Tissue clearing combined with high-resolution confocal imaging is a cutting-edge approach for dissecting the three-dimensional (3D) architecture of tissues and deciphering cellular spatial interactions under physiological and pathological conditions. Deciphering the spatial interaction of leptin receptor-expressing (LepR⁺) stromal cells with other compartments in the bone marrow is crucial for a deeper understanding of the stem cell niche and the skeletal tissue. In this study, we introduce an optimized protocol for the 3D analysis of skeletal tissues, enabling the visualization of hematopoietic and stromal cells, especially LepR⁺ stromal cells, within optically cleared bone hemisections. Our method preserves the 3D tissue architecture and is extendable to other hematopoietic sites such as calvaria and vertebrae. The protocol entails tissue fixation, decalcification, and cryosectioning to reveal the marrow cavity. Completed within approximately 12 days, this process yields highly transparent tissues that maintain genetically encoded or antibody-stained fluorescent signals. The bone hemisections are compatible with diverse antibody labeling strategies. Confocal microscopy of these transparent samples allows for qualitative and quantitative image analysis using Aivia or Bitplane Imaris software, assessing a spectrum of parameters. With proper storage, the fluorescent signal in the stained and cleared bone hemisections remains intact for at least 2–3 months. This protocol is robust, straightforward to implement, and highly reproducible, offering a valuable tool for tissue architecture and cellular interaction studies.

Osteocytes are the main cells in mineralized bone tissue. Elevated osteocyte apoptosis has been observed in lytic bone lesions of patients with multiple myeloma. However, their precise contribution to bone metastasis remains unclear. Here, we investigated the pathogenic mechanisms driving melanoma-induced osteocyte death. Both in vivo models and in vitro assays were combined with untargeted RNA sequencing approaches to explore the pathways governing melanoma-induced osteocyte death. We could show that ferroptosis is the primary mechanism behind osteocyte death in the context of melanoma bone metastasis. HMOX1 was identified as a crucial regulatory factor in this process, directly involved in inducing ferroptosis and affecting osteocyte viability. We uncover a non-canonical pathway that involves excessive autophagy-mediated ferritin degradation, highlighting the complex relationship between autophagy and ferroptosis in melanoma-induced osteocyte death. In addition, HIF1α pathway was shown as an upstream regulator, providing a potential target for modulating HMOX1 expression and influencing autophagy-dependent ferroptosis. In conclusion, our study provides insight into the pathogenic mechanisms of osteocyte death induced by melanoma bone metastasis, with a specific focus on ferroptosis and its regulation. This would enhance our comprehension of melanoma-induced osteocyte death.

Fibrotic remodeling of nucleus pulposus (NP) leads to structural and mechanical anomalies of intervertebral discs that prone to degeneration, leading to low back pain incidence and disability. Emergence of fibroblastic cells in disc degeneration has been reported, yet their nature and origin remain elusive. In this study, we performed an integrative analysis of multiple single-cell RNA sequencing datasets to interrogate the cellular heterogeneity and fibroblast-like entities in degenerative human NP specimens. We found that disc degeneration severity is associated with an enrichment of fibrocyte phenotype, characterized by CD45 and collagen I dual positivity, and expression of myofibroblast marker α-smooth muscle actin. Refined clustering and classification distinguished the fibrocyte-like populations as subtypes in the NP cells - and immunocytes-clusters, expressing disc degeneration markers HTRA1 and ANGPTL4 and genes related to response to TGF-β. In injury-induced mouse disc degeneration model, fibrocytes were found recruited into the NP undergoing fibrosis and adopted a myofibroblast phenotype. Depleting the fibrocytes in CD11b-DTR mice in which myeloid-derived lineages were ablated by diphtheria toxin could markedly attenuate fibrous modeling and myofibroblast formation in the NP of the degenerative discs, and prevent disc height loss and histomorphological abnormalities. Marker analysis supports that disc degeneration progression is dependent on a function of CD45⁺COL1A1⁺ and αSMA⁺ cells. Our findings reveal that myeloid-derived fibrocytes play a pivotal role in NP fibrosis and may therefore be a target for modifying disc degeneration and promoting its repair.

Mechanical stress modulates bone formation and organization of the extracellular matrix (ECM), the interaction of which affects heterotopic ossification (HO). However, the mechanically sensitive cell populations in HO and the underlying mechanism remain elusive. Here, we show that the mechanical protein Polysyctin-1 (PC1, Pkd1) regulates CTSK lineage tendon-derived mesenchymal stem cell (TDMSC) fate and ECM organization, thus affecting HO progression. First, we revealed that CTSK lineage TDMSCs are the major source of osteoblasts and fibroblasts in HO and are responsive to mechanical cues via single-cell RNA sequencing analysis and experiments with a lineage tracing mouse model. Moreover, we showed that PC1 mediates the mechanosignal transduction of CTSK lineage TDMSCs to regulate osteogenic and fibrogenic differentiation and alters the ECM architecture by facilitating TAZ nuclear translocation. Conditional gene depletion of Pkd1 or Taz in CTSK lineage cells and pharmaceutical intervention in the PC1-TAZ axis disrupt osteogenesis, fibrogenesis and ECM organization, and consequently attenuate HO progression. These findings suggest that mechanically sensitive CTSK-lineage TDMSCs contribute to heterotopic ossification through PC1-TAZ signaling axis mediated cell fate determination and ECM organization.

Gain-of-function mutations in fibroblast growth factor receptor (FGFR) genes lead to chondrodysplasia and craniosynostoses. FGFR signaling has a key role in the formation and repair of the craniofacial skeleton. Here, we analyzed the impact of Fgfr2- and Fgfr3-activating mutations on mandibular bone formation and endochondral bone repair after non-stabilized mandibular fractures in mouse models of Crouzon syndrome (Crz) and hypochondroplasia (Hch). Bone mineralization of the calluses was abnormally high in Crz mice and abnormally low in Hch mice. The latter model presented pseudarthrosis and impaired chondrocyte differentiation. Spatial transcriptomic analyses of the Hch callus revealed abnormally low expression of Col11, Col1a, Dmp1 genes in mature chondrocytes. We found that the expression of genes involved in autophagy and apoptosis (Smad1, Comp, Birc2) was significantly perturbed and that the Dusp3, Dusp9, and Socs3 genes controlling the mitogen-activated protein kinase pathway were overexpressed. Lastly, we found that treatment with a tyrosine kinase inhibitor (BGJ398, infigratinib) or a C-type natriuretic peptide (BMN111, vosoritide) fully rescued the defective endochondral bone repair observed in Hch mice. Taken as a whole, our findings show that FGFR3 is a critical orchestrator of bone repair and provide a rationale for the development of potential treatments for patients with FGFR3-osteochondrodysplasia.

Reproductive hormones associated with the hypothalamic-pituitary-gonadal (HPG) axis are closely linked to bone homeostasis. In this study, we demonstrate that Gonadotropin inhibitory hormone (GnIH, one of the key reproductive hormones upstream of the HPG axis) plays an indispensable role in regulating bone homeostasis and maintaining bone mass. We find that deficiency of GnIH or its receptor Gpr147 leads to a significant reduction in bone mineral density (BMD) in mice primarily by enhancement of osteoclast activation in vivo and in vitro. Mechanistically, GnIH/Gpr147 inhibits osteoclastogenesis by the PI3K/AKT, MAPK, NF-κB and Nfatc1 signaling pathways. Furthermore, GnIH treatment was able to alleviate bone loss in aging, ovariectomy (OVX) or LPS-induced mice. Moreover, the therapy using green light promotes the release of GnIH and rescues OVX-induced bone loss. In humans, serum GnIH increases and bone resorption markers decrease after green light exposure. Therefore, our study elucidates that GnIH plays an important role in maintaining bone homeostasis via modulating osteoclast differentiation and demonstrates the potential of GnIH therapy or green light therapy in preventing osteoporosis.

Craniometaphyseal dysplasia (CMD), a rare craniotubular disorder, occurs in an autosomal dominant (AD) or autosomal recessive (AR) form. CMD is characterized by hyperostosis of craniofacial bones and metaphyseal flaring of long bones. Many patients with CMD suffer from neurological symptoms. The pathogenesis of CMD is not fully understood. Treatment is limited to craniofacial surgery. Here, we report a knock in (KI) mouse model for AR CMD carrying a Cx43_R239Q mutation. Cx43 ^KI/KI mice replicate typical features of AR CMD, including thickening of craniofacial bones, club-shaped femurs, and widened diaphyseal cortical bones. Female Cx43 ^KI/KI mice display remarkably more bone overgrowth than male Cx43 ^KI/KI mice as they age. In contrast to Cx43 ^+/+ littermates, Cx43 ^KI/KI mice exhibit periosteal bone deposition and increased osteoclast (OC) numbers in the endosteum of long bones. Although formation of resting OCs in Cx43 ^+/+ and Cx43 ^KI/KI mice is comparable, the actively resorbing Cx43 ^KI/KI OCs have reduced resorption on bone chips. Cx43 ^KI/KI mice display reduced osteocyte dendrites. RNA from Cx43 ^KI/KI femoral cortical bones show reduced expression levels of Sost, Tnf-α, IL-1β, Esr1, Esr2, and a lower Rankl/Opg ratio. Moreover, the Cx43_R239Q mutation results in altered spatial expression of Cx43 protein and mild reduction of gap junction and hemichannel activity. The distinct phenotype seen in Cx43 ^KI/KI mice but not in Cx43 ablation models suggests that Cx43 loss-of-function is unlikely the main cause of AR CMD. Additional studies are required to investigate new roles of CMD-mutant Cx43.

Intervertebral disc degeneration is a degenerative disease where inflammation and immune responses play significant roles. Macrophages, as key immune cells, critically regulate inflammation through polarization into different phenotypes. In recent years, the role of macrophages in inflammation-related degenerative diseases, such as intervertebral disc degeneration, has been increasingly recognized. Macrophages construct the inflammatory microenvironment of the intervertebral disc and are involved in regulating intervertebral disc cell activities, extracellular matrix metabolism, intervertebral disc vascularization, and innervation, profoundly influencing the progression of disc degeneration. To gain a deeper understanding of the inflammatory microenvironment of intervertebral disc degeneration, this review will summarize the role of macrophages in the pathological process of intervertebral disc degeneration, analyze the regulatory mechanisms involving macrophages, and review therapeutic strategies targeting macrophage modulation for the treatment of intervertebral disc degeneration. These insights will be valuable for the treatment and research directions of intervertebral disc degeneration.

Plp1-lineage Schwann cells (SCs) of peripheral nerve play a critical role in vascular remodeling and osteogenic differentiation during the early stage of bone healing, and the abnormal plasticity of SCs would jeopardize the bone regeneration. However, how Plp1-lineage cells respond to injury and initiate the vascularized osteogenesis remains incompletely understood. Here, by employing single-cell transcriptional profiling combined with lineage-specific tracing models, we uncover that Plp1-lineage cells undergoing injury-induced glia-to-MSCs transition contributed to osteogenesis and revascularization in the initial stage of bone injury. Importantly, our data demonstrated that the Sonic hedgehog (Shh) signaling was responsible for the transition process initiation, which was strongly activated by c-Jun/SIRT6/BAF170 complex-driven Shh enhancers. Collectively, these findings depict an injury-specific niche signal-mediated Plp1-lineage cells transition towards Gli1⁺ MSCs and may be instructive for approaches to promote bone regeneration during aging or other bone diseases.

Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional endocytic receptor whose dysfunction is linked to developmental dysplasia of the hip, osteoporosis and osteoarthritis. Our work addresses the critical question of how these skeletal pathologies emerge. Here, we show the abundant expression of LRP1 in skeletal progenitor cells at mouse embryonic stage E10.5 and onwards, especially in the perichondrium, the stem cell layer surrounding developing limbs essential for bone formation. Lrp1 deficiency in these stem cells causes joint fusion, malformation of cartilage/bone template and markedly delayed or lack of primary ossification. These abnormalities, which resemble phenotypes associated with Wnt signalling pathways, result in severe and persistent skeletal defects including a severe deficit in hip joint and patella, and markedly deformed and low-density long bones leading to dwarfism and impaired mobility. Mechanistically, we show that LRP1 regulates core non-canonical Wnt/planar cell polarity (PCP) components that may explain the malformation of long bones. LRP1 directly binds to Wnt5a, facilitates its cell-association and endocytic degradation and recycling. In the developing limbs, LRP1 partially colocalises with Wnt5a and its deficiency alters abundance and distribution of Wnt5a and Vangl2. Finally, using Xenopus as a model system, we show the regulatory role for LRP1 in Wnt/PCP signalling. We propose that in skeletal progenitors, LRP1 plays a critical role in formation and maturity of multiple bones and joints by regulating Wnt signalling, providing novel insights into the fundamental processes of morphogenesis and the emergence of skeletal pathologies.

The death of osteoblasts induced by glucocorticoid (GC)-mediated oxidative stress plays a crucial role in the development of steroid-induced osteonecrosis of the femoral head (SIONFH). Improving bone formation driven by osteoblasts has shown promising outcomes in the prognosis of SIONFH. Isovitexin has demonstrated antioxidant properties, but its therapeutic effects on GC-induced oxidative stress and SIONFH remain unexplored. In this study, we analyzed clinical samples obtained from SIONFH patients using proteomic and bioinformatic approaches. We found an imbalance in mitochondrial homeostasis and ferroptosis-induced impairment of osteogenic capacity in SIONFH. Subsequently, we investigated the cause-and-effect relationship between mitochondria and ferroptosis, as well as the regulatory role of mitophagy in maintaining mitochondrial homeostasis and controlling ferroptosis. We then identified the critical involvement of SIRT3 in modulating mitochondrial homeostasis and ferroptosis. Furthermore, molecular docking and co-immunoprecipitation confirmed the strong interaction between SIRT3 and BNIP3. Strikingly, restoring SIRT3 expression significantly inhibited pathological mitophagy mediated by the BNIP3/NIX pathway. Additionally, we discovered that Isovitexin, by promoting SIRT3 expression, effectively regulated mitophagy, preserved mitochondrial homeostasis in osteoblasts, suppressed ferroptosis, and restored osteogenic capacity, leading to remarkable improvements in SIONFH. These findings reveal the effects and molecular mechanisms of Isovitexin on SIONFH and highlight the potential of targeting SIRT3 as a promising strategy to suppress mitophagy-mediated ferroptosis in osteoblasts and against SIONFH.

Healthy aging is a common goal for humanity and society, and one key to achieving it is the rejuvenation of senescent resident stem cells and empowerment of aging organ regeneration. However, the mechanistic understandings of stem cell senescence and the potential strategies to counteract it remain elusive. Here, we reveal that the aging bone microenvironment impairs the Golgi apparatus thus diminishing mesenchymal stem cell (MSC) function and regeneration. Interestingly, replenishment of cell aggregates-derived extracellular vesicles (CA-EVs) rescues Golgi dysfunction and empowers senescent MSCs through the Golgi regulatory protein Syntaxin 5. Importantly, in vivo administration of CA-EVs significantly enhanced the bone defect repair rate and improved bone mass in aging mice, suggesting their therapeutic value for treating age-related osteoporosis and promoting bone regeneration. Collectively, our findings provide insights into Golgi regulation in stem cell senescence and bone aging, which further highlight CA-EVs as a potential rejuvenative approach for aging bone regeneration.

Rheumatoid arthritis (RA) is an autoimmune disease characterized by inflammation and abnormal osteoclast activation, leading to bone destruction. We previously demonstrated that the large extracellular loop (LEL) of Tm4sf19 is important for its function in osteoclast differentiation, and LEL-Fc, a competitive inhibitor of Tm4sf19, effectively suppresses osteoclast multinucleation and prevent bone loss associated with osteoporosis. This study aimed to investigate the role of Tm4sf19 in RA, an inflammatory and abnormal osteoclast disease, using a mouse model of collagen-induced arthritis (CIA). Tm4sf19 expression was observed in macrophages and osteoclasts within the inflamed synovium, and Tm4sf19 expression was increased together with inflammatory genes in the joint bones of CIA-induced mice compared with the sham control group. Inhibition of Tm4sf19 by LEL-Fc demonstrated both preventive and therapeutic effects in a CIA mouse model, reducing the CIA score, swelling, inflammation, cartilage damage, and bone damage. Knockout of Tm4sf19 gene or inhibition of Tm4sf19 activity by LEL-Fc suppressed LPS/IFN-γ-induced TLR4-mediated inflammatory signaling in macrophages. LEL-Fc disrupted not only the interaction between Tm4sf19 and TLR4/MD2, but also the interaction between TLR4 and MD2. μCT analysis showed that LEL-Fc treatment significantly reduced joint bone destruction and bone loss caused by hyperactivated osteoclasts in CIA mice. Taken together, these findings suggest that LEL-Fc may be a potential treatment for RA and RA-induced osteoporosis by simultaneously targeting joint inflammation and bone destruction caused by abnormal osteoclast activation.

Clinical translation of tissue-engineered advanced therapeutic medicinal products is hindered by a lack of patient-dependent and independent in-process biological quality controls that are reflective of in vivo outcomes. Recent insights into the mechanism of native bone repair highlight a robust path dependence. Organoid-based bottom-up developmental engineering mimics this path-dependence to design personalized living implants scaffold-free, with in-build outcome predictability. Yet, adequate (noninvasive) quality metrics of engineered tissues are lacking. Moreover, insufficient insight into the role of donor variability and biological sex as influencing factors for the mechanism toward bone repair hinders the implementation of such protocols for personalized bone implants. Here, male and female bone-forming organoids were compared to non-bone-forming organoids regarding their extracellular matrix composition, transcriptome, and secreted proteome signatures to directly link in vivo outcomes to quality metrics. As a result, donor variability in bone-forming callus organoids pointed towards two distinct pathways to bone, through either a hypertrophic cartilage or a fibrocartilaginous template. The followed pathway was determined early, as a biological sex-dependent activation of distinct progenitor populations. Independent of donor or biological sex, a cartilage-to-bone transition was driven by a common panel of secreted factors that played a role in extracellular matrix remodeling, mineralization, and attraction of vasculature. Hence, the secreted proteome is a source of noninvasive biomarkers that report on biological potency and could be the missing link toward data-driven decision-making in organoid-based bone tissue engineering.

Exosomes have shown good potential in ischemic injury disease treatments. However, evidence about their effect and molecular mechanisms in osteonecrosis of femoral head (ONFH) treatment is still limited. Here, we revealed the cell biology characters of ONFH osteonecrosis area bone tissue in single cell scale and thus identified a novel ONFH treatment approach based on M2 macrophages-derived exosomes (M2-Exos). We further show that M2-Exos are highly effective in the treatment of ONFH by modulating the phenotypes communication between neutrophil and endothelium including neutrophil extracellular traps formation and endothelial phenotype transition. Additionally, we identified that M2-Exos’ therapeutic effect is attributed to the high content of miR-93-5p and constructed miR-93-5p overexpression model in vitro and in vivo based on lentivirus and adeno-associated virus respectively. Then we found miR-93-5p can not only reduce neutrophil extracellular traps formation but also improve angiogenic ability of endothelial cells. These results provided a new theoretical basis for the clinical application of ONFH therapeutic exosomes.

An increasing number of studies have characterized the bone as an endocrine organ, and that bone secreted factors may not only regulate local bone remodeling, but also other tissues and whole-body metabolic functions. The precise nature of these regulatory factors and their roles at bridging the bone, bone marrow adipose tissue, extramedullary body fat and whole-body energy homeostasis are being explored. In this study, we report that KIAA1199, a secreted factor produced from bone and bone marrow, previously described as an inhibitor of bone formation, also plays a role at promoting adipogenesis. KIAA1199-deficient mice exhibit reduced bone marrow adipose tissue, subcutaneous and visceral fat tissue mass, blood cholesterol, triglycerides, free fatty acids and glycerol, as well as improved insulin sensitivity in skeletal muscle, liver and fat. Moreover, these mice are protected from the detrimental effects of high-fat diet feeding, with decreased obesity, lower blood glucose and glucose tolerance, as well as decreased adipose tissue inflammation, insulin resistance and hepatic steatosis. In human studies, plasma levels of KIAA1199 or its expression levels in adipose tissue are positively correlated with insulin resistance and blood levels of cholesterol, triglycerides, free fatty acids, glycerol, fasting glucose and HOMA-IR. Mechanistically, KIAA1199 mediates its effects on adipogenesis through modulating osteopontin-integrin and AKT / ERK signaling. These findings provide evidence for the role of bone secreted factors on coupling bone, fat and whole-body energy homeostasis.

Sclerosteosis, an ultra-rare disorder characterised by high bone mass (HBM) and skeletal overgrowth, leads to facial paralysis, hearing loss and raised intracranial pressure, which is currently managed only through high-risk surgery. Sclerosteosis is caused by SOST mutations and loss of functional sclerostin, a protein that suppresses osteogenesis by antagonising Wnt/β-catenin signalling. Herein, using in vitro and in vivo approaches, we explore whether LGK974, another potent Wnt inhibitor that targets porcupine (PORCN, Wnt-specific acyltransferase), is a promising sclerosteosis therapeutic. In vitro assays showed that 100 nmol/L LGK974 significantly reduced osteoblast alkaline phosphatase (ALP) activity/mineralisation, decreased Wnt/osteoblast marker (Axin2, Runx2 and Ocn) expression, and downregulated ossification and the Wnt signalling pathway, without affecting osteoclast numbers/resorption. To assess in vivo effects, 6-week-old male and female Sost deficient (Sost^-/-) mice received LGK974 for 4 weeks and right hindlimbs were subjected to 20 N peak loading to assess mechanoadaptive interactions. µCT revealed significant reductions in vertebral trabecular number and lower cortical bone volume in loaded and non-loaded tibiae in male and female LGK974-treated Sost^-/- mice. Interestingly, the target engagement biomarker Axin2 was only significantly reduced in male vertebrae, which may indicate differences in male and female response to LGK974. This study also shows that PORCN inhibition may effectively limit characteristic HBM and skeletal overgrowth in sclerosteosis patients at sites with severe pathology.

Osteogenesis is the process of bone formation mediated by the osteoblasts, participating in various bone-related physiological processes including bone development, bone homeostasis and fracture healing. It exhibits temporal and spatial interconnectivity with angiogenesis, constructed by multiple forms of cell communication occurring between bone and vascular endothelial cells. Molecular regulation among different cell types is crucial for coordinating osteogenesis and angiogenesis to facilitate bone remodeling, fracture healing, and other bone-related processes. The transmission of signaling molecules and the activation of their corresponding signal pathways are indispensable for various forms of cell communication. This communication acts as a “bridge” in coupling osteogenesis to angiogenesis. This article reviews the modes and processes of cell communication in osteogenesis-angiogenesis coupling over the past decade, mainly focusing on interactions among bone-related cells and vascular endothelial cells to provide insights into the mechanism of cell communication of osteogenesis-angiogenesis coupling in different bone-related contexts. Moreover, clinical relevance and applications are also introduced in this review.

Neural EGFL-like 2 (NELL2) is a secreted protein known for its regulatory functions in the nervous and reproductive systems, yet its role in bone biology remains unexplored. In this study, we observed that NELL2 was diminished in the bone of aged and ovariectomized (OVX) mice, as well as in the serum of osteopenia and osteoporosis patients. In vitro loss-of-function and gain-of-function studies revealed that NELL2 facilitated osteoblast differentiation and impeded adipocyte differentiation from stromal progenitor cells. In vivo studies further demonstrated that the deletion of NELL2 in preosteoblasts resulted in decreased cancellous bone mass in mice. Mechanistically, NELL2 interacted with the FNI-type domain located at the C-terminus of Fibronectin 1 (Fn1). Moreover, we found that NELL2 activated the focal adhesion kinase (FAK)/AKT signaling pathway through Fn1/integrin β1 (ITGB1), leading to the promotion of osteogenesis and the inhibition of adipogenesis. Notably, administration of NELL2-AAV was found to ameliorate bone loss in OVX mice. These findings underscore the significant role of NELL2 in osteoblast differentiation and bone homeostasis, suggesting its potential as a therapeutic target for managing osteoporosis.

Chondrocyte senescence is a critical pathological hallmark of osteoarthritis (OA). Aberrant mechanical stress is considered a pivotal determinant in chondrocyte aging; however, the precise underlying mechanism remains elusive. Our findings demonstrate that SPI1 plays a significant role in counteracting chondrocyte senescence and inhibiting OA progression. SPI1 binds to the PERK promoter, thereby promoting its transcriptional activity. Importantly, PERK, rather than GCN2, facilitates eIF2α phosphorylation, activating the mitochondrial unfolded protein response (UPR^mt) and impeding chondrocyte senescence. Deficiency of SPI1 in mechanical overload-induced mice leads to diminished UPR^mt activation and accelerated OA progression. Intra-articular injection of adenovirus vectors overexpressing SPI1 and PERK effectively mitigates cartilage degeneration. In summary, our study elucidates the crucial regulatory role of SPI1 in the pathogenesis of chondrocyte senescence by activating UPR^mt signaling through PERK, which may present a novel therapeutic target for treating OA.

SPI1 alleviates the progression of OA by inhibiting mechanical stress-induced chondrocyte senescence through mitochondrial UPR signaling.

Osteoarthritis (OA) is a degenerative joint disease with significant clinical and societal impact. Traditional diagnostic methods, including subjective clinical assessments and imaging techniques such as X-rays and MRIs, are often limited in their ability to detect early-stage OA or capture subtle joint changes. These limitations result in delayed diagnoses and inconsistent outcomes. Additionally, the analysis of omics data is challenged by the complexity and high dimensionality of biological datasets, making it difficult to identify key molecular mechanisms and biomarkers. Recent advancements in artificial intelligence (AI) offer transformative potential to address these challenges. This review systematically explores the integration of AI into OA research, focusing on applications such as AI-driven early screening and risk prediction from electronic health records (EHR), automated grading and morphological analysis of imaging data, and biomarker discovery through multi-omics integration. By consolidating progress across clinical, imaging, and omics domains, this review provides a comprehensive perspective on how AI is reshaping OA research. The findings have the potential to drive innovations in personalized medicine and targeted interventions, addressing longstanding challenges in OA diagnosis and management.

Bone has long been acknowledged as a fundamental structural entity that provides support and protection to the body’s organs. However, emerging research indicates that bone plays a crucial role in the regulation of systemic metabolism. This is achieved through the secretion of a variety of hormones, cytokines, metal ions, extracellular vesicles, and other proteins/peptides, collectively referred to as bone-derived factors (BDFs). BDFs act as a medium through which bones can exert targeted regulatory functions upon various organs, thereby underscoring the profound and concrete implications of bone in human physiology. Nevertheless, there remains a pressing need for further investigations to elucidate the underlying mechanisms that inform the effects of bone on other body systems. This review aims to summarize the current findings related to the roles of these significant modulators across different organs and metabolic contexts by regulating critical genes and signaling pathways in vivo. It also addresses their involvement in the pathogenesis of various diseases affecting the musculoskeletal system, circulatory system, glucose and lipid metabolism, central nervous system, urinary system, and reproductive system. The insights gained from this review may contribute to the development of innovative therapeutic strategies through a focused approach to bone secretomes. Continued research into BDFs is expected to enhance our understanding of bone as a multifunctional organ with diverse regulatory roles in human health.

With the deepening of epigenetic research, studies have shown that N⁶-methyladenosine (m⁶A) is closely related to the development of rheumatoid arthritis (RA), but the mechanism is still unclear. In the study, we collected synovial tissues from normal controls and patients with osteoarthritis (OA) or RA. The levels of m⁶A and inflammation were analyzed by immunofluorescence staining and western blotting. The roles of IGF2BP3 in cell proliferation and inflammatory activation were explored using transfection and RNA immunoprecipitation assays. IGF2BP3^−/− mice were generated and used to establish an arthritis mouse model by transferring serum from adult arthritis K/BxN mice. We found m⁶A levels were markedly increased in RA patients and mouse models, and the expression of IGF2BP3 was upregulated in individuals with RA and related to the levels of inflammatory markers. IGF2BP3 played an important part in RA-fibroblast-like synoviocytes (FLS) by promoting cell proliferation, migration, invasion, inflammatory cytokine release and inhibiting autophagy. In addition, IGF2BP3 inhibited autophagy to reduce ROS production, thereby decreasing the inflammatory activation of macrophages. More importantly, RASGRF1-mediated mTORC1 activation played a crucial role in the ability of IGF2BP3 to promote cell proliferation and inflammatory activation. In an arthritis model of IGF2BP3^−/− mice, IGF2BP3 knockout inhibited RA-FLS proliferation and inflammatory infiltration, and further ameliorated RA joint injury. Our study revealed an important role for IGF2BP3 in RA progression. The targeted inhibition of IGF2BP3 reduced cell proliferation and inflammatory activation and limited RA development, providing a potential strategy for RA therapy.

Osteoarthritis (OA) is a prevalent degenerative joint disorder marked by chronic pain, inflammation, and cartilage loss, with current treatments limited to symptom relief. G protein-coupled receptors (GPCRs) play a pivotal role in OA progression by regulating inflammation, chondrocyte survival, and matrix homeostasis. However, their multifaceted signaling, via G proteins or β-arrestins, poses challenges for precise therapeutic targeting. Biased agonism, where ligands selectively activate specific GPCR pathways, emerges as a promising approach to optimize efficacy and reduce side effects. This review examines biased signaling in OA-associated GPCRs, including cannabinoid receptors (CB₁, CB₂), chemokine receptors (CCR2, CXCR4), protease-activated receptors (PAR-2), adenosine receptors (A₁R, A_2AR, A_2BR, A₃R), melanocortin receptors (MC₁R, MC₃R), bradykinin receptors (B₂R), prostaglandin E₂ receptors (EP-2, EP-4), and calcium-sensing receptors (CaSR). We analyze ligands in clinical trials and explore natural products from Traditional Chinese Medicine as potential biased agonists. These compounds, with diverse structures and bioactivities, offer novel therapeutic avenues. By harnessing biased agonism, this review underscores the potential for developing targeted, safer OA therapies that address its complex pathology, bridging molecular insights with clinical translation.

The SH2 domain-containing protein tyrosine phosphatase 2 (SHP2, also known as PTP2C), encoded by PTPN11, is ubiquitously expressed and has context-specific effects. It promotes RAS/MAPK signaling downstream of receptor tyrosine kinases, cytokine receptors, and extracellular matrix proteins, and was shown in various lineages to modulate cell survival, proliferation, differentiation, and migration. Over the past decade, PTPN11 inactivation in chondrocytes was found to cause metachondromatosis, a rare disorder characterized by multiple enchondromas and osteochondroma-like lesions. Moreover, SHP2 inhibition was found to mitigate osteoarthritis pathogenesis in mice, and abundant but incomplete evidence suggests that SHP2 is crucial for cartilage development and adult homeostasis, during which its expression and activity are tightly regulated transcriptionally and posttranslationally, and by varying sets of functional partners. Fully uncovering SHP2 actions and regulation in chondrocytes is thus fundamental to understanding the mechanisms underlying both rare and common cartilage diseases and to designing effective disease treatments. We here review current knowledge, highlight recent discoveries and controversies, and propose new research directions to answer remaining questions.

While bulk RNA sequencing and single-cell RNA sequencing have shed light on cellular heterogeneity and potential molecular mechanisms in the musculoskeletal system in both physiological and various pathological states, the spatial localization of cells and molecules and intercellular interactions within the tissue context require further elucidation. Spatial transcriptomics has revolutionized biological research by simultaneously capturing gene expression profiles and in situ spatial information of tissues, gradually finding applications in musculoskeletal research. This review provides a summary of recent advances in spatial transcriptomics and its application to the musculoskeletal system. The classification and characteristics of data acquisition techniques in spatial transcriptomics are briefly outlined, with an emphasis on widely-adopted representative technologies and the latest technological breakthroughs, accompanied by a concise workflow for incorporating spatial transcriptomics into musculoskeletal system research. The role of spatial transcriptomics in revealing physiological mechanisms of the musculoskeletal system, particularly during developmental processes, is thoroughly summarized. Furthermore, recent discoveries and achievements of this emerging omics tool in addressing inflammatory, traumatic, degenerative, and tumorous diseases of the musculoskeletal system are compiled. Finally, challenges and potential future directions for spatial transcriptomics, both as a field and in its applications in the musculoskeletal system, are discussed.

Age-related osteoporosis poses a significant challenge in musculoskeletal health; a condition characterized by reduced bone density and increased fracture susceptibility in older individuals necessitates a better understanding of underlying molecular and cellular mechanisms. Emerging evidence suggests that osteocytes are the pivotal orchestrators of bone remodeling and represent novel therapeutic targets for age-related bone loss. Our study uses the prematurely aged Polg^D257A/D257A (PolgA) mouse model to scrutinize age- and sex-related alterations in musculoskeletal health parameters (frailty, grip strength, gait data), bone and particularly the osteocyte lacuno-canalicular network (LCN). Moreover, a new quantitative in silico image analysis pipeline is used to evaluate the alterations in the osteocyte network with aging. Our findings underscore the pronounced degenerative changes in the musculoskeletal health parameters, bone, and osteocyte LCN in PolgA mice as early as 40 weeks, with more prominent alterations evident in aged males. Our findings suggest that the PolgA mouse model serves as a valuable model for studying the cellular mechanisms underlying age-related bone loss, given the comparable aging signs and age-related degeneration of the bone and the osteocyte network observed in naturally aging mice and elderly humans.

The discontinuation of denosumab [antibody targeting receptor activator of nuclear factor kappa B ligand (RANKL)] therapy may increase the risk of multiple vertebral fractures; however, the underlying pathophysiology is largely unknown. In patients who underwent discontinuation after multiple injections of denosumab, the levels of tartrate-resistant acid phosphatase 5b increased compared to pretreatment levels, indicating a phenomenon known as “overshoot.” The rate of decrease in bone mineral density during the withdrawal period was higher than the rate of decrease associated with aging, suggesting that the physiological bone metabolism had broken down. Overshoot and significant bone loss were also observed in mice receiving continuous administration of anti-RANKL antibody after treatment was interrupted, resembling the original pathology. In mice long out of overshoot, bone resorption recovered, but osteoblast numbers and bone formation remained markedly reduced. The bone marrow exhibited a significant reduction in stem cell (SC) antigen 1- and platelet-derived growth factor receptor alpha-expressing osteoblast progenitors (PαS cells) and alkaline phosphatase-positive early osteoblasts. Just before the overshoot phase, the osteoclast precursor cell population expands and RANKL-bearing extracellular vesicles (EVs) became abundant in the serum, leading to robust osteoclastogenesis after cessation of anti-RANKL treatment. Thus, accelerated bone resorption due to the accumulation of RANKL-bearing EVs and long-term suppression of bone formation uncoupled from bone resorption leads to the severe bone loss characteristic of denosumab discontinuation.

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-Runx2^cKO) mouse model. Fgfr3-Runx2^cKO 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.

Debate regarding the premature aging of knee joints in acquired immune deficiency syndrome (AIDS) patients has remained contentious, with conjectures pointing towards its correlation with distinct antiviral regimes. Protease inhibitors (PIs) stand as a prominent class of antiviral agents frequently utilized in AIDS management and have been significantly linked to premature senescence. This study aimed to investigate whether PI-containing regimens would accelerate osteoarthritis (OA) development and explore the molecular mechanisms underlying this association. A retrospective cohort of 151 HIV-infected individuals, categorized into PI and non-PI groups, was established. Patients in PI group exhibited lower KOOS and a higher prevalence of radiological knee OA than those in non-PI group. Additionally, 25 anti-HIV drugs were screened and among all antiviral drugs, lopinavir had the most detrimental impact on cartilage anabolism, accelerating cartilage senescence and promoting mouse OA development. Mechanistically, lopinavir accelerated cellular senescence by inhibiting Zmpste24 and interfering nuclear membrane stability, which leads to decreased binding between nuclear membrane-binding protein Usp7 and Mdm2 and activates Usp7/Mdm2/p53 pathway. Zmpste24 overexpression reduces OA severity in mice. These findings suggest that PI-containing regimens accelerate cartilage senescence and OA development through Zmpste24 inhibition, which provides new insights into the selection of HIV regimens.

Musculoskeletal disorders, including osteoarthritis, rheumatoid arthritis, osteoporosis, bone fracture, intervertebral disc degeneration, tendinopathy, and myopathy, are prevalent conditions that profoundly impact quality of life and place substantial economic burdens on healthcare systems. Traditional bulk transcriptomics, genomics, proteomics, and metabolomics have played a pivotal role in uncovering disease-associated alterations at the population level. However, these approaches are inherently limited in their ability to resolve cellular heterogeneity or to capture the spatial organization of cells within tissues, thus hindering a comprehensive understanding of the complex cellular and molecular mechanisms underlying these diseases. To address these limitations, advanced single-cell and spatial omics techniques have emerged in recent years, offering unparalleled resolution for investigating cellular diversity, tissue microenvironments, and biomolecular interactions within musculoskeletal tissues. These cutting-edge techniques enable the detailed mapping of the molecular landscapes in diseased tissues, providing transformative insights into pathophysiological processes at both the single-cell and spatial levels. This review presents a comprehensive overview of the latest omics technologies as applied to musculoskeletal research, with a particular focus on their potential to revolutionize our understanding of disease mechanisms. Additionally, we explore the power of multi-omics integration in identifying novel therapeutic targets and highlight key challenges that must be overcome to successfully translate these advancements into clinical applications.

Itaconate, a macrophage-specific anti-inflammatory metabolite, has recently emerged as a critical regulator in rheumatoid arthritis pathogenesis. We found that itaconate is a TNF-α responsive metabolite significantly elevated in the serum and synovial fluid of rheumatoid arthritis patients and we demonstrated that itaconate is primarily produced by inflammatory macrophages rather than osteoclasts or osteoblasts. In TNF-transgenic and Irg1 ^−/− hybrid mice, a more severe bone destruction phenotype was observed. Administration of itaconate prevents excessive activation of osteoclasts by inhibiting Tet2 enzyme activity. Furthermore, exogenous administration of itaconate or its derivative, 4-octyl-itaconate, inhibits arthritis progression and mitigates bone destruction, offering a potential therapeutic strategy for rheumatoid arthritis. This study elucidates that TNF-α drives macrophage-derived itaconate production to epigenetically suppress osteoclast hyperactivation through Tet2 inhibition, establishing itaconate and its derivative OI as novel therapeutic agents against rheumatoid arthritis -associated bone destruction.

Rheumatoid arthritis (RA) is a systemic autoimmune disease in which synovial fibroblasts (SFs) maintain chronic inflammation by secreting proinflammatory mediators, leading to joint destruction. While the role of proinflammatory mediators in this process is well-established, the contribution of non-inflammatory regulators in SFs to joint pathology remains poorly understood. In this study, we investigated the non-inflammatory role of SFs in RA using a co-culture model, and found that SFs from RA patients promote apoptosis of human chondrocytes. Mechanistic investigations reveal that SFs can secrete small extracellular vesicles (sEVs), which are taken up by chondrocytes and induce chondrocyte apoptosis in both normal chondrocytes and chondrocytes from patients with RA. sEV-derived miRNA 15-29148 are identified as key signaling molecules mediating the apoptosis effects of chondrocytes. Further studies reveal that SF-derived miRNA 15-29148 targeting CIAPIN1 results in increased chondrocyte apoptosis. We further demonstrate that SF-derived miRNA 15-29148 is transferred to chondrocytes, exacerbating cartilage damage in vivo. Moreover, chondrocyte-specific aptamer-modified polyamidoamine nanoparticles not only ameliorated RA but also prevented its onset. This study suggests that, in RA, the secretion of specific sEV-miRNAs from SFs plays a crucial role in promoting chondrocyte apoptosis, potentially through non-inflammatory regulation, and that sEV-miRNA inhibition in SFs may represent an early preventive treatment strategy for cartilage degradation in RA.

Intervertebral disc degeneration (IDD) is a progressive and dynamic process in which the senescence-associated secretory phenotype (SASP) of nucleus pulposus cells (NPC) plays a significant role. While impaired chaperone-mediated autophagy (CMA) has been associated with inflammation and cellular senescence, its specific involvement in the self-perpetuating feedback loop of NPC senescence remains poorly understood. Through LAMP2A knockout in NPC, we identified a significant upregulation of DYRK1A, a core mediator of premature senescence in Down syndrome. Subsequent validation established DYRK1A as the critical driver of premature senescence in CMA-deficient NPC. Combinatorial transcription factor analysis revealed that under IL1B stimulation or CMA inhibition, elevated DYRK1A promoted FOXC1 phosphorylation and nuclear translocation, initiating transcriptional activation of cell cycle arrest. Intriguingly, CMA impairment concurrently enhanced glutamine metabolic flux in senescent NPC, thereby augmenting their survival fitness. Transcriptomic profiling demonstrated that CMA reactivation in senescent NPC facilitated fate transition from senescence to apoptosis, mediated by decreased glutamine flux via GLUL degradation. Therefore, CMA exerts protective effects against IDD by maintaining equilibrium between premature senescence and senolysis. This study elucidates CMA’s regulatory role in SASP-mediated senescence amplification circuits, providing novel therapeutic insights for IDD and other age-related pathologies.

Rheumatoid arthritis (RA) is a prevalent and debilitating inflammatory disease that significantly impairs functional capacity and quality of life. RA accelerates musculoskeletal aging, leading to complications such as muscle degeneration and sarcopenia. Recent research has identified myopenia as a condition of significant muscle loss associated with illness, distinct from the muscle wasting seen in other chronic diseases like cancer cachexia or heart failure. In RA, myopenia is characterized by muscle depletion without concurrent significant fat loss, and it can affect individuals of all ages. While inflammation plays a central role, it is not the sole factor contributing to the high incidence of muscle wasting in RA. In subsequent discussions, secondary sarcopenia will be considered alongside myopenia, as both involve muscle wasting decline primarily due to disease. This review summarizes recent findings on the impact of RA-related myopenia and secondary sarcopenia on functional capacity, explores its underlying mechanisms, and discusses contemporary strategies to mitigate the process of musculoskeletal aging in RA patients.

Membrane-initiated estrogen receptor α (mERα) signaling has been shown to affect bone mass in murine models. However, it remains unknown which cell types mediate the mERα-dependent effects on bone. In this study, we generated a novel mouse model with a conditional C451A mutation in Esr1, which enables selective knockout of the palmitoylation site essential for the membrane localization of ERα (C451A^f/f). First, we used Runx2-Cre mice to generate Runx2-C451A^f/f mice with conditional inactivation of mERα signaling in Runx2-expressing osteoblast lineage cells. No significant changes were observed in body weight, weights of estrogen-responsive organs, or serum concentrations of estradiol between female Runx2-C451A^f/f and homozygous C451A^f/f littermate controls. High-resolution microcomputed tomography analysis showed a consistent decrease in cortical bone mass in the tibia, femur, and vertebra L5 of Runx2-C451A^f/f mice and three-point bending analysis of humerus revealed an impaired mechanical bone strength in Runx2-C451A^f/f female mice compared to controls. Additionally, primary osteoblast cultures from mice lacking mERα signaling showed impaired differentiation compared to controls. In contrast, conditional inactivation of mERα signaling in hematopoietic cells, by transplantation of bone marrow from mice lacking mERα signaling in all cells to adult wildtype female mice, did not result in any skeletal alterations. In conclusion, this study demonstrates that mERα signaling in osteoblast lineage cells plays a crucial role in the regulation of cortical bone in female mice and shows that mERα inactivation in hematopoietic cells of adult female mice is dispensable for bone regulation.

The effectiveness of cranial suture expansion therapy hinges on the timely and adequate regeneration of bone tissue in response to mechanical stimuli. To optimize clinical outcomes and prevent post-expansion relapse, we delved into the underlying mechanisms governing bone remodeling during the processes of suture expansion and relapse. Our findings revealed that in vitro stretching bolstered mesenchymal stem cells’ antioxidative and osteogenic capacity by orchestrating mitochondrial activities, which governed by force-induced endoplasmic reticulum (ER) stress. Nonetheless, this signal transduction occurred through the activation of protein kinase R-like ER kinase (PERK) at the ER-mitochondria interface, rather than ER-mitochondria calcium flow as previously reported. Subsequently, PERK activation triggered TFEB translocation to the nucleus, thus regulating mitochondrial dynamics transcriptionally. Assessment of the mitochondrial pool during expansion and relapse unveiled a sequential, two-phase regulation governed by the ER stress/p-PERK/TFEB signaling cascade. Initially, PERK activation facilitated TFEB nuclear localization, stimulating mitochondrial biogenesis through PGC1-α, thereby addressing energy demands during the initial phase. Subsequently, TFEB shifted focus towards ensuring adequate mitophagy for mitochondrial quality maintenance during the remodeling process. Premature withdrawal of expanding force disrupted this sequential regulation, leading to compromised mitophagy and the accumulation of dysfunctional mitochondria, culminating in suboptimal bone regeneration and relapse. Notably, pharmacological activation of mitophagy effectively mitigated relapse and attenuated bone loss, while its inhibition impeded anticipated bone growth in remodeling progress. Conclusively, we elucidated the ER stress/p-PERK/TFEB signaling orchestrated sequential mitochondria biogenesis and mitophagy under mechanical stretch, thus ensuring antioxidative capacity and osteogenic potential of cranial suture tissues.

Peripheral neuropathy is a common complication in diabetes, affecting around 50% of the diabetic population. Co-occurrence of diabetic peripheral neuropathy (DPN) and diabetic bone disease has led to the hypothesis that DPN influences bone metabolism, although little experimental evidence has yet supported this premise. To investigate, mice were fed a high-fat diet (HFD) followed by phenotyping of skeletal-innervating neurons and bone architectural parameters. Results showed that HFD feeding resulted in a marked decrease in skeletal innervation (69%–41% reduction in Beta-III-Tubulin-stained nerves, 38% reduction in CGRP-stained nerves in long bone periosteum). These changes in skeletal innervation were associated with significant alterations in bone mass and in cortical and trabecular bone microarchitecture of long bones. Single-cell RNA sequencing (scRNA-Seq) of sensory neurons and bone tissue was next utilized to reconstruct potential nerve-to-bone signaling interactions, including implication of sensory nerve-derived neurotrophins (Bdnf), neuropeptides (Gal, Calca and Calcb), and other morphogens (Vegfa, Pdgfa, and Angpt2). Moreover, scRNA-Seq identified marked shifts in periosteal cell transcriptional changes within HFD-fed conditions, including a reduction in cell proliferation, an increase in adipogenic differentiation markers, and reductions in WNT, TGFβ, and MAPK signaling activity. When isolated, periosteal cells from HFD-fed mice showed deficits in proliferative and osteogenic differentiation potential. Moreover, these cellular changes in proliferation and differentiation capacity were restored by treatment of HFD-exposed periosteal cells to sensory neuron-conditioned medium. In summary, HFD modeling of type 2 diabetes results in skeletal polyneuropathy. Moreover, the combination of multi-tissue scRNA-Seq and isolated in vitro studies strengthen the case for altered nerve-to-bone signaling in diabetic bone disease.

The treatment of severe diabetic foot remains a clinical challenge. While it is established that bone can exert systemic effects through the secretion of osteokines on other organs, whether this endocrine function can be harnessed to promote diabetic wound healing remains unexplored. Here, we investigate the impact of a bone injury strategy on diabetic wound healing, leveraging the body’s innate regenerative capacity to stimulate osteokine release and influence remote skin wound repair. This study demonstrates that the tibial defect significantly accelerates ipsilateral diabetic foot skin wound healing. Mechanistically, we identify osteokines, platelet-derived growth factor-BB (PDGF-BB), as the key to initiating this process. Bone defect triggers a substantial release of PDGF-BB, which reaches the skin wound site via peripheral circulation. At the skin wound site, PDGF-BB mediates the secretion of keratinocyte growth factor (KGF) from fibroblasts via the PDGFRβ signaling pathway, thereby promoting the rapid re-epithelialization of epidermal cells through a paracrine pathway. Additionally, elevated PDGF-BB levels enhance the regeneration of CD31^hi Emcn^hi blood vessels within the wound. Importantly, we demonstrate the therapeutic potential of osteokines by showing that a collagen hydrogel loaded with osteokines promotes wound healing in diabetic mice. Our findings reveal a clear link between bone and skin wound healing, providing a therapeutic inspiration for chronic wounds that are difficult to treat locally.

Osteoarthritis (OA) is one of the most common degenerative and age-related diseases in joints, which affects 654 million people worldwide. Current therapies could not fundamentally reverse the pathologic process of OA due to the complex pathogenesis. Although OA mechanisms have been investigated on a large scale over the past decade, the OA pathology correlated with aging-associated changes is still largely unrevealed. Therefore, in-depth analysis of the aging microenvironment and aging-related molecular mechanisms in OA may offer additional strategies for clinical prevention and treatment. In this review, we discuss the potential pathogenesis of OA in light of aging-associated changes and summarize three main components of the aging microenvironment of the OA joint: immune homeostatic imbalance, cellular senescence, and stem cell exhaustion, which could be induced by aging and further exacerbate OA progression. Additionally, it is emphasized that immune homeostatic imbalance appears before established OA, which occurs in the early stage and is the therapeutic window of opportunity for better clinical outcomes. Importantly, we evaluate recent therapeutic targets and promising interventions against these components, as well as the challenges and prospects for precise and individualized therapies of OA patients, which we believe would guide the construction of novel combined strategies targeting aging-related factors against OA for better treatments in the future.

Osteoarthritis (OA) is a widespread joint disorder that has emerged as a significant global healthcare challenge. Over the past decade, advancements in material science and medicine have transformed the development of functional materials aimed at addressing the complex issues associated with the diagnosis and treatment of OA. This review synthesizes the latest advancements in various types of intelligent micro-structured materials and their design principles. By examining the exceptional structural characteristics of materials with unique properties such as tailored attributes, controllability, biocompatibility, and bioactivity, we emphasize the design of composite materials for precise and early intervention in OA. This is achieved through advanced imaging techniques and machine learning-based analysis, alongside the customization of micro-structured material properties to align with the biological and mechanical requirements of specific joint tissues. This review offers an in-depth analysis of the transformative potential of advanced technologies and artificial intelligence (AI) in the development of innovative solutions for OA diagnosis and therapy. It aims to inform future research and inspire the creation of next-generation smart materials with unprecedented performance, thereby enhancing our capabilities in the prevention and treatment of OA.

Osteoarthritis (OA) and intervertebral disc degeneration (IVDD) are degenerative musculoskeletal disorders characterized by degeneration of cartilaginous tissues and inflammation. While inflammation is implicated in the pathogenesis of OA and IVDD, and cytosolic phospholipase A2 (cPLA2) is a key mediator of inflammation, direct evidence linking cPLA2 to chondrocyte homeostasis and cartilage degeneration is lacking. This study aims to investigate the role of cPLA2 in chondrocytes and its contribution to the development of cartilage degenerative conditions such as OA and IVDD. Here, single-cell RNA sequencing was used to examine cPLA2 expression in chondrocytes. To explore its importance in chondrocytes and OA/IVDD, various cell-based assays and genetically modified mouse models with age-related and surgically induced OA/IVDD were employed. Furthermore, the therapeutic potential of fexofenadine, an over-the-counter drug recently identified as a cPLA2 inhibitor, was explored in these models. cPLA2 is predominantly expressed in prehypertrophic chondrocytes, characterized by elevated levels of cartilage degeneration markers and senescence-related genes. Genetic deletion and pharmacological inhibition of cPLA2 reduced inflammation induced catabolic activity and senescence in chondrocytes, as well as cartilage degeneration in various OA and IVDD models. This study identifies cPLA2 as a pivotal driver of cartilage degeneration and senescence in OA and IVDD, highlighting its potential as a dual-action therapeutic target that suppresses both inflammation and senescence to preserve cartilage integrity. These findings position cPLA2 as a promising candidate for developing disease-modifying therapies for cartilage degenerative conditions such as OA and IVDD.

The formation of traumatic heterotopic ossification (HO) is an abnormal repair process after soft tissue injury. Recent studies establish the involvement of immune cells and cellular metabolism in the tissue healing process; however, their role in HO remains unknown. Here, by using murine burn/tenotomy model in vivo and tendon stem/progenitor cells (TSPCs) osteogenic differentiation model in vitro, together with techniques including transgenic knockout, gene knockdown, transcriptome and proteome sequencings, mass spectrometry, co-immunoprecipitation, seahorse, etc., we reveal a novel p21-activated kinase 4 (Pak4) mediated crosstalk where the necroptotic macrophages arouse TSPCs with reduced fatty acid β-oxidation (FAO), to promote aberrant osteogenic differentiation during HO formation. Necroptosis blockade with Mlkl knockout (C57BL/6JGpt-Mlkl^em1Cd1679/Gpt) significantly reduces HO than WT mice. Extracellular vesicle (EVs) secreted from necroptotic bone marrow-derived macrophages (BMDMs, NecroMφ-EVs) are determined to motivate FAO reduction in TSPCs and result in higher osteogenic activity. Pak4 conditional knockout (C57BL/6JGpt-Pak4^em1Cflox/Gpt) in macrophage significantly increases FAO and reduces HO than Flox mice, as well as local injection of PAK4^−/−-EVs (NecroMφ-EVs with Pak4 knockout) than NecroMφ-EVs, and the protective effects are reversed after transfection of Fabp3^S122D, a phosphomimetic mutant of S122 on fatty acid binding protein 3 (Fabp3) phosphorylation site. Mechanically, after soft tissue injury, macrophages infiltrate, and necroptosis occurs, accompanied by paracrine EVs-derived Pak4, which binds directly to Fabp3 and phosphorylates it at the S122 site in TSPCs, results in reduced FAO, finally osteogenic behavior, and HO formation. This study adds perceptiveness into abnormal regeneration-based theory for traumatic HO and raises treatment strategy development.

Chronic lower back pain (LBP) is the leading cause of disability worldwide. Due to its close relationship with intervertebral disc (IVD) degeneration (IVDD), research has historically focused more on understanding the mechanism behind IVDD while clinical efforts prioritize pain management. More recently, there has been a shift toward understanding LBP as a distinct pathological entity. This review synthesizes current knowledge on discogenic LBP, combining known pathophysiology, molecular mechanisms, risk factors, diagnostic challenges, and available experimental models. IVDD is a complex, multifactorial process involving biochemical, mechanical, and inflammatory changes within the disc, leading to structural breakdown and potential discogenic pain. Key mechanisms include extracellular matrix degradation, upregulation of inflammatory mediators, immune cell infiltration, and aberrant nerve and vascular ingrowth. However, not all cases of IVDD result in LBP, highlighting the need for further investigation into the cellular, molecular, and biomechanical factors contributing to symptom development. Current diagnostic tools and experimental models for studying discogenic LBP remain limited, impeding the development of targeted treatments. Existing therapies primarily focus on symptom management rather than addressing underlying disease mechanisms.

Ossification of the posterior longitudinal ligament (OPLL) is a degenerative disease characterized by progressive ectopic bone formation process, which can lead to severe neurological impairments and reduced quality of life. While the etiology of OPLL is generally considered multifactorial, there is no consensus regarding these contributing factors including genetic, endocrine, biomechanical, immune and lifestyle factors. Through accumulating evidence from multidisciplinary investigations, the pathophysiological connection between OPLL and endocrine-metabolic dysregulation is becoming increasingly clear. Nevertheless, comprehensive understanding of the relationship between the two is hindered by several problems, such as methodological limitations and inadequate mechanistic studies. This review takes a deep dive into the possible factors contributing to OPLL from all aspects of metabolism, including glucose metabolism, lipid metabolism, bone and mineral metabolism, leptin, vitamin, growth hormone/IGF-1 and sex hormones, highlighting their potential roles in the onset and progression of OPLL. Clarifying the etiology of OPLL and elucidating the underlying pathogenesis are crucial for advancing both early intervention strategies and therapeutic approaches in clinical management. Therefore, the endocrine and metabolic disorders in OPLL patients should become a focus of future research.

Circadian rhythm disorders are associated with dysfunction in inflammatory diseases, and targeted regulation of the circadian rhythm could serve as an intervention strategy. RORα/γ, as core components of circadian clock genes, positively modulate the key circadian molecule BMAL1. In this study, Gala-SR, a potent small-molecule compound designed to effectively regulate circadian rhythms, was synthesized through a monosaccharide modification prodrug strategy via a hydrolysable conjugation of galactose onto SR1078, an unique synthetic agonist of RORα/γ. Compared with SR1078, Gala-SR exhibited significantly greater aqueous solubility, cytocompatibility, pharmacokinetic characteristics and efficacy in the targeted activation of RORα. Importantly, Gala-SR ameliorated rhythm disorders by enhancing amplitude of the circadian rhythm both in vitro and in vivo. In circadian rhythm disordered mice with periodontitis, Gala-SR restored local circadian rhythm and mitigated inflammation in periodontal tissue in a circadian clock-dependent manner, and alleviated alveolar bone loss. Our study demonstrates that Gala-SR exhibits great promise in restoration of circadian rhythm and could potentially serve as a targeted therapeutic intervention for treating inflammatory diseases arising from disruptions in circadian rhythm. This work provides a feasible paradigm for the development and translational application of small molecule modulators targeting circadian rhythms.

Hypertrophic chondrocytes (HCs) could transform into osteoblastic lineage cells while the pathophysiological implications of HC transformation remain largely unknown. Here, we generated a mouse line utilizing Col10a1-Cre to induce DTA expression to genetically ablate HCs and their descendants. Col10a1-Cre; R26^DTA/+ mice displayed dwarf phenotype, abnormal spongy bone, and significantly delayed drill-hole injuries healing, suggesting an indispensable role of HC lineage extension in bone growth and injury repair. Intriguingly, single-cell RNA sequencing analysis revealed the most significant loss of a cell cluster expressing multiple angiogenic factors (Pro-Angiogenic Descendants of HCs, PADs) among cells derived from Col10a1-Cre; R26^DTA/+ and control femurs. In silico analysis of cell-cell communication supported Thrombospondin 4 (THBS4) as a specific angiogenic factor mediating the crosstalk between PADs and vascular endothelial cells. Concordantly, analyses using immunostaining combined with tissue clearing revealed that PADs physically contacted with endothelial cells, whereas Col10a1-Cre; R26^DTA/+ mice showed defective metaphyseal and cortical vessel formation and post-injury angiogenesis along with a significant loss of THBS4. Moreover, in vitro assays showed that supplying THBS4 was sufficient to promote proliferation and tube formation of endothelial cells and rescue defective angiogenesis of Col10a1-Cre; R26^DTA/+ metatarsal explants. Collectively, these findings demonstrate a critical role of PADs in bone growth and injury repair by secreting THBS4 to regulate angiogenesis.

Osteoporosis is a prevalent metabolic bone disorder that develops when osteoclast-mediated bone resorption chronically exceeds osteoblast-driven bone formation. The molecular pathways that govern osteogenic dysfunction and connect cellular metabolism to differentiation regulation remain poorly characterized. Here, we identify Sirtuin 5 (Sirt5) as a pivotal osteogenic regulator through bioinformatic screening and functional validation in Sirt5-knockout mice. Mechanistically, Sirt5 governs mitochondrial homeostasis by desuccinylating Solute Carrier Family 25 Member 4 (Slc25a4) at lysine 147 (K147), as demonstrated by quantitative succinylome profiling and site-directed mutagenesis. This site-specific desuccinylation triggers Slc25a4 degradation, attenuating mitochondrial oxidative stress and promoting osteoblast differentiation. Crucially, Slc25a4-K147 succinylation drives osteoporosis progression, while Sirt5-mediated desuccinylation at this site confers protection. Our work reveals the Sirt5-Slc25a4-K147 axis as a novel regulatory mechanism coupling mitochondrial metabolism to bone homeostasis, offering a therapeutic target for osteoporosis intervention.

Bone marrow lesions (BML) are early signs of osteoarthritis (OA) and are strongly correlated with the deterioration of cartilage lesions. Single-cell RNA sequencing (scRNA-seq) analyses were performed on BM from non-BML and BML areas and articular cartilage from intact and damaged areas to explore BML landscape and BML-cartilage crosstalk. We revealed the immune landscape of BM in non-BML and BML, and the transition to pro-inflammatory states of clusters in BMLs, such as classical monocytes and non-classical monocytes. Non-classical monocytes have high inflammation, OA gene signatures, and senescence scores, and are potential primary clusters promoting OA progression. Histological signs of OA related to the cellular landscape in damaged cartilage were identified, including PreFC exhaustion. The BM-cartilage crosstalk at the cell-cell interaction (CCIs) level and the TNF signal transmitted by non-classical monocytes are the critical CCIs in BML-induced cartilage damage, and PreFC is one of the primary receivers of the signal. We further validated the higher senescence level of non-classical monocyte and FC-2 in OA mice, compared with classical monocyte and PreFC, respectively. Transcription factor 7 like 2 (TCF7L2) was identified as a shared transcription factor in the senescence of monocytes and chondrocytes, facilitating the development of the senescence-associated secretory phenotype (SASP). Therefore, senescent non-classical monocytes promote BMLs and inflammation and senescence of chondrocytes by modulating BML–cartilage crosstalk in OA, with TCF7L2 serving as a regulator.

Bone healing is integral to orthopedic research, focusing on the restoration of bone function through a complex interplay of inflammatory responses, soft callus formation, hard callus development, and the final remodeling phase. While the natural progression of bone healing is a finely tuned process, it can be disrupted by inflammatory dysregulation, ranging from chronic inflammation to acute inflammatory anomalies, and by the depletion of essential repair substances under both chronic and acute conditions. Current strategies to enhance bone healing employ a multifaceted approach, including biochemical modulation of the local microenvironment through essential nutrient supplementation (e.g., calcium and vitamin D), biomechanical optimization via improved internal fixation stability, and advanced regenerative techniques incorporating bioactive factors, stem cell therapies, and functional biomaterials. Despite these efforts, challenges persist in the precise characterization of the local microenvironment and the precise control of in vivo bioactive molecule delivery. This review comprehensively summarizes the current research progress in bone healing, providing significant reference for understanding the mechanisms of bone healing and for guiding further research. It is expected to lay the theoretical foundation for the development of more effective therapeutic strategies for bone healing.

Survival of motor neuron (SMN) protein encoded by SMN1 gene, is the essential and ubiquitously expressed protein in all tissues. Prior studies demonstrated that SMN deficiency impaired bone development, but the underlying mechanism of abnormal endochondral ossification remains obscure. Here, we showed SMN is involved in hypertrophic chondrocytes differentiation through regulating RNA splicing and protein degradation via analyzing single cell RNA-sequencing data of hypertrophic chondrocytes. Of note, SMN loss induced dwarfism and delayed endochondral ossification in Smn1 depletion-severe spinal muscular atrophy (SMA) mouse model and Smn1 chondrocyte conditional knockdown mouse. Histological analysis revealed that SMN deficiency expanded the zone of hypertrophic chondrocytes in the growth plates, but delayed turnover from hypertrophic to ossification zone. Widespread changes in endochondral ossification related gene expression and alternative splicing profiles were identified via RNA sequencing of growth plate cartilages from SMA mice on postnatal day 4. Importantly, Mass spectrometry-based proteomics analysis elucidated Y-box-binding protein 1 (YBX1) as a vital SMN-binding factor, was decreased in SMA mice. YBX1 knockdown reproduced the aberrant gene expression and splicing changes observed in SMA growth plate cartilages. Comparing the binding proteins of SMN and YBX1 revealed TNF receptor-associated factor 6 (TRAF6), which promoted ubiquitination degradation of YBX1. By conditionally deleting Smn1 in chondrocytes of WT mice and overexpressing Smn1 in chondrocytes of SMA mice, we proved that SMN expression in chondrocytes is critical for hypertrophic chondrocyte-mediated endochondral ossification. Collectively, these results demonstrate that SMN deficiency contributes to rapid systemic bone dysplasia syndrome by promoting TRAF6-induced ubiquitination degradation of YBX1 in growth plate cartilages of SMA mice.

The global aging crisis has increased the prevalence of skeletal disorders, necessitating innovative therapeutic strategies. This review employs the brain-bone axis (BBA) framework to examine the role of the sympathetic nervous system (SNS) in bone metabolism. The research systematically elucidates the molecular mechanisms by which the SNS mediates signaling pathways through neurofibers and neurotransmitters, such as norepinephrine, dopamine, neuropeptide Y, and leptin, regulating interactions between bone-related cells to maintain skeletal homeostasis. It also identifies the pathological associations between the dysregulation of these pathways and the progression of bone-related conditions, such as osteoporosis, osteoarthritis, and intervertebral disc degeneration. By integrating current evidence, we identify novel therapeutic targets within the BBA and propose neuro-centric intervention strategies to mitigate skeletal diseases. This review deepens the understanding of neuro-skeletal interactions and lays a foundation for innovative treatments for bone-related pathologies.

Weight loss, whether resulting from disease-related conditions or intentional interventions, has been increasingly recognized as a significant risk factor for compromised skeletal integrity. While moderate weight reduction may yield metabolic benefits, rapid or sustained weight loss is frequently associated with decreased bone mineral density, deterioration of bone microarchitecture, and heightened fracture risk. The mechanisms underlying weight loss–induced bone loss are complex and multifactorial. Emerging evidence highlights a range of contributing factors, including reduced mechanical loading, increased bone marrow adiposity, hormonal and endocrine alterations, nutritional deficiencies, and disruptions in energy metabolism. These mechanisms are intricately interconnected, ultimately impairing bone remodeling and homeostatic balance. In this review, we provide a comprehensive analysis of the current literature on the mechanistic pathways, clinical consequences, and therapeutic strategies related to weight loss–induced bone loss. We further differentiate the skeletal effects of disease-associated versus intervention-induced weight loss, with a focus on their distinct molecular underpinnings. Our goal is to offer novel insights into the optimization of bone health management in the context of weight loss, guided by a translational medicine perspective.

Osteoclast-development patterns and their alterations across Ankylosing Spondylitis (AS) conditions are mysterious, making AS treatment difficult. Our study aims to clarify osteoclast-precursor (OCP) development patterns from monocytes and their variations under AS conditions. We performed single-cell transcriptomics in peripheral blood mononuclear cells (PBMCs) from healthy donors and AS patients in the early, aggravated and remission stages. After monocytic reclustering, OCP-development patterns and the alterations upon AS onset and different outcomes were revealed based on single-cell trajectory. The trajectories revealed two monocyte states with strong OCP features, and AS pathogenesis was characterized by their reduction. Ribosome synthesis was considered the essential function for the development towards OCP-featured states, and this function and its representative molecule, RPS17, showed a decreasing trend with AS onset and outcomes. Histology assessment showed that RPS17 underexpression participated in AS inflammatory osteogenesis and ankylosing destruction. Conditional knockout of RPS17 ameliorated ovariectomy-induced bone loss and enhanced osteoclastogenesis, and RPS17 overexpression improved the phenotype of AS-like mice. Importantly, local injection of RPS17-overexpressed monocytic OCPs markedly ameliorated the joint alterations of AS-like mice without promoting bone loss; this was associated with enhanced osteoclastogenesis adjacent to the articular surface and T-cell-suppressive property in monocytic OCPs. Overall, the evolution of monocytes towards OCP-lineage fate mainly depends on ribosome synthesis, and OCP-development disorder participates in AS lesions due to a reduction in RPS17-dependent ribosome synthesis. Notably, RPS17-overexpressed monocytic OCPs have translational potential in preventing and treating AS peripheral lesions.

The bone marrow microenvironment is critical for the maintenance and functionality of stem/progenitor cells, which are essential for bone development and regeneration. However, the composition and potential use of bone marrow interstitial fluid have not been well explored. In this study, we report the role of neonatal bovine bone marrow interstitial fluid (NBIF) in enhancing the bone regeneration capacity of human bone marrow mesenchymal stem cells (hBMSCs). Unlike adult bovine bone marrow interstitial fluid (ABIF), NBIF-fed hBMSCs exhibit enhanced self-renewal and osteogenic potential and bone marrow homing ability, along with transcriptome changes as compared to hBMSCs cultured in standard fetal bovine serum (FBS) supplemented medium. Mass spectrometry analysis reveals that multiple secreted factors associated with tissue repair and bone development are enriched in NBIF compared to FBS and ABIF. The combined use of NBIF-enriched Nerve Growth Factor (NGF), Lactoferrin (LTF), and High Mobility Group Protein B1 (HMGB1), together with Insulin-Like Growth Factor 1(IGF1) for culturing hBMSCs in the presence of FBS can enhance osteogenic potential and bone marrow homing ability, mimicking NBIF’s effects. These findings highlight the role of interstitial fluid in the bone marrow microenvironment and its potential to optimize stem cell-based therapies.

Neurofibromatosis type 1 (NF1) is a genetic disorder affecting 1 in 3 000 people due to heterozygous mutations in the NF1 gene. Patients with NF1 can develop multiple symptoms, such as neurofibromas, skin hyperpigmentation, and bone abnormalities, including tibial pseudarthrosis and spine deformity. Here, we aimed to elucidate the cellular origin and pathogenic mechanism of NF1 spine deformity. We explored the Prss56-Nf1 knockout (KO) mouse model that recapitulates neurofibromas and pseudarthrosis by carrying Nf1 gene inactivation in Prss56-expressing boundary cap cells, a neural crest subset, and their derivatives. Micro-CT analyses showed that Prss56-Nf1 KO mice exhibit spine deformity from 12 months of age, associated with vertebral anomalies reminiscent of patients with NF1. Fate mapping revealed a significant increase in OSX⁺ osteoblasts of the Prss56 lineage in vertebrae of Prss56-Nf1 KO mice. Increased traced Nf1-deficient cells correlated with increased vertebral bone volume and kyphosis spine curvature. Finally, we showed that treating Prss56-Nf1 KO mice with RAS-MAPK pathway inhibitors prevented spine deformity. Overall, the Prss56-Nf1 KO mouse model unravels the role of osteoblasts from the Prss56 lineage as the cellular origin of NF1 spine deformity and highlights RAS-MAPK pathway inhibition as a promising therapeutic strategy for preventing NF1 spine deformity.

Evidence on the association between osteoporosis and dementia is not fully clear. This study aimed to investigate the potential association between osteoporosis and the subsequent risk of dementia among older adults. We performed a cohort study of 176 150 community-dwelling older adults aged ≥65 years and free of cognitive impairment between 2018 and 2022 using integrated healthcare data from Shenzhen, China. Diagnoses of osteoporosis, osteoporotic fractures, and dementia were identified through linked outpatient and inpatient medical records and death registration records. Multivariate Cox proportional hazards models were used to estimate the adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) of incident dementia associated with osteoporosis and osteoporotic fractures. The mean (SD) age of the total study population was 70.7 (5.4) years, and 9 605 had a previous diagnosis of osteoporosis. Over a median follow-up of 2.2 (IQR: 1.8–4.3, maximum: 5.5) years, corresponding to 505 423 person-years at risk, 1 367 incident all-cause dementia cases, including 617 Alzheimer’s disease and 298 vascular dementia cases, occurred. Physician-diagnosed osteoporosis was associated with a higher risk of all-cause dementia (HR: 1.80, 95% CI: 1.53–2.12). The increased dementia risk tended to be more prominent among patients with osteoporotic fractures (HR: 2.43, 95% CI: 1.83–3.23) than those without (HR: 1.63, 95% CI: 1.35–1.97). Results were similar for Alzheimer’s disease and vascular dementia. This study provides evidence that older adults with osteoporosis, especially those with osteoporotic fractures, have an elevated risk of incident dementia. Effective prevention and management of osteoporosis among the older population may be promising to mitigate the dual burden of osteoporosis and dementia.

Osteoarthritis (OA) is a common degenerative joint disease with complex risk factors, and its underlying mechanism remains unclear. The disease has a subtle onset and mild early symptoms, and its progression is irreversible. Current treatments do not offer a complete cure. Therefore, developing new therapies, early prevention strategies, and reliable biomarkers is essential to reduce the disease burden and improve the quality of life for OA patients. Extracellular vesicles, with their natural biocompatibility and low immunogenicity, have shown great potential in drug delivery and acellular therapies. To provide a complete understanding of the current research and future prospects of extracellular vesicles in OA, this study used bibliometric analysis and Latent Dirichlet Allocation (LDA) methods to systematically evaluate international collaborations, research hotspots, and emerging trends in the field. Our aim is to offer a scientific basis and reference for innovative OA treatment strategies and the clinical application of extracellular vesicles.

Osteosarcoma (OS) is the most frequent primary bone sarcomas with high recurrence and poor prognosis. Emerging evidence indicates that membraneless organelles stress granules (SGs), whose assemblies are driven by scaffold protein G3BP1, are extensively involved in tumor, especially in OS. However, how SGs behave and communicate with organelles, particularly nucleoli and mitochondria, during drug challenges remain unknown. This study revealed that chemotherapeutic drugs activated the cysteine protease asparagine endopeptidase (AEP) to specifically cleave the SG core protein G3BP1 at N258/N309 in OS and malignant glioma. tG3BP1-Ns modulated SG dynamics by competitively binding to full-length G3BP1. Strikingly, tG3BP1-Cs, containing a conserved RNA recognition motif CCUBSCUS, sequestered mRNAs of ribosomal proteins and oxidative phosphorylation genes in the nucleoli and mitochondria to repress translation and oxidative stress. Moreover, the inhibition of AEP promoted the tumor-suppressing effect of chemotherapeutic drugs, whereas AEP-cleaved G3BP1 rescue reversed the effect in both OS and glioma models. Cancerous tissues exhibited high levels of AEP and G3BP1 truncations, which were strongly associated with poor prognosis. Accordingly, this study proposed a new paradigm and potential therapeutic targets to address chemotherapy sensitivity conferred by AEP-cleaved G3BP1-mediated SGs/nucleoli/mitochondria coordination.

Intervertebral disc degeneration (IVDD) is the primary contributor to a range of spinal diseases. Dynamin-related protein 1 (Drp1)-mediated mitochondrial fission has recently been identified as a new cause of nucleus pulposus cell (NPC) death and IVDD, but the underlying mechanisms remain unclear. Although the effects of Drp1 phosphorylation in IVDD have been studied, it is currently unknown if small ubiquitin-like modifications (SUMOylation) of Drp1 regulate IVDD. This study aimed to investigate the functions and mechanisms of mitochondria-anchored protein ligase (MAPL), a mitochondrial SUMO E3 ligase, during IVDD progression. The expression of genes related to SUMOylation and mitochondrial dynamics in TNF-α-stimulated NPCs was analysed via RNA sequencing. The levels of total and mitochondrial SUMO1 conjugates were elevated with MAPL upregulation in TNF-α-treated NPCs. Additionally, mitochondrial fragmentation and dysfunction were induced by TNF-α stimulation. MAPL overexpression promoted mitochondrial SUMOylation and SUMO1 modification of Drp1, thereby facilitating the mitochondrial translocation of Drp1 and mitochondrial fission. MAPL-induced ROS accumulation and ΔΨm loss led to increased NPC apoptosis. Mutation of the SUMO-acceptor lysine residues of Drp1 hindered its SUMOylation and rescued the mitochondrial phenotypes caused by MAPL. SENP5 overexpression phenocopied MAPL silencing, negatively modulating the SUMO1 modification of Drp1 and mitochondrial fission in NPCs. In a rat IVDD model, forced expression of MAPL by using an adeno-associated virus (AAV) vector aggravated IVD tissue damage, whereas the knockdown of MAPL delayed IVDD progression. Our findings highlight the importance of SUMOylation in IVDD. The inhibition of MAPL-mediated Drp1 SUMOylation alleviates mitochondrial fission and limits IVDD development, providing a potential strategy for IVDD treatment.

The delicate balance between bone formation by osteoblasts and bone resorption by osteoclasts maintains bone homeostasis. Nuclear receptors (NRs) are now understood to be crucial in bone physiology and pathology. However, the function of the Farnesoid X receptor (FXR), a member of the NR family, in regulating bone homeostasis remains incompletely understood. In this study, in vitro and in vivo models revealed delayed bone development and an osteoporosis phenotype in mice lacking FXR in bone marrow mesenchymal stem cells (BMSCs) and osteoblasts due to impaired osteoblast differentiation. Mechanistically, FXR could stabilize RUNX2 by inhibiting Thoc6-mediated ubiquitination, thereby promoting osteogenic activity in BMSCs. Moreover, activated FXR could directly bind to the Thoc6 promoter, suppressing its expression. The interaction between RUNX2 and Thoc6 was mediated by the Runt domain of RUNX2 and the WD repeat of Thoc6. Additionally, Obeticholic acid (OCA), an orally available FXR agonist, could ameliorate bone loss in an ovariectomy (OVX)-induced osteoporotic mouse model. Taken together, our findings suggest that FXR plays pivotal roles in osteoblast differentiation by regulating RUNX2 stability and that targeting FXR may be a promising therapeutic approach for osteoporosis.

Osteoporosis is a known risk factor for rotator cuff tears (RCTs), but the causal correlation and underlying mechanisms remain unclear. This study aims to evaluate the impact of osteoporosis on RCT risk and investigate their genetic associations. Using data from the UK Biobank (n = 457 871), cross-sectional analyses demonstrated that osteoporosis was significantly associated with an increased risk of RCTs (adjusted OR [95% CI] = 1.38 [1.25–1.52]). A longitudinal analysis of a subset of patients (n = 268 117) over 11 years revealed that osteoporosis increased the risk of RCTs (adjusted HR [95% CI] = 1.56 [1.29–1.87]), which is notably varied between sexes in sex-stratified analysis. Causal inference methods, including propensity score matching, inverse probability weighting, causal random forest and survival random forest models further confirmed the causal effect, both from cross-sectional and longitudinal perspectives. A colocalization analysis across multiple datasets identified six candidate loci, including the successfully replicated PKDCC rs12996954 variant, which may help explain the shared genetic basis between osteoporosis and RCTs. In conclusion, osteoporosis significantly increases the risk of RCTs, emphasizing the importance of osteoporosis management in preventing RCTs. The identification of shared genetic loci provides new insights into their potential pathogenic mechanisms.

Piezo1, a key mechanosensor in bone homeostasis, plays a crucial role in fracture healing. However, the mechanisms through which Piezo1 regulates chondrocytes and affects endochondral ossification remain poorly understood. This study aimed to investigate the regulatory mechanisms of Piezo1 in chondrocytes during endochondral ossification. Using lineage tracing, we identified chondrocyte-to-osteoblast transdifferentiation during endochondral ossification, which was impaired by chondrocyte-specific Piezo1 knockout. Piezo1 deficiency disrupted mitochondrial bioenergetics, characterized by diminished membrane potential, reduced adenosine triphosphate (ATP) synthesis, suppressed oxygen consumption rates (basal and maximal respiration), and elevated mitochondrial superoxide generation, thereby impairing endochondral ossification during fracture healing. Single-cell RNA sequencing revealed upregulated Lars2 expression in hypertrophic chondrocytes following Piezo1 knockout. Inhibition of Lars2 in chondrocytes normalized mitochondrial dynamics-related markers (MFN1, MFN2, OPA1, DRP1) and restored mitochondrial functional homeostasis. This intervention concurrently reversed Piezo1 knockout-induced suppression of osteogenic markers (Col1, ALP, OCN, OPN, RUNX2), thereby enhancing fracture repair. Protein interaction analyses confirmed direct binding between β-catenin and Lars2. Mechanistically, Piezo1 governs Lars2 expression via β-catenin signaling. Our findings demonstrate that Piezo1 activation via Yoda1 enhances mitochondrial bioenergetics and accelerates fracture repair through the β-catenin/Lars2 axis, offering novel insights and therapeutic avenues for fracture treatment.

Polytrauma with significant bone and volumetric muscle loss presents substantial clinical challenges. Although immune responses significantly influence fracture healing post-polytrauma, the cellular and molecular underpinnings of polytrauma-induced immune dysregulation require further investigation. While previous studies examined either injury site tissue or systemic tissue (peripheral blood), our study uniquely investigated both systemic and local immune cells at the same time to better understand polytrauma-induced immune dysregulation and associated impaired bone healing. Using single-cell RNA sequencing (scRNA-seq) in a rat polytrauma model, we analyzed blood, bone marrow, and the local defect soft tissue to identify potential cellular and molecular targets involved in immune dysregulation. We identified a trauma-associated immunosuppressive myeloid (TIM) cell population that drives systemic immune dysregulation, immunosuppression, and potentially impaired bone healing. We found CD1d as a global marker for TIM cells in polytrauma. In the local defect tissue, we observed Spp1⁺ monocytes/macrophages mediating inflammatory, fibrotic, and impaired adaptive immune responses. Finally, our findings highlighted increased signaling via Anxa1-Fpr2 and Spp1-Cd44 axes. This comprehensive analysis enhances our understanding of immune dysregulation-mediated nonunion following traumatic injury and provides biomarkers that could function as treatment targets.

Osteogenesis imperfecta (OI) is a group of diseases caused by defects in type I collagen processing which result in skeletal fragility. While these disorders have been regarded as defects in osteoblast function, the role of matrix-embedded osteocytes in OI pathogenesis remains largely unknown. Homozygous human SP7 (c.946  C >  T, R316C) mutation results in a recessive form of OI characterized by fragility fractures, low bone mineral density and osteocyte dendrite defects. To better understand how the OI-causing R316C mutation affects the function of SP7, we generated Sp7^R342C knock-in mice. Consistent with patient phenotypes, Sp7^R342C/R342C mice demonstrate increased cortical porosity and reduced cortical bone mineral density. Sp7^R342C/R342C mice show osteocyte dendrite defects, increased osteocyte apoptosis, and intracortical bone remodeling with ectopic intracortical osteoclasts and elevated osteocyte Tnfsf11 expression. Remarkably, these defects in osteocyte function contrast to only mild changes in mature osteoblast function, suggesting that this Sp7 mutation selectively interferes with the function of Sp7 in osteocytes and mature osteoblasts, but not during early stages of osteoblast differentiation. Osteocyte morphology changes in Sp7^R342C/R342C mice were not restored by inhibiting osteoclast formation, indicating that dendrite defects lie upstream of high intracortical osteoclast activity in this model. Moreover, transcriptomic profiling reveals that the expression of a core set osteocyte-enriched genes is highly dysregulated by the R342C mutation. Thus, this supports a model in which osteocyte dysfunction can drive OI pathogenesis and provides a valuable resource to test novel therapeutic approaches and to understand the osteocyte-specific role of SP7 in bone remodeling.

Circadian rhythm is ubiquitous in nature. Circadian clock genes such as Bmal1 and Clock form a multi-level transcription-translation feedback network, and regulate a variety of physiological and pathological processes, including bone and cartilage metabolism. Deletion of the core clock gene Bmal1 leads to pathological bone alterations, while the phenotypes are not consistent. Studies have shown that multiple signaling pathways are involved in the process of Bmal1 regulating bone and cartilage metabolism, but the exact regulatory mechanisms remain unclear. This paper reviews the signaling pathways by which Bmal1 regulates bone/cartilage metabolism, the upstream regulatory factors that control Bmal1, and the current Bmal1 knockout mouse models for research. We hope to provide new insights for the prevention and treatment of bone/cartilage diseases related to circadian rhythms.