Bone homeostasis requires a delicate balance between the activities of bone-resorbing osteoclasts and bone-forming osteoblasts. Various molecules coordinate osteoclast function with that of osteoblasts; however, molecules that mediate osteoclast-osteoblast interactions by simultaneous signal transduction in both cell types have not yet been identified. Here we show that osteoclasts express the NFATc1 target gene Efnb2 (encoding ephrinB2), while osteoblasts express the receptor EphB4, along with other ephrin-Eph family members. Using gain- and loss-of-function experiments, we demonstrate that reverse signaling through ephrinB2 into osteoclast precursors suppresses osteoclast differentiation by inhibiting the osteoclastogenic c-Fos-NFATc1 cascade. In addition, forward signaling through EphB4 into osteoblasts enhances osteogenic differentiation, and overexpression of EphB4 in osteoblasts increases bone mass in transgenic mice. These data demonstrate that ephrin-Eph bidirectional signaling links two major molecular mechanisms for cell differentiation--one in osteoclasts and the other in osteoblasts--thereby maintaining bone homeostasis.

Fra1 transgenic (Tg) mice develop osteosclerosis and exhibit altered expression of bone matrix proteins. We found that expression of Thbs1 and Thbs2 was reduced in Fra1 Tg osteoblasts. Fra1 Tg and non-osteosclerotic Thbs1 −/− Thbs2−/− mice share an edge-to-edge bite. Therefore, reduced expression of thrombospondins may contribute to craniofacial dysmorphism independently of osteosclerosis.Introduction: Tg mice overexpressing Fra1, a component of the transcription factor activator protein-1 (AP-1), show progressive osteosclerosis caused by cell autonomous abnormalities in osteoblasts. The expression of several bone matrix proteins, including matrix gla protein, is dysregulated in Fra1 Tg osteoblasts. Materials and Methods:In osteoblastogenic cultures, altered bone matrix production by Fra1 overexpression was monitored using Alizarin red staining, quantitative RT-PCR, and Western blotting. Responsiveness to ovariectomy was examined by bone histomorphometry. Craniofacial parameters were measured on radiographs and using CT. Results: Thrombospondin-1 (Thbs1) and thrombospondin-2 (Thbs2) were reduced in Fra1 Tg osteoblasts differentiated in vitro and in bones from Fra1 Tg mice. Despite alterations in bone matrix proteins, ovariectomy induces high turnover bone loss in Fra1 Tg mice as in wildtype mice. Fra1 Tg mice, as well as Thbs1

Background Idiopathic osteonecrosis of the femoral head (ONFH) frequently occurs after liver transplantation (LT) because of lifelong administration of corticosteroids or immunosuppressants and often requires total hip arthroplasty (THA). This study examines patient characteristics and short-term outcomes of THA after LT. Methods We observed 9 hips in 7 patients who underwent THA from August 2015 to December 2017 for ONFH after LT (group L). Cementless implants were inserted in all hips. Medical records were retrospectively reviewed to reveal reasons for LT, type of donor, and period from LT to THA. Preoperative laboratory data, operative time, intraoperative blood loss, complication rates, and Harris Hip Score were compared with a control group of 27 cementless THAs in 27 patients with ONFH. Results Causative diseases were liver cirrhosis (n = 4), type B fulminant hepatitis (n = 1), congenital biliary atresia (n = 1), and iatrogenic biliary tract injury (n = 1). Four livers were from living donors and 3 from cadavers. Mean time from LT to THA was 10.4 (1-20) years. Preoperative blood test showed a significant decrease in platelet count (178 vs 268 [∗10 3 /μl]) and rise in total bilirubin (1.1 vs 0.7 [mg/dL]) in group L. There was no significant difference in operative time (86 vs 100 [minutes]), but intraoperative blood loss (303 vs 163 [mL]) increased significantly in group L. There were no significant differences in complication incidence or Harris Hip Score between the 2 groups. Conclusion THA after LT requires caution because risks for bleeding increase. However, short-term outcomes appear to be equivalent to normal THA.

Osteoclasts and foreign body giant cells (FBGCs) are derived from common progenitors and share properties such as multi-nucleation capacity induced by cell-cell fusion; however, mechanisms underlying lineage determination between these cells remain unclear. Here we show that, under inflammatory conditions, osteoclasts are stimulated in a manner similar to M1 macrophages, while formation of FBGCs, which exhibit M2-like phenotypes, is inhibited in a manner similar to that seen in M1/M2 macrophage polarization. FBGC/osteoclast polarization was inhibited by conditional knockout of tumor necrosis factor receptor associated factor 6 (Traf6) in adults in vivo and in vitro. Traf6-null mice were previously reported to die soon after birth, but we found that Traf6 deletion in adults did not cause lethality but rather inhibited osteoclast activation and prevented FBGC inhibition under inflammatory conditions. Accordingly, basal osteoclastogenesis was significantly inhibited by Traf6 deletion in vivo and in vitro and accompanied by increased bone mass. Lipopolysaccharide-induced osteoclast formation and osteolysis were significantly inhibited in Traf6 conditional knockout mice. Our results suggest that Traf6 plays a crucial role in regulating M1 osteoclast and M2 FBGC polarization and is a potential therapeutic target in blocking FBGC inhibition, antagonizing osteolysis in inflammatory conditions, and increasing bone mass without adverse effects in adults.

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