Aging attenuates the infrapatellar fat pad's paracrine capacity to promote extracellular matrix production in aged chondrocytes

Objective: To elucidate the age and sex-specific impact of the IFP secretome on aged chondrocytes, and uncover the exact mechanism driving these effects. To explore mediators of the IFP on chondrocytes, we performed mass-spectrometry proteomics of CM from young versus aged male IFP and mass spectrometry on aged male chondrocytes treated with CM of the same experimental groups. Network analysis interrogated age-dependent changes in communication between IFP-derived secreted proteins and chondrocytes. Results: Col2 and aggrecan expression were significantly increased in chondrocytes co-cultured with young IFP tissue compared to aged IFP tissue, and young CM compared to aged CM. There was no significant increase in SOX9 under any of the different conditions. Similar trends were seen in female experiments. Multi-omics results combined with network analysis suggested IFPs secrete succinate co-A ligase related factors that regulate Hox-dependent spliceosome gene expression in chondrocytes in an age-dependent manner. Conclusions: This study suggests that a young IFP secretome enhances ECM production by aged chondrocytes, in part through enhanced mitochondrial activity. However, this beneficial effect decreases with age. Overall design: Design: Using male C57BL6 mice, aged chondrocytes were co-cultured with young (n=5) or aged (n=5) IFPs in a transwell system for 72 hours. To establish the direct effects of the IFP secretome on chondrocyte integrity, we incubated aged chondrocytes with untreated control media, conditioned media (CM) from young IFP (n=5), or CM from aged IFP (n=5) for the same duration. Outcomes for both experiments included SOX9, type II collagen (Col2), and aggrecan. Experiments were repeated with female models.