Characterizing the signaling network that controls MEF2 transcription factors is crucial for understanding skeletal and cardiac muscle gene expression. Glycogen synthase kinase 3 beta (GSK3 beta) regulates MEF2 activity indirectly through reciprocal regulation of p38MAPK. Cross-talk between GSK3 beta and p38MAPK regulates MEF2 activity in skeletal and cardiac muscle. Understanding cross-talk in the signaling network converging at MEF2 control has therapeutic implications in cardiac and skeletal muscle pathology. Glycogen synthase kinase 3 beta (GSK3 beta) is a known regulator of striated muscle gene expression suppressing both myogenesis and cardiomyocyte hypertrophy. Since myocyte enhancer factor 2 (MEF2) proteins are key transcriptional regulators in both systems, we assessed whether MEF2 is a target for GSK3 beta. Pharmacological inhibition of GSK3 beta resulted in enhanced MEF2A/D expression and transcriptional activity in skeletal myoblasts and cardiac myocytes. Even though in silico analysis revealed GSK3 beta consensus (S/T)XXX(S/T) sites on MEF2A, a subsequent in vitro kinase assay revealed that MEF2A is only a weak substrate. However, we did observe a post-translational modification in MEF2A in skeletal myoblasts treated with a GSK3 beta inhibitor which coincided with increased p38MAPK phosphorylation, a potent MEF2A activator, indicating that GSK3 beta inhibition may de-repress p38MAPK. Heart specific excision of GSK3 beta in mice also resulted in up-regulation of p38MAPK activity. Interestingly, upon pharmacological p38MAPK inhibition (SB203580), GSK3 beta inhibition loses its effect on MEF2 transcriptional activity suggesting potent cross-talk between the two pathways. Thus we have documented that cross-talk between p38MAPK and GSK3 beta signaling converges on MEF2 activity having potential consequences for therapeutic modulation of cardiac and skeletal muscle gene expression. (c) 2012 Elsevier Ltd. All rights reserved.
