Human B creatine kinase gene expression in C2C12 cells is regulated by protein interactions involving the first exon

While the activation of muscle specific genes is well characterized, the mechanism mediating the regulation during muscle development of embryonic genes is poorly understood. To begin to investigate this area, the transcriptional regulation of the human brain creatine kinase (BCK) gene was characterized during in vitro myogenesis of C2C12 muscle cells. Initial analyses identified the first exon as important for high level expression in general and for the decrease in expression in response to C2C12 differentiation. In vivo competition confirmed the functional importance of the first exon. Electrophoretic mobility shift assays using portions of the exon showed that two separate proteins bound to sequences +1 to +27 and +25 to +57, respectively. The +1 to +27 interacting factor is present in skeletal but not cardiac muscle and thus may function as a skeletal muscle determining factor. The +25 to +57 interacting factor functions as a positive effector in myoblasts. This region also imparts the differentiation dependent decrease in expression via either modification of the myoblast factor or due to the presence of an additional factor. Site directed mutagenesis show that base pair +26 is critical for appropriate down regulation of BCK.