Activin A marks a novel progenitor cell population during fracture healing and reveals a therapeutic strategy

Bone fraction healing is effective in most individuals, but can be subpar and not easily treatable in patients such as the elderly, requiring new remedies. Here we tested whether activin A promotes fracture repair, inspired by recent studies showing that the protein stimulates ectopic bone formation in other systems. Using a standard mouse tibia fracture model, we found that activin A became very abundant in the developing callus soon after fracture. Single cell RNA-seq and immunostaining showed that initially, expression of the activin A-encoding gene Inhba characterized inflammatory cells and periosteal mesenchymal lineage cells. Over time, Inhba expression became prominent in a unique, highly-proliferative progenitor cell (PPC) population that displayed a myofibroblast character and was at the center of a developmental trajectory bifurcation toward cartilage and bone cells in callus. Systemic administration of a neutralizing activin A monoclonal antibody delayed fracture healing, whereas local implantation of recombinant activin A enhanced it. Cells engaged in healing produced phosphorylated SMAD2 through which activin A normally signals intracellularly, and were also positive for aSMA, a recognized marker of myofibroblasts. In vitro gain- and loss-of-function assays showed that activin A directly stimulated chondrogenesis, osteogenesis and myofibroblast differentiation in periosteal progenitors. Our study reveals that activin A is a positive regulator of fracture repair and may represent a new therapeutic tool to enhance it. Its action involves a prominent and transient PPC population with a myofibroblastic character that would rapidly gear up to bring about repair in a timely and effective manner. mesenchymal cell atlas during fracture healing process