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World J Orthop. Nov 18, 2026; 17(11): 125372
Published online Nov 18, 2026. doi: 10.5312/wjo.125372
Single-cell RNA-sequencing analysis reveals cellular states and intercellular communication of human lumbar facet-joint degeneration
Xiang-Yuan Chen, Wen Luo, Yue-Ying Zhang, Hui-Ying Li, Ping Wang, Dan-Dan Wang, College of Clinical and Basic Medical Sciences, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan 250117, Shandong Province, China
Xiang-Yuan Chen, Yuan-Zhen Chen, Dan-Dan Wang, Neck-Shoulder and Lumbocrural Pain Hospital of Shandong First Medical University, Jinan 250062, Shandong Province, China
Yuan-Zhen Chen, Department of Pain Management, The Second Affiliated Hospital of Shandong First Medical University, Tai’an 271000, Shandong Province, China
Lei Wang, Department of Orthopaedics, The 960th Hospital of The People’s Liberation Army Joint Logistics Support Force, Jinan 250031, Shandong Province, China
Zhen-Tao Man, Wei Li, Department of Joint Surgery, Shandong Provincial Hospital Affiliated to Shandong First Medical University, Jinan 250021, Shandong Province, China
ORCID number: Dan-Dan Wang (0000-0001-5774-2871).
Co-first authors: Xiang-Yuan Chen and Wen Luo.
Co-corresponding authors: Dan-Dan Wang and Wei Li.
Author contributions: Chen XY and Luo W have played important and indispensable roles in the experimental design as co-first authors; Chen XY, Luo W, Chen YZ and Wang L performed the experiment; Chen XY, Zhang YY, Wang P, and Wang DD analyzed the data; Chen YZ, Wang L and Li HY collected samples and analyzed clinical data; Li W and Wang DD have played important and indispensable roles in the manuscript preparation as co-corresponding authors; Man ZT, Li W and Wang DD designed the research and revised the manuscript; Chen XY, Luo W, and Wang DD wrote the paper.
AI contribution statement: During the preparation of this manuscript, AI-assisted tools were used solely for language refinement and grammar correction to improve the clarity and readability of the text. No AI tools were used for any other aspect of this research.
Supported by the National Key R&D Program of China, No. 2022YFF0713100; and the Natural Science Foundation of Shandong Province, No. ZR2026MS1334.
Institutional review board statement: This study involving human participants was conducted in accordance with the Declaration of Helsinki and was approved by the Ethics Committee of Shandong First Medical University and Shandong Academy of Medical Sciences (No. R202204290099). Informed consent was received from all study participants.
Conflict-of-interest statement: All authors declare no conflict of interest in publishing the manuscript.
Data sharing statement: Raw single-cell RNA sequencing data of all samples were uploaded to the Genome Sequence Archive (GSA) of the National Genomics Data Center (PRJCA016822), which are publicly accessible at https://ngdc.cncb.ac.cn/gsa-human/s/E2WBojr6.
Corresponding author: Dan-Dan Wang, Additional Professor, College of Clinical and Basic Medical Sciences, Shandong First Medical University and Shandong Academy of Medical Sciences, No. 6699 Qingdao Road, Huaiyin District, Jinan 250117, Shandong Province, China. ddwang@sdfmu.edu.cn
Received: July 6, 2026
Revised: August 20, 2026
Accepted: October 30, 2026
Published online: November 18, 2026
Processing time: 127 Days and 12.6 Hours

Abstract
BACKGROUND

Facet joint osteoarthritis (FJOA) is a chronic degenerative joint disease, which can lead to severe low back pain. However, the pathophysiological mechanisms underlying the degenerative progression in human lumbar FJOA remain undefined.

AIM

To investigate the cellular heterogeneity, potential therapeutic markers, and underlying mechanisms of human lumbar facet joint (LFJ) degeneration.

METHODS

We determined the cell landscapes of human LFJ cartilage using single-cell RNA sequencing.

RESULTS

The proportions of proliferative chondrocytes and proinflammatory chondrocytes were significantly increased in mild degenerated LFJ samples, while those of fibrocartilage chondrocytes, effector chondrocytes, and homeostatic chondrocytes were increased in severe degenerated LFJs. Importantly, proinflammatory chondrocytes were found in mild degenerated LFJs, but not in severe degenerated LFJs. We also found that severe degenerated LFJs exhibited a significant increase in macrophages of five distinct subclusters: (1) Mφ-F13A1; (2) Mφ-TCN2; (3) Mφ-COL27A1; (4) Mφ-CD52; and (5) Mφ-IL3RA. Mφ-IL3RA was only identified in severe degenerated LFJs, implying its importance in the degenerative process. Cell crosstalk analysis revealed signaling pathways and changes in intercellular interactions during LFJ degeneration. Intercellular interactions between chondrocytes and macrophages were significantly greater in severe degenerated LFJs. Further analysis of the ligand-receptor interactions revealed that the increased crosstalk between chondrocytes and macrophages in severe degenerated LFJs was mainly associated with inflammation-remodeling-related and tissue-remodeling-related signaling pathways. These findings also showed progressively increased communication strength with degeneration, suggesting that chondrocyte-macrophage crosstalk becomes increasingly active during LFJ cartilage degeneration.

CONCLUSION

Our findings deepen the understanding and extend distinguishing features of cellular heterogeneity and intercellular interactions of LFJ cartilage, potentially contributing to new therapeutic strategies for human FJOA.

Key Words: Lumbar facet joint; Osteoarthritis; Chondrocytes; Single-cell RNA sequencing; Cellular heterogeneity; Intercellular crosstalk

Core Tip: This study provides a single-cell transcriptomic landscape of human lumbar facet joint (LFJ) cartilage degeneration across different degeneration stages. We identified stage-specific changes in chondrocyte and macrophage subpopulations and characterized their potential interactions during disease progression. Notably, proinflammatory chondrocytes were enriched in mild degenerated LFJs, whereas distinct macrophage populations emerged with advanced degeneration. Cell-cell communication analysis further revealed enhanced chondrocyte-macrophage crosstalk during degeneration. These findings provide new insights into the cellular heterogeneity and potential intercellular mechanisms underlying LFJ degeneration.



INTRODUCTION

Lumbar facet joint osteoarthritis (FJOA) is a prevalent degenerative disease of the spine, accounting for 15%-41% of patients with chronic low back pain (LBP)[1,2]. FJOA can lead to a series of LBP symptoms and neurological problems[3]. People of various ages who experience LBP may suffer mobility challenges and reduced quality of life, which in turn have enormous impacts on healthcare systems and economic development[4,5].

The lumbar facet joints (LFJs) are synovial joints in the posterior aspect of the spine that are covered with hyaline cartilage and enclosed by synovium and a joint capsule[6]. LFJ degeneration is considered the leading cause of FJOA and facetogenic chronic LBP[7,8]. It has been reported that degeneration of the facet joints can occur as early as 15 years of age[7]. Like appendicular joints, LFJ exhibits a similar pathological process of degenerative changes involving cartilage degeneration, subchondral bone erosion, joint-space narrowing, osteophyte formation, and synovial inflammation[4]. Many recent studies have used single-cell RNA sequencing (scRNA-seq) to explore articular chondrocytes, subchondral bone, synovium, and immune cells, revealing the heterogeneity and pathologic mechanisms of human osteoarthritis[9-12].

Previous molecular studies specific to LFJ degeneration have identified several key pathogenic pathways. For instance, transcriptomic analyses have suggested the aberrant activation of the Wnt and nuclear factor kappa B signaling pathways in FJOA[13,14]. Furthermore, cellular senescence and the upregulation of associated genes have been implicated in the progression of FJOA degeneration[15]. While these studies provide crucial molecular insights, they often rely on bulk tissue analysis, which masks cellular heterogeneity. Concurrently, scRNA-seq has been revolutionary in exploring appendicular osteoarthritis, revealing the heterogeneity and pathologic mechanisms of articular chondrocytes, subchondral bone, synovium, and immune cells[16-20]. However, a direct translation of these findings to the LFJ is inadequate due to the joint’s unique biomechanical and biological environment. Critically, the cellular landscape and intercellular crosstalk underlying human LFJ degeneration remain largely undefined at single-cell resolution. Therefore, a comprehensive scRNA-seq atlas is needed to investigate the cellular constituents and their dynamic interactions during LFJ degeneration.

Given the complexity and poor understanding of human FJOA, we used scRNA-seq to identify cell populations and to explore the heterogeneous landscapes and mechanisms of intercellular crosstalk that occur during the degeneration of LFJ. Our results allowed us to uncover the endogenous heterogeneity and novel phenotypes of chondrocytes, and to elucidate the pathophysiological changes and mechanisms of human LFJ degeneration and FJOA progression. These findings provide potential avenues for future diagnostic and therapeutic investigations for human FJOA and facetogenic spine disorders.

MATERIALS AND METHODS
Human cartilaginous tissues of LFJ

Six patients with lumbar degenerative diseases who underwent lumbar posterior surgery were recruited from the 960th Hospital of The People’s Liberation Army Joint Logistics Support Force. The LFJ degeneration grades of all six participants were confirmed using magnetic resonance and computed tomography images in accordance with the Weishaupt grading criteria[21]: (1) Grade 0, normal facet joint space (2-4 mm width); (2) Grade 1, narrowing of the facet joint space (< 2 mm) and/or small osteophytes and/or mild hypertrophy of the articular process; (3) Grade 2, narrowing of the facet joint space and/or moderate osteophytes and/or moderate hypertrophy of the articular process and/or mild subarticular bone erosions; and (4) Grade 3, narrowing of the facet joint space and/or large osteophytes and/or severe hypertrophy of the articular process and/or severe subarticular bone erosions and/or subchondral cysts. The detailed clinical characteristics of the patients are listed in Supplementary Table 1. The cartilaginous LFJ tissues were immediately immersed in tissue storage solution (Miltenyi Biotec, Germany) and then sent to OE Biotech Co. Ltd. (Shanghai, China) for subsequent scRNA-seq.

Under sterile conditions, the freshly collected cartilaginous LFJ tissues were washed twice with pre-cooled RPMI 1640 medium containing 0.04% bovine serum albumin (BSA). The tissues were then mechanically dissociated into approximately 0.5 mm3 fragments using surgical scissors and transferred into freshly prepared enzymatic digestion solution. The digestion mixture contained RPMI 1640 (Corning, cat. No. 10-040-CVR), 0.04% BSA (MACS, cat. No. 1000076), and 0.2% collagenase II (Gibco, cat. No. 17101015). The samples were incubated at 37 °C for 30-60 minutes with gentle inversion every 5-10 minutes. The digested cell suspension was filtered through a BD 40 μm cell strainer (Falcon, cat. No. 352340) 1-2 times. The filtrate was centrifuged at 300 ×g for 5 minutes at 4 °C. The cell pellet was resuspended in appropriate medium, mixed with an equal volume of red blood cell lysis buffer (Miltenyi, cat. No. 130-094-183), and incubated at 4 °C for 10 minutes. After centrifugation at 300 ×g for 5 minutes, the supernatant was discarded. The pellet was washed once with medium followed by another centrifugation at 300 ×g for 5 minutes, and the final supernatant was removed. Finally, the cells were re-suspended with 1 mL RPMI 1640 medium (Corning, cat. No. 10-040-CVR) with 0.04% BSA added. Single-cell suspension concentration and cell viability were then evaluated by Luna-FL cell counter (Logos Biosystems, Korea) or Trypan Blue staining method.

Cartilaginous tissue sections used for immunohistochemistry were also obtained from two other patients who required lumbar posterior surgery for lumbar degenerative diseases. The detailed clinical characteristics of these two patients are also listed in Supplementary Table 1.

The scRNA-seq library construction and sequencing

The scRNA-seq libraries of human LFJ cartilage were prepared using the Chromium Next GEM Single Cell 3ʹ Reagent Kit v3.1 (10× Genomics, America), in accordance with the manufacturer’s protocols. Briefly, the cells were isolated and resuspended in Master Mix from the Reagent Kit at a concentration of 700-1200 cells/µL (viability ≥ 85%). The gel beads, Cell Mix, and partitioning oil were loaded onto the Single Cell Chip G. The GEMs microdroplet was immediately performed on the 10× Chromium microfluidic platform. A sufficient mass of barcoded, full-length complementary DNA was amplified by PCR to construct libraries. The amplified complementary DNA was fragmented and end-repaired, and double-sided size selection was conducted using SPRIselect reagents. After quality control, the libraries were sequenced on an Illumina Nova 6000 PE150 platform to generate 150-bp paired-end reads (OE Biotech Co. Ltd, Shanghai, China).

The scRNA-seq raw data processing

The Cell Ranger software pipeline (version 5.0.0) provided by 10× Genomics was used for alignment and quantification of the raw scRNA-seq data from each sample. We processed the unique molecular identifier (UMI) count matrix using the R package Seurat (version 4.0.0)[22]. For quality control, cells with the following criteria were included for downstream analyses: (1) The nCount_RNA ≥ 1000; (2) The nFeature_RNA ≥ 200; (3) The log10GenesPerUMI > 0.7; and (4) The mitochondrial UMI < 10%. The DoubletFinder package (version 2.3.0) was used to identify potential doublets[23]. Finally, a total of 30418 cells were included in downstream analyses. Library size normalization was performed using the NormalizeData function, the most variable genes were selected using the FindVariableGenes function, and cells were clustered on the basis of their gene expression profile using the FindClusters function, all in Seurat[22]. To remove batch effects in the scRNA-seq data, the mutual nearest neighbors correction was performed using the R package batchelor[24]. Cells were visualized using a two-dimensional Uniform Manifold Approximation and Projection (UMAP) algorithm with the RunUMAP function[24]. The cell types were identified on the basis of marker gene expression using the FindAllMarkers function (test.use = bimod) in Seurat, and the R package SingleR, with the reference transcriptomic datasets from the Human Primary Cell Atlas[24-26]. Following secondary clustering of chondrocytes, the resulting subclusters were annotated according to their differentially expressed genes (DEGs) and characteristic transcriptional features. Proinflammatory chondrocytes (ProinCs) were defined as a transcriptionally distinct chondrocyte subcluster characterized by a coordinated inflammatory gene signature, particularly the preferential expression of CXCL3, CXCL1, CXCL2, and IL6. DEGs were identified using the FindMarkers function (test.use = MAST)[24-26]. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses of DEGs were performed using the R package clusterProfiler[27].

Macrophage subcluster identification and annotation

Macrophages were extracted from the integrated single-cell dataset and subjected to secondary unsupervised clustering based on their transcriptomic profiles. The resulting subclusters were identified according to cluster-specific DEGs and canonical macrophage markers. Subcluster annotation was further supported by the expression of characteristic marker genes and functional enrichment analysis.

Cell-cell communication analysis

Cell-cell communication networks among chondrocytes and macrophages were inferred using the CellChat R package (version 2.1.2) based on ligand-receptor interactions. First, we imported the normalized expression matrix to create the cellchat object with the create CellChat function. Secondly, the data was preprocessed with the identifyOverExpressedGenes, identifyOverExpressedInteractions and projectData functions using the default parameters. The computeCommunProb, filterCommunication (min.cells = 10) and computeCommunProb Pathway functions were then used to determine any potential ligand-receptor interactions. Finally, the cell communication network was aggregated using the aggregateNet function. The interacting receptors and ligands identified in different cell clusters are listed in Supplementary Table 2.

RNA velocity pseudotime trajectory analysis

Monocle2 (version 2) was used to determine the cell lineage of subclusters[28]; specifically, reduced-dimension clustering was performed using the reduceDimension function, while the orderCells function was used to infer the cell lineage differentiation trajectory. The spliced reads and unspliced reads for each gene in every cell were re-counted by velocy.py based on the data output from Cell Ranger[29]. The velocity of each cell for every gene was calculated using the velocity.R package (version 6.0), and projected onto the UMAP for visualization[29].

Single-cell communication

The cellular communications between cell subclusters were further analyzed by degeneration grade. The CellPhoneDB[30] (version 4.1.0) was used to identify biologically relevant ligand-receptor interactions from single-cell transcriptomics (scRNA-seq) data. A ligand or a receptor was defined as “expressed” in a particular cell type if 10% of the cells of that type had non-zero read counts for the ligand/receptor encoding gene. Statistical significance was then assessed by randomly shuffling the cluster labels of all cells and repeating the above steps, which generated a null distribution for each LR pair in each pairwise comparison between two cell types. After running 1000 permutations, P values were calculated with the normal distribution curve generated from the permuted LR pair interaction scores. To define networks of cell-cell communication, any two cell types where the ligand was expressed in the former cell type and the receptor in the latter was linked. R packages Igraph (version 1.2.4.1) and Circlize (version 0.4.8) were used to display the cell-cell communication networks.

Embedding of LFJ cartilaginous tissues

Place the tissue in a decalcifying reagent (Servicebio, Wuhan, Hubei Province, China) and replace the fresh solution daily until the tissue becomes soft. Rinse with tap water for 24 hours. Dehydrate the tissue using a gradient alcohol solution. After the tissue block is dehydrated with alcohol, it needs to be cleared. Add transparent reagent (Servicebio, Wuhan, Hubei Province, China) to clear the tissue. The cleared tissue block is then immersed in 3 cylinders of paraffin (60 °C) for wax impregnation. The wax-impregnated tissue block is wrapped around the paraffin block. The temperature of the embedding wax should be slightly higher than the wax impregnation temperature to ensure that the tissue block is completely integrated with the embedding paraffin.

Immunohistochemistry assay

Cartilage tissue sections were deparaffinized with dimethylbenzene and sequentially rehydrated with 100%, 95%, 90%, 80%, and 70% ethanol. After washing with phosphate-buffered saline (PBS), the sections were boiled in buffer containing 0.01 M sodium citrate-hydrochloric acid (pH 6.0) at 98 °C for 20 minutes for antigen retrieval. After three washes with PBS, the sections were rinsed in 3% peroxidase quenching solution (Invitrogen, America). Then 3% bovine serum albumin (Servicebio, Wuhan, Hubei Province, China) was used to block endogenous peroxidase. The sections were then incubated with primary antibody (Supplementary Table 3) at 4 °C overnight. After three washes with PBS, the sections were incubated with a biotinylated secondary antibody (Zhongshan Golden Bridge Biotech, Beijing, China) at room temperature for 30 minutes. The visualization signal was developed using 3, 3′-diaminobenzidine solution (Zhongshan Golden Bridge Biotech, Beijing, China). The sections were then counterstained with hematoxylin at room temperature for 15 minutes. All sections were observed and captured using a pathological section scanner (Pannoramic MIDI, 3D HISTECH, Hungary).

Statistical analysis

The Wilcoxon test was used to analyze differences in the percentages of subclusters between mildly and severely degenerated LFJ samples and to compare the relative expression of marker genes between subclusters. All statistical analyses were performed using the R software, and P value < 0.05 was considered statistically significant.

RESULTS
Single-cell profiling reveals cellular heterogeneity in human LFJ

To fully characterize the cellular heterogeneity in human LFJ, we profiled LFJ cartilage from six patients with lumbar degenerative disease (degeneration grades I, II, and III grouped as FJ1, FJ2, and FJ3, respectively) using scRNA-seq (Figure 1A and Table 1). After quality control and extensive filtration, 30418 individual cells were selected for sequencing and subsequent analysis. A total of 2646 DEGs were identified to classify cell populations in human LFJ cartilage (Supplementary Table 4). The following 15 putative cell populations were identified in LFJ samples (Figure 1B): (1) Chondrocytes; (2) Endothelial cells; (3) Granulocyte-monocyte progenitors; (4) Hematopoietic stem cells (HSCs); (5) Macrophages; (6) Monocytes; (7) T cells; (8) B cells; (9) Dendritic cells (DCs); (10) Mast cells; (11) Neutrophils; (12) Plasmacytoid DCs (pDCs); (13) Pericytes; (14) Plasma cells; and (15) Erythroid cells. The expression levels of the top 10 marker genes in each cluster revealed clear separation between clusters (Figure 1C).

Figure 1
Figure 1 Single-cell RNA sequencing analysis of human lumbar facet joint cartilage. A: Schematic workflow of the experimental strategy to isolate cells from human lumbar facet joint cartilage for subsequent single-cell RNA sequencing; B: Uniform Manifold Approximation and Projection showing the single-cell transcriptomic landscape of human lumbar facet joint cartilage. Fifteen clusters were visualized; C: Heatmap of the top marker genes of each cluster (defined in B); D: Distribution of cell clusters (defined in B) in human FJ1, 2 and 3 (degeneration grade I, II and III) cartilage samples. LFJ: Lumbar facet joint; DC: Dendritic cell; HSC: Hematopoietic stem cell; pDC: Plasmacytoid dendritic cell.
Table 1 Lumbar facet joint degeneration grade.
Sample
LFJ degeneration grade
Group
Age
Gender
FJ1_1IFJ139Male
FJ2_1IIFJ265Male
FJ3_1IIIFJ368Female
FJ1_2IFJ151Male
FJ2_2IIFJ269Female
FJ3_2IIIFJ378Female
Immunohistochemistry
FJ1_3I-49Male
FJ3_3III-65Female

The chondrocytes exhibited high expression of the integrin-related gene ITGBL1, the extracellular secretion-related and interaction-related genes FBLN1, FNDC1, and SULF1, and the collagen binding activity-related gene LRRC15 (Figure 1C and Supplementary Figure 1A). Macrophages were identified in LFJs of all degeneration grades using the markers LILRB5, C1QC, FPR3, MSR1, and FOLR2 (Figure 1C and Supplementary Figure 1B). A violin plot was used to illustrate the expression of representative genes of chondrocytes and macrophages (Supplementary Figure 1). Monocytes were identified by the markers CD300E, LGALS2, CLEC10A, CLEC7A, and EPB41 L3. HSCs exhibited high expression of AVP, HTR1F, and CRYGD, and endothelial cells exhibited high expression of CD300 LG, TM4SF18, KDR, TEK, and ADGRL4. Erythroid cells were identified by expression of CTSE, ART4, and TSPO2. Other cells involved in immune regulation, such as granulocyte-monocyte progenitors, T cells, B cells, DCs, mast cells, neutrophils, plasma cells, and pDCs, were also identified on the basis of marker gene expression (Figure 1C and Supplementary Table 4).

Additionally, we analyzed the proportion and distribution of each cell cluster (Figure 1D) in mild degenerated (FJ1) and severe degenerated (FJ2 and FJ3) LFJs. Compared with mild degenerated LFJs, the percentages of neutrophils, HSCs, and B cells were lower, while those of chondrocytes, macrophages, monocytes, and T cells were significantly higher, in severe degenerated LFJs.

Identification of cartilage chondrocyte populations and cell lineage trajectories during human LFJ degeneration

To determine the heterogeneity of LFJ cartilage and the functions of chondrocyte subpopulations during LFJ degeneration, we performed a secondary analysis of chondrocytes. This revealed eight subclusters with different functions (Figure 2A): (1) Four empirically defined populations, namely proliferative chondrocytes (proCs), prehypertrophic chondrocytes (preHTCs), hypertrophic chondrocytes (HTCs), and fibrocartilage chondrocytes (FCs); and (2) ProinCs four other populations, namely effector chondrocytes (ECs), regulatory chondrocytes (RegCs), homeostatic chondrocytes (HomCs). A total of 1697 DEGs were identified among these eight chondrocyte subclusters (Supplementary Table 5). The heatmap shown in Figure 2B illustrates the expression levels of the top 10 representative marker genes for each subcluster, while the violin plots in Figure 2C show the expression levels of representative genes for each chondrocyte subpopulation. Among the eight chondrocyte subclusters, proinCs exhibited a distinct inflammatory transcriptional signature. Specifically, proinCs showed preferential expression of several inflammation-related genes, including CXCL3, CXCL1, CXCL2, and IL6, with CXCL3 showing particularly marked expression in this population compared with the other chondrocyte subclusters (Figure 2B and C). Therefore, proinCs were annotated based on this coordinated inflammatory transcriptional signature. The results of immunohistochemical assays further validated the presence of these chondrocyte subclusters in LFJ cartilage samples of different degeneration grades (Figure 2D).

Figure 2
Figure 2 Identification of lumbar facet joint cartilage chondrocyte subpopulations and their marker-gene expression in different degeneration grades of human lumbar facet joint cartilage. A: Uniform Manifold Approximation and Projection plot showing the single-cell transcriptomic landscape of human lumbar facet joint chondrocytes. Eight subpopulations were identified; B: Heatmap of the top marker genes of each cluster (defined in A); C: Violin plot of representative genes of each chondrocyte population (defined in A); D: Immunohistochemistry staining of representative markers of the indicated cell clusters in different degenerated lumbar facet joint cartilage samples. Scale bar 100 μm. UMAP: Uniform Manifold Approximation and Projection; ProCs: Proliferative chondrocytes; PreHTCs: Prehypertrophic chondrocytes; HTCs: Hypertrophic chondrocytes; FCs: Fibrocartilage chondrocytes; ECs: Effector chondrocytes.

The GO and KEGG pathway enrichment analysis showed that the proCs were primarily enriched in the regulation of chondrocyte proliferation and metabolism (Figure 3A and B), as expected, while preHTCs were preferentially focused on differentiation, skeletal-muscle regeneration, biosynthesis, and secretion. HTCs were preferentially involved in chondrocyte differentiation, cell aggregation, ribosomes, and RNA transport. FCs were focused on the JAK-STAT, tumor necrosis factor, nuclear factor kappa B, and mitogen-activated protein kinase signaling pathways, as well as protein complex remodeling. ECs had the capacity for collagen-activated signaling pathway activity and metabolic processes, extracellular structure organization, and protein digestion and absorption. HomCs preferred ribosome biogenesis, RNA transport, and peptide biosynthesis. RegCs were mainly focused on membrane organization, regulation of cell activation, response to hydrostatic pressure, and the cGMP-PKG, calcium, and Notch signaling pathways. The proinCs were preferentially enriched in the Toll-like receptor and phosphatidylinositol signaling pathways, and cell adhesion molecules.

Figure 3
Figure 3 Distribution and trajectories of lumbar facet joint cartilage chondrocyte subpopulations during the degeneration process of human lumbar facet joint. A: Gene ontology functions of eight lumbar facet joint (LFJ) cartilage chondrocyte subpopulations; B: Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of eight LFJ cartilage chondrocyte subpopulations; C: Distribution of chondrocyte subclusters in mild (FJ1) and severe (FJ2-3) degenerated LFJs; D: RNA velocity-based pseudotime analysis inferred from Uniform Manifold Approximation and Projection embedding of LFJ chondrocyte subclusters. ProCs: Proliferative chondrocytes; PreHTCs: Prehypertrophic chondrocytes; HTCs: Hypertrophic chondrocytes; FCs: Fibrocartilage chondrocytes; ECs: Effector chondrocytes.

The proportion and distribution of the chondrocyte subclusters in mild and severe degenerated LFJs are shown in Figure 3C. There were no significant variations in the distributions of preHTCs, HTCs, and RegCs. By contrast, the proportions of proCs and proinCs were significantly higher in mild degenerated LFJs than in severe degenerated LFJs, indicating a more pronounced process of chondrocyte proliferation and immune response regulation in the mild degenerated tissues (Figure 3C). Additionally, the proportions of FCs, ECs, and HomCs were higher in severe degenerated LFJs, suggesting that chondrocytes differentiated as LFJ degeneration progressed (Figure 3C). Immunohistochemical results were also consistent with these results (Figure 2D).

To further explore the relationship and cell lineage trajectories of the chondrocyte subclusters, we performed RNA velocity analysis. The pseudotime trajectory axis aligned well with the chondrocyte subclusters. As shown in Figure 3D, the proCs were followed by preHTCs and HTCs, and most FCs were mainly distributed at the end of the pseudotime trajectory. ECs and RegCs were distributed along the pseudotime trajectory axis, and proinCs were mainly positioned at the beginning, but not at the end, of the trajectory. HomCs formed a small branch at approximately the midline of the axis, suggesting a potential differentiation branch of chondrocytes.

Identification of macrophage subpopulations and functional definition in LFJ degeneration

Results shown in Figure 1D indicated that the proportion of macrophages was significantly higher in severe degenerated LFJs than in mild degenerated LFJs. Further investigation led to the identification of five distinct macrophage subclusters (Mφ-F13A1, Mφ-TCN2, Mφ-COL27A1, Mφ-CD52, and Mφ-IL3RA) based on their transcriptomic profiles and cluster-specific DEGs (Figure 4 and Supplementary Table 6). Mφ-F13A1 expressed F13A1, LYVE1, AREG, CCL8, and NR4A3, while Mφ-TCN2 showed high expression of TCN2, LINC01094, BHLHE41, MYO1G, and ADAM8. Mφ-COL27A1 was characterized by COL27A1, AK5, WWC1, F5, and STEAP1 expression, whereas Mφ-CD52 exhibited high expression of CD52, LGALS2, IL4I1, FBP1, and LSP1. Mφ-IL3RA was distinguished by preferential expression of IL3RA, together with NCS1, SEMA6B, NEU4, and AC007326.5. These distinct marker profiles supported the annotation of the five macrophage subpopulations.

Figure 4
Figure 4 Identification of macrophage subpopulations and their marker-gene expression in different degeneration grades of the human lumbar facet joint. A: Uniform Manifold Approximation and Projection plot showing the single-cell transcriptomic landscape of macrophages. Five subclusters were identified. Numbers 1-5 represent Mφ-F13A1, Mφ-TCN2, Mφ-COL27A1, Mφ-CD52, and Mφ-IL3RA, respectively; B: Dotplot of top ten marker genes for five macrophage subclusters (defined in A); C: Violin plot of representative genes of macrophage subclusters (defined in A); D: Representative immunohistochemistry staining of CD68 of macrophage subclusters in different degenerated lumbar facet joint cartilage samples. Scale bar 100 μm; E: Gene enrichment for these subclusters (defined in A) in comparisons with the classical cell type M1 and M2 macrophage. Bubble size represents the proportion of upregulated and downregulated genes of subclusters based on the gene signature on the X axis. The color of the circle represents the directional false discovery rate. Red, enrichment of upregulated genes in the specific cell subsets; blue, depletion of downregulated genes.

The immunohistochemical assays of the macrophage marker CD68 confirmed the presence of macrophages in degenerated LFJs (Figure 4D). Furthermore, the subclusters resembled M1 and M2 macrophage signatures based on the enrichment of classical M1/M2 macrophage subsets (Figure 4E). Mφ-F13A1 showed more pro-inflammatory functions of M1 macrophages, whereas Mφ-IL3RA showed more anti-inflammatory and tissue-repair functions of M2 macrophages.

To further investigate the functions of the macrophage subclusters in LFJ degeneration, gene set variation analysis of the GO and KEGG signaling pathways was performed. Further analysis revealed distinct transcriptional and functional characteristics across the five macrophage subclusters. Mφ-F13A1 exhibited a relatively pro-inflammatory transcriptional profile, whereas Mφ-TCN2 was associated with phagocytosis and cell adhesion-related processes. Mφ-COL27A1 showed enrichment of mitochondrial and oxidative phosphorylation-related programs, while Mφ-CD52 was associated with metabolic processes including cGMP and biotin metabolism. In contrast, Mφ-IL3RA exhibited enrichment in neutrophil regulation, mineral absorption, and mTOR, FoxO, EGFR, and PI3K-AKT signaling pathways (Figure 4E and Figure 5A and B). These findings indicate that the five macrophage subpopulations possess distinct functional characteristics during LFJ degeneration.

Figure 5
Figure 5 Distribution and trajectories of macrophage subpopulations during the degeneration process of human lumbar facet joint. A: Gene ontology functions of macrophage subpopulations. Number 1-5 represents Mφ-F13A1, Mφ-TCN2, Mφ-COL27A1, Mφ-CD52 and Mφ-IL3RA respectively; B: Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis of macrophage subpopulations; C: Distribution of macrophage subclusters in mild and severe degenerated lumbar facet joint cartilage samples; D: Representative immunohistochemistry staining of IL3RA of macrophage subclusters in different degenerated lumbar facet joint cartilage samples. Scale bar 100 μm; E: RNA velocity-based pseudotime analysis inferred from Uniform Manifold Approximation and Projection embedding of macrophage subclusters.

The proportion and distribution of each macrophage subcluster in mild and severe degenerated LFJs are shown in Figure 5C. In severe degenerated LFJs, the proportions of Mφ-F13A1 and Mφ-TCN2 were increased, while the proportion of Mφ-CD52 was significantly decreased. Notably, Mφ-IL3RA was not detected in mild degenerated LFJs, but was identified in severe degenerated LFJs, indicating that it may play a crucial role in severe degeneration (Figure 5C and D). RNA velocity analysis showed a clear directional flow from Mφ-CD52 toward Mφ-TCN2, then toward Mφ-F13A1 and Mφ-IL3RA (Figure 5E). Based on the functional characteristics of macrophages, these findings suggest that macrophages are likely involved in the processes of inflammation and tissue regeneration regulation during LFJ degeneration. Notably, the strength and number of chondrocyte-macrophage interactions increased progressively with cartilage degeneration. Compared with mild degenerated LFJs, severe degenerated LFJs exhibited enhanced intercellular communication, with grade III showing the most prominent changes. These findings suggest that chondrocyte-macrophage crosstalk is progressively amplified during the progression of LFJ degeneration. The observed stage-dependent changes in inflammatory and matrix-remodeling markers were broadly consistent with the predicted enhancement of chondrocyte-macrophage communication.

Cell crosstalk reveals mechanisms of intercellular interactions in LFJ degeneration

To further investigate the intercellular interactions between chondrocytes and other cell clusters during human LFJ degeneration, we performed cell-cell communication network analysis on the basis of the ligand-receptor interactions using the CellChat R package. The interacting receptors and ligands identified in different cell clusters are listed in Supplementary Table 2. The differential numbers of interactions between chondrocytes and inflammation-related cell clusters, such as macrophages, B cells, mast cells, erythroid cells, and T cells, were significantly higher in severe degenerated LFJs compared with those in mild degenerated LFJs (Figure 6A and B). Furthermore, the interaction strength between chondrocytes and macrophages was also significantly increased in severe degenerated LFJs compared with that in mild degenerated LFJs (Figure 6C and D). The differential numbers of interactions and interaction strength were decreased between chondrocytes and DCs, pDCs, HSCs, pericytes, and endothelial cells in severe degenerated LFJs (Figure 6). These results suggested that the cell-crosstalk interactions were weak in mild degenerated LFJs, becoming more frequent in severe degenerated LFJs. Macrophages may play important functions in the LFJ degeneration process.

Figure 6
Figure 6 Intercellular crosstalk networks between chondrocytes and other cell clusters in the degeneration process of human lumbar facet joint. A: Intercellular crosstalk network of the interaction number between cell clusters in severely degenerated lumbar facet joints (LFJs) compared with mildly degenerated LFJs. Dots indicate cell clusters. The thickness of the directed line indicates the relative quantity of significant ligand-receptor pairs between any two pairs of cell clusters. Red represents the interaction amounts increased. Blue represents the interaction amounts decreased; B: Heatmap of the interaction number between cell clusters. The colored bar at the top and right represent the sum of receptor and ligand signals in each cell cluster, respectively; C: Intercellular crosstalk network of the interaction strength between cell clusters in severe degenerated LFJs comparing with mild degenerated LFJs. Red represents the interaction strength increased. Blue represents the interaction strength decreased; D: Heatmap of the interaction strength between cell clusters in severe degenerated LFJs comparing with mild degenerated LFJs.

The signaling pathways of intercellular interactions were further explored (Supplementary Figure 2). The signaling pathways involving collagen, fibronectin 1 (FN1), osteopontin (SPP1), laminin, C-X-C motif chemokine ligand (CXCL), CD99, midkine, and galectin were found to be active in all LFJ degeneration grades (Figure 7A). The signaling pathway network showed that the communication of chondrocytes with other cells mainly relies on the signaling pathways of collagen, FN1, CXCL, laminin, amyloid precursor protein, CD99, SPP1, pleiotrophin, notch, thrombospondin, macrophage migration inhibitory factor, and midkine (Figure 7A and Supplementary Figure 3). As shown in Supplementary Figure 4, in severe degenerated LFJs, the collagen and FN1 signaling pathways were significantly increased. Chondrocytes were regulated by other cells mainly through the galectin and CD46 signaling pathways (Supplementary Figure 4). The angiopoietin signaling pathway was active in mild degenerated LFJs, but not in severe degenerated LFJs (Supplementary Figure 4). Interestingly, the SPP1 signaling pathway was mainly active between macrophages and chondrocytes in mild degenerated LFJs, and was increasingly involved in interactions between chondrocytes and all cell clusters in severe degenerated LFJs (Figure 7B). Moreover, the CXCL and laminin signaling pathways were significantly increased between chondrocytes and other cell clusters in mild degenerated LFJs (Figure 7C). Collectively, these results revealed a critical intercellular communication network between chondrocytes and other cells during LFJ degeneration, providing a fundamental basis and research directions for treating FJOA.

Figure 7
Figure 7 Overview of the signaling pathways between cell clusters in the degeneration process of human lumbar facet joint. A: Signaling pathways between cell clusters in different degenerated grades of lumbar facet joints (LFJs); B: SPP1 signaling pathways in different degenerated LFJs; C: C-X-C motif chemokine ligand and LAMININ signaling pathways in different degenerated LFJs. CXCL: C-X-C motif chemokine ligand.
DISCUSSION

As a frequent form of LFJ syndrome and a common cause of LBP, FJOA imposes an enormous health and economic burden on society[31]. Considering that neither conservative strategies nor invasive surgical procedures have provided permanent pain relief[4], new strategies for the diagnosis and treatment of FJOA are urgently needed. Because of the limited research on FJOA, the cellular heterogeneity of LFJ and the complex pathophysiological process of LFJ degeneration have not been completely elucidated. Here, we described the entire cellular landscape of LFJ cartilage, identifying subclusters of chondrocytes and macrophages with distinct distribution profiles in LFJs of different degeneration grades. Cell lineage trajectories were used to predict the transition between chondrocyte types and their differentiation paths as LFJ degenerated. Our study findings deepen the understanding of the cellular heterogeneity and intercellular interactions of LFJ degeneration, and the investigation of intercellular communications offers a potential foundation for understanding molecular pathological mechanisms and provides a theoretical framework for experimental research of LFJ degeneration during the progression of FJOA.

Articular cartilage homeostasis is of critical importance for the physiological functioning of facet joints. FJOA is primarily characterized by the degeneration of small joint cartilage followed by inflammation and degradation[4]. Cartilage, comprising chondrocytes and extracellular matrix, is considered a physiologically non-self-renewing avascular tissue[32]. To better understand the pathophysiological mechanisms underlying the role of chondrocytes in the development of FJOA, we characterized the heterogeneity and functions of chondrocyte populations in human LFJ cartilage. Previous studies on chondrocyte subtypes in the knee joint have reported four populations: (1) ProCs; (2) PreHTCs; (3) HTCs; and (4) FCs[33]. Zhao et al[15] found that dasatinib and quercetin delayed chondrocyte senescence during FJOA progression, thereby alleviating the progression of reversible FJOA. We also found subpopulations of ECs, RegCs, and HomCs, which have similar cell biological functions as those previously reported[34]. The discovery of these populations in LFJ, particularly the inflammatory-prone proinCs, underscores the complex cellular ecosystem within the joint and provides a foundational atlas for future functional studies aimed at deciphering their precise roles in matrix remodeling, homeostasis, and cell-cell communication. Interestingly, in the chondrocyte subcluster of proinCs that preferentially expressed the Toll-like receptor signaling pathway, phosphatidylinositol signaling pathway activity and cell adhesion molecules were also observed. The cellular functional enrichment and differential signature markers of proinCs implicated this subcluster in the regulation of inflammation during LFJ degeneration.

The identification of proinCs in this study warrants careful interpretation. Rather than being defined by a single canonical marker, proinCs were characterized by a coordinated inflammatory transcriptional program, as evidenced by the preferential expression of CXCL3, CXCL1, CXCL2, and IL6, together with the enrichment of Toll-like receptor and phosphatidylinositol signaling pathways. These features collectively support the annotation of proinCs as an inflammation-associated chondrocyte subcluster. Nevertheless, because the annotation of proinCs was primarily based on transcriptomic profiles, these cells should be interpreted as a transcriptomically defined inflammatory chondrocyte subcluster or cellular state rather than a definitively established independent lineage. Further spatial and functional studies will be required to determine whether this transcriptional state corresponds to a stable and biologically distinct chondrocyte population and to clarify its contribution to LFJ degeneration.

The distribution of chondrocyte populations and their differential trajectories at different stages offers insights into the changing characteristics of cartilage and chondrocytes during the LFJ degeneration process. As expected, the proportion of proCs was significantly higher in mild degenerated LFJs than in severe degenerated LFJs, whereas the proportion of preHTCs showed the opposite trend. The differentiation trajectory of chondrocytes was basically consistent with previous studies of osteoarthritis, exhibiting a route from proCs to preHTCs and HTCs, and finally to FCs[9,33]. There were no significant differences in the distributions of HomCs, FCs, and RegCs during the different degeneration stages. The trajectories of ECs and RegCs were distributed along the pseudotime axis, indicating that their roles are interwoven throughout the entire progression of LFJ degeneration. By contrast, we found a higher proportion of proinCs in mildly degenerated LFJs than in severely degenerated LFJs. Additionally, proinCs were mainly distributed at the beginning of the inferred trajectory rather than at the terminal stage, predicting that this inflammatory chondrocyte state may be more closely associated with the early stage of LFJ degeneration. The prominence of proinCs during early degeneration may reflect an initial inflammatory response to joint stress. These findings raise the possibility that modulating proinC-associated inflammatory signaling during early degeneration may represent a potential research direction, although this hypothesis requires functional validation.

Inflammatory reactions and immune system changes are known to be important components of facetogenic LBP and the pathogenesis of osteoarthritis[35,36]. Research on osteoarthritis has revealed that the innate and adaptive immune cell types that contribute to the disease pathogenesis include macrophages, neutrophils, B cells, T cells, and DCs[37,38]. Data from scRNA-seq studies have revealed changes in the immune landscape in osteoarthritis[39]; however, the immune landscape and precise role of immunocyte subtypes in the initiation or progression of LFJ degeneration remain poorly understood. Here, we illustrated the immune landscape and changes in immunocyte subtypes during different degenerative stages of LFJ. Our findings indicated that immune cell subpopulations play distinct roles in the mild and severe stages of LFJ degeneration. These trajectory analyses represented statistical inferences based on mRNA splicing kinetics rather than deterministic biological fate maps. Furthermore, the robustness of macrophage trajectories (terminating in the Mφ-F13A1 and Mφ-IL3RA clusters) warranted more critical discussion, as these conclusions were derived from a small number of cells and patients.

Macrophages reportedly act as immune cells with critical importance in the symptomatology and structural progression of OA[40,41]. Activated macrophages polarize into either M1 or M2 subtypes, producing inflammatory cytokines and chemokines that regulate OA development[40]. Our study also found macrophage infiltration in degenerated LFJs, to a greater extent in severe degenerated LFJs than in mild degenerated LFJs. Further investigation of the functional characteristics and marker genes of macrophage subclusters indicated that, as the LFJ degeneration grade increased, macrophage subpopulations varied in proportions and polarization directions. Notably, Mφ-IL3RA was found in severe degenerated but not mild degenerated LFJs, indicating that Mφ-IL3RA may play an important role in the process of severe LFJ degeneration. We speculate that the emergence of Mφ-IL3RA in severe degeneration represents a shift in the immune landscape towards a more chronic or tissue-destructive state. Given the role of IL3RA (CD123) in immune cell recruitment and activation[34], this macrophage subset might potentiate a sustained inflammatory loop or directly contribute to pathological tissue remodeling and pain sensitization in advanced FJOA. Further experimental and theoretical foundations for understanding LFJ degeneration may be provided by investigating or intervening in the functions of Mφ-IL3RA during the severe degeneration stages of LFJ. Our study predicts stage-specific therapeutic direction. The early enrichment of proinCs suggests that modulating their activity could intervene in initial FJOA. Conversely, the appearance of Mφ-IL3RA in severely degenerated LFJs indicates that it contributes to the progression of LFJ degeneration. The cell-cell communication networks we uncovered offer additional opportunities for disrupting pathogenic crosstalk.

Cellular communications and molecular interactions among chondrocytes, immune cells, and various other cell types in the joint microenvironment may influence intra-articular degradation and degeneration, or repair and regeneration, and are crucial to understanding the pathogenesis and development of FJOA[39]. Research has shown that paracrine interactions between immunocytes and chondrocytes, such as the secretion of inflammatory cytokines, growth factors, matrix metalloproteinases, and tissue inhibitors of metalloproteinases, lead to cartilage degeneration and play a vital role in the development of OA[41]. Our study showed that the intercellular interactions between chondrocytes and other cells exhibited varying quantities and strength across different degeneration grades, with a notable enhancement of chondrocyte-macrophage interactions specifically in severe degenerated LFJs, which provides insights into the inflammatory pathological processes underlying LFJ degeneration. Moreover, in mildly degenerated LFJs, the SPP1 signaling pathway in interactions between macrophages and chondrocytes was significantly enhanced but decreased in severely degenerated LFJs. We anticipate that the detailed cellular and molecular interactions provided in this study will improve the understanding of the mechanism and functions of the intra-articular microenvironment in driving progressive LFJ degeneration. Nevertheless, these cellular and molecular biological mechanisms, and the crosstalk between chondrocytes and other cell types, require further investigation.

CONCLUSION

This exploratory study provides a single-cell atlas of human LFJ cartilage and identifies stage-associated chondrocyte and macrophage subpopulations, along with predicted intercellular communication patterns. The enhanced chondrocyte-macrophage crosstalk identified during degeneration provides new insights into the cellular mechanisms underlying LFJ degeneration and offers testable hypotheses for future investigation.

Limitations and future perspectives

This study has several limitations. First, the sample size, while informative for this initial investigation, remains modest. Furthermore, the analysis was confined to cartilage tissue, omitting the synovium and subchondral bone which are relevant to joint degeneration. Most critically, the functional roles of the novel cell clusters we identified are computationally inferred and await experimental validation in FJOA models. In conclusion, our study findings illustrate the landscape and heterogeneity of LFJ cartilage at single-cell resolution, showing the development of major cell subclusters and functional distinctions at the different degeneration grades of LFJ. Cell communications between chondrocytes and other cell clusters in the microenvironment further revealed the signaling pathways and mechanisms of LFJ degeneration. These findings expand our knowledge of the biological basis and pathophysiology of this widespread disorder, opening possibilities for the re-establishment of homeostasis to prevent LFJ degeneration and facilitating the development of improved diagnostic and treatment strategies for FJOA.

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Footnotes

Peer review: Externally peer reviewed.

Peer-review model: Single blind

Specialty type: Orthopedics

Country of origin: China

Peer-review report’s classification

Scientific quality: Grade A

Novelty: Grade A

Creativity or innovation: Grade B

Scientific significance: Grade B

P-Reviewer: Li B, PhD, Associate Professor, China S-Editor: Luo ML L-Editor: A P-Editor: Wang WB

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