MATURATION OF THE ACETYLCHOLINE-RECEPTOR IN SKELETAL-MUSCLE - REGULATION OF THE ACHR GAMMA-TO-EPSILON SWITCH

During the development of the mammalian neuromuscular junction, acetyl choline receptors (AChRs) become localized to the postsynaptic muscle membrane. As this process nears completion, the fetal form of the rece ptor, containing a gamma subunit (composition alpha(2) beta gamma delt a) is gradually replaced by an epsilon subunit-containing adult form ( alpha(2) beta epsilon delta). To understand how this transition is con trolled, we compared the expression and regulation of the AChR gamma a nd epsilon subunits in developing, adult, and cultured muscles. Immuno staining with subunit-specific antibodies showed that replacement of g amma subunit- by epsilon subunit-containing AChRs occurs largely durin g the first postnatal week in fast-twitch muscles, and occurs homogene ously throughout individual endplates. In the slow-twitch soleus, howe ver, this transition is delayed, and in the multiply innervated slow f ibers of extraocular muscle, gamma subunit expression persists into ad ulthood. The transcriptional bases of the AChR subunit transition, and of these intermuscular variations, were demonstrated in mice bearing transgenes containing promoter elements from the AChR gamma and epsilo n subunit genes, each coupled to a nuclear-localized beta-galactosidas e (nlacZ) reporter. We show that transgene expression is stimulated by the nerve-derived inducer of AChR expression, ARIA, in myotubes cultu red from gamma-nlacZ as well as epsilon-nlacZ mice. However, the expre ssion of gamma-nlacZ, but not epsilon-nlacZ, is increased by treatment of myotubes with TTX, and the ARIA sensitivity of gamma-nlacZ is depe ndent on the electrical state of the myotube. Thus, the promoters of t he gamma and epsilon subunit genes may integrate ARIA- and activity-de pendent signals in different ways to generate their complementary patt erns of expression. (C) 1996 Academic Press, Inc.