Postmitotic osteoclast precursors are mononuclear cells which express macrophage-associated phenotypes

Histochemical assessment of osteoclast-like giant cells in Rankl-/- mice

OBJECTIVES

We examined mice with gene deletion of Receptor activator of nuclear factor-κB (Rank) ligand (Rankl) to histologically clarify whether they contained progenitor cells committed to osteoclastic differentiation up to the stage requiring RANK/RANKL signaling.

METHODS

The tibiae and femora of ten-week-old male wild-type, c-fos^-/-, and Rankl^-/- mice were used for immunohistochemistry and transmission electron microscopy (TEM).

RESULTS

In Rankl^-/- mice, we observed osteoclast-like giant cells, albeit in low numbers, with single or two nuclei, engulfing the mineralized extracellular matrix. TEM revealed that these giant cells contained large numbers of mitochondria, vesicles/vacuoles, and clear zone-like structures but no ruffled borders. They often engulfed fragmented bony/cartilaginous components of the extracellular matrix that had been degraded. Additionally, osteoclast-like giant cells exhibited immunoreactivity for vacuolar H⁺-ATPase, galectin-3, and siglec-15 but not for tartrate-resistant acid phosphatase, cathepsin K, or MMP-9, all of which are classical hallmarks of osteoclasts. Furthermore, osteoclast-like giant cells were ephrinB2-positive as they were near EphB4-positive osteoblasts that are also positive for alkaline phosphatase and Runx2 in Rankl^-/- mice. Unlike Rankl^-/- mice, c-fos^-/- mice lacking osteoclast progenitors and mature osteoclasts had no ephrinB2-positive osteoclast-like cells or alkaline phosphatase-positive/Runx2-reactive osteoblasts. This suggests that similar to authentic osteoclasts, osteoclast-like giant cells might have the potential to activate osteoblasts in Rankl^-/- mice.

CONCLUSIONS

It seems plausible that osteoclast-like giant cells may have acquired some osteoclastic traits and the ability to resorb mineralized matrices even when the absence of RANK/RANKL signaling halted the osteoclastic differentiation cascade.

Contributions of Embryonic HSC-Independent Hematopoiesis to Organogenesis and the Adult Hematopoietic System

During ontogeny, the establishment of the hematopoietic system takes place in several phases, separated both in time and location. The process is initiated extra-embryonically in the yolk sac (YS) and concludes in the main arteries of the embryo with the formation of hematopoietic stem cells (HSC). Initially, it was thought that HSC-independent hematopoietic YS cells were transient, and only required to bridge the gap to HSC activity. However, in recent years it has become clear that these cells also contribute to embryonic organogenesis, including the emergence of HSCs. Furthermore, some of these early HSC-independent YS cells persist into adulthood as distinct hematopoietic populations. These previously unrecognized abilities of embryonic HSC-independent hematopoietic cells constitute a new field of interest. Here, we aim to provide a succinct overview of the current knowledge regarding the contribution of YS-derived hematopoietic cells to the development of the embryo and the adult hematopoietic system.

Monocyte/Macrophage Lineage Cells From Fetal Erythromyeloid Progenitors Orchestrate Bone Remodeling and Repair

A third of the population sustains a bone fracture, and the pace of fracture healing slows with age. The slower pace of repair is responsible for the increased morbidity in older individuals who sustain a fracture. Bone healing progresses through overlapping phases, initiated by cells of the monocyte/macrophage lineage. The repair process ends with remodeling. This last phase is controlled by osteoclasts, which are bone-specific multinucleated cells also of the monocyte/macrophage lineage. The slower rate of healing in aging can be rejuvenated by macrophages from young animals, and secreted proteins from macrophage regulate undifferentiated mesenchymal cells to become bone-forming osteoblasts. Macrophages can derive from fetal erythromyeloid progenitors or from adult hematopoietic progenitors. Recent studies show that fetal erythromyeloid progenitors are responsible for the osteoclasts that form the space in bone for hematopoiesis and the fetal osteoclast precursors reside in the spleen postnatally, traveling through the blood to participate in fracture repair. Differences in secreted proteins between macrophages from old and young animals regulate the efficiency of osteoblast differentiation from undifferentiated mesenchymal precursor cells. Interestingly, during the remodeling phase osteoclasts can form from the fusion between monocyte/macrophage lineage cells from the fetal and postnatal precursor populations. Data from single cell RNA sequencing identifies specific markers for populations derived from the different precursor populations, a finding that can be used in future studies. Here, we review the diversity of macrophages and osteoclasts, and discuss recent finding about their developmental origin and functions, which provides novel insights into their roles in bone homeostasis and repair.

Stepwise cell fate decision pathways during osteoclastogenesis at single-cell resolution

Osteoclasts are the exclusive bone-resorbing cells, playing a central role in bone metabolism, as well as the bone damage that occurs under pathological conditions^1,2. In postnatal life, haematopoietic stem-cell-derived precursors give rise to osteoclasts in response to stimulation with macrophage colony-stimulating factor and receptor activator of nuclear factor-κB ligand, both of which are produced by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes^1-3. However, the precise mechanisms underlying cell fate specification during osteoclast differentiation remain unclear. Here, we report the transcriptional profiling of 7,228 murine cells undergoing in vitro osteoclastogenesis, describing the stepwise events that take place during the osteoclast fate decision process. Based on our single-cell transcriptomic dataset, we find that osteoclast precursor cells transiently express CD11c, and deletion of receptor activator of nuclear factor-κB specifically in CD11c-expressing cells inhibited osteoclast formation in vivo and in vitro. Furthermore, we identify Cbp/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 2 (Cited2) as the molecular switch triggering terminal differentiation of osteoclasts, and deletion of Cited2 in osteoclast precursors in vivo resulted in a failure to commit to osteoclast fate. Together, the results of this study provide a detailed molecular road map of the osteoclast differentiation process, refining and expanding our understanding of the molecular mechanisms underlying osteoclastogenesis.

Erythromyeloid progenitors give rise to a population of osteoclasts that contribute to bone homeostasis and repair

Osteoclasts are multinucleated cells of the monocyte/macrophage lineage that degrade bone. Here, we used lineage tracing studies-labelling cells expressing Cx3cr1, Csf1r or Flt3-to identify the precursors of osteoclasts in mice. We identified an erythromyeloid progenitor (EMP)-derived osteoclast precursor population. Yolk-sac macrophages of EMP origin produced neonatal osteoclasts that can create a space for postnatal bone marrow haematopoiesis. Furthermore, EMPs gave rise to long-lasting osteoclast precursors that contributed to postnatal bone remodelling in both physiological and pathological settings. Our single-cell RNA-sequencing data showed that EMP-derived osteoclast precursors arose independently of the haematopoietic stem cell (HSC) lineage and the data from fate tracking of EMP and HSC lineages indicated the possibility of cell-cell fusion between these two lineages. Cx3cr1+ yolk-sac macrophage descendants resided in the adult spleen, and parabiosis experiments showed that these cells migrated through the bloodstream to the remodelled bone after injury.

Atorvastatin inhibits osteoclastogenesis and arrests tooth movement

INTRODUCTION

In addition to their cholesterol-lowering effects, the statin class of drugs appears to enhance osteogenesis and suppress bone resorption, which could be a clinical concern during orthodontic treatment. In this animal study, we aimed to determine whether atorvastatin (ATV) affects orthodontic tooth movement (OTM) through osteoclast inhibition. Furthermore, we analyzed the potential adverse effects of ATV on long-bone turnover and endochondral ossification.

METHODS

Rats were administered ATV (15 mg/kg) or saline solution via gavage (n = 12 animals/group), starting 2 weeks before initial OTM. Tooth displacement was measured after 7, 14, and 21 days. Histologic sections of the maxilla and femur were obtained after 14 and 21 days of OTM and stained (hematoxylin and eosin; TRAP assay) for histomorphometric analysis.

RESULTS

ATV was associated with significant (P <0.05) reductions in OTM and osteoclast counts. Independently of drug administration, OTM increased the number of osteoclasts and reduced the bone-volume ratio compared with the control maxillae without OTM. Long-term statin administration did not appear to affect femoral endochondral ossification.

CONCLUSIONS

This experimental study showed that the long-term use of ATV can significantly promote osteoclast inhibition and slow the OTM in the first week in rats. Under physiologic conditions, the drug did not affect bone turnover and endochondral ossification.

Osteoclasts-Key Players in Skeletal Health and Disease

The differentiation of osteoclasts (OCs) from early myeloid progenitors is a tightly regulated process that is modulated by a variety of mediators present in the bone microenvironment. Once generated, the function of mature OCs depends on cytoskeletal features controlled by an αvβ3-containing complex at the bone-apposed membrane and the secretion of protons and acid-protease cathepsin K. OCs also have important interactions with other cells in the bone microenvironment, including osteoblasts and immune cells. Dysregulation of OC differentiation and/or function can cause bone pathology. In fact, many components of OC differentiation and activation have been targeted therapeutically with great success. However, questions remain about the identity and plasticity of OC precursors and the interplay between essential networks that control OC fate. In this review, we summarize the key principles of OC biology and highlight recently uncovered mechanisms regulating OC development and function in homeostatic and disease states.

Involvement of Receptor Activator of Nuclear Factor-κB Ligand (RANKL)-induced Incomplete Cytokinesis in the Polyploidization of Osteoclasts

Osteoclasts are specialized polyploid cells that resorb bone. Upon stimulation with receptor activator of nuclear factor-κB ligand (RANKL), myeloid precursors commit to becoming polyploid, largely via cell fusion. Polyploidization of osteoclasts is necessary for their bone-resorbing activity, but the mechanisms by which polyploidization is controlled remain to be determined. Here, we demonstrated that in addition to cell fusion, incomplete cytokinesis also plays a role in osteoclast polyploidization. In in vitro cultured osteoclasts derived from mice expressing the fluorescent ubiquitin-based cell cycle indicator (Fucci), RANKL induced polyploidy by incomplete cytokinesis as well as cell fusion. Polyploid cells generated by incomplete cytokinesis had the potential to subsequently undergo cell fusion. Nuclear polyploidy was also observed in osteoclasts in vivo, suggesting the involvement of incomplete cytokinesis in physiological polyploidization. Furthermore, RANKL-induced incomplete cytokinesis was reduced by inhibition of Akt, resulting in impaired multinucleated osteoclast formation. Taken together, these results reveal that RANKL-induced incomplete cytokinesis contributes to polyploidization of osteoclasts via Akt activation.

Using a novel microRNA delivery system to inhibit osteoclastogenesis

Previously, we developed a novel microRNA (miRNA) delivery system based on bacteriophage MS2 virus-like particles (MS2 VLPs). In this current study, we used this system to transport miR-146a into human peripheral blood mononuclear cells (PBMCs), and demonstrated the inhibition of osteoclastogenesis in precursors. Two cytokines, receptor activator of NF-κB ligand (RANKL), and macrophage-colony stimulating factor (M-CSF) were used to induce osteoclastogenesis. MS2 VLPs were transfected into PBMCs. qRT-PCR was applied to measure expression levels of miR-146a and osteoclast (OC)-specific genes. Western blot (WB) was conducted to evaluate miR-146a downstream target proteins: epidermal growth factor receptor (EGFR) and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6). The formation and activity of OCs were assessed by cytochemical staining and bone resorption assay, respectively. In PBMCs treated with MS2-miR146a VLPs, qRT-PCR assays showed increased expression of miR-146a (p < 0.01) and decreased expression of all four OC-specific genes (p < 0.05). WB results indicated decreased expression of EGFR (p < 0.01) and TRAF6 (p < 0.05). The number of OCs decreased markedly and bone resorption assay demonstrated inhibited activity. This miR-146a delivery system could be applied to induce overexpression of miR-146a and to inhibit the differentiation and function of OCs.

M-CSF priming of osteoclast precursors can cause osteoclastogenesis-insensitivity, which can be prevented and overcome on bone

Osteoclasts and macrophages share progenitors that must receive decisive lineage signals driving them into their respective differentiation routes. Macrophage colony stimulation factor M-CSF is a common factor; bone is likely the stimulus for osteoclast differentiation. To elucidate the effect of both, shared mouse bone marrow precursor myeloid blast was pre-cultured with M-CSF on plastic and on bone. M-CSF priming prior to stimulation with M-CSF and osteoclast differentiation factor RANKL resulted in a complete loss of osteoclastogenic potential without bone. Such M-CSF primed cells expressed the receptor RANK, but lacked the crucial osteoclastogenic transcription factor NFATc1. This coincided with a steeply decreased expression of osteoclast genes TRACP and DC-STAMP, but an increased expression of the macrophage markers F4/80 and CD11b. Compellingly, M-CSF priming on bone accelerated the osteoclastogenic potential: M-CSF primed cells that had received only one day M-CSF and RANKL and were grown on bone already expressed an array of genes that are associated with osteoclast differentiation and these cells differentiated into osteoclasts within 2 days. Osteoclastogenesis-insensitive precursors grown in the absence of bone regained their osteoclastogenic potential when transferred to bone. This implies that adhesion to bone dictates the fate of osteoclast precursors. Common macrophage-osteoclast precursors may become insensitive to differentiate into osteoclasts and regain osteoclastogenesis when bound to bone or when in the vicinity of bone.

Liposomal clodronate inhibition of osteoclastogenesis and osteoinduction by submicrostructured beta-tricalcium phosphate

Bone graft substitutes such as calcium phosphates are subject to the innate inflammatory reaction, which may bear important consequences for bone regeneration. We speculate that the surface architecture of osteoinductive β-tricalcium phosphate (TCP) stimulates the differentiation of invading monocyte/macrophages into osteoclasts, and that these cells may be essential to ectopic bone formation. To test this, porous TCP cubes with either submicron-scale surface architecture known to induce ectopic bone formation (TCPs, positive control) or micron-scale, non-osteoinductive surface architecture (TCPb, negative control) were subcutaneously implanted on the backs of FVB strain mice for 12 weeks. Additional TCPs samples received local, weekly injections of liposome-encapsulated clodronate (TCPs + LipClod) to deplete invading monocyte/macrophages. TCPs induced osteoclast formation, evident by positive tartrate resistant acid phosphatase (TRAP) cytochemical staining and negative macrophage membrane marker F4/80 immunostaining. No TRAP positive cells were found in TCPb or TCPs + LipClod, only F4/80 positive macrophages and foreign body giant cells. TCPs stimulated subcutaneous bone formation in all implants, while no bone could be found in TCPb or TCPs + LipClod. In agreement, expression of bone and osteoclast gene markers was upregulated in TCPs versus both TCPb and TCPs + LipClod, which were equivalent. In summary, submicron-scale surface structure of TCP induced osteoclastogenesis and ectopic bone formation in a process that is blocked by monocyte/macrophage depletion.

Hyperglycemia induced and intrinsic alterations in type 2 diabetes-derived osteoclast function

Periodontal disease-associated alveolar bone loss is a comorbidity of type-2-diabetes, where the roles of osteoclasts are poorly understood.

OBJECTIVE

To evaluate osteoclast differentiation and function in the context of type-2-diabetes.

MATERIALS AND METHODS

Bone marrow-derived osteoclasts from db/db mice, a model of type-2-diabetes, as well as human osteoclasts derived from peripheral blood of individuals with type-2-diabetes were evaluated for differentiation, resorption, and soluble mediator expression.

RESULTS

While db/db mice were hyperglycemic at time of cell harvest, human participants were glycemically controlled. Although db/db cultures resulted in a higher number of larger osteoclasts, individual cell receptor activator of nuclear factor kappaB ligand (RANKL)-mediated bone resorption was similar to that observed in diabetes-free osteoclasts. Osteoclasts derived from individuals with type-2-diabetes differentiated similarly to controls with again no difference in bone resorbing capacity. Murine and human type-2-diabetes cultures both displayed inhibition of lipopolysaccharide (LPS)-induced deactivation and increased pro-osteoclastogenic mediator expression.

CONCLUSIONS

Hyperglycemia plays a role in aberrant osteoclast differentiation leading to an increased capacity for bone resorption. Osteoclasts derived from murine models of and individuals with type-2-diabetes are unable to be inhibited by LPS, again leading to increased capacity for bone resorption. Here, environmental and intrinsic mechanisms associated with the increased alveolar bone loss observed in periodontal patients with type-2-diabetes are described.

Augmented LPS responsiveness in type 1 diabetes-derived osteoclasts

Bone abnormalities are frequent co-morbidities of type 1 diabetes (T1D) and are principally mediated by osteoblasts and osteoclasts which in turn are regulated by immunologic mediators. While decreased skeletal health in T1D involves alterations in osteoblast maturation and function, the effect of altered immune function on osteoclasts in T1D-associated bone and joint pathologies is less understood. Here T1D-associated osteoclast-specific differentiation and function in the presence and absence of inflammatory mediators was characterized utilizing bone marrow-derived osteoclasts (BM-OCs) isolated from non-obese diabetic (NOD) mice, a model for spontaneous autoimmune diabetes with pathology similar to individuals with T1D. Differentiation and osteoclast-mediated bone resorption were evaluated along with cathepsin K, MMP-9, and immune soluble mediator expression. The effect of lipopolysaccharide (LPS), a pro-inflammatory cytokine cocktail, and NOD-derived conditioned supernatants on BM-OC function was also determined. Although NOD BM-OCs cultures contained smaller osteoclasts, they resorbed more bone concomitant with increased cathepsin K, MMP-9, and pro-osteoclastogenic mediator expression. NOD BM-OCs also displayed an inhibition of LPS-induced deactivation that was not a result of soluble mediators produced by NOD BM-OCs, although a pro-inflammatory milieu did enhance NOD BM-OCs bone resorption. Together these data indicate that osteoclasts from a T1D mouse model hyper-respond to RANK-L resulting in excessive bone degradation via enhanced cathepsin K and MMP-9 secretion concomitant with an increased expression of pro-osteoclastic soluble mediators. Our data also suggest that inhibition of LPS-induced deactivation in NOD-derived BM-OC cultures is most likely due to NOD osteoclast responsiveness rather than LPS-induced expression of soluble mediators.

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.

Characterization and identification of subpopulations of mononuclear preosteoclasts induced by TNF-α in combination with TGF-β in rats

Osteoclasts are unique multinucleated cells formed by fusion of preosteoclasts derived from cells of the monocyte/macrophage lineage, which are induced by RANKL. However, characteristics and subpopulations of osteoclast precursor cells are poorly understood. We show here that a combination of TNF-α, TGF-β, and M-CSF efficiently generates mononuclear preosteoclasts but not multinucleated osteoclasts (MNCs) in rat bone marrow cultures depleted of stromal cells. Using a rat osteoclast-specific mAb, Kat1, we found that TNF-α and TGF-β specifically increased Kat1⁺c-fms⁺ and Kat1⁺c-fms⁻ cells but not Kat1⁻c-fms⁺ cells. Kat1⁻c-fms⁺ cells appeared in early stages of culture, but Kat1⁺c-fms⁺ and Kat1⁺c-fms⁻ cells increased later. Preosteoclasts induced by TNF-α, TGF-β, and M-CSF rapidly differentiated into osteoclasts in the presence of RANKL and hydroxyurea, an inhibitor of DNA synthesis, suggesting that preosteoclasts are terminally differentiated cells. We further analyzed the expression levels of genes encoding surface proteins in bone marrow macrophages (BMM), preosteoclasts, and MNCs. Preosteoclasts expressed itgam (CD11b) and chemokine receptors CCR1 and CCR2; however, in preosteoclasts the expression of chemokine receptors CCR1 and CCR2 was not up-regulated compared to their expression in BMM. However, addition of RANKL to preosteoclasts markedly increased the expression of CCR1. In contrast, expression of macrophage antigen emr-1 (F4/80) and chemokine receptor CCR5 was down-regulated in preosteoclasts. The combination of TNF-α, TGF-β, and M-CSF induced Kat1⁺CD11b⁺ cells, but these cells were also induced by TNF-α alone. In addition, MIP-1α and MCP-1, which are ligands for CCR1 and CCR2, were chemotactic for preosteoclasts, and promoted multinucleation of preosteoclasts. Finally, we found that Kat1⁺c-fms⁺ cells were present in bone tissues of rats with adjuvant arthritis. These data demonstrate that TNF-α in combination with TGF-β efficiently generates preosteoclasts in vitro. We delineated characteristics that are useful for identifying and isolating rat preosteoclasts, and found that CCR1 expression was regulated in the fusion step in osteoclastogenesis.

Thiocolchicoside suppresses osteoclastogenesis induced by RANKL and cancer cells through inhibition of inflammatory pathways: a new use for an old drug

BACKGROUND AND PURPOSE

Most patients with cancer die not because of the tumour in the primary site, but because it has spread to other sites. Common tumours, such as breast, multiple myeloma, and prostate tumours, frequently metastasize to the bone. To search for an inhibitor of cancer-induced bone loss, we investigated the effect of thiocolchicoside, a semi-synthetic colchicoside derived from the plant Gloriosa superba and clinically used as a muscle relaxant, on osteoclastogenesis induced by receptor activator of NF-κB ligand (RANKL) and tumour cells.

EXPERIMENTAL APPROACH

We used RAW 264.7 (murine macrophage) cells, a well-established system for osteoclastogenesis, and evaluated the effect of thiocolchicoside on RANKL-induced NF-κB signalling and osteoclastogenesis as well as on osteoclastogenesis induced by tumour cells.

KEY RESULTS

Thiocolchicoside suppressed osteoclastogenesis induced by RANKL, and by breast cancer and multiple myeloma cells. Inhibition of the NF-κB pathway was responsible for this effect since the colchicoside inhibited RANKL-induced NF-κB activation, activation of IκB kinase (IKK) and suppressed inhibitor of NF-κBα (IκBα) phosphorylation and degradation, an inhibitor of NF-κB. Furthermore, an inhibitor of the IκBα kinase γ or NF-κB essential modulator, the regulatory component of the IKK complex, demonstrated that the NF-κB signalling pathway is mandatory for osteoclastogenesis induced by RANKL.

CONCLUSIONS AND IMPLICATIONS

Together, these data suggest that thiocolchicoside significantly suppressed osteoclastogenesis induced by RANKL and tumour cells via the NF-κB signalling pathway. Thus, thiocolchicoside, a drug that has been used for almost half a century to treat muscle pain, may also be considered as a new treatment for bone loss.

Adenosine A1 receptors (A1Rs) play a critical role in osteoclast formation and function

Adenosine regulates a wide variety of physiological processes via interaction with one or more G-protein-coupled receptors (A(1)R, A(2A)R, A(2B)R, and A(3)R). Because A(1)R occupancy promotes fusion of human monocytes to form giant cells in vitro, we determined whether A(1)R occupancy similarly promotes osteoclast function and formation. Bone marrow cells (BMCs) were harvested from C57Bl/6 female mice or A(1)R-knockout mice and their wild-type (WT) littermates and differentiated into osteoclasts in the presence of colony stimulating factor-1 and receptor activator of NF-kappaB ligand in the presence or absence of the A(1)R antagonist 1,3-dipropyl-8-cyclopentyl xanthine (DPCPX). Osteoclast morphology was analyzed in tartrate-resistant acid phosphatase or F-actin-stained samples, and bone resorption was evaluated by toluidine blue staining of dentin. BMCs from A(1)R-knockout mice form fewer osteoclasts than BMCs from WT mice, and the A(1)R antagonist DPCPX inhibits osteoclast formation (IC(50)=1 nM), with altered morphology and reduced ability to resorb bone. A(1)R blockade increased ubiquitination and degradation of TRAF6 in RAW264.7 cells induced to differentiate into osteoclasts. These studies suggest a critical role for adenosine in bone homeostasis via interaction with adenosine A(1)R and further suggest that A(1)R may be a novel pharmacologic target to prevent the bone loss associated with inflammatory diseases and menopause.

A novel osteoclast precursor cell line, 4B12, recapitulates the features of primary osteoclast differentiation and function: enhanced transfection efficiency before and after differentiation

Osteoclasts are bone-resorbing multinucleated cells differentiated from monocyte/macrophage lineage precursors. A novel osteoclast precursor cell line, 4B12 was established from Mac-1(+)c-Fms(+)RANK(+) cells from calvaria of 14-day-old mouse embryos using immunofluorescence and cell-sorting methods. Like M-CSF-dependent bone marrow macrophages (M-BMMs), M-CSF is required for 4B12 cells to differentiate into TRAP-positive multinucleated cells [TRAP(+) MNCs] in the presence of RANKL. Bone-resorbing osteoclasts differentiated from 4B12 cells on dentine slices possess both a clear zone and ruffled borders and express osteoclast-specific genes. Bone-resorbing activity, but not TRAP, was enhanced in the presence of IL-1alpha. The number of TRAP(+) MNCs and the number of pits formed from 4B12 cells on dentine slices was fourfold higher than that from M-BMMs. 4B12 cells were identified as macrophages with Mac-1 and F4/80, yet lost these markers upon differentiation into osteoclasts as determined by confocal laser scanning microscopy. The 4B12 cells do not have the potential to differentiate into dendritic cells indicating commitment to the osteoclast lineage. 4B12 cells are readily transfectable with siRNA transfection before and after differentiation. These data show that 4B12 cells faithfully replicate the properties of primary cells and are a useful and powerful model for analyzing the molecular and cellular regulatory mechanisms of osteoclastogenesis and osteoclast function.

Regulated proteolysis of nonmuscle myosin IIA stimulates osteoclast fusion

The nonmuscle myosin IIA heavy chain (Myh9) is strongly associated with adhesion structures of osteoclasts. In this study, we demonstrate that during osteoclastogenesis, myosin IIA heavy chain levels are temporarily suppressed, an event that stimulates the onset of cell fusion. This suppression is not mediated by changes in mRNA or translational levels but instead is due to a temporary increase in the rate of myosin IIA degradation. Intracellular activity of cathepsin B is significantly enhanced at the onset of osteoclast precursor fusion, and specific inhibition of its activity prevents myosin IIA degradation. Further, treatment of normal cells with cathepsin B inhibitors during the differentiation process reduces cell fusion and bone resorption capacity, whereas overexpression of cathepsin B enhances fusion. Ongoing suppression of the myosin IIA heavy chain via RNA interference results in formation of large osteoclasts with significantly increased numbers of nuclei, whereas overexpression of myosin IIA results in less osteoclast fusion. Increased multinucleation caused by myosin IIA suppression does not require RANKL. Further, knockdown of myosin IIA enhances cell spreading and lessens motility. These data taken together strongly suggest that base-line expression of nonmuscle myosin IIA inhibits osteoclast precursor fusion and that a temporary, cathepsin B-mediated decrease in myosin IIA levels triggers precursor fusion during osteoclastogenesis.

Identification of cell cycle-arrested quiescent osteoclast precursors in vivo

Osteoclasts are multinucleated cells that resorb bone. Although osteoclasts originate from the monocyte/macrophage lineage, osteoclast precursors are not well characterized in vivo. The relationship between proliferation and differentiation of osteoclast precursors is examined in this study using murine macrophage cultures treated with macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-kappaB (RANK) ligand (RANKL). Cell cycle-arrested quiescent osteoclast precursors (QuOPs) were identified as the committed osteoclast precursors in vitro. In vivo experiments show that QuOPs survive for several weeks and differentiate into osteoclasts in response to M-CSF and RANKL. Administration of 5-fluorouracil to mice induces myelosuppression, but QuOPs survive and differentiate into osteoclasts in response to an active vitamin D(3) analogue given to those mice. Mononuclear cells expressing c-Fms and RANK but not Ki67 are detected along bone surfaces in the vicinity of osteoblasts in RANKL-deficient mice. These results suggest that QuOPs preexist at the site of osteoclastogenesis and that osteoblasts are important for maintenance of QuOPs.

Identification of cell cycle-arrested quiescent osteoclast precursors in vivo

How are sites suitable for osteoclastogenesis determined? We addressed this issue using in vivo and in vitro experimental systems. We first examined the formation of osteoclasts in ectopic bone induced by BMP-2. When collagen disks which contained BMP-2 (BMP-2-disks) or vehicle (control-disks) were implanted into wild-type mice, osteoclasts and osteoblasts appeared in the BMP-2-disks, but not in the control disks. RANKL-deficient (RANKL(-/-)) mice exhibited osteopetrosis, with an absence of osteoclasts. BMP-2 and control disks were implanted into RANKL(-/-) mice, which were intraperitoneally injected with RANKL. Osteoclasts formed in the BMP-2-disks, but not in the control disks. In the BMP-2-disks, osteoclasts were observed in the vicinity of osteoblasts. Cell cycle-arrested quiescent osteoclast precursors (QOP) were identified as the committed osteoclast precursors in vitro. Experiments in vivo showed that QOPs survived for several weeks, and differentiated into osteoclasts in response to M-CSF and RANKL. QOPs were identified as RANK and c-Fms double-positive cells, and detected along bone surfaces in the vicinity of osteoblasts in RANKL(-/-) mice. QOPs were also observed in the ectopic bone induced by BMP-2 implanted into RANKL(-/-) mice, suggesting that QOPs were circulating. These results imply that osteoblasts support the homing of QOPs to bone tissues. In response to bone-resorbing stimuli, QOPs promptly differentiate into osteoclasts. Therefore, the distribution of QOPs appears to determine the correct site of osteoclastic development.

RGS10-null mutation impairs osteoclast differentiation resulting from the loss of [Ca2+]i oscillation regulation

Increased osteoclastic resorption leads to many bone diseases, including osteoporosis and rheumatoid arthritis. While rapid progress has been made in characterizing osteoclast differentiation signaling pathways, how receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL) evokes essential [Ca2+]i oscillation signaling remains unknown. Here, we characterized RANKL-induced signaling proteins and found regulator of G-protein signaling 10 (RGS10) is predominantly expressed in osteoclasts. We generated RGS10-deficient (RGS10-/-) mice that exhibited severe osteopetrosis and impaired osteoclast differentiation. Our data demonstrated that ectopic expression of RGS10 dramatically increased the sensitivity of osteoclast differentiation to RANKL signaling; the deficiency of RGS10 resulted in the absence of [Ca2+]i oscillations and loss of NFATc1; ectopic NFATc1 expression rescues impaired osteoclast differentiation from deletion of RGS10; phosphatidylinositol 3,4,5-trisphosphate (PIP3) is essential to PLCgamma activation; and RGS10 competitively interacts with Ca2+/calmodulin and PIP3 in a [Ca2+]i-dependent manner to mediate PLCgamma activation and [Ca2+]i oscillations. Our results revealed a mechanism through which RGS10 specifically regulates the RANKL-evoked RGS10/calmodulin-[Ca2+]i oscillation-calcineurin-NFATc1 signaling pathway in osteoclast differentiation using an in vivo model. RGS10 provides a potential therapeutic target for the treatment of bone diseases.

Assessment of cell proliferation during mandibular distraction osteogenesis in the maturing rat

INTRODUCTION

The cellular mechanisms controlling distraction osteogenesis are not well understood. The purpose of this study was to examine the role of cell proliferation in the regulation of mandibular distraction osteogenesis.

METHODS

Unilateral mandibular ramus osteotomies were performed on 125 3-month-old Sprague-Dawley rats. The rats were randomized into 4 distraction rate groups and distracted for 5 days after 3-day latency. Rats (7 or 8 from each rate group) were killed at 4 time points. The rats received 5-bromo-2-deoxyuridine (BrdU) injections (40 mg per kilogram, i.p.) at day 3 (end of latency).

RESULTS

Both intramembranous and endochondral ossification was seen in the osteogenesis area. BrdU+ mesenchymal progenitor cells were significantly higher at day 10 (P <.05) and were found most numerously around the sagittal middle portion of the gap (P <.01). The greatest numbers of BrdU+ osteocytes were seen at day 38 (P <.05). Both BrdU+ osteoclasts and chondrocytes peaked at day 24.

CONCLUSIONS

Mesenchymal progenitor cells are mostly recruited in the early consolidation period, but they decrease in the middle and late consolidation periods during mandibular distraction osteogenesis. The rapid rate might suppress or sustain the proliferation and differentiation of mesenchymal progenitor cells during mandibular distraction osteogenesis. BrdU+ cells can survive throughout the entire experimental period of 5 weeks.

Biomechanical and molecular regulation of bone remodeling

Bone is a dynamic tissue that is constantly renewed. The cell populations that participate in this process--the osteoblasts and osteoclasts--are derived from different progenitor pools that are under distinct molecular control mechanisms. Together, these cells form temporary anatomical structures, called basic multicellular units, that execute bone remodeling. A number of stimuli affect bone turnover, including hormones, cytokines, and mechanical stimuli. All of these factors affect the amount and quality of the tissue produced. Mechanical loading is a particularly potent stimulus for bone cells, which improves bone strength and inhibits bone loss with age. Like other materials, bone accumulates damage from loading, but, unlike engineering materials, bone is capable of self-repair. The molecular mechanisms by which bone adapts to loading and repairs damage are starting to become clear. Many of these processes have implications for bone health, disease, and the feasibility of living in weightless environments (e.g., spaceflight).

Identification of multiple osteoclast precursor populations in murine bone marrow

Murine BM was fractionated using a series of hematopoietic markers to characterize its osteoclast progenitor populations. We found that the early osteoclastogenic activity in total BM was recapitulated by a population of cells contained within the CD11b(-/low) CD45R- CD3- CD115high fraction.

INTRODUCTION

Osteoclasts are of hematopoietic origin and they have been shown to share the same lineage as macrophages. We further characterized the phenotype of osteoclast progenitor populations in murine bone marrow (BM) by analyzing their cell surface markers.

MATERIALS AND METHODS

We used fluorescence-activated cell sorting (FACS) to identify the subsets of BM cells that contained osteoclast progenitors. We fractionated BM according to several markers and cultured the sorted populations for a period of 2-6 days with macrophage-colony stimulating factor (M-CSF) and RANKL. The numbers of multinucleated osteoclast-like cells (OCLs) that formed in the cultures were counted.

RESULTS

We found that the CD45R- CD11b(-/low) population recapitulated the early osteoclastogenic activity of total BM. In addition, although previous experiments indicated that osteoclastogenic activity was enriched within the CD45R+ population, we found that highly purified CD45R+ BM was incapable of differentiating into osteoclasts in vitro. We also found that CD45R- CD11b(high) BM cells were an inefficient source of osteoclast progenitors. However, CD11b was transiently upregulated by cells of the CD45R- CD11b(-/low) fraction early (within 24 h) during culture with M-CSF. Finally, further fractionation of BM using CD115 and CD117 showed that, as osteoclast precursor cells matured, they downregulate CD117 but remain CD115+. Curiously, pure populations of CD117- (CD115high) cells isolated fresh from BM have low osteoclastogenic activity in vitro.

CONCLUSIONS

We provided a refined analysis of the precise subpopulations of murine BM that are capable of differentiating into OCLs in vitro when treated with M-CSF and RANKL.

Osteostat/tumor necrosis factor superfamily 18 inhibits osteoclastogenesis and is selectively expressed by vascular endothelial cells

Vascular endothelial cells (EC) participate in the process of bone formation through the production of factors regulating osteoclast differentiation and function. In this study, we report the selective expression in primary human microvascular EC of Osteostat/TNF superfamily 18, a ligand of the TNF superfamily. Osteostat protein is detectable in human microvascular EC and is highly up-regulated by IFN-alpha and IFN-beta. Moreover, an anti-Osteostat antibody strongly binds to the vascular endothelium in human tissues, demonstrating that the protein is present in the EC layers surrounding blood vessels. Functional in vitro assays were used to define Osteostat involvement in osteoclastogenesis. Both recombinant and membrane-bound Osteostat inhibit differentiation of osteoclasts from monocytic precursor cells. Osteostat suppresses the early stage of osteoclastogenesis via inhibition of macrophage colony-stimulating factor-induced receptor activator of NF-kappaB (RANK) expression in the osteoclast precursor cells. This effect appears to be specific for the differentiation pathway of the osteoclast lineage, because Osteostat does not inhibit lipopolysaccharide-induced RANK expression in monocytes and dendritic cells, or activation-induced RANK expression in T cells. These findings demonstrate that Osteostat is a novel regulator of osteoclast generation and substantiate the major role played by the endothelium in bone physiology.

Organization of transcriptional regulatory machinery in osteoclast nuclei: compartmentalization of Runx1

The osteoclast is a highly polarized multinucleated cell that resorbs bone. Using high resolution immunofluorescence microscopy, we demonstrated that all nuclei of an osteoclast are transcriptionally active. Each nucleus within the osteoclast contains punctately organized microenvironments where regulatory complexes that support transcriptional and post-transcriptional control reside. Functional equivalency of osteoclast nuclei is reflected by similar representation of regulatory proteins that support ribosomal RNA synthesis (nucleolin), mRNA transcription (RNA polymerase II, bromouridine triphosphate), processing of gene transcripts (SC35), signal transduction (NF-kappaB), and phenotypic gene expression (Runx1). Our results establish that gene regulatory machinery is architecturally associated and compartmentalized within intranuclear microenvironments of the multiple nuclei of osteoclasts to support physiologically responsive modifications in cellular structural and functional properties.

Osteoporosis with increased osteoclastogenesis in hematopoietic cell-specific STAT3-deficient mice

Hematopoietic cell-specific disruption of the STAT3 gene induces hyperproliferation of cells of the myeloid lineage. Osteoclasts (OC), the bone-resorbing cells, are generated from the same precursors as monocyte/macrophages. STAT3 mutant mice exhibit decreased bone density, bone volume, and increased numbers of TRAP-positive OC. In vitro generation of OC showed significantly greater numbers of OC in mutant mice. The increased numbers of Mac1+ cells and c-kit+ cells were detected by FACS analysis, suggesting an increased number of OC precursors. Treatment of splenocytes with CSF-1 and RANKL significantly increased the Mac-1+RANK+ cells, with much higher levels observed in cells from mutant mice compared with littermate controls. Besides enhanced number of Mac1+ OC precursors, we also identified an OC-generating Mac1- c-kit+ population in mutant mice which was absent in littermate controls. The Mac1- c-kit- cells did not generate OC. Finally, we determined that protein expression and mRNA level of c-fos, a transcription factor which is essential for OC differentiation, were enhanced in OC precursors of mutant mice, which may contribute to the osteopenic phenotype.

Essential role of p38 mitogen-activated protein kinase in cathepsin K gene expression during osteoclastogenesis through association of NFATc1 and PU.1

The receptor activator of NF-kappaB ligand (RANKL) induces various osteoclast-specific marker genes during osteoclast differentiation mediated by mitogen-activated protein (MAP) kinase cascades. However, the results of transcriptional programming of an osteoclast-specific cathepsin K gene are inconclusive. Here we report the regulatory mechanisms of RANKL-induced cathepsin K gene expression during osteoclastogenesis in a p38 MAP kinase-dependent manner. The reporter gene analysis with sequential 5'-deletion constructs of the cathepsin K gene promoter indicates that limited sets of the transcription factors such as NFATc1, PU.1, and microphthalmia transcription factor indeed enhance synergistically the gene expression when overexpressed in RAW264 cells. In addition, the activation of p38 MAP kinase is required for the maximum enhancement of the gene expression. RANKL-induced NFATc1 forms a complex with PU.1 in nuclei of osteoclasts following the nuclear accumulation of NFATc1 phosphorylated by the activated p38 MAP kinase. These results suggest that the RANKL-induced cathepsin K gene expression is cooperatively regulated by the combination of the transcription factors and p38 MAP kinase in a gradual manner.

Adult human bone marrow-derived mesenchymal progenitor cells are capable of adhesion-independent survival and expansion

OVERVIEW

We show the existence of adult human mesenchymal progenitor cells (hMPCs) that can proliferate, in a cytokine-dependent manner, as individual cells in stirred suspension cultures (SSC) while maintaining their ability to form functional differentiated mesenchymal cell types.

MATERIALS AND METHODS

Ficolled human bone marrow (BM)-derived cells were grown in SSC (and adherent controls) in the presence and absence of exogenously added cytokines. Phenotypic, gene expression, and functional assays for hematopoietic and nonhematopoietic cell populations were used to kinetically track cell production. Limiting-dilution analysis was used to relate culture-produced cells to input cell populations.

RESULTS

Cytokine cocktail influenced total and progenitor cell expansion, as well as the types of cells generated upon plating. Flow cytometric analysis of CD117, CD123, and CD45 expression showed that cytokine supplementation influenced SSC output. The concomitant growth of CD45(+) and CD45(-) cells in the cultures that exhibited the greatest hMPC expansions suggests that the growth of these cells may benefit from interactions with hematopoietic cells. Functional assays demonstrated that the SSC-derived cells (input CFU-O number: 1990+/-377) grown in the presence of SCF+IL-3 resulted, after 21 days, in the generation of a significantly greater number (p<0.05) of bone progenitors (33,700+/-8763 CFU-O) than similarly initiated adherent cultures (214+/-75 CFU-O). RT-PCR analysis confirmed that the SSC-derived cells grown in osteogenic conditions express bone-specific genes (Cbfa1/Runx2, bone sialoprotein, and osteocalcin).

CONCLUSIONS

Our approach not only provides an alternative strategy to expand adult BM-derived nonhematopoietic progenitor cell numbers in a scalable and controllable bioprocess, but also questions established biological paradigms concerning the properties of connective-tissue stem and progenitor cells.

Osteoclast formation from circulating precursors in osteoporosis

OBJECTIVE

An imbalance between bone formation and bone resorption is thought to underlie the pathogenesis of reduced bone mass in osteoporosis. Bone resorption is carried out by osteoclasts. which are formed from marrow-derived cells that circulate in the monocyte fraction. The aim of this study was to determine the role of osteoclast formation in the pathogenesis of bone loss in osteoporosis.

METHODS

The proportion of circulating osteoclast precursors and their relative sensitivity to the osteoclastogenic effects of M-CSF. 1,25(OH)2D3 and RANKL were assessed in primary osteoporosis patients and normal controls.

RESULTS

Although there was no difference in the number of circulating osteoclast precursors in osteoporosis patients and normal controls. osteoclasts formed from osteoporosis patients exhibited substantially increased resorptive activity relative to normal controls. Although no increased sensitivity to the osteoclastogenic effects of 1,25(OH)D3 or M-CSF was noted, increased bone resorption was found in osteoporosis peripheral blood mononuclear cell (PBMC) cultures to which these factors were added.

CONCLUSION

Our findings suggest that osteoclast functional activity rather than formation is increased in primary involutional osteoporosis and that dexamethasone acts to increase osteoclast formation.

Lipopolysaccharide promotes the survival of osteoclasts via Toll-like receptor 4, but cytokine production of osteoclasts in response to lipopolysaccharide is different from that of macrophages

Lipopolysaccharide is a pathogen that causes inflammatory bone loss. Monocytes and macrophages produce proinflammatory cytokines such as IL-1, TNF-alpha, and IL-6 in response to LPS. We examined the effects of LPS on the function of osteoclasts formed in vitro in comparison with its effect on bone marrow macrophages, osteoclast precursors. Both osteoclasts and bone marrow macrophages expressed mRNA of Toll-like receptor 4 (TLR4) and CD14, components of the LPS receptor system. LPS induced rapid degradation of I-kappaB in osteoclasts, and stimulated the survival of osteoclasts. LPS failed to support the survival of osteoclasts derived from C3H/HeJ mice, which possess a missense mutation in the TLR4 gene. The LPS-promoted survival of osteoclasts was not mediated by any of the cytokines known to prolong the survival of osteoclasts, such as IL-1beta, TNF-alpha, and receptor activator of NF-kappaB ligand. LPS stimulated the production of proinflammatory cytokines such as IL-1beta, TNF-alpha, and IL-6 in bone marrow macrophages and peritoneal macrophages, but not in osteoclasts. These results indicate that osteoclasts respond to LPS through TLR4, but the characteristics of osteoclasts are quite different from those of their precursors, macrophages, in terms of proinflammatory cytokine production in response to LPS.

New alternatively spliced form of galectin-3, a member of the beta-galactoside-binding animal lectin family, contains a predicted transmembrane-spanning domain and a leucine zipper motif

Osteoclasts or their precursors interact with the glycoprotein-enriched matrix of bone during extravasation from the vasculature, and upon attachment prior to resorption. Reverse transcriptase-PCR studies showed that two new alternatively spliced forms of chicken galectin-3, termed Gal-3TM1 and Gal-3TR1, were enriched and preferentially expressed in highly purified chicken osteoclast-like cells. Gal-3TM1 and Gal-3TR1 mRNA were also detected in chicken intestinal tissue, but not in kidney, liver, or lung. Gal-3TM1 and Gal-3TR1 messages both contain an open reading frame encoding a predicted 70-amino acid TM1 sequence inserted between the N-terminal Gly/Pro repeat domain and the carbohydrate recognition domain (exons 3 and 4). Gal-3TR1 mRNA contains an additional 241-bp sequence, which encodes a truncated open reading frame between the 4th and 5th exons, and, whose translation is expected to terminate within the carbohydrate recognition domain encompassing exons 4, 5, and 6. Immunoblotting and affinity chromatography showed that purified osteoclast preparations and intestinal homogenates contained a 36-kDa lactose-binding galectin. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analyses on chymotryptic peptides from the 36-kDa lectin confirmed its identity as Gal-3TM1. The TM1 insert contains a single transmembrane-spanning region and a leucine zipper-like stalk domain that is predicted to position the intact carbohydrate recognition domain of Gal-3TM1 on the exterior surface of the plasma membrane. Immunofluorescent staining of chicken osteoclasts confirmed the expression of Gal-3TM1 at the plasma membrane. Gal-3TM1 is the first example of a galectin superfamily member capable of being expressed as a soluble protein and as a transmembrane protein.

The generation of highly enriched osteoclast-lineage cell populations

Osteoclasts form when hematopoietic cells are stimulated by macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-kappaB ligand (RANKL) or tumor necrosis factor-alpha (TNFalpha). Osteoclast precursors derive from M-CSF-dependent proliferating hematopoietic cells but cannot yet be purified from mixed populations. M-CSF stimulation of bone marrow cells results in large numbers of nonadherent, proliferating macrophage precursors. These rapidly form adherent bone marrow macrophages (BMM). BMM and their precursors can be isolated free from mesenchymal and lymphocytic cells. BMM precursors derived from CBA-strain mouse bone marrow, when cocultured with ST2 cells (which express RANKL and M-CSF), formed numerous mononuclear osteoclasts, which resorbed bone and expressed tartrate-resistant acid phosphatase (TRAP) and calcitonin receptors (CTR). Addition of approximately 10 BMM precursors to ST2 cultures resulted in over 80% of these cocultures forming functional osteoclasts, suggesting that they are a highly enriched source of osteoclast progenitors. Supporting this, recombinant RANKL/M-CSF-stimulated BMM precursors formed populations in which all cells expressed TRAP. While only a small proportion of these cells (8.6%) expressed CTR, with transforming growth factor-beta (TGFbeta) present RANKL/M-CSF-stimulated BMM precursors formed almost pure (98.4%) CTR-positive osteoclasts after 7 days. This suggests that TGFbeta stimulated the maturation rate of these cells. Passaged or viably frozen BMM precursors gave rise to BMM that also all formed osteoclasts lineage cells after RANKL/M-CSF stimulation. These data suggest that BMM precursors derived from CBA mice are an expanded pool of osteoclast progenitors. These can be employed to generate osteoclast populations of high purity and in large numbers when stimulated by TGFbeta, which greatly augments the osteoclastogenic effects of RANKL.

The role of osteoclast differentiation in aseptic loosening

The major cause of orthopaedic implant loosening is thought to be accelerated osteoclastic bone resorption due to the action of cytokines produced in response to phagocytosis of implant-derived wear particles. This accelerated osteoclastic bone resorption could be due to increases in any of the following processes: recruitment of osteoclast precursors to the local microenvironment, differentiation of precursors into mature multinucleated osteoclasts. activation of mature osteoclasts, and/or survival of osteoclasts. Our studies have focused on differentiation and survival to complement work by others who have focused on recruitment of precursors and activation. Taken together, our studies and those of other investigators provide strong evidence that increased recruitment of osteoclast precursors and their subsequent differentiation play major roles in wear particle-induced osteolysis. In contrast, increased osteoclast activation and survival appear to play minor roles. These studies suggest that development of therapeutic interventions that reduce either recruitment or differentiation of osteoclast precursors would improve the performance of orthopaedic implants.

Cellular and humoral mechanisms of osteoclast formation and bone resorption in Gorham-Stout disease

Gorham-Stout disease (GSD) is a rare, massively osteolytic condition which is associated with increased vascularity and an increase in osteoclast numbers. To determine the cellular and humoral mechanisms underlying the increase in osteoclast numbers and osteolysis in GSD, this study analysed circulating osteoclast precursor numbers and sensitivity to osteoclastogenic factors in a GSD patient and age/sex-matched controls. Monocytes were cultured with M-CSF (25 ng/ml) and RANKL (30 ng/ml) and osteoclast formation was assessed in terms of the formation of TRAP(+) and VNR(+) multinucleated cells and the extent of lacunar resorption. There was no increase in the proportion of circulating osteoclast precursors in GSD relative to controls, but lacunar resorption was consistently greater in GSD monocyte cultures. Increased osteoclast formation in GSD was noted when monocytes were incubated with IL-1beta (1 ng/ml), IL-6/sIL-6R (100 ng/ml), and TNFalpha (10 ng/ml). An increase in osteoclast differentiation and bone resorption was also noted in control monocyte cultures in the presence of GSD serum. These results indicate that the increase in osteoclast formation in GSD is due not to an increase in the number of circulating osteoclast precursors, but rather to an increase in the sensitivity of these precursors to humoral factors which promote osteoclast formation and bone resorption.

Regulation of receptor activator of NF-kappaB ligand-induced tartrate-resistant acid phosphatase gene expression by PU.1-interacting protein/interferon regulatory factor-4. Synergism with microphthalmia transcription factor

The receptor activator of NF-kappaB ligand induces the expression of tartrate-resistant acid phosphatase (TRAP) and transcription factor, PU.1-interacting protein (Pip), during osteoclastogenesis. In this paper, we have examined the role of transcription factors in the regulation of TRAP gene expression employing reporter constructs containing the promoter region of TRAP gene. Transient transfection of RAW264 cells with sequential 5'-deletions of mouse TRAP gene promoter-luciferase fusion constructs indicated that at least two sites are required for the full promoter activity. Deletion and site-directed mutation studies revealed that M-box and interferon regulatory factor element sites are critical for TPAP gene expression in the cell, suggesting that microphthalmia transcription factor (MITF) and Pip could induce the gene expression independently. Moreover, the overexpression of MITF and Pip functionally stimulated TRAP promoter activity in a synergistic manner. Analysis of the deletion mutants of Pip protein indicated that both N-terminal DNA-binding and C-terminal regulatory domains are indispensable to the promoter-enhancing activity. Subcellular localization of green fluorescence protein-fused Pip and its mutant proteins indicated that the C-terminal domain is required for the translocation of Pip into the nucleus. These results suggest that Pip regulates and acts synergistically with MITF to induce the promoter activity of TRAP gene.

Osteoclasts differentiate from resident precursors in an in vivo model of synchronized resorption: a temporal and spatial study in rats

Osteoclasts differentiate from mononucleated precursors expressing monocyte markers, which gradually evolve to preosteoclasts expressing the osteoclast phenotype. Although the role of osteogenic cells in these changes has been well documented in vitro, their contribution in vivo has not been established. In this study, a synchronized wave of resorption was activated along the mandibular periosteum. The periosteum adjacent to the bone surface studied was separated by a computer-assisted technique into an osteogenic alkaline phosphatase-positive compartment and an outer nonosteogenic compartment. Specific markers (nonspecific esterase [NSE], tartrate-resistant acid phosphatase [TRAP], and ED1 antibody, a marker of the monocyte-macrophage lineage) were used to follow osteoclast differentiation quantitatively as a function of time after activation of resorption, from day 0 to day 4 (peak of resorption in this model). Local cell proliferation was assessed in parallel. Between day 0 and day 3, the thickness of the osteogenic compartment decreased by 50% (p < 0.0002). In the osteogenic compartment, proliferating cell numbers fell by 80% at 12 day, NSE(+) cells (located farthest from the bone surface) increased 3. 9-fold on day 4 vs. day 0 (p < 0.005), ED1(+) cells decreased between day 0 and day 2 (p < 0.02) before returning to their initial value, and TRAP(+) cells increased 2.7-fold between day 1 and day 3 (p < 0.0005). Resorption was absent in the site studied on day 0, but on day 4 there were 20.5 osteoclast nuclei per millimeter of bone surface. The cell ratio changed from 30.3 NSE(+) and ED1(+) (some of which were also TRAP(+)) cells per millimeter on day 0 to 37.6 mononucleated cells plus 20.5 osteoclast nuclei on day 4. In the nonosteogenic compartment, an entry of ED1(+)/NSE(-) was observed on 12 day (+23 cells, p < 0.02 vs. day 0). This was followed by a return of ED1(+) cell numbers to the control level on day 1, and a transient increase in NSE(+) cells (+47% on day 2 vs. day 1, p < 0.02). TRAP(+) cells were never seen in this compartment. Proliferating cell numbers did not change throughout the study. Our results strongly suggest that the osteoclasts present on day 4 differentiated from the pool of TRAP(+), ED1(+), and NSE(+) cells present at the site on day 0. The osteogenic compartment was gradually replenished by cells migrating from the nonosteogenic compartment, which was supplemented by ED1(+) cells recruited from the circulation early after activation. Moreover, osteogenic cells appeared to be as crucial in vivo for the acquisition of the TRAP phenotype as previously shown in vitro.

Involvement of p38 mitogen-activated protein kinase signaling pathway in osteoclastogenesis mediated by receptor activator of NF-kappa B ligand (RANKL)

The receptor activator of NF-kappaB ligand (RANKL) induces osteoclast differentiation from bone marrow cells in the presence of macrophage colony-stimulating factor. We found that treatment of bone marrow cells with SB203580 inhibited osteoclast differentiation via inhibition of the RANKL-mediated signaling pathway. To elucidate the role of p38 mitogen-activated protein (MAP) kinase pathway in osteoclastogenesis, we employed RAW264 cells which could differentiate into osteoclast-like cells following treatment with RANKL. In a dose-dependent manner, SB203580 but not PD98059, inhibited RANKL-induced differentiation. Among three MAP kinase families tested, this inhibition profile coincided only with the activation of p38 MAP kinase. Expression in RAW264 cells of the dominant negative form of either p38alpha MAP kinase or MAP kinase kinase (MKK) 6 significantly inhibited RANKL-induced differentiation of the cells. These results indicate that activation of the p38 MAP kinase pathway plays an important role in RANKL-induced osteoclast differentiation of precursor bone marrow cells.

Sequential requirements for SCL/tal-1, GATA-2, macrophage colony-stimulating factor, and osteoclast differentiation factor/osteoprotegerin ligand in osteoclast development

OBJECTIVE

Osteoclasts are of hematopoietic origin. The mechanism by which hematopoietic stem cells are specified to the osteoclast lineage is unclear. To understand the process of generation and differentiation of this lineage of cells, we performed in vitro studies on the differentiation of embryonic stem cells.

MATERIALS AND METHODS

We examined the potential of mutant embryonic stem cell lines harboring targeted deletions of the GATA-1, FOG, SCL/tal-1, or GATA-2 genes to differentiate into osteoclasts and determined when these molecules function in osteoclast development.

RESULTS

The lack of GATA-1 or FOG did not affect osteoclastogenesis. In contrast, SCL/tal-1-null embryonic stem cells generated no osteoclasts. In the case of the loss of GATA-2, a small number of osteoclasts were generated. GATA-2-null osteoclasts were morphologically normal and the terminal maturation was not disturbed, but a defect was observed in the generation of osteoclast progenitors. Experiments using specific inhibitors that block the signaling through macrophage colony-stimulating factor and osteoclast differentiation factor/osteoprotegerin ligand suggested that GATA-2 seems to act earlier in osteoclastogenesis than these cytokines. Interestingly, macrophage colony-forming units were not severely reduced by the loss of GATA-2 compared to osteoclast progenitors.

CONCLUSION

These results indicate that osteocalsts need SCL/tal-1 at an early point in development, and that GATA-2 is required for generation of osteoclast progenitors but not for the later stages when macrophage colony-stimulating factor and osteoclast differentiation factor/ osteoprotegerin ligand are needed. We also demonstrated that osteoclast progenitors behave as a different population than macrophage colony-forming units.

Fusomorphogenesis: cell fusion in organ formation

Cell fusion is a universal process that occurs during fertilization and in the formation of organs such as muscles, placenta, and bones. Very little is known about the molecular and cellular mechanisms of cell fusion during pattern formation. Here we review the dynamic anatomy of all cell fusions during embryonic and postembryonic development in an organism. Nearly all the cell fates and cell lineages are invariant in the nematode C. elegans and one third of the cells that are born fuse to form 44 syncytia in a reproducible and stereotyped way. To explain the function of cell fusion in organ formation we propose the fusomorphogenetic model as a simple cellular mechanism to efficiently redistribute membranes using a combination of cell fusion and polarized membrane recycling during morphogenesis. Thus, regulated intercellular and intracellular membrane fusion processes may drive elongation of the embryo as well as postembryonic organ formation in C. elegans. Finally, we use the fusomorphogenetic hypothesis to explain the role of cell fusion in the formation of organs like muscles, bones, and placenta in mammals and other species and to speculate on how the intracellular machinery that drive fusomorphogenesis may have evolved.

Bone loss. Factors that regulate osteoclast differentiation: an update

Osteoclast activation is a critical cellular process for pathological bone resorption, such as erosions in rheumatoid arthritis (RA) or generalized bone loss. Among many factors triggering excessive osteoclast activity, cytokines such as IL-1 or tumour necrosis factor (TNF)-alpha play a central role. New members of the TNF receptor ligand family (namely receptor activator of nuclear factor-kappa B [RANK] and RANK ligand [RANKL]) have been discovered whose cross-interaction is mandatory for the differentiation of osteoclasts from hemopoietic precursors, in both physiological and pathological situations. Osteoprotegerin, a decoy receptor which blocks this interaction, decreases osteoclast activity and could have a fascinating therapeutic potential in conditions associated with upregulated bone resorption.

Commitment and differentiation of osteoclast precursor cells by the sequential expression of c-Fms and receptor activator of nuclear factor kappaB (RANK) receptors

Osteoclasts are terminally differentiated cells derived from hematopoietic stem cells. However, how their precursor cells diverge from macrophagic lineages is not known. We have identified early and late stages of osteoclastogenesis, in which precursor cells sequentially express c-Fms followed by receptor activator of nuclear factor kappaB (RANK), and have demonstrated that RANK expression in early-stage of precursor cells (c-Fms(+)RANK(-)) was stimulated by macrophage colony-stimulating factor (M-CSF). Although M-CSF and RANKL (ligand) induced commitment of late-stage precursor cells (c-Fms(+)RANK(+)) into osteoclasts, even late-stage precursors have the potential to differentiate into macrophages without RANKL. Pretreatment of precursors with M-CSF and delayed addition of RANKL showed that timing of RANK expression and subsequent binding of RANKL are critical for osteoclastogenesis. Thus, the RANK-RANKL system determines the osteoclast differentiation of bipotential precursors in the default pathway of macrophagic differentiation.

Osteoclast-Derived Zinc Finger (OCZF) Protein With POZ Domain, a Possible Transcriptional Repressor, Is Involved in Osteoclastogenesis

The differentiation of osteoclasts is regulated by transcription factors expressed in cells of osteoclast lineage. We isolated here a potential transcription factor from a cDNA library of an enriched population of preosteoclasts and osteoclasts. The cDNA encodes a protein with N-terminal POZ domain and C-terminalKrüppel-like zinc fingers. We designate this protein as osteoclast-derived zinc finger (OCZF). OCZF was found to be rat homologue of mouse leukemia/lymphoma-related factor (LRF). Northern blot and in situ hybridization analysis showed OCZF mRNA at a high level in osteoclasts and kidney cells. OCZF had a nuclear targeting sequence and was localized in the nucleus of transfected cells. In addition, OCZF specifically bound to the guanine-rich consensus sequences of Egr-1 and c-Krox. Transient transfection assays indicate that OCZF can repress transcription activity like other POZ domain proteins. Furthermore, antisense but not sense phosphorothioate oligodeoxynucleotides (ODNs) for OCZF cDNA suppressed the formation of osteoclast-like multinucleated cells (MNCs) in bone marrow culture, whereas the same ODNs did not significantly affect the formation of macrophage polykaryons and mononuclear preosteoclast-like cells (POCs). These results suggest that OCZF is a unique transcription factor that plays an important role in the late stage of osteoclastogenesis.

Osteoclast-Derived Zinc Finger (OCZF) Protein With POZ Domain, a Possible Transcriptional Repressor, Is Involved in Osteoclastogenesis

The differentiation of osteoclasts is regulated by transcription factors expressed in cells of osteoclast lineage. We isolated here a potential transcription factor from a cDNA library of an enriched population of preosteoclasts and osteoclasts. The cDNA encodes a protein with N-terminal POZ domain and C-terminalKrüppel-like zinc fingers. We designate this protein as osteoclast-derived zinc finger (OCZF). OCZF was found to be rat homologue of mouse leukemia/lymphoma-related factor (LRF). Northern blot and in situ hybridization analysis showed OCZF mRNA at a high level in osteoclasts and kidney cells. OCZF had a nuclear targeting sequence and was localized in the nucleus of transfected cells. In addition, OCZF specifically bound to the guanine-rich consensus sequences of Egr-1 and c-Krox. Transient transfection assays indicate that OCZF can repress transcription activity like other POZ domain proteins. Furthermore, antisense but not sense phosphorothioate oligodeoxynucleotides (ODNs) for OCZF cDNA suppressed the formation of osteoclast-like multinucleated cells (MNCs) in bone marrow culture, whereas the same ODNs did not significantly affect the formation of macrophage polykaryons and mononuclear preosteoclast-like cells (POCs). These results suggest that OCZF is a unique transcription factor that plays an important role in the late stage of osteoclastogenesis.

Characterization of immortalized osteoclast precursors developed from mice transgenic for both bcl-X(L) and simian virus 40 large T antigen

We recently developed an immortalized osteoclast (OCL) precursor cell line that forms large numbers of OCLs. This cell line was derived from mice doubly transgenic for bcl-X(L) and large T antigen that was targeted to cells in the OCL lineage (bcl-X(L)/Tag cells). We have now characterized these cells in terms of their surface and enzymatic phenotype, responsiveness to osteotropic factors, and differentiation potential. The bcl-X(L)/Tag cells expressed interleukin-1 receptors 1 and 2, gelatinase B (MMP9), as well as Mac-1, CD16/CD32 (Fcgamma receptors), CD45.2 (common leukocyte marker), CD86 (costimulatory molecule expressed on B cells, follicular dendritic cells, and thymic epithelium), major histocompatibility complex I, and nonspecific esterase when cocultured with MC3T3E1 cells. However, they did not express the antigens for F4/80 (mature macrophage/dendritic cell marker) by immunostaining. Treatment of bcl-X(L)/Tag cells, cocultured with MC3T3E 1 cells, with the combination of 1,25-dihydroxyvitamin D3 and dexamethasone induced high levels of OCL formation. The bcl-X(L)/Tag cells formed large numbers of OCLs when cultured with RANK ligand and macrophage colony-stimulating factor in the absence of feeder cells. In the absence of RANK ligand and a feeder cell layer, 100% of the cells differentiated into F4/80-positive cells. However, neither PTH nor PTH-related protein enhanced OCL formation by bcl-X(L)/Tag cells even when they were cocultured with primary osteoblasts, suggesting that they differ from primary mouse bone marrow cells in their responsiveness to PTH/PTH-related protein. Thus, bcl-X(L)/Tag cells have many of the properties of primary mouse OCL precursors and should be very useful for studies of OCL differentiation and divergence of OCL precursors from the macrophage lineage.

CD9 molecule expressed on stromal cells is involved in osteoclastogenesis

Osteoclasts are derived from hematopoietic stem cells and their development is dependent on the products of stromal cells. CD9, a member of the tetraspan transmembrane-superfamily, is expressed on both hematopoietic cells and stromal cells. Addition of antagonistic rat anti-mouse CD9 antibody (KMC8.8) to cultures inhibited osteoclastogenesis on established stromal cell layers. When rat bone marrow cells depleted of adherent stromal cells were cultured on mouse stromal cells, numerous tartrate-resistant acid phosphatase-positive multinuclear cells were observed, and KMC8.8, which recognizes mouse but not rat CD9, completely prevented the generation of osteoclasts, suggesting that the CD9 expressed on the stromal cell is essential for osteoclastogenesis. Possibly for the same reason, KMC8.8 pretreatment of the mouse macrophage-like cell line C7, which is able to differentiate into mature osteoclasts, did not inhibit subsequent C7 cell differentiation, whereas the addition of KMC8.8 to cocultures of C7 cells with stromal cells inhibited the differentiation of C7 cells into osteoclasts. Moreover, we found that blockage of a signal via CD9 on stromal cells reduced transcription of the osteoclast differentiation factor (Odf) gene, which, together with macrophage colony-stimulating factor, is essential for osteoclastogenesis. These results revealed that CD9 molecules on stromal cells play a critical role in osteoclast development, possibly by modulating the expression of Odf.

Stable association of PYK2 and p130(Cas) in osteoclasts and their co-localization in the sealing zone

Bone resorption is initiated by osteoclast attachment to the mineralized matrix, cytoskeletal reorganization, cellular polarization, and the formation of the sealing zone. The present study examines the interaction between PYK2 and p130(Cas) (Crk-associated substrate), suggested to be part of the signaling pathway initiated by osteoclast adhesion. Using murine osteoclast-like cells (OCLs) and their mononuclear precursors (pOCs), generated in a co-culture of bone marrow and osteoblastic MB1.8 cells, we show that: 1) p130(Cas) is tyrosine-phosphorylated upon adhesion of pOCs to vitronectin or ligation of beta3 integrins; 2) p130(Cas) colocalizes with PYK2 and the cytoskeletal proteins F-actin, vinculin, and paxillin in the podosomal-rich ring-like structures of OCLs plated on glass and in the sealing zone in actively resorbing OCLs on bone; 3) p130(Cas) and PYK2 form a stable complex in pOCs, independent of tyrosine phosphorylation of either molecule, and this complex is present in Src (-/-) OCLs, in which neither protein is phosphorylated or associated with the osteoclast adhesion structure; 4) the association of p130(Cas) and PYK2 is mediated by the SH3 domain of p130(Cas) and the C-terminal domain of PYK2. These findings suggest that p130(Cas) and its association with PYK2 may play an important role in the adhesion-dependent signaling that leads to cytoskeletal reorganization and formation of the sealing zone during osteoclast activation.