Cotranslational association of mRNA transcripts encoding heteromeric GABA-A receptors.

Abstract

Type A γ-aminobutyric receptors (GABAR) are pentameric ligand gated ion channels that mediate fast inhibition in the adult brain and are drug targets for barbiturates, benzodiazepines, intravenous anesthetics, and neurosteroids. Consistent with their essential role in regulating neuronal excitability, dysregulation of GABAR activity contributes to anxiety, autism, depression, epilepsy, substance abuse and schizophrenia. GABARs are heteropentamers that can be assembled from α(1-6), β(1-3), γ(1-3), δ, ε, θ and π subunits. The abundance, type, and function of GABARs at synaptic and extra-synaptic sites directly controls the strength of GABAergic transmission. Surprisingly, our understanding of how different GABAR subtypes, each comprising a unique complement of subunits, are assembled is rudimentary. Moreover, how the appropriate levels of these receptors are maintained to control excitatory/inhibitory balance remains unclear. In preliminary experiments, when expressed in HEK293 cells, we find that mRNA transcripts encoding the GABA-A receptor α1, β2 and γ2 subunits are physically associated and can be co-immunoprecipitated with nascent GABAR protein using an antibody against the GABAR α1 subunit suggesting that hetero-oligomeric assembly of GABARs is mediated by a complex comprising the mRNAs encoding each of the subunits. The transcript association occurred only when GABAR subunits were co-expressed and not when lysates from cells independently expressing different subunits were mixed. When the potassium channel, hERG, was co-expressed with GABARs, hERG transcript was not co-immunoprecipitated. The GABAR transcript association was also observed when GABARs were immunoprecipitated from mouse brain cortex. Our data suggests a new paradigm for heteromeric assembly of GABARs.