Single-cell RNA-seq analysis identifies unique chondrocyte subsets and reveals involvement of ferroptosis in human intervertebral disc degeneration

Screening of NSAIDs library identifies Tinoridine as a novel ferroptosis inhibitor for potential intervertebral disc degeneration therapy

Low back pain (LBP) may profoundly impact the quality of life across the globe, and intervertebral disc degeneration (IVDD) is the major cause of LBP; however, targeted pharmaceutical interventions for IVDD are still lacking. Ferroptosis is a novel form of iron-dependent programmed cell death. Studies have showed that ferroptosis may closely associate with IVDD; thus, targeting ferroptosis may have great potential for IVDD therapy. Non-steroidal anti-inflammatory drugs (NSAIDs) are the first-line medications for LBP, while nuclear factor-erythroid 2-related factor-2 (Nrf2) is a key inhibitory protein for ferroptosis. In the current study, we conducted a molecular docking screening between NSAIDs library and Nrf2 protein. Tinoridine was shown to have a high binding affinity to Nrf2. The in vitro study in nucleus pulposus (NP) cells showed that Tinoridine may promote the expression and activity of Nrf2, it may also rescue RSL3-induced ferroptosis in NP cells. Knockdown of Nrf2 reverses the protective effect of Tinoridine on RSL3-induced ferroptosis in NP cells, suggesting that the inhibitory effect of Tinoridine on ferroptosis is through Nrf2. In vivo study demonstrated that Tinoridine may attenuate the progression of IVDD in rats. As NSAIDs are already clinically used for LBP therapy, the current study supports Tinoridine's application from the view of ferroptosis inhibition.

The role of oxidative stress in intervertebral disc degeneration: Mechanisms and therapeutic implications

Oxidative stress is one of the main driving mechanisms of intervertebral disc degeneration(IDD). Oxidative stress has been associated with inflammation in the intervertebral disc, cellular senescence, autophagy, and epigenetics of intervertebral disc cells. It and the above pathological mechanisms are closely linked through the common hub reactive oxygen species(ROS), and promote each other in the process of disc degeneration and promote the development of the disease. This reveals the important role of oxidative stress in the process of IDD, and the importance and great potential of IDD therapy targeting oxidative stress. The efficacy of traditional therapy is unstable or cannot be maintained. In recent years, due to the rise of materials science, many bioactive functional materials have been applied in the treatment of IDD, and through the combination with traditional drugs, satisfactory efficacy has been achieved. At present, the research review of antioxidant bioactive materials in the treatment of IDD is not complete. Based on the existing studies, the mechanism of oxidative stress in IDD and the common antioxidant therapy were summarized in this paper, and the strategies based on emerging bioactive materials were reviewed.

Advancing skeletal health and disease research with single-cell RNA sequencing

Orthopedic conditions have emerged as global health concerns, impacting approximately 1.7 billion individuals worldwide. However, the limited understanding of the underlying pathological processes at the cellular and molecular level has hindered the development of comprehensive treatment options for these disorders. The advent of single-cell RNA sequencing (scRNA-seq) technology has revolutionized biomedical research by enabling detailed examination of cellular and molecular diversity. Nevertheless, investigating mechanisms at the single-cell level in highly mineralized skeletal tissue poses technical challenges. In this comprehensive review, we present a streamlined approach to obtaining high-quality single cells from skeletal tissue and provide an overview of existing scRNA-seq technologies employed in skeletal studies along with practical bioinformatic analysis pipelines. By utilizing these methodologies, crucial insights into the developmental dynamics, maintenance of homeostasis, and pathological processes involved in spine, joint, bone, muscle, and tendon disorders have been uncovered. Specifically focusing on the joint diseases of degenerative disc disease, osteoarthritis, and rheumatoid arthritis using scRNA-seq has provided novel insights and a more nuanced comprehension. These findings have paved the way for discovering novel therapeutic targets that offer potential benefits to patients suffering from diverse skeletal disorders.

Single-cell RNA sequencing reveals the MIF/ACKR3 receptor-ligand interaction between neutrophils and nucleus pulposus cells in intervertebral disc degeneration

OBJECTIVES

To unravel the heterogeneity and function of microenvironmental neutrophils during intervertebral disc degeneration (IDD).

METHODS

Single-cell RNA sequencing (scRNA-seq) was utilized to dissect the cellular landscape of neutrophils in intervertebral disc (IVD) tissues and their crosstalk with nucleus pulposus cells (NPCs). The expression levels of macrophage migration inhibitory factor (MIF) and ACKR3 in IVD tissues were detected. The MIF/ACKR3 axis was identified and its effects on IDD were investigated in vitro and in vivo.

RESULTS

We sequenced here 71520 single cells from 5 control and 9 degenerated IVD samples using scRNA-seq. We identified a unique cluster of neutrophils abundant in degenerated IVD tissues that highly expressed MIF and was functionally enriched in extracellular matrix organization (ECMO). Cell-to-cell communication analyses showed that this ECMO-neutrophil subpopulation was closely interacted with an effector NPCs subtype, which displayed high expression of ACKR3. Further analyses revealed that MIF was positively correlated with ACKR3 and functioned via directly binding to ACKR3 on effector NPCs. MIF inhibition attenuated degenerative changes of NPCs and extracellular matrix, which could be partially reversed by ACKR3 overexpression. Clinically, a significant correlation of high MIF/ACKR3 expression with advanced IDD grade was observed. Furthermore, we also found a positive association between MIF+ ECMO-neutrophil counts and ACKR3+ effector NPCs density as well as higher expression of the MIF/ACKR3 signaling in areas where these two cell types were neighbors.

CONCLUSIONS

These data suggest that ECMO-neutrophil promotes IDD progression by their communication with NPCs via the MIF/ACKR3 axis, which may shed light on therapeutic strategies.

Shared and Compartment-Specific Processes in Nucleus Pulposus and Annulus Fibrosus During Intervertebral Disc Degeneration

Elucidating how cell populations promote onset and progression of intervertebral disc degeneration (IDD) has the potential to enable more precise therapeutic targeting of cells and mechanisms. Single-cell RNA-sequencing (scRNA-seq) is performed on surgically separated annulus fibrosus (AF) (19,978; 26,983 cells) and nucleus pulposus (NP) (20,884; 24,489 cells) from healthy and diseased human intervertebral discs (IVD). In both tissue types, depletion of cell subsets involved in maintenance of healthy IVD is observed, specifically the immature cell subsets - fibroblast progenitors and stem cells - indicative of an impairment of normal tissue self-renewal. Tissue-specific changes are also identified. In NP, several fibrotic populations are increased in degenerated IVD, indicating tissue-remodeling. In degenerated AF, a novel disease-associated subset is identified, which expresses disease-promoting genes. It is associated with pathogenic biological processes and the main gene regulatory networks include thrombospondin signaling and FOXO1 transcription factor. In NP and AF cells thrombospondin protein promoted expression of genes associated with TGFβ/fibrosis signaling, angiogenesis, and nervous system development. The data reveal new insights of both shared and tissue-specific changes in specific cell populations in AF and NP during IVD degeneration. These identified mechanisms and molecules are novel and more precise targets for IDD prevention and treatment.

Single-cell RNA-seq analysis reveals the Wnt/Ca2+ signaling pathway with inflammation, apoptosis in nucleus pulposus degeneration

BACKGROUND

Increasing studies have shown degeneration of nucleus pulposus cells (NPCs) as an critical part of the progression of intervertebral disc degeneration (IVDD). However, there are relatively few studies on single-cell transcriptome contrasts in human degenerated NPCs. Moreover, differences in Wnt/Ca2+ signaling in human degenerated nucleus pulposus cells have not been elucidated. The aim of this study is to investigate the differential expression of Wnt/Ca2+ signaling pathway between normal and degenerated nucleus pulposus cells in humans and try to investigate its mechanism.

METHODS

We performed bioinformatics analysis using our previously published findings to construct single cell expression profiles of normal and degenerated nucleus pulposus. Then, in-depth differential analysis was used to characterize the expression of Wnt/Ca2+ signaling pathway between normal and degenerated nucleus pulposus cells in humans.

RESULTS

The obtained cell data were clustered into five different chondrocytes clusters, which chondrocyte 4 and chondrocyte 5 mainly accounted for a high proportion in degenerated nucleus pulposus tissues, but rarely in normal nucleus pulposus tissues. Genes associated within the Wnt/Ca2+ signaling pathway, such as Wnt5B, FZD1, PLC (PLCB1), CaN (PPP3CA) and NAFATC1 are mainly present in chondrocyte 3, chondrocyte 4 and chondrocyte 5 from degenerated nucleus pulposus tissues. In addition, as a receptor that activates Wnt signaling pathway, LRP5 is mainly highly expressed in chondrocyte 5 of degenerated nucleus pulposus cells. Six genes, ANGPTL4, PTGES, IGFBP3, GDF15, TRIB3 and TNFRSF10B, which are associated with apoptosis and inflammatory responses, and are widespread in chondrocyte 4 and chondrocyte 5, may be closely related to degenerative of nucleus pulposus cells.

CONCLUSIONS

Single-cell RNA sequencing revealed differential expression of Wnt/Ca2+ signaling in human normal and degenerated nucleus pulposus cells, and this differential expression may be closely related to the abundance of chondrocyte 4 and chondrocyte 5 in degenerated nucleus pulposus cells. In degenerated nucleus pulposus cells, LRP5 activate Wnt5B, which promotes nucleus pulposus cell apoptosis and inflammatory response by regulating the Wnt/Ca2+ signaling pathway, thereby promoting disc degeneration. ANGPTL4, IGFBP3, PTGES in chondrocyte 4 and TRIB3, GDF15, TNFRSF10B in chondrocyte 5 may play an important role in this process.

Exploration of the mode of death and potential death mechanisms of nucleus pulposus cells

Intervertebral disc degeneration (IVDD) is a common chronic orthopaedic disease in orthopaedics that imposes a heavy economic burden on people and society. Although it is well established that IVDD is associated with genetic susceptibility, ageing and obesity, its pathogenesis remains incompletely understood. Previously, IVDD was thought to occur because of excessive mechanical loading leading to destruction of nucleus pulposus cells (NPCs), but studies have shown that IVDD is a much more complex process associated with inflammation, metabolic factors and NPCs death and can involve all parts of the disc, characterized by causing NPCs death and extracellular matrix (ECM) degradation. The damage pattern of NPCs in IVDD is like that of some programmed cell death, suggesting that IVDD is associated with programmed cell death. Although apoptosis and pyroptosis of NPCs have been studied in IVDD, the pathogenesis of intervertebral disc degeneration can still not be fully elucidated by using only traditional cell death modalities. With increasing research, some new modes of cell death, PANoptosis, ferroptosis and senescence have been found to be closely related to intervertebral disc degeneration. Among these, PANoptosis combines essential elements of pyroptosis, apoptosis and necroptosis to form a highly coordinated and dynamically balanced programmed inflammatory cell death process. Furthermore, we believe that PANoptosis may also crosstalk with pyroptosis and senescence. Therefore, we review the progress of research on multiple deaths of NPCs in IVDD to provide guidance for clinical treatment.

Recent Advances in Molecular Pathways and Therapeutic Implications for Peptic Ulcer Management: A Comprehensive Review

Peptic ulcers, recognized for their erosive impact on the gastrointestinal mucosa, present a considerable challenge in gastroenterology. Epidemiological insights underscore the global prevalence of peptic ulcers, affecting 5-10+% of individuals, with a yearly incidence of 0.3 to 1.9 cases per thousand. Recent decades have witnessed a decline in complications, attributed to improved diagnostics and therapeutic advancements. The review deepens into H. pylori-associated and NSAID-induced ulcers, emphasizing their distinct prevalence in developing and industrialized nations, respectively. Despite advancements, managing peptic ulcers remains challenging, notably in H. pylori-infected individuals facing recurrence and the rise of antibiotic resistance. The pathophysiology unravels the delicate balance between protective and destructive factors, including the intricate molecular mechanisms involving inflammatory mediators such as TNF-α, ILs, and prostaglandins. Genetic and ethnic factors, rare contributors, and recent molecular insights further enhance our understanding of peptic ulcer development. Diagnostic approaches are pivotal, with upper gastrointestinal endoscopy standing as the gold standard. Current treatment strategies focus on H. pylori eradication, NSAID discontinuation, and proton pump inhibitors. Surgical options become imperative for refractory cases, emphasizing a comprehensive approach. Advances include tailored H. pylori regimens, the emergence of vonoprazan, and ongoing vaccine development. Challenges persist, primarily in antibiotic resistance, side effects of acid suppressants, and translating natural compounds into standardized therapies. Promising avenues include the potential H. pylori vaccine and the exploration of natural compounds, with monoterpenes showing therapeutic promise. This review serves as a compass, guiding healthcare professionals, researchers, and policymakers through the intricate landscape of peptic ulcer management.

SSR1 and CKAP4 as potential biomarkers for intervertebral disc degeneration based on integrated bioinformatics analysis

Background

Intervertebral disc degeneration (IDD) is a significant cause of low back pain and poses a significant public health concern. Genetic factors play a crucial role in IDD, highlighting the need for a better understanding of the underlying mechanisms.

Aim

The aim of this study was to identify potential IDD-related biomarkers using a comprehensive bioinformatics approach and validate them in vitro.

Materials and Methods

In this study, we employed several analytical approaches to identify the key genes involved in IDD. We utilized weighted gene coexpression network analysis (WGCNA), MCODE, LASSO algorithms, and ROC curves to identify the key genes. Additionally, immune infiltrating analysis and a single-cell sequencing dataset were utilized to further explore the characteristics of the key genes. Finally, we conducted in vitro experiments on human disc tissues to validate the significance of these key genes in IDD.

Results

we obtained gene expression profiles from the GEO database (GSE23130 and GSE15227) and identified 1015 DEGs associated with IDD. Using WGCNA, we identified the blue module as significantly related to IDD. Among the DEGs, we identified 47 hub genes that overlapped with the genes in the blue module, based on criteria of |logFC| ≥ 2.0 and p.adj <0.05. Further analysis using both MCODE and LASSO algorithms enabled us to identify five key genes, of which CKAP4 and SSR1 were validated by GSE70362, demonstrating significant diagnostic value for IDD. Additionally, immune infiltrating analysis revealed that monocytes were significantly correlated with the two key genes. We also analyzed a single-cell sequencing dataset, GSE199866, which showed that both CKAP4 and SSR1 were highly expressed in fibrocartilage chondrocytes. Finally, we validated our findings in vitro by performing real time polymerase chain reaction (RT-PCR) and immunohistochemistry (IHC) on 30 human disc samples. Our results showed that CKAP4 and SSR1 were upregulated in degenerated disc samples. Taken together, our findings suggest that CKAP4 and SSR1 have the potential to serve as disease biomarkers for IDD.

Ferroptosis: A Frontier in Osteoporosis

Reduced bone mass and degeneration of the microarchitecture of bone tissue are the hallmarks of osteoporosis, a bone metabolic disease that increases skeletal fragility and fracture susceptibility. Osteoporosis is primarily caused by unbalanced bone remodeling, in which bone synthesis is outpaced by bone resorption caused by osteoclasts. Along with the bone-building vitamins calcium and vitamin D, typical medications for treating osteoporosis include bisphosphonates and calcitonin. The present therapies effectively stop osteoclast activation that is too high, however they come with varying degrees of negative effects. Numerous factors can contribute to osteoporosis, which is characterized by a loss of bone mass and density due to the deterioration of the bone's microstructure, which makes the bone more fragile. As a result, it is a systemic bone condition that makes patients more likely to fracture. Interest in the function of ferroptosis in the pathophysiology of osteoporosis is developing. In this review, we go through the shape of the cell, the fundamental mechanisms of ferroptosis, the relationship between osteoclasts and osteoblasts, the association between ferroptosis and diabetic osteoporosis, steroid-induced osteoporosis, and the relationship between ferroptosis and postmenopausal osteoporosis. The functions of ferroptosis and osteoporosis in cellular function, signaling cascades, pharmacological inhibition, and gene silencing have been better understood thanks to recent advances in biomedical research.

Analysis of joint protein expression profile in anterior disc displacement of TMJ with or without OA

OBJECTIVE

Anterior disc displacement (ADD) is a common clinical issue and may cause osteoarthritis (OA). However, the research of protein changes in synovial fluid as disease development marker and potential treatment clue is still insufficient.

MATERIALS AND METHODS

We conducted the high-resolution mass spectrometry (MS) of synovial fluid collected from 60 patients with normal disk position to ADD and ADD with osteoarthritis (OA). The proteins with significant changes among the 3 groups were analyzed by biological information and further validated by in primary rat condyle chondrocytes and OA animal model.

RESULTS

FGL2, THBS4, TNC, FN1, OMD etc. were significantly increased in ADD without OA (p < 0.05), which reflected the active extracellular matrix and collagen metabolism. FGFR1, FBLN2, GRB2 etc. were significantly increased in ADD with OA group (p < 0.05), which revealed an association with apoptosis and ferroptosis. Proteins such as P4HB, CBLN4, FHL1, VIM continuously increase in the whole disease progress (p < 0.05). Both the in vitro and in vivo results are consistent with protein changes detected in MS profile.

CONCLUSION

This study firstly provides the expression changes of proteins from normal disc condyle relationship toward ADD with OA, which can be selected and studied further as disease progress marker and potential treatment targets.

Serglycin secreted by late-stage nucleus pulposus cells is a biomarker of intervertebral disc degeneration

Intervertebral disc degeneration is a natural process during aging and a leading cause of lower back pain. Here, we generate a comprehensive atlas of nucleus pulposus cells using single-cell RNA-seq analysis of human nucleus pulposus tissues (three males and four females, age 41.14 ± 18.01 years). We identify fibrotic late-stage nucleus pulposus cells characterized by upregulation of serglycin expression which facilitate the local inflammatory response by promoting the infiltration of inflammatory cytokines and macrophages. Finally, we discover that daphnetin, a potential serglycin ligand, substantially mitigates the local inflammatory response by downregulating serglycin expression in an in vivo mouse model, thus alleviating intervertebral disc degeneration. Taken together, we identify late-stage nucleus pulposus cells and confirm the potential mechanism by which serglycin regulates intervertebral disc degeneration. Our findings indicate that serglycin is a latent biomarker of intervertebral disc degeneration and may contribute to development of diagnostic and therapeutic strategies.

Bioinformatics Analysis and Identification of Ferroptosis-Related Hub Genes in Intervertebral Disc Degeneration

Approximately 80% of individuals encounter lower back pain (LBP), a prevalent clinical issue largely attributed to intervertebral disc degeneration (IDD). Ferroptosis is an iron-dependent lipid peroxidation-driven cell death, and there is growing evidence that ferroptosis plays an important role in various human diseases. However, the underlying mechanism of ferroptosis in IDD remains unclear. This study aims to reveal the potential hub genes and related pathways of ferroptosis in the pathogenesis and progression of IDD. In this study, we analyzed three microarray datasets from the GEO database. Additionally, we downloaded ferroptosis-related genes from FerrDb-V2 and extracted apoptosis-related genes from UniProt as a control to show the specificity of ferroptosis. Weighted gene co-expression network analysis (WGCNA) was performed to identify the IDD-related module genes. Then, ferroptosis-related genes and apoptosis-related genes were separately overlapped with the IDD-related module genes, resulting in the identification of 35 ferroptosis-related module genes (FRMG) and 142 apoptosis-related module genes (ARMG). Furthermore, we performed functional enrichment analysis and protein-protein interaction network, and Cytoscape along with CytoHubba was used to identify the hub genes. Finally, logistic regression models were constructed and identified two hub FRMGs (PTEN and EGFR) and one hub ARMG (CTNNB1), which could distinguish IDD patients from controls (P < 0.05). The areas under the ROC curves were 0.792 and 0.730, respectively, suggesting that ferroptosis is more specific than apoptosis in IDD. In conclusion, this study provided fresh perspectives on ferroptosis in the pathogenesis and progression of IDD that can be used to evaluate potential biomarker genes and therapeutic targets.

The pathological mechanisms of circRNAs in mediating intervertebral disc degeneration

Lower back pain (LBP) is a worldwide health problem associated with significant economic and social burden. Intervertebral disc degeneration (IVDD) is a leading cause of LBP. Several studies show that the death of nucleus pulposus cells (NPCs), abnormal metabolism of the extracellular matrix (ECM), and inflammatory response are the key mechanisms behind the pathogenesis of IVDD. Circular RNAs (circRNAs) are key regulators of gene expression and play a significant role in regulating NPCs death, ECM homeostasis, and inflammatory response by acting as microRNAs (miRNAs) sponges in IVDD. However, the regulatory role of circRNAs in mediating IVDD remains unknown. This review comprehensively describes the normal anatomic structure and function of IVD, the pathogenesis of IVDD, the characteristics, synthesis, mechanisms, and function of circRNAs. Moreover, we highlighted the 23 circRNAs that mediate ECM metabolism, 16 circRNAs that mediate NPCs apoptosis, circ_0004354 and circ_0040039 that mediate NPCs pyroptosis, and 5 circRNAs that mediate inflammatory response in IVDD. In addition, this review presents suggestions for future studies, such as the need for further investigation on ferroptosis-related circRNAs in IVDD. This review could provide novel insights into the pathogenesis and treatment of IVDD.

Spatially multicellular variability of intervertebral disc degeneration by comparative single-cell analysis

Previous studies have revealed cellular heterogeneity in intervertebral discs (IVDs). However, the cellular and molecular alteration patterns of cell populations during degenerative progression remain to be fully elucidated. To illustrate the cellular and molecular alteration of cell populations in intervertebral disc degeneration (IDD), we perform single cell RNA sequencing on cells from four anatomic sites of healthy and degenerative goat IVDs. EGLN3+ StressCs, TGFBR3+ HomCs and GPRC5A+ RegCs exhibit the characteristics associated with resistance to stress, maintaining homeostasis and repairing, respectively. The frequencies and signatures of these cell clusters fluctuate with IDD. Notably, the chondrogenic differentiation programme of PROCR+ progenitor cells is altered by IDD, while notochord cells turn to stemness exhaustion. In addition, we characterise CAV1+ endothelial cells that communicate with chondrocytes through multiple signalling pathways in degenerative IVDs. Our comprehensive analysis identifies the variability of key cell clusters and critical regulatory networks responding to IDD, which will facilitate in-depth investigation of therapeutic strategies for IDD.

Role of ferroptosis and immune infiltration in intervertebral disc degeneration: novel insights from bioinformatics analyses

Background: Intervertebral disc degeneration (IVDD), which contributes to stenosis of the spinal segment, commonly causes lower back pain. The process of IVDD degradation entails gradual structural adjustments accompanied by extreme transformations in metabolic homeostasis. However, the molecular and cellular mechanisms associated with IVDD are poorly understood. Methods: The RNA-sequencing datasets GSE34095 and GSE56081 were obtained from the Gene Expression Omnibus (GEO) database. Ferroptosis-related differentially expressed genes (DEGs) were identified from these gene sets. The protein-protein interaction (PPI) network was established and visualized using the STRING database and Cytoscape software, and the key functional modules of ferroptosis-related genes were identified. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses were performed on the DEGs. Weighted gene co-expression network analysis (WGCNA), immune infiltration analysis in the GEO database, and other GSE series were used as validation datasets. The xCELL algorithm was performed to investigate the immune cell infiltration differences between the degenerated IVDD and control groups. Results: The major genes involved in nucleus pulposus tissue immune infiltration and ferroptosis-related genes were mined by bioinformatics analysis. A total of 3,056 DEGs were obtained between the IVDD tissue and control groups. The DEGs were enriched in the cell cycle; apoptosis; necroptosis; and the PI3K-Akt, Hippo, and HIF-1 signaling pathways. PCR and Western blot techniques were utilized to confirm the differential ferroptosis-related genes. The results indicated that the protein expression levels of NCOA4 and PCBP1 were elevated, while the protein expression level of GPX4 was reduced in NPCs following IL-1β treatment. Our study has found that severe disc tissue degeneration leads to a noteworthy increase in the expression of CD8A in naive T cells, CCR7 in memory CD4+ cells, GZMB in natural killer (NK) cells, and CD163 and CD45 in macrophages. Conclusion: Our data demonstrate that ferroptosis occurs in IVDD, suggesting that ferroptosis may also increase IVDD improvement by triggering immune infiltration. This work was conducted to further understand IVDD pathogenesis and identify new treatment strategies.

Single-cell sequencing: New insights for intervertebral disc degeneration

Over the past decade, single-cell RNA sequencing (scRNA-seq) has revolutionized research on biological mechanisms of diseases. Moreover, this technique has been utilized to identify and characterize unique cell types and subpopulations, thereby illuminating cellular heterogeneity. The true value of scRNA-seq lies in its ability to detect transcriptional alterations or perturbed pathways within specific cell types under pathological conditions. In the context of intervertebral disc degeneration (IVDD), the pathophysiological foundation is largely rooted in inflammation. The primary target cells of IVDD are nucleus pulposus cells, annulus fibrosus cells, cartilage endplate cells, and macrophages. The advancements in scRNA-seq technology have triggered remarkable progress in IVDD treatment, leading to breakthroughs in the identification of cell subsets, functional analysis, novel therapeutic targets, and the differentiation and development of various cell types. This review is the first of its kind to introduce the application of scRNA-seq techniques in IVDD, with a focus on the most recent scRNA-seq studies that have defined the populations of various cell types and specific cell-cell interactions in IVDD. Furthermore, we highlight several promising future research directions for scRNA-seq in IVDD.

Stiff Substrate Induces Nucleus Pulposus Cell Ferroptosis via YAP and N-Cadherin Mediated Mechanotransduction

Increased tissue stiffness is associated with various pathological processes, such as fibrosis, inflammation, and aging. The matrix stiffness of the nucleus pulposus (NP) tissues increases gradually during intervertebral disc degeneration (IDD), while the mechanism through which NP cells sense and react to matrix stiffness remains unclear. In this study, the results indicate that ferroptosis is involved in stiff substrate-induced NP cell death. The expression of acyl-CoA synthetase long-chain family member 4 (ACSL4) increases in NP cells of the stiff group, which mediates lipid peroxidation and ferroptosis in NP cells. In addition, stiff substrate activates the hippo signaling cascade and induces the nuclear translocation of yes-associated protein (YAP). Interestingly, inhibition of YAP is efficient to reverse the increase of ACSL4 expression caused by matrix stiffness. Furthermore, stiff substrate suppresses the expression of N-cadherin in NP cells. N-cadherin overexpression can inhibit YAP nuclear translocation via the formation of the N-cadherin/β-catenin/YAP complex, and reverse matrix stiffness-induced ferroptosis in NP cells. Finally, the effects of YAP inhibition and N-cadherin overexpression on IDD progression are further illustrated in animal models. These findings reveal a new mechanism of mechanotransduction in NP cells, providing novel insights into the development of therapies for the treatment of IDD.

The clock transcription factor BMAL1 is a key regulator of extracellular matrix homeostasis and cell fate in the intervertebral disc

The circadian clock in mammals temporally coordinates physiological and behavioural processes to anticipate daily rhythmic changes in their environment. Chronic disruption to circadian rhythms (e.g., through ageing or shift work) is thought to contribute to a multitude of diseases, including degeneration of the musculoskeletal system. The intervertebral disc (IVD) in the spine contains circadian clocks which control ∼6% of the transcriptome in a rhythmic manner, including key genes involved in extracellular matrix (ECM) homeostasis. However, it remains largely unknown to what extent the local IVD molecular clock is required to drive rhythmic gene transcription and IVD physiology. In this work, we identified profound age-related changes of ECM microarchitecture and an endochondral ossification-like phenotype in the annulus fibrosus (AF) region of the IVD in the Col2a1-Bmal1 knockout mice. Circadian time series RNA-Seq of the whole IVD in Bmal1 knockout revealed loss of circadian patterns in gene expression, with an unexpected emergence of 12 h ultradian rhythms, including FOXO transcription factors. Further RNA sequencing of the AF tissue identified region-specific changes in gene expression, evidencing a loss of AF phenotype markers and a dysregulation of ECM and FOXO pathways in Bmal1 knockout mice. Consistent with an up-regulation of FOXO1 mRNA and protein levels in Bmal1 knockout IVDs, inhibition of FOXO1 in AF cells suppressed their osteogenic differentiation. Collectively, these data highlight the importance of the local molecular clock mechanism in the maintenance of the cell fate and ECM homeostasis of the IVD. Further studies may identify potential new molecular targets for alleviating IVD degeneration.

Identification of cancer stemness and M2 macrophage-associated biomarkers in lung adenocarcinoma

Objective

Cancer stemness and M2 macrophages are intimately linked to the prognosis of lung adenocarcinoma (LUAD). For this reason, this investigation sought to identify the key genes relevant to cancer stemness and M2 macrophages, explore the relationship between these genes and clinical characteristics, and determine the potential mechanism.

Methods

LUAD transcriptomic data was analyzed from The Cancer Genome Atlas (TCGA) as well as the Gene Expression Omnibus databases. Differential expression analysis was performed to discern abnormally expressed genes between LUAD and control samples in TCGA cohort. The Cell type Identification by Estimating Relative Subsets of RNA Transcripts (CIBERSORT) algorithm was applied to determine varyingly infiltrated immune cells in LUAD compared with the control samples in TCGA cohort. Weighted correlation network analysis (WGCNA) was performed to identify genes associated with mRNA expression-based stemness index (mRNAsi) and M2 macrophages. Least absolute shrinkage and selection operator (LASSO), RandomForest (RF) and support vector machine-recursive feature elimination (SVM-RFE) machine learning methods were conducted to detect gene signatures. Global survival evaluation (Kaplan-Meier curve) was applied to investigate the relationship between gene signatures and the survival time of LUAD patients. Receiver operating characteristic (ROC) curves were produced to define biomarkers relevant to diagnosis. Gene Set Enrichment Analysis (GSEA) was performed to probe the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways related to diagnostic biomarkers. The public single-cell dataset of LUAD (GSE123902) was used to investigate the expression differences of diagnostic biomarkers in various cell types in LUAD. Real-time quantitative PCR (qRT-PCR) was performed to confirm key genes in lung adenocarcinoma cells.

Results

A total of 5,410 differentialy expressed genes (DEGs) as well as 15 differentially infiltrated immune cells were identified between LUAD and control sepcimens in TCGA cohort. Thirty-seven DEGs were associated with both M2 macrophages and mRNAsi according to the WGCNA analysis. Sixteen common gene signatures were obtained using three diverse algorithms. CBFA2T3, DENND3 and FCAMR were correlated to overall and disease-free survival of LUAD patients. ROC curves revealed that CBFA2T3 and DENND3 expression accurately classified LUAD and control samples. The results of single cell related analysis showed that two diagnostic biomarkers expressions were differed between the different tissue sources in M2-like macrophages. QRT-PCR demonstrated the mRNA expressions of CBFA2T3 and DENND3 were upregulated in lung adenocarcinoma cells A549 and H2122.

Conclusion

Our study identified CBFA2T3 and DENND3 as key genes associated with mRNAsi and M2 macrophages in LUAD and investigated the potential molecular mechanisms underlying this relationship.

Integrated analysis of single-cell and bulk RNA sequencing data identifies the characteristics of ferroptosis in lumbar disc herniation

Lumbar disc herniation (LDH) is the most common condition associated with low back pain, and it adversely impacts individuals' health. Ferroptosis has recently emerged as a novel factor in the pathogenesis of LDH; however, the specific impacts of ferroptosis on LDH have not been fully elucidated. Ferroptosis-related differentially expressed genes (FRDEGs) were identified from the transcriptomic datasets of LDH. Gene set enrichment analysis (GSEA) was conducted to identify biological mechanism and related pathways. LASSO and SVM-RFE algorithms were applied to detect signature genes. Function of the signature gene was confirmed by RT-qPCR. The CIBERSORT algorithm was used to compare immune infiltration between LDH and normal samples. Correlation analysis between MYB and immune cells was analyzed using the Pearson method. Additionally, we used scRNA-seq to dissect cell clusters and cellular interactions. AUCell scoring was used to analyze the ferroptosis scores of different cell types. We found that MYB, a highly expressed ferroptosis-related gene, was associated with LDH By leveraging bioinformatics analysis. In immune infiltration analysis, the abundance of monocytes and macrophages varied significantly between the LDH group and disc spondylolisthesis (DS) group. MYB was correlated with most immune cells. GSEA revealed MYB was significantly enriched in immune-related pathways. Furthermore, scRNA-seq analysis revealed the presence of eight distinct cell types. AUCell analysis showed that macrophages had a high ferroptosis score. Cell trajectory analysis revealed that chondrocyte 1 was at the beginning of the trajectory, while calcification inhibiting chondrocytes and fibrochondrocytes accumulated along the middle and tail end of the trajectory, respectively. Cell-cell communication analysis identified chondrocyte 1 had an extensive communication network with other clusters and interacted with nucleus pulposus through collagen signaling pathway. Our analysis demonstrated that MYB may be a potential therapeutic target for LDH. This study provides a resource for the orthopedics community that will facilitate additional discoveries directedly toward understanding the pathogenesis process of LDH.

Single-cell RNA-seq analysis reveals that immune cells induce human nucleus pulposus ossification and degeneration

Background and aims

Determining the transcriptomes and molecular mechanism underlying human degenerative nucleus pulposus (NP) is of critical importance for treating intervertebral disc degeneration (IDD). Here, we aimed to elucidate the detailed molecular mechanism of NP ossification and IDD using single-cell RNA sequencing.

Methods

Single-cell RNA-seq and bioinformatic analysis were performed to identify NP cell populations with gene signatures, biological processes and pathways, and subpopulation analysis, RNA velocity analysis, and cell-to-cell communication analysis were performed in four IDD patients. We also verified the effects of immune cells on NP ossification using cultured NP cells and a well-established rat IDD model.

Results

We identified five cell populations with gene expression profiles in degenerative NP at single-cell resolution. GO database analysis showed that degenerative NP-associated genes were mainly enriched in extracellular matrix organization, immune response, and ossification. Gene set enrichment analysis showed that rheumatoid arthritis signaling, antigen processing and presentation signaling were activated in the blood cell cluster. We revealed that stromal cells, which are progenitor cells, differentiated toward an ossification phenotype and delineated interactions between immune cells (macrophages and T cells) and stromal cells. Immune factors such as TNF-α, CD74 and CCL-3 promoted the differentiation of stromal cells toward an ossification phenotype in vitro. Blocking TNF-α with a specific inhibitor successfully reversed NP ossification and modified NP morphology in vivo.

Conclusion

Our study revealed an increase in macrophages and T cells in degenerative NP, which induced stromal cell differentiation toward an ossification phenotype, and contributed to the identification of a novel therapeutic target to delay IDD.

The role of ferroptosis in intervertebral disc degeneration

Nucleus pulposus, annulus fibrosus, and cartilage endplate constitute an avascular intervertebral disc (IVD), which is crucial for spinal and intervertebral joint mobility. As one of the most widespread health issues worldwide, intervertebral disc degeneration (IVDD) is recognized as a key contributor to back and neck discomfort. A number of degenerative disorders have a strong correlation with ferroptosis, a recently identified novel regulated cell death (RCD) characterized by an iron-dependent mechanism and a buildup of lipid reactive oxygen species (ROS). There is growing interest in the part ferroptosis plays in IVDD pathophysiology. Inhibiting ferroptosis has been shown to control IVDD development. Several studies have demonstrated that in TBHP-induced oxidative stress models, changes in ferroptosis marker protein levels and increased lipid peroxidation lead to the degeneration of intervertebral disc cells, which subsequently aggravates IVDD. Similarly, IVDD is significantly relieved with the use of ferroptosis inhibitors. The purpose of this review was threefold: 1) to discuss the occurrence of ferroptosis in IVDD; 2) to understand the mechanism of ferroptosis and its role in IVDD pathophysiology; and 3) to investigate the feasibility and prospect of ferroptosis in IVDD treatment.

Circ-STC2 promotes the ferroptosis of nucleus pulposus cells via targeting miR-486-3p/TFR2 axis

BACKGROUND

Low back pain (LBP) has become the second leading cause of disability worldwide, which has brought great economic burden to people. It is generally believed that intervertebral disc degeneration (IDD) is the main cause of LBP. This study aimed to explore the role of circ-STC2 in the pathogenesis of IDD.

METHODS

Nucleus pulposus cells (NPCs) were treated with T-Butyl Hydrogen Peroxide (TBHP) to establish IDD model in vitro. RT-qPCR was performed to detect mRNA expressions. The cell viability was detected with CCK-8 assay. The levels of lactate dehydrogenase (LDH), malondialdehyde (MDA), Fe2+ and glutathione (GSH) of NPCs were measured by corresponding kits. The protein expressions were determined by western blot. Dual-luciferase reporter and RNA pull-down assays were conducted to verify the relationship between circ-STC2 or transferrin recepto 2 (TFR2) and miR-486-3p.

RESULTS

Circ-STC2 and TFR2 expressions were up-regulated in IDD tissues, and miR-486-3p expression was down-regulated. Knockdown of circ-STC2 promoted the cell viability and inhibited the ferroptosis of the NPCs. The GSH levels, and glutathione peroxidase 4 (GPX4) and solute carrier family 7 member 11 (SLC7A11) protein expressions were increased, the LDH, MDA and Fe2+ levels and achaete-scute complexlike 4 (ASCL4) protein expressions were decreased after circ-STC2 knockdown. Knockdown of miR-486-3p abrogated the si-circ-STC2 effects and overexpression of TFR2 reversed the miR-486-3p mimic effects.

CONCLUSIONS

Circ-STC2 inhibits the cell viability, induced the ferroptosis of the TBHP treated NPCs via targeting miR-486-3p/TFR2 axis.

Inhibiting Heat Shock Protein 90 Attenuates Nucleus Pulposus Fibrosis and Pathologic Angiogenesis Induced by Macrophages via Down-Regulating Cell Migration-Inducing Protein

Intervertebral disc (IVD) degeneration (IVDD) is usually accompanied by nucleus pulposus (NP) fibrosis and pathologic angiogenesis, which are possibly associated with macrophage infiltration. Previous research indicates a destructive role of macrophages and the protective effect of inhibiting heat shock protein 90 (HSP90) in IVDD. Herein, the effects of inhibiting HSP90 on NP fibrosis and pathologic angiogenesis induced by macrophages were investigated further. Single-cell RNA-sequencing analysis was used to classify fibrotic NP cell (NPC) clusters and healthy NPC clusters in human NP tissues. The fibrotic NPC clusters were possibly associated with angiogenesis-related biological processes. Immunostaining showed the spatial association between blood vessel ingrowth and macrophage infiltration, as well as elevated levels of cell migration-inducing protein (CEMIP) and vascular endothelial growth factor A in severely degenerated human IVD tissues. Particularly, HSP90 inhibitor tanespimycin (17-AAG) ameliorated macrophage-induced fibrotic phenotype of NPCs via inhibiting CEMIP. M2, but not M1, macrophages promoted the pro-angiogenic ability of endothelial cells, which was attenuated by 17-AAG or HSP90 siRNA. Reversing the fibrotic phenotype of NPCs by Cemip siRNA also mitigated the pro-angiogenic effects of M2-conditioned medium-treated NPCs. Moreover, the murine IVDD model supported the 17-AAG-induced amelioration of NP fibrosis and endothelial cell invasion in IVD tissues. In conclusion, inhibiting HSP90 attenuated two interrelated pathologic processes, NP fibrosis and pathologic angiogenesis, induced by macrophages via down-regulating CEMIP.

Single-cell transcriptomics reveals heterogeneity and intercellular crosstalk in human intervertebral disc degeneration

The complexity of the human intervertebral disc (IVD) has hindered the elucidation of the microenvironment and mechanisms underlying IVD degeneration (IVDD). Here we determined the landscapes of nucleus pulposus (NP), annulus fibrosus (AF), and immunocytes in human IVD by scRNA-seq. Six NP subclusters and seven AF subclusters were identified, whose functional differences and distribution during different stages of degeneration (Pfirrmann I-V) were investigated. We found MCAM⁺ progenitor in AF, as well as CD24⁺ progenitor and MKI67⁺ progenitor in NP, forming a lineage trajectory from CD24⁺/MKI67⁺ progenitors to EffectorNP_⅓ during IVDD. There is a significant increase in monocyte/macrophage (Mφ) in degenerated IVDs (p = 0.044), with Mφ-SPP1 exclusively found in IVDD but not healthy IVDs. Further analyses of the intercellular crosstalk network revealed interactions between major subpopulations and changes in the microenvironment during IVDD. Our results elucidated the unique characteristics of IVDD, thereby shedding light on therapeutic strategies.

Polydopamine Nanoparticles Targeting Ferroptosis Mitigate Intervertebral Disc Degeneration Via Reactive Oxygen Species Depletion, Iron Ions Chelation, and GPX4 Ubiquitination Suppression

Intervertebral disc degeneration (IVDD)-induced lower back pain (LBP) is a common problem worldwide. The underlying mechanism is partially accredited to ferroptosis, based on sequencing analyses of IVDD patients from the gene expression omnibus (GEO) databases. In this study, it is shown that polydopamine nanoparticles (PDA NPs) inhibit oxidative stress-induced ferroptosis in nucleus pulposus (NP) cells in vitro. PDA NPs scavenge reactive oxygen species (ROS), chelate Fe²⁺ to mitigate iron overload, and regulate the expression of iron storage proteins such as ferritin heavy chain (FHC), ferritin, and transferrin receptor (TFR). More importantly, PDA NPs co-localize with glutathione peroxidase 4 (GPX4) around the mitochondria and suppress ubiquitin-mediated degradation, which in turn exerts a protective function via the transformation and clearance of phospholipid hydroperoxides. PDA NPs further down-regulate malondialdehyde (MDA) and lipid peroxide (LPO) production; thus, antagonizing ferroptosis in NP cells. Moreover, PDA NPs effectively rescue puncture-induced degeneration in vivo by targeting ferroptosis and inhibiting GPX4 ubiquitination, resulting in the upregulation of antioxidant pathways. The findings offer a new tool to explore the underlying mechanisms and a novel treatment strategy for IVDD-induced LBP.

Hesperidin mitigates oxidative stress-induced ferroptosis in nucleus pulposus cells via Nrf2/NF-κB axis to protect intervertebral disc from degeneration

Intervertebral disc degeneration (IVDD), a widely known contributor to low back pain (LBP), has been proved to be a global health challenging conundrum. Hesperidin (hesperetin-7-O-rutinoside, HRD) is a flavanone glycoside that belongs to the subgroup of citrus flavonoids with therapeutic effect on various diseases due to its anti-inflammatory, antioxidant properties. However, the effect of HRD on IVDD remains elusive. The human nucleus pulposus tissues were harvested for isolating human nucleus pulposus (HNP) cells to verify the expression of Nrf2. The biological effect of HRD on HNP cells were assessed in vitro, and the in vivo therapeutic effects of HRD were assessed in mice. Firstly, we found that the expression of Nrf2 was decreased with the progression of degeneration in degenerated human nucleus pulposus tissue. Subsequently, we confirmed that HRD could mitigate oxidative stress-induced ferroptosis in nucleus pulposus cells via enhancing the expression of Nrf2 axis and suppressing the NF-κB pathway to protect intervertebral disc from degeneration in vitro. Finally, the therapeutic effects of HRD were confirmed in vivo. The current study proved for the first time that HRD may protect HNP cells from degeneration by suppressing ferroptosis in an oxidative stress-dependent via enhancing the expression of Nrf2 and suppressing the NF-κB pathway. The evidence will provide a possible basis for future targeted treatment for IVDD.

The deubiquitinase USP11 ameliorates intervertebral disc degeneration by regulating oxidative stress-induced ferroptosis via deubiquitinating and stabilizing Sirt3

Increasing studies have reported that intervertebral disc degeneration (IVDD) is the main contributor and independent risk factor for low back pain (LBP), it would be, therefore, enlightening that investigating the exact pathogenesis of IVDD and developing target-specific molecular drugs in the future. Ferroptosis is a new form of programmed cell death characterized by glutathione (GSH) depletion, and inactivation of the regulatory core of the antioxidant system (glutathione system) GPX4. The close relationship of oxidative stress and ferroptosis has been studied in various of diseases, but the crosstalk between of oxidative stress and ferroptosis has not been explored in IVDD. At the beginning of the current study, we proved that Sirt3 decreases and ferroptosis occurs after IVDD. Next, we found that knockout of Sirt3 (Sirt3^-/-) promoted IVDD and poor pain-related behavioral scores via increasing oxidative stress-induced ferroptosis. The (immunoprecipitation coupled with mass spectrometry) IP/MS and co-IP demonstrated that USP11 was identified to stabilize Sirt3 via directly binding to Sirt3 and deubiquitinating Sirt3. Overexpression of USP11 significantly ameliorate oxidative stress-induced ferroptosis, thus relieving IVDD by increasing Sirt3. Moreover, knockout of USP11 in vivo (USP11^-/-) resulted in exacerbated IVDD and poor pain-related behavioral scores, which could be reversed by overexpression of Sirt3 in intervertebral disc. In conclusion, the current study emphasized the importance of the interaction of USP11 and Sirt3 in the pathological process of IVDD via regulating oxidative stress-induced ferroptosis, and USP11-mediated oxidative stress-induced ferroptosis is identified as a promising target for treating IVDD.

Integrated proteome sequencing, bulk RNA sequencing and single-cell RNA sequencing to identify potential biomarkers in different grades of intervertebral disc degeneration

Low back pain (LBP) is a prevalent health problem worldwide that affects over 80% of adults during their lifetime. Intervertebral disc degeneration (IDD) is a well-recognized leading cause of LBP. IDD is classified into five grades according to the Pfirrmann classification system. The purpose of this study was to identify potential biomarkers in different IDD grades through an integrated analysis of proteome sequencing (PRO-seq), bulk RNA sequencing (bRNA-seq) and single-cell RNA sequencing (scRNA-seq) data. Eight cases of grade I-IV IDD were obtained. Grades I and II were considered non-degenerative discs (relatively normal), whereas grades III and IV were considered degenerative discs. PRO-seq analysis was performed to identify differentially expressed proteins (DEPs) in various IDD grades. Variation analysis was performed on bRNA-seq data to differentiate expressed genes (DEGs) in normal and degenerated discs. In addition, scRNA-seq was performed to validate DEGs in degenerated and non-degenerated nucleus pulposus (NP). Machine learning (ML) algorithms were used to screen hub genes. The receiver operating characteristic (ROC) curve was used to validate the efficiency of the screened hub genes to predict IDD. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed to analyze function enrichment and signaling pathways. Protein-protein interaction (PPI) network was used to prioritize disease-related proteins. SERPINA1, ORM2, FGG and COL1A1 were identified through PRO-seq as the hub proteins involved in regulating IDD. ML algorithms selected ten hub genes, including IBSP, COL6A2, MMP2, SERPINA1, ACAN, FBLN7, LAMB2, TTLL7, COL9A3, and THBS4 in bRNA-seq. Since serine protease inhibitor clade A member 1 (SERPINA1) was the only common gene, its accuracy in degenerated and non-degenerated NP cells was validated using scRNA-seq. Then, the rat degeneration model of caudal vertebra was established. The expression of SERPINA1 and ORM2 was detected using immunohistochemical staining of human and rat intervertebral discs. The results showed that SERPINA1 was poorly expressed in the degenerative group. We further explored the potential function of SERPINA1 by Gene Set Enrichment Analysis (GSEA) and cell-cell communication. Therefore, SERPINA1 can be used as a biomarker to regulate or predict the progress of disc degeneration.

The Role of Ferroptosis in the Pathogenesis of Osteoarthritis

Osteoarthritis (OA) is the most common type of arthritis. Its high prevalence, especially in the elderly, and its negative impact on physical function make it a leading cause of disability in the elderly. Joint pain as well joint stiffness are the common classic signs of OA. Chondrocyte death together with loss of articular cartilage integrity are the main pathologic changes in OA. Non-steroidal anti-inflammatory drugs (NSAIDs) and glucocorticoids are commonly used for the management of OA; still, their effectiveness is limited, and no therapeutic strategy is able to fully stop OA progression. Ferroptosis is a kind of cell death, distinct from apoptosis and necroptosis, caused by iron-dependent peroxidation of membrane phospholipids that terminates cell life by disintegrating all plasma membranes. It has been suggested that ferroptosis has a critical role in decreased viability of chondrocytes in OA, and here, we review recent findings regarding the pathologic pathways that lead to chondrocyte ferroptosis, and discuss the possible therapeutic utility of ferroptosis inhibition in OA.

Constructing intervertebral disc degeneration animal model: A review of current models

Low back pain is one of the top disorders that leads to disability and affects disability-adjusted life years (DALY) globally. Intervertebral disc degeneration (IDD) and subsequent discogenic pain composed major causes of low back pain. Recent studies have identified several important risk factors contributing to IDD's development, such as inflammation, mechanical imbalance, and aging. Based on these etiology findings, three categories of animal models for inducing IDD are developed: the damage-induced model, the mechanical model, and the spontaneous model. These models are essential measures in studying the natural history of IDD and finding the possible therapeutic target against IDD. In this review, we will discuss the technical details of these models, the duration between model establishment, the occurrence of observable degeneration, and the potential in different study ranges. In promoting future research for IDD, each animal model should examine its concordance with natural IDD pathogenesis in humans. We hope this review can enhance the understanding and proper use of multiple animal models, which may attract more attention to this disease and contribute to translation research.

The Potential Roles of Ferroptosis in Pathophysiology and Treatment of Musculoskeletal Diseases—Opportunities, Challenges, and Perspectives

Ferroptosis is different from other forms of cell death, such as apoptosis, autophagy, pyroptosis, and cuproptosis, mainly involving iron metabolism and lipid peroxidation. Ferroptosis plays an important role in various disease, such as malignant tumors, neuron-degenerative diseases, and cardiovascular diseases, and has become the focus of current research. Both iron overload and lipid peroxide accumulation contribute to the occurrence, development, and treatment of musculoskeletal diseases, such as osteoporosis, osteoarthritis, osteosarcoma, intervertebral disc degeneration, and spinal cord injury. For a better understanding of the potential roles ferroptosis may play in pathophysiology and treatment of common musculoskeletal disorders, this article briefly reviewed the relationship and possible mechanisms. Through an investigation of ferroptosis’ role in musculoskeletal diseases’ occurrence, development, and treatment, ferroptosis could offer new opportunities for clinical diagnosis and treatment.

Fighting age-related orthopedic diseases: focusing on ferroptosis

Ferroptosis, a unique type of cell death, is characterized by iron-dependent accumulation and lipid peroxidation. It is closely related to multiple biological processes, including iron metabolism, polyunsaturated fatty acid metabolism, and the biosynthesis of compounds with antioxidant activities, including glutathione. In the past 10 years, increasing evidence has indicated a potentially strong relationship between ferroptosis and the onset and progression of age-related orthopedic diseases, such as osteoporosis and osteoarthritis. Therefore, in-depth knowledge of the regulatory mechanisms of ferroptosis in age-related orthopedic diseases may help improve disease treatment and prevention. This review provides an overview of recent research on ferroptosis and its influences on bone and cartilage homeostasis. It begins with a brief overview of systemic iron metabolism and ferroptosis, particularly the potential mechanisms of ferroptosis. It presents a discussion on the role of ferroptosis in age-related orthopedic diseases, including promotion of bone loss and cartilage degradation and the inhibition of osteogenesis. Finally, it focuses on the future of targeting ferroptosis to treat age-related orthopedic diseases with the intention of inspiring further clinical research and the development of therapeutic strategies.

Exploring the Mechanism of Microfracture in the Treatment of Porcine Full-Thickness Cartilage Defect

BACKGROUND

Microfracture has the most extensive clinical application because of its advantages of a single operation, unified process, and low operation cost. Because research on the repair mechanism of microfractures in the treatment of cartilage defects is not in-depth, this study aimed to elucidate the mechanism.

PURPOSE

To identify the characteristic cell subsets at different repair stages after microfracture, systematically analyze the repair process of the defect area after microfracture, and investigate the mechanism of fibrocartilage repair.

STUDY DESIGN

Descriptive laboratory study.

METHODS

Full-thickness articular cartilage defects and microfractures was established in the right knee of Bama miniature pigs. Single-cell transcriptional assays were used to identify the characteristics of cells isolated from healthy articular cartilage and regenerated tissues.

RESULTS

Microfractures induced mature fibrous repair in the full-thickness cartilage defect six months after surgery, while early stages of repair occurred within six weeks. Based on single-cell sequencing results, eight subsets and specific marker genes were identified. Two processes may occur after microfracture: normal hyaline cartilage regeneration and abnormal fibrocartilage repair. Regulatory chondrocytes, proliferative chondrocytes and cartilage progenitor cells (CPCs) may play important roles in the normal regeneration process. During abnormal repair, CPCs and skeletal stem cells may have different functions, and macrophages and endothelial cells may play important regulatory roles in the formation of fibrochondrocytes.

CONCLUSIONS

Using single-cell transcriptome sequencing, this study investigated the tissue regeneration process and identified key cell subsets after microfracture.

CLINICAL RELEVANCE

These results provide future targets for optimizing the repair effect of microfracture.

Intervertebral disc degeneration and osteoarthritis: a common molecular disease spectrum

Intervertebral disc degeneration (IDD) and osteoarthritis (OA) affecting the facet joint of the spine are biomechanically interdependent, typically occur in tandem, and have considerable epidemiological and pathophysiological overlap. Historically, the distinctions between these degenerative diseases have been emphasized. Therefore, research in the two fields often occurs independently without adequate consideration of the co-dependence of the two sites, which reside within the same functional spinal unit. Emerging evidence from animal models of spine degeneration highlight the interdependence of IDD and facet joint OA, warranting a review of the parallels between these two degenerative phenomena for the benefit of both clinicians and research scientists. This Review discusses the pathophysiological aspects of IDD and OA, with an emphasis on tissue, cellular and molecular pathways of degeneration. Although the intervertebral disc and synovial facet joint are biologically distinct structures that are amenable to reductive scientific consideration, substantial overlap exists between the molecular pathways and processes of degeneration (including cartilage destruction, extracellular matrix degeneration and osteophyte formation) that occur at these sites. Thus, researchers, clinicians, advocates and policy-makers should consider viewing the burden and management of spinal degeneration holistically as part of the OA disease continuum.

G3BP1 coordinates lysophagy activity to protect against compression-induced cell ferroptosis during intervertebral disc degeneration

Lysophagy is a form of selective autophagy to remove unwanted lysosomes. However, its role in the pathogenesis of intervertebral disc degeneration (IDD) remains unclear. We intended to investigate the relationship between lysophagy and ferroptosis, as well as the potential involved molecules during IDD. Human nucleus pulposus (NP) cells were obtained from clinical patients. The protein levels, protein colocalization and cellular reactive oxygen species levels were assessed by western blotting, immunofluorescence analysis, immunoprecipitation and flow cytometry, respectively. The in vivo experiments were conducted based on the needle puncture-induced IDD model in rats. Compression pressure induces the lysophagy inactivation and lysosomal damage, resulting in iron overload and ferroptosis in human NP cells. Notably, Ras GTPase-activating protein-binding proteins 1 (G3BP1) resides at lysosomes to coordinate lysophagy activity mainly via the function of G3BP1/TSC2 complex. Dysfunction of G3BP1/TSC2 complex accelerates the lysosomal damage and ferroptosis in NP cells. Besides, inhibition of mTOR signalling ameliorates lysosomal damage and protects against cell ferroptosis. The in vivo experiments also demonstrate that the G3BP1/mTOR signalling is involved in the progression of IDD. These findings illustrate the relationship between lysophagy and compression-induced cell ferroptosis. It also indicates the positive role of G3BP1 and may provide potential targets for IDD treatment.

Single-cell RNA sequencing in orthopedic research

Although previous RNA sequencing methods have been widely used in orthopedic research and have provided ideas for therapeutic strategies, the specific mechanisms of some orthopedic disorders, including osteoarthritis, lumbar disc herniation, rheumatoid arthritis, fractures, tendon injuries, spinal cord injury, heterotopic ossification, and osteosarcoma, require further elucidation. The emergence of the single-cell RNA sequencing (scRNA-seq) technique has introduced a new era of research on these topics, as this method provides information regarding cellular heterogeneity, new cell subtypes, functions of novel subclusters, potential molecular mechanisms, cell-fate transitions, and cell‒cell interactions that are involved in the development of orthopedic diseases. Here, we summarize the cell subpopulations, genes, and underlying mechanisms involved in the development of orthopedic diseases identified by scRNA-seq, improving our understanding of the pathology of these diseases and providing new insights into therapeutic approaches.

A single-cell transcriptional atlas reveals resident progenitor cell niche functions in TMJ disc development and injury

The biological characteristics of the temporomandibular joint disc involve complex cellular network in cell identity and extracellular matrix composition to modulate jaw function. The lack of a detailed characterization of the network severely limits the development of targeted therapies for temporomandibular joint-related diseases. Here we profiled single-cell transcriptomes of disc cells from mice at different postnatal stages, finding that the fibroblast population could be divided into chondrogenic and non-chondrogenic clusters. We also find that the resident mural cell population is the source of disc progenitors, characterized by ubiquitously active expression of the NOTCH3 and THY1 pathways. Lineage tracing reveals that Myh11⁺ mural cells coordinate angiogenesis during disc injury but lost their progenitor characteristics and ultimately become Sfrp2⁺ non-chondrogenic fibroblasts instead of Chad⁺ chondrogenic fibroblasts. Overall, we reveal multiple insights into the coordinated development of disc cells and are the first to describe the resident mural cell progenitor during disc injury.

Matrix Stiffness Activating YAP/TEAD1-Cyclin B1 in Nucleus Pulposus Cells Promotes Intervertebral Disc Degeneration

Intervertebral disc degeneration is a leading cause of disability in the elderly population. Rigid extracellular matrix is a critical pathological feature of disc degeneration, leading to aberrant nucleus pulposus cells (NPCs) proliferation. However, the underlying mechanism is unclear. Here, we hypothesize that increased matrix stiffness induces proliferation and thus degenerative phenotypes of NPCs through YAP/TEAD1 signaling pathway. We established hydrogel substrates to mimic stiffness of degenerated human nucleus pulposus tissues. RNA-sequencing identified differentially expressed genes between primary rat NPCs cultured on rigid and soft hydrogels. Dual luciferase assay and gain- and loss-function experiments evaluated the correlation between YAP/TEAD1 and Cyclin B1. Furthermore, single-cell RNA-sequencing of human NPCs was performed to identify specific cell clusters with high YAP expression. Matrix stiffness increased in severely degenerated human nucleus pulposus tissues (p < 0.05). Rigid substrate enhanced rat NPCs proliferation mainly through Cyclin B1, which was directly targeted and positively regulated by YAP/TEAD1. Depletion of YAP or Cyclin B1 arrested G2/M phase progression of rat NPCs and reduced fibrotic phenotypes including MMP13 and CTGF (p < 0.05). Fibro NPCs with high YAP expression were identified in human tissues and responsible for fibrogenesis during degeneration. Furthermore, inhibition of YAP/TEAD interaction by verteporfin suppressed cell proliferation and alleviated degeneration in the disc needle puncture model (p < 0.05). Our results demonstrate that elevated matrix stiffness stimulates fibro NPCs proliferation through YAP/TEAD1-Cyclin B1 axis, indicating a therapeutic target for disc degeneration.

Application of Single-Cell and Spatial Omics in Musculoskeletal Disorder Research

Musculoskeletal disorders, including fractures, scoliosis, heterotopic ossification, osteoporosis, osteoarthritis, disc degeneration, and muscular injury, etc., can occur at any stage of human life. Understanding the occurrence and development mechanism of musculoskeletal disorders, as well as the changes in tissues and cells during therapy, might help us find targeted treatment methods. Single-cell techniques provide excellent tools for studying alterations at the cellular level of disorders. However, the application of these techniques in research on musculoskeletal disorders is still limited. This review summarizes the current single-cell and spatial omics used in musculoskeletal disorders. Cell isolation, experimental methods, and feasible experimental designs for single-cell studies of musculoskeletal system diseases have been reviewed based on tissue characteristics. Then, the paper summarizes the latest findings of single-cell studies in musculoskeletal disorders from three aspects: bone and ossification, joint, and muscle and tendon disorders. Recent discoveries about the cell populations involved in these diseases are highlighted. Furthermore, the therapeutic responses of musculoskeletal disorders, especially single-cell changes after the treatments of implants, stem cell therapies, and drugs are described. Finally, the application potential and future development directions of single-cell and spatial omics in research on musculoskeletal diseases are discussed.

Ferrostatin-1 Inhibits Toll-Like Receptor 4/NF-κB Signaling to Alleviate Intervertebral Disc Degeneration in Rats

Ferrostatin-1 (Fer-1) is an inhibitor of ferroptosis and has been documented to be implicated in the development of intervertebral disc degeneration (IDD). This study intends to explore the role of Fer-1 in IDD via the toll-like receptor 4 (TLR4)/NF-κB signaling pathway. Through the Gene Expression Omnibus database, IDD-related gene expression microarray GSE124272 and high-throughput sequencing data set GSE175710 were obtained. Then, differentially expressed genes in IDD were identified, followed by implementation of protein-protein interaction network analysis and receiver operating characteristic curve analysis. Main pathways in IDD were obtained through Gene Ontology and Kyoto Encyclopedia of Genes and Genomes functional analyses, and target genes of Fer-1 were obtained through PubChem and PharmMapper websites. Finally, GPX4, FTH, and TLR4 expression was determined in a constructed IDD rat model. Three key co-expression modules involved in IDD were obtained through Weighted Gene Co-Expression Network Analysis. Thirteen differentially expressed genes were found to be associated with IDD, and eight key genes (TLR4, BCL2A1, CXCL1, IL1R1, NAMPT, SOCS3, XCL1, and IRAK3) were found to affect IDD. These eight key genes had the diagnostic potential for IDD. The NF-κB signaling pathway played a predominant role in IDD development. From network pharmacologic analysis, Fer-1 might suppress ferroptosis and ameliorate IDD via the TLR4/NF-κB signaling pathway, and the in vivo animal experiment further verified it. Fer-1 down-regulates TLR4 to inactivate NF-κB signaling pathway, suppressing ferroptosis and finally alleviating IDD in rats.

Identifification and validation of ferroptosis signatures and immune infifiltration characteristics associated with intervertebral disc degeneration

Ferroptosis and immune infiltration play an important role in the pathogenesis of intervertebral disc degeneration (IDD). However, there is still a lack of comprehensive analysis on the interaction between ferroptosis-related genes (FRGs) and immune microenvironment in IDD patients. Therefore, this study aims to explore the correlation between FRGs characteristics and immune infiltration in the progression of IDD. The expression profiles (GSE56081 and GSE70362) and FRGs were downloaded from the comprehensive gene expression omnibus (GEO) and FerrDb database, respectively, and the differences were analyzed using R. The intersection of IDD related differential genes (DEGs) and FRGs was taken as differentially expressed FRGs (DE-FRGs) and GO and KEGG enrichment analysis was conducted. Then, we used least absolute shrinkage and selection operator (LASSO) regression algorithm and support vector machine (SVM) algorithm to screen feature genes and draw ROC curve judge the diagnostic value of key DE-FRGs. Then CIBERSORT algorithm is used to evaluate the infiltration of immune cells and analyze the correlation between key DE-FRGs and immune infiltration. Based on the analysis results, we conducted single gene GSEA analysis on key DE-FRGs. RT-PCR and immunohistochemistry further verified the clinical value of the results of biochemical analysis and screening. Seven key DE-FRGs were screened, including the upregulated genes NOX4 and PIR, and the downregulated genes TIMM9, ATF3, ENPP2, FADS2 and TFAP2A. Single gene GSEA analysis further elucidates the role of DE-FRGs in IDD associated with ferroptosis. Correlation analysis showed that seven key DE-FRGs were closely related to immune infiltration in the development of IDD. Finally, RT-PCR and immunohistochemical staining showed that NOX4, ENPP2, FADS2 and TFAP2A were statistically significant differences. In this study, we explored the connection between ferroptosis related characteristics and immune infiltration in IDD, and confirmed that NOX4, ENPP2, FADS2, and TFAP2A may become biomarkers and potential therapeutic targets for IDD.

Identification and validation of ferroptosis-related gene signature in intervertebral disc degeneration

Lower back pain (LBP) is a leading cause of disability in the elderly and intervertebral disc degeneration (IDD) is the major contributor to LBP. Ferroptosis is a newly discovered programmed cell death, characterized by iron-dependent lethal lipid peroxidation. Growing evidence has shown that ferroptosis plays important roles in various human diseases. However, the underlying mechanism of ferroptosis in IDD remains elusive. This study is aimed to uncover the key roles of ferroptosis in the pathogenesis and progression of IDD comprehensively. To investigate the ferroptosis related differentially expressed genes (FRDEGs) in IDD, we analyzed the microarray data from the Gene Expression Omnibus (GEO) database. Then we performed functional enrichment analysis and protein-protein interaction (PPI) network analysis, and screened out the hub FRDEGs. To further evaluate the predictive value of these hub FRDEGs, we performed ROC analysis based on the GSE124272 dataset. A total of 80 FRDEGs were identified, including 20 downregulated and 60 upregulated FRDEGs. The FRDEGs were primarily involved in the biological processes of response to chemical, and response to stress. KEGG pathway enrichment analysis showed that the FRDEGs were mainly involved in ferroptosis, TNF signaling pathway, HIF-1 signaling pathway, NOD-like receptor signaling pathway, and IL-17 signaling pathway. Ten hub OSRDEGs were obtained according to the PPI analysis, including HMOX1, KEAP1, MAPK1, HSPA5, TXNRD1, IL6, PPARA, JUN, HIF1A, DUSP1. The ROC analysis and RT-qPCR validation results suggested that most of the hub FRDEGs might be potential signature genes for IDD. This study reveals that ferroptosis might provide promising strategy for the diagnosis and treatment of IDD.

Systematic study of single-cell isolation from musculoskeletal tissues for single-sell sequencing

Background

The single-cell platform provided revolutionary way to study cellular biology. Technologically, a sophistic protocol of isolating qualified single cells would be key to deliver to single-cell platform, which requires high cell viability, high cell yield and low content of cell aggregates or doublets. For musculoskeletal tissues, like bone, cartilage, nucleus pulposus and tendons, as well as their pathological state, which are tense and dense, it’s full of challenge to efficiently and rapidly prepare qualified single-cell suspension. Conventionally, enzymatic dissociation methods were wildly used but lack of quality control. In the present study, we designed the rapid cycling enzymatic processing method using tissue-specific enzyme cocktail to treat different human pathological musculoskeletal tissues, including degenerated nucleus pulposus (NP), ossifying posterior longitudinal ligament (OPLL) and knee articular cartilage (AC) with osteoarthritis aiming to rapidly and efficiently harvest qualified single-cell suspensions for single-cell RNA-sequencing (scRNA-seq).

Results

We harvested highly qualified single-cell suspensions from NP and OPLL with sufficient cell numbers and high cell viability using the rapid cycling enzymatic processing method, which significantly increased the cell viability compared with the conventional long-time continuous digestion group (P < 0.05). Bioanalyzer trace showed expected cDNA size distribution of the scRNA-seq library and a clear separation of cellular barcodes from background partitions were verified by the barcode-rank plot after sequencing. T-SNE visualization revealed highly heterogeneous cell subsets in NP and OPLL. Unfortunately, we failed to obtain eligible samples from articular cartilage due to low cell viability and excessive cell aggregates and doublets.

Conclusions

In conclusion, using the rapid cycling enzymatic processing method, we provided thorough protocols for preparing single-cell suspensions from human musculoskeletal tissues, which was timesaving, efficient and protective to cell viability. The strategy would greatly guarantee the cell heterogeneity, which is critical for scRNA-seq data analysis. The protocol to treat human OA articular cartilage should be further improved.

Method for the Isolation of "RNA-seq-Quality" RNA from Human Intervertebral Discs after Mortar and Pestle Homogenization

The problem of isolating high-quality total RNA from intervertebral discs has no recognized solution yet. This is due to the extremely low content of live cells in the samples and the voluminous intercellular matrix. A variety of published protocols focused on isolating RNA from articular cartilage have recommended the use of expensive equipment, enzymatic matrix cleavage, or cell culture. In our study, we used a combination of the traditional QIAzol protocol (Qiagen, Germany) and RNEasy column purification (Qiagen, Germany) to obtain high-quality RNA from post-surgical intervertebral disc fragments. Only a mortar and a pestle were used for grinding, making our method particularly accessible. The isolated RNA with a RIN of ~7 is suitable for studying the expression profile of chondrocytes in situ. RNA-seq analysis of three samples demonstrated cell type ratios to be mostly relevant to intervertebral disc tissues, with over 70% of the chondrocytes of the three subtypes having an admixture of blood-related cells.

Targeting Ferroptosis Holds Potential for Intervertebral Disc Degeneration Therapy

Intervertebral disc degeneration (IVDD) is a common pathological condition responsible for lower back pain, which can significantly increase economic and social burdens. Although considerable efforts have been made to identify potential mechanisms of disc degeneration, the treatment of IVDD is not satisfactory. Ferroptosis, a recently reported form of regulated cell death (RCD), is characterized by iron-dependent lipid peroxidation and has been demonstrated to be responsible for a variety of degenerative diseases. Accumulating evidence suggests that ferroptosis is implicated in IVDD by decreasing viability and increasing extracellular matrix degradation of nucleus pulposus cells, annulus fibrosus cells, or endplate chondrocytes. In this review, we summarize the literature regarding ferroptosis of intervertebral disc cells and discuss its molecular pathways and biomarkers for treating IVDD. Importantly, ferroptosis is verified as a promising therapeutic target for IVDD.

Is intervertebral disc degeneration associated with reduction in serum ferritin?

OBJECTIVE

Ferritin autophagy is characterized by intracellular ferroptosis and selective ferritin degradation. However, the role of ferritin in the development of intervertebral disc degeneration (IDD) has not been elucidated. The study aimed to investigate the role of serum iron metabolism markers, especially serum ferritin (SF), in IDD.

METHODS

217 patients who came to the spine surgery department of our hospital for low back pain were recruited, and blood samples were collected for routine examination after admission. The cumulative grade was also calculated by summing up the Pfirrmann grade of all lumbar discs.

RESULTS

Correlation analysis showed that cumulative grade was correlated with SF (r = - 0.185, p = 0.006), not with serum iron (SI), transferrin saturation (TS), unsaturated iron-binding capacity (UIBC) and total iron-binding capacity (TIBC) (all p > 0.05). In addition, SF levels in the low severity IDD were significantly higher than high severity IDD in cumulative grade (p = 0.003) and single disc grade. No statistically significant difference was found in the other four indicators. A statistically significant difference was observed between the high (cumulative grade > 17) and low score (cumulative grade ≤ 17) groups in terms of age. According to the ROC curve, the cut-off value of SF levels was 170.5. Patients with SF < 170.5 ng/mL had severe disc degeneration. The sensitivity and specificity were 0.635 and 0.602, respectively.

CONCLUSION

This study preliminarily showed that SF was negatively correlated with the degree of IDD and can be used to predict IDD severity.