Rad is temporally regulated within myogenic progenitor cells during skeletal muscle regeneration

The successful use of myogenic progenitor cells for therapeutic applications requires an understanding of the intrinsic and extrinsic cues involved in their regulation. Herein we demonstrate the expression pattern and transcriptional regulation of Rad, a prototypical member of a family of novel Ras-related GTPases, during mammalian development and skeletal muscle regeneration. Rad was identified using microarray analysis, which revealed robust upregulation of its expression during skeletal muscle regeneration. Our current findings demonstrate negligible Rad expression with resting adult skeletal muscle; however, after muscle injury, Rad is expressed within the myogenic progenitor cell population. Rad expression is significantly increased and localized to the myogenic progenitor cell population during the early phases of regeneration and within the newly regenerated myofibers during the later phases of regeneration. Immunohistochemical analysis demonstrated that Rad and MyoD are coexpressed within the myogenic progenitor cell population of regenerating skeletal muscle. This expression profile of Rad during skeletal muscle regeneration is consistent with the proposed roles for Rad in the inhibition of L-type C[a.sup.2+] channel activity and the inhibition of Rho/RhoA kinase activity. We also have demonstrated that known myogenic transcription factors (MEF2, MyoD, and Myf-5) can increase the transcriptional activity of the Rad promoter and that this ability is significantly enhanced by the presence of the C[a.sup.2+]-dependent phosphatase calcineurin. Furthermore, this enhanced transcriptional activity appears to be dependent on the presence of a conserved NFAT binding motif within the Rad promoter. Taken together, these data define Rad as a novel factor within the myogenic progenitor cells of skeletal muscle and identify key regulators of its transcriptional activity. satellite cell; myoblast; stem cell; transcriptional regulation; RGK protein