Bone Abstracts

Communication between osteoblasts and osteoclasts is crucial for bone remodeling. Thrombin and its thrombin receptor (TR; PAR-1) are expressed in osteoclasts and osteoblasts, respectively. To date, the physiological roles of thrombin and TR in bone metabolism have not been elucidated. Therefore, we fully characterized the bone phenotype of mice lacking the thrombin receptor.

We performed bone microarchitectural analyses of the femurs of 10–12 week old wild type (WT) and TR knockout (KO) mice, using three-dimensional micro-computed tomography (μCT). Serum analyses was done for RANKL and OPG levels and in the urine of these mice, we measured the bone resorption marker deoxypyridinoline crosslinks (DPD). Murine osteoblasts (MC-3T3) were cultured to study the effect of thrombin on RANKL and OPG production as well as on osteoblast signaling pathways, including the p42/44 ERK, PLC-β and PKC, using U0126, U73122 and chelerythrine, respectively, as specific inhibitors.

Using μCT, we found increased trabecular and cortical bone mass in TR KO mouse femurs compared to WT littermates. Trabecular bone thickness and connectivity were significantly enhanced. Increased bone mineral density (BMD) and decreased urinary DPD concentration in TR KO mice indicated a role for TR on both inorganic and organic phases of bone. Moreover, TR KO cortical bone expands and has a higher moment of inertia, implying stronger bone. Preliminary histological analyses did not reveal any abnormalities in the morphology of the femurs. Serum analysis showed a decrease in RANKL and an increase in OPG levels. In vitro experiments demonstrated a TR-dependent stimulatory effect of thrombin on RANKL mRNA expression and subsequent RANKL secretion into osteoblast culture medium. This effect was blocked by a p42/p44-ERK inhibitor.

We conclude that the thrombin/TR system maintains normal bone remodeling by activating RANKL and limiting OPG synthesis by osteoblasts through the p42/44-ERK signaling pathway. TR deficiency inhibits osteoclastogenesis, resulting in osteopetrosis.