Ca2+-ATPase inhibitors and Ca2+-ionophore induce osteoclast-like cell formation in the cocultures of mouse bone marrow cells and calvarial cells

Engineering approaches to manipulate osteoclast behavior for bone regeneration

Extensive research has delved into the multifaceted roles of osteoclasts beyond their traditional function in bone resorption in recent years, uncovering their significant influence on bone formation. This shift in understanding has spurred investigations into engineering strategies aimed at leveraging osteoclasts to not only inhibit bone resorption but also facilitate bone regeneration. This review seeks to comprehensively examine the mechanisms by which osteoclasts impact bone metabolism. Additionally, it explores various engineering methodologies, including the modification of bioactive material properties, localized drug delivery, and the introduction of exogenous cells, assessing their potential and mechanisms in aiding bone repair by targeting osteoclasts. Finally, the review proposes current limitations and future routes for manipulating osteoclasts through biological and material cues to facilitate bone repair.

Deletion of phospholipase D1 decreases bone mass and increases fat mass via modulation of Runx2, β-catenin-osteoprotegerin, PPAR-γ and C/EBPα signaling axis

In osteoporosis, mesenchymal stem cells (MSCs) prefer to differentiate into adipocytes at the expense of osteoblasts. Although the balance between adipogenesis and osteogenesis has been closely examined, the mechanism of commitment determination switch is unknown. Here we demonstrate that phospholipase D1 (PLD1) plays a key switch in determining the balance between bone and fat mass. Ablation of Pld1 reduced bone mass but increased fat in mice. Mechanistically, Pld1^/- MSCs inhibited osteoblast differentiaion with diminished Runx2 expression, while osteoclast differentiation was accelerated in Pld1^-/- bone marrow-derived macrophages. Pld1^-/- osteoblasts showed decreased expression of osteogenic makers. Increased number and resorption activity of osteoclasts in Pld1^-/- mice were corroborated with upregulation of osteoclastogenic markers. Moreover, Pld1^-/- osteoblasts reduced β-catenin mediated-osteoprotegerin (OPG) with increased RANKL/OPG ratio which resulted in accelerated osteoclast differentiation. Thus, low bone mass with upregulated osteoclasts could be due to the contribution of both osteoblasts and osteoclasts during bone remodeling. Moreover, ablation of Pld1 further increased bone loss in ovariectomized mice, suggesting that PLD1 is a negative regulator of osteoclastogenesis. Furthermore, loss of PLD1 increased adipogenesis, body fat mass, and hepatic steatosis along with upregulation of PPAR-γ and C/EBPα. Interestingly, adipocyte-specific Pld1 transgenic mice rescued the compromised phenotypes of fat mass and adipogenesis in Pld1 knockout mice. Collectively, PLD1 regulated the bifurcating pathways of mesenchymal cell lineage into increased osteogenesis and decreased adipogenesis, which uncovered a previously unrecognized role of PLD1 in homeostasis between bone and fat mass.

Finely-Tuned Calcium Oscillations in Osteoclast Differentiation and Bone Resorption

Calcium (Ca²⁺) plays an important role in regulating the differentiation and function of osteoclasts. Calcium oscillations (Ca oscillations) are well-known phenomena in receptor activator of nuclear factor kappa B ligand (RANKL)-induced osteoclastogenesis and bone resorption via calcineurin. Many modifiers are involved in the fine-tuning of Ca oscillations in osteoclasts. In addition to macrophage colony-stimulating factors (M-CSF; CSF-1) and RANKL, costimulatory signaling by immunoreceptor tyrosine-based activation motif-harboring adaptors is important for Ca oscillation generation and osteoclast differentiation. DNAX-activating protein of 12 kD is always necessary for osteoclastogenesis. In contrast, Fc receptor gamma (FcRγ) works as a key controller of osteoclastogenesis especially in inflammatory situation. FcRγ has a cofactor in fine-tuning of Ca oscillations. Some calcium channels and transporters are also necessary for Ca oscillations. Transient receptor potential (TRP) channels are well-known environmental sensors, and TRP vanilloid channels play an important role in osteoclastogenesis. Lysosomes, mitochondria, and endoplasmic reticulum (ER) are typical organelles for intracellular Ca²⁺ storage. Ryanodine receptor, inositol trisphosphate receptor, and sarco/endoplasmic reticulum Ca²⁺ ATPase on the ER modulate Ca oscillations. Research on Ca oscillations in osteoclasts has still many problems. Surprisingly, there is no objective definition of Ca oscillations. Causality between Ca oscillations and osteoclast differentiation and/or function remains to be examined.

Tusc2/Fus1 regulates osteoclast differentiation through NF-κB and NFATc1

Tumor suppressor candidate 2 (Tusc2, also known as Fus1) regulates calcium signaling, and Ca²⁺-dependent nuclear factor of activated T-cells (NFAT) and nuclear factor kappa B (NF-κB) pathways, which play roles in osteoclast differentiation. However, the role of Tusc2 in osteoclasts remains unknown. Here, we report that Tusc2 positively regulates the differentiation of osteoclasts. Overexpression of Tusc2 in osteoclast precursor cells enhanced receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation. In contrast, small interfering RNA-mediated knockdown of Tusc2 strongly inhibited osteoclast differentiation. In addition, Tusc2 induced the activation of RANKL-mediated NF-κB and calcium/calmodulin-dependent kinase IV (CaMKIV)/cAMP-response element (CRE)-binding protein CREB signaling cascades. Taken together, these results suggest that Tusc2 acts as a positive regulator of RANKL-mediated osteoclast differentiation.

Osteoclast precursor differentiation by MCPIP via oxidative stress, endoplasmic reticulum stress, and autophagy

Osteoclasts (OCs) are responsible for bone resorption in inflammatory joint diseases. Monocyte chemotactic protein-1 (MCP-1) has been shown to induce differentiation of monocytes to OC precursors, but nothing is known about the underlying mechanisms. Here, we elucidate how MCPIP, induced by MCP-1, mediates this differentiation. Knockdown of MCPIP abolished MCP-1-mediated expression of OC markers, tartrate-resistant acid phosphatase, and serine protease cathepsin K. Expression of MCPIP induced p47(PHOX) and its membrane translocation, reactive oxygen species formation, and induction of endoplasmic reticulum (ER) stress chaperones, up-regulation of autophagy marker, Beclin-1, and lipidation of LC3, and induction of OC markers. Inhibition of oxidative stress attenuated ER stress and autophagy, and suppressed expression of OC markers. Inhibition of ER stress by a specific inhibitor or by knockdown of IRE1 blocked autophagy and induction of OC markers. ER stress inducers, tunicamycin and thapsigargin, induced expression of OC markers. Autophagy inhibition by 3'-methyladenine, LY294002, wortmannin or by knockdown of Beclin-1 or Atg 7 inhibited MCPIP-induced expression of OC markers. These results strongly suggest that MCP-1-induced differentiation of OC precursor cells is mediated via MCPIP-induced oxidative stress that causes ER stress leading to autophagy, revealing a novel mechanistic insight into the role of MCP-1 in OCs differentiation.

The small molecule harmine regulates NFATc1 and Id2 expression in osteoclast progenitor cells

Small molecule compounds that potently affect osteoclastogenesis could be useful as chemical probes for elucidating the mechanisms of various biological phenomena and as effective therapeutic strategies against bone resorption. An osteoclast progenitor cell-based high-throughput screening system was designed to target activation of NFAT, which is a key event for osteoclastogenesis. Orphan ligand library screening using this system identified the β-carboline derivative harmine, which is a highly potent inhibitor of dual-specificity tyrosine-phosphorylation regulated kinase 1A (DYRK1A), to be an NFAT regulator in osteoclasts. RAW264.7 cells highly expressed DYRK1A protein, and in vitro phosphorylation assay demonstrated that harmine directly inhibited the DYRK1A-mediated phosphorylation (in-activation) of NFATc1. Harmine promoted the dephosphorylation (activation) of NFATc1 in RAW264.7 cells within 24h, and it significantly increased the expression of NFATc1 in RAW264.7 cells and mouse primary bone marrow macrophages (BMMs) both in the presence and absence of RANKL stimulation. Although harmine promoted NFATc1 expression and stimulated target genes for osteoclastogenesis, cell-cell fusion and the formation of TRAP-positive multinucleated osteoclasts from RAW264.7 cells and BMMs was significantly inhibited by harmine treatment. Meanwhile, harmine remarkably promoted the expression of inhibitor of DNA binding/differentiation-2 (Id2), which is a negative regulator for osteoclastogenesis, in RAW264.7 cells and BMMs. An Id2-null-mutant showed slightly increased osteoclast formation from BMMs, and the harmine-mediated inhibition of osteoclast formation was abolished in the BMMs of Id2-null-mutant mice. These results suggest that harmine is a potent activator of NFATc1 that interferes with the function of DYRK1A in osteoclast precursors and also up-regulates Id2 protein, which may dominantly inhibit expression pathways associated with cell-cell fusion, thereby leading to the disruption of the fusion events mediating osteoclastogenesis. The small molecule harmine is therefore expected to provide an experimental tool for investigating signaling cascades in osteoclastogenesis, especially those centered on DYRK1A-mediated NFATc1 and Id2 regulation.

Effects of soluble silicon compound and deep-sea water on biochemical and mechanical properties of bone and the related gene expression in mice

Silicon has been known as an essential element for bone formation. The silicon contents of sea water increase with increasing of depth: 1.8 ppm Si in deep-sea water (DW) at 612 m in depth versus 0.06 ppm in surface sea water (SW). The effects of soluble silicon (Si) and DW from which NaCl was eliminated were studied in comparison with tap water (TW) and SW in cell cultures and in animal experiments using the control strain of senescence accelerated mouse, SAMR1. Si at 10 ppm as sodium metasilicate or 10% DW in the alpha-MEM medium stimulated cellular viability, marker enzymes of osteoblast and osteoclast cell lines, and the (45)CaCl(2) uptake in those cells in comparison with the medium control. After weanling SAMR1 were maintained for 6 months on a diet containing 200 ppm Si and 39% of DW and SW, DW and Si improved bone biochemical indices such as femoral weight, mineral and collagen content, and marker enzymes of bone formation and resorption as well as mechanical properties as compared to TW. In the femoral bone marrow of SAMR1, the mRNA expression of bone morphogenetic protein-2 (BMP-2), interleukin-11 (IL-11), and runt-related transcription factor 2 (Runx 2), which stimulate osteoblast development as well as type I procollagen (COL1A1) mRNA, were significantly increased in both DW and Si groups. The expressions of both osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL) were also elevated, resulting in distinct increases of the OPG/RANKL ratio in both DW and Si groups. The results indicated that a soluble silicate and deep-sea water as its natural material stimulated cell growth in both osteoblasts and osteoclasts in cell culture and promoted bone metabolic turnover in favor of bone formation through stimulation of the related mRNA expression in animal experiments.

Biglycan deficiency increases osteoclast differentiation and activity due to defective osteoblasts

Bone mass is maintained by a fine balance between bone formation by osteoblasts and bone resorption by osteoclasts. Although osteoblasts and osteoclasts have different developmental origins, it is generally believed that the differentiation, function, and survival of osteoclasts are regulated by osteogenic cells. We have previously shown that the extracellular matrix protein, biglycan (Bgn), plays an important role in the differentiation of osteoblast precursors. In this paper, we showed that Bgn is involved in regulating osteoclast differentiation through its effect on osteoblasts and their precursors using both in vivo and in vitro experiments. The in vivo osteolysis experiment showed that LPS (lipopolisaccharide)-induced osteolysis occurred more rapidly and extensively in bgn deficient mice compared to wild type (WT) mice. To further understand the mechanism of action, we determined the effects of Bgn on 1alpha, 25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3))-induced osteoclast differentiation and bone resorption in an co-culture of calvariae-derived pre-osteoblasts and osteoclast precursors derived from spleen or bone marrow. Time course and dose response experiments showed that tartrate-resistant acid phosphatase-positive multinuclear cells appeared earlier and more extensively in the co-cultures containing calvarial cells from bgn deficient mice than WT mice, regardless of the genotype of osteoclast precursors. The osteoblast abnormality that stimulated osteoclast formation appeared to be independent of the differential production of soluble RANKL and OPG and, instead, due to a decrease in osteoblast maturation accompanied by increase in osteoblastic proliferation. In addition to the imbalance between differentiation and proliferation, there was a differential decrease in secretory leukocyte protease inhibitor (slpi) in bgn deficient osteoblasts treated with 1,25-(OH)(2)D(3). These findings point to a novel molecular factor made by osteoblasts that could potentially be involved in LPS-induced osteolysis.

Osteoclast and its roles in calcium metabolism and bone development and remodeling

Osteoclasts are multinucleated cells responsible for bone resorption and play important roles in normal skeletal development, in the maintenance of its integrity throughout life, and in calcium metabolism. During bone resorption, the cytoskeleton of osteoclasts undergoes extensive reorganization, with polarization and formation of ruffled borders to secrete acid and formation of sealing zone to prevent leakage. The differentiation and function of osteoclasts are in turn regulated by osteoblasts, stromal cells, and bone. They are also subjected to negative feedback regulation by extracellular and intracellular calcium concentrations.

Thapsigargin modulates osteoclastogenesis through the regulation of RANKL-induced signaling pathways and reactive oxygen species production

The mechanism by which TG modulates osteoclast formation and apoptosis is not clear. In this study, we showed a biphasic effect of TG on osteoclast formation and apoptosis through the regulation of ROS production, caspase-3 activity, cytosolic Ca2+, and RANKL-induced activation of NF-kappaB and AP-1 activities.

INTRODUCTION

Apoptosis and differentiation are among the consequences of changes in intracellular Ca2+ levels. In this study, we investigated the effects of the endoplasmic reticular Ca2+-ATPase inhibitor, thapsigargin (TG), on osteoclast apoptosis and differentiation.

MATERIALS AND METHODS

Both RAW264.7 cells and primary spleen cells were used to examine the effect of TG on RANKL-induced osteoclastogenesis. To determine the action of TG on signaling pathways, we used reporter gene assays for NF-kappaB and activator protein-1 (AP-1) activity, Western blotting for phospho-extracellular signal-related kinase (ERK), and fluorescent probes to measure changes in levels of intracellular calcium and reactive oxygen species (ROS). To assess rates of apoptosis, we measured changes in annexin staining, caspase-3 activity, and chromatin and F-actin microfilament structure.

RESULTS

At concentrations that caused a rapid rise in intracellular Ca2+, TG increased caspase-3 activity and promoted apoptosis in osteoclast-like cells (OLCs). Low concentrations of TG, which were insufficient to measurably alter intracellular Ca2+, unexpectedly suppressed caspase-3 activity and enhanced RANKL-induced osteoclastogenesis. At these lower concentrations, TG potentiated ROS production and RANKL-induced NF-kappaB activity, but suppressed RANKL-induced AP-1 activity and had little effect on ERK phosphorylation.

CONCLUSION

Our novel findings of a biphasic effect of TG are incompletely explained by our current understanding of TG action, but raise the possibility that low intensity or local changes in subcellular Ca2+ levels may regulate intracellular differentiation signaling. The extent of cross-talk between Ca2+ and RANKL-mediated intracellular signaling pathways might be important in determining whether cells undergo apoptosis or differentiate into OLCs.

Prostaglandin E2 induced the differentiation of osteoclasts in mouse osteoblast-depleted bone marrow cells

Prostaglandin (PG) E(2) is a known bone absorbing agent that acts on osteoblasts to facilitate osteoclastogenesis by increasing the secretion of RANKL. In the present study, we investigated the direct action of PGE(2) on osteoclastic progenitors that differentiate into TRAP-positive multinucleated cells. The hematopoietic stem cell obtained from murine bone marrow was purified by a Sephadex G-10 column, and cultured in the presence of CSF-1 and RANKL to facilitate cell differentiation. The introduction of low-density PGE(2) into the culture resulted in a drastic increase of TRAP-positive multinucleated cells, whereas the addition of high-density PGE(2) had the opposite effect. PCR analysis revealed increased level of EP3 mRNA in undifferentiated cells and reduced level after the development of osteoclast; EP1, EP2 and EP4 were constitutively expressed throughout the differentiation. Investigation of intracellular signaling verified that low-density PGE(2) suppressed PKA activity in undifferentiated cells, suggesting that PGE(2) acts on the osteoclastic cell lineage to facilitate cell differentiation by suppressing PKA in the presence of RANKL.

Regulatory mechanisms of osteoblast and osteoclast differentiation

Bone is continuously destroyed and reformed to maintain constant bone volume and calcium homeostasis in vertebrates throughout their lives. Osteoblasts and osteoclasts are specialized cells responsible for bone formation and resorption, respectively. Recent developments in bone cell biology have greatly changed our conceptions of the regulatory mechanisms of the differentiation of osteoblasts and osteoclasts. Bone morphogenetic proteins (BMPs) play critical roles in osteoblast differentiation. The discovery of Smad-mediated signals revealed the precise functions of BMPs in osteoblast differentiation. Transcription factors, Runx2 and Osterix, are found to be essential molecules for inducing osteoblast differentiation, as indicated by the fact that both Runx2-null mice and Osterix-null mice have neither bone tissue nor osteoblasts. Smad transcriptional factors are shown to interact with other transcription regulators, including Runx2. Also, the recent discovery of receptor activator of NF-kappaB ligand (RANKL)-RANK interaction confirms the well-known hypothesis that osteoblasts play an essential role in osteoclast differentiation. Osteoblasts express RANKL as a membrane-associated factor. Osteoclast precursors that express RANK, a receptor for RANKL, recognize RANKL through the cell-cell interaction and differentiate into osteoclasts. Recent studies have shown that lipopolysaccharide and inflammatory cytokines such as tumor necrosis factor receptor-alpha and interleukin I directly regulate osteoclast differentiation and function through a mechanism independent of the RANKL-RANK interaction. Transforming growth factor-beta super family members and interferon-gamma are also shown to be important regulators in osteoclastogenesis. These findings have opened new areas for exploring the molecular mechanisms of osteoblast and osteoclast differentiation.

Intracellular calcium and protein kinase C mediate expression of receptor activator of nuclear factor-kappaB ligand and osteoprotegerin in osteoblasts

Receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) produced by osteoblasts/stromal cells are involved as positive and negative regulators in osteoclast formation. Three independent signals have been proposed to induce RANKL expression in osteoblasts/stromal cells: vitamin D receptor-, cAMP-, and gp130-mediated signals. We previously reported that intracellular calcium-elevating compounds such as ionomycin, cyclopiazonic acid, and thapsigargin induced osteoclast formation in cocultures of mouse bone marrow cells and primary osteoblasts. Increases in calcium concentration in culture medium also induced osteoclast formation in cocultures. Treatment of primary osteoblasts with these compounds or with high calcium medium stimulated the expression of both RANKL and OPG messenger RNAs (mRNAs). 1,2-Bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid)-tetra(acetoxymethyl)ester, an intracellular calcium chelator, suppressed both ionomycin-induced osteoclast formation in cocultures and expression of RANKL and OPG mRNAs in primary osteoblasts. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, also stimulated osteoclast formation in these cocultures and the expression of RANKL and OPG mRNAs in primary osteoblasts. Protein kinase C inhibitors such as calphostin and staurosporin suppressed ionomycin- and PMA-induced osteoclast formation in cocultures and expression of RANKL and OPG mRNAs in primary osteoblasts. Ionomycin stimulated RANKL mRNA expression in ST2 and MC3T3-G2/PA6 cells, but not in MC3T3-E1 or NIH-3T3 cells. These effects were closely correlated with osteoclast formation in response to ionomycin in cocultures with these stromal cell lines. OPG strongly inhibited osteoclast formation induced by calcium-elevating compounds and PMA in cocultures, suggesting that RANKL expression in osteoblasts is a rate-limiting step for osteoclast induction. Forskolin, an activator of cAMP signals, also stimulated osteoclast formation in cocultures. Forskolin enhanced RANKL mRNA expression but suppressed OPG mRNA expression in primary osteoblasts. These results suggest that the calcium/protein kinase C signal in osteoblasts/stromal cells is the fourth signal for inducing RANKL mRNA expression, which, in turn, stimulates osteoclast formation.

Syntheses of (+/-)-shinflavanone and its structural analogues as potent inhibitors of bone resorption pits formation

The first total syntheses of (+/-)-shinflavanone and its structural analogues were achieved. (+/-)-Shinflavanone, appears to be a strong inhibitor of bone resorption pits formation by osteoclast-like cell induced by 1alpha, 25-dihydroxy vitamine D3 (IC50 = 0.70 microg/mL).