Activin A promotes bone fracture repair and acts through a novel myofibroblastic cell population in callus

Insufficient bone fracture repair represents a significant clinical burden, and identification of novel therapeutics enhancing repair would have substantial clinical and societal impact. Activin A is a TGF-{beta} protein superfamily member known to stimulate ectopic bone formation, but its roles in fracture repair and its therapeutic potentials remain unclear. Using two mouse tibia fracture repair models, here we mapped activin A expression at the tissue and single cell levels, tested its requirement for normal repair and evaluated its ability to enhance repair when provided exogenously. Activin A was minimally expressed in periosteum of intact bones but was markedly upregulated in developing callus soon after fracture. Single cell RNA-sequencing revealed that the activin A-encoding gene Inhba marked a unique, highly proliferative progenitor cell (PPC) population with a myofibroblast character which emerged over repair time and lay at the center of a developmental trajectory bifurcation producing cartilage and bone cells within callus. Systemic administration of a neutralizing activin A antibody impaired fracture repair and its endochondral and intramembranous phases, whereas local delivery of recombinant activin A enhanced repair. Activin A delivery also induced SMAD2 phosphorylation in vivo and increased the fraction of SMA+ myofibroblasts within fracture callus. Gain- and loss-of-function experiments in vitro showed that activin A directly stimulated myofibroblast differentiation, chondrogenesis and osteogenesis in periosteal progenitor cells. Together, these data identify a unique population of Inhba-expressing proliferative progenitor cells that give rise to chondrocytes and osteoblasts during fracture healing and establish activin A as a potential new therapeutic tool to enhance it. One Sentence SummaryDeficits in bone fracture repair remain a clinical challenge and the present study provides evidence for the therapeutic potentials of activin A