Osteoblast-specific constitutive activation of PTH/PTHrP receptor enhances marrow-ablation-induced osteoprogenitor activities

Objectives: Bone is incessantly formed to maintain bone mass, and bone-forming osteoblasts are provided by proliferation and differentiation of osteoprogenitors. Parathyroid hormone (PTH) activates bone formation, but because of the complexity of cells in the osteoblast lineage, how these osteoprogenitors are regulated by PTH in vivo is not completely understood. Therefore we conducted experiments to understand how signals by PTH in differentiated osteoblasts regulate osteoprogenitors in vivo. Methods: Femoral bone marrow ablation (using endodontic K-file) was conducted for Col1a1-caPPR transgenic mice, in which constitutive active PTH/PTHrP receptor (caPPR) was expressed under osteoblast-specific Col1a1 promoter. Results: Col1a1-caPPR transgenic mice (TG) showed x2.5 higher trabecular bone volume compared to wild-type mice (WT), normal hematopoietic marrow space and areas of cortical trabeculation under steady-state conditions. After bone marrow ablation, stromal cells recruited from bone surface extensively proliferated in marrow cavity in TG, compared to limited proliferation in WT. Whereas de novo bone formation was restricted to the ablated area in WT, the entire marrow cavity was obliterated by newly-formed bone in TG. This was associated with trabeculation of cortical bone at large periphery that was integrated into reticular bones of marrow cavity. Bone mineral density was significantly increased after ablation in TG. Bone marrow cell culture in osteogenic medium revealed that alkaline phosphatase (ALP)-positive area was markedly increased in TG, although the frequency of ALP-positive colony was not significantly varied. Furthermore, mRNA expression of Wnt-signaling molecules such as LRP5, Wnt7b and Wnt10b were upregulated after marrow ablation in TG bone marrow cells, suggesting their roles in the observed phenomena. Conclusions: These results indicate that constitutive activation of PTH/PTHrP receptor in differentiated osteoblasts enhanced recruitment, proliferation and differentiation of osteoprogenitors in response to bone marrow ablation.