TGF‐β/Smad2 signalling regulates enchondral bone formation of Gli1(+) periosteal cells during fracture healing

OBJECTIVES: Most bone fracture heals through enchondral bone formation that relies on the involvement of periosteal progenitor cells. However, the identity of periosteal progenitor cells and the regulatory mechanism of their proliferation and differentiation remain unclear. The aim of this study was to investigate whether Gli1‐Cre(ERT2) can identify a population of murine periosteal progenitor cells and the role of TGF‐β signalling in periosteal progenitor cells on fracture healing. MATERIALS AND METHODS: Double heterozygous Gli1‐CreER(T2);Rosa26‐tdTomato(flox/wt) mice were sacrificed at different time points for tracing the fate of Gli1(+) cells in both intact and fracture bone. Gli1‐CreER(T2)‐mediated Tgfbr2 knockout (Gli1‐CreER(T2);Tgfbr2(flox/flox)) mice were subjected to fracture surgery. At 4, 7, 10, 14 and 21 days post‐surgery, tibia samples were harvested for tissue analyses including μCT, histology, real‐time PCR and immunofluorescence staining. RESULTS: Through cell lineage‐tracing experiments, we have revealed that Gli1‐Cre(ER)(T2) can be used to identify a subpopulation of periosteal progenitor cells in vivo that persistently reside in periosteum and contribute to osteochondral elements during fracture repair. During the healing process, TGF‐β signalling is continually activated in the reparative Gli1(+) periosteal cells. Conditional knockout of Tgfbr2 in these cells leads to a delayed and impaired enchondral bone formation, at least partially due to the reduced proliferation and chondrogenic and osteogenic differentiation of Gli1(+) periosteal cells. CONCLUSIONS: TGF‐β signalling plays an essential role on fracture repair via regulating enchondral bone formation process of Gli1(+) periosteal cells.