Functional studies and distribution define a family of transmembrane AMPA receptor regulatory proteins

Molecular anatomy of the postsynaptic density

The postsynaptic density (PSD) is a structure composed of both membranous and cytoplasmic proteins localized at the postsynaptic plasma membrane of excitatory synapses. Biochemical and molecular biological studies have identified a number of proteins present in the PSD. Glutamate receptors are important constituents of the PSD and membrane proteins involved in synaptic signal transduction and cell adhesion are also essential components. Scaffolding proteins containing multiple protein interaction motifs are thought to provide the framework of the PSD through their interactions with both membrane proteins and the cytoplasmic proteins. Among the cytoplasmic signaling molecules, calcium-calmodulin-dependent protein kinase II stands out as a major component of the PSD and its dynamic translocation to the PSD in response to neuronal activity is crucial in synaptic signal transduction. Recent advancements in molecular biological, structural and electrophysiological techniques have enabled us to directly measure the number, distribution and interactions of PSD molecules with high sensitivity and precision. In this review, I describe the structure and molecular composition of the PSD as well as the molecular interactions between the major constituents. This information will be combined with recent quantitative analyses of the PSD protein contents per synapse, in order to provide a current view of the PSD molecular architecture and its dynamics.

TARPs and the AMPA Receptor Trafficking Paradox

AMPA receptors (AMPARs) conduct fast, excitatory currents that depolarize neurons and trigger action potentials. AMPARs took on new importance when it was shown that AMPAR transport can increase or decrease the number of AMPARs at synapses and give rise to synapse plasticity, including long-term potentiation (LTP) and long-term depression (LTD). This review considers how transmembrane AMPAR regulatory proteins (TARPs), a novel family of AMPAR auxiliary subunits, have changed our view of AMPAR transport and raised some perplexing questions.

Linkage and association analysis of CACNG3 in childhood absence epilepsy

Childhood absence epilepsy (CAE) is an idiopathic generalised epilepsy characterised by absence seizures manifested by transitory loss of awareness with 2.5-4 Hz spike-wave complexes on ictal EEG. A genetic component to aetiology is established but the mechanism of inheritance and the genes involved are not fully defined. Available evidence suggests that genes encoding brain expressed voltage-gated calcium channels, including CACNG3 on chromosome 16p12-p13.1, may represent susceptibility loci for CAE. The aim of this work was to further evaluate CACNG3 as a susceptibility locus by linkage and association analysis. Assuming locus heterogeneity, a significant HLOD score (HLOD = 3.54, alpha = 0.62) was obtained for markers encompassing CACNG3 in 65 nuclear families with a proband with CAE. The maximum non-parametric linkage score was 2.87 (P < 0.002). Re-sequencing of the coding exons in 59 patients did not identify any putative causal variants. A linkage disequilibrium (LD) map of CACNG3 was constructed using 23 single nucleotide polymorphisms (SNPs). Transmission disequilibrium was sought using individual SNPs and SNP-based haplotypes with the pedigree disequilibrium test in 217 CAE trios and the 65 nuclear pedigrees. Evidence for transmission disequilibrium (P < or = 0.01) was found for SNPs within a approximately 35 kb region of high LD encompassing the 5'UTR, exon 1 and part of intron 1 of CACNG3. Re-sequencing of this interval was undertaken in 24 affected individuals. Seventy-two variants were identified: 45 upstream; two 5'UTR; and 25 intronic SNPs. No coding sequence variants were identified, although four variants are predicted to affect exonic splicing. This evidence supports CACNG3 as a susceptibility locus in a subset of CAE patients.

Redundancy of Cav2.1 channel accessory subunits in transmitter release at the mouse neuromuscular junction

Ca(v)2.1 (P/Q-type) channels possess a voltage-sensitive pore-forming alpha(1) subunit that can associate with the accessory subunits alpha(2)delta, beta and gamma. The primary role of Ca(v)2.1 channels is to mediate transmitter release from nerve terminals both in the central and peripheral nervous system. Whole-cell voltage-clamp studies in in vitro expression systems have indicated that accessory channel subunits can have diverse modulatory effects on membrane expression and biophysical properties of Ca(v)2.1 channels. However, there is only limited knowledge on whether similar modulation also occurs in the specific presynaptic environment in vivo and, hence, whether accessory subunits influence neurotransmitter release. Ducky, lethargic and stargazer are mutant mice that lack functional alpha(2)delta-2, beta(4) and gamma(2) accessory Ca(v) channel subunits, respectively. The neuromuscular junction (NMJ) is a peripheral synapse, where transmitter release is governed exclusively by Ca(v)2.1 channels, and which can be characterized electrophysiologically with relative experimental ease. In order to investigate a possible synaptic influence of accessory subunits in detail, we electrophysiologically measured acetylcholine (ACh) release at NMJs of these three mutants. Surprisingly, we did not find any changes compared to wild-type littermates, other than a small reduction (25%) of evoked ACh release at ducky NMJs. This effect is most likely due to the approximately 40% reduced synapse size, associated with the reduced size of ducky mice, rather than resulting directly from reduced Ca(v)2.1 channel function due to alpha(2)delta-2 absence. We conclude that alpha(2)delta-2, beta(4), and gamma(2) accessory subunits are redundant for the transmitter release-mediating function of presynaptic Ca(v)2.1 channels at the mouse NMJ.

Synapse-specific and developmentally regulated targeting of AMPA receptors by a family of MAGUK scaffolding proteins

Trafficking of AMPA receptors (AMPA-Rs) to and from synapses controls the strength of excitatory synaptic transmission. However, proteins that cluster AMPA-Rs at synapses remain poorly understood. Here we show that PSD-95-like membrane-associated guanylate kinases (PSD-MAGUKs) mediate this synaptic targeting, and we uncover a remarkable functional redundancy within this protein family. By manipulating endogenous neuronal PSD-MAGUK levels, we find that both PSD-95 and PSD-93 independently mediate AMPA-R targeting at mature synapses. We also reveal unanticipated synapse heterogeneity as loss of either PSD-95 or PSD-93 silences largely nonoverlapping populations of excitatory synapses. In adult PSD-95 and PSD-93 double knockout animals, SAP-102 is upregulated and compensates for the loss of synaptic AMPA-Rs. At immature synapses, PSD-95 and PSD-93 play little role in synaptic AMPA-R clustering; instead, SAP-102 dominates. These studies establish a PSD-MAGUK-specific regulation of AMPA-R synaptic expression that establishes and maintains glutamatergic synaptic transmission in the mammalian central nervous system.

Stargazin interacts functionally with the AMPA receptor glutamate-binding module

Neuronal AMPA receptors comprise pore forming glutamate receptor (GluR) proteins and auxiliary transmembrane AMPA receptor regulatory (TARP) subunits. TARPs traffic AMPA receptors to synapses and regulate channel gating. Both intracellular and extracellular regions in TARPs regulate AMPA receptors; however, the details for these interactions remain unknown. Here, we employ site-directed mutagenesis to determine functional interactions between GluR1 and the prototypical TARP, stargazin. We find that a point mutation in the glutamate-binding region of GluR1 corresponding to the Lurcher allele of GluRdelta2, abolishes stargazin's effects on receptor trafficking and channel gating. A point mutation that prevents receptor desensitization modulates the effects of stargazin on channel gating but preserves receptor trafficking. These studies identify a functional interaction of stargazin with the extracellular glutamate-binding domain of AMPA receptors.

Abundant distribution of TARP gamma-8 in synaptic and extrasynaptic surface of hippocampal neurons and its major role in AMPA receptor expression on spines and dendrites

Transmembrane alpha-amino-3-hydroxyl-5-isoxazolepropionate (AMPA) receptor regulatory proteins (TARPs) play pivotal roles in AMPA receptor trafficking and gating. Here we examined cellular and subcellular distribution of TARP gamma-8 in the mouse brain. Immunoblot and immunofluorescence revealed the highest concentration of gamma-8 in the hippocampus. Immunogold electron microscopy demonstrated dense distribution of gamma-8 on the synaptic and extrasynaptic surface of hippocampal neurons with very low intracellular labeling. Of the neuronal surface, gamma-8 was distributed at the highest level on asymmetrical synapses of pyramidal cells and interneurons, whereas their symmetrical synapses selectively lacked immunogold labeling. Then, the role of gamma-8 in AMPA receptor expression was pursued in the hippocampus using mutant mice defective in the gamma-8 gene. In the mutant cornu ammonis (CA)1 region, synaptic and extrasynaptic AMPA receptors on dendrites and spines were severely reduced to 35-37% of control levels, whereas reduction was mild for extrasynaptic receptors on somata (74%) and no significant decrease was seen for intracellular receptors within spines. In the mutant CA3 region, synaptic AMPA receptors were reduced mildly at asymmetrical synapses in the stratum radiatum (67% of control level), and showed no significant decrease at mossy fiber-CA3 synapses. Therefore, gamma-8 is abundantly distributed on hippocampal excitatory synapses and extrasynaptic membranes, and plays an important role in increasing the number of synaptic and extrasynaptic AMPA receptors on dendrites and spines, particularly, in the CA1 region. Variable degrees of reduction further suggest that other TARPs may also mediate this function at different potencies depending on hippocampal subregions, input sources and neuronal compartments.

Isoform-specific early trafficking of AMPA receptor flip and flop variants

Flip and flop splice variants of AMPA receptor subunits are expressed in distinct but partly overlapping patterns and impart different desensitization kinetics to cognate receptor channels. In the absence of specific antibodies, isoform-specific differences in trafficking or localization of native flip and flop subunits remain uncharacterized. We report that in several transfected cell lines, transport of homomeric glutamate receptor (GluR)-D(flop) receptors is largely blocked at the endoplasmic reticulum (ER) exit, whereas GluR-D(flip) undergoes complex glycosylation and reaches the plasma membrane at >10x higher levels than GluR-D(flop), as determined by immunofluorescence, patch-clamp recordings and biochemical assays. The transport difference between flip and flop is independent of activity, is primarily determined by amino acid residue 780 (Leu in flop, Val in flip), and is manifested even in the secretion of the soluble ligand-binding domain, suggesting it is independent of oligomerization. Coexpression with stargazin or with the flip isoform rescues the surface expression of GluR-D(flop) near to the level exhibited by GluR-D(flip). Our results demonstrate that the extracellular flip/flop region, via interactions with ER luminal splice form-specific protein(s), plays a hitherto unappreciated and important role in AMPA-receptor trafficking.

Stargazin mutation impairs cerebellar synaptogenesis, synaptic maturation and synaptic protein distribution

Stargazin mutation results in absence epilepsy and cerebellar ataxia in stargazer (stg) mice. We have previously discovered defects of AMPA receptor function, failure of BDNF expression and immature morphology specifically in the cerebellar cortex of stg mice. To further characterize the nature of synaptic abnormalities, we examined the ultrastructure of cerebellar granule cell output synapses and measured the expression levels of several synaptic proteins in different brain regions of stg mutant. Electron microscopic examination revealed a number of immature features in the molecular layer of the mutant cerebellar cortex, including the presence of desmosoid plaques, concentric profiles of parallel fibers, smaller presynaptic terminal and fewer synaptic vesicles. Quantitative measurement showed a significantly lower number of synapses and smaller area of presynaptic terminals in adult stg cerebellum when compared with age-matched wildtype. Immunoblotting analysis of the SNARE proteins revealed selective reduction of the levels of synaptobrevin and synaptophysin in synaptosomes from stg cerebellum. The expression levels of synapsins were not altered in stg cerebellum, but showed a significant upregulation in stg cerebral cortex and hippocampus. Our results suggest that, despite the relatively normal gross morphology of cerebellum, stargazin mutation results in abnormal ultrastructure of cerebellar synapses, and stargazin-induced regional failure of BDNF expression may be responsible for abnormal SNARE protein distribution and partially attributes to the defects in the synaptic ultrastructure.

New role for spinal Stargazin in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated pain sensitization after inflammation

Considerable evidence has demonstrated that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor blockade has an antinociceptive effect on inflammatory pain. Stargazin (STG) is the first transmembrane protein known to associate with AMPA receptors and regulate their synaptic targeting. However, it is not known whether STG is involved in inflammatory pain processing by regulating AMPA receptor function. In the present study, we investigated the effect of knockdown of spinal STG on AMPA receptor-mediated pain sensitization after inflammation. Antisense technology was employed to knock down STG expression in the spinal cord. We show that STG was expressed and interacted with AMPA receptor subunit GluR2 in the spinal cord. Intrathecally injected STG antisense oligodeoxyribonucleotide (ODN) specifically decreased STG expression in the lumbar spinal cord and dose dependently inhibited formalin-induced inflammatory pain in the second phase. More important was our finding for the first time that this specific STG antisense ODN diminished AMPA (0.1 mug)-enhanced formalin pain and lost its effect if pretreated with AMPA receptor antagonist CNQX. Our results demonstrate a new role for STG in central sensitization of inflammatory pain by interacting with AMPA receptors in the spinal cord.

Aberrant GABA(A) receptor expression in the dentate gyrus of the epileptic mutant mouse stargazer

Stargazer (stg) mutant mice fail to express stargazin [transmembrane AMPA receptor regulatory protein gamma2 (TARPgamma2)] and consequently experience absence seizure-like thalamocortical spike-wave discharges that pervade the hippocampal formation via the dentate gyrus (DG). As in other seizure models, the dentate granule cells of stg develop elaborate reentrant axon collaterals and transiently overexpress brain-derived neurotrophic factor. We investigated whether GABAergic parameters were affected by the stg mutation in this brain region. GABA(A) receptor (GABAR) alpha4 and beta3 subunits were consistently upregulated, GABAR delta expression appeared to be variably reduced, whereas GABAR alpha1, beta2, and gamma2 subunits and the GABAR synaptic anchoring protein gephyrin were essentially unaffected. We established that the alpha4 betagamma2 subunit-containing, flunitrazepam-insensitive subtype of GABARs, not normally a significant GABAR in DG neurons, was strongly upregulated in stg DG, apparently arising at the expense of extrasynaptic alpha4 betadelta-containing receptors. This change was associated with a reduction in neurosteroid-sensitive GABAR-mediated tonic current. This switch in GABAR subtypes was not reciprocated in the tottering mouse model of absence epilepsy implicating a unique, intrinsic adaptation of GABAergic networks in stg. Contrary to previous reports that suggested that TARPgamma2 is expressed in the dentate, we find that TARPgamma2 was neither detected in stg nor control DG. We report that TARPgamma8 is the principal TARP isoform found in the DG and that its expression is compromised by the stargazer mutation. These effects on GABAergic parameters and TARPgamma8 expression are likely to arise as a consequence of failed expression of TARPgamma2 elsewhere in the brain, resulting in hyperexcitable inputs to the dentate.

Organelles and trafficking machinery for postsynaptic plasticity

Neurons are among the largest and most complex cells in the body. Their immense size and intricate geometry pose many unique cell-biological problems. How is dendritic architecture established and maintained? How do neurons traffic newly synthesized integral membrane proteins over such long distances to synapses? Functionally, protein trafficking to and from the postsynaptic membrane has emerged as a key mechanism underlying various forms of synaptic plasticity. Which organelles are involved in postsynaptic trafficking, and how do they integrate and respond to activity at individual synapses? Here we review what is currently known about long-range trafficking of newly synthesized postsynaptic proteins as well as the local rules that govern postsynaptic trafficking at individual synapses.

Remodeled cardiac calcium channels

Cardiac calcium channels play a pivotal role in the proper functioning of cardiac cells. In response to various pathologic stimuli, they become remodeled, changing how they function, as they adapt to their new environment. Specific features of remodeled channels depend upon the particular disease state. This review will summarize what is known about remodeled cardiac calcium channels in three disease states: hypertrophy, heart failure and atrial fibrillation. In addition, it will review the recent advances made in our understanding of the function of the various molecular building blocks that contribute to the proper functioning of the cardiac calcium channel.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Stargazin and other transmembrane AMPA receptor regulating proteins interact with synaptic scaffolding protein MAGI-2 in brain

The spatial coordination of neurotransmitter receptors with other postsynaptic signaling and structural molecules is regulated by a diverse array of cell-specific scaffolding proteins. The synaptic trafficking of AMPA receptors by the stargazin protein in some neurons, for example, depends on specific interactions between the C terminus of stargazin and the PDZ [postsynaptic density-95 (PSD-95)/Discs large/zona occludens-1] domains of membrane-associated guanylate kinase scaffolding proteins PSD-93 or PSD-95. Stargazin [Cacng2 (Ca2+ channel gamma2 subunit)] is one of four closely related proteins recently categorized as transmembrane AMPA receptor regulating proteins (TARPs) that appear to share similar functions but exhibit distinct expression patterns in the CNS. We used yeast two-hybrid screening to identify MAGI-2 (membrane associated guanylate kinase, WW and PDZ domain containing 2) as a novel candidate interactor with the cytoplasmic C termini of the TARPs. MAGI-2 [also known as S-SCAM (synaptic scaffolding molecule)] is a multi-PDZ domain scaffolding protein that interacts with several different ligands in brain, including PTEN (phosphatase and tensin homolog), dasm1 (dendrite arborization and synapse maturation 1), dendrin, axin, beta- and delta-catenin, neuroligin, hyperpolarization-activated cation channels, beta1-adrenergic receptors, and NMDA receptors. We confirmed that MAGI-2 coimmunoprecipitated with stargazin in vivo from mouse cerebral cortex and used in vitro assays to localize the interaction to the C-terminal -TTPV amino acid motif of stargazin and the PDZ1, PDZ3, and PDZ5 domains of MAGI-2. Expression of stargazin recruited MAGI-2 to cell membranes and cell-cell contact sites in transfected HEK-293T cells dependent on the presence of the stargazin -TTPV motif. These experiments identify MAGI-2 as a strong candidate for linking TARP/AMPA receptor complexes to a wide range of other postsynaptic molecules and pathways and advance our knowledge of protein interactions at mammalian CNS synapses.

Role of AMPA receptors in synaptic plasticity

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are the principal molecular units for fast excitatory synaptic transmission in the central nervous system. The glutamate-mediated transmission efficiency of synaptic AMPA receptors is influenced by their subunit composition (GluR-A to GluR-D), post-transcriptional and post-translational modifications, the number of synaptic AMPA receptors, and auxiliary proteins. Functional AMPA receptors are located predominantly in the post-synapse but are also found at extra-synaptic sites and occasionally in the pre-synapse. Thus, many factors influence the tasks of AMPA receptors in neuronal signal transmission. At hippocampal synaptic connections, AMPA receptor functions have been well studied in vitro and in the mouse; however, it is unlikely that these observations can be generalized to all glutamatergic synapses in the brain.

Different Domains of the AMPA Receptor Direct Stargazin-mediated Trafficking and Stargazin-mediated Modulation of Kinetics

Stargazin is an accessory protein of AMPA receptors that enhances surface expression and also affects the biophysical properties of the receptor. AMPA receptor domains necessary for either of these two processes have not yet been identified. Here, we used confocal imaging and electrophysiology of heterologously expressed, fluorophore-tagged GluR1, GluR2, and stargazin to study surface expression and desensitization kinetics. Stargazin-mediated trafficking was sensitive to the nature of the AMPA receptor cytoplasmic domain. The insertion of YFP after residue 15 of the truncated cytoplasmic tail of GluR1i perturbed stargazin-mediated trafficking of the receptor but not its modulation of desensitization kinetics. This construct also failed to permit fluorescence resonance energy transfer (FRET) with stargazin in the endoplasmic reticulum (ER), whereas FRET between fluorophore-tagged stargazin and non-truncated AMPA receptors demonstrated a specific interaction between these proteins, both in the ER and the plasma membrane. Rather than encoding a specific binding site, the fluorophore-tagged C terminus may restrict access to one or more ER retention sites. Although perturbations of the C terminus impeded stargazin-mediated trafficking to the plasma membrane, the effects of stargazin on the biophysical properties of AMPA receptors (i.e. modulation of desensitization) remained intact. These data provide strong evidence that the AMPA receptor domains required for stargazin modulation of gating and trafficking are separable.

AMPA and NMDA glutamate receptor trafficking: multiple roads for reaching and leaving the synapse

Glutamate receptor trafficking in and out of synapses is one of the core mechanisms for rapid changes in the number of functional receptors during synaptic plasticity. Recent data have shown that the fast gain and loss of receptors from synaptic sites are accounted for by endocytic/exocytic processes and by their lateral diffusion in the plane of the membrane. These events are interdependent and regulated by neuronal activity and interactions with scaffolding proteins. We review here the main cellular steps for AMPA and NMDA receptor synthesis, traffic within intracellular organelles, membrane exocytosis/endocytosis and surface trafficking. We focus on new findings that shed light on the regulation of receptor cycling events and surface trafficking and the way that this might reshape our thinking about the specific regulation of receptor accumulation at synapses.

Reconstitution of invertebrate glutamate receptor function depends on stargazin-like proteins

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) are a major subtype of ionotropic glutamate receptors (iGluRs) that mediate rapid excitatory synaptic transmission in the vertebrate brain. Putative AMPARs are also expressed in the nervous system of invertebrates. In Caenorhabditis elegans, the GLR-1 receptor subunit is expressed in neural circuits that mediate avoidance behaviors and is required for glutamate-gated current in the AVA and AVD interneurons. Glutamate-gated currents can be recorded from heterologous cells that express vertebrate AMPARs; however, when C. elegans GLR-1 is expressed in heterologous cells, little or no glutamate-gated current is detected. This finding suggests that other receptor subunits or auxiliary proteins are required for function. Here, we identify Ce STG-1, a C. elegans stargazin-like protein, and show that expression of Ce STG-1 together with GLR-1 and the CUB-domain protein SOL-1 reconstitutes glutamate-gated currents in Xenopus oocytes. Ce STG-1 and homologues cloned from Drosophila (Dro STG1) and Apis mellifera (Apis STG1) have evolutionarily conserved functions and can partially substitute for one another to reconstitute glutamate-gated currents from rat, Drosophila, and C. elegans. Furthermore, we show that Ce STG-1 and Apis STG1 are primarily required for function independent of possible roles in promoting the surface expression of invertebrate AMPARs.

Stargazin controls the pharmacology of AMPA receptor potentiators

Glutamate is the major excitatory neurotransmitter in brain, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) mediate the majority of postsynaptic depolarization. AMPAR ion channels display rapid gating, and their deactivation and desensitization determine the timing of synaptic transmission. AMPAR potentiators slow channel deactivation and desensitization, and these compounds represent exciting therapies for mental and neurodegenerative diseases. Previous studies showed that the AMPAR potentiators cyclothiazide and 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide display a preference for flip and flop alternatively spliced versions of glutamate receptor subunits, respectively. Here, we find that the AMPAR auxiliary subunit stargazin changes this pharmacology and makes both spliced forms of glutamate receptor subunit 1 sensitive to both classes of potentiator. Stargazin also enhances the effect of AMPAR potentiators on channel deactivation. This work demonstrates that stargazin controls AMPAR potentiator pharmacology, which has important implications for development of AMPAR potentiators as therapeutic agents.

Learning from stargazin: the mouse, the phenotype and the unexpected

The stargazin gene (also referred to as Cacng2) has been identified by forward genetics in a spontaneous mouse mutant with ataxic gait, upward head-elevating movements (hence the name stargazer for the mouse) and episodes of spike-wave discharges. Stargazin is related to the gamma-1 subunit of skeletal muscle voltage-dependent calcium channel (VDCC), and a deficit in its role as auxiliary VDCC subunit was proposed to underlie the epileptic phenotype of the mouse; yet, a conclusive demonstration of stargazin function in VDCC regulation is still lacking. In contrast, stargazin and its three closely related isoforms gamma-3, gamma-4 and gamma-8 were shown to function as auxiliary subunits for a very different ion channel - the AMPA-type glutamate receptor - prominently regulating early intracellular transport, synaptic targeting and anchoring, and ion channel functions of this major excitatory receptor in the brain.

The relationship between agonist potency and AMPA receptor kinetics

AMPA-type glutamate receptors are tetrameric ion channels that mediate fast excitatory synaptic transmission in the mammalian brain. When agonists occupy the binding domain of individual receptor subunits, this domain closes, triggering rearrangements that couple agonist binding to channel opening. Here we compare the kinetic behavior of GluR2 channels activated by four different ligands, glutamate, AMPA, quisqualate, and 2-Me-Tet-AMPA, full agonists that vary in potency by up to two orders of magnitude. After reduction of desensitization with cyclothiazide, deactivation decays were strongly agonist dependent. The time constants of decay increased with potency, and slow components in the multiexponential decays became more prominent. The desensitization decays of agonist-activated currents also contained multiple exponential components, but they were similar for the four agonists. The time course of recovery from desensitization produced by each agonist was described by two sigmoid components, and the speed of recovery varied substantially. Recovery was fastest for glutamate and slowest for 2-Me-Tet-AMPA, and the amplitude of the slow component of recovery increased with agonist potency. The multiple kinetic components appear to arise from closed-state transitions that precede channel gating. Stargazin increases the slow kinetic components, and they likely contribute to the biexponential decay of excitatory postsynaptic currents.

Building a synapse: genetic analysis of glutamatergic neurotransmission

Ionotropic glutamate receptors (iGluRs) are a critical component of the vertebrate central nervous system and mediate the majority of rapid excitatory neurotransmission. However, iGluRs are not self-regulating molecules and require additional proteins in order to function properly. Understanding the molecular architecture of functional glutamatergic synapses is therefore an important challenge in neurobiology. To address this question, we combine the techniques of genetics, molecular biology and electrophysiology in the nematode Caenorhabditis elegans. To date, genetic analysis has identified a number of genes required to build a glutamatergic synapse, including the CUB-domain transmembrane protein, SOL-1, which is thought to act as an auxiliary subunit that directly modifies iGluR function. Identifying and characterizing new proteins, such as SOL-1, in the relatively simple nervous system of the worm can contribute to our understanding of how more complex vertebrate nervous systems function.

A functional AMPA receptor-calcium channel complex in the postsynaptic membrane

Ca(2+) channels play critical roles in the regulation of synaptic activity. In contrast to the well established function of voltage-activated Ca(2+) channels in the presynaptic membrane for neurotransmitter release, some studies are just beginning to elucidate the functions of the Ca(2+) channels in the postsynaptic membrane. In this study, we demonstrated the functional association of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors with the neuronal Ca(2+) channels. A series of biochemical studies showed the specific association of Ca(v)2.1 (alpha(1A)-class) and Ca(v)2.2 (alpha(1B)-class) with AMPA receptors in the postsynaptic membrane. Our electrophysiological and Ca(2+) imaging analyses of recombinant Ca(v)2.1 and AMPA receptors also showed functional coupling of the two channels. Considering the critical roles of postsynaptic intracellular concentration of Ca(2+) ([Ca(2+)](i)) increase and AMPA receptor trafficking for long-term potentiation (LTP) and long-term depression (LTD), the functional association of Ca(2+) channels with the AMPA receptors may provide new insights into the mechanism of synaptic plasticity.

Differential localization and regulation of stargazin-like protein, gamma-8 and stargazin in the plasma membrane of hippocampal and cortical neurons

Transmembrane AMPA receptor regulatory proteins (TARPs), including stargazin/gamma-2, are associated with AMPA receptors and participate in their surface delivery and anchoring at the postsynaptic membrane. TARPs may also act as a positive modulator of the AMPA receptor ion channel function; however, little is known about other TARP members except for stargazin/gamma-2. We examined the synaptic localization of stargazin/gamma-2 and gamma-8 by immunoelectron microscopy and biochemical analysis. The analysis of sodium dodecyl sulfate-digested freeze-fracture replica labeling revealed that stargazin/gamma-2 was concentrated in the postsynaptic area, whereas gamma-8 was distributed both in synaptic and extra-synaptic plasma membranes of the hippocampal neuron. When a synaptic plasma membrane-enriched brain fraction was treated with Triton X-100 and separated by sucrose density gradient ultracentrifugation, a large proportion of NMDA receptor and stargazin/gamma-2 was accumulated in raft-enriched fractions, whereas AMPA receptor and gamma-8 were distributed in both the raft-enriched fractions and other Triton-insoluble fractions. Phosphorylation of stargazin/gamma-2 and gamma-8 was regulated by different sets of kinases and phosphatases in cultured cortical neurons. These results suggested that stargazin/gamma-2 and gamma-8 have distinct roles in postsynaptic membranes under the regulation of different intracellular signaling pathways.

Auxiliary Subunits Assist AMPA-Type Glutamate Receptors

Glutamate, the major excitatory neurotransmitter in the brain, acts primarily on two types of ionotropic receptors: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and N-methyl-d-aspartate (NMDA) receptors. Work over the past decade indicates that regulated changes in the number of synaptic AMPA receptors may serve as a mechanism for information storage. Recent studies demonstrate that a family of small transmembrane AMPA receptor regulatory proteins (TARPs) controls both AMPA receptor trafficking and channel gating. TARPs provide the first example of auxiliary subunits of ionotropic receptors. Here we review the pivotal role that TARPs play in the life cycle of AMPA receptors.

Stargazin reduces desensitization and slows deactivation of the AMPA-type glutamate receptors

The AMPA-type glutamate receptors mediate the majority of the fast excitatory synaptic transmission and critically contribute to synaptic plasticity in the brain, hence the existence of numerous trafficking proteins dedicated to regulation of their synaptic delivery and turnover. Stargazin (also termed gamma2) is a member of a recently identified protein family termed transmembrane AMPA receptor regulatory proteins (TARPs). TARPs physically associate with AMPA receptors and participate in their surface delivery and anchoring at the postsynaptic membrane. Here, we report that next to its trafficking roles, stargazin may also act as a positive allosteric modulator of AMPA receptor ion channel function. Coexpression of stargazin with AMPA receptor subunits, either in Xenopus oocytes or in human embryonic kidney 293 cells, significantly reduced receptor desensitization in response to glutamate. Receptor deactivation rates were also slowed, and the recovery from desensitization was accelerated. Structurally, based on the data showing a tight correlation between desensitization and the stability of the AMPA receptor intradimer interface, we propose that binding of stargazin may stabilize the receptor conformation. Functionally, our data suggest that AMPA receptors complexed with stargazin (and possibly also with other TARPs) at the postsynaptic membrane are significantly more responsive to synaptically released glutamate compared with AMPA receptors lacking stargazin/TARP interaction. The putative existence of such two states of synaptic AMPA receptors, with and without stargazin/TARP binding, may provide a novel mechanism for regulation of excitatory synaptic strength during development and/or in synaptic plasticity in the adult brain.

Stargazin modulates native AMPA receptor functional properties by two distinct mechanisms

AMPA receptors play a central role in basal excitatory synaptic transmission as well as synaptic maturation and plasticity. The transmembrane AMPA receptor regulatory protein (TARP) stargazin (gamma2) serves multiple roles in trafficking and stabilizing synaptic AMPA receptors and may be incorporated as an auxiliary subunit. We wanted to determine whether stargazin altered channel function of neuronal AMPA receptors. Transfection of cultured hippocampal neurons with stargazin produced two distinct effects on AMPA receptor functional properties: a sixfold reduction in glutamate-evoked desensitization and a twofold increase in the relative size of responses to the partial agonist kainate. Kinetic and dose-response analyses suggest that the effect of stargazin on glutamate desensitization results from an allosteric interaction that destabilizes the desensitized state of the receptor and that potentiation of kainate responses reflects increased efficacy rather than a change in affinity. These functional effects were also observed in human embryonic kidney 293 cells transfected with various heteromeric and homomeric AMPA receptors, with distinct subunit-dependent effects on glutamate desensitization, kainate efficacy, and trafficking. Two regions of stargazin mediate its functional effects: the C-terminal intracellular domain seems to be more important for effects on glutamate-evoked desensitization and receptor trafficking, whereas the first extracellular domain makes a larger contribution to effects on kainate efficacy. These data indicate that TARPs are involved both in trafficking and direct modulation of channel function and, as auxiliary subunits of neuronal AMPA receptors, must be considered in the functional heterogeneity of neuronal AMPA receptors.

SOL-1 is an auxiliary subunit that modulates the gating of GLR-1 glutamate receptors in Caenorhabditis elegans

Most rapid excitatory synaptic signaling in the brain is mediated by postsynaptic ionotropic glutamate receptors (iGluRs) that are gated open by the neurotransmitter glutamate. In Caenorhabditis elegans, sol-1 encodes a CUB-domain transmembrane protein that is required for currents that are mediated by the GLR-1 iGluR. Mutations in sol-1 do not affect GLR-1 expression, localization, membrane insertion, or stabilization at synapses, suggesting that SOL-1 is required for iGluR function. Here, we provide evidence that SOL-1 is an auxiliary subunit that modulates the gating of GLR-1 receptors. We show that mutant variants of GLR-1 with altered gating partially restore glutamate-gated current and GLR-1-dependent behaviors in sol-1 mutants. Domain analysis of SOL-1 indicates that extracellular CUB domain 3 is required for function and that a secreted variant partially restores glutamate-gated currents and behavior. Also, we show that endogenous glutamatergic synaptic currents are absent in sol-1 mutants. Our data suggest that GLR-1 iGluRs are not simply stand-alone molecules and require the SOL-1 auxiliary protein to promote the open state of the receptor. Our analysis presents the possibility that glutamatergic signaling in other organisms may be similarly modified by SOL-1-like transmembrane proteins.

Three-dimensional structure of an AMPA receptor without associated stargazin/TARP proteins

Most excitatory synaptic transmissions in the central nervous system are mediated by the neurotransmitter glutamate. Binding of glutamate released from the presynaptic membrane causes glutamate receptors in the postsynaptic membrane to open, which results in a transient depolarization of the postsynaptic membrane. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) subtype of glutamate receptors is responsible for the majority of excitatory postsynaptic currents and is thought to play a central role in synaptic plasticity. Because modulation of glutamate receptors is believed to be involved in the basic mechanism underlying information storage in the brain, the molecular architecture of native AMPA receptors (AMPA-Rs) is of great interest. Previously, we have shown that AMPA-Rs purified from the brain are tightly associated with members of the stargazin/TARP (transmembrane AMPA receptor regulatory protein) family of membrane proteins [Nakagawa et al., Nature 433 (2005), pp. 545-549]. Here, we present a three-dimensional (3D) density map of the hetero-tetrameric AMPA-R without associated stargazin/TARP proteins as determined by cryo-negative stain single-particle electron microscopy. In the absence of stargazin/TARP proteins, the density representing the transmembrane region of the AMPA-R particles is substantially smaller, corroborating our previous analysis that was based solely on projection images.

Interaction between SAP97 and PSD-95, two Maguk proteins involved in synaptic trafficking of AMPA receptors

Synapse-associated protein 97 (SAP97) and postsynaptic density 95 (PSD-95) are closely related membrane-associated guanylate kinase homologs (Maguks) implicated in the synaptic targeting and anchoring of alpha-amino-5-methyl-3-hydroxy-4-isoxazolepropionic acid (AMPA)-selective glutamate receptors. Prompted by accumulating evidence for an oligomeric nature of Maguks, we examined the potential of SAP97 and PSD-95 to form heteromeric complexes. SAP97 and PSD-95 coimmunoprecipitated from rat brain detergent extracts and subsequent glutathione S-transferase pull-down and immunoprecipitation experiments showed that the interaction is mediated by binding of the N-terminal segment of SAP97 (SAP97(NTD)) to the Src homology 3 domain of PSD-95 (PSD-95(SH3)). In cultured hippocampal neurons, expression of green fluorescent protein-tagged PSD-95 triggered accumulation of SAP97 in synaptic spines, which was totally inhibited by coexpression of PSD-95(SH3). Furthermore, overexpression of green fluorescent protein-PSD-95 induced dendritic clustering of GluR-A subunit-containing AMPA receptors, which was strongly inhibited by cotransfection with SAP97(NTD) and PSD-95(SH3) constructs. Our results demonstrated a direct interaction between SAP97 and PSD-95 and suggested that this association may play a functional role in the trafficking and clustering of AMPA receptors.

Neurotransmitter release regulated by a MALS-liprin-alpha presynaptic complex

Synapses are highly specialized intercellular junctions organized by adhesive and scaffolding molecules that align presynaptic vesicular release with postsynaptic neurotransmitter receptors. The MALS/Veli–CASK–Mint-1 complex of PDZ proteins occurs on both sides of the synapse and has the potential to link transsynaptic adhesion molecules to the cytoskeleton. In this study, we purified the MALS protein complex from brain and found liprin-α as a major component. Liprin proteins organize the presynaptic active zone and regulate neurotransmitter release. Fittingly, mutant mice lacking all three MALS isoforms died perinatally with difficulty breathing and impaired excitatory synaptic transmission. Excitatory postsynaptic currents were dramatically reduced in autaptic cultures from MALS triple knockout mice due to a presynaptic deficit in vesicle cycling. These findings are consistent with a model whereby the MALS–CASK–liprin-α complex recruits components of the synaptic release machinery to adhesive proteins of the active zone.

Spatial diversity in gene expression for VDCCγ subunit family in developing and adult mouse brains

The gamma subunit of voltage-dependent Ca2+ channels (VDCCs) is characterized by molecular diversity and regulation of AMPA-type glutamate receptors as well as VDCCs. In the present study, we examined expressions for the VDCCgamma1-8 subunit mRNAs in developing and adult mouse brains by in situ hybridization. In adult brains, the gamma2 and gamma7 subunit mRNAs were widely expressed in various grey matter regions with the highest level in cerebellar Purkinje cells and granule cells. The gamma3 and gamma8 subunit mRNAs predominated in the telencephalon, with the latter being at striking levels in the hippocampus. The gamma4 subunit mRNA was enriched in the olfactory bulb, striatum, thalamus and hypothalamus. The gamma5 subunit mRNA was abundant in the olfactory bulb, hippocampal CA2, thalamus, inferior colliculus and Bergmann glia. Transcripts of these subunits were detected in embryonic brains: some showed well-preserved spatial patterns (gamma2, gamma5, gamma7 and gamma8), while others underwent developmental up- (gamma3) or down-regulation (gamma4). In contrast, the gamma1 and gamma6 subunit mRNAs were negative or very low throughout brain development. Therefore, the present study has revealed spatial diversity in gene expression for individual VDCCgamma subunits, presumably reflecting functional diversity of this protein family and their differential involvement in neural function.

TARP γ-8 controls hippocampal AMPA receptor number, distribution and synaptic plasticity

Synaptic plasticity involves activity-dependent trafficking of AMPA-type glutamate receptors. Numerous cytoplasmic scaffolding proteins are postulated to control AMPA receptor trafficking, but the detailed mechanisms remain unclear. Here, we show that the transmembrane AMPA receptor regulatory protein (TARP) gamma-8, which is preferentially expressed in the mouse hippocampus, is important for AMPA receptor protein levels and extrasynaptic surface expression. By controlling the number of AMPA receptors, gamma-8 is also important in long-term potentiation, but not long-term depression. This study establishes gamma-8 as a critical protein for basal AMPA receptor expression and localization at extrasynaptic sites in the hippocampus and raises the possibility that TARP-dependent control of AMPA receptors during synapse development and plasticity may be widespread.

Rapid activity-driven SNARE-dependent trafficking of nicotinic receptors on somatic spines

Rapid trafficking of glutamate receptors contributes importantly to synaptic plasticity, but whether similar trafficking extends to other ionotropic receptors is unknown. Nicotinic acetylcholine receptors containing alpha7 subunits are widely expressed in the nervous system and allow calcium influx. Because of this, alpha7-containing receptors regulate diverse events, depending on the signaling pathways available. We find that the receptors codistribute with target soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) postsynaptically and that nicotinic stimulation rapidly induces SNARE-dependent vesicular endocytosis accompanied by receptor internalization. At the same time, a SNARE-dependent process recruits receptors to the cell surface from internal pools. Overall, the trafficking does not markedly change the number of surface receptors or their combined whole-cell response to nicotine. SNARE-dependent trafficking is needed, however, for the receptors to remain capable of activating the transcription factor cAMP response element-binding protein and attendant gene expression when repeatedly challenged. Thus, trafficking appears to be essential for maintaining functional coupling between alpha7-receptor responses and downstream signaling.

Interaction with the unfolded protein response reveals a role for stargazin in biosynthetic AMPA receptor transport

The transmembrane protein stargazin enhances levels of functional AMPA receptors at the neuronal plasma membrane and at synapses. To clarify the mechanism for this effect, we studied trafficking of the AMPA receptor subunit glutamate receptor 1 (GluR1) in transfected COS7 cells. GluR1 expressed poorly on the surface of these cells and was primarily retained in the endoplasmic reticulum (ER). Stargazin expression strongly increased the surface fraction of GluR1. This effect was not reduced by a dominant-negative dynamin mutant, suggesting that stargazin does not inhibit AMPA receptor endocytosis. Interestingly, upregulation of ER chaperones as part of the unfolded protein response (UPR) both mimicked and occluded the effect of stargazin, suggesting a role for stargazin in ER processing of AMPA receptors. Consistent with this idea, we detected UPR induction in cerebellar granule cells lacking stargazin. Finally, residual AMPA receptor currents in stargazin-deficient neurons were suppressed by inhibition of the UPR. These findings uncover a role for stargazin in AMPA receptor trafficking through the early compartments of the biosynthetic pathway. Furthermore, they provide evidence for modulation of AMPA receptor trafficking by the UPR.

Differential Regulation of AMPA Receptor Subunit Trafficking by Palmitoylation of Two Distinct Sites

Modification of AMPA receptor function is a major mechanism for the regulation of synaptic transmission and underlies several forms of synaptic plasticity. Post-translational palmitoylation is a reversible modification that regulates localization of many proteins. Here, we report that palmitoylation of the AMPA receptor regulates receptor trafficking. All AMPA receptor subunits are palmitoylated on two cysteine residues in their transmembrane domain (TMD) 2 and in their C-terminal region. Palmitoylation on TMD 2 is upregulated by the palmitoyl acyl transferase GODZ and leads to an accumulation of the receptor in the Golgi and a reduction of receptor surface expression. C-terminal palmitoylation decreases interaction of the AMPA receptor with the 4.1N protein and regulates AMPA- and NMDA-induced AMPA receptor internalization. Moreover, depalmitoylation of the receptor is regulated by activation of glutamate receptors. These data suggest that regulated palmitoylation of AMPA receptor subunits modulates receptor trafficking and may be important for synaptic plasticity.

Molecular constituents of neuronal AMPA receptors

Dynamic regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) underlies aspects of synaptic plasticity. Although numerous AMPAR-interacting proteins have been identified, their quantitative and relative contributions to native AMPAR complexes remain unclear. Here, we quantitated protein interactions with neuronal AMPARs by immunoprecipitation from brain extracts. We found that stargazin-like transmembrane AMPAR regulatory proteins (TARPs) copurified with neuronal AMPARs, but we found negligible binding to GRIP, PICK1, NSF, or SAP-97. To facilitate purification of neuronal AMPAR complexes, we generated a transgenic mouse expressing an epitope-tagged GluR2 subunit of AMPARs. Taking advantage of this powerful new tool, we isolated two populations of GluR2 containing AMPARs: an immature complex with the endoplasmic reticulum chaperone immunoglobulin-binding protein and a mature complex containing GluR1, TARPs, and PSD-95. These studies establish TARPs as the auxiliary components of neuronal AMPARs.

Glutamate receptor ion channels

Glutamate receptor ion channels mediate excitatory responses at the majority of CNS synapses. They are the only ligand-gated ion channels for which multiple high-resolution crystal structures have been solved. Highlights of information gained from mechanistic studies based on the crystal structures of their ligand-binding domains include explanations for strikingly diverse phenomena. These include the basis for subtype-specific agonist selectivity; mechanisms for desensitization and allosteric modulation; and mechanisms for partial agonist activity. In addition, multiple lines of evidence, including low-resolution electron microscopic studies, suggest that native AMPA receptors combine with an auxiliary subunit which regulates activity and trafficking. Functional studies suggest that glutamate receptor gating is distinct from that of structurally related voltage-gated ion channels.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Postsynaptic scaffold proteins at non-synaptic sites. The role of postsynaptic scaffold proteins in motor-protein-receptor complexes

Synapse-associated proteins that are located at the postsynaptic density (PSD) have recently been shown to have a structural role at non-synaptic locations. Here, they act as adaptor proteins between neurotransmitter receptors and the microtubule- or microfilament-based motor-protein complexes that are responsible for transport to the PSD. The use of a common set of proteins that contain multiple domains for protein-protein interactions as both intracellular transport adaptors and synaptic scaffold proteins might contribute to the transport specificity and postsynaptic integration of receptors that underlie synapse formation and plasticity.

The alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptor trafficking regulator "stargazin" is related to the claudin family of proteins by Its ability to mediate cell-cell adhesion

Mutations in the Cacng2 gene encoding the neuronal transmembrane protein stargazin result in recessively inherited epilepsy and ataxia in "stargazer" mice. Functional studies suggest a dual role for stargazin, both as a modulatory gamma subunit for voltage-dependent calcium channels and as a regulator of post-synaptic membrane targeting for alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors. Co-immunoprecipitation experiments demonstrate that stargazin can bind proteins of either complex in vivo, but it remains unclear whether it can associate with both complexes simultaneously. Cacng2 is one of eight closely related genes (Cacng1-8) encoding proteins with four transmembrane segments, cytoplasmic termini, and molecular masses between 25 and 44 kDa. This group of Cacng genes constitutes only one branch of a larger monophyletic assembly dominated by over 20 genes encoding proteins known as claudins. Claudins regulate cell adhesion and paracellular permeability as fundamental components of non-neuronal tight junctions. Because stargazin is structurally similar to claudins, we hypothesized that it might also have retained claudin-like functions inherited from a common ancestor. Here, we report that expression of stargazin in mouse L-fibroblasts results in cell aggregation comparable with that produced by claudins, and present evidence that the interaction is heterotypic and calcium dependent. The data suggest that the cell adhesion function of stargazin preceded its current role in neurons as a regulator of either voltage-dependent calcium channels or AMPA receptors. We speculate these complexes may have co-opted the established presence of stargazin at sites of close cell-cell contact to facilitate their own evolving intercellular signaling functions.

The expanding role of PSD-95: a new link to addiction

One feature of addiction is the enhanced locomotor response known as behavioral sensitization that occurs with prolonged exposure to psychostimulants. In a recent study, Yao et al. have analyzed four distinct animal models that share this common phenotype. Evaluation of gene expression in the striatum reveals that postsynaptic density (PSD)-95, a synaptic scaffolding protein, is downregulated in each model, suggesting an essential role for this protein in addiction.

Stargazin modulates AMPA receptor gating and trafficking by distinct domains

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors mediate fast excitatory synaptic transmission in the brain. These ion channels rapidly deactivate and desensitize, which determine the time course of synaptic transmission. Here, we find that the AMPA receptor interacting protein, stargazin, not only mediates AMPA receptor trafficking but also shapes synaptic responses by slowing channel deactivation and desensitization. The cytoplasmic tail of stargazin determines receptor trafficking, whereas the ectodomain controls channel properties. Stargazin alters AMPA receptor kinetics by increasing the rate of channel opening. Disrupting the interaction of stargazin ectodomain with hippocampal AMPA receptors alters the amplitude and shape of synaptic responses, establishing a crucial function for stargazin in controlling the efficacy of synaptic transmission in the brain.

Synaptic mechanisms of the cerebellar cortical network

The cerebellar cortex forms a defined array of relatively simple neural networks. Considerable progress has been made towards understanding the synaptic mechanisms of these networks. It has been revealed that inhibitory GABA receptors and distinct types of glutamate receptor cooperatively and differentially regulate and integrate glutamate-mediated transmission within cerebellar cortical circuits. In addition, molecular assembly of receptor-interacting scaffolding proteins and signaling molecules is dynamically controlled and underlies the functional synaptic organization of cerebellar cortical circuits. This article discusses some of the recent progress in this area, focusing on newly explored aspects of the synaptic mechanisms of glutamate-mediated transmission in the cerebellar cortical network.

Bidirectional synaptic plasticity regulated by phosphorylation of stargazin-like TARPs

Synaptic plasticity involves protein phosphorylation cascades that alter the density of AMPA-type glutamate receptors at excitatory synapses; however, the crucial phosphorylated substrates remain uncertain. Here, we show that the AMPA receptor-associated protein stargazin is quantitatively phosphorylated and that stargazin phosphorylation promotes synaptic trafficking of AMPA receptors. Synaptic NMDA receptor activity can induce both stargazin phosphorylation, via activation of CaMKII and PKC, and stargazin dephosphorylation, by activation of PP1 downstream of PP2B. At hippocampal synapses, long-term potentiation and long-term depression require stargazin phosphorylation and dephosphorylation, respectively. These results establish stargazin as a critical substrate in the bidirectional control of synaptic strength, which is thought to underlie aspects of learning and memory.

AMPA receptor synaptic targeting regulated by stargazin interactions with the Golgi-resident PDZ protein nPIST

Regulation of AMPA receptors (AMPARs) at synapses plays a critical role in alterations of synaptic strength in the brain. Stargazin, an AMPAR-interacting protein, is critical for clustering and regulation of synaptic AMPARs. Stargazin interacts with AMPARs via its extracellular domain and with PDZ [postsynaptic density-95 (PSD-95)/Discs large (Dlg)/zona occludens-1 (ZO-1)] proteins via its C-terminal PDZ-binding motif, and these interactions are necessary for stargazin and AMPAR synaptic targeting. By studying the expression of stargazin mutant constructs in cultured hippocampal neurons, we identified a novel domain corresponding to residues 243-283 within the cytoplasmic C terminus of stargazin that is also required for stargazin and AMPAR synaptic clustering. To identify proteins that interact with this stargazin synaptic clustering domain, we performed a yeast two-hybrid assay and found that this stargazin domain binds to nPIST (neuronal isoform of protein-interacting specifically with TC10), a Golgi-enriched protein implicated in trafficking of transmembrane proteins. Using in situ hybridization, immunohistochemistry, coimmunoprecipitation studies, and biochemical fractionation, we found that stargazin and nPIST colocalize and interact in the brain. Finally, by studying AMPAR clustering in transfected hippocampal neurons, we found that overexpression of nPIST enhances AMPAR synaptic clustering, whereas transfection of a dominant-negative nPIST construct attenuates AMPAR synaptic clustering. These studies identify a novel stargazin domain necessary for synaptic clustering of AMPARs and suggest that nPIST and stargazin interactions play a critical role in AMPAR trafficking to the synapse.

A targeted mutation in Cacng4 exacerbates spike-wave seizures in stargazer (Cacng2) mice

The voltage-dependent calcium channel gamma4 subunit protein, CACNG4, is closely related to the gamma2 subunit, CACNG2. Both are expressed primarily in the brain and share 53% amino acid identity. The Cacng2 gene is disrupted in the stargazer mouse, with its distinctive phenotype including ataxia, frequent absence seizure episodes, and head elevation. A disruption within Cacng4 was engineered to assess its particular function. The homozygous Cacng4-targeted mutant mouse appeared normal with no ataxic gait or absence seizures, suggesting that other members of the gamma subunit family might functionally compensate for the absence of CACNG4. To test this hypothesis, the targeted Cacng4 mutation was combined with alleles of Cacng2. Absence seizures were observed in combination with the stargazer 3J mutation, which itself does not have seizures, and increased seizure activity was observed in combination with the waggler allele. Furthermore, within the corticothalamic loop, where absence seizures arise, CACNG4 expression is restricted to the thalamus. Our studies show that the CACNG4 protein has seizure suppressing activity, but this effect is revealed only when CACNG2 expression is also compromised, suggesting that CACNG subunits have in vivo overlapping functions.

Quaternary structure, protein dynamics, and synaptic function of SAP97 controlled by L27 domain interactions

Single-particle electron microscopy (EM) combined with biochemical measurements revealed the molecular shape of SAP97 and a monomer-dimer transition that depended on the N-terminal L27 domain. Overexpression of SAP97 drove GluR1 to synapses, potentiated AMPA receptor (AMPAR) excitatory postsynaptic currents (EPSCs), and occluded LTP. Synaptic potentiation and GluR1 delivery were dissociable by L27 domain mutants that inhibit multimerization of SAP97. Loss of potentiation was correlated with faster turnover of monomeric SAP97 mutants in dendritic spines. We propose that L27-mediated interactions of SAP97 with itself or other proteins regulate the synaptic delivery of AMPARs. RNAi knockdown of endogenous PSD-95 depleted surface GluR1 and impaired AMPA EPSCs. In contrast, RNAi knockdown of endogenous SAP97 reduced surface expression of both GluR1 and GluR2 and inhibited both AMPA and NMDA EPSCs. Thus SAP97 has a broader role than its close relative, PSD-95, in the maintenance of synaptic function.