Multiple signaling pathways cooperate to activate skeletal muscle differentiation

Vertebrate skeletal muscle differentiation is a delicate and highly coordinated process in which multiple factors and intracellular signaling pathways participate. Two major families of transcription factors, the MRFs and MEF2s, are found to play important roles and act as end points for diverse intracellular signaling pathways. MRFs include Myf5, MyoD, myogenin and MRF4. In the first study, we identified that myogenin and MEF2C as important transcriptional targets under the control of three intracellular signaling pathways, namely, the p38 MAPK-, calcineurin-, and CaMK-mediated signaling pathways. In the second study, we demonstrated that full activation of MEF2s could only be achieved through cooperation of multiple pathways. And these signaling pathways seemed to target different domains of MEF2s. Ordered assembly or disassembly sequence was required for the activation of MEF2s. The histone deacetylases (HDACs) are a family of proteins acting as suppressors of MEF2s. We found that the p38 MAPK could directly bind and phosphorylate HDAC4 to derepress the repression effect of HDAC4 on MEF2s. We also provided some interesting mechanistic insights into the role of JNK in myogenesis. Akt is a key mediator of the IGF/ PI3K-mediated signaling pathway in stimulating muscle differentiation and muscle cell hypertrophy. mTOR serves as a direct substrate of Akt and is also involved in muscle differentiation and hypertrophy. However, the Akt/mTOR-responsive genes that bring about the observed phenotypes still remain unclear. In the third study, PCR-based suppression subtractive hybridization (SSH) was employed to identify the differentially displayed genes responsive to the Akt/mTOR signaling. Confirmed by Northern blot analysis, one of the ATF/CREB family members, ATF4, was found to be significantly induced by Akt/mTOR. A 500-bp fragment of ATF4 proximal promoter was shown to contain sufficient regulatory elements for specific response to the PI3K/Akt-mediated signaling pathway