Differential cell surface expression of GABA-A receptor alpha 1, alpha 6 and beta 2 and beta 3 subunits in cultured mouse cerebellar granule cells - influence of cAMP-activated signalling

Abstract

In this study we have used mature, primary cultured mouse cerebellar granule cells (CGCs) to initiate our studies on the mechanisms governing neuronal trafficking of GABAA receptors (GABARs). Initially the steady-state distribution of GABAR α1, α6, β2 and β3 subunits between the cell surface and cell interior was quantified. Cell surface proteins were modified with a membrane-impermeable cross-linking agent, bis(sulfosuccinimidyl)suberate (BS3) or the proteolytic enzyme, chymotrypsin. The proportion of unmodified (intracellular) and modified (cell surface) subunits was quantified by immunoblotting. We found that 51% of α6, 74% of α1, and 83% of β2/3 were expressed at the cell surface, thus identifying a sizeable intracellular pool of α6 in contrast to the low levels of intracellular α1 and β2/3. Chronic activation of protein kinase A (PKA) in CGCs in vitro, post-transcriptionally up-regulated expression of α1, β2 and β3 but not α6. This was paralleled by an increase in the BZ-S subtype of [3H]Ro15–4513 binding sites. GABAR α1 was increased at the cell surface and in the cell interior, β2 was increased almost exclusively at the cell surface whilst β3 was increased almost exclusively in the cell interior. The intracellular pool of α6 was not affected. Thus, GABAR subunits are subject to differentially regulated trafficking, affording yet greater scope for GABAR diversity and plasticity.