APEX1, a transcriptional hub for endochondral ossification and fracture repair

Abstract

After injury, bone tissue initiates a reparative response to restore its structure and function. The failure to initiate or delay this response could result in fracture nonunion. The molecular mechanisms underlying the occurrence of fracture nonunion are not yet established. We propose that hypoxia-triggered signaling pathways, mediated by reactive oxygen species (ROS) homeostasis, control Bmp2 expression and fracture healing initiation. The excessive ROS leads to oxidative stress and, ultimately, fracture nonunion. In this study, we silenced Apex1, the final ROS signaling transducer that mediates the activation of key transcription factors by their cysteines oxidoreduction, evaluating its role during endochondral ossification and fracture repair. Silencing Apex1 in limb bud mesenchyme results in transient metaphyseal dysplasia derived from impaired chondrocyte differentiation. During bone regeneration, Apex1 silencing induces a fracture nonunion phenotype, characterized by delayed fracture repair initiation, impaired periosteal response, and reduced chondrocyte and osteoblast differentiation. This compromised chondrocyte differentiation hampers callus vascularization and healing progression. Our findings highlight a critical mechanism where hypoxia-driven ROS signaling in mesenchymal progenitors through APEX1 is essential for fracture healing initiation.

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Funding

Universidad de Navarra (University of Navarra)(Asociación de Amigos de la Universidad de Navarra)

Fellowship CIMA AC from “Fundación para la Investigación Médica Aplicada”

Sara Borrell grant (CD22/00027) from the Instituto Carlos III and NextGenerationEU

"la Caixa" Foundation (Caixa Foundation)(Ref. HR23-00084)

PID2022-104776RB-100 and CB16/11/00399 (CIBER CV) from MCIN/AEI/10.13039/501100011033

Ministerio de Ciencia, Innovación y Universidades, co-financed by European Regional Development Fund-FEDER “A way to make Europe” (PID2023-153309OB-I00); Ministerio de Ciencia, Innovación y Universidades through Instituto de Salud Carlos III and European Regional Development Funds “A way to make Europe” (PI17/00136, PI20/00076), European Union Horizon 2020 program (grant agreement #874889, HEALIKICK)

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