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Endogenous activin A in ectopic bone formation
Heterotopic ossification (HO) is the formation of ectopic bone in soft tissues at sites of injury-induced inflammation. Similar to the development of normal endochondral bone, HO is initiated by a local mass of chondrocytes that progress through chondrogenesis, osteogenesis, and mineralization to form bone tissue. Using mouse models of both subcutaneous and intramuscular HO formation and single-cell RNA sequencing, Mundy et al. found that inflammatory cells and skeletal progenitor cells initially recruited to sites of HO formation expressed Inhba, which encodes the TGF-β superfamily member activin A. Treating mice with an activin A–neutralizing antibody reduced the number of chondrogenic cells at HO sites and inhibited HO formation. These results demonstrate that this ligand plays an important role in the physiological progression in these mouse models of HO and suggest that interfering with activin A signaling may be effective in patients.
Abstract
Heterotopic ossification (HO) is a common, potentially debilitating pathology that is instigated by inflammation caused by tissue damage or other insults, which is followed by chondrogenesis, osteogenesis, and extraskeletal bone accumulation. Current remedies are not very effective and have side effects, including the risk of triggering additional HO. The TGF-β family member activin A is produced by activated macrophages and other inflammatory cells and stimulates the intracellular effectors SMAD2 and SMAD3 (SMAD2/3). Because HO starts with inflammation and because SMAD2/3 activation is chondrogenic, we tested whether activin A stimulated HO development. Using mouse models of acquired intramuscular and subdermal HO, we found that blockage of endogenous activin A by a systemically administered neutralizing antibody reduced HO development and bone accumulation. Single-cell RNA-seq analysis and developmental trajectories showed that the antibody treatment reduced the recruitment of Sox9+ skeletal progenitors, many of which also expressed the gene encoding activin A (Inhba), to HO sites. Gain-of-function assays showed that activin A enhanced the chondrogenic differentiation of progenitor cells through SMAD2/3 signaling, and inclusion of activin A in HO-inducing implants enhanced HO development in vivo. Together, our data reveal that activin A is a critical upstream signaling stimulator of acquired HO in mice and could represent an effective therapeutic target against forms of this pathology in patients.
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