Senescent Synovial Intimal Fibroblasts Aggravate Osteoarthritis by Regulating Macrophage Polarization and Chondrocyte Phenotype Through ANGPTL4-α5β1 Axis.

Synovial intimal fibroblasts (SIF) display more marked premature senescence compared to other synovial cell types in osteoarthritis.

• Finding was established using multiplex immunofluorescence, gene regulatory network reconstruction, and single-cell RNA sequencing analyses of synovial tissue.
• A specific senescent subpopulation within SIF was identified and characterized.
• The accumulation of senescent cells in the synovium was found to precede chondrocyte senescence and cartilage degradation.
• The senescence-associated secretory phenotype (SASP) was characterized within the synovium.

Transcription factors EGR1 and ATF3 regulate senescence-related pathways in senescent synovial intimal fibroblasts.

• EGR1 and ATF3 were identified through gene regulatory network reconstruction as key regulators.
• These transcription factors were specifically implicated in the senescent SIF subpopulation.
• Functional experiments were conducted in vivo and in vitro to elucidate the mechanisms of fibroblast senescence.

Senescent SIF promote M1 macrophage polarization through paracrine secretion of ANGPTL4.

• ANGPTL4 was identified as the key paracrine factor secreted by senescent SIF that drives macrophage polarization toward the pro-inflammatory M1 phenotype.
• The mechanism involves the ANGPTL4-α5β1 integrin axis.
• Both paracrine secretion and direct cell-cell contact between senescent SIF and macrophages were identified as mechanisms of intercellular communication.

Senescent SIF promote cartilage degeneration and alter chondrocyte phenotype through ANGPTL4 secretion.

• ANGPTL4 secreted by senescent SIF was shown to act via the α5β1 integrin axis on chondrocytes.
• A series of in vivo and in vitro functional experiments were conducted to demonstrate effects on chondrocyte phenotype.
• OA incidence is strongly correlated with aging, and synovial cell senescence was shown to play a key role in OA pathogenesis.

Senescent SIF may facilitate OA progression through direct cell-cell contact with macrophages in addition to paracrine mechanisms.

• Direct cell-cell contact between senescent SIF and macrophages was identified as a distinct mechanism from paracrine ANGPTL4 secretion.
• This finding was supported by in vivo and in vitro functional experiments.
• Both contact-dependent and contact-independent (paracrine) mechanisms were characterized in the study.