SUN-140 TMEPAI Inhibits SMAD 2/3 Mediated Muscle Wasting

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.