Inhibition of myostatin- and activin-mediated SMAD2/3 signaling using ligand traps, such as soluble receptors, ligand-targeting propeptides and antibodies, or follistatin can increase skeletal muscle mass in healthy mice and ameliorate wasting in models of cancer cachexia and muscular dystrophy. However, clinical translation of these extracellular approaches targeting myostatin and activin has been hindered by the challenges of achieving efficacy without potential effects in other tissues. Toward the goal of developing tissue-specific myostatin/activin interventions, we explored the ability of transmembrane prostate androgen-induced (TMEPAI), an inhibitor of transforming growth factor-β (TGF-β1)-mediated SMAD2/3 signaling, to promote growth, and counter atrophy, in skeletal muscle. In this study, we show that TMEPAI can block activin A, activin B, myostatin and GDF-11 activity in vitro . To determine the physiological significance of TMEPAI, we employed Adeno-associated viral vector (AAV) delivery of a TMEPAI expression cassette to the muscles of healthy mice, which increased mass by as much as 30%, due to hypertrophy of muscle fibers. To demonstrate that TMEPAI mediates its effects via inhibition of the SMAD2/3 pathway, tibialis anterior (TA) muscles of mice were co-injected with AAV vectors expressing activin A and TMEPAI. In this setting, TMEPAI blocked skeletal muscle wasting driven by activin-induced phosphorylation of SMAD3. In a model of cancer cachexia associated with elevated circulating activin A, delivery of AAV:TMEPAI into TA muscles of mice bearing C26 colon tumors ameliorated the muscle atrophy normally associated with cancer progression. Collectively, the findings indicate that muscle-directed TMEPAI gene delivery can inactivate the activin/myostatin-SMAD3 pathway to positively regulate muscle mass in healthy settings and models of disease.
Inhibition of myostatin and activin activity using ligand traps, such as soluble receptors, follistatin and propeptides, can markedly increase skeletal muscle mass in healthy mice and ameliorate wasting in models of cancer cachexia and muscular dystrophy. Though effective, clinical translation of these approaches has been hindered by off-target effects. Toward the goal of developing tissue-specific myostatin/activin interventions, we explored the ability of transmembrane prostate androgen-induced (TMEPAI) to promote growth of skeletal muscle. TMEPAI, a transcriptional target of activin in muscle, is a known inhibitor of TGF-β1-mediated SMAD 2/3 signalling. In this study we show that TMEPAI also blocks activin A, activin B, myostatin and GDF-11 in vitro activity. Adeno-associated viral (AAV) gene delivery of TMEPAI into healthy mice increased local muscle mass by as much as 30%. Increased muscle mass was attributed to hypertrophy of fibres in TMEPAI-expressing muscles, and was coincident with an upregulation in markers of protein synthesis (pAkt, pMTOR, p70S6K). The ability of TMEPAI to block activation of the canonical activin/myostatin-SMAD 2/3 axis, was demonstrated by co-injecting AAV6:activin A and AAV6:TMEPAI into healthy mice. In this setting, TMEPAI blocked activin-induced phosphorylation of SMAD3 and associated skeletal muscle wasting. Finally, delivery of AAV6:TMEPAI into tibialis anterior muscles of mice bearing C26 tumours prevented muscle atrophy normally associated with this model. The results support that viral gene delivery of TMEPAI can effectively increase muscle mass via inactivation of the activin/myostatin-SMAD 2/3 pathway.
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