Aberrant development of motor axons and neuromuscular synapses in MyoD-null mice.
The expression of gamma and epsilon subunits of the acetylcholine receptor from mammalian skeletal muscle is regulated independently during myogenic differentiation and innervation. Genomic DNA fragments containing 5'-flanking sequences of the epsilon-subunit and gamma-subunit genes were characterised by a series of 5' deletions fused to the chloramphenicol-acetyltransferase gene and transiently expressed by transfection of primary cultures of rat muscle cells and non-muscle cells. A 6.3-kb epsilon-subunit fragment can be reduced to yield a 270-bp fragment that confers 5-10-times higher expression levels in muscle cells compared to in non-muscle cells. The region composed of nucleotides -185 to -128 increases the transcriptional activity moderately while the 14-bp palindrome containing a single E box at nucleotides -88 to -83 may interact with the promoter but has no enhancer properties in muscle cells. From a 1.1-kb genomic fragment of the gamma-subunit gene, 167 bp were sufficient for muscle-specific expression. Two promoter-proximal E-box elements enhance promoter activity in muscle and mediate transactivation by myogenic factors. Myogenin and myf5 were much more efficient than MRF4 or MyoD1 which exerted only little transactivation. Cotransfection experiments show that increased expression of Id in primary muscle cells inhibits chloramphenicol-acetyltransferase expression mediated by the gamma-subunit gene promoter and support the view that myogenic factors play an important role in the transcriptional regulation of the gamma-subunit gene.