The Muller Cell:A Functional Element of the Retina

GLIA The Muller cell: a functional element of the retina

Eric Newman and AndreasReichenbach

Mullercellsare the principalglialcellsof the retin~ assumingmanyof the functionscarriedout by astrocytes,oligodendrocytesand ependymalcellsin other CNS regions.Miillercellsexpress numerousvoltage-gatedchannelsand neurotransmitterreceptors,whichrecognizea varietyof neuronalsignalsand triggercelldepolarizationand 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 Mulleecell Na+-HCOJ- co-transport and carbonicanhydrase.The two-way communicationbetweenMullercellsandretinalneuronsindicatesthat Mullercellsplayan active role in retinalfunction.

TrendsNeurosci. (1996) 19, 307-312

Muller-cellphysiology F THE TWO principal types of macroglial cells 0 in the brain, oligodendrocytes are completely Muller cells, like other glial cells, express a wide absentin the retinae of most speciesand astrocytesare variety of voltage-gatedion channels&lz.Their mem- present only in mammalian species,and then only in brane conductance is dominated by inward-rectifier the nerve fiber layer. In their stead, the Muller cell K+channels913,which give these cells an extremely serves as the principal glial cell of the retina. It per- low membrane resistance, ranging from 10 to 21 Mfl forms many of the functions subservedby astrocytes, in differentspecies14. The inwar&rectifier K+channels oligodendrocytes and ependymal cells in other are distributedin a highly non-uniform manner over regions of the CNS. the cell surface. In amphibian species, 80-90?40of all Just as the retina has proveda valuableand access- channels are localized to the endfoot at the retinal ible tissue for elucidating the cellular and network surface,whilein mammalianspecieswith vascularized propertiesof neurons, the Mullercell has servedas an retinae, channels are localizedto the surfaceendfoot, important model system for investigations of glial soma, and, in the cat, apical microvilli14.Muller cells cells. Duringthe past quarter century, the physiologi- of various species also possess delayed-rectifier,fast- cal and morphological propertiesof Mullercells have inactivating and Ca2+-activatedK+ channels, Na+ been studied extensively. (See Refs 1-3 for recent channels, and Ca2+channels8,11,12.These channels reviews.)The amphibian retina, with its large, easily probably do not modulate cell membrane potential dissociatedMuller cells, has been a particularlyuseful significantly, however, as their cellular conduc- preparationin studiesof cell function. tance are much smaller than that of the K+inward- Muller-cellmorphology rectifier channel. AmphibianMuller cells are coupled together by gap junctions, while mammalian cells Miiller cells are radial glial cells which span the are coupled to astrocytes lying at the surface of the entire depth of the neural retina&6(Fig. 1). They are retina*5. present in the retinae of all vertebrate species. Mullercells also expressmany types of neurotrans- EricNewmanis at Radiatingfrom the soma (in the inner nuclearlayer)is mitter receptors, including a GABA~receptor16,17and the Deptof an inwardly directed process that terminates in an severaltypes of glutamatereceptors18-20.They possess Physiology, expanded endfoot at the inner border of the retina, high-affinity uptake carriers for glutamate21-23and Universityof adjacent to the vitreous humor. Also projecting from GABA(Refs21,24). Minnesota, the soma is an outwardlydirectedprocessthat ends in Mtillercells of the salamanderexpressa number of Minneapolis, the photoreceptor layer. Microvilli project from this acid–basetransport systems,includingan electrogenic MN55455, USA, apical process into the subretinal space surrounding Na+–HC03-co-transporter,an anion exchanger and a and Andreas the photoreceptors. In mammalian species with Na+–H+exchangerz5,Z6.They also have high levels of Reichenbach is at vascularizedretinae, en passant endfeet contact and carbonic anhydrase,an enzyme that plays an impor- the PaulFlechsig surround blood vessels within the retina while the tant role in pH regulationby catalyzingthe hydration Institutefor Brain endfeet of some cells next to the vitreous humor of COZto H+and HC03- (Refs27,28). Research,Deptof terminate on surface blood vessels. Secondary How do these physiological properties contribute Neurophysiology, processes branching from the main trunk of Muller to glio-neuronal communication within the retina? LeipzigUniversity, cells form extensive sheaths that surround neuronal Doesneuronalactivity resultin changesin Muller-cell D-04109Leipzi& cell bodies, dendrites,and, in the optic-fiberlayer,the function? Can Muller cells modulate neuronal activ- Federa[Republicof axons of ganglion cells. ity? These questionswill be exploredin this review. Germany.

Neurotransmitters Neurotransmittersreleasedfrom neurons constitute a second signaling mechanism by which neuronal activitycan modulateMullercells. Mullercells express a varietyof receptors,includingthose for amino acids, catecholamines, neuroactive peptides, hormones and growth factors16-Z0,J*S6.Generally, these receptors dis- play high binding affinities and pharmacological properties similar to those described in neurons. In most cases where electrophysiological studies have been performed,ligand binding elicits cell depolarization, causedby direct opening of ion channels, as in the case of the GABA~receptor in the skate and baboon1617,or by second-messenger systems. For example, increases in glutamate, dopamine18’36and CAP thrombin10are all believed to depolarizeMuller cells by a secondmessenger-linkedreduction in K+conductance in the cell. Carban dioxide Active retinal neurons (particularlyphotoreceptors in the dark) release C02, leading to substantial increases in extracellular Pco . Such increases can result in rapid intracellular a~idification in Muller cells. This acidification is generated largely by the action of the enzymecarbonic anhydrase28,and might modulate several pH-dependent functions of the Muller cell, including carrier-mediated glutamate uptake37, acid–base transport, and gap-junctional coupling. SignalingwithinMiiller cells

Calcium Neurotransmittersand ions releasedby neurons can activate second-messenger systems within Muller cells. One prominent example is the elevation of [Ca2+]iwithin Muller cells, which can be triggeredby K+-induceddepolarizationas well as by activation of ligand-gatedreceptors1933.In dissociated salamander Muller cells, raising IK+]Oresults in an influx of Ca2+ (Ref. 33), presumably through voltage-gated Ca2+ channels, while in cultured rabbit cells, glutamate elicits a Ca2+influx through non-NMDAreceptors19.In Fig. 1. Drawing of Muller cell in the mammalian retina. Neuronal the absence of external Ca2+,stimulation of salaman- somata and processesare ensheathed by the processesof a Miller cell der Mullercells leadsto the releaseof Ca2+from inter- (shaded blue). Abbreviations: A, amacrine cel~ B, bipolar cell; C, cone nal storesandto increasesin [Ca2+]iwhich begin in the photoreceptor cel~’CAP, capillary; E~ Mii//er-ce// endfoot; G, ganglion apicalend of the cell and travelin a wave-likemanner cell; H, horizontal cel~ M, Mti//er cel~ MV, Mti//er-ce// microvilli; 1?,rod towardsthe cell endfoot33(Fig. 2). These intracellular photoreceptor cell. Modifiedfrom Ref.7. Ca2+waves can be stimulated by elevated K+,glutamate and ATP,as well as by caffeine and ryanodine.It is interesting to speculate that these Ca2+waves pro- Recognitionof neuronalsignalsby Mullercells vide a second pathway,independent of the neuronal Potassium network,for signalsto be relayedfrom the outerto the Many substances released from active neurons, inner retina. including K+,neurotransmittersand metabolizes(such In addition to the release of Ca2+from internal as COZ),can potentially modulate Muller-cellbehav- stores, several other second-messenger systems can ior. Important signalingfunctions have been ascribed be activated by signals derived from neurons. These to increases in the extracellular K+ concentration include the inositol phosphate and adenylate ([K+]O)Z’.Increasesin IK+]Oresult in rapidcell depolar- cyclase-cAMPsystems34r35. ization in Miillercells30and in sloweractivation of the Control of neuronalmicroenvironmentby MuHer (Na+,K+)-ATPase31.Localizedincreasesin IK+]Ogenerate cells K+spatial-bufferingcurrentswithin Mullercells, leading to the redistribution of excess extracellular K+ Glial cells can modulate neuronal activity by con- and to the generation of field potentials within extra- trolling the concentration of neuroactive substances cellularspace32.Enhanced~+]0has alsobeen shownto in the extracellularfluid bathing CNS cells. The con- stimulate glycogenolysisin mammalian Miiller cells7, centrations of neurotransmittersand K+,for example, and can triggerincreasesin the intracellularCa2+con- are regulatedby glial-cell homeostatic mechanisms. centration ([Caz+]i)1933. Studies on Miiller cells have provided some of the

308 71NSVoL 19, ~0, 8,1996 E. Newman and A. Reichenbach – Mtiller cells GLIA

clearest examplesof glial-cellcontrol of the neuronal microenvironment. Potassium One of the most thoroughlycharacterizedfunctions of Muller cells is their regulation of IK+]Oin the retina3f29.Stimulation of the retina with light resultsin neuronal activation and increases in IK+]Oin the two retinal synaptic layers (the inner and outer plexiform layers)so.These light-elicited increases in IK+]Omust 0 33 82 139 205283 376485 be cleared rapidly in order to limit fluctuations in ntdCa’+ neuronal excitability. Muller cells remove excess K+ from extracellularspaceby severalmechanisms. Asin Fig. 2. Calcium wave in a dissociated salamander Mtiller cell. TheC&+wove,elicitedby other glial cells, K+is taken up and temporarilystored additionof 100 rw rpnodinein theabsenceof extrace//u/arCd+, begins ot the opico/ end of in Mullercells, with influx occurring by both passive the cc// and truvek towards the cc// endfoot. The intrucelkdar Cc/+ concentration is imaged (K+ and Cl- uptake) and active [(Na+,K+)-ATPase]using the Cd+-indicator dye fura-2. Images were obtained at 7s intervak. FromRef.33. processes31.Potassium is also removed from extracellular space by a spatial-buffering mechanism32: uptakecarriersfor many transmittersand arebelieved increases in IK+]Owithin the two plexiform layers to regulate extracellular transmitter levels in the depolarizeMullercells and leadto K+effluxfrom other retina. Muller cells processes ramify extensively in Mi.iller-cellregions (Fig. 3). The resulting K+spatial- the two synaptic layers of the retina, suggestingthe bufferingcurrent effectivelyredistributesextracellular importance of these cells for removing neurotrans- K+from regions where IK+]Ois initially high to regions mitters from extracellularspace. where it is low. Glutamate Spatial-buffering currents pass through inward- The high-affinity glutamate carrier of Miiller cells rectifiing K+channels, the predominant channel in has been studied extensively21-23.Expression of one Miiller cells913.Unlike other voltage-gatedchannels, such carrier, the L-glUtamate–L-aSPaflate transporter these channels are open at the resting membrane (GLAST), has been demonstrated in rat Muller cells43. potential. The voltage-and K+-dependentpropertiesof these channels serveto augment the spatial-buffering currents; in regionswhere IK+]Ois raised,channel conductance is increased. Neurotransmittersand other factors can modulate the conductance of Muller-cell K+channelslo’18’36,suggestingthat IK+]Oregulation by Miillercells is under neuronal control. Even a modest decrease in K+ inward-rectifier conductance would reduceK+spatial-bufferingcurrentsand diminish ~+]0 regulationby Mullercells. In amphibian species,a largefraction of all inward- rectifyingK+channels in Mullercells is localizedto the endfoot at the retinal surface14’38.Thus, K+spatial- bufferingcurrent preferentiallyexits from Mullercells at the endfoot. The result of this specializedform of spatialbuffering,termed ‘K+siphoning’32,is that most excess K+releasedby active neurons is transferredto the vitreous humor, which acts as a large K+sink3940. The pattern of the spatial-bufferingcurrent is more complex in mammalian species, with excess K+ directed to both the vitreous humor and the fluid space surroundingthe photoreceptors41(the subretinal space; Fig. 3). Modeling studies of amphibian and mammalian retinae indicate that K+siphoning is 1.6-3.7 times more effective in clearing excess K+from the retina than is K+diffision through extracellularspace31,3z,42. vitreous humor Experimental studies demonstrate directly that K+ v siphoning is a key mechanism for regulatingretinal Fig. 3. Potassium spatial-buffering in MiWer cells. Potassium, IK+]O.The light-elicited increases in IK+]Omore than released from active neurons into the inner p/exiform layer, is removed double in the frog40,and more than triple in the cat41 from extracellular space by a fC-@honing current flow through the when K+-siphoning currents are blocked by Ba2+ Mtiller cell. The excess /C entering the cell generates a depolarization (Fig.4). and drivesout an equo/ amount of ~ from other cc// regions. In vascu- /arized mammalion retinae, P efflux occursfrom several cell regions FJeurotransrnitters having a high density of F channek: (1) the endfaot at the inner sur- Glial cells play an important role in removing face of the retina; (2) the apical endaf the cel~ and (3) the endfeet ter- neurotransmitters from extracellular space following minating on blood vessels.Heavy arrows indicate K+fluxesinta and out their release from synaptic terminals. This uptake is of the Miller cell. Abbreviations: IPL, inner plexiform laye~ SRI, sub- essentialfor terminating synaptictransmissionas well retinal space; V~, cellmembrane potential. A second IK+]Oincrease, in as for preventingthe spreadof transmittersawayfrom the outer plexifarm laye~ has been amitted for clarity. The apicol end the synaptic cleft. Muller cells possess high-affinity of 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+ Fig. 4. Mii//er cells regulate[fC]@in the retina. (A) Light-elicited increasesin the extrace//u/ar it concentration ([K’]O)are recordedwithin the innerplexiform/oyerofthe cat retina.WhenMti//er-ce//~ @annek are blocked by the addition of 3 m~ B&+, thus reducing spatia/- buffering currents, the light-elicited increase in [fC]Omore than triples. (B) The /ight-e/icited decrease in[~]0 in the subretinal space more than quadruples when B&+ is added, indicating that spatial-buffering currents norma//y transfer excessf6 from the retina to the subretinal space B as well as to the vitreous humor. The timecourse of the light stimulus, 4s in duration, is indicated in the bottom trace. Modified from Ref.41.

0.5mV [ This pH shift is thought to be regulated by Muller J I v cells3’2b.SalamanderMuller cells possess a Na+–HCO~- co-transport system which has a stoichiometry of approximately three HCOJ- transported along with The Mtiller-cell glutamate carrier has a complex one Na+(Ref. 25). Because the transporter is electro- stoichiometry and questions concerning its details genic, cell depolarizationresults in an efflux of acid remain. In one scheme, the inward transport of one equivalents26 . Thu5, during periods of neuronal activ- glutamate molecule and two Na+is coupled to the ity, when Muller cells are depolarizedby increased outward transport of one K+and one OH- (Ref. 44). IK+]O,the activity of the Na+-HCO~co-transport sys- Because the transporter is electrogenic, glutamate tem will acidifyextracellularspace (Fig.6). This acidi- uptakeis voltage-dependent;cell depolarizationslows fication helps to muffle extracellularpH variationsby down or even reversesuptakeof the excitatory amino partially neutralizingthe neuronally generatedextra- acid4s(Fig.5). In addition, becauseOH-is transported cellularalkalinization. along with glutamate,uptakeis pH dependentand is Muller cells might also regulateextracellularpH in accompariiedby an extracellularalkalinization44. the retina by facilitatingthe removalof COZproduced GABA by neuronalactivity. Excessretinal CO, willbe rapidly Muller cells, including those of rabbit and mouse, converted to HC03- and H+by the enzyme carbonic also possess a high-affinity uptake system for GABA anhydrase, which is found both within Muller cells (Refs 21,46). Expressionof the GABAcarrier, GAT-3, and on the cell surface27’28.The HCOj- might then be has been demonstratedin Mullercells of the rat47.The transportedto. the vitreous humor by Na+-HCOq-co- transporter is electrogenic, and is believed to have a transporters,which, in the salamander,are localized stoichiometry of two Na+plus one Cl- plus one GABA preferentiallyat the cell endfoot252G.This homeostatic molecule, all transported inwardly24’48.GABAis the mechanism, termed ‘COZsiphoning’28for its resem- primary inhibitory transmitter of the retina, and is blance to K+siphoning, has yet to be demonstrated releasedby horizontalcellsand amacrinecells.In some experimentally.Such a scheme is consistent, however, mammalian species, including the rabbit, horizontal with the observation that in the frog inhibitors of cells lack a GABAtransporter49,and the responsibility carbonic anhydraseresultin an enhancement of light- for removing GABAfrom the extracellularspace pre- elicited pHvariationswithin the retinaso. sumablyfalls largelyupon Mullercells. Muller-celimodulationof neuronalactivity pHandCO, Neuronalactivity, triggeredby light, generatesa sig- Muller cells, by controlling the concentration of nificant extracellular alkalinization in the retinaso. neuroactivesubstancesin extracellularspace, can significantly modulate neuronal activity. A reduction in the uptakeof glutamateor GABAwill leadto enhancement of synaptic transmission. The accumulation of K+in extracellularspace,resultingfrom a reduction in K+removalby Mullercells, will also leadto changesin neuronal excitability.Variationsin pH can modulate neuronalactivityaswell.Eventhe smalldepolarization- inducedextracellularacidificationgeneratedby Muller cells can have a dramaticinhibitory effect on synaptic ~loPA transmission.For example,an acidification of 0.05 pH 0.5 s units in the salamanderretina producesa 24°h reduc- w tion in synaptictransmissionbetweenphotoreceptors and second-orderneurons51. Neuratransmitterrelease Muller cells, in addition to influencing neuronal Fig.5. Depolarization-elicited release of glutamate generated by reversal of the MiJller-cell activity by regulatingthe levels of substances in the glutamate uptake carrier. (A) Releaseof glutamate from a AWer cell (right) is monitored by neuronal microenvironment, might control neuronal recording glutamate-elicited currents from an adjacent Purkinje cell (/efl). (B) Depolarizing a Miller cell from -60 to +20mV (top trace)elicits an inward current in the Purkinje cell activity more directly. When depolarizedsufficiently, (middle trace). The Purkinje-ce// current is generated by activation of its glutamate-containing glutamate uptake by salamander Muller cells is receptors. When extracelkslar IC is omitted (bottom trace) reversed glutamate transport by reversedand glutamateis actuallyreleasedinto extra- IvhWercells is blocked and no glutamate current is recorded in the Purkinje cell. Photograph cellular space45(Fig. 5). This releasemight contribute in A, courtesyof BrianBillupsand DavidAttwell;B,modifiedfrom Ref.37. to 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 dissociated salamander Mti//erce/l. Scale bar, 20pm. (B) Image of extracel/u/arpH for the cellshownin ~ measured by imaging the pH-indicator dye BCECF(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 at the 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 release of GABAfrom rat Miiller cells’z. It should be noted, however,that the releaseof glutamateand GABAfrom Miiller cells has been demonstratedonly in cells that havebeen pre-loadedwith the transmitters.It remains to be shown that transmitter release occurs under in vivo conditions. Nitric oxide (NO) synthase is expressedin Muller cells of salamanderand fish53,although releaseof NO from Muller cells has yet to be demonstrated. It is interesting to speculatethat the Ca2+wavesobserved in Mtillercells might triggerthe releaseof neurotrans- 2 Reichenbach, A. and Robinson, S.Il. (1995) in Neuroglia (Kettenmann, H. and Ransom, B.R., eds), pp. 58-84, Oxford mitters, as is apparently the case for the release of UniversityPress glutamate from cultured astrocytess’. Such ‘glio- 3 Newman, E.A. (1996) TheNeuroscientist2, 110-119 transmitters’ could potentially modulate neuronal 4 Uga, S. and Smelser, G.K. (1973) invest. Ophthalmol. 12, 434-448 activity. S Reichenbach, A. et al. (1988) Z. Mikroskop. Anat. Forsch. 102, Metabolic sufi~oti 721-755 Retinal neurons are nourished by Muller cells. 6 Reichenbach, A. et al. (1989) Anat. Embryol. 180, 71-79 7 Reichenbach, A. et al. (1993) J. Chenr. Neuroanat. 6, 201-213 Glycogen stores in the retina are restricted to Muller 8 Newman, E.A. (1985) Nature317, 809-811 cells”. Glycogenolysisin Muller cells is stimulatedby 9 Newman, E.A. (1993) J Neurosci. 13, 3333-3345 neuronal activity’, and the direct transfer of lactate 10 Pure, D.G. and Stuenkel, E.L. (1995)/. PhysioL 485, 337-348 from Mi.illercells to neurons has been observedin the 11 Chao, T.L et al. (1994) PfliigersArch. 426, 51-60 56 Muller cells might also regulate blood 12 Chao, T.I. et al. (1994) G2ia10, 173-185 guinea pig . 13 Brew, H. et aL (1986) Nature324, 466-468 flow in retinal vessels in response to changes in 14 Newman, E.A. (1987) J. Neurosci. 7, 2423-2432 neuronal activity. Retinal blood vessels are almost 15 Robinson, S.R.et al. (1993) Science262, 1072-1074 16 Malchow, R.P., Qian, H. and Ripps,H. (1989) Proc.NatL Acad. completely surroundedby Muller-cellendfeet which Sci. U. S. A. 86, 4326-4330 can release K+,acid equivalents, and perhapsNO (all 17 Reichelt,W. et al. (1996) Neurosci..Lett.203, 159-162 vasodilators)when Mullercells depolarize2c’53’57. 18 Schwartz, E.A. (1993) Neuron 10, 1141-1149 Moreover, glial uptake of glutamate and GABAare 19 Wakakura, M. and Yamamoto, N. (1994) Vis. Res. 34, 1105-1109 important initial steps in the process of transmitter 20 Pure, D.G. (1995) Prog.Retinal Res. 15, 89-101 recycling. Glutamine synthetase, an enzyme that 21 Ehinger, B. (1977) Exp. Eye Res. 25, 221-234 transamidates glutamate to glutamine, is localized 22 Brew, H. and Attwell, D. (1987) Nature327, 707-709 23 Schwartz, E.A. and Tachibana, M. (1990) J. Physiol. 426,43-80 exclusively to Muller cells in the retinaz’. The gluta- 24 Biedermann, B., Eberhardt, W. and Reichelt, W. (1994) mine synthesizedby Mi.illercells is recycled back to NeuroReport5, 438-440 retinal neurons, where it servesas a precursorfor the 25 Newman, E.A. (1991) J. Neurosci. 11, 3972-3983 26 Newman, E.A. (1996) J. iVeurosci.16, 159-168 synthesisof additionalneurotransmitter.Inhibition of 27 Linser, P.J., Sorrentino, M. and Moscona, A.A. (1984) the glial enzyme in the rabbit causes a complete loss Dev. Brain Res. 13, 65-71 of neuronal finctionsB, demonstratingthe crucialrole 28 Newman, E.A. (1994) G2ia11, 291-299 that Muller cells play in neurotransmission in the 29 Newman, E.A. (1995) in Neuroglia (Kettenmann, H. and Ransom, B.R., eds), pp. 717–731, Oxford University Press retina. 30 Karwosld, C.J. and Proenza, L.M. (1977) J. Neurophysiol, 40, Acknowledgements Concludingremarks 244259 31 Reichenbach, A. et al. (1992) Can. J. Physiol. Pharrnacol. 70, We thankJanice I. S239-S247 Gepnerand To date, research on Muller cells has been con- 32 Newman, E.A., Frambach, D.A. and Odette, L.L. (1984) cerned largelywith their cellular propertiesand their Science225, 1174-1175 KathleenR. Zahs contributions to the regulation of the retinal micro- 33 Keirstead,S.A.and Miller, R.F. (1995) GZia 14, 1422 for theirhelpfil environment. These distinctive glial cells recognizea 34 Koh, S-W.M., Kyritsis,A. and Chader, G.J. (1984) J. Neurochem. commentson the 43, 199-203 variety of neuronal signalsand actively control levels 35 Osborne, N.N. and Ghazi, H. (1990) Prog. Retinal Res. 9, manuscript.This of K+,H+and neurotransmittersin extracellularspace. 101-134 workwas supported In the coming years, research will provideadditional 36 Biedermann, B. et al. (1995) NeuroReport6, 609-612 inpart by National 37 Billups,B. and AtWell, D. (1996) Nature379, 171-174 Institutesof Health insights into the role of Mi.illercells in modulating 38 Newman, E.A. (1985) J. Neurosci.5, 2225-2239 neuronal activity and retinal function. 39 Karwoski,C.J., Lu, H-K.and Newman, E.A. (1989) Science244, grantEY04077 578-580 (EN)and the Selectedreferences 40 Oakley, B.I. et aL (1992) Exp. EyeRes. 55, 539-550 Deutsche 1 Newman, E.A. (1994) in The Principles and Practice of 41 Frishman, L.J. et al. (1992) J. Neurophysiol.67, 1201-1212 Ophthab?dogy, Basic Sciences (Vol. 1) (Albert, D.M. and 42 Eberhardt, W. and Reichenbach, A. (1987) Neuroscience 22, Forschungsgemein- Jakobiec,F.A.,eds),pp.398-419, Saunders 687-696 schaft (AR).

TINS Vol. 19, No. 8, 1996 311 GLIA E. Newman and A. Reichenbach – Muller cells

43 Derouiche,A. and Rauen,T. (1995) J. Neurosci.Res. 42, 131-143 Vis.Res. 29, 1069–1077 44 Bouvier, M. et al. (1992) Nature 360, 471-474 51 Barnes, S., Merchant, V. and Mahmud, F. (1993) Proc. NatL 45 Szatkowski,M., Barbour, B. and Attwell,D. (1990) Nature348, Acad. .Sci.U..S.A. 90, 10081-10085 443-446 52 Sarthy, P.V. (1983) J. Neurosci. 3, 2494-2503 46 Sarthy, P.V. (1982) J. Neurosci.Methods 5, 77-82 53 Liepe, B.A. et aL (1994) J. Neurosci. 14, 7641-7654 47 Honda, S., Yamamoto, M. and Saito, N. (1995) Mol. Brain Res. 54 Parpura, V. et al. (1994) Nature 369, 744-747 33, 319-325 55 Kuwabara, T. and Cogan, D.G. (1961) Arch. Ophthalmol. 66, 48 Attwell, D., Barbour, B. and Szatkowski,M. (1993) Neuron 11, 680-688 401-407 56 Poitry-Yamate, C.L., Poitry, S. and Tsacopoulos, M. (1995) 49 Redbum, D.A. and Madtes, P.J. (1986) ]. Comp. Neurol. 243, J. Neurosci. 15, 5179-5191 41-57 57 Paulson, O.B. and Newman, E.A. (1987) Scierrce237, 896-898 50 Borgula, G.A., Karwoski, C.J. and Steinberg, R.H. (1989) 58 Pow, D.V. and Robinson, S.R. (1994) Neurosci. 60, 355-366

Microglia: a sensor for pathological events in the CNS

Georg W. Kreutzberg

The most characteristicfeatureof microglialcellsis their rapid activationin 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 scavengercellsbut alsoperformvariousotherfunctionsin tissuerepairand neuralregeneration. They form a network of immune alert resident macrophageswith a capacityfor immune surveillanceand control. Activated microgliacan destroy invadingmicro-organisms,remove potentiallydeleteriousdebris,promotetissuerepairbysecretinggrowthfactorsandthusfacilitate thereturnto tissuehomeostasis.An understandingofintercellularsignalingpathwaysfor microglia proliferationandactivationcouldform a rationalbasisfor targetedinterventionon glialreactions to injuriesin the CNS. Trends Neurosci. (1996) 19, 312-318

HEREIS HARDLYANYPATHOLOGYin the brain very subtle alterations in their microenvironment, Twithout an involvement of glial cells. The reactiv- such as imbalances in ion homeostasis that precede ity of microglia provides a striking example of this pathological changes that are detectable histologi- principle’. Resting microglia show a downregulated Callyll. It is possible that the unique collection of immunophenotype adapted to the specialized membranechannelsof microglia,includingan inward- microenvironment of the CNS.However,their ability rectifyingK+channel,is instrumentalin this responsive- to respondquicklyto a varietyof signaling molecules ness1213.Furthermore, microglia have receptors for suggeststhat their apparent quiescence represents a CNS signaling molecules such as ATP (Refs 14,15), state of vigilance to changes in their extracellular calcitonin gene-related peptide (CGRP)lC,ACh and milieu. noradrenaline17,and can react both with changes in The important role of microglia in various patho- their extracellularionic milieu14,15,17 and by activation logical conditions was first recognized by del Rio- of transcriptional mechanismslc. Their ability to Hortega, who also coined their name’. Their nature respond selectively to molecules involved in and identity havelong been debatedbut it is now gen- neurotransmissionallowsthem in their ‘resting’ state erallyacceptedthat they areontogeneticallyrelatedto to monitor the physiologicalintegrity of their micro- GeorgW. cells of the mononuclear phagocytelineage,unlike all environment continuously and to react rapidlyin the Kreutzbergis at other cell types in the CNS (Refs 3,4). There is, how- event of pathological disturbances. Our ignorance of the Deptof ever, a minority view based on in vitro experiments the physiological function of microglial cells in the Neuromorphology, that postulatesthat microgliabelong to the true gliaof normal brain must be understood also in the light Max-Planck- neuroectodermallineages. of our inability to measure their functional changes Instituteof One of the characteristicsof microglia is their acti- in vivo. Psychiatry, vation at a very early stage in responseto injurylc-lO. The rapid transformation of microglia from a rest- D-82152 Microglia activation often precedes reactions of any ing to an activated state has been clearly recognized Martkried near other cell type in the brain. They respondnot only to for almosta century(Fig.1)1’20.While the morphology Munich,Germany. changes in the brain’s structuralintegrity but also to of activated microglia was found to be extremely