 Oakley, B.I. et aL (1992) Exp. Eye Res. 55, 539-55041 Frishman, L.J. et al. (1992) J. Neurophysiol.67, 1201-121242 Eberhardt, W. and Reichenbach, A. (1987) Neuroscience 22,

687-696

TINS Vol. 19, No. 8, 1996

AcknowledgementsWe thank Janice I.GepnerandKathleenR. Zahsfor theirhelpfilcommentson themanuscript.Thiswork was supportedinpart by NationalInstitutesof HealthgrantEY04077(EN)and theDeutscheForschungsgemein-schaft (AR).

311

GLIA E. Newman and A. Reichenbach – Muller cells

GeorgW.Kreutzbergis at

the Dept ofNeuromorphology,

Max-Planck-InstituteofPsychiatry,

D-82152Martkried near

Munich,Germany.

312

43 Derouiche,A. and Rauen,T. (1995) J. Neurosci.Res. 42, 131-14344 Bouvier, M. et al. (1992) Nature 360, 471-47445 Szatkowski,M., Barbour, B. and Attwell,D. (1990) Nature348,

443-44646 Sarthy, P.V. (1982) J. Neurosci.Methods 5, 77-8247 Honda, S., Yamamoto, M. and Saito, N. (1995) Mol. Brain Res.

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401-40749 Redbum, D.A. and Madtes, P.J. (1986) ]. Comp. Neurol. 243,

41-5750 Borgula, G.A., Karwoski, C.J. and Steinberg, R.H. (1989)

Vis.Res. 29, 1069–107751 Barnes, S., Merchant, V. and Mahmud, F. (1993) Proc. NatL

Acad. .Sci.U..S.A. 90, 10081-1008552 Sarthy, P.V. (1983) J. Neurosci. 3, 2494-250353 Liepe, B.A. et aL (1994) J. Neurosci. 14, 7641-765454 Parpura, V. et al. (1994) Nature 369, 744-74755 Kuwabara, T. and Cogan, D.G. (1961) Arch. Ophthalmol. 66,

680-68856 Poitry-Yamate, C.L., Poitry, S. and Tsacopoulos, M. (1995)

J. Neurosci. 15, 5179-519157 Paulson, O.B. and Newman, E.A. (1987) Scierrce237, 896-89858 Pow, D.V. and Robinson, S.R. (1994) Neurosci. 60, 355-366

Microglia: a sensor for pathological events inthe CNSGeorg W. Kreutzberg

The most characteristicfeatureof microglialcellsis their rapidactivationin responseto even

minor pathologicalchangesin the CNS. Microgliaactivationisa keyfactorin the defenceof the

neuralparenchymaagainstinfectiousdiseases,inflammation,trauma,ischaemia,braintumours

and neurodegeneration.Microglia activation occurs as a graded response in vivo. The

transformationofmicrogliaintopotentiallycytotoxiccellsisunderstrictcontrolandoccursmainly

in responseto neuronal or terminal degeneration,or both. Activated microgliaare mainly

scavengercellsbutalsoperformvariousotherfunctionsintissuerepairandneuralregeneration.

They form a network of immune alert residentmacrophageswith a capacityfor immune

surveillanceand control. Activatedmicrogliacan destroy invadingmicro-organisms,remove

potentiallydeleteriousdebris,promotetissuerepairbysecretinggrowthfactorsandthusfacilitate

thereturntotissuehomeostasis.An understandingofintercellularsignalingpathwaysfor microglia

proliferationandactivationcouldform a rationalbasisfor targetedinterventionon glialreactions

to injuriesinthe CNS.Trends Neurosci. (1996) 19, 312-318

THEREIS HARDLYANYPATHOLOGYin the brainwithout an involvement of glial cells. The reactiv-

ity of microglia provides a striking example of thisprinciple’. Resting microglia show a downregulatedimmunophenotype adapted to the specializedmicroenvironment of the CNS.However,their abilityto respondquicklyto a varietyof signaling moleculessuggests that their apparent quiescence represents astate of vigilance to changes in their extracellularmilieu.

The important role of microglia in various patho-logical conditions was first recognized by del Rio-Hortega, who also coined their name’. Their natureand identity have long been debatedbut it is now gen-erallyacceptedthat they are ontogeneticallyrelatedtocells of the mononuclear phagocytelineage,unlike allother cell types in the CNS (Refs 3,4). There is, how-ever, a minority view based on in vitro experimentsthat postulatesthat microgliabelong to the true gliaofneuroectodermallineages.

One of the characteristicsof microglia is their acti-vation at a very early stage in response to injurylc-lO.Microglia activation often precedes reactions of anyother cell type in the brain. They respondnot only tochanges in the brain’s structural integrity but also to

very subtle alterations in their microenvironment,such as imbalances in ion homeostasis that precedepathological changes that are detectable histologi-Callyll. It is possible that the unique collection ofmembranechannels of microglia,includingan inward-rectifyingK+channel, is instrumentalin this responsive-ness1213.Furthermore, microglia have receptors forCNS signaling molecules such as ATP (Refs 14,15),calcitonin gene-related peptide (CGRP)lC,ACh andnoradrenaline17,and can react both with changes intheir extracellularionic milieu14,15,17 and by activation

of transcriptional mechanismslc. Their ability torespond selectively to molecules involved inneurotransmissionallowsthem in their ‘resting’ stateto monitor the physiological integrity of their micro-environment continuously and to react rapidlyin theevent of pathological disturbances. Our ignorance ofthe physiological function of microglial cells in thenormal brain must be understood also in the lightof our inability to measure their functional changesin vivo.

The rapid transformation of microglia from a rest-ing to an activated state has been clearly recognizedfor almosta century(Fig.1)1’20.While the morphologyof activated microglia was found to be extremely

TINS Vol. 19, NO. 8, 1996 Copyright 01996, Elsevier Science Ltd. All rights reserved. 0166- 2236/96/$15.00 PII: S0166-2236(96)1 OO49-7