Molecular neurobiology of glutamate receptors.

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Annu. Rev. Physiol. 1992. 54:507-36 Copyright © 1992 by Annual Reviews Inc. All rights reserved

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GLUTAMATE RECEPTORS Gregory

P.

l Gasic and Michael Hollmann

Molecular Neurobiology Laboratory, The Salk Institute, 10010 N. Torrey Pines Road La Jolla, California 92037 KEY WORDS:

excitatory amino acid receptors. ligand-gated ion channels. expression cloning.

kainate-AMPA and NMDA receptors

INTRODUCTION

Excitatory amino acid receptors are currently regarded as the principal neurotransmitter receptors that mediate synaptic excitation in the vertebrate central nervous system (CNS). The first demonstration that acidic amino acids could act as excitatory neurotransmitters dates to the early 1950s and arose from experiments in which monosodium glutamate topically applied to motor cortex caused tonic convulsions (9 1 ) . Subsequently, experiments with single spinal cord and CNS neurons illustrated the direct depolarizing action of L-glutamate (Glu) and L-aspartate (70). Despite the considerable enthusiasm generated by the initial flurry of discoveries, a long period of skepticism about the possible transmitter role for these amino acids followed. GIu was deemed to have "too ubiquitous" a distribution and a rather high concentration in the brain to be a neurotransmitter. Today, Glu essentially satisfies the four main criteria for classification as a neurotransmitter: (a) presynaptic localization; (b) specific release by physiological stimuli in concentrations sufficiently high enough to elicit a postsynaptic response; (c) identical action to the endogenous transmitter including response to antagonists; and (d) the existence of mech anisms to terminate transmitter action rapidly (70, 1 52) . This review focuses primarily on the physiology and molecular biology of the ionotropic non-NMDA receptors and the metabotropic ACPD receptor(s) IHoward Hughes Medical Institute, La Jolla, California 92037 507

0066-4278/92/03 1 5-0507$02.00

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since several of these receptor subunits have been characterized at the molecu lar level. Due to space limitations, some important physiological studies may not be cited. Properties of the cloned GluR subunits expressed in oocytes and mammalian cell lines are compared and contrasted to GluR receptors ex pressed in neurons. The computational potential of the NMDA receptor is briefly described and difficulties in cloning this receptor are discussed.


The Pharmacological Basis of /onotropic and G Protein-Coupled Glutamate Receptors On pharmacological grounds, glutamate receptors have been grouped (132, 1 84) into five distinct SUbtypes named after their most selective agonists: (a) NMDA (N-methyl-D-aspartate); (b) KA (kainate); (c) AMPA (alpha-amino3-hydroxy-5-methyl-4-isoxazole propionate) formerly known as quisqua late (QUIS) receptors; (d) L-AP4 (2-amino-4-phosphonobutyrate); and (e) ACPD (trans- l amino-cyclopentane- l ,3 dicarboxylate) . Competitive antago nists are available for the NMDA subtype (D-2-amino-5-phosphopentanoic acid, D-AP5) for at least one of the ACPD receptors (L-2-amino-3phosphopropionate, L-AP3) and for both KA and AMPA receptors (6-cyano7-nitroquinoxaline-2,3-dione, CNQX). The ionotropic glutamate receptor -

(iGluR) subtypes, NMDA, KA, and AMPA represent ligand-gated ion chan nels, which are activated on a fast time scale (msec). The other two receptor

subtypes are cQupled to G proteins and operate on a time scale of several hundred milliseconds to seconds. The L-AP4 subtype is deduced from the specific depressant effect of L-AP4 on some excitatory amino acid pathways in retina, spinal cord, and brain. Experiments utilizing retinal depolarizing bipolar cells suggest that the L-AP4 receptor in these cells acts via a G protein

to increase the hydrolysis of cGMP, which leads to the closure of ion channels conducting an inward current ( 1 5 1 , 1 68). The fifth subtype, ACPD or metabotropic GluR (mGluR), is a G protein-coupled receptor that is linked to inositol phosphate/diacylglycerol formation and subsequent release of cal cium from internal stores (64, 74, 166, 167, 172). These traditional pharma cological categories require modifications in the light of recent molecular and

physiological data.

The Biological Role of Glutamate Receptors in Activity-Dependent Synaptogenesis and Motor and Associative Memory More than forty years ago, Hebb formulated his neu rophysiological postulate of learning, which proposed that repeated and persistent coactivity at pre- and postsynaptic neurons results in selective strengthening of the synapses be tween them (93). Stent (171) and Changeux & Danchin (31) extended Hebb's postulate to include the complement: asynchronous activity at pre- and post-


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synaptic neurons leads to a selective weakening or elimination of synapses. Such a "hebbian mechanism" appears to be essential for some forms of long-term potentiation (LTP), the selective stabilization of synchronously active pre- and postsynaptic neurons during development and the maintenance of topographic maps (42-46). GluRs have been implicated as major partici pants in the establishment of synaptic networks, in processing sensory in formation, coordinating movements patterns, and in associative and motor memory ( 10-1 2 , 58, 92, 109- 1 1 1 , 130, 158 , 164). During development, globally-organized crude developmental cues are thought to determine broad areas of the eNS. Next, neuronal networks are specified during an activity-dependent phase in which the entire neuronal repertoire of cell-cell contacts within the developing neuropil is established by rounds of trial and error axon sprouting, synapse formation, and synapse withdrawal or stabilization (37 , 46, 130) . Activity-dependent, competitive synaptic interactions, which stabilize some axon branches and dendrites at the expense of others, involve GluRs ( 10-12, 41 , 45 , 73 , 1 30) . Block or activa tion of the NMDA receptor in developing Rana retinotectal projections alters retinal ganglion cell arbor structure (38-40) , whereas in the Xenopus visual system, NMDA treatment restores plasticity of binocular maps beyond the critical period in development ( 179) . Synaptic activity may also maintain specific neural pathways by regulating the production of neurotrophic factors . Non-NMDA receptor activation in the hippocampal pyramidal neurons reg ulates the rnRNA levels of brain-derived neurotrophic factor (BNDF, 190) and nerve growth factor (NGF, 75 , 190) , two growth factors essential to neuronal survival. The NMDA receptor plays a crucial role in LTP in hippocampal CA l (42, 43 , 45 , 47). Although in vivo, retrieval of established memories is not affected by treatment with the NMDA receptor competitive antagonist, D APV, activation of NMDA receptors is essential for some kinds of learning, especially those dependent on spatial orientation of an animal in its environ ment ( 1 48, 1 49). GluRs have also been implicated in cerebellar long-term depression (LTD), which is favored as a memory element for cerebellar motor learning ( 1 10). Conjunctive stimulation of climbing and parallel fiber path ways, but neither pathway alone, produces LTD in Purkinje cell output (109). The induction of LTD in cerebellar Purkinje cell cultures requires activation of iGluRs (AMPA) and mGluRs (QUIS) , together with depolarization in the presence of external Ca2+ ( 1 29) . Further experiments are needed to determine what other cellular messengers (e.g. NO) are involved in LTP and LTD (77) . The Pathophysiological Involvement of Glutamate Receptors Dysfunction of glutamatergic pathways has been implicated in several CNS disorders such as epilepsy (36, 50, 61 , 69 , 76) , Huntington's disease (2) ,


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Parkinson's disease (24, 82, 1 1 8 , 120, 1 78) , amyotrophic lateral sclerosis ( 157, 1 70, 1 87), lathyrism ( 142), AIDS encephalopathy and dementia com plex (63, 80, 1 3 1 ), and Alzheimer's disease (30, 49, 1 14, 142). GluR involvement in initiation and propagation of seizures (6 1 , 69) and in massive neuronal cell death during periods of ischemia and hypoglycemia (33, 34) is well established. NMDA receptors mediate the sensation of pain (59, 60). Less well understood is the role of the GluRs in the above neurodegenerative disorders (1, 1 42), although GluR-mediated excitotoxicity appears to be an essential element. NMDA receptor antagonists confer protection of substantia nigra from MPP+ ( l -methyl-4-phenyl-pyridinium ion) neurotoxicity in an experimental model of Parkinson's disease (82, 178). The neural destruction associated with AIDS dementia may be caused by excessive Ca2+ influx via the NMDA receptor and act synergistically with either the HIY glycoprotein (gp 1 20) , or a product of virus-infected macrophages (63, 80, 1 3 1 ) . A deficiency in glutamatergic neurotransmission resulting in a disturbed balance between glutamatergic and dopaminergic systems within the neostriatum may play a key role in the pathophysiology of schizophrenia (26-28, 90, 1 24, 153, 1 8 1) . The finding that GluRs regulate dopamine release and other aspects of dopamine neuron function (29, 143, 144) supports this provocative hypoth esis. Although linkage with a known genetic disorder has not been established to date, these receptors are principal candidates for maintaining if not initiat ing several of these disease processes. The same plasticity of the hebbian synapse, which allows selective synapse stabilization in development and changes in synaptic efficacy during LTP or LTD, may impart vulnerability of cognitive and motor functions to environmental insults (e.g. stress, trauma, heavy metal ions, HIY infection) and to genetically programmed onslaughts (Huntington's disease, schizophrenia) . GLUTAMATE RECEPTOR PHYSIOLOGY B.C. (BEFORE CLONING)

At many vertebrate CNS synapses, the release of Glu produces an excitatory postsynaptic current (epsc) with two components ( 1 3, 44, 7 1 , 96, 97, 1 16, 1 4 1 ) . Non-NMDA iGluRs are responsible for the portion of the epsc charac terized by a rapid onset and decay, while NMDA receptors mediate the component with a slow rise time and decay of several hundred milliseconds ( 1 3 , 43, 7 1 , 1 4 1 , 1 65) . The slow decay of the NMDA-mediated epsc appears to be the result of the persistence of bound Glu and the long open state of the ion channel ( 1 27). The mGluRs may function predominantly in more long term aspects of cellular control by operating via G proteins and several second messenger systems ( 1 66) . Further experiments are necessary to assign defini tive physiological functions to these receptors .

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Physiology of the Ionotropic Non-NMDA Glutamate Receptors AMPA, quisqualate (QUIS), and GIu activate non-NMDA GluRs on CNS and sensory ganglion neurons, which exhibit a rapid and strongly desensitiz ing response (117, 139, 1 73, ] 77) . In CNS neurons, KA and domoate produce a sustained response by opening non-NMDA channels, whereas in sensory ganglion neurons these agonists produce a desensitizing response (44, 1 4 1 ) . Despite the lack of an antagonist that can differentiate between KA and AMPA receptors, KA and AMPA were classified in pharmacologically dis tinct categories. Support for this distinction came from the existence of high affinity binding sites for KA and AMPA with disparate anatomical distribu tions (48, 146, 1 88 , 1 89) . Ligand-binding sites, however, should not be equated with functional receptors because they only indicate the presence of a ligand-binding entity that may not necessarily be linked to an effector moiety. In cerebellar granule cells and spinal cord neurons, conventional microelec trode recording techniques with physiological extracellular solutions detected currents activated by KA and QUIS, which reverse at membrane potentials slightly more negative than 0 mV (44) . The finding of a similar reversal potential for the muscle nicotinic acetylcholine receptor (nAchR) suggests the existence of a mixed ion conductance mechanism. Whole cell recordings with patch electrodes demonstrated that these channels are relatively nonselective between Na+ and K+ (or Cs+) (6, 44, 14 1 ) . Initially, no significant Ca2+ permeability was found for non-NMDA receptor channels (44, 1 4 1 ) . In current-voltage (I-V) plots derived from recording hippocampal pyramidal neurons, the QUIS trace shows some outward rectification (106) . KA re sponses in the same population of hippocampal neurons can be differentiated into two groups: type I, observed in the majority of cells, is characterized by outward rectification and impermeability to Ca2+ , while type 11, observed in ostensibly younger hippocampal neurons, exhibits a remarkable inward rectification and fluxes significant amounts of Ca2+ (106) . In these pyramidal neurons with thick dendrite-like processes, KA and Glu, but not QUIS, activate inwardly rectifying channels that flux Ca2+ with about 35% of the Ca2+ conductance of an NMDA receptor channel (106). Salamander retinal bipolar cells also possess non-NMDA receptors with significant Ca2+ con ductances activated by Glu, KA, and QUIS (79). The I-V relation, however, is almost linear, and external Mg2+ diminishes the steady state currents through the channel, but not the peak current (79). Calcium imaging studies with astrocytes have also detected Ca2+ fluxes that are activated by KA and blocked by CNQX (8 1 ) . Therefore, GIu-activated Ca2+ conductance is not exclusive to the NMDA receptor subtype. The physiological significance of this non-NMDA receptor Ca2+ conductance still remains to be determined . QUIS- and KA-activated channels have distinct single channel conduc-


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tances in several neurons. The predominant conductance state for QUIS-acti vated channels is about 8 pS (6, 44) . Recently, a rapidly inactivating (3 ms) high conductance state (35 pS) , which cannot be blocked with the NMDA antagonist D-APV, was described for QUIS ( 1 73) . This channel may generate the fast QUIS receptor-mediated epsc, which reverses near 0 mY, has a negligible voltage-dependence, inactivates with approximately 3 ms time constant, and recovers quickly from the desensitized state (44) . In contrast, KA primarily gates low conductance channels in hippocampal, cortex, spinal cord, and cerebellar neurons (6, 52) . Fluctuation analysis of recordings from outside-out patches shows a principal conductance of 4 pS with a lifetime of 0 . 5-3 ms for KA (6, 1 65). Noise analysis suggests that KA activates a < 1 pS ( 140 fS) channel in some preparations (44, 1 65). Although the antagonism of KA responses by QUIS, Glu, and AMPA is well documented ( 108, 1 17, 1 39, 156), the makeup of the receptor(s) acti vated by these agonists is not as clear. QUIS, KA, and Glu initially were proposed to exert their effects via a common receptor channel complex where QUIS cross-desensitizes responses to KA (44, 1 17, 141). Rapid desensitiza tion of responses to GIu , QUIS, and AMPA, but not KA, and the antagonism of KA responses by these agents observed in hippocampal neurons support this model (117). A.variant hypothesis contends that AMPA, QUIS, and Glu serve as partial agoriists at a receptor fully activated by KA and rapidly desensitized by QUIS . On the basis of experiments with hippocampal, retinal ganglion, and superior colliculus neurons, other groups have proposed an alternative "two receptor model" where AMPA and QUIS act as antagonists or weak partial agonists at KA-preferring receptor and as full agonists at a rapidly desensitizing AMPA-preferring receptor (126, 1 56, 1 73) . A block of the KA response in mammalian eNS neurons by high con centrations of QUIS, AMPA, and Glu is well documented (108), whereas the opposite approach, examining the effect of high concentrations of KA on the rapidly desensitizing response to QUIS or AMPA, has only recently received some attention ( 1 55). KA and domoate decrease the rate of onset of de sensitization produced by QUIS or AMPA (155). This observation is not easily accomodated by the two receptor model, which posits the existence of a separate AMPA-preferring receptor that rapidly desensitizes. Although the assumption could be made that the affinity of KA and domoate was nearly identical at both the AMPA- and KA-preferring receptors, and that they act as pure antagonists for the former and strong agonists for the latter, a simpler model for a single receptor type (103, 1 55) appears to be more attractive . A cyclic model for desensitization analogous to that proposed by Katz & Thesleff (11 4a) for the nAchR has been advanced to resolve the KAIAMPA receptor dichotomy ( 1 55). In this paradigm, KA and AMPA bind to the same receptor and compete for the agonist recognition site. Both the relative and


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absolute affinities of each agonist for the resting and desensitized states of the receptor are different. Application of KA or AMPA initially induces a con formational change in the receptor, which propagates to open the ion channel. Because AMPA-type agonists have a higher affinity for the desensitized state of the receptor, at equilibrium they drive the majority of receptors into a desensitized state . In contrast, KA-type agonists have a higher affinity for the resting state of the receptor, and at equilibrium they activate the receptor ion channel of a large number of receptors. The net effect is that AMPA drives the majority of receptors into the desensitized state, while KA maintains them in the open state for a longer period. Although these data demonstrate a clear interaction between KA and AMPA at what appears to be a single receptor ( 155), a model where the receptor passes through an open state before desensitizing cannot be excluded. This subject (95) receives further con sideration in the molecular biology section of our review . The Computational Potential of NMDA Receptor Properties A detailed description of NMDA receptor physiology (4, 7, 35, 43, 5 1-56, 1 12, 1 13 , 1 27 , 1 34 , 1 61 , 1 62, 176) is already available in several excellent reviews (44, 1 1 1 , 138, 140, 1 4 1 , 145, 174 , 175). Discussed below is the unique computational potential of the NMDA receptor ( 14) . NMDA receptor involvement in neuronal computations was first demonstrated in the Lamprey spinal cord neurons involved in pattern formation for locomotion, where NMDA application produces an oscillatory activity (87 , 9 1 , 98) . They have also been implicated computationally in the generation of respiratory rhythms in mammals (72), the jamming avoidance response of weakly electric fish (65, 66), and the signal processing in visual area 1 7 (73). Initially, NMDA receptors were thought not to participate in regular synaptic transmission because the NMDA receptor antagonist, D-APV, had no effect on synaptic potential-evoked stimulation of sensory afferents. Salt, however, discovered that the response to natural sensory, but not electrical, stimulation of afferents was blocked by D-APV (164) . When the unique characteristics of the NMDA receptor(s) are considered in the context of neuronal computation, this appar ent paradox is resolved. Three characteristics distinguish NMDA receptors from other ligand-gated ion channels in the brain: (a) requirement for glycine (or a glycine-like endogenous molecule) as a coagonist (16, 1 7 , 174-- 176) ; (b) voltage dependent control by Mg2+ ions of channel opening (7 , 134); and (c) a lengthy open state for the ion channel that lasts several hundreds of milli seconds ( 1 27). A significant Ca2+ conductance through NMDA channels is also important, but it is not unique . How each of these properties contributes to modulation of synaptic strength and firing synchronization, essential for computation in neural circuits, is considered below.


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Although the molecule that interacts with the glycine-bidding site on the NMDA receptor in vivo remains to be determined, an absolute requirement for this coagonist provides a means for regulation of the NMDA receptor's contribution to synaptic transmission. Such a modulation could be used computationally. An antagonist of glycine binding has been demonstrated to block NMDA receptor function in Lamprey locomotion (86). More informa tion, however, needs to be obtained about the nature of the in vivo coagonist, whether or not the binding site is normally saturated, and how levels of the cotransmitter are regulated. The computational use of the calcium influx through the NMDA receptor is better understood. In Lamprey spinal cord neurons, Grillner and colleagues (87 , 92) proposed that calcium influx through the NMDA receptor channel causes calcium-activated potassium channels to open and thereby contributes to locomotion pattern-generating voltages. By activating potassium con ductances via a calcium messenger in these spinal cord neurons (87, 92, 98), the NMDA receptor plays a dynamic role as a pacemaker for locomotion. Calcium entry into the neuron via the NMDA receptor can also initiate changes in synaptic efficacy that are longer lasting (LTP). The amount of calcium entering dendritic spines presumably through NMDA receptors ( 159) on hippocampal neurons is a function of how well the synapse's activity correlates with the activation of other excitatory synapses on that neuron (14). NMDA receptor activity can be used to calculate a cross-correlation function, where the level of calcium elevation represents the degree of correlation in synaptic activity. Calcium influx through NMDA receptors on the dendrites in the Schaeffer collateral CAl pathway initiates a cascade of postsynaptic messengers whose major effect is to increase the probability of transmitter release as determined by quantal analysis (15, 1 35 , 1 36) . In both Lamprey and hippocampal systems, modulation of intracellular calcium via the opening of the NMDA receptor channel has a computational role, e.g. to synchronize neuronal firing in the former and to increase long term synaptic strength in the latter cases. The voltage-dependence of NMDA receptor channel activation provides another means for regulating synaptic strength. At resting membrane poten tials (-70 mY) and in physiological concentrations of Mg2+, low con centrations of NMDA do not cause depolarization because extracellular Mg2+ blocks the channel. Simultaneous neuronal activity depolarizes the neuron by 40 mV and expels the Mg2+ from the channel. This voltage-dependent Mg2+ block has the computational role of integrating the entire synaptic input of the neuron over the interval the NMDA receptor is activated. Because a single, brief application of GIu to the NMDA receptor produces a lengthy activated state of several hundred milliseconds, whose transition to the closed state is independent of the conducting state of the channel ( 1 27) ,

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temporal summation of synaptic inputs over this interval contributes signifi cantly. This property permits the resolution of the paradox posed by Salt's data that natural stimulation was more effective than single shock stimulation of sensory afferents (164) . Single electrical pulses do not depolarize neurons sufficiently to relive the Mg2+ block at the NMDA receptor, whereas, repetitive (natural) stimuli are temporally summed, the neurons depolarize, and the activated NMDA receptor channels open. Further studies are required to determine whether NMDA receptor desensitization and modulation by phosphorylation contribute to regulation of synaptic strength. Modulation of Ionotropic Glutamate Receptors by Phosphorylation For the model voltage-gated ion channels, two major regulatory strategies are employed: direct interaction of ion channels with GTP-binding proteins (G proteins), and phosphorylation of ion channels or their associated proteins by protein kinases ( 160). In both cases, receptor occupancy by specific ligands leads to a G protein activation. The G protein functions as a direct link between the receptor and the ion channel complex, or as an intermediate in the cascade that involves second messenger production, protein kinase activation, and phosphorylation of the complex. Regulation of neuronal excitability by direct G protein activation or by phosphorylation of the voltage-gated Na + , K+, and Ca2+ channels is extensively documented (25 , 160, 167). For ligand-gated ion channels, only the modulation of the muscle nicotinic AChR function by phosphorylation is well established (104) . Knapp & Dowling demonstrated that dopamine acting via cAMP greatly enhances ionic conductances gated by KA and Glu in teleost retinal cone horizontal cells ( 1 2 1 ) . Single-channel recordings from cell-attached patches indicate that the frequency of 5-10 pS unitary events, but not their amplitude, increased as much as 150% following dopamine application to the rest of the' cell ( 1 22) . Dopamine increased the probability of channel opening for KA and Glu without altering the mean current conducted by an individual channel ( 1 22) . Noise analysis indicated that dopamine treatment did not alter the number of functional channels responding to KA . The duration of channel openings increased about 20-30% . Dopamine, acting via phosphorylation ( 1 28) of the Glu receptor complex, potentiates the activity of horizontal cell Glu receptors by altering the kinetics of the ion channel in favor of the open state ( 1 22) . Receptor complex phosphorylation is the basis of an analogous regulatory mechanism in cultured neonatal rat hippocampal pyramidal cells (83, 1 83) . Whole cell current responses to KA and Glu were enhanced by forskolin (FSK) . Single channel studies with the outside-out mode of the standard patch method revealed that FSK, acting via the cAMP-dependent protein kinase A


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(PKA), increased the frequency of opening and the mean open time of the non-NMDA receptor channels. Miniature excitatory postsynaptic currents under whole cell voltage-clamp increased in amplitude and decay time after FSK treatment, which indicated that junctional KA receptors were also regulated by PKA (83). Although NMDA receptors in hippocampal neurons require intracellular ATP for maintenance of ionic currents (1 33), and a protein kinase inhibitor has been shown to directly affect NMDA receptor channels (3), an FSK effect on the NMDA receptor channel was not observed, but could not be ruled out (83). Extension of these studies may determine what role direct GluR phosphorylation plays in altering synaptic efficacy.

Physiology of Metabotropic Glutamate Receptors The G protein-coupled mGluRs function via phosphoinositide hydrolysis. trans-ACPD specifically activates mGluRs and causes an increase in neuronal firing in thalamic, spinal cord, and hippocampal neurons ( 1 66) . The increase in observed neuronal firing may be the result of protein kinase C (PKC) mediated block of the Ca2+ -activated K+ channels responsible for after hyperpolarization. Alternatively, the K+ channeJ block in hippocampal neurons may be the result of mGluR-mediated inhibition of the voltage-gated Ca2+ channel conductances, which are enhanced by gamma S-GTP and insensitive to PKC inhibitors ( 166). Since NMDA, KA, AMPA, and L-AP4 were unable to achieve the same effect, the results suggest a direct G protein coupling between mGluRs and voltage-gated Ca2+ channels. Although furth er experiments are necessary to assign definitive function, these receptors have been implicated in modulation of synaptic plasticity and in neuronal degeneration ( 1 66) . IONOTROPIC AND G PROTEIN-COUPLED GLUTAMATE RECEPTORS: CLONING BY FUNCTIONAL EXPRESSION AND ELECTROPHYSIOLOGICAL STUDIES WITH CLONED RECEPTOR SUBUNITS

Within the past two years, molecular cloning strategies, which rely solely on the screening for functional receptors of cloned cDNA transcripts injected into Xenopus oocytes, have yielded clones for the first ionotropic (100) and metabotropic (102, 1 37) GluRs. The deduced primary structures of these receptors have raised doubts about the applicability to all ligand-gated ion channels of the superfamily hypothesis (which postulates a common pro genitor for all ligand-gated ion channels, 8, 19) and support the classification of the mGluR as a member of a new G protein coupled receptor subfamily. Subsequent cloning efforts have uncovered a plethora of iGluRs whose numbers exceed those predicted by pharmacological classification. Intense

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efforts are underway to clone and elucidate the primary structure of the NMDA receptor subclass. At the same time, experiments with cloned GluR subunits are discovering the basis for functional diversity (9, 78) of the KAJAMPA receptor subclass, thus paving the way for an analysis of the stoichiometry, subunit composition, and structure of these receptors at the synapse.


Molecular Cloning of Glutamate Receptors and Primary Structure Despite numerous attempts, protein fractionation approaches based on ligand binding, which made it possible to purify and clone the nAchR, GlyR, and GABAAR channels, failed to obtain a purified functional Glu-gated ion channel (19). Proteins that bind Glu (22, 23, 32, 67) and KA or domoate (84, 89, 154) have been purified, although their function in vivo is unclear. Employing a functional assay in Xenopus oocytes (88), the first iGluR and mGluR were cloned (100). Expression Cloning of the First Glutamate Receptor The cloning of the first Glu-gated channel, GluRI (originally called GluRK-1) (100) by functional expression in Xenopus oocytes circumvented the difficulties of the orthodox protein biochemical strategies. In this approach (Figure 1), cDNAs cloned directionally into lambda phages were used to derive sense transcripts from pools of phage clones to voltage-record agonist-induced depolarizations in transcript-injected Xenopus oocytes (op timally, electrophysiology is about four orders of mr.gnitude more sensitive than ligand-binding assays, 62), and to successively fractionate those pools of clones yielding GluR responses in oocytes. After the sequence of the first iGluR had been determined, cloning of additional subunits on the basis of homology screening by several laboratories (Heinemann, Seeburg, Axel, Mishina) ensued. The GluRl cDNA, which is larger than any other ligand-gated ion channel subunit previously cloned, encodes a protein of 99 .8 kd. When expressed in oocytes, GluRl subunits apparently form homomeric channels gated by Glu, KA, domoate ( 100), quisqualate (QUIS) (2 1 , 101), but not NMDA. The overall sequence similarity between this GluRl subunit and the nAchR, GlyR, and GABAAR subunits (100, 101) is small. Hydropathy analysis of the deduced amino acid sequences, however, does reveal structural similarities among these channels (see Figure 2, which shows at least four putative membrane spanning domains, M 1-M4, and two models for the transmem brane structure) (78, 1 0 1 ). The highest sequence conservation is observed for the threonine/serine-rich M2 domain (different sequences in Models I and 11), proposed to form the ion-conducting pore ( 101). All the.1igand-gated channels cloned to date also display a conserved proline in M 1 [at position 12 for

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Injection of rat forebrain poly(A)+ RNA into Xenopus oocytes, and

voltage recording of oocyte responses to glutamate receptor agonists


~ 10�M Glu

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3O�M NMDA 10�MGIy

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30�M KA

2min

10�M AP4

Construction of a directional cDNA expression library from the rat forebrain poly(A)+ RNA, using the phage vector lambda·ZAP, 18 individually amplified sublibraries of -47,000 clones each were established, comprising of total of-850,OOO independent clones.

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In vitro synthesis of RNA from each individual sublibrary, and test in oocytes of the pool of these transcripts, with agonists for all subtypes of glutamate receptors >

gL

2min

KA DO

IV.

Assay in oocytes of all individual sublibrary transcripts

Subdivision of the pool of -47,000 clones from sublibrary testing positive for kainate receptor into 100 pools of - 470 independent clones Plating of phages in positive pool at low density to allow picking of individual phage clones

. V.

Identification of a single clone coding for a kainate/ampa receptor

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2 min

Cloning of the first glutamate receptor by functional expression in oocytes.

nAchR and GluR (Model I only) and position 8 for GlyR and GABAAR]. They all have a 15 amino acid cysteine loop near the amino tenninus, but this feature is only partially conserved in GluRI and lacking in the other GluRs . (21, 101). - Functional Expression Cloning of the Metabotropic Receptor Two teams of researchers, working independently, cloned two apparently identical mGluR subunits from rat brain. They used an expression cloning strategy that assayed for Cl-currents in Xenopus oocytes activated by Ca2T

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A

200

400

600

800

Amino Acid Residues


B

out

membrane in

Figure 2 (A) A Hydropathy plot of GluRI (amino acid residues-18-889) obtained with the algorithm of Kyte & Doolittle (125) using a 15 residue window. The bars (I, II, IIA, III, and IV) denote hydrophobic regions that may span the membrane. (8) Two models of the possible transmembrane arrangements in GluRl-GluR4, assuming four transmembrane spanning do mains. The "Y" marks in the extracellular domain represent possible glycosylation sites. The "P" marks between III and IV represent possible casein kinase II (open bar) and C kinase (solid bar) sites.

released from internal stores upon mGluR activation. Masu et al ( 137) constructed a directional library in the lambda-GEM-2 vector using a sucrose gradient fraction of cerebellar polyA + RNA, which yielded potent mGluR expression when injected into Xenopus oocytes. The approach is essentially similar to that illustrated in Figure 1. Houamed et al (102) prepared a cDNA library from rat cerebellum polyA + RNA by directionally inserting cDNA into a plasmid vector, pVEGT. Both groups obtained mGluR clones with the same amino acid sequence. Expressed in Xenopus oocytes, the mGluR is coupled to a pertussis toxin sensitive G protein. Activation of this G protein results in an increase in inositol triphosphate formation and intracellular Ca2+ mobilization (102, 137). The predicted polypeptide of the mGluR from rat cerebellum consists of


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GASIC & HOLLMANN

1,199 amino acids with a calculated molecular mass of 133,299 daltons. It is one third larger than the iGluRs and much larger than other members of the G protein-coupled receptor family (137). Overall, this receptor does not share significant sequence similarity with any member of the G protein-coupled receptor family nor with iGluRs cloned thus far. Hydropathy analysis, howev er, revealed the presence of eight (including the signal peptide) hydrophobic segments of 20-25 uncharged residues and two large hydrophilic segments, 570 residues at the amino, and 360 residues at the carboxy terminus (102, 137). The mGluR structure more closely resembles that of the G protein coupled receptors for large glycoprotein hormones, which possess a large amino-terminal extracellular domain attached to a region composed of seven membrane-spanning domains (102, 137). By analogy to other G protein coupled receptors, the topology of the predicted mGluR is deduced. The receptor contains a signal peptide at the amino terminus 0-20), a large putative extracellular domain, a cluster of seven hydrophobic segments that probably represent the seven membrane spanning domains, and a large puta tive cytoplasmic domain (102, 137). Determination of the precise topology of this receptor, however, requires additional structural and biochemical experiments.

Cloning of Additional Glutamate-Gated Subunits on the Basis of Sequence Identity to GluRI Since cloning of receptors by functional expression is a tedious and painstak ing process, additional members of the GluR family were pursued and obtained by low stringency screening of cDNA libraries from different re gions of the rat brain. Polymerase chain reaction (peR) with degenerate primers based on GluRI sequences was another strategy employed to obtain related members of the family. On the basis of sequence identity, several GluR subunits (GluR2-GluR6) have been molecularly cloned and expressed. Pairwise comparisons of the deduced amino acid sequences between GluRI-GluR4 demonstrate a se quence identity of about 70% while only 40% similarity is found when GluRI-GluR4 are compared with GluR5-GluR6 (see Table 1). For compari son, Table 2 illustrates the sequence identity between the GluRs and KA binding proteins (KBPs) from frog (KBP-f), chick (KBP-c), and rat (KA-l ).

Distinction Between KA-(or Glu) Binding Proteins and Functional Glutamate Receptors A clear distinction should be drawn between characterizing actual functional Glu receptor channels (18, 19, 57, 68, 100, 101, 115, 169), and establishing a specific high affinity ligand-binding site for Glu or its analogues in anatomical studies (187, 188), in studies with proteins purified on the basis of ligand binding (84, 85, 89, 182), or in studiescusing cDNA clones expressed in cell

GLUTAMATE RECEPTORS Table 1

521

Percent amino acid sequence identity between cloned

ionotropic glutamate receptor subunits GluRI GluRI

100

GluR2

GluR2

GluR3

GluR4

GluR5

70

69

68

40

41

100

73

72

40

41

73

41

42

100

41

40

100

GluR3 G luR4

100

GluR5

81 100

GluR6


GluR6

lines (184). Whereas the ligand-binding proteins generally have nanomolar affinities for Glu or its analogues as exemplified by the KA-binding proteins (KBPs) purified and cloned from frog and chick cerebellum (84, 89) and rat hippocampus (185), the Glu-gated channels function in the micromolar range. The frog and chick KBPs are proteins of �52 kd, only about half the size of the GluRs (see Table 3), and have approximately 35% sequence identity with the carboxy-terminal half of the GluRs . The 105-kd protein KA- l is about the same size as the GluRs (see Table 3), but also shows only 35% sequence identity with the GluRs . It binds KA with high affinity but not AMPA (185), see Table 3). None of the KBPs has any significant sequence identity with the other cloned ligand-gated channels (85 , 182). Hydropathy analysis predicts that the KBPs may have four membrane-spanning domains. The cysteine loop, M1 proline, and M2 threonine/serines, however, are absent. Since significant sequence identity does not guarantee similar function, whether or not these KBPs are subunits of ligand-gated ion channels, second messenger linked receptors (L86), or serve another important function remains to be established.

Table 2

Percent amino acid sequence identity between cloned

ionotropic glutamate receptor subunits and kainate-binding proteins GluRI GluRI GluR5

100

GluR5

GluR6

41

41

38

37

35

100

81

42

38

42

43

40

44

100

56

35

100

34

GluR6 KBP-f

100

KBP-f

KBP-c KA-I

KBP-c

KA-I

100

GluR2. GluR3 and GluR4 are virtually identical to GluRI in sequency identity with the other proteins. KBP-f: KA binding protein cloned from frog brain (182). KBP-c: KA binding protein cloned from chick brain

protein cloned from rat brain (185).

(85).

KA-l; KA binding


Table 3

Structural and functional properties of cloned ionotropic glutamate receptors and binding protein subunits Number of Length

Number

of aa in mature form

Deduced M,(K)

potential

phosphory-

Dom KA AMPA Glu Quis

agonist

TMD

lation sites

( ....M)

potencies

Number of

signal

sylation

putative

sites

glyco-

Receptors (subunits fonning functional ion channels) GluRI GluR2

GluR3

GluR4

889

99.8

18

6

4

862

7/3/0/1

96.4

21

4

4

+

866

98.0

22

5

7/3/0/1

4

7/3/0/1

+

881

GluRS

98.4

21

5

890

4

30

8

6/3/0/1

GluR6

100.9

4

853

96.2

S/2/0/0

31

8

4

7/31110

1.8'

+

n.r.d n.r.

Binding proteins (proteins without ion channel function)

KBP-f

470

KBP-c

464

KA-I

936

Kd

Sequence of

glyco-

peptide

EC,o

potential

for ion channel activation

of

39'

1.3b

130b

36b

n.r,d

n.r.d

+

+

1.0'

+

+

n.r.e

9.3c 0.15'

+ + + +

31e

S2.S

17

2

4

n.r.

n.r,s

n.r.

n.r.

23

2

41110/1

51.8

4

4/21112

n.r.

n.r.

n.r.

4

S/OIOIO

n.r. n.r,i

IOS.2

20

n.r.

n.r.i

n.r.

+ + +

KA

Quis > Glu > KA

*

Quis > Glu > KA

n.df

Quis > Glu > KA Quis > Glu > KA

* *

n.r.d lie

KA > Quis > Glu

n .r. n, r.

n.r.

" ud."

n.r.

n.r.

Glu

n.r.

Sequence of

for ligand binding

n.d.

ligand

AMPA (fLM)

+ 12'

affinities

Quis > Glu > KA

+ +

n .d.

n.d.

n.d.

+

ud.

5.7g

ud .'

KA

4.7'

ud.'

KA »

ud" #

»

Quis > n.d.b#

Glu

Quis > Glu

Phosphorylation sites (nlnlnln) are given in the sequence casein kinase II, protein kinase C, cAMP-dependent protein kinase, and tyrosine kinase. n.d. = no data available; ud. = n.r. = no response; * = essentially identical to GluR2 (liS); + = activates channels (binds ligands), but no EC,o (kd ) available; # = ligand binding was shown for the solubilized and purified protein used to isolate the cDNA clone, but has not yet been demonstrated for the cDNA clone. Sites were determined using the "motifs" algorithm of the Univ. Wisconsin software package (58a) and represent onl� those sites that are compatible with the receptor tOJlOlog� sbown in Model II (Figure 2). References: '(100); b(150 ); c(l63); d(18); '(68); '(lIS); 8(182); h(85); '(185).

undetectable;

GLUTAMATE RECEPTORS

523


Ionotropic Glutamate Receptor Nomenclature When the first Glu receptor was cloned in 1989, it was designated GIuR-KI on the basis of agonist efficacy (l00) . As more subunits were cloned, expressed, and found to be activated by both KA and AMPA, the designation was changed to GluRI to avoid any pharmacological bias, and each additional cloned member was given the next number in the sequence (GluR2-GluR6) ( 1 8, 2 1 , 68). Keinanen et al (1 15) recloned GluR-K I and independently isolated related GIuR subunits on the basis of sequence identity to GluR-Kl . They determined by expression in mammalian kidney cells that these sub units, including GluR-Kl , bound AMPA with a nanomolar Kd• They decided to rename GluR I GluR-A and called GluR2, GluR3, and GluR4, GluR-B, GIuR-C, and GluR-D, respectively (1 15). They also introduced the collective name "AMPA-selective receptors" for GluRI -GluR4. Table 4 illustrates the different designations used by several laboratories for the equivalent iGluRs. A uniform nomenclature for these subunits needs to be established in order to avoid confusion. EXPRESSION STUDIES OF CLONED IONOTROPIC GLUTAMATE RECEPTOR SUBUNITS IN XENOPUS OOCYTES AND MAMMALIAN KIDNEY CELLS

Sequence Identity and Functional Diversity of Subunits When expressed individually in Xenopus oocytes or in the human embryonic kidney cells, GluR I -GluR4 respond to Glu, KA, and AMPA with marked cell depolarizations ( 1 1 5), while GluR5 responds weakly only to GIu ( 1 8) . Ligand-binding studies with low concentrations o f [3H]-AMPA and eH1KA using recombinantly expressed receptors (GluRI -GluR4) observed only specific [3H]-AMPA binding (e.g. for GluR2 [3H]-AMPA bound to a single class of sites with a Kd of about 12 nM; QUlS>Glu>KA for inhibition of binding) ( 1 15). Despite this nanomolar AMPA binding, these receptor chan nels respond to Glu, QUlS, KA, and AMPA only at much higher con centrations. Low-affinity (micromolar) binding, however, cannot be excluded because of technical limitations of these studies. These results blur previous pharmacological distinctions between the KA and AMPA subclasses. Addi tional support for a continuum of KA-AMPA receptors comes from electro physiological studies in rat DRG neurons (103) . Table 3 draws together some of the properties of the cloned GluR subunits and the KBPs. Recently, Egebjerg et al (68) cloned and characterized a GluR subunit, designated GluR6, which is activated by KA, QUIS, and Glu, but not AMPA. When expressed in Xenopus oocytes, this subunit has at least one order of magnitude greater apparent affinity for KA than the GluRI-GluR4 subunits (see Table 4). Furthermore, AMPA (100 /LM) neither activated the receptor

524

GASIC & HOLLMANN Nomenclatures of the cloned ionotropic glutamate receptor subunits employed by several laboratories


Table 4

Heinemann'

Seeburgb

Axelc

G1uRI GluR2 G1uR3 G1uR4 GluR5 GluR6

G1uR-A G1uR-B G1uR-C G1uR-D

G1uR-KI G1uR-K2 G1uR-K3

Mishinad GluRl G1uR2

GluRI-GluR4 (GluR-A-GluR-D) each exist as two splicing variants, designated as flip and flop by Seeburg's group, who first discovered these variants. GluR5 also has two splice variants designated GluR5.1 and GluR5.2. (18). KA-I is another subunit cloned recently in the Seeburg laboratory, which binds KA but does not function as an ion channel . • Hollmann et al (100); Boulter et al (21); Bettler et al (18); Egebjerg et al (68). bKeinanen et al (115); Sommer et al (169); Monyer et al (147). C Nakanishi et al (150). d Sakimura et al (163).

nor interfered with receptor activation by KA (68) . This finding suggests that AMPA is only capable of interacting with a subset of GluRs that respond to KAiQUIS. Continuous application of agonist to GluR6 results in rapid de sensitization, which was decreased effectively by pretreatment with 10 JLM Concanavalin A . GluRI-GluR4, which do not rapidly desensitize in response to KA, exhibit up to tenfold larger responses to KA when pretreated with 10 JLM Concanavalin A (M. Hollmann, unpublished observations) . Homomeric GluR6 receptors exhibit an outwardly rectifying current/voltage (I-V) relationship. Coexpression of GluR6 with GluRI -GluR4 subunits apparently does not produce heteromers with synergistic properties (J. Egebjerg, un published observations) as observed with heteromers containing GluRl GluR4 subunits (21 ) . This evidence argues in favor of a KA-selective GluR that is composed of GluR6 or GluR6-related subunits.

Glutamate Receptor Subunit Heteromers: Synergism, Rectification, and Ion Permeability Coexpression of certain GluR subunits (combinations in which GluR2 is one component) in Xenopus oocytes of mammalian kidney cells produces a synergistic effect in the observed GIu-gated currents with a two to fourfold potentiation (2 1 , 1 1 5, 1 50 , 1 63) Expression of different combinations of subunits yields disparate I-V relationships and rectification characteristics (2 1 , 1 1 5) . For example, GluR-2 expressed in combination with GluRI or GluR3 produces macroscopic currents with an almost linear I-V relation .

(2 1 ) ,which resembles that observed upon expression of hippocampal poly A +

GLUTAMATE RECEPTORS

525

RNA in oocytes and in pyramidal neurons (57). These observations suggest that in situ heteromeric combinations of GluR subunits generate functional receptor diversity. Ca2+ permeability was long thought to be a distinct property of NMDA


receptors ( 1 4 1 ) . Recent evidence, however, suggests that a subset of KN

AMPA receptors on hippocampal neurons and astrocytes can flux Ca2+ (8 1 , 106). Hollmann et al found that Ca2+ permeability depends on GluR subunit composition when they tested GluR subunits, expressed individually or in combinations, in oocytes (99) . Oocytes expressing GluR I , GluR3 , and GluRl plus GluR3 , but not GluRl plus GluR2, or GluR3 plus GluR2, respond to KA and AMPA with inward Ca2+ currents (99) . Thus in neurons that express certain combinations of GluR subunits, Glu can trigger Ca2+ influx via non-NMDA receptors . Alternative Splicing of Glutamate Receptor Genes Generates Another Level of Functional Diversity Alternative splicing of adjacent exons of the GluRI -GluR4 genes to produce different subunits introduces additional functional receptor diversity, pharma cological complexity (169), and cell specificity in expression (147). A 38amino acid stretch encoded by adjacent exons of the respective gene and preceding putative transmembrane domain 4 has been demonstrated to exist in two alternative forms, designated flip and flop (169) . [A more flexible nomenclature for alternatively spliced variants of a given receptor subunit gene is GluRX. Y (X running number of receptor subunit; Y running number of splice variant) as described in Bettler et al ( 1 8) . ] Among the ligand-gated channels, the GluRs appear to be unique because alternative splicing of all the GluR I -GluR4 subunits results in functional diversity (78). GluR5 has two splice variants (GluR5 . 1 , GluRS . 2; GluR5 . 1 has a I S-amino acid insertion not present in GluR5 . 2). However, no functional difference has been found ( 1 8) . Generally, saturating conditions of Glu o r K A evoke similar currents for the flip versions of GluRI-GluR4, whereas KA-evoked currents are greater than Glu-mediated currents for the flop versions ( 1 69) . This disparity was observed for both the desensitizing and steady state components of the current response to Glu. Glu activated channels four to five times more effectively when applied to the flip versions . In situ, GluRs may be composed of heteromeric assemblies of different subunits that contain either flip or flop modules. Therefore, one type of receptor could mediate the fast desensi tizing component, while the other type could give rise to the steady state component. On the basis of sequence identity, agonist preferences, and the ability to =

=

526

GASIC & HOLLMANN

form functional heteromers , the functional GluR subunits can be tentatively divided in two groups: GluRI-GluR4, which are AMPA-selective, but also respond to KA, and GluR6 (GluR5 and other subunits may also be in this group), which are KA-preferring, but do not respond to AMPA. Additional experiments are needed to determine the biological significance of these two classes.


Site-Directed Mutagenesis of Glutamate Receptor Subunits The first experiments to determine the molecular basis of electrophysiological differences between the iGluR subunits have made use of site-directed mutagenesis . As discussed above, homomeric channels composed of GluR l , GluR3 , or GluR4 all display strong inwardly rectifying I-V relation, whereas GluR2 has a linear or outwardly rectifying I-V relation (99). Coexpression of any of these subunits with GluR2 suppresses the strong inward rectification of these channels and abolishes their Ca2+ permeability (99). To determine the molecular basis responsible for the permeation properties of these channels, Verdoom et al (180) and Hume et al (105) expressed GluR subunits with single amino acid substitutions in mammalian cells and Xenopus oocytes, respectively. Subunit chimeras between GluR2 and GluR3 ( l 05). as well as GluRl and GluR2 (20) were constructed and their functional properties determined. These experiments indicated that a region extending from amino acid residue 350 to 750 determined the shape of the I-V curve (20, 105). The most striking difference between GluR2 and GluRl or GluR3 in this highly conserved region is a glutamine (Q)/arginine (R) charge difference in the putative second transmembrane region (see Figure 2; region IIA). GluR2 has a positively charged R at amino acid position 586 (counting from the putaili('e, initiator methionine this would be position 607), whereas GluRl (Q582), GluR3 (Q590), or GluR4 (Q587) have a neutral Q in the homologous position (Figure 3). The single amino acid mutant GluR2 (R586Q, where R is replaced by Q) exhibited strong inward rectification, whereas the mutants GluR4 (Q587R) or GluR3 (Q590R) had an outwardly rectifying I-V relation ( 105 , 1 80). Hume et al carried these experiments one step further and showed that the arginine to glutamine mutation in GluR2 (R586Q) conferred a significant Ca2+ con ductance to the channel, whereas the glutamine to arginine mutation in GluR3 (Q590R) abolished the Ca2+ conductance (105). This single amino acid position in the GluRI-GluR4 group subunits appears to be responsible for both the shape of the I-V relation ( l OS, 180) and the Ca2+ permeability of the channel (105). The finding that this amino acid position (Figure 3) plays a central role in regulating ion permeation through these AMPA-preferring GluRs may have

GLUTAMATE RECEPTORS

527

II A GluR1

Q 582

GluR2

R 586

GluR3 GluR4


Consensus

Q 590 Q 58?

NEFGIFNSLWFSLGAFM QGCD ISPRSLS •

asparagine; E, glutamate; F, phenyalanine; G, glycine; I, isoleucine; L, leucine; W, tryptophan; A, alanine; M, methionine; R, arginine; Q, glutamine; C, cysteine; D, aspartate; P, proline

N,

S, serine;

Figure 3

Conserved and variable amino acid positions in hydrophobic region IIA (see Figure 2) of GluRI -GluR4 that influence channel ion permeability.

interesting implications for the topological structure of these non-NMDA receptor channels. The domain that contains this key amino acid position is likely to be in the vicinity of the ion permeation path. In Figure 2, this position is located in the hydrophobic stretch designated IIA. In Model I, domain IIA would be located in the outer vestibule of the channel, whereas in Model II, it would map to the second transmembrane domain. In the nAChR, transmembrane 2 appears to line the ion channel ( 1 9 , 1 07) . Therefore, simply on the basis of proximity, Model II is favored. The largest perturbations of nAchR ion permeation are caused by mutations of the intermediate anionic ring ( 1 23). If the control of ion permeation in GluRs is mediated by similar rings, the;.-Model II" may be the better topological representation . PERSPECTIVE

The molecular cloning , characterization, and expression of recombinant ionotropic and metabotropic GluR subunits has opened a new approach to the study of these receptors that complements traditional electrophysiological studies . Multiple structural genes and alternative splicing of these iGluR genes give rise to a functional diversity far greater than predicted by tradition al phannacological categories. The first studies employing site directed mutagenesis of iGluR subunits have begun to elucidate the molecular determi nants of ion permeation in these channels. Future studies must tackle the problems of iGluR topology, stoichiometry, modulation, and regulation of expression at the synapse. Single cell peR experiments may determine which GluR subunits are expressed in a patch clamp-characterized neuron. The number of different GluR subunit combinations expressed by that neuron,


528

GASIC & HOLLMANN

however, will be more difficult to ascertain. Despite homology screening and functional expression efforts in several laboratories, the NMDA receptor subtype has not been cloned to date. Contrary to expectations, the NMDA receptor subunits may not share significant sequence identity with presently cloned iGluR subunits, which would render homology screening strategies ineffective. Also, this complex and interesting GluR may be composed of multiple large subunits that must be simultaneously expressed to assemble into a functional NMDA receptor. Finally, as cloning efforts characterize additional GluR subunits, imaginative experiments will need to determine the molecular basis of GluR channel physiology and to elucidate the diverse biological functions of GluRs at several important synapses. ACKNOWLEDGMENTS

We thank Chuck Stevens and Steve Heinemann for their generous support and encouragement . Jim Boulter, Ray Dingledine, Jan Egebjerg, Rick Huganir, Rich Hume, Joanna Jen, Stuart Lipton, Jim McNamara, Carl Moll, Jean Philippe Pin, Scott Rogers , and Jane Sullivan provided valuable discussions and critical reading of the manuscript. We thank Susan Dollick for manuscript preparation. This work was supported by fellOWShips of Howard Hughes Medical Institute to G.P.G. and the Deutsche Forschungsgemeinschaft to M.H.

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11

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