Spatial and temporal changes in myosin heavy chain gene expression in skeletal muscle development

Seven myosin heavy chains (MyHC) are expressed in mammalian skeletal muscle in spatially and temporally regulated patterns. The timing, distribution, and quantitation of MyHC expression during development and early postnatal life of the mouse are reported here. The three adult fast MyHC RNAs (IIa, I Ib, and IId/x) are expressed in the mouse embryo and each mRNA has a distin ct temporal and spatial distribution. In situ hybridization analysis demons trates expression of IIb mRNA by 14.5 dpc, which proceeds developmentally i n a rostral to caudal pattern. IId/x and IIa mRNAs are detectable 2 days la ter. Ribonuclease protection assays demonstrate that the three adult fast g enes are expressed at approximately equal levels relative to each other in the embryo but at quite low levels relative to the two developmental isofor ms, embryonic and perinatal. Just after birth major changes in the relative proportions of different MyHC RNAs and protein occur. In all cases, RNA ex pression and protein expression appear coincident. The changes in MyHC RNA and protein expression are distinct in different muscles and are restricted in some cases to particular regions of the muscle and do not always reflec t their distribution in the adult. (C) 1999 Academic Press.