A specific subtype of osteoclasts secretes factors inducing nodule formation by osteoblasts

Osteoclast-derived extracellular miR-106a-5p promotes osteogenic differentiation and facilitates bone defect healing

Small extracellular vesicles (sEVs) are considered to play critical roles in intercellular communications during normal and pathological processes since they are enriched with miRNAs and other signal molecules. In bone remodeling, osteoclasts generate large amounts of sEVs. However, there is very few research studying whether and how osteoclast-derived sEVs (OC-sEVs) affect surrounding cells. In our study, microarray analysis identified miR-106a-5p as highly enriched in OC-sEV. Further experiments confirmed that OC-sEVs inhibited Fam134a through miR-106a-5p and significantly promoted bone mesenchymal stem cell (BMSC) osteogenic mineralization in vitro. Next, we prepared an sEV-modified demineralized bone matrix (DBM) as scaffold treating calvarial defect mouse model to evaluate the pro-osteogenic activities of the scaffold. In vivo results indicated that DBM modified with miR-106a-5p-sEVs showed an enhanced capacity for bone regeneration. This important finding further emphasizes that sEV-mediated miR-106a-5p transfer plays a critical role in osteogenesis and indicates a novel communication mode between osteoclasts and BMSCs.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

Friend or Foe? Essential Roles of Osteoclast in Maintaining Skeletal Health

Heightened activity of osteoclast is considered to be the culprit in breaking the balance during bone remodeling in pathological conditions, such as osteoporosis. As a “foe” of skeletal health, many antiosteoporosis therapies aim to inhibit osteoclastogenesis. However, bone remodeling is a dynamic process that requires the subtle coordination of osteoclasts and osteoblasts. Severe suppression of osteoclast differentiation will impair bone formation because of the coupling effect. Thus, understanding the complex roles of osteoclast in maintaining proper bone remodeling is highly warranted to develop better management of osteoporosis. This review aimed to determine the varied roles of osteoclasts in maintaining skeletal health and to highlight the positive roles of osteoclasts in maintaining normal bone remodeling. Generally, osteoclasts interact with osteocytes to initiate targeted bone remodeling and have crosstalk with mesenchymal stem cells and osteoblasts via secreted factors or cell-cell contact to promote bone formation. We believe that a better outcome of bone remodeling disorders will be achieved when proper strategies are made to coordinate osteoclasts and osteoblasts in managing such disorders.

Osteoclasts degrade fibrinogen scaffolds and induce mesenchymal stem/stromal osteogenic differentiation

Fibrinogen (Fg) is a pro-inflammatory protein with pro-healing properties. Previous work showed that fibrinogen 3D scaffolds (Fg-3D) promote bone regeneration, but the cellular players were not identified. Osteoclasts are bone resorbing cells that promote bone remodeling in close crosstalk with osteoblasts. Herein, the capacity of osteoclasts differentiated on Fg-3D to degrade the scaffolds and promote osteoblast differentiation was evaluated in vitro. Fg-3D scaffolds were prepared by freeze-drying and osteoclasts were differentiated from primary human peripheral blood monocytes. Results obtained showed osteoclasts expressing the enzymes cathepsin K and tartrate resistant acid phosphatase colonizing Fg-3D scaffolds. Osteoclasts were able to significantly degrade Fg-3D, reducing the scaffold's area, and increasing D-dimer concentration, a Fg degradation product, in their culture media. Osteoclast conditioned media from the first week of differentiation promoted significantly stronger human primary mesenchymal stem/stromal cell (MSC) osteogenic differentiation, evaluated by alkaline phosphatase activity. Moreover, week 1 osteoclast conditioned media promoted earlier MSC osteogenic differentiation, than chemical osteogenesis inductors. TGF-β1 was found increased in osteoclast conditioned media from week 1, when compared to week 3 of differentiation. Taken together, our results suggest that osteoclasts are able to differentiate and degrade Fg-3D, producing factors like TGF-β1 that promote MSC osteogenic differentiation.

Osteoclasts Provide Coupling Signals to Osteoblast Lineage Cells Through Multiple Mechanisms

Bone remodeling is essential for the repair and replacement of damaged and old bone. The major principle underlying this process is that osteoclast-mediated resorption of a quantum of bone is followed by osteoblast precursor recruitment; these cells differentiate to matrix-producing osteoblasts, which form new bone to replace what was resorbed. Evidence from osteopetrotic syndromes indicate that osteoclasts not only resorb bone, but also provide signals to promote bone formation. Osteoclasts act upon osteoblast lineage cells throughout their differentiation by facilitating growth factor release from resorbed matrix, producing secreted proteins and microvesicles, and expressing membrane-bound factors. These multiple mechanisms mediate the coupling of bone formation to resorption in remodeling. Additional interactions of osteoclasts with osteoblast lineage cells, including interactions with canopy and reversal cells, are required to achieve coordination between bone formation and resorption during bone remodeling.

Coupling factors involved in preserving bone balance

Coupling during bone remodeling refers to the spatial and temporal coordination of bone resorption with bone formation. Studies have assessed the subtle interactions between osteoclasts and osteoblasts to preserve bone balance. Traditionally, coupling research related to osteoclast function has focused on bone resorption activity causing the release of growth factors embedded in the bone matrix. However, considerable evidence from in vitro, animal, and human studies indicates the importance of the osteoclasts themselves in coupling phenomena, and many osteoclast-derived coupling factors have been identified. These include sphingosine-1-phosphate, vesicular-receptor activator of nuclear factor-κB, collagen triple helix repeat containing 1, and cardiotrophin-1. Interestingly, neuronal guidance molecules, such as slit guidance ligand 3, semaphorin (SEMA) 3A, SEMA4D, and netrin-1, originally identified as instructive cues allowing the navigation of growing axons to their targets, have been shown to be involved in the intercellular cross-talk among bone cells. This review discusses osteoclast-osteoblast coupling signals, including recent advances and the potential roles of these signals as therapeutic targets for osteoporosis and as biomarkers predicting human bone health.

Monocyte-derived extracellular vesicles stimulate cytokine secretion and gene expression of matrix metalloproteinases by mesenchymal stem/stromal cells

Intercellular communication is essential in bone remodelling to ensure that new bone is formed with only temporary bone loss. Monocytes (MCs) and osteoclasts actively take part in controlling bone remodelling by providing signals that promote osteogenic differentiation of mesenchymal stem/stromal cells (MSCs). Extracellular vesicles (EVs) have attracted attention as regulators of bone remodelling. EVs facilitate intercellular communication by transferring a complex cargo of biologically active molecules to target cells. In the present study, we evaluated the potency of EVs from MCs and osteoclasts to induce a lineage-specific response in MSCs. We analysed gene expression and protein secretion by both adipose tissue-derived MSCs and bone marrow-derived MSCs after stimulation with EVs from lipopolysaccharide-activated primary human MCs and (mineral-resorbing) osteoclasts. Isolated EVs were enriched in exosomes (EVs of endosomal origin) and were free of cell debris. MC- and osteoclast-derived EVs were taken up by adipose tissue-derived MSCs. EVs from activated MCs promoted the secretion of cytokines by MSCs, which may represent an immunomodulatory mechanism. MC-derived EVs also upregulated the expression of genes encoding for matrix metalloproteinases. Therefore, we hypothesize that MCs facilitate tissue remodelling through EV-mediated signalling. We did not observe a significant effect of osteoclast-derived EVs on gene expression or protein secretion in MSCs. EV-mediated signalling might represent an additional mode of cell-cell signalling during the transition from injury and inflammation to bone regeneration and play an important role in the coupling between bone resorption and bone formation.

DATABASE

Gene expression data are available in the GEO database under the accession number GSE102401.

Osteoclasts degrade bone and cartilage knee joint compartments through different resorption processes

Background

Osteoclasts have been strongly implicated in osteoarthritic cartilage degradation, at least indirectly via bone resorption, and have been shown to degrade cartilage in vitro. The osteoclast resorption processes required to degrade subchondral bone and cartilage—the remodeling of which is important in the osteoarthritic disease process—have not been previously described, although cathepsin K has been indicated to participate. In this study we profile osteoclast-mediated degradation of bovine knee joint compartments in a novel in vitro model using biomarkers of extracellular matrix (ECM) degradation to assess the potential of osteoclast-derived resorption processes to degrade different knee joint compartments.

Methods

Mature human osteoclasts were cultured on ECMs isolated from bovine knees—articular cartilage, cortical bone, and osteochondral junction ECM (a subchondral bone-calcified cartilage mixture)—in the presence of inhibitors: the cystein protease inhibitor E-64, the matrix metalloproteinase (MMP) inhibitor GM6001, or the vacuolar-type H⁺-ATPase (V-ATPase) inhibitor diphyllin. Biomarkers of bone (calcium and C-terminal type I collagen (CTX-I)) and cartilage (C2M) degradation were measured in the culture supernatants. Cultures without osteoclasts were used as background samples. Background-subtracted biomarker levels were normalized to the vehicle condition and were analyzed using analysis of variance with Tukey or Dunnett’s T3 post hoc test, as applicable.

Results

Osteochondral CTX-I release was inhibited by E-64 (19% of vehicle, p = 0.0008), GM6001 (51% of vehicle, p = 0.013), and E-64/GM6001 combined (4% of vehicle, p = 0.0007)—similarly to bone CTX-I release. Diphyllin also inhibited osteochondral CTX-I release (48% of vehicle, p = 0.014), albeit less than on bone (4% of vehicle, p < 0.0001). Osteochondral C2M release was only inhibited by E-64 (49% of vehicle, p = 0.07) and GM6001 (14% of vehicle, p = 0.006), with complete abrogation when combined (0% of vehicle, p = 0.004). Cartilage C2M release was non-significantly inhibited by E-64 (69% of vehicle, p = 0.98) and was completely abrogated by GM6001 (0% of vehicle, p = 0.16).

Conclusions

Our study supports that osteoclasts can resorb non-calcified and calcified cartilage independently of acidification. We demonstrated both MMP-mediated and cysteine protease-mediated resorption of calcified cartilage. Osteoclast functionality was highly dependent on the resorbed substrate, as different ECMs required different osteoclast processes for degradation. Our novel culture system has potential to facilitate drug and biomarker development aimed at rheumatic diseases, e.g. osteoarthritis, where pathological osteoclast processes in specific joint compartments may contribute to the disease process.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1564-5) contains supplementary material, which is available to authorized users.

Cathepsin K Inhibitors for Osteoporosis: Biology, Potential Clinical Utility, and Lessons Learned

Cathepsin K is a cysteine protease member of the cathepsin lysosomal protease family. Although cathepsin K is highly expressed in osteoclasts, lower levels of cathepsin K are also found in a variety of other tissues. Secretion of cathepsin K from the osteoclast into the sealed osteoclast-bone cell interface results in efficient degradation of type I collagen. The absence of cathepsin K activity in humans results in pycnodysostosis, characterized by increased bone mineral density and fractures. Pharmacologic cathepsin K inhibition leads to continuous increases in bone mineral density for ≤5 years of treatment and improves bone strength at the spine and hip. Compared with other antiresorptive agents, cathepsin K inhibition is nearly equally efficacious for reducing biochemical markers of bone resorption but comparatively less active for reducing bone formation markers. Despite multiple efforts to develop cathepsin K inhibitors, potential concerns related to off-target effects of the inhibitors against other cathepsins and cathepsin K inhibition at nonbone sites, including skin and perhaps cardiovascular and cerebrovascular sites, prolonged the regulatory approval process. A large multinational randomized, double-blind phase III study of odanacatib in postmenopausal women with osteoporosis was recently completed. Although that study demonstrated clinically relevant reductions in fractures at multiple sites, odanacatib was ultimately withdrawn from the regulatory approval process after it was found to be associated with an increased risk of cerebrovascular accidents. Nonetheless, the underlying biology and clinical effects of cathepsin K inhibition remain of considerable interest and could guide future therapeutic approaches for osteoporosis.

Unique Regenerative Mechanism to Replace Bone Lost During Dietary Bone Depletion in Weanling Mice

The present study was undertaken to determine the mechanism whereby calcitropic hormones and mesenchymal stem cell progeny changes are involved in bone repletion, a regenerative bone process that restores the bone lost to calcium deficiency. To initiate depletion, weanling mice with a mixed C57BL/6 (75%) and CD1 (25%) genetic background were fed a calcium-deficient diet (0.01%) for 14 days. For repletion, the mice were fed a control diet containing 1.2% calcium for 14 days. Depletion decreased plasma calcium and increased plasma parathyroid hormone, 1,25(OH)2D (calcitriol), and C-terminal telopeptide of type I collagen. These plasma parameters quickly returned toward normal on repletion. The trabecular bone volume and connectivity decreased drastically during depletion but were completely restored by the end of repletion. This bone repletion process largely resulted from the development of new bone formation. When bromodeoxyuridine (BrdU) was administered in the middle of depletion for 3 days and examined by fluorescence-activated cell sorting at 7 days into repletion, substantial increases in BrdU incorporation were seen in several CD105 subsets of cells of osteoblastic lineage. When BrdU was administered on days 1 to 3 of repletion and examined 11 days later, no increases in BrdU were seen in these subsets. Additionally, osteocytes that stained positively for BrdU were increased during depletion. In conclusion, the results of the present study have established a unique regenerative mechanism to initiate bone repair during the bone insult. Calcium homeostatic mechanisms and the bone repletion mechanism are opposing functions but are simultaneously orchestrated such that both endpoints are optimized. These results have potential clinical relevance for disease entities such as type 2 osteoporosis.

Osteoclast differentiation from human blood precursors on biomimetic calcium-phosphate substrates

The design of synthetic bone grafts to foster bone formation is a challenge in regenerative medicine. Understanding the interaction of bone substitutes with osteoclasts is essential, since osteoclasts not only drive a timely resorption of the biomaterial, but also trigger osteoblast activity. In this study, the adhesion and differentiation of human blood-derived osteoclast precursors (OCP) on two different micro-nanostructured biomimetic hydroxyapatite materials consisting in coarse (HA-C) and fine HA (HA-F) crystals, in comparison with sintered stoichiometric HA (sin-HA, reference material), were investigated. Osteoclasts were induced to differentiate by RANKL-containing supernatant using cell/substrate direct and indirect contact systems, and calcium (Ca⁺⁺) and phosphorus (P⁵⁺) in culture medium were measured. We observed that OCP adhered to the experimental surfaces, and that osteoclast-like cells formed at a rate influenced by the micro- and nano-structure of HA, which also modulate extracellular Ca⁺⁺. Qualitative differences were found between OCP on biomimetic HA-C and HA-F and their counterparts on plastic and sin-HA. On HA-C and HA-F cells shared typical features of mature osteoclasts, i.e. podosomes, multinuclearity, tartrate acid phosphatase (TRAP)-positive staining, and TRAP5b-enzyme release. However, cells were less in number compared to those on plastic or on sin-HA, and they did not express some specific osteoclast markers. In conclusion, blood-derived OCP are able to attach to biomimetic and sintered HA substrates, but their subsequent fusion and resorptive activity are hampered by surface micro-nano-structure. Indirect cultures suggest that fusion of OCP is sensitive to topography and to extracellular calcium.

STATEMENT OF SIGNIFICANCE

The novelty of the paper is the differentiation of human blood-derived osteoclast precursors, instead of mouse-derived macrophages as used in most studies, directly on biomimetic micro-nano structured HA-based surfaces, as triggered by osteoblast-produced factors (RANKL/OPG), and influenced by chemistry and topography of the substrate(s). Biomimetic HA-surfaces, like those obtained in calcium phosphate cements, are very different from the conventional calcium phosphate ceramics, both in terms of topography and ion exchange. The role of these factors in modulating precursors' differentiation and activity is analysed. The system is closely reproducing the physiological process of attachment of host cells and further maturation to osteoclasts toward resorption of the substrate, which occurs in vivo after filling bone defects with the calcium phosphate grafts.

Protein degradation fragments as diagnostic and prognostic biomarkers of connective tissue diseases: understanding the extracellular matrix message and implication for current and future serological biomarkers

The aim of this review is to discuss the potential usefulness of novel biochemical markers of connective tissues: neo-epitopes of extracellular matrix proteins generated by post-translational modifications by tissue proteinases. As each modification results from a specific local physiological or pathobiological process, the identification of specific proteinase-mediated cleavage products of tissue-specific proteins may produce a unique disease-specific biochemical marker. The authors present a novel interpretation of the process of tissue degradation described by neo-epitope fragments of the interstitial and basement membrane matrix in fibrotic disease, and the diagnostic and prognostic potential of such markers. Moreover, the authors highlight the importance of matrix protein fragments not only as markers of tissue remodeling, but also as players in tissue remodeling, due to their signaling properties.

Osteoclast TGF-β Receptor Signaling Induces Wnt1 Secretion and Couples Bone Resorption to Bone Formation

Osteoblast-mediated bone formation is coupled to osteoclast-mediated bone resorption. These processes become uncoupled with age, leading to increased risk for debilitating fractures. Therefore, understanding how osteoblasts are recruited to sites of resorption is vital to treating age-related bone loss. Osteoclasts release and activate TGF-β from the bone matrix. Here we show that osteoclast-specific inhibition of TGF-β receptor signaling in mice results in osteopenia due to reduced osteoblast numbers with no significant impact on osteoclast numbers or activity. TGF-β induced osteoclast expression of Wnt1, a protein crucial to normal bone formation, and this response was blocked by impaired TGF-β receptor signaling. Osteoclasts in aged murine bones had lower TGF-β signaling and Wnt1 expression in vivo. Ex vivo stimulation of osteoclasts derived from young or old mouse bone marrow macrophages showed no difference in TGF-β-induced Wnt1 expression. However, young osteoclasts expressed reduced Wnt1 when cultured on aged mouse bone chips compared to young mouse bone chips, consistent with decreased skeletal TGF-β availability with age. Therefore, osteoclast responses to TGF-β are essential for coupling bone resorption to bone formation, and modulating this pathway may provide opportunities to treat age-related bone loss.

Serum Amyloid A3 Secreted by Preosteoclasts Inhibits Parathyroid Hormone-stimulated cAMP Signaling in Murine Osteoblasts

Continuous parathyroid hormone (PTH) blocks its own osteogenic actions in marrow stromal cell cultures by inducing Cox2 and receptor activator of nuclear factor κB ligand (RANKL) in the osteoblastic lineage cells, which then cause the hematopoietic lineage cells to secrete an inhibitor of PTH-stimulated osteoblast differentiation. To identify this inhibitor, we used bone marrow macrophages (BMMs) and primary osteoblasts (POBs) from WT and Cox2 knock-out (KO) mice. Conditioned medium (CM) from RANKL-treated WT, but not KO, BMMs blocked PTH-stimulated cAMP production in POBs. Inhibition was reversed by pertussis toxin (PTX), which blocks Gαi/o activation. Saa3 was the most highly differentially expressed gene in a microarray comparison of RANKL-treated WT versus Cox2 KO BMMs, and RANKL induced Saa3 protein secretion only from WT BMMs. CM from RANKL-stimulated BMMs with Saa3 knockdown did not inhibit PTH-stimulated responses in POBs. SAA added to POBs inhibited PTH-stimulated cAMP responses, which was reversed by PTX. Selective agonists and antagonists of formyl peptide receptor 2 (Fpr2) suggested that Fpr2 mediated the inhibitory actions of Saa3 on osteoblasts. In BMMs committed to become osteoclasts by RANKL treatment, Saa3 expression peaked prior to appearance of multinucleated cells. Flow sorting of WT marrow revealed that Saa3 was secreted only from the RANKL-stimulated B220(-) CD3(-)CD11b(-/low) CD115(+) preosteoclast population. We conclude that Saa3 secretion from preosteoclasts, induced by RANKL in a Cox2-dependent manner, inhibits PTH-stimulated cAMP signaling and osteoblast differentiation via Gαi/o signaling. The induction of Saa3 by PTH may explain the suppression of bone formation when PTH is applied continuously and may be a new therapeutic target for osteoporosis.

Elevated Lifetime Lead Exposure Impedes Osteoclast Activity and Produces an Increase in Bone Mass in Adolescent Mice

The heavy metal lead (Pb) has a deleterious effect on skeletal health. Because bone mass is maintained through a balance of bone formation and resorption, it is important to understand the effect of Pb levels on osteoblastic and osteoclastic activity. Pb exposure is associated with low bone mass in animal models and human populations; however, the correlation between Pb dosing and corresponding bone mass has been poorly explored. Thus, mice were exposed to increasing Pb and at higher levels (500 ppm), there was unexpectedly an increase in femur-tibial bone mass by 3 months of age. This is contrary to several studies alluded to earlier. Increased bone volume (BV) was accompanied by a significant increase in cortical thickness of the femur and trabecular bone that extended beyond the epiphyseal area into the marrow cavity. Subsequent evaluations revealed an increase in osteoclast numbers with high Pb exposure, but a deficiency in osteoclastic activity. These findings were substantiated by observed increases in levels of the resorption-altering hormones calcitonin and estrogen. In addition we found that pro-osteoclastic nuclear factor-kappa beta (NF-κB) pathway activity was dose dependently elevated with Pb, both in vivo and in vitro. However, the ability of osteoclasts to resorb bone was depressed in the presence of Pb in media and within test bone wafers. These findings indicate that exposure to high Pb levels disrupts early life bone accrual that may involve a disruption of osteoclast activity. This study accentuates the dose dependent variation in Pb exposure and consequent effects on skeletal health.

Novel targets for the prevention of osteoporosis - lessons learned from studies of metabolic bone disorders

INTRODUCTION

Osteoporosis is a major health care problem, and whereas efficacious treatments for vertebral fracture reduction are available for osteoporosis patients, these therapies are still limited with respect to capacity for restoration of bone loss, as well as efficacy on non-vertebral fractures, such as hip fractures, which are the source of morbidity and mortality.

AREAS COVERED

Studies of rare bone diseases in humans, such as osteopetrosis, sclerosteosis, pycnodysostosis and more, have shed light on a series of drug targets in bone that have the potential to result in therapies for osteoporosis with novel mechanisms of action, and the potential to improve the standard of care substantially. We focus on how they are separated from classic treatments for osteoporosis, in terms of novel modes of action, additional beneficial effects on bone turnover and importantly also safety. We focus on the status of anti-sclerostin antibodies, novel parathyroid hormone-related protein analogs, inhibitors of cathepsin K and ClC-7 in osteoclasts, all of which are currently in development.

EXPERT OPINION

There is a good possibility that the treatment of osteoporosis will be greatly improved within the coming years; however, with numerous effective and safe drugs already available careful attention to the safety of these novel candidates is crucial.

PDGF-BB secreted by preosteoclasts induces angiogenesis during coupling with osteogenesis

Osteogenesis during bone modeling and remodeling is coupled with angiogenesis. A recent study shows that the specific vessel subtype, strongly positive for CD31 and Endomucin (CD31^hiEmcn^hi), couples angiogenesis and osteogenesis. We found that preosteoclasts secrete platelet derived growth factor-BB (PDGF-BB), inducing CD31^hiEmcn^hi vessels during bone modeling and remodeling. Mice with depletion of PDGF-BB in tartrate-resistant acid phosphatase positive (TRAP⁺) cell lineage (Pdgfb^–/–) show significantly lower trabecular and cortical bone mass, serum and bone marrow PDGF-BB concentrations, and CD31^hiEmcn^hi vessels compared to wild-type mice. In the ovariectomized (OVX) osteoporotic mouse model, concentrations of serum and bone marrow PDGF-BB and CD31^hiEmcn^hi vessels are significantly decreased. Inhibition of cathepsin K (CTSK) increases preosteoclast numbers, resulting in higher levels of PDGF-BB to stimulate CD31^hiEmcn^hi vessels and bone formation in OVX mice. Thus, pharmacotherapies that increase PDGF-BB secretion from preosteoclasts offer a novel therapeutic target for osteoporosis to promote angiogenesis for bone formation.

A novel microRNA regulates osteoclast differentiation via targeting protein inhibitor of activated STAT3 (PIAS3)

MicroRNAs (miRNAs) involve in the regulation of a wide range of physiological processes. Recent studies suggested that miRNAs might play a role in osteoclast differentiation. Here, we identify a new miRNA (miR-9718) in primary mouse osteoclasts that promotes osteoclast differentiation by repressing protein inhibitor of activated STAT3 (PIAS3) at the post-transcriptional level. MiR-9718 was found to be transcribed during osteoclastogenesis, which was induced by macrophage colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). Overexpression of miR-9718 in RAW 264.7 cells promoted M-CSF and RANKL-induced osteoclastogenesis, whereas inhibition of miR-9718 attenuated it. PIAS3 was predicted to be a target of miR-9718. Luciferase reporter gene validated the prediction. Transfection of pre-miR-9718 in RAW 264.7 cells induced by both M-CSF and RANKL inhibited expression of PIAS3 protein, while the mRNA levels of PIAS3 were not attenuated. In vivo, our study showed that silencing of miR-9718 using a specific antagomir inhibited bone resorption and increased bone mass in mice receiving ovariectomy (OVX) and in sham-operated control mice. Thus, our study showed that miR-9718 played an important role in osteoclast differentiation via targeting PIAS3 both in vitro and in vivo.

A comparison of osteoclast-rich and osteoclast-poor osteopetrosis in adult mice sheds light on the role of the osteoclast in coupling bone resorption and bone formation

Osteopetrosis due to lack of acid secretion by osteoclasts is characterized by abolished bone resorption, increased osteoclast numbers, but normal or even increased bone formation. In contrast, osteoclast-poor osteopetrosis appears to have less osteoblasts and reduced bone formation, indicating that osteoclasts are important for regulating osteoblast activity. To illuminate the role of the osteoclast in controlling bone remodeling, we transplanted irradiated skeletally mature 3-month old wild-type mice with hematopoietic stem cells (HSCs) to generate either an osteoclast-rich or osteoclast-poor adult osteopetrosis model. We used fetal liver HSCs from (1) oc/oc mice, (2) RANK KO mice, and (3) compared these to wt control cells. TRAP5b activity, a marker of osteoclast number and size, was increased in the oc/oc recipients, while a significant reduction was seen in the RANK KO recipients. In contrast, the bone resorption marker CTX-I was similarly decreased in both groups. Both oc/oc and Rank KO recipients developed a mild osteopetrotic phenotype. However, the osteoclast-rich oc/oc recipients showed higher trabecular bone volume (40 %), increased bone strength (66 %), and increased bone formation rate (54 %) in trabecular bone, while RANK KO recipients showed only minor trends compared to control recipients. We here show that maintaining non-resorbing osteoclasts, as opposed to reducing the osteoclasts, leads to increased bone formation, bone volume, and ultimately higher bone strength in vivo, which indicates that osteoclasts are sources of anabolic molecules for the osteoblasts.

Fibrodysplasia ossificans progressiva-related activated activin-like kinase signaling enhances osteoclast formation during heterotopic ossification in muscle tissues

Fibrodysplasia ossificans progressiva is characterized by extensive ossification within muscle tissues, and its molecular pathogenesis is responsible for the constitutively activating mutation (R206H) of the bone morphogenetic protein type 1 receptor, activin-like kinase 2 (ALK2). In this study, we investigated the effects of implanting ALK2 (R206H)-transfected myoblastic C2C12 cells into nude mice on osteoclast formation during heterotopic ossification in muscle and subcutaneous tissues. The implantation of ALK2 (R206H)-transfected C2C12 cells with BMP-2 in nude mice induced robust heterotopic ossification with an increase in the formation of osteoclasts in muscle tissues but not in subcutaneous tissues. The implantation of ALK2 (R206H)-transfected C2C12 cells in muscle induced heterotopic ossification more effectively than that of empty vector-transfected cells. A co-culture of ALK2 (R206H)-transfected C2C12 cells as well as the conditioned medium from ALK2 (R206H)-transfected C2C12 cells enhanced osteoclast formation in Raw264.7 cells more effectively than those with empty vector-transfected cells. The transfection of ALK2 (R206H) into C2C12 cells elevated the expression of transforming growth factor (TGF)-β, whereas the inhibition of TGF-β signaling suppressed the enhanced formation of osteoclasts in the co-culture with ALK2 (R206H)-transfected C2C12 cells and their conditioned medium. In conclusion, this study demonstrated that the causal mutation transfection of fibrodysplasia ossificans progressiva in myoblasts enhanced the formation of osteoclasts from its precursor through TGF-β in muscle tissues.

Decline in calcitonin receptor expression in osteocytes with age

We have previously shown that co-administration of the transient osteoclast inhibitor, salmon calcitonin (sCT), blunts the anabolic effect of parathyroid hormone (PTH) in young rats and increases osteocytic expression of the bone formation inhibitor sclerostin (Sost). To determine whether this also occurs in adult animals, we co-administered sCT with PTH to 6-month-old sham-operated (SHAM) and ovariectomised (OVX) rats. While sCT reduced the stimulatory effect of PTH on serum amino-terminal propeptide of type 1 procollagen levels, in contrast to its influence in young rats, sCT did not reduce the anabolic effect of PTH on femoral bone mineral density, tibial trabecular bone volume or bone formation rate in 6-month-old SHAM or OVX rats. Quantitative real-time PCR analysis of femoral metaphyses collected 1 and 4 h after a single PTH injection confirmed a significant increase in mRNA levels for interleukin 6 (Il6) and ephrinB2 (EfnB2), and a significant reduction in Sost and dentin matrix protein-1 (Dmp1) in response to PTH. However, in contrast to observations in young rats, these effects were not modified by co-administration of sCT, nor did sCT significantly modify Sost, Dmp1, or matrix extracellular phosphoglycoprotein (Mepe) mRNA levels. Furthermore, while CT receptor (CTR) mRNA (Calcr) was readily detected in GFP+ osteocytes isolated from young (3-week-old) DMP1-GFP mice, Calcr levels in osteocytes declined as mice aged, reaching levels that were undetectable in long bone at 49 weeks of age. These data indicate that osteocyte-mediated responses to CT are most likely to be of physiological relevance in young rodents.

Coupling the activities of bone formation and resorption: a multitude of signals within the basic multicellular unit

Coupling between bone formation and bone resorption refers to the process within basic multicellular units in which resorption by osteoclasts is met by the generation of osteoblasts from precursors, and their bone-forming activity, which needs to be sufficient to replace the bone lost. There are many sources of activities that contribute to coupling at remodeling sites, including growth factors released from the matrix, soluble and membrane products of osteoclasts and their precursors, signals from osteocytes and from immune cells and signaling taking place within the osteoblast lineage. Coupling is therefore a process that involves the interaction of a wide range of cell types and control mechanisms. As bone remodeling occurs at many sites asynchronously throughout the skeleton, locally generated activities comprise very important control mechanisms. In this review, we explore the potential roles of a number of these factors, including sphingosine-1-phosphate, semaphorins, ephrins, interleukin-6 (IL-6) family cytokines and marrow-derived factors. Their interactions achieve the essential tight control of coupling within individual remodeling units that is required for control of skeletal mass.

Osteoclast-derived coupling factors in bone remodeling

In the bone remodeling process that takes place throughout the skeleton at bone multicellular units, intercellular communication processes are crucial. The osteoblast lineage has long been known to program osteoclast formation and hence resorption, but the preservation of bone mass and integrity requires tight control of remodeling. This needs local controls that ensure availability of mesenchymal precursors and the provision of local signals that promote differentiation through the osteoblast lineage. Some signals can come from growth factors released from resorbed bone matrix, and there is increasing evidence that the osteoclast lineage itself produces factors that can either enhance or inhibit osteoblast differentiation and hence bone formation. A number of such factors have been identified from predominantly in vitro experiments. The coupling of bone formation to resorption is increasingly recognized as a complex, dynamic process that results from the input of many local factors of cell and matrix origin that can either promote or inhibit bone formation.

Mechanisms of osteoclast-dependent bone formation

Should we believe that osteoclasts are only involved in bone resorption? What about their contribution to bone formation? In this article I will review evidence that bone formation can be regulated by osteoclasts. Why is this? Likely because in the physiologic condition of bone remodeling, bone resorption and formation are balanced, and there is no better way to control this equilibrium than through a concerted action between the two cell types. Although the influence of osteoblasts on osteoclastic bone resorption is well documented and consolidated over time, what osteoclasts do to regulate osteoblast activity is still matter of intense investigation. The original hypothesis that all is in the osteoblast-seeking factors stored in the bone matrix, released and activated during bone resorption, is now being challenged by several studies, suggesting that osteoclasts are also capable of producing 'clastokines' that regulate osteoblast performance. Indeed, several of them have been demonstrated to orchestrate osteoclast-osteoblast activities. However, we are probably still at the dawn of a new era, and future work will tell us whether any of these clastokines can be exploited to stimulate bone formation and rebalance bone remodeling in skeletal diseases.

Lentiviral gene transfer of TCIRG1 into peripheral blood CD34(+) cells restores osteoclast function in infantile malignant osteopetrosis

Infantile malignant osteopetrosis (IMO) is a rare, lethal, autosomal recessive disorder characterized by non-functional osteoclasts. More than 50% of the patients have mutations in the TCIRG1 gene, encoding for a subunit of the osteoclast proton pump. The aim of this study was to restore the resorptive function of IMO osteoclasts by lentiviral mediated gene transfer of the TCIRG1 cDNA. CD34(+) cells from peripheral blood of five IMO patients and from normal cord blood were transduced with lentiviral vectors expressing TCIRG1 and GFP under a SFFV promoter, expanded in culture and differentiated on bone slices to mature osteoclasts. qPCR analysis and western blot revealed increased mRNA and protein levels of TCIRG1, comparable to controls. Vector corrected IMO osteoclasts generated increased release of Ca(2+) and bone degradation product CTX-I into the media as well as increased formation of resorption pits in the bone slices, while non-corrected IMO osteoclasts failed to resorb bone. Resorption was approximately 70-80% of that of osteoclasts generated from cord blood. Furthermore, transduced CD34(+) cells successfully engrafted in NSG-mice. In conclusion we provide the first evidence of lentiviral-mediated correction of a human genetic disease affecting the osteoclastic lineage.

Prostaglandin E2 acts via bone marrow macrophages to block PTH-stimulated osteoblast differentiation in vitro

Intermittent PTH is the major anabolic therapy for osteoporosis while continuous PTH causes bone loss. PTH acts on the osteoblast (OB) lineage to regulate bone resorption and formation. PTH also induces cyclooxygenase-2 (COX-2), producing prostaglandin E2 (PGE(2)) that can act on both OBs and osteoclasts (OCs). Because intermittent PTH is more anabolic in Cox-2 knockout (KO) than wild type (WT) mice, we hypothesized COX-2 might contribute to the effects of continuous PTH by suppressing PTH-stimulated differentiation of mesenchymal stem cells into OBs. We compared effects of continuous PTH on bone marrow stromal cells (BMSCs) and primary OBs (POBs) from Cox-2 KO mice, mice with deletion of PGE(2) receptors (Ptger(4) and Ptger(2) KO mice), and WT controls. PTH increased OB differentiation in BMSCs only in the absence of COX-2 expression or activity. In the absence of COX-2, PTH stimulated differentiation if added during the first week of culture. In Cox-2 KO BMSCs, PTH-stimulated differentiation was prevented by adding PGE(2) to cultures. Co-culture of POBs with M-CSF-expanded bone marrow macrophages (BMMs) showed that the inhibition of PTH-stimulated OB differentiation required not only COX-2 or PGE(2) but also BMMs. Sufficient PGE(2) to mediate the inhibitory effect was made by either WT POBs or WT BMMs. The inhibitory effect mediated by COX-2/PGE(2) was transferred by conditioned media from RANKL-treated BMMs and could be blocked by osteoprotegerin, which interferes with RANKL binding to its receptor on OC lineage cells. Deletion of Ptger(4), but not Ptger(2), in BMMs prevented the inhibition of PTH-stimulated OB differentiation. As expected, PGE(2) also stimulated OB differentiation, but when given in combination with PTH, the stimulatory effects of both were abrogated. These data suggest that PGE(2), acting via EP4R on BMMs committed to the OC lineage, stimulated secretion of a factor or factors that acted to suppress PTH-stimulated OB differentiation. This suppression of OB differentiation could contribute to the bone loss seen with continuous PTH in vivo.

Autosomal dominant osteopetrosis revisited: lessons from recent studies

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

miR-148a regulates osteoclastogenesis by targeting V-maf musculoaponeurotic fibrosarcoma oncogene homolog B

MicroRNAs (miRNAs) play crucial roles in bone metabolism. In the present study, we found that miR-148a is dramatically upregulated during osteoclastic differentiation of circulating CD14+ peripheral blood mononuclear cells (PBMCs) induced by macrophage colony stimulating factor (M-CSF) and receptor activator of nuclear factor-κB ligand (RANKL). Overexpression of miR-148a in CD14+ PBMCs promoted osteoclastogenesis, whereas inhibition of miR-148a attenuated osteoclastogenesis. V-maf musculoaponeurotic fibrosarcoma oncogene homolog B (MAFB) is a transcription factor negatively regulating RANKL-induced osteoclastogenesis. miR-148a directly targeted MAFB mRNA by binding to the 3' untranslated region (3'UTR) and repressed MAFB protein expression. In vivo, our study showed that silencing of miR-148a using a specific antagomir-inhibited bone resorption and increased bone mass in mice receiving ovariectomy (OVX) and in sham-operated control mice. Furthermore, our results showed that miR-148a levels significantly increased in CD14+ PBMCs from lupus patients and resulted in enhanced osteoclastogenesis, which contributed to the lower bone mineral density (BMD) in lupus patients compared with normal controls. Thus, our study provides a new insight into the roles of miRNAs in osteoclastogenesis, and contributes to a new therapeutic pathway for osteoporosis.