Jagged ligand expression by alpha-SMA progenitors is required for normal fracture healing.

Abstract

Fracture healing is a tightly regulated process dependent on coordinated activation and differentiation of mesenchymal progenitor cells and their progeny through spatiotemporally regulated signaling pathways. Canonical Notch signaling has been implicated in governing mesenchymal progenitor fate in both calvarial and long bone healing. Endogenous Notch ligands, particularly Jagged-1 (Jag1), are highly expressed during the bone healing process, yet the role of Jag1 and its homolog, Jagged-2 (Jag2), during fracture healing have not been fully examined. Herein, we utilized temporally controlled, inducible transgenic mouse models to disrupt Jag1 and Jag2 in all cells of the callus (Rosa-CreERT2) and more specifically in early osteochondral progenitor cells (alpha-SMA-CreERT2) during traumatic tibial fracture healing. Absence of Jag1 and Jag2 in all cells or only in the progenitors impaired bone regeneration with distinct phenotypic differences in callus composition, mineral deposition, and cartilage resolution. Rosa-CreERT2-Jag1/Jag2-double knock out mice showed diminished callus bone volume and prolonged chondrogenesis without altering overall callus size, whereas lineage-specific deletion in alpha-SMA+ osteochondral progenitors resulted in disproportionate callus expansion, delayed cartilage clearance and altered mineral deposition. These findings identify Jag1 and Jag2 ligands as regulators of chondrogenesis and cartilage remodeling in alpha-SMA+ progenitors and underscore the spatiotemporal specificity of Notch signaling in coordinating endochondral ossification. Targeted activation of Jagged-mediated Notch signaling in osteochondral progenitors may represent a promising therapeutic strategy to enhance skeletal regeneration and accelerate endochondral repair in cases of nonunion or delayed fracture healing.