Abstracts

Rationale: : Gamma-aminobutyric acid type A (GABAA) receptors mediate the majority of rapid inhibitory synaptic transmission in the CNS. Mutations in the GABAA receptor β3 subunit and deletions of chromosome 15q, which contain the β3 subunit gene GABRB3 are frequently associated with epilepsy as well as other neurodevelopmental abnormalities like autism and Angelman syndrome. Deletion of β3 subunits in GABRB3 knockout mice results in ~90% neonatal lethality, and the surviving GABRB3 null mice have been proposed to be both human Angelman syndrome and autism models due to their neurodevelopmental abnormalities and phenotypic presentations. The GABAA receptor GABRB3 subunit is essential for pentameric GABAA receptor assembly and trafficking. Any alteration of GABRB3 subunit expression would change GABAA receptor expression, composition, distribution and membrane diffusion in a neuronal milieu during brain development when multiple GABAA receptor subunits coexists. Methods: We have characterized GABAA receptor expression, distribution and mobility in GABRB3 knockout mice. Considering all patients reportedly carrying GABRB3 subunit mutations are heterozygous, we decided to focus on the expression of GABAA receptor subunits in both mouse brain and cultured neurons from heterozygous GABRB3 KO mice. To do so, we employed live brain slice surface biotinylation, single Quantum dot imaging and patch-clamp recording in live hippocampal neurons. Results: We demonstrated that both surface and total expression of GABRB3 subunits were reduced in heterozygous mice. Moreover, γ2 subunit expression was reduced and β2 subunits had altered glycosylation in heterozygous mice. Using single quantum dot imaging, we found that surface GABAA receptor in heterozygous GABRB3 KO mice had slowed mobility and a larger fraction of them located outside of synapses. Whole-cell patch-clamp recording of hippocampal GABAergic interneurons from both heterozygous and homozygous mutant GABRB3 knockout mice showed reduced peak current amplitudes and increased zinc inhibition. The heterozygous GABRB3 knockout mice exhibited absence-like seizures in both electrographical and behavioral examination. Intriguingly, GABRB3 knockout mice also had reduced latency to fall during rotarod testing, suggesting impaired cerebellar function. Conclusions: Our study suggests that impairment in GABRB3 subunit alters expression, post-translational modification, distribution and mobility of other wildtype GABAA receptor subunits, which significantly impacts GABAA receptor function and synaptic transmission, resulting in a complex neurobehavioral phenotype