FORSKOLIN STABILIZES EPSILON-SUBUNIT CONTAINING ACETYLCHOLINE-RECEPTORS

Fetal muscle-like acetylcholine receptors (AChRs) are composed of alph a, beta, gamma, delta subunits (gamma-AChRs) and have a rapid half lif e (t(1/2)), whereas adult muscle-like AChRs are composed of alpha, bet a, epsilon, delta subunits (epsilon-AChRs) and have a slow t(1/2). Two populations of AChRs, a slowly degrading population and a rapidly deg rading population, have been shown to coexist in the postsynaptic memb rane after denervation [In: Penn et al. (Eds.), Myasthenia Gravis and Related Disorders s, Vol. 681, NY Acad. Sci., 1993, pp. 155-164]. Trea tment of rat myotubes or mouse diaphragm muscle in organ culture with forskolin or cAMP analogues causes an increase in the t (1/2) of the s lowly degrading population of AChRs with no apparent effect on the rap idly degrading population of increase in the t AChRs(19). In this stud y, we have investigated the effect of forskolin on the cell surface ha lf-lives of mouse gamma-AChRs, epsilon-AChRs and alpha beta delta comp lexes stably expressed in mouse fibroblasts. Forskolin had no signific ant effect on the t(1/2) of gamma-AChRs or alpha beta delta complexes. The effect of forskolin on surface AChRs (alpha beta gamma delta) exp ressed in the C2 muscle cell line was similar to its effect on gamma-A ChRs expressed in fibroblasts. In contrast, forskolin stabilized the e psilon-AChRs by similar to 2 fold. We show that the epsilon-subunit is phosphorylated in vivo, phosphorylation of epsilon increases with for skolin treatment, and the forskolin effect is reversible. Although the precise role of epsilon-subunit phosphorylation is yet to be determin ed, our results support the hypothesis that the slowly degrading popul ation of AChRs consists of epsilon-AChRs and the rapidly degrading pop ulation of AChRs consists of gamma-AChRs.