The expansion of myogenic progenitors (MPs) in the adult muscle stem cell niche is critical for the regeneration of skeletal muscle. Activation of quiescent MPs depends on the dismantling of the basement membrane and increased access to growth factors such as fibroblast growth factor-2 (FGF2). Here, we demonstrate using microRNA (miRNA) profiling in mouse and human myoblasts that the capacity of FGF2 to stimulate myoblast proliferation is mediated by miR-29a. FGF2 induces miR-29a expression and inhibition of miR-29a using pharmacological or genetic deletion decreases myoblast proliferation. Next generation RNA sequencing from miR-29a knockout myoblasts (Pax7 CE/1 ; miR-29a flox/flox ) identified members of the basement membrane as the most abundant miR-29a targets. Using gain-and loss-of-function experiments, we confirm that miR-29a coordinately regulates Fbn1, Lamc1, Nid2, Col4a1, Hspg2 and Sparc in myoblasts in vitro and in MPs in vivo. Induction of FGF2 and miR-29a and downregulation of its target genes precedes muscle regeneration during cardiotoxin (CTX)-induced muscle injury. Importantly, MP-specific tamoxifen-induced deletion of miR-29a in adult skeletal muscle decreased the proliferation and formation of newly formed myofibers during both CTX-induced muscle injury and after a single bout of eccentric exercise. Our results identify a novel miRNAbased checkpoint of the basement membrane in the adult muscle stem cell niche. Strategies targeting miR-29a might provide useful clinical approaches to maintain muscle mass in disease states such as ageing that involve aberrant FGF2 signaling. STEM CELLS 2016;34:768-780 SIGNIFICANCE STATEMENTSkeletal muscle mass and function is critical for the maintenance of health, and the decline of muscle mass during aging inversely correlates with mortality. Adult muscle stem cells provide an important target for strategies to maintain muscle mass, but the molecular mechanisms that control the activation of these stem cells in vivo are still incompletely understood. We demonstrate that microRNA-29a is a novel downstream target of FGF2, and that miR-29a mediates the dismantling of the basement membrane in the adult muscle stem cell niche that is required for the proliferation of myogenic progenitors during muscle regeneration. We propose the FGF2/miR-29a pathway as a novel target to prevent a decrease in muscle mass in disease states such as ageing where FGF2 signaling is overly activated.

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