Hypophosphatasia (HPP) is an inherited error-of-metabolism caused by loss-of-function mutations in ALPL-encoded tissue-nonspecific alkaline phosphatase (TNAP). HPP has wide-ranging severity, including a clinical subtype called odontohypophosphatasia (odonto HPP), which selectively affects craniofacial structures. Dentoalveolar defects in HPP can affect enamel, dentin, and alveolar bone, and deficient acellular cementum contributes to tooth loss. Global Alpl knockout phenocopies effects of severe HPP, but early lethality precludes longer-term studies. Aiming to create a mouse model replicating dentoalveolar effects of HPP, we used Wnt1Cre2 mice to conditionally delete Alpl in ectomesenchymal cells that make dentin, cementum, periodontal ligament (PDL), and alveolar bone. We compared appendicular and craniofacial skeletal effects of Wnt1Cre2 to Prx1Cre conditional Alpl ablation in limb bud mesenchyme. We also tested alveolar bone socket healing in Wnt1Cre2; Alplfl/fl conditional knockout mice and the effect of TNAP-Fc-D10 enzyme replacement therapy (ERT) on socket healing. Prx1Cre; Alplfl/fl mice exhibited 38 % reduced circulating alkaline phosphatase (ALP) and long bone defects, but no craniofacial phenotypes. Wnt1Cre2; Alplfl/fl mice featured 60 % reduced ALP and profound mineralization defects in dentin, cementum, and alveolar bone, but no appendicular skeleton changes. Defects were noted in neural crest-derived intersphenoid synchondrosis of the cranial base and mandibular condyle of Wnt1Cre2; Alplfl/fl mice. Extraction of maxillary molars in Wnt1Cre2; Alplfl/fl mice revealed profound alveolar bone healing defects that were partially rescued by ERT. Cranial neural crest deletion of Alpl resulted in a mouse model phenocopying odonto HPP that can be used to investigate mechanisms underlying pathologies as well as interventions.

By 2050, the global aging population is predicted to reach 1.5 billion, highlighting the need to enhance the quality of life of the elderly population. Osteoporotic fractures are projected to affect one in three women and one in five men over age 50. Initial treatments for osteoporosis in postmenopausal women include antiresorptive agents such as bisphosphonates, strontium ranelate, estrogen replacement therapy (ERT) and selective estrogen receptor modulators (SERMs). However, these do not rebuild bone, limiting their effectiveness. Denosumab, an FDA-approved antiresorptive monoclonal antibody, also has drawbacks including high costs, biannual subcutaneous injections, slow healing, impaired bone growth and side effects like eczema, flatulence, cellulitis, osteonecrosis of the jaw (ONJ) and an increased risk of spinal fractures after discontinuation of treatment. Nutraceuticals, particularly phytoestrogens, are gaining attention for their health benefits and safety in osteoporosis prevention, management and treatment. Phytoestrogens are plant metabolites similar to mammalian estrogens and include isoflavones, coumestans, lignans, stilbenes, and flavonoids. They interact with estrogen receptor isoforms ERα and ERβ, acting as agonists or antagonists based on concentration and bioavailability. Their tissue-selective activities are particularly significant: anti-estrogenic effects in reproductive tissues may lower the risk of hormone-related cancers (such as ovarian, uterine, breast and prostate), while estrogenic effects on bone could contribute to the preservation of bone mineral density.Phytoestrogens are, thus, used in managing breast and prostate cancers, cardiovascular diseases, menopause and osteoporosis. The present review focuses on the botanical origin, classification, sources and mechanism(s) of action of major phytoestrogens, their potential in prevention and management of osteoporosis and the requirement for additional clinical trials to achieve more definitive outcomes in order to confirm their efficacy and dosage safety.

The cranial base synchondroses, comprised of opposite-facing bidirectional chondrocyte layers, drive anteroposterior cranial base growth. In humans, RUNX2 haploinsufficiency causes cleidocranial dysplasia associated with deficient midfacial growth. However, how RUNX2 regulates chondrocytes in the cranial base synchondroses remains unknown. To address this, we inactivated Runx2 in postnatal synchondrosis chondrocytes using a tamoxifen-inducible Fgfr3-creER (Fgfr3-Runx2cKO) mouse model. Fgfr3-Runx2cKO mice displayed skeletal dwarfism and reduced anteroposterior cranial base growth associated with premature synchondrosis ossification due to impaired chondrocyte proliferation, accelerated hypertrophy, apoptosis, and osteoclast-mediated cartilage resorption. Lineage tracing reveals that Runx2-deficient Fgfr3+ cells failed to differentiate into osteoblasts. Notably, Runx2-deficient chondrocytes showed an elevated level of FGFR3 and its downstream signaling components, pERK1/2 and SOX9, suggesting that RUNX2 downregulates FGFR3 in the synchondrosis. This study unveils a new role of Runx2 in cranial base chondrocytes, identifying a possible RUNX2-FGFR3-MAPK-SOX9 signaling axis that may control cranial base growth.

A large number of studies have shown that osteoporosis is closely related to bone immunology. The purpose of this study is to conduct bibliometrics and visual analysis of the fields related to osteoimmunology and osteoporosis from 2013 to 2022 and to summarize the research hotspots and trends in this field.

We searched the Web of Science core collection database for articles on osteoimmunology and osteoporosis published between 2013 and 2022. Vosviewer 1.6.18 and CiteSpace.6.2. R4 were used to analyze the retrieved data.

A total of 3218 articles on osteoimmunology and osteoporosis were included in this study. A total of 76 countries, 347 institutions, and 502 authors were included in the articles examined in this study. The main research countries were China, the United States, and South Korea. Shanghai Jiaotong University, Harvard University, and the University of California system were the main research institutions. The author who published the most papers was Xu, Jiake.

This study is the first to summarize the global research trends in the field of osteoimmunology and osteoporosis from 2013 to 2022. That helps researchers quickly understand the research hotspots and directions in this field.

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

Osteoarthritis, a progressive and degenerative joint disease, disrupts the integrity of the entire joint structure, underscoring the urgency of identifying more effective therapeutic strategies and innovative targets. Among these, exercise therapy is considered a key component in the early management of osteoarthritis, functioning by stimulating the secretion of myokines from the skeletal muscle system. Irisin, a myokine predominantly secreted by skeletal muscle during exercise and encoded by the FNDC5 gene, has garnered attention for its regulatory effects on bone health. Emerging evidence suggests that irisin may play a protective role in osteoarthritis by promoting tissue homeostasis, enhancing subchondral bone density and microstructure, and inhibiting chondrocyte apoptosis. By improving chondrocyte viability, preserving extracellular matrix integrity, and maintaining homeostasis in osteoblasts, osteoclasts, and osteocytes, irisin emerges as a promising therapeutic target for osteoarthritis. This review delves into the role of irisin in osteoarthritis pathogenesis, highlighting its influence on cartilage and bone metabolism as well as its dynamic relationship with exercise. Additionally, this review suggests that further exploration on its specific molecular mechanisms, optimization of drug delivery systems, and strategic utilization of exercise-induced benefits will be pivotal in unlocking the full potential of irisin as a novel intervention for osteoarthritis.

Chronic inflammatory diseases, such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), multiple sclerosis (MS), atherosclerosis, and inflammatory bowel disease (IBD), pose major global health concerns. These disorders are marked by persistent inflammation, immune system dysfunction, tissue injury, and fibrosis, ultimately leading to severe organ dysfunction and diminished quality of life. Osteopontin (OPN), a multifunctional extracellular matrix protein, plays a crucial role in immune regulation, inflammation, and tissue remodeling. It promotes immune cell recruitment, stimulates pro-inflammatory cytokine production, and contributes to fibrosis through interactions with integrins and CD44 receptors. Additionally, OPN activates key inflammatory pathways, including NF-κB, MAPK, and PI3K/Akt, further aggravating tissue damage in chronic inflammatory conditions. Our review highlights the role of OPN in chronic inflammation, its potential as a biomarker, and its therapeutic implications. We explore promising preclinical approaches, such as monoclonal antibodies, small molecule inhibitors, and natural compounds like curcumin, which have demonstrated potential in mitigating OPN-driven inflammation. However, challenges persist in selectively targeting OPN while maintaining its essential physiological roles, including bone remodeling and wound healing. Our review offers insights into therapeutic strategies and future research directions.

Magnesium is essential for bone development and mineralization and may influence osteoporosis progression. However, its relationship with low bone mineral density (BMD) and fracture risk is not well understood. This study aimed to identify the primary risk factors and the effect of magnesium deficiency on bone density in osteoporosis patients.

The study involved 162 adults categorized into normal, osteopenia, and osteoporosis groups, plus 50 healthy individuals. BMD of the lumbar spine (L1-L4) and femur neck, body mass index, and T-scores were assessed via dual-energy X-ray absorptiometry, while serum magnesium, 25-(OH) Vitamin D3, inflammatory markers, and other clinical tests were measured. The results showed significant variations in BMD, T-scores, magnesium, and vitamin 25(OH)D levels.

Notably, osteoporosis patients exhibited a substantial decline in mean BMD along with an increase in mean T-scores. They also had significantly lower serum levels of magnesium, vitamin 25(OH)D, and calcium, compared to other groups, while parathyroid hormone levels slightly increased. Inflammatory markers were significantly elevated in osteoporosis patients. Magnesium and vitamin 25(OH)D showed an inverse relationship with T-scores and a direct positive correlation with BMD and bone mineral content. Additionally, a negative correlation between magnesium and inflammatory markers was observed. The findings highlighted a strong correlation between magnesium deficiency and osteoporosis, with a more significant odds ratio compared to factors like 25(OH)D, PTH, BMD, T-score, and calcium.

Magnesium deficiency has a more pronounced impact on bone health than vitamin D deficiency. Thus, magnesium deficiency emerges as a major risk factor for osteoporosis progression and a predictor of fracture incidence in patients with osteoporosis or osteopenia.

Multiple myeloma (MM) is a clonal plasma cell proliferative malignancy characterized by a debilitating bone disease. Osteolytic destruction, a hallmark of MM, is driven by increased osteoclast number and exacerbated bone resorption, primarily fueled by the excessive production of RANKL, the master regulator of osteoclast formation, within the tumor niche. We previously reported that osteocytes, the most abundant cells in the bone niche, promote tumor progression and support MM bone disease by overproducing RANKL. However, the molecular mechanisms underlying RANKL dysregulation in osteocytes in the context of MM bone disease are not entirely understood. Here, we present evidence that MM-derived CCL3 induces upregulation of RANKL expression in both human and murine osteocytes. Through a combination of in vitro, ex vivo, and in vivo models and clinical data, we demonstrate that genetic or pharmacologic inhibition of CCL3 prevents RANKL upregulation in osteocytes and attenuates the bone loss induced by MM cells. Mechanistic studies revealed that MM-derived CCL3 triggers the secretion of HMGB1 by osteocytes, a process required for osteocytic RANKL upregulation by MM cells. These findings identify a previously unknown CCL3-HMGB1 signaling axis in the MM tumor niche that drives bone resorption by promoting RANKL overproduction in osteocytes.

Mitochondrial (mt)ROS, insufficient NAD+, and cellular senescence all contribute to the decrease in bone formation with aging. ROS can cause senescence and decrease NAD+, but it remains unknown whether these mechanisms mediate the effects of ROS in vivo. Here, we generated mice lacking the mitochondrial antioxidant enzyme Sod2 in osteoblast lineage cells targeted by Osx1-Cre and showed that Sod2ΔOsx1 mice had low bone mass. Osteoblastic cells from these mice had impaired mitochondrial respiration and attenuated NAD+ levels. Administration of an NAD+ precursor improved mitochondrial function in vitro but failed to rescue the low bone mass of Sod2ΔOsx1 mice. Single-cell RNA-sequencing of bone mesenchymal cells indicated that ROS had no significant effects on markers of senescence but disrupted parathyroid hormone signaling, iron metabolism, and proteostasis. Our data supports the rationale that treatment combinations aimed at decreasing mtROS and senescent cells and increasing NAD+ should confer additive effects in delaying age-associated osteoporosis.

Bone resorption by osteoclasts is a critical step in bone remodeling, a process important for maintaining bone homeostasis and repairing injured bone. We previously identified a bone marrow mesenchymal subpopulation, marrow adipogenic lineage precursors (MALPs), and showed that its production of RANKL stimulates bone resorption in young mice using Adipoq-Cre. To exclude developmental defects and to investigate the role of MALPs-derived RANKL in adult bone, we generated inducible reporter mice (Adipoq-CreER Tomato) and RANKL deficient mice (Adipoq-CreER RANKLflox/flox, iCKO). Single cell-RNA sequencing data analysis and lineage tracing revealed that Adipoq+ cells contain not only MALPs but also some mesenchymal progenitors capable of osteogenic differentiation. In situ hybridization showed that RANKL mRNA is only detected in MALPs, but not in osteogenic cells. RANKL deficiency in MALPs induced at 3 months of age rapidly increased trabecular bone mass in long bones as well as vertebrae due to diminished bone resorption but had no effect on the cortical bone. Ovariectomy (OVX) induced trabecular bone loss at both sites. RANKL depletion either before OVX or at 6 weeks post OVX protected and restored trabecular bone mass. Furthermore, bone healing after drill-hole injury was delayed in iCKO mice. Together, our findings demonstrate that MALPs play a dominant role in controlling trabecular bone resorption and that RANKL from MALPs is essential for trabecular bone turnover in adult bone homeostasis, postmenopausal bone loss, and injury repair.

Bone remodeling is a dynamic and continuous process involving three components: bone formation mediated by osteoblasts, bone resorption mediated by osteoclasts, and bone formation-resorption balancing regulated by osteocytes. Excessive osteocyte death is found in various bone diseases, such as postmenopausal osteoporosis (PMOP), and osteoclasts are found increased and activated at osteocyte death sites. Currently, apart from apoptosis and necrosis as previously established, more forms of cell death are reported, including necroptosis, ferroptosis and pyroptosis. These forms of cell death play important role in the development of inflammatory diseases and bone diseases. Increasing studies have revealed that various forms of osteocyte death promote osteoclast formation via different mechanism, including actively secreting pro-inflammatory and pro-osteoclastogenic cytokines, such as tumor necrosis factor alpha (TNF-α) and receptor activator of nuclear factor-kappa B ligand (RANKL), or passively releasing pro-inflammatory damage associated molecule patterns (DAMPs), such as high mobility group box 1 (HMGB1). This review summarizes the established and potential mechanisms by which various forms of osteocyte death regulate osteoclast formation, aiming to provide better understanding of bone disease development and therapeutic target.

Over the past decade, advancements in cell line development and genetic tools have led to a wealth of new insights into the effects of parathyroid hormone (PTH), particularly on osteocytes-cells deeply embedded within the bone's mineral matrix. These cells were once believed to be inactive bystanders, with little, if any, role in skeletal and mineral homeostasis. This concept of passive osteocytes has been challenged in recent years and osteocytes are now recognized for their crucial functions in skeletal mechanotransduction, bone modeling and remodeling, mineral ion regulation, and hematopoiesis. Moreover, osteocytes are key targets of PTH, and studies utilizing genetically modified mice, in which the PTH receptor is either deleted or overexpressed, have shed light on the hormone's complex effects on these cells. Several signaling molecules, including salt kinase inhibitors and histone deacetylases, have been identified as part of PTH's intracellular signaling cascade. In addition to its effects on bone metabolism, PTH has also been implicated in regulating bone energy metabolism, skeletal aging, and hematopoiesis. This review summarizes both classical and emerging effects of PTH on osteocytes, highlights the limitations of current research, and offers perspectives for future investigations in the field.

To explore the effects of Icaritin (ICA) on the regulation of osteocyte exosomal miRNAs and to promote the understanding of the potential molecular mechanisms involved in bone repair by ICA.

MLO-Y4 cells were treat with PBS or 10 µM ICA for 24 h and the supernatant was collected. Exosomes were isolated and purified according to standard methods, and identified by transmission electron microscopy, nanoparticle tracking analysis and protein blotting. Exosomal miRNAs were analysed by RNA sequencing.

Osteocyte exosomes were successfully isolated and characterised. MiRNA sequencing showed that two known exosomal miRNAs (miR-128-3p, miR-30a-5p) were significantly up-regulated and two were significantly down-regulated (miR-5112, miR-1285) after ICA intervention.

Based on the findings, ICA regulates several miRNAs of osteocytes, which deepen our understanding of the therapeutic effects and mechanisms of ICA on skeletal diseases.

Osteocytes are the most abundant cell type in bone tissue, of which the impact on bone homeostasis is still not clear. This study explored the impact of icaritin on osteocytes and their derived exosomes. By doing so, we hope to contribute to the understanding the therapeutic potential of ICA and osteocytes in maintaining bone health and treating conditions such as osteoporosis.

Our study elucidates the crucial role of the liver in bone homeostasis through the p53-miR17 family (miR17-miR20/miR20-miR106/miR93-miR106)-Rankl axis. We demonstrate the enhanced hepatocyte Rankl expression in inflammaging conditions, such as aging, ovariectomized (OVX) mice, and elderly humans. Mice with hepatocyte-specific Rankl deletion exhibit significant resistance to bone mass loss associated with aging, lipopolysaccharide (LPS)-induced inflammation, or estrogen deficiency, compared with controls. Our study highlights hepatocytes as the primary source of Rankl in the liver and serum under these conditions. We identify the p53-miR17 family axis as a crucial regulator for hepatocyte Rankl expression, with p53 inhibiting the miR17 family transcription. Through bioinformatics analysis and in vitro validation, we identify Rankl mRNA as a direct target of the miR17 family. Targeting this axis via CasRx-mediated mRNA editing or miRNA interference significantly attenuates bone mass loss in mice. Our investigation underscores the pivotal significance and therapeutic potential of modulating the p53-miR17 family-Rankl axis in the treatment of inflammaging-associated osteoporosis.

The complement cascade plays an important role in the inflammation amplification and tissue destruction of periodontitis. Importantly, complement C3 was proved to be the central element of complement cascade. Thus, targeting inhibition of C3 has become one of the focuses of treatment method development and exploration.

The siRNAs targeting C3 were designed and screened for in vitro potency. The selected siRNA was conjugated to GalNAc (GalNAc-C3 siRNA) for liver-specific delivery. The mouse model of periodontitis was established by silk ligation. Stereomicroscopy, Micro-CT, histological and histochemical assessment, and immunofluorescence staining were performed to evaluate the level of bone destructive and osteoclast activity. The influence of GalNAc-C3 siRNA on inflammatory reactions was determined by qRT-PCR, ELISA, and flow cytometry.

GalNAc-C3 siRNA showed great in vivo potency and durability to silence hepatic C3 mRNA expression. GalNAc-C3 siRNA treatment could effectively inhibit the production of inflammatory cytokines (IL-17A, TNF-α, IL-6, and IFN-γ) and restrain Th17 differentiation. Importantly, the expression of RANKL and differentiation of osteoclast were inhibited by GalNAc-C3 siRNA.

GalNAc-C3 siRNA could efficiently play a role in bone protection by inhibiting inflammatory responses and osteoclast activities. This therapeutic siRNA may become an effective treatment strategy for periodontitis.

Phospholipase C β (PLCβ) is involved in diverse biological processes, including inflammatory responses and neurogenesis; however, its role in bone cell function is largely unknown. Among the PLCβ isoforms (β1-β4), we found that PLCβ4 was the most highly upregulated during osteoclastogenesis. Here we used global knockout and osteoclast lineage-specific PLCβ4 conditional knockout (LysM-PLCβ4-/-) mice as subjects and demonstrated that PLCβ4 is a crucial regulator of receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclast differentiation. The deletion of PLCβ4, both globally and in the osteoclast lineage, resulted in a significant reduction in osteoclast formation and the downregulation of osteoclast marker genes. Notably, male LysM-PLCβ4-/- mice presented greater bone mass and fewer osteoclasts in vivo than their wild-type littermates, without altered osteoblast function. Mechanistically, we found that PLCβ4 forms a complex with p38 mitogen-activated protein kinase (MAPK) and MAPK kinase 3 (MKK3) in response to RANKL-induced osteoclast differentiation, thereby modulating p38 activation. An immunofluorescence assay further confirmed the colocalization of PLCβ4 with p38 after RANKL exposure. Moreover, p38 activation rescued impaired osteoclast formation and restored the reduction in p38 phosphorylation caused by PLCβ4 deficiency. Thus, our findings reveal that PLCβ4 controls osteoclastogenesis via the RANKL-dependent MKK3-p38 MAPK pathway and that PLCβ4 may be a potential therapeutic candidate for bone diseases such as osteoporosis.

Mechanical and thermal cell damage can occur due to invasive procedures related to drilling, the insertion of dental implants, and periodontal treatments. Necrotic cells release the content of their cytoplasm and membrane fragments, thereby signaling the need for repair, which includes bone resorption by osteoclasts and inflammation. Here we screened lysates from human gingival fibroblasts, HSC2 and TR146 oral squamous carcinoma cell lines, as well as murine IDG-SW3 osteocytic and RAW264.7 macrophage cell lines for their potential to modulate in vitro osteoclastogenesis in murine bone marrow cultures. We also tested the impact of necrotic lysates on modulating the expression of inflammatory cues in murine ST2 bone marrow stromal cells. We report here that independent of human or murine origin, all cell lysates significantly reduced in vitro osteoclastogenesis in bone marrow cultures, as indicated by the expression of the osteoclast marker genes cathepsin K and tartrate-resistant acid phosphatase and the respective histochemical staining in multinucleated cells. We also found that lysates from HSC2 and TR146 cells significantly pushed the expression of CCL2, CCL5, CXCL1, IL1, and IL6 in ST2 cells. These findings suggest that oral cell lysates reduce in vitro osteoclastogenesis, but only damaged oral squamous carcinoma cells can force murine stromal cells to produce an inflammatory environment.

Osteoporosis is a metabolic bone disease primarily caused by a decreased bone formation and increased bone resorption. Osteoclasts are a special class of terminally differentiated cells that play an important role in normal bone remodeling and bone loss in osteoporosis as well as in a variety of osteolytic diseases. Osteoclasts can be differentiated from monocyte-macrophage cells of the hematopoietic system; they are the key cells in bone resorption. Osteoclast formation and differentiation are regulated by various cytokines and transcription factors. In this review, we summarize recent advances in research on the regulation of osteoclast differentiation and function by factors such as M-CSF, RANKL, AP-1, NFATC1, MITF, and PU.1. Understanding these cytokines and transcription factors can not only help identify targets for osteoclast differentiation but also aid in intervening in the treatment of osteoclast-related diseases.

Bone remodeling is regulated by the interaction between receptor activator of nuclear factor kappa-B ligand (RANKL) and its receptor RANK on osteoblasts and osteoclasts, respectively. Osteoprotegerin (OPG) is secreted from osteoblasts and inhibits osteoclast differentiation by acting as a decoy receptor for RANKL. Despite its importance, the mechanism underlying the secretion of OPG remains poorly understood. Here, we applied a method of video-rate bioluminescence imaging using a fusion protein with Gaussia luciferase (GLase) and visualized the secretion of OPG from living mouse osteoblastic MC3T3-E1 cells. The bioluminescence imaging revealed that the secretion of OPG fused to GLase (OPG-GLase) occurred frequently and widely across the cell surface. Notably, co-expression of RANKL significantly reduced the secretion of OPG-GLase, indicating an inhibitory role of RANKL on OPG secretion within cells. Further imaging and biochemical analyses using deletion mutants of OPG and RANKL, as well as RANKL mutants that cause autosomal recessive osteopetrosis, demonstrated the essential role of protein-protein interaction between OPG and RANKL in the inhibition of OPG secretion. Treatment with proteasome inhibitors resulted in increased levels of OPG in both culture medium and cell lysates. However, the fold-increase of OPG was similar regardless of the presence or absence of RANKL, suggesting that the regulation of OPG secretion by RANKL is independent of proteasome activity. This report visualized the secretion of OPG from living cells and provided evidence for a novel intracellular inhibitory effect of RANKL on OPG secretion.

Tumor necrosis factor-alpha (TNF-α) is a significant cytokine that regulates bone resorption under inflammatory conditions. However, its mechanism of action in osteocytes remains unclear. In this study, highly purified osteocytes were isolated from dentin matrix protein 1 (DMP1)-Topaz mice using cell sorter. RNA sequencing (RNA-seq) revealed that TNF-α stimulation increased C-X-C motif chemokine ligand 10 (CXCL10) gene expression in osteocytes. Although CXCL10 did not affect osteoclast differentiation in vitro, it enhanced the migration of osteoclast precursors. Additionally, in the transwell co-culture system, TNF-α induced the migration of osteoclast precursors. However, this effect was attenuated by a CXCL10-neutralizing antibody. In vivo, mice were administered supracalvarial injections of TNF-α with or without the CXCL10-neutralizing antibody for 5 days. The percentage of CXCL10-positive osteocytes increased after TNF-α administration. Additionally, osteoclast formation and bone resorption were assessed. CXCL10-neutralizing antibody-treated calvariae exhibited a significantly lower number of osteoclasts and bone resorption than those treated with TNF-α alone. These results indicated that TNF-α-induced CXCL10, which affects the migration of osteocyte-derived osteoclast precursors, may enhance TNF-α-triggered osteoclast formation and bone resorption in vivo.

Indole-3-propionic acid (IPA), a metabolite produced by gut microbiota through tryptophan metabolism, has recently been identified as playing a pivotal role in bone metabolism. IPA promotes osteoblast differentiation by upregulating mitochondrial transcription factor A (Tfam), contributing to increased bone density and supporting bone repair. Simultaneously, it inhibits the formation and activity of osteoclasts, reducing bone resorption, possibly through modulation of the nuclear factor-κB (NF-κB) pathway and downregulation of osteoclast-associated factors, thereby maintaining bone structural integrity. Additionally, IPA provides indirect protection to bone health by regulating host immune responses and inflammation via activation of receptors such as the Aryl hydrocarbon Receptor (AhR) and the Pregnane X Receptor (PXR). This review summarizes the roles and signaling pathways of IPA in bone metabolism and its impact on various bone metabolic disorders. Furthermore, we discuss the therapeutic potential and limitations of IPA in treating bone metabolic diseases, aiming to offer novel strategies for clinical management.

Bone remodeling is a continuous cyclic process that maintains and regulates bone structure and strength. The disturbance of bone remodeling leads to a series of bone metabolic diseases. Recent studies have shown that citrate, an intermediate metabolite of the tricarboxylic acid (TCA) cycle, plays an important role in bone remodeling. But the exact mechanism is still unclear. In this study, we focused on the systemic regulatory mechanism of citrate on bone remodeling, and found that citrate is involved in bone remodeling in multiple ways. The participation of citrate in oxidative phosphorylation (OXPHOS) facilitates the generation of ATP, thereby providing substantial energy for bone formation and resorption. Osteoclast-mediated bone resorption releases citrate from bone mineral salts, which is subsequently released as an energy source to activate the osteogenic differentiation of stem cells. Finally, the differentiated osteoblasts secrete into the bone matrix and participate in bone mineral salts formation. As a substrate of histone acetylation, citrate regulates the expression of genes related to bone formation and bone reabsorption. Citrate is also a key intermediate in the metabolism and synthesis of glucose, fatty acids and amino acids, which are three major nutrients in the organism. Citrate can also be used as a biomarker to monitor bone mass transformation and plays an important role in the diagnosis and therapeutic evaluation of bone remodeling disorders. Citrate imbalance due to citrate transporter could result in the supression of osteoblast/OC function through histone acetylation, thereby contributing to disorders in bone remodeling. Therefore, designing drugs targeting citrate-related proteins to regulate bone citrate content provides a new direction for the drug treatment of diseases related to bone remodeling disorders.

This study assessed the novel concept that osteoclast-derived Grem1 has regulatory functions in the skeletal response to calcium stress using an osteoclastic Grem1 conditional knockout (cKO) mouse model. The calcium stress was initiated by feeding cKO mutants and wildtype (WT) littermates a calcium-deficient diet for 2 weeks. Deletion of Grem1 in mature osteoclasts did not affect developmental bone growth nor basal bone turnover. In response to calcium depletion, male cKO mutants showed greater increases in osteoclastic resorption and trabecular bone loss than male WT littermates, indicating an enhanced skeletal sensitivity to calcium depletion in male mutants. The enhanced sensitivity to calcium depletion was sex-dependent, as female cKO mutants showed lower increases in osteoclastic resorption and bone loss than female WT littermates as well as male cKO mutants. The sex disparity in osteoclastic resorption response to calcium stress was intrinsic to osteoclasts since osteoclasts of male but not female cKO mutants showed greater in vitro bone resorption activity than osteoclasts of WT littermates of respective sex. Male cKO mutants displayed smaller bone formation response to calcium depletion than male WT littermates, while female mutants showed bigger bone formation response than female WT littermates, indicating that cKO mutants also displayed sex disparity in bone formation response. The sex disparity in bone formation response was not caused by intrinsic differences in osteoblasts but might be due to sex-dependent differential osteoclastic release of osteogenic factors. In summary, osteoclast-derived gremlin-1 has complicated and sex-dependent regulatory roles in skeletal response to calcium stress.

Osteocytes reside within the bone matrix and produce both paracrine and endocrine factors that influence the skeleton and other tissues. Despite their abundance and physiological importance, osteocytes have been challenging to study in vitro because they are difficult to extract and purify and do not retain their phenotype in standard culture conditions. New techniques for this purpose are emerging. This chapter will describe two methods and adaptations we use to study osteocytes: (1) isolating and purifying primary osteocytes from murine bone, with and without hematopoietic lineage depletion and (2) differentiating cultured osteoblasts or osteoblast-lineage cell lines (including cell lines termed "osteocytic") until they reach a stage of osteocytic gene expression. We will also discuss the limitations of these methods and possible directions for future improvements.

Iron overload leads to apoptosis and increased expression of receptor activator of nuclear factor kappa-Β ligand (RANKL) in osteocytes, which in turn accelerates osteoclastogenesis. Since osteocytes are the main RANKL producers, we hypothesized that apoptotic osteocytes increase RANKL expression in osteocytes, which in turn stimulates osteoclastogenesis and bone resorption. In this study, alendronate or IG9402, a bisphosphonate (BP) analog which does not inhibit bone resorption, inhibited iron overload-induced osteocyte apoptosis and increased RANKL expression. Both BPs also prevented osteoblast apoptosis but did not inhibit the increase in osteoblastic RANKL. Alendronate, but not IG9402, prevented the increase in osteoclastogenesis and serum levels of the bone resorption marker C-telopeptide of type I collagen (CTX) in iron-overloaded mice. Alendronate also prevented the iron overload-induced reduction in femoral bone mineral density, disruption of bone microstructure, and weakness of bone strength. Although IG9402 did not prevent bone loss due to iron overload, it partially prevented reduction of strength, suggesting that osteocyte viability contributes to the maintenance of bone strength. In conclusion, although osteocyte apoptosis in the presence of iron overload leads to an increase in osteocytic RANKL production. However, blocking these events was not sufficient to inhibit iron overload-induced osteoclastogenesis and bone loss.

RANKL and its cognate receptor RANK are crucial regulators of bone metabolism in physiological as well as in pathological conditions. Here we go through the works that unveiled the paramount role of this signaling pathway. We focus on the RANKL cytokine, whose alterations are responsible for rare and common bone diseases. We describe recent insights on the regulation of RANKL expression, which provide new hints for the pharmacological regulation of this molecule. Based on the multiple functions exerted by RANKL (within and outside the bone tissue), we advise caution regarding the potential unintended consequences of its inhibition.

RANK pathway has attracted increasing interest as a promising target in breast cancer, given the availability of denosumab, an anti-RANKL drug. RANK signaling mediates progesterone-driven regulation of mammary gland development and favors breast cancer initiation by controlling mammary cell proliferation and stem cell fate. RANK activation promotes luminal mammary epithelial cell senescence, acting as an initial barrier to tumorigenesis but ultimately facilitating tumor progression and metastasis. Comprehensive analyses have demonstrated that RANK protein expression is an independent biomarker of poor prognosis in postmenopausal and estrogen receptor-negative breast cancer patients. RANK pathway also has multiple roles in immunity and inflammation, regulating innate and adaptive responses. In the tumor microenvironment, RANK and RANKL are expressed by different immune cell populations and contribute to the regulation of tumor immune surveillance, mainly driving immunosuppressive effects.Herein, we discuss the preventive and therapeutic potential of targeting RANK signaling in breast cancer given its tumor cell intrinsic and extrinsic effects. RANKL inhibition has been shown to induce mammary tumor cell differentiation and an antitumor immune response. Moreover, loss of RANK signaling increases sensitivity of breast cancer cells to chemotherapy, targeted therapies such as HER2 and CDK4/6 inhibitors, and immunotherapy. Finally, we describe clinical trials of denosumab for breast cancer prevention, such as those ongoing in women with high risk of developing breast cancer, large phase III clinical trials where the impact of adjuvant denosumab on disease-free survival has been assessed, and window trials to evaluate the immunomodulatory effects of denosumab in breast cancer and other solid tumors.

Osteoporosis is characterized by excessive osteoclast activity leading to bone loss, decreased bone mineral density, and increased susceptibility to fractures. Through in vivo/vitro experiments, along with network pharmacology analysis, we aimed to explore the underlying mechanisms of Isoginkgetin (IGG) in inhibiting osteoclastogenesis, providing valuable insights for further research in the future. Firstly, we ascertained the safe concentration of IGG stimulation on BMMs, followed by a systematic exploration of the concentration gradient at which IGG inhibited osteoclastogenesis using TRAP analysis. An osteoporosis model was established to further validate the in vitro experimental findings by combining Micro-CT and immunohistochemical analysis. The results show that IGG did not exhibit cytotoxicity or proliferative effects on BMMs at concentrations equal to or less than 10 μM. Additionally, IGG inhibited the activity of osteoclastogenesis and bone resorption function at lower concentrations. RT-PCR and Western Blot results demonstrated that IGG could downregulate genes and proteins associated with osteoclastogenesis. The Western Blot results also showed that IGG inhibited the phosphorylation expression of P38, ERK, and P65 in the MAPK and NF-κB pathways. At the same time, it rescued the degradation of IκB-α at 15 and 60 min. IGG can also impact the relative expression levels of oxidative proteins such as SOD-1, HO-1, and catalase, thereby influencing cellular equilibrium and stress levels, ultimately inhibiting the formation of mature OC. In vivo experiments demonstrated that IGG alleviated bone loss caused by osteoclasts and improved relevant parameters of trabecular bone. So, IGG effectively attenuated osteoclastogenesis, and improved bone density, thereby portraying its role in osteoporosis management.

Tissue regeneration involves dynamic dialogue between and among different cells and their surrounding matrices. Bone regeneration is specifically governed by reciprocity between osteoblasts and osteoclasts within the bone microenvironment. Osteoclast-directed resorption and osteoblast-directed formation of bone are essential to bone remodeling, and the crosstalk between these cells is vital to curating a sequence of events that culminate in the creation of bone tissue. Among bone biomaterial strategies, many have investigated the use of different material cues to direct the development and activity of osteoblasts. However, less attention has been given to exploring features that similarly target osteoclast formation and activity, with even fewer strategies demonstrating or integrating biomaterial-directed modulation of osteoblast-osteoclast coupling. This review aims to describe various biomaterial cues demonstrated to influence osteoclastogenesis and osteoclast function, emphasizing those that enhance a material construct's ability to achieve bone healing and regeneration. Additionally discussed are approaches that influence the communication between osteoclasts and osteoblasts, particularly in a manner that takes advantage of their coupling. Deepening our understanding of how biomaterial cues may dictate osteoclast differentiation, function, and influence on the microenvironment may enable the realization of bone-replacement interventions with enhanced integrative and regenerative capacities.

Bone metabolic diseases such as osteoporosis are caused by disruption of the metabolic balance between osteoblasts and osteoclasts. Thousands of papers have been published on osteoporosis and bone metabolizing cells. The purpose of this study is to draw the publication trend of highly cited literature in this field through bibliometrics and to explore the research hotspot analysis.

This paper provides a comprehensive analysis of the impact of countries/regions, research institutions, authors, keywords, relevant journals, and references in the field of osteoporosis and bone metabolic cells research, with a specific focus on the theme of "Osteoporosis and bone metabolic cells". Furthermore, utilizing bibliometric methods, the study aims to offer valuable insights and references for future research endeavors, as well as clinical prevention and treatment strategies in this domain.

The Web of Science (WOS) Core Collection database was examined in order to identify articles with high citation counts from 2013 to 31 October 2023. The citation counts, authors, year of publication, source, journal, geographical origin, subject, article type, and level of evidence were further analyzed using the R bibliometric package. The VOSviewer software was utilized to visualize word co-occurrence in a total of 251 articles.

Our search strategy included 251 highly cited articles published between 2013 and 2023 in the field of osteoporosis and bone metabolic cells. The number of publications in this field remains consistently high, indicating ongoing research interest. Notably, the United States has made significant achievements and contributions in this area. Xie Hui, Cao Xu, and Goodman, Stewart are among the main contributors to these advancements. Nature medicine has the highest journal impact factor of 82.9, highlighting its prominence. The journal of bone and mineral research ranks first with 1,322 citations. Keyword research topics in this field include osteoclast differentiation, osteoblast differentiation, and mesenchymal stem cells. Through citation analysis, we found that 195 articles have been cited more than 100 times, demonstrating their significance and impact.

This study analyzed the relationship between osteoporosis and bone metabolic cells using a bibliometric method. The results of these analyses can help researchers gain a more direct and scientific understanding of trends in the field. Additionally, it can provide guidance in identifying hot research directions and offer new ideas for the prevention and treatment of osteoporosis.

Periodontitis arises from imbalanced host-microbe interactions, leading to dysbiosis and destructive inflammation. The host's innate and adaptive immune responses produce pro-inflammatory mediators that stimulate destructive events, which cause loss of alveolar bone and connective tissue attachment. There is no consensus on the factors that lead to a conversion from gingivitis to periodontitis, but one possibility is the proximity of the inflammation to the bone, which promotes bone resorption and inhibits subsequent bone formation during coupled bone formation. Conversely, orthodontic tooth movement is triggered by the mechanical force applied to the tooth, resulting in bone resorption on the compression side and new bone formation on the tension side. However, the environment around orthodontic brackets readily retains dental plaque and may contribute to inflammation and bone remodeling. The immune, epithelial, stromal, endothelial and bone cells of the host play an important role in setting the stage for bone remodeling that occurs in both periodontitis and orthodontic tooth movement. Recent advancements in single-cell RNA sequencing have provided new insights into the roles and interactions of different cell types in response to challenges. In this review, we meticulously examine the functions of key cell types such as keratinocytes, leukocytes, stromal cells, osteocytes, osteoblasts, and osteoclasts involved in inflammation- and mechanical force-driven bone remodeling. Moreover, we explore the combined effects of these two conditions: mechanical force-induced bone remodeling combined with periodontal disease (chronic inflammation) and periodontally accelerated osteogenic orthodontics (acute transient inflammation). This comprehensive review enhances our understanding of inflammation- and mechanical force-induced bone remodeling.

As the most predominant form of arthritis, osteoarthritis (OA) is featured with irreversible progress and involvement of the whole joint. Since OA onset, abnormal mechanical load initiates excessive osteoclastogenesis, evolving a rapid turnover of subchondral bone, cyst creation, synovitis, cartilage degradation, and ultimately resulting in joint failure. Additionally, aberrant vascularization and nociceptive pain are invoked by osteoclast-induced angiogenesis and sensory innervation in the subchondral bone. Rhizoma anemarrhenae (Zhimu) has been extensively demonstrated to show multiple pharmacological effects including anti-inflammation, anti-aging, and immunomodulation. Herein, Broussonin a (BRA), Markogein (MAN), and Isosakuranetin (ISN) derived from Rhizoma anemarrhenae, were initially discovered for their affinity with Bone marrow mononuclear cell (BMMC) membranes using the Cell membrane chromatography/Time of flight mass spectrometry (CMC/TOFMS) method, while only ISN exerted a significant inhibitory effect on RANKL-induced osteoclastogenesis in BMMC in vitro. Intriguingly, we disclosed that ISN blunted the overactivation of Tartrate-resistant acid phosphatase positive (TRAP+) osteoclasts in subchondral bone in OA mice, as indicated by enhanced bone volume/total volume (BV/TV), trabecular number (Tb.N), and trabeculae thickness (Tb.Th), as well as diminished trabecular pattern factor (Tb.pf). Treatment with ISN also impaired aberrant angiogenesis and nociceptive reaction in the subchondral bone marrow. Moreover, ISN hindered the loss of articular cartilage proteoglycan and lowered the Osteoarthritis Research Society International (OARSI) grade, boosting the expression amount of Aggrecan (ACAN) and Collagen II (COL II) positive cells while reducing Matrix metalloproteinase 13 (MMP-13) positive cells. For mechanisms, We verified that ISN hampered subchondral osteoclastogenesis by blocking nuclear factor kappa light chain enhancer of activated B cells (NF-κB) signaling and C-X-C Motif Chemokine Ligand 2 (CXCL2) stimulation. Taken together, we reveal that ISN impedes the progression of OA by preventing hyperactivated subchondral osteoclastogenesis via suppressing the NF-κB/CXCL2 axis.

Sickle cell disease (SCD) is a severe hematological disorder characterized by erythrocyte sickling that causes significant morbidity and mortality. Skeletal complications of SCD include a high incidence of bone loss, especially in vertebrae, leading to fragility fractures that contribute to disease burden. Whether hydroxyurea (HU), a front-line therapy for SCD ameliorates bone disease has not been established. To investigate HU action on SCD-related vertebral defects, we used HU-treated "Townes" mice, an SCD animal model and performed high-resolution micro-computed tomography (µCT) imaging to resolve bone volume and micro-architectural structure of cortical and trabecular bone, the two major compartments contributing to bone mass and strength. Our data revealed that cortical bone was significantly diminished in the vertebrae of skeletally mature (representing adults) and immature (representing children) SCD mice, while only mature mice lost trabecular bone mass. Administration of HU ameliorated cortical bone loss in mature SCD mice, but paradoxically promoted trabecular bone decline in both groups. We further investigated the mechanisms of HU action in wild-type C57BL6/J mice. HU caused dose-dependent trabecular bone loss due to diminished osteoclast and osteoblast function, indicative of a low bone turnover state. Mechanistic investigations in vitro revealed that HU impeded osteoblast-progenitor proliferation and early differentiation, and diminished osteoclastogenic cytokine production, blunting osteoclast formation as well as the activity of mature osteoclasts. HU further, suppressed mitochondrial, but not glycolytic energy metabolism in both differentiating osteoblasts and differentiated osteoclasts. Collectively, these findings reveal that despite ameliorating cortical bone loss, HU inhibits trabecular bone formation and resorption, by suppressing mitochondrial energy metabolism and blunting the differentiation and/or activity of osteoblasts and osteoclasts. Together HU drives a low bone turnover state culminating in trabecular bone loss. Further investigation into HU's impact on bone in SCD patients is warranted for understanding and managing skeletal complications in this population.

Selective serotonin reuptake inhibitor correlates with decreased bone mineral density and impedes orthodontic tooth movement. The present study aimed to examine the effects of fluoxetine on osteoclast differentiation and function. Human peripheral blood mononuclear cells (hPBMCs) and murine RAW264.7 cells were cultured with RANKL to stimulate osteoclast differentiation. The resulting multinucleated cells displayed characteristics of mature osteoclasts. Fluoxetine at 0.01-1 μM did not impact cellular viability or oxidative stress. However, 10 μM fluoxetine significantly reduced clonal growth, cell viability, and increased cytotoxicity and lipid peroxidation in RAW 264.7 cells. Further, application of 0.1 μM fluoxetine potently suppressed osteoclast differentiation of both RAW264.7 and hPBMCs, with reduced osteoclast numbers and downregulation of osteoclastic genes matrix metalloproteinase-9, cathepsin K, and integrin β3 at mRNA and protein levels. Fluoxetine also disrupted F-actin ring formation essential for osteoclast resorptive function. Mechanistically, fluoxetine inhibited NF-kB signaling by reducing phosphorylation of pathway members IκBα and p65, preventing IκBα degradation and blocking p65 nuclear translocation. In conclusion, this study demonstrates fluoxetine suppressing osteoclast differentiation in association with disrupting NF-kB activation, providing insight into orthodontic treatment planning for patients taking fluoxetine.

Basic, translational and clinical research over the past few decades has provided new understanding on the mechanisms by which activation of the receptor of parathyroid hormone (parathyroid hormone 1 receptor (PTH1R)) regulates bone physiology and pathophysiology. A fundamental change in the field emerged upon the recognition that osteocytes, which are permanent residents of bone and the most abundant cells in bone, are targets of the actions of natural and synthetic ligands of PTH1R (parathyroid hormone and abaloparatide, respectively), and that these cells drive essential actions related to bone remodelling. Among the numerous genes regulated by PTH1R in osteocytes, SOST (which encodes sclerostin, the WNT signalling antagonist and inhibitor of bone formation) has a critical role in bone homeostasis and changes in its expression are associated with several bone pathologies. The bone fragility syndrome induced by diabetes mellitus is accompanied by increased osteocyte apoptosis and changes in the expression of osteocytic genes, including SOST. This Review will discuss advances in our knowledge of the role of osteocytes in PTH1R signalling and the new opportunities to restore bone health in diabetes mellitus by targeting the osteocytic PTH1R-sclerostin axis.

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.

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.

Cortical bone loss is intricately associated with ageing and coincides with iron accumulation. The precise role of ferroptosis, characterized by iron overload and lipid peroxidation, in senescent osteocytes remains elusive. We found that ferroptosis was a crucial mode of osteocyte death in cortical bone during ageing. Using a single-cell transcriptome analysis, we identified activating transcription factor 3 (ATF3) as a critical driver of osteocyte ferroptosis. Elevated ATF3 expression in senescent osteocytes promotes iron uptake by upregulating transferrin receptor 1 while simultaneously inhibiting solute carrier family 7-member 11-mediated cystine import. This process leads to an iron overload and lipid peroxidation, culminating in ferroptosis. Importantly, ATF3 inhibition in aged mice effectively alleviated ferroptosis in the cortical bone and mitigated cortical bone mass loss. Taken together, our findings establish a pivotal role of ferroptosis in cortical bone loss in older adults, providing promising prevention and treatment strategies for osteoporosis and fractures.