Protein Kinase Cζ Regulates Cdk5/p25 Signaling during Myogenesis

Atypical protein kinase C zeta (PKC zeta) is emerging as a mediator of differentiation. Here, we describe a novel role for PKC zeta in myogenic differentiation, demonstrating that PKC zeta activity is indispensable for differentiation of both C2C12 and mouse primary myoblasts. PKC zeta was found to be associated with and to regulate the Cdk5/p35 signaling complex, an essential factor for both neuronal and myogenic differentiation. Inhibition of PKC zeta activity prevented both myotube formation and simultaneous reorganization of the nestin intermediate filament cytoskeleton, which is known to be regulated by Cdk5 during myogenesis. p35, the Cdk5 activator, was shown to be a specific phosphorylation target of PKC zeta. PKC zeta-mediated phosphorylation of Ser-33 on p35 promoted calpain-mediated cleavage of p35 to its more active and stable fragment, p25. Strikingly, both calpain activation and the calpain-mediated cleavage of p35 were shown to be PKC zeta-dependent in differentiating myoblasts. Overall, our results identify PKC zeta as a controller of myogenic differentiation by its regulation of the phosphorylation-dependent and calpain-mediated p35 cleavage, which is crucial for the amplification of the Cdk5 activity that is required during differentiation.