Inefficient cell-surface expression of hybrid complexes formed by the co-assembly of neuronal nicotinic acetylcholine receptor and serotonin receptor subunits

Abstract

Previous studies have demonstrated that relatively low levels of α4β2 neuronal nicotinic acetylcholine receptors (nAChRs) are expressed on the cell surface of transfected mammalian cell lines but that surface expression levels can be dramatically up-regulated by co-expression of these subunits with chimeric subunits containing the N-terminal portion of the neuronal nAChR α4 or β2 subunits together with the C-terminal domain of the 5-HT 3A subunit. Recent work has also suggested that the nAChR α4 subunit can co-assemble in a ‘promiscuous’ manner with the serotonin receptor 5-HT 3A subunit to form functional hybrid receptors. In this study we have examined whether co-assembly of either α4 or β2 with 5-HT 3A itself (rather than with the α4/5-HT 3A or β2/5-HT 3A subunit chimeras) can also facilitate cell surface expression of α4 and β2 subunits in transfected mammalian cells. Evidence has been obtained by immunoprecipitation, cell-surface antibody binding and radioligand binding which indicates that the 5-HT 3A can co-assemble with both the α4 and β2 nAChR subunits. We conclude, however, that co-assembly of 5-HT 3A with either α4 or β2 does not result in efficient cell surface expression of the nAChR subunits and that co-assembled hybrid (nAChR subunit + 5-HT 3R subunit) receptor complexes are largely retained within the cell.