Coupling Of Bone Formation Research Articles

Overexpression of H1 calponin in osteoblast lineage cells leads to a decrease in bone mass by disrupting osteoblast function and promoting osteoclast formation

H1 calponin (CNN1) is known as a smooth muscle-specific, actin-binding protein which regulates smooth muscle contractive activity. Although previous studies have shown that CNN1 has effect on bone, the mechanism is not well defined. To investigate the role of CNN1 in maintaining bone homeostasis, we generated transgenic mice overexpressing Cnn1 under the control of the osteoblast-specific 3.6-kb Col1a1 promoter. Col1a1-Cnn1 transgenic mice showed delayed bone formation at embryonic stage and decreased bone mass at adult stage. Morphology analyses showed reduced trabecular number, thickness and defects in bone formation. The proliferation and migration of osteoblasts were decreased in Col1a1-Cnn1 mice due to alterations in cytoskeleton. The early osteoblast differentiation of Col1a1-Cnn1 mice was increased, but the late stage differentiation and mineralization of osteoblasts derived from Col1a1-Cnn1 mice were significantly decreased. In addition to impaired bone formation, the decreased bone mass was also associated with enhanced osteoclastogenesis. Tartrate-resistant acid phosphatase (TRAP) staining revealed increased osteoclast numbers in tibias of 2-month-old Col1a1-Cnn1 mice, and increased numbers of osteoclasts co-cultured with Col1a1-Cnn1 osteoblasts. The ratio of RANKL to OPG was significantly increased in Col1a1-Cnn1 osteoblasts. These findings reveal a novel function of CNN1 in maintaining bone homeostasis by coupling bone formation to bone resorption.

REMODELING SIMULATION ESTIMATES CHANGES IN MECHANICAL PROPERTIES OF OSTEOPOROTIC BONES

Osteoporosis causes bone fracture due to changes in the stress distribution of the trabecular bone with structural change as well as a decrease in bone mass. It arises from an imbalance in bone remodeling which involves the coupling of bone formation and resorption by osteoblast/osteoclast [Cowin SC, 1991]. Therefore, an evaluation of mechanical properties of the trabecular bone with its morphological change by remodeling is necessary to estimate the long-term fracture risk. In this study, we simulated bone remodeling of a pig trabecular bone model and estimated morphology of healthy and osteoporotic cases using our proposed mathematical model of bone remodeling [Adachi T, 2001]. We also evaluated fracture risks using a cumulative histogram of high stress. Potential of evaluation of change in mechanical properties of the trabecular bone considering its remodeling is discussed.

Runx2 promotes both osteoblastogenesis and novel osteoclastogenic signals in ST2 mesenchymal progenitor cells

We profiled the global gene expression of a bone marrow-derived mesenchymal pluripotent cell line in response to Runx2 expression. Besides osteoblast differentiation, Runx2 promoted the osteoclastogenesis of co-cultured splenocytes. This was attributable to the upregulation of many novel osteoclastogenic genes and the downregulation of anti-osteoclastogenic genes. In addition to being a master regulator for osteoblast differentiation, Runx2 controls osteoblast-driven osteoclastogenesis. Previous studies profiling gene expression during osteoblast differentiation had limited focus on Runx2 or paid little attention to its role in mediating osteoblast-driven osteoclastogenesis. ST2/Rx2(dox), a bone marrow-derived mesenchymal pluripotent cell line that expresses Runx2 in response to Doxycycline (Dox), was used to profile Runx2-induced gene expression changes. Runx2-induced osteoblast differentiation was assessed based on alkaline phosphatase staining and expression of classical marker genes. Osteoclastogenic potential was evaluated by TRAP staining of osteoclasts that differentiated from primary murine splenocytes co-cultured with the ST2/Rx2(dox) cells. The BeadChip™ platform (Illumina) was used to interrogate genome-wide expression changes in ST2/Rx2(dox) cultures after treatment with Dox or vehicle for 24 or 48h. Expression of selected genes was also measured by RT-qPCR. Dox-mediated Runx2 induction in ST2 cells stimulated their own differentiation along the osteoblast lineage and the differentiation of co-cultured splenocytes into osteoclasts. The latter was attributable to the stimulation of osteoclastogenic genes such as Sema7a, Ltc4s, Efnb1, Apcdd1, and Tnc as well as the inhibition of anti-osteoclastogenic genes such as Tnfrsf11b (OPG), Sema3a, Slco2b1, Ogn, Clec2d (Ocil), Il1rn, and Rspo2. Direct control of osteoblast differentiation and concomitant indirect control of osteoclast differentiation, both through the activity of Runx2 in pre-osteoblasts, constitute a novel mechanism of coordination with a potential crucial role in coupling bone formation and resorption.

Bone and calcium metabolism in menopause transition

Menopause, or estrogen deficiency, predispose women towards fragility fractures resulting from impaired bone strength, as the composite consequence of : (1) increased bone resorption caused by proliferation and activation of osteoclasts ; (2) dissociation of coupling bone formation with resorption ; (3) deterioration of bone quality ; (4) decreased calcium absorption from intestines and resorption from renal tubules. The current explanation of each phenomenon is described in this article, although the exact mechanisms have not been unanimously defined.

Potential for Bisphosphonate Treatment in Legg-Calve-Perthes Disease

The observation that the pathogenesis of femoral head deformity in Legg-Calve-Perthes disease (LCPD) relates to bone resorption without subsequent coupled bone formation leads quickly to the hypothesis that antiresorptive agents may provide a useful adjunctive avenue for therapy. Robust bone catabolism in the absence of anabolism can only lead to femoral head deformity. The published data on bisphosphonate use in models of LCPD was reviewed. Multiple animal studies support the further investigation of bisphosphonates and other anticatabolic agents in LCPD. Clinical data is only now starting to be gathered on the basis of animal and safety data.Rodent studies of traumatic and spontaneous osteonecrosis confirm that femoral head shape can be preserved by systemic bisphosphonate administration. Large animal piglet studies also show better preservation of the femoral head shape with systemic and local bisphosphonate administration, and also illustrate that systemic therapy requires initial revascularization of the necrotic head before its distribution within the head-a potential drawback of systemic therapy. Timing of effective dosing is likely to be very important. If the goal of treatment is to prevent deformity, then the window of therapy may be limited to an early stage of the disease before the significant collapse of the head.Although limiting bone resorption and preserving femoral head shape with bisphosphonates or other anticatabolic drugs may be of use, anabolic stimulation to speed up healing and new bone formation may also be desirable. Physical factors such as weight relief and activity modification may be important as the mechanical properties of the necrotic femoral head are significantly compromised during healing. The inflammatory nature of early LCPD is likely to also play a role in pathogenesis. These factors cannot be addressed by the use of antiosteoclastic therapy alone. Further basic science and clinical studies are required to clarify the role of bisphosphonates as adjunctive therapy in LCPD.

Is Nitroglycerin a Novel and Inexpensive Treatment for Osteoporosis?

Is Nitroglycerin a Novel and Inexpensive Treatment for Osteoporosis?

Review of osteoimmunology and the host response in endodontic and periodontal lesions

Both lesions of endodontic origin and periodontal diseases involve the host response to bacteria and the formation of osteolytic lesions. Important for both is the upregulation of inflammatory cytokines that initiate and sustain the inflammatory response. Also important are chemokines that induce recruitment of leukocyte subsets and bone-resorptive factors that are largely produced by recruited inflammatory cells. However, there are differences also. Lesions of endodontic origin pose a particular challenge since that bacteria persist in a protected reservoir that is not readily accessible to the immune defenses. Thus, experiments in which the host response is inhibited in endodontic lesions tend to aggravate the formation of osteolytic lesions. In contrast, bacteria that invade the periodontium appear to be less problematic so that blocking arms of the host response tend to reduce the disease process. Interestingly, both lesions of endodontic origin and periodontitis exhibit inflammation that appears to inhibit bone formation. In periodontitis, the spatial location of the inflammation is likely to be important so that a host response that is restricted to a subepithelial space is associated with gingivitis, while a host response closer to bone is linked to bone resorption and periodontitis. However, the persistence of inflammation is also thought to be important in periodontitis since inflammation present during coupled bone formation may limit the capacity to repair the resorbed bone.

Levels of osteoprotegerin (OPG) and receptor activator for nuclear factor kappa B ligand (RANKL) in serum: Are they of any help?

The coupling of bone formation and resorption is mediated through the OPG/RANK/RANKL system. OPG and RANKL are mainly produced by osteoblasts but also a variety of other tissues. The binding of RANKL to RANK, its natural receptor which is expressed by osteoclasts, accelerates bone resorption. OPG acts as decoy receptor and prevents the interaction of RANKL with RANK and therefore leads to a decrease in activity, survival and proliferation of osteoclasts. Since assays for measurements of serum OPG and RANKL have become commercially available, intense research focused on serum OPG/RANKL levels in context with underlying disease, age, co-morbidities, bone density, and fractures has derived. This review aims to provide an overview if and to which extent serum OPG and RANKL levels may reflect bone metabolism in patients with osteoporosis and metabolic bone disease.

Regulation of interleukin-6 expression in human osteoblastic cells in vitro.

Interleukin-6 (IL-6) is a potent stimulator of bone resorption which has been demonstrated in a variety of in vivo and in vitro models. We investigated the regulation of IL-6 secretion in primary human osteoblastic cells (HOC) in vitro by cytokines known to play an important role in coupling bone formation to bone resorption. HOC were isolated from healthy adults who underwent selective orthopedic surgery and treated with cytokines released in the bone microenvironment during coupling i.e Interleukin-1beta (IL-1beta), Tumor Necrosis Factor alpha (TNFalpha), Transforming Growth Factor beta1 and 2 (TGFbeta 1 and 2) and Endothelin-1 (ET-1). Furthermore, we determined whether systemically-acting steroid hormones of gonadal and adrenal origin as well as glucocorticoids affect the local regulation of IL-6 secretion in primary HOC. To examine the effects of different steroid hormones on IL-6 production, HOC were exposed to estradiol (E2), dihydrotestosterone (DHT), dehydroepiandrosterone (DHEA) and dexamethasone (Dexa) with and without a subsequent treatment of the HOC populations with cytokines. We observed that (1) IL-1beta and TNFalpha induced IL-6 in a dose and time-dependent fashion, (2) TGFbeta 1 and 2 enhanced basal and IL-1beta and TNFalpha induced IL-6 expression, (3) ET-1 elicited a dose-dependent stimulatory effect on IL-6 expression. (4) E2, DHT and DHEA alone and in combination with IL-1beta and TNFalpha elicited no reproducible dose-dependent effect on IL-6 production, whereas Dexa inhibited basal and IL-1beta and TNFalpha induced IL-6 expression dose dependently. In conclusion, IL-1beta, TNFalpha, TGFbeta 1 and 2 and ET-1 may participate in the regulation of bone resorption by stimulating IL-6 expression in HOC. Dexa inhibits the constitutive and cytokine stimulated IL-6 expression, whereas there is no in vitro evidence that sex steroids exert a major inhibitory effect on the osteoblastic secretion of IL-6 as demonstrated in a primary human bone cell model.

Emerging treatments for postmenopausal osteoporosis – focus on denosumab

The pathway of the receptor activator of the nuclear factor kappaB ligand (RANKL), RANK and osteoprotegerin (OPG) plays a central role in coupling bone formation and resorption during normal bone turnover and in a wide spectrum of diseases characterized by disturbed bone remodeling, increased bone resorption and bone destruction (osteoporosis, Paget's disease of bone, rheumatoid arthritis [RA], metastatic bone disease). Clinical trials indicate that denosumab, a RANKL-specific recombinant humanized monoclonal antibody, is effective in suppressing bone resorption, resulting in increase in bone mineral density (BMD) in post-menopausal women with low BMD, and has the potential to prevent progression of erosions in RA and of skeletal-related events in metastatic bone disease. The effects on fracture reduction in postmenopausal osteoporosis are awaited from the recently finished FREEDOM study. In clinical trials with denosumab, overall adverse events were similar to placebo or comparators, indicating a favorable safety profile in these diseases, which until now have been available up to 4 years, but data on long-term safety will be needed.

A Physical Mechanism for Coupling Bone Resorption and Formation in Adult Human Bone

During skeletal remodeling, pre-osteoclasts and pre-osteoblasts are targeted to critical sites of the bone to resorb and reconstruct bone matrix, respectively. Coordination of site-specific recruitment of these two cell types is a prerequisite to maintain the specific architecture of each bone within strict limits throughout adult life. Here, we determined that the bone marrow microanatomy adjacent to remodeling areas is a central player in this process. By using histomorphometry and multiple immunostainings, we demonstrated in biopsies exhibiting coupled bone resorption and formation that osteoclasts and osteoblasts on the bone surface were always covered by a canopy of flat cells expressing osteoblast markers. In contrast, in biopsies in which this canopy was disrupted, bone formation was deficient. Three-dimensional visualizations revealed that this canopy covered the entire remodeling site and was associated with capillaries, thereby forming a previously unrecognized microanatomical entity. Furthermore, pre-osteoclasts were positioned along these capillaries. These findings led to a model that implicates vasculature in the site-specific recruitment of osteoclasts and osteoblasts and embraces the current knowledge on the molecular mechanism of bone remodeling.

Activation of the Acquired Immune Response Reduces Coupled Bone Formation in Response to a Periodontal Pathogen

Osteoimmunolgy involves the interaction of the immune system with skeletal elements. This interaction can lead to the formation of osseous lesions. To investigate how the acquired immune response could contribute to osteolytic lesions, we injected the periodontal pathogen Porphyromonas gingivalis adjacent to calvarial bone with or without prior immunization against the bacterium. Activation of the acquired immune response increased osteoclastogenesis and decreased coupled bone formation. The latter was accompanied by an increase in nuclear translocation of the transcription factor FOXO1 in vivo, increased apoptosis of bone-lining cells measured by the TUNEL assay and number of activated caspase-3 positive cells and a decrease in bone lining cell density. Further studies were conducted with MC3T3 osteoblastic cells. Apoptosis and increased FOXO1 DNA binding activity were induced when a combination of cytokines was tested, IL-beta, TNF-alpha, and IFN-gamma. Knockdown of FOXO1 by small interfering RNA significantly reduced cytokine stimulated apoptosis, cleaved caspase-3/7 activity and decreased mRNA levels of the proapoptotic genes, TNF-alpha, FADD, and caspase-3, -8, and -9. These results indicate that activation of the acquired immunity by a periodontal pathogen reduces the coupling of bone formation and resorption. This may occur by enhancing bone lining cell apoptosis through a mechanism that involves increased FOXO1 activation. These studies give insight into inflammatory bone diseases such as periodontal disease and arthritis were the formation of lytic lesions occurs in conjunction with deficient bone formation and activation of an acquired immune response.

Expression and Synthesis of Bone Morphogenetic Proteins by Osteoclasts: A Possible Path to Anabolic Bone Remodeling

Skeletal remodeling is a finely orchestrated process coupling bone formation to bone resorption. The dynamics of coupling is regulated by the microenvironment at the bone remodeling site, which in turn is influenced by the intercellular communication between cells like osteoclasts and osteoblasts. Understanding the dynamics of coupling is important in devising new therapeutic approaches to the treatment of skeletal diseases characterized by disturbances in the bone remodeling process. In this study, we report the localization of bone morphogenetic proteins (BMPs) in osteoclasts generated from primary cocultures of bone marrow cells from mouse femur and tibia with mouse calvarial osteoblasts, using immunocytochemistry and in situ hybridization. Positive staining was seen in osteoclasts for BMP-2, -4, -6, and -7. Real-time PCR was used to quantitatively confirm the expression of transcripts for BMP-2, BMP-4, and BMP-6 mRNA in murine osteoclasts. Finally, the presence of BMP-2, -4, -6, and-7 proteins was confirmed in osteoclast lysates by Western blotting. Overall, our data suggest a possible direct role for osteoclasts in promoting bone formation via expression and synthesis of BMPs, which then would play an important role in promoting the recruitment, proliferation, and differentiation of osteoblasts at bone resorption sites.

Coupling between osteogenesis and angiogenesis

Recent progresses clear show that osteogenesis, which requires a coupling between angiogenesis and osteogenesis, is not a cell autonomous process. The new findings suggest that pericytes, arising from a vessel‐associated stem cell progenitor, represent an osteoprogenitor capable of bone formation in a “right” microenvironment. Second, hypoxia environment where bone cells are located greatly affect osteogenesis. Recent studies showed that hypoxia‐inducible factor 1α (HIF‐1α), a transcriptional regulator of vascular endothelial growth factor (VEGF) expression, represents rate‐limiting components of osteogenic‐angiogenic coupling and trabecular bone formation. Last but not the least, the bone does not have a lymphatic system to transport the excessive fluid back to the circulation system. Our studies using a scanning electronic microscope combining with a resin‐casted‐acid etching method reveal that the entire blood vessel system in bone, including arterioles, venules and capillaries are connected with dendritic osteocytes. We propose that osteo‐canalicular system, in addition to its mechanical function, may play a role in facilitating fluid flow back to circulation system. (This work is partially supported by NIH grants: AR51587 and DE 016977).

Smad4 is required for maintaining normal murine postnatal bone homeostasis

Transforming growth factor beta (TGFbeta) is a multifunctional cytokine involved in skeletal development. Smad4 is the central intracellular mediator of TGFbeta signaling. Our previous studies reveal that Smad4 is required for maintaining the normal development of chondrocytes in the growth plate. However, its biological function during postnatal bone remodeling is largely unknown. To investigate the role of Smad4 in maintaining bone homeostasis, we disrupted the Smad4 gene in differentiated osteoblasts using the Cre-loxP system. The Smad4 mutant mice exhibited lower bone mass up to 6 months of age. The proliferation and function of the mutant osteoblasts were significantly decreased. Bone mineral density, bone volume, bone formation rate and osteoblast numbers were remarkably reduced in Smad4 mutants. Intriguingly, the trabecular bone volume in Smad4 mutant mice older than 7 months was higher than that of controls whereas the calvarial and cortical bone remained thinner than in controls. This correlated with reduced bone resorption possibly caused by downregulation of TGFbeta1 and alteration of the ligand receptor activator of NF-kappaB (RANKL)-osteoprotegerin (OPG) axis. These studies demonstrate essential roles of Smad4-mediated TGFbeta signaling in coupling bone formation and bone resorption and maintaining normal postnatal bone homeostasis.

Relationships of a Transforming Growth Factor-β2 Single Nucleotide Polymorphism and Messenger Ribonucleic Acid Abundance with Bone and Production Traits in Chickens

Osteoporosis is a serious problem for the laying hen industry with economic, production, and welfare consequences. Transforming growth factor-β2 (TGFβ2) has been implicated as an important factor in coupling bone resorption and formation in bone remodeling. The current study was designed to determine if TGFβ2 was associated with variation in bone mineralization in chickens, using 2 complementary experimental approaches. First, an intronic single nucleotide polymorphism (SNP) present in TGFβ2 was investigated in an F2 population to determine its association with bone, growth, and egg traits of importance to the layer and broiler industries. The TGFβ2 SNP was significantly associated (P < 0.05) with bone mineral density and content. However, these associations became nonsignificant when BW was included as a covariate in analyses. The TGFβ2 SNP was also significantly associated (P < 0.05) with BW from 1 to 6 wk of age and egg production from 46 to 55 wk of age. To further explore the relationship between TGFβ2 and bone strength, bone marrow TGFβ2 mRNA abundance was compared between broiler and layer chickens at 15, 35, and 60 wk of age. Bone and egg traits were measured along with mRNA abundance at each age and found to differ significantly between lines. The TGFβ2 mRNA abundance was approximately 4-fold greater in broiler compared with layer hens at 15 wk of age but was similar between lines at later ages. Thus, even though the TGFβ2 SNP will likely not be an effective marker for improving bone strength independently of changes in BW, further research is warranted to investigate the relationship of TGFβ2 mRNA abundance to bone strength in laying hens.

Remodeling of Mineralized Tissues, Part II: Control and Pathophysiology

Based on a review of literature, the bone remodeling concepts, as defined by Harold Frost, are extended to the molecular level to help explain common bone physiopathology. Remodeling of mineralized tissue is an inflammatory response to accumulated tissue damage. Inflammation activates (A) the localized cell population, which attracts circulating osteoclast precursors, and initiates foci of vascular invasion. Coordination of these cybernetic events results in formation of a cutting/filling cone (cortical bone) or a hemicutting/filling cone (trabecular bone). Damaged bone is resorbed (R) creating a self-limited resorption cavity that is then filled by bone formation (F). A genetic mechanism (RANK/RANKL/OPG) is proposed for coupling bone formation to resorption during the remodeling process. Following surgery and/or initiation of orthodontic tooth movement, a regional acceleratory response (RAP) occurs throughout the affected alveolar process. Undermining resorption during the initial stage of tooth movement is analogous to initiation of a bone remodeling cycle (A→R→F). Understanding the cell dynamics of the ARF sequence is fundamental for appreciating common remodeling disorders such as osteoporosis, Paget's disease, hypo- and hyperparathyroidism, metastases, and external apical root resorption. Depending on the physiopathologic context, remodeling may enhance or limit the orthodontic options for management of a malocclusion.

Calcineurin/NFAT Signaling in Osteoblasts Regulates Bone Mass

Development and repair of the vertebrate skeleton requires the precise coordination of bone-forming osteoblasts and bone-resorbing osteoclasts. In diseases such as osteoporosis, bone resorption dominates over bone formation, suggesting a failure to harmonize osteoclast and osteoblast function. Here, we show that mice expressing a constitutively nuclear NFATc1 variant (NFATc1(nuc)) in osteoblasts develop high bone mass. NFATc1(nuc) mice have massive osteoblast overgrowth, enhanced osteoblast proliferation, and coordinated changes in the expression of Wnt signaling components. In contrast, viable NFATc1-deficient mice have defects in skull bone formation in addition to impaired osteoclast development. NFATc1(nuc) mice have increased osteoclastogenesis despite normal levels of RANKL and OPG, indicating that an additional NFAT-regulated mechanism influences osteoclastogenesis in vivo. Calcineurin/NFATc signaling in osteoblasts controls the expression of chemoattractants that attract monocytic osteoclast precursors, thereby coupling bone formation and bone resorption. Our results indicate that NFATc1 regulates bone mass by functioning in both osteoblasts and osteoclasts.

Antioxidant Vitamin Supplements and Markers of Bone Turnover in a Community Sample of Nonsmoking Women

Whereas several epidemiological studies suggest that low dietary intake of vitamins C and E is linked to increased hip fracture in smokers and antioxidants (dietary and endogenous) are reduced in elderly osteoporotic women, none has demonstrated an effect of supplemental antioxidants on bone turnover. In an observational study of 533 randomly selected women, we investigated the associations among the use of antioxidant supplements, vitamins C and E, serum levels of biochemical markers of bone turnover (C-telopeptide [CTx] and bone-specific alkaline phosphatase [BSAP]), and whole body bone mineral density (BMD). Twenty-two women were identified as current users of supplemental vitamin C or E. Duration of antioxidant supplement use was negatively associated with age-adjusted and weight-adjusted serum CTx, such that mean CTx levels (natural log transformed) were 0.022 units lower for each year of exposure. No significant differences were detected for adjusted serum BSAP or whole body BMD. Our results suggest that antioxidant vitamin E or C supplements may suppress bone resorption in nonsmoking postmenopausal women. Coupling of bone formation and resorption may explain the absence of an effect on bone formation markers, given evidence of enhanced effects of antioxidants on osteoblast differentiation; this warrants further investigation. This work adds to the growing body of evidence that antioxidants may play a role in preventing osteoporosis.

Short-Range Intercellular Calcium Signaling in Bone

The regulation of bone turnover is a complex and finely tuned process. Many factors regulate bone remodeling, including hormones, growth factors, cytokines etc. However, little is known about the signals coupling bone formation to bone resorption, and how mechanical forces are translated into biological effects in bone. Intercellular calcium waves are increases in intracellular calcium concentration in single cells, subsequently propagating to adjacent cells, and can be a possible mechanism for the coupling of bone formation to bone resorption. The aim of the present studies was to investigate whether bone cells are capable of communicating via intercellular calcium signals, and determine by which mechanisms the cells propagate the signals. First, we found that osteoblastic cells can propagate intercellular calcium transients upon mechanical stimulation, and that there are two principally different mechanisms for this propagation. One mechanism involves the secretion of a nucleotide, possibly ATP, acting in an autocrine action to purinergic P2Y2 receptors on the neighboring cells, leading to intracellular IP3 generation and subsequent release of calcium from intracellular stores. The other mechanism involves the passage of a small messenger through gap junctions to the cytoplasm of the neighboring cells, inducing depolarization of the plasma membrane with subsequent opening of membrane bound voltage-operated calcium channels. Next, we found that osteoblasts can propagate these signals to osteoclasts as well. We demonstrated that paracrine action of ATP was responsible for the wave propagation, but now the purinergic P2X7 receptor was involved. Thus, the studies demonstrate that calcium signals can be propagated not only among osteoblasts, but also between osteoblasts and osteoclasts in response to mechanical stimulation. Thus, intercellular calcium signaling can be a mechanism by which mechanical stimuli on bone are translated into biological signals in bone cells, and propagated through the network of cells in bone. Further, the observations offer new pharmacological targets for the modulation of bone turnover, and perhaps even for the treatment of bone metabolic disorders.