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

A Sub-Clone of RAW264.7-Cells Form Osteoclast-Like Cells Capable of Bone Resorption Faster than Parental RAW264.7 through Increased De Novo Expression and Nuclear Translocation of NFATc1

The murine macrophage cell line RAW264.7 is extensively used as a progenitor to study osteoclast (OC) differentiation. RAW264.7 is a heterogeneous cell line, containing sub-clones with different abilities to form OCs. The aim of this study was to identify characteristics within the heterogeneous RAW264.7 cells that define sub-clones with an augmented ability to form bone-resorbing OCs (H9), as well as sub-clones representing non-OCs (J8). RAW264.7 sub-clones were isolated by single cell cloning. Selection was based on TRAP/cathepsin K expression in sub-clone cultures without added RANKL. Sub-clones before and after differentiation with RANKL were assayed for multiple OC-characteristics. Sub-clone H9 cells presented a higher expression of OC-markers in cultures without added RANKL compared to the parental RAW264.7. After 6 days of RANKL stimulation, sub-clone H9 cells had equal expression levels of OC-markers with RAW264.7 and formed OCs able to demineralize hydroxyapatite. However, sub-clone H9 cells displayed rapid differentiation of OC already at Day 2 compared to Day 4 from parental RAW264.7, and when cultured on plastic and on bone they were more efficient in resorption. This rapid differentiation was likely due to high initial expression/nuclear translocation of OC master transcription factor, NFATc1. In contrast to H9, J8 cells expressed initially very low levels of OC-markers, and they did not respond to RANKL-stimulation by developing OC-characteristics/OC-marker expression. Hence, H9 is an additional clone suitable for experimental setup requiring rapid differentiation of large numbers of OCs.

Monocyte-Specific Knockout of C/ebpα Results in Osteopetrosis Phenotype, Blocks Bone Loss in Ovariectomized Mice, and Reveals an Important Function of C/ebpα in Osteoclast Differentiation and Function

CCAAT/enhancer-binding protein α (C/ebpα) is critical for osteoclastogenesis by regulating osteoclast (OC) lineage commitment and is also important for OC differentiation and function in vitro. However, the role of C/ebpα in postnatal skeletal development has not been reported owing to lethality in C/ebpα-/- mice from hypoglycemia within 8 hours after birth. Herein, we generated conditional knockout mice by deleting the C/ebpα gene in monocyte via LysM-Cre to examine its role in OC differentiation and function. C/ebpαf/f LysM-Cre mice exhibited postnatal osteopetrosis due to impaired osteoclastogenesis, OC lineage priming defects, as well as defective OC differentiation and activity. Furthermore, our ex vivo analysis demonstrated that C/ebpα conditional deletion significantly reduced OC differentiation, maturation, and activity while mildly repressing macrophage development. At the molecular level, C/ebpα deficiency significantly suppresses the expressions of OC genes associated with early stages of osteoclastogenesis as well as genes associated with OC differentiation and activity. We also identified numerous C/ebpα critical cis-regulatory elements on the Cathepsin K promoter that allow C/ebpα to significantly upregulate Cathepsin K expression during OC differentiation and activity. In pathologically induced mouse model of osteoporosis, C/ebpα deficiency can protect mice against ovariectomy-induced bone loss, uncovering a central role for C/ebpα in osteolytic diseases. Collectively, our findings have further established C/ebpα as a promising therapeutic target for bone loss by concurrently targeting OC lineage priming, differentiation, and activity. © 2017 American Society for Bone and Mineral Research.

C/ebpα controls osteoclast terminal differentiation, activation, function, and postnatal bone homeostasis through direct regulation of Nfatc1

Osteoclast lineage commitment and differentiation have been studied extensively, although the mechanism by which transcription factor(s) control osteoclast terminal differentiation, activation, and function remains unclear. CCAAT/enhancer-binding protein α (C/ebpα) has been reported to be a key regulator of osteoclast cell lineage commitment, yet C/ebpα's roles in osteoclast terminal differentiation, activation and function, and bone homeostasis, under physiological or pathological conditions, have not been studied because newborn C/ebpα-null mice die within several hours after birth. Furthermore, the function of C/ebpα in osteoclast terminal differentiation, activation, and function is largely unknown. Herein, we generated and analyzed an osteoclast-specific C/ebpα conditional knockout (CKO) mouse model via Ctsk-Cre mice and found that C/ebpα-deficient mice exhibited a severe osteopetrosis phenotype due to impaired osteoclast terminal differentiation, activation, and function, including mildly reduced osteoclast number, impaired osteoclast polarization, actin formation, and bone resorption, which demonstrated the novel function of C/ebpα in cell function and terminal differentiation. Interestingly, C/ebpα deficiency did not affect bone formation or monocyte/macrophage development. Our results further demonstrated that C/ebpα deficiency suppressed the expression of osteoclast functional genes, e.g. encoding cathepsin K (Ctsk), Atp6i (Tcirg1), and osteoclast regulator genes, e.g. encoding c-fos (Fos), and nuclear factor of activated T-cells 1 (Nfatc1), while having no effect on Pu.1 (Spi1) expression. Promoter activity mapping and ChIP assay defined the critical cis-regulatory element (CCRE) in the promoter region of Nfatc1, and also showed that the CCREs were directly associated with C/ebpα, which enhanced the promoter's activity. The deficiency of C/ebpα in osteoclasts completely blocked ovariectomy-induced bone loss, indicating that C/ebpα is a promising new target for the treatment of osteolytic diseases. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Actin-binding protein coronin 1A controls osteoclastic bone resorption by regulating lysosomal secretion of cathepsin K

Osteoclasts degrade bone matrix proteins via the secretion of lysosomal enzymes. However, the precise mechanisms by which lysosomal components are transported and fused to the bone-apposed plasma membrane, termed ruffled border membrane, remain elusive. Here, we identified coronin 1A as a negative regulator of exocytotic release of cathepsin K, one of the most important bone-degrading enzymes in osteoclasts. The modulation of coronin 1A expression did not alter osteoclast differentiation and extracellular acidification, but strongly affected the secretion of cathepsin K and osteoclast bone-resorption activity, suggesting the coronin 1A-mediated regulation of lysosomal trafficking and protease exocytosis. Further analyses suggested that coronin 1A prevented the lipidation-mediated sorting of the autophagy-related protein LC3 to the ruffled border and attenuated lysosome-plasma membrane fusion. In this process, the interactions between coronin 1A and actin were crucial. Collectively, our findings indicate that coronin 1A is a pivotal component that regulates lysosomal fusion and the secretion pathway in osteoclast-lineage cells and may provide a novel therapeutic target for bone diseases.

Gα13 negatively controls osteoclastogenesis through inhibition of the Akt-GSK3β-NFATc1 signalling pathway

Many positive signalling pathways of osteoclastogenesis have been characterized, but negative signalling pathways are less well studied. Here we show by microarray and RNAi that guanine nucleotide-binding protein subunit α13 (Gα13) is a negative regulator of osteoclastogenesis. Osteoclast-lineage-specific Gna13 conditional knockout mice have a severe osteoporosis phenotype. Gna13-deficiency triggers a drastic increase in both osteoclast number and activity (hyper-activation), mechanistically through decreased RhoA activity and enhanced Akt/GSK3β/NFATc1 signalling. Consistently, Akt inhibition or RhoA activation rescues hyper-activation of Gna13-deficient osteoclasts, and RhoA inhibition mimics the osteoclast hyperactivation resulting from Gna13-deficiency. Notably, Gα13 gain-of-function inhibits Akt activation and osteoclastogenesis, and protects mice from pathological bone loss in disease models. Collectively, we reveal that Gα13 is a master endogenous negative switch for osteoclastogenesis through regulation of the RhoA/Akt/GSK3β/NFATc1 signalling pathway, and that manipulating Gα13 activity might be a therapeutic strategy for bone diseases.

Molecular mechanism of thiazolidinedione-mediated inhibitory effects on osteoclastogenesis

Thiazolidinediones are synthetic peroxisome proliferator-activated receptor γ agonists used to treat type 2 diabetes mellitus. Clinical evidence indicates that thiazolidinediones increase fracture risks in type 2 diabetes mellitus patients, but the mechanism by which thiazolidinediones augment fracture risks is not fully understood. Several groups recently demonstrated that thiazolidinediones stimulate osteoclast formation, thus proposing that thiazolidinediones induce bone loss in part by prompting osteoclastogenesis. However, numerous other studies showed that thiazolidinediones inhibit osteoclast formation. Moreover, the molecular mechanism by which thiazolidinediones modulate osteoclastogenesis is not fully understood. Here we independently address the role of thiazolidinediones in osteoclastogenesis in vitro and furthermore investigate the molecular mechanism underlying the in vitro effects of thiazolidinediones on osteoclastogenesis. Our in vitro data indicate that thiazolidinediones dose-dependently inhibit osteoclastogenesis from bone marrow macrophages, but the inhibitory effect is considerably reduced when bone marrow macrophages are pretreated with RANKL. In vitro mechanistic studies reveal that thiazolidinediones inhibit osteoclastogenesis not by impairing RANKL-induced activation of the NF-κB, JNK, p38 and ERK pathways in bone marrow macrophages. Nonetheless, thiazolidinediones inhibit osteoclastogenesis by suppressing RANKL-induced expression of NFATc1 and c-Fos, two key transcriptional regulators of osteoclastogenesis, in bone marrow macrophages. In addition, thiazolidinediones inhibit the RANKL-induced expression of osteoclast genes encoding matrix metalloproteinase 9, cathepsin K, tartrate-resistant acid phosphatase and carbonic anhydrase II in bone marrow macrophages. However, the ability of thiazolidinediones to inhibit the expression of NFATc1, c-Fos and the four osteoclast genes is notably weakened in RANKL-pretreated bone marrow macrophages. These in vitro studies have not only independently demonstrated that thiazolidinediones exert inhibitory effects on osteoclastogenesis but have also revealed crucial new insights into the molecular mechanism by which thiazolidinediones inhibit osteoclastogenesis.

RANKL-mediated osteoclast formation from murine RAW 264.7 cells

Extensive research efforts over the years have provided us with great insights into how bone-resorbing osteoclasts (OCs) develop and function and, based on such work, valuable antiresorptive therapies have been developed to help combat the excessive bone loss that occurs in numerous skeletal disorders. The RAW 264.7 murine cell line has proven to be an important tool for in vitro studies of OC formation and function, having particular advantages over the use of OCs generated from primary bone marrow cell populations or directly isolated from murine bones. These include their ready access and availability, simple culture for this pure macrophage/pre-OC population, sensitive and rapid development into highly bone-resorptive OCs expressing hallmark OC characteristics following their RANKL stimulation, abundance of RAW cell-derived OCs that can be generated to provide large amounts of study material, relative ease of transfection for genetic and regulatory manipulation, and close correlation in characteristics, gene expression, signaling, and developmental or functional processes between RAW cell-derived OCs and OCs either directly isolated from murine bones or formed in vitro from primary bone marrow precursor cells. Here, we describe methods for the culture and RANKL-mediated differentiation of RAW cells into bone-resorptive OCs as well as procedures for their enrichment, characterization, and general use in diverse analytical assays.

Rac1 and Rac2 in osteoclastogenesis: a cell immortalization model

Cell lines generated from primary cells with a particular gene deletion are useful for examining the function of the specific deleted genes and provide the opportunity to genetically rescue the lost genes using standard gene transfection techniques. In the present study, bone marrow monocytes from wild-type (WT), Rac1 null, and Rac2 null mice were primed with macrophage colony-stimulating factor and soluble receptor activator of NF-kappaB ligand to generate preosteoclasts. This was followed by transduction of a retrovirus containing simian virus 40 large T-antigen and a neomycin-resistant cassette. Seven to 19 immortalized cell lines from each genotype were established. Among them, WT2, Rac1 null-D9, and Rac2 null-A2 were characterized to verify that osteoclastogenesis and osteoclast functions were identical to the parental primary cells. Results showed that immortalized WT2 cells were able to differentiate into mature, multinucleated, functional, tartrate-resistant acid phosphatase-positive osteoclasts. Immortal Rac1 null cells, as with their primary cell counterparts, displayed a severe defect in osteoclastogenesis and function. Transfection of the Rac1 gene into Rac1 null cells was sufficient to rescue osteoclastogenesis. We believe this method of generating immortalized preosteoclasts will provide a key tool for studying the signaling mechanisms involved in osteoclastogenesis.

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.

Atp6v0d2 is an essential component of the osteoclast-specific proton pump that mediates extracellular acidification in bone resorption

Bone resorption relies on the extracellular acidification function of vacuolar (V-) ATPase proton pump(s) present in the plasma membrane of osteoclasts. The exact configuration of osteoclast-specific V-ATPases remains largely unknown. In this study, we found that Atp6v0d2 (d2), an isoform of the d subunit in the V-ATPase, showed 5-fold higher expression than that of Atp6v0d1 (d1) in mature osteoclasts, indicating a potential function in osteoclastic bone resorption. When d2 was depleted at an early stage of RANKL-induced osteoclast differentiation in vitro, formation of multinucleated cells was severely impaired. However, depletion of d2 at a late differentiation stage did not affect osteoclast fusion but did abolish the activity of extracellular acidification and bone resorption of mature osteoclasts. We also showed the association of the two tagged-proteins d2 and a3 when co-expressed in mammalian cells with a co-immunoprecipitation assay. Moreover, glutathione-S-transferase (GST) pull-down assay showed the direct interaction of d2 with the N terminus of Atp6v0a3 (a3), which is the functionally identified osteoclast-specific component of V-ATPase. Therefore, our results show the dual function of d2 as a regulator of cell fusion in osteoclast differentiation and as an essential component of the osteoclast-specific proton pump that mediates extracellular acidification in bone resorption.

Signaling networks that control the lineage commitment and differentiation of bone cells

Osteoblasts and osteoclasts are the two major bone cells involved in the bone remodeling process. Osteoblasts are responsible for bone formation while osteoclasts are the bone-resorbing cells. The major event that triggers osteogenesis and bone remodeling is the transition of mesenchymal stem cells into differentiating osteoblast cells and monocyte/macrophage precursors into differentiating osteoclasts. Imbalance in differentiation and function of these two cell types will result in skeletal diseases such as osteoporosis, Paget's disease, rheumatoid arthritis, osteopetrosis, periodontal disease, and bone cancer metastases. Osteoblast and osteoclast commitment and differentiation are controlled by complex activities involving signal transduction and transcriptional regulation of gene expression. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of the multiple factors and signaling networks that control the differentiation process at a molecular level. This review summarizes recent advances in studies of signaling transduction pathways and transcriptional regulation of osteoblast and osteoclast cell lineage commitment and differentiation. Understanding the signaling networks that control the commitment and differentiation of bone cells will not only expand our basic understanding of the molecular mechanisms of skeletal development but will also aid our ability to develop therapeutic means of intervention in skeletal diseases.

Atp6v1c1 is an essential component of the osteoclast proton pump and in F-actin ring formation in osteoclasts

Bone resorption relies on the extracellular acidification function of V-ATPase (vacuolar-type proton-translocating ATPase) proton pump(s) present in the plasma membrane of osteoclasts. The exact configuration of the osteoclast-specific ruffled border V-ATPases remains largely unknown. In the present study, we found that the V-ATPase subunit Atp6v1c1 (C1) is highly expressed in osteoclasts, whereas subunits Atp6v1c2a (C2a) and Atp6v1c2b (C2b) are not. The expression level of C1 is highly induced by RANKL [receptor activator for NF-kappaB (nuclear factor kappaB) ligand] during osteoclast differentiation; C1 interacts with Atp6v0a3 (a3) and is mainly localized on the ruffled border of activated osteoclasts. The results of the present study show for the first time that C1-silencing by lentivirus-mediated RNA interference severely impaired osteoclast acidification activity and bone resorption, whereas cell differentiation did not appear to be affected, which is similar to a3 silencing. The F-actin (filamentous actin) ring formation was severely defected in C1-depleted osteoclasts but not in a3-depleted and a3(-/-) osteoclasts. C1 co-localized with microtubules in the plasma membrane and its vicinity in mature osteoclasts. In addition, C1 co-localized with F-actin in the cytoplasm; however, the co-localization chiefly shifted to the cell periphery of mature osteoclasts. The present study demonstrates that Atp6v1c1 is an essential component of the osteoclast proton pump at the osteoclast ruffled border and that it may regulate F-actin ring formation in osteoclast activation.

Specificity of RGS10A as a key component in the RANKL signaling mechanism for osteoclast differentiation

Significant progress has been made in studies of the mechanisms by which RANKL induces terminal osteoclast differentiation. However, many crucial details in the RANKL-evoked signaling pathway for osteoclast differentiation remain to be defined. We characterized genes specifically expressed in osteoclasts by differential screening of a human osteoclastoma cDNA library, and found that the regulator of G-protein signaling 10A (RGS10A), but not the RGS10B isoform, was specifically expressed in human osteoclasts. The expression of RGS10A is also induced by RANKL in osteoclast precursors and is prominently expressed in mouse osteoclast-like cells. RGS10A silencing by RNA interference blocked intracellular [Ca2+]i oscillations, the expression of NFAT2, and osteoclast terminal differentiation in both bone marrow cells and osteoclast precursor cell lines. Reintroduction of RGS10A rescued the impaired osteoclast differentiation. RGS10A silencing also resulted in premature osteoclast apoptosis. RGS10A silencing affected the RANKL-[Ca2+]i oscillation-NFAT2 signaling pathway but not other RANKL-induced responses. Our data demonstrate that target components of RGS10A are distinct from those of RGS12 in the RANKL signaling mechanism. Our results thus show the specificity of RGS10A as a key component in the RANKL-evoked signaling pathway for osteoclast differentiation, which may present a promising target for therapeutic intervention.

Novel pycnodysostosis mouse model uncovers cathepsin K function as a potential regulator of osteoclast apoptosis and senescence

Pycnodysostosis is a genetic bone disease featuring the unique bone homeostasis disorders of osteolysis and osteopetrosis in the same organism. The pathomechanism for pycnodysostosis has been largely unknown due to the unavailability of a pycnodysostosis mouse model with all the traits of the disease. We generated cathepsin K(-/-) mouse strains in the 129/Sv and C57BL/6J backgrounds and found that, only in the 129/Sv background, cathepsin K(-/-) mice exhibit many characteristics of the human pycnodysostosis-like phenotype. Our data indicated that 129/Sv cathepsin K(-/-) osteoclasts (OCs) lacked normal apoptosis and senescence and exhibited over-growth both in vitro and in vivo. These abnormalities resulted in an unusually high OC number, which is consistent with a recent case study of human pycnodysostosis. Our results show that cathepsin K function has different effects around the skeleton due to site-specific variations in bone homeostasis, such as phenotypes of osteopetrosis in tibiae and osteolysis in calvariae as a result of cathepsin K mutation. Our data demonstrated that the expression levels of p19, p53 and p21 were significantly reduced in 129/Sv cathepsin K(-/-) OCs and forced expression of cathepsin K in pre-OCs induced premature senescence and increased expression of p19, p53 and p21. This is the first evidence that cathepsin K plays a key role in OC apoptosis and senescence, revealing the importance of OC senescence in bone homeostasis. The finding of this novel cathepsin K function provides insight into the pathomechanism of pycnodysostosis and may provide new drug targets for diseases involved in OC-related abnormal bone homeostasis.

RGS12 is essential for RANKL-evoked signaling for terminal differentiation of osteoclasts in vitro

How RANKL evokes [Ca(2+)](i) oscillations and leads to osteoclast differentiation is unclear. We identified a new signaling protein, RGS12, and found that RGS12 is essential for [Ca(2+)](i) oscillations and osteoclast differentiation induced by RANKL. RGS12 may play a critical role in the RANKL-evoked PLCgamma-calcium channels-[Ca(2+)](i) oscillation-NFAT2 pathway.

INTRODUCTION

RANKL-induced [Ca(2+)](i) oscillations play a switch-on role in NFAT2 expression and osteoclast differentiation. However, RANKL evokes [Ca(2+)](i) oscillations and leads to osteoclast differentiation by an unknown mechanism. In this study, we identified a new RANKL-induced signaling protein, regulator of G signaling protein 12 (RGS12), and investigated its effect on osteoclast differentiation in vitro.

MATERIALS AND METHODS

We used a genome-wide screening approach to identify genes that are specifically or prominently expressed in osteoclasts. To study the role of the RGS12 in osteoclast differentiation, we used vector and lentivirus-based RNAi gene silencing technology to silence the RGS12 gene in the monocyte progenitor cell lines and primary bone marrow-derived monocytes (BMMs). The interaction between RGS12 and N-type calcium channels was elucidated using co-immunoprecipitation and immunoblotting.

RESULTS

We found that RGS12 was prominently expressed in osteoclast-like cells (OLCs) induced by RANKL. This result was further confirmed at both the mRNA and protein level in human osteoclasts and mouse OLCs. Silence of RGS12 expression using vector and lentivirus based RNA interference (RNAi) impaired phosphorylation of phospholipase C (PLC)gamma and blocked [Ca(2+)](i) oscillations, NFAT2 expression, and osteoclast differentiation in RANKL-induced RAW264.7 cells and BMMs. We further found that N-type calcium channels were expressed in OLCs after RANKL stimulation and that RGS12 directly interacted with the N-type calcium channels.

CONCLUSIONS

These results reveal that RGS12 is essential for the terminal differentiation of osteoclasts induced by RANKL. It is possible that RGS12 regulates osteoclast differentiation through a PLC gamma-calcium channel-[Ca(2+)](i) oscillation-NFAT2 pathway.

B and T lymphocytes are the primary sources of RANKL in the bone resorptive lesion of periodontal disease

Receptor activator of nuclear factor-kappaB (RANKL)-mediated osteoclastogenesis plays a pivotal role in inflammatory bone resorption. The aim of this study was to identify the cellular source of RANKL in the bone resorptive lesions of periodontal disease. The concentrations of soluble RANKL, but not its decoy receptor osteoprotegerin, measured in diseased tissue homogenates were significantly higher in diseased gingival tissues than in healthy tissues. Double-color confocal microscopic analyses demonstrated less than 20% of both B cells and T cells expressing RANKL in healthy gingival tissues. By contrast, in the abundant mononuclear cells composed of 45% T cells, 50% B cells, and 5% monocytes in diseased gingival tissues, more than 50 and 90% of T cells and B cells, respectively, expressed RANKL. RANKL production by nonlymphoid cells was not distinctly identified. Lymphocytes isolated from gingival tissues of patients induced differentiation of mature osteoclast cells in a RANKL-dependent manner in vitro. However, similarly isolated peripheral blood B and T cells did not induce osteoclast differentiation, unless they were activated in vitro to express RANKL; emphasizing the osteoclastogenic potential of activated RANKL-expressing lymphocytes in periodontal disease tissue. These results suggest that activated T and B cells can be the cellular source of RANKL for bone resorption in periodontal diseased gingival tissue.

Establishment and characterization of new osteoclast progenitor cell lines derived from osteopetrotic and wild type mice

Malignant infantile osteopetrosis is a rare and lethal disease characterized by the absence of bone resorption due to inactive osteoclasts (OCLs). Among the murine models of osteopetrosis, the Tcirg1oc/oc mouse is the most resembling to the human pathology. In the majority of patients as in Tcirg1oc/oc mouse, the gene involved is the Tcirg1 gene, encoding the a3 subunit of the vacuolar proton pump. However, to date, no osteoclastic cell lines from osteopetrotic mice are available to facilitate the study of either OCL differentiation in osteopetrosis or the factors involved in the control of Tcirg1 gene expression. Heterozygotes Tcirg1+/oc mice were crossed with p53+/- mice to obtain homozygotes p53-/-Tcirg1oc/oc and p53-/-Tcirg1+/+ animals. The p53-/-Tcirg1oc/oc mice display the same bone and hematological phenotype as the original Tcirg1oc/oc mice. From the bone marrow of these mice, we have derived cell lines named POC-MGoc/oc and POC-MG+/+. These cell lines express standard osteoclastogenic markers and differentiate into OCLs in the presence of RANK-L and M-CSF. Furthermore, both cell lines can be transduced by a lentiviral vector with a high efficiency and without alteration of their OCL differentiation potential. Therefore, these cell lines provide valuable new tools to study the differentiation and function of osteoclasts in normal and resorption defective conditions.

IL-1alpha stimulates cathepsin K expression in osteoclasts via the tyrosine kinase-NF-kappaB pathway

Interleukin-1alpha (IL-1alpha) is a powerful activator of osteoclast cells. However, the underlying mechanism for this activation is unknown. In this study, we reveal that IL-1alpha up-regulates the expression of cathepsin K protein, a key protease in bone resorption, by five-fold. Northern blot analysis and promoter analysis show that this induction occurs at the transcriptional level, in a dose-responsive and time-dependent manner. No increase in expression occurs in the presence of either pyrrolidine dithiocarbamate (PDTC), a selective inhibitor of NF-kappaB, or Genistein, a protein tyrosine kinase inhibitor, suggesting that IL-1alpha up-regulation may be via the tyrosine kinase-NF-kappaB pathway to regulate cathepsin K expression. Antisense oligonucleotides to p65, but not the p50 subunit of NF-kappaB, suppress the IL-1alpha-induced expression of cathepsin K. We therefore conclude that IL-1alpha up-regulates cathepsin K gene expression at the transcription level, and this regulation may be via the tyrosine-kinase-NF-kappaB pathway.

Selective blockade of voltage-gated potassium channels reduces inflammatory bone resorption in experimental periodontal disease

The effects of the potassium channel (Kv1.3) blocker kaliotoxin on T-cell-mediated periodontal bone resorption were examined in rats. Systemic administration of kaliotoxin abrogated the bone resorption in conjunction with decreased RANKL mRNA expression by T-cells in gingival tissue. This study suggests a plausible therapeutic approach for inflammatory bone resorption by targeting Kv1.3.

INTRODUCTION

Kv1.3 is a critical potassium channel to counterbalance calcium influx at T-cell receptor activation. It is not known if Kv1.3 also regulates RANKL expression by antigen-activated T-cells, and consequently affects in vivo bone resorption mediated by activated T-cells.

MATERIALS AND METHODS

Actinobacillus actinomycetemcomitans 29-kDa outer membrane protein-specific Th1-clone cells were used to evaluate the expression of Kv1.3 (using reverse transcriptase-polymerase chain reaction [RT-PCR] and Western blot analyses) and the effects of the potassium channel blocker kaliotoxin (0-100 nM) on T-cell activation parameters ([3H]thymidine incorporation assays and ELISA) and expression of RANKL and osteoprotegerin (OPG; flow cytometry, Western blot, and RT-PCR analyses). A rat periodontal disease model based on the adoptive transfer of activated 29-kDa outer membrane protein-specific Th1 clone cells was used to analyze the effects of kaliotoxin in T-cell-mediated alveolar bone resorption and RANKL and OPG mRNA expression by gingival T-cells. Stimulated 29-kDa outer membrane protein-specific Th1 clone cells were transferred intravenously on day 0 to all animals used in the study (n = 7 animals per group). Ten micrograms of kaliotoxin were injected subcutaneously twice per day on days 0, 1, 2, and 3, after adoptive transfer of the T-cells. The control group of rats was injected with saline as placebo on the same days as injections for the kaliotoxin-treated group. The MOCP-5 osteoclast precursor cell line was used in co-culture studies with fixed 29-kDa outer membrane protein-specific Th1-clone cells to measure T-cell-derived RANKL-mediated effects on osteoclastogenesis and resorption pit formation assays in vitro. Statistical significance was evaluated by Student's t-test.

RESULTS

Kaliotoxin decreased T-cell activation parameters of 29-kDa outer membrane protein-specific Th1 clone cells in vitro and in vivo. Most importantly, kaliotoxin administration resulted in an 84% decrease of the bone resorption induced in the saline-treated control group. T-cells recovered from the gingival tissue of kaliotoxin-treated rats displayed lower ratios of RANKL and OPG mRNA expression than those recovered from the control group. The ratio of RANKL and osteoprotegerin protein expression and induction of RANKL-dependent osteoclastogenesis by the activated T-cells were also markedly decreased after kaliotoxin treatments in vitro.

CONCLUSION

The use of kaliotoxin or other means to block Kv1.3 may constitute a potential intervention therapy to prevent alveolar bone loss in periodontal disease.

An assessment of ADAMs in bone cells: absence of TACE activity prevents osteoclast recruitment and the formation of the marrow cavity in developing long bones

ADAMs (A Disintegrin And Metalloprotease domain) are metalloprotease-disintegrin proteins that have been implicated in cell adhesion, protein ectodomain shedding, matrix protein degradation and cell fusion. Since such events are critical for bone resorption and osteoclast recruitment, we investigated whether they require ADAMs. We report here which ADAMs we have identified in bone cells, as well as our analysis of the generation, migration and resorptive activity of osteoclasts in developing metatarsals of mouse embryos lacking catalytically active ADAM 17 [TNFalpha converting enzyme (TACE)]. The absence of TACE activity still allowed the generation of cells showing an osteoclastic phenotype, but prevented their migration into the core of the diaphysis and the subsequent formation of marrow cavity. This suggests a role of TACE in the recruitment of osteoclasts to future resorption sites.

Molecular cloning, developmental expression, promoter analysis and functional characterization of the mouse CNBP gene

Striking conservation in various organisms suggests that cellular nucleic acid-binding protein (CNBP) plays a fundamental biological role across different species. However, the regulated expression and physiological properties of the CNBP gene are unknown. In this study, we report the molecular cloning, promoter characterization, developmental expression and functional analysis of the mouse CNBP gene. The gene contains five exons and is localized to chromosome 6 in the region corresponding to band 6 D1-D2. Primer extension assay indicates that the transcription start site is located 230 bp upstream of the initiator Met codon. Our promoter analysis indicates that strong transcription enhancer and silencer regions lie within the 1.6 kb proximal region of the promoter and the upstream -3.0 to -1.6 kb region, respectively. The promoter activity is 10 fold higher in embryonic carcinoma cells than that in fibroblast, as determined by CAT assay. Consistent with its function as a transcription factor, CNBP protein is located in the nucleus of cells. During mouse embryogenesis, CNBP is expressed in the anterior region of the early embryo and in the limb, tail and craniofacial region. Overexpression of CNBP strongly stimulates cell proliferation and increases c-myc promoter activity. Our finds suggest that CNBP may play an important role in cell proliferation and tissue patterning during anterior-posterior axis, craniofacial and limb development by targeting c-Myc.

c-myc is required for osteoclast differentiation

The role of the receptor activator of nuclear factor kappaB (NF-kappaB) ligand (RANKL)-a tumor necrosis factor (TNF)-related cytokine-in osteoclast formation has been established clearly. However, the downstream signaling pathways activated by this cytokine remain largely unknown. To identify genes that play a role in osteoclastogenesis, we used RAW 264.7 mouse monocytes as a model system for the differentiation of multinucleated osteoclasts from mononucleated precursors. RAW 264.7 cells were induced with RANKL to form multinucleated giant osteoclast-like cells (OCLs) that expressed a number of osteoclast-specific markers and were able to form resorption pits on both calcium phosphate films and bone slices. This system was used to identify genes that are regulated by RANKL and may play a role in osteoclast differentiation. The proto-oncogene c-myc was strongly up-regulated in RANKL-induced OCLs but was absent in undifferentiated cells. Expression of Myc partners Max and Mad, on the other hand, was constant during OCL differentiation. We expressed a dominant negative Myc in RAW 264.7 cells and were able to block RANKL-induced OCL formation. Northern Blot analysis revealed a delay and a significant reduction in the level of messenger RNA (mRNA) for tartrate-resistant acid phosphatase (TRAP) and cathepsin K. We conclude that c-myc is a downstream target of RANKL and its expression is required for RANKL-induced osteoclastogenesis.

Characterization of mouse Atp6i gene, the gene promoter, and the gene expression

Solubilization of bone mineral by osteoclasts depends on the formation of an acidic extracellular compartment through the action of a V-type ATPase. We previously cloned a gene encoding a putative osteoclast-specific proton pump subunit, termed OC-116 kDa, approved mouse Atp6i (ATPase, H+ transporting, [vacuolar proton pump] member I). The function of Atp6i as osteoclast-specific proton pump subunit was confirmed in our mouse knockout study. However, the transcription regulation of Atp6i remains largely unknown. In this study, the gene encoding mouse Atp6i and the promoter have been isolated and completely sequenced. In addition, the temporal and spatial expressions of Atp6i have been characterized. Intrachromosomal mapping studies revealed that the gene contains 20 exons and 19 introns spanning approximately 11 kilobases (kb) of genomic DNA. Alignment of the mouse Atp6i gene exon sequence and predicted amino acid sequence to that of the human reveals a strong homology at both the nucleotide (82%) and the amino acid (80%) levels. Primer extension assay indicates that there is one transcription start site at 48 base pairs (bp) upstream of the initiator Met codon. Analysis of 4 kb of the putative promoter region indicates that this gene lacks canonical TATA and CAAT boxes and contains multiple putative transcription regulatory elements. Northern blot analysis of RNAs from a number of mouse tissues reveals that Atp6i is expressed predominantly in osteoclasts, and this predominant expression was confirmed by reverse-transcription polymerase chain reaction (RT-PCR) assay and immunohistochemical analysis. Whole-mount in situ hybridization shows that Atp6i expression is detected initially in the headfold region and posterior region in the somite stage of mouse embryonic development (E8.5) and becomes progressively restricted to anterior regions and the limb bud by E9.5. The expression level of Atp6i is largely reduced after E10.5. This is the first report of the characterizations of Atp6i gene, its promoter, and its gene expression patterns during mouse development. This study may provide valuable insights into the function of Atp6i, its osteoclast-selective expression, regulation, and the molecular mechanisms responsible for osteoclast activation.

Interferon-gamma down-regulates gene expression of cathepsin K in osteoclasts and inhibits osteoclast formation

The cytokine, IFN-gamma, has been shown in vitro to inhibit bone resorption, but the mechanisms responsible for this inhibition have not been clearly defined. Cathepsin K is a major protease responsible for bone resorption. IFN-gamma may inhibit bone resorption through down-regulation of osteoclast genes, including cathepsin K. To test the hypothesis, we investigated the effect of IFN-gamma on cathepsin K expression in the MOCP-5 and wild-type mouse bone marrow co-culture systems by Northern blot as well as osteoclast formation at different stages of differentiation. The results show that IFN-gamma down-regulates mRNA levels of cathepsin K in a time- and dose-dependent manner. Consequently, cathepsin K protein production is also reduced by IFN-gamma. Moreover, our results indicate that IFN-gamma inhibits osteoclast formation only early in osteoclast differentiation. IL-6 and TNFalpha did not significantly affect cathepsin K gene expression in osteoclasts. However, IL-1alpha stimulated gene expression. In conclusion, our data suggest that the actions of IFN-gamma on osteoclastic bone resorption may be mediated by its effects on both osteoclast formation at an early stage and osteoclast gene expression in mature osteoclasts.

Identification and characterization of the new osteoclast progenitor with macrophage phenotypes being able to differentiate into mature osteoclasts

Osteoclasts are thought to belong to a macrophage lineage. However, the nature of common precursors of osteoclasts and macrophages remains to be investigated. We have characterized the differentiation potential of mouse bone marrow macrophages into mature osteoclasts. Monocyte macrophage-colony-stimulating factor (M-CSF) stimulated the proliferation of bone marrow macrophages in a dose-dependent manner and these M-CSF-dependent bone marrow macrophage (MDBM) cells efficiently differentiated into the tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts in the presence of soluble RANKL (sRANKL) and M-CSF in the in vitro culture. The macrophage-like cell line TMC16 was established from tsA58 (temperature-sensitive SV40 large T-antigen) transgenic mice in the same manner to the preparation of MDBM cells and also differentiated into mature osteoclasts. During this differentiation in vitro, the morphology of the cells changed from spindle to round and smaller (termed pOC) on day 2 and to multinuclear (termed multinucleated cells [MNCs]) on day 4. The surface expression of macrophage marker CD14 was down-regulated and that of CD43 was up-regulated on pOC, analyzed by flow cytometry. RNA analysis revealed that osteoclast marker genes such as calcitonin receptor (CTR), carbonic anhydrase II (CAII), cathepsin K (cath K), MMP9, and TRAP were strongly expressed in MNCs and weakly in pOC whereas MDBM cells did not express these genes. However, the osteopontin (OPN) gene was strongly expressed in MDBM cells and this expression became weakened after differentiation into pOC. The TMC16 cell line weakly expressed cath K, TRAP, and OPN, suggesting that the TMC16 cell line is immortalized at a stage slightly differentiated from MDBM cells. Furthermore, cell sorting analysis revealed that osteoclast early progenitors in bone marrow cells are preferentially present in the Mac-1- F4/80dull population, which differentiated into MDBM cells (the osteoclast progenitor) expressing Mac-1+ F4/80int, suggesting that M-CSF plays roles of a differentiation factor as well as a growth factor for osteoclast early progenitors. These results showed the transition of morphology, surface markers, and gene expression from the early to mature stage in osteoclast differentiation. We propose three differentiation stages in the osteoclast lineage: the pro-osteoclast (spindle-shaped macrophage cells), the pre-osteoclast (small round mononucleated TRAP-positive cells), and the mature osteoclast (multinucleated TRAP-positive cells) stage.

Osteoclast lineage commitment of bone marrow precursors through expression of membrane-bound TRANCE

Osteoclast formation from hemopoietic precursors is induced by TRANCE (also called RANKL, ODF, and OPGL), a membrane-bound ligand expressed by bone marrow stromal cells. Because soluble recombinant TRANCE is a suboptimal osteoclastogenic stimulus, and to eliminate the need for such dependence on stromal cells, membrane-bound TRANCE was expressed in hematopoietic precursors using retroviral gene transfer. Four TRANCE-expressing osteoclast cell lines were established that continuously generate large numbers of multinucleated cells and express tartrate-resistant acid phosphatase and calcitonin receptors. The multinuclear cells are long-lived and either fuse continuously with each other and with mononuclear cells to form enormous syncytia, or separate to form daughter multinuclear cells. When formed on bone, but not on plastic, the majority of multinuclear cells develop actin rings on bone, and resorb bone, suggesting that bone matrix may provide additional signals that facilitate osteoclastic functional maturation. Surprisingly, multinuclear cells originate from fusion of proliferating mononuclear cells that strongly express the mature macrophage markers F4/80 and Fc receptor, which are not expressed by osteoclasts. These results indicate that osteoclasts can be derived from F4/80-positive and Fc receptor-positive cells, and that TRANCE induces osteoclastic differentiation partly by suppressing the macrophage phenotype.

Atp6i-deficient mice exhibit severe osteopetrosis due to loss of osteoclast-mediated extracellular acidification

Solubilization of bone mineral by osteoclasts depends on the formation of an acidic extracellular compartment through the action of a V-proton pump that has not yet been characterized at the molecular level. We previously cloned a gene (Atp6i, for V-proton pump, H+ transporting (vacuolar proton pump) member I) encoding a putative osteoclast-specific proton pump subunit, termed OC-116kD (ref. 4). Here we show that targeted disruption of Atp6i in mice results in severe osteopetrosis. Atp6i-/- osteoclast-like cells (OCLs) lose the function of extracellular acidification, but retain intracellular lysosomal proton pump activity. The pH in Atp6i-/- liver lysosomes and proton transport in microsomes of Atp6i-/- kidney are identical to that in wild-type mice. Atp6i-/- mice exhibit a normal acid-base balance in blood and urine. Our results demonstrate that Atp6i is unique and necessary for osteoclast-mediated extracellular acidification.

Cell biology of the osteoclast

The osteoclast is a hematopoietic cell derived from CFU-GM and branches from the monocyte-macrophage lineage early during the differentiation process. The marrow microenvironment appears critical for osteoclast formation due to production of RANK ligand, a recently described osteoclast differentiation factor, by marrow stromal cells in response to a variety of osteotropic factors. In addition, factors such as osteoprotegerin, a newly described inhibitor of osteoclast formation, as well as secretory products produced by the osteoclast itself and other cells in the marrow enhance or inhibit osteoclast formation. The identification of the role of oncogenes such as c-fos and pp60 c-src in osteoclast differentiation and bone resorption have provided important insights in the regulation of normal osteoclast activity. Current research is beginning to delineate the signaling pathways involved in osteoclastic bone resorption and osteoclast formation in response to cytokines and hormones. The recent development of osteoclast cell lines may make it possible for major advances to our understanding of the biology of the osteoclast to be realized in the near future.

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.

Characterization of mouse cathepsin K gene, the gene promoter, and the gene expression

Cathepsin K, a lysosomal cysteine protease, is abundantly and selectively expressed in osteoclasts and has a specialized role in osteoclast-mediated bone resorption. In contrast to function studies, transcription regulation of cathepsin K remains largely unknown. In this study, the gene encoding mouse cathepsin K and the promoter have been isolated and completely sequenced. In addition, the temporal and spatial expressions of cathepsin K have been characterized. Intrachromosomal mapping studies revealed that the gene contains eight exons and seven introns spanning approximately 10.6 kb of genomic DNA, a genomic organization that was highly conserved with respect to its human homology. Analysis of the 9 kb 5' flanking region indicates that this gene lacks canonical TATA and CAAT boxes and contains multiple putative transcription regulatory elements which are also present in the comparable position of 5' flanking region of human cathepsin K gene. Mouse cathepsin K was found to be a single-copy gene. Northern blot analysis of RNAs from a number of mouse tissues revealed that cathepsin K mRNA is selectively expressed in osteoclast. The selective expression of cathepsin K was confirmed by anticathepsin K immunohistochemical staining. The sequence of cathepsin K expression was linked to osteoclast differentiation in vivo and in vitro by a tartrate-resistant acid phosphatase-anticathepsin K dual immunostaining technique. Cathepsin K is initially expressed at the preosteoclast stage and throughout the mature osteoclast stage. The primer extension assay indicated a major transcription start site 58 bp upstream of the initiator Met codon. The characterization of the cathepsin K gene, its promoter, and the temporal and spatial expression may provide valuable insights into its osteoclast-specific expression and the molecular mechanisms responsible for osteoclast activation.