Maturation of the acetylcholine receptor in skeletal muscle: Regulation of the AChR γ-to-ε switch

Abstract

During the development of the mammalian neuromuscular junction, acetylcholine receptors (AChRs) become localized to the postsynaptic muscle membrane. As this process nears completion, the fetal form of the receptor, containing a γ subunit (composition (α2βγδ) is gradually replaced by an ε subunit-containing adult form (α2βεδ). To understand how this transition is controlled, we compared the expression and regulation of the AChR γ and ε subunits in developing, adult, and cultured muscles. Immunostaining with subunit-specific antibodies showed that replacement of γ subunit- by ε subunit-containing AChRs occurs largely during the first postnatal week in fast-twitch muscles, and occurs homogeneously throughout individual endplates. In the slow-twitch soleus, however, this transition is delayed, and in the multiply innervated slow fibers of extraocular muscle, γ subunit expression persists into adulthood. 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 γ and ε subunit genes, each coupled to a nuclear-localized β-galactosidase (nlacZ) reporter. We show that transgene expression is stimulated by the nerve-derived inducer of AChR expression, ARIA, in myotubes cultured from γ-nlacZ as well as ε-nlacZ mice. However, the expression of γ-nlacZ, but not ε-nlacZ, is increased by treatment of myotubes with TTX, and the ARIA sensitivity of γ-nlacZ is dependent on the electrical state of the myotube. Thus, the promoters of the γ and ε subunit genes may integrate ARIA- and activity-dependent signals in different ways to generate their complementary patterns of expression.