Osteocyte-specific WNT1 regulates osteoblast function during bone homeostasis

Metabolic reprogramming in skeletal cell differentiation

The human skeleton is a multifunctional organ made up of multiple cell types working in concert to maintain bone and mineral homeostasis and to perform critical mechanical and endocrine functions. From the beginning steps of chondrogenesis that prefigures most of the skeleton, to the rapid bone accrual during skeletal growth, followed by bone remodeling of the mature skeleton, cell differentiation is integral to skeletal health. While growth factors and nuclear proteins that influence skeletal cell differentiation have been extensively studied, the role of cellular metabolism is just beginning to be uncovered. Besides energy production, metabolic pathways have been shown to exert epigenetic regulation via key metabolites to influence cell fate in both cancerous and normal tissues. In this review, we will assess the role of growth factors and transcription factors in reprogramming cellular metabolism to meet the energetic and biosynthetic needs of chondrocytes, osteoblasts, or osteoclasts. We will also summarize the emerging evidence linking metabolic changes to epigenetic modifications during skeletal cell differentiation.

A mechanism of action-reflective, dual cell-based bioassay for determining the bioactivity of sclerostin-neutralizing antibodies

Osteoporosis is a major threat to the elderly worldwide. The Wnt signaling pathway plays a critical role in bone development and homeostasis. Sclerostin, a Wnt ligand inhibitor, competes with Wnt ligands for low-density lipoprotein receptor-related protein 5 or 6 (LRP5/6) on osteoblasts, thereby suppressing bone formation. Sclerostin-neutralizing monoclonal antibodies (mAbs) have emerged as a potential bone-forming therapy for osteoporosis. A cell-based bioassay which determines the relative activity of a product, related to its mechanism of action, is of great importance from drug discovery to quality control and batch release. Currently used cell-based bioassays for sclerostin-neutralizing mAbs usually use Wnt1 or Wnt3a to stimulate the Wnt pathway; sclerostin is a direct inhibitor of Wnt1 but not Wnt3a. Wnt1 is a highly hydrophobic protein that binds to the producing cell membrane and acts in a juxtacrine manner to stimulate the Wnt pathway in neighboring cells. Bioassays for drugs that induce Wnt1 signaling should be performed in a juxtacrine manner. Here, we present a mechanism of action-reflective, dual cell-based reporter gene assay. In this assay, Wnt1 producer cells are co-cultured with cells containing the Wnt reporter genes, Wnt1 on the producer cells activates the Wnt signaling pathway in the reporter cells that are in direct cell-to-cell contact, and sclerostin-neutralizing mAbs specifically and effectively antagonize the sclerostin-mediated Wnt reporter gene suppression. This bioassay demonstrates good specificity, accuracy, linearity, and precision and is suitable for quality control, stability testing, batch release, and biosimilarity assessment of sclerostin-neutralizing mAbs.

Piezo1 expression in mature osteocytes is dispensable for the skeletal response to mechanical loading

Mechanical loading is essential for bone growth and homeostasis, with osteocytes considered the primary mechanosensors. Deleting the mechanosensitive ion channel Piezo1 from Dmp1-Cre-targeted cells, which include both osteoblasts and osteocytes, resulted in reduced bone mass and impaired skeletal responses to mechanical stimuli. To specifically isolate Piezo1's role in osteocytes, we used Sost-Cre mice to generate mice with Piezo1 deletion exclusively in mature osteocytes. These mice exhibited lower bone mineral density, decreased cancellous bone mass, and reduced cortical thickness with decrease periosteal expansion. However, unlike mice lacking Piezo1 in both osteoblasts and osteocytes, those with Piezo1 deletion in mature osteocytes responded normally to mechanical loading. These findings suggest that Piezo1 expression in mature osteocytes contributes to bone maintenance under normal physiological condition, but is dispensable for the skeletal response to mechanical loading.

O-GlcNAcylation in the osteoblast lineage-boosting the complexity of Wnt-stimulated bone formation

The molecular mechanisms explaining the osteogenic influence of Wnt molecules are still not fully clarified. A study in this issue shows that O-GlcNAcylation is required for the osteoanabolic effects of Wnt stimulation.

Multi-omics analyses reveal aberrant differentiation trajectory with WNT1 loss-of-function in type XV osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a group of severe genetic bone disorders characterized by congenital low bone mass, deformity, and frequent fractures. Type XV OI is a moderate to severe form of skeletal dysplasia caused by WNT1 variants. In this cohort study from southern China, we summarized the clinical phenotypes of patients with WNT1 variants and found that the proportion of type XV patients was around 10.3% (25 out of 243) with a diverse spectrum of phenotypes. Functional assays indicated that variants of WNT1 significantly impaired its secretion and effective activity, leading to moderate to severe clinical manifestations, porous bone structure, and enhanced osteoclastic activities. Analysis of proteomic data from human skeleton indicated that the expression of SOST (sclerostin) was dramatically reduced in type XV patients compared to patients with COL1A1 quantitative variants. Single-cell transcriptome data generated from human tibia samples of patients diagnosed with type XV OI and leg-length discrepancy, respectively, revealed aberrant differentiation trajectories of skeletal progenitors and impaired maturation of osteocytes with loss of WNT1, resulting in excessive CXCL12+ progenitors, fewer mature osteocytes, and the existence of abnormal cell populations with adipogenic characteristics. The integration of multi-omics data from human skeleton delineates how WNT1 regulates the differentiation and maturation of skeletal progenitors, which will provide a new direction for the treatment strategy of type XV OI and relative low bone mass diseases such as early onset osteoporosis.

Update on the Genetics of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous heritable skeletal dysplasia characterized by bone fragility and deformity, growth deficiency, and other secondary connective tissue defects. OI is now understood as a collagen-related disorder caused by defects of genes whose protein products interact with collagen for folding, post-translational modification, processing and trafficking, affecting bone mineralization and osteoblast differentiation. This review provides the latest updates on genetics of OI, including new developments in both dominant and rare OI forms, as well as the signaling pathways involved in OI pathophysiology. There is a special emphasis on discoveries of recessive mutations in TENT5A, MESD, KDELR2 and CCDC134 whose causality of OI types XIX, XX, XXI and XXI, respectively, is now established and expends the complexity of mechanisms underlying OI to overlap LRP5/6 and MAPK/ERK pathways. We also review in detail new discoveries connecting the known OI types to each other, which may underlie an eventual understanding of a final common pathway in OI cellular and bone biology.

The role of calcium channels in osteoporosis and their therapeutic potential

Osteoporosis, a systemic skeletal disorder marked by diminished bone mass and compromised bone microarchitecture, is becoming increasingly prevalent due to an aging population. The underlying pathophysiology of osteoporosis is attributed to an imbalance between osteoclast-mediated bone resorption and osteoblast-mediated bone formation. Osteoclasts play a crucial role in the development of osteoporosis through various molecular pathways, including the RANK/RANKL/OPG signaling axis, cytokines, and integrins. Notably, the calcium signaling pathway is pivotal in regulating osteoclast activation and function, influencing bone resorption activity. Disruption in calcium signaling can lead to increased osteoclast-mediated bone resorption, contributing to the progression of osteoporosis. Emerging research indicates that calcium-permeable channels on the cellular membrane play a critical role in bone metabolism by modulating these intracellular calcium pathways. Here, we provide an overview of current literature on the regulation of plasma membrane calcium channels in relation to bone metabolism with particular emphasis on their dysregulation during the progression of osteoporosis. Targeting these calcium channels may represent a potential therapeutic strategy for treating osteoporosis.

Osteomodulin deficiency in mice causes a specific reduction of transversal cortical bone size

Skeletal growth, modeling, and remodeling are regulated by various molecules, one of them being the recently identified osteoanabolic factor WNT1. We have previously reported that WNT1 transcriptionally activates the expression of Omd, encoding Osteomodulin (OMD), in a murine mesenchymal cell line, which potentially explained the skeletal fragility of mice with mutational WNT1 inactivation, since OMD has been shown to regulate type I collagen fibril formation in vitro. In this study we confirmed the strong induction of Omd expression in a genome-wide expression analysis of transfected cells, and we obtained further evidence for Omd being a direct target gene of WNT1. To assess the in vivo relevance of this regulation, we crossed Omd-deficient mice with a mouse line harboring an inducible, osteoblast-specific Wnt1 transgene. After induction of Wnt1 expression for 1 or 3 weeks, the osteoanabolic potency of WNT1 was not impaired despite the Omd deficiency. Since current knowledge regarding the in vivo physiological function of OMD is limited, we next focused on skeletal phenotyping of wild-type and Omd-deficient littermates, in the absence of a Wnt1 transgene. Here we did not observe an impact of Omd deficiency on trabecular bone parameters by histomorphometry and μCT either. Importantly, however, male and female Omd-deficient mice at the ages of 12 and 24 weeks displayed a slender bone phenotype with significantly smaller long bones in the transversal dimension, while the longitudinal bone growth remained unaffected. Although mechanical testing revealed no significant changes explained by impaired bone material properties, atomic force microscopy of the femoral bone surface of Omd-deficient mice revealed moderate changes at the nanostructural level, indicating altered regulation of collagen fibril formation and aggregation. Taken together, our data demonstrate that, although OMD is dispensable for the osteoanabolic effect of WNT1, its deficiency in mice specifically modulates transversal cortical bone morphology.

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.

Spatial Multi-omics Reveals the Role of the Wnt Modulator, Dkk2, in Palatogenesis'

Multiple genetic and environmental etiologies contribute to the pathogenesis of cleft palate, which is the most common of the inherited disorders of the craniofacial complex. Insights into the molecular mechanisms regulating osteogenic differentiation and patterning in the palate during embryogenesis are limited and needed for the development of innovative diagnostics and cures. This study used the Pax9-/- mouse model with a consistent phenotype of cleft secondary palate to investigate the role of Pax9 in the process of palatal osteogenesis. Although prior research has identified the upregulation of Wnt pathway modulators Dkk1 and Dkk2 in Pax9-/- palate mesenchyme, limitations of spatial resolution and technology restricted a more robust analysis. Here, data from single-nucleus transcriptomics and chromatin accessibility assays validated by in situ highly multiplex targeted single-cell spatial profiling technology suggest a distinct relationship between Pax9+ and osteogenic populations. Loss of Pax9 results in spatially restricted osteogenic domains bounded by Dkk2, which normally interfaces with Pax9 in the mesenchyme. Moreover, the loss of Pax9 leads to a disruption in the normal osteodifferentiaion of palatal osteogenic mesenchymal cells. These results suggest that Pax9-dependent Wnt signaling modulators influence osteogenic programming during palate formation, potentially contributing to the observed cleft palate phenotype.

A novel WNT10A variant impairs the homeostasis of alveolar bone mesenchymal stem cells

OBJECTIVES

To explore the influence of a novel WNT10A variant on bone mineral density, proliferation, and osteogenic differentiation capacities of alveolar bone mesenchymal stem cells in humans.

SUBJECTS AND METHODS

Whole-exome sequencing and Sanger sequencing were utilized to detect gene variants in a family with non-syndromic tooth agenesis (NSTA). The panoramic mandibular index was calculated on the proband with WNT10A variant and normal controls to evaluate bone mineral density. Alveolar bone mesenchymal stem cells from the proband with a novel WNT10A variant and normal controls were isolated and cultured, then proliferation and osteogenic differentiation capacities were evaluated and compared.

RESULTS

We identified a novel WNT10A pathogenic missense variant (c.353A > G/p. Tyr118Cys) in a family with NSTA. The panoramic mandibular index of the proband implied a reduction in bone mineral density. Moreover, the proliferation and osteogenic differentiation capacities of alveolar bone mesenchymal stem cells from the proband with WNT10A Tyr118Cys variant were significantly decreased.

CONCLUSIONS

Our findings broaden the spectrum of WNT10A variants in patients with non-syndromic oligodontia, suggest an association between WNT10A and the proliferation and osteogenic differentiation of alveolar bone mesenchymal stem cells, and demonstrate that WNT10A is involved in maintaining jaw bone homeostasis.

Genetic Evaluation for Monogenic Disorders of Low Bone Mass and Increased Bone Fragility: What Clinicians Need to Know

PURPOSE OF REVIEW

The purpose of this review is to outline the principles of clinical genetic testing and to provide practical guidance to clinicians in navigating genetic testing for patients with suspected monogenic forms of osteoporosis.

RECENT FINDINGS

Heritability assessments and genome-wide association studies have clearly shown the significant contributions of genetic variations to the pathogenesis of osteoporosis. Currently, over 50 monogenic disorders that present primarily with low bone mass and increased risk of fractures have been described. The widespread availability of clinical genetic testing offers a valuable opportunity to correctly diagnose individuals with monogenic forms of osteoporosis, thus instituting appropriate surveillance and treatment. Clinical genetic testing may identify the appropriate diagnosis in a subset of patients with low bone mass, multiple or unusual fractures, and severe or early-onset osteoporosis, and thus clinicians should be aware of how to incorporate such testing into their clinical practices.

Mechanical induction of osteoanabolic Wnt1 promotes osteoblast differentiation via Plat

Physical activity-induced mechanical stimuli play a crucial role in preserving bone mass and structure by promoting bone formation. While the Wnt pathway is pivotal for mediating the osteoblast response to loading, the exact mechanisms are not fully understood. Here, we found that mechanical stimulation induces osteoblastic Wnt1 expression, resulting in an upregulation of key osteogenic marker genes, including Runx2 and Sp7, while Wnt1 knockdown using siRNA prevented these effects. RNAseq analysis identified Plat as a major target through which Wnt1 exerts its osteogenic influence. This was corroborated by Plat depletion using siRNA, confirming its positive role in osteogenic differentiation. Moreover, we demonstrated that mechanical stimulation enhances Plat expression, which, in turn leads to increased expression of osteogenic markers like Runx2 and Sp7. Notably, Plat depletion by siRNA prevented this effect. We have established that Wnt1 regulates Plat expression by activating β-Catenin. Silencing Wnt1 impairs mechanically induced β-Catenin activation, subsequently reducing Plat expression. Furthermore, our findings showed that Wnt1 is essential for osteoblasts to respond to mechanical stimulation and induce Runx2 and Sp7 expression, in part through the Wnt1/β-Catenin/Plat signaling pathway. Additionally, we observed significantly reduced Wnt1 and Plat expression in bones from ovariectomy (OVX)-induced and age-related osteoporotic mouse models compared with non-OVX and young mice, respectively. Overall, our data suggested that Wnt1 and Plat play significant roles in mechanically induced osteogenesis. Their decreased expression in bones from OVX and aged mice highlights their potential involvement in post-menopausal and age-related osteoporosis, respectively.

Spatial Multiomics Reveal the Role of Wnt Modulator, Dkk2, in Palatogenesis

Multiple genetic and environmental etiologies contribute to the pathogenesis of cleft palate, which constitutes the most common among the inherited disorders of the craniofacial complex. Insights into the molecular mechanisms regulating osteogenic differentiation and patterning in the palate during embryogenesis are limited and needed for the development of innovative diagnostics and cures. This study utilized the Pax9-/- mouse model with a consistent phenotype of cleft secondary palate to investigate the role of Pax9 in the process of palatal osteogenesis. While prior research had identified upregulation of Wnt pathway modulators Dkk1 and Dkk2 in Pax9-/- palate mesenchyme, limitations of spatial resolution and technology restricted a more robust analysis. Here, data from single-nucleus transcriptomics and chromatin accessibility assays validated by in situ highly multiplex targeted single-cell spatial profiling technology suggest a distinct relationship between Pax9+ and osteogenic populations. Loss of Pax9 results in spatially restricted osteogenic domains bounded by Dkk2, which normally interfaces with Pax9 in the mesenchyme. These results suggest that Pax9-dependent Wnt signaling modulators influence osteogenic programming during palate formation, potentially contributing to the observed cleft palate phenotype.

Wnt16 Promotes Vascular Smooth Muscle Contractile Phenotype and Function via Taz (Wwtr1) Activation in Male LDLR-/- Mice

Wnt16 is expressed in bone and arteries, and maintains bone mass in mice and humans, but its role in cardiovascular physiology is unknown. We show that Wnt16 protein accumulates in murine and human vascular smooth muscle (VSM). WNT16 genotypes that convey risk for bone frailty also convey risk for cardiovascular events in the Dallas Heart Study. Murine Wnt16 deficiency, which causes postnatal bone loss, also reduced systolic blood pressure. Electron microscopy demonstrated abnormal VSM mitochondrial morphology in Wnt16-null mice, with reductions in mitochondrial respiration. Following angiotensin-II (AngII) infusion, thoracic ascending aorta (TAA) dilatation was greater in Wnt16-/- vs Wnt16+/+ mice (LDLR-/- background). Acta2 (vascular smooth muscle alpha actin) deficiency has been shown to impair contractile phenotype and worsen TAA aneurysm with concomitant reductions in blood pressure. Wnt16 deficiency reduced expression of Acta2, SM22 (transgelin), and other contractile genes, and reduced VSM contraction induced by TGFβ. Acta2 and SM22 proteins were reduced in Wnt16-/- VSM as was Ankrd1, a prototypic contractile target of Yap1 and Taz activation via TEA domain (TEAD)-directed transcription. Wnt16-/- VSM exhibited reduced nuclear Taz and Yap1 protein accumulation. SiRNA targeting Wnt16 or Taz, but not Yap1, phenocopied Wnt16 deficiency, and Taz siRNA inhibited contractile gene upregulation by Wnt16. Wnt16 incubation stimulated mitochondrial respiration and contraction (reversed by verteporfin, a Yap/Taz inhibitor). SiRNA targeting Taz inhibitors Ccm2 and Lats1/2 mimicked Wnt16 treatment. Wnt16 stimulated Taz binding to Acta2 chromatin and H3K4me3 methylation. TEAD cognates in the Acta2 promoter conveyed transcriptional responses to Wnt16 and Taz. Wnt16 regulates cardiovascular physiology and VSM contractile phenotype, mediated via Taz signaling.

Identifying Rare Genetic Determinants for Improved Polygenic Risk Prediction of Bone Mineral Density and Fracture Risk

Osteoporosis and fractures severely impact the elderly population. Polygenic risk scores for bone mineral density have demonstrated potential clinical utility. However, the value of rare genetic determinants in risk prediction has not been assessed. With whole-exome sequencing data from 436,824 UK Biobank participants, we assigned White British ancestry individuals into a training data set (n = 317,434) and a test data set (n = 74,825). In the training data set, we developed a common variant-based polygenic risk score for heel ultrasound speed of sound (SOS). Next, we performed burden testing to identify genes harboring rare determinants of bone mineral density, targeting influential rare variants with predicted high deleteriousness. We constructed a genetic risk score, called ggSOS, to incorporate influential rare variants in significant gene burden masks into the common variant-based polygenic risk score. We assessed the predictive performance of ggSOS in the White British test data set, as well as in populations of non-White British European (n = 18,885), African (n = 7165), East Asian (n = 2236), South Asian (n = 9829), and other admixed (n = 1481) ancestries. Twelve genes in pivotal regulatory pathways of bone homeostasis harbored influential rare variants associated with SOS (p < 5.5 × 10-7 ), including AHNAK, BMP5, CYP19A1, FAM20A, FBXW5, KDM5B, KREMEN1, LGR4, LRP5, SMAD6, SOST, and WNT1. Among 4013 (5.4%) individuals in the test data set carrying these variants, a one standard deviation decrease in ggSOS was associated with 1.35-fold (95% confidence interval [CI] 1.16-1.57) increased hazard of major osteoporotic fracture. However, compared with a common variant-based polygenic risk score (C-index = 0.641), ggSOS had only marginally improved prediction accuracy in identifying at-risk individuals (C-index = 0.644), with overlapping confidence intervals. Similarly, ggSOS did not demonstrate substantially improved predictive performance in non-European ancestry populations. In summary, modeling the effects of rare genetic determinants may assist polygenic prediction of fracture risk among carriers of influential rare variants. Nonetheless, improved clinical utility is not guaranteed for population-level risk screening. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Wnt1 induces osteoblastic changes in a well-established osteolytic skeletal metastatic model derived from breast cancer

BACKGROUND

Osteoblastic skeletal metastasis is frequently observed in prostate cancer. An effective therapy has not been developed due to the unclear molecular mechanism. The Wnt family is involved in various biological phenomena including bone metabolism. There is no direct evidence that the family causes osteoblastic skeletal metastasis.

AIMS

The present study aims to evaluate whether overexpressed Wnt induces osteoblastic bone metastasis in a well-established osteolytic bone metastatic model.

METHODS AND RESULTS

The breast cancer-derived 5a-D-Luc-ZsGreen cells were transfected with Wnt1, Wnt3A, and Wnt5A expression vectors, producing stably highly expressing cells. These cells were intracardially transplanted in nude mice. Bone metastasis development was confirmed by fluorescence imaging. Hind-limb bones including metastasis were dissected and visualized through micro-CT imaging. After imaging, sections were stained with hematoxylin and eosin (H&E), and immunohistochemically stained with an anti-SATB2 antibody. Luminescent imaging confirmed mice with bone metastases in the hind limbs. Micro-CT imaging found an osteoblastic change only in bone metastasis of mice transplanted with Wnt1-expressing cells. This was confirmed on H&E-stained sections. SATB2 immunostaining showed differentiated osteoblasts were at the site of bone metastases in the diaphysis. SATB2 in the Wnt/β-catenin pathway activated by overexpressed Wnt1 could induce osteoblastic change.

CONCLUSION

Our findings provided direct evidence Wnt1 is involved in osteoblastic bone metastasis development. Our model would be a powerful tool for further elucidating molecular mechanisms underlying the disease and developing effective therapies.

Consequences of Aging on Bone

With the aging of the global population, the incidence of musculoskeletal diseases has been increasing, seriously affecting people's health. As people age, the microenvironment within skeleton favors bone resorption and inhibits bone formation, accompanied by bone marrow fat accumulation and multiple cellular senescence. Specifically, skeletal stem/stromal cells (SSCs) during aging tend to undergo adipogenesis rather than osteogenesis. Meanwhile, osteoblasts, as well as osteocytes, showed increased apoptosis, decreased quantity, and multiple functional limitations including impaired mechanical sensing, intercellular modulation, and exosome secretion. Also, the bone resorption function of macrophage-lineage cells (including osteoclasts and preosteoclasts) was significantly enhanced, as well as impaired vascularization and innervation. In this study, we systematically reviewed the effect of aging on bone and the within microenvironment (including skeletal cells as well as their intracellular structure variations, vascular structures, innervation, marrow fat distribution, and lymphatic system) caused by aging, and mechanisms of osteoimmune regulation of the bone environment in the aging state, and the causal relationship with multiple musculoskeletal diseases in addition with their potential therapeutic strategy.

Dominant negative variants in KIF5B cause osteogenesis imperfecta via down regulation of mTOR signaling

BACKGROUND

Kinesin motor proteins transport intracellular cargo, including mRNA, proteins, and organelles. Pathogenic variants in kinesin-related genes have been implicated in neurodevelopmental disorders and skeletal dysplasias. We identified de novo, heterozygous variants in KIF5B, encoding a kinesin-1 subunit, in four individuals with osteogenesis imperfecta. The variants cluster within the highly conserved kinesin motor domain and are predicted to interfere with nucleotide binding, although the mechanistic consequences on cell signaling and function are unknown.

METHODS

To understand the in vivo genetic mechanism of KIF5B variants, we modeled the p.Thr87Ile variant that was found in two patients in the C. elegans ortholog, unc-116, at the corresponding position (Thr90Ile) by CRISPR/Cas9 editing and performed functional analysis. Next, we studied the cellular and molecular consequences of the recurrent p.Thr87Ile variant by microscopy, RNA and protein analysis in NIH3T3 cells, primary human fibroblasts and bone biopsy.

RESULTS

C. elegans heterozygous for the unc-116 Thr90Ile variant displayed abnormal body length and motility phenotypes that were suppressed by additional copies of the wild type allele, consistent with a dominant negative mechanism. Time-lapse imaging of GFP-tagged mitochondria showed defective mitochondria transport in unc-116 Thr90Ile neurons providing strong evidence for disrupted kinesin motor function. Microscopy studies in human cells showed dilated endoplasmic reticulum, multiple intracellular vacuoles, and abnormal distribution of the Golgi complex, supporting an intracellular trafficking defect. RNA sequencing, proteomic analysis, and bone immunohistochemistry demonstrated down regulation of the mTOR signaling pathway that was partially rescued with leucine supplementation in patient cells.

CONCLUSION

We report dominant negative variants in the KIF5B kinesin motor domain in individuals with osteogenesis imperfecta. This study expands the spectrum of kinesin-related disorders and identifies dysregulated signaling targets for KIF5B in skeletal development.

RNA N6-methyladenosine demethylase FTO inhibits glucocorticoid-induced osteoblast differentiation and function in bone marrow mesenchymal stem cells

Excess glucocorticoids (GCs) have been reported as key factors that impair osteoblast (OB) differentiation and function. However, the role of RNA N6-methyladenosine (m6 A) in this process has not yet been elucidated. In this study, we report that both the mRNA and protein expression of fat mass and obesity-associated gene (FTO), a key m6 A demethylase, were dose-dependently downregulated during OB differentiation by dexamethasone (DEX) in bone marrow mesenchymal stem cells (BMSCs), and FTO was gradually increased during OB differentiation. Meanwhile, FTO knockdown suppressed OB differentiation and mineralization, whereas overexpression of wide-type FTO, but not mutant FTO (mutated m6 A demethylase active site), reversed DEX-induced osteogenesis impairment. Interfering with FTO inhibited proliferation and the expression of Ki67 and Pcna in BMSCs during OB differentiation, whereas forced expression of wide-type FTO improved DEX-induced inhibition of BMSCs proliferation. Moreover, FTO knockdown reduced the mRNA stability of the OB marker genes Alpl and Col1a1, and FTO-modulated OB differentiation via YTHDF1 and YTHDF2. In conclusion, our results suggest that FTO inhibits the GCs-induced OB differentiation and function of BMSCs.

The role of cells and signal pathways in subchondral bone in osteoarthritis

Osteoarthritis (OA) is mainly caused by ageing, strain, trauma, and congenital joint abnormalities, resulting in articular cartilage degeneration. During the pathogenesis of OA, the changes in subchondral bone (SB) are not only secondary manifestations of OA, but also an active part of the disease, and are closely associated with the severity of OA. In different stages of OA, there were microstructural changes in SB. Osteocytes, osteoblasts, and osteoclasts in SB are important in the pathogenesis of OA. The signal transduction mechanism in SB is necessary to maintain the balance of a stable phenotype, extracellular matrix (ECM) synthesis, and bone remodelling between articular cartilage and SB. An imbalance in signal transduction can lead to reduced cartilage quality and SB thickening, which leads to the progression of OA. By understanding changes in SB in OA, researchers are exploring drugs that can regulate these changes, which will help to provide new ideas for the treatment of OA.

Klotho reduces the risk of osteoporosis in postmenopausal women: a cross-sectional study of the National Health and Nutrition Examination Survey (NHANES)

BACKGROUND

Osteoporosis (OP) is one of the diseases that endanger the health of the elderly population. Klotho protein is a hormone with anti-aging effects. A few studies have discussed the relationship between Klotho and OP. However, there is still a lack of research on larger populations. This study aims to evaluate the association between OP and Klotho in American postmenopausal women.

METHODS

This is a retrospective study. We searched the National Health and Nutrition Examination Survey (NHANES) database and collected data of 3 survey cycles, finally involving 871 postmenopausal women over 50 years old in the present study. All participants took dual-energy X-ray absorptiometry examination and serum Klotho testing at the time of investigation. After adjusting the possible confounding variables, a multivariate regression model was employed to estimate the relationship between OP and Klotho proteins. Besides, the P for trend and restricted cubic spline (RCS) were applied to examine the threshold effect and calculate the inflection point.

RESULTS

Factors influencing the occurrence of OP included age, ethnicity, body mass index and Klotho levels. Multivariate regression analysis indicated that the serum Klotho concentration was lower in OP patients than that in participants without OP (OR[log2Klotho] = 0.568, P = 0.027). The C-index of the prediction model built was 0.765, indicating good prediction performance. After adjusting the above-mentioned four variables, P values for trend showed significant differences between groups. RCSs revealed that when the Klotho concentration reached 824.09 pg/ml, the risk of OP decreased drastically.

CONCLUSION

Based on the analysis of the data collected from the NHANES database, we propose a correlation between Klotho and postmenopausal OP. A higher serum Klotho level is related to a lower incidence of OP. The findings of the present study can provide guidance for research on diagnosis and risk assessment of OP.

Regulation of the Osteocyte Secretome with Aging and Disease

As the most numerous and long-lived of all bone cells, osteocytes have essential functions in regulating skeletal health. Through the lacunar-canalicular system, secreted proteins from osteocytes can reach cells throughout the bone. Furthermore, the intimate connectivity between the lacunar-canalicular system and the bone vasculature allows for the transport of osteocyte-secreted factors into the circulation to reach the entire body. Local and endocrine osteocyte signaling regulates physiological processes such as bone remodeling, bone mechanoadaptation, and mineral homeostasis. However, these processes are disrupted by impaired osteocyte function induced by aging and disease. Dysfunctional osteocyte signaling is now associated with the pathogenesis of many disorders, including chronic kidney disease, cancer, diabetes mellitus, and periodontitis. In this review, we focus on the targeting of bone and extraskeletal tissues by the osteocyte secretome. In particular, we highlight the secreted osteocyte proteins, which are known to be dysregulated during aging and disease, and their roles during disease progression. We also discuss how therapeutic or genetic targeting of osteocyte-secreted proteins can improve both skeletal and systemic health.

Reciprocal Effect of Environmental Stimuli to Regulate the Adipogenesis and Osteogenesis Fate Decision in Bone Marrow-Derived Mesenchymal Stem Cells (BM-MSCs)

Mesenchymal stem cells derived from bone marrow (BM-MSCs) can differentiate into adipocytes and osteoblasts. Various external stimuli, including environmental contaminants, heavy metals, dietary, and physical factors, are shown to influence the fate decision of BM-MSCs toward adipogenesis or osteogenesis. The balance of osteogenesis and adipogenesis is critical for the maintenance of bone homeostasis, and the interruption of BM-MSCs lineage commitment is associated with human health issues, such as fracture, osteoporosis, osteopenia, and osteonecrosis. This review focuses on how external stimuli shift the fate of BM-MSCs towards adipogenesis or osteogenesis. Future studies are needed to understand the impact of these external stimuli on bone health and elucidate the underlying mechanisms of BM-MSCs differentiation. This knowledge will inform efforts to prevent bone-related diseases and develop therapeutic approaches to treat bone disorders associated with various pathological conditions.

Synthetic Multivalent Disulfide-Constrained Peptide Agonists Potentiate Wnt1/β-Catenin Signaling via LRP6 Coreceptor Clustering

Wnt ligands are critical for tissue homeostasis and form a complex with LRP6 and frizzled coreceptors to initiate Wnt/β-catenin signaling. Yet, how different Wnts achieve various levels of signaling activation through distinct domains on LRP6 remains elusive. Developing tool ligands that target individual LRP6 domains could help elucidate the mechanism of Wnt signaling regulation and uncover pharmacological approaches for pathway modulation. We employed directed evolution of a disulfide constrained peptide (DCP) to identify molecules that bind to the third β-propeller domain of LRP6. The DCPs antagonize Wnt3a while sparing Wnt1 signaling. Using PEG linkers with different geometries, we converted the Wnt3a antagonist DCPs to multivalent molecules that potentiated Wnt1 signaling by clustering the LRP6 coreceptor. The mechanism of potentiation is unique as it occurred only in the presence of extracellular secreted Wnt1 ligand. While all DCPs recognized a similar binding interface on LRP6, they displayed different spatial orientations that influenced their cellular activities. Moreover, structural analyses revealed that the DCPs exhibited new folds that were distinct from the parent DCP framework they were evolved from. The multivalent ligand design principles highlighted in this study provide a path for developing peptide agonists that modulate different branches of cellular Wnt signaling.

Ubiquitin-specific peptidases: Players in bone metabolism

Osteoporosis is an ageing-related disease, that has become a major public health problem and its pathogenesis has not yet been fully elucidated. Substantial evidence suggests a strong link between overall age-related disease progression and epigenetic modifications throughout the life cycle. As an important epigenetic modification, ubiquitination is extensively involved in various physiological processes, and its role in bone metabolism has attracted increasing attention. Ubiquitination can be reversed by deubiquitinases, which counteract protein ubiquitination degradation. As the largest and most structurally diverse cysteinase family of deubiquitinating enzymes, ubiquitin-specific proteases (USPs), comprising the largest and most structurally diverse cysteine kinase family of deubiquitinating enzymes, have been found to be important players in maintaining the balance between bone formation and resorption. The aim of this review is to explore recent findings highlighting the regulatory functions of USPs in bone metabolism and provide insight into the molecular mechanisms governing their actions during bone loss. An in-deep understanding of USPs-mediated regulation of bone formation and bone resorption will provide a scientific rationale for the discovery and development of novel USP-targeted therapeutic strategies for osteoporosis.

Fibulin2: a negative regulator of BMSC osteogenic differentiation in infected bone fracture healing

Bone fracture remains a common occurrence, with a population-weighted incidence of approximately 3.21 per 1000. In addition, approximately 2% to 50% of patients with skeletal fractures will develop an infection, one of the causes of disordered bone healing. Dysfunction of bone marrow mesenchymal stem cells (BMSCs) plays a key role in disordered bone repair. However, the specific mechanisms underlying BMSC dysfunction caused by bone infection are largely unknown. In this study, we discovered that Fibulin2 expression was upregulated in infected bone tissues and that BMSCs were the source of infection-induced Fibulin2. Importantly, Fibulin2 knockout accelerated mineralized bone formation during skeletal development and inhibited inflammatory bone resorption. We demonstrated that Fibulin2 suppressed BMSC osteogenic differentiation by binding to Notch2 and inactivating the Notch2 signaling pathway. Moreover, Fibulin2 knockdown restored Notch2 pathway activation and promoted BMSC osteogenesis; these outcomes were abolished by DAPT, a Notch inhibitor. Furthermore, transplanted Fibulin2 knockdown BMSCs displayed better bone repair potential in vivo. Altogether, Fibulin2 is a negative regulator of BMSC osteogenic differentiation that inhibits osteogenesis by inactivating the Notch2 signaling pathway in infected bone.

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.

Conditional Loss of Nmp4 in Mesenchymal Stem Progenitor Cells Enhances PTH-Induced Bone Formation

Activation of bone anabolic pathways is a fruitful approach for treating severe osteoporosis, yet FDA-approved osteoanabolics, eg, parathyroid hormone (PTH), have limited efficacy. Improving their potency is a promising strategy for maximizing bone anabolic output. Nmp4 (Nuclear Matrix Protein 4) global knockout mice exhibit enhanced PTH-induced increases in trabecular bone but display no overt baseline skeletal phenotype. Nmp4 is expressed in all tissues; therefore, to determine which cell type is responsible for driving the beneficial effects of Nmp4 inhibition, we conditionally removed this gene from cells at distinct stages of osteogenic differentiation. Nmp4-floxed (Nmp4fl/fl ) mice were crossed with mice bearing one of three Cre drivers including (i) Prx1Cre+  to remove Nmp4 from mesenchymal stem/progenitor cells (MSPCs) in long bones; (ii) BglapCre+  targeting mature osteoblasts, and (iii) Dmp1Cre+  to disable Nmp4 in osteocytes. Virgin female Cre+  and Cre- mice (10 weeks of age) were sorted into cohorts by weight and genotype. Mice were administered daily injections of either human PTH 1-34 at 30 μg/kg or vehicle for 4 weeks or 7 weeks. Skeletal response was assessed using dual-energy X-ray absorptiometry, micro-computed tomography, bone histomorphometry, and serum analysis for remodeling markers. Nmp4fl/fl ;Prx1Cre+  mice virtually phenocopied the global Nmp4-/- skeleton in the femur, ie, a mild baseline phenotype but significantly enhanced PTH-induced increase in femur trabecular bone volume/total volume (BV/TV) compared with their Nmp4fl/fl ;Prx1Cre- controls. This was not observed in the spine, where Prrx1 is not expressed. Heightened response to PTH was coincident with enhanced bone formation. Conditional loss of Nmp4 from the mature osteoblasts (Nmp4fl/fl ;BglapCre+ ) failed to increase BV/TV or enhance PTH response. However, conditional disabling of Nmp4 in osteocytes (Nmp4fl/fl ;Dmp1Cre+ ) increased BV/TV without boosting response to hormone under our experimental regimen. We conclude that Nmp4-/- Prx1-expressing MSPCs drive the improved response to PTH therapy and that this gene has stage-specific effects on osteoanabolism. © 2022 American Society for Bone and Mineral Research (ASBMR).

An Update on Animal Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous disorder characterized by bone fragility, multiple fractures, bone deformity, and short stature. In recent years, the application of next generation sequencing has triggered the discovery of many new genetic causes for OI. Until now, more than 25 genetic causes of OI and closely related disorders have been identified. However, the mechanisms of many genes on skeletal fragility in OI are not entirely clear. Animal models of OI could help to understand the cellular, signaling, and metabolic mechanisms contributing to the disease, and how targeting these pathways can provide therapeutic targets. To date, a lot of animal models, mainly mice and zebrafish, have been described with defects in 19 OI-associated genes. In this review, we summarize the known genetic causes and animal models that recapitulate OI with a main focus on engineered mouse and zebrafish models. Additionally, we briefly discuss domestic animals with naturally occurring OI phenotypes. Knowledge of the specific molecular basis of OI will advance clinical diagnosis and potentially stimulate targeted therapeutic approaches.

Loading-induced bone formation is mediated by Wnt1 induction in osteoblast-lineage cells

Mechanical loading on the skeleton stimulates bone formation. Although the exact mechanism underlying this process remains unknown, a growing body of evidence indicates that the Wnt signaling pathway is necessary for the skeletal response to loading. Recently, we showed that Wnts produced by osteoblast lineage cells mediate the osteo-anabolic response to tibial loading in adult mice. Here, we report that Wnt1 specifically plays a crucial role in mediating the mechano-adaptive response to loading. Independent of loading, short-term loss of Wnt1 in the Osx-lineage resulted in a decreased cortical bone area in the tibias of 5-month-old mice. In females, strain-matched loading enhanced periosteal bone formation in Wnt1F/F controls, but not in Wnt1F/F; OsxCreERT2 knockouts. In males, strain-matched loading increased periosteal bone formation in both control and knockout mice; however, the periosteal relative bone formation rate was 65% lower in Wnt1 knockouts versus controls. Together, these findings show that Wnt1 supports adult bone homeostasis and mediates the bone anabolic response to mechanical loading.

Early-Onset Osteoporosis: Rare Monogenic Forms Elucidate the Complexity of Disease Pathogenesis Beyond Type I Collagen

Early-onset osteoporosis (EOOP), characterized by low bone mineral density (BMD) and fractures, affects children, premenopausal women and men aged <50 years. EOOP may be secondary to a chronic illness, long-term medication, nutritional deficiencies, etc. If no such cause is identified, EOOP is regarded primary and may then be related to rare variants in genes playing a pivotal role in bone homeostasis. If the cause remains unknown, EOOP is considered idiopathic. The scope of this review is to guide through clinical and genetic diagnostics of EOOP, summarize the present knowledge on rare monogenic forms of EOOP, and describe how analysis of bone biopsy samples can lead to a better understanding of the disease pathogenesis. The diagnostic pathway of EOOP is often complicated and extensive assessments may be needed to reliably exclude secondary causes. Due to the genetic heterogeneity and overlapping features in the various genetic forms of EOOP and other bone fragility disorders, the genetic diagnosis usually requires the use of next-generation sequencing to investigate several genes simultaneously. Recent discoveries have elucidated the complexity of disease pathogenesis both regarding genetic architecture and bone tissue-level pathology. Two rare monogenic forms of EOOP are due to defects in genes partaking in the canonical WNT pathway: LRP5 and WNT1. Variants in the genes encoding plastin-3 (PLS3) and sphingomyelin synthase 2 (SGMS2) have also been found in children and young adults with skeletal fragility. The molecular mechanisms leading from gene defects to clinical manifestations are often not fully understood. Detailed analysis of patient-derived transiliac bone biopsies gives valuable information to understand disease pathogenesis, distinguishes EOOP from other bone fragility disorders, and guides in patient management, but is not widely available in clinical settings. Despite the great advances in this field, EOOP remains an insufficiently explored entity and further research is needed to optimize diagnostic and therapeutic approaches. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Bisphosphonate-based hydrogel mediates biomimetic negative feedback regulation of osteoclastic activity to promote bone regeneration

The intricate dynamic feedback mechanisms involved in bone homeostasis provide valuable inspiration for the design of smart biomaterial scaffolds to enhance in situ bone regeneration. In this work, we assembled a biomimetic hyaluronic acid nanocomposite hydrogel (HA-BP hydrogel) by coordination bonds with bisphosphonates (BPs), which are antiosteoclastic drugs. The HA-BP hydrogel exhibited expedited release of the loaded BP in response to an acidic environment. Our in vitro studies showed that the HA-BP hydrogel inhibits mature osteoclastic differentiation of macrophage-like RAW264.7 cells via the released BP. Furthermore, the HA-BP hydrogel can support the initial differentiation of primary macrophages to preosteoclasts, which are considered essential during bone regeneration, whereas further differentiation to mature osteoclasts is effectively inhibited by the HA-BP hydrogel via the released BP. The in vivo evaluation showed that the HA-BP hydrogel can enhance the in situ regeneration of bone. Our work demonstrates a promising strategy to design biomimetic biomaterial scaffolds capable of regulating bone homeostasis to promote bone regeneration.

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HA-BP hydrogel can mediate the expedited release of BP in response to the acidic microenvironment created by osteoclasts.

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HA-BP hydrogel supports preosteoclastic differentiation, but inhibits the further osteoclastic maturation.

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The implantation of HA-BP hydrogel in critical-sized bone defects significantly promotes in situ bone regeneration in vivo.

Evidence for the major contribution of remodeling-based bone formation in sclerostin-deficient mice

Bone formation by osteoblasts is achieved through remodeling-based bone formation (RBBF) and modeling-based bone formation (MBBF). The former is when bone formation occurs after osteoclastic bone resorption to maintain bone mass and calcium homeostasis. The latter is when new bone matrices are added on the quiescent bone surfaces. Administration of anti-sclerostin neutralizing antibody promotes MBBF in ovariectomized rats and postmenopausal women. However, it remains to be elucidated which mode of bone formation mainly occurs in Sost-deficient mice under physiological conditions. Here, we show that two-thirds of bone formation involves RBBF in 12-week-old Sost-deficient mice (C57BL/6 background). Micro-computed tomography and histomorphometric analyses showed that the trabecular bone mass in Sost-KO mice was higher than that in Sost^+/- mice. In contrast, the osteoclast number remained unchanged in Sost-KO mice, but the bone resorption marker TRAP5b in serum was slightly higher in those mice. Treatment with anti-RANKL antibody increased the trabecular bone mass of Sost^+/- or Sost-KO mice. Bone formation markers such as osteoid surfaces, the mineral apposition rate, and bone formation rate were almost completely suppressed in Sost^+/- mice treated with anti-RANKL antibody compared with vehicle-treated Sost^+/- mice. In Sost-KO mice, treatment with anti-RANKL antibody suppressed those parameters by more than half. These findings indicate that RBBF accounts for most of the bone formation in Sost^+/- mice, whereas approximately two-thirds of bone formation is estimated to be remodeling-based in 12-week-old Sost-deficient mice. Furthermore, anti-RANKL antibody may be useful for detecting MBBF on trabecular bone.

Skeletal Aging

Aging represents the single greatest risk factor for chronic diseases, including osteoporosis, a skeletal fragility syndrome that increases fracture risk. Optimizing bone strength throughout life reduces fracture risk. Factors critical for bone strength include nutrition, physical activity, and vitamin D status, whereas unhealthy lifestyles, illnesses, and certain medications (eg, glucocorticoids) are detrimental. Hormonal status is another important determinant of skeletal health, with sex steroid concentrations, particularly estrogen, having major effects on bone remodeling. Aging exacerbates bone loss in both sexes and results in imbalanced bone resorption relative to formation; it is associated with increased marrow adiposity, osteoblast/osteocyte apoptosis, and accumulation of senescent cells. The mechanisms underlying skeletal aging are as diverse as the factors that determine the strength (and thus fragility) of bone. This review updates our current understanding of the epidemiology, pathophysiology, and treatment of osteoporosis and provides an overview of the underlying hallmark mechanisms that drive skeletal aging.

Early-Onset Osteoporosis

Osteoporosis is a skeletal disorder with enhanced bone fragility, usually affecting the elderly. It is very rare in children and young adults and the definition is not only based on a low BMD (a Z-score < - 2.0 in growing children and a Z-score ≤ - 2.0 or a T-score ≤ - 2.5 in young adults) but also on the occurrence of fragility fractures and/or the existence of underlying chronic diseases or secondary factors such as use of glucocorticoids. In the absence of a known chronic disease, fragility fractures and low BMD should prompt extensive screening for secondary causes, which can be found in up to 90% of cases. When fragility fractures occur in childhood or young adulthood without an evident secondary cause, investigations should explore the possibility of an underlying monogenetic bone disease, where bone fragility is caused by a single variant in a gene that has a major role in the skeleton. Several monogenic forms relate to type I collagen, but other forms also exist. Loss-of-function variants in LRP5 and WNT1 may lead to early-onset osteoporosis. The X-chromosomal osteoporosis caused by PLS3 gene mutations affects especially males. Another recently discovered form relates to disturbed sphingolipid metabolism due to SGMS2 mutations, underscoring the complexity of molecular pathology in monogenic early-onset osteoporosis. Management of young patients consists of treatment of secondary factors, optimizing lifestyle factors including calcium and vitamin D and physical exercise. Treatment with bone-active medication should be discussed on a personalized basis, considering the severity of osteoporosis and underlying disease versus the absence of evidence on anti-fracture efficacy and potential harmful effects in pregnancy.

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.

Genetic Parameter Estimation and Whole Sequencing Analysis of the Genetic Architecture of Chicken Keel Bending

Bone health is particularly important for high-yielding commercial layer chickens. The keel of poultry is an extension of the abdomen side of the sternum along the sagittal plane and is one of the most important bones. In this study, the keel phenotype of White Leghorns laying hen flocks showed significant individual differences. To clarify its genetic mechanism, we first estimated the heritability of keel bend (KB) in White Leghorn, recorded the production performance of the chicken flock, examined the blood biochemical indexes and bone quality in KB and keel normal (KN) chickens, and performed whole-genome pooled sequencing in KB and KN chickens. We then performed selection elimination analysis to determine the genomic regions that may affect the keel phenotypes. The results show that KB is a medium heritability trait. We found that cage height had a significant effect on the KB (p < 0.01). At 48 weeks, there were significant differences in the number of eggs, the number of normal eggs, and eggshell strength (p < 0.05). The content of parathyroid hormone was lower (p < 0.01) and that of calcitonin was higher (p < 0.01) in KB chickens than in KN chickens. The differences in bone mineral density, bone strength, and bone cortical thickness of the humerus and femur were extremely significant (p < 0.01), with all being lower in KB chickens than in KN chickens. In addition, the bones of KB chickens contained more fat organization. A total of 128 genes were identified in selective sweep regions. We identified 10 important candidate genes: ACP5, WNT1, NFIX, CNN1, CALR, FKBP11, TRAPPC5, MAP2K7, RELA, and ENSGALG00000047166. Among the significantly enriched Kyoto Encyclopedia of Genes and Genomes pathways found, we identifed two bone-related pathways, one involving “osteoclast differentiation” and the other the “MAPK signaling pathway.” These results may help us better understand the molecular mechanism of bone traits in chickens and other birds and provide new insights for the genetic breeding of chickens.

Aged bone matrix-derived extracellular vesicles as a messenger for calcification paradox

Adipocyte differentiation of bone marrow mesenchymal stem/stromal cells (BMSCs) instead of osteoblast formation contributes to age- and menopause-related marrow adiposity and osteoporosis. Vascular calcification often occurs with osteoporosis, a contradictory association called “calcification paradox”. Here we show that extracellular vesicles derived from aged bone matrix (AB-EVs) during bone resorption favor BMSC adipogenesis rather than osteogenesis and augment calcification of vascular smooth muscle cells. Intravenous or intramedullary injection of AB-EVs promotes bone-fat imbalance and exacerbates Vitamin D3 (VD3)-induced vascular calcification in young or old mice. Alendronate (ALE), a bone resorption inhibitor, down-regulates AB-EVs release and attenuates aging- and ovariectomy-induced bone-fat imbalance. In the VD3-treated aged mice, ALE suppresses the ovariectomy-induced aggravation of vascular calcification. MiR-483-5p and miR-2861 are enriched in AB-EVs and essential for the AB-EVs-induced bone-fat imbalance and exacerbation of vascular calcification. Our study uncovers the role of AB-EVs as a messenger for calcification paradox by transferring miR-483-5p and miR-2861.

This study uncovers the role of extracellular vesicles from bone matrix as a messenger in the development of osteoporosis and vascular calcification (calcification paradox) during skeletal aging and menopause by transferring miR-483-5p and miR-2861.

The Mechanosensory Role of Osteocytes and Implications for Bone Health and Disease States

Bone homeostasis is a dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. This process is primarily controlled by the most abundant and mechanosensitive bone cells, osteocytes, that reside individually, within chambers of porous hydroxyapatite bone matrix. Recent studies have unveiled additional functional roles for osteocytes in directly contributing to local matrix regulation as well as systemic roles through endocrine functions by communicating with distant organs such as the kidney. Osteocyte function is governed largely by both biochemical signaling and the mechanical stimuli exerted on bone. Mechanical stimulation is required to maintain bone health whilst aging and reduced level of loading are known to result in bone loss. To date, both in vivo and in vitro approaches have been established to answer important questions such as the effect of mechanical stimuli, the mechanosensors involved, and the mechanosensitive signaling pathways in osteocytes. However, our understanding of osteocyte mechanotransduction has been limited due to the technical challenges of working with these cells since they are individually embedded within the hard hydroxyapatite bone matrix. This review highlights the current knowledge of the osteocyte functional role in maintaining bone health and the key regulatory pathways of these mechanosensitive cells. Finally, we elaborate on the current therapeutic opportunities offered by existing treatments and the potential for targeting osteocyte-directed signaling.

Novel insights into the coupling of osteoclasts and resorption to bone formation

Bone remodeling consists of resorption by osteoclasts (OCs) and formation by osteoblasts (OBs). Precise coordination of these activities is required for the resorbed bone to be replaced with an equal amount of new bone in order to maintain skeletal mass throughout the lifespan. This coordination of remodeling processes is referred to as the "coupling" of resorption to bone formation. In this review, we discuss the essential role for OCs in coupling resorption to bone formation, mechanisms for this coupling, and how coupling becomes less efficient or disrupted in conditions of bone loss. Lastly, we provide perspectives on targeting coupling to treat human bone disease.

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.

Osteogenesis Imperfecta: Mechanisms and Signaling Pathways Connecting Classical and Rare OI Types

Osteogenesis imperfecta (OI) is a phenotypically and genetically heterogeneous skeletal dysplasia characterized by bone fragility, growth deficiency, and skeletal deformity. Previously known to be caused by defects in type I collagen, the major protein of extracellular matrix, it is now also understood to be a collagen-related disorder caused by defects in collagen folding, posttranslational modification and processing, bone mineralization, and osteoblast differentiation, with inheritance of OI types spanning autosomal dominant and recessive as well as X-linked recessive. This review provides the latest updates on OI, encompassing both classical OI and rare forms, their mechanism, and the signaling pathways involved in their pathophysiology. There is a special emphasis on mutations in type I procollagen C-propeptide structure and processing, the later causing OI with strikingly high bone mass. Types V and VI OI, while notably different, are shown to be interrelated by the interferon-induced transmembrane protein 5 p.S40L mutation that reveals the connection between the bone-restricted interferon-induced transmembrane protein-like protein and pigment epithelium-derived factor pathways. The function of regulated intramembrane proteolysis has been extended beyond cholesterol metabolism to bone formation by defects in regulated membrane proteolysis components site-2 protease and old astrocyte specifically induced-substance. Several recently proposed candidate genes for new types of OI are also presented. Discoveries of new OI genes add complexity to already-challenging OI management; current and potential approaches are summarized.

Wnt/β-catenin signalling: function, biological mechanisms, and therapeutic opportunities

The Wnt/β-catenin pathway comprises a family of proteins that play critical roles in embryonic development and adult tissue homeostasis. The deregulation of Wnt/β-catenin signalling often leads to various serious diseases, including cancer and non-cancer diseases. Although many articles have reviewed Wnt/β-catenin from various aspects, a systematic review encompassing the origin, composition, function, and clinical trials of the Wnt/β-catenin signalling pathway in tumour and diseases is lacking. In this article, we comprehensively review the Wnt/β-catenin pathway from the above five aspects in combination with the latest research. Finally, we propose challenges and opportunities for the development of small-molecular compounds targeting the Wnt signalling pathway in disease treatment.

Osteoblast-Specific Wnt Secretion Is Required for Skeletal Homeostasis and Loading-Induced Bone Formation in Adult Mice

Wnt signaling is critical to many aspects of skeletal regulation, but the importance of Wnt ligands in the bone anabolic response to mechanical loading is not well established. Recent transcriptome profiling studies by our laboratory and others show that mechanical loading potently induces genes encoding Wnt ligands, including Wnt1 and Wnt7b. Based on these findings, we hypothesized that mechanical loading stimulates adult bone formation by inducing Wnt ligand expression. To test this hypothesis, we inhibited Wnt ligand secretion in adult (5 months old) mice using a systemic (drug) and a bone-targeted (conditional gene knockout) approach, and subjected them to axial tibial loading to induce lamellar bone formation. Mice treated with the Wnt secretion inhibitor WNT974 exhibited a decrease in bone formation in non-loaded bones as well as a 54% decline in the periosteal bone formation response to tibial loading. Next, osteoblast-specific Wnt secretion was inhibited by dosing 5-month-old Osx-CreERT2; WlsF/F mice with tamoxifen. Within 1 to 2 weeks of Wls deletion, skeletal homeostasis was altered with decreased bone formation and increased resorption, and the anabolic response to loading was reduced 65% compared to control (WlsF/F ). Together, these findings show that Wnt ligand secretion is required for adult bone homeostasis, and furthermore establish a role for osteoblast-derived Wnts in mediating the bone anabolic response to tibial loading. © 2021 American Society for Bone and Mineral Research (ASBMR).

Case report: Early-onset osteoporosis in a patient carrying a novel heterozygous variant of the WNT1 gene

The WNT1 gene is crucial for bone development and homeostasis. Homozygous mutations in WNT1 cause severe bone fragility known as osteogenesis imperfecta type XV. Moreover, heterozygous WNT1 mutations have been found in adults with early-onset osteoporosis. We identified a 35 year-old Caucasian woman who experienced multiple vertebral fractures two months after her second pregnancy. There was no history of risk factors for secondary osteoporosis or family history of osteoporosis. Dual-energy X-ray absorptiometry confirmed a marked reduction of bone mineral density (BMD) at the lumbar spine (0.734 g/cm², Z-score -2.8), femoral neck (0.48 g/cm², Z-score -3.5), and total hip (0.589 g/cm², Z-score -3.0). Blood tests excluded secondary causes of bone fragility. Genetic analysis revealed a heterozygous missense mutation (p.Leu370Val) in the WNT1 gene. Varsome classified it as a variant of uncertain significance. However, the fact that the Leucine residue at position 370 is highly conserved among vertebrate species and the variant has a very low allelic frequency in the general population would exclude the possibility of a polymorphism. The patient was treated for two years with teriparatide therapy associated with calcium and vitamin D supplements. During the follow-up period she did not report further clinical fractures. After 24 months of teriparatide, BMD increased at lumbar spine (+14.6%), femoral neck (+8.3%) and total hip (+4.9%) compared to baseline. We confirm that the heterozygous WNT1 mutation could cause a variable bone fragility and low turnover osteoporosis. We suggest that teriparatide is one of the most appropriate available therapies for this case.

Neuronal Induction of Bone-Fat Imbalance through Osteocyte Neuropeptide Y

A differentiation switch of bone marrow mesenchymal stem/stromal cells (BMSCs) from osteoblasts to adipocytes contributes to age- and menopause-associated bone loss and marrow adiposity. Here it is found that osteocytes, the most abundant bone cells, promote adipogenesis and inhibit osteogenesis of BMSCs by secreting neuropeptide Y (NPY), whose expression increases with aging and osteoporosis. Deletion of NPY in osteocytes generates a high bone mass phenotype, and attenuates aging- and ovariectomy (OVX)-induced bone-fat imbalance in mice. Osteocyte NPY production is under the control of autonomic nervous system (ANS) and osteocyte NPY deletion blocks the ANS-induced regulation of BMSC fate and bone-fat balance. γ-Oryzanol, a clinically used ANS regulator, significantly increases bone formation and reverses aging- and OVX-induced osteocyte NPY overproduction and marrow adiposity in control mice, but not in mice lacking osteocyte NPY. The study suggests a new mode of neuronal control of bone metabolism through the ANS-induced regulation of osteocyte NPY.