A Transcriptional Hub for Endochondral ossification adn Fracture Repair.

After injury, bone tissue initiates a reparative response that will recover its structure and function, failure to initiate or delay this response results in fracture nonunion. The molecular mechanisms underlying fracture nonunion appearance are not yet stablished. We proposed that the signaling pathways derived from hypoxia and mediated by reactive oxygen species (ROS) homeostasis control Bmp2 expression and fracture healing initiation, having potential to induce fracture nonunion when excessive ROS develops into oxidative stress. Here, we silenced Apex1, the final transducer of ROS signaling and responsible of the activation of key transcription factors by oxidoreduction of cysteines, evaluating its role during endochondral ossification and fracture repair. Silencing Apex1 in limb bud mesenchyme results in transient metaphyseal dysplasia derived of impaired chondrocyte differentiation. During bone regeneration, silencing Apex1 generates a fracture nonunion phenotype characterized by delayed fracture healing initiation with impaired periosteal response and reduced chondrocyte and osteoblasts differentiation. In addition, compromised chondrocyte differentiation hampers fracture healing progression and callus vascularization. Our data supports a molecular mechanism of fracture healing initiation dependent of hypoxia derived ROS signaling in mesenchymal progenitors. Overall design: Bulk RNA-seq for fracture calluses of wild-type and Apex1 KO mice at different days post fracture(-dpf) 3-, 6-, 10- and 14-dpf)