Our subsequent functional analyses demonstrate that mature osteoclasts, whose platelet-derived growth factor bb (PDGF-bb) expression is reduced by siRNAs, exhibit a reduced capability of attracting osteoblasts. properties of the skeleton. Two major cell types, the bone-forming osteoblasts and the bone-resorbing osteoclasts contribute to this process, which occurs continuously throughout life. The balance between bone formation and degradation is normally tightly controlled but it becomes deregulated, shifting towards more degradation under pathological conditions or during aging, thereby leading to osteoporosis[1],[2]. This tight balance implies the existence of mechanisms coordinating the differentiation of osteoblasts and osteoclasts as well as their migration to locations where they function. The mechanisms by which osteoblasts control the differentiation of hematopoietic osteoclast precursors towards mature multinucleated cells, i.e. osteoclastogenesis, are now established[3]. Osteoblasts and stromal cells control bone degradation by expressing the Macrophage-Colony Stimulating Factor (M-CSF) required for the proliferation of osteoclast precursors and the Receptor for Activation of NF-kB Ligand (RANKL), a TNF family member triggering their differentiation[1],[3][7]. The mechanisms controlling the differentiation of mesenchymal stromal cells (MSCs) towards osteoblasts, i.e osteoblastogenesis, are less understood. During the past years, several signaling molecules have been implicated in bone development and osteogenesis[8]. These include fibroblast growth factors, parathyroid hormone complexes[9], bone morphogenetic proteins (BMPs)[10], Canonical Wnt[11][14], Indian hedgehog[15]and epidermal growth factor[16]. However, the cell types producing these factors have remained elusive. There is evidence however, that osteoclasts can control osteoblast function. Ephrin B2 expressed by osteoclasts and its Ephrin B4 receptor expressed by osteoblasts allow bidirectional signaling[17]. Signaling through Ephrin B4 into osteoblasts enhances osteogenic differentiation whereas signaling through Ephrin B2 into osteoclast precursors suppresses osteoclast differentiation. It has also been reported that the v-ATPase V0subunit D2 is not only involved in osteoclast fusion but also regulates the secretion by osteoclasts of still unidentified factors that inhibit the differentiation of osteoblast precursors into mature cells[18]. During development, osteoblasts must colonize the cartilage that will be replaced by bone. During adulthood, bone remodeling and repair require the migration of osteoblasts to bone areas that need to be rebuilt. This latter process also requires the mobilization of their progenitors residing together with MSCs into niches of the bone marrow[19],[20]. A plethora of growth factors used as recombinant proteins, in particular BMPs, platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF) and leukemia inhibitory factor (LIF), have been shown Rabbit Polyclonal to P2RY13 to exhibitin vitrochemotactic activities towards various cell types including osteoblasts and SIB 1757 their progenitors[21],[22]. However, it is still unclear which cell types in the bone matrix secrete the factors able to attract bone rebuilding cells, which growth factors can function in this context and how the migration of bone rebuilding cells is coordinated with bone digestion. We tested here the hypothesis that SIB 1757 osteoclasts can control osteoblast chemotaxis. Usingin vitrocell systems of osteoclastogenesis and osteoblasto-genesis, we show that mature osteoclasts secrete PDGF-bb, which is recognized by PDGF receptor (PDGFR-) on SIB 1757 the surface of osteoblasts and regulates osteoblast chemotaxis. Thus, thisin vitrostudy provides the first molecular basis for further investigating the importance of the PDGF-bb/ PDGFR- signaling pathway in bone SIB 1757 remodelingin vivo. == Results == In order to test whether osteoclasts can signal to osteoblasts, we usedin vitrocell systems of osteoclastogenesis or osteoblastogenesis. First, we used the mouse monocyte/macrophage-like Raw264.7 cells, which differentiatein vitrotowards mature osteoclasts upon stimulation with the osteoclastogenic cytokine RANKL. Raw264.7 cell-derived osteoclasts conform to conventional assays for osteoclast function as primary osteoclasts isolated from bones. Primary osteoclast precursors from mouse bone marrow and derived osteoclasts were also used. Second, we used the mouse pre-osteoblastic MC3T3-E1 cells that differentiatein vitrotowards mature osteoblasts upon stimulation with chemical cocktails containing dexamethasone, ascorbic acid and -glycerophosphate. The MC3T3-E1-derived osteoblasts express the typical markers of osteoblasts isolated from bones. == Osteoclasts secrete factors attracting osteoblasts == We first tested whether the conditioned medium of Raw264.7 cells or Raw264.7-derived osteoclasts could attract pre-osteoblastic MC3T3-E1 cells or MC3T3-E1-cell derived osteoblasts. For this, we used the Boyden chamber assay to measure chemotaxis.Figure 1Ashows that the chemotactic activity of Raw264.7 cells towards MC3T3-E1 cells was rather low. However, their chemotactic activity increased with time when they were differentiated towards osteoclasts upon stimulation with RANKL. After 4 days of.