Factors regulating skeletal muscle development : cell culture and transgenic mouse studies

The final steps of vertebrate skeletal muscle development involve withdrawal of determined muscle precursors from the cell cycle, expression of muscle-specific genes encoding myofibril components, and cell fusion to form terminally differentiated multinucleated myotubes. A great deal of this process has been adapted to and studied in cell culture for decades. While much has been learned about how a cell's environment influences differentiation decisions and what those decisions involve, only in the last few years has progress been made in understanding how the observed drastic changes in gene expression which accompany differentiation are regulated in the nucleus. This progress was spurred by the cloning of MyoD, a nuclear protein which can convert a variety of nonmyogenic cell types to skeletal muscle. Much evidence suggests that MyoD and its three relatives, myogenin, Myf-5, and MRF4-herculin-Myf-6, play critical roles in the transcriptional activation of skeletal muscle differentiation genes. The studies presented in this thesis have addressed several important aspects of murine skeletal muscle development. Herculin, a novel member of the MyoD family of myogenic regulators, was cloned and characterized. Of the four known MyoD family members, herculin is the most abundant in adult skeletal muscle and therefore may be crucial for maintaining and/or enacting the mature muscle phenotype. Also, the c-myc proto-oncogene was shown to inhibit the ability of both MyoD and myogenin to initiate myogenic differentiation, even under conditions which normally promote it. This result is relevant for rationalizing, in part, why proliferating myoblasts which express MyoD do not spontaneously differentiate: c-myc is normally expressed in proliferating myoblasts but is down-regulated upon differentiation, perhaps allowing MyoD to become fully transcriptionally active. Finally, transgenic mice which express MyoD ectopically in the heart were produced. Transgenic hearts have morphological abnormalities and express myogenin and sarcomeric genes usually specific to skeletal muscle. This was the first demonstration that MyoD can function during murine embryogenesis and, more generally, that targeted misexpression of a tissue-specific regulator during mammalian development can activate genes normally transcribed in an unrelated tissue