Roles of chemokine receptor CX3CR1 in maintaining murine bone homeostasis through the regulation of both osteoblasts and osteoclasts

Abstract

Chemokines have recently been reported to be involved in pathological bone destruction. However, the physiological roles of chemokines in bone metabolism in vivo have not been well documented. We analyzed the bone phenotypes in Cx3cr1-deficient mice. The mice exhibited slight but significant increases in trabecular and cortical thickness, reduced numbers of osteoclasts and increased rates of osteoid formation. Although the morphometric parameters showed marginal differences, the Cx3cr1-deficient bones showed an elevated expression of Osterix/SP7, an essential transcriptional factor for osteoblasts, while a late marker, Osteocalcin/Bglap, was downregulated. The levels of various osteoclastic markers, such as receptor activator of NF-κB (Rank)/Tnfrsf11a, receptor activator of NF-κB ligand (RANKL)/Tnfsf11, tartrate-resistant acid phosphatase 5b (Trap5b)/Acp5b, Cathepsin K/Ctsk, Mmp3 and Mmp13, were significantly decreased in the Cx3cr1-deficient bones. Cultured Cx3cr1-deficient osteoblastic cells showed inverse temporal patterns of osteoblastic marker expression and reduced calcium deposition. Further in vitro studies and immunofluorescence staining against CX3CR1 and CX3CL1 suggested a role for the CX3CR1-CX3CL1axis in an early stage of osteoblast differentiation, possibly through their trans- and cis-interactions. Cultured Cx3cr1-deficient pre-osteoclasts showed impaired differentiation, mainly due to a deficiency of the CD115+CD11blo osteoclastogenic population of myeloid-lineage precursors. The treatment of bone marrow-derived osteoclastic cultures with recombinant CX3CL1 at different time points suggested that the CX3CR1-CX3CL1 axis favors the maintenance of osteoclastic precursors, but not differentiated osteoclasts. The current observations uncovered novel roles of the CX3CR1-CX3CL1 axis in the differentiation of both osteoblasts and osteoclasts.