Fine-scale transgenic mapping of the MyoD core enhancer: MyoD is regulated by distinct but overlapping mechanisms in myotomal and non-myotomal muscle lineages.

Skeletal muscle lineage determination is regulated by the myogenic regulatory genes, MyoD and Myf-5. Previously, we identified a 258 bp core enhancer element 20 kb 5' of the MyoD gene that regulates MyoD gene activation in mouse embryos. To elucidate the cis control mechanisms that regulate MyoD transcription, we have mutagenized the entire core enhancer using linker-scanner mutagenesis, and have tested the transcriptional activity of enhancer mutants using lacZ reporter gene expression in transgenic mouse embryos. In total, 83 stable transgenic lines representing 17 linker-scanner mutations were analyzed in midgestational mouse embryos. Eight linker-scanner mutations resulted in a partial or complete loss of enhancer activity, demonstrating that MyoD is primarily under positive transcriptional control. Six of these mutations reduced or abolished transgene expression in all skeletal muscle lineages, indicating that activation of MyoD expression in trunk, limb and head musculature is regulated, in part, by shared transcriptional mechanisms. Interestingly, however, two adjacent linker-scanner mutations (LS-14 and LS-15) resulted in a dramatic reduction in transgene expression specifically in myotomes at 11.5 days. At later stages, transgene expression was absent or greatly reduced in myotomally derived muscles including epaxial muscles (deep back muscles) and hypaxial muscles of the body wall (intercostal muscles, abdominal wall musculature). In contrast, head muscles, as well as muscles of the body derived from migrating muscle progenitor cells (e.g. limb, diaphragm), were unaffected by these mutations. In Pax-3-mutant mice, LS-14 and LS-15 transgene expression was eliminated in the body, but was unaffected in the head, yielding an identical expression pattern to the endogenous MyoD gene in mice mutant for both Myf-5 and Pax-3. These data support the hypothesis that LS-14 and LS-15 define the core enhancer targets for Myf-5-dependent activation of MyoD in myotomal muscles.