Disruption of Lrp4 function by genetic deletion or pharmacological blockade increases bone mass and serum sclerostin levels

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.

The Relationship between Sclerostin and Kidney Transplantation Mineral Bone Disorders: A Molecule of Controversies

Kidney transplantation is the most effective treatment option for most patients with end-stage kidney disease due to reduced mortality, decreased cardiovascular events and increased quality of life compared to patients treated with dialysis. However, kidney transplantation is not devoid of both acute and chronic complications including mineral bone disorders (MBD) which are already present in patients with chronic kidney disease (CKD) before kidney transplantation. The natural history of MBD after kidney transplantation is variable and new markers are needed to define MBD after kidney transplantation. One of these promising molecules is sclerostin. The main action of sclerostin is to inhibit bone formation and mineralization by blocking osteoblast differentiation and function. In kidney transplant recipients (KTRs), various studies have shown that sclerostin is associated with graft function, bone parameters, vascular calcification, and arterial stiffness although non-uniformly. Furthermore, data for inhibition of sclerostin with monoclonal antibody romosozumab for treatment of osteoporosis is available for general population but not in KTRs which osteoporosis is highly prevalent. In this narrative review, we have summarized the studies investigating the change of sclerostin before and after kidney transplantation, the relationship between sclerostin and laboratory parameters, bone metabolism and vascular calcification in the context of kidney transplantation. We also pointed out the uncertainties, explained the causes of divergent findings and suggest further potential study topics regarding sclerostin in kidney transplantation.

Effect of α7 nAChR-autophagy axis of deciduous tooth pulp stem cells in regulating IL-1β in the process of physiological root resorption of deciduous teeth

Physiological root resorption of deciduous teeth is a normal phenomenon occurring during the developmental stages of children. Previous research has indicated the pivotal role of the inflammatory microenvironment in this process, although the specific mechanisms remain unclear. This study is aimed at elucidating the involvement of the alpha7 nicotinic acetylcholine receptors (α7 nAChR)-autophagy axis in the regulation of the inflammatory microenvironment during physiological root resorption in deciduous teeth. Samples were collected from deciduous teeth at various stages of physiological root resorption, and deciduous dental pulp stem cells (DDPSCs) were isolated and cultured during the mid-phase of root resorption. The findings revealed a substantial infiltration of the pulp of deciduous teeth at the mid-phase of root resorption, characterized by elevated expression levels of α7 nAChR and IL-1β. Significantly increased IL-1β and α7 nAChR expressions were observed in DDPSCs during the mid-phase of root resorption, with α7 nAChR demonstrating a regulatory effect on IL-1β. Moreover, evidence suggested that mechanical stress may act as a trigger, regulating autophagy and IL-1 expression via α7 nAChR. In conclusion, mechanical stress was identified as a regulator of autophagy in DDPSCs through α7 nAChR, influencing the expression of IL-1β and contributing to the formation of the inflammatory microenvironment. This mechanism plays a crucial role in the physiological root resorption of deciduous teeth. KEY MESSAGES: The pulp of deciduous teeth at mid-phase of root resorption was heavily infiltrated with high expression of α7nAChR and IL-1β. α7 nAChR acts as an initiating factor to regulate IL-1β through autophagy in DDPSCs. Mechanical stress can regulate autophagy of DDPSCs through α7 nAChR and thus affect IL-1β expression and inflammatory microenvironment formation in physiological root resorption in deciduous teeth.

Induction of osteoblast apoptosis stimulates macrophage efferocytosis and paradoxical bone formation

Apoptosis is crucial for tissue homeostasis and organ development. In bone, apoptosis is recognized to be a main fate of osteoblasts, yet the relevance of this process remains underexplored. Using our murine model with inducible Caspase 9, the enzyme that initiates intrinsic apoptosis, we triggered apoptosis in a proportion of mature osteocalcin (OCN+) osteoblasts and investigated the impact on postnatal bone development. Osteoblast apoptosis stimulated efferocytosis by osteal macrophages. A five-week stimulation of OCN+ osteoblast apoptosis in 3-week-old male and female mice significantly enhanced vertebral bone formation while increasing osteoblast precursors. A similar treatment regimen to stimulate osterix+ cell apoptosis had no impact on bone volume or density. The vertebral bone accrual following stimulation of OCN+ osteoblast apoptosis did not translate in improved mechanical strength due to disruption of the lacunocanalicular network. The observed bone phenotype was not influenced by changes in osteoclasts but was associated with stimulation of macrophage efferocytosis and vasculature formation. Phenotyping of efferocytic macrophages revealed a unique transcriptomic signature and expression of factors including VEGFA. To examine whether macrophages participated in the osteoblast precursor increase following osteoblast apoptosis, macrophage depletion models were employed. Depletion of macrophages via clodronate-liposomes and the CD169-diphtheria toxin receptor mouse model resulted in marked reduction in leptin receptor+ and osterix+ osteoblast precursors. Collectively, this work demonstrates the significance of osteoblast turnover via apoptosis and efferocytosis in postnatal bone formation. Importantly, it exposes the potential of targeting this mechanism to promote bone anabolism in the clinical setting.

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 inhibition in rare bone diseases: Molecular understanding and therapeutic perspectives

Sclerostin emerges as a novel target for bone anabolic therapy in bone diseases. Osteogenesis imperfecta (OI) and X-linked hypophosphatemia (XLH) are rare bone diseases in which therapeutic potential of sclerostin inhibition cannot be ignored. In OI, genetic/pharmacologic sclerostin inhibition promoted bone formation of mice, but responses varied by genotype and age. Serum sclerostin levels were higher in young OI-I patients, while lower in adult OI-I/III/IV. It's worth investigating whether therapeutic response of OI to sclerostin inhibition could be clinically predicted by genotype and age. In XLH, preclinical/clinical data suggested factors other than identified FGF23 contributing to XLH. Higher levels of circulating sclerostin were detected in XLH. Sclerostin inhibition promoted bone formation in Hyp mice, while restored phosphate homeostasis in age-/gender-dependent manner. The role of sclerostin in regulating phosphate metabolism deserves investigation. Sclerostin/FGF23 levels of XLH patients with/without response to FGF23-antibody warrants study to develop precise sclerostin/FGF23 inhibition strategy or synergistic/additive strategy. Notably, OI patients were associated with cardiovascular abnormalities, so were XLH patients receiving conventional therapy. Targeting sclerostin loop3 promoted bone formation without cardiovascular risks. Further, blockade of sclerostin loop3-LRP4 interaction while preserving sclerostin loop2-ApoER2 interaction could be a potential precise sclerostin inhibition strategy for OI and XLH with cardiovascular safety. The Translational Potential of this Article. Preclinical data on the molecular understanding of sclerostin inhibition in OI and therapeutic efficacy in mouse models of different genotypes, as well as clinical data on serum sclerostin levels in patients with different phenotypes of OI, were reviewed and discussed. Translationally, it would facilitate to develop clinical prediction strategies (e.g. based on genotype and age, not just phenotype) for OI patients responsive to sclerostin inhibition. Both preclinical and clinical data suggested sclerostin as another factor contributing to XLH, in addition to the identified FGF23. The molecular understanding and therapeutic effects of sclerostin inhibition on both promoting bone anabolism and improving phosphate homostasis in Hyp mice were reviewed and discussed. Translationaly, it would facilitate the development of precise sclerostin/FGF23 inhibition strategy or synergistic/additive strategy for the treatment of XLH. Cardiovascular risk could not be ruled out during sclerostin inhibition treatment, especially for OI and XLH patients with cardiovascular diseases history and cardiovascular abnormalities. Studies on the role of sclerostin in inhiting bone formation and protecting cardiovascular system were reviewed and discussed. Translationaly, blockade of sclerostin loop3-LRP4 interaction while preserving sclerostin loop2-ApoER2 interaction could be a potential precise sclerostin inhibition strategy for OI and XLH with cardiovascular safety.

Refining the identity of mesenchymal cell types associated with murine periosteal and endosteal bone

Single-cell RNA-seq has led to novel designations for mesenchymal cells associated with bone as well as multiple designations for what appear to be the same cell type. The main goals of this study were to increase the amount of single-cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. We created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-Cxcl12 abundant reticular (CAR), osteo-CAR, preosteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by periostin expression. Osteo-X, osteo-CAR, and preosteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in preosteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single-cell RNA-seq datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

Severe Osteoporosis With Pathogenic LRP5 Variant

A 24-year-old female patient was diagnosed with osteoporosis after presenting with numerous fractures throughout her childhood and adolescence. Risk factors included chronic constipation, severe vitamin D deficiency, and long-term high-dose steroid use for severe eczema. Metabolic bone disorder clinical exome screening (limited panel of metabolic bone disorders and gastrointestinal disorders) was undertaken and revealed a class 4 likely pathogenic variant in the LRP5 gene known to cause osteoporosis. Optimal treatment for patients with this variant is not well defined. A literature review of the condition and potential treatment options is discussed.

An Additional Lrp4 High Bone Mass Mutation Mitigates the Sost-Knockout Phenotype in Mice by Increasing Bone Remodeling

Pathogenic variants disrupting the binding between sclerostin (encoded by SOST) and its receptor LRP4 have previously been described to cause sclerosteosis, a rare high bone mass disorder. The sclerostin-LRP4 complex inhibits canonical WNT signaling, a key pathway regulating osteoblastic bone formation and a promising therapeutic target for common bone disorders, such as osteoporosis. In the current study, we crossed mice deficient for Sost (Sost-/-) with our p.Arg1170Gln Lrp4 knock-in (Lrp4KI/KI) mouse model to create double mutant Sost-/-;Lrp4KI/KI mice. We compared the phenotype of Sost-/- mice with that of Sost-/-;Lrp4KI/KI mice, to investigate a possible synergistic effect of the disease-causing p.Arg1170Trp variant in Lrp4 on Sost deficiency. Interestingly, presence of Lrp4KI alleles partially mitigated the Sost-/- phenotype. Cellular and dynamic histomorphometry did not reveal mechanistic insights into the observed phenotypic differences. We therefore determined the molecular effect of the Lrp4KI allele by performing bulk RNA sequencing on Lrp4KI/KI primary osteoblasts. Unexpectedly, mostly genes related to bone resorption or remodeling (Acp5, Rankl, Mmp9) were upregulated in Lrp4KI/KI primary osteoblasts. Verification of these markers in Lrp4KI/KI, Sost-/- and Sost-/-;Lrp4KI/KI mice revealed that sclerostin deficiency counteracts this Lrp4KI/KI effect in Sost-/-;Lrp4KI/KI mice. We therefore hypothesize that models with two inactivating Lrp4KI alleles rather activate bone remodeling, with a net gain in bone mass, whereas sclerostin deficiency has more robust anabolic effects on bone formation. Moreover, these effects of sclerostin and Lrp4 are stronger in female mice, contributing to a more severe phenotype than in males and more detectable phenotypic differences among different genotypes.

The impacts of oral and gut microbiota on alveolar bone loss in periodontitis

Periodontitis, a chronic infectious disease, primarily arises from infections and the invasion of periodontal pathogens. This condition is typified by alveolar bone loss resulting from host immune responses and inflammatory reactions. Periodontal pathogens trigger aberrant inflammatory reactions within periodontal tissues, thereby exacerbating the progression of periodontitis. Simultaneously, these pathogens and metabolites stimulate osteoclast differentiation, which leads to alveolar bone resorption. Moreover, a range of systemic diseases, including diabetes, postmenopausal osteoporosis, obesity and inflammatory bowel disease, can contribute to the development and progression of periodontitis. Many studies have underscored the pivotal role of gut microbiota in bone health through the gut-alveolar bone axis. The circulation may facilitate the transfer of gut pathogens or metabolites to distant alveolar bone, which in turn regulates bone homeostasis. Additionally, gut pathogens can elicit gut immune responses and direct immune cells to remote organs, potentially exacerbating periodontitis. This review summarizes the influence of oral microbiota on the development of periodontitis as well as the association between gut microbiota and periodontitis. By uncovering potential mechanisms of the gut-bone axis, this analysis provides novel insights for the targeted treatment of pathogenic bacteria in periodontitis.

A framework for defining mesenchymal cell types associated with murine periosteal and endosteal bone

Single-cell RNA sequencing has led to numerous novel designations for mesenchymal cell types associated with bone. Consequently, there are now multiple designations for what appear to be the same cell type. In addition, existing datasets contain relatively small numbers of mature osteoblasts and osteocytes and there has been no comparison of periosteal bone cells to those at the endosteum and trabecular bone. The main goals of this study were to increase the amount of single cell RNA sequence data for osteoblasts and osteocytes, to compare cells from the periosteum to those inside bone, and to clarify the major categories of cell types associated with murine bone. To do this, we created an atlas of murine bone-associated cells by harmonizing published datasets with in-house data from cells targeted by Osx1-Cre and Dmp1-Cre driver strains. Cells from periosteal bone were analyzed separately from those isolated from the endosteum and trabecular bone. Over 100,000 mesenchymal cells were mapped to reveal 11 major clusters designated fibro-1, fibro-2, chondrocytes, articular chondrocytes, tenocytes, adipo-CAR, osteo-CAR, pre-osteoblasts, osteoblasts, osteocytes, and osteo-X, the latter defined in part by Postn expression. Osteo-X, osteo-CAR, and pre-osteoblasts were closely associated with osteoblasts at the trabecular bone surface. Wnt16 was expressed in multiple cell types from the periosteum but not in any cells from endocortical or cancellous bone. Fibro-2 cells, which express markers of skeletal stem cells, localized to the periosteum but not trabecular bone in adult mice. Suppressing bone remodeling eliminated osteoblasts and altered gene expression in pre-osteoblasts but did not change the abundance or location of osteo-X or osteo-CAR cells. These results provide a framework for identifying bone cell types in murine single cell RNA sequencing datasets and suggest that osteoblast progenitors reside near the surface of remodeling bone.

A variant in the LDL receptor-related protein encoding gene LRP4 underlying polydactyly and phalangeal synostosis in a family of Pakistani origin

A family of Pakistani origin, segregating polydactyly, and phalangeal synostosis in an autosomal dominant manner, has been investigated and presented in the present report. Whole-exome sequencing (WES), followed by segregation analysis using Sanger sequencing, revealed a heterozygous missense variant [c.G1696A, p.(Gly566Ser)] in the LRP4 gene located on human chromosome 11p11.2. Homology protein modeling revealed the mutant Ser566 generated new interactions with at least four other amino acids and disrupted protein folding and function. Our findings demonstrated the first direct evidence of involvement of LRP4 in causing polydactyly and phalangeal synostosis in the same family. This study highlighted the importance of inclusion of LRP4 gene in screening individuals presenting polydactyly in hands and feet, and phalangeal synostosis in the same family.

A novel multifunctional radioprotective strategy using P7C3 as a countermeasure against ionizing radiation-induced bone loss

Radiotherapy is a critical component of cancer care but can cause osteoporosis and pathological insufficiency fractures in surrounding and otherwise healthy bone. Presently, no effective countermeasure exists, and ionizing radiation-induced bone damage continues to be a substantial source of pain and morbidity. The purpose of this study was to investigate a small molecule aminopropyl carbazole named P7C3 as a novel radioprotective strategy. Our studies revealed that P7C3 repressed ionizing radiation (IR)-induced osteoclastic activity, inhibited adipogenesis, and promoted osteoblastogenesis and mineral deposition in vitro. We also demonstrated that rodents exposed to clinically equivalent hypofractionated levels of IR in vivo develop weakened, osteoporotic bone. However, the administration of P7C3 significantly inhibited osteoclastic activity, lipid formation and bone marrow adiposity and mitigated tissue loss such that bone maintained its area, architecture, and mechanical strength. Our findings revealed significant enhancement of cellular macromolecule metabolic processes, myeloid cell differentiation, and the proteins LRP-4, TAGLN, ILK, and Tollip, with downregulation of GDF-3, SH2B1, and CD200. These proteins are key in favoring osteoblast over adipogenic progenitor differentiation, cell matrix interactions, and shape and motility, facilitating inflammatory resolution, and suppressing osteoclastogenesis, potentially via Wnt/β-catenin signaling. A concern was whether P7C3 afforded similar protection to cancer cells. Preliminarily, and remarkably, at the same protective P7C3 dose, a significant reduction in triple-negative breast cancer and osteosarcoma cell metabolic activity was found in vitro. Together, these results indicate that P7C3 is a previously undiscovered key regulator of adipo-osteogenic progenitor lineage commitment and may serve as a novel multifunctional therapeutic strategy, leaving IR an effective clinical tool while diminishing the risk of adverse post-IR complications. Our data uncover a new approach for the prevention of radiation-induced bone damage, and further work is needed to investigate its ability to selectively drive cancer cell death.

Genetic insight into the putative causal proteins and druggable targets of osteoporosis: a large-scale proteome-wide mendelian randomization study

Background: Osteoporosis is a major causative factor of the global burden of disease and disability, characterized by low bone mineral density (BMD) and high risks of fracture. We aimed to identify putative causal proteins and druggable targets of osteoporosis. Methods: This study utilized the largest GWAS summary statistics on plasma proteins and estimated heel BMD (eBMD) to identify causal proteins of osteoporosis by mendelian randomization (MR) analysis. Different GWAS datasets were used to validate the results. Multiple sensitivity analyses were conducted to evaluate the robustness of primary MR findings. We have also performed an enrichment analysis for the identified causal proteins and evaluated their druggability. Results: After Bonferroni correction, 67 proteins were identified to be causally associated with estimated BMD (eBMD) (p < 4 × 10^-5). We further replicated 38 of the 67 proteins to be associated with total body BMD, lumbar spine BMD, femoral neck BMD as well as fractures, such as RSPO3, IDUA, SMOC2, and LRP4. The findings were supported by sensitivity analyses. Enrichment analysis identified multiple Gene Ontology items, including collagen-containing extracellular matrix (GO:0062023, p = 1.6 × 10^-10), collagen binding (GO:0005518, p = 8.6 × 10^-5), and extracellular matrix structural constituent (GO:0005201, p = 2.7 × 10^-5). Conclusion: The study identified novel putative causal proteins for osteoporosis which may serve as potential early screening biomarkers and druggable targets. Furthermore, the role of plasma proteins involved in collagen binding and extracellular matrix in the development of osteoporosis was highlighted. Further studies are warranted to validate our findings and investigate the underlying mechanism.

Polymorphism of LRP4 Gene (rs9667108) among Post Menopause Women with Osteoporosis

Background

Many studies have been done to identify the factors that influence the development and progression of osteoporosis. One genetic factor is polymorphisms of LRP4 gene. Regarding the lack of comprehensive study on polymorphisms of LRP4 gene in the north of Iran, mainly Mazandaran Province, we decided to investigate the polymorphism of this gene in postmenopausal women with osteoporosis.

Methods

This case-control study has been conducted at GhaemShahr Valiasr Hospital on 100 female patients with osteoporosis (average age of 58.1) and 90 healthy females without osteoporosis (average age of 55.2). After sampling and extraction of genomic DNA via of the salt deposition method, the genotype and SNP (rs9667108) polymorphism of LRP4 gene were evaluated with the PCR-RFLP method. Restriction enzymes cut the PCR products. In order to identify patients, their bone mineral density was tested by the DEXA method. The results of digestion (digestion enzyme) were analyzed by MedCalc, SPSS software, Hardy-Weinberg equilibrium, and Chi2.

Results

The statistical analysis has shown the significant relationship between SNP (rs9667108) polymorphism and the risk of osteoporosis disease in patients and control groups (P<0.05). In SNP (rs9667108), the GC genotype, compared to GG, increased the risk of disease significantly (1.556 time). Similarly, CC genotype, compared to GG genotype, increased the risk of this disease by 2.091 time.

Conclusion

The existence of mutation in the LRP4 gene could increase susceptibility to osteoporosis disease. Moreover, determining this patient's genotype in SNP (rs9667108) can be used to identify individuals who are in endanger osteoporosis.

Endocrine Functions of Sclerostin

Sclerostin, the product of the SOST gene has primarily been studied for its profound impact on bone mass. By interacting with LRP5 and LRP6, the glycoprotein suppresses the propagation of Wnt signals to β-catenin and thereby suppresses new bone formation. In this review, we discuss emerging data which suggest that sclerostin also acts outside the skeleton to influence metabolism. In humans, serum sclerostin levels are associated with body mass index and indices of metabolic function. Likewise, genetic mouse models of Sost gene deficiency indicate sclerostin influences adipocyte development and insulin signaling. These data raise the possibility that sclerostin neutralization may be effective at treating two epidemic conditions: osteoporosis and obesity.

Role of Wnt signaling and sclerostin in bone and as therapeutic targets in skeletal disorders

Wnt signaling and its bone tissue-specific inhibitor sclerostin are key regulators of bone homeostasis. The therapeutic potential of anti-sclerostin antibodies (Scl-Abs), for bone mass recovery and fragility fracture prevention in low bone mass phenotypes, has been supported by animal studies. The Scl-Ab romosozumab is currently used for osteoporosis treatment.

INTRODUCTION

Wnt signaling is a key regulator of skeletal development and homeostasis; germinal mutations affecting genes encoding components, inhibitors, and enhancers of the Wnt pathways were shown to be responsible for the development of rare congenital metabolic bone disorders. Sclerostin is a bone tissue-specific inhibitor of the Wnt/β-catenin pathway, secreted by osteocytes, negatively regulating osteogenic differentiation and bone formation, and promoting osteoclastogenesis and bone resorption.

PURPOSE AND METHODS

Here, we reviewed current knowledge on the role of sclerostin and Wnt pathways in bone metabolism and skeletal disorders, and on the state of the art of therapy with sclerostin-neutralizing antibodies in low-bone-mass diseases.

RESULTS

Various in vivo studies on animal models of human low-bone-mass diseases showed that targeting sclerostin to recover bone mass, restore bone strength, and prevent fragility fracture was safe and effective in osteoporosis, osteogenesis imperfecta, and osteoporosis pseudoglioma. Currently, only treatment with romosozumab, a humanized monoclonal anti-sclerostin antibody, has been approved in human clinical practice for the treatment of osteoporosis, showing a valuable capability to increase BMD at various skeletal sites and reduce the occurrence of new vertebral, non-vertebral, and hip fragility fractures in treated male and female osteoporotic patients.

CONCLUSIONS

Preclinical studies demonstrated safety and efficacy of therapy with anti-sclerostin monoclonal antibodies in the preservation/restoration of bone mass and prevention of fragility fractures in low-bone-mass clinical phenotypes, other than osteoporosis, to be validated by clinical studies for their approved translation into prevalent clinical practice.

Heterozygous LRP1 deficiency causes developmental dysplasia of the hip by impairing triradiate chondrocytes differentiation due to inhibition of autophagy

Developmental dysplasia of the hip (DDH) is one of the most common congenital skeletal malformations; however, its etiology remains unclear. Here, we conducted whole-exome sequencing and identified likely pathogenic variants in the LRP1 (low-density lipoprotein receptor-related protein 1) gene in two families and seven unrelated patients. We found that the timing of triradiate cartilage development was brought forward 1 or 2 wk earlier in the LRP-deficient mice, which leads to malformation of the acetabulum and femoral head. Furthermore, Lrp1 deficiency caused a significant decrease of chondrogenic ability in vitro. Our study reveals a critical role of LRP1 in the etiology and pathogenesis of DDH, opening an avenue for its treatment.

Developmental dysplasia of the hip (DDH) is one of the most common congenital skeletal malformations; however, its etiology remains unclear. Here, we conducted whole-exome sequencing in eight DDH families followed by targeted sequencing of 68 sporadic DDH patients. We identified likely pathogenic variants in the LRP1 (low-density lipoprotein receptor-related protein 1) gene in two families and seven unrelated patients. All patients harboring the LRP1 variants presented a typical DDH phenotype. The heterozygous Lrp1 knockout (KO) mouse (Lrp1^+/−) showed phenotypes recapitulating the human DDH phenotypes, indicating Lrp1 loss of function causes DDH. Lrp1 knockin mice with a missense variant corresponding to a human variant identified in DDH (Lrp1^R1783W) also presented DDH phenotypes, which were milder in heterozygotes and severer in homozygotes than those of the Lrp1 KO mouse. The timing of triradiate cartilage development was brought forward 1 or 2 wk earlier in the LRP-deficient mice, which leads to malformation of the acetabulum and femoral head. Furthermore, Lrp1 deficiency caused a significant decrease of chondrogenic ability in vitro. During the chondrogenic induction of mice bone marrow stem cells and ATDC5 (an inducible chondrogenic cell line), Lrp1 deficiency caused decreased autophagy levels with significant β-catenin up-regulation and suppression of chondrocyte marker genes. The expression of chondrocyte markers was rescued by PNU-74654 (a β-catenin antagonist) in an shRNA-Lrp1–expressed ATDC5 cell. Our study reveals a critical role of LRP1 in the etiology and pathogenesis of DDH, opening an avenue for its treatment.

Skeletal Effects of Inducible ERα Deletion in Osteocytes in Adult Mice

Estrogen is known to regulate bone metabolism in both women and men, but substantial gaps remain in our knowledge of estrogen and estrogen receptor alpha (ERα) regulation of adult bone metabolism. Studies using global ERα-knockout mice were confounded by high circulating sex-steroid levels, and osteocyte/osteoblast-specific ERα deletion may be confounded by ERα effects on growth versus the adult skeleton. Thus, we developed mice expressing the tamoxifen-inducible CreERT2 in osteocytes using the 8-kilobase (kb) Dmp1 promoter (Dmp1CreERT2 ). These mice were crossed with ERαfl//fl mice to create ERαΔOcy mice, permitting inducible osteocyte-specific ERα deletion in adulthood. After intermittent tamoxifen treatment of adult 4-month-old mice for 1 month, female, but not male, ERαΔOcy mice exhibited reduced spine bone volume fraction (BV/TV (-20.1%, p = 0.004) accompanied by decreased trabecular bone formation rate (-18.9%, p = 0.0496) and serum P1NP levels (-38.9%, p = 0.014). Periosteal (+65.6%, p = 0.004) and endocortical (+64.1%, p = 0.003) expansion were higher in ERαΔOcy mice compared to control (Dmp1CreERT2 ) mice at the tibial diaphysis, reflecting the known effects of estrogen to inhibit periosteal apposition and promote endocortical formation. Increases in Sost (2.1-fold, p = 0.001) messenger RNA (mRNA) levels were observed in trabecular bone at the spine in ERαΔOcy mice, consistent with previous reports that estrogen deficiency is associated with increased circulating sclerostin as well as bone SOST mRNA levels in humans. Further, the biological consequences of increased Sost expression were reflected in significant overall downregulation in panels of osteoblast and Wnt target genes in osteocyte-enriched bones from ERαΔOcy mice. These findings thus establish that osteocytic ERα is critical for estrogen action in female, but not male, adult bone metabolism. Moreover, the reduction in bone formation accompanied by increased Sost, decreased osteoblast, and decreased Wnt target gene expression in ERαΔOcy mice provides a direct link in vivo between ERα and Wnt signaling. © 2022 American Society for Bone and Mineral Research (ASBMR).

Positive and Negative Regulators of Sclerostin Expression

Sclerostin is secreted from osteocytes, binds to the Wnt co-receptor Lrp5/6, and affects the interaction between Wnt ligands and Lrp5/6, which inhibits Wnt/β-catenin signals and suppresses bone formation. Sclerostin plays an important role in the preservation of bone mass by functioning as a negative regulator of bone formation. A sclerostin deficiency causes sclerosteosis, which is characterized by an excess bone mass with enhanced bone formation in humans and mice. The expression of sclerostin is positively and negatively regulated by many factors, which also govern bone metabolism. Positive and negative regulators of sclerostin expression and their effects are introduced and discussed herein based on recent and previous findings, including our research.

Sclerostin: From Molecule to Clinical Biomarker

Sclerostin, a glycoprotein encoded by the SOST gene, is mainly produced by mature osteocytes and is a critical regulator of bone formation through its inhibitory effect on Wnt signaling. Osteocytes are differentiated osteoblasts that form a vast and highly complex communication network and orchestrate osteogenesis in response to both mechanical and hormonal cues. The three most commonly described pathways of SOST gene regulation are mechanotransduction, Wnt/β-catenin, and steroid signaling. Downregulation of SOST and thereby upregulation of local Wnt signaling is required for the osteogenic response to mechanical loading. This review covers recent findings concerning the identification of SOST, in vitro regulation of SOST gene expression, structural and functional properties of sclerostin, pathophysiology, biological variability, and recent assay developments for measuring circulating sclerostin. The three-dimensional structure of human sclerostin was generated with the AlphaFold Protein Structure Database applying a novel deep learning algorithm based on the amino acid sequence. The functional properties of the 3-loop conformation within the tertiary structure of sclerostin and molecular interaction with low-density lipoprotein receptor-related protein 6 (LRP6) are also reviewed. Second-generation immunoassays for intact/biointact sclerostin have recently been developed, which might overcome some of the reported methodological obstacles. Sclerostin assay standardization would be a long-term objective to overcome some of the problems with assay discrepancies. Besides the use of age- and sex-specific reference intervals for sclerostin, it is also pivotal to use assay-specific reference intervals since available immunoassays vary widely in their methodological characteristics.

Marein Prevented LPS-Induced Osteoclastogenesis by Regulating the NF-κB Pathway In Vitro

Many bone diseases such as osteolysis, osteomyelitis, and septic arthritis are caused by gram-negative bacterial infection, and lipopolysaccharide (LPS), a bacterial product, plays an essential role in this process. Drugs that inhibit LPS-induced osteoclastogenesis are urgently needed to prevent bone destruction in infective bone diseases. Marein, a major bioactive compound of Coreopsis tinctoria, possesses anti-oxidative, anti-inflammatory, anti-hypertensive, anti-hyperlipidemic, and anti-diabetic effects. In this study, we measured the effect of marein on RAW264.7 cells by CCK-8 assay and used TRAP staining to determine osteoclastogenesis. The levels of osteoclast-related genes and NF-κB-related proteins were then analyzed by western blot, and the levels of pro-inflammatory cytokines were quantified by ELISA. Our results showed that marein inhibited LPS-induced osteoclast formation by osteoclast precursor RAW264.7 cells. The effect of marein was related to its inhibitory function on expressions of pro-inflammatory cytokines and osteoclast-related genes containing RANK, TRAF6, MMP-9, CK, and CAII. Additionally, marein leads to markedly inhibited NF-κB signaling pathway activation in LPS-induced RAW264.7 cells. Concurrently, when the NF-κB signaling pathway was inhibited, osteoclast formation and pro-inflammatory cytokine expression were decreased. Collectively, marein could inhibit LPS-induced osteoclast formation in RAW264.7 cells via regulating the NF-κB signaling pathway. Our data demonstrate that marein might be a potential drug for bacteria-induced bone destruction disease. Our findings provide new insights into LPS-induced bone disease.

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.

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

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

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.

The osteocyte as a signaling cell

Osteocytes, former osteoblasts encapsulated by mineralized bone matrix, are far from being passive and metabolically inactive bone cells. Instead, osteocytes are multifunctional and dynamic cells capable of integrating hormonal and mechanical signals and transmitting them to effector cells in bone and in distant tissues. Osteocytes are a major source of molecules that regulate bone homeostasis by integrating both mechanical cues and hormonal signals that coordinate the differentiation and function of osteoclasts and osteoblasts. Osteocyte function is altered in both rare and common bone diseases, suggesting that osteocyte dysfunction is directly involved in the pathophysiology of several disorders affecting the skeleton. Advances in osteocyte biology initiated the development of novel therapeutics interfering with osteocyte-secreted molecules. Moreover, osteocytes are targets and key distributors of biological signals mediating the beneficial effects of several bone therapeutics used in the clinic. Here we review the most recent discoveries in osteocyte biology demonstrating that osteocytes regulate bone homeostasis and bone marrow fat via paracrine signaling, influence body composition and energy metabolism via endocrine signaling, and contribute to the damaging effects of diabetes mellitus and hematologic and metastatic cancers in the skeleton.

microRNA-136-5p from bone marrow mesenchymal stem cell-derived exosomes facilitates fracture healing by targeting LRP4 to activate the Wnt/β-catenin pathway

Aims

Exosomes derived from bone marrow mesenchymal stem cells (BMSCs) have been reported to be a promising cellular therapeutic approach for various human diseases. The current study aimed to investigate the mechanism of BMSC-derived exosomes carrying microRNA (miR)-136-5p in fracture healing.

Methods

A mouse fracture model was initially established by surgical means. Exosomes were isolated from BMSCs from mice. The endocytosis of the mouse osteoblast MC3T3-E1 cell line was analyzed. CCK-8 and disodium phenyl phosphate microplate methods were employed to detect cell proliferation and alkaline phosphatase (ALP) activity, respectively. The binding of miR-136-5p to low-density lipoprotein receptor related protein 4 (LRP4) was analyzed by dual luciferase reporter gene assay. HE staining, tartrate-resistant acid phosphatase (TRAP) staining, and immunohistochemistry were performed to evaluate the healing of the bone tissue ends, the positive number of osteoclasts, and the positive expression of β-catenin protein, respectively.

Results

miR-136-5p promoted fracture healing and osteoblast proliferation and differentiation. BMSC-derived exosomes exhibited an enriched miR-136-5p level, and were internalized by MC3T3-E1 cells. LRP4 was identified as a downstream target gene of miR-136-5p. Moreover, miR-136-5p or exosomes isolated from BMSCs (BMSC-Exos) containing miR-136-5p activated the Wnt/β-catenin pathway through the inhibition of LRP4 expression. Furthermore, BMSC-derived exosomes carrying miR-136-5p promoted osteoblast proliferation and differentiation, thereby promoting fracture healing.

Conclusion

BMSC-derived exosomes carrying miR-136-5p inhibited LRP4 and activated the Wnt/β-catenin pathway, thus facilitating fracture healing.

Cite this article: Bone Joint Res 2021;10(12):744–758.

Bone-derived sclerostin and Wnt/β-catenin signaling regulate PDGFRα+ adipoprogenitor cell differentiation

The Wnt signaling antagonist, sclerostin, is a potent suppressor of bone acquisition that also mediates endocrine communication between bone and adipose. As a result, Sost^-/- mice exhibit dramatic increases in bone formation but marked decreases in visceral and subcutaneous adipose that are secondary to alterations in lipid synthesis and utilization. While interrogating the mechanism by which sclerostin influences adipocyte metabolism, we observed paradoxical increases in the adipogenic potential and numbers of CD45^- :Sca1⁺ :PDGFRα⁺ adipoprogenitors in the stromal vascular compartment of fat pads isolated from male Sost^-/- mice. Lineage tracing studies indicated that sclerostin deficiency blocks the differentiation of PDGFRα⁺ adipoprogenitors to mature adipocytes in association with increased Wnt/β-catenin signaling. Importantly, osteoblast/osteocyte-specific Sost gene deletion mirrors the accumulation of PDGFRα⁺ adipoprogenitors, reduction in fat mass, and improved glucose metabolism evident in Sost^-/- mice. These data indicate that bone-derived sclerostin regulates multiple facets of adipocyte physiology ranging from progenitor cell commitment to anabolic metabolism.

LRP4 promotes migration and invasion of gastric cancer under the regulation of microRNA-140-5p

BACKGROUND

Low-density lipoprotein receptor-related protein 4 (LRP4) has been reported to be implicated in multiple types of cancers. However, the significance of LRP4 in gastric cancer (GC) remains poorly elucidated. Therefore, it's urgent to investigate the importance and underlying mechanisms of LRP4 in GC.

OBJECTIVE

To investigate the clinical roles of LRP4 in GC.

METHODS

The LRP4 mRNA and miR-140-5p was measured by qRT-PCR. The protein expression was determined Western blot. Kaplan-Meier survival curves and Cox proportional hazard regression models were performed to evaluate prognosis.

RESULTS

We demonstrated that LRP4 mRNA and protein was up-regulated in GC tissues for the first time. Its high expression was significantly correlated with malignant clinical features including TNM stage and lymph-node metastasis and poor prognosis for GC patients. LRP4 promotes migration, invasion and epithelial-mesenchymal transition (EMT) progress of GC cells. Mechanically, LRP4 regulated PI3K/AKT in GC cells. AKT inhibitors reversed the effects of LRP4. Finally, LRP4 was regulated by miR-140-5p in GC.

CONCLUSIONS

Our findings showed that LRP4 has an important function in GC progression and promotes GC migration, invasion and EMT by regulating PI3K/AKT under regulation of miR-140-5p, providing a potential therapeutic target for GC.

Role of Osteocytes in Cancer Progression in the Bone and the Associated Skeletal Disease

PURPOSE OF REVIEW

The goal of this manuscript is to review the current knowledge on the role of osteocytes in cancer in the bone, discuss the potential of osteocytes as a therapeutic target, and propose future research needed to understand the crosstalk between cancer cells and osteocytes in the tumor niche.

RECENT FINDINGS

Numerous studies have established that cancer cells manipulate osteocytes to facilitate invasion and tumor progression in bone. Moreover, cancer cells dysregulate osteocyte function to disrupt physiological bone remodeling, leading to the development of bone disease. Targeting osteocytes and their derived factors has proven to effectively interfere with the progression of cancer in the bone and the associated bone disease. Osteocytes communicate with cancer cells and are also part of the vicious cycle of cancer in the bone. Additional studies investigating the role of osteocytes on metastases to the bone and the development of drug resistance are needed.

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.

Mechanical loading prevents bone destruction and exerts anti-tumor effects in the MOPC315.BM.Luc model of myeloma bone disease

Bone continually adapts to changing external loading conditions via (re)modeling (modeling and remodeling) processes. While physical activity is known to beneficially enhance bone mass in healthy individuals, little is known in how physical stimuli affect osteolytic bone destruction in patients suffering from multiple myeloma bone disease. Multiple myeloma (MM) is caused by malignant plasma cells in the bone marrow, shifting the balance in bone remodeling towards massive resorption. We hypothesized that in vivo tibial mechanical loading has anabolic effects in mice with locally injected MOPC315.BM.Luc cells. Conventional microCT analysis revealed enhanced cortical bone mass and microstructure in loaded compared to nonloaded mice. State-of-the-art time-lapse microCT based image analysis demonstrated bone (re)modeling processes at the endosteal and periosteal surfaces as the underlying causes of increased bone mass. Loading prevented the progression and development of osteolytic destruction. Physical stimuli also diminished local MM cell growth and dissemination evidenced by quantification of MM cell-specific immunoglobulin A levels in the serum of mice and by bioluminescence analysis. These data indicate that mechanical loading not only rescues the bone phenotype, but also exerts cell-extrinsic anti-myeloma effects in the MOPC315.BM.Luc model. In conclusion, the use of physical stimuli should be further investigated as an anabolic treatment for osteolytic bone destruction in patients with MM.

Sclerostin and Osteocalcin: Candidate Bone-Produced Hormones

In addition to its structural role, the skeleton serves as an endocrine organ that controls mineral metabolism and energy homeostasis. Three major cell types in bone - osteoblasts, osteoclasts, and osteocytes - dynamically form and maintain bone and secrete factors with systemic activity. Osteocalcin, an osteoblast-derived factor initially described as a matrix protein that regulates bone mineralization, has been suggested to be an osteoblast-derived endocrine hormone that regulates multiple target organs including pancreas, liver, muscle, adipose, testes, and the central and peripheral nervous system. Sclerostin is predominantly produced by osteocytes, and is best known as a paracrine-acting regulator of WNT signaling and activity of osteoblasts and osteoclasts on bone surfaces. In addition to this important paracrine role for sclerostin within bone, sclerostin protein has been noted to act at a distance to regulate adipocytes, energy homeostasis, and mineral metabolism in the kidney. In this article, we aim to bring together evidence supporting an endocrine function for sclerostin and osteocalcin, and discuss recent controversies regarding the proposed role of osteocalcin outside of bone. We summarize the current state of knowledge on animal models and human physiology related to the multiple functions of these bone-derived factors. Finally, we highlight areas in which future research is expected to yield additional insights into the biology of osteocalcin and sclerostin.

LRP4 LDLα repeats of astrocyte enhance dendrite arborization of the neuron

The low-density lipoprotein receptor-related protein 4 (LRP4) is essential for inducing the neuromuscular junction (NMJ) formation in muscle fibers, and LRP4 plays a critical role in dendritic development and synaptogenesis in the central nervous system (CNS). As a single transmembrane protein, LRP4 contains an enormously sizeable extracellular domain (ECD), containing multiple LDLα repeats in the N-terminal of ECD. LRP4 only with extracellular domain acts as a similar mechanism of full-length LRP4 in muscles to stimulate acetylcholine receptor clustering. In this study, we elucidated that LDLα repeats of LRP4 maintained the body weight and survival rate. Dendritic branches of the pyramidal neurons in Lrp4-null mice with LRP4 LDLα repeats residue were more than in Lrp4-null mice without residual LRP4 domain. Supplement with conditioned medium from LRP4 LDLα overexpression cells, the primary culture pyramidal neurons achieved strong dendritic arborization ability. Besides, astrocytes with LRP4 LDLα repeats residue could promote pyramidal neuronal dendrite arborization in the primary co-cultured system. These observations signify that LRP4 LDLα repeats play a prominent underlying role in dendrite arborization.

Bone and Muscle Crosstalk in Aging

Osteoporosis and sarcopenia are two age-related diseases that affect the quality of life in the elderly. Initially, they were thought to be two independent diseases; however, recently, increasing basic and clinical data suggest that skeletal muscle and bone are both spatially and metabolically connected. The term "osteosarcopenia" is used to define a condition of synergy of low bone mineral density with muscle atrophy and hypofunction. Bone and muscle cells secrete several factors, such as cytokines, myokines, and osteokines, into the circulation to influence the biological and pathological activities in local and distant organs and cells. Recent studies reveal that extracellular vesicles containing microRNAs derived from senescent skeletal muscle and bone cells can also be transported and aid in regulating bone-muscle crosstalk. In this review, we summarize the age-related changes in the secretome and extracellular vesicle-microRNAs secreted by the muscle and bone, and discuss their interactions between muscle and bone cells during aging.

Molecular Basis for Craniofacial Phenotypes Caused by Sclerostin Deletion

Some genetic disorders are associated with distinctive facial features, which can aid in diagnosis. While considerable advances have been made in identifying causal genes, relatively little progress has been made toward understanding how a particular genotype results in a characteristic craniofacial phenotype. An example is sclerosteosis/van Buchem disease, which is caused by mutations in the Wnt inhibitor sclerostin (SOST). Affected patients have a high bone mass coupled with a distinctive appearance where the mandible is enlarged and the maxilla is foreshortened. Here, mice carrying a null mutation in Sost were analyzed using quantitative micro-computed tomographic (µCT) imaging and histomorphometric analyses to determine the extent to which the size and shape of craniofacial skeleton were altered. Sost^-/- mice exhibited a significant increase in appositional bone growth, which increased the height and width of the mandible and reduced the diameters of foramina. In vivo fluorochrome labeling, histology, and immunohistochemical analyses indicated that excessive bone deposition in the premaxillary suture mesenchyme curtailed overall growth, leading to midfacial hypoplasia. The amount of bone extracellular matrix produced by Sost^-/- cells was significantly increased; as a consequence, osteoid seams were evident throughout the facial skeleton. Collectively, these analyses revealed a remarkable fidelity between human characteristics of sclerosteosis/van Buchem disease and the Sost^-/- phenotype and provide clues into the conserved role for sclerostin signaling in modulating craniofacial morphology.

Therapeutic and Mechanistic Approaches of Tridax Procumbens Flavonoids for the Treatment of Osteoporosis

Homeostasis of bone is closely regulated by the balanced activities between the bone resorbing activity of osteoclast cells and bone-forming ability of osteoblast cells. Multinucleated osteoclasts degrade bone matrix and involve in the dynamic bone remodelling in coordination with osteoblasts. Disruption of this regulatory balance between these cells or any imbalance in bone remodelling caused by a higher rate of resorption over construction of bone results in a decrease of bone matrix including bone mineral density (BMD). These osteoclast-dominant effects result in a higher risk of bone crack and joint demolition in several bone-related diseases, including osteoporosis and rheumatoid arthritis (RA). Tridax procumbens is a very interesting perennial plant and its secondary metabolites called here T. procumbens flavonoids (TPFs) are well-known phytochemical agents owing to various therapeutic practices such as anti-inflammatory, anti-anaemic and anti-diabetic actions. This review designed to focus the systematic convention concerning the medicinal property and mechanism of actions of TPFs for the management of bone-related diseases. Based on the current literature, the review offers evidence-based information of TPFs for basic researchers and clinicians for the prevention and treatment of bone related diseases, including osteoporosis. It also emphasizes the medical significance for more research to comprehend the cellular signalling pathways of TPFs for the regulation of bone remodelling and discusses the possible promising ethnobotanical resource that can convey the preclinical and clinical clues to develop the next generation therapeutic agents for the treatment of bonerelated disorders.

Associations of serum sclerostin with bone mineral density, markers of bone metabolism and thalassaemia characteristics in adult patients with transfusion-dependent beta-thalassaemia

Aim of the study: To assess serum sclerostin in transfusion-dependent beta-thalassaemia patients versus healthy controls and to examine its associations with bone mineral density, bone metabolism markers and beta thalassaemia alterations.Material and methods: Sixty-two transfusion-dependent beta-thalassaemia (TDßT) patients and 30 healthy controls were evaluated for serum sclerostin, osteocalcin, beta-cross laps, osteoprotegerin and serum level of receptor activator of nuclear factor kappa-Β ligand (sRANKL). Bone mineral density was measured at the lumbar spine and femoral neck. Thalassaemia characteristics were collected from the patients' medical records.Results: A significantly higher sclerostin level (median 565.50 pmol/L) was observed in the transfusion-dependent beta-thalassaemia patients vs. the healthy controls (median 48.65 pmol/L, p < .001). Sclerostin showed significant associations with the Z-scores at the lumbar spine and femoral neck, osteocalcin, beta-cross laps, osteoprotegerin, sRANKL, pretransfusion haemoglobin, liver iron concentration and female gonadal state. Significantly higher levels of sclerostin were observed in splenectomized TDßT patients and in those with fragility fractures. Age, sex, body mass index, disease severity, serum ferritin, cardiac T2* and male gonadal state did not show significant associations with sclerostin.Conclusion: Sclerostin may play a role in the bone pathophysiology of beta-thalassaemia patients and could serve as a marker of severe osteoporosis.KEY MЕSSAGESSerum sclerostin is more than 10-fold higher in adult patients with transfusion-dependent beta-thalassaemia compared to healthy controls.Serum sclerostin is negatively associated with bone mineral density and the bone synthesis markers and positively with the bone resorption indices.Serum sclerostin is significantly associated with pre-transfusion haemoglobin, liver iron concentration, splenectomy status and fragility fracture events in adult patients with transfusion-dependent beta-thalassaemia.Serum sclerostin could serve as a marker of severe osteoporosis in beta-thalassaemia patients.

New Targets and Emergent Therapies for Osteoporosis

The 11 existing FDA-approved osteoporosis drug treatments include hormone replacement therapy, 2 SERMs (raloxifene and bazedoxifene), 5 inhibitors of bone-resorbing osteoclasts (4 bisphosphonates and anti-RANKL denosumab), 2 parathyroid hormone analogues (teriparatide and abaloparatide), and 1 WNT signaling enhancer (romosozumab). These therapies are effective and provide multiple options for patients and physicians. As the genomic revolution continues, potential novel targets for future drug development are identified. This review takes a wide perspective to describe potentially rewarding topics to explore, including knowledge of genes and pathways involved in bone cell metabolism, the utility of animal models, targeting drugs to bone, and ongoing advances in drug design and delivery.

The Regulation of Bone Metabolism and Disorders by Wnt Signaling

Wnt, a secreted glycoprotein, has an approximate molecular weight of 40 kDa, and it is a cytokine involved in various biological phenomena including ontogeny, morphogenesis, carcinogenesis, and maintenance of stem cells. The Wnt signaling pathway can be classified into two main pathways: canonical and non-canonical. Of these, the canonical Wnt signaling pathway promotes osteogenesis. Sclerostin produced by osteocytes is an inhibitor of this pathway, thereby inhibiting osteogenesis. Recently, osteoporosis treatment using an anti-sclerostin therapy has been introduced. In this review, the basics of Wnt signaling, its role in bone metabolism and its involvement in skeletal disorders have been covered. Furthermore, the clinical significance and future scopes of Wnt signaling in osteoporosis, osteoarthritis, rheumatoid arthritis and neoplasia are discussed.

Lrp4 Mediates Bone Homeostasis and Mechanotransduction through Interaction with Sclerostin In Vivo

Wnt signaling plays a key role in regulating bone remodeling. In vitro studies suggest that sclerostin's inhibitory action on Lrp5 is facilitated by the membrane-associated receptor Lrp4. We generated an Lrp4 R1170W knockin mouse model (Lrp4^KI), based on a published mutation in patients with high bone mass (HBM). Lrp4^KI mice have an HBM phenotype (assessed radiographically), including increased bone strength and formation. Overexpression of a Sost transgene had osteopenic effects in Lrp4-WT but not Lrp4^KI mice. Conversely, sclerostin inhibition had blunted osteoanabolic effects in Lrp4^KI mice. In a disuse-induced bone wasting model, Lrp4^KI mice exhibit significantly less bone loss than wild-type (WT) mice. In summary, mice harboring the Lrp4-R1170W missense mutation recapitulate the human HBM phenotype, are less sensitive to altered sclerostin levels, and are protected from disuse-induced bone loss. Lrp4 is an attractive target for pharmacological targeting aimed at increasing bone mass and preventing bone loss due to disuse.

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Missense mutation in the third beta-propeller of Lrp4 improve bone properties

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The R1170W mutation in Lrp4 interferes with sclerostin inhibition in vivo

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The R1170W Lrp4 mutation alters the bone wasting effects of mechanical disuse

Reactive Oxygen Species in Osteoclast Differentiation and Possible Pharmaceutical Targets of ROS-Mediated Osteoclast Diseases

Reactive oxygen species (ROS) and free radicals are essential for transmission of cell signals and other physiological functions. However, excessive amounts of ROS can cause cellular imbalance in reduction–oxidation reactions and disrupt normal biological functions, leading to oxidative stress, a condition known to be responsible for the development of several diseases. The biphasic role of ROS in cellular functions has been a target of pharmacological research. Osteoclasts are derived from hematopoietic progenitors in the bone and are essential for skeletal growth and remodeling, for the maintenance of bone architecture throughout lifespan, and for calcium metabolism during bone homeostasis. ROS, including superoxide ion (O₂⁻) and hydrogen peroxide (H₂O₂), are important components that regulate the differentiation of osteoclasts. Under normal physiological conditions, ROS produced by osteoclasts stimulate and facilitate resorption of bone tissue. Thus, elucidating the effects of ROS during osteoclast differentiation is important when studying diseases associated with bone resorption such as osteoporosis. This review examines the effect of ROS on osteoclast differentiation and the efficacy of novel chemical compounds with therapeutic potential for osteoclast related diseases.

Reptiles in Space Missions: Results and Perspectives

Reptiles are a rare model object for space research. However, some reptile species demonstrate effective adaptation to spaceflight conditions. The main scope of this review is a comparative analysis of reptile experimental exposure in weightlessness, demonstrating the advantages and shortcomings of this model. The description of the known reptile experiments using turtles and geckos in the space and parabolic flight experiments is provided. Behavior, skeletal bones (morphology, histology, and X-ray microtomography), internal organs, and the nervous system (morphology, histology, and immunohistochemistry) are studied in the spaceflight experiments to date, while molecular and physiological results are restricted. Therefore, the results are discussed in the scope of molecular data collected from mammalian (mainly rodents) specimens and cell cultures in the parabolic and orbital flights and simulated microgravity. The published data are compared with the results of the gecko model studies after the 12–44.5-day spaceflights with special reference to the unique peculiarities of the gecko model for the orbital experiments. The complex study of thick-toed geckos after three spaceflights, in which all geckos survived and demonstrated effective adaptation to spaceflight conditions, was performed. However, future investigations are needed to study molecular mechanisms of gecko adaptation in space.

TG-interacting factor 1 (Tgif1)-deficiency attenuates bone remodeling and blunts the anabolic response to parathyroid hormone

Osteoporosis is caused by increased bone resorption and decreased bone formation. Intermittent administration of a fragment of Parathyroid hormone (PTH) activates osteoblast-mediated bone formation and is used in patients with severe osteoporosis. However, the mechanisms by which PTH elicits its anabolic effect are not fully elucidated. Here we show that the absence of the homeodomain protein TG-interacting factor 1 (Tgif1) impairs osteoblast differentiation and activity, leading to a reduced bone formation. Deletion of Tgif1 in osteoblasts and osteocytes decreases bone resorption due to an increased secretion of Semaphorin 3E (Sema3E), an osteoclast-inhibiting factor. Tgif1 is a PTH target gene and PTH treatment failed to increase bone formation and bone mass in Tgif1-deficient mice. Thus, our study identifies Tgif1 as a novel regulator of bone remodeling and an essential component of the PTH anabolic action. These insights contribute to a better understanding of bone metabolism and the anabolic function of PTH.

Lrp4 expression by adipocytes and osteoblasts differentially impacts sclerostin's endocrine effects on body composition and glucose metabolism

Sclerostin exerts profound local control over bone acquisition and also mediates endocrine communication between fat and bone. In bone, sclerostin's anti-osteoanabolic activity is enhanced by low-density lipoprotein receptor-related protein 4 (Lrp4), which facilitates its interaction with the Lrp5 and Lrp6 Wnt co-receptors. To determine whether Lrp4 similarly affects sclerostin's endocrine function, we examined body composition as well as glucose and fatty acid metabolism in mice rendered deficient of Lrp4 in the adipocyte (AdΔLrp4) or the osteoblast (ObΔLrp4). AdΔLrp4 mice exhibit a reduction in adipocyte hypertrophy and improved glucose and lipid homeostasis, marked by increased glucose and insulin tolerance and reduced serum fatty acids, and mirror the effect of sclerostin deficiency on whole-body metabolism. Indeed, epistasis studies place adipocyte-expressed Lrp4 and sclerostin in the same genetic cascade that regulates adipocyte function. Intriguingly, ObΔLrp4 mice, which exhibit dramatic increases in serum sclerostin, accumulate body fat and develop impairments in glucose tolerance and insulin sensitivity despite development of a high bone mass phenotype. These data indicate that expression of Lrp4 by both the adipocyte and osteoblast is required for normal sclerostin endocrine function and that the impact of sclerostin deficiency on adipocyte physiology is distinct from the effect on osteoblast function.

Bone Measurements by Peripheral Quantitative Computed Tomography in Rodents

This chapter provides information for the in vivo use of peripheral quantitative computed tomography in rats and mice to determine bone density and cortical geometric data, including suggestions for study design, instrument setting, and data interpretation. This update also provides guidance for the use of pQCT to extract muscle and fat cross-sectional area information from the bone scans.

Osteocyte regulation of bone and blood

This past decade has witnessed a renewed interest in the function and biology of matrix-embedded osteocytes and these cells have emerged as master regulators of bone homeostasis. They secrete two very powerful proteins, sclerostin, a Wnt-inhibitor, that suppresses bone formation, and receptor-activator of NF-kB ligand (RANKL), a cytokine required for osteoclastogenesis. Neutralizing antibodies against these proteins are currently used for the treatment of osteoporosis. Recent studies however, ascribed yet another function to osteocytes: the control of hematopoiesis and the HSPC niche, directly and through secreted factors. In the absence of osteocytes there is an increase in HSC mobilization and abnormal lymphopoiesis whereas in the absence of G_sα signaling in these cells there is an increase of myeloid cells. How exactly osteocytes control hematopoiesis or the HSPC niche is still not completely understood. In this review we summarize the actions of osteocytes in bone and then analyze the effects of these cells on hematopoiesis. Future directions and gaps in current knowledge are further discussed.

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.

Sclerostin/Receptor Related Protein 4 and Ginkgo Biloba Extract Alleviates β-Glycerophosphate-Induced Vascular Smooth Muscle Cell Calcification By Inhibiting Wnt/β-Catenin Pathway

BACKGROUND

Abnormal mineral metabolism in patients with chronic kidney disease (CKD) may lead to vascular calcification, which is markedly associated with adverse events, including ischemic cardiac diseases and all-cause cardiovascular mortality. Thus, preventing and treating vascular calcification play an important role in improving the prognosis of CKD patients.

OBJECTIVES

To investigate the potential functions of sclerostin and low-density lipoprotein receptor-related protein 4 (Lrp4) in alleviating the β-glycerophosphate (β-GP)-induced vascular smooth muscle cell (VSMC) calcification, and the protective effect of Ginkgo biloba extract (GBE).

METHODS

VSMC were extracted from Sprague-Dawley rat aorta and cultured in medium. The VSMCs were divided into 3 groups: (1) Negative control group, (2) β-GP group, in which the VSMCs were treated with β-GP, and (3) GBE and β-GP group, where the VSMCs were treated with both β-GP and GBE. The calcium nodules within the cells were examined by using Alizarin red S staining. The mRNA expression levels of β-catenin and bone gamma-carboxyglutamic-acid-containing proteins (BGP) were detected by real-time PCR. The protein levels of sclerostin and Lrp4 were determined by Western blot.

RESULTS

Alizarin red S staining showed that the VSMCs in β-GP group had a distinct orange-red precipitate when compared with VSMCs in the negative control group, while the orange-red precipitate of the GBE and β-GP group was significantly reduced compared to the β-GP group. Real-time PCR showed that the mRNA levels of β-catenin and BGP in VSMCs of β-GP group were significantly higher than those of the negative control group (p < 0.05); while they were significantly reduced in VSMCs of the GBE and β-GP group (p < 0.05). Western blot results showed that the expression of sclerostin in the β-GP group was significantly higher than that in the control group (p < 0.05), whereas Lrp4 was significantly lower than in control group (p < 0.05). Sclerostin in GBE and β-GP group was significantly reduced (p < 0.05), but Lrp4 was significantly elevated when compared with that of the β-GP group (p < 0.05).

CONCLUSION

β-GP induced VSMC calcification by activating the Wnt/β-catenin signaling pathway. Sclerostin and Lrp4 were involved in β-GP-induced VSMC calcification and play an important role. GBE could alleviate VSMC calcification induced by β-GP through inhibiting the Wnt/β-catenin signaling pathway.