Changes in conformation and subcellular distribution of alpha4beta2 nicotinic acetylcholine receptors revealed by chronic nicotine treatment and expression of subunit chimeras.

Chronic exposure to nicotine, as occurs during tobacco smoking, is one of several factors that have been reported to cause an upregulation of neuronal nicotinic acetylcholine receptors (nAChRs). Here, the influence of both chronic exposure to nicotine (10 microm, 24 hr) and the coexpression of subunit chimeras has been examined in cultured cell lines expressing recombinant alpha4beta2 nAChRs, a major nicotinic receptor subtype expressed in the mammalian brain. Evidence is presented which demonstrates that both chronic exposure to nicotine and the coexpression of subunit chimeras upregulates levels of receptor expressed on the cell surface. Immunoblotting data indicate that neither chronic nicotine treatment nor coexpressed subunit partners greatly affect the level of total subunit protein. This finding, together with radioligand and antibody binding studies conducted on both intact and permeabilized cells, reveals that receptor upregulation corresponds to an increase in the proportion of total receptor expressed on the cell surface. It is also apparent that nicotine-induced nAChR upregulation is very strongly dependent on subunit composition and subunit domains. An important aspect of this study is that direct evidence has been obtained indicating that both chronic exposure to nicotine and coexpressed subunit partners can influence subunit conformation. The influence of chronic nicotine treatment on subunit folding may help to explain the phenomenon of nicotine-induced receptor upregulation. The finding that subunit conformation can be influenced by coassembled subunit partners is in agreement with models of receptor assembly which propose that subunit folding continues after initial subunit-subunit interactions.