Different mechanisms regulate muscle-specific AChR gamma- and epsilon-subunit gene expression.

Five different subunits, alpha, beta, gamma, delta and epsilon, constitute the acetylcholine receptors from mammalian skeletal muscle. Their corresponding mRNA levels are regulated differentially. In particular, mRNAs encoding the gamma- and epsilon-subunits, which specify two AChR isoforms, show a reciprocal behaviour during synapse formation and maturation. We have isolated 5' flanking sequences of the gamma- and epsilon-subunit genes that confer muscle-specific expression upon transient transfection of primary cultures of rat muscle cells. The gamma-subunit gene fragment contains two adjacent CANNTG sequence motifs that are essential for muscle-specific transcriptional activity suggesting transactivation by helix-loop-helix proteins. The epsilon-subunit gene fragment carries only a single CANNTG consensus motif which is not required for expression in transfected muscle cells. This sequence motif is, however, necessary to repress transcriptional activity in non-muscle cells and thus may control the muscle-specific expression of the epsilon-subunit gene. The results suggest that CANNTG motifs together with their 3' and 5' flanking nucleotides provide binding sites for both activating as well as repressing trans-acting factors. These elements could thus contribute to the muscle-specific expression of AChR subunit genes.