bHLH transcription factor MyoD affects myosin heavy chain expression pattern in a muscle-specific fashion

A strong correlative pattern between MyoD gene expression and myosin heavy chain IIB (MHC IIB) gene expression exists. To test whether this correlativ e relationship is causative, MHC gene expression in muscles from MyoD(-/-) mice was analyzed. The MHC IIB gene was not detectable in the MyoD(-/-) dia phragm, whereas the MHC IIB protein made up 10.0 +/- 1.7% of the MHC protei n pool in the wild-type (WT) mouse diaphragm. Furthermore, the MHC IIA prot ein was not detectable in the MyoD(-/-) biceps brachii, and the MHC IIB pro tein was overexpressed in the masseter. To examine whether MyoD is required for the upregulation of the MHC IIB gene within slow muscle after disuse, MyoD(-/-) and WT hindlimb musculature was unweighted. MyoD(-/-) exhibited a diminished response in the upregulation of the MHC IIB mRNA within the sol eus muscle as a result of the hindlimb unweighting. Collectively, these dat a suggest that MyoD plays a role in the MHC profile in a muscle-specific fa shion.