Maintain Bone Homeostasis Research Articles

Leucine-rich repeat-containing G-protein-coupled receptor 4: a new receptor of RANKL for osteoclast regulation

Bones are constantly remodelled through the coordinate work by the bone-forming osteoblasts and bone-resorbing osteoclasts. Physiologically, the osteoblast-mediated bone formation and osteoclast-mediated bone resorption are coupled for maintaining bone homeostasis. However, such coupling is disrupted in pathological conditions such as osteoporosis that is caused by the hyperactivated bone resorption or insufficient bone formation (1).

Effects of gallotannin on osteoclastogenesis and the p38 MAP kinase pathway

PurposeOsteoclasts are multinucleated giant cells that specialize in bone resorption and work together with bone-forming osteoblasts to maintain bone homeostasis. However, excessive osteoclast activation accounts for bone diseases, such as osteoporosis and periodontitis. In previous studies, natural small-molecule compounds have been shown to regulate osteoclastogenesis and osteoclast functions. Here we demonstrate that gallotannin, a hydrolyzable plant tannin, suppresses osteoclast differentiation. MethodsWe first used an ex vivo bone marrow culture system containing both osteoclast precursors and surrounding cells, thereby resembling physiological conditions, to evaluate the suppressive effect of gallotannin. We also used a RANKL-induced osteoclastogenesis assay containing only osteoclast precursors to confirm the suppressive effect of gallotannin in the absence of effects from other cells. ResultsThe suppressive effect of gallotannin was associated with the reduced RANKL-mediated induction of NFATc1, a critical transcription factor involved in osteoclast differentiation. We further confirmed that gallotannin reduced the p38 MAPK pathway activation, which is mediated by M-CSF and RANKL. This pathway suppression might underlie the suppression of NFATc1 production and subsequent reduction in osteoclast differentiation. ConclusionOur data indicate that the natural small-molecule compound gallotannin might be useful as a novel anti-bone resorptive agent.

Compressive force-produced CCN2 induces osteocyte apoptosis through ERK1/2 pathway

Osteocytes produce various factors that mediate the onset of bone formation and resorption and play roles in maintaining bone homeostasis and remodeling in response to mechanical stimuli. One such factor, CCN2, is thought to play a significant role in osteocyte responses to mechanical stimuli, but its function in osteocytes is not well understood. Here, we showed that CCN2 induces apoptosis in osteocytes under compressive force loading. Compressive force increased CCN2 gene expression and production, and induced apoptosis in osteocytes. Application of exogenous CCN2 protein induced apoptosis, and a neutralizing CCN2 antibody blocked loading-induced apoptosis. We further examined how CCN2 induces loaded osteocyte apoptosis. In loaded osteocytes, extracellular signal-regulated kinase 1/2 (ERK1/2) was activated, and an ERK1/2 inhibitor blocked loading-induced apoptosis. Furthermore, application of exogenous CCN2 protein caused ERK1/2 activation, and the neutralizing CCN2 antibody inhibited loading-induced ERK1/2 activation. Therefore, this study demonstrated for the first time to our knowledge that enhanced production of CCN2 in osteocytes under compressive force loading induces apoptosis through activation of ERK1/2 pathway. © 2014 American Society for Bone and Mineral Research.

Use of Hes1-GFP reporter mice to assess activity of the Hes1 promoter in bone cells under chronic inflammation

Notch signaling plays a critical role in maintaining bone homeostasis partially by controlling the formation of osteoblasts from mesenchymal stem cells (MSCs). We reported that TNF activates Notch signaling in MSCs which inhibits osteoblast differentiation in TNF transgenic (TNF-Tg) mice, a mouse model of chronic inflammatory arthritis. In the current study, we used Hes1-GFP and Hes1-GFP/TNF-Tg mice to study the distribution and dynamic change of Notch active cells in normal and inflammatory bone loss and mechanisms mediating their enhanced proliferation. We found that Hes1-GFP+ cells are composed of cells expressing mesenchymal, hematopoietic and endothelial surface markers. CD45−/Hes1-GFP+ cells express high levels of mesenchymal markers and form CFU-F and CFU-ALP colonies. Expansion of CFU-F colonies is associated with a rapid increase in Hes1-GFP+ cell numbers and their GFP intensity. The GFP signal is lost when a CFU-F colony differentiates into an ALP+ osteoblast colony. TNF increases the numbers of CD45−/Hes1-GFP+ cells, which are stained negatively for osteoblast marker osteocalcin and localized adjacent to endosteal and trabecular bone surfaces. CD45−/Hes1-GFP+ cells in Hes1-GFP/TNF-Tg mice have increased BrdU incorporation and PDGFRβ levels. TNF increases the number of proliferating Hes1-GFP+ cells, which is prevented by a specific PDGFRβ inhibitor. Notch inhibition blocks TNF-mediated PDGFRβ expression and cell proliferation. Thus, TNF-induced MSC proliferation is mediated by PDGFRβ signal, which works at downstream of Notch. Hes1-GFP mice can be used to assess the activation status of Notch in bone cells.

Mechanical stress regulates osteogenic differentiation and RANKL/OPG ratio in periodontal ligament stem cells by the Wnt/β-catenin pathway

BackgroundThe balance between osteoblastic and osteoclastic activity is critical in orthodontic tooth movement (OTM). Mesenchymal stem cells (MSCs) play an important role in maintaining bone homeostasis, and periodontal ligament stem cells (PDLSCs) are tissue-specific MSCs in the periodontal ligament. However, whether PDLSCs are required for periodontal tissue remodeling during OTM is not fully understood. MethodsHere, we used PDGFRα and Nestin to trace PDLSCs during OTM in rats. We treat human PDLSCs with 100kpa static pressure for 1h or 12h in vitro, and examined the phenotypic changes and expression of RANKL and OPG in these cells. ResultsIn vivo, we found that positive signals of PDGFRα and Nestin in the PDL gradually increased and then decreased on the pressure side to which pressure was applied. In vitro, the osteogenic differentiation of PDLSCs was significantly increased after force treatment for 1h relative to 12h. In contrast, the expression ratio of RANKL/OPG was reduced at 1h and significantly increased at 12h. Furthermore, we found that the Wnt/β-catenin pathway was dynamically activated in the PDL and in PDLSCs after mechanical stimulation. Importantly, the canonical Wnt pathway inhibitor DKK1 blocked the osteogenesis effect and rescued the ratio of RANKL/OPG in PDLSCs under force treatment for 1h. ConclusionsOur findings reveal that PDLSCs participate in OTM and that the Wnt/β-catenin pathway maintains bone homeostasis during tooth movement by regulating the balance between osteoblastic and osteoclastic activity. General significanceWe describe a novel potential mechanism related to tooth movement.

Osteoporosis: Therapeutic Options

The definition of osteoporosis was originally formulated at a conference of the World Health Organization (WHO) in 1993 as 'a systemic skeletal disease characterized by decreased bone mass and altered micro-architecture of bone tissue, leading to enhanced bone fragility and risk of fractures'. Osteoporosis is characterized by low bone mineral density (BMD) and loss of the structural and bio-mechanical properties that are required to maintain bone homeostasis. This review aims to address the currently available options in prevention and treatment of osteoporosis. Management of osteoporosis includes non-pharmacological treatment - diet rich of calcium and vitamin D, healthy lifestyle, proper exercise plan, and pharmacological therapy. Combination of non-pharmacological and pharmacological treatment options have to be considered for prevention of osteoporosis and minimization of the risk of fractures. Given the heterogeneity of osteoporosis syndrome and lack of significant number of comparative studies, the choice of a pharmacological agents should be individualized.

CLEC5A is critical for dengue virus-induced osteoclast activation and bone homeostasis.

Osteoclasts are bone tissue macrophages critical to maintain bone homeostasis. However, whether osteoclasts are susceptible to flaviviral infections and involved in dengue virus (DV)-induced disease pathogenesis is still unknown. In this study, we found that osteoclasts were preferentially susceptible to DV infection and produced similar amounts of cytokines and infectious virions as macrophages. Interestingly, DV-induced cytokine secretion and nuclear translocation of the transcription factor NFATc1 in osteoclast via the Syk-coupled myeloid C-type lectin member 5A (CLEC5A). Moreover, DV caused transient inflammatory reaction in bone tissue and upregulated osteolytic activity to release C-telopeptide of type I collagen (CTX-1) from bone tissue. Furthermore, DV-induced osteolytic activity was attenuated in CLEC5A-deficient mice, and administration of antagonistic anti-CLEC5A mAb inhibited DV-activated osteolytic activity and reduced CTX-1 serum level in vivo. This observation suggests that osteoclasts serve as a novel target for DV, and transient upregulation of osteolytic activity may contribute to the clinical symptoms in dengue patients.Key messagesCultured osteoclasts were susceptible to DV infection.Osteoclasts produced similar amounts of cytokines and infectious virions as macrophages.DV induced nuclear translocation of NFATc1 in osteoclast via CLEC5A.DV caused transient inflammatory reaction in bone tissue and upregulated osteolytic activity.Antagonistic anti-CLEC5A mAb inhibited DV-activated osteolytic activity in vivo.Electronic supplementary materialThe online version of this article (doi:10.1007/s00109-016-1409-0) contains supplementary material, which is available to authorized users.

Sirtuin 3 (SIRT3) maintains bone homeostasis by regulating AMPK-PGC-1β axis in mice.

The mitochondrial sirtuin 3 (SIRT3) is involved in suppressing the onset of multiple pathologies, including cardiovascular disease, fatty liver, age-related hearing loss, and breast cancer. But a physiological role of SIRT3 in bone metabolism is not known. Here we show that SIRT3 is a key regulatory molecule to maintain bone homeostasis. Mice deficient in SIRT3 exhibited severe osteopenia owing to increased numbers of osteoclasts. Osteoclast precursors from Sirt3−/− mice underwent increased osteoclastogenesis in response to receptor activator of nuclear factor-κB ligand (RANKL), an essential cytokine for osteoclast differentiation. SIRT3 expression from RANKL induction depended on the transcription coactivator PGC-1β (peroxisome proliferator-activated receptor-γ co-activator-1β) and the nuclear receptor ERRα (estrogen receptor-related receptor α), and that SIRT3 inhibited the differentiation by interfering with the RANKL-induced expression of PGC-1β. Thus an auto-regulatory feedback mechanism operates to induce its own inhibitor SIRT3 by PGC-1β. Moreover, Sirt3−/− osteoclast precursors reduced AMP-activated protein kinase (AMPK) phosphorylation through down-regulating the expression of AMPK. Our results suggest that a mitochondrial SIRT3 is an intrinsic inhibitor for RANKL-mediated osteoclastogenesis.

A Comprehensive Review of Immunoreceptor Regulation of Osteoclasts.

Osteoclasts require coordinated co-stimulation by several signaling pathways to initiate and regulate their cellular differentiation. Receptor activator for NF-κB ligand (RANKL or TNFSF11), a tumor necrosis factor (TNF) superfamily member, is the master cytokine required for osteoclastogenesis with essential co-stimulatory signals mediated by immunoreceptor tyrosine-based activation motif (ITAM)-signaling adaptors, DNAX-associated protein 12kDa size (DAP12) and FcεRI gamma chain (FcRγ). The ITAM-signaling adaptors do not have an extracellular ligand-binding domain and, therefore, must pair with ligand-binding immunoreceptors to interact with their extracellular environment. DAP12 pairs with a number of different immunoreceptors including triggering receptor expressed on myeloid cells 2 (TREM2), myeloid DAP12-associated lectin (MDL-1), and sialic acid-binding immunoglobulin-type lectin 15 (Siglec-15); while FcRγ pairs with a different set of receptors including osteoclast-specific activating receptor (OSCAR), paired immunoglobulin receptor A (PIR-A), and Fc receptors. The ligands for many of these receptors in the bone microenvironment remain unknown. Here, we will review immunoreceptors known to pair with either DAP12 or FcRγ that have been shown to regulate osteoclastogenesis. Co-stimulation and the effects of ITAM-signaling have turned out to be complex, and now include paradoxical findings that ITAM-signaling adaptor-associated receptors can inhibit osteoclastogenesis and immunoreceptor tyrosine-based inhibitory motif (ITIM) receptors can promote osteoclastogenesis. Thus, co-stimulation of osteoclastogenesis continues to reveal additional complexities that are important in the regulatory mechanisms that seek to maintain bone homeostasis.

Effect of Porphyromonas gingivalis lipopolysaccharide (Pg-LPS) on the expression of EphA2 in osteoblasts and osteoclasts

Porphyromonas gingivalis (Pg) as the major pathogenic bacterium of chronic periodontitis can cause alveolar bone resorption. Lipopolysaccharide (LPS) is its main virulence factor. The Eph family plays an important role in maintaining bone homeostasis. In this study, the effects of P. gingivalis lipopolysaccharide (Pg-LPS) on the expression of EphA2 in osteoblasts and osteoclasts were investigated. MC3T3-E1 cells and RAW264.7 cells were separately cultured in osteoblast-conditioned medium and osteoclast-conditioned medium to induce their differentiation into osteoblasts and osteoclasts, respectively. MC3T3-E1 cells were treated with 1 μg/mL of Pg-LPS 3, 7, and 14 d later, while RAW264.7 cells were treated with 10 μg/mL of Pg-LPS 1, 3, and 5 d later. The results have shown that Pg-LPS increased the expression of EphA2 both in osteoblasts and osteoclasts, decreased the expression of osteogenic-related genes (ALP, Sp7), and increased the expression of osteoclast-related genes (MMP9, c-fos, ACP5, CtsK, and NFATc1). Tartrate-resistant acid phosphatase (TRAP) staining illustrated that Pg-LPS promoted osteoclast differentiation and decreased the activity of alkaline phosphatase. Therefore, analysis indicates that, when treated with Pg-LPS, the expression of EphA2 is upregulated while the activity of osteoblasts and osteoclasts was reduced and increased, respectively. Our data suggest that EphA2 is closely related to the formation of osteoblasts and resorption of osteoclast and is likely to play an role in bone resorption induced in chronic periodontitis. These findings may provide information on new targets for prevention and treatment of chronic periodontitis.

Macrophage Polarization and Bone Formation: A review.

The contribution of inflammation to bone loss is well documented in arthritis and other diseases with an emphasis on how inflammatory cytokines promote osteoclastogenesis. Macrophages are the major producers of cytokines in inflammation, and the factors they produce depend upon their activation state or polarization. In recent years, it has become apparent that macrophages are also capable of interacting with osteoblasts and their mesenchymal precursors. This interaction provides growth and differentiation factors from one cell that act on the other and visa versa-a concept akin to the requirement for a feeder layer to grow hemopoietic cells or the coupling that occurs between osteoblasts and osteoclasts to maintain bone homeostasis. Alternatively, activated macrophages are the most likely candidates to promote bone formation and have also been implicated in the tissue repair process in other tissues. In bone, a number of factors, including oncostatin M, have been shown to promote osteoblast formation both in vitro and in vivo. This review discusses the different cell types involved, cellular mediators, and how this can be used to direct new bone anabolic approaches.

ID: 226: DCIR maintains bone homeostasis by regulating IFN-γ production in T cells

Dendritic cell immunoreceptor (DCIR) is a C-type lectin receptor with a canonical signal motif for initiating inhibitory signals in the cytoplasmic region. In accordance with the structural insights, Dcir−/− mice spontaneously developed autoimmune-like diseases like sialadenitis and swelling and redness of joints. Thus, DCIR is a pivotal regulator for maintaining the homeostasis of the immune system. Furthermore, Dcir−/− mice not only developed ankylosis, characterized by aberrant cartilage and bone formation, but also increased bone volume, accompanied with the higher number of osteoclasts and osteoblasts (OBs). To understand the role of DCIR in bone metabolism, we dissected the molecular mechanism affecting the bone volume in Dcir−/− mice. Flow cytometry analyses revealed that Dcir−/− mice increased IFN- γ -positive T cells in peripheral blood and lymph nodes. Dcir−/− dendritic cells had higher potency to shape IFN- γ -positive T cells in the co-culture with wild-type T cells. X-ray histomorphometric analysis revealed that both Rag2−/− Dcir−/− and Ifng−/− Dcir−/− negated the increase of bone volume, suggesting that T cell-secreting IFN- γ be a pathogenic in the increased bone mass. In in vitro OB culture system, we identified IFN- γ as an osteogenic factor capable of facilitating mineralization of OBs, indicating that IFN- γ released from T cells acts on OBs. These findings demonstrate that Dcir−/− mice had increased IFN- γ -positive T cells, which accelerates mineralization of OBs, leading to the increased bone volume. Collectively, DCIR plays a key role in maintaining bone homeostasis and IFN- γ functions as an enhancing factor in bone formation.

DCIR maintains bone homeostasis by regulating IFN-γ production in T cells.

Dendritic cell immunoreceptor (DCIR) is a C-type lectin receptor mainly expressed in DCs. Dcir (-/-) mice spontaneously develop autoimmune enthesitis and ankylosis accompanied by fibrocartilage proliferation and ectopic ossification. However, the mechanisms of new bone/cartilage formation in Dcir (-/-) mice remain to be elucidated. In this study, we show that DCIR maintains bone homeostasis by regulating IFN-γ production under pathophysiological conditions. DCIR deficiency increased bone volume in femurs and caused aberrant ossification in joints, whereas these symptoms were abolished in Rag2(-/-)Dcir(-/-) mice. IFN-γ-producing T cells accumulated in lymph nodes and joints of Dcir(-/-) mice, and purified Dcir(-/-) DCs enhanced IFN-γ(+) T cell differentiation. The ankylotic changes and bone volume increase were suppressed in the absence of IFN-γ. Thus, IFN-γ is a positive chondrogenic and osteoblastogenic factor, and DCIR is a crucial regulator of bone metabolism; consequently, both factors are potential targets for therapies directed against bone metabolic diseases.

Calcium, Is It Better to Have Less?--Global Health Perspectives.

Appropriate calcium intake is necessary for the accrual and maintenance of bone mass. A significant proportion of the world's population does not have adequate calcium intake, and thus, supplementation plays a key role in maintaining bone homeostasis and other aspects of health. Since a series of reports from the Auckland calcium study and meta-analysis indicated that calcium supplementation was associated with an increased risk for adverse cardiovascular events, concern over the safety of calcium supplementation has grown; however, considerable inconsistencies in the reproducibility were found and questions regarding the study methodologies have been raised. In addition, since the increased adverse cardiovascular events by calcium supplementation were observed in calcium-replete subjects, it should be clarified whether the same risk profile would be observed in countries with low calcium intake. Dietary calcium intake varies widely across the world; cardiovascular event risk factors and outcomes also vary and appear to be the opposite of calcium intake levels. Furthermore, the effects of calcium supplementation were shown to depend on dietary calcium intake, with a better bone mineral density response for low calcium intake subjects compared to that in calcium-replete subjects. Based on these evidences, the risk-benefit ratio of calcium supplement is likely to be different in different region of the world. Therefore, accumulation of evidence to establish population-specific guidelines for calcium supplementation is warranted before extrapolating the results obtained from a limited number of studies to the other people with different age, gender, ethnicity and risk profile across the world.

MicroRNA-194 promotes osteoblast differentiation via downregulating STAT1

Osteoblast differentiation is a vital process in maintaining bone homeostasis in which various transcriptional factors, signaling molecules, and microRNAs (miRNAs) are involved. Recently, signal transducer and activator of transcription 1 (STAT1) has been found to play an important role in regulating osteoblast differentiation. Here, we identified that STAT1 expression was regulated by miR-194. Using mouse bone mesenchymal stem cells (BMSCs), we found that miR-194 expression was significantly increased following osteoblast differentiation induction. Overexpression of miR-194 by lentivirus-mediated gene transfer markedly increased osteoblast differentiation, whereas inhibition of miR-194 significantly suppressed osteoblast differentiation of BMSCs. Using a dual-luciferase reporter assay, a direct interaction between miR-194 and the 3′-untranslated region (UTR) of STAT1 was confirmed. Additionally, miR-194 regulated mRNA and protein expression of STAT1 in BMSCs. Further analysis showed that miR-194 overexpression promoted the nuclear translocation of runt-related transcription factor 2 (Runx2), which is critical for osteoblast differentiation. In contrast, inhibition of miR-194 blocked the nuclear translocation of Runx2. Moreover, overexpression of STAT1 significantly blocked Runx2 nuclear translocation and osteoblast differentiation mediated by miR-194 overexpression. Taken together, our data suggest that miR-194 regulates osteoblast differentiation through modulating STAT1-mediated Runx2 nuclear translocation.

Osteoblast-induced osteoclast apoptosis by fas ligand/FAS pathway is required for maintenance of bone mass.

The interplay between osteoblasts and osteoclasts has a crucial role in maintaining bone homeostasis. In this study, we reveal that osteoblasts are capable of inducing osteoclast apoptosis by FAS ligand (FASL)/FAS signaling. Conditional knockout of FASL in osteoblasts results in elevated osteoclast numbers and activity, along with reduced bone mass, suggesting that osteoblast-produced FASL is required to maintain physiological bone mass. More interestingly, we show that osteoblasts from ovariectomized (OVX) osteoporotic mice exhibit decreased FASL expression that results from the IFN-γ- and TNF-α-activated NF-κB pathway, leading to reduced osteoclast apoptosis and increased bone resorption. Systemic administration of either IFN-γ or TNF-α ameliorates the osteoporotic phenotype in OVX mice and rescues FASL expression in osteoblasts. In addition, ovariectomy induces more significant bone loss in FASL conditional knockout mice than in control group with increased osteoclast activity in which the levels of RANKL and OPG remain unchanged. Taken together, this study suggests that osteoblast-induced osteoclast apoptosis via FASL/FAS signaling is a previously unrecognized mechanism that has an important role in the maintenance of bone mass in both physiological conditions and OVX osteoporosis.

Inhibited Wnt signaling causes age-dependent abnormalities in the bone matrix mineralization in the Apert syndrome FGFR2(S252W/+) mice.

Apert syndrome (AS) is a type of autosomal dominant disease characterized by premature fusion of the cranial sutures, severe syndactyly, and other abnormalities in internal organs. Approximately 70% of AS cases are caused by a single mutation, S252W, in fibroblast growth factor receptor 2 (FGFR2). Two groups have generated FGFR2 knock-in mice Fgfr2 S252W/+ that exhibit features of AS. During the present study of AS using the Fgfr2 S252W/+ mouse model, an age-related phenotype of bone homeostasis was discovered. The long bone mass was lower in 2 month old mutant mice than in age-matched controls but higher in 5 month old mutant mice. This unusual phenotype suggested that bone marrow-derived mesenchymal stem cells (BMSCs), which are vital to maintain bone homeostasis, might be involved. BMSCs were isolated from Fgfr2 S252W/+ mice and found that S252W mutation could impair osteogenic differentiation BMSCs but enhance mineralization of more mature osteoblasts. A microarray analysis revealed that Wnt pathway inhibitors SRFP1/2/4 were up-regulated in mutant BMSCs. This work provides evidence to show that the Wnt/β-catenin pathway is inhibited in both mutant BMSCs and osteoblasts, and differentiation defects of these cells can be ameliorated by Wnt3a treatment. The present study suggested that the bone abnormalities caused by deregulation of Wnt pathway may underlie the symptoms of AS.

Spaceflight osteoporosis: current state and future perspective.

Osteoporosis is one of the established major consequences of long-duration spaceflights in astronauts seriously undermining their health after their returning on Earth. Indeed, astronauts typically lose more bone mass during one month than postmenopausal women on Earth lose in one year. To date, countermeasures mainly consist in exercise and supplementation while pharmacological treatment as those used in postmenopausal women are not routine. However, it is evident that exercise and supplementation alone are not enough to maintain bone homeostasis. In this paper we describe the current countermeasures for bone loss during long-term spaceflight, review the modern treatment which are successfully employed to prevent osteoporosis on Earth and that could be quickly used also for astronauts and finally focus on the recent cellular and molecular understanding of bone homeostasis which might provide the basis for the development of future targeted therapies.

From osteoblast to osteoclast: new insights of Yin-Yang theory in bone remodeling

Yin-Yang theory plays critical role in traditional Chinese medicine (TCM) science. However, there are many competing interpretations of Yin- Yang theory in the context with the development in biology, and no consensus has been established. Here we first propose that osteoblast should be regarded as Yin, while osteoclast should be considered as Yang in bone remodeling compartment (BRC). Our conceptions are consistent with the following key findings: 1)osteoblast and osteoclast both derive from the embryonic ectoderm; 2)osteoblast and osteoclast work collaboratively in BRC to maintain bone homeostasis; 3)the activities of osteoblast and osteoclast are coupled to remain dynamic balance; 4)osteoblast and osteoclast exert their function in ceaseless successions. Moreover, we argue that Yin-Yang relationships exist between osteoblast and osteoclast: osteoblast secretes the receptor activator for nuclear factor-κB ligand (RANKL) and the monocyte/macrophage colony-stimulating factor (M-CSF) and osteoprotegerin (OPG) to positively or negatively regulate osteoclast differentiation and function. Meanwile, osteoclast and bone resorption in turn release cytokines, such as transforming growth factor-β (TGF-β), insulin-like growth factor-1 (IGF-1) from bone matrix, to regulate osteoblast differentiation and function. Next, the unlimited division of Yin-Yang can be applied to divide sublevel of Yin-Yang inside osteoblast or osteoclast. Finally, Yin-Yang relationship of osteoblast and osteoclast is relative. Therefore, we come into the conclusion that the relationships between osteoblast and osteoclast as established in contemporary biology reflect the classic Yin-Yang in bone remodeling. The new Yin-Yang concepts of osteoblast and osteoclast may strengthen basic theory and clinical practice in TCM.

Macrophages promote osteoblastic differentiation in-vivo: implications in fracture repair and bone homeostasis.

Macrophages are activated in inflammation and during early phases of repair processes. Interestingly, they are also present in bone during development, but their function during this process is unclear. Here, we explore the function of macrophages in bone development, growth, and repair using transgenic mice to constitutively or conditionally deplete macrophages. Depletion of macrophages led to early skeletal growth retardation and progressive osteoporosis. By 3 months of age, macrophage-deficient mice displayed a 25% reduction in bone mineral density and a 70% reduction in the number of trabecular bone compared to control littermates. Despite depletion of macrophages, functional osteoclasts were still present in bones, lining trabecular bone and the endosteal surface of the cortical bone. Furthermore, ablation of macrophages led to a 60% reduction in the number of bone marrow mesenchymal progenitor cells and a decrease in the ability of these cells to differentiate to osteoblasts. When macrophages were depleted during fracture repair, bone union was impaired. Calluses from macrophage-deficient animals were smaller, and contained less bone and more fibrotic tissue deposition. Taken together, this shows that macrophages are crucial for maintaining bone homeostasis and promoting fracture repair by enhancing the differentiation of mesenchymal progenitors.