Synthesis of membrane- and matrix-bound colony-stimulating factor-1 by cultured osteoblasts

Csf1 from marrow adipogenic precursors is required for osteoclast formation and hematopoiesis in bone

Colony-stimulating factor 1 (Csf1) is an essential growth factor for osteoclast progenitors and an important regulator for bone resorption. It remains elusive which mesenchymal cells synthesize Csf1 to stimulate osteoclastogenesis. We recently identified a novel mesenchymal cell population, marrow adipogenic lineage precursors (MALPs), in bone. Compared to other mesenchymal subpopulations, MALPs expressed Csf1 at a much higher level and this expression was further increased during aging. To investigate its role, we constructed MALP-deficient Csf1 CKO mice using AdipoqCre. These mice had increased femoral trabecular bone mass, but their cortical bone appeared normal. In comparison, depletion of Csf1 in the entire mesenchymal lineage using Prrx1Cre led to a more striking high bone mass phenotype, suggesting that additional mesenchymal subpopulations secrete Csf1. TRAP staining revealed diminished osteoclasts in the femoral secondary spongiosa region of Csf1 CKOAdipoq mice, but not at the chondral-osseous junction nor at the endosteal surface of cortical bone. Moreover, Csf1 CKOAdipoq mice were resistant to LPS-induced calvarial osteolysis. Bone marrow cellularity, hematopoietic progenitors, and macrophages were also reduced in these mice. Taken together, our studies demonstrate that MALPs synthesize Csf1 to control bone remodeling and hematopoiesis.

Autophagy Plays Multiple Roles in the Soft-Tissue Healing and Osseointegration in Dental Implant Surgery-A Narrative Review

Dental endo-osseous implants have become a widely used treatment for replacing missing teeth. Dental implants are placed into a surgically created osteotomy in alveolar bone, the healing of the soft tissue lesion and the osseointegration of the implant being key elements to long-term success. Autophagy is considered the major intracellular degradation system, playing important roles in various cellular processes involved in dental implant integration. The aim of this review is an exploration of autophagy roles in the main cell types involved in the healing and remodeling of soft tissue lesions and implant osseointegration, post-implant surgery. We have focused on the autophagy pathway in macrophages, endothelial cells; osteoclasts, osteoblasts; fibroblasts, myofibroblasts and keratinocytes. In macrophages, autophagy modulates innate and adaptive immune responses playing a key role in osteo-immunity. Autophagy induction in endothelial cells promotes apoptosis resistance, cell survival, and protection against oxidative stress damage. The autophagic machinery is also involved in transporting stromal vesicles containing mineralization-related factors to the extracellular matrix and regulating osteoblasts' functions. Alveolar bone remodeling is achieved by immune cells differentiation into osteoclasts; autophagy plays an important and active role in this process. Autophagy downregulation in fibroblasts induces apoptosis, leading to better wound healing by improving excessive deposition of extracellular matrix and inhibiting fibrosis progression. Autophagy seems to be a dual actor on the scene of dental implant surgery, imposing further research in order to completely reveal its positive features which may be essential for clinical efficacy.

The Autophagy in Osteoimmonology: Self-Eating, Maintenance, and Beyond

It has been long realized that the immune and skeletal systems are closely linked. This crosstalk, also known as osteoimmunology, is a primary process required for bone health. For example, the immune system acts as a key regulator in osteoclasts-osteoblasts coupling to maintain the balanced bone remodeling. Osteoimmunology is achieved through many cellular and molecular processes, among which autophagy has recently been found to play an indispensable role. Autophagy is a highly conserved process in eukaryotic cells, by which the cytoplasm components such as dysfunctional organelles are degraded through lysosomes and then returned to the cytosol for reuse. Autophagy is present in all cells at basal levels to maintain homeostasis and to promote cell survival in response to cellular stress conditions such as nutrition deprivation and hypoxia. Autophagy is a required process in immune cell activation/polarization and osteoclast differentiation, which protecting cells from oxidative stress. The essential of autophagy in osteogenesis is its involvement in osteoblast differentiation and mineralization, especially the role of autophagosome in extracellular calcium transportation. The modulatory feature of autophagy in both immune and skeleton systems suggests its crucial roles in osteoimmunology. Furthermore, autophagy also participates in the maintenance of bone marrow hematopoietic stem cell niche. The focus of this review is to highlight the role of autophagy in the immune-skeleton interactions and the effects on bone physiology, as well as the future application in translational research.

Paracrine and endocrine actions of bone-the functions of secretory proteins from osteoblasts, osteocytes, and osteoclasts

The skeleton is a dynamic organ that is constantly remodeled. Proteins secreted from bone cells, namely osteoblasts, osteocytes, and osteoclasts exert regulation on osteoblastogenesis, osteclastogenesis, and angiogenesis in a paracrine manner. Osteoblasts secrete a range of different molecules including RANKL/OPG, M-CSF, SEMA3A, WNT5A, and WNT16 that regulate osteoclastogenesis. Osteoblasts also produce VEGFA that stimulates osteoblastogenesis and angiogenesis. Osteocytes produce sclerostin (SOST) that inhibits osteoblast differentiation and promotes osteoclast differentiation. Osteoclasts secrete factors including BMP6, CTHRC1, EFNB2, S1P, WNT10B, SEMA4D, and CT-1 that act on osteoblasts and osteocytes, and thereby influenceaA osteogenesis. Osteoclast precursors produce the angiogenic factor PDGF-BB to promote the formation of Type H vessels, which then stimulate osteoblastogenesis. Besides, the evidences over the past decades show that at least three hormones or "osteokines" from bone cells have endocrine functions. FGF23 is produced by osteoblasts and osteocytes and can regulate phosphate metabolism. Osteocalcin (OCN) secreted by osteoblasts regulates systemic glucose and energy metabolism, reproduction, and cognition. Lipocalin-2 (LCN2) is secreted by osteoblasts and can influence energy metabolism by suppressing appetite in the brain. We review the recent progresses in the paracrine and endocrine functions of the secretory proteins of osteoblasts, osteocytes, and osteoclasts, revealing connections of the skeleton with other tissues and providing added insights into the pathogenesis of degenerative diseases affecting multiple organs and the drug discovery process.

Synovial fibroblasts spread rheumatoid arthritis to unaffected joints

Active rheumatoid arthritis originates from few joints but subsequently affects the majority of joints. Thus far, the pathways of the progression of the disease are largely unknown. As rheumatoid arthritis synovial fibroblasts (RASFs) which can be found in RA synovium are key players in joint destruction and are able to migrate in vitro, we evaluated the potential of RASFs to spread the disease in vivo. To simulate the primary joint of origin, we implanted healthy human cartilage together with RASFs subcutaneously into severe combined immunodeficient (SCID) mice. At the contralateral flank, we implanted healthy cartilage without cells. RASFs showed an active movement to the naive cartilage via the vasculature independent of the site of application of RASFs into the SCID mouse, leading to a marked destruction of the target cartilage. These findings support the hypothesis that the characteristic clinical phenomenon of destructive arthritis spreading between joints is mediated, at least in part, by the transmigration of activated RASFs.

Cell surface colony-stimulating factor 1 can be cleaved by TNF-alpha converting enzyme or endocytosed in a clathrin-dependent manner

CSF-1 is a hemopoietic growth factor, which plays an essential role in macrophage and osteoclast development. Alternative splice variants of CSF-1 are synthesized as soluble or membrane-anchored molecules, although membrane CSF-1 (mCSF-1) can be cleaved from the cell membrane to become soluble CSF-1. The activities involved in this proteolytic processing, also referred to as ectodomain shedding, remain poorly characterized. In the present study, we examined the properties of the mCSF-1 sheddase in cell-based assays. Shedding of mCSF-1 was up-regulated by phorbol ester treatment and was inhibited by the metalloprotease inhibitors GM6001 and tissue inhibitor of metalloproteases 3. Moreover, the stimulated shedding of mCSF-1 was abrogated in fibroblasts lacking the TNF-alpha converting enzyme (TACE, also known as a disintegrin and metalloprotease 17) and was rescued by expression of wild-type TACE in these cells, strongly suggesting that the stimulated shedding is TACE dependent. Additionally, we observed that mCSF-1 is predominantly localized to intracellular membrane compartments and is efficiently internalized in a clathrin-dependent manner. These results indicate that the local availability of mCSF-1 is actively regulated by ectodomain shedding and endocytosis. This mechanism may have important implications for the development and survival of monocyte lineage cells.

Molecular interactions between breast cancer cells and the bone microenvironment drive skeletal metastases

Breast cancer cells preferentially spread to bone. Bone metastases are currently incurable and therefore better treatments need to be developed. Metastasis is an inefficient, multi-step process. Specific aspects of both breast cancer cells and the bone microenvironment contribute to the development of bone metastases. Breast cancers express chemokine receptors, integrins, cadherins, and bone-resorbing and bone-forming factors that contribute to the successful and preferential spread of tumor to bone. Bone is rich in growth factors and cell types that make it a hospitable environment for breast cancer growth. Once breast cancer cells enter the bone, a highly complex vicious cycle develops, in which breast cancer cells secrete factors that act on bone cells and other cells within the bone (stem cells, T cells, platelets, adipocytes, fibroblasts, and endothelial cells), causing them to secrete factors that act on adjacent cancer cells. The steps in the metastatic cascade and the vicious cycle within bone offer unique targets for adjuvant treatments to treat and cure bone metastases.

Targeted expression of csCSF-1 in op/op mice ameliorates tooth defects

OBJECTIVE

The aim of this study was to characterize the tooth phenotype of CSF-1-deficient op/op mice and determine whether expression of csCSF-1 in these mice has a role in primary tooth matrix formation.

DESIGN

Ameloblasts and odontoblasts, isolated from wt/wt frozen sections using laser capture microdissection, were analysed for csCSF-1, sCSF-1 and CSF-1R mRNA by RT-PCR. Mandibles, excised from 8 days op/op and wt/wt littermates, were examined for tooth morphology as well as amelogenin and DMP1 expression using in situ hybridisation. op/opCS transgenic mice, expressing csCSF-1 in teeth and bone using the osteocalcin promoter, were generated. Skeletal X-rays and histomorphometry were performed; teeth were analysed for morphology and matrix proteins.

RESULTS

Normal dental cells in vivo express both CSF-1 isoforms and CSF-1R. Compared to wt/wt, op/op teeth prior to eruption showed altered dental cell morphology and dramatic reduction in DMP1 transcripts. op/opCS mice showed marked resolution of osteopetrosis, tooth eruption and teeth that resembled amelogenesis imperfecta-like phenotype. At 3 weeks, op/op teeth showed severe enamel and dentin defects and barely detectable amelogenin and DMP1. In op/opCS mice, DMP1 in odontoblasts increased to near normal and dentin morphology was restored; amelogenin also increased. Enamel integrity improved in op/opCS, although it was thinner than wt enamel.

CONCLUSIONS

Results demonstrate that ameloblasts and odontoblasts are a source and potential target of CSF-1 isoforms in vivo. Expression of csCSF-1 within the tooth microenvironment is essential for normal tooth morphogenesis and may provide a mechanism for coordinating the process of tooth eruption with endogenous matrix formation.

Concerted action of Smad and CREB-binding protein regulates bone morphogenetic protein-2-stimulated osteoblastic colony-stimulating factor-1 expression

Bone remodeling depends upon proper osteoblast and osteoclast function. Bone morphogenetic protein-2 (BMP-2) stimulates differentiation of osteoblasts from pluripotent precursors. Osteoclast formation depends on the concerted action of osteoblast-derived receptor activator of NF-kappaB ligand and colony-stimulating factor-1 (CSF-1). BMP-2 stimulates receptor activator of NF-kappaB ligand expression. However, the effect of BMP-2 on CSF-1 expression has not been studied. We investigated the role of BMP-2 in CSF-1 expression in osteogenic C2C12 cells. Incubation of C2C12 cells with BMP-2 supported osteoclastogenesis of spleen cells with a concomitant increase in expression of CSF-1 mRNA and protein. To determine the mechanism, we identified a BMP-responsive element between -627 bp and -509 bp in the CSF-1 promoter. DNase I footprint analysis revealed the presence of consensus Smad binding motif in this region. Electrophoretic mobility shift assay showed BMP-2-stimulated binding of proteins to this motif. Mutation of core sequence as well as its 5'- and 3'-flanking sequences abolished the DNA-protein interaction resulting in inhibition of CSF-1 transcription. Supershift analysis detects the presence of Smads 1, 5, and 4 and the transcriptional coactivator CREB-binding protein in the BMP-responsive element-protein complex. In addition, Smads 1 and 5 alone or in combination with Smad 4 increased CSF-1 transcription. Furthermore, CREB-binding protein markedly increased transcription of CSF-1. These data represent the first evidence that BMP-2 increases the osteoclastogenic CSF-1 expression by a transcriptional mechanism using the canonical Smad pathway and provide a mechanism for BMP-2-induced osteoclast differentiation.

Incomplete restoration of colony-stimulating factor 1 (CSF-1) function in CSF-1-deficient Csf1op/Csf1op mice by transgenic expression of cell surface CSF-1

The primary macrophage growth factor, colony-stimulating factor 1 (CSF-1), is expressed as a secreted glycoprotein or proteoglycan found in the circulation or as a biologically active cell surface glycoprotein (csCSF-1). To investigate the in vivo roles of csCSF-1, we created mice that exclusively express csCSF-1, in a normal tissue-specific and developmental manner, by transgenic expression of csCSF-1 in the CSF-1-deficient osteopetrotic (Csf1(op)/Csf1(op)) background. The gross defects of Csf1(op)/Csf1(op) mice, including growth retardation, failure of tooth eruption, and abnormal male and female reproductive functions were corrected. Macrophage densities in perinatal liver, bladder, sublinguinal salivary gland, kidney cortex, dermis, and synovial membrane were completely restored, whereas only partial or no restoration was achieved in adult liver, adrenal gland, kidney medulla, spleen, peritoneal cavity, and intestine. Residual osteopetrosis, significantly delayed trabecular bone resorption in the subepiphyseal region of the long bone, and incomplete correction of the hematologic abnormalities in the peripheral blood, bone marrow, and spleens of CSF-1-deficient mice were also found in mice exclusively expressing csCSF-1. These data suggest that although csCSF-1 alone is able to normalize several aspects of development in Csf1(op)/Csf1(op) mice, it cannot fully restore in vivo CSF-1 function, which requires the presence of the secreted glycoprotein and/or proteoglycan forms.

Osteoblast-specific targeting of soluble colony-stimulating factor-1 increases cortical bone thickness in mice

The soluble and membrane-bound forms of CSF-1 are synthesized by osteoblasts and stromal cells in the bone microenvironment. Transgenic mice, generated to selectively express sCSF-1 in bone, showed increased cortical thickness in the femoral diaphysis caused by new bone formation along the endosteal surface. The ability of sCSF-1 to enhance bone cell activity in vivo is potentially relevant for increasing cortical bone in a variety of disorders.

INTRODUCTION

The soluble form of colony-stimulating factor-1 (sCSF-1) and the membrane-bound form of CSF-1 (mCSF-1) have been shown to support osteoclastogenesis in vitro; however, the effect of each peptide on bone remodeling in vivo is unclear. To determine the effect of sCSF-1, selectively expressed in bone, the skeletal phenotype of transgenic mice harboring the human sCSF-1 cDNA under the control of the osteocalcin promoter was assessed.

METHODS

At 5 and 14 weeks, mice were analyzed for CSF-1 protein levels, weighed, and X-rayed, and femurs were removed for peripheral quantitative computed tomography, histology, and histomorphometry.

RESULTS

High levels of human sCSF-1 were detected in bone extracts and, to a lesser extent, in plasma. Adult transgenic mice showed normal body weight and increased circulating monocytic cells. At 5 weeks, the femoral diaphysis was similar in CSF-1T and wt/wt littermates. However, by 14 weeks, the femoral diaphysis in CSF-1T mice showed increased cortical thickness and bone mineral density. In contrast to the diaphysis, the femoral metaphysis of CSF-1T mice showed normal cancellous bone comparable with wt/wt littermates at each time point. Histological sections demonstrated increased woven bone along the endosteal surface of the diaphysis and intracortical remodeling. Fluorochrome-labeling analysis confirmed endocortical bone formation in CSF-1T, with a 3.1-fold increase in the percentage of double-labeled surfaces and a 3.6-fold increase in the bone formation rate compared with wt/wt mice. Although remodeling resulted in a slightly porous cortex, sCSF-1 preferentially stimulated endocortical bone formation, leading to increased cortical thickness.

CONCLUSIONS

These findings indicate that sCSF-1 is a key determinant of bone cell activity in the corticoendosteal envelope.

Analysis of the mouse CSF-1 gene promoter in a transgenic mouse model

CSF-1 stimulates monocyte and osteoclast populations. However, the molecular mechanisms involved in regulating CSF-1 gene expression are unclear. To identify regulatory regions that control normal CSF-1 gene expression, a -774/+183-bp fragment of the murine CSF-1 promoter was analyzed in vitro and in vivo. Transcriptional activity was high in cultured osteoblasts that express CSF-1 mRNA compared to ARH-77 B cells that lack CSF-1 gene expression. Transient transfection of osteoblasts with promoter deletion constructs showed that the -774-bp fragment conferred the highest transcriptional activity and contained activator and repressor sequences. To assess the ability of the CSF-1 promoter to confer normal tissue expression of CSF-1, transgenic mice containing the -774/+183-bp region driving the E. coli beta-galactosidase (lacZ) reporter gene were generated. beta-Gal analysis of whole tissue extracts showed transgene expression in all tissues tested except liver and kidney. At the cellular level, the pattern of beta-gal expression in the spleen, thymus, bone, lung, and testes of adult transgenic mice mimicked normal endogenous CSF-1 mRNA expression in non-transgenic littermates detected by in situ hybridization. This region also directed appropriate transgene expression to sites in other tissues known to synthesize CSF-1, with the exception of the liver and kidney. These findings indicate that the -774-bp fragment contains cis-acting elements sufficient to direct CSF-1 gene expression in many tissues. CSF-1 promoter/lacZ mice may be useful for studying the transcriptional mechanisms involved in regulating CSF-1 gene expression in tissues throughout development.

PDGF BB and bFGF stimulate DNA synthesis and upregulate CSF-1 and MCP-1 gene expression in dental follicle cells

CSF-1 and MCP-1, released by dental follicle cells, stimulate the influx of monocytes into the follicle sac and enhance the formation of osteoclasts that, in turn, resorb alveolar bone for the eruption pathway. PDGF and bFGF, released by cells adjacent to the follicle or by activated monocytes, are prime candidates that may regulate CSF-1 and MCP-1 gene expression. The present study demonstrates that PDGF and bFGF are mitogens for dental follicle cells and stimulate CSF-1 and MCP-1 mRNA, but with different time course kinetics. Peak induction of CSF-1 mRNA was observed at 6-8h, while maximal MCP-1 induction was observed at 2h. These findings suggest that MCP-1 is an early chemotactic signal for monocytes and that subsequent release of CSF-1 may act synergistically with MCP-1 to enhance monocyte influx. Further understanding of the molecular mechanisms by which cytokines regulate CSF-1 and MCP-1 may lead to more effective treatment regimens for disorders associated with abnormal tooth eruption.

The osteopetrotic mutation toothless (tl) is a loss-of-function frameshift mutation in the rat Csf1 gene: Evidence of a crucial role for CSF-1 in osteoclastogenesis and endochondral ossification

The toothless (tl) mutation in the rat is a naturally occurring, autosomal recessive mutation resulting in a profound deficiency of bone-resorbing osteoclasts and peritoneal macrophages. The failure to resorb bone produces severe, unrelenting osteopetrosis, with a highly sclerotic skeleton, lack of marrow spaces, failure of tooth eruption, and other pathologies. Injections of CSF-1 improve some, but not all, of these. In this report we have used polymorphism mapping, sequencing, and expression studies to identify the genetic lesion in the tl rat. We found a 10-base insertion near the beginning of the open reading of the Csf1 gene that yields a truncated, nonfunctional protein and an early stop codon, thus rendering the tl rat CSF-1(null). All mutants were homozygous for the mutation and all carriers were heterozygous. No CSF-1 transcripts were identified in rat mRNA that would avoid the mutation via alternative splicing. The biology and actions of CSF-1 have been elucidated by many studies that use another naturally occurring mutation, the op mouse, in which a single base insertion also disrupts the reading frame. The op mouse has milder osteoclastopenia and osteopetrosis than the tl rat and recovers spontaneously over the first few months of life. Thus, the tl rat provides a second model in which the functions of CSF-1 can be studied. Understanding the similarities and differences in the phenotypes of these two models will be important to advancing our knowledge of the many actions of CSF-1.

Biological characterization of uncleavable plasma membrane-anchored human macrophage colony-stimulating factor

The cell-surface form of human macrophage colony-stimulating factor (CSF-1(256), M-CSFalpha) is a plasma membrane-anchored transmembrane protein from which the soluble CSF-1 is released by ectodomain proteolytic cleavage. We have previously generated two forms of cell surface CSF-1 which failed to undergo the cleavage by deleting residues 161-165 or residues 159-165 in the extracellular juxtamembrane region (1). To determine the biologic significance of the ectodomain cleavage, we compared the biosynthesis and biologic activities of uncleavable mutant CSF-1 forms with those of the cleavable wild-type (WT) CSF-1. We found that the uncleavable CSF-1 forms were able to accumulate on cell surface at about threefold higher level than the cleavable WT CSF-1 did. We further demonstrated that the uncleavable plasma membrane-anchored forms of CSF-1 were biologically active in mediating the proliferation of CSF-1-dependent cells as well as the intercellular adhesion between CSF-1 receptor-bearing cells and CSF-1 expressing cells. Furthermore, the adhesive activity of uncleavable CSF-1 forms was about twofold stronger than that of WT CSF-1, which indicated that the ectodomain cleavage system plays an important role in regulating the biologic activities of membrane-anchored CSF-1.

Induction of colony-stimulating factor expression following Staphylococcus or Salmonella interaction with mouse or human osteoblasts

Staphylococcus aureus and Salmonella spp. are common causes of bone diseases; however, the immune response during such infections is not well understood. Colony-stimulating factors (CSF) have a profound influence on osteoclastogenesis, as well as the development of immune responses following infection. Therefore, we questioned whether interaction of osteoblasts with two very different bacterial pathogens could affect CSF expression by these cells. Cultured mouse and human osteoblasts were exposed to various numbers of S. aureus or Salmonella dublin bacteria, and a comprehensive analysis of granulocyte-macrophage (GM)-CSF, granulocyte (G)-CSF, macrophage (M)-CSF, and interleukin-3 (IL-3) mRNA expression and cytokine secretion was performed. Expression of M-CSF and IL-3 mRNAs by mouse osteoblasts was constitutive and did not increase significantly following bacterial exposure. In contrast, GM-CSF and G-CSF mRNA expression by mouse osteoblasts was dramatically upregulated following interaction with either viable S. aureus or Salmonella. This increased mRNA expression also translated into high levels of GM-CSF and G-CSF secretion by mouse and human osteoblasts following bacterial exposure. Viable S. aureus and Salmonella induced maximal levels of CSF mRNA expression and cytokine secretion compared to UV-killed bacteria. Furthermore, GM-CSF and G-CSF mRNA expression could be induced in unexposed osteoblasts separated by a permeable Transwell membrane from bacterially exposed osteoblasts. M-CSF secretion was increased in cultures of exposed human osteoblasts but not in exposed mouse osteoblast cultures. Together, these studies are the first to define CSF expression and suggest that, following bacterial exposure, osteoblasts may influence osteoclastogenesis, as well as the development of an immune response, via the production of these cytokines.

Importance of membrane- or matrix-associated forms of M-CSF and RANKL/ODF in osteoclastogenesis supported by SaOS-4/3 cells expressing recombinant PTH/PTHrP receptors

SaOS-4/3, a subclone of the human osteosarcoma cell line SaOS-2, established by transfecting the human parathyroid hormone/parathyroid hormone-related protein (PTH/PTHrP) receptor complementary DNA (cDNA), supported osteoclast formation in response to PTH in coculture with mouse bone marrow cells. Osteoclast formation supported by SaOS-4/3 cells was completely inhibited by adding either osteoprotegerin (OPG) or antibodies against human macrophage colony-stimulating factor (M-CSF). Expression of messenger RNAs (mRNAs) for receptor activator of NF-kappaB ligand/osteoclast differentiation factor (RANKL/ODF) and both membrane-associated and secreted forms of M-CSF by SaOS-4/3 cells was up-regulated in response to PTH. SaOS-4/3 cells constitutively expressed OPG mRNA, expression of which was down-regulated by PTH. To elucidate the mechanism of PTH-induced osteoclastogenesis, SaOS-4/3 cells were spot-cultured for 2 h in the center of a culture well and then mouse bone marrow cells were uniformly plated over the well. When the spot coculture was treated for 6 days with both PTH and M-CSF, osteoclasts were induced exclusively inside the colony of SaOS-4/3 cells. Osteoclasts were formed both inside and outside the colony of SaOS-4/3 cells in coculture treated with a soluble form of RANKL/ODF (sRANKL/sODF) in the presence of M-CSF. When the spot coculture was treated with sRANKL/sODF, osteoclasts were formed only inside the colony of SaOS-4/3 cells. Adding M-CSF alone failed to support osteoclast formation in the spot coculture. PTH-induced osteoclast formation occurring inside the colony of SaOS-4/3 cells was not affected by the concentration of M-CSF in the culture medium. Mouse primary osteoblasts supported osteoclast formation in a similar fashion to SaOS-4/3 cells. These findings suggest that the up-regulation of RANKL/ODF expression is an essential step for PTH-induced osteoclastogenesis, and membrane- or matrix-associated forms of both M-CSF and RANKL/ ODF are essentially involved in osteoclast formation supported by osteoblasts/stromal cells.

Role of NFkappaB in the regulation of macrophage colony stimulating factor by tumor necrosis factor-alpha in ST2 bone stromal cells

Expression of MCSF in bone is important to the regulation of osteoclastogenesis. We show here that tumor necrosis factor-alpha (TNFalpha) increases the production of both soluble (sMCSF) and membrane-bound (mMCSF) macrophage colony stimulating factor by ST2 bone stromal cells. Treatment of ST2 cells with TNFalpha caused sMCSF levels to increase by 394+/-5% from basal; mMCSF rose by 316+/-66% from 30+/-10 per 100,000 cells in the same time. These increases were consistent with increased expression of mRNAs encoding both isoforms. Increases in MCSF mRNA are also seen after stimulation with dexamethasone. To investigate the potential role of NFkappaB in this TNFalpha effect, we treated cells with sodium salicylate (NaS), an inhibitor of NFkappaB translocation. NaS decreased TNFalpha-stimulated NFkappaB activation by 50% as assessed by EMSA. Despite inhibition of NFkappaB signaling, NaS enhanced TNFalpha-stimulated MCSF secretion and did not prevent TNFalpha-stimulated increases in sMCSF mRNA, suggesting that NFkappaB was not involved in TNFalpha effect on the gene. TNFalpha failed to stimulate transcription of a 774 nucleotide MCSF promoter-luciferase reporter transfected into ST2 cells which contained the NFkappaB consensus sequence. Deletion of the seven nucleotides containing the NFkappaB homology response sequence from the MCSF promoter increased basal gene transcription by twofold. TNFalpha thus contributes to an osteoclastogenic environment through upregulation of bone expression of both MCSF isoforms. Our data suggests that NFkappaB is not the major signaling pathway through which this occurs.

Expression of colony-stimulating factor-1 in vivo during the formation of osteoclasts

Colony-stimulating factor-1 (CSF-1), originally described as a growth factor for macrophages, is essential for the proliferation and differentiation of the cells of the osteoclast lineage. The cytokine is synthesized either as a secreted or a membrane-bound protein, which are encoded by four transcripts. The aim of the present study was to investigate the expression of CSF-1 in vivo at the mRNA level. Transcripts encoding CSF-1 were determined in total RNA from fetal murine metatarsals of different ages by a quantitative reverse-transcription polymerase chain reaction assay. Within the investigated period of time, the bone rudiments contain cells of the osteoclastic lineage representing well-defined differentiation stages. We found that only low levels of transcripts encoding CSF-1 could be detected in metatarsals from 15-day-old fetuses. Transcript levels increased slowly during the following days to reach a maximum in the rudiments from 18-day-old fetuses. After birth, in newborn animals, transcript levels were lowered again. While in rudiments from 15-day-old fetuses a considerable portion of the transcripts encoded the membrane-bound molecule, a transcript encoding the secreted form of the cytokine was the predominant species during the following days. These results suggest that the maintenance of proliferating and postmitotic osteoclast precursors requires low levels of CSF-1 only. Highest levels of locally synthesized CSF-1 are required, however, during the initial recruitment and activation of osteoclasts. After birth, levels of CSF-1 transcripts decrease again, suggesting that newly synthesized CSF-1 may be replaced by protein released from the mineralized matrix during resorption. In conclusion, the present data further strengthen the notion that CSF-1 produced locally acts in a paracrine fashion during the formation of osteoclasts.

The cell-surface form of colony-stimulating factor-1 is regulated by osteotropic agents and supports formation of multinucleated osteoclast-like cells

Colony-stimulating factor-1 (CSF-1) is a hematopoietic growth factor that is released by osteoblasts and is recognized to play a critical role in bone remodeling in vivo and in vitro. CSF-1 is synthesized as a soluble or cell-surface protein. It is unclear, however, whether human osteoblasts express both molecular forms of CSF-1, and whether these isoforms can independently mediate osteoclastogenesis. In the present study, using a combination of quantitative reverse transcriptase polymerase chain reaction, flow cytometry, and Western immunoblot analysis, we have demonstrated that human osteoblast-like cells as well as primary human osteoblasts express the cell-surface form of CSF-1 both constitutively and in response to parathyroid hormone and tumor necrosis factor. Furthermore, using an in vitro co-culture system, we have shown that cell-surface CSF-1 alone is sufficient to support osteoclast formation. These findings may be especially significant in view of evidence that direct cell-to-cell contact is critical for osteoclast formation, and suggest that differential regulation of expression of the CSF-1 isoforms may influence osteoclast function modulated by osteotropic hormones.

The effects of colony-stimulating factor-1 on the number and ultrastructure of osteoclasts in toothless (tl) rats and osteopetrotic (op) mice

The role of colony-stimulating factor-1 (CSF-1 or M-CSF) in osteoclast development is illustrated by observations that administration of exogenous CSF-1 increases osteoclast number and improves the skeletal sclerosis of two osteopetrotic mutations, toothless (tl) in the rat and osteopetrotic (op) in the mouse. We examined the effects of CSF-1 treatment on the number and ultrastructure of osteoclasts in the tibial metaphysis of normal and mutant animals of both stocks to understand the similarities and differences between these two mutations. Osteoclasts from normal animals of both stocks were abundant and possessed the ultrastructural features of active cells. These included apical areas in contact with mineralized surfaces with tightly apposed clear zones, extensive ruffled borders, and a vacuolated cytoplasm with numerous mitochondria. In toothless rats osteoclasts were difficult to locate and those present had poorly defined ruffled borders, fewer cytoplasmic vacuoles, and a basal membrane with both smooth and ruffled areas. Large lipid accumulations were often found near tl osteoclasts. Osteoclasts in op mice were difficult to find, but more numerous than in tl rats. Unlike tl osteoclasts, those of op mice possessed very well developed ruffled borders, small clear zones, and large electron-dense cytoplasmic inclusions. These cells also had unusual basal membranes with both smooth and ruffled regions. CSF-1 treatment increased the number of osteoclasts in both mutant stocks, normalizing the numbers in op mice, but not tl rats. CSF-1 injections caused dramatic changes in the morphology of tl osteoclasts, including increased incidence and size of ruffled borders and cytoplasmic vacuolization. The growth factor had little effect on ruffled borders or clear zones in op mice. Interestingly, mutant osteoclasts of both stocks exhibited a ruffled basal membrane in response to CSF-1 treatment. This increase in membrane ruffling may reflect the ability of CSF-1 to promote rapid formation of osteoclasts from mononuclear precursors in a more permissive microenvironment. Our data indicate that CSF-1 is not required for the development of at least some osteoclasts. The differences in response to CSF-1 treatment which we report lead us to speculate that additional factors may be involved in osteoclastogenesis.

Osteopontin expression by osteoclast and osteoblast progenitors in the murine bone marrow: demonstration of its requirement for osteoclastogenesis and its increase after ovariectomy

Osteoclast development requires cell-to-cell contact between hematopoietic osteoclast progenitors and bone marrow stromal/osteoblastic support cells. Based on this, we hypothesized that osteopontin, an adhesion protein produced by osteoclasts and osteoblasts, plays a role in osteoclastogenesis. Using in situ hybridization, we demonstrate that cells expressing the osteopontin messenger RNA (mRNA) appear after 3 days of culturing murine bone marrow cells. The number of these cells increases thereafter, reaching a peak on day 5. In the same cultures, cells expressing alkaline phosphatase (AP) or tartrate resistant acid phosphatase (TRAP), phenotypic markers for osteoblastic and osteoclast-like cells, respectively, appeared subsequent to the appearance of the osteopontin-positive cells. By means of a combination of in situ hybridization and histostaining, it was shown that the osteopontin mRNA was localized in 30-50% of the AP-positive or the TRAP-positive, as well as in nonspecific esterase (NSE)-positive, cells. The number of cells expressing both the osteopontin mRNA and either one of the three phenotypic markers was significantly increased in bone marrow cultures from estrogen-deficient mice, as compared with controls. Conversely, the number of all three populations of double positive cells was decreased in cultures treated with a specific antimouse rabbit osteopontin antibody or an RGD peptide. These findings indicate that osteopontin is expressed during the early stages of the differentiation of osteoclast and osteoblast progenitors in the bone marrow and that its cell adhesion properties are required for osteoclastogenesis.

A novel culture system to generate osteoclasts and bone resorption using porcine bone marrow cells: role of M-CSF

A novel osteoclast generation and bone resorption assay system is described in which enhanced osteoclastic generation and bone resorption is induced in porcine bone marrow cell cultures cultured in low-serum medium supplemented with fibroblastic cell (L929) conditioned medium (CM). Numerous osteoclasts, which could be identified by TRAP staining and specific labelling with 121F antibody, were generated in a time-dependent and L929-CM concentration-dependent fashion. A specific antibody against murine M-CSF/CSF-1 abolished osteoclast formation indicating that M-CSF is the essential component of the L929-CM driven osteoclast generation. Culturing on devitalized bone slices resulted in extensive osteoclast-mediated resorption as visualized microscopically. After 16 days in culture, practically the entire bone slice surface was excavated by the osteoclastic cells. Bone resorption could be monitored with time using a novel enzyme-linked immunoassay measuring type I collagen N-telopeptides in culture supernatants. Release of collagen fragments from the slices was paralleled by osteoclastic secretion of TRAP. Salmon calcitonin significantly inhibited collagen fragment and TRAP release. 1,25-Dihydroxyvitamin D3 greatly promoted osteoclast generation and subsequent bone resorption, but its presence was not essential for this process to occur.