miR133

miR-133 is a family of microRNAs enriched in muscle tissue and categorized as myomiRs, along with miR-1 and miR-206. The mature miR-133 species include miR-133a and miR-133b. In humans, miR-133a and miR-133b are encoded by separate gene loci and can be co-expressed with miR-1 in muscle-specific bicistronic transcription units regulated by myogenic factors such as MyoD and MEF2. Primary transcripts are processed by Drosha into precursor hairpins and by Dicer into mature miR-133, which is loaded into the RNA-induced silencing complex to guide target recognition through its seed sequence.

Biological functions of miR-133 include promoting myoblast proliferation and suppressing differentiation in skeletal muscle, a role that contrasts with miR-1 and miR-206, which tend to promote differentiation. Targets proposed for miR-133 include serum response factor (SRF) and other regulators of the actin cytoskeleton and cell cycle; repression of SRF is thought to contribute to delaying differentiation and maintaining proliferative capacity. In cardiac muscle, miR-133 participates in the regulation of hypertrophic growth and remodeling; changes in its expression have been observed after myocardial injury and in heart disease. miR-133 may also influence fibrosis in some contexts by targeting CTGF and related pathways.

Clinical and research relevance: Altered miR-133 expression has been reported in muscular dystrophy, myocardial infarction, and other cardiac conditions. It is studied as a potential biomarker and therapeutic target, though challenges remain for clinical application.

See also: microRNA, myomiR.