Mirk/dyrk1B decreases the nuclear accumulation of class II histone deacetylases during skeletal muscle differentiation

Mirk/dyrk1B is a member of the dyrk/minibrain family of serine/threonine kinases that mediate the transition from growth to differentiation in lower eukaryotes and mammals. Depletion of endogenous Mirk from C2C12 myoblasts by RNA interference blocks skeletal muscle differentiation (Deng, X., Ewton, D., Pawlikowski, B., Maimone, M., and Friedman, E. (2003) J. Biol. Chem. 278, 41347-41354). We now demonstrate that knockdown of Mirk blocks transcription of the muscle regulatory factor myogenin. Co-expression of Mirk with MEF2C, but not MyoD or Myf5, enhanced activation of the myogenin promoter in a Mirk kinase-dependent manner. Mirk activated MEF2 not through direct phosphorylation of MEF2 but by phosphorylation of its inhibitors, the class II histone deacetylases (HDACs). MEF2 is sequestered by class II HDACs such as HDAC5 and MEF2-interacting transcriptional repressor (MITR). Mirk antagonized the inhibition of MEF2C by MITR, whereas kinase-inactive Mirk was ineffective. Mirk phosphorylates class II HDACs at a conserved site within the nuclear localization region, reducing their nuclear accumulation in a dose-dependent and kinase-dependent manner. Moreover, less mutant MITR phosphomimetic at the Mirk phosphorylation site localized in the nucleus than wildtype MITR. Regulation of class II HDACs occurs by multiple mechanisms. Others have shown that calcium signaling leads to phosphorylation of HDACs at 14-3-3-binding sites, blocking their association with MEF2 within the nucleus. Mirk provides another level of regulation. Mirk is induced within the initial 24 h of myogenic differentiation and enables MEF2 to transcribe the myogenin gene by decreasing the nuclear accumulation of class II HDACs.