Resident macrophages support embryonic development and tissue homeostasis and repair. The mechanisms that control their differentiation remain unclear. We report here that Erythro-Myeloid Progenitors generate pre-macrophages (pMacs) that simultaneously colonize the whole embryo from embryonic day (E)9.5 in a chemokine-receptor dependent manner. The core macrophage program initiated in pMacs is rapidly diversified as expression of transcriptional regulators becomes tissue-specific in early macrophages. This process appears essential for macrophage specification and maintenance, as inactivation of Id3 impairs the development of liver macrophages and results in selective Kupffer cell deficiency in adults. We propose that macrophage differentiation is an integral part of organogenesis as colonization of organ anlagen by pMacs is followed by their specification into tissue macrophages, hereby generating the macrophage diversity observed in postnatal tissues.

The signaling molecule Wnt regulates bone homeostasis through β-catenin-dependent canonical and β-catenin-independent noncanonical pathways. Impairment of canonical Wnt signaling causes bone loss in arthritis and osteoporosis; however, it is unclear how noncanonical Wnt signaling regulates bone resorption. Wnt5a activates noncanonical Wnt signaling through receptor tyrosine kinase-like orphan receptor (Ror) proteins. We showed that Wnt5a-Ror2 signaling between osteoblast-lineage cells and osteoclast precursors enhanced osteoclastogenesis. Osteoblast-lineage cells expressed Wnt5a, whereas osteoclast precursors expressed Ror2. Mice deficient in either Wnt5a or Ror2, and those with either osteoclast precursor-specific Ror2 deficiency or osteoblast-lineage cell-specific Wnt5a deficiency showed impaired osteoclastogenesis. Wnt5a-Ror2 signals enhanced receptor activator of nuclear factor-κB (RANK) expression in osteoclast precursors by activating JNK and recruiting c-Jun on the promoter of the gene encoding RANK, thereby enhancing RANK ligand (RANKL)-induced osteoclastogenesis. A soluble form of Ror2 acted as a decoy receptor of Wnt5a and abrogated bone destruction in mouse arthritis models. Our results suggest that the Wnt5a-Ror2 pathway is crucial for osteoclastogenesis in physiological and pathological environments and represents a therapeutic target for bone diseases, including arthritis.

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Wnt, a secreted glycoprotein, has an approximate molecular weight of 40 kDa, and it is a cytokine involved in various biological phenomena including ontogeny, morphogenesis, carcinogenesis, and maintenance of stem cells. The Wnt signaling pathway can be classified into two main pathways: canonical and non-canonical. Of these, the canonical Wnt signaling pathway promotes osteogenesis. Sclerostin produced by osteocytes is an inhibitor of this pathway, thereby inhibiting osteogenesis. Recently, osteoporosis treatment using an anti-sclerostin therapy has been introduced. In this review, the basics of Wnt signaling, its role in bone metabolism and its involvement in skeletal disorders have been covered. Furthermore, the clinical significance and future scopes of Wnt signaling in osteoporosis, osteoarthritis, rheumatoid arthritis and neoplasia are discussed.

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Metabolic reprogramming occurs in response to the cellular environment to mediate differentiation, but the fundamental mechanisms linking metabolic processes to differentiation programs remain to be elucidated. During osteoclast differentiation, a shift toward more oxidative metabolic processes occurs. In this study we identified the de novo DNA methyltransferase 3a (Dnmt3a) as a transcription factor that couples these metabolic changes to osteoclast differentiation. We also found that receptor activator of nuclear factor-κB ligand (RANKL), an essential cytokine for osteoclastogenesis, induces this metabolic shift towards oxidative metabolism, which is accompanied by an increase in S-adenosylmethionine (SAM) production. We found that SAM-mediated DNA methylation by Dnmt3a regulates osteoclastogenesis via epigenetic repression of anti-osteoclastogenic genes. The importance of Dnmt3a in bone homeostasis was underscored by the observations that Dnmt3a-deficient osteoclast precursor cells do not differentiate efficiently into osteoclasts and that mice with an osteoclast-specific deficiency in Dnmt3a have elevated bone mass due to a smaller number of osteoclasts. Furthermore, inhibition of DNA methylation by theaflavin-3,3'-digallate abrogated bone loss in models of osteoporosis. Thus, this study reveals the role of epigenetic processes in the regulation of cellular metabolism and differentiation, which may provide the molecular basis for a new therapeutic strategy for a variety of bone disorders.

Osteoclasts are multinucleated cells that resorb bone. Although osteoclasts originate from the monocyte/macrophage lineage, osteoclast precursors are not well characterized in vivo. The relationship between proliferation and differentiation of osteoclast precursors is examined in this study using murine macrophage cultures treated with macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB (RANK) ligand (RANKL). Cell cycle–arrested quiescent osteoclast precursors (QuOPs) were identified as the committed osteoclast precursors in vitro. In vivo experiments show that QuOPs survive for several weeks and differentiate into osteoclasts in response to M-CSF and RANKL. Administration of 5-fluorouracil to mice induces myelosuppression, but QuOPs survive and differentiate into osteoclasts in response to an active vitamin D3 analogue given to those mice. Mononuclear cells expressing c-Fms and RANK but not Ki67 are detected along bone surfaces in the vicinity of osteoblasts in RANKL-deficient mice. These results suggest that QuOPs preexist at the site of osteoclastogenesis and that osteoblasts are important for maintenance of QuOPs.

The aim of the present study was to analyze the levels of osteocalcin, deoxypyridinoline (Dpd) and interleukin-1beta as markers of bone metabolism in peri-implant crevicular fluid (PICF) from peri-implantitis patients. PICF was sampled from a total of 34 endosseous titanium implants from 16 patients; nine females (mean age 52.8, range 40-62 years) and seven males (mean age 56.0, range 36-66 years). The implants had been in place for a period of 9-112 months (mean; 35.8 months) since the loading. These sites were categorized as six peri-implantitis, eight peri-implant mucositis and 20 healthy implant. PICF volume from peri-implantitis sites was significantly higher than mucositis and healthy implant sites (P < 0.01). Osteocalcin levels in PICF from mucositis sites were significantly higher than healthy implants (P < 0.05), whereas peri-implantitis sites were not significantly different from either mucositis or healthy implant sites. Dpd could not be detected in any of the samples examined. IL-1beta levels in PICF from peri-implantitis sites were significantly higher than levels from peri-implant mucositis (P < 0.05) and healthy implant sites (P < 0.01). In conclusion, osteocalcin in PICF may reflect increased local bone turnover around implants. Further, IL-1beta should be a useful marker for peri-implant inflammation.