Apoptotic osteocytes and the control of targeted bone resorption

Post-blast histological changes to three animal bones exposed to close-range chemical detonation

A range of investigative practices to aid explosive-related death investigations currently exist, although the use of histopathological bone samples to diagnose blast exposure and the distance of individuals from the blast source has not been previously reported. Forensic histopathology has been used effectively on soft tissue samples to define blast-related injuries effectively, analysing human organs such as the lungs, brain, liver, and skeletal muscles, providing important and useful forensic pathology interpretations. However, no studies currently exist examining the post-blast histological changes in human or animal bones subjected to blasts for forensic pathology practice, despite the opportunity that hard tissue bone samples present, given their significantly lower rate of decomposition over soft tissue. This study presents the first evidence-based findings on the post-blast histological changes in three animal bones when exposed to close-range chemical detonation (C4). The study's qualitative findings highlight critical changes in the tissue architecture of three different animal bone sources due to blast effects with range from the blast source. This emphasises the potential use of histopathological bone sample analysis in future blast-related death investigations, while providing ideas to further explore this work using larger-scale experiments and post-blast case studies in aid of applying this work to human samples and forensic pathology practice.

AOP Report: Development of an adverse outcome pathway for deposition of energy leading to bone loss

Bone loss, commonly seen in osteoporosis, is a condition that entails a progressive decline of bone mineral density and microarchitecture, often seen in post-menopausal women. Bone loss has also been widely reported in astronauts exposed to a plethora of stressors and in patients with osteoporosis following radiotherapy for cancer. Studies on mechanisms are well documented but the causal connectivity of events to bone loss development remains incompletely understood. Herein, the adverse outcome pathway (AOP) framework was used to organize data and develop a qualitative AOP beginning from deposition of energy (the molecular initiating event) to bone loss (the adverse outcome). This qualitative AOP was developed in collaboration with bone loss research experts to aggregate relevant findings, supporting ongoing efforts to understand and mitigate human system risks associated with radiation exposures. A literature review was conducted to compile and evaluate the state of knowledge based on the modified Bradford Hill criteria. Following review of 2029 studies, an empirically supported AOP was developed, showing the progression to bone loss through many factors affecting the activities of bone-forming osteoblasts and bone-resorbing osteoclasts. The structural, functional, and quantitative basis of each proposed relationship was defined, for inference of causal changes between key events. Current knowledge and its gaps relating to dose-, time- and incidence-concordance across the key events were identified, as well as modulating factors that influence linkages. The new priorities for research informed by the AOP highlight areas for improvement to enable development of a quantitative AOP used to support risk assessment strategies for space travel or cancer radiotherapy.

BIGH3 mediates apoptosis and gap junction failure in osteocytes during renal cell carcinoma bone metastasis progression

Renal cell carcinoma (RCC) bone metastatis progression is driven by crosstalk between tumor cells and the bone microenvironment, which includes osteoblasts, osteoclasts, and osteocytes. RCC bone metastases (RCCBM) are predominantly osteolytic and resistant to antiresorptive therapy. The molecular mechanisms underlying pathologic osteolysis and disruption of bone homeostasis remain incompletely understood. We previously reported that BIGH3/TGFBI (transforming growth factor-beta-induced protein ig-h3, shortened to BIGH3 henceforth) secreted by colonizing RCC cells drives osteolysis by inhibiting osteoblast differentiation, impairing healing of osteolytic lesions, which is reversible with osteoanabolic agents. Here, we report that BIGH3 induces osteocyte apoptosis in both human RCCBM tissue specimens and in a preclinical mouse model. We also demonstrate that BIGH3 reduces Cx43 expression, blocking gap junction (GJ) function and osteocyte network communication. BIGH3-mediated GJ inhibition is blocked by the lysosomal inhibitor hydroxychloroquine (HCQ), but not osteoanabolic agents. Our results broaden the understanding of pathologic osteolysis in RCCBM and indicate that targeting the BIGH3 mechanism could be a combinational strategy for the treatment of RCCBM-induced bone disease that overcomes the limited efficacy of antiresorptives that target osteoclasts.

TNF-α promotes osteocyte necroptosis by upregulating TLR4 in postmenopausal osteoporosis

Postmenopausal osteoporosis (PMOP) is a common kind of osteoporosis that is associated with excessive osteocyte death and bone loss. Previous studies have shown that TNF-α-induced osteocyte necroptosis might exert a stronger effect on PMOP than apoptosis, and TLR4 can also induce cell necroptosis, as confirmed by recent studies. However, little is known about the relationship between TNF-α-induced osteocyte necroptosis and TLR4. In the present study, we showed that TNF-α increased the expression of TLR4, which promoted osteocyte necroptosis in PMOP. In patients with PMOP, TLR4 was highly expressed at skeletal sites where exists osteocyte necroptosis, and high TLR4 expression is correlated with enhanced TNF-α expression. Osteocytes exhibited robust TLR4 expression upon exposure to necroptotic osteocytes in vivo and in vitro. Western blotting and immunofluorescence analyses demonstrated that TNF-α upregulated TLR4 expression in vitro, which might further promote osteocyte necroptosis. Furthermore, inhibition of TLR4 by TAK-242 in vitro effectively blocked osteocyte necroptosis induced by TNF-α. Collectively, these results suggest a novel TLR4-mediated process of osteocyte necroptosis, which might increase osteocyte death and bone loss in the process of PMOP.

Osteoclast-specific Plastin 3 knockout in mice fail to develop osteoporosis despite dramatic increased osteoclast resorption activity

PLS3 loss-of-function mutations in humans and mice cause X-linked primary osteoporosis. However, it remains largely unknown how PLS3 mutations cause osteoporosis and which function PLS3 plays in bone homeostasis. A recent study showed that ubiquitous Pls3 KO in mice results in osteoporosis. Mainly osteoclasts were impacted in their function However, it has not been proven if osteoclasts are the major cell type affected and responsible for osteoporosis development in ubiquitous Pls3 KO mice. Here, we generated osteoclast-specific Pls3 KO mice. Additionally, we developed a novel polyclonal PLS3 antibody that showed specific PLS3 loss in immunofluorescence staining of osteoclasts in contrast to previously available antibodies against PLS3, which failed to show PLS3 specificity in mouse cells. Moreover, we demonstrate that osteoclast-specific Pls3 KO causes dramatic increase in resorptive activity of osteoclasts in vitro. Despite these findings, osteoclast-specific Pls3 KO in vivo failed to cause any osteoporotic phenotype in mice as proven by micro-CT and three-point bending test. This demonstrates that the pathomechanism of PLS3-associated osteoporosis is highly complex and cannot be reproduced in a system singularly focused on one cell type. Thus, the loss of PLS3 in alternative bone cell types might contributes to the osteoporosis phenotype in ubiquitous Pls3 KO mice.

Role of Apoptosis in the Pathogenesis of Osteoarthritis: An Explicative Review

Apoptosis is a complex regulatory, active cell death process that plays a role in cell development, homeostasis, and ageing. Cancer, developmental defects, and degenerative diseases are all pathogenic disorders caused by apoptosis dysregulation. Osteoarthritis (OA) is by far the most frequently diagnosed joint disease in the aged, and it is characterized by the ongoing breakdown of articular cartilage, which causes severe disability. Multiple variables regulate the anabolic and catabolic pathways of the cartilage matrix, which either directly or indirectly contribute to cartilage degeneration in osteoarthritis. Articular cartilage is a highly specialized tissue made up of an extracellular matrix of cells that are tightly packed together. As a result, chondrocyte survival is crucial for the preservation of an optimal cartilage matrix, and chondrocyte characteristics and survival compromise may result in articular cartilage failure. Inflammatory cytokines can either promote or inhibit apoptosis, the process of programmed cell death. Pro-apoptotic cytokines like TNF-α can induce cell death, while anti-apoptotic cytokines like IL-4 and IL-10 protect against apoptosis. The balance between these cytokines plays a critical role in determining cell fate and has implications for tissue damage and disease progression. Similarly, they contribute to the progression of OA by disrupting the metabolic balance in joint tissues by promoting catabolic and anabolic pathways. Their impact on cell joints, as well as the impacts of cell signalling pathways on cytokines and inflammatory substances, determines their function in osteoarthritis development. Apoptosis is evident in osteoarthritic cartilage; however, determining the relative role of chondrocyte apoptosis in the aetiology of OA is difficult, and the rate of apoptotic chondrocytes in osteoarthritic cartilage is inconsistent. The current study summarises the role of apoptosis in the development of osteoarthritis, the mediators, and signalling pathways that trigger the cascade of events, and the other inflammatory features involved.

Mesenchymal stem cell-derived apoptotic bodies alleviate alveolar bone destruction by regulating osteoclast differentiation and function

Periodontitis is caused by overactive osteoclast activity that results in the loss of periodontal supporting tissue and mesenchymal stem cells (MSCs) are essential for periodontal regeneration. However, the hypoxic periodontal microenvironment during periodontitis induces the apoptosis of MSCs. Apoptotic bodies (ABs) are the major product of apoptotic cells and have been attracting increased attention as potential mediators for periodontitis treatment, thus we investigated the effects of ABs derived from MSCs on periodontitis. MSCs were derived from bone marrows of mice and were cultured under hypoxic conditions for 72 h, after which ABs were isolated from the culture supernatant using a multi-filtration system. The results demonstrate that ABs derived from MSCs inhibited osteoclast differentiation and alveolar bone resorption. miRNA array analysis showed that miR-223-3p is highly enriched in those ABs and is critical for their therapeutic effects. Targetscan and luciferase activity results confirmed that Itgb1 is targeted by miR-223-3p, which interferes with the function of osteoclasts. Additionally, DC-STAMP is a key regulator that mediates membrane infusion. ABs and pre-osteoclasts expressed high levels of DC-STAMP on their membranes, which mediates the engulfment of ABs by pre-osteoclasts. ABs with knock-down of DC-STAMP failed to be engulfed by pre-osteoclasts. Collectively, MSC-derived ABs are targeted to be engulfed by pre-osteoclasts via DC-STAMP, which rescued alveolar bone loss by transferring miR-223-3p to osteoclasts, which in turn led to the attenuation of their differentiation and bone resorption. These results suggest that MSC-derived ABs are promising therapeutic agents for the treatment of periodontitis.

Utility of HR-pQCT in detecting training-induced changes in healthy adult bone morphology and microstructure

Healthy bone adjusts its traits in an exceptionally coordinated, compensatory process. Recent advancements in skeletal imaging via High-Resolution Peripheral Quantitative Computed Tomography (HR-pQCT) allows for the in vivo 3-dimensional and longitudinal quantification of bone density, microarchitecture, geometry, and parameters of mechanical strength in response to varying strain stimuli including those resulting from exercise or military training. Further, the voxel size of 61 microns has the potential to capture subtle changes in human bone in as little as 8 weeks. Given the typical time course of bone remodeling, short-term detection of skeletal changes in bone microstructure and morphology is indicative of adaptive bone formation, the deposition of new bone formation, uncoupled from prior resorption, that can occur at mechanistically advantageous regions. This review aims to synthesize existing training-induced HR-pQCT data in three distinct populations of healthy adults excluding disease states, pharmacological intervention and nutritional supplementation. Those included are: 1) military basic or officer training 2) general population and 3) non-osteoporotic aging. This review aims to further identify similarities and contrasts with prior modalities and cumulatively interpret results within the scope of bone functional adaptation.

Explanted Skull Flaps after Decompressive Hemicraniectomy Demonstrate Relevant Bone Avitality-Is Their Reimplantation Worth the Risk?

BACKGROUND

Reimplantations of autologous skull flaps after decompressive hemicraniectomies (DHs) are associated with high rates of postoperative bone flap resorption (BFR). We histologically assessed the cell viability of explanted bone flaps in certain periods of time after DH, in order to conclude whether precursors of BRF may be developed during their storage.

METHODS

Skull bone flaps explanted during a DH between 2019 and 2020 were stored in a freezer at either -23 °C or -80 °C. After their thawing process, the skulls were collected. Parameters of bone metabolism, namely PTH1 and OPG, were analyzed via immunohistochemistry. H&E stain was used to assess the degree of avital bone tissue, whereas the repeated assays were performed after 6 months.

RESULTS

A total of 17 stored skull flaps (8 at -23 °C; 9 at -80 °C) were analyzed. The duration of cryopreservation varied between 2 and 17 months. A relevant degree of bone avitality was observed in all skull flaps, which significantly increased at the repeated evaluation after 6 months (p < 0.001). Preservation at -23 °C (p = 0.006) as well as longer storage times (p < 0.001) were identified as prognostic factors for higher rates of bone avitality in a linear mixed regression model.

CONCLUSIONS

Our novel finding shows a clear benefit from storage at -80° C, which should be carefully considered for the future management and storage of explanted skull flaps. Our analysis also further revealed a significant degree of bone avitality, a potential precursor of BFR, in skull flaps stored for several weeks. To this end, we should reconsider whether the reimplantation of autologous skull flaps instead of synthetic skull flaps is still justified.

Identification of an intronic enhancer regulating RANKL expression in osteocytic cells

The bony skeleton is continuously renewed throughout adult life by the bone remodeling process, in which old or damaged bone is removed by osteoclasts via largely unknown mechanisms. Osteocytes regulate bone remodeling by producing the osteoclast differentiation factor RANKL (encoded by the TNFSF11 gene). However, the precise mechanisms underlying RANKL expression in osteocytes are still elusive. Here, we explored the epigenomic landscape of osteocytic cells and identified a hitherto-undescribed osteocytic cell-specific intronic enhancer in the TNFSF11 gene locus. Bioinformatics analyses showed that transcription factors involved in cell death and senescence act on this intronic enhancer region. Single-cell transcriptomic data analysis demonstrated that cell death signaling increased RANKL expression in osteocytic cells. Genetic deletion of the intronic enhancer led to a high-bone-mass phenotype with decreased levels of RANKL in osteocytic cells and osteoclastogenesis in the adult stage, while RANKL expression was not affected in osteoblasts or lymphocytes. These data suggest that osteocytes may utilize a specialized regulatory element to facilitate osteoclast formation at the bone surface to be resorbed by linking signals from cellular senescence/death and RANKL expression.

Oscillating Fluid Flow Activated Osteocyte Lysate-Based Hydrogel for Regulating Osteoblast/Osteoclast Homeostasis to Enhance Bone Repair

As major regulators on bone formation/resorption in response to mechanical stimuli, osteocytes have shown great promise for restoring bone injury. However, due to the unmanageable and unabiding cell functions in unloading or diseased environments, the efficacy of osteogenic induction by osteocytes has been enormously limited. Herein, a facile method of oscillating fluid flow (OFF) loading for cell culture is reported, which enables osteocytes to initiate only osteogenesis and not the osteolysis process. After OFF loading, multiple and sufficient soluble mediators are produced in osteocytes, and the collected osteocyte lysates invariably induce robust osteoblastic differentiation and proliferation while restraining osteoclast generation and activity under unloading or pathological conditions. Mechanistic studies confirm that elevated glycolysis and activation of the ERK1/2 and Wnt/β-catenin pathways are the major contributors to the initiation of osteoinduction functions induced by osteocytes. Moreover, an osteocyte lysate-based hydrogel is designed to establish a stockpile of "active osteocytes" to sustainably deliver bioactive proteins, resulting in accelerated healing through regulation of endogenous osteoblast/osteoclast homeostasis.

Osteocyte apoptosis and cellular micropetrosis signify skeletal aging in type 1 diabetes

Bone fragility is a profound complication of type 1 diabetes mellitus (T1DM), increasing patient morbidity. Within the mineralized bone matrix, osteocytes build a mechanosensitive network that orchestrates bone remodeling; thus, osteocyte viability is crucial for maintaining bone homeostasis. In human cortical bone specimens from individuals with T1DM, we found signs of accelerated osteocyte apoptosis and local mineralization of osteocyte lacunae (micropetrosis) compared with samples from age-matched controls. Such morphological changes were seen in the relatively young osteonal bone matrix on the periosteal side, and micropetrosis coincided with microdamage accumulation, implying that T1DM drives local skeletal aging and thereby impairs the biomechanical competence of the bone tissue. The consequent dysfunction of the osteocyte network hampers bone remodeling and decreases bone repair mechanisms, potentially contributing to the enhanced fracture risk seen in individuals with T1DM. STATEMENT OF SIGNIFICANCE: Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease that causes hyperglycemia. Increased bone fragility is one of the complications associated with T1DM. Our latest study on T1DM-affected human cortical bone identified the viability of osteocytes, the primary bone cells, as a potentially critical factor in T1DM-bone disease. We linked T1DM with increased osteocyte apoptosis and local accumulation of mineralized lacunar spaces and microdamage. Such structural changes in bone tissue suggest that T1DM speeds up the adverse effects of aging, leading to the premature death of osteocytes and potentially contributing to diabetes-related bone fragility.

Osteocytes in bone aging: Advances, challenges, and future perspectives

Osteocytes play a critical role in maintaining bone homeostasis and in regulating skeletal response to hormones and mechanical loading. Substantial evidence have demonstrated that osteocytes and their lacunae exhibit morphological changes in aged bone, indicating the underlying involvement of osteocytes in bone aging. Notably, recent studies have deciphered aged osteocytes to have characteristics such as impaired mechanosensitivity, accumulated cellular senescence, dysfunctional perilacunar/canalicular remodeling, and degenerated lacuna-canalicular network. However, detailed molecular mechanisms of osteocytes remain unclear. Nonetheless, osteocyte transcriptomes analyzed via advanced RNA sequencing (RNA-seq) techniques have identified several bone aging-related genes and signaling pathways, such as Wnt, Bmp/TGF, and Jak-STAT. Moreover, inflammation, immune dysfunction, energy shortage, and impaired hormone responses possibly affect osteocytes in age-related bone deterioration. In this review, we summarize the hallmarks of aging bone and osteocytes and discuss osteocytic mechanisms in age-related bone loss and impaired bone quality. Furthermore, we provide insights into the challenges faced and their possible solutions when investigating osteocyte transcriptomes. We also highlight that single-cell RNA-seq can decode transcriptomic messages in aged osteocytes; therefore, this technique can promote novel single cell-based investigations in osteocytes once a well-established standardized protocol specific for osteocytes is developed. Interestingly, improved understanding of osteocytic mechanisms have helped identify promising targets and effective therapies for aging-related osteoporosis and fragile fractures.

Role of the Interaction of Tumor Necrosis Factor-α and Tumor Necrosis Factor Receptors 1 and 2 in Bone-Related Cells

Tumor necrosis factor-α (TNF-α) is a pleiotropic cytokine expressed by macrophages, monocytes, and T cells, and its expression is triggered by the immune system in response to pathogens and their products, such as endotoxins. TNF-α plays an important role in host defense by inducing inflammatory reactions such as phagocytes and cytocidal systems activation. TNF-α also plays an important role in bone metabolism and is associated with inflammatory bone diseases. TNF-α binds to two cell surface receptors, the 55kDa TNF receptor-1 (TNFR1) and the 75kDa TNF receptor-2 (TNFR2). Bone is in a constant state of turnover; it is continuously degraded and built via the process of bone remodeling, which results from the regulated balance between bone-resorbing osteoclasts, bone-forming osteoblasts, and the mechanosensory cell type osteocytes. Precise interactions between these cells maintain skeletal homeostasis. Studies have shown that TNF-α affects bone-related cells via TNFRs. Signaling through either receptor results in different outcomes in different cell types as well as in the same cell type. This review summarizes and discusses current research on the TNF-α and TNFR interaction and its role in bone-related cells.

Could Bone Biomarkers Predict Bone Turnover after Kidney Transplantation?—A Proof-of-Concept Study

Aim: Bone disease after kidney transplant (KT) results from multiple factors, including previous bone and mineral metabolism disturbances and effects of transplant-related medications. New biomolecules have been recently associated with the development and progression of the chronic kidney disease–associated bone and mineral disorder (CKD-MBD). These include sclerostin and the soluble receptor activator of nuclear factor-kB ligand (sRANKL). Methods: To better understand the role of biomarkers in post-transplant bone disease, this study was designed to prospectively evaluate and correlate results from the histomorphometric analysis of bone biopsies after KT with emerging serum biomarkers of the CKD-MBD: sclerostin, Dickkopf-related protein 1 (Dkk-1), sRANKL and osteo-protegerin (OPG). Results: Our data shows a significant increase in plasma levels of bioactive sclerostin after KT accompanied by a significant reduction in plasma levels of Dkk-1, suggesting a promotion of the inhibition of bone formation by osteoblasts through the activation of these inhibitors of the Wnt signaling pathway. In addition, we found a significant increase in plasma levels of free sRANKL after KT accompanied by a significant reduction in plasma levels of its decoy receptor OPG, suggesting an enhanced bone resorption by osteoclasts mediated by this mechanism. Conclusions: Taken together, these results suggest that the loss of bone volume observed after KT could be explain mainly by the inhibition of bone formation mediated by sclerostin accompanied by an enhanced bone resorption mediated by sRANKL.

Recombinant Proteins-Based Strategies in Bone Tissue Engineering

The increase in fracture rates and/or problems associated with missing bones due to accidents or various pathologies generates socio-health problems with a very high impact. Tissue engineering aims to offer some kind of strategy to promote the repair of damaged tissue or its restoration as close as possible to the original tissue. Among the alternatives proposed by this specialty, the development of scaffolds obtained from recombinant proteins is of special importance. Furthermore, science and technology have advanced to obtain recombinant chimera’s proteins. This review aims to offer a synthetic description of the latest and most outstanding advances made with these types of scaffolds, particularly emphasizing the main recombinant proteins that can be used to construct scaffolds in their own right, i.e., not only to impregnate them, but also to make scaffolds from their complex structure, with the purpose of being considered in bone regenerative medicine in the near future.

Mechanobiology of Bone Metastatic Cancer

PURPOSE OF REVIEW

In this review, we provide an overview of what is currently known about the impacts of mechanical stimuli on metastatic tumor-induced bone disease (TIBD). Further, we focus on the role of the osteocyte, the skeleton's primary mechanosensory cell, which is central to the skeleton's mechanoresponse, sensing and integrating local mechanical stimuli, and then controlling the downstream remodeling balance as appropriate.

RECENT FINDINGS

Exercise and controlled mechanical loading have anabolic effects on bone tissue in models of bone metastasis. They also have anti-tumorigenic properties, in part due to offsetting the vicious cycle of osteolytic bone loss as well as regulating inflammatory signals. The impacts of metastatic cancer on the mechanosensory function of osteocytes remains unclear. Increased mechanical stimuli are a potential method for mitigating TIBD.

MicroRNAs and osteocytes

MicroRNAs, identified in the early 1990s, are believed to regulate approximately 30% of the human genome. The role of microRNA in bone cells was first reported in 2007 in a manuscript showing that microRNA-223 is essential for osteoclast differentiation in vitro, and a few studies reported a role of microRNAs in osteoblasts the same year. The first report of microRNA actions in osteocytes was published in 2010, in which it was demonstrated that the microRNA cluster 23a~27a~24-2 regulates osteocyte differentiation. Since then, few studies have described the role of these 18-25-nucleotide non-coding RNAs on osteocyte biology, reporting osteocytes both as producers and as targets of the actions of microRNAs. We review here the current knowledge on the effects of microRNAs on osteocyte biology.

The role of inhibition of osteocyte apoptosis in mediating orthodontic tooth movement and periodontal remodeling: a pilot study

BACKGROUND

Orthodontic tooth movement (OTM) has been shown to induce osteocyte apoptosis in alveolar bone shortly after force application. However, how osteocyte apoptosis affects orthodontic tooth movement is unknown. The goal of this study was to assess the effect of inhibition of osteocyte apoptosis on osteoclastogenesis, changes in the alveolar bone density, and the magnitude of OTM using a bisphosphonate analog (IG9402), a drug that affects osteocyte and osteoblast apoptosis but does not affect osteoclasts.

MATERIAL AND METHODS

Two sets of experiments were performed. Experiment 1 was used to specifically evaluate the effect of IG9402 on osteocyte apoptosis in the alveolar bone during 24 h of OTM. For this experiment, twelve mice were divided into two groups: group 1, saline administration + OTM_24-h (n=6), and group 2, IG9402 administration + OTM_24-h (n=6). The contralateral unloaded sides served as the control. The goal of experiment 2 was to evaluate the role of osteocyte apoptosis on OTM magnitude and osteoclastogenesis 10 days after OTM. Twenty mice were divided into 4 groups: group 1, saline administration without OTM (n=5); group 2, IG9402 administration without OTM (n=5); group 3, saline + OTM_10-day (n=6); and group 4, IG9402 + OTM_10-day (n=4). For both experiments, tooth movement was achieved using Ultra Light (25g) Sentalloy Closed Coil Springs attached between the first maxillary molar and the central incisor. Linear measurements of tooth movement and alveolar bone density (BVF) were assessed by MicroCT analysis. Cell death (or apoptosis) was assessed by terminal dUTP nick-end labeling (TUNEL) assay, while osteoclast and macrophage formation were assessed by tartrate-resistant acid phosphatase (TRAP) staining and F4/80+ immunostaining.

RESULTS

We found that IG9402 significantly blocked osteocyte apoptosis in alveolar bone (AB) at 24 h of OTM. At 10 days, IG9402 prevented OTM-induced loss of alveolar bone density and changed the morphology and quality of osteoclasts and macrophages, but did not significantly affect the amount of tooth movement.

CONCLUSION

Our study demonstrates that osteocyte apoptosis may play a significant role in osteoclast and macrophage formation during OTM, but does not seem to play a role in the magnitude of orthodontic tooth movement.

Quercetin and vitamin E alleviate ovariectomy-induced osteoporosis by modulating autophagy and apoptosis in rat bone cells

Osteoporosis is the most prevalent metabolic bone disease and one of the most important postmenopausal consequences. The aim of this study was to investigate the effects of quercetin (Q) and vitamin E (vitE) on ovariectomy-induced osteoporosis. Animals were ovariectomized and treated with Q (15 mg/kg/day), vitE (60 mg/kg/day), estradiol (10 µg/kg/day), and Q (7.5 mg/kg/day) + vitE (30 mg/kg/day) for 10 weeks by gavage, and osteoporosis markers and messenger RNA (mRNA) expression of autophagy and apoptosis-related genes were analyzed in serum and tibia of rats. Data indicated that ovariectomy resulted in development of osteoporosis as demonstrated by reduction in serum calcium, bone weight, bone volume, trabeculae volume, and the total number of osteocytes and osteoblasts, and increase in the total number of osteoclasts and serum osteocalcin. Total mRNA expressions of LC3, beclin1, and caspase 3 were also increased and bcl2 expression was decreased in the tibia. By reversing these changes, treatment with Q and vitE markedly improved osteoporosis. In conclusion, Q, and to a lesser extent, vitE, prevented osteoporosis by regulating the total number of bone cells, maybe through regulating autophagy and apoptosis.

Maternal High-Fat Diet Induces Long-Lasting Defects in Bone Structure in Rat Offspring Through Enhanced Osteoclastogenesis

Maternal stressors during the prenatal and perinatal periods are associated with increased susceptibility for and severity of chronic disease phenotypes in adult offspring. In this study, we used a rat model of maternal high-fat diet (HFD) exposure during pregnancy and lactation to investigate the impact on skeletal homeostasis in offspring. In the distal femur, young male and female offspring (up to 3 weeks of age) from dams fed a HFD exhibited marked increases in trabecular bone volume relative to offspring from dams fed a chow diet, but this was followed by sustained bone loss. By 15 weeks of age, male offspring of HFD fed dams exhibited a 33% reduction in trabecular bone volume fraction that histomorphometric analyses revealed was due to a nearly threefold increase in the abundance of bone-resorbing osteoclasts, while there were no differences between female control and HFD offspring by 15 weeks of age. The osteoblastic differentiation of male offspring-derived bone marrow stromal cells was not affected by maternal diet. However, osteoclastic precursors isolated from the male offspring of HFD fed dams exhibited enhanced differentiation in vitro, forming larger osteoclasts with higher expression of the fusion marker DC-STAMP. This effect appears to be mediated by a cell autonomous increase in the sensitivity of precursors to RANKL. Taken together, these results suggest that maternal stressors like HFD exposure have persistent consequences for the skeletal health of offspring that may ultimately lead to a predisposition for osteopenia/osteoporosis.

Osteocyte apoptosis: the roles and key molecular mechanisms in resorption-related bone diseases

Vital osteocytes have been well known to function as an important orchestrator in the preservation of robustness and fidelity of the bone remodeling process. Nevertheless, some key pathological factors, such as sex steroid deficiency and excess glucocorticoids, and so on, are implicated in inducing a bulk of apoptotic osteocytes, subsequently resulting in resorption-related bone loss. As much, osteocyte apoptosis, under homeostatic conditions, is in an optimal state of balance tightly controlled by pro- and anti-apoptotic mechanism pathways. Importantly, there exist many essential signaling proteins in the process of osteocyte apoptosis, which has a crucial role in maintaining a homeostatic environment. While increasing in vitro and in vivo studies have established, in part, key signaling pathways and cross-talk mechanism on osteocyte apoptosis, intrinsic and complex mechanism underlying osteocyte apoptosis occurs in various states of pathologies remains ill-defined. In this review, we discuss not only essential pro- and anti-apoptotic signaling pathways and key biomarkers involved in these key mechanisms under different pathological agents, but also the pivotal role of apoptotic osteocytes in osteoclastogenesis-triggered bone loss, hopefully shedding new light on the attractive and proper actions of pharmacotherapeutics of targeting apoptosis and ensuing resorption-related bone diseases such as osteoporosis and fragility fractures.

Bone, a Secondary Growth Site of Breast and Prostate Carcinomas: Role of Osteocytes

Bone is the primarily preferred site for breast and prostate cancer to metastasize. Bone metastases are responsible for most deaths related to breast and prostate cancer. The bone's particular microenvironment makes it conducive for the growth of cancer cells. Studies on bone metastasis have focused on the interaction between cancer cells and the bone microenvironment. Osteocytes, the most common cell type of bone tissue, have received little attention in bone metastasis, although they are master signal sensors, integrators, and skeleton transducers. They play an important role in regulating bone mass by acting on both osteoblasts and osteoclasts, through the release of proteins such as sclerostin, Dickkopf-1 (DKK-1), and fibroblast growth factor 23 (FGF23). Osteocytes have been extensively re-evaluated, in light of their multiple functions: with different experimental approaches, it has been shown that, indeed, osteocytes are actively involved in the colonization of bone tissue by cancer cells. The present review focuses on recent research on the role that osteocytes play in bone metastasis of breast and prostate cancers. Moreover, the studies here summarized open up perspectives for new therapeutic approaches focused on modulating the activity of osteocytes to improve the condition of the bone metastatic patients. A better understanding of the complex interactions between cancer cells and bone-resident cells is indispensable for identifying potential therapeutic targets to stop tumor progression and prevent bone metastases.

Methionine Enkephalin Suppresses Osteocyte Apoptosis Induced by Compressive Force through Regulation of Nuclear Translocation of NFATc1

Mechanical stress stimulates bone remodeling, which occurs through bone formation and resorption, resulting in bone adaptation in response to the mechanical stress. Osteocytes perceive mechanical stress loaded to bones and promote bone remodeling through various cellular processes. Osteocyte apoptosis is considered a cellular process to induce bone resorption during mechanical stress-induced bone remodeling, but the underlying molecular mechanisms are not fully understood. Recent studies have demonstrated that neuropeptides play crucial roles in bone metabolism. The neuropeptide, methionine enkephalin (MENK) regulates apoptosis positively and negatively depending on cell type, but the role of MENK in osteocyte apoptosis, followed by bone resorption, in response to mechanical stress is still unknown. Here, we examined the roles and mechanisms of MENK in osteocyte apoptosis induced by compressive force. We loaded compressive force to mouse parietal bones, resulting in a reduction of MENK expression in osteocytes. A neutralizing connective tissue growth factor (CTGF) antibody inhibited the compressive force-induced reduction of MENK. An increase in osteocyte apoptosis in the compressive force-loaded parietal bones was inhibited by MENK administration. Nuclear translocation of NFATc1 in osteocytes in the parietal bones was enhanced by compressive force. INCA-6, which inhibits NFAT translocation into nuclei, suppressed the increase in osteocyte apoptosis in the compressive force-loaded parietal bones. NFATc1-overexpressing MLO-Y4 cells showed increased expression of apoptosis-related genes. MENK administration reduced the nuclear translocation of NFATc1 in osteocytes in the compressive force-loaded parietal bones. Moreover, MENK suppressed Ca²⁺ influx and calcineurin and calmodulin expression, which are known to induce the nuclear translocation of NFAT in MLO-Y4 cells. In summary, this study shows that osteocytes expressed MENK, whereas the MENK expression was suppressed by compressive force via CTGF signaling. MENK downregulated nuclear translocation of NFATc1 probably by suppressing Ca²⁺ signaling in osteocytes and consequently inhibiting compressive force-induced osteocyte apoptosis, followed by bone resorption. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Uptake of osteoblast-derived extracellular vesicles promotes the differentiation of osteoclasts in the zebrafish scale

Differentiation of osteoclasts (OCs) from hematopoietic cells requires cellular interaction with osteoblasts (OBs). Due to the difficulty of live-imaging in the bone, however, the cellular and molecular mechanisms underlying intercellular communication involved in OC differentiation are still elusive. Here, we develop a fracture healing model using the scale of trap:GFP; osterix:mCherry transgenic zebrafish to visualize the interaction between OCs and OBs. Transplantation assays followed by flow cytometric analysis reveal that most trap:GFPhigh OCs in the fractured scale are detected in the osterix:mCherry+ fraction because of uptake of OB-derived extracellular vesicles (EVs). In vivo live-imaging shows that immature OCs actively interact with osterix:mCherry+ OBs and engulf EVs prior to convergence at the fracture site. In vitro cell culture assays show that OB-derived EVs promote OC differentiation via Rankl signaling. Collectively, these data suggest that EV-mediated intercellular communication with OBs plays an important role in the differentiation of OCs in bone tissue.

Elevations in Cortical Porosity Occur Prior to Significant Rise in Serum Parathyroid Hormone in Young Female Mice with Adenine-Induced CKD

Chronic kidney disease (CKD) leads to significant bone loss primarily through the development of cortical porosity. In both patients and animal models of CKD, sustained elevations in serum parathyroid hormone (PTH) are associated with cortical porosity. In this study, we aimed to track the progression of cortical porosity and increased PTH utilizing the adenine-induced CKD model. Young female mice (8 weeks) were given 0.2% adenine to induce CKD. Tissues were collected from groups of adenine and age-matched control mice after 2, 6, and 10 weeks. Serum blood urea nitrogen was elevated at all time points in adenine mice, but serum PTH was only statistically elevated at the 10-week time point. Cortical porosity was sevenfold higher in 6-week adenine mice compared to age-matched controls and 14-fold higher in 10-week adenine mice vs. controls. Additionally, osteocyte receptor activator of nuclear factor κB ligand (RANKL) was elevated in adenine-fed mice, while annexin V, an early marker of cellular apoptosis, was mildly decreased in osteocytes in adenine-fed mice. Based on these results, we hypothesize high serum PTH signals to osteocytes prolonging their lifespan resulting in sustained RANKL which drives osteoclastic bone resorption in the cortex. In conclusion, our data show time-dependent elevations in serum PTH and cortical porosity in adenine-induced CKD mice and demonstrate changes in osteocyte RANKL and apoptosis which may contribute to the development of cortical pores.

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

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

Osteocytic miR21 deficiency improves bone strength independent of sex despite having sex divergent effects on osteocyte viability and bone turnover

Osteocytes play a critical role in mediating cell-cell communication and regulating bone homeostasis, and osteocyte apoptosis is associated with increased bone resorption. miR21, an oncogenic microRNA, regulates bone metabolism by acting directly on osteoblasts and osteoclasts, but its role in osteocytes is not clear. Here, we show that osteocytic miR21 deletion has sex-divergent effects in bone. In females, miR21 deletion reduces osteocyte viability, but suppresses bone turnover. Conversely, in males, miR21 deletion increases osteocyte viability, but stimulates bone turnover and enhances bone structure. Further, miR21 deletion differentially alters osteocyte cytokine production in the two sexes. Interestingly, despite these changes, miR21 deletion increases bone mechanical properties in both sexes, albeit to a greater extent in males. Collectively, our findings suggest that miR21 exerts both sex-divergent and sex-equivalent roles in osteocytes, regulating osteocyte viability and altering bone metabolism through paracrine actions on osteoblasts and osteoclasts differentially in males vs females, whereas, influencing bone mechanical properties independent of sex.

Mechanisms of palmitate-induced lipotoxicity in osteocytes

BACKGROUND

Lipotoxicity is defined as cellular toxicity observed in the presence of an abnormal accumulation of fat and adipocyte-derived factors in non-fat tissues. Palmitic acid (PA), an abundant fatty acid in the bone marrow and particularly in osteoporotic bones, affects osteoblastogenesis and osteoblast function, decreasing their survival through induction of apoptosis and dysfunctional autophagy. In this study, we hypothesized that PA also has a lipotoxic effect on osteocytes in vitro.

METHODS

Initially, we tested the effect of PA on osteocyte-derived factors DKK1, sclerostin and RANKL. Then, we tested whether PA affects survival and causes apoptosis in osteocytes. Subsequently, we investigated the effect of PA on autophagy by detecting the membrane component LC3-II (Western blot) and staining it and lysosomes with Lysotracker Red dye.

RESULTS

PA decreases RANKL, DKK1 and sclerostin expression in osteocytes. In addition, we found that PA induces apoptosis and reduces osteocyte survival. PA also caused autophagy failure identified by a significant increase in LC3-II and a reduced number of autophagosomes/lysosomes in the cytoplasm.

CONCLUSION

In addition to the effects of PA on RANKL, DKK1 and sclerostin expression, which could have significant deleterious impact on bone cell coupling and bone turnover, PA also induced apoptosis and reduced autophagy in osteocytes. Considering that apoptosis and cell dysfunction are two common changes occurring in the osteocytes of osteoporotic bone, our findings suggest that PA could play a role in the pathogenesis of the disease. Suppression of these effects could bring new potential targets for therapeutic interventions in the future.

Age- and sex-dependent role of osteocytic pannexin1 on bone and muscle mass and strength

Pannexins (Panxs), glycoproteins that oligomerize to form hemichannels on the cell membrane, are topologically similar to connexins, but do not form cell-to-cell gap junction channels. There are 3 members of the family, 1-3, with Panx1 being the most abundant. All Panxs are expressed in bone, but their role in bone cell biology is not completely understood. We now report that osteocytic Panx1 deletion (Panx1Δot) alters bone mass and strength in female mice. Bone mineral density after reaching skeletal maturity is higher in female Panx1Δot mice than in control Panx1fl/fl mice. Further, osteocytic Panx1 deletion partially prevented aging effects on cortical bone structure and mechanical properties. Young 4-month-old female Panx1Δot mice exhibited increased lean body mass, even though pannexin levels in skeletal muscle were not affected; whereas no difference in lean body mass was detected in male mice. Furthermore, female Panx1-deficient mice exhibited increased muscle mass without changes in strength, whereas Panx1Δot males showed unchanged muscle mass and decreased in vivo maximum plantarflexion torque, indicating reduced muscle strength. Our results suggest that osteocytic Panx1 deletion increases bone mass in young and old female mice and muscle mass in young female mice, but has deleterious effects on muscle strength only in males.

Screening of key candidate genes and pathways for osteocytes involved in the differential response to different types of mechanical stimulation using a bioinformatics analysis

This study aimed to predict the key genes and pathways that are activated when different types of mechanical loading are applied to osteocytes. mRNA expression datasets (series number of GSE62128 and GSE42874) were obtained from Gene Expression Omnibus database (GEO). High gravity-treated osteocytic MLO-Y4 cell-line samples from GSE62128 (Set1), and fluid flow-treated MLO-Y4 samples from GSE42874 (Set2) were employed. After identifying the differentially expressed genes (DEGs), functional enrichment was performed. The common DEGs between Set1 and Set2 were considered as key DEGs, then a protein-protein interaction (PPI) network was constructed using the minimal nodes from all of the DEGs in Set1 and Set2, which linked most of the key DEGs. Several open source software programs were employed to process and analyze the original data. The bioinformatic results and the biological meaning were validated by in vitro experiments. High gravity and fluid flow induced opposite expression trends in the key DEGs. The hypoxia-related biological process and signaling pathway were the common functional enrichment terms among the DEGs from Set1, Set2 and the PPI network. The expression of almost all the key DEGs (Pdk1, Ccng2, Eno2, Egln1, Higd1a, Slc5a3 and Mxi1) were mechano-sensitive. Eno2 was identified as the hub gene in the PPI network. Eno2 knockdown results in expression changes of some other key DEGs (Pdk1, Mxi1 and Higd1a). Our findings indicated that the hypoxia response might have an important role in the differential responses of osteocytes to the different types of mechanical force.

Grapefruit juice exerts anti-osteoporotic activities in a prednisolone-induced osteoporosis rat femoral fracture model, possibly via the RANKL/OPG axis

This study aimed to shed light on the protective and therapeutic anti-osteoporotic effects and mechanisms of action of grapefruit juice (GFJ) on prednisolone-induced osteoporosis a rat femoral fracture model. We found that treating rats with GFJ before and/or after prednisolone-induced osteoporosis resulted in increased bone density, total mineral content, and calcium content to counteract the osteoporotic effects of prednisolone. In parallel, the histological and ultrastructural results of the GFJ-treated groups correlated well with enhanced breaking strength of femurs subjected to a constant load. Furthermore, GFJ treatment before and after prednisolone-induced osteoporosis decreased plasma alkaline phosphatase and tartrate-resistant acid phosphatase activities and increased the level of insulin-like growth factor 1. Mechanistically, our immunohistochemistry study showed that GFJ ameliorated prednisolone-induced osteocalcin depletion, decreased receptor activator of nuclear factor kappa-B ligand (RANKL) expression, and increased osteoprotegerin (OPG) expression. GFJ showed a beneficial anti-osteoporotic effect against prednisolone-induced osteoporosis in rats, possibly via the RANKL/OPG axis, suggesting that GFJ might be a good candidate for developing anti-osteoporotic drugs.

Histological ageing of fractures in infants: a practical algorithm for assessing infants suspected of accidental or non-accidental injury

Aims

This study is the first to systematically document histological features of fractures of known age in infants (≦12 months). It has been used to develop a tabulated database specifically to guide histopathologists to age fractures in children considered to have suffered accidental or non‐accidental injury (NAI). Currently in the United Kingdom there are insufficient pathologists with experience in histological ageing of fractures to meet the medicolegal need for this examination. This study provides a practical tool that will allow those skilled paediatric and forensic pathologists currently involved in assessing infants for evidence of accidental or non‐accidental injury a basis for extending their assessment into this area of unmet need.

Methods and results

One hundred and sixty‐nine fractures of known age at death were obtained from 52 anonymised infants over a period of 32 years (1985–2016 inclusive). Sections stained using haematoxylin and eosin (H&E) and Martius scarlet blue (MSB) were used to identify specific histological features and to relate them to fracture age. In 1999 the data were entered into a tabulated database for fractures accumulated between from 1985 to 1998 inclusive. Thereafter cases were added, and at 2‐yearly intervals the accumulated data were audited against the previous database and adjustments made.

Conclusions

This paper describes the final data set from the 2017 audit. The study was terminated at the end of 2016, as there had been no material changes in the data set for three consecutive audits.

Release of CXCL12 From Apoptotic Skeletal Cells Contributes to Bone Growth Defects Following Dexamethasone Therapy in Rats

Dexamethasone (Dex) is known to cause significant bone growth impairment in childhood. Although previous studies have suggested roles of osteocyte apoptosis in the enhanced osteoclastic recruitment and local bone loss, whether it is so in the growing bone following Dex treatment requires to be established. The current study addressed the potential roles of chemokine CXCL12 in chondroclast/osteoclast recruitment and bone defects following Dex treatment. Significant apoptosis was observed in cultured mature ATDC5 chondrocytes and IDG-SW3 osteocytes after 48 hours of 10^-6  M Dex treatment, and CXCL12 was identified to exhibit the most prominent induction in Dex-treated cells. Conditioned medium from the treated chondrocytes/osteocytes enhanced migration of RAW264.7 osteoclast precursor cells, which was significantly inhibited by the presence of the anti-CXCL12 neutralizing antibody. To investigate the roles of the induced CXCL12 in bone defects caused by Dex treatment, young rats were orally gavaged daily with saline or Dex at 1 mg/kg/day for 2 weeks, and received an intraperitoneal injection of anti-CXCL12 antibody or control IgG (1 mg/kg, three times per week). Aside from oxidative stress induction systemically, Dex treatment caused reductions in growth plate thickness, primary spongiosa height, and metaphysis trabecular bone volume, which are associated with induced chondrocyte/osteocyte apoptosis and enhanced chondroclast/osteoclast recruitment and osteoclastogenic differentiation potential. CXCL12 was induced in apoptotic growth plate chondrocytes and metaphyseal bone osteocytes. Anti-CXCL12 antibody supplementation considerably attenuated Dex-induced chondroclast/osteoclast recruitment and loss of growth plate cartilage and trabecular bone. CXCL12 neutralization did not affect bone marrow osteogenic potential, adiposity, and microvasculature. Thus, CXCL12 was identified as a potential molecular linker between Dex-induced skeletal cell apoptosis and chondroclastic/osteoclastic recruitment, as well as growth plate cartilage/bone loss, revealing a therapeutic potential of CXCL12 functional blockade in preventing bone growth defects during/after Dex treatment. © 2018 American Society for Bone and Mineral Research.

The Role of Osteocytes in Inflammatory Bone Loss

Osteoimmunology investigations to-date have demonstrated the significant interactions between bone surface cells, osteoclasts and osteoblasts, and immune cells. However, there is a paucity of knowledge on osteocytes, cells embedded in the bone matrix, and their role in inflammation and inflammatory bone loss. Osteocytes communicate through various mechanisms; directly via dendritic processes and through secretion of proteins that can influence the formation and activity of osteoblasts and osteoclasts. Some osteocyte proteins (e.g., interleukin-6 and RANKL) also have roles within the immune system. In the context of mechanical loading/unloading, the regulatory role of osteocytes is well understood. More recent data on osteocytes in various inflammatory models suggest they may also aid in orchestrating inflammation-induced changes in bone turnover. In inflammatory conditions, osteocytes express multiple pro-inflammatory cytokines which are associated with increases in bone resorption and declines in bone formation. Cytokines are known to also influence cell population growth, maturation, and responsiveness via various signaling modalities, but how they influence osteocytes has not been greatly explored. Furthermore, osteocytes may play regulatory roles in orchestrating bone's response to immunological changes in inflammatory conditions. This review will address what is known about osteocyte biology in physiological conditions and in response to varying immunological conditions, as well as highlight key areas of interest for future investigations.

Understanding the Bone in Cancer Metastasis

The bone is the third most common site of metastasis for a wide range of solid tumors including lung, breast, prostate, colorectal, thyroid, gynecologic, and melanoma, with 70% of metastatic prostate and breast cancer patients harboring bone metastasis.¹ Unfortunately, once cancer spreads to the bone, it is rarely cured and is associated with a wide range of morbidities including pain, increased risk of fracture, and hypercalcemia. This fact has driven experts in the fields of bone and cancer biology to study the bone, and has revealed that there is a great deal that each can teach the other. The complexity of the bone was first described in 1889 when Stephen Paget proposed that tumor cells have a proclivity for certain organs, where they "seed" into a friendly "soil" and eventually grow into metastatic lesions. Dr. Paget went on to argue that although many study the "seed" it would be paramount to understand the "soil." Since this original work, significant advances have been made not only in understanding the cell-autonomous mechanisms that drive metastasis, but also alterations which drive changes to the "soil" that allow a tumor cell to thrive. Indeed, it is now clear that the "soil" in different metastatic sites is unique, and thus the mechanisms that allow tumor cells to remain in a dormant or growing state are specific to the organ in question. In the bone, our knowledge of the components that contribute to this fertile "soil" continues to expand, but our understanding of how they impact tumor growth in the bone remains in its infancy. Indeed, we now appreciate that the endosteal niche likely contributes to tumor cell dormancy, and that osteoclasts, osteocytes, and adipocytes can impact tumor cell growth. Here, we discuss the bone microenvironment and how it impacts cancer cell seeding, dormancy, and growth. © 2018 American Society for Bone and Mineral Research.

Early bone tissue aging in human auditory ossicles is accompanied by excessive hypermineralization, osteocyte death and micropetrosis

Within the mineralized bone, osteocytes form a multifunctional mechanosensitive network orchestrating bone remodelling. A preserved osteocyte population is a crucial determinant of bone quality. In human auditory ossicles, the early decrease in osteocyte numbers but maintained integrity remains an unexplained phenomenon that might serve for sound transmission from air to the labyrinth. Here we analysed the frequency, size and composition of osteocyte lacunae in the auditory ossicles of 22 individuals from early postnatal period to old age. Mineralization of the bone matrix was determined using backscattered electron imaging. No signs of bone remodelling were observed above the age of 1 year. We detected characteristics of early bone tissue aging, such as decrease in osteocytes, lower total lacunar density and lacunar area, as well as high matrix mineralization accompanied by distinct accumulation of micropetrotic lacunae and decreased indentation depths. The majority of these changes took place in the first months and years of life, while afterwards only minor reorganization was present. With osteocyte apoptosis potentially being a consequence of low mechanical stimuli, the early loss of osteocytes without initiation of bone remodelling indicates an adaptive response conserving the architecture of the auditory ossicles and ensuring stable sound transmission throughout life.

Mechanically-Loaded Breast Cancer Cells Modify Osteocyte Mechanosensitivity by Secreting Factors That Increase Osteocyte Dendrite Formation and Downstream Resorption

Advanced breast cancer predominantly metastasizes to the skeleton, at which point patient prognosis significantly declines concomitant with bone loss, pain, and heightened fracture risk. Given the skeleton's sensitivity to mechanical signals, increased mechanical loading is well-documented to increase bone mass, and it also inhibited bone metastatic tumor formation and progression in vivo, though the underlying mechanisms remain under investigation. Here, we focus on the role of the osteocyte because it is the primary skeletal mechanosensor and in turn directs the remodeling balance between formation and resoprtion. In particular, osteocytic dendrites are important for mechanosensing, but how this function is altered during bone metastatic breast cancer is unknown. To examine how breast cancer cells modulate dendrite formation and function, we exposed osteocytes (MLO-Y4) to medium conditioned by breast cancer cells (MDA-MB231) and to applied fluid flow (2 h per day for 3 days, shear stress 1.1 Pa). When loading was applied to MLOs, dendrite formation increased despite the presence of tumor-derived factors while overall MLO cell number was reduced. We then exposed MLOs to fluid flow as well as media conditioned by MDAs that had been similarly loaded. When nonloaded MLOs were treated with conditioned media from loaded MDAs, their dendrite formation increased in a manner similar to that observed due to loading alone. When MLOs simultaneously underwent loading and treatment with loaded conditioned media, dendrite formation was greatest. To understand potential molecular mechanisms, we then investigated expression of genes related to osteocyte maturation and dendrite formation (E11) and remodeling (RANKL, OPG) as well as osteocyte apoptosis. E11 expression increased with loading, consistent with increased dendrite formation. Though loaded conditioned media decreased MLO cell number, apoptosis was not detected via TUNEL staining, suggesting an inhibition of growth instead. OPG expression was inhibited while RANKL expression was unaffected, leading to an overall increase in the RANKL/OPG ratio with conditioned media from loaded breast cancer cells. Taken together, our results suggest that skeletal mechanical loading stimulates breast cancer cells to alter osteocyte mechanosensing by increasing dendrite formation and downstream resorption.

Effects of estrogen status in osteocyte autophagy and its relation to osteocyte viability in alveolar process of ovariectomized rats

Estrogen maintains osteocyte viability, whereas its deficiency induces osteocyte apoptosis. As autophagy is important for osteocyte viability, we hypothesized whether the anti-apoptotic effect of estrogen is related to autophagy in osteocytes. Thirty adult female rats were sham-operated (SHAM) or ovariectomized (OVX). After three weeks, twelve rats of SHAM and OVX groups were killed before treatment (basal period), whereas the remaining rats received estrogen (OVXE) or vehicle (OVX) for 45 days. Fragments of maxilla containing alveolar process of the first molars were embedded in paraffin or Araldite. Paraffin-sections were stained with hematoxylin/eosin for histomorphometry, or subjected to the silver impregnation method for morphological analysis of osteocyte cytoplasmic processes. Autophagy was analyzed by immunohistochemical detections of beclin-1, MAP-LC3α and p62, whereas apoptosis was evaluated by immunohistochemical detections of cleaved caspase-3 and BAX, TUNEL (Terminal deoxynucleotidyl transferase dUTP nick end labeling) method and by ultrastructural analysis. Araldite-semithin sections were subjected to the Sudan-black method for detection of lipids. OVX-basal group showed high frequency of caspase-3-, TUNEL- and p62-positive osteocytes accompanied with low frequency of beclin-1- and MAP-LC3α-positive osteocytes. At 45 days, OVXE group exhibited higher number of osteocytes, higher frequency of beclin-1- and MAP-LC3α-positive osteocytes, and lower frequency of caspase-3, BAX-, TUNEL- and p62-positive osteocytes than OVX group. Significant reduction in bone area was observed in the OVX compared to OVXE and SHAM groups. The highest frequency of Sudan-Black-positive osteocytes and osteocytes with scarce cytoplasmic processes, or showing apoptotic features were mainly observed in OVX groups. Our results indicate that estrogen deficiency decreases autophagy and increases apoptosis, whereas estrogen replacement enhances osteocyte viability by inhibiting apoptosis and maintaining autophagy in alveolar process osteocytes. These results suggest that the anti-apoptotic effect of estrogen may be, at least in part, related to autophagy regulation in osteocytes.

Alpinumisoflavone protects against glucocorticoid-induced osteoporosis through suppressing the apoptosis of osteoblastic and osteocytic cells

The long-term use of glucocorticoids is found to cause osteoporosis. This study is designed to evaluate the protective effect of alpinumisoflavone (AIF), a naturally occurring flavonoid compound, on dexamethasone(Dex)-induced osteoporosis. We use a rat model to investigate the apoptosis of osteoblastic and osteocytic cells. The results indicate that AIF effectively protects against dexamethasone-induced osteoporosis. Moreover, AIF effectively reversed dexamethasone-induced apoptosis in osteoblastic and osteocytic cells through inhibiting ROS overproduction and regulating the Nrf2 pathway. In conclusion, the AIF activated Nrf2 signaling pathway was observed to suppress Dex-induced ROS production in osteoblastic and osteocytic cells, which may explain its anti-osteoporotic effects against dexamethasone-induced osteoporosis.

Mechanisms of bone destruction in multiple myeloma

Osteolytic bone disease is a frequent complication of multiple myeloma, resulting in skeletal complications that are a significant cause of morbidity and mortality. It is the result of an increased activity of osteoclasts, which is not followed by reactive bone formation by osteoblasts. Recent studies have revealed novel molecules and pathways that are implicated in osteoclast activation and osteoblast inhibition. Among them, the most important include the receptor activator of nuclear factor-kappa B ligand/osteoprotegerin pathway, the macrophage inflammatory proteins and the activin-A that play a crucial role in osteoclast stimulation in myeloma, while the wingless-type (Wnt) signalling inhibitors (sclerostin and dickkopf-1) along with the growth factor independence-1 are considered the most important factors for the osteoblast dysfunction of myeloma patients. Finally, the role of osteocytes, which is the key cell for normal bone remodelling, has also revealed during the last years through their interaction with myeloma cells that leads to their apoptosis and the release of RANKL and sclerostin maintaining bone loss in these patients. This review focuses on the latest available data for the mechanisms of bone destruction in multiple myeloma.

Matrix Metalloproteinases in Bone Resorption, Remodeling, and Repair

Matrix metalloproteinases (MMPs) are the major protease family responsible for the cleavage of the matrisome (global composition of the extracellular matrix (ECM) proteome) and proteins unrelated to the ECM, generating bioactive molecules. These proteins drive ECM remodeling, in association with tissue-specific and cell-anchored inhibitors (TIMPs and RECK, respectively). In the bone, the ECM mediates cell adhesion, mechanotransduction, nucleation of mineralization, and the immobilization of growth factors to protect them from damage or degradation. Since the first description of an MMP in bone tissue, many other MMPs have been identified, as well as their inhibitors. Numerous functions have been assigned to these proteins, including osteoblast/osteocyte differentiation, bone formation, solubilization of the osteoid during bone resorption, osteoclast recruitment and migration, and as a coupling factor in bone remodeling under physiological conditions. In turn, a number of pathologies, associated with imbalanced bone remodeling, arise mainly from MMP overexpression and abnormalities of the ECM, leading to bone osteolysis or bone formation. In this review, we will discuss the functions of MMPs and their inhibitors in bone cells, during bone remodeling, pathological bone resorption (osteoporosis and bone metastasis), bone repair/regeneration, and emergent roles in bone bioengineering.

Disruption of the Cx43/miR21 pathway leads to osteocyte apoptosis and increased osteoclastogenesis with aging

Skeletal aging results in apoptosis of osteocytes, cells embedded in bone that control the generation/function of bone forming and resorbing cells. Aging also decreases connexin43 (Cx43) expression in bone; and osteocytic Cx43 deletion partially mimics the skeletal phenotype of old mice. Particularly, aging and Cx43 deletion increase osteocyte apoptosis, and osteoclast number and bone resorption on endocortical bone surfaces. We examined herein the molecular signaling events responsible for osteocyte apoptosis and osteoclast recruitment triggered by aging and Cx43 deficiency. Cx43‐silenced MLO‐Y4 osteocytic (Cx43^def) cells undergo spontaneous cell death in culture through caspase‐3 activation and exhibit increased levels of apoptosis‐related genes, and only transfection of Cx43 constructs able to form gap junction channels reverses Cx43^def cell death. Cx43^def cells and bones from old mice exhibit reduced levels of the pro‐survival microRNA miR21 and, consistently, increased levels of the miR21 target phosphatase and tensin homolog (PTEN) and reduced phosphorylated Akt, whereas PTEN inhibition reduces Cx43^def cell apoptosis. miR21 reduction is sufficient to induce apoptosis of Cx43‐expressing cells and miR21 deletion in miR21^fl/fl bones increases apoptosis‐related gene expression, whereas a miR21 mimic prevents Cx43^def cell apoptosis, demonstrating that miR21 lies downstream of Cx43. Cx43^def cells release more osteoclastogenic cytokines [receptor activator of NFκB ligand (RANKL)/high‐mobility group box‐1 (HMGB1)], and caspase‐3 inhibition prevents RANKL/HMGB1 release and the increased osteoclastogenesis induced by conditioned media from Cx43^def cells, which is blocked by antagonizing HMGB1‐RAGE interaction. These findings identify a novel Cx43/miR21/HMGB1/RANKL pathway involved in preventing osteocyte apoptosis that also controls osteoclast formation/recruitment and is impaired with aging.

Osteocyte Alterations Induce Osteoclastogenesis in an In Vitro Model of Gaucher Disease

Gaucher disease (GD) is caused by mutations in the glucosylceramidase β (GBA 1) gene that confer a deficient level of activity of glucocerebrosidase (GCase). This deficiency leads to the accumulation of the glycolipid glucocerebroside in the lysosomes of cells, mainly in the monocyte/macrophage lineage. Its mildest form is Type I GD, characterized by non-neuronopathic involvement. Bone compromise is the most disabling aspect of the Gaucher disease. However, the pathophysiological aspects of skeletal alterations are not yet fully understood. The bone tissue homeostasis is maintained by a balance between resorption of old bone by osteoclasts and new bone formation by osteoblasts. A central player in this balance is the osteocyte as it controls both processes. We studied the involvement of osteocytes in an in vitro chemical model of Gaucher disease. The osteocyte cell line MLO-Y4 was exposed to conduritol-β-epoxide (CBE), an inhibitor of GCase, for a period of 7, 14 and 21 days. Conditioned media from CBE-treated osteocytes was found to induce osteoclast differentiation. GCase inhibition caused alterations in Cx43 expression and distribution pattern and an increase in osteocyte apoptosis. Osteoclast differentiation involved osteocyte apoptotic bodies, receptor activator of nuclear factor κ-B ligand (RANKL) and soluble factors. Thus, our results indicate that osteocytes may have a role to play in the bone pathophysiology of GD.

Mechanical Loading Attenuates Radiation-Induced Bone Loss in Bone Marrow Transplanted Mice

Exposure of bone to ionizing radiation, as occurs during radiotherapy for some localized malignancies and blood or bone marrow cancers, as well as during space travel, incites dose-dependent bone morbidity and increased fracture risk. Rapid trabecular and endosteal bone loss reflects acutely increased osteoclastic resorption as well as decreased bone formation due to depletion of osteoprogenitors. Because of this dysregulation of bone turnover, bone’s capacity to respond to a mechanical loading stimulus in the aftermath of irradiation is unknown. We employed a mouse model of total body irradiation and bone marrow transplantation simulating treatment of hematologic cancers, hypothesizing that compression loading would attenuate bone loss. Furthermore, we hypothesized that loading would upregulate donor cell presence in loaded tibias due to increased engraftment and proliferation. We lethally irradiated 16 female C57Bl/6J mice at age 16 wks with 10.75 Gy, then IV-injected 20 million GFP(+) total bone marrow cells. That same day, we initiated 3 wks compression loading (1200 cycles 5x/wk, 10 N) in the right tibia of 10 of these mice while 6 mice were irradiated, non-mechanically-loaded controls. As anticipated, before-and-after microCT scans demonstrated loss of trabecular bone (-48.2% Tb.BV/TV) and cortical thickness (-8.3%) at 3 wks following irradiation. However, loaded bones lost 31% less Tb.BV/TV and 8% less cortical thickness (both p<0.001). Loaded bones also had significant increases in trabecular thickness and tissue mineral densities from baseline. Mechanical loading did not affect donor cell engraftment. Importantly, these results demonstrate that both cortical and trabecular bone exposed to high-dose therapeutic radiation remain capable of an anabolic response to mechanical loading. These findings inform our management of bone health in cases of radiation exposure.

Changes in bone macro- and microstructure in diabetic obese mice revealed by high resolution microfocus X-ray computed tomography

High resolution microfocus X-ray computed tomography (HR-microCT) was employed to characterize the structural alterations of the cortical and trabecular bone in a mouse model of obesity-driven type 2 diabetes (T2DM). C57Bl/6J mice were randomly assigned for 14 weeks to either a control diet-fed (CTRL) or a high fat diet (HFD)-fed group developing obesity, hyperglycaemia and insulin resistance. The HFD group showed an increased trabecular thickness and a decreased trabecular number compared to CTRL animals. Midshaft tibia intracortical porosity was assessed at two spatial image resolutions. At 2 μm scale, no change was observed in the intracortical structure. At 1 μm scale, a decrease in the cortical vascular porosity of the HFD bone was evidenced. The study of a group of 8 week old animals corresponding to animals at the start of the diet challenge revealed that the decreased vascular porosity was T2DM-dependant and not related to the ageing process. Our results offer an unprecedented ultra-characterization of the T2DM compromised skeletal micro-architecture and highlight an unrevealed T2DM-related decrease in the cortical vascular porosity, potentially affecting the bone health and fragility. Additionally, it provides some insights into the technical challenge facing the assessment of the rodent bone structure using HR-microCT imaging.

Osteocytic signalling pathways as therapeutic targets for bone fragility

Osteocytes are differentiated osteoblasts that become surrounded by matrix during the process of bone formation. Acquisition of the osteocyte phenotype is achieved by profound changes in gene expression that facilitate adaptation to the changing cellular environment and constitute the molecular signature of osteocytes. During osteocytogenesis, the expression of genes that are characteristic of the osteoblast are altered and the expression of genes and/or proteins that impart dendritic cellular morphology, regulate matrix mineralization and control the function of cells at the bone surface are ordely modulated. The discovery of mutations in human osteocytic genes has contributed, in a large part, to our understanding of the role of osteocytes in bone homeostasis. Osteocytes are targets of the mechanical force imposed on the skeleton and have a critical role in integrating mechanosensory pathways with the action of hormones, which thereby leads to the orchestrated response of bone to environmental cues. Current, therapeutic approaches harness this accumulating knowledge by targeting osteocytic signalling pathways and messengers to improve skeletal health.