Increased Bone Turnover and Possible Accelerated Fracture Healing in a Murine Model With an Increased Circulating C-Type Natriuretic Peptide

Recent studies have revealed that C-type natriuretic peptide (CNP) is a potent stimulator of endochondral bone growth. Nevertheless, the effect of CNP on bone turnover has not yet been well studied. To elucidate this issue, we investigated the bone phenotype of a mouse model with elevated plasma CNP concentrations (SAP-CNP-Tg mice) in the present study. Microcomputed tomography (CT) analysis revealed less bone in femurs, but not in lumber vertebrae, of young adult SAP-CNP-Tg mice than that of wild-type mice. Bone histomorphometry of the tibiae from 8-week-old SAP-CNP-Tg mice showed enhanced osteoblastic and osteoclastic activities, in accordance with elevated serum levels of osteocalcin and tartrate-resistant acid phosphatase-5b, respectively. Next we performed an open and stabilized femoral fracture using 8-week-old SAP-CNP-Tg mice and compared the healing process with age-matched wild-type mice. An immunohistochemical study revealed that CNP and its receptors, natriuretic peptide receptor-B and natriuretic peptide clearance receptor, are expressed in hard calluses of wild-type mice, suggesting a possible role of CNP/natriuretic peptide receptor-B signaling in fracture repair, especially in bone remodeling stage. On micro-CT analysis, a rapid decrease in callus volume was observed in SAP-CNP-Tg mice, followed by a generation of significantly higher new bone volume with a tendency of increased bone strength. In addition, a micro-CT analysis also showed that bone remodeling was accelerated in SAP-CNP-Tg mice, which was also evident from increased serum osteocalcin and tartrate-resistant acid phosphatase-5b levels in SAP-CNP-Tg mice at the remodeling stage of fracture repair. These results indicate that CNP activates bone turnover and remodeling in vivo and possibly accelerates fracture healing in our mouse model.