Discovering novel regulators of muscle cell differentiation and maintenance: A transcriptome approach
Skeletal muscle is a major tissue type comprising a large portion of the adult mass and accounting for most of an individual's daily energy consumption. Muscle diseases occur due to genetic-, injury, and aging-related factors. When the function of the muscle tissue is impaired it results in many metabolism- and motility-related defects. One of the therapeutic avenues for treating muscle diseases is cell-based therapies, where functional muscle cells will be generated and amplified in culture, and finally delivered into patient tissue. However, generation of functional muscle cells require a detailed understanding of how muscle cells differentiate in the first place. As transcription factors are the master regulators of cell differentiation, uncovering the functional roles of the transcription factors that are expressed at the early stages of muscle differentiation is critical. Our recent spatiotemporal microarray study in zebrafish has identified functionally uncharacterized transcriptional regulators that are expressed at initial stages of myogenesis. Cited3 is one such transcriptional coactivator, which is expressed in the precursors of the oxidative muscle fibers. Here, we identified that the expression of Cited3 is activated by Hedgehog signaling. We knocked the expression of Cited3 down by injecting two different antisense oligonucleotides. Loss of Cited3 increased the number of apoptotic muscle cells, impaired muscle cell differentiation and growth, and eventually lead to total immotility. Overexpression of Cited3 significantly rescued all the Cited3 loss-of-function phenotypes. In situ hybridization for various muscle-specific genes placed Cited3 into a gene regulatory network, where it acts downstream of MyoD/Myf5 but upstream of Mef2c. This is the first report demonstrates that Cited3 functions in muscle differentiation and its absence lead to muscle cell death and immotility.