Plasticity of GABAA receptor diffusion dynamics at the axon initial segment.
ABSTRACT: The axon initial segment (AIS), a site of action potential initiation, undergoes activity-dependent homeostatic repositioning to fine-tune neuronal activity. However, little is known about the behavior of GABAA receptors (GABAARs) at synapses made onto the axon and especially the AIS. Here, we study the clustering and lateral diffusion of GABAARs in the AIS under baseline conditions, and find that GABAAR lateral mobility is lower in the AIS than dendrites. We find differences in axonal clustering and lateral mobility between GABAARs containing the ?1 or ?2 subunits, which are known to localize differentially to the AIS. Interestingly, we find that chronic activity driving AIS repositioning does not alter GABAergic synapse location along the axon, but decreases GABAAR cluster size at the AIS. Moreover, in response to chronic depolarization, GABAAR diffusion is strikingly increased in the AIS, and not in dendrites, and this is coupled with a decrease in synaptic residency time of GABAARs at the AIS. We also demonstrate that activation of L-type voltage-gated calcium channels is important for regulating GABAAR lateral mobility at the AIS during chronic depolarization. Modulation of GABAAR diffusion dynamics at the AIS in response to prolonged activity may be a novel mechanism for regulating GABAergic control of information processing.
Project description:The axon initial segment (AIS) is the site of action potential (AP) initiation in most neurons and is thus a critical site in the regulation of neuronal excitability. Normal function within the discrete AIS compartment requires intricate molecular machinery to ensure the proper concentration and organization of voltage-gated and ligand-gated ion channels; in humans, dysfunction at the AIS due to channel mutations is commonly associated with epileptic disorders. In this review, we will examine the molecular mechanisms underlying the formation of the only synapses found at the AIS: synapses containing ?-aminobutyric type A receptors (GABAARs). GABAARs are heteropentamers assembled from 19 possible subunits and are the primary mediators of fast synaptic inhibition in the brain. Although the total GABAAR population is incredibly heterogeneous, only one specific GABAAR subtype-the ?2-containing receptor-is enriched at the AIS. These AIS synapses are innervated by GABAergic chandelier cells, and this inhibitory signaling is thought to contribute to the tight control of AP firing. Here, we will summarize the progress made in understanding the regulation of GABAAR synapse formation, concentrating on post-translational modifications of subunits and on interactions with intracellular proteins. We will then discuss subtype-specific synapse formation, with a focus on synapses found at the AIS, and how these synapses influence neuronal excitation.
Project description:The lateral mobility of neurotransmitter receptors has been shown to tune synaptic signals. Here we report that GABAA receptors (GABAARs) can diffuse between adjacent dendritic GABAergic synapses in long-living desensitized states, thus laterally spreading "activation memories" between inhibitory synapses. Glutamatergic activity limits this inter-synaptic diffusion by trapping GABAARs at excitatory synapses. This novel form of activity-dependent hetero-synaptic interplay is likely to modulate dendritic synaptic signaling.
Project description:The axon initial segment (AIS) is enriched in specific adaptor, cytoskeletal, and transmembrane molecules. During AIS establishment, a membrane diffusion barrier is formed between the axonal and somatodendritic domains. Recently, an axonal periodic pattern of actin, spectrin, and ankyrin forming 190-nm-spaced, ring-like structures has been discovered. However, whether this structure is related to the diffusion barrier function is unclear. Here, we performed single-particle tracking time-course experiments on hippocampal neurons during AIS development. We analyzed the mobility of lipid-anchored molecules by high-speed single-particle tracking and correlated positions of membrane molecules with the nanoscopic organization of the AIS cytoskeleton. We observe a strong reduction in mobility early in AIS development. Membrane protein motion in the AIS plasma membrane is confined to a repetitive pattern of ?190-nm-spaced segments along the AIS axis as early as day in vitro 4, and this pattern alternates with actin rings. Mathematical modeling shows that diffusion barriers between the segments significantly reduce lateral diffusion along the axon.
Project description:GABAergic dysfunction underlies many neurodevelopmental and psychiatric disorders. GABAergic synapses exhibit several forms of plasticity at both pre- and postsynaptic levels. NMDA receptor (NMDAR)-dependent inhibitory long-term potentiation (iLTP) at GABAergic postsynapses requires an increase in surface GABAARs through promoted exocytosis; however, the regulatory mechanisms and the neuropathological significance remain unclear. Here we report that the autism-related protein PX-RICS is involved in GABAAR transport driven during NMDAR-dependent GABAergic iLTP. Chemically induced iLTP elicited a rapid increase in surface GABAARs in wild-type mouse hippocampal neurons, but not in PX-RICS/RICS-deficient neurons. This increase in surface GABAARs required the PX-RICS/GABARAP/14-3-3 complex, as revealed by gene knockdown and rescue studies. iLTP induced CaMKII-dependent phosphorylation of PX-RICS to promote PX-RICS-14-3-3 assembly. Notably, PX-RICS/RICS-deficient mice showed impaired amygdala-dependent fear learning, which was ameliorated by potentiating GABAergic activity with clonazepam. Our results suggest that PX-RICS-mediated GABAAR trafficking is a key target for GABAergic plasticity and its dysfunction leads to atypical emotional processing underlying autism.
Project description:The function and pharmacology of ?-aminobutyric acid type A receptors (GABAARs) are of great physiological and clinical importance and have long been thought to be determined by the channel pore-forming subunits. We discovered that Shisa7, a single-passing transmembrane protein, localizes at GABAergic inhibitory synapses and interacts with GABAARs. Shisa7 controls receptor abundance at synapses and speeds up the channel deactivation kinetics. Shisa7 also potently enhances the action of diazepam, a classic benzodiazepine, on GABAARs. Genetic deletion of Shisa7 selectively impairs GABAergic transmission and diminishes the effects of diazepam in mice. Our data indicate that Shisa7 regulates GABAAR trafficking, function, and pharmacology and reveal a previously unknown molecular interaction that modulates benzodiazepine action in the brain.
Project description:In the adult brain GABAA receptors (GABAARs) mediate the majority of synaptic inhibition that provides inhibitory balance to excitatory drive and controls neuronal output. In the immature brain GABAAR signaling is critical for neuronal development. However, the cell-autonomous role of GABAARs in synapse development remains largely unknown. We have employed the CRISPR-CAS9 technology to genetically eliminate GABAARs in individual hippocampal neurons and examined GABAergic and glutamatergic synapses. We found that development of GABAergic synapses, but not glutamatergic synapses, critically depends on GABAARs. By combining different genetic approaches, we have also removed GABAARs and two ionotropic glutamate receptors, AMPA receptors (AMPARs) and NMDA receptors (NMDARs), in single neurons and discovered a striking dichotomy. Indeed, while development of glutamatergic synapses and spines does not require signaling mediated by these receptors, inhibitory synapse formation is crucially dependent on them. Our data reveal a critical cell-autonomous role of GABAARs in inhibitory synaptogenesis and demonstrate distinct molecular mechanisms for development of inhibitory and excitatory synapses.
Project description:Neurosteroids are synthesized within the brain and act as endogenous anxiolytic, anticonvulsant, hypnotic, and sedative agents, actions that are principally mediated via their ability to potentiate phasic and tonic inhibitory neurotransmission mediated by ?-aminobutyric acid type A receptors (GABAARs). Although neurosteroids are accepted allosteric modulators of GABAARs, here we reveal they exert sustained effects on GABAergic inhibition by selectively enhancing the trafficking of GABAARs that mediate tonic inhibition. We demonstrate that neurosteroids potentiate the protein kinase C-dependent phosphorylation of S443 within ?4 subunits, a component of GABAAR subtypes that mediate tonic inhibition in many brain regions. This process enhances insertion of ?4 subunit-containing GABAAR subtypes into the membrane, resulting in a selective and sustained elevation in the efficacy of tonic inhibition. Therefore, the ability of neurosteroids to modulate the phosphorylation and membrane insertion of ?4 subunit-containing GABAARs may underlie the profound effects these endogenous signaling molecules have on neuronal excitability and behavior.
Project description:The GABAA receptor (GABAAR) is a major target of antiseizure drugs (ASDs). A variety of agents that act at GABAARs s are used to terminate or prevent seizures. Many act at distinct receptor sites determined by the subunit composition of the holoreceptor. For the benzodiazepines, barbiturates, and loreclezole, actions at the GABAAR are the primary or only known mechanism of antiseizure action. For topiramate, felbamate, retigabine, losigamone and stiripentol, GABAAR modulation is one of several possible antiseizure mechanisms. Allopregnanolone, a progesterone metabolite that enhances GABAAR function, led to the development of ganaxolone. Other agents modulate GABAergic "tone" by regulating the synthesis, transport or breakdown of GABA. GABAAR efficacy is also affected by the transmembrane chloride gradient, which changes during development and in chronic epilepsy. This may provide an additional target for "GABAergic" ASDs. GABAAR subunit changes occur both acutely during status epilepticus and in chronic epilepsy, which alter both intrinsic GABAAR function and the response to GABAAR-acting ASDs. Manipulation of subunit expression patterns or novel ASDs targeting the altered receptors may provide a novel approach for seizure prevention.
Project description:?-aminobutyric acid type A receptors (GABAARs) are the major mediators of synaptic inhibition in the brain. Aberrant GABAAR activity or regulation is observed in various neurodevelopmental disorders, neurodegenerative diseases and mental illnesses, including epilepsy, Alzheimer's and schizophrenia. Benzodiazepines, anesthetics and other pharmaceutics targeting these receptors find broad clinical use, but their inherent lack of receptor subtype specificity causes unavoidable side effects, raising a need for new or adjuvant medications. In this review article, we introduce a new strategy to modulate GABAeric signaling: targeting the intracellular protein interactors of GABAARs. Of special interest are scaffolding, anchoring and supporting proteins that display high GABAAR subtype specificity. Recent efforts to target gephyrin, the major intracellular integrator of GABAergic signaling, confirm that GABAAR-associated proteins can be successfully targeted through diverse molecules, including recombinant proteins, intrabodies, peptide-based probes and small molecules. Small-molecule artemisinins and peptides derived from endogenous interactors, that specifically target the universal receptor binding site of gephyrin, acutely affect synaptic GABAAR numbers and clustering, modifying neuronal transmission. Interference with GABAAR trafficking provides another way to modulate inhibitory signaling. Peptides blocking the binding site of GABAAR to AP2 increase the surface concentration of GABAAR clusters and enhance GABAergic signaling. Engineering of gephyrin binding peptides delivered superior means to interrogate neuronal structure and function. Fluorescent peptides, designed from gephyrin binders, enable live neuronal staining and visualization of gephyrin in the post synaptic sites with submicron resolution. We anticipate that in the future, novel fluorescent probes, with improved size and binding efficiency, may find wide application in super resolution microscopy studies, enlightening the nanoscale architecture of the inhibitory synapse. Broader studies on GABAAR accessory proteins and the identification of the exact molecular binding interfaces and affinities will advance the development of novel GABAAR modulators and following in vivo studies will reveal their clinical potential as adjuvant or stand-alone drugs.
Project description:Molecular mechanisms underlying plasticity at brain inhibitory synapses remain poorly characterized. Increased postsynaptic clustering of GABAA receptors (GABAARs) rapidly strengthens inhibition during inhibitory long-term potentiation (iLTP). However, it is unclear how synaptic GABAAR clustering is maintained to sustain iLTP. Here, we identify a role for miR376c in regulating the translation of mRNAs encoding the synaptic ?1 and ?2 GABAAR subunits, GABRA1 and GABRG2, respectively. Following iLTP induction, transcriptional repression of miR376c is induced through a calcineurin-NFAT-HDAC signaling pathway and promotes increased translation and clustering of synaptic GABAARs. This pathway is essential for the long-term expression of iLTP and is blocked by miR376c overexpression, specifically impairing inhibitory synaptic strength. Finally, we show that local de novo synthesis of synaptic GABAARs occurs exclusively in dendrites and in a miR376c-dependent manner following iLTP. Together, this work describes a local post-transcriptional mechanism that regulates inhibitory synaptic plasticity via miRNA control of dendritic protein synthesis.