ActRII or BMPR ligands inhibit skeletal myoblast differentiation, and BMPs promote heterotopic ossification in skeletal muscles in mice

Prior studies suggested that canonical Activin Receptor II (ActRII) and BMP receptor (BMPR) ligands can have opposing, distinct effects on skeletal muscle depending in part on differential downstream SMAD activation. It was therefore of interest to test ActRII ligands versus BMP ligands in settings of muscle differentiation and in vivo. In human skeletal muscle cells, both ActRII ligands and BMP ligands inhibited myogenic differentiation: ActRII ligands in a SMAD2/3-dependent manner, and BMP ligands via SMAD1/5. Surprisingly, a neutralizing ActRIIA/B antibody mitigated the negative effects of both classes of ligands, indicating that some BMPs act at least partially through the ActRII receptors in skeletal muscle. Gene expression analysis showed that both ActRII and BMP ligands repress muscle differentiation genes in human myoblasts and myotubes. In mice, hepatic BMP9 over-expression induced liver toxicity, caused multi-organ wasting, and promoted a pro-atrophy gene signature despite elevated SMAD1/5 signaling in skeletal muscle. Local overexpression of BMP7 or BMP9, achieved by intramuscular AAV delivery, induced heterotopic ossification. Elevated SMAD1/5 signaling with increased expression of BMP target genes was also observed in sarcopenic muscles of old rats. The canonical ActRII ligand-SMAD2/3 and BMP ligand-SMAD1/5 axes can both block human myoblast differentiation. Our observations further demonstrate the osteoinductive function of BMP ligands while pointing to a potential relevancy of blocking the BMP-SMAD1/5 axis in the setting of therapeutic anti-ActRIIA/B inhibition.