Osteocyte-driven bone remodeling

Idiopathic Acquired Osteosclerosis in a Middle-Aged Woman With Systemic Lupus Erythematosus

Widely distributed osteosclerosis is an unusual radiographic finding with multiple causes. A 42-year-old premenopausal Spanish woman gradually acquired dense bone diffusely affecting her axial skeleton and focally affecting her proximal long bones. Systemic lupus erythematosus (SLE) diagnosed in adolescence had been well controlled. She had not fractured or received antiresorptive therapy, and she was hepatitis C virus antibody negative. Family members had low bone mass. Lumbar spine bone mineral density (BMD) measured by dual-photon absorptiometry (DPA) at age 17 years, while receiving glucocorticoids, was 79% the average value of age-matched controls. From ages 30 to 37 years, dual-energy X-ray absorptiometry (DXA) BMD Z-scores steadily increased in her lumbar spine from +3.8 to +7.9, and in her femoral neck from -1.4 to -0.7. Serum calcium and phosphorus levels were consistently normal, 25-hydroxyvitamin D (25OHD) <20 ng/mL, and parathyroid hormone (PTH) sometimes slightly increased. Her reduced estimated glomerular filtration rate (eGFR) was 38 to 55 mL/min. Hypocalciuria likely reflected positive mineral balance. During increasing BMD, turnover markers (serum bone-specific alkaline phosphatase [ALP], procollagen type 1 N propeptide [P1NP], osteocalcin [OCN], and carboxy-terminal cross-linking telopeptide of type 1 collagen [CTx], and urinary amino-terminal cross-linking telopeptide of type 1 collagen [NTx and CTx]) were 1.6- to 2.8-fold above the reference limits. Those of bone formation seemed increased more than those of resorption. FGF-23 was slightly elevated, perhaps from kidney disease. Serum osteoprotegerin (OPG) and TGFβ1 levels were normal, but sclerostin (SOST) and receptor activator of nuclear factor kappa-B ligand (RANKL) were elevated. Serum multiplex biomarker profiling confirmed a high level of SOST and RANKL, whereas Dickkopf-1 (DKK-1) seemed low. Matrix metalloproteinases-3 (MMP-3) and -7 (MMP-7) were elevated. Iliac crest biopsy revealed tetracycline labels, no distinction between thick trabeculae and cortical bone, absence of peritrabecular fibrosis, few osteoclasts, and no mastocytosis. Then, for the past 3 years, BMD Z-scores steadily decreased. Skeletal fluorosis, mastocytosis, myelofibrosis, hepatitis C-associated osteosclerosis, multiple myeloma, and aberrant phosphate homeostasis did not explain her osteosclerosis. Mutation analysis of the LRP5, LRP4, SOST, and osteopetrosis genes was negative. Microarray showed no notable copy number variation. Perhaps her osteosclerosis reflected an interval of autoimmune-mediated resistance to SOST and/or RANKL. © 2016 American Society for Bone and Mineral Research.

High serum levels of sclerostin and Dickkopf-1 are associated with acute ischaemic stroke

BACKGROUND AND AIMS

Sclerostin and Dickkopf-1 (Dkk-1) are potent antagonists of Wnt signalling and might therefore play important roles in cardiovascular disease. We investigated whether serum sclerostin and Dkk-1 levels are associated with acute ischaemic stroke and specific stroke subtypes.

METHODS

Serum levels of sclerostin and Dkk-1 were measured by ELISA on day 1 and on day 6 after stroke in 62 patients with large artery atherosclerotic (LAA) stroke, on day 1 after stroke in 62 age- and gender-matched patients with small-artery occlusion (SAO) stroke and on admission in 62 healthy controls. Stroke severity was determined based on the National Institutes of Health Stroke Scale (NIHSS) and by measuring stroke volume on diffusion-weighted imaging. Outcome was measured by the modified Rankin Scale (mRS) on day 90.

RESULTS

Compared with controls, serum sclerostin and Dkk-1 levels were significantly higher in both patients with LAA stroke and with SAO stroke, and no difference was detected between the stroke subtypes. Sclerostin and Dkk-1 levels remained stable between the first and sixth day after stroke in the patients with LAA stroke. Receiver operating characteristic curve analysis was used to evaluate sclerostin and Dkk-1 as markers of a high risk of stroke and produced area under curve values of 0.773 and 0.776. Adjusted logistic regression showed that serum sclerostin and Dkk-1 levels remained as independent markers of stroke. No correlations were found between sclerostin or Dkk-1 levels and stroke severity or stroke outcome.

CONCLUSIONS

High serum levels of sclerostin and Dkk-1 are associated with acute ischaemic stroke.

Parallel mechanisms suppress cochlear bone remodeling to protect hearing

Bone remodeling, a combination of bone resorption and formation, requires precise regulation of cellular and molecular signaling to maintain proper bone quality. Whereas osteoblasts deposit and osteoclasts resorb bone matrix, osteocytes both dynamically resorb and replace perilacunar bone matrix. Osteocytes secrete proteases like matrix metalloproteinase-13 (MMP13) to maintain the material quality of bone matrix through perilacunar remodeling (PLR). Deregulated bone remodeling impairs bone quality and can compromise hearing since the auditory transduction mechanism is within bone. Understanding the mechanisms regulating cochlear bone provides unique ways to assess bone quality independent of other aspects that contribute to bone mechanical behavior. Cochlear bone is singular in its regulation of remodeling by expressing high levels of osteoprotegerin. Since cochlear bone expresses a key PLR enzyme, MMP13, we examined whether cochlear bone relies on, or is protected from, osteocyte-mediated PLR to maintain hearing and bone quality using a mouse model lacking MMP13 (MMP13(-/-)). We investigated the canalicular network, collagen organization, lacunar volume via micro-computed tomography, and dynamic histomorphometry. Despite finding defects in these hallmarks of PLR in MMP13(-/-) long bones, cochlear bone revealed no differences in these markers, nor hearing loss as measured by auditory brainstem response (ABR) or distortion product oto-acoustic emissions (DPOAEs), between wild type and MMP13(-/-) mice. Dynamic histomorphometry revealed abundant PLR by tibial osteocytes, but near absence in cochlear bone. Cochlear suppression of PLR corresponds to repression of several key PLR genes in the cochlea relative to long bones. These data suggest that cochlear bone uniquely maintains bone quality and hearing independent of MMP13-mediated osteocytic PLR. Furthermore, the cochlea employs parallel mechanisms to inhibit remodeling by osteoclasts and osteoblasts, and by osteocytes, to protect hearing. Understanding the cellular and molecular mechanisms that confer site-specific control of bone remodeling has the potential to elucidate new pathways that are deregulated in skeletal disease.

The anti-metastatic micro-environment of the bone: Importance of osteocyte Cx43 hemichannels

Bone metastases of tumor cells are a common and life-threatening feature of a variety of late-stage cancers, including breast cancers. However, until now, much less has been known about the intrinsic anti-metastatic properties of the bones and how these could be exploited to prevent or treat bone metastases. Very recently, native Cx43 hemichannels present in osteocytes have been identified as important anti-metastatic signaling complexes by establishing high local extracellular ATP levels. Moreover, bisphosphonate drugs, applied as adjuvant therapies in the treatment of breast cancer patients and bone diseases, are known to display anti-metastatic properties. Now, it became clear that these compounds exert their effects through osteocyte Cx43 hemichannels, thereby triggering their opening and promoting ATP release in the extracellular micro-environment. Hence, endogenous osteocyte Cx43 hemichannels emerge as important and promising therapeutic targets for the prevention of bone metastases and/or clinical treatment of bone-metastasized breast cancers.

A Single Chance to Contact Multiple Targets: Distinct Osteocyte Morphotypes Shed Light on the Cellular Mechanism Ensuring the Robust Formation of Osteocytic Networks

The formation of the complex osteocytic network relies on the emission of long cellular processes involved in communication, mechanical strain sensing, and bone turnover control. Newly deposited osteocytic processes rapidly become trapped within the calcifying matrix, and, therefore, they must adopt their definitive conformation and contact their targets in a single morphogenetic event. However, the cellular mechanisms ensuring the robustness of this unique mode of morphogenesis remain unknown. To address this issue, we examined the developing calvaria of the amphibian Xenopus tropicalis by confocal, two-photon, and super-resolution imaging, and described flattened osteocytes lying within a woven bone structured in lamellae of randomly oriented collagen fibers. While most cells emit peripheral and perpendicular processes, we report two osteocytes morphotypes, located at different depth within the bone matrix and exhibiting distinct number and orientation of perpendicular cell processes. We show that this pattern is conserved with the chick Gallus gallus and suggest that the cellular microenvironment, and more particularly cell-cell contact, plays a fundamental role in the induction and stabilization of osteocytic processes. We propose that this intrinsic property might have been evolutionarily selected for its ability to robustly generate self-organizing osteocytic networks harbored by the wide variety of bone shapes and architectures found in extant and extinct vertebrates.

Osteoblast-specific Notch2 inactivation causes increased trabecular bone mass at specific sites of the appendicular skeleton

Notch signaling is a key pathway controlling various cell fate decisions during embryogenesis and adult life. It is activated by binding of specific ligands to four different Notch receptors that are subsequently cleaved by presenilins to release an intracellular domain that enters the nucleus and activates specific transcription factors. While the skeletal analysis of various mouse models with activated or inactivated Notch signaling has demonstrated a general impact of this pathway on bone remodeling, the more recent identification of NOTCH2 mutations in individuals with Hajdu-Cheney syndrome (HCS) has highlighted its human relevance. Since HCS is primarily characterized by skeletal defects, these latter findings led us to analyze the specific role of Notch2 in skeletal remodeling. After observing Notch2 expression in osteoblasts and osteoclasts, we utilized Runx2-Cre and Lyz2-Cre mice to inactivate Notch2 in cells of the osteoblast or osteoclast lineage, respectively. Whereas Notch2(fl/fl)/Lyz2-Cre mice did not display significant alterations of skeletal growth, bone mass or remodeling, Notch2(fl/fl)/Runx2-Cre mice progressively developed skeletal abnormalities in long bones. More specifically, these mice displayed a striking increase of trabecular bone mass in the proximal femur and the distal tibia at 6 and 12months of age. Whereas undecalcified sectioning of the respective regions did not reveal impaired osteocyte differentiation as a potential trigger for the observed phenotype, ex vivo experiments with bone marrow cells identified an increased osteogenic capacity of Notch2(fl/fl)/Runx2-Cre cultures. Collectively, our findings demonstrate that Notch2 physiologically regulates bone remodeling by inhibiting trabecular bone formation in the appendicular skeleton. Understanding the underlying mechanisms may help to improve diagnosis and therapy of HCS.

Bone loss in chronic kidney disease: Quantity or quality?

Chronic kidney disease (CKD) patients experience bone loss and fracture because of a specific CKD-related systemic disorder known as CKD-mineral bone disorder (CKD-MBD). The bone turnover, mineralization, and volume (TMV) system describes the morphological bone lesions in renal osteodystrophy related to CKD-MBD. Bone turnover and bone volume are defined as high, normal, or low, and bone mineralization is classified as normal or abnormal. All types of bone histology related to TMV are responsible for both bone quantity and bone quality losses in CKD patients. This review focuses on current bone quantity and bone quality losses in CKD patients and finally discusses potential therapeutic measures.

Inflammation, fracture and bone repair

The reconstitution of lost bone is a subject that is germane to many orthopedic conditions including fractures and non-unions, infection, inflammatory arthritis, osteoporosis, osteonecrosis, metabolic bone disease, tumors, and periprosthetic particle-associated osteolysis. In this regard, the processes of acute and chronic inflammation play an integral role. Acute inflammation is initiated by endogenous or exogenous adverse stimuli, and can become chronic in nature if not resolved by normal homeostatic mechanisms. Dysregulated inflammation leads to increased bone resorption and suppressed bone formation. Crosstalk among inflammatory cells (polymorphonuclear leukocytes and cells of the monocyte-macrophage-osteoclast lineage) and cells related to bone healing (cells of the mesenchymal stem cell-osteoblast lineage and vascular lineage) is essential to the formation, repair and remodeling of bone. In this review, the authors provide a comprehensive summary of the literature related to inflammation and bone repair. Special emphasis is placed on the underlying cellular and molecular mechanisms, and potential interventions that can favorably modulate the outcome of clinical conditions that involve bone repair.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Time-dependent cellular and transcriptional changes in the osteoblast lineage associated with sclerostin antibody treatment in ovariectomized rats

Inhibition of sclerostin with sclerostin antibody (Scl-Ab) has been shown to stimulate bone formation, decrease bone resorption, and increase bone mass in both animals and humans. To obtain insight into the temporal cellular and transcriptional changes in the osteoblast (OB) lineage associated with long-term Scl-Ab treatment, stereological and transcriptional analyses of the OB lineage were performed on lumbar vertebrae from aged ovariectomized rats. Animals were administered Scl-Ab 3 or 50mg/kg/wk or vehicle (VEH) for up to 26weeks (d183), followed by a treatment-free period (TFP). At 50mg/kg/wk, bone volume (BV/total volume [TV]) increased through d183 and declined during the TFP. Bone formation rate (BFR/bone surface [BS]) and total OB number increased through d29, then progressively declined, coincident with a decrease in total osteoprogenitor (OP) numbers from d29 through d183. Analysis of differentially expressed genes (DEGs) from microarray analysis of mRNA isolated from laser capture microdissection samples enriched for OB, lining cells, and osteocytes (OCy) revealed modules of genes that correlated with BFR/BS, BV/TV, and osteoblastic surface (Ob.S)/BS. Expression change of canonical Wnt target genes was similar in all three cell types at d8, including upregulation of Twist1 and Wisp1. At d29, the pattern of Wnt target gene expression changed in the OCy, with Twist1 returning to VEH level, sustained upregulation of Wisp1, and upregulation of several other Wnt targets that continued into the TFP. Predicted activation of pathways recognized to integrate with and regulate canonical Wnt signaling were also activated at d29 in the OCy. The most significantly affected pathways represented transcription factor signaling known to inhibit cell cycle progression (notably p53) and mitogenesis (notably c-Myc). These changes occurred at the time of peak BFR/BS and continued as BFR/BS declined during treatment, then trended toward VEH level in the TFP. Concurrent with this transcriptional switch was a reduction in OP numbers, an effect that would ultimately limit bone formation. This study confirms that the initial transcriptional response in response to Scl-Ab is activation of canonical Wnt signaling and the data demonstrate that there is induction of additional regulatory pathways in OCy with long-term treatment. The interactions between Wnt and p53/c-Myc signaling may be key in limiting OP populations, thus contributing to self-regulation of bone formation with continued Scl-Ab administration.

Rapamycin reduces severity of senile osteoporosis by activating osteocyte autophagy

SUMMARY

Osteocyte is the orchestrator of bone remolding and decline in osteocyte autophagy is involved in senile osteoporosis. Our results suggested that rapamycin, at least in part by activating osteocyte autophagy, reduced the severity of age-related bone changes in trabecular bone of old male rats.

INTRODUCTION

Previous literatures have showed that osteocyte is the orchestrator of bone remolding and age-related decline in osteocyte number is associated with senile osteoporosis. Autophagy is an important cellular protective mechanism which can preserve osteocyte viability and failure of autophagy in osteocyte with age has been linked to senile osteoporosis. The purpose of this study was to explore whether rapamycin, one activator of autophagy, has protective effects on senile osteoporosis through inducing osteocyte autophagy.

METHODS

Fifty-two 24-month-old male Sprague-Dawley (SD) rats were randomly divided into two groups. Rapamycin (1 mg/kg weight/day) or DMSO vehicle control was administered intraperitoneally for 12 weeks. BMD and bone microstructure were determined by Micro-CT. Fluorochrome labeling of the bones was performed to measure the mineral apposition rate (MAR). TRAP staining was performed to evaluate osteoclast number. The plasma levels of bone turnover markers were also analyzed. The effects of rapamycin on osteocyte autophagy were determined by immunohistochemistry, Western blot, and q-PCR. TUNEL was used to determine the prevalence of osteocyte apoptosis.

RESULTS

Micro-CT evaluation demonstrated that rapamycin had a protective effect on age-related bone loss in trabecular bone. Besides, rapamycin resulted in an obvious increase of MAR and a decrease of osteoclast number in contrast to the control group. Furthermore, rapamycin also induced autophagy in osteocyte demonstrated by increased LC3-positive osteocyte and increased LC3 turnover. In addition, rats treated with rapamycin exhibited decreased apoptosis of osteocyte determined by TUNEL.

CONCLUSIONS

These results suggested that rapamycin, at least in part by activating osteocyte autophagy, reduced the severity of age-related bone changes in trabecular bone of old male rats. Therefore, rapamycin might be a feasible therapeutic approach for senile osteoporosis.

Deficiency of circadian clock protein BMAL1 in mice results in a low bone mass phenotype

The circadian clock is an endogenous time keeping system that controls the physiology and behavior of many organisms. The transcription factor Brain and Muscle ARNT-like Protein 1 (BMAL1) is a component of the circadian clock and necessary for clock function. Bmal1(-/-) mice display accelerated aging and many accompanying age associated pathologies. Here, we report that mice deficient for BMAL1 have a low bone mass phenotype that is absent at birth and progressively worsens over their lifespan. Accelerated aging of these mice is associated with the formation of bony bridges occurring across the metaphysis to the epiphysis, resulting in shorter long bones. Using micro-computed tomography we show that Bmal1(-/-) mice have reductions in cortical and trabecular bone volume and other micro-structural parameters and a lower bone mineral density. Histology shows a deficiency of BMAL1 results in a reduced number of active osteoblasts and osteocytes in vivo. Isolation of bone marrow derived mesenchymal stem cells from Bmal1(-/-) mice demonstrate a reduced ability to differentiate into osteoblasts in vitro, which likely explains the observed reductions in osteoblasts and osteocytes, and may contribute to the observed osteopenia. Our data support the role of the circadian clock in the regulation of bone homeostasis and shows that BMAL1 deficiency results in a low bone mass phenotype.

Experimental studies of bone mechanoadaptation: bridging in vitro and in vivo studies with multiscale systems

Despite advancements in technology and science over the last century, the mechanisms underlying Wolff's law-bone structure adaptation in response to physical stimuli-remain poorly understood, limiting the ability to effectively treat and prevent skeletal diseases. A challenge to overcome in the study of the underlying mechanisms of this principle is the multiscale nature of mechanoadaptation. While there exist in silico systems that are capable of studying across these scales, experimental studies are typically limited to interpretation at a single dimension or time point. For instance, studies of single-cell responses to defined physical stimuli offer only a limited prediction of the whole bone response, while overlapping pathways or compensatory mechanisms complicate the ability to isolate critical targets in a whole animal model. Thus, there exists a need to develop experimental systems capable of bridging traditional experimental approaches and informing existing multiscale theoretical models. The purpose of this article is to review the process of mechanoadaptation and inherent challenges in studying its underlying mechanisms, discuss the limitations of traditional experimental systems in capturing the many facets of this process and highlight three multiscale experimental systems which bridge traditional approaches and cover relatively understudied time and length scales in bone adaptation.

Isolation and Functional Analysis of an Immortalized Murine Cementocyte Cell Line, IDG-CM6

The dental cementum covering the tooth root is similar to bone in several respects but remains poorly understood in terms of development and differentiation of cementoblasts, as well as the potential function(s) of cementocytes residing in the cellular cementum. It is not known if the cementocyte is a dynamic actor in cementum metabolism, comparable to the osteocyte in the bone. Cementocytes exhibit irregular spacing and lacunar shape, with fewer canalicular connections compared with osteocytes. Immunohistochemistry and quantitative PCR (qPCR) revealed that the in vivo expression profile of cementocytes paralleled that of osteocytes, including expression of dentin matrix protein 1 (Dmp1/DMP1), Sost/sclerostin, E11/gp38/podoplanin, Tnfrsf11b (osteoprotegerin [OPG]), and Tnfsf11 (receptor activator of NF-κB ligand [RANKL]). We used the Immortomouse(+/-); Dmp1-GFP(+/-) mice to isolate cementocytes as Dmp1-expressing cells followed by immortalization using the interferon (IFN)-γ-inducible promoter driving expression of a thermolabile large T antigen to create the first immortalized line of cementocytes, IDG-CM6. This cell line reproduced the expression profile of cementocytes observed in vivo, including alkaline phosphatase activity and mineralization. IDG-CM6 cells expressed higher levels of Tnfrsf11b and lower levels of Tnfsf11 compared with IDG-SW3 osteocytes, and under fluid flow shear stress, IDG-CM6 cells significantly increased OPG while decreasing RANKL, leading to a significantly increased OPG/RANKL ratio, which would inhibit osteoclast activation. These studies indicate similarities yet potentially important differences in the function of cementocytes compared with osteocytes and support cementocytes as mechanically responsive cells.

Bidirectional Notch Signaling and Osteocyte-Derived Factors in the Bone Marrow Microenvironment Promote Tumor Cell Proliferation and Bone Destruction in Multiple Myeloma

In multiple myeloma, an overabundance of monoclonal plasma cells in the bone marrow induces localized osteolytic lesions that rarely heal due to increased bone resorption and suppressed bone formation. Matrix-embedded osteocytes comprise more than 95% of bone cells and are major regulators of osteoclast and osteoblast activity, but their contribution to multiple myeloma growth and bone disease is unknown. Here, we report that osteocytes in a mouse model of human MM physically interact with multiple myeloma cells in vivo, undergo caspase-3-dependent apoptosis, and express higher RANKL (TNFSF11) and sclerostin levels than osteocytes in control mice. Mechanistic studies revealed that osteocyte apoptosis was initiated by multiple myeloma cell-mediated activation of Notch signaling and was further amplified by multiple myeloma cell-secreted TNF. The induction of apoptosis increased osteocytic Rankl expression, the osteocytic Rankl/Opg (TNFRSF11B) ratio, and the ability of osteocytes to attract osteoclast precursors to induce local bone resorption. Furthermore, osteocytes in contact with multiple myeloma cells expressed high levels of Sost/sclerostin, leading to a reduction in Wnt signaling and subsequent inhibition of osteoblast differentiation. Importantly, direct contact between osteocytes and multiple myeloma cells reciprocally activated Notch signaling and increased Notch receptor expression, particularly Notch3 and 4, stimulating multiple myeloma cell growth. These studies reveal a previously unknown role for bidirectional Notch signaling that enhances MM growth and bone disease, suggesting that targeting osteocyte-multiple myeloma cell interactions through specific Notch receptor blockade may represent a promising treatment strategy in multiple myeloma.

The role of selenium in insulin-like growth factor I receptor (IGF-IR) expression and regulation of apoptosis in mouse osteoblasts

Selenium (Se) is an essential component for animals and human beings. The chemoprotective role of Se, via the regulation of the cell cycle, stimulation of apoptosis and activation of some cytokines among others, is well known; however, the comprehensive effects of Se on the expression of IGF-IR and its regulation of apoptosis have not been investigated. Thus the aim of this study was to report on the effects that different concentrations of Se extert on body weight, blood serum IGF-IR levels and histopathology in mice; and on IGF-IR expression, proliferation and apoptosis in mouse osteoblasts. In vivo experiments showed a significant decrease in body weight, serum levels of IGF-IR and prominent toxicant effects on the liver, kidney, heart and spleen following the administration of defined concentrations of Se for 30 d. However, moderate levels (0.1 mg/kg) of Se gradually improved weight and serum IGF-IR. In vitro osteoblast experiments revealed that at concentrations of 5 × 10(-6) and 10(-5) mol/L Se, MTT activity decreased in comparison with control cells. Cell cycle, TEM and caspase-3 activity supported these observations including an increase in the sub-G1 phase and notable apoptosis in osteoblasts, along with a decrease in the expression of mRNA and protein levels of IGF-IR. Moreover, the MTT activity, mRNA and protein levels of IGF-IR in osteoblasts were decreased and caspase-3 activity was increased in siRNA groups as compared with non-siRNA groups. These data suggest that Se significantly affects IGF-IR expression, and that it contributes to the proliferation and regulation of apoptosis in osteoblasts.

Osteoblast Lineage Cells Play an Essential Role in Periodontal Bone Loss Through Activation of Nuclear Factor-Kappa B

Bacterial pathogens stimulate periodontitis, the most common osteolytic disease in humans and the most common cause of tooth loss in adults. Previous studies identified leukocytes and their products as key factors in this process. We demonstrate for the first time that osteoblast lineage cells play a critical role in periodontal disease. Oral infection stimulated nuclear localization of NF-κB in osteoblasts and osteocytes in the periodontium of wild type but not transgenic mice that expressed a lineage specific dominant negative mutant of IKK (IKK-DN) in osteoblast lineage cells. Wild-type mice were also susceptible to bacteria induced periodontal bone loss but transgenic mice were not. The lack of bone loss in the experimental group was linked to reduced RANKL expression by osteoblast lineage cells that led to diminished osteoclast mediated bone resorption and greater coupled new bone formation. The results demonstrate that osteoblast lineage cells are key contributors to periodontal bone loss through an NF-κB mediated mechanism.

ENDOCRINOLOGY AND ADOLESCENCE: Osteoporosis in children: diagnosis and management

Osteoporosis in children can be primary or secondary due to chronic disease. Awareness among paediatricians is vital to identify patients at risk of developing osteoporosis. Previous fractures and backaches are clinical predictors, and low cortical thickness and low bone density are radiological predictors of fractures. Osteogenesis Imperfecta (OI) is a rare disease and should be managed in tertiary paediatric units with the necessary multidisciplinary expertise. Modern OI management focuses on functional outcomes rather than just improving bone mineral density. While therapy for OI has improved tremendously over the last few decades, this chronic genetic condition has some unpreventable, poorly treatable and disabling complications. In children at risk of secondary osteoporosis, a high degree of suspicion needs to be exercised. In affected children, further weakening of bone should be avoided by minimising exposure to osteotoxic medication and optimising nutrition including calcium and vitamin D. Early intervention is paramount. However, it is important to identify patient groups in whom spontaneous vertebral reshaping and resolution of symptoms occur to avoid unnecessary treatment. Bisphosphonate therapy remains the pharmacological treatment of choice in both primary and secondary osteoporosis in children, despite limited evidence for its use in the latter. The duration and intensity of treatment remain a concern for long-term safety. Various new potent antiresorptive agents are being studied, but more urgently required are studies using anabolic medications that stimulate bone formation. More research is required to bridge the gaps in the evidence for management of paediatric osteoporosis.

Skeletal muscle Ca(2+) mishandling: Another effect of bone-to-muscle signaling

Our appreciation of crosstalk between muscle and bone has recently expanded beyond mechanical force-driven events to encompass a variety of signaling factors originating in one tissue and communicating to the other. While the recent identification of new 'myokines' has shifted some focus to the role of muscle in this partnership, bone-derived factors and their effects on skeletal muscle should not be overlooked. This review summarizes some previously known mediators of bone-to-muscle signaling and also recent work identifying a new role for bone-derived TGF-β as a cause of skeletal muscle weakness in the setting of cancer-induced bone destruction. Oxidation of the ryanodine receptor/calcium release channel (RyR1) in skeletal muscle occurs via a TGF-β-Nox4-RyR1 axis and leads to calcium mishandling and decreased muscle function. Multiple points of potential therapeutic intervention were identified, from preventing the bone destruction to stabilizing the RYR1 calcium channel. This new data reinforces the concept that bone can be an important source of signaling factors in pathphysiological settings.

Osteocytes and Skeletal Pathophysiology

For many years, osteocytes have been the forgotten bone cells and considered as inactive spectators buried in the bone matrix. We now know that osteocytes detect and respond to mechanical and hormonal stimuli to coordinate bone resorption and bone formation. Osteocytes are currently considered a major source of molecules that regulate the activity of osteoclasts and osteoblasts, such as RANKL and sclerostin; and genetic and pharmacological manipulations of either molecule markedly affect bone homeostasis. Besides playing a role in physiological bone homeostasis, accumulating evidence supports the notion that dysregulation of osteocyte function and alteration of osteocyte life-span underlies the pathophysiology of skeletal disorders characterized by loss bone mass and increased bone fragility, as well as the damaging effects of cancer in bone. In this review, we highlight some of these investigations and discuss novel observations that demonstrate that osteocytes, far from being passive cells entombed in the bone, are critical for bone function and maintenance.

Impaired bone remodeling and its correction by combination therapy in a mouse model of mucopolysaccharidosis-I

Mucopolysaccharidosis-I (MPS-I) is a lysosomal storage disease (LSD) caused by inactivating mutations of IDUA, encoding the glycosaminoglycan-degrading enzyme α-l-iduronidase. Although MPS-I is associated with skeletal abnormalities, the impact of IDUA deficiency on bone remodeling is poorly defined. Here we report that Idua-deficient mice progressively develop a high bone mass phenotype with pathological lysosomal storage in cells of the osteoblast lineage. Histomorphometric quantification identified shortening of bone-forming units and reduced osteoclast numbers per bone surface. This phenotype was not transferable into wild-type mice by bone marrow transplantation (BMT). In contrast, the high bone mass phenotype of Idua-deficient mice was prevented by BMT from wild-type donors. At the cellular level, BMT did not only normalize defects of Idua-deficient osteoblasts and osteocytes but additionally caused increased osteoclastogenesis. Based on clinical observations in an individual with MPS-I, previously subjected to BMT and enzyme replacement therapy (ERT), we treated Idua-deficient mice accordingly and found that combining both treatments normalized all histomorphometric parameters of bone remodeling. Our results demonstrate that BMT and ERT profoundly affect skeletal remodeling of Idua-deficient mice, thereby suggesting that individuals with MPS-I should be monitored for their bone remodeling status, before and after treatment, to avoid long-term skeletal complications.

Connecting the Dots: Linking Osteocyte Activity and Therapeutic Modulation of Sclerostin by Extending a Multiscale Systems Model

The goal of this work was to extend a mathematical, multiscale systems model of bone function, remodeling, and health in order to explore hypotheses related to therapeutic modulation of sclerostin and quantitatively describe purported osteocyte activity within bone remodeling events. A pharmacokinetic model with first-order absorption and dual elimination pathways was used to describe the kinetics of romosozumab, a monoclonal antibody (mAb) against sclerostin. To describe total circulating sclerostin, an extended indirect response model of inhibition of offset was developed. These models were subsequently linked to the systems model, with sclerostin signaling changes in resorption and formation through established osteocyte-mediated mechanisms. The model proposes relative contributions of the osteocyte to the RANKL pool, a major player in feedback signaling, and is used to explore hypotheses surrounding attenuation of anabolic activity after multiple doses of sclerostin mAbs, a phenomenon whose mechanism is poorly understood.

Effect of electrical stimulation-induced muscle force and streptomycin treatment on muscle and trabecular bone mass in early-stage disuse musculoskeletal atrophy

OBJECTIVES

The aim was to determine whether daily muscle electrical stimulation (ES) and streptomycin treatment would have positive or negative effects on trabecular bone mass in disuse rats.

METHODS

Seven-week-old male F344 rats were randomly divided into five groups of eight animals each: an age-matched control group (CON); a sciatic denervation group (DN); a DN + direct electrical stimulation group (DN+ES); a DN + streptomycin treatment group (DN+SM); and a DN+ES+SM group. The tibialis anterior (TA) muscles in all ES groups were stimulated with 16mA at 10Hz for 30 min/day, six days/week, for one week. Bone volume and structure were evaluated using micro-CT, and histological examinations of the tibiae were performed.

RESULTS

Direct ES significantly reduced the disuse-induced trabecular bone loss. Osteoid thickness were also significantly greater in the ES groups than in the DN group. Micro CT and histomorphological parameters were significantly lower in the DN+ES+SM group than in the DN+ES group, while there were no significant differences between the DN and DN+SM groups.

CONCLUSIONS

These results suggest that ES-induced muscle force reduced trabecular bone loss, and streptomycin treatment did not induce bone loss, but attenuated the effects of ES-induced muscle force on reducing the loss of disused bone.

Activation of AMP-activated protein kinase protects against homocysteine-induced apoptosis of osteocytic MLO-Y4 cells by regulating the expressions of NADPH oxidase 1 (Nox1) and Nox2

BACKGROUND

Elevated plasma homocysteine (Hcy) level is associated with the risk of osteoporotic fracture. While Hcy increases oxidative stress, AMP-activated protein kinase (AMPK) activation ameliorates it. This study aimed to investigate whether Hcy induces apoptosis of osteocytic MLO-Y4 cells through regulating expressions of oxidant and anti-oxidant enzymes and determine the effects of AMPK activation by 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) and metformin on the Hcy-induced apoptosis of the cells.

RESULTS

DNA fragment ELISA and TUNEL staining assays showed that Hcy treatments (0.1-5.0 mM) induced apoptosis of MLO-Y4 cells in a dose-dependent manner. The detrimental effect of Hcy was partly but significantly reversed by an antioxidant (N-acetylcysteine) and NADPH oxidase (Nox) inhibitors (apocynin and diphenyleneiodonium). In addition, treatment with AICAR (0.05-0.1 mM) and metformin (10-100 μM) ameliorated Hcy-induced apoptosis of the cells. The favorable effect of metformin on Hcy-induced apoptosis was completely canceled by an AMPK inhibitor Ara-A. Hcy increased the expression levels of Nox1 and Nox2, while it had no effects on the expressions of Nox4 or the anti-oxidant enzymes, superoxide dismutase 1 and 2. Hcy-induced increases in the expressions of Nox1 and Nox2 decreased significantly by treatments with AICAR.

CONCLUSION

These findings suggest that Hcy induces apoptosis of osteocytes by increasing the expressions of Nox1 and Nox2, and AMPK activation by AICAR and metformin effectively prevents the detrimental reactions. Thus, AMPK activation may be a potent therapeutic candidate for preventing Hcy-induced osteocyte apoptosis and the resulting bone fragility.

Role of Wnt/β-catenin and RANKL/OPG in bone healing of diabetic Charcot arthropathy patients

BACKGROUND AND PURPOSE

Charcot neuropathy is characterized by bone destruction in a foot leading to deformity, instability, and risk of amputation. Little is known about the pathogenic mechanisms. We hypothesized that the bone-regulating Wnt/β-catenin and RANKL/OPG pathways have a role in Charcot arthropathy.

PATIENTS AND METHODS

24 consecutive Charcot patients were treated by off-loading, and monitored for 2 years by repeated foot radiography, MRI, and circulating levels of sclerostin, dickkopf-1, Wnt inhibitory factor-1, Wnt ligand-1, OPG, and RANKL. 20 neuropathic diabetic controls and 20 healthy controls served as the reference.

RESULTS

Levels of sclerostin, Dkk-1 and Wnt-1, but not of Wif-1, were significantly lower in Charcot patients than in the diabetic controls at inclusion. Dkk-1 and Wnt-1 levels responded to off-loading by increasing. Sclerostin levels were significantly higher in the diabetic controls than in the other groups whereas Wif-1 levels were significantly higher in the healthy controls than in the other groups. OPG and RANKL levels were significantly higher in the Charcot patients than in the other groups at inclusion, but decreased to the levels in healthy controls at 2 years. OPG/RANKL ratio was balanced in all groups at inclusion, and it remained balanced in Charcot patients on repeated measurement throughout the study.

INTERPRETATION

High plasma RANKL and OPG levels at diagnosis of Charcot suggest that there is high bone remodeling activity before gradually normalizing after off-loading treatment. The consistently balanced OPG/RANKL ratio in Charcot patients suggests that there is low-key net bone building activity by this pathway following diagnosis and treatment. Inter-group differences at diagnosis and changes in Wnt signaling following off-loading treatment were sufficiently large to be reflected by systemic levels, indicating that this pathway has a role in bone remodeling and bone repair activity in Charcot patients. This is of particular clinical relevance considering the recent emergence of promising drugs that target this system.

Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells

Bone tissue is continuously remodeled through the concerted actions of bone cells, which include bone resorption by osteoclasts and bone formation by osteoblasts, whereas osteocytes act as mechanosensors and orchestrators of the bone remodeling process. This process is under the control of local (e.g., growth factors and cytokines) and systemic (e.g., calcitonin and estrogens) factors that all together contribute for bone homeostasis. An imbalance between bone resorption and formation can result in bone diseases including osteoporosis. Recently, it has been recognized that, during bone remodeling, there are an intricate communication among bone cells. For instance, the coupling from bone resorption to bone formation is achieved by interaction between osteoclasts and osteoblasts. Moreover, osteocytes produce factors that influence osteoblast and osteoclast activities, whereas osteocyte apoptosis is followed by osteoclastic bone resorption. The increasing knowledge about the structure and functions of bone cells contributed to a better understanding of bone biology. It has been suggested that there is a complex communication between bone cells and other organs, indicating the dynamic nature of bone tissue. In this review, we discuss the current data about the structure and functions of bone cells and the factors that influence bone remodeling.

Role of osteocytes in multiple myeloma bone disease

PURPOSE OF REVIEW

Despite the increased knowledge of osteocyte biology, the contribution of this most abundant bone cell to the development and progression of multiple myeloma in bone is practically unexplored.

RECENT FINDINGS

Multiple myeloma bone disease is characterized by exacerbated bone resorption and the presence of osteolytic lesions that do not heal because of a concomitant reduction in bone formation. Osteocytes produce molecules that regulate both bone formation and resorption. Recent findings suggest that the life span of osteocytes is compromised in multiple myeloma patients with bone lesions. In addition, multiple myeloma cells affect the transcriptional profile of osteocytes by upregulating the production of pro-osteoclastogenic cytokines, stimulating osteoclast formation and activity. Further, patients with active multiple myeloma have elevated circulating levels of sclerostin, a potent inhibitor of bone formation which is specifically expressed by osteocytes in bone.

SUMMARY

Understanding the contribution of osteocytes to the mechanisms underlying the skeletal consequences of multiple myeloma bone disease has the potential to provide important new therapeutic strategies that specifically target multiple myeloma-osteocyte interactions.

Modulating Bone Resorption and Bone Formation in Opposite Directions in the Treatment of Postmenopausal Osteoporosis

Bone remodeling, the fundamental process for bone renewal, is targeted by treatments of osteoporosis to correct the imbalance between bone resorption and bone formation and reduce the risk of fractures and associated clinical consequences. Currently available therapeutics affect bone resorption and bone formation in the same direction and either decrease (inhibitors of bone resorption) or increase (parathyroid hormone [PTH] peptides) bone remodeling. Studies of patients with rare bone diseases and genetically modified animal models demonstrated that bone resorption and bone formation may not necessarily be coupled, leading to identification of molecular targets in bone cells for the development of novel agents for the treatment of osteoporosis. Application of such agents to the treatment of women with low bone mass confirmed that bone resorption and bone formation can be modulated in different directions and so far two new classes of therapeutics for osteoporosis have been defined with distinct mechanisms of action. Such treatments, if combined with a favorable safety profile, will offer new therapeutic options and will improve the management of patients with osteoporosis.

Inhibition of osteocyte apoptosis prevents the increase in osteocytic receptor activator of nuclear factor κB ligand (RANKL) but does not stop bone resorption or the loss of bone induced by unloading

Apoptosis of osteocytes and osteoblasts precedes bone resorption and bone loss with reduced mechanical stimulation, and receptor activator of NF-κB ligand (RANKL) expression is increased with unloading in mice. Because osteocytes are major RANKL producers, we hypothesized that apoptotic osteocytes signal to neighboring osteocytes to increase RANKL expression, which, in turn, increases osteoclastogenesis and bone resorption. The traditional bisphosphonate (BP) alendronate (Aln) or IG9402, a BP analog that does not inhibit resorption, prevented the increase in osteocyte apoptosis and osteocytic RANKL expression. The BPs also inhibited osteoblast apoptosis but did not prevent the increase in osteoblastic RANKL. Unloaded mice exhibited high serum levels of the bone resorption marker C-telopeptide fragments of type I collagen (CTX), elevated osteoclastogenesis, and increased osteoclasts in bone. Aln, but not IG9402, prevented all of these effects. In addition, Aln prevented the reduction in spinal and femoral bone mineral density, spinal bone volume/tissue volume, trabecular thickness, mechanical strength, and material strength induced by unloading. Although IG9402 did not prevent the loss of bone mass, it partially prevented the loss of strength, suggesting a contribution of osteocyte viability to strength independent of bone mass. These results demonstrate that osteocyte apoptosis leads to increased osteocytic RANKL. However, blockade of these events is not sufficient to restrain osteoclast formation, inhibit resorption, or stop bone loss induced by skeletal unloading.

Effect of recent spinal cord injury on wnt signaling antagonists (sclerostin and dkk-1) and their relationship with bone loss. A 12-month prospective study

Spinal cord injury (SCI) has been associated with a marked increase in bone loss and bone remodeling, especially short-term after injury. The absence of mechanical load, mediated by osteocyte mechanosensory function, seems to be a causative factor related to bone loss in this condition. However, the pathogenesis and clinical management of this process remain unclear. Therefore, the aim of the study was to analyze the effect of recent SCI on the Wnt pathway antagonists, sclerostin and Dickkopf (Dkk-1), and their relationship with bone turnover and bone mineral density (BMD) evolution. Forty-two patients (aged 35 ± 14yrs) with a recent (<6months) complete SCI were prospectively included. Sclerostin and Dkk-1, bone turnover markers (bone formation: PINP, bone ALP; resorption: sCTx) and BMD (lumbar spine, proximal femur, total body and lower extremities [DXA]) were assessed at baseline and at 6 and 12 months. The results were compared with a healthy control group. 22/42 patients completed the 12-month follow-up. At baseline, SCI patients showed a marked increase in bone markers (PINP and sCTx), remaining significantly increased at up to 6 months of follow-up. Additionally, they presented significantly increased Dkk-1 values throughout the study, whereas sclerostin values did not significantly change. BMD markedly decreased at the proximal femur (-20.2 ± 5.4%, p < 0.01), total body (-5.7 ± 2.2%, p = 0.02) and lower extremities (-13.1 ± 4.5%, p = 0.01) at 12 months. Consequently, 59% of patients developed densitometric osteoporosis at 12 months. Patients with higher Dkk-1 values (>58 pmol/L) at baseline showed higher sublesional BMD loss. In conclusion, this study shows that short-term after SCI there is a marked increase in bone turnover and bone loss, the latter associated with an increase in Dkk-1 serum levels. The persistence of increased levels of this Wnt antagonist throughout the study and their relationship with the magnitude of bone loss suggests a contributory role of this mediator in this process.

Bone Loss Triggered by the Cytokine Network in Inflammatory Autoimmune Diseases

Bone remodeling is a lifelong process in vertebrates that relies on the correct balance between bone resorption by osteoclasts and bone formation by osteoblasts. Bone loss and fracture risk are implicated in inflammatory autoimmune diseases such as rheumatoid arthritis, ankylosing spondylitis, inflammatory bowel disease, and systemic lupus erythematosus. The network of inflammatory cytokines produced during chronic inflammation induces an uncoupling of bone formation and resorption, resulting in significant bone loss in patients with inflammatory autoimmune diseases. Here, we review and discuss the involvement of the inflammatory cytokine network in the pathophysiological aspects and the therapeutic advances in inflammatory autoimmune diseases.

Glutathione, N-acetylcysteine and lipoic acid down-regulate starvation-induced apoptosis, RANKL/OPG ratio and sclerostin in osteocytes: involvement of JNK and ERK1/2 signalling

Osteocyte apoptosis due to microdamage and/or oxidative stress is related to increased local bone turnover and resorption observed in various bone diseases. Previous data on osteoblasts and osteoclasts have linked reactive oxygen species and antioxidants to bone remodelling. This study performs a comprehensive analysis on the effect of antioxidants such as glutathione (GSH), N-acetylcysteine and lipoic acid (LA) on starvation-induced osteocyte apoptosis and on cytokines involved in bone remodelling such as the receptor activator kB ligand (RANKL), osteoprotegerin (OPG) and sclerostin. For this study, apoptosis was induced by serum starvation in a murine osteocyte-like cell line MLO-Y4; this condition mimics in part osteocyte apoptosis due to microdamage. The results show that starvation-induced apoptosis and expression of RANKL, OPG and sclerostin are redox regulated processes. All antioxidants are able to inhibit the apoptosis due to starvation. They down-regulate the expression and the release of RANKL, the expression of sclerostin and RANKL/OPG ratio, whereas they only in part up-regulate OPG expression. Antioxidants mediate their effect on starvation-induced apoptosis by JNK signalling and on cytokine expression by both JNK and ERK1/2 activities. This study shows the possible involvement of biological antioxidants such as GSH and LA on redox regulated mechanisms related to apoptosis and expression of cytokines involved in bone remodelling. Moreover, it suggests that both JNK and ERK1/2 may be useful biological targets for drugs affecting bone diseases associated with increased oxidative stress.

Osteocytes mediate the anabolic actions of canonical Wnt/β-catenin signaling in bone

Osteocytes, >90% of the cells in bone, lie embedded within the mineralized matrix and coordinate osteoclast and osteoblast activity on bone surfaces by mechanisms still unclear. Bone anabolic stimuli activate Wnt signaling, and human mutations of components along this pathway underscore its crucial role in bone accrual and maintenance. However, the cell responsible for orchestrating Wnt anabolic actions has remained elusive. We show herein that activation of canonical Wnt signaling exclusively in osteocytes [dominant active (da)βcat(Ot) mice] induces bone anabolism and triggers Notch signaling without affecting survival. These features contrast with those of mice expressing the same daß-catenin in osteoblasts, which exhibit decreased resorption and perinatal death from leukemia. daßcat(Ot) mice exhibit increased bone mineral density in the axial and appendicular skeleton, and marked increase in bone volume in cancellous/trabecular and cortical compartments compared with littermate controls. daßcat(Ot) mice display increased resorption and formation markers, high number of osteoclasts and osteoblasts in cancellous and cortical bone, increased bone matrix production, and markedly elevated periosteal bone formation rate. Wnt and Notch signaling target genes, osteoblast and osteocyte markers, and proosteoclastogenic and antiosteoclastogenic cytokines are elevated in bones of daßcat(Ot) mice. Further, the increase in RANKL depends on Sost/sclerostin. Thus, activation of osteocytic β-catenin signaling increases both osteoclasts and osteoblasts, leading to bone gain, and is sufficient to activate the Notch pathway. These findings demonstrate disparate outcomes of β-catenin activation in osteocytes versus osteoblasts and identify osteocytes as central target cells of the anabolic actions of canonical Wnt/β-catenin signaling in bone.

New simulation model for bone formation markers in osteoporosis patients treated with once-weekly teriparatide

Daily 20-μg and once-weekly 56.5-μg teriparatide (parathyroid hormone 1–34) treatment regimens increase bone mineral density (BMD) and prevent fractures, but changes in bone turnover markers differ between the two regimens. The aim of the present study was to explain changes in bone turnover markers using once-weekly teriparatide with a simulation model. Temporary increases in bone formation markers and subsequent decreases were observed during once-weekly teriparatide treatment for 72 weeks. These observations support the hypothesis that repeated weekly teriparatide administration stimulates bone remodeling, replacing old bone with new bone and leading to a reduction in the active remodeling surface. A simulation model was developed based on the iterative remodeling cycle that occurs on residual old bone. An increase in bone formation and a subsequent decrease were observed in the preliminary simulation. For each fitted time point, the predicted value was compared to the absolute values of the bone formation and resorption markers and lumbar BMD. The simulation model strongly matched actual changes in bone turnover markers and BMD. This simulation model indicates increased bone formation marker levels in the early stage and a subsequent decrease. It is therefore concluded that remodeling-based bone formation persisted during the entire treatment period with once-weekly teriparatide.

Acute-phase protein serum amyloid A3 is a novel paracrine coupling factor that controls bone homeostasis

Serum amyloid A (A-SAA/Saa3) was shown before to affect osteoblastic metabolism. Here, using RT-quantitative PCR and/or immunoblotting, we show that expression of mouse Saa3 and human SAA1 and SAA2 positively correlates with increased cellular maturation toward the osteocyte phenotype. Expression is not detected in C3H10T1/2 embryonic fibroblasts but is successively higher in preosteoblastic MC3T3-E1 cells, late osteoblastic MLO-A5 cells, and MLO-Y4 osteocytes, consistent with findings using primary bone cells from newborn mouse calvaria. Recombinant Saa3 protein functionally inhibits osteoblast differentiation as reflected by reductions in the expression of osteoblast markers and decreased mineralization in newborn mouse calvaria. Yet, Saa3 protein enhances osteoclastogenesis in mouse macrophages/monocytes based on the number of multinucleated and tartrate-resistant alkaline phosphatase-positive cells and Calcr mRNA expression. Depletion of Saa3 in MLO osteocytes results in the loss of the mature osteocyte phenotype. Recombinant osteocalcin, which is reciprocally regulated with Saa3 at the osteoblast/osteocyte transition, attenuates Saa3 expression in MLO-Y4 osteocytes. Mechanistically, Saa3 produced by MLO-Y4 osteocytes is integrated into the extracellular matrix of MC3T3-E1 osteoblasts, where it associates with the P2 purinergic receptor P2rx7 to stimulate Mmp13 expression via the P2rx7/MAPK/ERK/activator protein 1 axis. Our data suggest that Saa3 may function as an important coupling factor in bone development and homeostasis.

Sclerostin: recent advances and clinical implications

PURPOSE OF REVIEW

Discovery of the Wnt signaling pathway and understanding the central role of osteocyte in skeletal homeostasis have been the major advances in skeletal biology over the past decade. Sclerostin, secreted mainly (but not exclusively) by osteocytes, has emerged as a key player in skeletal homeostasis. This review highlights the most relevant recent advances.

RECENT FINDINGS

Sclerostin by inhibiting Wnt signaling pathway decreases bone formation and osteoblast differentiation and promotes osteoblast apoptosis. Ability to measure serum sclerostin levels better clarified the role of sclerostin in various physiologic and pathologic states. Early clinical trials with antibodies to sclerostin have produced robust increases in bone mineral density, and fracture prevention trials are underway.

SUMMARY

Since the discovery of Wnt signaling pathway and sclerostin's association with high bone mass, there has been a remarkable progress. Clinical trials with fracture endpoints, already underway, should expand osteoanabolic therapeutic horizon in the very near future. Measurement of sclerostin levels in a number of conditions has advanced our knowledge about pathophysiology of skeletal and nonskeletal disorders in an altogether new light.

Osteocyte control of bone remodeling: is sclerostin a key molecular coordinator of the balanced bone resorption-formation cycles?

Osteocytes, entrapped within a newly mineralized bone matrix, possess a unique cellular identity due to a specialized morphology and a molecular signature. These features endow them to serve as a bone response mechanism for mechanical stress in their microenvironment. Sclerostin, a primarily osteocyte product, is widely considered as a mechanotranduction key molecule whose expression is suppressed by mechanical loading, or it is induced by unloading. This review presents a model suggesting that sclerostin is major mediator for integrating mechanical, local, and hormonal signals, sensed by the osteocytes, in controlling the remodeling apparatus. This central role is achieved through interplay between two opposing mechanisms: (1) unloading-induced high sclerostin levels, which antagonize Wnt-canonical-β-catenin signaling in osteocytes and osteoblasts, permitting simultaneously Wnt-noncanonical and/or other pathways in osteocytes and osteoclasts, directed at bone resorption; (2) mechanical loading results in low sclerostin levels, activation of Wnt-canonical signaling, and bone formation. Therefore, adaptive bone remodeling occurring at a distinct bone compartment is orchestrated by altered sclerostin levels, which regulate the expression of the other osteocyte-specific proteins, such as RANKL, OPG, and proteins encoded by "mineralization-related genes" (DMP1, PHEX, and probably FGF23). For example, under specific terms, sclerostin regulates differential RANKL and OPG production, and creates a dynamic RANKL/OPG ratio, leading either to bone formation or resorption. It also controls the expression of PHEX, DMP1, and most likely FGF23, leading to either bone matrix mineralization or its inhibition. Such opposing up- or down-regulation of remodeling phases allows osteocytes to function as an "external unit", ensuring transition from bone resorption to bone formation.Mini Abstract: The osteocyte network plays a central role in directing bone response either to mechanical loading, or to unloading, leading correspondingly to bone formation or resorption. This review shows a key role of the osteocyte-produced sclerostin as a major mediator of the molecular mechanisms involved in the process of adaptive bone remodeling.

Bone mineralization is regulated by signaling cross talk between molecular factors of local and systemic origin: the role of fibroblast growth factor 23

Body phosphate homeostasis is regulated by a hormonal counter-balanced intestine-bone-kidney axis. The major systemic hormones involved in this axis are parathyroid hormone (PTH), 1,25-dihydroxyvitamin-D, and fibroblast growth factor-23 (FGF23). FGF23, produced almost exclusively by the osteocytes, is a phosphaturic hormone that plays a major role in regulation of the bone remodeling process. Remodeling composite components, bone mineralization and resorption cycles create a continuous influx-efflux loop of the inorganic phosphate (Pi) through the skeleton. This "bone Pi loop," which is formed, is controlled by local and systemic factors according to phosphate homeostasis demands. Although FGF23 systemic actions in the kidney, and for the production of PTH and 1,25-dihydroxyvitamin-D are well established, its direct involvement in bone metabolism is currently poorly understood. This review presents the latest available evidence suggesting two aspects of FGF23 bone local activity: (a) Regulation of FGF23 production by both local and systemic factors. The suggested local factors include extracellular levels of Pi and pyrophosphate (PPi), (the Pi/PPi ratio), and another osteocyte-derived protein, sclerostin. In addition, 1,25-dihydroxyvitamin-D, synthesized locally by bone cells, may contribute to regulation of FGF23 production. The systemic control is achieved via PTH and 1,25-dihydroxyvitamin-D endocrine functions. (b) FGF23 acts as a local agent, directly affecting bone mineralization. We support the assumption that under balanced physiological conditions, sclerostin, by para- autocrine signaling, upregulates FGF23 production by the osteocyte. FGF23, in turn, acts as a mineralization inhibitor, by stimulating the generation of the major mineralization antagonist-PPi.

Relevance of Wnt signaling for osteoanabolic therapy

The Wnt signaling pathway is long known to play fundamental roles in various aspects of embryonic development, but also in several homeostatic processes controlling tissue functions in adults. The complexity of this system is best underscored by the fact that the mammalian genome encodes for 19 different Wnt ligands, most but not all of them acting through an intracellular stabilization of β-catenin, representing the key molecule within the so-called canonical Wnt signaling pathway. Wnt ligands primarily bind to 10 different serpentine receptors of the Fzd family, and this binding can be positively or negatively regulated by additional molecules present at the surface of the respective target cells. One of these molecules is the transmembrane protein Lrp5, which has been shown to act as a Wnt co-receptor. In 2001, Lrp5, and thereby Wnt signaling, entered center stage in the research area of bone remodeling, a homeostatic process controlling bone mass, whose disturbance causes osteoporosis, one of the most prevalent disorders worldwide. More specifically, it was found that inactivating mutations of the human LRP5 gene cause osteoporosis-pseudoglioma syndrome, a rare genetic disorder characterized by impaired bone formation and persistence of hyaloid vessels in the eyeballs. In addition, activating LRP5 mutations were identified in individuals with osteosclerosis, a high bone mass condition characterized by excessive bone formation. Especially explained by the lack of cost-effective osteoanabolic treatment options, these findings had an immediate impact on the research regarding the bone-forming cell type, i.e. the osteoblast, whose differentiation and function is apparently controlled by Wnt signaling. This review summarizes the most important results obtained in a large number of studies, involving tissue culture experiments, mouse models and human patients. While there are still many open questions regarding the precise molecular interactions controlling Wnt signaling in osteoblasts, it is obvious that understanding this pathway is a key to optimize the therapeutic strategies for treating various skeletal disorders, including osteoporosis.

Relevance of Wnt signaling for osteoanabolic therapy

The Wnt signaling pathway is long known to play fundamental roles in various aspects of embryonic development, but also in several homeostatic processes controlling tissue functions in adults. The complexity of this system is best underscored by the fact that the mammalian genome encodes for 19 different Wnt ligands, most but not all of them acting through an intracellular stabilization of β-catenin, representing the key molecule within the so-called canonical Wnt signaling pathway. Wnt ligands primarily bind to 10 different serpentine receptors of the Fzd family, and this binding can be positively or negatively regulated by additional molecules present at the surface of the respective target cells. One of these molecules is the transmembrane protein Lrp5, which has been shown to act as a Wnt co-receptor. In 2001, Lrp5, and thereby Wnt signaling, entered center stage in the research area of bone remodeling, a homeostatic process controlling bone mass, whose disturbance causes osteoporosis, one of the most prevalent disorders worldwide. More specifically, it was found that inactivating mutations of the human LRP5 gene cause osteoporosis-pseudoglioma syndrome, a rare genetic disorder characterized by impaired bone formation and persistence of hyaloid vessels in the eyeballs. In addition, activating LRP5 mutations were identified in individuals with osteosclerosis, a high bone mass condition characterized by excessive bone formation. Especially explained by the lack of cost-effective osteoanabolic treatment options, these findings had an immediate impact on the research regarding the bone-forming cell type, i.e. the osteoblast, whose differentiation and function is apparently controlled by Wnt signaling. This review summarizes the most important results obtained in a large number of studies, involving tissue culture experiments, mouse models and human patients. While there are still many open questions regarding the precise molecular interactions controlling Wnt signaling in osteoblasts, it is obvious that understanding this pathway is a key to optimize the therapeutic strategies for treating various skeletal disorders, including osteoporosis.

MicroRNAs in the control of metastatic bone disease

Bone metastasis is a common and devastating complication of late-stage breast and prostate cancer. Complex interactions between tumor cells, bone cells, and a milieu of components in their microenvironment contribute to the osteolytic, osteoblastic, or mixed lesions present in patients with metastasis to bone. In the past decade microRNAs (miRNAs) have emerged as key players in cancer progression, but the importance of miRNAs in regulating cancer metastasis to bone is only now being appreciated. We emphasize here important concepts of bone biology and miRNAs in the context of breast and prostate cancer, and focus on recent advances that have improved our understanding of the role of specific miRNAs with direct involvement in metastatic bone disease.

Apoptotic osteocytes and the control of targeted bone resorption

Studies from the 1950s and 1960s already recognize the fact that osteocytes, although long living cells, die, as evidenced by accumulation of osteocytic lacunae devoid of cells. More recently, it was demonstrated that these cells die by apoptosis. The rate of osteocyte apoptosis is regulated by the age of the bone, as well as by systemic hormones, local growth factors, cytokines, pharmacological agents, and mechanical forces. Apoptotic osteocytes, in turn, recruit osteoclasts to initiate targeted bone resorption. This results in the removal of "dead" bone and may improve the mechanical properties of the skeleton. However, the molecular regulators of osteocyte survival and targeted bone remodeling are not completely known. In this review, the current knowledge on the molecular mechanism that lead to osteocyte death or survival, and the signals that mediate targeted bone resorption is discussed.