Complex dynamics of osteoclast formation and death in long-term cultures

Loss of ninein interferes with osteoclast formation and causes premature ossification

Ninein is a centrosome protein that has been implicated in microtubule anchorage and centrosome cohesion. Mutations in the human NINEIN gene have been linked to Seckel syndrome and to a rare form of skeletal dysplasia. However, the role of ninein in skeletal development remains unknown. Here, we describe a ninein knockout mouse with advanced endochondral ossification during embryonic development. Although the long bones maintain a regular size, the absence of ninein delays the formation of the bone marrow cavity in the prenatal tibia. Likewise, intramembranous ossification in the skull is more developed, leading to a premature closure of the interfrontal suture. We demonstrate that ninein is strongly expressed in osteoclasts of control mice, and that its absence reduces the fusion of precursor cells into syncytial osteoclasts, whereas the number of osteoblasts remains unaffected. As a consequence, ninein-deficient osteoclasts have a reduced capacity to resorb bone. At the cellular level, the absence of ninein interferes with centrosomal microtubule organization, reduces centrosome cohesion, and provokes the loss of centrosome clustering in multinucleated mature osteoclasts. We propose that centrosomal ninein is important for osteoclast fusion, to enable a functional balance between bone-forming osteoblasts and bone-resorbing osteoclasts during skeletal development.

Modelling the Effects of Growth and Remodelling on the Density and Structure of Cancellous Bone

A two-stage model is proposed for investigating remodelling characteristics in bone over time and distance to the growth plate. The first stage comprises a partial differential equation (PDE) for bone density as a function of time and distance from the growth plate. This stage clarifies the contributions to changes in bone density due to remodelling and growth processes and tracks the rate at which new bone emanates from the growth plate. The second stage consists of simulating the remodelling process to determine remodelling characteristics. Implementing the second stage requires the rate at which bone moves away from the growth plate computed during the first stage. The second stage is also needed to confirm that remodelling characteristics predicted by the first stage may be explained by a realistic model for remodelling and to compute activation frequency. The model is demonstrated on microCT scans of tibia of juvenile female rats in three experimental groups: sham-operated control, oestrogen deprived, and oestrogen deprived followed by treatment. Model predictions for changes in bone density and remodelling characteristics agree with the literature. In addition, the model provides new insight into the role of treatment on the density of new bone emanating from the growth plate and provides quantitative descriptions of changes in remodelling characteristics beyond what has been possible to ascertain by experimentation alone.

Integration of osteoclastogenesis through addition of PBMCs in human osteochondral explants cultured ex vivo

The preservation of tissue specific cells in their native 3D extracellular matrix in bone explants provides a unique platform to study remodeling. Thus far, studies involving bone explant cultures showed a clear focus on achieving bone formation and neglected osteoclast activity and resorption. To simulate the homeostatic bone environment ex vivo, both key elements of bone remodeling need to be represented. This study aimed to assess and include osteoclastogenesis in human osteochondral explants through medium supplementation with RANKL and M-CSF and addition of peripheral blood mononuclear cells (PBMCs), providing osteoclast precursors. Osteochondral explants were freshly harvested from human femoral heads obtained from hip surgeries and cultured for 20 days in a two-compartment culture system. Osteochondral explants preserved viability and cellular abundance over the culture period, but histology demonstrated that resident osteoclasts were no longer present after 4 days of culture. Quantitative extracellular tartrate resistant acid phosphatase (TRAP) analysis confirmed depletion of osteoclast activity on day 4 even when stimulated with RANKL and M-CSF. Upon addition of PBMCs, a significant upregulation of TRAP activity was measured from day 10 onwards. Evaluation of bone loss trough μCT registration and measurement of extracellular cathepsin K activity revealed indications of enhanced resorption upon addition of PBMCs. Based on the results we suggest that an external source of osteoclast precursors, such as PBMCs, needs to be added in long-term bone explant cultures to maintain osteoclastic activity, and bone remodeling.

Osteoclastogenesis of human peripheral blood, bone marrow, and cord blood monocytes

Osteoclasts are multinucleated bone resorbing cells that can be differentiated from human monocytes in vitro. There are few studies comparing osteoclastogenesis of different monocyte sources. We compared monocytes from human bone marrow (BM), peripheral blood (PB), and umbilical cord blood (CB) and their osteoclastogenic potential by culturing them with RANKL (20 and 80 ng/ml) and M-CSF (10 ng/ml) for 14 days. We also cultured cells without growth factors, as umbilical cord blood monocytes have been reported to be able to fuse spontaneously into osteoclasts. The data was analysed on d4, d8, d11, and d14. After culture with RANKL and M-CSF, all types of cell cultures developed TRACP -positive multinuclear cells that were able to form resorption pits on human bone slices. Only occasional multinuclear cells and small infrequent resorbed areas could be found in PB and CB-derived cultures without growth factors. BM-derived cells formed greater resorption areas than PB- and CB-derived monocytes. The greatest monocyte population in BM samples were intermediate (CD14⁺⁺CD16⁺) and in PB and CB classical monocytes (76.3% and 54.4%, respectively). In conclusion, our data demonstrates that bone resorbing osteoclasts can be differentiated from BM, PB and CB. However, the osteoclast precursor origin can affect the osteoclast properties and function.

The Effect of Osteoblast Isolation Methods from Adult Rats on Osteoclastogenesis in Co-Cultures

Co-cultures of osteoblasts and osteoclasts are on the rise because they enable a more complex study. Diseases such as osteoporosis are related to a higher age. Thus, cell isolation from adult individuals is necessary. Osteoblasts can be isolated from the rat femur by three methods: explant culture, explant culture with enzymatic pre-treatment, or enzymatic treatment. The isolation methods yield different populations of osteoblasts which, in a co-culture with peripheral blood mononuclear cells, might result in differences in osteoclastogenesis. Therefore, we examined the differences in osteogenic markers, cell proliferation, and the metabolic activity of isolated osteoblast-like cells in a growth and differentiation medium. We then evaluated the effect of the isolated populations of osteoblast-like cells on osteoclastogenesis in a subsequent co-culture by evaluating osteoclast markers, counting formed osteoclast-like cells, and analyzing their area and number of nuclei. Co-cultures were performed in the presence or absence of osteoclastogenic growth factors, M-CSF and RANKL. It was discovered that enzymatic isolation is not feasible in adult rats, but explant culture and explant culture with enzymatic pre-treatment were both successful. Explant culture with enzymatic pre-treatment yielded cells with a higher proliferation than explant culture in a growth medium. The differentiation medium reduced differences in proliferation during the culture. Some differences in metabolic activity and ALP activity were also found between the osteoblast-like cells isolated by explant culture or by explant culture with enzymatic pre-treatment, but only on some days of cultivation. According to microscopy, the presence of exogenous growth factors supporting osteoclastogenesis in co-cultures was necessary for the formation of osteoclast-like cells. In this case, the formation of a higher number of osteoclast-like cells with a larger area was observed in the co-culture with osteoblast-like cells isolated by explant culture compared to the explant culture with enzymatic pre-treatment. Apart from this observation, no differences in osteoclast markers were noted between the co-cultures with osteoblast-like cells isolated by explant culture and the explant culture with enzymatic pre-treatment. The TRAP and CA II activity was higher in the co-cultures with exogenous growth than that in the co-cultures without exogenous growth factors on day 7, but the opposite was true on day 14. To conclude, explant culture and explant culture with enzymatic pre-treatment are both suitable methods to yield osteoblast-like cells from adult rats capable of promoting osteoclastogenesis in a direct co-culture with peripheral blood mononuclear cells. Explant culture with enzymatic pre-treatment yielded cells with a higher proliferation. The explant culture yielded osteoblast-like cells which induced the formation of a higher number of osteoclast-like cells with a larger area compared to the explant culture with enzymatic pre-treatment when cultured with exogenous M-CSF and RANKL.

Integrated computational and in vivo models reveal Key Insights into macrophage behavior during bone healing

Myeloid-derived monocyte and macrophages are key cells in the bone that contribute to remodeling and injury repair. However, their temporal polarization status and control of bone-resorbing osteoclasts and bone-forming osteoblasts responses is largely unknown. In this study, we focused on two aspects of monocyte/macrophage dynamics and polarization states over time: 1) the injury-triggered pro- and anti-inflammatory monocytes/macrophages temporal profiles, 2) the contributions of pro- versus anti-inflammatory monocytes/macrophages in coordinating healing response. Bone healing is a complex multicellular dynamic process. While traditional in vitro and in vivo experimentation may capture the behavior of select populations with high resolution, they cannot simultaneously track the behavior of multiple populations. To address this, we have used an integrated coupled ordinary differential equations (ODEs)-based framework describing multiple cellular species to in vivo bone injury data in order to identify and test various hypotheses regarding bone cell populations dynamics. Our approach allowed us to infer several biological insights including, but not limited to,: 1) anti-inflammatory macrophages are key for early osteoclast inhibition and pro-inflammatory macrophage suppression, 2) pro-inflammatory macrophages are involved in osteoclast bone resorptive activity, whereas osteoblasts promote osteoclast differentiation, 3) Pro-inflammatory monocytes/macrophages rise during two expansion waves, which can be explained by the anti-inflammatory macrophages-mediated inhibition phase between the two waves. In addition, we further tested the robustness of the mathematical model by comparing simulation results to an independent experimental dataset. Taken together, this novel comprehensive mathematical framework allowed us to identify biological mechanisms that best recapitulate bone injury data and that explain the coupled cellular population dynamics involved in the process. Furthermore, our hypothesis testing methodology could be used in other contexts to decipher mechanisms in complex multicellular processes.

Myeloid-derived monocytes/macrophages are key cells for bone remodeling and injury repair. However, their temporal polarization status and control of bone-resorbing osteoclasts and bone-forming osteoblasts responses is largely unknown. In this study, we focused on two aspects of monocyte/macrophage population dynamics: 1) the injury-triggered pro- and anti-inflammatory monocytes/macrophages temporal profiles, 2) the contributions of pro- versus anti-inflammatory monocytes/macrophages in coordinating healing response. In order to test various hypotheses regarding bone cell populations dynamics, we have integrated a coupled ordinary differential equations-based framework describing multiple cellular species to in vivo bone injury data. Our approach allowed us to infer several biological insights including: 1) anti-inflammatory macrophages are key for early osteoclast inhibition and pro-inflammatory macrophage suppression, 2) pro-inflammatory macrophages are involved in osteoclast bone resorptive activity, whereas osteoblasts promote osteoclast differentiation, 3) Pro-inflammatory monocytes/macrophages rise during two expansion waves, which can be explained by the anti-inflammatory macrophages-mediated inhibition phase between the two waves. Taken together, this mathematical framework allowed us to identify biological mechanisms that recapitulate bone injury data and that explain the coupled cellular population dynamics involved in the process.

Spatial extension of a bone remodeling dynamics model and its finite element analysis

There are many works dealing with the dynamics of bone remodeling, proposing increasingly complex and complete models. In the recent years, the efforts started to focus on developing models that not only reproduce the temporal evolution, but also include the spatial aspects of this phenomenon. In this work, we propose the spatial extension of an existing model that includes the dynamics of osteocytes. The spatial dependence is modeled in terms of a linear diffusion, as proposed in previous works dealing with related problems. The resulting model is then written in its variational form, and fully discretized using the well-known finite element method and a combination of the implicit and explicit Euler schemes. The numerical algorithm is then analyzed, proving some a priori error estimates and its linear convergence. Finally, we extend the examples already published for the temporal model to one and two dimensions, showing the dynamics of the solution in the spatial domain.

'Educated' Osteoblasts Reduce Osteoclastogenesis in a Bone-Tumor Mimetic Microenvironment

Breast cancer (BC) metastases to bone disrupt the balance between osteoblasts and osteoclasts, leading to excessive bone resorption. We identified a novel subpopulation of osteoblasts with tumor-inhibitory properties, called educated osteoblasts (EOs). Here we sought to examine the effect of EOs on osteoclastogenesis during tumor progression. We hypothesized that EOs affect osteoclast development in the bone-tumor niche, leading to suppressed pre-osteoclast fusion and bone resorption. Conditioned media (CM) was analyzed for protein expression of osteoclast factors receptor activator of nuclear factor kappa-β ligand (RANKL), osteoprotegerin (OPG), and tumor necrosis factor alpha (TNFα) via ELISA. EOs were co-cultured with pre-osteoclasts on a bone mimetic matrix to assess osteoclast resorption. Pre-osteoclasts were tri-cultured with EOs plus metastatic BC cells and assessed for tartrate-resistance acid phosphatase (TRAP)-positive, multinucleated (≥3 nuclei), mature osteoclasts. Tumor-bearing murine tibias were stained for TRAP to determine osteoclast number in-vivo. EO CM expressed reduced amounts of soluble TNFα and OPG compared to naïve osteoblast CM. Osteoclasts formed in the presence of EOs were smaller and less in number. Upon co-culture on a mimetic bone matrix, a 50% reduction in the number of TRAP-positive osteoclasts formed in the presence of EOs was observed. The tibia of mice inoculated with BC cells had less osteoclasts per bone surface in bones with increased numbers of EO cells. These data suggest EOs reduce osteoclastogenesis and bone resorption. The data imply EOs provide a protective effect against bone resorption in bone metastatic BC.

Role of Altered Metabolic Microenvironment in Osteolytic Metastasis

Metastatic bone disease is generally incurable and leads to pathological fractures, pain, hypercalcemia, spinal cord compression and decreased mobility. The skeleton is the major site of bone metastases from solid cancers, including breast and prostate carcinoma. Bone metastasis is facilitated by activation of bone-resorbing osteoclasts, terminally differentiated multinucleated cells formed by fusion from monocytic precursors. Cancer cells are known to produce specific factors that stimulate osteoclast differentiation and function. Of interest, cancer cells are also known to alter their own bioenergetics increasing the use of glycolysis for their survival and function. Such change in energy utilization by cancer cells would result in altered levels of cell-permeable metabolites, including glucose, lactate, and pyruvate. Osteoclast resorption is energy-expensive, and we have previously demonstrated that during differentiation osteoclasts actively adapt to their bioenergetics microenvironment. We hypothesize that altered bioenergetics state of cancer cells will also modify the bioenergetics substrate availability for the tissue-resident bone cells, potentially creating a favorable milieu for pathological osteolysis. The goals of this review are to analyze how metastasizing cancer cells change the availability of energy substrates in bone microenvironment; and to assess how the altered bioenergetics may affect osteoclast differentiation and activity.

Analysis of a bone remodeling model with myeloma disease arising in cellular dynamics

In this work we study a bone remodeling model for the evolution of the myeloma disease. The biological problem is written as a coupled nonlinear system consisting of parabolic partial differential equations. They are written in terms of the concentrations of osteoblasts and osteoclasts, the density of the relative bone and the concentration of the tumor cells. Then, we deal with the numerical analysis of this variational problem, introducing a numerical approximation by using the finite element method and a hybrid combination of both implicit and explicit Euler schemes. We perform some a priori error estimates and show a few numerical simulations to demonstrate the accuracy of the approximation. Finally, we present the comparison with previous works and the behavior of the solution in two-dimensional examples.

Interaction of Brucella abortus with Osteoclasts: a Step toward Understanding Osteoarticular Brucellosis and Vaccine Safety

Osteoarticular disease is a frequent complication of human brucellosis. Vaccination remains a critical component of brucellosis control, but there are currently no vaccines for use in humans, and no in vitro models for assessing the safety of candidate vaccines in reference to the development of bone lesions currently exist. While the effect of Brucella infection on osteoblasts has been extensively evaluated, little is known about the consequences of osteoclast infection. Murine bone marrow-derived macrophages were derived into mature osteoclasts and infected with B. abortus 2308, the vaccine strain S19, and attenuated mutants S19vjbR and B. abortus ΔvirB2 While B. abortus 2308 and S19 replicated inside mature osteoclasts, the attenuated mutants were progressively killed, behavior that mimics infection kinetics in macrophages. Interestingly, B. abortus 2308 impaired the growth of osteoclasts without reducing resorptive activity, while osteoclasts infected with B. abortus S19 and S19vjbR were significantly larger and exhibited enhanced resorption. None of the Brucella strains induced apoptosis or stimulated nitric oxide or lactose dehydrogenase production in mature osteoclasts. Finally, infection of macrophages or osteoclast precursors with B. abortus 2308 resulted in generation of smaller osteoclasts with decreased resorptive activity. Overall, Brucella exhibits similar growth characteristics in mature osteoclasts compared to the primary target cell, the macrophage, but is able to impair the maturation and alter the resorptive capacity of these cells. These results suggest that osteoclasts play an important role in osteoarticular brucellosis and could serve as a useful in vitro model for both analyzing host-pathogen interactions and assessing vaccine safety.

Activin A receptor type 1-mediated BMP signaling regulates RANKL-induced osteoclastogenesis via canonical SMAD-signaling pathway

Bone morphogenetic proteins (BMPs) are important mediators of osteoclast differentiation. Although accumulating evidence has implicated BMPs in osteoblastogenesis, the mechanisms by which BMPs regulate osteoclastogenesis remain unclear. Activin A receptor type 1 (ACVR1) is a BMP type 1 receptor essential for skeletal development. Here, we observed that BMP-7, which preferentially binds to ACVR1, promotes osteoclast differentiation, suggesting ACVR1 is involved in osteoclastogenesis. To investigate this further, we isolated osteoclasts from either Acvr1-floxed mice or mice with constitutively-activated Acvr1 (caAcvr1) carrying tamoxifen-inducible Cre driven by a ubiquitin promotor and induced Cre activity in culture. Osteoclasts from the Acvr1-floxed mice had reduced osteoclast numbers and demineralization activity, whereas those from the caAcvr1-mutant mice formed large osteoclasts and demineralized pits, suggesting that BMP signaling through ACVR1 regulates osteoclast fusion and activity. It is reported that BMP-2 binds to BMPR1A, another BMP type 1 receptor, whereas BMP-7 binds to ACVR1 to activate SMAD1/5/9 signaling. Here, Bmpr1a-disrupted osteoclasts displayed reduced phospho-SMAD1/5/9 (pSMAD1/5/9) levels when induced by BMP-2, whereas no impacts on pSMAD1/5/9 were observed when induced by BMP-7. In contract, Acvr1-disrupted osteoclasts displayed reduced pSMAD1/5/9 levels when induced either by BMP-2 or BMP-7, suggesting that ACVR1 is the major receptor for transducing BMP-7 signals in osteoclasts. Indeed, LDN-193189 and LDN-212854, which specifically block SMAD1/5/9 phosphorylation, inhibited osteoclastogenesis of caAcvr1-mutant cells. Moreover, increased BMP signaling promoted nuclear translocation of nuclear factor-activated T-cells 1 (NFATc1), which was inhibited by LDN treatments. Taken together, ACVR1-mediated BMP-SMAD signaling activates NFATc1, a regulatory protein crucial for receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis.

Protection Against Arthritis by the Parasitic Worm Product ES-62, and Its Drug-Like Small Molecule Analogues, Is Associated With Inhibition of Osteoclastogenesis

The immunomodulatory actions of parasitic helminth excretory-secretory (ES) products that serendipitously protect against development of chronic inflammatory disorders are well established: however, knowledge of the interaction between ES products and the host musculoskeletal system in such diseases is limited. In this study, we have focused on ES-62, a glycoprotein secreted by the rodent filarial nematode Acanthocheilonema viteae that is immunomodulatory by virtue of covalently attached phosphorylcholine (PC) moieties, and also two synthetic drug-like PC-based small molecule analogues (SMAs) that mimic ES-62's immunomodulatory activity. We have previously shown that each of these molecules prevents development of pathology in collagen-induced arthritis (CIA), a model of the musculoskeletal disease rheumatoid arthritis (RA) and reflecting this, we now report that ES-62 and its SMAs, modify bone remodeling by altering bone marrow progenitors and thus impacting on osteoclastogenesis. Consistent with this, we find that these molecules inhibit functional osteoclast differentiation in vitro. Furthermore, this appears to be achieved by induction of anti-oxidant response gene expression, thereby resulting in reduction of the reactive oxygen species production that is necessary for the increased osteoclastogenesis witnessed in musculoskeletal diseases like RA.

Regulation of Osteoclast Growth and Fusion by mTOR/raptor and mTOR/rictor/Akt

Osteoclasts are giant bone cells formed by fusion from monocytes and uniquely capable of a complete destruction of mineralized tissues. Previously, we have demonstrated that in energy-rich environment not only osteoclast fusion index (the number of nuclei each osteoclast contains), but also cytoplasm volume per single nucleus was increased. The goal of this study was to investigate the regulation of metabolic sensor mTOR during osteoclast differentiation in energy-rich environment simulated by addition of pyruvate. We have found that in the presence of pyruvate, the proportion of mTOR associated with raptor increased, while mTOR-rictor-mediated Akt phosphorylation decreased. Inhibition of mTOR with rapamycin (10 nM) significantly interfered with all aspects of osteoclastogenesis. However, rapamycin at 1 nM, which preferentially targets mTOR-raptor complex, was only effective in control cultures, while in the presence of pyruvate osteoclast fusion index was successfully increased. Inhibition of Akt drastically reduced osteoclast fusion, however in energy-rich environment, osteoclasts of comparable size were formed through increased cytoplasm growth. These data suggest that mTOR-rictor mediated Akt signaling regulates osteoclast fusion, while mTOR-raptor regulation of protein translation contributes to fusion-independent cytoplasm growth. We demonstrate that depending on the bioenergetics microenvironment osteoclastogenesis can adjust to occur through preferential multinucleation or through cell growth, implying that attaining large cell size is part of the osteoclast differentiation program.

Surrogate Outcome Measures of In Vitro Osteoclast Resorption of β Tricalcium Phosphate

Introduction of porosity to calcium phosphate scaffolds for bone repair has created a new challenge when measuring bioresorption in vitro, rendering traditional outcome measures redundant. The aim of this study is to identify a surrogate endpoint for use with 3D scaffolds. Murine RAW 264.7 cells are cultured on dense discs of β-tricalcium phosphate in conditions to stimulate osteoclast (OC) formation. Multinucleated OCs are visible from day 6 with increases at days 8 and 10. Resorption pits are first observed at day 6 with much larger pits visible at days 8, 10, and 12. The concentration of calcium ions in the presence of cells is significantly higher than cell-free cultures at days 3 and 9. Using linear regression analysis, Ca ion release could account for 35.9% of any subsequent change in resorption area. The results suggest that Ca ion release is suitable to measure resorption of a beta-tricalcium phosphate ceramic substrate in vitro. This model could replace the more accepted resorption pit assay in circumstances where quantification of pits is not possible, e.g., when characterizing 3D tissue engineered bone scaffolds.

RAW 264.7 co-cultured with ultra-high molecular weight polyethylene particles spontaneously differentiate into osteoclasts: an in vitro model of periprosthetic osteolysis

Wear-particle osteolysis affects prosthesis survival leading to implant loosening up to 70% of revisions. Therapeutic strategies are increasing, however alternative testing methods to experimentally evaluate such treatments are lacking. The aim of this study was to reproduce an in vitro osteolysis model recapitulating the events that, starting from the exposure of macrophages to polyethylene, lead to the establishment of osteoclastogenesis and inflammation. Responses to polyethylene, at 3 and 7 days, in a macrophage cell line, RAW 264.7, were determined by DNA quantification, immunofluorescence, pit assay, gene expression, cytokine production and NF-kB activation. Results showed that 3 days exposure to particles could induce a significant production of Tumor Necrosis Factor alpha (p < 0.0005) and Prostaglandin E₂ (p < 0.005) compared to controls. Particles also induced macrophages to spontaneously differentiate into mature and active osteoclasts, in terms of identification of multinucleated cells by Phalloidin staining and by the analysis of osteoclast-specific gene markers. In particular, at 3 days polyethylene induced a significant up-regulation of Nuclear Factor of Activated T-cells, cytoplasmic 1, Receptor Activator of Nuclear factor Kappa-B and Receptor Activator of Nuclear Factor Kappa-B Ligand genes (p < 0.0005) compared to controls. At protein level, the particles induced a significant increase of Receptor Activator of Nuclear Factor Kappa-B Ligand at day 7 over controls (p < 0.0005). Osteoclasts were capable to resorb bone even in absence of differentiating factors. The possible mechanism, beside spontaneous osteoclastogenesis mediated by wear debris, was identified in an autocrine up-regulation of Receptor activator of nuclear factor kappa-B ligand gene expression and protein synthesis. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 510-520, 2017.

Sympathetic Neurotransmitters Modulate Osteoclastogenesis and Osteoclast Activity in the Context of Collagen-Induced Arthritis

Excessive synovial osteoclastogenesis is a hallmark of rheumatoid arthritis (RA). Concomitantly, local synovial changes comprise neuronal components of the peripheral sympathetic nervous system. Here, we wanted to analyze if collagen-induced arthritis (CIA) alters bone marrow-derived macrophage (BMM) osteoclastogenesis and osteoclast activity, and how sympathetic neurotransmitters participate in this process. Therefore, BMMs from Dark Agouti rats at different CIA stages were differentiated into osteoclasts in vitro and osteoclast number, cathepsin K activity, matrix resorption and apoptosis were analyzed in the presence of acetylcholine (ACh), noradrenaline (NA) vasoactive intestinal peptide (VIP) and assay-dependent, adenylyl cyclase activator NKH477. We observed modulation of neurotransmitter receptor mRNA expression in CIA osteoclasts without affecting protein level. CIA stage-dependently altered marker gene expression associated with osteoclast differentiation and activity without affecting osteoclast number or activity. Neurotransmitter stimulation modulated osteoclast differentiation, apoptosis and activity. VIP, NA and adenylyl cyclase activator NKH477 inhibited cathepsin K activity and osteoclastogenesis (NKH477, 10(-6) M NA) whereas ACh mostly acted pro-osteoclastogenic. We conclude that CIA alone does not affect metabolism of in vitro generated osteoclasts whereas stimulation with NA, VIP plus specific activation of adenylyl cyclase induced anti-resorptive effects probably mediated via cAMP signaling. Contrary, we suggest pro-osteoclastogenic and pro-resorptive properties of ACh mediated via muscarinic receptors.

Characterization of biomimetic calcium phosphate labeled with fluorescent dextran for quantification of osteoclastic activity

Bone resorbing osteoclasts represent an important therapeutic target for diseases associated with bone and joint destruction, such as rheumatoid arthritis, periodontitis, and osteoporosis. The quantification of osteoclast resorptive activity in vitro is widely used for screening new anti-resorptive medications. The aim of this paper was to develop a simplified semi-automated method for the quantification of osteoclastic resorption using fluorescently labeled biomimetic mineral layers which can replace time intensive, often subjective and clearly non-sustainable use of translucent slices of tusks from vulnerable or endangered species such as the elephant. Osteoclasts were formed from RAW 264.7 mouse monocyte cell line using the pro-resorptive cytokine receptor activator of nuclear factor kappa-B ligand (RANKL). We confirmed that fluorescent labeling did not interfere with the biomimetic features of hydroxyapatite, and developed an automated method for quantifying osteoclastic resorption. Correlation between our assay and traditional manual measurement techniques was found to be very strong (R(2)=0.99). In addition, we modified the technique to provide depth and volume data of the resorption pits by confocal imaging at defined depths. Thus, our method allows automatic quantification of total osteoclastic resorption as well as additional data not obtainable by the current tusk slice technique offering a better alternative for high throughput screening of potential antiresorptives.

Autocrine signaling is a key regulatory element during osteoclastogenesis

Osteoclasts are responsible for bone destruction in degenerative, inflammatory and metastatic bone disorders. Although osteoclastogenesis has been well-characterized in mouse models, many questions remain regarding the regulation of osteoclast formation in human diseases. We examined the regulation of human precursors induced to differentiate and fuse into multinucleated osteoclasts by receptor activator of nuclear factor kappa-B ligand (RANKL). High-content single cell microscopy enabled the time-resolved quantification of both the population of monocytic precursors and the emerging osteoclasts. We observed that prior to induction of osteoclast fusion, RANKL stimulated precursor proliferation, acting in part through an autocrine mediator. Cytokines secreted during osteoclastogenesis were resolved using multiplexed quantification combined with a Partial Least Squares Regression model to identify the relative importance of specific cytokines for the osteoclastogenesis outcome. Interleukin 8 (IL-8) was identified as one of RANKL-induced cytokines and validated for its role in osteoclast formation using inhibitors of the IL-8 cognate receptors CXCR1 and CXCR2 or an IL-8 blocking antibody. These insights demonstrate that autocrine signaling induced by RANKL represents a key regulatory component of human osteoclastogenesis.

Reproducible quantification of osteoclastic activity: characterization of a biomimetic calcium phosphate assay

Osteoclasts are responsible for bone and joint destruction in rheumatoid arthritis, periodontitis, and osteoporosis. Animal tusk slice assays are standard for evaluating the effect of therapeutics on these cells. However, in addition to batch-to-batch variability inherent to animal tusks, their use is clearly not sustainable. Our objective was to develop and characterize a biomimetic calcium phosphate assay based on the use of phase pure hydroxyapatite coated as a thin film on the surface of culture plates, to facilitate the reproducible quantification of osteoclast resorptive activity. Osteoclasts were formed from RAW 264.7 mouse monocyte cell line using a pro-resorptive cytokine RANKL (50 ng/mL). No change in substrate appearance was noted after culture with media without cells, or undifferentiated monocytes. Only in the presence of osteoclasts localized areas of calcium phosphate dissolution were observed. The total area resorbed positively correlated with the osteoclast numbers (R(2) = 0.99). The resorbed area was significantly increased by the addition of RANKL, and decreased after application of known inhibitors of osteoclast resorptive activity, calcitonin (10 μM), or alendronate (100 μM). Thus, calcium phosphate coated substrates allow reliable monitoring of osteoclast resorptive activity and offer an alternative to animal tusk slice assays.

Evidence for mitoxantrone-induced block of inwardly rectifying K(+) channels expressed in the osteoclast precursor RAW 264.7 cells differentiated with lipopolysaccharide

BACKGROUND/AIMS

Mitoxanthrone (MX) is an anthracenedione antineoplastic agent. Whether this drug and other related compounds have any effects on ion currents in osteoclasts remains largely unclear.

METHODS

In this study, the effects of MX and other related compounds on inwardly rectifying K(+) current (I(K(IR))) were investigated in RAW 264.7 osteoclast precursor cells treated with lipopolysaccharide.

RESULTS

The I(K(IR))in these cells are blocked by BaCl(2) (1 mM). MX (1-100 µM) decreased the amplitude of I(K(IR)) in a concentration-dependent manner with an IC(50) value of 6.4 µM. MX also slowed the time course of I(K(IR)) inactivation elicited by large hyperpolarization. Doxorubicin (10 µM), 17β-estradiol (10 µM) and tertiapin (1 µM) decreased the I(K(IR)) amplitude in these cells. In bafilomycin A(1)-treated cells, MX-mediated block of I(K(IR)) still existed. In cell-attached configuration, when the electrode was filled with MX (10 µM), the activity of inwardly rectifying K(+) (Kir) channels was decreased with no change in single-channel conductance. MX-mediated reduction of channel activity is accompanied by a shortening of mean open time. Under current-clamp conditions, addition of MX resulted in membrane depolarization. Therefore, MX can interact with the Kir channels to decrease the I(K(IR)) amplitude and to depolarize the membrane in these cells.

CONCLUSION

The block by this drug of Kir2.1 channels appears to be one of the important mechanisms underlying its actions on the resorptive activity of osteoclasts, if similar results occur in vivo. Targeting at Kir channels may be clinically useful as an adjunctive regimen to anti-cancer drugs (e.g., MX or doxorubicin) in influencing the resorptive activity of osteoclasts.

Fluid flow-induced calcium response in early or late differentiated osteoclasts

Intracellular calcium oscillation caused by receptor activator of nuclear factor kappa-B ligand has been demonstrated to promote the differentiation of osteoclasts. Osteoclasts are recruited on the surface of trabeculae, and are exposed to fluid flow caused by the deformation of the bone matrix. However, the roles of fluid shear stress (FSS) on calcium response during the differentiation process of osteoclasts are still unknown. In the current study, the formation of tartrate-resistant acid phosphatase-positive, multinucleated osteoclasts from RAW264.7 macrophage cells were induced by co-culturing them with the conditioned medium from MC3T3-E1 osteoblasts. The in situ observations showed a high correlation between the area and the nuclear number of osteoclasts. The cells were stimulated by FSS at different levels (1 or 10 dyne/cm(2)) before (0 day) or after being induced for 4 or 8 days. The mechanically-induced calcium response was recorded and analyzed. The results indicated a different property of calcium oscillation for the osteoclasts in different fusion stages (i.e., more calcium-responsive peaks appeared in small osteoclasts than those in the larger ones). The rates of calcium influx decreased and the time of recovery in osteoclast cytosol increased along with the fusion of osteoclasts. In addition, increasing the FSS level enhanced the calcium oscillation of osteoclasts at early induction (4 days). However, this effect was weakened at the late induction (8 days). The present work could help provide understanding regarding the mechanism of the involvement of calcium in mechanically induced bone remodeling.

Towards a new spatial representation of bone remodeling

Irregular bone remodeling is associated with a number of bone diseases such as osteoporosis and multiple myeloma. Computational and mathematical modeling can aid in therapy and treatment as well as understanding fundamental biology. Different approaches to modeling give insight into different aspects of a phenomena so it is useful to have an arsenal of various computational and mathematical models. Here we develop a mathematical representation of bone remodeling that can effectively describe many aspects of the complicated geometries and spatial behavior observed. There is a sharp interface between bone and marrow regions. Also the surface of bone moves in and out, i.e. in the normal direction, due to remodeling. Based on these observations we employ the use of a level-set function to represent the spatial behavior of remodeling. We elaborate on a temporal model for osteoclast and osteoblast population dynamics to determine the change in bone mass which influences how the interface between bone and marrow changes. We exhibit simulations based on our computational model that show the motion of the interface between bone and marrow as a consequence of bone remodeling. The simulations show that it is possible to capture spatial behavior of bone remodeling in complicated geometries as they occur in vitro and in vivo. By employing the level set approach it is possible to develop computational and mathematical representations of the spatial behavior of bone remodeling. By including in this formalism further details, such as more complex cytokine interactions and accurate parameter values, it is possible to obtain simulations of phenomena related to bone remodeling with spatial behavior much as in vitro and in vivo. This makes it possible to perform in silica experiments more closely resembling experimental observations.

General analysis of mathematical models for bone remodeling

Bone remodeling is regulated by pathways controlling the interplay of osteoblasts and osteoclasts. In this work, we apply the method of generalized modeling to systematically analyse a large class of models of bone remodeling. Our analysis shows that osteoblast precursors can play an important role in the regulation of bone remodeling. Further, we find that the parameter regime most likely realized in nature lies close to bifurcation lines, marking qualitative changes in the dynamics. Although proximity to a bifurcation facilitates adaptive responses to changing external conditions, it entails the danger of losing dynamical stability. Some evidence implicates such dynamical transitions as a potential mechanism leading to forms of Paget's disease.

Mathematical modeling in bone biology: from intracellular signaling to tissue mechanics

Although conceptual and experimental models are historically well incorporated in bone biology studies, mathematical modeling has been much less-frequently utilized. This review aims to introduce mathematical modeling to readers who are not familiar with the concept underlying this methodology, to outline how mathematical models can help to improve current understanding of bone biology and to discuss examples where mathematical modeling was used to provide new insights into important questions of bone biology.

A mathematical model of bone remodeling dynamics for normal bone cell populations and myeloma bone disease

BACKGROUND

Multiple myeloma is a hematologic malignancy associated with the development of a destructive osteolytic bone disease.

RESULTS

Mathematical models are developed for normal bone remodeling and for the dysregulated bone remodeling that occurs in myeloma bone disease. The models examine the critical signaling between osteoclasts (bone resorption) and osteoblasts (bone formation). The interactions of osteoclasts and osteoblasts are modeled as a system of differential equations for these cell populations, which exhibit stable oscillations in the normal case and unstable oscillations in the myeloma case. In the case of untreated myeloma, osteoclasts increase and osteoblasts decrease, with net bone loss as the tumor grows. The therapeutic effects of targeting both myeloma cells and cells of the bone marrow microenvironment on these dynamics are examined.

CONCLUSIONS

The current model accurately reflects myeloma bone disease and illustrates how treatment approaches may be investigated using such computational approaches.

REVIEWERS

This article was reviewed by Ariosto Silva and Mark P. Little.

Breast cancer-derived factors stimulate osteoclastogenesis through the Ca2+/protein kinase C and transforming growth factor-beta/MAPK signaling pathways

Breast cancer commonly metastasizes to bone where its growth depends on the action of bone-resorbing osteoclasts. We have previously shown that breast cancer cells secrete factors able to directly stimulate osteoclastogenesis from receptor activator of nuclear factor kappaB ligand (RANKL)-primed precursors and that transforming growth factor-beta (TGFbeta) plays a permissive role in this process. Now, we evaluate the signaling events triggered in osteoclast precursors by soluble factors produced by MDA-MB-231 human breast carcinoma cells. In mouse bone marrow cultures and RAW 264.7 murine monocytic cells, MDA-MB-231-derived factors increased osteoclast number, size, and nucleation. These factors failed to induce Smad2 phosphorylation, and short interfering RNAs against Smad4 did not affect their ability to induce osteoclastogenesis. In contrast, MDA-MB-231 factors induced phosphorylation of p38 and ERK1/2, and pharmacological inhibitors against p38 (SB203580) and MEK1/2 (PD98059) impeded the osteoclastogenic effects of cancer-derived factors. Neutralizing antibodies against TGFbeta attenuated p38 activation, whereas activation of ERK1/2 was shortened in duration, but not decreased in amplitude. ERK1/2 phosphorylation induced by cancer-derived factors was blocked by MEK1/2 inhibitor, but not by Ras (manumycin A) or Raf (GW5074) inhibitors. Inhibition of protein kinase Calpha using Gö6976 prevented both ERK1/2 phosphorylation and osteoclast formation in response to MDA-MB-231-derived factors. Using microspectrofluorimetry of fura-2-AM-loaded osteoclast precursors, we have found that cancer-derived factors, similar to RANKL, induced sustained oscillations in cytosolic free calcium. The calcium chelator BAPTA prevented calcium elevations and osteoclast formation in response to MDA-MB-231-derived factors. Thus, we have shown that breast cancer-derived factors induce osteoclastogenesis through the activation of calcium/protein kinase Calpha and TGFbeta-dependent ERK1/2 and p38 signaling pathways.

Bone morphogenetic protein 2 enhances PGE(2)-stimulated osteoclast formation in murine bone marrow cultures

Bone morphogenetic protein 2 (BMP-2) is used clinically to stimulate bone formation and accelerate fracture repair. Adding prostaglandin (PG) E(2) or PGE(2) receptor agonists to BMP-2 has been proposed to improve BMP-2 efficacy. However, this may enhance bone resorption, since PGE(2) can increase receptor activator of NF-kappaB ligand (RANKL) expression and decrease osteoprotegerin (OPG) expression in osteoblasts, and the RANKL:OPG ratio is critical for osteoclast formation. We used bone marrow (BM) cultures and BM macrophage (BMM) cultures from outbred CD1 mice to examine effects on osteoclast formation of BMP-2 and PGE(2). In BM cultures, which contain both osteoblastic and osteoclastic lineage cells, BMP-2 (100 ng/ml) alone did not increase osteoclast formation but enhanced the peak response to PGE(2) by 1.6-9.6-fold. In BMM cultures, which must be treated with RANKL because they do not contain osteoblastic cells, BMP-2 did not increase osteoclast formation, with or without PGE(2). Our results suggest that BMP-2 can increase osteoclast formation in response to PGE(2) by increasing the RANKL:OPG ratio in osteoblasts, which may have therapeutic implications for the use of BMP-2.