Osteoclast size heterogeneity in rat long bones is associated with differences in adhesive ligand specificity

Effective Orthodontic Tooth Movement via an Occlusion-Activated Electromechanical Synergistic Dental Aligner

Malocclusion is a prevalent dental health problem plaguing over 56% worldwide. Mechanical orthodontic aligners render directional teeth movement extensively used for malocclusion treatment in the clinic, while mechanical regulation inefficiency prolongs the treatment course and induces adverse complications. As a noninvasive physiotherapy, an appropriate electric field plays a vital role in tissue metabolism engineering. Here, we propose an occlusion-activated electromechanical synergistic dental aligner that converts occlusal energy into a piezo-excited alternating electric field for accelerating orthodontic tooth movement. Within an 18-day intervention, significantly facilitated orthodontic results were obtained from young and aged Sprague-Dawley rats, increasing by 34% and 164% in orthodontic efficiency, respectively. The different efficiencies were attributed to age-distributed periodontal tissue status. Mechanistically, the electromechanical synergistic intervention modulated the microenvironment, enhanced osteoblast and osteoclast activity, promoted alveolar bone metabolism, and ultimately accelerated tooth movement. This work holds excellent potential for personalized and effective treatment for malocclusions, which would vastly reduce the suffering of the long orthodontic course.

Mechanical Heterogeneity in the Bone Microenvironment as Characterized by Atomic Force Microscopy

Bones are structurally heterogeneous organs with diverse functions that undergo mechanical stimuli across multiple length scales. Mechanical characterization of the bone microenvironment is important for understanding how bones function in health and disease. Here, we describe the mechanical architecture of cortical bone, the growth plate, metaphysis, and marrow in fresh murine bones, probed using atomic force microscopy in physiological buffer. Both elastic and viscoelastic properties are found to be highly heterogeneous with moduli ranging over three to five orders of magnitude, both within and across regions. All regions include extremely compliant areas, with moduli of a few pascal and viscosities as low as tens of Pa·s. Aging impacts the viscoelasticity of the bone marrow strongly but has a limited effect on the other regions studied. Our approach provides the opportunity to explore the mechanical properties of complex tissues at the length scale relevant to cellular processes and how these impact aging and disease.

Pisidium coreanum Inhibits Multinucleated Osteoclast Formation and Prevents Estrogen-Deficient Osteoporosis

Mollusks have served as important sources of human food and medicine for a long time. Raw Pisidium coreanum, a freshwater bivalve of the phylum Mollusca, is used in traditional therapies in parts of Asia. However, the therapeutic effects of Pisidium coreanum on bone diseases are not known. We investigated the functional roles of Pisidium coreanum in osteoporotic bone diseases. Pisidium coreanum inhibited the differentiation of bone marrow-derived monocytic cells into mature osteoclasts in vitro. The ovariectomized mice that received oral administration of Pisidium coreanum showed improvements in both trabecular and cortical bones. This preventive activity of Pisidium coreanum against bone loss was due to limited osteoclast maturation with reduced osteoclast surface extent in trabecular bone tissue. The formation of large multinucleated osteoclasts in vitro was significantly decreased in response to Pisidium coreanum, consistent with the reduced expression levels of osteoclast markers and fusion-related genes, such as NFATc1, p65, integrinαvβ3, DC-STAMP, OC-STAMP, Atp6v0d2, FAK, CD44, and MFR. These data suggest that Pisidium coreanum inhibits osteoclast differentiation by negatively regulating the fusion of mononuclear osteoclast precursors. Thus, our data demonstrate the ability of Pisidium coreanum to effectively prevent estrogen-deficient osteoporosis through inhibition of multinucleated osteoclast formation.

Long Chain Omega-3 Polyunsaturated Fatty Acid Supplementation Protects Against Adriamycin and Cyclophosphamide Chemotherapy-Induced Bone Marrow Damage in Female Rats

Although bone marrow and bone toxicities have been reported in breast cancer survivors, preventative strategies are yet to be developed. Clinical studies suggest consumption of long chain omega-3 polyunsaturated fatty acids (LCn3PUFA) can attenuate age-related bone loss, and recent animal studies also revealed benefits of LCn3PUFA in alleviating bone marrow and bone toxicities associated with methotrexate chemotherapy. Using a female rat model for one of the most commonly used anthracycline-containing breast cancer chemotherapy regimens (adriamycin + cyclophosphamide) (AC) chemotherapy, this study investigated potential effects of daily LCn3PUFA consumption in preserving bone marrow and bone microenvironment during chemotherapy. AC treatment for four cycles significantly reduced bone marrow cellularity and increased marrow adipocyte contents. It increased trabecular bone separation but no obvious changes in bone volume or bone cell densities. LCn3PUFA supplementation (375 mg/100 g/day) attenuated AC-induced bone marrow cell depletion and marrow adiposity. It also partially attenuated AC-induced increases in trabecular bone separation and the cell sizes and nuclear numbers of osteoclasts formed ex vivo from bone marrow cells isolated from AC-treated rats. This study suggests that LCn3PUFA supplementation may have beneficial effects in preventing bone marrow damage and partially protecting the bone during AC cancer chemotherapy.

Natural uranium impairs the differentiation and the resorbing function of osteoclasts

BACKGROUND

Uranium is a naturally occurring radionuclide ubiquitously present in the environment. The skeleton is the main site of uranium long-term accumulation. While it has been shown that natural uranium is able to perturb bone metabolism through its chemical toxicity, its impact on bone resorption by osteoclasts has been poorly explored. Here, we examined for the first time in vitro effects of natural uranium on osteoclasts.

METHODS

The effects of uranium on the RAW 264.7 monocyte/macrophage mouse cell line and primary murine osteoclastic cells were characterized by biochemical, molecular and functional analyses.

RESULTS

We observed a cytotoxicity effect of uranium on osteoclast precursors. Uranium concentrations in the μM range are able to inhibit osteoclast formation, mature osteoclast survival and mineral resorption but don't affect the expression of the osteoclast gene markers Nfatc1, Dc-stamp, Ctsk, Acp5, Atp6v0a3 or Atp6v0d2 in RAW 274.7 cells. Instead, we observed that uranium induces a dose-dependent accumulation of SQSTM1/p62 during osteoclastogenesis.

CONCLUSIONS

We show here that uranium impairs osteoclast formation and function in vitro. The decrease in available precursor cells, as well as the reduced viability of mature osteoclasts appears to account for these effects of uranium. The SQSTM1/p62 level increase observed in response to uranium exposure is of particular interest since this protein is a known regulator of osteoclast formation. A tempting hypothesis discussed herein is that SQSTM1/p62 dysregulation contributes to uranium effects on osteoclastogenesis.

GENERAL SIGNIFICANCE

We describe cellular and molecular effects of uranium that potentially affect bone homeostasis.

Osteoblasts of calvaria induce higher numbers of osteoclasts than osteoblasts from long bone

Several studies have demonstrated the existence of functional differences between osteoclasts harbored in different bones. The mechanisms involved in the occurrence of such a heterogeneity are not yet understood. Since cells of the osteoblast lineage play a critical role in osteoclastogenesis, osteoclast heterogeneity may be due to osteoblasts that differ at the different bone sites. In the present study we evaluated possible differences in the capacity of calvaria and long bone osteoblasts to induce osteoclastogenesis. Osteoblasts were isolated from calvaria and long bone of mice and co-cultured with osteoclast precursors obtained from bone marrow of both types of bone, spleen and peripheral blood. Irrespective of the source of the precursors, a significantly higher number of TRACP-positive multinucleated cells were formed with calvaria osteoblasts. The expression of osteoclastogenesis related genes was analyzed by qPCR. OPG was significantly higher expressed by long bone osteoblasts. The RANKL/OPG ratio and TNF-α gene expression were significantly higher in calvaria osteoblast cultures. OPG added to the culture system inhibited osteoclastogenesis in both groups. Blocking TNF-α had no effect on osteoclastogenesis. Calvaria and long bone osteoblasts were pre-stimulated with VitD3 for 5days. Subsequently, osteoclast precursors were added to these cultures. After a co-culture of 6days, it was shown that VitD3 pre-stimulation of long bone osteoblasts strongly improved their capacity to induce osteoclast formation. This coincided with an increased ratio of RANKL/OPG. Taken together, the data demonstrated differences in the capacity of calvaria and long bone osteoblasts to induce osteoclastogenesis. This appeared to be due to differences in the expression of RANKL and OPG. VitD3 pre-stimulation improved the ability of long bone osteoblasts to induce osteoclast formation. Our findings demonstrate bone-site specific differences in osteoblast-mediated formation of osteoclasts. The data may suggest that the heterogeneity of osteoclasts is partially due to the way the osteoblasts induce their formation.

Osteoprotegerin exposure at different stages of osteoclastogenesis differentially affects osteoclast formation and function

This study aimed to investigate the effects of osteoprotegerin (OPG), a decoy receptor for receptor activator for nuclear factor κB ligand (RANKL), during the various stages of osteoclast differentiation, and additionally investigate its effects on osteoclast adhesion and activity. RAW264.7 murine monocytic cells were incubated with macrophage colony-stimulating factor and RANKL for 1, 3, 5, or 7 days, followed by an additional 24-h incubation in the presence or absence of OPG (80 ng/mL). We examined osteoclast differentiation and adhesion capacity using the tartrate-resistant acid phosphatase (TRAP) assay and immunofluorescence microscopy, and additionally examined cell growth in real time using the xCELLigence system. Furthermore, the expression levels of TRAP, RANK, integrin β3, matrix metalloproteinase 9, cathepsin K, carbonic anhydrase II, and vesicular-type H(+)-ATPase A1 were examined using western blotting. OPG exposure on day 1 enhanced the osteoclast growth curve as well as adhesion, and increased RANK and integrin β3 expression. In contrast, exposure to OPG at later time points (days 3-7) inhibited osteoclast differentiation, adhesion structure formation, and protease expression. In conclusion, the biological effects of OPG exposure at the various stages of osteoclast differentiation were varied, and included the enhanced adhesion and survival of preosteoclasts, the block of differentiation from the early to the terminal stages of osteoclastogenesis, and suppression of mature osteoclast activation following OPG exposure during the terminal differentiation stage, suggesting that the effects of OPG exposure differ based on the stage of differentiation.

Thrombin inhibits osteoclast differentiation through a non-proteolytic mechanism

Thrombin stimulates expression of interleukin 6 and cyclooxygenase 2 by osteoblasts, both of which enhance osteoblast-mediated osteoclast differentiation by increasing the ratio of receptor activator of nuclear factor κB ligand (RANKL) expression to that of osteoprotegerin (OPG) in osteoblasts. We hypothesised that thrombin would also increase this ratio and thereby stimulate osteoclast differentiation in mixed cultures of osteoblastic cells and osteoclast precursors. In primary mouse osteoblasts, but not in bone marrow stromal cells, thrombin increased the ratio of RANKL to OPG expression. Thrombin inhibited differentiation of osteoclasts, defined as tartrate-resistant acid phosphatase (TRAP)-positive cells with three or more nuclei, in mouse bone marrow cultures treated with osteoclastogenic hormones; this effect was not mediated by the major thrombin receptor, protease-activated receptor 1, nor did it require thrombin's proteolytic activity. Thrombin also caused a decrease in the number of TRAP-positive cells with fewer than three nuclei. Thrombin (active or inactive) also inhibited osteoclast differentiation and bone resorption, respectively, in cultures of mouse spleen cells and human peripheral blood mononuclear cells induced to undergo osteoclastogenesis by treatment with RANKL and macrophage colony-stimulating factor. Osteoclast differentiation in spleen cells was inhibited when they were exposed to thrombin from days 0 to 3 or 3 to 5 of culture but not days 5 to 7 when most fusion occurred. Thrombin inhibited expression of RANK by spleen cells. These observations indicate that, although thrombin stimulates production of osteoclastogenic factors by osteoblastic cells, it inhibits the early stages of RANKL-induced osteoclast differentiation through a direct effect on osteoclast precursors that does not require thrombin's proteolytic activity.

Osteoclast fusion and fission

Osteoclasts are specialized multinucleated cells with the unique capacity to resorb bone. Despite insight into the various steps of the interaction of osteoclast precursors leading to osteoclast formation, surprisingly little is known about what happens with the multinucleated cell itself after it has been formed. Is fusion limited to the short period of its formation, or do osteoclasts have the capacity to change their size and number of nuclei at a later stage? To visualize these processes we analyzed osteoclasts generated in vitro with M-CSF and RANKL from mouse bone marrow and native osteoclasts isolated from rabbit bones by live cell microscopy. We show that osteoclasts fuse not only with mononuclear cells but also with other multinucleated cells. The most intriguing finding was fission of the osteoclasts. Osteoclasts were shown to have the capacity to generate functional multinucleated compartments as well as compartments that contained apoptotic nuclei. These compartments were separated from each other, each giving rise to a novel functional osteoclast or to a compartment that contained apoptotic nuclei. Our findings suggest that osteoclasts have the capacity to regulate their own population in number and function, probably to adapt quickly to changing situations.

Imaging an adapted dentoalveolar complex

Adaptation of a rat dentoalveolar complex was illustrated using various imaging modalities. Micro-X-ray computed tomography for 3D modeling, combined with complementary techniques, including image processing, scanning electron microscopy, fluorochrome labeling, conventional histology (H&E, TRAP), and immunohistochemistry (RANKL, OPN) elucidated the dynamic nature of bone, the periodontal ligament-space, and cementum in the rat periodontium. Tomography and electron microscopy illustrated structural adaptation of calcified tissues at a higher resolution. Ongoing biomineralization was analyzed using fluorochrome labeling, and by evaluating attenuation profiles using virtual sections from 3D tomographies. Osteoclastic distribution as a function of anatomical location was illustrated by combining histology, immunohistochemistry, and tomography. While tomography and SEM provided past resorption-related events, future adaptive changes were deduced by identifying matrix biomolecules using immunohistochemistry. Thus, a dynamic picture of the dentoalveolar complex in rats was illustrated.

Structural and functional changes in the alveolar bone osteoclasts of estrogen-treated rats

This study investigated structural and functional features of apoptotic alveolar bone osteoclasts in estrogen-treated rats. For this purpose, 15 female rats 22 days old were divided into three groups: Estrogen (EG), Sham (SG) and Control (CG). The rats of EG received daily intramuscular injection of estrogen for 7 days. The SG received only the oil vehicle. Maxillary fragments containing alveolar bone were removed and processed for light and transmission electron microscopy. Area (OcA) and number of nuclei (OcN) and bone resorption surface per TRAP-positive osteoclasts (BS/OC) were obtained. Vimentin, caspase-3 and MMP-9 immunoreactions, TUNEL/TRAP and MMP-9/TUNEL combined reactions were performed. In EG, the OcA, OcN and BS/Oc were reduced. Moreover, osteoclasts showed cytoplasm immunolabelled by caspase-3 and a different pattern of vimentin expression in comparison with CG and SG. MMP-9 expression was not affected by estrogen and the TUNEL-positive osteoclasts were MMP-9-immunolabelled. In EG, ultrastructural images showed that apoptotic osteoclasts did not exhibit ruffled borders or clear zones and were shedding mononucleated portions. TRAP-positive structures containing irregular and dense chromatin were partially surrounded by fibroblast-like cells. In conclusion, the reduction in the BS/Oc may be due to reduction in OcA and OcN; these effects seem to be related to vimentin disarrangement rather than to an interference of estrogen with osteoclast MMP-9 expression. Osteoclast apoptosis involves caspase-3 activity and vimentin degradation; these cells release portions containing one apoptotic nucleus and, subsequently, undergo fragmentation, giving rise to apoptotic bodies.

Localization and expression of prothrombin in rodent osteoclasts and long bones

The serum protein prothrombin (PT) is proteolytically converted to thrombin during the coagulation cascade by the cell-associated prothrombinase complex. In vitro, RANKL-differentiated osteoclasts express tissue factor and coagulation factor Xa, which convert PT to thrombin (Karlstrom et al. Biochem Biophys Res Commun 394:593-599, 2010). The present study investigated the localization of PT in bone as well as the expression of PT mRNA in bone and osteoclasts. Herein, immunoblot analysis detected PT and smaller proteolytically cleaved fragments with sizes consistent with the action of prothrombinase in a protein fraction extracted with guanidine-HCl EDTA from mouse tibia. Light microscopic and ultrastructural immunohistochemistry demonstrated the presence of PT in the newly formed bone matrix of the metaphysis. Furthermore, fluorescent immunohistochemistry on metaphyseal trabecular bone showed that PT colocalized with MMP-9-expressing subepiphyseal osteoclasts, whereas cathepsin K-expressing osteoclasts were closely associated with PT of the bone matrix. RT-qPCR analysis revealed that PT mRNA was detected in tibia. Expression of PT mRNA in the tibia was 0.2% of the level in the liver. In addition, PT mRNA expression was increased by RANKL-induced differentiation of bone marrow macrophages to osteoclasts. The results demonstrate that PT is synthesized and proteolytically processed in bone. Furthermore, PT is present mainly in the newly formed bone matrix of the metaphyseal trabecular bone compartment in close association to osteoclasts. In addition, MMP-9-positive osteoclasts contain PT, and PT expression is increased during osteoclastogenesis.

Jaw and long bone marrows have a different osteoclastogenic potential

Osteoclasts, the multinucleated bone-resorbing cells, arise through fusion of precursors from the myeloid lineage. However, not all osteoclasts are alike; osteoclasts at different bone sites appear to differ in numerous respects. We investigated whether bone marrow cells obtained from jaw and long bone differed in their osteoclastogenic potential. Bone marrow cells from murine mandible and tibiae were isolated and cultured for 4 and 6 days on plastic or 6 and 10 days on dentin. Osteoclastogenesis was assessed by counting the number of TRAP(+) multinucleated cells. Bone marrow cell composition was analyzed by FACS. The expression of osteoclast- and osteoclastogenesis-related genes was studied by qPCR. TRAP activity and resorptive activity of osteoclasts were measured by absorbance and morphometric analyses, respectively. At day 4 more osteoclasts were formed in long bone cultures than in jaw cultures. At day 6 the difference in number was no longer observed. The jaw cultures, however, contained more large osteoclasts on plastic and on dentin. Long bone marrow contained more osteoclast precursors, in particular the myeloid blasts, and qPCR revealed that the RANKL:OPG ratio was higher in long bone cultures. TRAP expression was higher for the long bone cultures on dentin. Although jaw osteoclasts were larger than long bone osteoclasts, no differences were found between their resorptive activities. In conclusion, bone marrow cells from different skeletal locations (jaw and long bone) have different dynamics of osteoclastogenesis. We propose that this is primarily due to differences in the cellular composition of the bone site-specific marrow.

Long bone osteoclasts display an augmented osteoclast phenotype compared to calvarial osteoclasts

Osteoclasts are multinucleated cells specialized in degrading bone and characterized by high expression of the enzymes tartrate-resistant acid phosphatase (TRAP) and cathepsin K (CtsK). Recent studies show that osteoclasts exhibit phenotypic differences depending on their anatomical site of action. Using immunohistochemistry, RT-qPCR, FPLC chromatography and immunoblotting, we compared TRAP expression in calvaria and long bone. TRAP protein and enzyme activity levels were higher in long bones compared to calvaria. In addition, proteolytic processing of TRAP was more extensive in long bones than calvaria which correlated with higher cysteine proteinase activity and protein expression of CtsK. These two types of bones also exhibited a differential expression of monomeric TRAP and CtsK isoforms. Analysis of CtsK(-/-) mice revealed that CtsK is involved in proteolytic processing of TRAP in calvaria. Moreover, long bone osteoclasts exhibited higher expression of not only TRAP and CtsK but also of the membrane markers CD68 and CD163. The results suggest that long bone osteoclasts display an augmented osteoclastic phenotype with stronger expression of both membranous and secreted osteoclast proteins.

RANKL induces components of the extrinsic coagulation pathway in osteoclasts

Prothrombin is converted to thrombin by factor Xa in the cell-associated prothrombinase complex. Prothrombin is present in calcified bone matrix and thrombin exerts effects on osteoblasts as well as on bone resorption by osteoclasts. We investigated whether (1) osteoclasts display factor Xa-dependent prothrombinase activity and (2) osteoclasts express critical regulatory components upstream of the prothrombinase complex. The osteoclast differentiation factor RANKL induced formation of multinucleated TRAP positive cells concomitant with induction of prothrombinase activity in cultures of RAW 264.7 cells and bone marrow osteoclast progenitors. Expression analysis of extrinsic coagulation factors revealed that RANKL enhanced protein levels of factor Xa as well as of coagulation factor III (tissue factor). Inhibition assays indicated that factor Xa and tissue factor were involved in the control of prothrombinase activity in RANKL-differentiated osteoclasts, presumably at two stages (1) conversion of prothrombin to thrombin and (2) conversion of factor X to factor Xa, respectively. Activation of the extrinsic coagulation pathway during osteoclast differentiation through induction of tissue factor and factor Xa by a RANKL-dependent pathway indicates a novel role for osteoclasts in converting prothrombin to thrombin.