Subunit interactions and AMPA receptor desensitization

Different gating mechanisms in glutamate receptor and K+ channels

The basic structural features of channel gating in glutamate receptors (GluRs) remain unknown. Here we used covalent modification of substituted cysteines and fast agonist application to study the contribution of the M3 segment in AMPA receptor GluR-A subunits to channel structure and gating. The pattern of accessibility of substituted cysteines to extracellularly applied methanethiosulfonate reagents and the rates of their modification by these reagents, measured in either the presence or absence of glutamate, indicate that M3 forms an alpha-helix that lines the pore of the channel and is involved in gating-related movements. The voltage dependence of modification rates places the tip of the M2 loop (the Q/R site) close to the middle of M3. All of these results are consistent with pore-forming domains in GluR and K+ channels having a similar structure but inverted membrane topology. Nevertheless, GluRs lack a glycine residue at a homologous structural position as the gating hinge glycine in K+ channels. Moreover, simultaneous substitution of the only two glycines in M3 of GluR-A with alanines produced channels with gating properties indistinguishable from wild type. Given the unique role of glycines in the flexibility ofalpha-helices, our results indicate that the M3 segment in GluR does not contain a glycine gating hinge and suggest that, in contrast to the homologous domain in K+ channels, M3 is rigid during gating. The different positioning and functional significance of glycines in a key structural domain may represent the basis for the distinct features of gating in GluR and K+ channels.

Structural basis for partial agonist action at ionotropic glutamate receptors

An unresolved problem in understanding neurotransmitter receptor function concerns the mechanism(s) by which full and partial agonists elicit different amplitude responses at equal receptor occupancy. The widely held view of 'partial agonism' posits that resting and active states of the receptor are in equilibrium, and partial agonists simply do not shift the equilibrium toward the active state as efficaciously as full agonists. Here we report findings from crystallographic and electrophysiological studies of the mechanism of activation of an AMPA-subtype glutamate receptor ion channel. In these experiments, we used 5-substituted willardiines, a series of partial agonists that differ by only a single atom. Our results show that the GluR2 ligand-binding core can adopt a range of ligand-dependent conformational states, which in turn control the open probability of discrete subconductance states of the intact ion channel. Our findings thus provide a structure-based model of partial agonism.

The mechanism of action of aniracetam at synaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors: indirect and direct effects on desensitization

The mechanism of action of aniracetam on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors was examined in outside-out patches and at glutamatergic synapses in neurons of the chick cochlear nucleus. A combination of rapid-flow analysis, using glutamate as an agonist, and kinetic modeling indicated that aniracetam slows both the rate of channel closing, and the microscopic rates of desensitization, even for partially liganded receptors. Little effect was observed on the rate of recovery from desensitization or on the response to the weakly desensitizing agonist kainate. Aniracetam's effects on receptor deactivation saturated at lower concentrations than its effects on desensitization, suggesting that cooperativity between homologous binding sites was required to regulate desensitization. Analysis of responses to paired pulses of agonist also indicated that AMPA receptors must desensitize partially even after agonist exposures too brief to permit rebinding. In the presence of aniracetam, evoked excitatory synaptic currents (EPSCs) and miniature EPSCs in low quantal-content conditions had decay times similar to the time course of receptor deactivation. Under these conditions, the time course of both transmitter release and clearance must be <1 to 2 ms. However, in high quantal-content conditions, the evoked EPSC in aniracetam decayed with a time course intermediate between deactivation and desensitization, suggesting that the time course of transmitter clearance is prolonged because of pooling of transmitter in the synaptic cleft. Moreover, by comparing the amounts of paired-pulse synaptic depression and patch desensitization prevented by aniracetam, we conclude that significant desensitization occurs in response to rebinding of transmitter to the AMPA receptors.

Distinguishing between presynaptic and postsynaptic mechanisms of short-term depression during action potential trains

Short-term facilitation and depression have a profound influence on transmission at many glutamatergic synapses, particularly during trains of stimuli. A major component of these processes is postsynaptic receptor desensitization. Both presynaptic and postsynaptic mechanisms can contribute to synaptic efficacy, but it is often difficult to define their respective contributions. Blockers of desensitization such as cyclothiazide (CTZ) can be used, but many of these drugs have nonspecific effects on transmitter release, complicating attempts to define synaptic effectiveness under physiological conditions. We describe and validate a new method to minimize desensitization during trains of synaptic stimuli that is based on the low-affinity competitive glutamate receptor antagonists gamma-D-glutamylglycine or kynurenic acid. A computational model of AMPA receptor kinetics shows that the mechanism can be accounted for by simple competitive antagonism of AMPA receptors, where the rapid off-rate of the antagonist permits re-equilibration between blocked and unblocked pools during the interstimulus interval. Our results at the calyx of Held show that desensitization makes little contribution to synaptic depression at frequencies below 10 Hz, but at higher frequencies it makes an important contribution, with accumulating desensitization masking short-term facilitation and causing an underestimation of quantal content. This novel method of protection from desensitization is compatible with physiological studies but cannot be used in conjunction with CTZ. Although presynaptic vesicle depletion makes the dominant contribution to short-term depression, our results show that AMPA receptor desensitization contributes to the depression at auditory synapses after hearing onset and in a frequency-dependent manner.

Probing the function, conformational plasticity, and dimer-dimer contacts of the GluR2 ligand-binding core: studies of 5-substituted willardiines and GluR2 S1S2 in the crystal

Numerous naturally occurring and synthetic alpha-amino acids act as agonists on (S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazole) propionic acid (AMPA) receptors but nevertheless display significant differences in their functional properties and modes of interaction. The 5-substituted willardiines are a series of compounds that exhibit a range of affinities, act as partial agonists, and give rise to intermediate levels of activation and desensitization. However, the molecular basis for the activities of 5-substituted willardiines has not been conclusively elaborated at the level of atomic resolution. Here we provide insight into the molecular basis of the potency and efficacy elicited by the 5-substituted willardiines on the basis of cocrystal structures with the GluR2 ligand-binding core. We also show that the crystallized ligand-binding core has an affinity for agonists similar to the ligand-binding core in solution. Analysis of multiple crystal lattices suggests modes by which the ligand-binding core dimers interact in the tetrameric receptor. These studies further our understanding of how subtle differences in the structures of agonists are correlated to changes in the conformation of residues and water molecules in the immediate binding pocket and to the degree of domain closure.

Kinetic properties of human AMPA-type glutamate receptors expressed in HEK293 cells

AMPA-type glutamate receptors (AMPAR) display a high variability in functional properties, which determine the time course of excitatory postsynaptic potentials. They are assembled as tetramers of GluR subunits 1-4 of different splice variants and nuclear edited isoforms. Presently, the kinetics of activation, desensitization and recovery from desensitization of human AMPARs (GluR1, 3 and 4 flip and flop, and GluR2 flip and flop in R and G edited forms, respectively) transiently expressed in HEK293 cells were studied with patch-clamp techniques and ultra fast agonist application. Activation time constants were identical for all receptors (0.13 ms). The GluR2 flip G variant showed the slowest desensitization (10.8 ms), GluR4 flip the fastest (1.6 ms). Recovery from desensitization varied between 3.1 ms (GluR4 flip) and 178 ms (GluR1 flip). To determine functional interactions between subunits in heteromeric receptors the GluR1 flip and the GluR2 flip R were coexpressed. The time constant of desensitization increased linearly from 2.5 ms (GluR1 flip homomers) to 6.8 ms (GluR2 flip R homomers) with the amount of GluR2 flip R cDNA transfected. Recovery followed a monoexponential time course and had a time constant of 178 ms in GluR1 flip homomeric expression. In all GluR1 flip/GluR2 flip heteromers and in GluR2 flip R homomers desensitization recovered with a time constant of approximately 50 ms. Thus, subunit interaction seems likely during recovery but not desensitization. Both parameters might influence the ability of AMPA receptors to mediate glutamate induced chronic excitotoxicity in neurodegenerative diseases.

Protein kinase C gamma associates directly with the GluR4 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor subunit. Effect on receptor phosphorylation

Ionotropic glutamate receptors mediate the majority of excitatory synaptic transmission in the brain and are thought to be involved in learning and memory formation. The activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-type glutamate receptors can be regulated by direct phosphorylation of their subunits, which affects the electrophysiological properties of the receptor, and the receptor association with numerous proteins that modulate membrane traffic and synaptic targeting of the receptor. In the present study we investigated the association of protein kinase C (PKC) gamma isoform with the GluR4 AMPA receptor subunit. PKC gamma was co-immunoprecipitated with GluR4 AMPA receptor subunit in rat cerebellum and in cultured chick retina cell extracts, and immunocytochemistry experiments showed co-localization of GluR4 and PKC gamma in cultured chick retinal neurons. Pull-down assays showed that native PKC gamma binds the GluR4 C-terminal membrane-proximal region, and recombinant PKC gamma was retained by GST-GluR4 C-terminal fusion protein, suggesting that the kinase binds directly to GluR4. Furthermore, GST-GluR4 C-terminal protein was phosphorylated on GluR4 Ser-482 by bound kinases, retained by the fusion protein, including PKC gamma. The GluR4 C-terminal segment that interacts with PKC gamma, which lacks the PKC phosphorylation sites, inhibited histone H1 phosphorylation by PKC, to the same extent as the PKC pseudosubstrate peptide 19-31, indicating that PKC gamma bound to GluR4 preferentially phosphorylates GluR4 to the detriment of other substrates. Additionally, PKC gamma expression in GluR4 transfected human embryonic kidney 293T cells increased the amount of plasma membrane-associated GluR4. Our results suggest that PKC gamma binds directly to GluR4, thereby modulating the function of GluR4-containing AMPA receptors.

Studies of NMDA receptor function and stoichiometry with truncated and tandem subunits

The subunits that compose eukaryotic glutamate ion channel receptors have three transmembrane domains (TMs) and terminate with intracellular tails that are important for controlling channel expression and localization. Truncation of NMDA receptor subunits before the final TM showed that this TM and intracellular tail region are necessary to form functional channels. However, it is shown here that these truncated subunits may be partially rescued by coexpressing the final TM and tail as a separate protein. The whole-cell currents so produced are somewhat lower than with full-length subunits, and they do not show the sag characteristic of currents from channels containing NR1 and NR2A subunits in the continued presence of an agonist. In addition, these truncated subunits were joined to full-length subunits to generate tandems. The functional expression of these tandems confirmed the tetrameric structure of NMDA receptors and also suggested that the subunits making up NMDA receptors are arranged as a dimer of dimers in the receptors with a 1-1-2-2 orientation of the subunits in the channel, and not in an alternating pattern of subunits around the pore. These results may redirect future studies into the mechanism of binding and gating in these receptors toward schemes including dimers, and may also be relevant to studies of glutamate receptor ion channels in general.

Amino-acid residues involved in glutamate receptor 6 kainate receptor gating and desensitization

The glutamate receptor (GluR) agonist-binding site consists of amino acid residues in the extracellular S1 and S2 segments in the N-terminal and M3-M4 loop regions, respectively. Molecular and atomic level structural analyses have identified specific S1 and S2 residues that interact directly with ligands, interact with one another in a dimeric configuration, and influence channel gating and desensitization properties of AMPA receptors. Other studies suggest that KA receptor gating and desensitization may differ mechanistically. In particular, a leucine (L) to tyrosine (Y) mutation in the S1 segment of AMPA receptors is sufficient to block desensitization, whereas KA receptors naturally contain a tyrosine residue at the equivalent position (Y751 in GluR6) but retain the fast-desensitizing phenotype. We hypothesized that KA receptor desensitization is preserved by a compensatory substitution in the S2 segment. We generated a series of GluR6 mutants that converted individual S2 domain residues to their AMPA receptor equivalents. Various S2 mutations had effects on the kinetics of desensitization and recovery from desensitization, but no single amino acid substitution was found to block desensitization, as in the L/Y mutant AMPA receptors, or to prevent desensitization to KA. Other mutations designed to neutralize residues thought to interact across the dimer interface had dramatic effects on channel gating and desensitization. These results are consistent with a close but imperfect structural homology between AMPA and KA receptors and support the role of conserved S1S2 domain interactions at the dimer interface in GluR channel function.

Activation of NR1/NR2B NMDA receptors

N-methyl-D-aspartate (NMDA) receptors are highly expressed in the central nervous system and are involved in excitatory synaptic transmission as well as synaptic plasticity. Despite considerable structural and biophysical research, the mechanism behind activation of the NMDA receptor is still poorly understood. By analyzing patch clamp recordings of one channel activated by glutamate, we determined the burst structure and open probability for recombinant rat NR1/NR2B receptors. We used partial agonists at the glutamate and glycine binding sites to show that at least two kinetically distinct subunit-associated conformational changes link co-agonist binding to the opening of the NMDA receptor pore. These data suggest that NR1 and NR2B subunits, respectively, undergo a fast and slow agonist-dependent conformational change that precedes opening of the pore. We propose a new working model of receptor activation that can account for macroscopic as well as microscopic NMDA receptor properties.

How AMPA receptor desensitization depends on receptor occupancy

AMPA-type glutamate receptors mediate fast excitatory transmission at many central synapses, and rapid desensitization of these receptors can shape the decay of synaptic currents and limit the fidelity of high-frequency synaptic transmission. Here we use a combination of fast glutamate application protocols and kinetic simulations to determine how AMPA receptor desensitization depends on the number of subunits occupied by glutamate. We show that occupancy of a single subunit is sufficient to desensitize AMPA-type channels and that receptors with one to four glutamates bound enter desensitization at similar rates. We find that recovery from desensitization follows a similar sigmoid time course for channels with two to four glutamates bound but is faster and exponential for singly occupied channels. The results suggest that desensitization, at intermediate and high glutamate concentrations, is accompanied by two conformational changes that slow glutamate dissociation. We propose a kinetic scheme that accurately predicts several types of experimental results and differs significantly from previous models in the assignment of affinities for binding to closed and desensitized states. We conclude that desensitization involves a rearrangement that stabilizes the binding domains of one subunit in each dimer in a partially closed conformation. This stabilization likely results from an interaction at the dimer-dimer interface between the binding domains of adjacent subunits.

Competitive antagonism of AMPA receptors by ligands of different classes: crystal structure of ATPO bound to the GluR2 ligand-binding core, in comparison with DNQX

Ionotropic glutamate receptors (iGluRs) constitute a family of ligand-gated ion channels that are essential for mediating fast synaptic transmission in the central nervous system. This study presents a high-resolution X-ray structure of the competitive antagonist (S)-2-amino-3-[5-tert-butyl-3-(phosphonomethoxy)-4-isoxazolyl]propionic acid (ATPO) in complex with the ligand-binding core of the receptor. Comparison with the only previous structure of the ligand-binding core in complex with an antagonist, 6,7-dinitro-2,3-quinoxalinedione (DNQX) (Armstrong, N.; Gouaux, E. Neuron 2000, 28, 165-181), reveals that ATPO and DNQX stabilize an open form of the ligand-binding core by different sets of interactions. Computational techniques are used to quantify the differences between these two ligands and to map the binding site. The isoxazole moiety of ATPO acts primarily as a spacer, and other scaffolds could potentially be used. Whereas agonists induce substantial domain closures compared to the apo structure, ATPO only induces minor conformational changes. These results are consistent with the hypothesis that domain closure is related to receptor activation. To facilitate the design of novel AMPA receptor antagonists, we present a modified model of the binding site that includes key residues involved in ligand recognition.

The selective activation of the glutamate receptor GluR5 by ATPA is controlled by serine 741

Only a few agonists exhibit selectivity between the AMPA and the kainate subtypes of the glutamate receptor. The most commonly used kainate receptor preferring agonist, (S)-2-amino-3-(5-tert-butyl-3-hydroxy-4-isoxazolyl)propionic acid [(S)-ATPA], is an (R,S)-2-amino-3-(5-methyl-3-hydroxy-4-isoxazolyl)propionic acid (AMPA) derivative in which the methyl group at the 5-position of the isoxazole ring has been replaced by a tert-butyl group. When characterized by the two-electrode voltage clamp method in Xenopus laevis oocytes, ATPA exhibits at least 50-fold higher potency on the kainate receptor subtype, GluR5, compared with the AMPA receptors. Through mutagenesis studies of GluR5 and the AMPA receptor subtype, GluR1, we demonstrate that this pronounced selectivity for ATPA can be ascribed to Ser741 in GluR5 and Met722 in GluR1. Examination of other aliphatic substitutions at the 5-position of the isoxazole ring revealed that (R,S)-2-amino-3-(5-isopropyl-3-hydroxy-4-isoxazolyl)propionic acid (isopropyl-AMPA) displayed a 6-fold higher potency for GluR5 than for GluR1, whereas the analogs, propyl-AMPA and isobutyl-AMPA, did not exhibit significantly different potencies. Our study suggests that the GluR5 selectivity was a result not only of steric interference between the bulky tert-butyl group in ATPA and the methionine (Met722) in GluR1 but also a serine-dependent stabilization of the active conformation of GluR5 induced by ATPA. The stabilization was agonist-dependent and observed only for ATPA and isopropyl-AMPA, not for other AMPA analogs with bulky substitutions at the 5-position of the isoxazole ring.

Mechanism of activation and selectivity in a ligand-gated ion channel: structural and functional studies of GluR2 and quisqualate

Glutamate is the major excitatory neurotransmitter in the mammalian brain. The (S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazole)propionic acid (AMPA)-subtype glutamate receptor, a ligand-gated ion channel, mediates most of the fast excitatory synaptic transmission in the mammalian central nervous system. Here we present electrophysiological, biochemical, and crystallographic data on the interactions between quisqualate and the GluR2 receptor ion channel and its corresponding ligand binding core. Quisqualate is a high-affinity, full agonist which like AMPA and glutamate elicits maximum peak current responses, and stabilizes the ligand binding core in a fully closed conformation, reinforcing the concept that full agonists produce similar conformational changes [Armstrong, N., and Gouaux, E. (2000) Neuron 28, 165-181]. Nevertheless, the mechanism of quisqualate binding is different from that of AMPA but similar to that of glutamate, illustrating that quisqualate is a faithful glutamate analogue. A detailed comparison of the three agonist complexes reveals distinct binding mechanisms, particularly in the region of a hydrophobic pocket that is proximal to the anionic gamma-substituents, and demonstrates the importance of agonist-water-receptor interactions. The hydrophobic pocket, which is predicted to vary in chemical character between receptor subtypes, probably plays an important role in determining receptor subtype specificity.

Age-related changes in tolerance to the marine algal excitotoxin domoic acid

During an incident of toxic mussel poisoning, the epileptogenic excitotoxin domoic acid (DOM) was associated with lasting neurological deficits mainly in older patients (), suggesting supersensitivity to excitotoxins is a feature of brain aging. Here, hippocampal slices from young (3 months) and aged (26-29 months) Sprague Dawley rats were assessed by CA1 field potential analysis before and after preconditioning with DOM. In naïve slices from young animals, DOM produced initial hyperexcitability followed by significant dose-dependent reductions in population spike amplitude during prolonged application. Following toxin washout, only small changes in neuronal activity were evident during a second application of DOM, suggesting that a resistance to the effects of DOM occurs in hippocampal slices which have undergone prior exposure to DOM. This inducible tolerance was not antagonized by the NMDA receptor blockers APV or MK-801, nor was it diminished by the group I, II or III mGluR blockers AIDA, CPPG and EGLU. Likewise, neither the AMPA/KA blocker CNQX nor the VSCC blocker nifedipine were effective in blocking tolerance induction in young slices. Field potential analysis revealed significant age-related reductions in CA1 EPSP strength, population spike amplitude and paired-pulse inhibition, but aged slices did not differ in sensitivity to DOM relative to young. However, aged CA1 failed to exhibit any tolerance to DOM following preconditioning, suggesting that a loss of inducible neuroprotective mechanisms may account for increased sensitivity to excitotoxins during aging.

Staggering of subunits in NMDAR channels

Functional N-methyl-D-aspartate receptors (NMDARs) are heteromultimers formed by NR1 and NR2 subunits. The M3 segment, as contributed by NR1, forms the core of the extracellular vestibule, including binding sites for channel blockers, and represents a critical molecular link between ligand binding and channel opening. Taking advantage of the substituted cysteine accessibility method along with channel block and multivalent coordination, we studied the contribution of the M3 segment in NR2C to the extracellular vestibule. We find that the M3 segment in NR2C, like that in NR1, contributes to the core of the extracellular vestibule. However, the M3 segments from the two subunits are staggered relative to each other in the vertical axis of the channel. Compared to NR1, homologous positions in NR2C, including those in the highly conserved SYTANLAAF motif, are located about four amino acids more externally. The staggering of subunits may represent a key structural feature underlying the distinct functional properties of NMDARs.

Purinergic P2X(2) receptor desensitization depends on coupling between ectodomain and C-terminal domain

The wild-type P2X(2) purinergic receptor (P2X(2a)R) and its splice form lacking the intracellular Val(370)-Gln(438) C-terminal sequence (P2X(2b)R) respond to ATP stimulation with comparable EC(50) values and peak current/calcium responses but desensitize in a receptor-specific manner. P2X(2a)R desensitizes slowly and P2X(2b)R desensitizes rapidly. We studied the effects of different agonists, and of substituting the ectodomain, on the pattern of calcium signaling by P2X(2a)R and P2X(2b)R. Both receptors showed similar EC(50) values (estimated from the peak calcium response) and IC(50) values (estimated from the rate of calcium signal desensitization) for agonists, in the order 2-MeS-ATP <or= ATP <or= ATPgammaS < BzATP << alphabeta-meATP, and the IC(50) values for agonists were shifted to the right compared with their EC(50) values. Furthermore, the ATP-induced receptor-subtype specific pattern of desensitization was mimicked by high- but not by low-efficacy agonists, suggesting a ligand-specific desensitization pattern. To test this hypothesis, we generated chimeric P2X(2a)R and P2X(2b)R containing the Val(60)-Phe(301) ectodomain sequence of P2X(3)R and Val(61)-Phe(313) ectodomain sequence of P2X(7)R instead the native Ile(66)-Tyr(310) sequence. The mutated P2X(2a)+X(3)R and P2X(2b)+X(3)R exhibited comparable EC(50) values for ATP, BzATP, and alphabeta-meATP in the submicromolar concentration range and desensitized in a receptor-specific and ligand-nonspecific manner. On the other hand, the chimeric P2X(2)+X(7)R exhibited decreased sensitivity for ATP and desensitized in a receptor-nonspecific manner. These results suggest that efficacy of agonists for the ligand-binding domain of P2X(2)Rs reflects the strength of desensitization controlled by their C-terminal structures.

Desensitization of homomeric alpha1 glycine receptor increases with receptor density

Variations in the number of receptors at glycinergic synapses are now established and are believed to contribute to inhibitory synaptic plasticity. However, the relation between glycine receptor (GlyR) kinetics and density is still unclear. We used outside-out patch-clamp recordings and fast-flow application techniques to resolve fast homomeric GlyRalpha1 kinetics and to determine how the functional properties of these receptors depend on their density and on the presence of the anchoring protein gephyrin. The expression of GlyRs in human embryonic kidney cells increased with time and was correlated with an increase in GlyR desensitization at 2 days after transfection. Cotransfection of homomeric GlyRalpha1 bearing the gephyrin-binding site with gephyrin also increased desensitization but at 1 day after transfection compared with transfections of homomeric GlyRalpha1 without gephyrin. This increase results from the occurrence of a fast desensitization component and short applications of a saturating concentration of glycine suffice to promote a rapidly entered desensitized closed state. The level of desensitization changed neither the EC(50) value nor the Hill coefficient of the glycine dose-response curves because the amplitude of the current was measured at the peak of the responses. These results demonstrate that variations in GlyR density during cluster formation result from a change in GlyR efficiency due to modifications in their desensitization properties.

RNA editing of neurotransmitter receptors in the mammalian brain

RNA editing refers to various posttranscriptional mechanisms that alter the nucleotide sequence of RNA. In the mammalian brain, RNA editing results in significant changes in the functional properties of receptors for the important neurotransmitters glutamate and serotonin. These changes result from site-specific deamination of single adenosines in the pre-messenger RNA encoding these receptors. Here, we review what is known about the mechanisms underlying this editing, the consequences of RNA editing for glutamate and serotonin receptor function, and recent studies on transgenic mice and human post-mortem tissue that have begun to elucidate the role of RNA editing in the intact mammalian brain.

The speeding of EPSC kinetics during maturation of a central synapse

Several factors contribute to the shape of excitatory postsynaptic currents (EPSCs) in CNS neurons, among them the kinetics of presynaptic release, transmitter clearance, and the properties and distribution of postsynaptic receptors. The decays of AMPA receptor-mediated EPSCs at rat cerebellar mossy fibre-granule cell (MF-gc) synapses follow a bi-exponential time-course. The fast component dominates the decay, accounting for 84-94% of the peak amplitude. Here we show that both components of decay, and also the risetimes, became faster during postnatal maturation. At adult, but not immature, synapses, the risetimes and decays of evoked multiquantal EPSCs were similar to those of monoquantal miniature (m)EPSCs. The faster risetimes at mature synapses reflected increased synchrony of multivesicular release, whereas the faster decays appeared to reflect changes in the properties of postsynaptic receptors. Inhibition of glutamate uptake was without effect on evoked EPSCs at both ages. Furthermore, after slowing receptor desensitization with cyclothiazide, the EPSCs at mature synapses decayed as slowly as EPSCs at immature synapses, suggesting that faster glutamate clearance does not account for the developmental speeding of EPSC decay. Our results support previous conclusions that glutamate clearance and receptor deactivation are important determinants of the fast decay component at immature synapses. Desensitization becomes increasingly important during development and plays a major role in shaping EPSC decay at mature synapses.

The expression of dominant-negative subunits selectively suppresses neuronal AMPA and kainate receptors

Glutamate-gated ion channels are widely expressed in neurons where they serve a host of cellular functions. An appealing, but yet unexplored, way to delineate the functions of particular glutamate receptor subtypes is to direct the expression of dominant-negative and gain-of-function mutant subunits. We tested the ability of two dominant-negative subunits, an alpha-amino-3-hydroxy-5-methyl-isoxazolproprionic acid receptor subunit and a kainate receptor subunit, to silence recombinant and neuronal glutamate receptors. Co-expression studies in non-neuronal cells indicated that the inclusion of a single mutant subunit was sufficient to silence the receptor. When expressed in cerebellar granule cells, the dominant-negative subunits silenced native channels in a subtype-specific fashion. Immunocytochemical staining of control and transfected neurons, as well as studies with a gain-of-function glutamate receptor-1 mutant, indicated that the mutant subunits were expressed at levels roughly equal to the total abundance of related native subunits, and both dominant-negatives suppressed native channel expression 60-65% when tested 24 h post-transfection. If co-assembly of the mutant subunits with related native subunits is combinatorial, this level of suppression gives receptor half-lives of approximately 20 h.

Heteromeric AMPA receptors assemble with a preferred subunit stoichiometry and spatial arrangement

AMPA receptors are thought to be a tetrameric assembly of the subunits GluR1-4. We have examined whether two coexpressed subunits (GluR1/2) combine at random to form channels, or preferentially assemble with a specific stoichiometry and spatial configuration. The subunits carried markers controlling ion permeation and desensitization, and these properties were monitored as a function of relative expression level and subunit composition. Homomeric receptors assembled stochastically while heteromeric receptors preferentially formed with a stoichiometry of two GluR1 and two GluR2 subunits, and with identical subunits positioned on opposite sides of the channel pore. This structure will predominate if GluR1 binds to GluR2 more rapidly during receptor assembly than other subunit combinations. The practical outcome of selective heteromeric assembly is a more homogenous receptor population in vivo.

Mutation of the 9' leucine in the GABA(A) receptor gamma2L subunit produces an apparent decrease in desensitization by stabilizing open states without altering desensitized states

A conserved leucine near the middle (9' position) of the second transmembrane domain of ligand-gated ion channels has been implicated in both gating and desensitization. Specifically, L9'S and L9'T mutations decreased agonist EC50, decreased apparent desensitization and prolonged deactivation in members of the LGIC superfamily, suggesting that this residue may regulate channel properties including desensitization. GABA(A) receptors desensitize in three phases, but in previous 9' leucine studies, only slow phases of desensitization were resolved. We used excised patches containing alpha1beta3gamma2L or alpha1beta3gamma2L(L9'S) GABA(A) receptors and combined single channel recording and concentration jump techniques to reevaluate the effects of this mutation on desensitization. Although desensitization extent was decreased in mutated channels, desensitization still occurred in three phases, suggesting that desensitized states may be intact. Interestingly, deactivation rate was slowed by the mutation, opposite to that expected if desensitization was attenuated. alpha1beta3gamma2L(L9'S) receptor single channels had increased open durations. Simulations revealed that stabilizing the open state (by decreasing the channel closing rate) could account for multiple macroscopic findings: left-shifted GABA EC50, smaller extent of desensitization, slower desensitization rate, and longer deactivation. We concluded that changes in efficacy can alter macroscopic desensitization without affecting desensitized states per se.