Loss of miR-29 in Myoblasts Contributes to Dystrophic Muscle Pathogenesis

Abstract

microRNAs (miRNAs) are noncoding RNAs that regulate gene expression in post-transcriptional fashion, and emerging studies support their importance in a multitude of physiological and pathological processes. Here, we describe the regulation and function of miR-29 in Duchenne muscular dystrophy (DMD) and its potential use as therapeutic target. Our results demonstrate that miR-29 expression is downregulated in dystrophic muscles of mdx mice, a model of DMD. Restoration of its expression by intramuscular and intravenous injection improved dystrophy pathology by both promoting regeneration and inhibiting fibrogenesis. Mechanistic studies revealed that loss of miR-29 in muscle precursor cells (myoblasts) promotes their transdifferentiation into myofibroblasts through targeting extracellular molecules including collagens and microfibrillar-associated protein 5 (Mfap5). We further demonstrated that miR-29 is under negative regulation by transforming growth factor-β (TGF-β) signaling. Together, these results not only identify TGF-β-miR-29 as a novel regulatory axis during myoblasts conversion into myofibroblasts which constitutes a novel contributing route to muscle fibrogenesis of DMD but also implicate miR-29 replacement therapy as a promising treatment approach for DMD.

MicroRNA-29 overexpression by adeno-associated virus suppresses fibrosis and restores muscle function in combination with micro-dystrophin

“…In the current study, we validated these findings in GAS and found that fibrosis was accompanied by a reduction of miR-29c transcripts. In addition, we provide further evidence that miR-29c/micro-dystrophin combination therapy reduces expression of collagens I and III and Fbn, direct targets of miR-29c (12,15), and Tgfb1, which is a negative regulator of miR-29c (9). Treatment lowers overall fibrosis, improves absolute and specific force, and protects against contraction-induced injury.…”

“…TGF-β1 is a known profibrotic cytokine that downregulates miR-29c and is responsible for conversion of myoblasts to myofibroblasts, with an increase in collagen and muscle fibrogenesis (9). qRT-PCR analysis shows that cotreated muscle had significantly lower levels of Tgfb1 compared with untreated muscle and either treatment alone ( Figure 4G).…”

“…Treatment was done 3 times per week for 2 consecutive weeks, with muscle collected 3 weeks after the initial injection. Staining for embryonic myosin heavy chain (E-MyHC) revealed an increase in the number of regenerating fibers validated by detection of an increase in myogenin, myoD, and Pax7 by immunoblotting (9).…”

“…A caveat to gene replacement alone is that it fails to address fibrosis and fat replacement resulting from muscle fiber loss. In contrast, the miR-29 family, comprised of miR-29a, b, and c, is implicated in fibrosis in a variety of organs including liver, lung, heart, kidney, and skeletal muscle, and provides a potential tool for translational therapy (5)(6)(7)(8)(9)(10)(11). miRs are noncoding, single-stranded RNAs, of 21-25 nucleotides in length that constitute a novel class of gene regulators.…”

“…Currently, a miR-29b mimic produced by MiRagen Therapeutics is in a phase 1 clinical trial to evaluate safety, tolerability, and pharmacokinetics in healthy volunteers (MRG-201;NCT02603224). As proof of principle, Wang and colleagues showed that delivery of miR-29 mimics to dystrophic muscle resulted in a reduction of Col1A1 and Col3A1 (9). In translating miR-29 delivery to the clinic for DMD, viral delivery of miR-29 using rAAV holds a distinct advantage in that it allows for one-time treatment rather than repeated intravenous delivery of oligos.…”