Endothelial cells, which are positive for Tie-2, have been reported to differentiate NLG919 molecular weight into chondrocytes and osteoblasts heterotopic cartilaginous and bone tissues in FOP but not in normal skeletal tissue [74] and [75]. In endothelial cells, over-expression of mutant ALK2 found in FOP induced their endothelial-to-mesenchymal transition and differentiation into chondrocytes, osteoblasts and adipocytes [75]. Treatment with BMP-4 or TGF-β1

also induced similar changes in human endothelial cells in vitro [75]. However, lineage-tracing experiments reveal that endothelial cells failed to contribute to either the chondrocytic or osteoblastic populations within the BMP-2-induced ectopic skeletal tissues in mice [76]. Both chondrocytes and osteoblasts induced by BMP-2 developed

from the non-myogenic Sca-1+ and PDGFRα+ residential mesenchymal cells of the skeletal muscle [76]. Taken together, the results suggest that both myogenic and non-myogenic skeletal muscle cells have functional BMP receptors, but only non-myogenic mesenchymal cells contribute to heterotopic bone formation in vivo. Myostatin (also called GDF-8) is a member of the TGF-β family that is specifically expressed in skeletal muscle tissue [77]. Myostatin−/− mice are larger than wild-type Paclitaxel mice due to the increased size of their hypertrophic skeletal muscles [77]. From zebrafish to humans, an increase in skeletal muscle mass in response to a loss-of-function mutation of myostatin has been demonstrated, indicating that myostatin acts as a negative regulator of skeletal muscle mass. Myostatin

binds to ActR-IIA and ActR-IIB type II receptors similarly to several BMPs, whereas, in contrast to osteogenic BMPs, myostatin binds to ALK4 and ALK5/TβR-I type I receptors similarly to non-osteogenic members of the TGF-β family [78]. Phosphorylated Smad1/5/8 was found in normal skeletal muscle tissue in mice, suggesting that BMP-like activity is physiologically activated in the muscles [79]. Over-expression of noggin, an antagonist of BMPs but not of myostatin, prevented the hypertrophy of skeletal muscle in myostatin-deficient mice, suggesting that BMP signaling plays a role in the myostatin-deficient phenotype [79]. BMP signaling via Smad1/5/8 and Smad4 suppresses the expression Vitamin B12 of MUSA-1, a ubiquitin ligase that induces the atrophy of skeletal muscle, and myostatin signaling via Smad2/3 inhibits the expression of MUSA-1 by competing with Smad4 [79]. Thus, BMPs are important physiological regulators of the skeletal muscle mass [80]. Since the original “BMP” activity was discovered a half century ago, BMPs have been shown to be critical factors in the development and regeneration of bone and cartilage. As Urist suggested in 1965, clinicians have repeatedly attempted to induce bone formation using BMPs. Recent findings suggested that BMPs are also involved in maintaining a normal skeletal muscle mass.