Arming the osteoclast

Serine/threonine phosphatases in osteoclastogenesis and bone resorption

Maintenance of optimal bone mass is controlled through the concerted functions of several cell types, including bone resorbing osteoclasts. Osteoclasts function to remove calcified tissue during developmental bone modeling, and degrade bone at sites of damage during bone remodeling. Changes to bone homeostasis can arise with alterations in osteoclastogenesis and/or catabolic activity that are not offset by anabolic activity; thus, factors that regulate osteoclastogenesis and bone resorption are of interest to further our understanding of basic bone biology, and as potential targets for therapeutic intervention. Several key cytokines, including RANKL and M-CSF, as well as co-stimulatory factors elicit kinase signaling cascades that promote osteoclastogenesis. These kinase cascades are offset by the action of protein phosphatases, including members of the serine/threonine phosphatase family. Here we review the functions of serine/threonine phosphatases and their control of osteoclast differentiation and function, while highlighting deficiencies in our understanding of this understudied class of proteins within the field.

Immune Function and Diversity of Osteoclasts in Normal and Pathological Conditions

Osteoclasts (OCLs) are key players in controlling bone remodeling. Modifications in their differentiation or bone resorbing activity are associated with a number of pathologies ranging from osteopetrosis to osteoporosis, chronic inflammation and cancer, that are all characterized by immunological alterations. Therefore, the 2000s were marked by the emergence of osteoimmunology and by a growing number of studies focused on the control of OCL differentiation and function by the immune system. At the same time, it was discovered that OCLs are much more than bone resorbing cells. As monocytic lineage-derived cells, they belong to a family of cells that displays a wide heterogeneity and plasticity and that is involved in phagocytosis and innate immune responses. However, while OCLs have been extensively studied for their bone resorption capacity, their implication as immune cells was neglected for a long time. In recent years, new evidence pointed out that OCLs play important roles in the modulation of immune responses toward immune suppression or inflammation. They unlocked their capacity to modulate T cell activation, to efficiently process and present antigens as well as their ability to activate T cell responses in an antigen-dependent manner. Moreover, similar to other monocytic lineage cells such as macrophages, monocytes and dendritic cells, OCLs display a phenotypic and functional plasticity participating to their anti-inflammatory or pro-inflammatory effect depending on their cell origin and environment. This review will address this novel vision of the OCL, not only as a phagocyte specialized in bone resorption, but also as innate immune cell participating in the control of immune responses.

DNMT and HDAC inhibitors modulate MMP-9-dependent H3 N-terminal tail proteolysis and osteoclastogenesis

BACKGROUND

MMP-9-dependent proteolysis of histone H3 N-terminal tail (H3NT) is an important mechanism for activation of gene expression during osteoclast differentiation. Like other enzymes targeting their substrates within chromatin structure, MMP-9 enzymatic activity toward H3NT is tightly controlled by histone modifications such as H3K18 acetylation (H3K18ac) and H3K27 monomethylation (H3K27me1). Growing evidence indicates that DNA methylation is another epigenetic mechanism controlling osteoclastogenesis, but whether DNA methylation is also critical for regulating MMP-9-dependent H3NT proteolysis and gene expression remains unknown.

RESULTS

We show here that treating RANKL-induced osteoclast progenitor (OCP) cells with the DNMT inhibitor 5-Aza-2'-deoxycytidine (5-Aza-CdR) induces CpG island hypomethylation and facilitates MMP-9 transcription. This increase in MMP-9 expression results in a significant enhancement of H3NT proteolysis and OCP cell differentiation. On the other hand, despite an increase in levels of H3K18ac, treatment with the HDAC inhibitor trichostatin A (TSA) leads to impairment of osteoclastogenic gene expression. Mechanistically, TSA treatment of OCP-induced cells stimulates H3K27ac with accompanying reduction in H3K27me1, which is a key modification to facilitate stable interaction of MMP-9 with nucleosomes for H3NT proteolysis. Moreover, hypomethylated osteoclastogenic genes in 5-Aza-CdR-treated cells remain transcriptionally inactive after TSA treatment, because H3K27 is highly acetylated and cannot be modified by G9a.

CONCLUSIONS

These findings clearly indicate that DNA methylation and histone modification are important mechanisms in regulating osteoclastogenic gene expression and that their inhibitors can be used as potential therapeutic tools for treating bone disorders.

MacroH2A1.2 inhibits prostate cancer-induced osteoclastogenesis through cooperation with HP1α and H1.2

Osteoclasts are multinuclear bone-resorbing cells that differentiate from hematopoietic precursor cells. Prostate cancer cells frequently spread to bone and secrete soluble signaling factors to accelerate osteoclast differentiation and bone resorption. However, processes and mechanisms that govern the expression of osteoclastogenic soluble factors secreted by prostate cancer cells are largely unknown. MacroH2A (mH2A) is a histone variant that replaces canonical H2A at designated genomic loci and establishes functionally distinct chromatin regions. Here we report that mH2A1.2, one of the mH2A isoforms, attenuates prostate cancer-induced osteoclastogenesis by maintaining the inactive state of genes encoding soluble factors in prostate cancer cells. Our functional analyses of soluble factors identify lymphotoxin beta (LTβ) as a major stimulator of osteoclastogenesis and an essential mH2A1.2 target for its anti-osteoclastogenic activity. Mechanistically, mH2A1.2 directly interacts with HP1α and H1.2 and requires them to inactivate LTβ gene in prostate cancer cells. Consistently, HP1α and H1.2 have an intrinsic ability to inhibit osteoclast differentiation in a mH2A1.2-dependent manner. Together, our data uncover a new and specific role for mH2A1.2 in modulating osteoclastogenic potential of prostate cancer cells and demonstrate how this signaling pathway can be exploited to treat osteolytic bone metastases at the molecular level.

H3K27me1 is essential for MMP-9-dependent H3N-terminal tail proteolysis during osteoclastogenesis

Background

MMP-9 plays a direct role in the activation of pro-osteoclastogenic genes by cleaving histone H3N-terminal tail (H3NT) and altering chromatin architecture. Although H3 acetylation at K18 has been shown to stimulate MMP-9 enzymatic activity toward H3NT, nothing is known about the influence of other H3NT modifications on this epigenetic reaction.

Results

We show that H3 monomethylation at lysine 27 (H3K27me1) is essential for MMP-9-dependent H3NT proteolysis during RANKL-induced osteoclast differentiation. Through the recognition of H3K27me1 mark, MMP-9 localizes and generates H3NT proteolysis at the genes encoding osteoclast differentiation factors. By using RNAi and small molecule inhibitor approaches, we also confirmed that G9a is the major methyltransferase to catalyze H3K27me1 for MMP-9-dependent H3NT proteolysis and trigger the expression of osteoclast-specific genes.

Conclusions

Our data establish new functions for G9a-mediated H3K27me1 in MMP-9-dependent H3NT proteolysis and demonstrate how histone modification can be exploited to regulate osteoclastogenic gene expression at the molecular level. Further studies are warranted to investigate the detailed mechanism by which G9a overexpression with concomitant dysregulation of osteoclastogenesis contributes to the pathogenesis of bone disorders.

Electronic supplementary material

The online version of this article (10.1186/s13072-018-0193-1) contains supplementary material, which is available to authorized users.

Alzheimer's disease and osteoporosis

Alzheimer's disease (AD) and osteoporosis are both common degenerative diseases in the elderly population. The incidence of both diseases increases with age and will be posing enormous societal burden worldwide. It may appear that AD and osteoporosis are two distinct diseases although many risk factors are shared. Previous observational studies have shown that patients with osteoporosis have higher risks of developing AD than those who do not have osteoporosis. Although osteoporosis, falls, and fractures are more often seen in patients with AD than other older adults, the association between these two diseases may be due to a pathophysiological link rather than one condition causing the other. Several in vitro and in vivo studies lend support to this notion. Patients with AD have excessive amyloid plaques in the brain, and the pathology may extend to peripheral organs and cause skeletal amyloid deposition, which would enhance receptor activator nuclear factor-kappa B ligand signaling and lead to greater osteoclast activities. Patients with osteoporosis may have Vitamin D deficiency or lower levels of Vitamin D binding protein, which protects against amyloid aggregation, thus linking Vitamin D deficiency and AD or osteoporosis and AD. Osteoporosis coexisting with AD provides a window to examine the amyloid hypothesis from peripheral tissues. Future studies are warranted to clarify the role of genetic background regarding Vitamin D levels, exposure to sunlight, estrogen replacement therapy, and physical activity in patients with both chronic diseases.

Signaling pathway STAT1 is strongly activated by IFN-β in the pathogenesis of osteoporosis

Background

Despite extensive research, the underlying pathological mechanisms of osteoporosis are not completely understood. Recent studies have indicated a distinct role for the IFN-β/STAT1 pathway in bone metabolism. An inhibitory effect of IFN-β on osteoclastogenesis has been detected and STAT1/2 has been shown to influence osteoblastic bone metabolism. So far, no data concerning the IFN-β/STAT1 pathways in osteoblasts and osteoclasts from osteoporotic and non-osteoporotic patients are available. The aim of the study was to analyze these pathways in both cell types.

Methods

Osteoblasts were isolated from the femoral heads of 12 osteoporotic and 11 non-osteoporotic patients and monocytes were differentiated into osteoclasts. After the differentiation period, cells were stimulated once with 20 and 100 ng/mL IFN-β for 4 days. Viability, activity, bone metabolism-related genes, and the proteins Fra1, SOCS1, STAT1, p-STAT1, and TRAF6 were analyzed.

Results

Viability, activity, and gene expressions were not affected by stimulating the osteoblasts. However, in osteoporotic osteoclasts, which display a significantly higher basal osteoclastic activity, the stimulation with IFN-β lead to significant inhibition. Further, an increased STAT1 activation was detected in both cell types with no significant differences between the groups. Regarding the phosphorylation of STAT1, no significant influence was detected in osteoblasts but the IFN-β stimulation led to a significant increase of p-STAT1 in osteoclasts of both groups.

Conclusions

IFN-β is a principal mediator in the pathogenesis of osteoporosis by inhibiting osteoclasts and inducing and activating STAT1. Our results also confirm this in cells from osteoporotic and non-osteoporotic patients. Strong inhibitory effects on the osteoclastogenesis of osteoporotic osteoclasts were detectable. Nevertheless, osteoblast activity was not negatively affected by IFN-β stimulation. These results may contribute to a better understanding of the underlying pathological signaling pathways of osteoporosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s40001-014-0074-4) contains supplementary material, which is available to authorized users.

Defective entry into mitosis 1 (Dim1) negatively regulates osteoclastogenesis by inhibiting the expression of nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1)

Bone remodeling is a continuous process of osteoblastic bone formation and osteoclastic bone resorption to maintain normal bone mass. NFATc1 is the master regulator of osteoclastogensis and transcriptionally activated by c-Fos and NF-κB in response to receptor activator of NF-κB ligand (RANKL) treatment. Defective entry into mitosis 1 (Dim1) is a nuclear protein that is implicated in pre-mRNA splicing and cell cycle progression, but the possible role of Dim1 in regulating other cellular processes remains unknown. Here, we demonstrate that Dim1 attenuates RANKL-induced osteoclastogenesis by targeting NFATc1 signaling pathway. Expression levels of Dim1 and NFATc1 are significantly increased during the formation of multinucleated osteoclasts. RNAi-mediated knockdown of Dim1 markedly enhances the expression of NFATc1 and its target genes, leading to the increase of RANKL-induced osteoclastogenesis in bone marrow-derived macrophages. Conversely, ectopic expression of Dim1 decreases RANKL-induced osteoclast differentiation by silencing NFATc1 and its target genes, further linking Dim1 to the dynamic regulation of osteoclastogenesis. Consistent with this notion, ChIP and interaction studies show that Dim1 directly associates with c-Fos and prevents c-Fos from binding to the NFATc1 promoter, resulting in targeted inactivation of the NFATc1 gene. Therefore, our studies reveal an unrecognized role for Dim1 as a master modulator of osteoclast differentiation, as well as the molecular mechanism underlying its repressive action toward osteoclastogensis.

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.

Cytoskeletal dysfunction dominates in DAP12-deficient osteoclasts

Despite evidence that DAP12 regulates osteoclasts, mice lacking the ITAM-bearing protein exhibit only mild osteopetrosis. Alternatively, Dap12(-/-) mice, also lacking FcRgamma, are severely osteopetrotic, suggesting that FcRgamma compensates for DAP12 deficiency in the bone-resorbing polykaryons. Controversy exists, however, as to whether these co-stimulatory molecules regulate differentiation of osteoclasts or the capacity of the mature cell to degrade bone. We find that Dap12(-/-) osteoclasts differentiate normally when generated on osteoblasts but have a dysfunctional cytoskeleton, impairing their ability to transmigrate through the osteoblast layer and resorb bone. To determine whether the FcRgamma co-receptor, OSCAR mediates osteoclast function in the absence of DAP12, we overexpressed OSCAR fused to FLAG (OSCAR-FLAG), in Dap12(-/-) osteoclasts. OSCAR-FLAG partially rescues the abnormal cytoskeleton of Dap12(-/-) osteoclasts grown on bone, but not those grown on osteoblasts. Thus, cytoskeletal dysfunction, and not arrested differentiation, is the dominant consequence of DAP12 deficiency in osteoclasts. The failure of osteoblasts to normalize Dap12(-/-) osteoclasts indicates that functionally relevant quantities of OSCAR ligand do not reside in bone-forming cells.

Transcriptionally active nuclei are selective in mature multinucleated osteoclasts

Multinucleation is indispensable for the bone-resorbing activity of mature osteoclasts. Although multinucleation is evident in mature osteoclasts and certain other cell types, putative regulatory networks among nuclei remain poorly characterized. To address this issue, transcriptional activity of each nucleus in a multinucleated osteoclast was assessed by detecting the distributions of nuclear proteins by immunocytochemistry and primary transcripts by RNA FISH. Patterns of epigenetic histone markers governing transcription as well as localization of tested nuclear receptor proteins appeared indistinguishable among nuclei in differentiated Raw264 cells and mouse mature osteoclasts. However, RNAPII-Ser5P/2P and NFATc1 proteins were selectively distributed in certain nuclei in the same cell. Similarly, the distributions of primary transcripts for osteoclast-specific genes (Nfatc1, Ctsk and Acp5) as well as a housekeeping gene (beta-tubulin) were limited in certain nuclei within individual cells. By fusing two Raw264 cell lines that stably expressed ZsGreen-NLS and DsRed-NLS proteins, transmission of nuclear proteins across all of the nuclei in a cell could be observed, presumably through the shared cytoplasm. Taken together, we conclude that although nuclear proteins are diffusible among nuclei, only certain nuclei within a multinucleated osteoclast are transcriptionally active.

Gene expression profiling in monocytes and SNP association suggest the importance of the STAT1 gene for osteoporosis in both Chinese and Caucasians

Osteoporosis is characterized mainly by low bone mineral density (BMD). Many cytokines and chemokines have been related with bone metabolism. Monocytes in the immune system are important sources of cytokines and chemokines for bone metabolism. However, no study has investigated in vivo expression of a large number of various factors simultaneously in human monocytes underlying osteoporosis. This study explored the in vivo expression pattern of general cytokines, chemokines, and their receptor genes in human monocytes and validated the significant genes by qRT-PCR and genetic association analyses. Expression profilings were performed in monocyte samples from 26 Chinese and 20 Caucasian premenopausal women with discordant BMD. Genome-wide association analysis with BMD variation was conducted in 1000 unrelated Caucasians. We selected 168 cytokines, chemokines, osteoclast-related factors, and their receptor genes for analyses. Significantly, the signal transducer and activator of transcription 1 (STAT1) gene was upregulated in the low versus the high BMD groups in both Chinese and Caucasians. We also revealed a significant association of the STAT1 gene with BMD variation in the 1000 Caucasians. Thus we conclude that the STAT1 gene is important in human circulating monocytes in the etiology of osteoporosis.

Estrogen action on bone marrow osteoclast lineage cells of postmenopausal women in vivo

In bone marrow aspirates from postmenopausal women, systemic estrogen treatment decreased differentiation of mononuclear progenitor cells toward a more mature osteoclast phenotype. This was not associated with changes in surface receptor for proresorptive cytokines.

INTRODUCTION

Although mechanisms by which estrogen (E) decreases bone resorption have been extensively studied in rodents, little information is available in humans.

METHODS

In bone marrow aspirates from 34 early postmenopausal women randomly assigned to receive 4 weeks of treatment (100 microg/day of transdermal 17beta-estradiol) or no treatment, we assessed osteoclast differentiation and surface receptors using flow cytometry with fluorescent-labeled specific antibodies.

RESULTS

E treatment decreased (P < 0.05) the proportion of bone marrow mononuclear cells (BMMNCs) expressing the calcitonin receptor (CTR), a late osteoclast phenotype marker. There was an increase in c-Fms concentration in osteoclast lineage cells (P < 0.05) and in the proportion of BMMNCs expressing TNFR2 (P < 0.05), but there were no significant effects on other surface receptors for proresorptive factors (RANK, TNFR1, TREM2, or OSCAR). Changes in serum CTx and TRAP 5b, markers for bone resorption, correlated directly (P < 0.05) with the proportion of BMMNCs expressing CTR and, for TRAP 5b only, TNFR2 and inversely with c-Fms concentration (all P < 0.05).

CONCLUSION

E reduces bone resorption, in part, by decreasing differentiation of BMMNCs into mature osteoclasts. This action cannot be explained by decreased concentrations of surface receptors for proresorptive factors. The roles of increases in c-Fms concentration and the proportion of TNFR2((+)) cells are unclear.

Glucocorticoid excess in mice results in early activation of osteoclastogenesis and adipogenesis and prolonged suppression of osteogenesis: a longitudinal study of gene expression in bone tissue from glucocorticoid-treated mice

OBJECTIVE

Glucocorticoid (GC) excess induces alterations in bone metabolism that weaken bone structure and increase fracture risk. The aim of this study was to identify genes associated with bone metabolism in GC-treated mice, by performing a microarray analysis.

METHODS

Long bones from mice exposed to GC excess were collected after 0, 7, 28, and 56 days of treatment, to measure bone microarchitecture and extract RNA for microarray analyses.

RESULTS

Bone loss in this animal model was confirmed by changes in bone turnover markers as well as bone architecture, as measured by microfocal computed tomography. GC excess induced an early up-regulation of genes involved in osteoclast activation, function, and adipogenesis, which peaked on day 7. The expression of genes associated with osteoclast cytoskeletal reorganization and genes associated with matrix degradation peaked on day 28. On day 28 and day 56, the expression of genes associated with osteoblast activation and maturation was decreased from baseline, while the expression of Wnt antagonists was increased. In addition, the expression of genes expressed in osteocytes associated with bone mineralization was significantly higher at the later time points, day 28 and day 56. Reverse transcription-polymerase chain reaction confirmed the results of microarray analysis in selected genes.

CONCLUSION

GC excess is associated with early activation of genes associated with osteoclastogenesis and adipogenesis and a later suppression of genes associated with osteogenesis and mineralization. Novel interventions with agents that modulate either Wnt signaling or mineralization may be effective in GC-induced osteoporosis.

In vivo genome-wide expression study on human circulating B cells suggests a novel ESR1 and MAPK3 network for postmenopausal osteoporosis

INTRODUCTION

Osteoporosis is characterized by low BMD. Studies have shown that B cells may participate in osteoclastogenesis through expression of osteoclast-related factors, such as RANKL, transforming growth factor beta (TGFB), and osteoprotegerin (OPG). However, the in vivo significance of B cells in human bone metabolism and osteoporosis is still largely unknown, particularly at the systematic gene expression level.

MATERIALS AND METHODS

In this study, Affymetrix HG-U133A GeneChip arrays were used to identify genes differentially expressed in B cells between 10 low and 10 high BMD postmenopausal women. Significance of differential expression was tested by t-test and adjusted for multiple testing with the Benjamini and Hochberg (BH) procedure (adjusted p </= 0.05).

RESULTS

Twenty-nine genes were downregulated in the low versus high BMD group. These genes were further analyzed using Ingenuity Pathways Analysis (Ingenuity Systems). A network involving estrogen receptor 1 (ESR1) and mitogen activated protein kinase 3 (MAPK3) was identified. Real-time RT-PCR confirmed differential expression of eight genes, including ESR1, MAPK3, methyl CpG binding protein 2 (MECP2), proline-serine-threonine phosphatase interacting protein 1 (PSTPIP1), Scr-like-adaptor (SLA), serine/threonine kinase 11 (STK11), WNK lysine-deficient protein kinase 1 (WNK1), and zinc finger protein 446 (ZNF446).

CONCLUSIONS

This is the first in vivo genome-wide expression study on human B cells in relation to osteoporosis. Our results highlight the significance of B cells in the etiology of osteoporosis and suggest a novel mechanism for postmenopausal osteoporosis (i.e., that downregulation of ESR1 and MAPK3 in B cells regulates secretion of factors, leading to increased osteoclastogenesis or decreased osteoblastogenesis).

Osteoclast precursor interaction with bone matrix induces osteoclast formation directly by an interleukin-1-mediated autocrine mechanism

Interleukin-1 (IL-1) and tumor necrosis factor (TNF) mediate bone resorption in a variety of diseases affecting bone. Like TNF, IL-1 is secreted by osteoclast precursors (OCPs), but unlike TNF, it does not induce osteoclast formation directly from OCPs in vitro. TNF induces IL-1 expression and activates c-Fos, a transcription factor required in OCPs for osteoclast formation. Here, we examined whether IL-1 can induce osteoclast formation directly from OCPs overexpressing c-Fos and whether interaction with bone matrix affects OCP cytokine expression. We infected OCPs with c-Fos or green fluorescent protein retrovirus, cultured them with macrophage colony-stimulating factor and IL-1 on bone slices or plastic dishes, and assessed osteoclast and resorption pit formation and expression of IL-1 by OCPs. We used a Transwell assay to determine whether OCPs secrete IL-1 when they interact with bone matrix. IL-1 induced osteoclast formation directly from c-Fos-expressing OCPs on plastic. c-Fos-expressing OCPs formed osteoclasts spontaneously on bone slices without addition of cytokines. OCPs on bone secreted IL-1, which induced osteoclast formation from c-Fos-expressing OCPs in the lower Transwell dishes. The bone matrix proteins dentin sialoprotein and osteopontin, but not transforming growth factor-beta, stimulated OCP expression of IL-1 and induced c-Fos-expressing OCP differentiation into osteoclasts. Osteoclasts eroding inflamed joints have higher c-Fos expression compared with osteoclasts inside bone. We conclude that OCPs expressing c-Fos may induce their differentiation directly into osteoclasts by an autocrine mechanism in which they produce IL-1 through interaction with bone matrix. TNF could induce c-Fos expression in OCPs at sites of inflammation in bone to promote this autocrine mechanism and thus amplify bone loss.

Osteoclast receptors and signaling

Osteoclasts are bone-resorbing cells derived from hematopoietic precursors of the monocyte-macrophage lineage. Besides the well known Receptor Activator of Nuclear factor-kappaB (RANK), RANK ligand and osteoprotegerin axis, a variety of factors tightly regulate osteoclast formation, adhesion, polarization, motility, resorbing activity and life span, maintaining bone resorption within physiological ranges. Receptor-mediated osteoclast regulation is rather complex. Nuclear receptors, cell surface receptors, integrin receptors and cell death receptors work together to control osteoclast activity and prevent both reduced or increased bone resorption. Here we will discuss the signal transduction pathways activated by the main osteoclast receptors, integrating their function and mechanisms of action.

Lymphocytes and the Dap12 adaptor are key regulators of osteoclast activation associated with gonadal failure

Bone resorption by osteoclasts is necessary to maintain bone homeostasis. Osteoclast differentiation from hematopoietic progenitors and their activation depend on M-CSF and RANKL, but also requires co-stimulatory signals acting through receptors associated with DAP12 and FcRgamma adaptors. Dap12 mutant mice (KDelta75) are osteopetrotic due to inactive osteoclasts but, surprisingly, these mice are more sensitive than WT mice to bone loss following an ovariectomy. Because estrogen withdrawal is known to disturb bone mass, at least in part, through lymphocyte interaction, we looked at the role of mature lymphocytes on osteoclastogenesis and bone mass in the absence of functional DAP12. Lymphocytes were found to stimulate an early osteoclast differentiation response from Dap12-deficient progenitors in vitro. In vivo, Rag1-/- mice lacking mature lymphocytes did not exhibit any bone phenotype, but lost their bone mass after ovariectomy like KDelta75 mice. KDelta75;Rag1-/- double mutant female mice exhibited a more severe osteopetrosis than Dap12-deficient animals but lost their bone mass after ovariectomy, like single mutants. These results suggest that both DAP12 and mature lymphocytes act synergistically to maintain bone mass under physiological conditions, while playing similar but not synergistic co-stimulatory roles in protecting bone loss after gonadal failure. Thus, our data support a role for lymphocytes during osteoclast differentiation and suggest that they may function as accessory cells when regular osteoclast function is compromised.

Osteoimmunological insight into bone damage in rheumatoid arthritis

Research into the bone destruction associated with rheumatoid arthritis has highlighted the importance of the interplay of the immune and skeletal systems. Arthritic bone destruction is attributable to the defective control of osteoclastogenesis by T cells. We revealed that excessive expression of receptor activator of NF-kappaB ligand (RANKL) and a paucity of interferon-gamma underlie the enhanced osteoclastogenesis in arthritis. The interdisciplinary research field called osteoimmunology has attracted further attention after identification of a number of unexpected bone phenotypes in mice lacking immunomodulatory molecules. Accumulating evidence suggests that the immune and skeletal systems share not only cytokines but also various signaling molecules, transcription factors, and membrane receptors. Thus, bone turns out to be a dynamic tissue that is constantly renewed, where the immune system participates to a hitherto unexpected extent. This emerging field will be of great importance for a better understanding and treatment of rheumatic diseases.

Identification of novel RANK polymorphisms and their putative association with low BMD among postmenopausal women

INTRODUCTION

Bone mineral density (BMD) is the major factor for determining bone strength, which is closely correlated to osteoporotic fracture risk and is largely determined by multiple genetic factors. The RANK (TNFRSF11A), receptor for RANKL, is a member of the tumor necrosis factor receptor (TNFR) superfamily and plays a central role in osteoclast development.

METHODS

In order to investigate the effects of RANK polymorphism on BMD and osteoporosis, we directly sequenced the RANK gene in 24 Korean individuals and identified 25 sequence variants. Eleven of these polymorphisms were selected and genotyped in a larger-scale study of postmenopausal women (n = 560). Areal BMD (g/cm(2)) of the anterior-posterior lumbar spine and the nondominant proximal femur were measured using dual-energy X-ray absorptiometry.

RESULTS

We found that two intronic polymorphisms in the RANK gene [RANK + 34863G > A (rs12458117) and RANK + 35928insdelC (new polymorphism found in this study) in intron 6] were significantly associated with the BMD of the lumbar spine, i.e., rare alleles were significantly associated with low BMD of the lumbar spine among Korean postmenopausal women (p = 0.04 and 0.02, respectively). These polymorphisms were also associated with low BMD of proximal femur sites, including Ward's triangle, trochanter, and total femur. Our results suggest that +34863G > A and +35928insdelC polymorphisms in RANK are possible genetic factors for low BMD in postmenopausal women.

The molecular understanding of osteoclast differentiation

Osteoclasts are multinucleated cells of monocyte/macrophage origin that degrade bone matrix. The differentiation of osteoclasts is dependent on a tumor necrosis factor (TNF) family cytokine, receptor activator of nuclear factor (NF)-kappaB ligand (RANKL), as well as macrophage colony-stimulating factor (M-CSF). Congenital lack of osteoclasts causes osteopetrosis, investigation of which has provided insights into the essential molecules for osteoclastogenesis, including TNF receptor-associated factor (TRAF) 6, NF-kappaB and c-Fos. In addition, genome-wide screening techniques have shed light on an additional set of gene products such as nuclear factor of activated T cells (NFAT) c1. Here we summarize the efforts to understand the sequential molecular events induced by RANKL during osteoclast differentiation. RANKL binds to its receptor RANK, which recruits adaptor molecules such as TRAF6. TRAF6 activates NF-kappaB, which is important for the initial induction of NFATc1. NFATc1 is activated by calcium signaling and binds to its own promoter, thus switching on an autoregulatory loop. An activator protein (AP)-1 complex containing c-Fos is required for the autoamplification of NFATc1, enabling the robust induction of NFATc1. Finally, NFATc1 cooperates with other transcriptional partners to activate osteoclast-specific genes. NFATc1 autoregulation is controlled by an epigenetic mechanism, which has profound implications for an understanding of the general mechanism of irreversible cell fate determination. From the clinical point of view, RANKL signaling pathway has promise as a strategy for suppressing the excessive osteoclast formation characteristic of a variety of bone diseases.

Osteoprotegerin as a potential therapy for osteoporosis

The discovery and characterization of the RANKL/RANK/OPG signaling pathway and the identification of its role in the pathogenesis of bone loss have provided the rationale for the development of drugs with the ability to modulate RANK-induced osteoclastogenesis. In vivo studies have identified interfering with the RANKL/RANK interaction as a potential therapeutic target in the management of osteoporosis. Two agents capable of blocking the binding of RANKL to RANK have been so far tested in clinical studies--osteoprotegerin (Fc-OPG fusion molecule) and the RANKL-antibody (AMG 162). Both have been found to have profound inhibitory effects on bone resorption, with AMG 162 appearing to be overall superior to OPG. Data are still very scarce, however, and much remains to be uncovered before novel strategies capable of modulating the RANKL/OPG signaling pathway could be safely and effectively used in the management of osteoporosis.

The role of the immune system in the pathophysiology of osteoporosis

The role of the immune system in the development of senile osteoporosis, which arises primarily through the effects of estrogen deficiency and secondary hyperparathyroidism, is slowly being unraveled. This review focuses on our current understanding of how the components of this complex-interlinked system are regulated and how these fit with previous models of senile and postmenopausal osteoporosis. There is certainly substantial evidence that bone remodeling is a tightly regulated, finely balanced process influenced by subtle changes in proinflammatory and inhibitory cytokines as well as hormones and cellular components that act primarily but not exclusively through the receptor activator of nuclear factor-kappaB (RANK)/RANK ligand/osteoprotegerin system. In addition, an acute or chronic imbalance in the system due to infection or inflammation could contribute to systemic (or local) bone loss and increase the risk of fracture. Although significant progress has been made, there remains much to be done in unraveling this complex interaction between the immune system and bone.

EBF2 Regulates Osteoblast-Dependent Differentiation of Osteoclasts

Communication between bone-depositing osteoblasts and bone-resorbing osteoclasts is required for bone development and homeostasis. Here, we identify EBF2, a member of the early B cell factor (EBF) family of transcription factors that is expressed in osteoblast progenitors, as a regulator of osteoclast differentiation. We find that mice homozygous for a targeted inactivation of Ebf2 show reduced bone mass and an increase in the number of osteoclasts. These defects are accompanied by a marked downregulation of the osteoprotegerin (Opg) gene, encoding a RANK decoy receptor. EBF2 binds to sequences in the Opg promoter and transactivates the Opg promoter in synergy with the Wnt-responsive LEF1/TCF:beta-catenin pathway. Taken together, these data identify EBF2 as a regulator of RANK-RANKL signaling and osteoblast-dependent differentiation of osteoclasts.

Interplay between interferon and other cytokine systems in bone metabolism

Interferons (IFNs) play crucial roles in the regulation of a wide variety of innate and adaptive immune responses. Type I interferons (IFN-alpha/beta) are central to the host defense against pathogens such as viruses, whereas type II interferon (IFN-gamma) mainly contributes to the T-cell-mediated regulation of the immune responses. Studies of bone destruction associated with rheumatoid arthritis have highlighted the importance of the interaction between the immune and skeletal systems. Recently, a new research area, termed osteoimmunology, has been spawned by a series of studies focusing on the signaling networks between IFN and other cytokines in bone metabolisms. It has been revealed that IFN-gamma interferes with the osteoclast differentiation induced by receptor activator of nuclear factor-kappaB ligand (RANKL), and this mechanism is critical for the suppression of pathological bone resorption associated with inflammation. In addition, RANKL induces the IFN-beta gene in osteoclast precursor cells, and this induction constitutes a critical aspect of the negative feedback regulation mechanisms of RANKL signaling to suppress excessive osteoclastogenesis. Furthermore, a novel function of signal transducer and activator of transcription 1 (Stat1), the essential transcription factor for both type I and type II IFN responses, was revealed in the regulation of osteoblast differentiation. Collectively, these studies unveil novel aspects of the IFN system and indicate the operation of the intricate signaling network among IFN and other cytokine systems in bone remodeling, which might offer a molecular basis for the treatment of bone diseases.

Osteoprotegerin as a potential therapy for osteoporosis

The discovery and characterization of the RANKL/RANK/OPG signaling pathway and the identification of its role in the pathogenesis of bone loss have provided the rationale for the development of drugs with the ability to modulate RANK-induced osteoclastogenesis. In vivo studies have identified interfering with the RANKL/RANK interaction as a potential therapeutic target in the management of osteoporosis. Two agents capable of blocking the binding of RANKL to RANK have been so far tested in clinical studies--osteoprotegerin (Fc-OPG fusion molecule) and the RANKL-antibody (AMG 162). Both have been found to have profound inhibitory effects on bone resorption, with AMG 162 appearing to be overall superior to OPG. Data are still very scarce, however, and much remains to be uncovered before novel strategies capable of modulating the RANKL/OPG signaling pathway could be safely and effectively used in the management of osteoporosis.

Double-stranded RNA-dependent protein kinase is required for bone calcification in MC3T3-E1 cells in vitro

In this study, we demonstrated that double-stranded RNA-dependent protein kinase (PKR) is required for the calcification of osteoblasts via the signal transducers and activators of transcription 1alpha (STAT1alpha) signaling in vitro. A dominant-negative mutant PKR cDNA, in which the amino acid lysine at 296 was replaced with arginine and which does not have catalytic activity, was transfected into mouse osteoblastic MC3T3-E1 cells; thereby, we established cells that stably expressed the PKR mutant gene (PKR-K/R). Phosphorylation of PKR was not stimulated by polyinosic-polycytidylic acid in the mutant cells. The PKR-K/R mutant cells exhibited up-regulated cell growth and had low alkaline phosphatase (ALP) activity. The PKR-K/R mutant cells were not able to form bone nodules in vitro. In the PKR-K/R mutant cells, runt-related gene 2 (Runx2)-mediated transcription decreased compared with the levels in the control cells. The expression of STAT1alpha protein increased and the protein was translocated to the nucleus in the PKR-K/R mutant cells. When the expression of STAT1alpha protein in PKR mutant cells was suppressed using RNAi, the activity of Runx2-mediated transcription recovered to the control level. Our results indicate that PKR is a stimulator of Runx2 transcription and is a negative modulator of STAT1alpha expression. Our findings also suggest that PKR plays important roles in the differentiation and calcification of osteoblasts by modulating STAT1alpha and/or Runx2 expression.

Inflammatory bone destruction and osteoimmunology

OBJECTIVES

The metabolism of hard tissue is influenced by the immune system. Research into the bone destruction associated with inflammatory diseases such as periodontal disease and rheumatoid arthritis has highlighted the importance of the interplay of the immune and skeletal systems. This interdisciplinary research field, called 'osteoimmunology', has become increasingly important for each system by itself as well as the biology linking them. The history and recent progress of this field are reviewed.

MATERIAL AND METHODS

'Osteoimmunology' was coined to describe the pioneering work on the T-cell regulation of osteoclastogenesis by the receptor activator of nuclear factor-kappaB ligand (RANKL) and interferon (IFN)-gamma. Accumulating evidence suggests that the immune and skeletal systems share not only cytokines but also various signaling molecules, transcription factors and membrane receptors. The contribution of T cells to the pathogenesis of inflammatory bone destruction is discussed, and our recent findings are summarized to illustrate how the osteoimmunological network functions.

RESULTS

RANKL is an osteoclastogenic cytokine that links bone and the immune system. Immunomodulatory cytokines such as IFNs also participate in the regulation of RANKL signaling and inflammatory bone loss. The transcription factor nuclear factor of activated T cells c1 (NFATc1) has been identified as a master switch regulator of osteoclastogenesis. In addition, immunoglobulin-like receptors are critically involved in bone homeostasis.

CONCLUSION

Bone turns out to be a dynamic tissue that is constantly renewed, where the immune system participates to a hitherto unexpected extent. This emerging field will be of great importance to a better understanding and treatment of diseases of the skeletal and immune systems, as well as to the fundamental biology underpinning both.

Association of the OSCAR promoter polymorphism with BMD in postmenopausal women

In an effort to identify genetic polymorphisms in potential candidate genes for osteoporosis, 10 variants were identified in the OSCAR gene using direct DNA sequencing, and 560 postmenopausal women were genotyped at five SNP loci, using the TaqMan method. The rare allele (G allele) of OSCAR-2322A>G (SNP in the 5' flanking region) showed significant association with lower BMD at various bone sites in postmenopausal women (n = 560).

INTRODUCTION

BMD is the major factor for determining bone strength and osteoporotic fracture risk and is determined by both environmental and multiple genetic factors. The osteoclast-associated receptor (OSCAR) plays a critical role in osteoclast differentiation and thus is an important candidate gene for the modulation of BMD.

MATERIALS AND METHODS

Through direct sequencing in 24 Korean individuals, 10 sequence variants were identified: 2 in the 5' flanking region, 7 in the exons (including 6 nonsynonymous single-nucleotide polymorphisms [SNPs]), and 1 in an intron. Five of these polymorphisms were selected for larger-scale genotyping in postmenopausal women (n = 560). Areal BMD (g/cm2) of the anterior-posterior lumbar spine and the nondominant proximal femur was measured using DXA (Lunar Expert XL and Hologic QDR 4500-A). Lateral thoracolumbar radiographs were obtained in all subjects.

RESULTS

Using multiple regression analysis and controlling for age, years since menopause, height, weight, and evaluation machine as covariates, the rare allele (G allele) of OSCAR-2322A>G showed significant association with lower BMD at various bone sites in postmenopausal women.

CONCLUSION

These findings suggest that the promoter variant in OSCAR gene (OSCAR-2322A>G) might be one of genetic determinants of BMD in postmenopausal women.

Brain and bone damage in KARAP/DAP12 loss-of-function mice correlate with alterations in microglia and osteoclast lineages

Human polycystic lipomembraneous osteodysplasia with sclerosing leukoencephalopathy, also known as Nasu-Hakola disease, has been described to be associated with mutations affecting the immunoreceptor tyrosine-based activation motif-bearing KARAP/DAP12 immunoreceptor gene. Patients present bone fragilities and severe neurological alterations leading to presenile dementia. Here we investigated whether the absence of KARAP/DAP12-mediated signals in loss-of-function (KDelta75) mice also leads to bone and central nervous system pathological features. Histological analysis of adult KDelta75 mice brains revealed a diffuse hypomyelination predominating in anterior brain regions. As this was not accompanied by oligodendrocyte degeneration or microglial cell activation it suggests a developmental defect of myelin formation. Interestingly, in postnatal KDelta75 mice, we observed a dramatic reduction in microglial cell numbers similar to in vitro microglial cell differentiation impairment. Our results raise the intriguing possibility that defective microglial cell differentiation might be responsible for abnormal myelin development. Histomorphometry revealed that bone remodeling is also altered, because of a resorption defect, associated with a severe block of in vitro osteoclast differentiation. In addition, we show that, among monocytic lineages, KARAP/DAP12 specifically controls microglial and osteoclast differentiation. Our results confirm that KARAP/DAP12-mediated signals play an important role in the regulation of both brain and bone homeostasis. Yet, important differences exist between the symptoms observed in Nasu-Hakola patients and KDelta75 mice.

Mechanistic insight into osteoclast differentiation in osteoimmunology

Recently a close relationship between the immune and skeletal systems or the interdisciplinary field called osteoimmunology has attracted much attention due to the observations that bone destruction is caused by an abnormal activation of the immune system in rheumatoid arthritis, and that mice lacking immunomodulatory molecules often exhibit an unexpected bone phenotype. Osteoclasts are cells of monocyte/macrophage origin that degrade the bone matrix. They are among the key players in the control of bone metabolism in health and disease. Receptor activator of NF-kappaB ligand (RANKL), a tumor necrosis factor (TNF) family cytokine, induces the differentiation of osteoclasts in the presence of macrophage-colony stimulating factor. RANKL activates TRAF6, c-Fos, and calcium signaling pathways, all of which are indispensable for the induction and activation of nuclear factor of activated T cells (NFAT) c1, the master transcription factor for osteoclastogenesis. The autoamplification of NFATc1 gene results in the efficient induction of osteoclast-specific genes. An AP-1 transcription factor complex containing c-Fos plays a crucial role in these processes, although results in conditional knockout mice show that Jun family members have a redundant role. The immunoreceptor tyrosine-based activation motif (ITAM) is an important signaling component for a number of receptors in the immune system including T-cell, B-cell, NK-cell, and Fc receptors, but its contribution to the skeletal system remains unclarified. In search for the calcium-mobilizing mechanism during osteoclastogenesis we determined that multiple immunoglobulinlike receptors associated with ITAM-harboring adaptors, Fc receptor common gamma chain (FcRgamma), and DNAX-activating protein (DAP) 12, are essential for osteoclastogenesis. In osteoclast precursor cells FcRgamma-associated receptors include osteoclast-associated receptor and paired immunoglobulinlike receptor A, while triggering receptor expressed in myeloid cells 2 and signal-regulatory protein beta1 preferentially associate with DAP12. In cooperation with RANKL these receptors activate phospholipase Cgamma and calcium signaling essential for the induction of NFATc1 through ITAM phosphorylation. Thus we have established the importance of the ITAM-mediated costimulatory signals in RANKL-induced osteoclast differentiation, which is analogous to the role of costimulatory signals in the immune system. Here we summarize recent advances in the study of signaling mechanism of osteoclast differentiation in the context of osteoimmunology.

Stat1-mediated cytoplasmic attenuation in osteoimmunology

Signal transducer and activator of transcription 1 (Stat1) is a critical mediator of gene transcription in type I interferon (IFN-alpha/beta) signaling that is essential for host defense against viruses. In the skeletal system, type I IFNs (IFN-alpha/beta) also play an important physiological role in the inhibition of receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast differentiation and bone resorption: mice deficient in IFN signaling exhibit decreased bone mass accompanied by the activation of osteoclastogenesis. On the other hand, an unexpected increase in bone mass was observed in Stat1-deficient mice, indicating that Stat1 has a hitherto unknown function in the regulation of bone formation. Indeed, Stat1 was found to have a unique, non-canonical function as a cytoplasmic attenuator of Runx2, a key transcription factor for osteoblast differentiation. Thus, the loss of Stat1 results in excessive activation of Runx2 and osteoblast differentiation, thereby tipping the balance in favor of bone formation over bone resorption. This is an interesting example in which a latent transcription factor attenuates the activity of another transcription factor in the cytoplasm, and reveals a novel regulatory mechanism of bone remodeling by immunomodulatory molecules. Here, we summarize recent advances in the study of Stat1 and IFNs in the context of osteoimmunology, including latest reports that question whether the inhibitory function of Stat1 in chondrocytes is responsible for dwarfism in achondroplasia.

The deficiency of immunoregulatory receptor PD-1 causes mild osteopetrosis

Recently, the involvement of immune responses in metabolic bone disease and/or local bone destruction has received much attention. Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), a member of the immunoglobulin (Ig) superfamily, negatively regulates T cell activation. The deficiency of CTLA-4 induces profound osteopenia with an increase in osteoclastogenesis, suggesting the important role of activated T cells in osteoclastogenesis. Programmed death-1 (PD-1) is the newly identified immunoregulatory receptor, which also belongs to the Ig superfamily. Both CTLA-4 and PD-1 are induced on activated T cells, however, there are no reports linking PD-1 with osteoclasts. In the present study, we have examined the bone phenotype in PD-1-deficient mice PD-1-/- and the role of PD-1 in osteoclastogenesis and osteoclast function. Both trabecular and cortical bone mineral densities of tibia were significantly increased, as observed in peripheral quantitative computed tomography (pQCT), at 12 weeks of age in PD-1-/- mice. Histomorphometric analysis of the PD-1-/- mice and the age-matched controls at 12 weeks of age showed a 2-fold increase in bone volume (BV/TV) with a 55% decrease in osteoclast number (N.Oc/BS). Bone formation indices were similar in both groups. The number of soluble receptor activator of nuclear factor kappaB ligand (sRANKL)-induced osteoclast-like cells (OCLs) derived from the PD-1-deficient splenocytes was significantly decreased (by 25%). On the other hand, PD-1 deficiency did not affect the bone-resorbing activity of mature osteoclasts. Our results suggest that PD-1 deficiency reduces osteoclastogenesis resulting in an osteopetrotic phenotype. Identical members of the Ig superfamily, CTLA-4 and PD-1, which negatively regulate immune responses, may differentially affect osteoclastogenesis and bone remodeling.