Protein Kinase A Represses Skeletal Myogenesis by Targeting Myocyte Enhancer Factor 2D

Abstract

Activation of protein kinase A (PKA) by elevation of the intracellular cyclic AMP (cAMP) level inhibits skeletal myogenesis. Previously, an indirect modulation of the myogenic regulatory factors (MRFs) was implicated as the mechanism. Because myocyte enhancer factor 2 (MEF2) proteins are key regulators of myogenesis and obligatory partners for the MRFs, here we assessed whether these proteins could be involved in PKA-mediated myogenic repression. Initially, in silico analysis revealed several consensus PKA phosphoacceptor sites on MEF2, and subsequent analysis by in vitro kinase assays indicated that PKA directly and efficiently phosphorylates MEF2D. Using mass spectrometric determination of phosphorylated residues, we document that MEF2D serine 121 and serine 190 are targeted by PKA. Transcriptional reporter gene assays to assess MEF2D function revealed that PKA potently represses the transactivation properties of MEF2D. Furthermore, engineered mutation of MEF2D PKA phosphoacceptor sites (serines 121 and 190 to alanine) rendered a PKA-resistant MEF2D protein, which efficiently rescues myogenesis from PKA-mediated repression. Concomitantly, increased intracellular cAMP-mediated PKA activation also resulted in an enhanced nuclear accumulation of histone deacetylase 4 (HDAC4) and a subsequent increase in the MEF2D-HDAC4 repressor complex. Collectively, these data identify MEF2D as a primary target of PKA signaling in myoblasts that leads to inhibition of the skeletal muscle differentiation program.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank Xu Guo from Applied Biosystems/MDS Sciex for assistance in performing nano-LC-MS/MS. We thank Tetsuaki Miyake for technical assistance. We also thank Lee Wong in the York University Core Molecular Biology Facility for DNA sequencing.

These studies were made possible by a grant from the Canadian Institute of Health Research (CIHR) to J.C.M. Salary support for R.L.S.P. was provided in part by a postdoctoral fellowship from the Muscular Dystrophy Association of Canada (MDAC) and CIHR. Infrastructural support was provided by the Ontario Research and Development Challenge Fund and Applied Biosystems/MDS SCIEX to K.W.M.S. Seneca College Office of Research and Innovation is acknowledged for providing release time support for J. W. Gordon.