Developmental Regulation of Acetylcholinesterase mRNA in the Mouse Diaphragm

Abstract

A single gene generates distinct catalytic subunits of acetylcholinesterase (AChE, EC 3.1.1.7), by alternative splicing of exons encoding C-terminal peptides (Massoulie et al., 1993). The T (“tailed”) subunits produce a variety of molecular forms, including collagen-tailed molecules and hydrophobic-tailed tetramers, the H (“hydrophobic”) subunits produce membrane-bound dimers, and the R (“readthrough”) subunits have only been inferred from the existence of cDNAs which retain the intervening sequence following the common exon in the genome (Li et al., 1993). In adult mammals, only mRNAs encoding the T subunit have been detected in muscles and brain (Legay et al., 1993). We have used in situ hybridization and the polymerase chain reaction (PCR) to analyze the nature and distribution of AChE mRNAs along myofibers in the mouse diaphragm, during development. While adult muscle only contains the T transcript, expressed by junctional nuclei (Jasmin et al., 1993), we were surprized to find that, from embryonic day 13 (E13) until birth, the diaphragm contains the three types of AChE mRNAs: T is already predominant at this stage, R represents a few percent of mRNA and H is detectable in still lower proportion. This is the first report of the existence of the R transcript in muscle, and we obtained similar results with the C2 myogenic cell line, both as myoblasts and differentiated myotubes. As early as E13, T mRNAs preferentially accumulate in the midline, where the first neuromuscular contacts are forming.