Forskolin stabilizes epsilon subunit-containing acetylcholine receptors

Fetal muscle-like acetylcholine receptors (AChRs) are composed of alpha, beta, gamma, delta subunits (gamma-AChRs) and have a rapid half life (t1/2), whereas adult muscle-like AChRs are composed of alpha, beta, epsilon, delta subunits (epsilon-AChRs) and have a slow t1/2. Two populations of AChRs, a slowly degrading population and a rapidly degrading population, have been shown to coexist in the postsynaptic membrane after denervation [In: Penn et al. (Eds.), Myasthenia Gravis and Related Disorders, Vol. 681, NY Acad. Sci., 1993, pp. 155-164]. Treatment of rat myotubes or mouse diaphragm muscle in organ culture with forskolin or cAMP analogues causes and increase in the t1/2 of the slowly degrading population of AChRs with no apparent effect on the rapidly degrading population of AChRs19. In this study, we have investigated the effect of forskolin on the cell surface half-lives of mouse gamma-AChRs, epsilon-AChRs and alpha beta delta complexes stably expressed in mouse fibroblasts. Forskolin had no significant effect on the t1/2 of gamma-AChRs or alpha beta delta complexes. The effect of forskolin on surface AChRs (alpha beta gamma delta) expressed in the C2 muscle cell line was similar to its effect on gamma-AChRs expressed in fibroblasts. In contrast, forskolin stabilized the epsilon-AChRs by approximately 2 fold. We show that the epsilon-subunit is phosphorylated in vivo, phosphorylation of epsilon increases with forskolin treatment, and the forskolin effect is reversible. Although the precise role of epsilon-subunit phosphorylation is yet to be determined, our results support the hypothesis that the slowly degrading population of AChRs consists of epsilon-AChRs and the rapidly degrading population of AChRs consists of gamma-AChRs.