No abstract

Emerging concepts suggest that macrophage functional phenotype is regulated by transcription factors that define alternative activation states. We found that RBP-J, the major nuclear transducer of Notch signaling, augmented TLR4-induced expression of key mediators of classically activated M1 macrophages and thus innate immune responses to L. monocytogenes. Notch-RBP-J signaling controlled expression of the transcription factor IRF8 that induced downstream M1-specific genes. RBP-J promoted IRF8 protein synthesis by selectively augmenting IRAK2-dependent TLR4 signaling to the MNK1 kinase and downstream translation initiation control through eIF4E. These results define a signaling network in which Notch-RBP-J and TLR signaling are integrated at the level of IRF8 protein synthesis and identify a mechanism by which heterologous signaling pathways can regulate TLR-induced inflammatory macrophage polarization.

Recent studies have identified a specialized subset of CD31hiEMCNhi vascular endothelium that positively regulates bone formation. However, it remains unclear how CD31hiEMCNhi endothelium levels are coupled to anabolic bone formation. Mice with an osteoblast-specific deletion of Shn3, which have markedly elevated bone formation, demonstrated an increase in CD31hiEMCNhi endothelium. Transcriptomic analysis identified SLIT3 as an osteoblast-derived, SHN3-regulated proangiogenic factor. Genetic deletion of Slit3 reduced skeletal CD31hiEMCNhi endothelium, resulted in low bone mass due to impaired bone formation and partially reversed the high bone mass phenotype of Shn3−/− mice. This coupling between osteoblasts and CD31hiEMCNhi endothelium is essential for bone healing, as shown by defective fracture repair in SLIT3-mutant mice and enhanced fracture repair in SHN3-mutant mice. Finally, administration of recombinant SLIT3 both enhanced bone-fracture healing and counteracted bone loss in a mouse model of postmenopausal osteoporosis. Thus, drugs that target the SLIT3 pathway may represent a new approach for vascular-targeted osteoanabolic therapy to treat bone loss.

Bone metabolism results from a balance between osteoclast-driven bone resorption and osteoblastmediated bone formation. Diseases such as periodontitis and rheumatoid arthritis are characterized by increased bone destruction due to enhanced osteoclastogenesis 1,2 . Here we report that interferon regulatory factor 8 (IRF8), a transcription factor expressed in immune cells, is a key regulatory molecule for osteoclastogenesis. IRF8 expression in osteoclast precursors was downregulated during the initial phase of osteoclast differentiation induced by receptor activator of nuclear factor κB ligand (RANKL, also called TRANCE, ODF, and OPGL), which is encoded by the Tnfsf11 gene. Mice deficient in IRF8 exhibited severe osteoporosis due to increased numbers of osteoclasts, and enhanced bone destruction following lipopolysaccharide (LPS) administration. Irf8 -/-osteoclast precursors underwent increased osteoclastogenesis in response to RANKL and tumor necrosis factor α (TNFα). IRF8 suppressed osteoclastogenesis by inhibiting the function and expression of nuclear factor of activated T cells c1 (NFATc1). Our results show that IRF8 inhibits osteoclast formation Correspondence should be addressed to M.T. (takami@dent.showa-u.ac.jp). AUTHOR CONTRIBUTIONS B.Z. performed most of the experiments with significant assistance from M.T. X.W. conducted the histological analysis. A.Y., T.K., X.H., and T.T. assisted with the experiments. K.O. provided the Irf8 −/− mice. B.Z. and M.T. designed the project and wrote the manuscript. Y.C. provided recombinant RANKL and contributed to manuscript preparation. L.B.I. oversaw the bone marrow chimera and human cell experiments and contributed to manuscript revision. H.T. oversaw the inflammatory bone destruction experiments and provided critical advice for the experiments. M.T. and R.K. supervised the project. COMPETING INTERESTS STATEMENTThe authors declare that they have no competing financial interests. [3][4][5] . Numerous studies have focused on these upregulated genes and their roles in osteoclastogenesis. On the other hand, the expression levels of various genes are simultaneously downregulated during osteoclastogenesis 6 . The biological significance of the downregulated expression of these genes following RANK activation, however, has not been fully elucidated. NIH Public AccessTo identify genes that show reduced expression levels in response to RANK signaling, we performed a genome-wide screening of mRNAs from osteoclast precursors and osteoclasts using a DNA microarray technique (data not shown). Among the identified genes, expression of the transcription factor Irf8 [also called interferon consensus sequence binding protein (ICSBP)] was found to be downregulated during the initial phase of osteoclastogenesis triggered by RANKL (data not shown). IRF8 is known to be specifically expressed in immune cells, including monocytes/macrophages, B lymphocytes, and activated T lymphocytes 7-9 . It is a member of the IRF family and has been shown to regulate myeloid cell development by...