CYCLIN-DEPENDENT KINASE-5, CDK5, IS A POSITIVE REGULATOR OF MYOGENESIS IN MOUSE C2 CELLS

We have examined the expression, activity and localization of cyclin d ependent kinase 5 (cdk5), during myogenesis. Cdk5 protein was found ex pressed in adult mouse muscle. In murine C2 cells, both the protein le vel and kinase activity of cdk5 showed a marked increase during early myogenesis with a peak between 36 and 48 hours of differentiation, dec reasing as myotubes fuse after 60 to 72 hours. This increase in cdk5 p rotein level was specific for differentiation and not simply related t o cell cycle arrest since it was not observed in fibroblasts grown for 48 hours in low serum medium. Indirect immunofluorescence using monos pecific purified anti-cdk5 antibodies showed a low level cytoplasmic s taining in proliferative myoblasts, a rapid increase in nuclear staini ng during the initial 12 hours of differentiation and a predominant nu clear staining in myotubes. Microinjection of plasmids encoding wild-t ype cdk5 into C2 myoblasts enhanced differentiation as assessed by bot h myogenin and troponin T expression after 48 hours of differentiation . In contrast, microinjection of plasmids encoding a dominant negative mutant of cdk5 inhibited the onset of differentiation. These data imp ly a previously unsuspected role for cdk5 protein kinase as a positive modulator of early myogenesis.