Abstract
Signaling at nerve cell synapses is a key determinant of proper brain function, and synaptic defects—or synaptopathies—are at the basis of many neurological and psychiatric disorders. Collybistin (CB), a brain-specific guanine nucleotide exchange factor, is essential for the formation of γ-aminobutyric acidergic (GABAergic) postsynapses in defined regions of the mammalian forebrain, including the hippocampus and basolateral amygdala. This process depends on a direct interaction of CB with the scaffolding protein gephyrin, which leads to the redistribution of gephyrin into submembranous clusters at nascent inhibitory synapses. Strikingly, synaptic clustering of gephyrin and GABAA type A receptors (GABAARs) in several brain regions, including the cerebral cortex and certain thalamic areas, is unperturbed in CB-deficient mice, indicating that the formation of a substantial subset of inhibitory postsynapses must be controlled by gephyrin-interacting proteins other than CB. Previous studies indicated that the α3 subunit of GABAARs (GABAAR-α3) binds directly and with high affinity to gephyrin. Here, we provide evidence (i) that a homooligomeric GABAAR-α3A343W mutant induces the formation of submembranous gephyrin clusters independently of CB in COS-7 cells, (ii) that gephyrin clustering is unaltered in the neuronal subpopulations endogenously expressing the GABAAR-α3 in CB-deficient brains, and (iii) that exogenous expression of GABAAR-α3 partially rescues impaired gephyrin clustering in CB-deficient hippocampal neurons. Our results identify an important role of GABAAR-α3 in promoting gephyrin-mediated and CB-independent formation of inhibitory postsynapses.
Highlights
Heteropentameric γ-aminobutyric acid (GABA)-gated chloride channels that belong to the Cys-loop ligand-gated ion channel superfamily [2]
In cells coexpressing GABA type A receptor (GABAAR)-α1WT, we consistently found no colocalization with intracellular GFP–gephyrin aggregates
We identified an important role of the α3 subunit of GABAARs in priming CB-independent and gephyrin-mediated formation of GABAergic synapses in specific regions of the mammalian forebrain
Summary
Heteropentameric GABA-gated chloride channels that belong to the Cys-loop ligand-gated ion channel superfamily [2]. In agreement with a previous study [26], the densities of gephyrin puncta were significantly reduced in the dendrites of cultured hippocampal CB KO neurons expressing mScarlet alone, as compared with CB WT controls (Fig. 6, A, B, and E; CB KO/mScarlet, 2.15 ± 0.87 puncta/40 μm versus WT/mScarlet, 12.9 ± 2.97 puncta/ 40 μm).
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