Intracellular calcium and myosin isoform transitions. Calcineurin and calcium-calmodulin kinase pathways regulate preferential activation of the IIa myosin heavy chain promoter.

Intracellular calcium levels can have profound effects on muscle biology via alterations in gene expression. In particular, intracellular calcium levels increase during muscle activation and are thought to underlie fast-to-slow shifts in muscle gene expression. In the present work, we determined that increased intracellular calcium has a significant effect on the activity of the adult fast myosin heavy chain (MyHC) promoters in the order of MyHC IIa>> IId/x > IIb. We have identified the pathways by which the calcium signal mediates increased activation of the MyHC IIa promoter. Inhibition of calcineurin or calcium-calmodulin kinase greatly attenuates ionophore-induced activation of the MyHC IIa promoter, whereas protein kinase C inhibitors have no effect. Inhibition and overexpression studies with members of the mitogen-activated protein kinase family reveal roles for MEK1/MEK2 and MEKK1, but not p38 or phosphatidylinositol 3-kinase. Downstream mediators of these effects are the activities of the MEF-2 and NFAT transcription factors, whose binding sites in the MyHC IIa promoter are required for calcium-induced activation of the MyHC IIa promoter.