Gephyrin Regulates the Cell Surface Dynamics of Synaptic GABA A Receptors

In mature neurons AMPA receptors cluster at excitatory synapses primarily on dendritic spines, whereas GABA A receptors cluster at inhibitory synapses mainly on the soma and dendritic shafts. The molecular mechanisms underlying the precise sorting of these receptors remain unclear. By directly studying the constitutive exocytic vesicles of AMPA and GABA A receptors in vitro and in vivo, we demonstrate that they are initially sorted into different vesicles in the Golgi apparatus and inserted into distinct domains of the plasma membrane. These insertions are dependent on distinct Rab GTPases and SNARE complexes. The insertion of AMPA receptors requires SNAP25-syntaxin1A/B-VAMP2 complexes, whereas insertion of GABA A receptors relies on SNAP23-syntaxin1A/B-VAMP2 complexes. These SNARE complexes affect surface targeting of AMPA or GABA A receptors and synaptic transmission. Our studies reveal vesicular sorting mechanisms controlling the constitutive exocytosis of AMPA and GABA A receptors, which are critical for the regulation of excitatory and inhibitory responses in neurons.AMPA receptor | GABA A receptor | constitutive exocytosis | TIRFM | SNARE I n the mammalian central nervous system, neurons receive excitatory and inhibitory signals at synapses. Specific receptors at postsynaptic membranes are activated by neurotransmitters released by presynaptic terminals. Most fast excitatory neurotransmission is mediated by AMPA receptors, the majority of which are heterotetramers of GluA1/GluA2 or GluA2/GluA3 subunits in the hippocampus (1). Fast synaptic inhibition is largely mediated by GABA A receptors, which are predominantly heteropentamers of two α subunits, two β subunits, and one γ or δ subunit in the hippocampus (2). Numerous studies have demonstrated AMPA receptors are selectively localized at excitatory synapses on dendritic spines, whereas GABA A receptors cluster at inhibitory synapses localized on dendritic shafts and the soma (3). This segregation of excitatory and inhibitory receptors requires highly precise sorting machinery to target receptors to distinct synapses opposing specific presynaptic terminals. However, it is still not clear whether the receptors are sorted before exocytosis into the plasma membrane or are differentially localized only after exocytosis. For example in a "plasma membrane sorting model," different receptors could be pooled into the same vesicle and inserted along the somatodendritic membrane. The initial sorting would occur on the plasma membrane, where inserted receptors would be segregated by lateral diffusion and stabilization at different postsynaptic zones. Alternatively, in a "vesicle sorting model," different receptors would first be sorted into different vesicles during intracellular trafficking processes and independently inserted to the plasma membrane, where receptors could be further targeted to specific zones and stabilized by synaptic scaffolds. To date there has been no direct evidence to support either model. However, a large body of literature suggests that the e...

Section: Resultsmentioning

confidence: 78%

Differential vesicular sorting of AMPA and GABA _A receptors

Chiu

Liu

et al. 2016

“…Pharmacological treatments provided additional information on the mechanisms controlling the distribution of GABA A Rs. We tested the role of MTs, which play a crucial role in axon growth and in directional sensing mechanisms (10,20) and interact, at least indirectly, with GABA A Rs (21,22). We found that the asymmetric redistribution of the receptors was suppressed when MTs were depolymerized with nocodazole (Fig.…”

Section: Resultsmentioning

confidence: 98%

“…The results of the experiments with taxol thus do not favor the hypothesis of the interactions with a motor. The directed movements were more likely due to the receptors binding to MT ends, possibly by means of other molecular partners (21,22), and being displaced during MT polymerization.…”

Section: Resultsmentioning

confidence: 99%

Asymmetric redistribution of GABA receptors during GABA gradient sensing by nerve growth cones analyzed by single quantum dot imaging

Bouzigues

Morel

Triller

et al. 2007

129

146

During development of the nervous system, the tip of a growing axon, the growth cone (GC), must respond accurately to stimuli that direct its growth. This axonal navigation depends on extracellular concentration gradients of numerous guidance cues, including GABA. GCs can detect even weak directional signals, yet the mechanisms underlying this sensitivity remain unclear. Past studies in other eukaryotic chemotactic systems have pointed to the role of the spatial reorganization of the transduction pathway in their sensitive response. Here we have developed a singlemolecule assay to observe individual GABAA receptors (GABAARs) in the plasma membrane of nerve GCs subjected to directional stimuli. We report that in the presence of an external GABA gradient GABAARs redistribute asymmetrically across the GC toward the gradient source. Single-particle tracking of GABAARs shows that the redistribution results from transient interactions between the receptors and the microtubules. Moreover, the relocalization is accompanied by an enhancement in the asymmetry of intracellular calcium concentration. Altogether, our results reveal a microtubule-dependent polarized reorganization of chemoreceptors at the cell surface and suggest that this polarization serves as an amplification step in GABA gradient sensing by nerve GCs.axonal guidance ͉ chemotaxis ͉ single molecule ͉ polarity

“…Accumulation of GABARs at synaptic sites is governed by lateral migration within the membrane, rather than insertion of newly synthesized receptors (26), where interactions with scaffolding proteins such as gephyrin stabilize postsynaptic receptors (51). Furthermore, GABAR insertion and endocytosis via AP2 adaptin occur predominantly at extrasynaptic sites, indicating that lateral diffusion and trafficking of receptors are critically important for regulating synaptic strength at GABAergic synapses (25).…”

Section: Discussionmentioning

confidence: 99%

ErbB4 reduces synaptic GABA _A currents independent of its receptor tyrosine kinase activity

Mitchell

Janssen

Karavanova

et al. 2013

ErbB4 signaling in the central nervous system is implicated in neuropsychiatric disorders and epilepsy. In cortical tissue, ErbB4 associates with excitatory synapses located on inhibitory interneurons. However, biochemical and histological data described herein demonstrate that the vast majority of ErbB4 is extrasynaptic and detergent-soluble. To explore the function of this receptor population, we used unbiased proteomics, in combination with electrophysiological, biochemical, and cell biological techniques, to identify a clinically relevant ErbB4-interacting protein, the GABA A receptor α1 subunit (GABAR α1). We show that ErbB4 and GABAR α1 are robustly coexpressed in hippocampal interneurons, and that ErbB4-null mice have diminished cortical GABAR α1 expression. Moreover, we characterize a Neuregulin-mediated ErbB4 signaling modality, independent of receptor tyrosine kinase activity, that couples ErbB4 to decreased postsynaptic GABAR currents on inhibitory interneurons. Consistent with an evolving understanding of GABAR trafficking, this pathway requires both clathrin-mediated endocytosis and protein kinase C to reduce GABAR inhibitory currents, surface GABAR α1 expression, and colocalization with the inhibitory postsynaptic protein gephyrin. Our results reveal a function of ErbB4, independent of its tyrosine kinase activity, that modulates postsynaptic inhibitory control of hippocampal interneurons and may provide a novel pharmacological target in the treatment of neuropsychiatric disorders and epilepsy.PKC | schizophrenia | hippocampus | parvalbumin | mIPSCs E rbB4 signaling regulates neuronal excitability (1, 2) and synaptic plasticity (3, 4) in the adult brain, and has been implicated in psychiatric disorders (5, 6) and epilepsy (2, 7). In the neocortex and hippocampus of rodents, monkeys, and humans, ErbB4 expression is restricted to GABAergic interneurons, and its expression is particularly high in parvalbumin-positive fastspiking (PV+) interneurons (8, 9). Of note, targeted ablation of ErbB4 specifically in PV+ interneurons recapitulates behavioral abnormalities of full ErbB4-null mice, highlighting the importance of ErbB4 signaling in this GABAergic interneuron subclass (10). Moreover, gamma oscillations, a type of high-frequency network activity that depends on synchronization of local circuits by PV+ interneurons, are augmented by Neuregulin (NRG)1 in vitro in an ErbB4-dependent manner (11).In the hippocampus, the GABA A receptor α1 subunit (GABAR α1), which imparts rapid decay kinetics (12), is also selectively expressed in subsets of inhibitory interneurons, especially in PV+ neurons (13,14). Furthermore, a mutation in GABRA1 has been linked to absence seizures (15), and heterozygous Gabra1-null mice show cortical absence epileptiform activity (16). Additionally, genome-wide linkage analyses have repeatedly identified a cluster of GABAR subunits, which includes GABRA1, as a schizophrenia (SCZ) susceptibility locus (17). Therefore, identifying mechanisms that acutely regulate α1-containing GABARs o...