Single-Cell Transcriptomic Profiling Reveals Superficial Zone Chondrocytes as a Catabolic and Senescent Focus in Osteoarthritis Cartilage.

ObjectiveTo compare zone-specific expression patterns of catabolic proteases and senescence-related genes across chondrocyte subpopulations in osteoarthritis (OA) cartilage.DesignSingle-cell RNA sequencing data from 72,616 chondrocytes (11 human cartilage samples; 6 normal, 5 OA; GSE220243) were analyzed. Following quality control and Harmony-based batch correction, cells were clustered into 27 subpopulations and assigned to superficial (SZ), middle (MZ), deep (DZ), or calcified (CZ) zones using zone-specific gene set module scores. Developmental origin scoring, catabolic gene expression ranking, senescence scoring, and signaling pathway analysis were performed across all clusters.ResultsUpon zonal assignment, the MZ constituted the largest compartment (73.6%), followed by the SZ (21.9%) and CZ (4.5%), with no cluster assigned a dominant DZ identity. MMP13 and ADAMTS5 showed their highest per-cell expression in SZ clusters, with MMP13 most prominent in a FibroC-COL1A2+ superficial cluster (c10). SZ chondrocytes harbored a constitutively encoded fibroblastic identity transcriptionally resembling perichondrial and synovial fibroblast programs, independent of OA status, with concurrent deficiencies in Wnt inhibitor (FRZB, DKK1, WIF1) and FGF chondroprotective signaling relative to MZ. Senescence was observed in terminally differentiated clusters of both SZ and MZ, where a constitutively senescence-prone niche in SZ associated with mTOR and HIF1A signaling was mechanistically distinct from the senescence program observed in the MZ clusters.ConclusionsThese findings identify the developmentally distinct SZ as a candidate catabolic and senescent focus in OA cartilage and a potential contributor to matrix degradation and target for zone-stratified therapeutic intervention.