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

Abstract

Five different subunits, α, β, γ, δ and ε, constitute the acetylcholine receptors from mammalian skeletal muscle. Their corresponding mRNA levels are regulated differentially. In particular, mRNAs encoding the γ- and ε-subunits, which specify two AChR isoforms, show a reciprocal behaviour during synapse formation and maturation. We have isolated 5′ flanking sequences of the γ- and ε-subunit genes that confer muscle-specific expression upon transient transfection of primary cultures of rat muscle cells. The γ-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 ε-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 ε-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.