Distinct modes of inhibition by sclerostin on bone morphogenetic protein and Wnt signaling pathways

Pathophysiology of bone remodelling cycle: Role of immune system and lipids

Osteoporosis is the most common skeletal disease worldwide, characterized by low bone mineral density, resulting in weaker bones, and an increased risk of fragility fractures. The maintenance of bone mass relies on the precise balance between bone synthesis and resorption. The close relationship between the immune and skeletal systems, called "osteoimmunology", was coined to identify these overlapping "scientific worlds", and its function resides in the evaluation of the mutual effects of the skeletal and immune systems at the molecular and cellular levels, in both physiological and pathological states. Lipids play an essential role in skeletal metabolism and bone health. Indeed, bone marrow and its skeletal components demand a dramatic amount of daily energy to control hematopoietic turnover, acquire and maintain bone mass, and actively being involved in whole-body metabolism. Statins, the main therapeutic agents in lowering plasma cholesterol levels, are able to promote osteoblastogenesis and inhibit osteoclastogenesis. This review is meant to provide an updated overview of the pathophysiology of bone remodelling cycle, focusing on the interplay between bone, immune system and lipids. Novel therapeutic strategies for the management of osteoporosis are also discussed.

Mechanical Analysis of Romosozumab's Effects on Bone Strength in a Rat Posterolateral Lumbar Fusion Model

PURPOSE

 This study aimed to evaluate the effects of romosozumab, a humanized monoclonal antibody, on bone healing and mechanical strength in a rat posterolateral lumbar fusion (PLF) model. The primary objective was to determine its potential in promoting bone union and enhancing the structural integrity of graft sites, addressing challenges such as pseudarthrosis and hardware failure in spinal surgeries. These complications are particularly common in osteoporotic patients, where compromised bone quality and reduced healing capacity significantly increase the risk of surgical failure. With an aging global population, osteoporosis-related complications in spinal surgery are expected to rise, necessitating novel interventions to improve outcomes.

MATERIALS AND METHODS

 Twenty male Sprague-Dawley rats were randomized into two groups: romosozumab-treated (R) and control (C). All animals underwent bilateral PLF surgery involving the placement of autogenous bone grafts harvested from the spinous process combined with a demineralized bone matrix between the transverse processes of the lumbar vertebrae. Subcutaneous injections of romosozumab (105 mg/1.17 mL) or saline were administered twice weekly for 10 weeks. Bone healing was assessed through micro-computed tomography (CT) imaging at baseline and 10 weeks post-surgery. Key metrics included the bone fusion rate, fused bone volume, and bone mineral density (BMD). Additionally, the mechanical strength of the fusion mass was evaluated using a three-point bending test to determine the force required to induce rupture.

RESULTS

 The R group exhibited significant improvements across all evaluated parameters compared to the C group. Fused bone volume in the R group was significantly greater at 10 weeks (826.7 ± 27.5 mm³) compared to the C group (652.6 ± 30.7 mm³, p < 0.05), reflecting a higher bone volume growth rate (158.1 ± 12.9% vs. 106.8 ± 10.4%, p < 0.05). BMD at the distal femoral diaphysis was also markedly increased in the R group (830.2 ± 11.1 mgHA/cm³) compared to the C group (725.5 ± 12.1 mgHA/cm³, p < 0.05). Mechanical testing revealed superior compressive strength in the R group, with a rupture force of 312.5 ± 43.2 N versus 209.3 ± 35.4 N in the C group (p < 0.05). These results demonstrate romosozumab's capacity to promote robust bone formation and significantly enhance the mechanical integrity of the fusion mass.

CONCLUSION

Romosozumab treatment significantly improved bone healing, mineral density, and mechanical strength in a rat PLF model, suggesting its potential as a therapeutic option for enhancing spinal surgery outcomes. By promoting rapid bone formation and increasing bone strength, romosozumab addresses critical challenges such as pseudarthrosis and pedicle screw loosening, which frequently compromise surgical success, especially in osteoporotic patients. These findings underscore the therapeutic promise of romosozumab not only in spinal surgery but also as a broader intervention for bone repair and healing. Further research is needed to explore its dose-response relationship, long-term safety, and efficacy in osteoporotic models. Moreover, the use of biochemical markers and microstructural analyses will help elucidate the underlying mechanisms of its action. With its demonstrated ability to enhance both structural and functional bone properties, romosozumab offers a promising avenue for advancing spinal surgery and improving patient outcomes.

Mapping the sclerostin-LRP4 binding interface identifies critical interaction hotspots in loops 1 and 3 of sclerostin

The interaction of sclerostin (Scl) with the low-density lipoprotein receptor-related protein 4 (LRP4) leads to a marked reduction in bone formation by inhibiting the Wnt/β-catenin pathway. To characterize the Scl-LRP4 binding interface, we sorted a combinatorial library of Scl variants and isolated variants with reduced affinity to LRP4. We identified Scl single-mutation variants enriched during the sorting process and verified their reduction in affinity toward LRP4-a reduction that was not a result of changes in the variants' secondary structure or stability. We found that Scl positions K75 (loop 1) and V136 (loop 3) are critical hotspots for binding to LRP4. Our findings establish the foundation for targeting these hotspots for developing novel therapeutic strategies to promote bone formation.

Bone Disease in Primary Hyperparathyroidism-Changes Occurring in Bone Metabolism and New Potential Treatment Strategies

Primary hyperparathyroidism (PHPT) is a common endocrinopathy, predominantly caused by a single parathyroid adenoma that is responsible for the excessive secretion of parathyroid hormone (PTH)-the hallmark of disease. Excess of this hormone causes remarkable changes in bone metabolism, including an increased level of bone remodeling with a predominance of bone resorption. Those changes lead to deterioration of bone structure and density, especially in cortical bone. The main treatment for PHPT is surgical removal of the adenoma, which normalizes PTH levels and terminates the progression of bone disease and leads to its regeneration. However, because not all the patients are suitable candidates for surgery, alternative therapies are needed. Current non-surgical treatments targeting bone disease secondary to PHPT include bisphosphonates and denosumab. Those antiresorptives prevent further bone loss, but they lack the ability to regenerate already degraded bone. There is ongoing research to find targeted drugs capable of halting resorption alongside stimulating bone formation. This review presents the advancements in understanding the molecular mechanisms responsible for bone disease in PHPT and assesses the efficacy of new potential therapeutic approaches (e.g., allosteric inhibitors of the PTH receptor, V-ATPase, or cathepsin inhibitors) aimed at mitigating bone loss and enhancing bone regeneration in affected patients.

An in-silico approach to the potential modulatory effect of taurine on sclerostin (SOST) and its probable role during osteoporosis

The cysteine-knot containing negative regulator of the Wnt (Wingless-related integration site) signaling pathway, sclerostin (SOST) is an emerging therapeutic target for osteoporosis. Its inhibition is responsible for the promotion of osteoblastogenesis. In this study, taurine, an amino sulfonic acid was used to study its mechanism of action for the inhibition of the SOST protein. Molecular docking and dynamic studies were performed as a part of the study whereby, it was observed that taurine binds to a probable allosteric pocket which allows it to modulate the structure of the SOST protein affecting all of the loops - loops 1, loop 2, and loop 3 - as well as the cysteine residues forming the cysteine-knot. The study also identified a set of seven taurine analogues that have better pharmacological activity than their parent compound using screening techniques. The conclusions derived from the study support that taurine has a probable antagonistic effect on the SOST protein directly through the modulation of HNQS motif and loops 2 and 3 and indirectly through its influence on the cysteine residues - 134, 165 and 167 C. Based on the results, it can be assumed that the binding of taurine with SOST protein probably reduces its binding affinity to the LRP6 protein greatly, while also inhibiting the target protein from anchoring to LRP4. Furthermore, it was noted that probable additional binding with any small molecule inhibitor (SMI) at the active site (PNAIG motif), in the presence of an already allosterically bound taurine, of the SOST protein would result in a complete potential antagonism of the target protein. Additionally, the study also uncovers the possible role of the GKWWRPS motif in providing stability to the PNAIG motif for the purpose of binding with LRP6.Communicated by Ramaswamy H. Sarma.

Probing the effects of single point mutations in the GKWWRPS motif on the PNAIG motif within Loop 2 of sclerostin (SOST) using in-silico techniques

Sclerostin (SOST), a Wnt signaling pathway inhibitor, is involved in the pathogenesis of skeletal disorders. This study investigated the impact of the GKWWRPS motif on the PNAIG motif in Loop 2 of SOST, which is accountable for the interactions with the LRP6 protein that triggers the down-regulation of the Wnt signaling pathway. Single amino acid mutations on the GKWWRPS motif, hypothesized to have a probable stabilization effect towards the PNAIG motif, led to a significant reduction in the primary interactions between the SOST and LRP6 proteins. Protein-protein docking and molecular dynamic studies were conducted to investigate the role of the motif. The study found that a solitary mutation in the GKWWRPS motif significantly reduced the primary interactions between SOST and LRP6 proteins, except for probable cold-spot residues. The study's findings establish the GKWWRPS motif as a promising target for therapeutic interventions. Based on the obtained results, it can be inferred that alterations implemented within the GKWWRPS motif could lead to the destabilization of the PNAIG motif, which would directly modulate the interactions between the SOST and LRP6 proteins. The present investigation thus presents novel opportunities in the field of anti-sclerostin interventions.

Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and disease

Wnts are secreted, lipid-modified proteins that bind to different receptors on the cell surface to activate canonical or non-canonical Wnt signaling pathways, which control various biological processes throughout embryonic development and adult life. Aberrant Wnt signaling pathway underlies a wide range of human disease pathogeneses. In this review, we provide an update of Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and diseases. The Wnt proteins, receptors, activators, inhibitors, and the crosstalk of Wnt signaling pathways with other signaling pathways are summarized and discussed. We mainly review Wnt signaling functions in bone formation, homeostasis, and related diseases, and summarize mouse models carrying genetic modifications of Wnt signaling components. Moreover, the therapeutic strategies for treating bone diseases by targeting Wnt signaling, including the extracellular molecules, cytosol components, and nuclear components of Wnt signaling are reviewed. In summary, this paper reviews our current understanding of the mechanisms by which Wnt signaling regulates bone formation, homeostasis, and the efforts targeting Wnt signaling for treating bone diseases. Finally, the paper evaluates the important questions in Wnt signaling to be further explored based on the progress of new biological analytical technologies.

Sclerostin modulates mineralization degree and stiffness profile in the fibrocartilaginous enthesis for mechanical tissue integrity

Fibrocartilaginous entheses consist of tendons, unmineralized and mineralized fibrocartilage, and subchondral bone, each exhibiting varying stiffness. Here we examined the functional role of sclerostin, expressed in mature mineralized fibrochondrocytes. Following rapid mineralization of unmineralized fibrocartilage and concurrent replacement of epiphyseal hyaline cartilage by bone, unmineralized fibrocartilage reexpanded after a decline in alkaline phosphatase activity at the mineralization front. Sclerostin was co-expressed with osteocalcin at the base of mineralized fibrocartilage adjacent to subchondral bone. In Scx-deficient mice with less mechanical loading due to defects of the Achilles tendon, sclerostin+ fibrochondrocyte count significantly decreased in the defective enthesis where chondrocyte maturation was markedly impaired in both fibrocartilage and hyaline cartilage. Loss of the Sost gene, encoding sclerostin, elevated mineral density in mineralized zones of fibrocartilaginous entheses. Atomic force microscopy analysis revealed increased fibrocartilage stiffness. These lines of evidence suggest that sclerostin in mature mineralized fibrochondrocytes acts as a modulator for mechanical tissue integrity of fibrocartilaginous entheses.

Comparison of the Clinical Efficacy of Anabolic Agents and Bisphosphonates in the Patients With Osteoporotic Vertebral Fracture: Systematic Review and Meta-analysis of Randomized Controlled Trials

OBJECTIVE

We investigated the clinical efficacy of anabolic agents compared with bisphosphonates (BPs) for the incidence of new osteoporotic vertebral fracture (OVF) and fracture healing of OVF in the patients with OVF via meta-analyses of randomized controlled trials (RCTs).

METHODS

Electronic databases, including PubMed, Embase, and Cochrane Library were searched for published RCTs till December 2022. The RCTs that recruited participants with osteoporosis at high-/very high-risk of fracture (a history of osteoporotic vertebral or hip fracture) or fresh OVF were included in this study. We assessed the risk of bias on every included RCTs, estimated relative risk (RR) for the incidence of new OVF and fracture healing of OVF, and overall certainty of evidence. Meta-analyses were performed by Cochrane review manager (RevMan) ver. 5.3. Cochrane risk of bias 2.0 and GRADEpro/GDT were applied for evaluating methodological quality and overall certainty of evidence, respectively.

RESULTS

Five hundred eighteen studies were screened, and finally 6 eligible RCTs were included in the analysis. In the patients with prevalent OVF, anabolic agents significantly reduced the incidence of new OVF (teriparatide and romosozumab vs. alendronate and risedronate [RR, 0.57; 95% confidence interval, 0.45-0.71; p < 0.00001; high-certainty of evidence]; teriparatide vs. risedronate [RR, 0.50; 95% confidence interval, 0.37-0.68; p < 0.0001; high-certainty of evidence]). However, there was no evidence of teriparatide compared to alendronate in fracture healing of OVF (RR, 1.23; 95% confidence interval, 0.95-1.60; p = 0.12; low-certainty of evidence).

CONCLUSION

In the patients with prevalent OVF, anabolic agents showed a significant superiority for preventing new OVF than BPs, with no significant evidence for promoting fracture healing of OVF. However, considering small number of RCTs in this study, additional studies with large-scale data are required to obtain more robust evidences.

Effect of gut hormones on bone metabolism and their possible mechanisms in the treatment of osteoporosis

Bone is a highly dynamic organ that changes with the daily circadian rhythm. During the day, bone resorption is suppressed due to eating, while it increases at night. This circadian rhythm of the skeleton is regulated by gut hormones. Until now, gut hormones that have been found to affect skeletal homeostasis include glucagon-like peptide-1 (GLP-1), glucagon-like peptide-2 (GLP-2), glucose-dependent insulinotropic polypeptide (GIP), and peptide YY (PYY), which exerts its effects by binding to its cognate receptors (GLP-1R, GLP-2R, GIPR, and Y1R). Several studies have shown that GLP-1, GLP-2, and GIP all inhibit bone resorption, while GIP also promotes bone formation. Notably, PYY has a strong bone resorption-promoting effect. In addition, gut microbiota (GM) plays an important role in maintaining bone homeostasis. This review outlines the roles of GLP-1, GLP-2, GIP, and PYY in bone metabolism and discusses the roles of gut hormones and the GM in regulating bone homeostasis and their potential mechanisms.

Beyond boundaries: unraveling innovative approaches to combat bone-metastatic cancers

Evidence demonstrated that bones, liver, and lungs are the most common metastasis sites in some human malignancies, especially in prostate and breast cancers. Bone is the third most frequent target for spreading tumor cells among these organs and tissues. Patients with bone-metastatic cancers face a grim prognosis characterized by short median survival time. Current treatments have proven insufficient, as they can only inhibit metastasis or tumor progression within the bone tissues rather than providing a curative solution. Gaining a more profound comprehension of the interplay between tumor cells and the bone microenvironment (BME) is of utmost importance in tackling this issue. This knowledge will pave the way for developing innovative diagnostic and therapeutic approaches. This review summarizes the mechanisms underlying bone metastasis and discusses the clinical aspects of this pathologic condition. Additionally, it highlights emerging therapeutic interventions aimed at enhancing the quality of life for patients affected by bone-metastatic cancers. By synthesizing current research, this review seeks to shed light on the complexities of bone metastasis and offer insights for future advancements in patient care.

The importance of BMPs and TGF-βs for endochondral bone repair - A longitudinal study in hip arthroplasty patients

Introduction

Osseointegration of hip implants, although a decade-long process, shows striking similarities with the four major phases of endochondral bone repair. In the current study we investigated the spatiotemporal involvement of bone morphogenic proteins (BMPs) and transforming growth factor betas (TGF-βs) throughout the process of bone repair leading to successfully osseointegrated hip implants.

Materials and methods

Twenty-four patients that had undergone primary total hip arthroplasty (THA) due to one-sided osteoarthritis (OA) were investigated during a period of 18 years (Y) with repeated measurements of plasma biomarkers as well as clinical and radiological variables. All implants were clinically and radiographically well anchored throughout the follow-up. Eighty-one healthy donors divided in three gender- and age-matched groups and twenty OA patients awaiting THA, served as controls. Plasma was analyzed for BMP-1, -2, -3, -4, -6, -7 -9 and TGF-β1, -β2, -β3 by use of a high-sensitivity and wide dynamic range electrochemiluminescence technique allowing for detection of minor changes.

Results

Spatiotemporal changes during the follow-up are presented in the context of the four phases of endochondral bone repair shown in earlier studies and transposed to the current study based on similarities in biomarker responses. Phase 1: Primary proinflammatory phase lasting from surgery until day 7, Phase 2: Chondrogenic phase from day 7 until 18 months postsurgery, Phase 3: Secondary proinflammatory and cartilage remodeling phase lasting from 18 months until 7Y, Phase 4: coupled bone remodeling from 7Y until 18Y postsurgery. BMP-1 increased sharply shortly after surgery and remained significantly above healthy during the chondrocyte recruitment, proliferation, and hypertrophy phases with a subsequent return to control level at 5Y postsurgery. BMP-2 was above healthy controls before surgery and 1 day after surgery before decreasing to control level and remaining there throughout the follow-up. BMP-3 was at control level from presurgery until 6M after surgery when it increased to a peak at 2Y during the cartilage hypertrophy phase followed by a gradual decrease to control level at 10Y during the phase of bone formation. In the following, BMP-3 decreased below controls to a nadir 15Y postsurgery during coupled bone remodeling. BMP-4 was at control level from presurgery until 10Y postsurgery when it increased to a sharp peak at 15Y after surgery followed by a return to the level of healthy at 18Y. BMP-6 did not differ from healthy during the follow-up. BMP-7 was at control level from presurgery until 1Y postsurgery before gradually increasing to a peak at 10Y during the early phase of osteogenesis with a gradual return to control level at 18Y during the phase of coupled bone remodeling. BMP-9 was above OA before surgery followed by a decrease to basal level on day 1 after surgery and a renewed increase to a plateau above controls lasting from 6 W until returning to the level of healthy at 18Y postsurgery, i.e., throughout the phases of cartilage formation, cartilage hypertrophy and remodeling, bone formation and coupled bone remodeling. TGF-β1 was above controls presurgery before decreasing to baseline shortly after surgery followed by a renewed increase at 6 M to a peak at 2Y during cartilage hypertrophy/remodeling followed by a gradual return to baseline at 10Y during early osteoblastogenesis. TGF-β2 was at control level from presurgery until the phase of cartilage remodeling at 5Y when it increased sharply to a peak at 7Y with a gradual return to baseline at 18Y postsurgery. TGF-β3 remained at control level throughout the study.

Conclusion

This study shows that the involvement of BMPs and TGF-βs in endochondral bone repair is a process of stepwise recruitment of individual biomarkers characterized by distinct, yet overlaping, spatiotemporal patterns that extend from the early phase of pre-chondrocyte recruitment until the late phase of coupled bone remodeling.

Could hypoxia rehabilitate the osteochondral diseased interface? Lessons from the interplay of hypoxia and purinergic signals elsewhere

The osteochondral unit comprises the articular cartilage (90%), subchondral bone (5%) and calcified cartilage (5%). All cells present at the osteochondral unit that is ultimately responsible for matrix production and osteochondral homeostasis, such as chondrocytes, osteoblasts, osteoclasts and osteocytes, can release adenine and/or uracil nucleotides to the local microenvironment. Nucleotides are released by these cells either constitutively or upon plasma membrane damage, mechanical stress or hypoxia conditions. Once in the extracellular space, endogenously released nucleotides can activate membrane-bound purinoceptors. Activation of these receptors is fine-tuning regulated by nucleotides' breakdown by enzymes of the ecto-nucleotidase cascade. Depending on the pathophysiological conditions, both the avascular cartilage and the subchondral bone subsist to significant changes in oxygen tension, which has a tremendous impact on tissue homeostasis. Cell stress due to hypoxic conditions directly influences the expression and activity of several purinergic signalling players, namely nucleotide release channels (e.g. Cx43), NTPDase enzymes and purinoceptors. This review gathers experimental evidence concerning the interplay between hypoxia and the purinergic signalling cascade contributing to osteochondral unit homeostasis. Reporting deviations to this relationship resulting from pathological alterations of articular joints may ultimately unravel novel therapeutic targets for osteochondral rehabilitation. At this point, one can only hypothesize how hypoxia mimetic conditions can be beneficial to the ex vivo expansion and differentiation of osteo- and chondro-progenitors for auto-transplantation and tissue regenerative purposes.

Assessment of sclerostin levels in the gingival crevicular fluid of patients with periodontitis: A clinico-biochemical crosssectional study

Background

Sclerostin, a glycoprotein, plays a key role in regulating bone mass. In this study, sclerostin levels in the gingival crevicular fluid (GCF) were assessed in patients with Stage III Grade C generalized periodontitis (SIII-GC) and Stage III Grade B generalized periodontitis (SIII-GB).

Methods

This cross-sectional study included 30 participants divided equally into three groups: group I (gingival health), group II (SIII-GC), and group III (SIII-GB). Clinical periodontal parameters like plaque index (PI), gingival bleeding index (GBI), probing pocket depth (PD), and clinical attachment level (CAL) were recorded. A sandwich ELISA was used to determine the sclerostin levels in GCF samples. One-way ANOVA and post hoc Tukey tests were used to analyze the clinical parameters and GCF sclerostin levels. The association between GCF sclerostin levels and periodontal parameters was assessed using Pearson's correlation coefficient (r).

Results

Patients in groups II and III had much higher sclerostin levels in their GCF than in group I (P≤0.05). In contrast, no significant difference in sclerostin levels was observed between the two diseased conditions (P=0.841). Concerning periodontal parameters, a statistically significant difference was observed between the three groups. There was a positive correlation between the periodontal clinical parameters and the expression levels of sclerostin in GCF (P≤0.05).

Conclusion

Increased expression of sclerostin in GCF in patients with periodontitis indicated that it could be considered a reliable biomarker of periodontal disease activity.

Physiopathology of Osteoporosis: Nursing Involvement and Management

Osteoporosis is a major public health problem today. We are facing an aging society where the average life expectancy continues to increase. Osteoporosis affects more than 30% of postmenopausal women due to hormonal changes that occur during this time. Postmenopausal osteoporosis is therefore of particular concern. The aim of this review is to identify the etiology, pathophysiology, diagnosis and treatment of this disease and lay the foundation for the role nurses should play in preventing postmenopausal osteoporosis. Several risk factors are associated with osteoporosis. In addition to age and sex, genetics, ethnicity, diet, or the presence of other disorders determine the development of this disease. The key factors include exercise, a balanced diet, and high levels of vitamin D. This is primarily from a solar source, and infancy is the time when future bone formation is greatest. There are now medications that can complement these preventive measures. The work of nursing staff is not only prevention, but also early detection and early treatment. In addition, imparting information and knowledge about the disease to the population is key to preventing an osteoporosis epidemic. In this study, a detailed description is provided of the biological and physiological disease, the preventive measures currently being researched, the information currently available to the population, and how health professionals address osteoporosis from a preventive perspective.

B4GALNT3 regulates glycosylation of sclerostin and bone mass

BACKGROUND

Global sclerostin inhibition reduces fracture risk efficiently but has been associated with cardiovascular side effects. The strongest genetic signal for circulating sclerostin is in the B4GALNT3 gene region, but the causal gene is unknown. B4GALNT3 expresses the enzyme beta-1,4-N-acetylgalactosaminyltransferase 3 that transfers N-acetylgalactosamine onto N-acetylglucosaminebeta-benzyl on protein epitopes (LDN-glycosylation).

METHODS

To determine if B4GALNT3 is the causal gene, B4galnt3^-/- mice were developed and serum levels of total sclerostin and LDN-glycosylated sclerostin were analysed and mechanistic studies were performed in osteoblast-like cells. Mendelian randomization was used to determine causal associations.

FINDINGS

B4galnt3^-/- mice had higher circulating sclerostin levels, establishing B4GALNT3 as a causal gene for circulating sclerostin levels, and lower bone mass. However, serum levels of LDN-glycosylated sclerostin were lower in B4galnt3^-/- mice. B4galnt3 and Sost were co-expressed in osteoblast-lineage cells. Overexpression of B4GALNT3 increased while silencing of B4GALNT3 decreased the levels of LDN-glycosylated sclerostin in osteoblast-like cells. Mendelian randomization demonstrated that higher circulating sclerostin levels, genetically predicted by variants in the B4GALNT3 gene, were causally associated with lower BMD and higher risk of fractures but not with higher risk of myocardial infarction or stroke. Glucocorticoid treatment reduced B4galnt3 expression in bone and increased circulating sclerostin levels and this may contribute to the observed glucocorticoid-induced bone loss.

INTERPRETATION

B4GALNT3 is a key factor for bone physiology via regulation of LDN-glycosylation of sclerostin. We propose that B4GALNT3-mediated LDN-glycosylation of sclerostin may be a bone-specific osteoporosis target, separating the anti-fracture effect of global sclerostin inhibition, from indicated cardiovascular side effects.

FUNDING

Found in acknowledgements.

Characterization of sclerostin's response within white adipose tissue to an obesogenic diet at rest and in response to acute exercise in male mice

Introduction: It is well established that sclerostin antagonizes the anabolic Wnt signalling pathway in bone, however, its physiological role in other tissues remains less clear. This study examined the effect of a high-fat diet (HFD) on sclerostin content and downstream markers of the Wnt signaling pathway (GSK3β and β-catenin) within subcutaneous inguinal white adipose tissue (iWAT), and visceral epididymal WAT (eWAT) depots at rest and in response to acute aerobic exercise. Methods: Male C57BL/6 mice (n = 40, 18 weeks of age) underwent 10 weeks of either a low-fat diet (LFD) or HFD. Within each diet group, mice were assigned to either remain sedentary (SED) or perform 2 h of endurance treadmill exercise at 15 m min^-1 with 5° incline (EX), creating four groups: LFD + SED (N = 10), LFD + EX (N = 10), HFD + SED (N = 10), and HFD + EX (N = 10). Serum and WAT depots were collected 2 h post-exercise. Results: Serum sclerostin showed a diet-by-exercise interaction, reflecting HFD + EX mice having higher concentration than HFD + SED (+31%, p = 0.03), and LFD mice being unresponsive to exercise. iWAT sclerostin content decreased post-exercise in both 28 kDa (-31%, p = 0.04) and 30 kDa bands (-36%, main effect for exercise, p = 0.02). iWAT β-catenin (+44%, p = 0.03) and GSK3β content were higher in HFD mice compared to LFD (+128%, main effect for diet, p = 0.005). Monomeric sclerostin content was abolished in eWAT of HFD mice (-96%, main effect for diet, p < 0.0001), was only detectable as a 30 kDa band in LFD mice and was unresponsive to exercise. β-catenin and GSK3β were both unresponsive to diet and exercise within eWAT. Conclusion: These results characterized sclerostin's content to WAT depots in response to acute exercise, which appears to be specific to a reduction in iWAT and identified a differential regulation of sclerostin's form/post-translational modifications depending on diet and WAT depot.

The Role of the Innate Immune System in Wear Debris-Induced Inflammatory Peri-Implant Osteolysis in Total Joint Arthroplasty

Periprosthetic osteolysis remains a leading complication of total hip and knee arthroplasty, often resulting in aseptic loosening of the implant and necessitating revision surgery. Wear-induced particulate debris is the main cause initiating this destructive process. The purpose of this article is to review recent advances in understanding of how wear debris causes osteolysis, and emergent strategies for the avoidance and treatment of this disease. A strong activator of the peri-implant innate immune this debris-induced inflammatory cascade is dictated by macrophage secretion of TNF-α, IL-1, IL-6, and IL-8, and PGE2, leading to peri-implant bone resorption through activation of osteoclasts and inhibition of osteoblasts through several mechanisms, including the RANK/RANKL/OPG pathway. Therapeutic agents against proinflammatory mediators, such as those targeting tumor necrosis factor (TNF), osteoclasts, and sclerostin, have shown promise in reducing peri-implant osteolysis in vitro and in vivo; however, radiographic changes and clinical diagnosis often lag considerably behind the initiation of osteolysis, making timely treatment difficult. Considerable efforts are underway to develop such diagnostic tools, therapies, and identify novel targets for therapeutic intervention.

Biocompatibility of ZrO2 vs. Y-TZP Alloys: Influence of Their Composition and Surface Topography

The osseointegration of implants is defined as the direct anatomical and functional connection between neoformed living bone and the surface of a supporting implant. The biological compatibility of implants depends on various parameters, such as the nature of the material, chemical composition, surface topography, chemistry and loading, surface treatment, and physical and mechanical properties. In this context, the objective of this study is to evaluate the biocompatibility of rough (Ra = 1 µm) and smooth (Ra = 0 µm) surface conditions of yttria–zirconia (Y-TZP) discs compared to pure zirconia (ZrO₂) discs by combining a classical toxicological test, morphological observations by SEM, and a transcriptomic analysis on an in vitro model of human Saos-2 bone cells. Similar cell proliferation rates were observed between ZrO₂ and Y-TZP discs and control cells, regardless of the surface topography, at up to 96 h of exposure. Dense cell matting was similarly observed on the surfaces of both materials. Interestingly, only 110 transcripts were differentially expressed across the human transcriptome, consistent with the excellent biocompatibility of Y-TZP reported in the literature. These deregulated transcripts are mainly involved in two pathways, the first being related to “mineral uptake” and the second being the “immune response”. These observations suggest that Y-TZP is an interesting candidate for application in implantology.

Influence of Renal Function on Pharmacokinetics, Pharmacodynamics, and Safety of a Single Dose of Romosozumab

We evaluated the pharmacokinetics, pharmacodynamics, and safety of a single subcutaneous dose of romosozumab 210 mg, a monoclonal antibody against sclerostin, in an open-label, parallel-group study in participants with severe (stage 4) renal impairment (RI; n = 8) or end-stage renal disease requiring hemodialysis (ESRD-RH; n = 8), or healthy participants with normal renal function (n = 8). Compared with the group with normal renal function, the mean romosozumab exposure was 31% and 43% higher as measured by C_max and AUC, respectively, in the severe RI group and similar to those in the ESRD-RH group. For all 3 groups, the maximum mean percent increase in procollagen type 1 N terminal propeptide (P1NP) and decrease in serum C-telopeptide (sCTX) levels from baseline were observed on day 15. Changes in P1NP and sCTX were of similar patterns in all 3 groups. The single dose of romosozumab 210 mg was well tolerated. Adverse events (AEs) were reported for 13 patients (7 patients with severe RI and 6 with ESRD-RH), with no deaths, AEs, or serious AEs leading to withdrawal. The incidence of subjects with postbaseline transient decreases in serum calcium (severe RI, n = 1; ESRD-RH, n = 5) and increases in intact parathyroid hormone (severe RI, n = 7; ESRD-RH, n = 7; healthy, n = 3) were greater in severe RI and ESRD-RH groups than in the healthy group. All reported events of hypocalcemia (severe RI, n = 1; ESRD-RH, n = 4) were asymptomatic. These results support the use of romosozumab without dose adjustment in patients with severe RI or ESRD-RH. This article is protected by copyright. All rights reserved.

Competitive blocking of LRP4-sclerostin binding interface strongly promotes bone anabolic functions

Induction of bone formation by Wnt ligands is inhibited when sclerostin (Scl), an osteocyte-produced antagonist, binds to its receptors, the low-density lipoprotein receptor-related proteins 5 or 6 (LRP5/6). Recently, it was shown that enhanced inhibition is achieved by Scl binding to the co-receptor LRP4. However, it is not clear if the binding of Scl to LRP4 facilitates Scl binding to LRP5/6 or inhibits the Wnt pathway in an LRP5/6-independent manner. Here, using the yeast display system, we demonstrate that Scl exhibits a stronger binding affinity for LRP4 than for LRP6. Moreover, we found stronger Scl binding to LRP6 in the presence of LRP4. We further show that a Scl mutant (SclN93A), which tightly binds LRP4 but not LRP6, does not inhibit the Wnt pathway on its own. We demonstrate that SclN93A competes with Scl for a common binding site on LRP4 and antagonizes Scl inhibition of the Wnt signaling pathway in osteoblasts in vitro. Finally, we demonstrate that 2 weeks of bi-weekly subcutaneous injections of SclN93A fused to the fragment crystallizable (Fc) domain of immunoglobulin (SclN93AFc), which retains the antagonistic activity of the mutant, significantly increases bone formation rate and enhances trabecular volumetric bone fraction, trabecular number, and bone length in developing mice. Our data show that LRP4 serves as an anchor that facilitates Scl-LRP6 binding and that inhibition of the Wnt pathway by Scl depends on its prior binding to LRP4. We further provide evidence that compounds that inhibit Scl-LRP4 interactions offer a potential strategy to promote anabolic bone functions.

Wnt Pathway Extracellular Components and Their Essential Roles in Bone Homeostasis

The Wnt pathway is involved in several processes essential for bone development and homeostasis. For proper functioning, the Wnt pathway is tightly regulated by numerous extracellular elements that act by both activating and inhibiting the pathway at different moments. This review aims to describe, summarize and update the findings regarding the extracellular modulators of the Wnt pathway, including co-receptors, ligands and inhibitors, in relation to bone homeostasis, with an emphasis on the animal models generated, the diseases associated with each gene and the bone processes in which each member is involved. The precise knowledge of all these elements will help us to identify possible targets that can be used as a therapeutic target for the treatment of bone diseases such as osteoporosis.

Interrelationships Between Sclerostin, Secondary Hyperparathyroidism, and Bone Metabolism in Patients on Hemodialysis

CONTEXT

Sclerostin is an osteocyte-derived inhibitor of bone formation and is increased in kidney failure, but its role in the pathogenesis of renal bone disease remains unknown.

OBJECTIVE

We aimed to explore the association of serum sclerostin with bone metabolism in patients undergoing hemodialysis, with a particular focus on parathyroid hormone (PTH)-dependent and PTH-independent pathways.

METHODS

This cross-sectional and prospective cohort study included 654 patients undergoing hemodialysis at 10 facilities in Japan. We employed multivariable linear regression to explore whether sclerostin levels were associated with metacarpal bone mineral density (BMD), intact PTH, bone alkaline phosphatase (BAP), and tartrate-resistant acid phosphatase-5b (TRACP-5b). We employed mediation analyses to explore whether and to what extent the association of PTH with bone turnover markers is mediated by sclerostin. We also compared sclerostin levels between patients with and without previous or incident fractures.

RESULTS

The median sclerostin level in hemodialysis patients was 3- to 4-fold higher than that in healthy individuals. Higher sclerostin levels were associated with higher metacarpal BMD and lower levels of intact PTH, BAP, and TRACP-5b. However, the relationships of sclerostin with bone turnover markers were substantially attenuated after adjustment for PTH. Mediation analysis suggested that the effects of PTH on bone turnover markers were mainly direct rather than mediated by sclerostin. Sclerostin levels were not associated with previous or incident fractures.

CONCLUSION

These findings suggest that in patients undergoing dialysis, sclerostin has only a limited role in bone metabolism and may not mediate the effect of PTH on bone turnover.

A systematic review and meta-analysis of efficacy and safety of Romosozumab in postmenopausal osteoporosis

The study was conducted to illustrate the effect of Romosozumab in postmenopausal osteoporosis patients. Romosozumab decreased the incidence of vertebral, nonvertebral, and clinical fractures significantly. In addition, decreased incidence of falls and increased bone mineral density at lumbar spine, total hip, and femoral neck was observed. Romosozumab is a monoclonal antibody that acts against the sclerostin pathway leading to enhanced bone formation and reduced bone resorption in patients with osteoporosis. Electronic search was performed on Medline (via PubMed), The Cochrane Central Register of Controlled Trials, and clinicaltrials.gov, till May 2020, for RCTs evaluating the effectiveness of Romosozumab in postmenopausal osteoporosis. RCTs evaluating the effect of Romosozumab on fractures and bone mineral density in postmenopausal osteoporosis patients. Meta-analysis was performed by Cochrane review manager 5 (RevMan) version 5.3. Cochrane risk of bias 2.0 tool and GRADE pro-GDT were applied for methodological quality and overall evidence quality, respectively. One hundred seventy-nine studies were screened, and 10 eligible studies were included in the analysis, with a total of 6137 patients in romosozumab group and 5732 patients in control group. Romosozumab significantly reduced the incidence of vertebral fractures [OR = 0.43 (95%CI = 0.35-0.52), High-quality evidence], nonvertebral fractures [OR = 0.78 (95%CI = 0.66-0.92), High quality], and clinical fractures [OR = 0.70 (95%CI = 0.60-0.82), High quality] at 24 months. Significant reduction in incidence risk of falls [OR = 0.87 (95%CI = 0.78-0.96), High quality] was observed with romosozumab. Bone mineral density was significantly increased in the romosozumab treated groups at lumbar spine [MD = 12.66 (95%CI = 12.66-12.67), High quality], total hip [MD = 5.69 (95%CI = 5.68 - 5.69), Moderate quality], and femoral neck [MD = 5.18 (95%CI = 5.18-5.19), Moderate quality] at 12 months. The total adverse events [RR = 0.98(95%CI = 0.96-1.01), Moderate quality] and serious adverse events [RR = 0.98(95%CI = 0.88-1.08), Moderate quality] with romosozumab were comparable to the control group. The current analysis with evidence on efficacy and safety of Romosozumab, authors opine to recommend the use of Romosozumab treatment for post-menopausal osteoporosis.Systematic review registration: PROSPERO registration number: CRD42019112196.

The Effects of Osteoporotic and Non-osteoporotic Medications on Fracture Risk and Bone Mineral Density

Osteoporosis is a highly prevalent bone disease affecting more than 37.5 million individuals in the European Union (EU) and the  United States of America (USA). It is characterized by low bone mineral density (BMD), impaired bone quality, and loss of structural and biomechanical properties, resulting in reduced bone strength. An increase in morbidity and mortality is seen in patients with osteoporosis, caused by the approximately 3.5 million new osteoporotic fractures occurring every year in the EU. Currently, different medications are available for the treatment of osteoporosis, including anti-resorptive and osteoanabolic medications. Bisphosphonates, which belong to the anti-resorptive medications, are the standard treatment for osteoporosis based on their positive effects on bone, long-term experience, and low costs. However, not only medications used for the treatment of osteoporosis can affect bone: several other medications are suggested to have an effect on bone as well, especially on fracture risk and BMD. Knowledge about the positive and negative effects of different medications on both fracture risk and BMD is important, as it can contribute to an improvement in osteoporosis prevention and treatment in general, and, even more importantly, to the individual's health. In this review, we therefore discuss the effects of both osteoporotic and non-osteoporotic medications on fracture risk and BMD. In addition, we discuss the underlying mechanisms of action.

Genotypic Analyses of the Sclerostin rs851056 and Dickkopf rs1569198 Polymorphisms in Mexican-Mestizo Postmenopausal Osteoporosis: A Case-Control Study

Background: The Wnt/β catenin pathway promotes bone mineralization stimulating proliferation, differentiation, and survival of osteoblasts; it also inhibits osteoclast differentiation and osteocyte activity. Sclerostin (SOST) and Dickkopf 1 (DKK1) are Wnt/β catenin pathway inhibitors. Genetic variability in the expression of SOST and DKK1 might be involved in the development of postmenopausal osteoporosis (OP). Aim: To determine whether the SOST rs851056 and DKK1 rs1569198 polymorphisms are associated with OP in Mexican-Mestizo postmenopausal women. Materials and Methods: Two hundred and eighty Mexican-Mestizo postmenopausal women were assessed for their bone mineral density by dual-energy X-ray absorptiometry (DXA). Patients were classified as OP or non-OP. Genomic DNA was extracted from peripheral blood leukocytes. Genetic polymorphisms were analyzed by quantitative polymerase chain reaction using TaqMan probes. Results: The frequency of OP was 40% among the study population. Osteoporotic patients were older (p < 0.001), had a higher frequency of smoking (p = 0.01), and lower body mass index (p < 0.001) compared with the non-osteoporotic patients. The genotypic frequencies of the rs851056 locus of the SOST gene were GG 19%, GC 45%, and CC 35%, whereas the genotypic frequencies of the rs1569198 locus of the DKK1 gene were GG 15%, GA 40%, and AA 44%. In relation to rs851056 locus of the SOST gene, no differences were observed between the OP and non-OP cohorts in the frequencies of the GC polymorphism (48.7% vs. 43.1%). Similarly, analyses of the DKK1 rs1569198 does not demonstrate differences in the GA genotypic frequencies between the OP and non-OP cohorts (42.5% vs. 38.9%). Conclusion: Polymorphisms SOST rs851056 and DKK1 rs1569198 polymorphisms are not associated with OP in Mexican-Mestizo postmenopausal women.

Sclerostin-Mediated Impaired Osteogenesis by Fibroblast-Like Synoviocytes in the Particle-Induced Osteolysis Model

Engineered biomaterials are envisioned to replace, augment, or interact with living tissues for improving the functional deformities associated with end-stage joint pathologies. Unfortunately, wear debris from implant interfaces is the major factor leading to periprosthetic osteolysis. Fibroblast-like synoviocytes (FLSs) populate the intimal lining of the synovium and are in direct contact with wear debris. This study aimed to elucidate the effect of Ti particles as wear debris on human FLSs and the mechanism by which they might participate in the bone remodeling process during periprosthetic osteolysis. FLSs were isolated from synovial tissue from patients, and the condition medium (CM) was collected after treating FLSs with sterilized Ti particles. The effect of CM was analyzed for the induction of osteoclastogenesis or any effect on osteogenesis and signaling pathways. The results demonstrated that Ti particles could induce activation of the NFκB signaling pathway and induction of COX-2 and inflammatory cytokines in FLSs. The amount of Rankl in the conditioned medium collected from Ti particle–stimulated FLSs (Ti CM) showed the ability to stimulate osteoclast formation. The Ti CM also suppressed the osteogenic initial and terminal differentiation markers for osteoprogenitors, such as alkaline phosphate activity, matrix mineralization, collagen synthesis, and expression levels of Osterix, Runx2, collagen 1α, and bone sialoprotein. Inhibition of the WNT and BMP signaling pathways was observed in osteoprogenitors after the treatment with the Ti CM. In the presence of the Ti CM, exogenous stimulation by WNT and BMP signaling pathways failed to stimulate osteogenic activity in osteoprogenitors. Induced expression of sclerostin (SOST: an antagonist of WNT and BMP signaling) in Ti particle–treated FLSs and secretion of SOST in the Ti CM were detected. Neutralization of SOST in the Ti CM partially restored the suppressed WNT and BMP signaling activity as well as the osteogenic activity in osteoprogenitors. Our results reveal that wear debris–stimulated FLSs might affect bone loss by not only stimulating osteoclastogenesis but also suppressing the bone-forming ability of osteoprogenitors. In the clinical setting, targeting FLSs for the secretion of antagonists like SOST might be a novel therapeutic approach for preventing bone loss during inflammatory osteolysis.

Sclerostin and its role as a bone modifying agent in periodontal disease

BACKGROUND

Periodontitis is a highly prevalent inflammatory disease affecting the periodontium that results from an imbalance between periodontopathogens and host mechanisms. Continuous progression of the disease may lead to tissue and bone destruction, eventually resulting in tooth loss. The extent of bone loss depends on the dysregulated host immune response. Various host-elicited molecules play a major role in disease progression. The discovery of the glycoprotein sclerostin and its role as a regulator of bone mass has led to a better understanding of bone metabolism.

HIGHLIGHT

Sclerostin, which is primarily expressed by osteocytes, is a negative regulator of bone formation. It is a potent antagonist of the canonical Wingless-related integration site (Wnt) pathway, which is actively involved in bone homeostasis. Sclerostin is known to stimulate bone resorption by altering the osteoprotegerin (OPG)/receptor activator of nuclear factor kappa- β ligand (RANKL) balance. Additionally, in periodontitis, activation of the inflammatory cascade also increases the synthesis of sclerostin.

CONCLUSION

The recently discovered sclerostin antibody has emerged as a positive therapeutic tool for the treatment of metabolic bone diseases. It has been reported to improve bone strength, bone formation, osseointegration around implants and lower the risk of bone fractures in various animal and human models. This review describes the properties and action of sclerostin, its role in periodontal diseases, and the advent and efficacy of sclerostin antibodies.

The Roles of Sclerostin in Immune System and the Applications of Aptamers in Immune-Related Research

Wnt signaling is one of the fundamental pathways that play a major role in almost every aspect of biological systems. In addition to the well-known influence of Wnt signaling on bone formation, its essential role in the immune system also attracted increasing attention. Sclerostin, a confirmed Wnt antagonist, is also proven to modulate the development and differentiation of normal immune cells, particularly B cells. Aptamers, single-stranded (ss) oligonucleotides, are capable of specifically binding to a variety of target molecules by virtue of their unique three-dimensional structures. With in-depth study of those functional nucleic acids, they have been gradually applied to diagnostic and therapeutic area in immune diseases due to their various advantages over antibodies. In this review, we focus on several issues including the roles of Wnt signaling and Wnt antagonist sclerostin in the immune system. For the sake of understanding, current examples of aptamers applications for the immune diseases are also discussed. At the end of this review, we propose our ideas for the future research directions.

Osteoporosis Treatment with Anti-Sclerostin Antibodies-Mechanisms of Action and Clinical Application

Osteoporosis is characterized by reduced bone mass and disruption of bone architecture, resulting in increased risk of fragility fractures and significant long-term disability. Although both anti-resorptive treatments and osteoanabolic drugs, such as parathyroid hormone analogues, are effective in fracture prevention, limitations exist due to lack of compliance or contraindications to these drugs. Thus, there is a need for novel potent therapies, especially for patients at high fracture risk. Romosozumab is a monoclonal antibody against sclerostin with a dual mode of action. It enhances bone formation and simultaneously suppresses bone resorption, resulting in a large anabolic window. In this opinion-based narrative review, we highlight the role of sclerostin as a critical regulator of bone mass and present human diseases of sclerostin deficiency as well as preclinical models of genetically modified sclerostin expression, which led to the development of anti-sclerostin antibodies. We review clinical studies of romosozumab in terms of bone mass accrual and anti-fracture activity in the setting of postmenopausal and male osteoporosis, present sequential treatment regimens, and discuss its safety profile and possible limitations in its use. Moreover, an outlook comprising future translational applications of anti-sclerostin antibodies in diseases other than osteoporosis is given, highlighting the clinical significance and future scopes of Wnt signaling in these settings.

Genetics and Genomics of SOST: Functional Analysis of Variants and Genomic Regulation in Osteoblasts

SOST encodes the sclerostin protein, which acts as a key extracellular inhibitor of the canonical Wnt pathway in bone, playing a crucial role in skeletal development and bone homeostasis. The objective of this work was to assess the functionality of two variants previously identified (the rare variant rs570754792 and the missense variant p.Val10Ile) and to investigate the physical interactors of the SOST proximal promoter region in bone cells. Through a promoter luciferase reporter assay we show that the minor allele of rs570754792, a variant located in the extended TATA box motif, displays a significant decrease in promoter activity. Likewise, through western blot studies of extracellular and intracellular sclerostin, we observe a reduced expression of the p.Val10Ile mutant protein. Finally, using a circular chromosome conformation capture assay (4C-seq) in 3 bone cell types (MSC, hFOB, Saos-2), we have detected physical interactions between the SOST proximal promoter and the ECR5 enhancer, several additional enhancers located between EVT4 and MEOX1 and a distant region containing exon 18 of DHX8. In conclusion, SOST presents functional regulatory and missense variants that affect its expression and displays physical contacts with far reaching genomic sequences, which may play a role in its regulation within bone cells.

Secreted BMP antagonists and their role in cancer and bone metastases

Bone morphogenetic proteins (BMPs) are multifunctional secreted cytokines that act in a highly context-dependent manner. BMP action extends beyond the induction of cartilage and bone formation, to encompass pivotal roles in controlling tissue and organ homeostasis during development and adulthood. BMPs signal via plasma membrane type I and type II serine/threonine kinase receptors and intracellular SMAD transcriptional effectors. Exquisite temporospatial control of BMP/SMAD signalling and crosstalk with other cellular cues is achieved by a series of positive and negative regulators at each step in the BMP/SMAD pathway. The interaction of BMP ligand with its receptors is carefully controlled by a diverse set of secreted antagonists that bind BMPs and block their interaction with their cognate BMP receptors. Perturbations in this BMP/BMP antagonist balance are implicated in a range of developmental disorders and diseases, including cancer. Here, we provide an overview of the structure and function of secreted BMP antagonists, and summarize recent novel insights into their role in cancer progression and bone metastasis. Gremlin1 (GREM1) is a highly studied BMP antagonist, and we will focus on this molecule in particular and its role in cancer. The therapeutic potential of pharmacological inhibitors for secreted BMP antagonists for cancer and other human diseases will also be discussed.

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.

The Role of Sclerostin in Bone and Ectopic Calcification

Sclerostin, a 22-kDa glycoprotein that is mainly secreted by the osteocytes, is a soluble inhibitor of canonical Wnt signaling. Therefore, when present at increased concentrations, it leads to an increased bone resorption and decreased bone formation. Serum sclerostin levels are known to be increased in the elderly and in patients with chronic kidney disease. In these patient populations, there is a high incidence of ectopic cardiovascular calcification. These calcifications are strongly associated with cardiovascular morbidity and mortality. Although data are still controversial, it is likely that there is a link between ectopic calcification and serum sclerostin levels. The main question, however, remains whether sclerostin exerts either a protective or deleterious role in the ectopic calcification process.

Downregulation of Sostdc1 in Testicular Sertoli Cells is Prerequisite for Onset of Robust Spermatogenesis at Puberty

An alarming decline in sperm count of men from several countries has become a major concern for the world community. Hormones act on testicular Sertoli cells (Sc) to regulate male fertility by governing the division and differentiation of germ cells (Gc). However, there is a limited knowledge about Sc specific gene(s) regulating the spermatogenic output of the testis. Sclerostin domain-containing 1 protein (Sostdc1) is a dual BMP/Wnt regulator is predominantly expressed in the Sc of infant testes which hardly show any sign of spermatogenesis. In order to investigate the role of Sostdc1 in spermatogenic regulation, we have generated transgenic (Tg) rats which induced persistent expression of Sostdc1 in mature Sc causing reduced sperm counts. Although Sc specific Sostdc1 did not affect the function of either Sc or Leydig cells (Lc) in the adult testis of Tg rat, we observed a selective augmentation of the BMP target genes via activated phospho smad 1/5/8 signaling in Gc leading to apoptosis. Here, for the first time, we have demonstrated that Sostdc1 is a negative regulator of spermatogenesis, and provided substantial evidence that down regulation of Sostdc1 during puberty is critically essential for quantitatively and qualitatively normal spermatogenesis governing male fertility.

ACVR1-knockout promotes osteogenic differentiation by activating the Wnt signaling pathway in mice

Osteogenic differentiation refers to the process of bone formation and remodeling, which is controlled by complex molecular mechanisms. Activin A receptor type I (ACVR1) is reported to be associated with osteogenic differentiation. However, the underlying molecular mechanism remains elusive. Therefore, this study evaluates the function of ACVR1 in osteogenic differentiation through the Wnt signaling pathway. The expression of osteocalcin (Oc) and osterix together with osteogenic differentiation and mineralization was examined in ACVR1-knockout (KO) mouse. Furthermore, the Wnt signaling pathway was inhibited in bone marrow stromal cells (BMSCs) of mice to explore the role of the Wnt signaling pathway in osteogenic differentiation by means of alkaline phosphatase (ALP) activity detection and evaluation of mineralized nodules and calcium content. Subsequently, the effect of ACVR1 on the Wnt signaling pathway was assessed by determining the expression of ACVR1, β-catenin, glycogen synthase kinase 3 β (GSK3β), dickkopf-related protein 1 (DKK1), and frizzled class receptor 1 (FZD1). Both their effects on osteogenic differentiation were further evaluated by determination of Oc, osterix, and Runx2 expression. AVCR1 KO mice exhibited increased Oc and osterix expression and promoted bone resorption and formation. ACVR1-knockout was observed to activate the Wnt signaling pathway with an increase of β-catenin and reductions in GSK3β, DKK1, and FZD1. With the inhibited Wnt signaling pathway expression of Oc, osterix, and Runx2 was decreased, and ALP activity, mineralized nodule, and calcium content in cellular matrix were decreased as well, indicating that inactivation of the Wnt signaling pathway reduced the differentiation of BMSCs into osteoclasts. These findings indicate that ACVR1-knockout promotes osteogenic differentiation by activating the Wnt signaling pathway in mice.

Osteoanabolic and dual action drugs

Teriparatide (TPTD) and abaloparatide (ABL) are the only osteoanabolic drugs available, at this time, for treatment of osteoporosis. TPTD is a 34-amino acid fragment that is identical in its primary sequence to the 34 amino acids of full-length human parathyroid hormone [hPTH(1-84)]. ABL is identical to parathyroid hormone-related peptide (PTHrP) through the first 22 residues with significantly different amino acids inserted thereafter, between residues 22 and 34. The osteoanabolic actions of PTH are due directly to its effects on cells of the osteoblast lineage and indirectly by stimulating IGF-I synthesis and suppressing sclerostin and associated enhancement of Wnt signalling. Both TPTD and ABL are ligands that bind to and activate the PTH receptor type 1 (PTHR1) receptor but they appear to do so differently: ABL favours the transient, more anabolic configuration of the receptor. Both TPTD and ABL reduce the risk of vertebral fractures and non-vertebral fractures. Both drugs are administered for a maximum of 24 months, and should be followed by an antiresorptive agent to maintain gains in bone mineral density (BMD). Romosozumab, a monoclonal antibody that binds to and inhibits sclerostin, appears to have dual actions by stimulating bone formation and reducing bone resorption. In the pivotal clinical trial, romosozumab, administered as a 210 mg monthly subcutaneous dose, significantly reduced new vertebral fractures and in a subsequent study reduced both vertebral and non-vertebral fractures.

Novel actions of sclerostin on bone

The discovery that two rare autosomal recessive high bone mass conditions were caused by the loss of sclerostin expression prompted studies into its role in bone homeostasis. In this article, we aim to bring together the wealth of information relating to sclerostin in bone though discussion of rare human disorders in which sclerostin is reduced or absent, sclerostin manipulation via genetic approaches and treatment with antibodies that neutralise sclerostin in animal models and in human. Together, these findings demonstrate the importance of sclerostin as a regulator of bone homeostasis and provide valuable insights into its biological mechanism of action. We summarise the current state of knowledge in the field, including the current understanding of the direct effects of sclerostin on the canonical WNT signalling pathway and the actions of sclerostin as an inhibitor of bone formation. We review the effects of sclerostin, and its inhibition, on bone at the cellular and tissue level and discuss new findings that suggest that sclerostin may also regulate adipose tissue. Finally, we highlight areas in which future research is expected to yield additional insights into the biology of sclerostin.

The Emerging Role of Bone Markers in Diagnosis and Risk Stratification of Patients With Coronary Artery Disease

Molecules that govern bone metabolism, such as osteoprotegerin (OPG) and osteopontin (OPN), have been isolated from other tissues, including blood vessels. Atherosclerosis and coronary artery disease (CAD) are leading causes of mortality worldwide. Despite novel biochemical and imaging techniques, early detection of CAD is still unsatisfactory. Experimental data indicate that bone turnover markers (BTMs) contribute to the development of atherosclerosis. This finding has sparked interest in their clinical use. This narrative review analyzed information from >50 human studies, which strongly suggest that OPG, OPN, and alkaline phosphatase (ALP) serum concentrations are altered in patients with CAD. Osteoprotegerin seems to be more useful for the detection of early disease, while OPN and ALP are recruited in vessels after the establishment of disease. Osteocalcin may be used as a flow cytometry marker for endothelial progenitor cells and can constitute a marker to monitor response to interventional treatments and risk of restenosis. However, most data derive from observational studies. Incorporation of BTMs in multifactorial computational algorithms could further determine their role in CAD diagnosis and prognosis together with other imaging techniques and biochemical markers.

How is mechanobiology involved in mesenchymal stem cell differentiation toward the osteoblastic or adipogenic fate?

Mechanobiology plays a major role in transducing physical cues from the dynamic cellular environment into biochemical modifications that promote cell-specific differentiation paths. Mesenchymal stem cells in the bone marrow or in other mesenchymal tissues will differentiate according to the expression of transcription factors (TFs) that govern their lineage commitment. The favoring of either osteogenic or adipogenic differentiation relies on TF expression as well as mechanical properties of the cells' niche that are translated into the activation of certain signaling pathways. Physical factors can induce significant shifts in bipotential lineage commitment between osteogenesis and adipogenesis. The stiffness of the extracellular matrix (ECM) surrounding a cell, varying greatly from rigid environments close to the bone surface to softer regions in the bone marrow, can influence the path of differentiation. Additionally, mechanical loading through exercise appears to favor osteogenesis whereas disuse conditions seem to promote adipogenesis.

Skeletal disease in a father and daughter with a novel monoallelic WNT1 mutation

Context

Most heritable causes of low bone mass in children occur due to mutations affecting type 1 collagen. We describe two related patients with low bone mass and fracture without mutations in the type 1 collagen genes.

Case description

We describe the index case of a 10-year-old girl with low-impact fractures in childhood and her 59-year-old father with traumatic fractures in adulthood, both with low bone mineral density. They were found to have the same heterozygous missense mutation in the WNT1 gene (p.Gly222Arg), occurring in a highly conserved WNT motif in close proximity to the Frizzled binding site.

Conclusions

The WNT-ligand WNT1, signaling through the canonical WNT-β­catenin pathway, plays a critical role in skeletal development, adult skeletal homeostasis, and bone remodeling. Biallelic mutations have been described and are associated with moderate to severe osteogenesis imperfecta, in some cases with extra-skeletal manifestations. Patients with monoallelic mutations, as in our case, seem to present with low bone mineral density and less severe disease. The phenotypic difference between biallelic and monoallelic mutations highlights that the aberrant protein in monoallelic mutations may exert a dominant negative effect on the wild type protein as heterozygous carriers in families with biallelic disease are usually asymptomatic. With better understanding of disorders associated with WNT1 mutations, therapies targeting this signaling pathway may offer therapeutic benefit.

Inhibitors of osteoblastogenesis in early human milk and maternal serum: evidence for protective properties of mother's milk on bone

Objective: Lactation is associated with a dramatic increase of maternal bone turnover, leading to a reversible bone loss. Early life nutrition may influence later osteoporosis risk. Proteins synthesized by the group of wingless (Wnt) genes are key mediators of osteoblastogenesis and bone formation. We aimed to investigate maternal milk and serum concentrations of the inhibitors of the Wnt signaling pathway, Dickkopf-1 (DKK-1) and sclerostin.Material and methods: In 80 women, maternal milk and serum concentrations of DKK-1 and sclerostin were determined by ELISA on the 3rd-4th day postpartum. Concentrations were associated with various maternal, gestational and neonatal characteristics.Results: DKK-1 and sclerostin were detectable in early milk [mean ± SD: 817.17 ± 259.61 pg/mL, median (range) 258.04 (2452.40-53.17) pg/mL, respectively] at significantly lower concentrations than in maternal serum [mean ± SD: 3375.36 ± 416.75 pg/mL, median (range) 16 200.54 (58 832.00-3012.60) pg/mL, respectively], (p < .000). Maternal milk sclerostin concentrations positively correlated with respective serum ones (r = 0.599, p = .000). Maternal serum and milk sclerostin concentrations positively correlated with maternal body mass index (r = 0.37, p = .001 and r  =0.38, p = .000, respectively), while maternal serum sclerostin concentrations were higher in primiparas (p = .002).Conclusion: DKK-1 and sclerostin are present in early human milk at significantly lower concentrations, compared with maternal serum, probably contributing to the short- and long-term benefits of mother's milk for bone health. Moreover, the large amounts of both substances in maternal serum may represent disruption of the Wnt cascade, contributing to the well-known lactation-associated bone loss, which seems to be greater in primiparas and obese mothers.

Sclerostin-Neutralizing Antibody Enhances Bone Regeneration Around Oral Implants

BACKGROUND

Dental implants are an important option for replacement of missing teeth. A major clinical challenge is how best to accelerate bone regeneration and reduce the healing time for functional restoration after implant placement. A sclerostin-neutralizing antibody (Scl-Ab) has been shown to enhance alveolar bone formation and fracture repair. The aim of this study was to investigate the effects of systemic administration of Scl-Ab on dental implant osseointegration and bone regeneration in an experimental alveolar ridge tooth extraction model.

MATERIALS AND METHODS

To investigate the effects of Scl-Ab on bone regeneration and dental implant osseointegration, an experimental alveolar bone osteotomy rat model was adopted. One month after extraction of maxillary right first molars, osteotomy defects were created at the coronal aspect of each of the extraction sites, and 1 × 2-mm custom titanium implants were installed into the osteotomies. Coincident with implant placement, Scl-Ab was administered subcutaneously at a dose of 25 mg/kg twice weekly for 10-28 days and compared with a vehicle control. Animals were sacrificed 10, 14, and 28 days after surgery, and maxillae were harvested and analyzed by microcomputed tomography (microCT), histology, and histomorphometry.

RESULTS

microCT analysis demonstrated that the maxillary bone volume fraction was approximately 2- to 2.5-fold greater in Scl-Ab-treated animals compared with vehicle alone at days 14 and 28. Consistent with those findings, two-dimensional bone fill percentages within the coronal osteotomy sites were highest in Scl-Ab treatment groups at 28 days. In addition, bone-implant contact at 28 days was approximately twofold greater in the Scl-Ab group compared with the vehicle control.

CONCLUSIONS

These results indicate that systemic Scl-Ab administration enhances osseointegration and bone regeneration around dental implants. This approach offers potential as a treatment modality for patients with low bone mass or bone defects to achieve more predictable bone regeneration at alveolar bone defects and to enhance dental implant osseointegration.

Functional reconstruction of critical-sized load-bearing bone defects using a Sclerostin-targeting miR-210-3p-based construct to enhance osteogenic activity

A considerable amount of research has focused on improving regenerative therapy strategies for repairing defects in load-bearing bones. The enhancement of tissue regeneration with microRNAs (miRNAs) is being developed because miRNAs can simultaneously regulate multiple signaling pathways in an endogenous manner. In this study, we developed a miR-210-based bone repair strategy. We identified a miRNA (miR-210-3p) that can simultaneously up-regulate the expression of multiple key osteogenic genes in vitro. This process resulted in enhanced bone formation in a subcutaneous mouse model with a miR-210-3p/poly-l-lactic acid (PLLA)/bone marrow-derived stem cell (BMSC) construct. Furthermore, we constructed a model of critical-sized load-bearing bone defects and implanted a miR-210-3p/β-tricalcium phosphate (β-TCP)/bone mesenchymal stem cell (BMSC) construct into the defect. We found that the load-bearing defect was almost fully repaired using the miR-210-3p construct. We also identified a new mechanism by which miR-210-3p regulates Sclerostin protein levels. This miRNA-based strategy may yield novel therapeutic methods for the treatment of regenerative defects in vital load-bearing bones by utilizing miRNA therapy for tissue engineering.

STATEMENT OF SIGNIFICANCE

The destroyed maxillofacial bone reconstruction is still a real challenge for maxillofacial surgeon, due to that functional bone reconstruction involved load-bearing. Base on the above problem, this paper developed a novel miR-210-3p/β-tricalcium phosphate (TCP)/bone marrow-derived stem cell (BMSC) construct (miR-210-3p/β-TCP/BMSCs), which lead to functional reconstruction of critical-size mandible bone defect. We found that the load-bearing defect was almost fully repaired using the miR-210-3p construct. In addition, we also found the mechanism of how the delivered microRNA activated the signaling pathways of endogenous stem cells, leading to the defect regeneration. This miRNA-based strategy can be used to regenerate defects in vital load-bearing bones, thus addressing a critical challenge in regenerative medicine by utilizing miRNA therapy for tissue engineering.

Transforming growth factor-β in stem cells and tissue homeostasis

TGF-β 1-3 are unique multi-functional growth factors that are only expressed in mammals, and mainly secreted and stored as a latent complex in the extracellular matrix (ECM). The biological functions of TGF-β in adults can only be delivered after ligand activation, mostly in response to environmental perturbations. Although involved in multiple biological and pathological processes of the human body, the exact roles of TGF-β in maintaining stem cells and tissue homeostasis have not been well-documented until recent advances, which delineate their functions in a given context. Our recent findings, along with data reported by others, have clearly shown that temporal and spatial activation of TGF-β is involved in the recruitment of stem/progenitor cell participation in tissue regeneration/remodeling process, whereas sustained abnormalities in TGF-β ligand activation, regardless of genetic or environmental origin, will inevitably disrupt the normal physiology and lead to pathobiology of major diseases. Modulation of TGF-β signaling with different approaches has proven effective pre-clinically in the treatment of multiple pathologies such as sclerosis/fibrosis, tumor metastasis, osteoarthritis, and immune disorders. Thus, further elucidation of the mechanisms by which TGF-β is activated in different tissues/organs and how targeted cells respond in a context-dependent way can likely be translated with clinical benefits in the management of a broad range of diseases with the involvement of TGF-β.

Role of the Wnt/β-Catenin Pathway in Renal Osteodystrophy

Vascular calcification and bone fragility are common and interrelated health problems that affect chronic kidney disease (CKD) patients. Bone fragility, which leads to higher risk of fracture and mortality, arises from the abnormal bone remodeling and mineralization that are seen in chronic kidney disease. Recently, sclerostin and Dickkopf-related protein 1 were suggested to play a significant role in CKD-related bone disease as they are known inhibitors of the Wnt pathway, thus preventing bone formation. This review focuses on new knowledge about the Wnt pathway in bone, how its function is affected by chronic kidney disease and how this affects bone structure. Expression of components and inhibitors of the Wnt pathway has been shown to be affected by the loss of kidney function, and a better understanding of the bone effects of Wnt pathway inhibitors could allow the development of new therapies to prevent bone fragility in this population.

The sclerostin-neutralizing antibody AbD09097 recognizes an epitope adjacent to sclerostin's binding site for the Wnt co-receptor LRP6

The glycoprotein sclerostin has been identified as a negative regulator of bone growth. It exerts its function by interacting with the Wnt co-receptor LRP5/6, blocks the binding of Wnt factors and thereby inhibits Wnt signalling. Neutralizing anti-sclerostin antibodies are able to restore Wnt activity and enhance bone growth thereby presenting a new osteoanabolic therapy approach for diseases such as osteoporosis. We have generated various Fab antibodies against human and murine sclerostin using a phage display set-up. Biochemical analyses have identified one Fab developed against murine sclerostin, AbD09097 that efficiently neutralizes sclerostin's Wnt inhibitory activity. In vitro interaction analysis using sclerostin variants revealed that this neutralizing Fab binds to sclerostin's flexible second loop, which has been shown to harbour the LRP5/6 binding motif. Affinity maturation was then applied to AbD09097, providing a set of improved neutralizing Fab antibodies which particularly bind human sclerostin with enhanced affinity. Determining the crystal structure of AbD09097 provides first insights into how this antibody might recognize and neutralize sclerostin. Together with the structure-function relationship derived from affinity maturation these new data will foster the rational design of new and highly efficient anti-sclerostin antibodies for the therapy of bone loss diseases such as osteoporosis.

Loss of mechanosensitive sclerostin may accelerate cranial bone growth and regeneration

OBJECTIVE

Cranial defects can result from trauma, infection, congenital malformations, and iatrogenic causes and represent a surgical challenge. The current standard of care is cranioplasty, with either autologous or allogeneic material. In either case, the intrinsic vascularity of the surrounding tissues allows for bone healing. The objective of this study was to determine if mechanotransductive gene manipulation would yield non-weight-bearing bone regeneration in a critical size calvarial defect in mice.

METHODS

A mouse model of Sost deletion in Sost knockout (KO) mice was created in which the osteocytes do not express sclerostin. A critical size calvarial defect (4 mm in diameter) was surgically created in the parietal bone in 8-week-old wild-type (n = 8) and Sost KO (n = 8) male mice. The defects were left undisturbed (no implant or scaffold) to simulate a traumatic calvariectomy model. Eight weeks later, the animals were examined at necropsy by planimetry, histological analysis of new bone growth, and micro-CT scanning of bone thickness.

RESULTS

Defects created in wild-type mice did not fill with bone over the study period of 2 months. Genetic downregulation of sclerostin yielded animals that were able to regenerate 40% of the initial critical size defect area 8 weeks after surgery. A thin layer of bone covered a significant portion of the original defect in all Sost KO animals. A statistically significant increase in bone volume (p < 0.05) was measured in Sost KO mice using radiodensitometric analysis. Immunohistochemical analysis also confirmed that this bone regeneration occurred through the Wnt pathway and originated from the edge of the defect; BMP signaling did not appear to be affected by sclerostin.

CONCLUSIONS

Mechanical loading is an important mechanism of bone formation in the cranial skeleton and is poorly understood. This is partially due to the fact that it is difficult to load bone in the craniomaxillofacial skeleton. This study suggests that modulation of the Wnt pathway, as is able to be done with monoclonal antibodies, is a potentially efficacious method for bone regeneration that requires further study.