Postnatal myosin heavy chain isoform expression in normal mice and mice null for IIb or IId myosin heavy chains

The patterns of myosin heavy chain (MyHC) isoform expression in the embryo and in the adult mouse are reasonably well characterized and quite distinct . However, little is known about the transition between these two states, w hich involves major decreases and increases in the expression of several My HC genes. In the present study, the expression of seven sarcomeric MyHCs wa s analyzed in the hindlimb muscles of wild-type mice and in mice null for t he MyHC IIb or IId/x genes at several time points from 1 day of postnatal l ife (dpn) to 20 dpn. In early postnatal life, the developmental isoforms (e mbryonic and perinatal) comprise >90% of the total MyHC expression, while t hree adult fast isoforms (IIa, IIb, and lid) comprise <1% of the total MyHC protein. However, between 5 and 20 dpn their expression increases to compr ise >90% of the total MyHC. Expression of each of the three adult fast isof orms occurs in a spatially and temporally distinct manner.We also show that alpha MyHC, which is almost exclusively expressed in the heart, is express ed in scattered fibers in all hindlimb muscles during postnatal development . Surprisingly, the timing and localization of expression of the MyHC isofo rms is unchanged in IIb and IId/x null mice, although the magnitude of expr ession is altered for some isoforms. Together these data provide a comprehe nsive overview of the postnatal expression pattern of the sarcomeric MyHC i soforms in the mouse hindlimb. (C) 2001 Academic Press.