Abstract
Connexin43 is widely expressed in mammalian tissues but is absent from skeletal muscle. Skeletal myoblast fusion in vitro requires the expression of connexin43 gap junction channels. However, gap junctions need to be absent in neonatal skeletal muscle for fine motor control and formation of mature neuromuscular junctions (NMJs). In this study I show that this downregulation is accomplished by a specific microRNA, miR206, that inhibits the translation of cx43 mRNA during myoblast fusion. Cx43 mRNA contains two binding sites for miR-206 in its 3'-UTR, both of which are required for efficient downregulation. The miR-206 gene is found as a single copy in most known vertebrate genomes, and the sequence of the mature microRNA is perfectly conserved in human, mouse, rat, dog, and zebrafish. In this work I show that miR-206 downregulates Cx43 expression during perinatal skeletal muscle development in vivo as well as during myoblast fusion in vitro. Proper development of singly innervated muscle fibers requires muscle contraction and NMJ terminal selection. This work details the mechanism by which initial downregulation of Cx43 occurs during myogenesis and highlights the tight control mechanisms that are required for the regulation of gap junctions during differentiation and development. Translational regulation of Cx43 by miR-206 is one of the few detailed demonstrations of a microRNA interacting with its mRNA target during development in vertebrates.