Different pathways regulate expression of the skeletal myosin heavy chain genes

Mammalian skeletal muscles are a mosaic of different fiber types largely de fined by differential myosin heavy chain (MyHC) expression. Little is known about the molecular mechanisms regulating expression of the MyHC gene fami ly members in different fiber types. In this work, we identified several ci s- and trans-elements that regulate expression of the three adult fast MyHC genes. Despite multiple DNA-binding motifs for well characterized muscle t ranscription factors upstream of all three fast MyHC genes, expression of M yoD/Myf-5, calcineurin, or NFAT3 had different effects on the three promote rs. MyoD or Myf-5 overexpression preferentially activated the M promoter, w hereas NFAT or activated calcineurin overexpression preferentially activate d the IIa promoter. Calcineurin had a 50-100-fold stimulatory effect on the IIa promoter, and the known downstream effectors of calcineurin (myocyte e nhancer factor-2 and NFAT) cannot completely account for this activation. F inally, we identified two elements critical for regulating MyHC-IId/x expre ssion: a 130-base pair enhancer element and a CArG-like element that inhibi ted IId/x promoter activity in vitro. Thus, we have found specific regulato ry pathways that are distinct for the three adult fast MyHC genes. These el ements are logical candidates for fiber-specific control of skeletal muscle gene expression in vivo.