Up-regulation of cell-surface α4β2 neuronal nicotinic receptors by lower temperature and expression of chimeric subunits

Abstract

The predominant nicotinic acetylcholine receptor (nAChR) expressed in vertebrate brain is a pentamer containing α4 and β2 subunits. In this study we have examined how temperature and the expression of subunit chimeras can influence the efficiency of cell-surface expression of the rat α4β2 nAChR. Functional recombinant α4β2 nAChRs, showing high affinity binding of nicotinic radioligands (K(d) = 41 ± 22 pM for [3H]epibatidine), are expressed in both stably and transiently transfected mammalian cell lines. Despite this, only very low levels of α4β2 nAChRs can be detected on the cell surface of transfected mammalian cells maintained at 37 °C. At 30 °C, however, cells expressing α4β2 nAChRs show a 12-fold increase in radioligand binding (with no change in affinity), and a 5-fold up-regulation in cell-surface receptors with no increase in total subunit protein. In contrast to 'wild-type; α4 and β2 subunits, chimeric nicotinic/serotonergic subunits ('α4χ' and 'β2χ') are expressed very efficiently on the cell surface (at 30 °C or 37 °C), either as hetero-oligomeric complexes (e.g. α4χ+β2 or α4χ+β2χ) or when expressed alone. Compared with α4β2 nAChRs, expression of complexes containing chimeric subunits typically results in up to 20-fold increase in nicotinic radioligand binding sites (with no change in affinity) and a similar increase in cell-surface receptor, despite a similar level of total chimeric and wild-type protein.