Compartmentalized transcription of acetylcholine receptor genes during motor endplate epigenesis

Abstract

In the adult motor endplate the acetylcholine receptor protein (AChR) is strictly localized under the motor nerve ending, whereas in the noninnervated myotube it is distributed all over the surface of the cell. The genesis of this anisotropic distribution involves a differential regulation of AChR gene transcription. In situ hybridization with AChR subunit probes discloses high levels of unspliced and mature mRNA all over differentiating myotubes. After the entry of the exploratory motor axons, the mRNA clusters located outside the endplate decrease in number and become restricted to the subneural "fundamental" nuclei. Denervation causes a reappearance of unspliced and mature mRNA in extrajunctional areas. A compartmentalized expression of AChR genes take place during endplate formation. Chronic paralysis of the embryo interferes with the disappearance of extrajunctional AChR that, thus, represents an electrical activity-dependent repression of AChR genes. The entry of Ca2+ ions through the sarcolemmal membrane during electrical activity and the activation of protein kinase C plausibly contribute to this membrane-to-gene regulation. The maintenance and late increase in AChR number at the endplate requires the intervention of an anterograde signal or signals, of neural origin. Several factors have been suggested to play a role in this process, such as an acetylcholine receptor-inducing activity (ARIA), ascorbic acid, or calcitonin gene-related peptide (CGRP), a peptide known to coexist with acetylcholine in spinal cord motoneurons. In cultured chick muscle cells, CGRP increases the concentration of surface AChR and alpha-subunit unspliced and mature mRNA and stimulates membrane-bound adenylate cyclase, suggesting that distinct second messengers are involved in the regulation of AChR biosynthesis by electrical activity and by CGRP. The data are interpreted in terms of a model in which it is assumed that (i) in the adult muscle fiber, different stages of gene expression occur in the nuclei in subneural and extrajunctional areas, and (ii) different second messengers elicited by neural factors or electrical activity regulate the state of transcription of these nuclei via trans-acting allosteric proteins binding to cis-acting DNA regulatory elements. The upstream flanking regions of several of the AChR subunit genes reveal ubiquitous DNA elements such as TATA and CAAT boxes, Sp1 binding sites and SV40 core enhancer sites, and muscle-specific MyoD (CANNTG) elements. The contribution of some of these elements to the differential regulation of the multiple AChR subunits is discussed.