Kbtbd5 is regulated by MyoD and restricted to the myogenic lineage.

BTB-BACK-Kelch (BBK) proteins play broad roles in cellular and molecular regulation. The role of BBK proteins in the skeletal muscle lineage and myogenesis remains an active area of research. Herein, we report a novel BBK gene, Kbtbd5, which we discovered and found to be restricted to the myogenic lineage. We observed that Kbtbd5 was absent in proliferating myoblasts and upregulated upon myogenic differentiation. In situ hybridization analysis revealed that Kbtbd5 was restricted to the skeletal muscle lineage during embryogenesis. We identified a conserved 1.2kb upstream region, which directs reporter expression to the developing skeletal muscle lineage. Transcriptional and mutagenesis assays demonstrated that the E-box motifs contribute to the Kbtbd5 promoter activity. We have also demonstrated the in vivo and in vitro binding between MRFs and the E-box motif in the 1.2kb promoter of the Kbtbd5 gene. Our studies have revealed that the Myod family can transactivate the 1.2kb-luc reporter through the E-box motifs. In addition, we have shown that Kbtbd5 can recruit the Cullin 3 complex in vivo. Using shRNA knockdown, our study has revealed that Kbtbd5 plays an important role in the myogenic differentiation. In summary, we have demonstrated that Kbtbd5 is the direct downstream target gene of the Myod family and regulates myogenic differentiation. Our results further support the notion that Kbtbd5 may serve as an adapter of Cul3 during myogenic differentiation.

Myogenic progenitor cells express filamin C in developing and regenerating skeletal muscle.

The regenerative capacity of skeletal muscle is due to the myogenic progenitor cell population that is resident in adult skeletal muscle. To enhance our understanding of this cell population, we examined the temporal-spatial expression pattern for filamin C during murine embryogenesis, adult muscle regeneration and in selected myopathic models of human disease. Using in situ hybridization, we observed filamin C to be restricted to mesodermal lineages including the developing heart and skeletal muscle during embryogenesis. Following cardiotoxin-induced muscle injury of adult skeletal muscle, filamin C expression was dynamically regulated in activated myogenic progenitor cells and newly regenerated myotubes. This expression pattern was further supported using RT-PCR analysis of filamin C expression in differentiating C2C12 myotubes. These results support the paradigm that the regulatory mechanisms of muscle regeneration largely recapitulate the fundamental events observed during muscle development and that filamin C may function in signal transduction or cellular migration of the myogenic progenitor cell population.