Purification and characterization of fully functional human osteoclast precursors

Fabrication, vascularization and osteogenic properties of a novel synthetic biomimetic induced membrane for the treatment of large bone defects

The induced membrane has been widely used in the treatment of large bone defects but continues to be limited by a relatively lengthy healing process and a requisite two stage surgical procedure. Here we report the development and characterization of a synthetic biomimetic induced membrane (BIM) consisting of an inner highly pre-vascularized cell sheet and an outer osteogenic layer using cell sheet engineering. The pre-vascularized inner layer was formed by seeding human umbilical vein endothelial cells (HUVECs) on a cell sheet comprised of a layer of undifferentiated human bone marrow-derived mesenchymal stem cells (hMSCs). The outer osteogenic layer was formed by inducing osteogenic differentiation of hMSCs. In vitro results indicated that the undifferentiated hMSC cell sheet facilitated the alignment of HUVECs and significantly promoted the formation of vascular-like networks. Furthermore, seeded HUVECs rearranged the extracellular matrix produced by hMSC sheet. After subcutaneous implantation, the composite constructs showed rapid vascularization and anastomosis with the host vascular system, forming functional blood vessels in vivo. Osteogenic potential of the BIM was evidenced by immunohistochemistry staining of osteocalcin, tartrate-resistant acid phosphatase (TRAP) staining, and alizarin red staining. In summary, the synthetic BIM showed rapid vascularization, significant anastomoses, and osteogenic potential in vivo. This synthetic BIM has the potential for treatment of large bone defects in the absence of infection.

MiR-126-5p regulates osteoclast differentiation and bone resorption in giant cell tumor through inhibition of MMP-13

Giant cell tumor (GCT) of bone is an aggressive skeletal tumor characterized by localized bone resorption. Matrix metalloproteinase-13 (MMP-13) is the principal proteinase expressed by the stromal cells of GCT (GCTSCs) and also considered to play a crucial role in formation of the osteolytic lesion in GCT. However, the exact mechanism of the regulation of MMP-13 expression in GCTSCs was unknown. In this study, we identified miR-126-5p was significantly downregulated in GCTSCs and affect osteoclast (OC) differentiation and bone resorption by repressing MMP-13 expression at the post-transcriptional level. Thus, our studies show that miR-126-5p plays an important physiological role in multinucleated giant cell formation and osteolytic lesion in GCT.

Role of Wnt signaling in the biology of the periodontium

Continuously erupting teeth have associated with them a continuously regenerating periodontal ligament, but the factors that control this amazing regenerative potential are unknown. We used genetic strategies to show that the periodontal ligament arises from the cranial neural crest. Despite their histological similarity, the periodontal ligament of continuously erupting incisor teeth differs dramatically from the periodontal ligament of molar teeth. The most notable difference was in the distribution of Wnt responsive cells in the incisor periodontal ligament, which coincided with regions of periodontal ligament cell proliferation. We discuss these findings in the context of dental tissue regeneration.

Role of protein-tyrosine phosphatases in regulation of osteoclastic activity

Osteoclasts, the primary cell type mediating bone resorption, are multinucleated, giant cells derived from hematopoietic cells of monocyte-macrophage lineage. Osteoclast activity is, in a large part, regulated by protein-tyrosine phosphorylation. While information about functional roles of several protein-tyrosine kinases (PTK), including c-Src, in osteoclastic resorption has been accumulated, little is known about the roles of protein-tyrosine phosphatases (PTPs) in regulation of osteoclast activity. Recent evidence implicates important regulatory roles for four PTPs (SHP-1, cyt-PTP-epsilon, PTP-PEST, and PTPoc) in osteoclasts. Cyt-PTP-epsilon, PTP-PEST, and PTP-oc are positive regulators of osteoclast activity, while SHP-1 is a negative regulator. Of these PTPs in osteoclasts, only PTP-oc is a positive regulator of c-Src PTK through dephosphorylation of the inhibitory phosphotyrosine-527 residue. Although some information about mechanisms of action of these PTPs to regulate osteoclast activity is reviewed in this article, much additional work is required to provide more comprehensive details about their functions in osteoclasts.

Collagenase expression and activity in the stromal cells from giant cell tumour of bone

The characteristic bone destruction in giant cell tumour of bone (GCT) is largely attributed to the osteoclast-like giant cells. However, experimental analyses of bone resorption by cells from GCT often fail to exclude the neoplastic spindle-like stromal cells, and several studies have demonstrated that bone resorption by GCT cells is increased in the presence of stromal cells. The spindle-like stromal cells from GCT may therefore actively contribute to the bone resorption observed in the tumour. Type I collagen, a major organic constituent of bone, is effectively degraded by three matrix metalloproteinases (MMPs) known as the collagenases: MMP-1, MMP-8 and MMP-13. We established primary cell cultures from nine patients with GCT and the stromal cell populations were isolated in culture. The production of collagenases by primary cultures of GCT stromal cells was determined through real-time PCR, western blot analysis and a multiplex assay system. Results show that the cells produce MMP-1 and MMP-13 but not MMP-8. Immunohistochemistry confirmed the presence of MMP-1 and MMP-13 in paraffin-embedded GCT tissue samples. Medium conditioned by the stromal cell cultures was capable of proteolytic activity as determined by MMP-1 and MMP-13-specific standardized enzyme activity assays. The spindle-like stromal cells from GCT may therefore actively participate in the bone destruction that is characteristic of the tumour.

RANK Expression as a cell surface marker of human osteoclast precursors in peripheral blood, bone marrow, and giant cell tumors of bone

RANK expression in vivo on hematopoietic subsets including pre-osteoclasts, identified by monoclonal antibodies, has not been described. We describe the lineages that express RANK in bone marrow, peripheral blood, and GCTs. We show that CD14(+)RANK(high) cells constitute a circulating pre-osteoclast pool.

INTRODUCTION

The expression of RANK by subsets of hematopoietic cells has not been adequately studied in humans. While attributed to the monocytoid lineage, the phenotype of the pre-osteoclast (pre-OC) with respect to RANK expression in vivo remains unclear. We tested monoclonal antibodies (MAbs) raised against the extracellular domain of recombinant human RANK for reactivity with normal peripheral blood (PB) and bone marrow (BM) mononuclear cells (PBMNCs and BMMNCs, respectively). We also tested reactivity with giant cell tumor cells (GCT), a confirmed source of pre-OC and mature OCs.

MATERIALS AND METHODS

Human PBMNCs, BMMNCs, and GCT cells were analyzed for reactivity with anti-RANK MAbs by flow cytometry in combination with hematopoietic lineage restricted markers. GCTs were also analyzed by immunofluorescence. CD14+ monocytoid cells were sorted by fluorescence-activated cell sorting (FACS) based on their relative RANK expression and cultured under OC-forming conditions.

RESULTS

RANK+ cells were detected similarly by three independent anti-RANK MAbs. One MAb (80736) immunoprecipitated RANK-RANKL complexes from surface-biotinylated GCT lysates. Using dual-color flow cytometry, RANK was detected on CD14+ (monocytoid), CD19+ (B-lymphoid), CD56+ (NK cell), and glycophorin A+ erythroid progenitors. Minor populations of both CD3+ T lymphocytes and BM CD34+ hematopoietic progenitors also expressed cell surface RANK. In GCTs, RANK expression was identified on mononuclear CD45(+)CD14(+)alphaVbeta3(+)c-Fms+ cells, likely to be committed pre-OC, and on multinucleated CD45(+)alphaVbeta3(+)TRACP(+) OCs. Importantly, sorted CD14(+)RANK(high) PBMNCs treated with recombinant RANKL and macrophage-colony stimulating factor (M-CSF) gave rise to approximately twice the number of osteoclasts than RANK(mid) or RANK(low) cells.

CONCLUSIONS

These results suggest that committed monocytoid RANK+ pre-OCs are represented in the marrow and circulate in the periphery, forming a pool of cells capable of responding rapidly to RANKL. The ability to reliably detect committed pre-OC in peripheral blood could have important clinical applications in the management of diseases characterized by abnormal osteoclastic activity.

cAMP-PKA signaling pathway regulates bone resorption mediated by processing of cathepsin K in cultured mouse osteoclasts

Cathepsin K (Cat K) is the major cysteine protease expressed in osteoclast and is thought to play a key role in matrix degradation during bone resorption. It is shown that the intracellular maturation of Cat K was prevented by the cAMP antagonist, Rp-cAMP, and the protein kinase A (PKA) inhibitors of KT5720 and H89. In contrast, forskolin, an adenylate cyclase agonist, rather induced Cat K processing and maturation in osteoclast. Furthermore, to determine whether Cat K processing and maturation signaling involves protein kinase C (PKC), mouse total bone cells were treated with calphostin C, a specific inhibitor of PKC, however, no effect was observed, indicating that PKC calphostin C did not affect to osteoclast-mediated Cat K processing and maturation in osteoclast. Thus, it is indicated that the cAMP-PKA signaling pathway regulate Cat K maturation in osteoclast. Since secreted proenzymes have the potential to reenter the cell via M6P receptor, to prevent this possibility, we tested cAMP antagonist Rp-cAMP and the PKA inhibitors KT5720 and H89 in the absence or presence of M6P. Inhibition of Cat K processing by Rp-cAMP, KT5720 or H89 was observed in a dose-dependent manner. Furthermore, the addition of M6P resulted in enhanced potency of Rp-cAMP, KT5720 and H89, which dose-dependently inhibited in vitro bone resorption with potency similar to that observed for inhibition of Cat K processing.

Cell lines and primary cell cultures in the study of bone cell biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. The major cell types of bone are osteoblasts, osteoclasts and chondrocytes. In the laboratory, primary cultures or cell lines established from each of these different cell types provide valuable information about the processes of skeletal development, bone formation and bone resorption, leading ultimately, to the formulation of new forms of treatment for common bone diseases such as osteoporosis.

Osteoclast-like cell formation by circulating myeloma B lymphocytes: role of RANK-L

Excessive bone resorption in multiple myeloma (MM), a malignancy of B lymphoid origin, is mediated through osteoclasts, which respond to local osteoclast-activating factors produced by tumor cells within the bone marrow microenvironment. Direct bone resorption by myeloma cells is investigated in the present study, since a connection between B lymphocytes and osteoclast differentiation pathways has been recently postulated in mice. Peripheral CD19+ B lymphocytes isolated from 10 myeloma patients with multiple osteolytic lesions and 10 healthy donors were cultured in the presence of M-CSF and RANK-L, two major osteoclast-activating factors. The TRAP expression and resorption of bone substrates were employed to evaluate osteoclast differentiation. MM patients were characterized by the presence of circulating B lymphocytes endowed with both phenotypical and functional properties of osteoclast-like cells in vitro when stimulated with RANK-L. The absence of these characteristics in B lymphocytes from healthy donors indicates that the transformation can be ascribed to the presence of clonogenic B cells in patients with MM. Clonotypic B lymphocytes may contribute to the pathogenesis of bone disease in MM by acting as RANK-L-dependent osteoclast progenitors.

Isolation of a human homolog of osteoclast inhibitory lectin that inhibits the formation and function of osteoclasts

Osteoclast inhibitory lectin (OCIL) is a newly recognized inhibitor of osteoclast formation. We identified a human homolog of OCIL and its gene, determined its regulation in human osteoblast cell lines, and established that it can inhibit murine and human osteoclast formation and resorption. OCIL shows promise as a new antiresorptive.

INTRODUCTION

Murine and rat osteoclast inhibitory lectins (mOCIL and rOCIL, respectively) are type II membrane C-type lectins expressed by osteoblasts and other extraskeletal tissues, with the extracellular domain of each, expressed as a recombinant protein, able to inhibit in vitro osteoclast formation.

MATERIALS AND METHODS

We isolated the human homolog of OCIL (hOCIL) from a human fetal cDNA library that predicts a 191 amino acid type II membrane protein, with the 112 amino acid C-type lectin region in the extracellular domain having 53% identity with the C-type lectin sequences of rOCIL and mOCIL. The extracellular domain of hOCIL was expressed as a soluble recombinant protein in E. coli, and its biological effects were determined.

RESULTS AND CONCLUSIONS

The hOCIL gene is 25 kb in length, comprised of five exons, and is a member of a superfamily of natural killer (NK) cell receptors encoded by the NK gene complex located on chromosome 12. Human OCIL mRNA expression is upregulated by interleukin (IL)-1alpha and prostaglandin E2 (PGE2) in a time-dependent manner in human osteogenic sarcoma MG63 cells, but not by dexamethasone or 1,25 dihydroxyvitamin D3. Soluble recombinant hOCIL had biological effects comparable with recombinant mOCIL on human and murine osteoclastogenesis. In addition to its capacity to limit osteoclast formation, OCIL was also able to inhibit bone resorption by mature, giant-cell tumor-derived osteoclasts. Thus, a human homolog of OCIL exists that is highly conserved with mOCIL in its primary amino acid sequence (C-lectin domain), genomic structure, and activity to inhibit osteoclastogenesis.

Guidelines for using in vitro methods to study the effects of phyto-oestrogens on bone

These guidelines review the relevant literature on the way plant phyto-oestrogens act on bone and the responsiveness of different bone cell systems to phyto-oestrogenic compounds. The primary emphasis is on the experimental conditions used, the markers available for assessing osteoblast and osteoclast function, and their expected sensitivity. Finally, we assess the published results to derive some general recommendations for in vitro experiments in this area of research.

Tissue culture models for studies of hormone and vitamin action in bone cells

Osteoporosis is a major health care concern and levies a serious financial burden on the world health care system. For this reason, many physicians and scientists are engaged in research to better understand and treat this disease. To this end, numerous in vitro bone cell models have been developed to explore the cellular and molecular mechanisms of skeletal biology and for the identification and characterization of new drug targets and therapies. In this chapter, we review many of these cellular models as tools to study the hormonal regulation of bone metabolism. In particular, we pay special attention to new human bone cell models, since these have the greatest relevance to osteoporosis research and drug discovery. These new models include (1) the use of peripheral blood mononuclear cells as progenitors of osteoclasts and primary cultures of mesenchymal stem cells as precursors of osteoblasts; (2) the development of conditionally immortalized preosteoclastic and osteoblastic cell lines using temperature-sensitive large T-antigens; and (3) the establishment of the first osteocytic cell lines. Thus, we now have at our disposal many good in vitro models to investigate the regulation of bone resorption and formation by hormones, vitamins and drugs. These models should accelerate our understanding of bone physiology and pathophysiology as well as our ability to develop important new therapies to prevent and treat skeletal diseases.

Beta1 and beta2 integrins mediate adhesion during macrophage fusion and multinucleated foreign body giant cell formation

An in vitro system of interleukin (IL)-4-induced human monocyte-derived macrophage fusion was used to investigate the cell/substrate adhesive mechanisms that support multinucleated foreign body giant cell (FBGC) formation. Monocytes were cultured for 3 days and IL-4 was added to induce macrophage fusion and FBGC formation by day 7. Functionally defined anti-integrin antibodies demonstrated that initial monocyte adhesion is mediated by beta2 integrins, whereas during the induction of macrophage fusion by IL-4, an additional dependence on beta1 integrins is acquired. The combination of anti-beta1 plus anti-beta2 was most effective, reducing macrophage/FBGC adhesion to 10% of controls. Consistent with integrin-mediated signaling, the tyrosine kinase inhibitor genistein and the phosphatidylinositol-3-kinase inhibitors wortmannin and LY294002 also attenuated macrophage/FBGC adhesion. Confocal microscopic analysis revealed that beta2 integrins are present on monocytes after initial adhesion and are strongly expressed on fusing macrophages, particularly in peripheral cell areas, and on FBGCs. In contrast, beta1 integrins are not detected on monocytes but begin to appear during macrophage development and are strongly expressed on fusing macrophages and FBGCs. For the first time, these results demonstrate the IL-4-induced acquisition of cooperation between beta1 and beta2 integrins in the cell/substrate adhesive interactions that are required for multinucleated FBGC formation.

Gene expression of glucocorticoid receptor alpha and beta in giant cell tumour of bone: evidence of glucocorticoid-stimulated osteoclastogenesis by stromal-like tumour cells

Glucocorticoids have been shown to increase bone resorption in vitro and in vivo, however, the mechanism(s) of this action are not fully understood. Given that human giant cell tumour of bone (GCT) is considered to arise from mesenchymal stromal cells and has the capacity to recruit and harbour macrophages and multinucleated osteoclasts, we have used GCT as a model for studying the effect of glucocorticoids on osteoclast formation. We have demonstrated, by RT-PCR and fluorescence in-situ hybridisation, that both glucocorticoid receptor alpha and beta (GRalpha and GRbeta) gene transcripts were present in the stromal-like tumour cells, macrophage-like cells (putative osteoclast precursors) and multinucleated osteoclast-like cells. Moreover, in the presence of 1,25(OH)(2)D(3), dexamethasone dose-dependently stimulated the formation of osteoclast-like cells from GCT-derived co-culture system of stromal-like tumour cells and macrophage-like cells. The stimulation of osteoclastogenesis by dexamethasone was coincident with the up-regulation of receptor activator of NF-kappaB ligand (RANKL) but down-regulation of osteoprotegerin (OPG) gene expression in stromal-like tumour cells. These data are consistent with the hypothesis that glucocorticoids increase bone resorption by promoting osteoclastogenesis which is at least in part due to the stimulation of RANKL and inhibition of OPG production in bone stromal cells.

Potent and selective cathepsin L inhibitors do not inhibit human osteoclast resorption in vitro

Cathepsins K and L are related cysteine proteases that have been proposed to play important roles in osteoclast-mediated bone resorption. To further examine the putative role of cathepsin L in bone resorption, we have evaluated selective and potent inhibitors of human cathepsin L and cathepsin K in an in vitro assay of human osteoclastic resorption and an in situ assay of osteoclast cathepsin activity. The potent selective cathepsin L inhibitors (K(i) = 0.0099, 0.034, and 0.27 nm) were inactive in both the in situ cytochemical assay (IC(50) > 1 micrometer) and the osteoclast-mediated bone resorption assay (IC(50) > 300 nm). Conversely, the cathepsin K selective inhibitor was potently active in both the cytochemical (IC(50) = 63 nm) and resorption (IC(50) = 71 nm) assays. A recently reported dipeptide aldehyde with activity against cathepsins L (K(i) = 0.052 nm) and K (K(i) = 1.57 nm) was also active in both assays (IC(50) = 110 and 115 nm, respectively) These data confirm that cathepsin K and not cathepsin L is the major protease responsible for human osteoclastic bone resorption.

Osteoprotegerin inhibits osteoclast formation and bone resorbing activity in giant cell tumors of bone

Osteolysis is a common complication of tumors that arise in, or metastasize to, bone. The recent discovery of key regulators of osteoclast formation and activity, including receptor activator of nuclear factor of kappaB ligand (RANKL), RANK, and osteoprotegerin (OPG), may facilitate new treatment regimes for certain tumors associated with excessive bone loss. We recently showed that the stromal cells of osteolytic giant cell tumors (GCT) of bone express high levels of mRNA encoding RANKL, relative to mRNA for the RANKL antagonist, OPG, compared with the expression patterns of other lytic and nonlytic bone tumors. In this study, we found that expression of RANKL and OPG mRNA continued by the stromal element of these tumors in a constitutive manner for at least 9 days in the absence of giant cells. Immunostaining of unfractionated GCT cultured in vitro revealed punctate cytoplasmic/membranous staining for RANKL and both cytoplasmic and extracellular matrix staining for OPG in stromal cells. Giant cells (osteoclasts) were negative for RANKL staining, but stained brightly for cytoplasmic OPG protein. We also investigated the functional relevance of these molecules for GCT osteolysis by adding recombinant OPG and RANKL to cultured GCT cells. We found that OPG treatment potently and dose-dependently inhibited resorption of bone slices by GCT, and could also inhibit the formation of multinucleated osteoclasts from precursors within the GCT. These effects of OPG were reversed by stoichiometric concentrations of exogenous RANKL. These data indicate that both the processes of osteoclast formation and activation in GCT are promoted by RANKL. Therefore, GCT represent a paradigm for the direct stimulation of osteoclast formation and activity by tumor stromal cells, in contrast to the mechanisms described for osteolytic breast tumors and multiple myeloma. The demonstration of these relationships is important in developing approaches to limit tumor-induced osteolysis.

Biosynthesis and processing of cathepsin K in cultured human osteoclasts

Cathepsin K (cat K) is the major cysteine protease expressed in osteoclasts and is thought to play a key role in matrix degradation during bone resorption. However, little is known regarding the synthesis, activation, or turnover of the endogenous enzyme in osteoclasts. In this study, we show that mature cat K protein and enzyme activity are localized within osteoclasts. Pulse-chase experiments revealed that, following the synthesis of pro cat K, intracellular conversion to the mature enzyme occurred in a time-dependent manner. Subsequently, the level of mature enzyme decreased. Little or no cat K was observed in the culture media at any timepoint. Pretreatment of osteoclasts with either chloroquine or monensin resulted in complete inhibition of the processing of newly synthesized cat K. In addition, pro cat K demonstrated susceptibility to treatment with N-glycosidase F, suggesting the presence of high-mannose-containing oligosaccharides. Treatment of osteoclasts with the PI3-kinase inhibitor, Wortmannin (WT), not only prevented the intracellular processing of cat K but also resulted in the secretion of proenzyme into the culture media. Taken together, these results suggest that the biosynthesis, processing, and turnover of cat K in human osteoclasts is constitutive and occurs in a manner similar to that of other known cysteine proteases. Furthermore, cat K is not secreted as a proenzyme, but is processed intracellularly, presumably in lysosomal compartments prior to the release of active enzyme into the resorption lacunae.

CKbeta-8 [CCL23], a novel CC chemokine, is chemotactic for human osteoclast precursors and is expressed in bone tissues

We have previously demonstrated that a tartrate-resistant acid phosphatase (TRAP)-positive subpopulation of mononuclear cells isolated from collagenase digests of human osteoclastoma tissue exhibits an osteoclast phenotype and can be induced to resorb bone. Using these osteoclast precursors as a model system, we have assessed the chemotactic potential of 16 chemokines. Three CC chemokines, the recently described CKbeta-8, RANTES, and MIP-1alpha elicited significant chemotactic responses. In contrast, 10 other CC chemokines (MIP-1beta, MCP-1, MCP-2, MCP-3, MCP-4, HCC-1, eotaxin-2, PARC, SLC, ELC) and 3 CXC chemokines (IL-8, GROalpha, SDF-1) were inactive. None of these chemokines showed any chemotactic activity for either primary osteoblasts derived from human bone explants or the osteoblastic MG-63 cell line. The identity of the osteoclast receptor that mediates the chemotactic response remains to be established. However, all three active chemokines have been reported to bind to CCR1 and cross-desensitization studies demonstrate that RANTES and MIP-1alpha can partially inhibit the chemotactic response elicited by CKbeta-8. CKbeta-8, the most potent of the active CC chemokines (EC(max) 0.1-0.3 nM), was further characterized with regard to expression in human bone and cartilage. Although expression is not restricted to these tissues, CKbeta-8 mRNA was shown to be highly expressed in osteoblasts and chondrocytes in human fetal bone by in situ hybridization. In addition, CKbeta-8 protein was shown to be present in human osteophytic tissue by immunolocalization. These observations suggest that CKbeta-8, and perhaps other chemokines, may play a role in the recruitment of osteoclast precursors to sites of bone resorption.

Expression of osteoclast differentiation signals by stromal elements of giant cell tumors

The mechanisms by which primary tumors of the bone cause bone destruction have not been elucidated. Unlike most other lytic bone tumors, osteoclastomas, otherwise known as giant cell tumors (GCT), contain osteoclast-like cells within the tumor stroma. A new member of the TNF-ligand superfamily member, osteoclast differentiation factor (ODF/OPGL/RANKL/TRANCE), was recently identified. ODF was shown to directly stimulate osteoclastogenesis, in the presence of M-CSF. In this study, the expression of ODF was examined in a number of tumor samples associated with bone lysis in vivo. In addition, we investigated expression of the ODF receptor on osteoclast precursors, RANK, as well as the ODF inhibitor osteoprotegerin (OPG), and another TNF-ligand superfamily member, TRAIL, previously shown to abrogate the inhibitory effects of OPG. We report here the novel finding that GCT stromal cells contain abundant ODF mRNA, whereas the giant cell population exclusively expresses RANK mRNA. These results are consistent with the osteoclast-mediated bone destruction by these tumors. We also report the expression of OPG and TRAIL mRNA in GCT samples. A comparison with other lytic and nonlytic tumors of bone showed that GCT express more ODF and TRAIL mRNA relative to OPG mRNA. In addition, GCT were found to express a number of cytokines previously reported to play central roles in osteoclastogenesis, namely, IL-1, -6, -11, -17, as well as TNF-alpha. Importantly, GCT were also found to express high levels of M-CSF mRNA, a cytokine shown to be an essential cofactor of ODF, and a survival factor for mature and developing osteoclasts. Furthermore, expression of these molecules by stromal cells isolated from GCT continued in vitro. Thus GCT constitutively express all of the signals that are currently understood to be necessary for the differentiation of osteoclasts from precursor cells.

Development and characterization of a human in vitro resorption assay: demonstration of utility using novel antiresorptive agents

A human in vitro resorption assay has been developed using osteoclastoma-derived osteoclasts and used to evaluate novel antiresorptive agents including antagonists of the alphavbeta3 integrin, and inhibitors of cathepsin K and the osteoclast ATPase. The potency of novel compounds in the in vitro resorption assay correlates with functional assays for each class of inhibitor: the human alphavbeta3-mediated cell adhesion assay for the vitronectin receptor antagonists (r2 = 0.82), the chick osteoclast vacuolar ATPase enzyme assay for the H+-ATPase inhibitors (r2 = 0.77) and the recombinant human cathepsin K enzyme assay for the cathepsin K inhibitors (r2 = 0.80). Cell suspensions, rich in osteoclasts, are prepared by collagenase digestion of the tumor tissue. These cells can be stored long-term in liquid nitrogen and upon thawing maintain their bone-resorbing phenotype. The cryopreserved cells can be cultured on bovine cortical bone for 24-48 h and resorption can be measured by either confocal microscopy or biochemical assays. The resorptive activity of osteoclasts derived from a number of tumors can be inhibited reproducibly using a number of mechanistically unique antiresorptive compounds. In addition, the measurement of resorption pits by laser confocal microscopy correlates with the release of type I collagen C-telopeptides or N-telopeptides, as measured by enzyme-linked immunosorbent assay. Resorption can be measured reproducibly using a 48-h incubation of osteoclasts on bone slices, or a 24-h incubation with bone particles. This in vitro human osteoclast resorption assay provides a robust system for the evaluation of inhibitors of osteoclastic function that may be developed for the treatment of metabolic bone diseases such as osteoporosis.

Human mesenchymal stem cells promote human osteoclast differentiation from CD34+ bone marrow hematopoietic progenitors

Interactions between osteoclast progenitors and stromal cells derived from mesenchymal stem cells (MSCs) within the bone marrow are important for osteoclast differentiation. In vitro models of osteoclastogenesis are well established in animal species; however, such assays do not necessarily reflect human osteoclastogenesis. We sought to establish a reproducible coculture model of human osteoclastogenesis using highly purified human marrow-derived MSCs (hMSCs) and CD34+ hematopoietic stem cells (HSCs). After 3 weeks, coculture of hMSCs and HSCs resulted in an increase in hematopoietic cell number with formation of multinucleated osteoclast-like cells (Ocls). Coculture of hMSCs with HSCs, transduced with a retroviral vector that expresses enhanced green fluorescent protein, produced enhanced green fluorescent protein+ Ocls, further demonstrating that Ocls arise from HSCs. These Ocls express calcitonin and vitronectin receptors and tartrate-resistant acid phosphatase and possess the ability to resorb bone. Ocl formation in this assay is cell contact dependent and is independent of added exogenous factors. Conditioned medium from the coculture contained high levels of interleukin (IL)-6, IL-11, leukemia inhibitory factor (LIF), and macrophage-colony stimulating factor. IL-6 and LIF were present at low levels in cultures of hMSCs but undetectable in cultures of HSCs alone. These data suggest that coculture with HSCs induce hMSCs to secrete cytokines involved in Ocl formation. Addition of neutralizing anti-IL-6, IL-11, LIF, or macrophage-colony stimulating factor antibodies to the coculture inhibited Ocl formation. hMSCs seem to support Ocl formation as undifferentiated progenitor cells, because treatment of hMSCs with dexamethasone, ascorbic acid, and beta-glycerophosphate (to induce osteogenic differentiation) actually inhibited osteoclastogenesis in this coculture model. In conclusion, we have developed a simple and reproducible assay using culture-expanded hMSCs and purified HSCs with which to study the mechanisms of human osteoclastogenesis.

Expression of cathepsin K messenger RNA in giant cells and their precursors in human osteoarthritic synovial tissues

OBJECTIVE

To investigate the expression of cathepsin K messenger RNA (mRNA) in the giant cells found in human osteoarthritic (OA) synovium and associated reparative connective tissues, and to compare this with mRNA expression of cathepsins B, L, and S, which are cysteine proteases known to be highly expressed by cells of the monocyte/macrophage lineage.

METHODS

Sections of human OA synovium were processed for in situ hybridization and probed for cathepsins K, B, L, and S. Serial sections were reacted for tartrate-resistant acid phosphatase (TRAP) and nonspecific esterase (NSE) activity, which are selective markers for the osteoclast and cells of the macrophage/monocyte lineage, respectively.

RESULTS

At 3 sites of monocyte infiltration/giant cell formation (granulation tissue, the intimal and subintimal synovial layers, and deep stroma extending to the periphery of osteophytic tissue), both TRAP-positive mono- and multinucleated cells and TRAP-negative, NSE-positive mononuclear precursors were identified. Cells containing both enzyme activities were also found, potentially indicating an intermediate stage of differentiation. The TRAP-positive mononuclear/giant cells, and the occasional NSE-positive precursor, expressed an intense signal for cathepsin K mRNA, but did not express cathepsins B, L, and S. In contrast, the deep zone of phagocytic-like cells adjacent to sites of ossification expressed high levels of mRNA for cathepsins L, B, and S as well as cathepsin K mRNA.

CONCLUSION

Giant cells that form within OA synovial tissue express high levels of cathepsin K mRNA. It appears that cathepsin K acts principally to digest the bone (and cartilage) fragments sheered from the joint surface during OA. The high TRAP activity and the undetectable expression of the macrophage-associated degradative proteases (cathepsins B, L, and S) by synovial giant cells strengthens the hypothesis that cathepsin K is the primary protease involved in bone degradation. At sites of synovial osteogenesis, a population of phagocytic-like cells expressed TRAP and cathepsins B, L, S, and K, and may represent blood-derived macrophages pushed toward an osteoclast phenotype.

Osteoclast markers accumulate on cells developing from human peripheral blood mononuclear precursors

Recent studies show that human osteoclasts develop in vitro from hematopoietic cells; however, special cultures conditions and/or cytokine mobilized peripheral blood are apparently required. Here, we report that cells expressing osteoclast markers differentiate from precursors present in nonmobilized peripheral blood mononuclear cells (PBMC), without the addition of stromal cells, growth factors, cytokines or steroids; and characterize their phenotype. Three days after establishing high-density PBMC cultures (1.5 x 10(6) cells/cm2), in serum-containing medium, small adherent colonies of tartrate resistant acid phosphatase positive (TRAP+) cells emerge, amidst massive monocyte cell death. These adherent cells have an eccentrically placed, round nucleus, and express low levels of TRAP and sodium fluoride-resistant- alpha-naphthyl-acetate-esterase (NaF-R-NSE). Over the next week, this cell population accumulates phenotypic markers of osteoclasts (vitronectin receptor [VR], calcitonin receptor, TRAP, cathepsin K protein, and mRNA) with increased nuclearity, covering the entire surface by 15 days. When cultured on bone, VR+, TRAP+ cells of low multinuclearity appear and cover up to 50% of the surface. Resorption lacunae can be observed by day 22. Although these pits are not nearly as numerous as the cells of preosteoclast phenotype, they do represent the activity of a subset of osteoclast-like cells that has achieved osteoclastic maturity under these culture conditions. Transcripts for osteoprotegerin ligand (OPGL), an osteoclast differentiation factor (also known as RANKL and TRANCE) are expressed, likely by adherent cells. Thus, an adherent population of cells, with preosteoclast/osteoclast phenotypic properties, arises selectively under simple culture conditions from normal PBMC. Further characterization of these cells should identify factors involved in the growth, terminal differentiation and activation of osteoclasts.

Rat osteoclast precursors in vivo express a vitronectin receptor and a chloride-bicarbonate exchanger

In vivo osteoclast precursors, which are mononuclear, were previously found to express TRAP (tartrate-resistant acid phosphatase) and CTR (calcitonin receptor), like multinucleated osteoclasts. In vitro, they were found to express, in addition, VNR (vitronectin receptor) and CBE (chloride-bicarbonate exchanger). In order to ascertain that osteoclast precursors in vivo express VNR and CBE like their in vitro counterparts, we used immunohistochemistry to localize these molecules in developing long bones of neonatal rats. Frozen sections of metatarsals and phalanges of 1-2 day-old rats were stained for TRAP and mineralization using histochemistry or were reacted with polyclonal antibodies specific for either the beta3 chain of the VNR or synthetic sequences of the CBE. Both mature, multinucleated osteoclasts within the forming marrow cavity of metatarsals (as shown previously) and mononuclear osteoclast precursors located outside the bony collar of the phalangeal calcified rudiment (as shown here for the first time) expressed both TRAP, VNR and CBE. These findings suggest that mononuclear osteoclast precursors express many of the phenotypical markers of multinucleated osteoclasts prior to their fusion and multinucleation which may allow them to resorb bone, as suggested by in vitro observations of pit formation by preosteoclasts cultured on resorbable substances.

Idoxifene: a novel selective estrogen receptor modulator prevents bone loss and lowers cholesterol levels in ovariectomized rats and decreases uterine weight in intact rats

Idoxifene, a novel selective estrogen receptor modulator, was tested for its effects on bone loss, serum cholesterol, and uterine wet weight and histology in the ovariectomized (Ovx) rat. Idoxifene (0.5 mg/kg x day) completely prevented loss of both lumbar and proximal tibial bone mineral density (BMD). In an intervention study, idoxifene (0.5 and 2.5 mg/kg x day) completely prevented further loss of both lumbar and proximal tibial BMD during a 2-month treatment period commencing 1 month after surgery, when significant loss of BMD had occurred in the Ovx control group. Idoxifene reduced total serum cholesterol, which was maximal at 0.5 mg/kg x day. Idoxifene alone displayed minimal uterotrophic activity in Ovx rats and inhibited the agonist activity of estrogen in intact rats. Histologically, myometrial and endometrial atrophy were observed in both idoxifene and vehicle-treated Ovx rats. In this report, we also provide molecular-based evidence to support the observations in vivo of a novel selective estrogen receptor modulator (SERM) mechanism of action in bone and endometrial cells. Idoxifene is an agonist through the estrogen response element (ERE) and exhibits similar postreceptor effects to estrogen in bone-forming osteoblasts. Idoxifene also stimulates osteoclast apoptosis, and these pleiotropic effects ultimately could contribute to the maintenance of bone homeostasis. However, idoxifene differs from estrogen in a tissue-specific manner. In human endometrial cells, where estrogen is a potent agonist through the ERE, idoxifene has negligible agonist activity. Moreover, idoxifene was able to block estrogen induced gene expression in endometrial cells, which is in agreement with the observation in the intact rat study. In the uterus, idoxifene has a pharmacologically favorable profile, lacking agonist and therefore growth-promoting activity. Together with its cholesterol lowering effect and lack of uterotrophic activity, these data suggest that idoxifene may be effective in the prevention of osteoporosis and other postmenopausal diseases without producing unwanted estrogenic effects on the endometrium.

Generation of mouse osteoclastogenic cell lines immortalized with SV40 large T antigen

Progress in the field of osteoclast gene regulation has been hampered significantly by the lack of such cell lines. In this study, mouse osteoclast precursor cells were elicited in an osteoclast-inductive coculture system and immortalized using SV40 large T antigen. One of the osteoclast precursor cell lines (MOCP-5) forms 95% tartrate-resistant acid phosphatase positive (TRAP+) multinuclear osteoclast-like cells (OCLs) in the coculture system. The yield of TRAP+ OCLs was 4.5-7x10(4) cells per 10 cm2 dish. Expression of SV40 large T antigen was visualized in the nucleus of MOCP-5 cells and OCLs by immunohistochemistry. MOCP-5 cells were positive for MoMa-2 antigen and nonspecific esterase but negative for F4/80 antigen. OCLs derived from MOCP-5 cells were positive for able to form extensive resorption bone pits on bone slices. The resorbing activity of the OCLs was comparable to that of authentic mouse osteoclasts. Pit formation was inhibited by salmon calcitonin (CT). Acid production by OCLs was demonstrated by vital staining with acridine orange. The OCLs expressed cathepsin K and CT receptors. MOCP-5 cells could be transfected by a construct that carries the beta-galactosidase gene. Transfected MOCP-5 cells expressing beta-galactosidase retain the ability to differentiate into OCLs, indicating a useful model for osteoclast gene regulation. To date, the MOCP-5 cell line has been maintained in continuous culture for 23 months and has maintained the capacity to differentiate into osteoclasts throughout this time. In summary, these data show that a stable immortalized osteoclast precursor cell line has been established and that the immortalization with SV40 large T oncogene does not prevent osteoclast precursor cell differentiation.

Induction of carbonic anhydrase II expression in osteoclast progenitors requires physical contact with stromal cells

Carbonic anhydrase II (CA II) expression is vital to normal osteoclast function. We and others have previously reported induction of CA II messenger RNA (mRNA) expression by 1,25(OH)2D3 in myelomonocytic cells and marrow culture. However, since 1,25(OH)2D3 stimulates osteoclast differentiation as well, we wished to separate direct effects of 1,25(OH)2D3 on the CA II gene from the differentiating effects of the hormone. Using primary murine mixed marrow cultures, we measured CA II mRNA expression by RT-PCR. 10 nM 1,25(OH)2D3 dose dependently induced expression of CA II mRNA (4.12 +/- 0.68-fold) at day 4 in culture compared with control with an ED50 of 0.25 nM. When nonadherent marrow cells containing osteoclast progenitors were depleted of stromal cells and exposed to 10 nM 1,25(OH)2D3, CA II mRNA expression was decreased by more than 60%. Coculture of progenitors with ST-2 stromal cells for 3 days with 10 nM 1,25(OH)2D3 stimulated CA II expression by 22 +/- 3.6-fold. 1,25(OH)2D3 stimulated CA II mRNA expression in progenitors separated from ST-2 cells by transwells was insignificant demonstrating that the two cell types must be in physical contact. PTH also stimulated CA II mRNA expression (4.91 +/- 0.01-fold) to a similar degree as seen with 1,25(OH)2D3 treatment. These results demonstrate that induction of CA II in osteoclast progenitors requires their physical communication with stromal cells and is inseparable from the osteoclast differentiation process.