Single-cell atlas of human infrapatellar fat pad and synovium implicates APOE signaling in osteoarthritis pathology

The inflammatory endotype in osteoarthritis: Reflections from the 2024 OARSI clinical trials symposium (CTS) with a special emphasis on feasibility for clinical development

Objective

The inflammatory endotype is arguably one of the most well-established endotype in osteoarthritis (OA). While endotyping holds promise for advancing drug development, numerous potential challenges must be considered, addressed and resolved before successful clinical outcomes can be achieved.

Design

Since 2017, the Osteoarthritis Research Society International (OARSI) has hosted the Clinical Trials Symposium (CTS). Each year, OARSI and the CTS steering committee encourage discussions on selected topics among a broad range of stakeholders, including regulators, drug developers, clinicians, clinical researchers, biomarker specialists, and basic scientists, with the aim of advancing drug development in the OA field.

Results

This report highlights the ongoing tension between academia's "blue ocean" strategy and the feasibility-driven approach of drug developers, all within the context of scientific efforts to find effective solutions. Understanding the needs, goals, constraints, and opportunities of all involved stakeholders is crucial for defining optimal drug development strategies for OA.

Conclusion

A multidisciplinary, collaborative approach is essential for developing effective OA treatments, balancing scientific discovery with regulatory and clinical feasibility.

Towards a consensus atlas of human and mouse adipose tissue at single-cell resolution

Adipose tissue (AT) is a complex connective tissue with a high relative proportion of adipocytes, which are specialized cells with the ability to store lipids in large droplets. AT is found in multiple discrete depots throughout the body, where it serves as the primary repository for excess calories. In addition, AT has an important role in functions as diverse as insulation, immunity and regulation of metabolic homeostasis. The Human Cell Atlas Adipose Bionetwork was established to support the generation of single-cell atlases of human AT as well as the development of unified approaches and consensus for cell annotation. Here, we provide a first roadmap from this bionetwork, including our suggested cell annotations for humans and mice, with the aim of describing the state of the field and providing guidelines for the production, analysis, interpretation and presentation of AT single-cell data.

Molecular features and diagnostic modeling of synovium- and IPFP-derived OA macrophages in the inflammatory microenvironment via scRNA-seq and machine learning

BACKGROUND

Osteoarthritis (OA) is the leading cause of degenerative joint disease, with total joint replacement as the only definitive cure. However, no disease-modifying therapy is currently available. Inflammation and fibrosis in the infrapatellar fat pad (IPFP) contribute to OA onset and progression. However, the cellular composition and molecular mechanisms in the IPFP microenvironment remain unclear. This study investigates the functions of OA-macrophages and their clinical significance.

METHODS

We analyzed single-cell RNA sequencing (scRNA-seq) data from normal and OA patients. Enrichment analysis revealed differences in biological pathways across cell types. Pseudotime and cell-cell communication analyses revealed the developmental trajectory and interactions of OA-macrophages with other cell types. Machine learning (ML) algorithms identified feature genes of OA-macrophages. An OAMGS diagnostic score was developed, and CIBERSORT was used to analyze immune infiltration and its association with immune cells. Rat OA and normal models were established, and feature gene expression was validated using immunofluorescence (IF) staining and quantitative reverse transcription PCR (RT-qPCR).

RESULTS

OA-macrophages play a central role in inflammation and fibrosis, enhancing leukocyte recruitment, chondrocyte apoptosis, and angiogenesis. They interact with chondrocytes, endothelial cells, and fibroblasts via CXCL and NF-κB signaling. High-dimensional weighted gene co-expression network analysis (hdWGCNA) identified 352 module genes linked to OA-macrophages. Machine learning developed a four-gene-based OAMGS score that accurately identifies OA-macrophages, with an AUC of 1 in the discovery cohort and 0.990 in an external cohort. Gene expression was validated in the OA model using RT-qPCR and IF.

CONCLUSION

This study identifies a macrophage subcluster elevated in OA patients. OA-macrophages play an immunoregulatory role and may serve as diagnostic markers. The OAMGS score, based on four genes, provides an accurate diagnostic tool and potential therapeutic target for OA.

Infrapatellar fat pad as a source of biomarkers and therapeutic target for knee osteoarthritis

BACKGROUND AND OBJECTIVE

Osteoarthritis (OA) is a multifactorial and highly prevalent disease in elderly adults; however, its pathogenesis, diagnosis, and treatment are unmet needs nowadays. Research efforts have focused on elucidating the molecular mechanisms involved in the pathogenesis, onset, and progression of OA to facilitate early detection and effective therapeutic approaches. Infrapatellar fat pad (IPFP) represents a promising novel source of OA biomarkers given that it is an active player in OA. This review aims to investigate the current literature regarding the potential of the IPFP as a source of diagnostic and prognostic biomarkers for OA as well as potential target for novel therapies.

METHODS

A literature search was conducted in the PubMed database in June 2024. We included cross-sectional and longitudinal studies based on IPFP from human OA patients, oriented in the identification of imaging, biochemical, and molecular biomarkers in the IPFP.

RESULTS

After screening and evaluation, we included a total of 61 studies. Most of the imaging publications (n = 47) on IPFP are based on magnetic resonance imaging (MRI) that revealed potential semiquantitative and quantitative imaging biomarkers linked to inflammation, fibrosis, pain, and joint degeneration imaging parameters. Biochemical and molecular studies (n = 14) pointed out an increase in interleukin-6 (IL-6), fatty acid-binding protein 4 (FABP4), adiponectin, and lysophosphatidylcholine (LysoPC) in the IPFP during OA progression.

CONCLUSIONS

Imaging, biochemical, and molecular studies indicate OA potential biomarkers in the IPFP related to inflammation, lipid dysregulation, and fibrosis. The combination of imaging and biochemical biomarkers could provide a better prediction of OA onset and the identification of OA progressors at an early stage. The IPFP study could also reveal potential therapeutic targets with the vision of better precision medicine.

Injury and obesity differentially and synergistically induce dysregulation of synovial immune cells in osteoarthritis

OBJECTIVES

The heterogeneity and phenotype of immune cells orchestrate many physiologic and pathologic processes. Recent evidence suggests that immune cells play critical roles in the progression of osteoarthritis (OA). We hypothesised that injury and obesity, two major risk factors for OA, affect the immunophenotype of the synovium, the primary reservoir of immune cells in the joint.

METHODS

Using single-cell transcriptomics, immunoprofiling, transgenic mouse models, and genetic fate mapping methods, we characterised the presence and fate of multiple populations of immune cells found in the knee joint capsule.

RESULTS

We found that joint injury and obesity differentially and synergistically alter the architectural, cellular, and molecular profiles of the synovial capsule. We observed fewer patrolling monocytes in obese animals and found a significantly higher influx of proinflammatory monocyte-derived macrophages in the first 3 days after joint injury in obese compared with that in control animals. We also showed a significant loss of barrier-forming synovial lining macrophages 3 days after destabilisation of medial meniscus surgery, with a significant restoration of their numbers in normal weight but not in obese mice in advanced stages of OA. Finally, we characterised the presence and changes of other immune cell subtypes, including T, B, and mast cells and neutrophils, as well as local synovial fluid cytokines associated with injury and obesity.

CONCLUSIONS

Our data revealed that injury and obesity independently and synergistically contribute to the dysregulation of the synovial immune landscape, providing new insight into their role in the pathogenesis of OA.

Stem and progenitor cells in the synovial joint as targets for regenerative therapy

Damage to articular cartilage, tendons, ligaments and entheses as a result of trauma, degeneration or inflammation in rheumatic diseases is prevalent. Regenerative medicine offers promising strategies for repairing damaged tissues, with the aim of restoring both their structure and function. While these strategies have traditionally relied on tissue engineering approaches using exogenous cells, interventions based on the activation of endogenous repair mechanisms are an attractive alternative. Key to advancing such approaches is a comprehensive understanding of the diversity of the stem and progenitor cells that reside in the adult synovial joint and how they function to repair damaged tissues. Advances in developmental biology have provided a lens through which to understand the origins, identities and functions of these cells, and insights into the roles of stem and progenitor cells in joint tissue repair, as well as their complex relationship with fibroblasts, have emerged. Integration of knowledge obtained through studies using advanced single-cell technologies will be crucial to establishing unified models of cell populations, lineage hierarchies and their molecular regulation. Ultimately, a more complete understanding of how cells repair tissues in adult life will guide the development of innovative pro-regenerative drugs, which are poised to enter clinical practice in musculoskeletal medicine.

Swimming exercise induces redox-lipid crosstalk to ameliorate osteoarthritis progression

Conventional pharmacotherapy exhibits limited efficacy in halting cartilage degeneration, whereas exercise interventions have demonstrated promising protective effects against osteoarthritis (OA), albeit with unclear underlying mechanisms. This study investigated the beneficial effects of swimming in mitigating local joint damage through the enhancement of systemic antioxidant capacity. We found that overexpression of superoxide dismutase 3 (SOD3) could promote the elimination of extracellular reactive oxygen species (ROS) and preserve the cartilage extracellular matrix (C-ECM). Conversely, genetic deletion of SOD3 accelerated the loss of C-ECM and contributed to OA due to an imbalance in extracellular oxidative stress. Further investigation revealed that SOD3 could interact with CCAAT/enhancer binding protein β (C/EBPβ), leading to the inhibition of apolipoprotein E (APOE) transcription and subsequent APOE-induced cholesterol transport. Ultimately, we developed targeted extracellular vesicles (EVs) with high cartilage affinity for efficient and precise delivery of SOD3. Overall, this study elucidated the potential of exercise for degenerative joint disorders through SOD3-mediated extracellular antioxidation and cholesterol redistribution.

The role of the immune system in osteoarthritis: mechanisms, challenges and future directions

Osteoarthritis (OA) is a chronic joint disease that has long been considered a simple wear-and-tear condition. Over the past decade, research has revealed that various inflammatory features of OA, such as low-grade peripheral inflammation and synovitis, contribute substantially to the pathophysiology of the disease. Technological advances in the past 5 years have revealed a large diversity of innate and adaptive immune cells in the joints, particularly in the synovium and infrapatellar fat pad. Notably, the presence of synovial lymphoid structures, circulating autoantibodies and alterations in memory T cell and B cell populations have been documented in OA. These data indicate a potential contribution of self-reactivity to the disease pathogenesis, blurring the often narrow and inaccurate line between chronic inflammatory and autoimmune diseases. The diverse immune changes associated with OA pathogenesis can vary across disease phenotypes, and a better characterization of their underlying molecular endotypes will be key to stratifying patients, designing novel therapeutic approaches and ultimately ameliorating treatment allocation. Furthermore, examining both articular and systemic alterations, including changes in the gut-joint axis and microbial dysbiosis, could open up novel avenues for OA management.

Unraveling the spatial and signaling dynamics and splicing kinetics of immune infiltration in osteoarthritis synovium

Introduction

Osteoarthritis (OA), a debilitating joint disorder characterized by synovial inflammation and immune myeloid cell infiltration, currently lacks a comprehensive spatial and transcriptional atlas. This study investigates the spatial dynamics, splicing kinetics, and signaling pathways that drive immune infiltration in OA synovium.

Methods

We integrated single-cell RNA sequencing (scRNA-seq) data from 8 OA and 4 healthy synovial samples with spatial transcriptomics using Spatrio. Spatial transition tensor (STT) analysis decoded multistable spatial homeostasis, while splicing kinetics and non-negative matrix factorization (NMF) identified gene modules. CellPhoneDB and pyLIGER mapped ligand-receptor interactions and transcriptional networks.

Results

Re-annotation of scRNA-seq data resolved synovial cells into 27 subclasses. Spatial analysis revealed OA-specific attractors (8 in OA vs. 6 in healthy samples), including immune myeloid (Attractor3) and lymphoid infiltration (Attractor4). Key genes OLR1 (myeloid homeostasis) and CD69 (T-cell activation) exhibited dysregulated splicing kinetics, driving inflammatory pathways. Myeloid-specific transcription factors (SPI1, MAF, NFKB1) and lymphoid-associated BCL11B were identified as regulators. Computational drug prediction nominated ZILEUTON as a potential inhibitor of ALXN5 to mitigate myeloid infiltration.

Discussion

This study delineates the spatial and transcriptional landscape of OA synovium, linking immune cell dynamics to localized inflammation. The identification of OLR1 and CD69 as spatial homeostasis drivers, alongside dysregulated signaling networks, offers novel therapeutic targets. These findings advance strategies to modulate immune infiltration and restore synovial homeostasis in OA.

Subchondral bone marrow adipose tissue lipolysis regulates bone formation in hand osteoarthritis

OBJECTIVE

Bone marrow adipose tissue (BMAT) is emerging as an important regulator of bone formation and energy metabolism. Lipolysis of BMAT releases glycerol and fatty acid substrates that are catabolized by osteoblasts. Here, we investigated whether BMAT lipolysis is involved in subchondral bone formation in hand osteoarthritis (OA).

METHODS

Subchondral BMAT lipolysis and bone marrow adipocyte (BMAd) morphology were studied in clinical specimens of carpometacarpal (CMC-1) and distal interphalangeal joint OA. BMAd size, osteoblast numbers and expression of lipolysis enzymes (ATGL, phospho-HSL, MGLL) were compared between regions of low and high bone formation. Free fatty acids, glycerol and bone biomarkers were measured in osteochondral explants.

RESULTS

Subchondral BMAd size was positively correlated with BMI (r = 0.60, [0.082,0.87]) and reduced in regions of high bone formation (-1149 µm2, [-1977,-726.2]). Osteoblast numbers were negatively correlated with BMAd size (r = -0.48, [-0.73,-0.12]). All lipolysis enzymes were expressed in both in BMAds and activated osteoblasts and the area percentages of ATGL (+2.26% [0.19,3.47]), phospho-HSL (+1.57% [0.31,6.48]) and MGLL (+4.04% [1.09,5.69]) were increased in regions of high bone formation. Secreted glycerol levels, but not free fatty acids, were correlated with bone formation markers pro-collagen type I (rho = 0.90) and alkaline phosphatase (rho = 0.78).

CONCLUSION

Our findings reveal a previously unrecognized role of BMAT lipolysis in regulating bone formation in hand OA, which may be modulated by BMI.

Sustained therapeutic effects of self-assembled hyaluronic acid nanoparticles loaded with α-Ketoglutarate in various osteoarthritis stages

Osteoarthritis (OA) is a prevalent degenerative disease characterized by irreversible destruction of articular cartilage, for which no current drugs are known to modify its progression. While intra-articular (IA) injections of hyaluronic acid (HA) offer temporary relief, their effectiveness and long-term benefits are debated. Alpha-ketoglutarate (αKG) has potential chondroprotective properties, but its use is limited by a short half-life and poor cartilage-targeting efficiency. Here, we developed self-assembled HA-αKG nanoparticles (NPs) to combine the benefits of both HA and αKG, showing stability, bioavailability, and sustained pH-responsive release in the knee joint. In both early and advanced OA stages in mice, HA, αKG, and HA-αKG NPs could relieve pain, enhance mobility, and reduce cartilage damage, with HA-αKG NPs demonstrating the best efficacy. Mechanistically, αKG not only promotes cartilage matrix synthesis but also inhibits degradation by activating the PERK-ATF4 signaling pathway to reduce endoplasmic reticulum stress (ERS) in chondrocytes. This study highlights the therapeutic potential of HA-αKG NPs for treating various OA stages, with efficient and sustained effects, suggesting rapid clinical adoption and high acceptability among clinicians and patients.

Osteoarthritis

Osteoarthritis is a heterogeneous whole-joint disease that can cause pain and is a leading cause of disability and premature work loss. The predominant disease risk factors - obesity and joint injury - are well recognized and modifiable. A greater understanding of the complex mechanisms, including inflammatory, metabolic and post-traumatic processes, that can lead to disease and of the pathophysiology of pain is helping to delineate mechanistic targets. Currently, management is primarily focused on alleviating the main symptoms of pain and obstructed function through lifestyle interventions such as self-management programmes, education, physical activity, exercise and weight management. However, lack of adherence to known effective osteoarthritis therapeutic strategies also contributes to the high global disease burden. For those who have persistent symptoms that are compromising quality of life and have not responded adequately to core treatments, joint replacement is an option to consider. The burden imparted by the disease causes a substantial impact on individuals affected in terms of quality of life. For society, this disease is a substantial driver of increased health-care costs and underemployment. This Primer highlights advances and controversies in osteoarthritis, drawing key insights from the current evidence base.

Progress in multi-omics studies of osteoarthritis

Osteoarthritis (OA), a ubiquitous degenerative joint disorder, is marked by pain and disability, profoundly impacting patients' quality of life. As the population ages, the global prevalence of OA is escalating. Omics technologies have become instrumental in investigating complex diseases like OA, offering comprehensive insights into its pathogenesis and progression by uncovering disease-specific alterations across genomics, transcriptomics, proteomics, and metabolomics levels. In this review, we systematically analyzed and summarized the application and recent achievements of omics technologies in OA research by scouring relevant literature in databases such as PubMed. These studies have shed light on new potential therapeutic targets and biomarkers, charting fresh avenues for OA diagnosis and treatment. Furthermore, in our discussion, we highlighted the immense potential of spatial omics technologies in unraveling the molecular mechanisms of OA and in the development of novel therapeutic strategies, proposing future research directions and challenges. Collectively, this study encapsulates the pivotal advances in current OA research and prospects for future investigation, providing invaluable references for a deeper understanding and treatment of OA. This review aims to synthesize the recent progress of omics technologies in the realm of OA, aspiring to furnish theoretical foundations and research orientations for more profound studies of OA in the future.

Single-cell transcriptomics reveals novel chondrocyte and osteoblast subtypes and their role in knee osteoarthritis pathogenesis

Research on treating knee osteoarthritis (KOA) is becoming more challenging due to a growing number of younger patients being affected. The pathogenesis of KOA is complex for being a multifactorial disease affecting the entire joint, with remodeling of subchondral bone playing a key role in the degeneration of the overlying cartilage. Therefore, this study constructed a bipedal postmenopausal KOA mouse model to better understand how the interplay between subchondral bone remodeling and cartilage degeneration contributes to KOA development. A single-cell atlas of the osteochondral composite tissue was established. Furthermore, three novel subtypes of chondrocytes, including Smoc2+ angiogenic chondrocytes, Angptl7+ angiogenic chondrocytes, and Col1a1+ osteogenic chondrocytes, were identified in femoral condyles of KOA mice. In addition, the Angptl7+ chondrocytes promoted angiogenesis in the subchondral bone of KOA mice by interacting with endothelial cells via the FGF2-FGFR2 signaling pathway. The number of H-type vessels was increased in the subchondral bone, recruiting osteoprogenitor cells and facilitating osteogenesis in KOA mice. Sparc+ osteoblasts have negatively regulated bone mineralization and osteoblastic differentiation, aggravated the pathological remodeling of subchondral bone, and promoted the progression of KOA. The above findings have offered new targets and opened up an avenue for the therapeutic intervention of KOA.

Cell and transcriptomic diversity of infrapatellar fat pad during knee osteoarthritis

OBJECTIVES

In this study, we employ a multiomic approach to identify major cell types and subsets, and their transcriptomic profiles within the infrapatellar fat pad (IFP), and to determine differences in the IFP based on knee osteoarthritis (KOA), sex and obesity status.

METHODS

Single-nucleus RNA sequencing of 82 924 nuclei from 21 IFPs (n=6 healthy control and n=15 KOA donors), spatial transcriptomics and bioinformatic analyses were used to identify contributions of the IFP to KOA. We mapped cell subclusters from other white adipose tissues using publicly available literature. The diversity of fibroblasts within the IFP was investigated by bioinformatic analyses, comparing by KOA, sex and obesity status. Metabolomics was used to further explore differences in fibroblasts by obesity status.

RESULTS

We identified multiple subclusters of fibroblasts, macrophages, adipocytes and endothelial cells with unique transcriptomic profiles. Using spatial transcriptomics, we resolved distributions of cell types and their transcriptomic profiles and computationally identified putative cell-cell communication networks. Furthermore, we identified transcriptomic differences in fibroblasts from KOA versus healthy control donor IFPs, female versus male KOA-IFPs and obese versus normal body mass index (BMI) KOA-IFPs. Finally, using metabolomics, we defined differences in metabolite levels in supernatants of naïve, profibrotic stimuli-treated and proinflammatory stimuli-treated fibroblasts from obese compared to normal BMI KOA-IFPs.

CONCLUSIONS

Overall, by employing a multiomic approach, this study provides the first comprehensive map of the cellular and transcriptomic diversity of human IFP and identifies IFP fibroblasts as key cells contributing to transcriptomic and metabolic differences related to KOA disease, sex or obesity.

CD34hi subset of synovial fibroblasts contributes to fibrotic phenotype of human knee osteoarthritis

Osteoarthritis (OA) shows various clinical manifestations depending on the status of its joint components. We aimed to identify the synovial cell subsets responsible for OA pathophysiology by comprehensive analyses of human synovium samples in single-cell resolution. Two distinct OA synovial tissue groups were classified by gene expression profiles in RNA-Seq: inflammatory and fibrotic. The inflammatory group exhibited high expression of inflammatory cytokines, histologically inflammatory infiltrate, and a more severe pain score. The fibrotic group showed higher expression of fibroblast growth factor (FGFs) and bone morphogenetic proteins (BMPs), showed histologically perivascular fibrosis, and showed a lower pain score. In single-cell RNA-Seq (scRNA-Seq) of synovial cells, MERTKloCD206lo macrophages and CD34hi fibroblasts were associated with the inflammatory and fibrotic groups, respectively. Among the 3 fibroblast subsets, CD34loTHY1lo and CD34loTHY1hi fibroblasts were influenced by synovial immune cells, whereas CD34hi fibroblasts were influenced by mural and endothelial cells. Particularly, in CD34hi fibroblast subsets, CD34hiCD70hi fibroblasts promoted proliferation of Tregs, potentially suppressing synovitis and protecting articular cartilage. Elucidation of the mechanisms underlying the regulation of these synovial cell subsets may lead to novel strategies for OA therapeutics.

Exercise induces dynamic changes in intra-articular metabolism and inflammation associated with remodeling of the infrapatellar fat pad in mice

We hypothesized that daily exercise promotes joint health by upregulating anti-inflammatory mediators via adaptive molecular and metabolic changes in the infrapatellar fat pad (IFP). We tested this hypothesis by conducting time-resolved analyses between 1 and 14 days of voluntary wheel running exercise in C57BL/6J mice. IFP structure and cellularity were evaluated by histomorphology, picrosirius red collagen staining, and flow cytometry analysis of stromal vascular fraction cells. Joint inflammation and metabolism were evaluated by multiplex gene expression analysis of synovium-IFP tissue and synovial fluid metabolomics, respectively. Exercise transiently increased cytokine and chemokine gene expression in synovium-IFP tissue, resolving within the first 5 days of exercise. The acute inflammatory response was associated with decreased adipocyte size and elevated CD45+Gr1+ myeloid cells, increased collagen content, and oxidized phospholipids. Exercise acutely altered synovial fluid metabolites, characterized by increased amino acids, peptides, bile acids, sphingolipids, dicarboxylic acids, and straight medium chain fatty acids and decreased hydroxy fatty acids and diacylglycerols. Between 5 and 14 days of exercise, inflammation, collagen, and adipocyte size returned to pre-exercise levels, and CD206+ immuno-regulatory macrophages increased. Thus, although the onset of new daily exercise transiently induced synovium-IFP inflammation and altered tissue structure, sustained daily exercise promoted IFP homeostasis.

Comprehensive Analysis of Programmed Cell Death-Related Genes in Diagnosis and Synovitis During Osteoarthritis Development: Based on Bulk and Single-Cell RNA Sequencing Data

Background

Synovitis is one of the key pathological feature driving osteoarthritis (OA) development. Diverse programmed cell death (PCD) pathways are closely linked to the pathogenesis of OA, but few studies have explored the relationship between PCD-related genes and synovitis.

Methods

The transcriptome expression profiles of OA synovial samples were obtained from the Gene Expression Omnibus (GEO) database. Using machine learning algorithms, Hub PCD-related differentially expressed genes (Hub PCD-DEGs) were identified. The expression of Hub PCD-DEGs was validated in human OA samples by qRT-PCR. A diagnostic model for OA was constructed based on the expression levels of Hub PCD-DEGs. Unsupervised consensus clustering analysis and weighted correlation network analysis (WGCNA) were employed to identify differential clustering patterns of PCD-related genes in OA patients. The molecular characteristics of Hub PCD-DEGs, their role in synovial immune inflammation, and their association with the immune microenvironment were investigated through functional enrichment analysis and ssGSEA immune infiltration analysis. Single-cell RNA sequencing analysis provided insights into the characteristics of distinct cell clusters in OA synovial tissues and their interactions with Hub PCD-DEGs.

Results

We identified five Hub PCD-DEGs: TNFAIP3, JUN, PPP1R15A, INHBB, and DDIT4. qRT-PCR analysis confirmed that all five genes were significantly downregulated in OA synovial tissue. The diagnostic model constructed based on these Hub PCD-DEGs demonstrated diagnostic efficiency in distinguishing OA tissues as well as progression of OA. Additionally, a correlation was observed between the expression levels of Hub PCD-DEGs, immune cell infiltration, and inflammatory cytokine levels. We identified two distinct PCD clusters, each exhibiting unique molecular and immunological characteristics. Single-cell RNA sequencing further revealed dynamic and complex cellular changes in OA synovial tissue, with differential expression of Hub PCD-DEGs across various immune cell types.

Conclusion

Our study suggests that PCD-related genes may be involved in development of OA synovitis. The five screened Hub PCD-DEGs (TNFAIP3, JUN, PPP1R15A, INHBB and DDIT4) could be explored as candidate biomarkers or therapeutic targets for OA.

Osteoarthritis year in review 2024: Biology

Osteoarthritis (OA) research is a fast-growing and extremely wide field, in which a substantial increase in knowledge has been achieved over the last year. It covers many different topics, however, a PubMed search using the terms 'osteoarthritis' and 'biology' resulted in only a limited number of studies that were published between April 2023 and April 2024. In order to identify OA-relevant studies that focus on mechanistic studies of biological processes at the tissue, cellular, and molecular level, the following keywords were included as search terms: tissue interactions, single cell sequencing, transcriptomics, extracellular matrix, signaling, ion channels, and pain. The final selection of publications presented in this 'year in review' was influenced by the personal preferences of the authors, and eventually three larger key themes emerged: 1) Joint tissue interactions covering meniscus, subchondral bone, fat tissue, synovium, and synovial fluid. 2) Degeneration of the cartilage extracellular matrix and generation of bioactive fragments. 3) Receptors, ion channels, signaling pathways, and cellular metabolism. Many of the studies summarized here identified novel potential targets for OA treatment, and promising results were already obtained addressing these targets in different animal models. It will be exciting to see which findings can be translated into future clinical studies and eventually lead to novel treatment approaches for human OA.

Clinical and omics biomarkers in osteoarthritis diagnosis and treatment

Osteoarthritis (OA) is a prevalent degenerative joint disease that significantly impacts the quality of life for hundreds of millions, and is a major cause of disability. Despite this, diagnostic and therapeutic options for OA are still limited. With advances in molecular biology, an increasing number of OA biomarkers have been identified, which not only enhances our understanding of OA pathogenesis, but also offers new approaches for OA diagnosis and treatment. This review discussed the research progress on traditional OA biomarkers, and analyzed the application of various omics, including genomics, transcriptomics, proteomics, and metabolomics, in the diagnosis and treatment of OA. Furthermore, we explored how integrating multi-omics methods can reveal interactions among different biomolecules and their roles in the development of OA. This emerging interdisciplinary approach not only provides a more comprehensive understanding of the fundamental biological characteristics of OA, but also aids in identifying new integrated biomarkers, thereby allowing for more accurate predictions of disease progression and treatment responses. The identification and development of biomarkers offer new perspectives in understanding OA, enhancing the specificity and sensitivity of biological diagnostic markers, providing a basis for the design of targeted drugs, and ultimately advancing the development of precision diagnosis and treatment strategies in clinical OA. This study provides an overview of both commonly used and emerging biomarkers of OA which is beneficial for a more accurate, timely, effective clinical diagnosis and treatment for OA.

Obesity-induced fibrosis in osteoarthritis: Pathogenesis, consequences and novel therapeutic opportunities

Osteoarthritis (OA) is a significant global burden, affecting more than half a billion people across the world. It is characterized by degeneration and loss of articular cartilage, synovial inflammation, and subchondral bone sclerosis, leading to pain and functional impairment. After age, obesity is a major modifiable risk factor for OA, and it has recently been identified as a chronic disease by the World Health Organization (WHO). Obesity is associated with high morbidity and mortality, imposing a significant cost on individuals and society. Obesity increases the risk of knee OA through increased joint loading, altered body composition, and elevated pro-inflammatory adipokines in the systemic circulation. Moreover, obesity triggers fibrotic processes in different organs and tissues, including those involved in OA. Fibrosis in OA refers to the abnormal accumulation of fibrous tissue within and around the joints. It can be driven by increased adiposity, low-grade inflammation, oxidative stress, and metabolic alterations. However, the clinical outcomes of fibrosis in OA are unclear. This review focuses on the link between obesity and OA, explores the mechanism of obesity-driven fibrosis, and examines potential therapeutic opportunities for targeting fibrotic processes in OA.

Systemic and joint adipose tissue lipids and their role in osteoarthritis

Osteoarthritis (OA) is a major disease whose prevalence increases with aging, sedentary lifestyles, and obesity. The association between obesity and OA has been well documented, but the precise mechanisms underlying this heightened risk remain unclear. While obesity imposes greater forces on joints, systemic fat-derived factors such as lipids or adipokine may potentially act on the pathophysiology of OA, but the exact role of these factors in weight-bearing and non-weight-bearing joints remains elusive. Intra-articular adipose tissues (IAAT) have gained significant attention for actively participating in OA pathogenesis by interacting with various joint tissues. Lipid content has been proposed as a diagnostic target for early OA detection and a potential source of biomarkers. Moreover, targeting a specific IAAT called infrapatellar fat pad (IFP) and its lipids hold promise for attenuating OA-associated inflammation. Conversely, bone marrow adipose tissue (BMAT), which was long thought to be an inert filling tissue, is now increasingly considered a dynamic tissue whose volume and lipid content regulate bone remodeling in pathological conditions. Given OA's ability to alter adipose tissues, particularly those within the joint (IFP and BMAT), and the influence of adipose tissues on OA pathogenesis, this review examines the lipids produced by OA-associated adipose tissues, shedding light on their potential role in OA pathophysiology and highlighting them as potential therapeutic targets.

Bone and Joint-on-Chip Platforms: Construction Strategies and Applications

Organ-on-a-chip, also known as "tissue chip," is an advanced platform based on microfluidic systems for constructing miniature organ models in vitro. They can replicate the complex physiological and pathological responses of human organs. In recent years, the development of bone and joint-on-chip platforms aims to simulate the complex physiological and pathological processes occurring in human bones and joints, including cell-cell interactions, the interplay of various biochemical factors, the effects of mechanical stimuli, and the intricate connections between multiple organs. In the future, bone and joint-on-chip platforms will integrate the advantages of multiple disciplines, bringing more possibilities for exploring disease mechanisms, drug screening, and personalized medicine. This review explores the construction and application of Organ-on-a-chip technology in bone and joint disease research, proposes a modular construction concept, and discusses the new opportunities and future challenges in the construction and application of bone and joint-on-chip platforms.

Integration of ATAC-Seq and RNA-Seq Reveals VDR-SELENBP1 Axis Promotes Adipogenesis of Porcine Intramuscular Preadipocytes

Intramuscular fat (IMF) content plays a crucial role in determining pork quality. Recent studies have highlighted transcriptional mechanisms controlling adipogenesis in porcine IMF. However, the changes in chromatin accessibility during adipogenic differentiation are still not well understood. In this study, we performed the assay for transposase-accessible chromatin with high-throughput sequencing (ATAC-seq) and transcriptome sequencing (RNA-Seq) analyses on porcine intramuscular preadipocytes to explore their adipogenic differentiation into mature adipocytes. We identified a total of 56,374 differentially accessible chromatin peaks and 4226 differentially expressed genes at day 0 and day 4 during adipogenic differentiation. A combined analysis of the ATAC-seq and RNA-seq data revealed that 1750 genes exhibited both differential chromatin accessibility and differential RNA expression during this process, including selenium-binding protein 1 (SELENBP1), PLIN1, ADIPOQ, and FASN. Furthermore, we found that vitamin D receptor (VDR) could bind to the promoter region of the SELENBP1 gene, activate SELENBP1 transcription, and ultimately promote lipid accumulation during adipogenic differentiation. This study provides a detailed overview of chromatin accessibility and gene expression changes during the adipogenic differentiation of porcine intramuscular preadipocytes. Moreover, we propose a novel regulatory mechanism involving the VDR-SELENBP1 signaling axis in adipogenic differentiation.

Unveiling the macrophage dynamics in osteoarthritic joints: From inflammation to therapeutic strategies

Osteoarthritis (OA) is an incurable, painful, and debilitating joint disease affecting over 500 million people worldwide. The OA joint tissues are infiltrated by various immune cells, particularly macrophages, which are able to induce or perpetuate inflammation. Notably, synovitis and its macrophage component represent a target of interest for developing treatments. In this review, we describe the latest advances in understanding the heterogeneity of macrophage origins, phenotypes, and functions in the OA joint and the effect of current symptomatic therapies on these cells. We then highlight the therapeutic potential of anticytokines/chemokines, nano- and microdrug delivery, and future strategies to modulate macrophage functions in OA.

Genetic predisposition to thyrotoxicosis and onset of knee osteoarthritis

OBJECTIVE

Thyroid hormones have actions on cartilage, whereas the association between thyroid hormone related diseases and osteoarthritis (OA) are unclear. This study aims to investigate the association between thyrotoxicosis and OA.

METHODS

Summary-level genetic data of thyrotoxicosis were obtained from FinnGen cohorts (nCase = 10,569, nControl = 762,037). Summary-level data of OA were obtained from a large-scale genome-wide association study of UK Biobank (nCase = 40,659, nControl = 756,338). Single nucleotide polymorphisms (SNPs) robustly associated with thyrotoxicosis or OA were used as genetic instruments. A two-sample bidirectional Mendelian randomization (MR) analysis was designed to assess the effect of genetic predisposition of thyrotoxicosis on OA risk, as well as the reverse their relationship. The causal effect was estimated by Inverse-variance weighted method, with weighted median and MR-Egger as supplementary methods.

RESULTS

Genetic predisposition of thyrotoxicosis was associated with the onset of knee OA (autoimmune hyperthyroidism: odds ratio [OR]: 1.05, 95% confidence interval [CI]: 1.03-1.07, FDR < 0.001; thyrotoxicosis: OR: 1.05, 95% CI: 1.02-1.08, FDR = 0.016; thyrotoxicosis with diffuse goitre: OR: 1.04, 95% CI: 1.02-1.07, FDR = 0.003; other and/or unspecified thyrotoxicosis: OR: 1.05, 95% CI: 1.02-1.09, FDR = 0.003), whereas thyrotoxicosis was not associated with hip OA. In reverse MR analysis, genetic predisposition to OA was not associated with thyrotoxicosis. No pleiotropy was identified in the MR analyses. Sensitivity analyses indicated the robustness of the MR estimates.

CONCLUSION

This study provides MR evidence supporting causal association of thyrotoxicosis with knee OA in European population, whereas OA may have no causal effects on thyrotoxicosis.

The biphasic role of the infrapatellar fat pad in osteoarthritis

Osteoarthritis (OA) is a progressive degenerative disease resulting in joint deterioration. It is a whole organ disease characterized by cartilage degeneration and varying degrees of synovitis, involving pathological changes in all joint tissues, such as cartilage, subchondral bone, ligaments, meniscus, synovium, and infrapatellar fat pad (IPFP). IPFP is the largest adipose tissue structure in the knee joint and is composed of fat cells, immune cells and blood vessels. Moreover, IPFP is located close to the cartilage and bone surface so that it may reduce the impact of loading and absorb forces generated through the knee joint, and may have a protective role in joint health. IPFP has been shown to release various cytokines and adipokines that play pro-inflammatory and pro-catabolic roles in cartilage, promoting OA progression. Intra-articular injections of IPFP-derived mesenchymal stem cells and exosomes have been shown to reduce pain and prevent OA progression in patients with knee OA. Previous studies have shown that IPFP has a biphasic effect on OA progression. This article reviews the latest research progress of IPFP, discusses the role and mechanism of IPFP in OA, provide new intervention strategies for the treatment of OA. This article will also discuss the handling of IPFP during the procedure of total knee arthroplasty.

Sex-specific effects of injury and beta-adrenergic activation on metabolic and inflammatory mediators in a murine model of post-traumatic osteoarthritis

OBJECTIVE

Metabolic processes are intricately linked to the resolution of innate inflammation and tissue repair, two critical steps for treating post-traumatic osteoarthritis (PTOA). Based on lipolytic and immunoregulatory actions of norepinephrine, we hypothesized that intra-articular β-adrenergic receptor (βAR) stimulation would suppress PTOA-associated inflammation in the infrapatellar fat pad (IFP) and synovium.

DESIGN

We used the βAR agonist isoproterenol to perturb intra-articular metabolism 3.5 weeks after applying a non-invasive single-load compression injury to knees of 12-week-old male and female mice. We examined the acute effects of intra-articular isoproterenol treatment relative to saline on IFP histology, multiplex gene expression of synovium-IFP tissue, synovial fluid metabolomics, and mechanical allodynia.

RESULTS

Injured knees developed PTOA pathology characterized by heterotopic ossification, articular cartilage loss, and IFP atrophy and fibrosis. Isoproterenol suppressed the upregulation of pro-fibrotic genes and downregulated the expression of adipose genes and pro-inflammatory genes (Adam17, Cd14, Icam1, Csf1r, and Casp1) in injured joints of female (but not male) mice. Analysis of published single-cell RNA-seq data identified elevated catecholamine-associated gene expression in resident-like synovial-IFP macrophages after injury. Injury substantially altered synovial fluid metabolites by increasing amino acids, peptides, sphingolipids, phospholipids, bile acids, and dicarboxylic acids, but these changes were not appreciably altered by isoproterenol. Intra-articular injection of either isoproterenol or saline increased mechanical allodynia in female mice, whereas neither substance affected male mice.

CONCLUSIONS

Acute βAR activation altered synovial-IFP transcription in a sex and injury-dependent manner, suggesting that women with PTOA may be more sensitive than men to treatments targeting sympathetic neural signaling pathways.

Crosstalk between bone and brain in Alzheimer's disease: Mechanisms, applications, and perspectives

Alzheimer's disease (AD) is a neurodegenerative disease that involves multiple systems in the body. Numerous recent studies have revealed bidirectional crosstalk between the brain and bone, but the interaction between bone and brain in AD remains unclear. In this review, we summarize human studies of the association between bone and brain and provide an overview of their interactions and the underlying mechanisms in AD. We review the effects of AD on bone from the aspects of AD pathogenic proteins, AD risk genes, neurohormones, neuropeptides, neurotransmitters, brain-derived extracellular vesicles (EVs), and the autonomic nervous system. Correspondingly, we elucidate the underlying mechanisms of the involvement of bone in the pathogenesis of AD, including bone-derived hormones, bone marrow-derived cells, bone-derived EVs, and inflammation. On the basis of the crosstalk between bone and the brain, we propose potential strategies for the management of AD with the hope of offering novel perspectives on its prevention and treatment. HIGHLIGHTS: The pathogenesis of AD, along with its consequent changes in the brain, may involve disturbing bone homeostasis. Degenerative bone disorders may influence the progression of AD through a series of pathophysiological mechanisms. Therefore, relevant bone intervention strategies may be beneficial for the comprehensive management of AD.

Development and Validation of Diagnostic Models for Transcriptomic Signature Genes for Multiple Tissues in Osteoarthritis

Background

Progress in research on expression profiles in osteoarthritis (OA) has been limited to individual tissues within the joint, such as the synovium, cartilage, or meniscus. This study aimed to comprehensively analyze the common gene expression characteristics of various structures in OA and construct a diagnostic model.

Methods

Three datasets were selected: synovium, meniscus, and knee joint cartilage. Modular clustering and differential analysis of genes were used for further functional analyses and the construction of protein networks. Signature genes with the highest diagnostic potential were identified and verified using external gene datasets. The expression of these genes was validated in clinical samples by Real-time (RT)-qPCR and immunohistochemistry (IHC) staining. This study investigated the status of immune cells in OA by examining their infiltration.

Results

The merged OA dataset included 438 DEGs clustered into seven modules using WGCNA. The intersection of these DEGs with WGCNA modules identified 190 genes. Using Least Absolute Shrinkage and Selection Operator (LASSO) and Random Forest algorithms, nine signature genes were identified (CDADC1, PPFIBP1, ENO2, NOM1, SLC25A14, METTL2A, LINC01089, L3HYPDH, NPHP3), each demonstrating substantial diagnostic potential (areas under the curve from 0.701 to 0.925). Furthermore, dysregulation of various immune cells has also been observed.

Conclusion

CDADC1, PPFIBP1, ENO2, NOM1, SLC25A14, METTL2A, LINC01089, L3HYPDH, NPHP3 demonstrated significant diagnostic efficacy in OA and are involved in immune cell infiltration.

Logic-Based Strategy for Spatiotemporal Release of Dual Extracellular Vesicles in Osteoarthritis Treatment

To effectively treat osteoarthritis (OA), the existing inflammation must be reduced before the cartilage damage can be repaired; this cannot be achieved with a single type of extracellular vesicles (EVs). Here, a hydrogel complex with logic-gates function is proposed that can spatiotemporally controlled release two types of EVs: interleukin 10 (IL-10)+ EVs to promote M2 polarization of macrophage, and SRY-box transcription factor 9 (SOX9)+ EVs to increase cartilage matrix synthesis. Following dose-of-action screening, the dual EVs are loaded into a matrix metalloporoteinase 13 (MMP13)-sensitive self-assembled peptide hydrogel (KM13E) and polyethylene glycol diacrylate/gelatin methacryloyl-hydrogel microspheres (PGE), respectively. These materials are mixed to form a "microspheres-in-gel" KM13E@PGE system. In vitro, KM13E@PGE abruptly released IL-10+ EVs after 3 days and slowly released SOX9+ EVs for more than 30 days. In vivo, KM13E@PGE increased the CD206+ M2 macrophage proportion in the synovial tissue and decreased the tumor necrosis factor-α and IL-1β levels. The aggrecan and SOX9 expressions in the cartilage tissues are significantly elevated following inflammation subsidence. This performance is not achieved using anti-inflammatory or cartilage repair therapy alone. The present study provides an injectable, integrated delivery system with spatiotemporal control release of dual EVs, and may inspire logic-gates strategies for OA treatment.

Role of joint adipose tissues in osteoarthritis

Osteoarthritis (OA) is the most common musculoskeletal disease, without any curative treatment. Obesity being the main modifiable risk factor for OA, much attention focused on the role of adipose tissues (AT). In addition to the involvement of visceral and subcutaneous AT via systemic ways, many arguments also highlight the involvement of local AT, present in joint tissues. Local AT include intra-articular AT (IAAT), which border the synovium, and bone marrow AT (BMAT) localized within marrow cavities in the bones. This review describes the known features and involvement of IAAT and BMAT in joint homeostasis and OA. Recent findings evidence that alteration in magnetic resonance imaging signal intensity of infrapatellar fat pad can be predictive of the development and progression of knee OA. IAAT and synovium are partners of the same functional unit; IAAT playing an early and pivotal role in synovial inflammation and fibrosis and OA pain. BMAT, whose functions have only recently begun to be studied, is in close functional interaction with its microenvironment. The volume and molecular profile of BMAT change according to the pathophysiological context, enabling fine regulation of haematopoiesis and bone metabolism. Although its role in OA has not yet been studied, the localization of BMAT, its functions and the importance of the bone remodelling processes that occur in OA argue in favour of a role for BMAT in OA.

Single nucleus and spatial transcriptomic profiling of healthy human hamstring tendon

The molecular and cellular basis of health in human tendons remains poorly understood. Among human tendons, hamstring tendon has markedly low pathology and can provide a prototypic healthy tendon reference. The aim of this study was to determine the transcriptomes and location of all cell types in healthy hamstring tendon. Using single nucleus RNA sequencing, we profiled the transcriptomes of 10 533 nuclei from four healthy donors and identified 12 distinct cell types. We confirmed the presence of two fibroblast cell types, endothelial cells, mural cells, and immune cells, and identified cell types previously unreported in tendons, including different skeletal muscle cell types, satellite cells, adipocytes, and undefined nervous system cells. The location of these cell types within tendon was defined using spatial transcriptomics and imaging, and potential transcriptional networks and cell-cell interactions were analyzed. We demonstrate that fibroblasts have the highest number of potential cell-cell interactions in our dataset, are present throughout the tendon, and play an important role in the production and organization of extracellular matrix, thus confirming their role as key regulators of hamstring tendon homeostasis. Overall, our findings underscore the complexity of the cellular networks that underpin healthy human tendon function and the central role of fibroblasts as key regulators of hamstring tendon tissue homeostasis.

Applications of Hydrogels in Osteoarthritis Treatment

This review critically evaluates advancements in multifunctional hydrogels, particularly focusing on their applications in osteoarthritis (OA) therapy. As research evolves from traditional natural materials, there is a significant shift towards synthetic and composite hydrogels, known for their superior mechanical properties and enhanced biodegradability. This review spotlights novel applications such as injectable hydrogels, microneedle technology, and responsive hydrogels, which have revolutionized OA treatment through targeted and efficient therapeutic delivery. Moreover, it discusses innovative hydrogel materials, including protein-based and superlubricating hydrogels, for their potential to reduce joint friction and inflammation. The integration of bioactive compounds within hydrogels to augment therapeutic efficacy is also examined. Furthermore, the review anticipates continued technological advancements and a deeper understanding of hydrogel-based OA therapies. It emphasizes the potential of hydrogels to provide tailored, minimally invasive treatments, thus highlighting their critical role in advancing the dynamic field of biomaterial science for OA management.

Single cell transcriptome profiling of infrapatellar fat pad highlights the role of interstitial inflammatory fibroblasts in osteoarthritis

OBJECTIVES

Osteoarthritis (OA) is a whole-joint disease in which the role of the infrapatellar fat pad (IFP) in its pathogenesis is unclear. Our study explored the cellular heterogeneity of IFP to understand OA and identify therapeutic targets.

METHODS

Single-cell and single-nuclei RNA sequencing were used to analyze 10 IFP samples, comprising 5 from OA patients and 5 from healthy controls. Analyses included differential gene expression, enrichment, pseudotime trajectory, and cellular communication, along with comparative studies with visceral and subcutaneous fats. Key subcluster and pathways were validated using multiplex immunohistochemistry.

RESULTS

The scRNA-seq performed on the IFPs of the OA and control group profiled the gene expressions of over 49,674 cells belonging to 11 major cell types. We discovered that adipose stem and progenitor cells (ASPCs), contributing to the formation of both adipocytes and synovial-lining fibroblasts (SLF). Interstitial inflammatory fibroblasts (iiFBs) were a subcluster of ASPCs that exhibit notable pro-inflammatory and proliferative characteristics. We identified four adipocyte subtypes, with one subtype showing a reduced lipid synthesis ability. Furthermore, iiFBs modulated the activities of macrophages and T cells in the IFP. Compared to subcutaneous and visceral adipose tissues, iiFBs represented a distinctive subpopulation of ASPCs in IFP that regulated cartilage proliferation through the MK pathway.

CONCLUSION

This study presents a comprehensive single-cell transcriptomic atlas of IFP, uncovering its complex cellular landscape and potential impact on OA progression. Our findings highlight the role of iiFBs in OA, especially through MK pathway, opening new avenues for understanding OA pathogenesis and developing novel targeted therapies.

Spermidine ameliorates osteoarthritis via altering macrophage polarization

OBJECTIVE

Spermidine (SPD) is an anti-aging natural substance, and it exerts effects through anti-apoptosis and anti-inflammation. However, the specific protective mechanism of SPD in osteoarthritis (OA) remains unclear. Here, we explored the role of SPD on the articular cartilage and the synovial tissue, and tested whether the drug would regulate the polarization of synovial macrophages by in vivo and in vitro experiments.

METHODS

By constructing an OA model in mice, we preliminarily explored the protective effect of SPD on the articular cartilage and the synovial tissue. Meanwhile, we isolated and cultured human primary chondrocytes and bone marrow-derived macrophages (BMDMs), and prepared a conditioned medium (CM) to explore the specific protective effect of SPD in vitro.

RESULTS

We found that SPD alleviated cartilage degeneration and synovitis, increased M2 polarization and decreased M1 polarization in synovial macrophages. In vitro experiments, SPD inhibited ERK MAPK and p65/NF-κB signaling in macrophages, and transformed macrophages from M1 to M2 subtypes. Interestingly, SPD had no direct protective effect on chondrocytes in vitro; however, the conditioned medium (CM) from M1 macrophages treated with SPD promoted the anabolism and inhibited the catabolism of chondrocytes. Moreover, this CM markedly suppressed IL-1β-induced p38/JNK MAPK signaling pathway activation in chondrocytes.

CONCLUSIONS

This work provides new perspectives on the role of SPD in OA. SPD does not directly target chondrocytes, but can ameliorate the degradation of articular cartilage through regulating M1/M2 polarization of synovial macrophages. Hence, SPD is expected to be the potential therapy for OA.