The Regulation of Myogenic Transcription Factors by MicroRNAs

Molecular biology has been a field of study with a constantly expanding frontier. Short RNAs in particular were once thought to be cellular debris. They are in fact critical regulators of other nucleic acids. Early studies showed that short RNAs are enriched for a specific length and sequence. A class of such short RNAs was called microRNAs, and their function is to bind and repress messenger RNA. MicroRNAs regulate cellular processes by repressing multiple genes. Studying the transcriptional regulation of microRNAs as well as the messenger RNAs that they target can give insight into how cells govern cell fate decisions. The genetics of skeletal muscle lineage commitment are at first glance relatively simple. MyoD overexpression is sufficient to convert fibroblasts into skeletal muscle myotubes. In vivo, there is a spectrum of differentiation that requires a large regulatory network to maintain the proper balance of progenitor and differentiating cells. The understanding of the mechanisms regulating transitions between various steps of differentiation remains incomplete. In this work, I will describe how microRNA plays a critical role in skeletal muscle differentiation by targeting repressors of myogenic transcription factors. Pax7 is a marker of myogenic stem cells, called satellite cells. The downregulation of Pax7 is a critical step to permit differentiation of satellite cells. In the first part, I will show that miR-206 and miR-486 give negative feedback from MyoD to Pax7. To prevent premature differentiation, MyoD is inhibited by other proteins besides Pax7. In the second part, I will describe how MyoD also promotes the transcription of miR-378. This microRNA represses the competitive inhibitor of MyoD, known as MyoR. v The expression of a number of structural proteins associated with terminal differentiation is dependent on the transcription factor Mef2c. In the third part I will demonstrate that the intracellular domain of Notch3 is a negative regulator of Mef2c. Furthermore, I will show that Notch3 is regulated by a combination of a unique enhancer and miR-206. In summation, I will show that microRNAs regulate multiple steps in myogenesis, setting up and reinforcing bistable switches between different stages of differentiation in myogenic stem cells.

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