AMPA Receptor Subunit-Specific Regulation by a Distinct Family of Type II TARPs

Structural dynamics in α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor gating

The ionotropic glutamate receptors (iGluRs) are comprised of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA), N-methyl-d-aspartate receptor, kainate, and delta subtypes and are pivotal in neuronal plasticity. Recent structural studies on AMPA receptors reveal intricate conformational changes during activation and desensitization elucidating the steps from agonist binding to channel opening and desensitization. Additionally, interactions with auxiliary subunits, including transmembrane AMPA-receptor regulatory proteins, germ-cell-specific gene 1-like protein, and cornichon homologs, intricately modulate AMPA receptors. We discuss the recent high-resolution structures of these complexes that unveil stoichiometry, subunit positioning, and differences in specific side-chain interactions that influence these functional modulations.

Dynamic dysregulation of transcriptomic networks in brainstem autonomic nuclei during hypertension development in the female spontaneously hypertensive rat

Neurogenic hypertension stems from an imbalance in autonomic function that shifts the central cardiovascular control circuits toward a state of dysfunction. Using the female spontaneously hypertensive rat and the normotensive Wistar-Kyoto rat model, we compared the transcriptomic changes in three autonomic nuclei in the brainstem, nucleus of the solitary tract (NTS), caudal ventrolateral medulla, and rostral ventrolateral medulla (RVLM) in a time series at 8, 10, 12, 16, and 24 wk of age, spanning the prehypertensive stage through extended chronic hypertension. RNA-sequencing data were analyzed using an unbiased, dynamic pattern-based approach that uncovered dominant and several subtle differential gene regulatory signatures. Our results showed a persistent dysregulation across all three autonomic nuclei regardless of the stage of hypertension development as well as a cascade of transient dysregulation beginning in the RVLM at the prehypertensive stage that shifts toward the NTS at the hypertension onset. Genes that were persistently dysregulated were heavily enriched for immunological processes such as antigen processing and presentation, the adaptive immune response, and the complement system. Genes with transient dysregulation were also largely region-specific and were annotated for processes that influence neuronal excitability such as synaptic vesicle release, neurotransmitter transport, and an array of neuropeptides and ion channels. Our results demonstrate that neurogenic hypertension is characterized by brainstem region-specific transcriptomic changes that are highly dynamic with significant gene regulatory changes occurring at the hypertension onset as a key time window for dysregulation of homeostatic processes across the autonomic control circuits.NEW & NOTEWORTHY Hypertension is a major disease and is the primary risk factor for cardiovascular complications and stroke. The gene expression changes in the central nervous system circuits driving hypertension are understudied. Here, we show that coordinated and region-specific gene expression changes occur in the brainstem autonomic circuits over time during the development of a high blood pressure phenotype in a rat model of human essential hypertension.

Enhanced TARP-γ8-PSD-95 coupling in excitatory neurons contributes to the rapid antidepressant-like action of ketamine in male mice

Ketamine produces rapid antidepressant effects at sub-anesthetic dosage through early and sustained activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), however, the exact molecular mechanism still remains unclear. Transmembrane AMPAR regulatory protein-γ8 (TARP-γ8) is identified as one of AMPAR auxiliary subunits, which controls assemblies, surface trafficking and gating of AMPARs. Here, we show that ketamine rescues both depressive-like behaviors and the decreased AMPARs-mediated neurotransmission by recruitment of TARP-γ8 at the postsynaptic sites in the ventral hippocampus of stressed male mice. Furthermore, the rapid antidepressant effects of ketamine are abolished by selective blockade of TARP-γ8-containing AMPAR or uncoupling of TARP-γ8 from PSD-95. Overexpression of TARP-γ8 reverses chronic stress-induced depressive-like behaviors and attenuation of AMPARs-mediated neurotransmission. Conversely, knockdown of TARP-γ8 in excitatory neurons prevents the rapid antidepressant effects of ketamine.

GSG1L-containing AMPA receptor complexes are defined by their spatiotemporal expression, native interactome and allosteric sites

Transmembrane AMPA receptor regulatory proteins (TARPs) and germ cell-specific gene 1-like protein (GSG1L) are claudin-type AMPA receptor (AMPAR) auxiliary subunits that profoundly regulate glutamatergic synapse strength and plasticity. While AMPAR-TARP complexes have been extensively studied, less is known about GSG1L-containing AMPARs. Here, we show that GSG1L's spatiotemporal expression, native interactome and allosteric sites are distinct. GSG1L generally expresses late during brain development in a region-specific manner, constituting about 5% of all AMPAR complexes in adulthood. While GSG1L can co-assemble with TARPs or cornichons (CNIHs), it also assembles as the sole auxiliary subunit. Unexpectedly, GSG1L acts through two discrete evolutionarily-conserved sites on the agonist-binding domain with a weak allosteric interaction at the TARP/KGK site to slow desensitization, and a stronger interaction at a different site that slows recovery from desensitization. Together, these distinctions help explain GSG1L's evolutionary past and how it fulfills a unique signaling role within glutamatergic synapses.

Modulation of GluA2-γ5 synaptic complex desensitization, polyamine block and antiepileptic perampanel inhibition by auxiliary subunit cornichon-2

Synaptic complexes of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs) with auxiliary subunits mediate most excitatory neurotransmission and can be targeted to treat neuropsychiatric and neurological disorders, including epilepsy. Here we present cryogenic-electron microscopy structures of rat GluA2 AMPAR complexes with inhibitory mouse γ5 and potentiating human cornichon-2 (CNIH2) auxiliary subunits. CNIH2 appears to destabilize the desensitized state of the complex by reducing the separation of the upper lobes in ligand-binding domain dimers. At the same time, CNIH2 stabilizes binding of polyamine spermidine to the selectivity filter of the closed ion channel. Nevertheless, CNIH2, and to a lesser extent γ5, attenuate polyamine block of the open channel and reduce the potency of the antiepileptic drug perampanel that inhibits the synaptic complex allosterically by binding to sites in the ion channel extracellular collar. These findings illustrate the fine-tuning of synaptic complex structure and function in an auxiliary subunit-dependent manner, which is critical for the study of brain region-specific neurotransmission and design of therapeutics for disease treatment.

GluR2Q and GluR2R AMPA Subunits are not Targets of lypd2 Interaction

A large family of prototoxin-like molecules endogenous to mammals, Ly6 proteins have been implicated in the regulation of cell signaling processes across multiple species. Previous work has shown that certain members of the Ly6 family are expressed in the brain and target nicotinic acetylcholine receptor and potassium channel function. Structural similarities between Ly6 proteins and alpha-neurotoxins suggest the possibility of additional ionotropic receptor targets. Here, we investigated the possibility of lypd2 as a novel regulator of AMPA receptor (AMPAR) function. In particular, we focused on potential interactions with the Q/R isoforms of the GluR2 subunit, which have profound impacts on AMPAR permeability to calcium during neuronal stimulation. We find that although lypd2 and GluR2 share overlapping expression patterns in the mouse hippocampus, there was no interaction between lypd2 and either GluR2Q or GluR2R isoform. These results underscore the importance of continuing to investigate novel targets for Ly6 interaction and regulation.

Expression and Development of TARP γ-4 in Embryonic Zebrafish

Fast excitatory synaptic transmission in the CNS is mediated by the neurotransmitter glutamate, binding to and activating AMPA receptors (AMPARs). AMPARs are known to interact with auxiliary proteins that modulate their behavior. One such family of proteins is the transmembrane AMPAR-related proteins, known as TARPs. Little is known about the role of TARPs during development or about their function in nonmammalian organisms. Here, we report on the presence of TARP γ-4 in developing zebrafish. We find that zebrafish express 2 forms of TARP γ-4: γ-4a and γ-4b as early as 12 h post-fertilization. Sequence analysis shows that both γ-4a and γ-4b shows great level of variation particularly in the intracellular C-terminal domain compared to rat, mouse, and human γ-4. RT-qPCR showed a gradual increase in the expression of γ-4a throughout the first 5 days of development, whereas γ-4b levels were constant until day 5 when levels increased significantly. Knockdown of TARP γ-4a and γ-4b via either splice-blocking morpholinos or translation-blocking morpholinos resulted in embryos that exhibited deficits in C-start escape responses, showing reduced C-bend angles. Morphant larvae displayed reduced bouts of swimming. Whole-cell patch-clamp recordings of AMPAR-mediated currents from Mauthner cells showed a reduction in the frequency of mEPCs but no change in amplitude or kinetics. Together, these results suggest that γ-4a and γ-4b are required for proper neuronal development.

Heterogeneity of Early Host Response to Infection with Four Low-Pathogenic H7 Viruses with a Different Evolutionary History in the Field

Once low-pathogenic avian influenza viruses (LPAIVs) of the H5 and H7 subtypes from wild birds enter into poultry species, there is the possibility of them mutating into highly pathogenic avian influenza viruses (HPAIVs), resulting in severe epizootics with up to 100% mortality. This mutation from a LPAIV to HPAIV strain is the main cause of an AIV’s major economic impact on poultry production. Although AIVs are inextricably linked to their hosts in their evolutionary history, the contribution of host-related factors in the emergence of HPAI viruses has only been marginally explored so far. In this study, transcriptomic sequencing of tracheal tissue from chickens infected with four distinct LP H7 viruses, characterized by a different history of pathogenicity evolution in the field, was implemented. Despite the inoculation of a normalized infectious dose of viruses belonging to the same subtype (H7) and pathotype (LPAI), the use of animals of the same age, sex and species as well as the identification of a comparable viral load in the target samples, the analyses revealed a heterogeneity in the gene expression profile in response to infection with each of the H7 viruses administered.

Structure and desensitization of AMPA receptor complexes with type II TARP γ5 and GSG1L

AMPA receptors (AMPARs) mediate the majority of excitatory neurotransmission. Their surface expression, trafficking, gating, and pharmacology are regulated by auxiliary subunits. Of the two types of TARP auxiliary subunits, type I TARPs assume activating roles, while type II TARPs serve suppressive functions. We present cryo-EM structures of GluA2 AMPAR in complex with type II TARP γ5, which reduces steady-state currents, increases single-channel conductance, and slows recovery from desensitization. Regulation of AMPAR function depends on its ligand-binding domain (LBD) interaction with the γ5 head domain. GluA2-γ5 complex shows maximum stoichiometry of two TARPs per AMPAR tetramer, being different from type I TARPs but reminiscent of the auxiliary subunit GSG1L. Desensitization of both GluA2-GSG1L and GluA2-γ5 complexes is accompanied by rupture of LBD dimer interface, while GluA2-γ5 but not GluA2-GSG1L LBD dimers remain two-fold symmetric. Different structural architectures and desensitization mechanisms of complexes with auxiliary subunits endow AMPARs with broad functional capabilities.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Role of AMPA receptor desensitization in short term depression - lessons from retinogeniculate synapses

Repetitive synapse activity induces various forms of short-term plasticity. The role of presynaptic mechanisms such as residual Ca²⁺ and vesicle depletion in short-term facilitation and short-term depression is well established. On the other hand, the contribution of postsynaptic mechanisms such as receptor desensitization and saturation to short-term plasticity is less well known and often ignored. In this review, I will describe short-term plasticity in retinogeniculate synapses of relay neurons of the dorsal lateral geniculate nucleus (dLGN) to exemplify the synaptic properties that facilitate the contribution of AMPA receptor desensitization to short-term plasticity. These include high vesicle release probability, glutamate spillover and, importantly, slow recovery from desensitization of AMPA receptors. The latter is strongly regulated by the interaction of AMPA receptors with auxiliary proteins such as CKAMP44. Finally, I discuss the relevance of short-term plasticity in retinogeniculate synapses for the processing of visual information by LGN relay neurons.

Auxiliary Subunits Control Function and Subcellular Distribution of AMPA Receptor Complexes in NG2 Glia of the Developing Hippocampus

Synaptic and axonal glutamatergic signaling to NG2 glia in white matter is critical for the cells' differentiation and activity dependent myelination. However, in gray matter the impact of neuron-to-NG2 glia signaling is still elusive, because most of these cells keep their non-myelinating phenotype throughout live. Early in postnatal development, hippocampal NG2 glia express AMPA receptors with a significant Ca²⁺ permeability allowing for plasticity of the neuron-glia synapses, but whether this property changes by adulthood is not known. Moreover, it is unclear whether NG2 glia express auxiliary transmembrane AMPA receptor related proteins (TARPs), which modify AMPA receptor properties, including their Ca²⁺ permeability. Through combined molecular and functional analyses, here we show that hippocampal NG2 glia abundantly express TARPs γ4, γ7, and γ8 as well as cornichon (CNIH)-2. TARP γ8 undergoes profound downregulation during development. Receptors of adult NG2 glia showed an increased sensitivity to blockers of Ca²⁺ permeable AMPA receptors, but this increase mainly concerned receptors located close to the soma. Evoked synaptic currents of NG2 glia were also sensitive to blockers of Ca²⁺ permeable AMPA receptors. The presence of AMPA receptors with varying Ca²⁺ permeability during postnatal maturation may be important for the cells' ability to sense and respond to local glutamatergic activity and for regulating process motility, differentiation, and proliferation.

Transcriptomic expression of AMPA receptor subunits and their auxiliary proteins in the human brain

Receptors to glutamate of the AMPA type (AMPARs) serve as the major gates of excitation in the human brain, where they participate in fundamental processes underlying perception, cognition and movement. Due to their central role in brain function, dysregulation of these receptors has been implicated in neuropathological states associated with a large variety of diseases that manifest with abnormal behaviors. The participation of functional abnormalities of AMPARs in brain disorders is strongly supported by genomic, transcriptomic and proteomic studies. Most of these studies have focused on the expression and function of the subunits that make up the channel and define AMPARs (GRIA1-GRIA4), as well of some accessory proteins. However, it is increasingly evident that native AMPARs are composed of a complex array of accessory proteins that regulate their trafficking, localization, kinetics and pharmacology, and a better understanding of the diversity and regional expression of these accessory proteins is largely needed. In this review we will provide an update on the state of current knowledge of AMPA receptors subunits in the context of their accessory proteins at the transcriptome level. We also summarize the regional expression in the human brain and its correlation with the channel forming subunits. Finally, we discuss some of the current limitations of transcriptomic analysis and propose potential ways to overcome them.

Auxiliary subunits of the AMPA receptor: The Shisa family of proteins

AMPA receptors mediate fast synaptic transmission in the CNS and can assemble with several types of auxiliary proteins in a spatio-temporal manner, from newly synthesized AMPA receptor tetramers to mature AMPA receptors in the cell membrane. As such, the interaction of auxiliary subunits with the AMPA receptor plays a major role in the regulation of AMPA receptor biogenesis, trafficking, and biophysical properties. Throughout the years, various 'families' of proteins have been identified and today the approximate full complement of AMPAR auxiliary proteins is known. This review presents the current knowledge on the most prominent AMPA-receptor-interacting auxiliary proteins, highlights recent results regarding the Shisa protein family, and provides a discussion on future research that might contribute to the discovery of novel pharmacological targets of auxiliary subunits.

Targeting receptor complexes: a new dimension in drug discovery

Targeting receptor proteins, such as ligand-gated ion channels and G protein-coupled receptors, has directly enabled the discovery of most drugs developed to modulate receptor signalling. However, as the search for novel and improved drugs continues, an innovative approach - targeting receptor complexes - is emerging. Receptor complexes are composed of core receptor proteins and receptor-associated proteins, which have profound effects on the overall receptor structure, function and localization. Hence, targeting key protein-protein interactions within receptor complexes provides an opportunity to develop more selective drugs with fewer side effects. In this Review, we discuss our current understanding of ligand-gated ion channel and G protein-coupled receptor complexes and discuss strategies for their pharmacological modulation. Although such strategies are still in preclinical development for most receptor complexes, they exemplify how receptor complexes can be drugged, and lay the groundwork for this nascent area of research.

Glutamatergic Receptor Trafficking and Delivery: Role of the Exocyst Complex

Cells comprise several intracellular membrane compartments that allow them to function properly. One of these functions is cargo movement, typically proteins and membranes within cells. These cargoes ride microtubules through vesicles from Golgi and recycling endosomes to the plasma membrane in order to be delivered and exocytosed. In neurons, synaptic functions employ this cargo trafficking to maintain inter-neuronal communication optimally. One of the complexes that oversee vesicle trafficking and tethering is the exocyst. The exocyst is a protein complex containing eight subunits first identified in yeast and then characterized in multicellular organisms. This complex is related to several cellular processes, including cellular growth, division, migration, and morphogenesis, among others. It has been associated with glutamatergic receptor trafficking and tethering into the synapse, providing the molecular machinery to deliver receptor-containing vesicles into the plasma membrane in a constitutive manner. In this review, we discuss the evidence so far published regarding receptor trafficking and the exocyst complex in both basal and stimulated levels, comparing constitutive trafficking and long-term potentiation-related trafficking.

AMPA receptor auxiliary subunits emerged during early vertebrate evolution by neo/subfunctionalization of unrelated proteins

In mammalian synapses, the function of ionotropic glutamate receptors is critically modulated by auxiliary subunits. Most of these specifically regulate the synaptic localization and electrophysiological properties of AMPA-type glutamate receptors (AMPARs). Here, we comprehensively investigated the animal evolution of the protein families that contain AMPAR auxiliary subunits (ARASs). We observed that, on average, vertebrates have four times more ARASs than other animal species. We also demonstrated that ARASs belong to four unrelated protein families: CACNG-GSG1, cornichon, shisa and Dispanin C. Our study demonstrates that, despite the ancient origin of these four protein families, the majority of ARASs emerged during vertebrate evolution by independent but convergent processes of neo/subfunctionalization that resulted in the multiple ARASs found in present vertebrate genomes. Importantly, although AMPARs appeared and diversified in the ancestor of bilateral animals, the ARAS expansion did not occur until much later, in early vertebrate evolution. We propose that the surge in ARASs and consequent increase in AMPAR functionalities, contributed to the increased complexity of vertebrate brains and cognitive functions.

TARPs Modulate Receptor-Mediated Paired-Pulse Depression and Recovery from Desensitization

Transmembrane AMPA receptor regulatory proteins (TARPs) are auxiliary AMPA receptor subunits that play a key role in receptor trafficking and in modulating receptor gating. The ability of TARPs to slow both deactivation and desensitization is isoform specific. However, TARP isoform-specific modulation of receptor properties remains uncharacterized. Here, we compare the isoform-specific effects of γ-2, γ-3, γ-4, and γ-8 TARPs on recovery from desensitization and responses to pairs of brief applications of glutamate. All four isoforms were able to reduce receptor-mediated paired-pulse depression and significantly speed recovery from desensitization in an isoform-specific manner. In the presence of TARPs, recovery time courses were observed to contain two components, fast and slow. The proportion of fast and slow components was determined by the TARP isoform. The time constant of recovery was also altered by the duration of glutamate application. When studies with TARP chimeras were performed, TARP extracellular loops were found to play a vital role in TARP modulation of recovery. Thus, isoform-specific differences in TARP modulation of recovery from desensitization influence receptor responses to repeated brief applications of glutamate, and these differences may impact frequency-dependent synaptic signaling in the mammalian central nervous system.SIGNIFICANCE STATEMENT AMPA receptors are major determinants of excitatory synaptic strength. The channel kinetics of AMPA receptors contribute to the kinetics of synaptic transmission. Transmembrane AMPA receptor regulatory proteins (TARPs) auxiliary subunits can modulate the decay kinetics of AMPA receptors. However, whether TARP isoforms specifically modulate receptor recovery is unclear. Here, we investigated the recovery kinetics of AMPA receptors by expressing various TARP isoforms and chimeras. We observed that the TARP isoforms and duration of glutamate application uniquely modulate time constants and the proportion of fast and slow components through a previously unidentified TARP domain. Given the impact of recovery kinetics on receptor responses to repetitive stimulation such as synaptic transmission, this work will be of great interest in the field of excitatory synaptic transmission research.

AMPA receptor structure and auxiliary subunits

Fast excitatory synaptic transmission in the mammalian brain is largely mediated by AMPA-type ionotropic glutamate receptors (AMPARs), which are activated by the neurotransmitter glutamate. In synapses, the function of AMPARs is tuned by their auxiliary subunits, a diverse set of membrane proteins associated with the core pore-forming subunits of the AMPARs. Each auxiliary subunit provides distinct functional modulation of AMPARs, ranging from regulation of trafficking to shaping ion channel gating kinetics. Understanding the molecular mechanism of the function of these complexes is key to decoding synaptic modulation and their global roles in cognitive activities, such as learning and memory. Here, we review the structural and molecular complexity of AMPAR-auxiliary subunit complexes, as well as their functional diversity in different brain regions. We suggest that the recent structural information provides new insights into the molecular mechanisms underlying synaptic functions of AMPAR-auxiliary subunit complexes.

AMPA receptors and their minions: auxiliary proteins in AMPA receptor trafficking

To correctly transfer information, neuronal networks need to continuously adjust their synaptic strength to extrinsic stimuli. This ability, termed synaptic plasticity, is at the heart of their function and is, thus, tightly regulated. In glutamatergic neurons, synaptic strength is controlled by the number and function of AMPA receptors at the postsynapse, which mediate most of the fast excitatory transmission in the central nervous system. Their trafficking to, at, and from the synapse, is, therefore, a key mechanism underlying synaptic plasticity. Intensive research over the last 20 years has revealed the increasing importance of interacting proteins, which accompany AMPA receptors throughout their lifetime and help to refine the temporal and spatial modulation of their trafficking and function. In this review, we discuss the current knowledge about the roles of key partners in regulating AMPA receptor trafficking and focus especially on the movement between the intracellular, extrasynaptic, and synaptic pools. We examine their involvement not only in basal synaptic function, but also in Hebbian and homeostatic plasticity. Included in our review are well-established AMPA receptor interactants such as GRIP1 and PICK1, the classical auxiliary subunits TARP and CNIH, and the newest additions to AMPA receptor native complexes.

TAK-137, an AMPA-R potentiator with little agonistic effect, has a wide therapeutic window

Activation of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPA-R) is a promising strategy to treat psychiatric and neurological diseases if issues of bell-shaped response and narrow safety margin against seizure can be overcome. Here, we show that structural interference at Ser743 in AMPA-R is a key to lower the agonistic effect of AMPA-R potentiators containing dihydropyridothiadiazine 2,2-dioxides skeleton. With this structural insight, TAK-137, 9-(4-phenoxyphenyl)-3,4-dihydropyrido[2,1-c][1,2,4]thiadiazine 2,2-dioxide, was discovered as a novel AMPA-R potentiator with a lower agonistic effect than an AMPA-R potentiator LY451646 ((R)-N-(2-(4'-cyanobiphenyl-4-yl)propyl)propane-2-sulfonamide) in rat primary neurons. TAK-137 induced brain-derived neurotrophic factor in neurons in rodents and potently improved cognition in both rats and monkeys. Compared to LY451646, TAK-137 had a wider safety margin against seizure in rats. TAK-137 enhanced neural progenitor proliferation over a broader range of doses in rodents. Thus, TAK-137 is a promising AMPA-R potentiator with potent procognitive effects and lower risks of bell-shaped response and seizure. These data may open the door for the development of AMPA-R potentiators as therapeutic drugs for psychiatric and neurological diseases.

Polyamine-mediated channel block of ionotropic glutamate receptors and its regulation by auxiliary proteins

Most excitatory neurotransmission in the mammalian brain is mediated by a family of plasma membrane-bound signaling proteins called ionotropic glutamate receptors (iGluRs). iGluRs assemble at central synapses as tetramers, forming a central ion-channel pore whose primary function is to rapidly transport Na⁺ and Ca²⁺ in response to binding the neurotransmitter l-glutamic acid. The pore of iGluRs is also accessible to bulkier cytoplasmic cations, such as the polyamines spermine, spermidine, and putrescine, which are drawn into the permeation pathway, but get stuck and block the movement of other ions. The degree of this polyamine-mediated channel block is highly regulated by processes that control the free cytoplasmic polyamine concentration, the membrane potential, or the iGluR subunit composition. Recently, an additional regulation by auxiliary proteins, most notably transmembrane AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor regulatory proteins (TARPs), cornichons, and neuropilin and tolloid-like proteins (NETOs), has been identified. Here, I review what we have learned of polyamine block of iGluRs and its regulation by auxiliary subunits. TARPs, cornichons, and NETOs attenuate the channel block by enabling polyamines to exit the pore. As a result, polyamine permeation occurs at more negative and physiologically relevant membrane potentials. The structural basis for enhanced polyamine transport remains unresolved, although alterations in both channel architecture and charge-screening mechanisms have been proposed. That auxiliary subunits can attenuate the polyamine block reveals an unappreciated impact of polyamine permeation in shaping the signaling properties of neuronal AMPA- and kainate-type iGluRs. Moreover, enhanced polyamine transport through iGluRs may have a role in regulating cellular polyamine levels.

The Role of Polyamine-Dependent Facilitation of Calcium Permeable AMPARs in Short-Term Synaptic Enhancement

Depending on subunit composition AMPA receptor channels can be subdivided into two groups: GluA2-containing calcium impermeable AMPARs, and GluA2-lacking calcium permeable, AMPARs. These two groups differ in a number of biophysical properties and, most likely, in their functional role at glutamatergic synapses. GluA2-lacking channels have received a lot of attention over the last two decades mainly due to high calcium permeability, which was suggested to play a significant role in the induction of long-term synaptic plasticity in healthy tissue and neuronal death under neuropathological conditions. However, calcium permeable AMPARs possess another property that can contribute substantially to frequency dependent dynamics of synaptic efficacy. In the closed state calcium permeable AMPARs are blocked by endogenous polyamines, however, repetitive activation leads to progressive relief from the block and to the facilitation of ion flux through these channels. Polyamine-dependent facilitation of AMPARs can contribute to short-term plasticity at synapses that have high initial release probability and express calcium permeable AMPARs. During synaptic transmission activity-dependent relief from polyamine block of postsynaptic calcium-permeable AMPARs either counteracts presynaptic short-term depression in a frequency-dependent manner or, under specific stimulation conditions, induces facilitation of a synaptic response. Taking into account the fact that expression of calcium permeable AMPARs is developmentally regulated, depends on network activity and increases in diseased brain states, polyamine-dependent facilitation of calcium permeable AMPARs is an important, entirely postsynaptic mechanism of synaptic gain regulation.

Phosphorylation of the AMPAR-TARP Complex in Synaptic Plasticity

Synaptic plasticity has been considered a key mechanism underlying many brain functions including learning, memory, and drug addiction. An increase or decrease in synaptic activity of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) complex mediates the phenomena as shown in the cellular models of synaptic plasticity, long-term potentiation (LTP), and depression (LTD). In particular, protein phosphorylation shares the spotlight in expressing the synaptic plasticity. This review summarizes the studies on phosphorylation of the AMPAR pore-forming subunits and auxiliary proteins including transmembrane AMPA receptor regulatory proteins (TARPs) and discusses its role in synaptic plasticity.

Synthesis, pharmacology and preclinical evaluation of 11C-labeled 1,3-dihydro-2H-benzo[d]imidazole-2-ones for imaging γ8-dependent transmembrane AMPA receptor regulatory protein

a-Amino-3-hydroxyl-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are implicated in the pathology of neurological diseases such as epilepsy and schizophrenia. As pan antagonists for this target are often accompanied with undesired effects at high doses, one of the recent drug discovery approaches has shifted to subtype-selective AMPA receptor (AMPAR) antagonists, specifically, via modulating transmembrane AMPAR regulatory proteins (TARPs). The quantification of AMPARs by positron emission tomography (PET) would help obtain insights into disease conditions in the living brain and advance the translational development of AMPAR antagonists. Herein we report the design, synthesis and preclinical evaluation of a series of TARP γ-8 antagonists, amenable for radiolabeling, for the development of subtype-selective AMPAR PET imaging agents. Based on the pharmacology evaluation, molecular docking studies and physiochemical properties, we have identified several promising lead compounds 3, 17-19 and 21 for in vivo PET studies. All candidate compounds were labeled with [¹¹C]COCl₂ in high radiochemical yields (13-31% RCY) and high molar activities (35-196 GBq/μmol). While tracers 30 ([¹¹C]17) &32 ([¹¹C]21) crossed the blood-brain barrier and showed heterogeneous distribution in PET studies, consistent with TARP γ-8 expression, high nonspecific binding prevented further evaluation. To our delight, tracer 31 ([¹¹C]3) showed good in vitro specific binding and characteristic high uptake in the hippocampus in rat brain tissues, which provides the guideline for further development of a new generation subtype selective TARP γ-8 dependent AMPAR tracers.

HBT1, a Novel AMPA Receptor Potentiator with Lower Agonistic Effect, Avoided Bell-Shaped Response in In Vitro BDNF Production

α-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor (AMPA-R) potentiators with brain-derived neurotrophic factor (BDNF)-induction potential could be promising as therapeutic drugs for neuropsychiatric and neurologic disorders. However, AMPA-R potentiators such as LY451646 have risks of narrow bell-shaped responses in pharmacological effects, including in vivo BDNF induction. Interestingly, LY451646 and LY451395, other AMPA-R potentiators, showed agonistic effects and exhibited bell-shaped responses in the BDNF production in primary neurons. We hypothesized that the agonistic property is related to the bell-shaped response and endeavored to discover novel AMPA-R potentiators with lower agonistic effects. LY451395 showed an agonistic effect in primary neurons, but not in a cell line expressing AMPA-Rs, in Ca²⁺ influx assays; thus, we used a Ca²⁺ influx assay in primary neurons and, from a chemical library, discovered two AMPA-R potentiators with lower agonistic effects: 2-(((5-methyl-3-(trifluoromethyl)-1H-pyrazol-1-yl)acetyl)amino)-4,5,6,7-tetrahydro-1-benzothiophene-3-carboxamide (HBT1) and (3S)-1-(4-tert-butylphenyl)-N-((1R)-2-(dimethylamino)-1-phenylethyl)-3-isobutyl-2-oxopyrrolidine-3-carboxamide (OXP1). In a patch-clamp study using primary neurons, HBT1 showed little agonistic effect, whereas both LY451395 and OXP1 showed remarkable agonistic effects. HBT1, but not OXP1, did not show remarkable bell-shaped response in BDNF production in primary neurons. HBT1 bound to the ligand-binding domain (LBD) of AMPA-R in a glutamate-dependent manner. The mode of HBT1 and LY451395 binding to a pocket in the LBD of AMPA-R differed: HBT1, but not LY451395, formed hydrogen bonds with S518 in the LBD. OXP1 may bind to a cryptic binding pocket on AMPA-R. Lower agonistic profile of HBT1 may associate with its lower risks of bell-shaped responses in BDNF production in primary neurons.

Structural Determinants of the γ-8 TARP Dependent AMPA Receptor Antagonist

The forebrain specific AMPA receptor antagonist, LY3130481/CERC-611, which selectively antagonizes the AMPA receptors associated with TARP γ-8, an auxiliary subunit enriched in the forebrain, has potent antiepileptic activities without motor side effects. We designated the compounds with such activities as γ-8 TARP dependent AMPA receptor antagonists (γ-8 TDAAs). In this work, we further investigated the mechanisms of action using a radiolabeled γ-8 TDAA and ternary structural modeling with mutational validations to characterize the LY3130481 binding to γ-8. The radioligand binding to the cells heterologously expressing GluA1 and/or γ-8 revealed that γ-8 TDAAs binds to γ-8 alone without AMPA receptors. Homology modeling of γ-8, based on the crystal structures of a distant TARP homologue, murine claudin 19, in conjunction with knowledge of two γ-8 residues previously identified as critical for the LY3130481 TARP-dependent selectivity provided the basis for a binding mode prediction. This allowed further rational mutational studies for characterization of the structural determinants in TARP γ-8 for LY3130481 activities, both thermodynamically as well as kinetically.

Auxiliary subunits of AMPA receptors: The discovery of a forebrain-selective antagonist, LY3130481/CERC-611

Drugs originate from the discovery of compounds, natural or synthetic, that bind to proteins (receptors, enzymes, transporters, etc.), the interaction of which modulates biological cascades that have potential therapeutic benefit. Rational strategies for identifying novel drug therapies are typically based on knowledge of the structure of the target proteins and the design of new chemical entities that modulate these proteins in a beneficial manner. The present review discusses a novel approach to drug discovery based on the identification and characterization of auxiliary proteins, the transmembrane AMPA receptor regulatory proteins (TARPs) that are associated with AMPA receptors. Utilizing these auxiliary proteins in compound screening led to the discovery of the TARP-dependent-AMPA forebrain selective receptor antagonist (TDAA), LY3130481/CERC-611 that is currently in clinical development for epilepsy.

Screening for AMPA receptor auxiliary subunit specific modulators

AMPA receptors (AMPAR) are ligand gated ion channels critical for synaptic transmission and plasticity. Their dysfunction is implicated in a variety of psychiatric and neurological diseases ranging from major depressive disorder to amyotrophic lateral sclerosis. Attempting to potentiate or depress AMPAR activity is an inherently difficult balancing act between effective treatments and debilitating side effects. A newly explored strategy to target subsets of AMPARs in the central nervous system is to identify compounds that affect specific AMPAR-auxiliary subunit complexes. This exploits diverse spatio-temporal expression patterns of known AMPAR auxiliary subunits, providing means for designing brain region-selective compounds. Here we report a high-throughput screening-based pipeline that can identify compounds that are selective for GluA2-CNIH3 and GluA2-stargazin complexes. These compounds will help us build upon the growing library of AMPAR-auxiliary subunit specific inhibitors, which have thus far all been targeted to TARP γ-8. We used a cell-based assay combined with a voltage-sensitive dye (VSD) to identify changes in glutamate-gated cation flow across the membranes of HEK cells co-expressing GluA2 and an auxiliary subunit. We then used a calcium flux assay to further validate hits picked from the VSD assay. VU0612951 and VU0627849 are candidate compounds from the initial screen that were identified as negative and positive allosteric modulators (NAM and PAM), respectively. They both have lower IC₅₀/EC₅₀s on complexes containing stargazin and CNIH3 than GSG1L or the AMPAR alone. We have also identified a candidate compound, VU0539491, that has NAM activity in GluA2(R)-CNIH3 and GluA2(Q) complexes and PAM activity in GluA2(Q)-GSG1L complexes.

CaMKII Phosphorylation of TARPγ-8 Is a Mediator of LTP and Learning and Memory

Protein phosphorylation is an essential step for the expression of long-term potentiation (LTP), a long-lasting, activity-dependent strengthening of synaptic transmission widely regarded as a cellular mechanism underlying learning and memory. At the core of LTP is the synaptic insertion of AMPA receptors (AMPARs) triggered by the NMDA receptor-dependent activation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). However, the CaMKII substrate that increases AMPAR-mediated transmission during LTP remains elusive. Here, we identify the hippocampus-enriched TARPγ-8, but not TARPγ-2/3/4, as a critical CaMKII substrate for LTP. We found that LTP induction increases TARPγ-8 phosphorylation, and that CaMKII-dependent enhancement of AMPAR-mediated transmission requires CaMKII phosphorylation sites of TARPγ-8. Moreover, LTP and memory formation, but not basal transmission, are significantly impaired in mice lacking CaMKII phosphorylation sites of TARPγ-8. Together, these findings demonstrate that TARPγ-8 is a crucial mediator of CaMKII-dependent LTP and therefore a molecular target that controls synaptic plasticity and associated cognitive functions.

TARP γ-2 and γ-8 Differentially Control AMPAR Density Across Schaffer Collateral/Commissural Synapses in the Hippocampal CA1 Area

The number of AMPA-type glutamate receptors (AMPARs) at synapses is the major determinant of synaptic strength and varies from synapse to synapse. To clarify the underlying molecular mechanisms, the density of AMPARs, PSD-95, and transmembrane AMPAR regulatory proteins (TARPs) were compared at Schaffer collateral/commissural (SCC) synapses in the adult mouse hippocampal CA1 by quantitative immunogold electron microscopy using serial sections. We examined four types of SCC synapses: perforated and nonperforated synapses on pyramidal cells and axodendritic synapses on parvalbumin-positive (PV synapse) and pravalbumin-negative interneurons (non-PV synapse). SCC synapses were categorized into those expressing high-density (perforated and PV synapses) or low-density (nonperforated and non-PV synapses) AMPARs. Although the density of PSD-95 labeling was fairly constant, the density and composition of TARP isoforms was highly variable depending on the synapse type. Of the three TARPs expressed in hippocampal neurons, the disparity in TARP γ-2 labeling was closely related to that of AMPAR labeling. Importantly, AMPAR density was significantly reduced at perforated and PV synapses in TARP γ-2-knock-out (KO) mice, resulting in a virtual loss of AMPAR disparity among SCC synapses. In comparison, TARP γ-8 was the only TARP expressed at nonperforated synapses, where AMPAR labeling further decreased to a background level in TARP γ-8-KO mice. These results show that synaptic inclusion of TARP γ-2 potently increases AMPAR expression and transforms low-density synapses into high-density ones, whereas TARP γ-8 is essential for low-density or basal expression of AMPARs at nonperforated synapses. Therefore, these TARPs are critically involved in AMPAR density control at SCC synapses.

SIGNIFICANCE STATEMENT

Although converging evidence implicates the importance of transmembrane AMPA-type glutamate receptor (AMPAR) regulatory proteins (TARPs) in AMPAR stabilization during basal transmission and synaptic plasticity, how they control large disparities in AMPAR numbers or densities across central synapses remains largely unknown. We compared the density of AMPARs with that of TARPs among four types of Schaffer collateral/commissural (SCC) hippocampal synapses in wild-type and TARP-knock-out mice. We show that the density of AMPARs correlates with that of TARP γ-2 across SCC synapses and its high expression is linked to high-density AMPAR expression at perforated type of pyramidal cell synapses and synapses on parvalbumin-positive interneurons. In comparison, TARP γ-8 is the only TARP expressed at nonperforated type of pyramidal cell synapses, playing an essential role in low-density or basal AMPAR expression.

Elucidation of AMPA receptor-stargazin complexes by cryo-electron microscopy

AMPA-subtype ionotropic glutamate receptors (AMPARs) mediate fast excitatory neurotransmission and contribute to high cognitive processes such as learning and memory. In the brain, AMPAR trafficking, gating, and pharmacology is tightly controlled by transmembrane AMPAR regulatory proteins (TARPs). Here, we used cryo-electron microscopy to elucidate the structural basis of AMPAR regulation by one of these auxiliary proteins, TARP γ2, or stargazin (STZ). Our structures illuminate the variable interaction stoichiometry of the AMPAR-TARP complex, with one or two TARP molecules binding one tetrameric AMPAR. Analysis of the AMPAR-STZ binding interfaces suggests that electrostatic interactions between the extracellular domains of AMPAR and STZ play an important role in modulating AMPAR function through contact surfaces that are conserved across AMPARs and TARPs. We propose a model explaining how TARPs stabilize the activated state of AMPARs and how the interactions between AMPARs and their auxiliary proteins control fast excitatory synaptic transmission.

Rediscovering area CA2: unique properties and functions

Hippocampal area CA2 has several features that distinguish it from CA1 and CA3, including a unique gene expression profile, failure to display long-term potentiation and relative resistance to cell death. A recent increase in interest in the CA2 region, combined with the development of new methods to define and manipulate its neurons, has led to some exciting new discoveries on the properties of CA2 neurons and their role in behaviour. Here, we review these findings and call attention to the idea that the definition of area CA2 ought to be revised in light of gene expression data.

α/β-Hydrolase domain-containing 6 (ABHD6) negatively regulates the surface delivery and synaptic function of AMPA receptors

In the brain, AMPA-type glutamate receptors are major postsynaptic receptors at excitatory synapses that mediate fast neurotransmission and synaptic plasticity. α/β-Hydrolase domain-containing 6 (ABHD6), a monoacylglycerol lipase, was previously found to be a component of AMPA receptor macromolecular complexes, but its physiological significance in the function of AMPA receptors (AMPARs) has remained unclear. The present study shows that overexpression of ABHD6 in neurons drastically reduced excitatory neurotransmission mediated by AMPA but not by NMDA receptors at excitatory synapses. Inactivation of ABHD6 expression in neurons by either CRISPR/Cas9 or shRNA knockdown methods significantly increased excitatory neurotransmission at excitatory synapses. Interestingly, overexpression of ABHD6 reduced glutamate-induced currents and the surface expression of GluA1 in HEK293T cells expressing GluA1 and stargazin, suggesting a direct functional interaction between these two proteins. The C-terminal tail of GluA1 was required for the binding between of ABHD6 and GluA1. Mutagenesis analysis revealed a GFCLIPQ sequence in the GluA1 C terminus that was essential for the inhibitory effect of ABHD6. The hydrolase activity of ABHD6 was not required for the effects of ABHD6 on AMPAR function in either neurons or transfected HEK293T cells. Thus, these findings reveal a novel and unexpected mechanism governing AMPAR trafficking at synapses through ABHD6.

Activity- and development-dependent down-regulation of TARPγ8 and GluA1 in cultured rat hippocampal neurons

AIM

Transmembrane AMPA receptor regulatory proteins (TARPs) regulate the trafficking and expression of AMPA receptors that are essential for the fast excitatory synaptic transmission and plasticity in the brain. This study aimed to investigate the activity-dependent regulation of TARPγ8 in cultured rat hippocampal neurons.

METHODS

Rat hippocampal neurons cultured for 7-8 DIV or 17-18 DIV were exposed to the AMPA receptor agonist AMPA at a non-toxic concentration (100 μmol/L) for 4 h. The protein levels of TARPγ8 and AMPA receptor subunits (GluA1 and GluA2) were measured using Western blotting analysis. AMPA-induced currents were recorded in the neurons using a whole-cell recording method.

RESULTS

Four-hour exposure to AMPA significantly decreased the protein levels of TARPγ8 and GluA1 in the neurons at 17-18 DIV, but did not change the protein level of TARPγ8 in the neurons cultured at 7-8 DIV. AMPA-induced down-regulation of TARPγ8 and GluA1 was largely blocked by the calpain inhibitor calpeptin (50 μmol/L), but not affected by the caspase inhibitor zVAD (50 μmol/L). Four-hour exposure to AMPA significantly decreased AMPA-induced currents in the neurons at 17-18 DIV, which was blocked by co-exposure to calpeptin (50 μmol/L).

CONCLUSION

The down-regulation of TARPγ8 and GluA1 protein levels and AMPA-induced currents in cultured rat hippocampal neurons is activity- and development-dependent, and mediated by endogenous calpain.

Auxiliary subunits of the CKAMP family differentially modulate AMPA receptor properties

AMPA receptor (AMPAR) function is modulated by auxiliary subunits. Here, we report on three AMPAR interacting proteins—namely CKAMP39, CKAMP52 and CKAMP59—that, together with the previously characterized CKAMP44, constitute a novel family of auxiliary subunits distinct from other families of AMPAR interacting proteins. The new members of the CKAMP family display distinct regional and developmental expression profiles in the mouse brain. Notably, despite their structural similarities they exert diverse modulation on AMPAR gating by influencing deactivation, desensitization and recovery from desensitization, as well as glutamate and cyclothiazide potency to AMPARs. This study indicates that AMPAR function is very precisely controlled by the cell-type specific expression of the CKAMP family members.

The brain processes and transmits information through large networks of cells called neurons. A neuron can pass the information it receives to other neurons by releasing chemicals called neurotransmitters across junctions known as synapses. These chemicals bind to receptor proteins on the surface of the neighboring neuron, which triggers changes that affect the activity of this neuron.

Glutamate is the most commonly used neurotransmitter in the brain and binds to receptor proteins called AMPA receptors. If a neuron frequently sends glutamate across a particular synapse, the number of AMPA receptors in the second neuron will increase in response. This makes signaling across the synapse easier – a process known as synaptic strengthening. The ability to change the strength of synapses is important for learning and memory.

Proteins called auxiliary subunits also bind to AMPA receptors and regulate their properties, and hence also affect the strength of the synapse. For instance, some auxiliary subunits increase the number of AMPA receptors at the synapse, while others have an effect on how the receptor protein works. In 2010, researchers identified a new auxiliary protein called CKAMP44 that modifies AMPA receptor activity. Now, Farrow, Khodosevich, Sapir, Schulmann et al. – including some of the researchers involved in the 2010 study – have identified three other auxiliary proteins that are similar to CKAMP44. Collectively, these four proteins are termed the CKAMP family.

The sequences of all four proteins were found to share many common features, especially in the regions that bind to the AMPA receptors. Like CKAMP44, the new members of the CKAMP family are only present in the brain, although each protein is produced in different brain regions. Further investigation revealed that each member of the CKAMP family affects the AMPA receptor channels in a different way.

Taken together, Farrow et al.’s results suggest that the different CKAMP family members allow the activity of the AMPA receptors to be precisely controlled. The next challenge is to understand in more detail how each CKAMP family member influences how AMPA receptors work.

Advances in the pharmacology of lGICs auxiliary subunits

Ligand-gated ion channels (LGICs) are cell surface integral proteins that mediate the fast neurotransmission in the nervous system. LGICs require auxiliary subunits for their trafficking, assembly and pharmacological modulation. Auxiliary subunits do not form functional homomeric receptors, but are reported to assemble with the principal subunits in order to modulate their pharmacological profiles. For example, nACh receptors are built at least by co-assemble of α and β subunits, and the neuronal auxiliary subunits β3 and α5 and muscle type β, δ, γ, and ϵ determine the agonist affinity of these receptors. Serotonergic 5-HT3B, 5-HT3C, 5-HT3D and 5-HT3E are reported to assemble with the 5-HT3A subunit to modulate its pharmacological profile. Functional studies evaluating the role of γ2 and δ auxiliary subunits of GABAA receptors have made important advances in the understanding of the action of benzodiazepines, ethanol and neurosteroids. Glycine receptors are composed principally by α1-3 subunits and the auxiliary subunit β determines their synaptic location and their pharmacological response to propofol and ethanol. NMDA receptors appear to be functional as heterotetrameric channels. So far, the existence of NMDA auxiliary subunits is controversial. On the other hand, Kainate receptors are modulated by NETO 1 and 2. AMPA receptors are modulated by TARPs, Shisa 9, CKAMP44, CNIH2-3 auxiliary proteins reported that controls their trafficking, conductance and gating of channels. P2X receptors are able to associate with auxiliary Pannexin-1 protein to modulate P2X7 receptors. Considering the pharmacological relevance of different LGICs auxiliary subunits in the present work we will highlight the therapeutic potential of these modulator proteins.

Zebrafish TARP Cacng2 is required for the expression and normal development of AMPA receptors at excitatory synapses

Fast excitatory synaptic transmission in the CNS is mediated by the neurotransmitter glutamate, binding to and activating AMPA receptors (AMPARs). AMPARs are known to interact with auxiliary proteins that modulate their behavior. One such family of proteins is the transmembrane AMPA receptor-related proteins, known as TARPs. Little is known about the role of TARPs during development, or about their function in non-mammalian organisms. Here we report the presence of TARPs, specifically the prototypical TARP, stargazin, in developing zebrafish. We find that zebrafish express two forms of stargazin, Cacng2a and Cacng2b from as early as 12-h post fertilization (hpf). Knockdown of Cacng2a and Cacng2b via splice-blocking morpholinos resulted in embryos that exhibited deficits in C-start escape responses, showing reduced C-bend angles, smaller tail velocities and aberrant C-bend turning directions. Injection of the morphants with Cacng2a or 2b mRNA rescued the morphological phenotype and the synaptic deficits. To investigate the effect of reduced Cacng2a and 2b levels on synaptic physiology, we performed whole cell patch clamp recordings of AMPA mEPSCs from zebrafish Mauthner cells. Knockdown of Cacng2a results in reduced AMPA currents and lower mEPSC frequencies, whereas knockdown of Cacng2b displayed no significant change in mEPSC amplitude or frequency. Non-stationary fluctuation analysis confirmed a reduction in the number of active synaptic receptors in the Cacng2a but not in the Cacng2b morphants. Together, these results suggest that Cacng2a is required for normal trafficking and function of synaptic AMPARs, while Cacng2b is largely non-functional with respect to the development of AMPA synaptic transmission.

Relative contribution of TARPs γ-2 and γ-7 to cerebellar excitatory synaptic transmission and motor behavior

Transmembrane AMPA receptor regulatory proteins (TARPs) play an essential role in excitatory synaptic transmission throughout the central nervous system (CNS) and exhibit subtype-specific effects on AMPA receptor (AMPAR) trafficking, gating, and pharmacology. The function of TARPs has largely been determined through work on canonical type I TARPs such as stargazin (TARP γ-2), absent in the ataxic stargazer mouse. Little is known about the function of atypical type II TARPs, such as TARP γ-7, which exhibits variable effects on AMPAR function. Because γ-2 and γ-7 are both strongly expressed in multiple cell types in the cerebellum, we examined the relative contribution of γ-2 and γ-7 to both synaptic transmission in the cerebellum and motor behavior by using both the stargazer mouse and a γ-7 knockout (KO) mouse. We found that the loss of γ-7 alone had little effect on climbing fiber (cf) responses in Purkinje neurons (PCs), yet the additional loss of γ-2 all but abolished cf responses. In contrast, γ-7 failed to make a significant contribution to excitatory transmission in stellate cells and granule cells. In addition, we generated a PC-specific deletion of γ-2, with and without γ-7 KO background, to examine the relative contribution of γ-2 and γ-7 to PC-dependent motor behavior. Selective deletion of γ-2 in PCs had little effect on motor behavior, yet the additional loss of γ-7 resulted in a severe disruption in motor behavior. Thus, γ-7 is capable of supporting a component of excitatory transmission in PCs, sufficient to maintain essentially normal motor behavior, in the absence of γ-2.

Auxiliary proteins promote modal gating of AMPA- and kainate-type glutamate receptors

The gating behavior of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors is modulated by association with the auxiliary proteins: transmembrane AMPA receptor regulatory proteins (TARPs) and neuropilin tolloid-like (Netos), respectively. Although the mechanisms underlying receptor modulation differ for both AMPA and kainate receptors, association with these auxiliary subunits results in the appearance of a slow component in the decay of ensemble responses to rapid applications of saturating concentrations of glutamate. We show here that these components arise from distinct gating behaviors, characterized by substantially higher open probability (Popen ), which we only observe when core subunits are associated with their respective auxiliary partners. We refer to these behaviors as gating modes, because individual receptors switch between the low- and high-Popen gating on a time-scale of seconds. At any given time, association of AMPA and kainate receptors with their auxiliary subunits results in a heterogeneous receptor population, some of which are in the high-Popen mode and others that display gating behavior similar to that seen for receptors formed from core subunits alone. While the switching between modes is infrequent, the presence of receptors displaying both types of gating has a large impact on both the kinetics and amplitude of ensemble currents similar to those seen at synapses.

Modulation of non-NMDA receptor gating by auxiliary subunits

During the past decade, considerable evidence has accumulated that non-NMDA glutamate receptors (both AMPA and kainate subtypes) are modulated by the association of the core tetrameric receptor with auxiliary proteins that are integral components of native receptor assemblies. This short review focuses on the effect of two types of auxiliary subunits on the biophysical properties and kinetic behaviour of AMPA and kainate receptors at the level of single receptor molecules. Type I transmembrane AMPA receptor proteins increase the number of AMPA receptor openings that result from a single receptor activation as well as the proportion of openings to conductance levels above 30 pS, resulting in larger peak ensemble currents that decay more slowly and bi-exponentially. Co-expression of Neto1 and 2 with pore-forming kainate receptor subunits also increases the duration of bursts and destabilizes desensitized states, resulting in a rapid component of recovery and clusters of bursts that produce a slow component in desensitization decays. The distinct gating seen in the presence of auxiliary subunits reflects slow switching between gating modes with different single-channel kinetics and open probability. At any given time, the relative proportions of receptors in each gating mode determine both the shape and the amplitude of synaptic currents.

Auxiliary subunits: shepherding AMPA receptors to the plasma membrane

Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated cation channels that mediate excitatory signal transmission in the central nervous system (CNS) of vertebrates. The members of the iGluR subfamily of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs) mediate most of the fast excitatory signal transmission, and their abundance in the postsynaptic membrane is a major determinant of the strength of excitatory synapses. Therefore, regulation of AMPAR trafficking to the postsynaptic membrane is an important constituent of mechanisms involved in learning and memory formation, such as long-term potentiation (LTP) and long-term depression (LTD). Auxiliary subunits play a critical role in the facilitation and regulation of AMPAR trafficking and function. The currently identified auxiliary subunits of AMPARs are transmembrane AMPA receptor regulatory proteins (TARPs), suppressor of lurcher (SOL), cornichon homologues (CNIHs), synapse differentiation-induced gene I (SynDIG I), cysteine-knot AMPAR modulating proteins 44 (CKAMP44), and germ cell-specific gene 1-like (GSG1L) protein. In this review we summarize our current knowledge of the modulatory influence exerted by these important but still underappreciated proteins.

Characterization of Human Hippocampal Neural Stem/Progenitor Cells and Their Application to Physiologically Relevant Assays for Multiple Ionotropic Glutamate Receptors

The hippocampus is an important brain region that is involved in neurological disorders such as Alzheimer disease, schizophrenia, and epilepsy. Ionotropic glutamate receptors-namely,N-methyl-D-aspartate (NMDA) receptors (NMDARs), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors (AMPARs), and kainic acid (KA) receptors (KARs)-are well known to be involved in these diseases by mediating long-term potentiation, excitotoxicity, or both. To predict the therapeutic efficacy and neuronal toxicity of drug candidates acting on these receptors, physiologically relevant systems for assaying brain region-specific human neural cells are necessary. Here, we characterized the functional differentiation of human fetal hippocampus-derived neural stem/progenitor cells-namely, HIP-009 cells. Calcium rise assay demonstrated that, after a 4-week differentiation, the cells responded to NMDA (EC50= 7.5 ± 0.4 µM; n= 4), AMPA (EC50= 2.5 ± 0.1 µM; n= 3), or KA (EC50= 33.5 ± 1.1 µM; n= 3) in a concentration-dependent manner. An AMPA-evoked calcium rise was observed in the absence of the desensitization inhibitor cyclothiazide. In addition, the calcium rise induced by these agonists was inhibited by antagonists for each receptor-namely, MK-801 for NMDA stimulation (IC50= 0.6 ± 0.1 µM; n= 4) and NBQX for AMPA and KA stimulation (IC50= 0.7 ± 0.1 and 0.7 ± 0.03 µM, respectively; n= 3). The gene expression profile of differentiated HIP-009 cells was distinct from that of undifferentiated cells and closely resembled that of the human adult hippocampus. Our results show that HIP-009 cells are a unique tool for obtaining human hippocampal neural cells and are applicable to systems for assay of ionotropic glutamate receptors as a physiologically relevant in vitro model.