Blimp1-mediated repression of negative regulators is required for osteoclast differentiation. Proc Natl Acad Sci U S A. 2010;107:3117-3122. [PMID: 20133620 DOI: 10.1073/pnas.0912779107]

Aryl hydrocarbon receptor catabolic activity in bone metabolism is osteoclast dependent in vivo

Bone mass is regulated by various molecules including endogenous factors as well as exogenous factors, such as nutrients and pollutants. Aryl hydrocarbon receptor (AhR) is known as a dioxin receptor and is responsible for various pathological and physiological processes. However, the role of AhR in bone homeostasis remains elusive because the cell type specific direct function of AhR has never been explored in vivo. Here, we show the cell type specific function of AhR in vivo in bone homeostasis. Systemic AhR knockout (AhRKO) mice exhibit increased bone mass with decreased resorption and decreased formation. Meanwhile, osteoclast specific AhRKO (AhR(ΔOc/ΔOc)) mice have increased bone mass with reduced bone resorption, although the mice lacking AhR in osteoblasts have a normal bone phenotype. Even under pathological conditions, AhR(ΔOc/ΔOc) mice are resistant to sex hormone deficiency-induced bone loss resulting from increased bone resorption. Furthermore, 3-methylcholanthrene, an AhR agonist, induces low bone mass with increased bone resorption in control mice, but not in AhR(ΔOc/ΔOc) mice. Taken together, cell type specific in vivo evidence for AhR functions indicates that osteoclastic AhR plays a significant role in maintenance of bone homeostasis, suggesting that inhibition of AhR in osteoclasts can be beneficial in the treatment of osteoporosis.

Miconazole inhibits receptor activator of nuclear factor-κB ligand-mediated osteoclast formation and function

Osteoclasts are responsible for bone erosion in diseases as diverse as osteoporosis, periodontitis, and rheumatoid arthritis. Antifungal products have received recent attention as potential therapeutic and preventative drugs in human disease. Since little is known about the action of miconazole, an antifungal imidazole, on bone metabolism, we investigated the effects of miconazole on osteoclast formation using mouse bone marrow macrophages (BMMs). Miconazole inhibited RANKL-induced osteoclast formation in a dose-dependent manner without cytotoxicity. Furthermore, miconazole inhibited the bone resorptive activity of osteoclasts. Miconazole suppressed RANKL-induced expression of c-Fos and NFATc1, two essential transcription factors for osteoclast differentiation. Miconazole seemed to inhibit osteoclast formation MAPK pathways as well as Blimp1 through MafB expression. Miconazole also inhibited RANKL-induced expression of the pro-inflammatory cytokines, COX-2 and iNOS. In accordance with the in vitro study, miconazole reduced lipopolysaccharide-induced osteoclast formation in vivo. Therefore, miconazole exerted an inhibitory effect on osteoclast formation in vitro and in vivo. It could be useful for the treatment of bone diseases associated with excessive bone resorption.

Development of an in vitro culture method for stepwise differentiation of mouse embryonic stem cells and induced pluripotent stem cells into mature osteoclasts

The development of methods for differentiation of embryonic stem cells (ESCs) and induced pluripotent stem cell (iPSCs) into functional cells have helped to analyze the mechanism regulating cellular processes and to explore cell-based assays for drug discovery. Although several reports have demonstrated methods for differentiation of mouse ESCs into osteoclast-like cells, it remains unclear whether these methods are applicable for differentiation of iPSCs to osteoclasts. In this study, we developed a simple method for stepwise differentiation of mouse ESCs and iPSCs into bone-resorbing osteoclasts based upon a monoculture approach consisting of three steps. First, based on conventional hanging-drop methods, embryoid bodies (EBs) were produced from mouse ESCs or iPSCs. Second, EBs were cultured in medium supplemented with macrophage colony-stimulating factor (M-CSF), and differentiated to osteoclast precursors, which expressed CD11b. Finally, ESC- or iPSC-derived osteoclast precursors stimulated with receptor activator of nuclear factor-B ligand (RANKL) and M-CSF formed large multinucleated osteoclast-like cells that expressed tartrate-resistant acid phosphatase and were capable of bone resorption. Molecular analysis showed that the expression of osteoclast marker genes such as Nfatc1, Ctsk, and Acp5 are increased in a RANKL-dependent manner. Thus, our procedure is simple and easy and would be helpful for stem cell-based bone research.

Emerging therapeutic targets for osteoporosis treatment

INTRODUCTION

To date, osteoporosis still remains a major public health burden especially for the aging populations. Over the last few decades treatments for osteoporosis have largely focused on anti-resorptive agents represented by bisphosphonates and estrogen therapy that dominated the market. Unsatisfactory efficacy, non-specificity and long-term safety of current therapies necessitate the need for new targets effectively preventing and treating of osteoporosis.

AREAS COVERED

This review expatiates on the mechanism of osteoporosis occurrence and bone remodeling cycle in detail. New targets of antiresorptive and anabolic agents based on the functions of osteoblasts and osteoclasts as well as associated signaling pathways are outlined.

EXPERT OPINION

Advanced understanding in the fields of bone remodeling, functions of osteoblasts, osteoclasts and osteocytes associated with osteoporosis occurrence offers the emerging bone-resorptive or bone-formative targets. Currently, molecules involving RANK-RANKL-OPG system and Wnt/β-catenin signaling pathway act as the most promising targets.

Analysis of NFATc1-centered transcription factor regulatory networks in osteoclast formation

Osteoclasts are bone-resorbing cells that differentiate from the macrophage/monocyte lineage. The master transcription factor NFATc1 has a central role in the process of this differentiation. Thus, it is important to understand the NFATc1-centered transcription factor regulatory networks (TFRNs) in terms of the mechanisms of NFATc1 expression and activation as well as the gene expression regulated by NFATc1. The Genome Network Project has provided a unique opportunity for the analysis of NFATc1-centered TFRNs in osteoclasts. Here we introduce a report on the application of the methods established by the project for osteoclast biology and a summarization of the results obtained in this project thus far.

Cell cycle-dependent Rho GTPase activity dynamically regulates cancer cell motility and invasion in vivo

The mechanism behind the spatiotemporal control of cancer cell dynamics and its possible association with cell proliferation has not been well established. By exploiting the intravital imaging technique, we found that cancer cell motility and invasive properties were closely associated with the cell cycle. In vivo inoculation of human colon cancer cells bearing fluorescence ubiquitination-based cell cycle indicator (Fucci) demonstrated an unexpected phenomenon: S/G2/M cells were more motile and invasive than G1 cells. Microarray analyses showed that Arhgap11a, an uncharacterized Rho GTPase-activating protein (RhoGAP), was expressed in a cell-cycle-dependent fashion. Expression of ARHGAP11A in cancer cells suppressed RhoA-dependent mechanisms, such as stress fiber formation and focal adhesion, which made the cells more prone to migrate. We also demonstrated that RhoA suppression by ARHGAP11A induced augmentation of relative Rac1 activity, leading to an increase in the invasive properties. RNAi-based inhibition of Arhgap11a reduced the invasion and in vivo expansion of cancers. Additionally, analysis of human specimens showed the significant up-regulation of Arhgap11a in colon cancers, which was correlated with clinical invasion status. The present study suggests that ARHGAP11A, a cell cycle-dependent RhoGAP, is a critical regulator of cancer cell mobility and is thus a promising therapeutic target in invasive cancers.

Identification and expression profiles of prdm1 in medaka Oryzias latipes

Mouse Prdm1, also known as Blimp1, plays important roles in maturation and survival of lymphoid cells, as well as in organogenesis of muscle, limb, sensor organs and primordial germ cells. The homologues of mouse prdm1 have been identified in a diverse of animals including zebrafish and fugu. Here, we report the identification and expression profiles of two homologues of prdm1, namely prdm1a and prdm1b in medaka, Oryzias latipes. The transcripts of prdm1a and prdm1b were detectable in all the tissues including immune organs such as gill, spleen, kidney, liver and intestine that we have checked on. The transcripts of prdm1a could be detected in the embryonic shield at mid-gastrula stage and later in the somite, eye, otic vesicle, branchial arches, fin, intestine and cloaca during embryogenesis using in situ hybridization. Moreover, the expression of prdm1a in the liver of both medaka and zebrafish could be up-regulated by the immune stimuli including lipopolysaccharide, polyI:C and the grass carp reovirus, similarly to the up-regulation of IL1B. These results indicate that Prdm1a may play important roles in embryogenesis and also in immune response in fish.

p130Cas, Crk-associated substrate, plays important roles in osteoclastic bone resorption

p130Cas, Crk-associated substrate (Cas), is an adaptor/scaffold protein that plays a central role in actin cytoskeletal reorganization. We previously reported that p130Cas is not tyrosine-phosphorylated in osteoclasts derived from Src-deficient mice, which are congenitally osteopetrotic, suggesting that p130Cas serves as a downstream molecule of c-Src and is involved in osteoclastic bone resorption. However, the physiological role of p130Cas in osteoclasts has not yet been confirmed because the p130Cas-deficient mice displayed embryonic lethality. Osteoclast-specific p130Cas conditional knockout (p130Cas(ΔOCL-) ) mice exhibit a high bone mass phenotype caused by defect in multinucleation and cytoskeleton organization causing bone resorption deficiency. Bone marrow cells from p130Cas(ΔOCL-) mice were able to differentiate into osteoclasts and wild-type cells in vitro. However, osteoclasts from p130Cas(ΔOCL-) mice failed to form actin rings and resorb pits on dentine slices. Although the initial events of osteoclast attachment, such as β3-integrin or Src phosphorylation, were intact, the Rac1 activity that organizes the actin cytoskeleton was reduced, and its distribution was disrupted in p130Cas(ΔOCL-) osteoclasts. Dedicator of cytokinesis 5 (Dock5), a Rho family guanine nucleotide exchanger, failed to associate with Src or Pyk2 in osteoclasts in the absence of p130Cas. These results strongly indicate that p130Cas plays pivotal roles in osteoclastic bone resorption.

Necessity of lysophosphatidic acid receptor 1 for development of arthritis

OBJECTIVE

Lysophosphatidic acid (LPA) is a bioactive lipid that binds to a group of cell surface G protein-coupled receptors (LPA receptors 1-6 [LPA1-6 ]) and has been implicated as an important mediator of angiogenesis, inflammation, and cancer growth. This study was undertaken to analyze the effects of LPA1 on the development of arthritis.

METHODS

Expression of LPA receptors on synovial tissue was analyzed by immunohistochemistry and quantitative reverse transcription-polymerase chain reaction. The effects of abrogation of LPA1 on collagen-induced arthritis (CIA) were evaluated using LPA1 -deficient mice or LPA1 antagonist. Migrating fluorescence-labeled CD11b+ splenocytes, which were transferred into the synovium of mice with CIA, were counted. CD4+ naive T cells were incubated under Th1-, Th2-, or Th17-polarizing conditions, and T helper cell differentiation was assessed. Osteoclast formation from bone marrow cells was examined.

RESULTS

LPA1 was highly expressed in the synovium of patients with rheumatoid arthritis (RA) compared with that of patients with osteoarthritis. LPA1 -deficient mice did not develop arthritis following immunization with type II collagen (CII). LPA1 antagonist also ameliorated murine CIA. Abrogation of LPA1 was associated with reductions in cell infiltration, bone destruction in the joints, and interleukin-17 production from CII-stimulated splenocytes. Infiltration of transferred CD11b+ macrophages from LPA1 -deficient mice into the synovium was suppressed compared with infiltration of macrophages from wild-type mice. LPA1 antagonist inhibited the infiltration of macrophages from wild-type mice. Differentiation into Th17, but not Th1 or Th2, and osteoclast formation were also suppressed under conditions of LPA1 deficiency or LPA1 inhibition in vitro.

CONCLUSION

Collectively, these results indicate that LPA/LPA1 signaling contributes to the development of arthritis via cellular infiltration, Th17 differentiation, and osteoclastogenesis. Thus, LPA1 may be a promising target molecule for RA therapy.

PRDM1/BLIMP1 is commonly inactivated in anaplastic large T-cell lymphoma

The commonest lesions in anaplastic large cell lymphomas are losses at 17p13 and at 6q21, concomitant in up to one-quarter of the cases. PRDM1 (BLIMP1) gene (6q21) is inactivated by multiple mechanisms and acts as a tumor suppressor gene in anaplastic large B-cell lymphoma.

Noncoordinate expression of J-chain and Blimp-1 define nurse shark plasma cell populations during ontogeny

B-lymphocyte-induced maturation protein 1 (Blimp-1) is the master regulator of plasma cell development, controlling genes such as those encoding J-chain and secretory Ig heavy chain. However, some mammalian plasma cells do not express J-chain, and mammalian B1 cells secrete "natural" IgM antibodies without upregulating Blimp-1. While these results have been controversial in mammalian systems, here we describe subsets of normally occurring Blimp-1(-) antibody-secreting cells in nurse sharks, found in lymphoid tissues at all ontogenic stages. Sharks naturally produce large amounts of both pentameric (classically "19S") and monomeric (classically "7S") IgM, the latter an indicator of adaptive immunity. Consistent with the mammalian paradigm, shark Blimp-1 is expressed in splenic 7S IgM-secreting cells, though rarely detected in the J-chain(+) cells producing 19S IgM. Although IgM transcript levels are lower in J-chain(+) cells, these cells nevertheless secrete 19S IgM in the absence of Blimp-1, as demonstrated by ELISPOT and metabolic labeling. Additionally, cells in the shark BM equivalent (epigonal) are Blimp-1(-). Our data suggest that, in sharks, 19S-secreting cells and other secreting memory B cells in the epigonal are maintained for long periods without Blimp-1, but like in mammals, Blimp-1 is required for terminating the B-cell program following an adaptive immune response in the spleen.

A tripartite transcription factor network regulates primordial germ cell specification in mice

Transitions in cell states are controlled by combinatorial actions of transcription factors. BLIMP1, the key regulator of primordial germ cell (PGC) specification, apparently acts together with PRDM14 and AP2γ. To investigate their individual and combinatorial functions, we first sought an in vitro system for transcriptional readouts and chromatin immunoprecipitation sequencing analysis. We then integrated this data with information from single-cell transcriptome analysis of normal and mutant PGCs. Here we show that BLIMP1 binds directly to repress somatic and cell proliferation genes. It also directly induces AP2γ, which together with PRDM14 initiates the PGC-specific fate. We determined the occupancy of critical genes by AP2γ-which, when computed altogether with those of BLIMP1 and PRDM14 (both individually and cooperatively), reveals a tripartite mutually interdependent transcriptional network for PGCs. We also demonstrate that, in principle, BLIMP1, AP2γ and PRDM14 are sufficient for PGC specification, and the unprecedented resetting of the epigenome towards a basal state.

Widespread but tissue-specific patterns of interferon-induced transmembrane protein 3 (IFITM3, FRAGILIS, MIL-1) in the mouse gastrula

Interferon-induced transmembrane protein 3 (IFITM3; FRAGILIS; MIL-1) is part of a larger family of important small interferon-induced transmembrane genes and proteins involved in early development, cell adhesion, and cell proliferation, and which also play a major role in response to bacterial and viral infections and, more recently, in pronounced malignancies. IFITM3, together with tissue-nonspecific alkaline phosphatase (TNAP), PRDM1, and STELLA, has been claimed to be a hallmark of segregated primordial germ cells (PGCs) (Saitou et al., 2002). However, whether IFITM3, like STELLA, is part of a broader stem/progenitor pool that builds the posterior region of the mouse conceptus (Mikedis and Downs, 2012) is obscure. To discover the whereabouts of IFITM3 during mouse gastrulation (~E6.5-9.0), systematic immunohistochemical analysis was carried out at closely spaced 2-4-h intervals. Results revealed diverse, yet consistent, profiles of IFITM3 localization throughout the gastrula. Within the putative PGC trajectory and surrounding posterior tissues, IFITM3 localized as a large cytoplasmic spot with or without staining in the plasma membrane. IFITM3, like STELLA, was also found in the ventral ectodermal ridge (VER), a posterior progenitor pool that builds the tailbud. The large cytoplasmic spot with plasma membrane staining was exclusive to the posterior region; the visceral yolk sac, non-posterior tissues, and epithelial tissues exhibited spots of IFITM3 without cell surface staining. Colocalization of the intracellular IFITM3 spot with the endoplasmic reticulum, Golgi apparatus, or endolysosomes was not observed. That relatively high levels of IFITM3 were found throughout the posterior primitive streak and its derivatives is consistent with evidence that IFITM3, like STELLA, is part of a larger stem/progenitor cell pool at the posterior end of the primitive streak that forms the base of the allantois and builds the fetal-umbilical connection, thus further obfuscating practical phenotypic distinctions between so-called PGCs and surrounding soma.

Negative regulation of osteoclast precursor differentiation by CD11b and β2 integrin-B-cell lymphoma 6 signaling

Negative regulation of osteoclastogenesis is important for bone homeostasis and prevention of excessive bone resorption in inflammatory and other diseases. Mechanisms that directly suppress osteoclastogenesis are not well understood. In this study we investigated regulation of osteoclast differentiation by the β2 integrin CD11b/CD18 that is expressed on myeloid lineage osteoclast precursors. CD11b-deficient mice exhibited decreased bone mass that was associated with increased osteoclast numbers and decreased bone formation. Accordingly, CD11b and β2 integrin signaling suppressed osteoclast differentiation by preventing receptor activator of NF-κB ligand (RANKL)-induced induction of the master regulator of osteoclastogenesis nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) and of downstream osteoclast-related NFATc1 target genes. CD11b suppressed induction of NFATc1 by the complementary mechanisms of downregulation of RANK expression and induction of recruitment of the transcriptional repressor B-cell lymphoma 6 (BCL6) to the NFATC1 gene. These findings identify CD11b as a negative regulator of the earliest stages of osteoclast differentiation, and provide an inducible mechanism by which environmental cues suppress osteoclastogenesis by activating a transcriptional repressor that makes genes refractory to osteoclastogenic signaling.

The transcription factor Jdp2 controls bone homeostasis and antibacterial immunity by regulating osteoclast and neutrophil differentiation

Jdp2 is an AP-1 family transcription factor that regulates the epigenetic status of histones. Previous in vitro studies revealed that Jdp2 is involved in osteoclastogenesis. However, the roles of Jdp2 in vivo and its pleiotropic functions are largely unknown. Here we generated Jdp2(-/-) mice and discovered its crucial roles not only in bone metabolism but also in differentiation of neutrophils. Jdp2(-/-) mice exhibited osteopetrosis resulting from impaired osteoclastogenesis. Jdp2(-/-) neutrophils were morphologically normal but had impaired surface expression of Ly6G, bactericidal function, and apoptosis. We also found that ATF3 was an inhibitor of neutrophil differentiation and that Jdp2 directly suppresses its expression via inhibition of histone acetylation. Strikingly, Jdp2(-/-) mice were highly susceptible to Staphylococcus aureus and Candida albicans infection. Thus, Jdp2 plays pivotal roles in in vivo bone homeostasis and host defense by regulating osteoclast and neutrophil differentiation.

Molecular mechanisms of triggering, amplifying and targeting RANK signaling in osteoclasts

Osteoclast differentiation depends on receptor activator of nuclear factor-κB (RANK) signaling, which can be divided into triggering, amplifying and targeting phases based on how active the master regulator nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) is. The triggering phase is characterized by immediate-early RANK signaling induced by RANK ligand (RANKL) stimulation mediated by three adaptor proteins, tumor necrosis factor receptor-associated factor 6, Grb-2-associated binder-2 and phospholipase C (PLC)γ2, leading to activation of IκB kinase, mitogen-activated protein kinases and the transcription factors nuclear factor (NF)-κB and activator protein-1 (AP-1). Mice lacking NF-κB p50/p52 or the AP-1 subunit c-Fos (encoded by Fos) exhibit severe osteopetrosis due to a differentiation block in the osteoclast lineage. The amplification phase occurs about 24 h later in a RANKL-induced osteoclastogenic culture when Ca(2+) oscillation starts and the transcription factor NFATc1 is abundantly produced. In addition to Ca(2+) oscillation-dependent nuclear translocation and transcriptional auto-induction of NFATc1, a Ca(2+) oscillation-independent, osteoblast-dependent mechanism stabilizes NFATc1 protein in differentiating osteoclasts. Osteoclast precursors lacking PLCγ2, inositol-1,4,5-trisphosphate receptors, regulator of G-protein signaling 10, or NFATc1 show an impaired transition from the triggering to amplifying phases. The final targeting phase is mediated by activation of numerous NFATc1 target genes responsible for cell-cell fusion and regulation of bone-resorptive function. This review focuses on molecular mechanisms for each of the three phases of RANK signaling during osteoclast differentiation.

New insights into osteoclastogenic signaling mechanisms

Bone is continuously renewed through a dynamic balance between bone resorption and formation. This process is the fundamental basis for the maintenance of normal bone mass and architecture. Osteoclasts play a crucial role in both physiological and pathological bone resorption, and receptor activator of nuclear factor-κB ligand (RANKL) is the key cytokine that induces osteoclastogenesis. Here we summarize the recent advances in the understanding of osteoclastogenic signaling by focusing on the investigation of RANKL signaling and RANKL-expressing cells in the context of osteoimmunology. The context afforded by osteoimmunology will provide a scientific basis for future therapeutic approaches to diseases related to the skeletal and immune systems.

Inhibition of osteoclast generation: a novel function of the bone morphogenetic protein 7/osteogenic protein 1

Monocytes have the potential to differentiate to either macrophages, dendritic cells, or to osteoclasts. The microenvironment, particularly cytokines, directs the monocyte differentiation. Receptors of NFκB (RANK) ligand, tumor necrosis factor (TNF) α, or interleukin- (IL-) 8 have be identified as inducers of osteoclastogenesis, whereas others, such as IL-10 or transforming growth factor (TGF)ß inhibit osteoclast generation or induce differentiation towards a dendritic cell type. We now describe that bone morphogenetic protein (BMP) 7/osteogenic protein- (OP-) 1 inhibited the differentiation of human CD14+ monocytes to osteoclasts. In the presence of BMP7/OP-1 the transcription factors c-Fos and NFATc1, though upregulated and translocated to the nucleus in response to either RANKL or IL-8, did not persist. In parallel, MafB, a transcription factor expressed by monocytes and required for differentiation to macrophages but inhibiting osteoclast generation, was preserved. Because both persistence of NFATc1 and downregulation of MafB are crucial for osteoclastogenesis, we conclude that BMP7/OP-1 inhibits the generation of osteoclasts by interfering with signalling pathways.

Alternative splicing regulates Prdm1/Blimp-1 DNA binding activities and corepressor interactions

Prdm1/Blimp-1 is a master regulator of gene expression in diverse tissues of the developing embryo and adult organism. Its C-terminal zinc finger domain mediates nuclear import, DNA binding, and recruitment of the corepressors G9a and HDAC1/2. Alternatively spliced transcripts lacking exon 7 sequences encode a structurally divergent isoform (Blimp-1Δexon7) predicted to have distinct functions. Here we demonstrate that the short Blimp-1Δexon7 isoform lacks DNA binding activity and fails to bind G9a or HDAC1/2 but retains the ability to interact with PRMT5. To investigate functional roles of alternative splicing in vivo, we engineered novel mouse strains via embryonic stem (ES) cell technology. Like null mutants, embryos carrying a targeted deletion of exon 7 and exclusively expressing Blimp-1Δexon7 die at around embryonic day 10.5 (E10.5) due to placental defects. In heterozygous Δexon7 mice, there is no evidence of dominant-negative effects. Mice carrying a knock-in allele with an exon 6-exon 7 fusion express full-length Blimp-1 only, develop normally, are healthy and fertile as adults, and efficiently generate mature plasma cells. These findings strongly suggest that the short Blimp-1Δexon7 isoform is dispensable. We propose that developmentally regulated alternative splicing is influenced by chromatin structure at the locus and fine-tunes Blimp-1's functional capabilities.

The Prdm family: expanding roles in stem cells and development

Members of the Prdm family are characterized by an N-terminal PR domain that is related to the SET methyltransferase domain, and multiple zinc fingers that mediate sequence-specific DNA binding and protein-protein interactions. Prdm factors either act as direct histone methyltransferases or recruit a suite of histone-modifying enzymes to target promoters. In this way, they function in many developmental contexts to drive and maintain cell state transitions and to modify the activity of developmental signalling pathways. Here, we provide an overview of the structure and function of Prdm family members and discuss the roles played by these proteins in stem cells and throughout development.

Bach1 regulates osteoclastogenesis in a mouse model via both heme oxygenase 1-dependent and heme oxygenase 1-independent pathways

OBJECTIVE

Reducing inflammation and osteoclastogenesis by heme oxygenase 1 (HO-1) induction could be beneficial in the treatment of rheumatoid arthritis (RA). However, the function of HO-1 in bone metabolism remains unclear. This study was undertaken to clarify the effects of HO-1 and its repressor Bach1 in osteoclastogenesis.

METHODS

In vitro osteoclastogenesis was compared in Bach1-deficient and wild-type mice. Osteoclasts (OCs) were generated from bone marrow-derived macrophages by stimulation with macrophage colony-stimulating factor and RANKL. Osteoclastogenesis was assessed by tartrate-resistant acid phosphatase staining and expression of OC-related genes. Intracellular signal pathways in OC precursors were also assessed. HO-1 short hairpin RNA (shRNA) was transduced into Bach1(-/-) mouse bone marrow-derived macrophages to examine the role of HO-1 in osteoclastogenesis. In vivo inflammatory bone loss was evaluated by local injection of tumor necrosis factor α (TNFα) into calvaria.

RESULTS

Transcription of HO-1 was down-regulated by stimulation with RANKL in the early stage of OC differentiation. Bach1(-/-) mouse bone marrow-derived macrophages were partially resistant to the RANKL-dependent HO-1 reduction and showed impaired osteoclastogenesis, which was associated with reduced expression of RANK and components of the downstream TNF receptor-associated factor 6/c-Fos/NF-ATc1 pathway as well as reduced expression of Blimp1. Treatment with HO-1 shRNA increased the number of OCs and expression of OC-related genes except for the Blimp1 gene during in vitro osteoclastogenesis from Bach1(-/-) mouse bone marrow-derived macrophages. TNFα-induced bone destruction was reduced in Bach1(-/-) mice in vivo.

CONCLUSION

The present findings demonstrate that Bach1 regulates osteoclastogenesis under inflammatory conditions, via both HO-1-dependent and HO-1-independent mechanisms. Bach1 may be worthy of consideration as a target for treatment of inflammatory bone loss in diseases including RA.

Conditional inactivation of Blimp1 in adult mice promotes increased bone mass

Bone resorption, which is regulated by osteoclasts, is excessively activated in bone destructive diseases such as osteoporosis. Thus, controlling osteoclasts would be an effective strategy to prevent pathological bone loss. Although several transcription factors that regulate osteoclast differentiation and function could serve as molecular targets to inhibit osteoclast formation, those factors have not yet been characterized using a loss of function approach in adults. Here we report such a study showing that inactivation of B-lymphocyte induced maturation protein 1 (Blimp1) in adult mice increases bone mass by suppressing osteoclast formation. Using an ex vivo assay, we show that osteoclast differentiation is significantly inhibited by Blimp1 inactivation at an early stage of osteoclastogenesis. Conditional inactivation of Blimp1 inhibited osteoclast formation and increased bone mass in both male and female adult mice. Bone resorption parameters were significantly reduced by Blimp1 inactivation in vivo. Blimp1 reportedly regulates immune cell differentiation and function, but we detected no immune cell failure following Blimp1 inactivation. These data suggest that Blimp1 is a potential target to promote increased bone mass and prevent osteoclastogenesis.

Regulators of osteoclast differentiation and cell-cell fusion

Osteoclasts are multinuclear giant cells derived from osteoclast/macrophage/dendritic cell common progenitor cells. The most characteristic feature of osteoclasts is multinucleation resulting from cell-cell fusion of mononuclear osteoclasts. Osteoclast cell-cell fusion is considered essential for re-organization of the cytoskeleton, such as the actin-ring and ruffled boarder to seal the resorbing area and to secret protons, respectively, to resorb bone; the fusion process is thus critical for osteoclast function. Various molecules, such as E-cadherin and macrophage fusion receptor (MFR), have been identified as regulators of osteoclast or macrophage cell-cell fusion. Laboratory production of osteoclasts used to be performed in a co-culture of osteoclast progenitors with osteoblastic cells, but recent advances in the identification of nuclear factor of kappa B ligand (RANKL) enabled the isolation of osteoclast-specific molecules involving osteoclast cell-cell fusion and differentiation regulators from purified osteoclast mRNA, since osteoclasts can be formed without osteoblasts. The essential cell-cell fusion regulator, dendritic cell-specific transmembrane protein (DC-STAMP), was isolated by a cDNA subtractive screen between mononuclear macrophages and RANKL-induced multinuclear osteoclasts. The cell-cell fusion of osteoclasts and foreign body giant cells (FBGCs) was completely abrogated in DC-STAMP-deficient mice in vivo and in vitro. Bone resorbing activity was significantly reduced but was still detected in DC-STAMP-deficient osteoclasts. DC-STAMP expression is positively regulated by two transcriptional factors: nuclear factor of activated T cells 1 (NFATc1) and c-Fos, both of which are essential for osteoclast differentiation. Furthermore, a novel osteoclastogenesis-regulating pathway involving two transcriptional repressors [B cell lymphoma 6 (Bcl6) and B lymphocyte-induced maturation protein 1 (Blimp1)] under RANKL stimulation has been discovered. The expression of osteoclastic genes such as DC-STAMP, NFATc1, and Cathepsin K, as well as osteoclast differentiation, was inhibited by Bcl6. Bcl6-deficient mice showed enhanced osteoclastogenesis and reduced bone mass, whereas osteoclast-specific Blimp1-conditional knockout mice showed elevated Bcl6 expression, osteoclastic gene expression, and osteoclast differentiation and increased bone mass. In this review, recent advances in our understanding of the regulators of osteoclast differentiation and cell-cell fusion are discussed.

New regulation mechanisms of osteoclast differentiation

Osteoclasts play a crucial role in both physiological and pathological bone resorption. It is, thus, of compelling importance to understand the molecular mechanisms of osteoclast regulation. Because receptor activator of nuclear factor-κB ligand (RANKL) is the key cytokine that induces osteoclast differentiation, we have focused on the investigation of RANKL signaling and RANKL-expressing cells. Here, we summarize the recent advances in the understanding of osteoclastogenic signaling and the cells that express RANKL in the context of osteoimmunology. The scope of osteoimmunology has been extended to now encompass a wide range of molecular and cellular interactions, and its framework provides a scientific basis for future therapeutic approaches to diseases related to the bone and/or immune systems.

JMJD5, a Jumonji C (JmjC) domain-containing protein, negatively regulates osteoclastogenesis by facilitating NFATc1 protein degradation

Osteoclastogenesis is a highly regulated process governed by diverse classes of regulators. Among them, nuclear factor of activated T-cells calcineurin-dependent 1 (NFATc1) is the primary osteoclastogenic transcription factor, and its expression is transcriptionally induced during early osteoclastogenesis by receptor activation of nuclear factor κB ligand (RANKL), an osteoclastogenic cytokine. Here, we report the novel enzymatic function of JMJD5, which regulates NFATc1 protein stability. Among the tested Jumonji C (JmjC) domain-containing proteins, decreased mRNA expression levels during osteoclastogenesis were found for JMJD5 in RAW264 cells stimulated by RANKL. To examine the functional role of JMJD5 in osteoclast differentiation, we established stable JMJD5 knockdown cells, and osteoclast formation was assessed. Down-regulated expression of JMJD5 led to accelerated osteoclast formation together with induction of several osteoclast-specific genes such as Ctsk and DC-STAMP, suggesting that JMJD5 is a negative regulator in osteoclast differentiation. Although JMJD5 was recently reported as a histone demethylase for histone H3K36me2, no histone demethylase activity was detected in JMJD5 in vitro or in living cells, even for other methylated histone residues. Instead, JMJD5 co-repressed transcriptional activity by destabilizing NFATc1 protein. Protein hydroxylase activity mediated by the JmjC domain in JMJD5 was required for the observed functions of JMJD5. JMJD5 induced the association of hydroxylated NFATc1 with the E3 ubiquitin ligase Von Hippel-Lindau tumor suppressor (VHL), thereby presumably facilitating proteasomal degradation of NFATc1 via ubiquitination. Taken together, the present study demonstrated that JMJD5 is a post-translational co-repressor for NFATc1 that attenuates osteoclastogenesis.

TNF-induced osteoclastogenesis and inflammatory bone resorption are inhibited by transcription factor RBP-J

Tumor necrosis factor (TNF) plays a key role in the pathogenesis of inflammatory bone resorption and associated morbidity in diseases such as rheumatoid arthritis and periodontitis. Mechanisms that regulate the direct osteoclastogenic properties of TNF to limit pathological bone resorption in inflammatory settings are mostly unknown. Here, we show that the transcription factor recombinant recognition sequence binding protein at the J(κ) site (RBP-J) strongly suppresses TNF-induced osteoclastogenesis and inflammatory bone resorption, but has minimal effects on physiological bone remodeling. Myeloid-specific deletion of RBP-J converted TNF into a potent osteoclastogenic factor that could function independently of receptor activator of NF-κB (RANK) signaling. In the absence of RBP-J, TNF effectively induced osteoclastogenesis and bone resorption in RANK-deficient mice. Activation of RBP-J selectively in osteoclast precursors suppressed inflammatory osteoclastogenesis and arthritic bone resorption. Mechanistically, RBP-J suppressed induction of the master regulator of osteoclastogenesis (nuclear factor of activated T cells, cytoplasmic 1) by attenuating c-Fos activation and suppressing induction of B lymphocyte-induced maturation protein-1, thereby preventing the down-regulation of transcriptional repressors such as IRF-8 that block osteoclast differentiation. Thus, RBP-J regulates the balance between activating and repressive signals that regulate osteoclastogenesis. These findings identify RBP-J as a key upstream negative regulator of osteoclastogenesis that restrains excessive bone resorption in inflammatory settings.

Stage-specific functions of leukemia/lymphoma-related factor (LRF) in the transcriptional control of osteoclast development

Cell fate determination is tightly regulated by transcriptional activators and repressors. Leukemia/lymphoma-related factor (LRF; encoded by Zbtb7a), known as a POK (POZ/BTB and Krüppel) family transcriptional repressor, is induced during the development of bone-resorbing osteoclasts, but the physiological significance of LRF in bone metabolism and the molecular mechanisms underlying the transcriptional regulation of osteoclastogenesis by LRF have not been elucidated. Here we show that LRF negatively regulates osteoclast differentiation by repressing nuclear factor of activated T cells c1 (NFATc1) induction in the early phase of osteoclast development, while positively regulating osteoclast-specific genes by functioning as a coactivator of NFATc1 in the bone resorption phase. The stage-specific distinct functions of LRF were demonstrated in two lines of conditional knockout mice in which LRF was deleted in the early or late phase of osteoclast development. Thus, this study shows that LRF plays stage-specific distinct roles in osteoclast differentiation, exemplifying the delicate transcriptional regulation at work in lineage commitment.

Lineage-committed osteoclast precursors circulate in blood and settle down into bone

Osteoclasts are derived from the monocyte/macrophage lineage, but little is known about osteoclast precursors in circulation. We previously showed that cell cycle-arrested quiescent osteoclast precursors (QOPs) were detected along bone surfaces as direct osteoclast precursors. Here we show that receptor activator of NF-κB (RANK)-positive cells isolated from bone marrow and peripheral blood possess characteristics of QOPs in mice. RANK-positive cells expressed c-Fms (receptors of macrophage colony-stimulating factor) at various levels, but scarcely expressed other monocyte/granulocyte markers. RANK-positive cells failed to exert phagocytic and proliferating activities, and differentiated into osteoclasts but not into dendritic cells. To identify circulating QOPs, collagen disks containing bone morphogenetic protein-2 (BMP disks) were implanted into mice, which were administered bromodeoxyuridine daily. Most nuclei of osteoclasts detected in BMP-2-induced ectopic bone were bromodeoxyuridine-negative. RANK-positive cells in peripheral blood proliferated more slowly and had a much longer lifespan than F4/80 (a macrophage marker)-positive macrophages. When BMP disks and control disks were implanted in RANK ligand-deficient mice, RANK-positive cells were observed in the BMP disks but not in the controls. F4/80-positive cells were distributed in both disks. Administration of FYT720, a sphingosine 1-phosphate agonist, promoted the egress of RANK-positive cells from hematopoietic tissues into bloodstream. These results suggest that lineage-determined QOPs circulate in the blood and settle in the bone.

Repression of the DNA-binding inhibitor Id3 by Blimp-1 limits the formation of memory CD8+ T cells

Blimp-1 is a transcriptional repressor that promotes the differentiation of CD8⁺ T cells into short-lived KLRG-1⁺ effector cells (SLEC), but how it operates remains poorly defined. Here we show that Blimp-1 binds and represses the Id3 promoter in SLEC. Repression of Id3 by Blimp-1 was dispensable for SLEC development but limited their capacity to persist as memory cells. Enforced expression of Id3 was sufficient to rescue SLEC survival and enhanced recall responses. Id3 function was mediated in part through inhibition of E2a transcriptional activity and induction of genes regulating genome stability. These findings identify a Blimp-1-Id3-E2a axis as a key molecular switch that determines whether effector CD8⁺ T cells are programmed to die or enter the memory pool.

MafB interacts with Gcm2 and regulates parathyroid hormone expression and parathyroid development

Serum calcium and phosphate homeostasis is critically regulated by parathyroid hormone (PTH) secreted by the parathyroid glands. Parathyroid glands develop from the bilateral parathyroid-thymus common primordia. In mice, the expression of transcription factor Glial cell missing 2 (Gcm2) begins in the dorsal/anterior part of the primordium on embryonic day 9.5 (E9.5), specifying the parathyroid domain. The parathyroid primordium then separates from the thymus primordium and migrates to its adult location beside the thyroid gland by E15.5. Genetic ablation of gcm2 results in parathyroid agenesis in mice, indicating that Gcm2 is essential for early parathyroid organogenesis. However, the regulation of parathyroid development at later stages is not well understood. Here we show that transcriptional activator v-maf musculoaponeurotic fibrosarcoma oncogene homologue B (MafB) is developmentally expressed in parathyroid cells after E11.5. MafB expression was lost in the parathyroid primordium of gcm2 null mice. The parathyroid glands of mafB(+/-) mice were mislocalized between the thymus and thyroid. In mafB(-/-) mice, the parathyroid did not separate from the thymus. Furthermore, in mafB(-/-) mice, PTH expression and secretion were impaired; expression levels of renal cyp27b1, one of the target genes of PTH, was decreased; and bone mineralization was reduced. We also demonstrate that although Gcm2 alone does not stimulate the PTH gene promoter, it associates with MafB to synergistically activate PTH expression. Taken together, our results suggest that MafB regulates later steps of parathyroid development, that is, separation from the thymus and migration toward the thyroid. MafB also regulates the expression of PTH in cooperation with Gcm2.

Negative regulation of osteoclastogenesis and bone resorption by cytokines and transcriptional repressors

Bone remodeling in physiological and pathological conditions represents a balance between bone resorption mediated by osteoclasts and bone formation by osteoblasts. Bone resorption is tightly and dynamically regulated by multiple mediators, including cytokines that act directly on osteoclasts and their precursors, or indirectly by modulating osteoblast lineage cells that in turn regulate osteoclast differentiation. The critical role of cytokines in inducing and promoting osteoclast differentiation, function and survival is covered by the accompanying review by Zwerina and colleagues. Recently, it has become clear that negative regulation of osteoclastogenesis and bone resorption by inflammatory factors and cytokines, downstream signaling pathways, and a newly described network of transcriptional repressors plays a key role in bone homeostasis by fine tuning bone remodeling and restraining excessive bone resorption in inflammatory settings. In this review we discuss negative regulators of osteoclastogenesis and mechanisms by which these factors suppress bone resorption.

The role of the BH3-only protein Noxa in bone homeostasis

Bone homeostasis is maintained by a dynamic balance between bone resorption by osteoclasts and bone formation by osteoblasts. Since excessive osteoclast activity is implicated in pathological bone resorption, understanding the mechanism underlying osteoclast differentiation, function and survival is of both scientific and clinical importance. Osteoclasts are monocyte/macrophage lineage cells with a short life span that undergo rapid apoptosis, the rate of which critically determines the level of bone resorption in vivo. However, the molecular basis of rapid osteoclast apoptosis remains obscure. Here we report the role of a BH3-only protein, Noxa (encoded by the Pmaip1 gene), in bone homeostasis using Noxa-deficient mice. Among the Bcl-2 family members, Noxa was selectively induced during osteoclastogenesis. Mice lacking Noxa exhibit a severe osteoporotic phenotype due to an increased number of osteoclasts. Noxa deficiency did not have any effect on the number of osteoclast precursor cells or the expression of osteoclast-specific genes, but led to a prolonged survival of osteoclasts. Furthermore, adenovirus-mediated Noxa overexpression remarkably reduced bone loss in a model of inflammation-induced bone destruction. This study reveals Noxa to be a crucial regulator of osteoclast apoptosis, and may provide a molecular basis for a new therapeutic approach to bone diseases.

Blimp1: driving terminal differentiation to a T

B lymphocyte maturation-induced protein-1 (Blimp1) is a transcriptional repressor expressed in diverse cell types. In the adaptive immune system, Blimp1 is expressed in lymphocytes that have undergone effector differentiation. Blimp1 is a master regulator of plasma cell differentiation and plays important roles in controlling T cell homeostasis and effector differentiation. Blimp1 can be induced by a variety of cytokines including IL-2, IL-4, IL-12, and IL-21 in addition to TCR and co-stimulatory signals. Blimp1-deficient mice develop spontaneous inflammatory disease mediated by infiltration of activated T cells into tissues. During immune responses Blimp1 is required for the differentiation of plasma cells as well as short-lived CD8(+) cytotoxic T cells. Mounting evidence suggests that Blimp1 plays a common role in the terminal differentiation of multiple cell subsets.

Maf promotes osteoblast differentiation in mice by mediating the age-related switch in mesenchymal cell differentiation

Aging leads to the disruption of the homeostatic balance of multiple biological systems. In bone marrow multipotent mesenchymal cells undergo differentiation into various anchorage-dependent cell types, including osteoblasts and adipocytes. With age as well as with treatment of antidiabetic drugs such as thiazolidinediones, mesenchymal cells favor differentiation into adipocytes, resulting in an increased number of adipocytes and a decreased number of osteoblasts, causing osteoporosis. The mechanism behind this differentiation switch is unknown. Here we show an age-related decrease in the expression of Maf in mouse mesenchymal cells, which regulated mesenchymal cell bifurcation into osteoblasts and adipocytes by cooperating with the osteogenic transcription factor Runx2 and inhibiting the expression of the adipogenic transcription factor Pparg. The crucial role of Maf in both osteogenesis and adipogenesis was underscored by in vivo observations of delayed bone formation in perinatal Maf(-/-) mice and an accelerated formation of fatty marrow associated with bone loss in aged Maf(+/-) mice. This study identifies a transcriptional mechanism for an age-related switch in cell fate determination and may provide a molecular basis for novel therapeutic strategies against age-related bone diseases.