Osteoclasts recycle via osteomorphs during RANKL-stimulated bone resorption.

Osteoclasts are large multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage-derived precursors that are thought to undergo apoptosis once resorption is complete. Here, by intravital imaging, we reveal that RANKL-stimulated osteoclasts have an alternative cell fate in which they fission into daughter cells called osteomorphs. Inhibiting RANKL blocked this cellular recycling and resulted in osteomorph accumulation. Single-cell RNA sequencing showed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and express a number of non-canonical osteoclast genes that are associated with structural and functional bone phenotypes when deleted in mice. Furthermore, genetic variation in human orthologs of osteomorph genes causes monogenic skeletal disorders and associates with bone mineral density, a polygenetic skeletal trait. Thus, osteoclasts recycle via osteomorphs, a cell type involved in the regulation of bone resorption that may be targeted for the treatment of skeletal diseases.

A novel sequential treatment approach between denosumab and romosozumab in patients with severe osteoporosis.

In severe osteoporosis, the optimal approach for sequential treatment between denosumab and romosozumab is unclear. We utilised a novel overlapping strategy in three patients with very-high fracture risk despite long-term denosumab which led to greater bone density improvements than previously reported with standard approaches. Larger confirmatory prospective studies are needed. PURPOSE/INTRODUCTION: In patients with severe osteoporosis, the optimal approach for sequential treatment between denosumab and romosozumab has not been established. The ideal strategy would maximise gains in bone mineral density (BMD) with romosozumab and effectively mitigate the risk of rebound increased bone turnover when sequencing from denosumab. Limited studies exploring the sequence from denosumab to romosozumab report only modest-to-no improvement in BMD and inadequate suppression of rebound bone turnover. METHODS: We describe three patients with severe osteoporosis and multiple fragility fractures despite long-term denosumab. A novel overlapping sequential treatment approach was utilised to maximise therapeutic benefit given these patients had a very high fracture risk. Romosozumab was commenced 3 months after the last denosumab dose. Instead of waiting until completion of romosozumab, denosumab was recommenced 6 months after commencing romosozumab in response to rising bone turnover markers. RESULTS: Patients experienced a ~ 5-22% increase in lumbar spine BMD, and one patient had an 8% increase in total hip BMD after 12 months romosozumab. Serum bone turnover markers demonstrated an anabolic effect of romosozumab occurred despite overlapping treatment with denosumab. Recommencement of denosumab suppressed an increase in bone resorption in all cases. No new vertebral fractures occurred during this treatment. CONCLUSIONS: A novel overlapping sequential treatment approach between denosumab and romosozumab produced greater improvements in lumbar spine and hip BMD than previously reported with standard approaches. Larger prospective controlled studies are needed to confirm these findings and establish the optimal use of romosozumab in patients pre-treated with denosumab to maximise BMD gains and minimise fracture risk.

Real-Time Live Imaging of Osteoclast Activation via Cathepsin K Activity in Bone Diseases.

Intravital fluorescence imaging of functional osteoclasts within their intact disease context provides valuable insights into the intricate biology at the microscopic level, facilitating the development of therapeutic approaches for osteoclast-associated bone diseases. However, there is a lack of studies investigating osteoclast activity within deep-seated bone lesions using appropriate fluorescent probes, despite the advantages offered by the multi-photon excitation system in enhancing deep tissue imaging resolution. In this study, we report on the intravital tracking of osteoclast activity in three distinct murine bone disease models. We utilized a cathepsin K (CatK)-responsive two-photon fluorogenic probe (CatKP1), which exhibited a notable fluorescence turn-on response in the presence of active CatK. By utilizing CatKP1, we successfully monitored a significant increase in osteoclast activity in hindlimb long bones and its attenuation through pharmacological intervention without sacrificing mice. Thus, our findings highlight the efficacy of CatKP1 as a valuable tool for unraveling pathological osteoclast behavior and exploring novel therapeutic strategies.

Osteoclast Recycling and the Rebound Phenomenon Following Denosumab Discontinuation.

PURPOSE OF REVIEW: Inhibition of receptor activator of nuclear factor kappa-B ligand (RANKL) with denosumab is an effective treatment in a number of conditions including osteoporosis where suppression of bone resorption is desired. However, denosumab discontinuation is associated with rebound increase in bone resorption and subsequent loss in bone mass and a rapid return to baseline fracture risk. We review recent data on the rebound increase in bone resorption following denosumab discontinuation and the potential mechanisms behind this phenomenon. RECENT FINDINGS: Osteoclasts have been considered to be highly specialised cells that undergo apoptosis after fulfilling their function of bone resorption. However, recent studies suggest that osteoclasts are longer lived cells which migrate through vasculature and are capable of undergoing fission into a novel cell type (the osteomorph) and re-fusion in a process termed osteoclast recycling. The life cycle of the osteoclast is more complex than previously appreciated. Osteoclast recycling provides a novel mechanistic framework to examine changes in osteoclast biology in response to treatment of bone diseases and provides an exciting new avenue towards personalised medicine.

A niche-dependent myeloid transcriptome signature defines dormant myeloma cells.

The era of targeted therapies has seen significant improvements in depth of response, progression-free survival, and overall survival for patients with multiple myeloma. Despite these improvements in clinical outcome, patients inevitably relapse and require further treatment. Drug-resistant dormant myeloma cells that reside in specific niches within the skeleton are considered a basis of disease relapse but remain elusive and difficult to study. Here, we developed a method to sequence the transcriptome of individual dormant myeloma cells from the bones of tumor-bearing mice. Our analyses show that dormant myeloma cells express a distinct transcriptome signature enriched for immune genes and, unexpectedly, genes associated with myeloid cell differentiation. These genes were switched on by coculture with osteoblastic cells. Targeting AXL, a gene highly expressed by dormant cells, using small-molecule inhibitors released cells from dormancy and promoted their proliferation. Analysis of the expression of AXL and coregulated genes in human cohorts showed that healthy human controls and patients with monoclonal gammopathy of uncertain significance expressed higher levels of the dormancy signature genes than patients with multiple myeloma. Furthermore, in patients with multiple myeloma, the expression of this myeloid transcriptome signature translated into a twofold increase in overall survival, indicating that this dormancy signature may be a marker of disease progression. Thus, engagement of myeloma cells with the osteoblastic niche induces expression of a suite of myeloid genes that predicts disease progression and that comprises potential drug targets to eradicate dormant myeloma cells.

Pretreatment with Pamidronate Decreases Bone Formation but Increases Callus Bone Volume in a Rat Closed Fracture Model.

Clinical concerns have been raised over prior exposure to bisphosphonates impairing fracture healing. To model this, groups of male Wistar rats were assigned to saline control or treatment groups receiving 0.15 mg/kg (low dose), 0.5 mg/kg (medium dose), and 5 mg/kg (high dose) Pamidronate (PAM) twice weekly for 4 weeks. At this point, closed fractures were made using an Einhorn apparatus, and bisphosphonate dosing was continued until the experimental endpoint. Specimens were analyzed at 2 and 6 weeks (N = 8 per group per time point). Twice weekly PAM dosing was found to have no effect on early soft callus remodeling at 2 weeks post fracture. At this time point, the highest dose PAM group gave significant increases in bone volume (+ 10%, p < 0.05), bone mineral content (+ 30%, p < 0.01), and bone mineral density (+ 10%, p < 0.01). This PAM dosing regimen showed more substantive effects on hard callus at 6 weeks post fracture, with PAM treatment groups showing + 46-79% increased bone volume. Dynamic bone labeling showed reduced calcein signal in the PAM-treated calluses (38-63%, p < 0.01) and reduced MAR (32-49%, p < 0.01), suggesting a compensatory reduction in bone anabolism. These data support the concept that bisphosphonates lead to profound decreases in bone turnover in fracture repair, however, this does not affect soft callus remodeling.

Inhibiting the osteocyte-specific protein sclerostin increases bone mass and fracture resistance in multiple myeloma.

Multiple myeloma (MM) is a plasma cell cancer that develops in the skeleton causing profound bone destruction and fractures. The bone disease is mediated by increased osteoclastic bone resorption and suppressed bone formation. Bisphosphonates used for treatment inhibit bone resorption and prevent bone loss but fail to influence bone formation and do not replace lost bone, so patients continue to fracture. Stimulating bone formation to increase bone mass and fracture resistance is a priority; however, targeting tumor-derived modulators of bone formation has had limited success. Sclerostin is an osteocyte-specific Wnt antagonist that inhibits bone formation. We hypothesized that inhibiting sclerostin would prevent development of bone disease and increase resistance to fracture in MM. Sclerostin was expressed in osteocytes from bones from naive and myeloma-bearing mice. In contrast, sclerostin was not expressed by plasma cells from 630 patients with myeloma or 54 myeloma cell lines. Mice injected with 5TGM1-eGFP, 5T2MM, or MM1.S myeloma cells demonstrated significant bone loss, which was associated with a decrease in fracture resistance in the vertebrae. Treatment with anti-sclerostin antibody increased osteoblast numbers and bone formation rate but did not inhibit bone resorption or reduce tumor burden. Treatment with anti-sclerostin antibody prevented myeloma-induced bone loss, reduced osteolytic bone lesions, and increased fracture resistance. Treatment with anti-sclerostin antibody and zoledronic acid combined increased bone mass and fracture resistance when compared with treatment with zoledronic acid alone. This study defines a therapeutic strategy superior to the current standard of care that will reduce fractures for patients with MM.

Dkk1 KO Mice Treated with Sclerostin Antibody Have Additional Increases in Bone Volume.

Dickkopf-1 (DKK1) and sclerostin are antagonists of the Wnt/ß-catenin pathway and decreased expression of either results in increased bone formation and mass. As both affect the same signaling pathway, we aimed to elucidate the redundancy and/or compensation of sclerostin and DKK1. Weekly sclerostin antibody (Scl-Ab) was used to treat 9-week-old female Dkk1 KO (Dkk1-/-:Wnt3+/-) mice and compared to Scl-Ab-treated wild-type mice as well as vehicle-treated Dkk1 KO and wild-type animals. While Wnt3 heterozygote (Wnt3+/-) mice show no bone phenotype, Scl-Ab and vehicle-treated control groups of this genotype were included. Specimens were harvested after 3 weeks for microCT, bone histomorphometry, anti-sclerostin immunohistochemistry, and biomechanical testing. Scl-Ab enhanced bone anabolism in all treatment groups, but with synergistic enhancement seen in the cancellous compartment of Dkk1 KO mice (bone volume + 55% Dkk1 KO p < 0.01; + 22% wild type p < 0.05). Scl-Ab treatment produced less marked increases in cortical bone of the tibiae, with anabolic effects similar across genotypes. Mechanical testing confirmed that Scl-Ab improved strength across all genotypes; however, no enhancement was seen within Dkk1 KO mice. Dynamic bone labeling showed that Scl-Ab treatment was associated with increased bone formation, regardless of genotype. Immunohistochemical staining for sclerostin protein indicated no differences in the Dkk1 KO mice, indicating that the increased Wnt signaling associated with DKK1 deficiency was not compensated by upregulation of sclerostin protein. These data suggest complex interactions between Wnt signaling factors in bone, but critically illustrate synergy between DKK1 deficiency and Scl-Ab treatment. These data support the application of dual-targeted therapeutics in the modulation of bone anabolism.

Homozygous Dkk1 Knockout Mice Exhibit High Bone Mass Phenotype Due to Increased Bone Formation.

Wnt antagonist Dkk1 is a negative regulator of bone formation and Dkk1 +/- heterozygous mice display a high bone mass phenotype. Complete loss of Dkk1 function disrupts embryonic head development. Homozygous Dkk1 -/- mice that were heterozygous for Wnt3 loss of function mutation (termed Dkk1 KO) are viable and allowed studying the effects of homozygous inactivation of Dkk1 on bone formation. Dkk1 KO mice showed a high bone mass phenotype exceeding that of heterozygous mice as well as a high incidence of polydactyly and kinky tails. Whole body bone density was increased in the Dkk1 KO mice as shown by longitudinal dual-energy X-ray absorptiometry. MicroCT analysis of the distal femur revealed up to 3-fold increases in trabecular bone volume and up to 2-fold increases in the vertebrae, compared to wild type controls. Cortical bone was increased in both the tibiae and vertebrae, which correlated with increased strength in tibial 4-point bending and vertebral compression tests. Dynamic histomorphometry identified increased bone formation as the mechanism underlying the high bone mass phenotype in Dkk1 KO mice, with no changes in bone resorption. Mice featuring only Wnt3 heterozygosity showed no evident bone phenotype. Our findings highlight a critical role for Dkk1 in the regulation of bone formation and a gene dose-dependent response to loss of DKK1 function. Targeting Dkk1 to enhance bone formation offers therapeutic potential for osteoporosis.

Adipose, Bone, and Myeloma: Contributions from the Microenvironment.

Researchers globally are working towards finding a cure for multiple myeloma (MM), a destructive blood cancer diagnosed yearly in ~750,000 people worldwide (Podar et al. in Expert Opin Emerg Drugs 14:99-127, 2009). Although MM targets multiple organ systems, it is the devastating skeletal destruction experienced by over 90 % of patients that often most severely impacts patient morbidity, pain, and quality of life. Preventing bone disease is therefore a priority in MM treatment, and understanding how and why myeloma cells target the bone marrow (BM) is fundamental to this process. This review focuses on a key area of MM research: the contributions of the bone microenvironment to disease origins, progression, and drug resistance. We describe some of the key cell types in the BM niche: osteoclasts, osteoblasts, osteocytes, adipocytes, and mesenchymal stem cells. We then focus on how these key cellular players are, or could be, regulating a range of disease-related processes spanning MM growth, drug resistance, and bone disease (including osteolysis, fracture, and hypercalcemia). We summarize the literature regarding MM-bone cell and MM-adipocyte relationships and subsequent phenotypic changes or adaptations in MM cells, with the aim of providing a deeper understanding of how myeloma cells grow in the skeleton to cause bone destruction. We identify avenues and therapies that intervene in these networks to stop tumor growth and/or induce bone regeneration. Overall, we aim to illustrate how novel therapeutic target molecules, proteins, and cellular mediators may offer new avenues to attack this disease while reviewing currently utilized therapies.

Sclerostin Antibody Increases Callus Size and Strength but does not Improve Fracture Union in a Challenged Open Rat Fracture Model.

Open fractures remain a challenge in orthopedics. Current strategies to intervene are often inadequate, particularly in severe fractures or when treatment is delayed. Sclerostin is a negative regulator of bone growth and sclerostin-neutralizing antibodies (Scl-Ab) can increase bone mass and strength. The application of these antibodies to improve orthopedic repair has shown varied results, and may be dependent on the location and severity of the bony injury. We examined Scl-Ab treatment within an established rat osteotomy model with periosteal stripping analogous to open fracture repair. In one study, Scl-Ab was given 25 mg/kg bi-weekly, either from the time of fracture or from 3 weeks post-fracture up to an end-point of 12 weeks. A second study treated only delayed union open fractures that did not show radiographic union by week 6 post-fracture. Outcome measures included radiographic union, microCT analysis of bone volume and architecture, and histology. In the first study, Scl-Ab given from either 0 or 3 weeks significantly improved callus bone volume (+52%, p < 0.05 and +58%, p < 0.01) at 12 weeks, as well as strength (+48%, p < 0.05 and +70%, p < 0.05). Despite these improvements, union rate was not changed. In the second study treating only established delayed fractures, bony callus volume was similarly increased by Scl-Ab treatment; however, this did not translate to increased biomechanical strength or union improvement. Sclerostin antibody treatment has limited effects on the healing of challenging open fractures with periosteal stripping, but shows the greatest benefits on callus size and strength with earlier intervention.

Sclerostin: an Emerging Target for the Treatment of Cancer-Induced Bone Disease.

PURPOSE OF REVIEW: This review provides a summary of the current knowledge on Sost/sclerostin in cancers targeting the bone, discusses novel observations regarding its potential as a therapeutic approach to treat cancer-induced bone loss, and proposes future research needed to fully understand the potential of therapeutic approaches that modulate sclerostin function. RECENT FINDINGS: Accumulating evidence shows that sclerostin expression is dysregulated in a number of cancers that target the bone. Further, new findings demonstrate that pharmacological inhibition of sclerostin in preclinical models of multiple myeloma results in a robust prevention of bone loss and preservation of bone strength, without apparent effects on tumor growth. These data raise the possibility of targeting sclerostin for the treatment of cancer patients with bone metastasis. Sclerostin is emerging as a valuable target to prevent the bone destruction that accompanies the growth of cancer cells in the bone. Further studies will focus on combining anti-sclerostin therapy with tumor-targeted agents to achieve both beneficial skeletal outcomes and inhibition of tumor progression.

Signaling between tumor cells and the host bone marrow microenvironment.

Tumor cells with high skeletal homing affinity express numerous cell surface receptors that bind ligands produced in bone. Upon arrival, these cells survive in the host environment, encompassed in close proximity to bone marrow cells. Interactions between tumor cells and cells of the host microenvironment are essential to not only tumor cell survival but also their activation and proliferation into environment-modifying tumors. Through the production of RANKL, PTHrP, cytokines, and integrins, activated tumor cells stimulate osteoclastogenesis, enhance bone resorption, and subsequently release matrix-bound proteins that further promote tumor growth and bone resorption. In addition, alterations in the TGF-ß/BMP and Wnt signaling pathways via tumor cell growth can either stimulate or suppress osteoblastic bone formation and function, leading to sclerotic or lytic bone disease, respectively. Hence, the presence of tumor cells in bone dysregulates bone remodeling, dramatically impairing skeletal integrity. Furthermore, through complex mechanisms, cells of the immune system interact with tumor cells to further impact bone remodeling. Lastly, with alterations in bone cell activity, the environment is permissive to promoting tumor growth further, suggesting an interdependence between tumor cells and bone cells in metastatic bone disease and multiple myeloma.

New Horizons: Translational Aspects of Osteomorphs.

Osteomorphs are a newly described osteoclast lineage cell in mice, which are suggested to play a significant role in the maintenance of bone resorption. Preclinical investigations revealed that osteomorphs are generated through the fission of multinucleated bone-resorbing osteoclasts and can also re-fuse with existing osteoclasts. Modifications to RANKL signaling have been shown to alter cycles of fission and re-fusion of osteomorphs in mice. These novel findings were also shown to contribute to the rebound phenomenon after cessation of anti-RANKL therapy in mice. Moreover, the absence of osteomorph-specific genes in mice exhibits bone structural and quality phenotypes. Given these insights, it could be speculated that osteomorphs play a significant role in bone homeostasis, bone metabolic diseases, and response to therapeutics. In this review, we discuss these potential translational roles for osteomorphs. Importantly, we highlight the need for future preclinical and clinical studies to verify the presence of osteomorphs in humans and explore further the translational implications of this discovery.

An in silico approach to elucidate the pathways leading to primary osteoporosis: age-related vs. postmenopausal.

Numerical models of bone remodelling have traditionally been used to perform in silico tests of bone loss in postmenopausal women and also to simulate the response to different drug treatments. These models simulate the menopausal oestrogen decline by altering certain signalling pathways. However, they do not consider the simultaneous effect that ageing can have on cell function and bone remodelling, and thus on bone loss. Considering ageing and oestrogen decline together is important for designing osteoporosis treatments that can selectively counteract one or the other disease mechanism. A previously developed bone cell population model was adapted to consider the effect of ageing through: (1) the decrease of TGF- ß contained in the bone matrix and (2) an increased production of sclerostin by non-skeletal cells. Oestrogen deficiency is simulated in three different ways: (a) an increase in RANKL expression, (b) a decrease in OPG production, and (c) an increase in the responsiveness of osteoclasts to RANKL. The effect of ageing was validated using the cross-sectional study of (Riggs et al. in J Bone Miner Res 19: 1945-1954, 2004) on BMD of trabecular bone of the vertebral body of men. The joint effect of ageing and oestrogen deficiency was validated using these same clinical results but in women. In ageing, the effect of the increasing production of sclerostin is more important than the decrease of TGF- ß , while the three mechanisms used to simulate the effect of oestrogen deficiency produce almost identical responses. The results show that an early menopause leads to a lower average density in the fifth decade, but after the sixth decade the average density is independent of the age at menopause. Treatment of osteoporosis with denosumab was also simulated to conclude that the drug is not very effective if started before 10 years after menopause or before age 60.

Cardiac Mass in a 78-Year-Old Patient With a History of Cancer: Diagnostic and Treatment Challenges.

Primary cardiac angiosarcoma is a rare, aggressive malignancy that commonly metastasizes to various organs. The presenting symptoms are typically nonspecific, so a comprehensive examination is required to confirm the diagnosis promptly. This case report describes the presentation of an older patient with a history of neoplasms. Echocardiography and biopsy were performed, but despite surgical intervention to resect a large right atrial mass, the patient died. A final diagnosis of primary angiosarcoma was made based on the resected specimen.

The performance of artificial intelligence chatbot large language models to address skeletal biology and bone health queries.

Artificial intelligence (AI) chatbots utilizing large language models (LLMs) have recently garnered significant interest due to their ability to generate humanlike responses to user inquiries in an interactive dialog format. While these models are being increasingly utilized to obtain medical information by patients, scientific and medical providers, and trainees to address biomedical questions, their performance may vary from field to field. The opportunities and risks these chatbots pose to the widespread understanding of skeletal health and science are unknown. Here we assess the performance of 3 high-profile LLM chatbots, Chat Generative Pre-Trained Transformer (ChatGPT) 4.0, BingAI, and Bard, to address 30 questions in 3 categories: basic and translational skeletal biology, clinical practitioner management of skeletal disorders, and patient queries to assess the accuracy and quality of the responses. Thirty questions in each of these categories were posed, and responses were independently graded for their degree of accuracy by four reviewers. While each of the chatbots was often able to provide relevant information about skeletal disorders, the quality and relevance of these responses varied widely, and ChatGPT 4.0 had the highest overall median score in each of the categories. Each of these chatbots displayed distinct limitations that included inconsistent, incomplete, or irrelevant responses, inappropriate utilization of lay sources in a professional context, a failure to take patient demographics or clinical context into account when providing recommendations, and an inability to consistently identify areas of uncertainty in the relevant literature. Careful consideration of both the opportunities and risks of current AI chatbots is needed to formulate guidelines for best practices for their use as source of information about skeletal health and biology.

Artificial intelligence chatbots are increasingly used as a source of information in health care and research settings due to their accessibility and ability to summarize complex topics using conversational language. However, it is still unclear whether they can provide accurate information for questions related to the medicine and biology of the skeleton. Here, we tested the performance of three prominent chatbots­ChatGPT, Bard, and BingAI­by tasking them with a series of prompts based on well-established skeletal biology concepts, realistic physician­patient scenarios, and potential patient questions. Despite their similarities in function, differences in the accuracy of responses were observed across the three different chatbot services. While in some contexts, chatbots performed well, and in other cases, strong limitations were observed, including inconsistent consideration of clinical context and patient demographics, occasionally providing incorrect or out-of-date information, and citation of inappropriate sources. With careful consideration of their current weaknesses, artificial intelligence chatbots offer the potential to transform education on skeletal health and science.

Temporal patterns of osteoclast formation and activity following withdrawal of RANKL inhibition.

Rebound bone loss following denosumab discontinuation is an important clinical challenge. Current treatment strategies to prevent this fail to suppress the rise and overshoot in osteoclast-mediated bone resorption. In this study, we use a murine model of denosumab treatment and discontinuation to show the temporal changes in osteoclast formation and activity during RANKL inhibition and withdrawal. We show that the cellular processes that drive the formation of osteoclasts and subsequent bone resorption following withdrawal of RANKL inhibition precede the rebound bone loss. Furthermore, a rise in serum TRAP and RANKL levels is detected before markers of bone turnover used in current clinical practice. These mechanistic advances may provide insight into a more defined window of opportunity to intervene with sequential therapy following denosumab discontinuation.

Stopping denosumab, a medication commonly used to improve bone mass by blocking formation of bone resorbing osteoclasts, leads to a rebound loss in the bone which was gained during treatment. Current strategies to prevent this bone loss fail in most cases as they are unable to prevent the rise and overshoot in bone resorption by osteoclasts. Thie stems from an incomplete understanding of how osteoclasts behave during denosumab treatment and after treatment is discontinued. We use a mouse model of this phenomenon to show how osteoclast formation and activity changes throughout this process. We show that increases in the processes that drive the formation of osteoclasts can be detected in the circulation before bone loss occurs. These findings could therefore provide insight into a targeted 'window of opportunity' to intervene and prevent the rebound bone loss following stopping denosumab in patients.