Proteinase-activated receptor-2 is required for normal osteoblast and osteoclast differentiation during skeletal growth and repair

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Proteinase-activated receptor-2 (PAR2) is a G-protein coupled receptor expressed by osteoblasts and monocytes. PAR2 is activated by a number of proteinases including coagulation factors and proteinases released by inflammatory cells. The aim of the current study was to investigate the role of PAR2 in skeletal growth and repair using wild type (WT) and PAR2 knockout (KO) mice. Micro computed tomography and histomorphometry were used to examine the structure of tibias isolated from uninjured mice at 50 and 90days of age, and from 98-day-old mice in a bone repair model in which a hole had been drilled through the tibias. Bone marrow was cultured and investigated for the presence of osteoblast precursors (alkaline phosphatase-positive fibroblastic colonies), and osteoclasts were counted in cultures treated with M-CSF and RANKL. Polymerase chain reaction (PCR) was used to determine which proteinases that activate PAR2 are expressed in bone marrow. Regulation of PAR2 expression in primary calvarial osteoblasts from WT mice was investigated by quantitative PCR. Cortical and trabecular bone volumes were significantly greater in the tibias of PAR2 KO mice than in those of WT mice at 50days of age. In trabecular bone, osteoclast surface, osteoblast surface and osteoid volume were significantly lower in KO than in WT mice. Bone marrow cultures from KO mice showed significantly fewer alkaline phosphatase-positive colony-forming units and osteoclasts compared to cultures from WT mice. Significantly less new bone and significantly fewer osteoclasts were observed in the drill sites of PAR2 KO mice compared to WT mice 7days post-surgery. A number of activators of PAR2, including matriptase and kallikrein 4, were found to be expressed by normal bone marrow. Parathyroid hormone, 1,25 dihydroxyvitamin D3, or interleukin-6 in combination with its soluble receptor down-regulated PAR2 mRNA expression, and fibroblast growth factor-2 or thrombin stimulated PAR2 expression. These results suggest that PAR2 activation contributes to determination of cells of both osteoblast and osteoclast lineages within bone marrow, and thereby participates in the regulation of skeletal growth and bone repair.