β-NGF promotes cartilage to bone conversion, accelerating endochondral fracture repair

During endochondral fracture repair, a myriad of biochemical and phenotypic changes occur at the chondro-osseuous junction that regulate cartilage to bone conversion. Osteogenic and angiogenic factors have long been studied for accelerating fracture repair. In our concise study, we focused on the neurotrophic factor nerve growth factor (NGF) and its receptor tropomyosin receptor kinase A (TRKA) as understudied therapeutic targets for accelerating endochondral fracture repair. We first characterized endogenous expression of NGF and TRKA during endochondral repair of tibial fractures. We then analyzed gene expression data from β-NGF stimulated hypertrophic cartilage and observed a promotion in endochondral ossification associated markers. Additional gene ontology analyses revealed promotion of genes associated with Wnt activation, PDGF binding, and integrin binding. Subsequent histological analyses of in vivo samples confirmed Wnt activation following local β-NGF injections via reporter mice. Finally, we tested the therapeutic efficacy of local β-NGF injections in mice, which resulted in a decrease of cartilage and increase of bone volume. Moreover, the newly formed bone contained higher trabecular number, connective density, and bone mineral density. Collectively, we demonstrate the ability for β-NGF to promote endochondral fracture repair in a murine model and uncover mechanisms that will serve to further understand the molecular switches that occur during endochondral ossification.