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

The predominant nicotinic acetylcholine receptor (nAChR) expressed in vertebrate brain is a pentamer containing alpha 4 and beta 2 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 alpha 4 beta 2 nAChR, Functional recombinant alpha 4 beta 2 nAChRs, showing high affinity binding of nicotinic radioligands (K(d) = 41 +/- 22 pM for [(3)H]epibatidine), are expressed in both stably and transiently transfected mammalian cell lines, Despite this, only very low levels of alpha 4 beta 2 nAChRs can be detected on the cell surface of transfected mammalian cells maintained at 37 degrees C. At 30 degrees C, however, cells expressing alpha 4 beta 2 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" alpha 4 and beta 2 subunits, chimeric nicotinic/serotonergic subunits ("alpha 4 chi" and "beta 2 chi") are expressed very efficiently on the cell surface (at 30 degrees C or 37 degrees C), either as hetero-oligomeric complexes (e.g. alpha 4 chi+beta 2 or alpha 4 chi+beta 2 chi) or when expressed alone. Compared with alpha 4 beta 2 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.