Injury-induced niche factors Cxcl12 and Shh/Ihh coordinate suture stem cell activation during calvarial bone regeneration

Stem cells reside in niches that determine how they respond to injury to expand in number, migrate to the injury site, and differentiate into cells to regenerate lost tissue. Suture stem cells (SuSCs) are important for homeostasis and regeneration of cranial bone and can be used as a model to understand stem cell regulation for bone regeneration at a distance. Using a mouse cranial bone injury model, we identified the chemokine Cxcl12 and the Hedgehog family ligands Shh and Ihh as injury-induced niche factors that coordinately promoted the proliferation, directional migration, and osteoblastic differentiation of the Gli1⁺ subset of SuSCs from the sagittal suture. Cxcl12 was constitutively produced in the SuSC niche, induced at the injury site, and activated its cognate receptor Cxcr4 on Gli1⁺ SuSCs to stimulate Gli1⁺ SuSC proliferation and migration to the injury site. Cxcl12-Cxcr4 signaling also induced the production of Shh and Ihh, which promoted Gli1⁺ SuSC proliferation and osteoblastic differentiation. Furthermore, expressing loss- or gain-of-function mutant forms of the G protein Gα_s, which cause inherited diseases characterized by cranial bone defects, led to aberrant Cxcl12, Shh, and Ihh signaling during regeneration and resulted in cranial bone phenotypes similar to those in human patients. Our results indicate that the injury-induced niche factors Cxcl12, Shh, and Ihh orchestrate SuSC activation and migration to promote injury repair and suggest that disrupting this system impairs regeneration and contributes to human disease.