ECM remodeling features in reparative chondrocytes during knee osteoarthritis.

Background: Knee osteoarthritis (KOA) is characterized by progressive cartilage degeneration and disruption of extracellular matrix (ECM) homeostasis. Chondrocytes are not a homogeneous or static population during disease progression but exhibit pronounced functional heterogeneity. Although single-cell transcriptomic studies have identified multiple chondrocyte states in osteoarthritic cartilage, how these states dynamically relate to ECM remodeling and disease progression remains incompletely understood. Methods: We integrated multiple publicly available single-cell RNA sequencing datasets of human knee cartilage to construct a unified cellular atlas and systematically compared chondrocyte states between control and KOA samples. Differential expression analysis, functional enrichment, pseudotime trajectory inference, and cell-cell communication analysis were applied to characterize ECM-related chondrocyte states and their dynamic transitions. Key signaling cues identified from single-cell analyses were further evaluated using Results: We observed a marked expansion of the previously described reparative chondrocyte population (RepC) in KOA cartilage. Rather than reflecting a simple increase in cell proportion, KOA-associated RepC exhibited enhanced ECM remodeling programs characterized by collagen reorganization and strengthened ECM-cell interactions. Pseudotime analysis positioned RepC downstream of proliferation chondrocytes and near a major branching region toward either regulator chondrocytes with further extension toward fibrocartilage chondrocytes or effector chondrocytes. In KOA, RepC was preferentially represented at mid-to-late pseudotime stages within the reconstructed trajectory framework. Cell-cell communication analysis suggested that RepC showed prominent inferred ECM-related interactions, particularly involving collagen and FN1-integrin pathways. Consistently, FN1 or TGF-β1 stimulation Conclusion: These findings highlight ECM remodeling features of reparative chondrocytes during KOA and support a state-centric view in which disproportionate representation of reparative states within the pseudotime trajectory framework is associated with maladaptive ECM remodeling in KOA.