Bone resorption is a vital physiological process that enables skeletal remodeling, maintenance, and adaptation to mechanical forces throughout life. While tightly regulated under the physiological state, its dysregulation contributes to pathological conditions such as osteoporosis, rheumatoid arthritis, and periodontitis. Periodontitis is a highly prevalent chronic inflammatory disease driven by dysbiotic biofilms that disrupt the oral microbiome, leading to the progressive breakdown of the periodontal ligament, cementum, and alveolar bone and ultimately resulting in tooth loss. This review outlines the molecular and cellular mechanisms underlying periodontitis, focusing on osteoclastogenesis, the differentiation and activation of osteoclasts, the primary mediators of bone resorption. Key transcriptional regulators, including NFATc1, c-Fos, and c-Src are discussed alongside major signaling pathways such as Mitogen Activated Protein Kinase (MAPK), Janus Tyrosine Kinase/Signal Transducer and Activator of Transcription (JAK/STAT), Nuclear Factor Kappa B (NF-κB), and Phosphoinositide 3-kinase (PI3K)/Akt, to elucidate their roles in the initiation and progression of periodontal bone loss. These pathways orchestrate the inflammatory response and osteoclast activity, underscoring their relevance in periodontitis and other osteolytic conditions. Hallmark features of periodontitis, including chronic inflammation, immune dysregulation, and tissue destruction are highlighted, with emphasis on current and emerging therapeutic strategies targeting these molecular pathways. Special attention is given to small molecules, biologics, and natural compounds that have the potential to modulate key signaling pathways. Although advances in understanding these mechanisms have identified promising therapeutic targets, translation into effective clinical interventions remains challenging. Continued research into regulating bone-resorptive signaling pathways is essential for developing more effective treatments for periodontitis and related inflammatory bone diseases.

Bone is highly innervated, and its regeneration is significantly nerve-dependent. Extensive evidence suggests that the nervous system plays an active role in bone metabolism and development by modulating osteoblast and osteoclast activity. However, the majority of research to date has focused on the direct effects of peripheral nerves and their neurotransmitters on bone regeneration. Emerging studies have begun to reveal a more intricate role of nerves in regulating the immune microenvironment, which is crucial for bone regeneration. This review summarizes how nerves influence bone regeneration through modulation of the immune microenvironment. We first discuss the changes in peripheral nerves during the regenerative process. We then describe conduction and paracrine pathways through which nerves affect the osteogenic immune microenvironment, emphasizing nerves, neural factors, and their impacts. Our goal is to deepen the understanding of the nerve-immune axis in bone regeneration. A better grasp of how nerves influence the osteogenic immune microenvironment may lead to new strategies that integrate the nervous, immune, and skeletal systems to promote bone regeneration.

Traumatic spinal cord injury (SCI) is a debilitating condition characterized by the impairment of neural circuits, leading to the loss of motor and sensory functions and accompanied by severe complications. Substantial research has reported the therapeutic potential of Omega-3 fatty acids for the central nervous system, particularly after traumatic SCI. Omega-3 fatty acids may contribute to improving SCI recovery through their anti-inflammatory, anti-oxidative, neurotrophic, and membrane integrity-preserving properties. These functions of Omega-3 fatty acids are primarily mediated via the activation of G protein-coupled receptor 120 (GPR120), commonly known as the fish oil-specific receptor. Advancements in understanding of the molecular mechanisms of GPR120’s recognition of Omega-3 fatty acids and its downstream signaling mechanisms has significantly promoted research on the pharmacological potential of Omega-3 fatty acids and the development of highly selective and high-affinity alternatives. This review aims to provide in-depth analysis of the comprehensive therapeutic potential of Omega-3 fatty acids for SCI and its accompanying complications, and the prospects for developing novel drugs based on the recognition of Omega-3 fatty acids by GPR120.

The continuous extension of human life expectancy and the global trend of population aging have contributed to a marked increase in the incidence of musculoskeletal diseases, with fractures and osteoporosis being prominent examples. Consequently, promoting bone regeneration is a crucial medical challenge that demands immediate attention. As early as the mid-20th century, researchers revealed that electrical stimulation could effectively promote the healing and regeneration of bone tissue. This is achieved by mimicking the endogenous electric field within bone tissue, which influences cellular behavior and molecular mechanisms. In recent years, electroactive hydrogels responsive to electric field stimulation have been developed and applied to regulate cell functions at different stages of bone regeneration. This paper elaborates on the regulatory effects of electrical stimulation on MSCs, macrophages, and vascular endothelial cells during the process of bone regeneration. It also involves the activation of relevant ion channels and signaling pathways. Subsequently, it comprehensively reviews various electric-field-responsive hydrogels developed in recent years, covering aspects such as material selection, preparation methods, characteristics, and their applications in bone regeneration. Ultimately, it provides an objective summary of the existing deficiencies in hydrogel materials and research, and looks ahead to future development directions.

Bone fractures represent a significant global healthcare burden. Although fractures typically heal on their own, some fail to regenerate properly, leading to nonunion, a condition that causes prolonged disability, morbidity, and mortality. The challenge of treating nonunion fractures is further complicated in patients with underlying bone disorders where systemic and local factors impair bone healing. Traditional treatment approaches, including autografts, allografts, xenografts, and synthetic biomaterials, face limitations such as donor site pain, immune rejection, and insufficient mechanical strength, underscoring the need for alternative strategies. Biologic therapies have emerged as promising tools to enhance bone regeneration by leveraging the body’s natural healing processes. This review explores the critical role of conventional and emerging biologics in fracture healing. We categorize biologic therapies into protein-based treatments, gene and transcript therapies, small molecules, peptides, and cell-based therapies, highlighting their mechanisms of action, advantages, and clinical relevance. Finally, we examine the potential applications of biologics in treating fractures associated with bone disorders such as osteoporosis, osteogenesis imperfecta, rickets, osteomalacia, Paget’s disease, and bone tumors. By integrating biologic therapies with existing biomaterial-based strategies, these innovative approaches have the potential to transform clinical management and improve outcomes for patients with difficult-to-heal fractures.

The transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) signaling pathways are pivotal regulators of cellular processes, playing indispensable roles in embryogenesis, postnatal development, and tissue homeostasis. These pathways are particularly critical within the skeletal system, as they coordinate osteogenesis, chondrogenesis, and bone remodeling through intricate molecular mechanisms. TGF-β/BMP signaling is primarily transduced via canonical Smad-dependent pathways (e.g., ligands, receptors, and intracellular Smads) and the non-canonical Smad-independent (e.g., p38 mitogen-activated protein kinase, MAPK) cascade. Both pathways converge on master transcriptional regulators, including Runx2 and Osterix, and their precise coordination is indispensable for skeletal development, maintenance, and repair. The dysregulation of TGF-β/BMP signaling contributes to a spectrum of skeletal dysplasia and bone pathologies. Advances in molecular genetics, particularly gene-targeting strategies and transgenic mouse models, have deepened our understanding of the spatiotemporal control of TGF-β/BMP signaling in bone and cartilage development. Moreover, emerging research underscores extensive crosstalk between TGF-β/BMP and other critical pathways, such as Wnt/β-catenin, mitogen-activated protein kinase (MAPK), parathyroid hormone (PTH)/PTH-related protein (PTHrP), fibroblast growth factors (FGF), Hedgehog, Notch, insulin-like growth factors (IGF)/insulin-like growth factors receptor (IGFR), Mammalian target of rapamycin (mTOR), and autophagy, forming an integrated regulatory network that ensures skeletal integrity. Our review synthesizes the current knowledge on the molecular components, regulatory mechanisms, and functional integration of TGF-β/BMP signaling in skeletal biology, with an emphasis on its roles in development, regeneration, and disease. By elucidating the molecular underpinnings of TGF-β/BMP pathways and their contextual interactions, we aim to highlight translational opportunities and novel therapeutic strategies for treating skeletal disorders.

After injury, bone tissue initiates a reparative response to restore its structure and function. The failure to initiate or delay this response could result in fracture nonunion. The molecular mechanisms underlying the occurrence of fracture nonunion are not yet established. We propose that hypoxia-triggered signaling pathways, mediated by reactive oxygen species (ROS) homeostasis, control Bmp2 expression and fracture healing initiation. The excessive ROS leads to oxidative stress and, ultimately, fracture nonunion. In this study, we silenced Apex1, the final ROS signaling transducer that mediates the activation of key transcription factors by their cysteines oxidoreduction, evaluating its role during endochondral ossification and fracture repair. Silencing Apex1 in limb bud mesenchyme results in transient metaphyseal dysplasia derived from impaired chondrocyte differentiation. During bone regeneration, Apex1 silencing induces a fracture nonunion phenotype, characterized by delayed fracture repair initiation, impaired periosteal response, and reduced chondrocyte and osteoblast differentiation. This compromised chondrocyte differentiation hampers callus vascularization and healing progression. Our findings highlight a critical mechanism where hypoxia-driven ROS signaling in mesenchymal progenitors through APEX1 is essential for fracture healing initiation.

The lymphatic system is widely distributed in skeletal muscles, joints, and skeletal tissues and plays a key role in maintaining immune homeostasis, regulating inflammatory responses, and tissue repair. In recent years, an increasing number of studies have shown that morphological and functional changes in lymphatic vessels are closely associated with the onset and progression of a variety of musculoskeletal disorders (MSDs), such as osteoarthritis (OA), fractures, and muscular dystrophy. However, the specific mechanisms of the lymphatic system’s role in these diseases have not been fully elucidated, and their potential clinical value remains to be thoroughly explored. In this review, we review the recent research progress on the structure, function, and pathophysiological role of the lymphatic system in the musculoskeletal system, and we focus on the association between lymphangiogenesis, dysfunction, and MSDs, and systematically summarize the therapeutic strategies targeting the lymphatic system. In addition, we summarize the limitations of current studies and propose key directions for future research, with a view to providing new ideas for basic research and clinical intervention in MSDs.

Nociceptive pain is a cardinal feature of traumatic and inflammatory bone diseases. However, whether and how nociceptors actively regulate the immune response during bone regeneration remains unclear. Here, we found that neutrophil-triggered nociceptive ingrowth functioned as negative feedback regulation to inflammation during bone healing. A unique Il4ra⁺Ccl2^high neutrophil subset drove intense postinjury TRPV1⁺ nociceptive ingrowth, which in return dissipated inflammation by activating the production of pro-resolving mediator lipoxin A4 (LXA4) in osteoblasts. Mechanistically, osteoblastic autophagy activated by nociceptor-derived calcitonin gene-related peptide (CGRP) suppressed the nuclear translocation of arachidonate 5-lipoxygenase (5-LOX) to favor the LXA4 biosynthesis. Moreover, in alveolar bone from patients with Type II diabetes, we found diminished nociceptive innervation correlated with reduced autophagy, increased inflammation, and impaired bone formation. Activating nociceptive nerves by spicy diet or topical administration of a clinical-approved TRPV1 agonist showed therapeutic benefits on alveolar bone healing in diabetic mice. These results reveal a critical neuroimmune interaction underlying the inflammation-regeneration balance during bone repairing and may lead to novel therapeutic strategies for inflammatory bone diseases.

Thrombospondin 1 and 2 (TSP1 and TSP2) are critical regulators of extracellular matrix (ECM) interactions, influencing cell differentiation and tissue repair. Recent discoveries from our laboratory and others highlight the importance of altered ECM alignment in influencing aberrant mesenchymal progenitor cell (MPC) differentiation and subsequent ectopic bone formation in trauma-induced heterotopic ossification (HO). However, the key regulators of this MPC to ECM interaction have yet to be elucidated. This study uncovers the role of matricellular TSP1 and TSP2 in MPC/ECM interaction as well as HO formation and progression. Using single-cell RNA sequencing, spatial transcriptomics, and in vivo models, we found that TSP1 is upregulated in tissue remodeling macrophages and MPCs at the injury site, while TSP2 is restricted to MPCs surrounding the HO anlagen. TSP1/2 double knockout (DKO) mice exhibited significantly reduced HO volume and disrupted ECM alignment. These findings highlight the crucial roles of TSP1 and TSP2 in musculoskeletal injury repair as well as HO formation and progression, supporting the potential to therapeutically target TSP1 and TSP2 to prevent HO.

Bone sialoprotein (BSP) is a major non-collagenous protein of the bone extracellular matrix and an important regulator of bone formation and resorption. BSP is produced by bone cells and chondrocytes and present in the bone matrix, cells, dentin and cartilage. However, its aberrant expression in primary tumour tissues and the sera of cancer patients with metastases implicates BSP in tumour biology and progression. The Arg-Gly-Asp (RGD) motif of BSP may be crucial not only for the attachment of metastasising cells to the bone surface but also for tumour growth, survival and activity. This review examines the structure and functions of BSP, including its roles in angiogenesis, bone formation, osteoclast differentiation and activity and cancer cell proliferation, survival, complement evasion, adhesion, migration and invasion. Growing evidence highlights BSP as a key mediator of tumour pathophysiology, skeletal metastasis development and associated bone remodelling. These processes are driven through RGD-integrin binding, the integrin/BSP/matrix metalloproteinase axis, integrin-independent signalling pathways, epithelial-to-mesenchymal transition and potentially post-translational modifications. A deeper understanding of BSP’s role in tumour progression may reinforce its potential as a prognostic and diagnostic tumour biomarker and aid the development of anti-BSP antibodies or targeted inhibitors for skeletal metastases and bone diseases.

During aging, the spine undergoes degenerative changes, particularly with vertebral endplate bone expansion and sclerosis, that are associated with nonspecific low back pain. We report that parathyroid hormone (PTH) treatment reduced vertebral endplate sclerosis and improved pain behaviors in three mouse models of spinal degeneration (aged, SM/J, and young lumbar spine instability mice). Aberrant innervation in the vertebral body and endplate during spinal degeneration was decreased with PTH treatment as quantified by PGP9.5⁺ and CGRP⁺ nerve fibers, as well as CGRP expression in dorsal root ganglia. The neuronal repulsion factor Slit3 significantly increased in response to PTH treatment mediated by transcriptional factor FoxA2. PTH type 1 receptor and Slit3 deletion in osteocalcin-expressing cells prevented PTH-reduction of endplate porosity and improvement in behavior tests. Altogether, PTH stimulated osteoblast production of Slit3, decreased aberrant sensory nerve innervation, and provided symptomatic relief of LBP associated with mouse spinal degeneration.

Advanced age impairs bone fracture healing; the underlying mechanism of this phenomenon remains unknown. We determined that apolipoprotein E (ApoE) increases with age and causes poor fracture healing. After deletion of hepatic ApoE expression (ΔApoE), 24-month-old ΔApoE mice displayed a 95% reduction in circulating ApoE levels and significantly improved fracture healing. ApoE treatment of aged BMSCs inhibited osteoblast differentiation in tissue culture models; RNA-seq, Western blot, immunofluorescence, and RT-PCR analyses indicated that the Wnt/β-catenin pathway is the target of this inhibition. Indeed, we showed that ApoE had no effect on cultures with stabilized β-catenin levels. Next, we determined that Lrp4 serves as the osteoblast cell surface receptor to ApoE, as expression of Lrp4 is required in ApoE-based inhibition of Wnt/β-catenin signaling and osteoblast differentiation. Importantly, we validated this ApoE-Lrp4-Wnt/β-catenin molecular mechanism in human osteoblast differentiation. Finally, we identified an ApoE-neutralizing antibody (NAb) and used it to treat aged, wildtype mice 3 days after fracture surgery resulting in fracture calluses with 35% more bone deposition. Our work here identifies novel liver-to-bone cross-talk and a noninvasive, translatable therapeutic intervention for aged bone regeneration.

Osteoarthritis (OA) is a degenerative skeletal condition marked by the loss of articular cartilage and changes to subchondral bone homeostasis. Treatments for OA beyond full joint replacement are lacking primarily due to gaps in molecular knowledge of the biological drivers of disease. Mass Spectrometry Imaging (MSI) enables molecular spatial mapping of the proteomic landscape of tissues. Histologic sections of human tibial plateaus from knees of human OA patients and cadaveric controls were treated with collagenase III to target extracellular matrix (ECM) proteins prior to MS Imaging of bone and cartilage proteins. Spatial MS imaging of the knee identified distinct areas of joint damage to the subchondral bone underneath areas of lost cartilage. This damaged bone signature extended underneath remaining cartilage in OA joints, indicating subchondral bone remodeling could occur before full thickness cartilage loss in OA. Specific ECM peptide markers from OA-affected medial tibial plateaus were compared to their healthier lateral halves from the same patient, as well as to healthy, age-matched cadaveric knees. Overall, 31 peptide candidates from ECM proteins, including Collagen alpha-1(I), Collagen alpha-1(III), and surprisingly, Collagen alpha-1(VI) and Collagen alpha-3(VI), exhibited significantly elevated abundance in diseased tissues. Additionally, highly specific hydroxyproline-containing collagen peptides, mainly from collagen type I, dominated OA subchondral bone directly under regions of lost cartilage but not areas where cartilage remained intact. A separate analysis of synovial fluid from a second cohort of OA patients found similar regulation of collagens and ECM proteins via LC-MS/MS demonstrating that markers of subchondral bone remodeling discovered by MALDI-MS may be detectable as biomarkers in biofluid samples. The identification of specific protein markers for subchondral bone remodeling in OA advances our molecular understanding of disease progression in OA and provides potential new biomarkers for OA detection and disease grading.

Cells actively sense and transduce microenvironmental mechanical inputs into chemical signals via cytoskeletal rearrangements. During these mechanosensation and mechanotransduction processes, the role of the actin cytoskeleton is well-understood, whereas the role of the tubulin cytoskeleton remains largely elusive. Here, we report the dynamic changes in microtubules in response to microenvironmental stiffness during chondrocyte mitosis. Mechanical stiffness was found to be coupled with microtubule generation, directing microtubule dynamics in mitotic chondrocytes. Refilin B was found to be a key regulator of microtubule assembly in chondrocytes in response to mechanical stiffness. It was found to play its role in microtubule formation via the p-Smad3 signaling pathway. Additionally, integrin-linked kinase (ILK), triggered by mechanical stiffness, was found to play an indispensable role in the process of microtubule dynamics mediated by refilin B. Our data emphasizes stiffness-mediated dynamic changes in the microtubules of chondrocytes in a quiescent state (G0) and at anaphase, which improves our understanding of the mechanical regulation of microtubule assembly during the chondrocyte cell cycle and provides insights into microenvironment mechanics during tissue maintenance, wound healing, and disease occurrence.

Craniofacial development relies on the migration of cranial neural crest cells (CNCCs) to the first and second pharyngeal arches, followed by their differentiation into various cell types during embryogenesis. Although the CNCC migration has been well-studied, the role of the niche in relation to CNCC remains unclear. Variants in FOXI3 have been implicated in craniofacial microsomia (CFM), yet the molecular mechanisms remain unexplored. FOXI3 is expressed in the ectoderm and auricle epidermis, but not in CNCCs or cartilage. Deletion of Foxi3 in the mouse CNCCs did not disrupt mandible and auricular development, further confirming that FOXI3 does not directly regulate CNCCs. However, Foxi3 deficiency in the ectoderm reduced the production of chondrogenesis-related cytokines derived from ectodermal cells, such as TGF-β1. This impairment affected CNCC proliferation through cell communication, subsequently altering the development of the mandible and auricle. These results emphasize the critical role of FOXI3 in establishing the microenvironment supporting CNCC function. Furthermore, FOXI3 directly regulates target genes associated with translation, thereby orchestrating cytokine production in epidermal cells. The validation using auricle sample from a CFM patient carrying FOXI3 mutation further supports our findings. These insights highlight the function of FOXI3 in creating the niche necessary for CNCC development and provide a basis for understanding the molecular mechanisms driving CFM pathogenesis.

Irreversible fibrotic scarring after rotator cuff tear (RCT) compromises the mechanical properties of the healing tendon, yet the underlying mechanisms remain poorly understood. Here, we analyzed the histological features of human RCT scars, characterized by disruption of tendon architecture, disorganized collagen fibrils, and imbalance in type I/III collagen ratios and fibril diameters. Using single-cell RNA sequencing of tendon stumps from patients with RCT, we deconvolved the cellular and molecular landscape of the fibrotic scarring microenvironment. Heterogenous pro-fibrotic subclusters were identified and validated to participate into scar formation, including tendon stem cell, senescent tenocyte, SOX9-driven pro-fibrotic macrophage, and pro-fibrotic endothelial cells undergoing endothelial-mesenchymal transition (EndoMT). Furthermore, we found that osteopontin and TGF-β signaling were key drivers of extracellular matrix deposition, and their blockade ameliorated fibrotic scarring after RCT. Collectively, our study dissected the dynamic scarring microenvironment in human RCT and highlights potential therapeutic targets for preventing pathological scar formation.

Respiratory inflammatory diseases disrupt bone metabolism and cause pathological bone loss. The lung-bone axis is established in chronic diseases like asthma and cystic fibrosis but is less studied in acute lung injury (ALI), recently implicated in COVID-19-induced bone loss. This study examined the effects of LPS-induced ALI on bone phenotype and explored the role of 2-N, 6-O sulfated chitosan (26SCS) in mitigating pneumonia-induced bone loss via inflammatory response modulation. Our findings show that 26SCS effectively reaches bone tissue after oral administration. It promotes macrophage polarization to the M2 phenotype, alleviating immune cascade reactions and inhibiting osteoclast-mediated bone resorption. Increased M2 macrophages support type H vessel formation, enhancing inflammatory bone vascularization. These effects foster a favorable osteogenic microenvironment and mitigate ALI-induced bone loss. While dexamethasone is effective in reducing inflammation, it can aggravate ALI-induced bone loss. Our research offers a therapeutic strategy targeting the lung-bone axis for inflammation-induced bone loss.

Endochondral ossification is a physiological process involving a sequential formation of cartilage and bone tissues. Classically, cartilage and bone formation have been considered independent processes at cellular level. However, the recently described multiple cell differentiation dynamics suggest that some bone cells are indeed the progeny of cartilage cells, or chondrocyte-derived osteoblasts. We hypothesized that the cartilage-to-bone phenotype transition is triggered by specific molecular events. First, the process was assessed in mouse bone tissue, and then, it was mimicked using in vivo cell implantation and in vitro serial differentiation protocols. Data indicates that cartilage cells transition to bone cell phenotype during postnatal physiological bone formation. This process can be reproduced using cartilage precursor cells coupled to specific implantation procedures or differentiation protocols. Gene expression profiling reveals that NOTCH, BMP and MAPK signaling pathways are relevant at the phenotype-switch, while the transcription factors Mesp1, Alx1, Grhl3 and Hmx3 are the feasible driver genes for chondrocyte-derived osteoblasts formation. Altogether, this report shows that endochondral ossification can be modeled using primary cell cultures and data indicate that this process is regulated by specific molecular events, previously described at skeleton morphogenesis during embryo development, and from now on also linkable to postnatal bone development and regeneration processes.

Mitochondrial regulation in mesenchymal stem cells (MSCs) serves as a critical determinant of bone formation and skeletal homeostasis. While dietary nitrate and its transporter Sialin are implicated in systemic homeostasis, their specific roles in MSCs' function remain unclear. Here, we demonstrate that Sialin deficiency impairs MSCs' function and disrupts bone homeostasis. Gain- and loss-of-function studies reveal that Sialin localizes to the mitochondrial membrane and promotes osteogenic differentiation by maintaining mitochondrial bioenergetic integrity. Mechanistically, Sialin recruits pSTAT3^S727 to mitochondria, forming a functional complex that activates mitochondrial bioenergy and stabilizes bone remodeling. Notably, dietary nitrate restores Sialin expression in aged mice, thereby enhancing MSCs' function and preventing osteoporosis. Our findings identify a nutrient-responsive signaling axis—nitrate-Sialin-pSTAT3^S727—that promotes osteogenic differentiation via mitochondrial homeostasis, offering a potential therapeutic strategy for age-related osteoporosis.

Obesity and type-2 diabetes, two interconnected and increasingly prevalent metabolic disorders, are associated with poor bone quality, higher fracture risk, and impaired fracture repair. The causes are not yet resolved but appear to relate to the impaired glucose homeostasis, altered bone material properties and remodeling, and compromised skeletal vascularization. Each of these features is impacted by hypoxia-inducible factor (HIF) signaling, which led us to hypothesize that HIF pathway modulation might be an effective strategy to concomitantly improve energy metabolism and bone health in conditions of metabolic stress. Here, we evaluated whether pharmacological HIF activation using the HIF-prolyl-hydroxylase-domain enzyme (PHD) inhibitor FG-4592 (Roxadustat) could protect mice against the adverse skeletal and metabolic consequences of high-fat diet (HFD)-induced obesity. We found that systemic FG-4592 treatment effectively prevented HFD-triggered body weight gain, glucose intolerance, and peripheral fat accumulation, associated with globally increased energy expenditure. Concomitantly, FG-4592 administration prevented the skeletal vascular damage, marrow fat accumulation, and bone formation deficits that were caused by HFD. Moreover, the HIF-activating drug also improved glucose metabolism and bone regeneration in a model of compromised fracture repair associated with overnutrition. Specifically, short-term FG-4592 treatment during fracture recovery reduced the body weight and fat mass of obese mice, improved glucose tolerance, and enhanced the fracture bridging capacity, along with promoting callus vascularization. These findings demonstrate that systemic hypoxia signaling stimulation using PHD inhibitors alleviates both the metabolic and skeletal consequences of diet-induced obesity in mice, highlighting its potential as a dual-action therapeutic strategy for enhancing glucose homeostasis and bone health/regeneration in disorders of obesity and metabolic dysfunction.

Estrogen deficiency after menopause accelerates bone loss by stimulating osteoclast formation and activity, but the molecular pathways that link estrogen signaling to osteoclast regulation remain incompletely defined. Here, we identify the sialyltransferase ST3GAL-I as a key mediator of RANKL-induced osteoclastogenesis. RANKL activates c-FOS to drive ST3GAL1 transcription, whereas estrogen-bound ERα competes with TRAF6 and suppresses this c-FOS–dependent induction. In a clinical cohort of pre-menopausal and post-menopausal women with or without osteoporosis, serum total and α-2,3-linked sialic acid levels increased with age and were highest in post-menopausal osteoporotic patients. Single-cell RNA sequencing of human bone revealed that osteoclasts form a prominent cluster only after menopause, where FOS, CTSK, and ST3GAL1 are strongly co-expressed, and the estrogen-responsive gene PGR is down-regulated. Additionally, in vivo experiments showed that sialidase treatment in estrogen-deficient models effectively reduced osteoclast-mediated bone loss, mimicking the effects of estradiol. These findings define a direct molecular link between loss of estrogen and activation of a FOS–ST3GAL1 sialylation pathway in osteoclasts, providing mechanistic insight into the enhanced bone resorption characteristic of post-menopausal osteoporosis.

Steroid-induced osteonecrosis of the femoral head (SONFH) is a debilitating condition resulting from the use of glucocorticoids, commonly prescribed for immune-related and inflammatory diseases. Understanding the mechanisms driving SONFH remains a significant challenge, complicating efforts to prevent and treat the condition. While genetic predispositions, impaired blood supply, and metabolic changes are recognized contributors, the complex interplay between these factors is not yet fully understood. Recent research has shed light on the pathogenesis of SONFH, exploring it from multiple perspectives, including tissue-level damage, cellular dysfunction, and molecular pathways. This review summarizes these recent advancements, providing an integrated understanding of the onset and progression of the condition. Additionally, it highlights emerging therapeutic strategies that potentially pave the way for more effective treatments in the future.

Skeletal aging associated with diverse age-related disorders is increasing due to unhealthy diets, stressful lifestyles, and rapid aging. Repair and regeneration of aging skeletons are a global issue. Despite the self-healing ability of bone and the availability of various treatment strategies, degenerative bone repair and regeneration face significant problems due to unbalanced bone remodeling and a lack of active treatment strategies. The development of smart materials has created opportunities for degenerative bone repair and regeneration. The smart materials are responsive to endogenous/exogenous stimuli with tailored structure and function, which can promote skeletal aging repair and regeneration. Thus, in this study, skeletal aging is recognized as the progressive state that begins from peak bone mass to pathophysiological state and disorder conditions. We have introduced and characterized skeletal aging from the perspectives of cell-matrix-microenvironment and macrostructure-function-mechanical properties, for which systemic smart drug delivery systems and local smart scaffolds are designed. The smart drug delivery systems undergo conformation change and phase transition upon stimuli to release drugs at time- and site-specific to promote aging bone repair. Smart scaffolds with versatility and mechanical strength can replace bone defects to provide a tissue repair and regeneration microenvironment. Endogenous disease microenvironments and/or external physical triggers stimulate scaffold activation, which release bioactive factors to accelerate bone regeneration. This manuscript discusses the manufacturing techniques of these smart materials and presents key challenges and future directions for clinical translation, emphasizing their potential for personalized treatment and targeted therapy of skeletal aging.

Bone regeneration is initiated after a bone injury, such as a bone fracture or tooth extraction. It is a highly complex biological process involving multiple cell types, signaling molecules, and molecular pathways. The hypoxic microenvironment in the early stage of bone regeneration poses challenges to cell status and the final outcome of bone regeneration. During this phase, two key regulators—HIF-1α (the critical mediator of hypoxia response) and BMAL1 (the central component of the circadian rhythm)—orchestrate the activities of bone-regenerating cells, ensuring proper cellular function and orderly progression of bone repair. Existing studies have shown that there is a close crosstalk between HIF-1α and BMAL1, including regulation of gene expression, protein interaction, and regulation of downstream pathways. In this review, we discuss the respective regulatory roles of HIF-1α and BMAL1 in bone regeneration and further summarize their interactions within cells. Additionally, we extend the discussion to their interactions in other bone-related diseases, and summarize the existing research directions and deficiencies, providing new insights for in-depth studies of the hypoxia response and circadian rhythm systems.

Osteoarthritis (OA) is an aging-related degenerative joint disease without effective therapies. In the early stage of OA, mild synovitis has been reported to induce cartilage lesions. A better understanding of crosstalk between synovial macrophages and chondrocytes are being developed to discover new OA therapeutics. Here, we identified that the extracellular vesicles (EVs) derived from synovial pro-inflammatory macrophages regulated the autophagy function of chondrocytes, induced the onset of cartilage degeneration in normal joints. Mechanistically, the active transfer of miR-155-5p via EVs from synovial pro-inflammatory macrophages to chondrocytes accelerates cartilage degeneration by suppressing GSK-3β/mTORC1 axis-mediated autophagy function during OA progression. Deleting miR-155 from synovial pro-inflammatory macrophages relieved cartilage lesions and synovitis in OA mice. On the other hand, Fragile X mental retardation protein (FMRP) selectively sorted miR-155-5p into EVs derived from synovial pro-inflammatory macrophages, and the levels of plasma EVs FMRP were closely related to OA progression, suggesting the potential candidate for diagnostic OA biomarkers. Based on these findings, we developed engineering EVs with MAP (pro-inflammatory macrophages-affinity peptide) derived from adipose-derived stromal cells (ADSCs) as the antagomiR-155-5p delivery vehicles which exhibited superior therapeutic effects on synovitis and injured cartilage in the surgery-induced OA rats. Furthermore, MAP-ADSCs-EVs were proved to target the polarization of synovial pro-inflammatory macrophages in the clinical OA samples. Collectively, our study indicates that plasma EVs FMRP and engineered MAP-ADSCs-EVs targeting synovial pro-inflammatory macrophages represent potential novel therapeutic strategy for the progression of OA.

Spinal cord injury (SCI) often causes long-term disability. But effective means to promote proper regeneration after SCI has so far failed to reach the clinic. Here, we report that fibrotic scar formation at injury sites prevents recovery after SCI and that the inhibition of fibrotic scar formation significantly improved SCI recovery in adult mice. We found that after SCI there is an elevation of macrophages, which are a primary source of activated transforming growth factor-β 1 (TGF-β1) that in turn recruits mesenchymal stromal/stem cells (MSCs) to induce their fibroblast differentiation, thus promoting scar formation. We also found that activated TGF-β1 acts on resident pericytes in the endothelial niche of the blood-spinal cord barrier to promote their differentiation into fibroblasts, which also contributes to scarring. Interrupting these pathways by selective genetic KOs or treatment with a TGF-β–neutralizing antibody inhibited scar formation and improved SCI functional recovery. Notably, we found that neonatal mice recover scarlessly after SCI and with no active TGF-β at the injury site. Together, these findings suggest that fibrotic scarring occurs due to elevated activation of TGF-β, and preventing such activation or neutralizing active TGF-β may be an approach to improve outcome after SCI.

The lymphatic system, traditionally regarded as a unidirectional conduit for interstitial fluid and immune cell transport, has recently been redefined through the discovery of lymphatic networks along the spinal axis. These spinal lymphatic vessels, encompassing the spinal cord, vertebral bones, and intervertebral discs, challenge long-standing anatomical dogmas and introduce new perspectives on the interplay between the central nervous system (CNS) and the vertebral column. This review systematically summarizes the distribution and dual functions of the spinal lymphatic system in regulating cerebrospinal fluid drainage, maintaining tissue homeostasis, and mediating immune responses. Furthermore, we highlight emerging evidence linking spinal lymphatic dysfunction to spinal pathologies, neurological disorders, and vertebral degeneration. Based on these findings, we propose that the spinal lymphatic system constitutes a previously underappreciated pathway integrating spinal cord and vertebral physiology, with potential implications for both disease progression and therapeutic intervention. While research on the cranial lymphatic system has rapidly advanced, the spinal lymphatic system remains comparatively underexplored. We hope this review will catalyze further investigation into spinal lymphatic biology and inform the development of novel therapeutic strategies targeting spinal and neurological diseases.

Heterotopic ossification (HO) is a debilitating disorder marked by ectopic bone formation in soft tissues, frequently triggered by inflammation after trauma. While macrophage-driven inflammation plays a critical role in HO pathogenesis, the molecular mechanisms governing its initiation, amplification and resolution remain elusive. Using a trauma/burn injury (TBI)-induced mouse model of HO, we identified rapid and sustained macrophage accumulation at the injury site during the early inflammatory phase, and macrophage depletion markedly suppressed HO formation. Transcriptomic profiling identified a pronounced upregulation of protein arginine methyltransferase 6 (PRMT6) in macrophages following injury. Genetic deletion or macrophage-targeted knockdown of Prmt6 reduced macrophage accumulation and significantly attenuated HO, without impairing tendon repair. Consistently, pharmacological inhibition of PRMT6 suppressed HO only when administered during the early inflammatory phase, indicating a restricted therapeutic window. Mechanistically, PRMT6 amplified macrophage chemotactic signaling by transcriptionally and epigenetically upregulating CCL2. Genetic disruption of macrophage-derived CCL2 phenocopied Prmt6 deficiency, whereas CCL2 supplementation rescued macrophage recruitment and partially restored HO in Prmt6-deficient mice. At the molecular level, PRMT6 formed a coactivation complex with NF-κB and catalyzed H3R17 asymmetric dimethylation at the Ccl2 promoter, thereby promoting sustained chemokine expression. Collectively, our findings identify PRMT6 as a central epigenetic amplifier of macrophage-driven inflammation that links early injury responses to ectopic bone formation. Targeting PRMT6 during the early inflammatory phase represents a promising strategy to prevent HO while preserving physiological tissue repair.

In addition to its role in hemostasis, Factor VIII (FVIII) has recently been shown to potentially impact angiogenesis, inflammation, osteopenia, and sarcopenia. This was explored here by studying the musculoskeletal development of FVIII knockout (FVIII^-/-) male mice. These animals developed an osteoporotic phenotype with significant bone microarchitectural alteration, reduced vascularization, and a lower osteoblastic population. Proteomic analyses revealed differentiating bone metabolism-related proteins between FVIII^-/- and wildtype mice. Weekly infusions of recombinant FVIII protein reversed this phenotype. Surprisingly, younger FVIII^-/- mice had heavier muscles with larger fibers, shifted from type IIx to type IIb, not reversed by FVIII treatment. Significant proteomic and metabolomic differences between wildtype and FVIII^-/- muscles were observed, some of which were reduced by FVIII treatment. This study provides the first comprehensive full-phenotypic characterization of bones and muscles in FVIII^-/- mice and demonstrates the benefits of FVIII supplementation to normalize their musculoskeletal phenotype.

Follicle-stimulating hormone (FSH), a gonadotropin that rises in post-menopausal females, activates its receptor FSHR to trigger bone loss via increasing bone resorption by osteoclasts. FSH stimulates CCAAT/enhancer binding protein beta (C/EBPβ) /asparagine endopeptidase (AEP) pathway, facilitating neural degeneration in the brain of mouse models with Alzheimer’s disease (AD). However, whether C/EBPβ/AEP pathway feeds back and modulates FSHβ bone resorption action remains elusive. Here we show that C/EBPβ acts as a transcription factor for fshb gene and directly binds its promoter, mediating its mRNA transcription in the pituitary gland. Knocking down C/EBPβ in primary pituitary cells significantly blunts GnRH (gonadotropin-releasing hormone)-induced FSHβ expression. Knockout of C/EBPβ also robustly diminishes FSHβ levels in mice. Inactivation of AEP, either by knockout of AEP or its small molecular inhibitor, antagonizes C/EBPβ and suppresses FSHβ levels, attenuating ovariectomy (OVX)-elicited osteoporosis. Markedly, a specific AEP inhibitor (#11a) displays comparable therapeutic effect as an FDA-approved drug teriparatide in OVX-induced osteoporosis. Hence, these findings support that C/EBPβ dictates FSHβ transcription and blocking AEP by its inhibitor represses C/EBPβ-mediated FSHβ levels, exerting prominent therapeutic efficacy toward osteoporosis.

Bone marrow adipocytes are known to have a critical role within the bone marrow niche. However, our understanding of bone marrow adipose tissue expansion with obesity and the role it plays in immune cell regulation and osteoclastogenesis is limited. Here, we showed the expansion of bone marrow adipocytes promoted osteoclast differentiation and subsequently led to obesity-related trabecular and cortical bone loss through a stimulatory effect of the PD-1/PD-L1 axis. Bone marrow adipocytes isolated from obese mice had increased Mcp-1 expression, a key regulator of osteoclastogenesis and myeloid cell accumulation. With the increase in bone marrow adipose tissue-derived Mcp-1, we found an increase in the number of PD-L1⁺ myeloid cells. While these cells inhibited activated T-cells, we found evidence of a stimulatory osteoclastogenic effect of PD-L1⁺ myeloid cells on PD-1-expressing osteoclast precursors. The inhibition of PD-1/PD-L1 signaling during early osteoclastogenesis prevented myeloid cell commitment and resulted in decreased cell fusion, supporting the role of PD-1/PD-L1 signaling in osteoclastogenesis. Using a bone marrow adipocyte depletion mouse model (BMAd-Pparg KO), we demonstrated that obese BMAd-Pparg KO mice had a reduced number of bone marrow PD-L1⁺ myeloid cells, accompanied by a decrease in PD-1⁺ osteoclast precursors. The reduction in these precursors resulted in fewer osteoclasts, subsequently leading to improved trabecular bone volume. Since osteoclasts are myeloid cell-derived, these results suggest that bone marrow adipocytes are critical for the commitment and differentiation of myeloid cells into osteoclasts. Targeting bone marrow adipogenesis could ameliorate enhanced osteoclastogenesis and provide a novel approach to treat obesity-related bone loss.

Obesity-induced expansion of BM adipocytes leads to PD-1/PD-L1-driven osteoclastogenesis and subsequent bone loss in obese, HFD-fed (OB-HFD) mice. After 12 weeks on a HFD, OB-HFD mice had a significant increase in BM adiposity and BMAT-derived Mcp-1 expression. The increase in BMAT-specific Mcp-1 expression was coupled with an increase in PD-1⁺ osteoclast (OC) precursors and PD-L1⁺ myeloid cells. In the context of obesity, the PD-1/PD-L1 axis has a stimulatory effect that enhances osteoclastogenesis and leads to trabecular and cortical bone loss. By depleting BM adipocytes with obesity, BMAT-derived Mcp-1 expression was decreased, as well as a decrease in PD-1⁺ OC precursors and PD-L1⁺ myeloid cells. This prevented obesity-related trabecular bone loss. Overall, this work demonstrated a strong correlation between BMAT expansion and PD-1/PD-L1-driven osteoclastogenesis as a mechanism for obesity-induced bone loss. (This image was created using BioRender).

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Craniofacial bone regeneration remains a major clinical challenge, yet the identity of orofacial mesenchymal stem/stromal cells (OMSCs) has not been fully elucidated. Here, we performed single-cell RNA sequencing (scRNA-seq) on mouse orofacial bone and identified multiple stromal cell clusters. Cell-cell communication mapping and trajectory inference uncovered the heterogeneity of OMSCs and functional divergence among subpopulations. We identified a previously unrecognized population, Smmhc-expressing mesenchymal stem/stromal cells (MSCs), at the earliest stage of the progenitor lineage trajectory. In vivo lineage tracing demonstrated that Smmhc⁺ MSCs are multipotent, giving rise to osteoblasts, osteocytes, periodontal ligament (PDL) cells, and dental pulp cells. Targeted ablation of Smmhc⁺ MSCs using Smmhc^CreER;iDTR mouse model led to impaired orofacial bone development and disrupted orofacial tissue homeostasis, characterized by reduced osteogenic differentiation and non-cell autonomous reduction of bone resorption. Collectively, this study establishes a cellular atlas of OMSCs and identifies Smmhc⁺ MSCs as a functionally indispensable subset for craniofacial bone homeostasis, orchestrating the dynamic balance between osteogenesis and bone resorption within the orofacial skeletal niche.

Developmental dysplasia of the hip (DDH), a morphological abnormality of the hip joint, is a well-recognized risk factor for hip osteoarthritis (OA). Much remains unknown about the genetic factors of DDH and its subtypes. To further understand its genetic basis, we conducted genome-wide association studies (GWASs) using a total of 1 085 Japanese DDH cases (including 788 hip dysplasia cases without dislocation and 297 cases with dislocated hip) and 24 000 controls. Additionally, we meta-analyzed with United Kingdom (UK) DDH GWAS and the largest hip OA GWAS to date. We identified three genome-wide significant novel loci, COL11A2, CALN1 and TRPM7, associated with hip dysplasia without dislocation. None of these signals were significant in dislocated hips, and additionally two of the signals had an opposite direction of association, suggesting distinct genetic architectures between the subtypes. The Japanese DDH GWAS identified five associated loci (VEGF-C, FOXC1, SMC2, SLC38A4, and TRPM7), and the trans-ancestry meta-analysis with UK revealed two loci (COL11A1 and GDF5) supported by strong trans-ancestry genetic correlation (r = 1.0). In total, nine loci were identified for DDH and its subtypes, with hip dysplasia without dislocation showing distinct genetic signals from hip dislocation. The meta-analysis of DDH and hip OA identified five novel signals for hip OA. Susceptibility loci and heritability enrichment analyses implicated pathways involving bone formation, collagen type XI trimer, and chondrocyte development, as well as their gene regulation, in DDH. These findings enhance understanding of the genetic architecture and biological pathways underlying DDH, providing new insights into its relationship with OA.

Osteoclasts are essential for bone resorption and interact with osteoblasts during bone remodeling. Ion channels and transporters located in the ruffled border or intracellular vesicles coordinate the transport of various ions and substrates, which is fundamental to the primary functions of osteoclasts. Numerous channels and transporters are implicated in bone metabolic disorders and genetic diseases. Among these, the voltage-gated chloride channel 7 (ClC-7) and vacuolar proton ATPases (V-ATPase) represent the most well-characterized examples in osteoclasts. Using the classification system of the Transporter Classification Database, we reviewed nearly 90 osteoclastic ion channels and transporters, categorizing them into six groups: ATPases, cation channels, anion channels, complex transporters, organic substance transporters, and ATP-binding cassette transporters. We summarized recent advances in their subcellular localization, transported substrates, associated diseases, and physiological roles in relevant biological functions and signaling pathways. Notably, transporters for hydrogen, chloride, phosphate, and calcium are particularly critical for osteoclast function. We also reviewed therapeutic candidates targeting these ion channels and discussed strategies for their future development. As transcriptome and other advanced techniques have identified more channels and transporters in osteoclasts, the diversity and unexplored functions of these molecules may exceed previous understanding. Increased attention to their widespread distributions and interactions could reveal new therapeutic targets for osteoclast-related and other bone disorders.

Standard clinical imaging metrics perform poorly in predicting skeletal fragility in chronic kidney disease (CKD), particularly due to the complex and heterogeneous cortical deterioration that characterizes advanced disease. Here, this study aimed to identify radiomic features derived from high-resolution peripheral quantitative computed tomography (HR-pQCT) in tibial cortical bone that distinguish CKD-related differences and may serve as markers of subtle cortical alterations undetected by conventional imaging. HR-pQCT image stacks were obtained from 72 participants (38 non-CKD and 34 with CKD stage 5D) at 7.3% (distal) and 30% (diaphyseal) proximally from the tibial endplate, resulting in a total of 24 192 slices. In non-CKD cases, features were largely derived from first-order statistics, while complex features from higher-order statistics were more prominent in CKD cases. Although conventional HR-pQCT outcomes, such as volumetric bone mineral density, showed limited ability to differentiate CKD from non-CKD cortical bone in our population of stage 5D patients, the top features, such as Minimum and Strength, provided a significant distinction between the two groups (P < 0.001, Effect size r = from 0.813 to 0.856). Our findings demonstrate that radiomic analysis identifies cortical bone differences associated with CKD that were not distinguished by conventional HR-pQCT metrics, highlighting its potential to improve bone quality assessment in this high-risk population.

Dynamic transitions of mature osteoblasts between active and quiescent states are essential for bone homeostasis and present a promising target for osteoanabolic therapy. However, these transitions remain poorly understood due to cellular heterogeneity and limited spatial context. Here, we employed spatially resolved osteoblast-traced transcriptomics, integrating an osteoblast-specific lineage tracing study and spatially resolved laser-activated cell sorting (SLACS), to profile osteoblast states on quiescent bone surfaces. This approach identified transforming growth factor-beta (TGF-β) signaling as a regulator of osteoblast activation. We further validated this role using single-cell RNA sequencing, in vitro functional assays, and in vivo. In a hindlimb unloading mouse model, dual inhibition of TGF-β and sclerostin enhanced bone mass and mitigated bone loss more effectively than sclerostin inhibition alone. These findings reveal a mechanistic role for TGF-β in regulating osteoblast dynamics and propose a dual-target therapeutic strategy that enhances the efficacy of anti-sclerostin treatment in osteoporosis.

Osteosarcoma is a highly malignant bone tumor which primarily affects the juvenile population and is characterized by high rate of recurrence and metastasis. RNA editing has emerged as a key process in cancer progression. Herein, we investigated the role of RNA editing enzyme ADAR2 (Adenosine Deaminase Acting on RNA 2) in osteosarcoma. We demonstrated that ADAR2 expression increases during osteoblast differentiation and inversely correlates with the aggressiveness of osteosarcoma cells. Interestingly, the overexpression of ADAR2 in osteosarcoma cell lines reduces their tumoral properties and promotes their differentiation in osteoblast-like cells, as shown by gene expression analysis and mineralization assays. These results were also confirmed by in vivo experiments; indeed, intratibial injection of ADAR2-overexpressing osteosarcoma cells in NSG mice resulted in less aggressive tumors compared to mice injected with pEmpty or pInactive ADAR2 E/A vector-transfected cells. To elucidate the mechanisms by which ADAR2 overexpression induces osteogenic terminal differentiation of osteosarcoma cells, we performed RNA-seq analysis of Saos-2 cells and identified IGFBP7 (Insulin-like Growth Factor Binding Protein 7) as the most highly edited transcript in ADAR2-overexpressing cells. We showed that the editing activity of ADAR2 on IGFBP7 abolishes its proliferative effect on osteosarcoma cells and triggers terminal differentiation. Overall, our results indicate that ADAR2 acts as a tumor suppressor in osteosarcoma and may represent a novel therapeutic target for this aggressive pediatric tumor.

Bone adhesives have emerged as promising alternatives for complex fracture fixation. However, discrepancies between material degradation rates and the physiological timeline of bone healing remain a critical limitation. Here, a polyurethane-based adhesive (TNC) was developed, synthesized from trimeric hexamethylene diisocyanate, nano-hydroxyapatite, and type I collagen. The TNC demonstrates strong initial adhesion to both wet and blood-contaminated bone surfaces and exhibits excellent biocompatibility. A distinguishing feature of TNC is its capacity to synchronize degradation with the stages of bone healing. During degradation, TNC forms a mineralized surface layer that releases calcium ions. The calcium ions activate cathepsin K, an enzyme integral to bone remodeling. This calcium-mediated mechanism accelerates TNC degradation by 1.9-fold during the remodeling phase compared to the initial phase. In a rat skull fracture model, TNC supported effective fracture stabilization and achieved favorable bone regeneration at 8 weeks after implantation. These findings demonstrate that TNC combines early mechanical stability with phase-specific degradability to facilitate bone regeneration in a temporally-controlled manner. The present work provides a framework for the development of bio-responsive bone adhesives that synchronize degradation behavior with healing phases for orthopedic applications.

Insufficient skeletal repair is the primary threat of health span and lifespan in elders with increasingly vast global burden; yet, to date, the knowledge of resolving this crisis remains limited. In this study, we addressed the specific mechanisms underlying aging-associated poor bone repair, which are driven by the mitochondrial DNA structures mitochondrial G-quadruplex (mtG4). We found that mtG4 is spatiotemporal-wisely accumulated within Pdgfra⁺ periosteal mesenchymal stromal/stem cells (PPM) both in healthy and premature aging, which substantially increases cellular senescence and the degenerative alterations of PPM. By utilizing transgenic lineage tracking, PPM organoids formation, mitochondrial transgenic mutation, organoids transplantation, and serial cellular molecular investigations, we reveal that mtG4 in PPM restricts vital mitochondrial genes’ transcription to cause mitochondrial dysfunction, which utterly leads to severe mitophagy and cell senescence. These senescent PPM demonstrates impaired stemness and disrupted fate determination, finally phenocopying aging-associated poor bone repair. This study decodes the mitochondrial genomic reasons for insufficient bone repair during aging, which offers insights for developing cell-type- and disease-specific senolytic therapies in the future.

Chondrocyte hypertrophy and mineralization are essential for endochondral ossification; however, the mechanisms underlying these processes remain incompletely understood. In this study, we have identified the facilitated role of ubiquitin-specific protease 26 (USP26) in endochondral ossification by stimulating chondrocyte hypertrophy and mineralization. Ultimately, this promotes skeletal development, bone fracture healing, and the occurrence of osteoarthritis. Mechanistically, USP26 decreases FBP2 undergoing K63-linked ubiquitination, leading to a reduction in the protein level of FBP2. This reduction promotes mitochondrial biogenesis and oxidative phosphorylation, thus facilitating chondrocyte hypertrophy and mineralization and aiding in the process of endochondral ossification. Furthermore, our study found that compression loading induces USP26 to initiate chondrocyte hypertrophy and mineralization through the phosphorylation of estrogen receptor-α at serine 118. These findings suggest that USP26, acting as a mechanosensor, facilitates chondrocyte hypertrophy and mineralization by maintaining mitochondrial biogenesis through the reduction of FBP2. Identifying USP26 as a potential therapeutic target for physiological skeletal growth, bone fracture healing, and osteoarthritis.

Genetic background is a major determinant of disc degeneration, a leading cause of chronic back pain and disability. Herein, we demonstrate that premature disc cell senescence contributes to early-onset degeneration in SM/J mice and test two systemic senotherapeutic strategies to mitigate it: Navitoclax (Nav.) and a cocktail of Dasatinib and Quercetin (DQ). While Nav. treatment did not improve severe degeneration in SM/J mice or senescence status, DQ-treated mice showed lower grades of degeneration and a decreased abundance of senescence markers, including p19ARF, p21, and the senescence-associated secretory phenotype (SASP). DQ improved disc cell viability and phenotype retention and retarded fibrosis of the nucleus pulposus tissue. Transcriptomic analysis revealed tissue-specific effects of the treatment, with cell cycle regulation and JNK signaling being commonly affected across different tissue types. A comparison of SM/J data with DQ-mediated aging-dependent amelioration of disc degeneration in C57BL/6 N mice identified Junb and Zfp36l1 signaling as shared DQ targets in the mouse disc. Notably, the in vitro inhibition studies of the JUN pathway in human degenerated NP cells mimicked the benefits of DQ, namely, a reduction in senescence and SASP. This study reinforces the efficacy of senolytic treatment in ameliorating local senescence and intervertebral disc fibrosis.

Elucidating the identity of enthesis-resident progenitors is critical for advancing regenerative strategies, particularly in the context of the long-standing question of how is fibrocartilage formed at tendon enthesis (bone-tendon interface) under mechanical loading. To address the question of cellular origins of entheseal fibrocartilage, we first employed spatial transcriptional and single cell sequencing to identify a novel population of Tnn⁺ progenitor cells and delineate their lineage trajectories across developmental stages. Subsequently, we used a diphtheria toxin mediated ablation model targeting these Tnn⁺ progenitors and demonstrated their functional importance, as ablation resulted in hypoplastic phenotypes characterized by impaired fibrocartilage maturation. Furthermore, comparative single-cell profiling between unloaded entheses and normal entheses revealed that tendon unloading significantly diminished both the abundance and chondrogenic potential of Tnn⁺ progenitors. Collectively, these findings resolve fundamental questions regarding enthesis morphogenesis and provide mechanistic insights into how mechanical loading orchestrates this critical developmental process.

The nervous system has emerged as a multi-scale regulator of bone biology, integrating central neural circuits with peripheral innervation to control skeletal homeostasis and repair. While bone remodeling is classically described as being governed by coupling between bone formation and resorption, neural signaling provides an additional hierarchical layer that links organism-level cues to local skeletal stem/progenitor cell niches. This review presents a mechanistic framework for the neuro–bone regulatory network across three hierarchical levels. First, we examine central regulation, in which hypothalamic circuits integrate hormonal and metabolic signals via circumventricular organs to modulate endocrine outputs such as parathyroid hormone (PTH), thereby establishing circadian rhythms and systemic control of bone metabolism. Second, we analyze peripheral neural communication, where sensory inputs triggered by injury or inflammation, along with autonomic efferent signaling, including β-adrenergic pathways, directly influence osteolineage and stromal cells. These signals recalibrate cellular metabolic states, differentiation programs, and regenerative responses, linking pain perception with tissue repair mechanisms. Third, we investigate the bone marrow niche, where distinct subtypes of nerve fibers release a diverse array of neurotransmitters and or neuromodulators that shape the microenvironment of skeletal stem and progenitor cells (SSPCs) as well as downstream osteoprogenitors, thereby regulating proliferation, lineage commitment, and quiescence. Collectively, these findings delineate an integrated model of neural regulation of bone spanning central, peripheral, and local niche levels, providing a foundation for testable hypotheses in neuro-osteobiology.

Sclerostin negatively regulates bone formation. The marketed antibody against sclerostin loop2 promoted bone formation but may have caused severe cardiovascular events in clinical use. In our published studies, sclerostin loop3 was found to be involved in inhibitory effects of sclerostin on bone formation, whereas cardiovascular protective effects of sclerostin in mice were independent of loop3. It is necessary to investigate how sclerostin loop3 participates in the inhibitory effects of sclerostin on bone formation to facilitate developing precise strategies that promote bone formation without increasing cardiovascular risk. In this study, sclerostin loop3 was identified to bind to LRP4, thereby facilitating binding of sclerostin to LRP6 in osteoblasts. Blockade of sclerostin loop3-LRP4 interaction by both Lrp4 mutation (Lrp4m) and blocking peptide (LRP4-Pep) diminished the antagonistic effect of sclerostin on Wnt/β-catenin signaling in osteoblasts in vitro. Consistently, Lrp4m promoted bone formation in Lrp4m mice in vivo. Mechanistically, osteoblast-conditional correction of Lrp4m to wild-type Lrp4 resulted in significantly lower bone formation than Lrp4m mice, indicating that the promotive effects of Lrp4m on bone formation acted in osteoblasts in vivo. Moreover, re-expression of sclerostin dramatically inhibited bone formation in sost^−/− mice, whilst the inhibitory effects of sclerostin were significantly weaker in sost^−/−.Lrp4m mice. Pharmacologically, LRP4-Pep diminished the inhibitory effects of sclerostin on bone formation in SOST^ki mice. Taken together, osteoblastic sclerostin loop3-LRP4 interaction, as an anchor, was required by sclerostin to bind to LRP6, thereby inhibiting bone formation. Translationally, blockade of sclerostin loop3-LRP4 interaction in osteoblasts would provide precise therapeutic strategies to promote bone formation without increasing cardiovascular risk.