The Calcium-Sensing Receptor Mediates Bone Turnover Regulated By Parathyroid Hormone In Adult Mice

To investigate the calcium-sensing receptor (CaSR) and parathyroid hormone (PTH) in the adult mice bone turnover interactions and its mechanism. We established CaSR and PTH double knockout mouse models (PTH-/-CaSR-/-) to comparative analyse the PTH gene knock (PTH-/-) and wild-type (WT) mice phenotypic differences. In addition, we gave subcutaneous injection of exogenous PTH (1-34), of80μg/kg weight,1times/day for28days, and the same volume of saline in control group to comparative observe the influence of PTH on vertebral bone turnover in mice. Compared with WT mice, littermates PTH-/-mice showed low calcium hyperphosphatemia, but PTH-/-CaSR-/-mice serum calcium and phosphorus were no significant changes. After injected exogenous PTH (1-34), the three genotypes mice were characterized by hypercalcemia and hypophosphatemia, however, the changes in WT and PTH-/-mice were greater than PTH-/-CaSR-/-mice. Therefore, we concluded that the knockout CaSR gene of PTH-/-mice could correct the PTH deletion caused by abnormal calcium.Both in the experimental group and control group, the bone mineral density (BMD), bone volume (BV), number of osteoblasts (N.OB), collagen I positive area(COL1), Alkaline phosphatase positive area(ALP), runt-related transcription factor2-mRNA(Runx2-mRNA), Osteocalcin-mRNA(OCN-mRNA) and ALP-mRNA were decreased in PTH-/-mice, and more obviously decreased in PTH-/-CaSR-/-mice compared to the same group of WT mice. Compared to the same group of PTH-/-mice, the above indicators were significantly decreased in PTH-/-CaSR-/-mice whether injected exogenous PTH (1-34) or not. Compared with control group in the same genotypes mice, however, the above indicators were significantly increased in three genotypes mice after injected exogenous PTH (1-34), and the changes in WT and PTH-/-mice were greater than PTH-/-CaSR-/-mice.Both in the experimental group and control group, the number of osteoclasts (N.Oc), Osteoclast area (OC.S/B.S) and Receptor activator of NF-κB ligand/Osteoprotegerin mRNA(RANKL/OPG mRNA) were decreased in PTH-/-CaSR-/-mice, and more obviously decreased in PTH-/-mice compared to the same group of WT mice. Compared to the same group of PTH-/-mice, the above indicators were significantly increased but decreased after injected exogenous PTH (1-34) in PTH-/-CaSR-/-mice. Compared with control group, however, the above indicators were significantly increased in three genotypes mice after injected exogenous PTH (1-34), and the changes in WT and PTH-/-mice were greater than PTH-/-CaSR-/-mice.Both in the experimental group and control group, the expressions of p21and p27were higer while the expression of cyclin D1was lower in PTH-/-mice, and more obvious in PTH-/-CaSR-/-mice compared to the same group of WT mice. Compared to the same group of PTH-/-mice, the expressions of p21and p27were higer while the expression of cyclin D1was lower in PTH-/-CaSR-/-mice. Compared with control group, however, the expressions of p21and p27were lower while cyclin Dlwere high in three genotypes mice after injected exogenous PTH (1-34), similarly, the changes in WT and PTH-/-mice were greater than PTH-/-CaSR-/-mice.PTH can promote bone formation and regulate bone turnover, however, it depends on the presence of the calcium-sensing receptor. Intermittent exogenous PTH injection can enhance the level of serum calcium, and stimulate bone formation by CaSR-mediated cell cycle regulation.The present study further explore the calcium-sensing receptor (CaSR) and parathyroid hormone (PTH) in the bone turnover interactions and its mechanism. It provide theoretical and experimental basis to improve calcium and promote bone formation drugs clinical efficacy. Moreover, it also provide a new perspective to the development of bone formation drug targeted to CaSR gene and PTH signal.