MYOD PROTEIN IS DIFFERENTIALLY ACCUMULATED IN FAST AND SLOW SKELETAL-MUSCLE FIBERS AND REQUIRED FOR NORMAL FIBER-TYPE BALANCE IN RODENTS

MyoD is a muscle-specific transcription factor involved in commitment of cells to myogenesis. MyoD mRNA levels differ between fast and slow muscles, suggesting that MyoD may regulate aspects of fibre type. Here we show that detectable MyoD protein becomes restricted during develo pment to the nuclei of the fastest classes of fibres in fast muscles. myoD(ml) mice, in which the myoD gene has been disrupted, show subtle shifts in fibre type of fast muscles toward a slower character, sugges ting that MyoD is involved in the maintenance of the fast IIB/IIX fibr e type. In contrast, slow muscle shifts to a faster phenotype in myoD( m1). Moreover, MD6.0-lacZ transgenic mice with the myoD promoter drivi ng lacZ, show highest beta-galactosidase activity in the fastest fibre s of fast muscles, but also express low levels in slow fibres of slow, but not fast, muscles, suggesting distinct regulation of gene express ion in slow fibres of fast and slow muscles. (C) 1997 Elsevier Science Ireland Ltd.