| Summary: | Thesis (Ph.D.)--Boston University === Skeletal muscle regenerates in response to disease or injury through the activation of quiescent muscle stem cells and their subsequent differentiation into multi-nucleated myotubes. Understanding the molecular mechanisms of regeneration is critical to exploit this pathway for use in tissue repair. Data shown here demonstrate that MEF2A plays an essential role in skeletal muscle regeneration in adult mice. Regenerating muscle from MEF2A knockout mice displays widespread necrosis, reduced myofiber cross-sectional area, and a significant reduction in Pax7-positive progenitors. The existence of activated progenitor cells that co-express MEF2A and Pax7 is also documented in adult regenerative myogenesis. MEF2A controls this process through its direct regulation of the largest known mammalian rnicroRNA (rniRNA) cluster, the Gtl2-Dio3 locus. All miRNAs (>40) within this cluster are coordinately downregulated in MEF2A-deficient regenerating muscle, and a subset of the Gtl2-Dio3 rniRNAs represses secreted Frizzled- related proteins (sFRPs), inhibitors of Wnt signaling. Consistent with downregulation of this rniRNA cluster, expression of sFRPs is upregulated and Wnt signaling is inhibited in MEF2A-deficient regenerating muscle. Furthermore, overexpression of Gtl2-Dio3 miRNAs, miR-433 and miR-410, restores myotube formation in MEF2A-deficient myoblasts. Thus, miRNA-mediated modulation of Wnt signaling by MEF2A is a requisite step for proper muscle regeneration, and represents an attractive pathway for enhancing regeneration of diseased muscle.
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