CXCL12 in late-stage osteoblasts and osteocytes is required for load induced bone formation in mice

Increased physical loading of the skeleton activates new bone formation ensuring its ability to meet mechanical demands over time; however, the capacity of bone to respond to mechanical stimulation diminishes with age. Osteocytes, the cells embedded and dispersed throughout mineralized bone matrix, are master regulators of mechanoadaptation through recruitment of new bone-forming cells, the osteoblasts, via signaling to osteoprogenitors located on bone surfaces. We previously demonstrated that in vivo and in vitro mechanical stimulation significantly upregulated the chemokine C-X-C Motif Chemokine Ligand 12 (CXCL12) and its receptor, CXCR4, in osteocytes and bone lining cells, and that CXCR4 antagonism with AMD3100 attenuated in vivo load-induced bone formation. Here, we extended this work by showing that ablation of CXCL12+ cells and deletion of cxcl12 in late-stage osteoblasts and osteocytes significantly attenuated in vivo load-induced bone formation in the mouse tibia. This bone loading phenotype was rescued by treatment with recombinant CXCL12. To address mechanism, we showed that in vitro deletion of cxcl12 and cxcr4, separately, in bone marrow stromal cells resulted in significantly reduced osteogenic differentiation. Furthermore, CXCL12 treatment enhanced GSK-3b phosphorylation and β-catenin translocation to the nucleus, the former of which was partially blocked by AMD3100. Finally, CXCL12 synergized Wnt signaling leading to significantly increased total β-catenin protein and Axin2 expression, a Wnt signaling target gene. These findings together demonstrate that CXCL12 expression in late-stage osteoblasts and osteocytes is essential for load-induced bone formation, in part, by regulating osteogenic differentiation through activation of the Wnt signaling pathway.