ATDC5 cells as a model of cartilage extracellular matrix neosynthesis, maturation and assembly

Fibrillar collagens and proteoglycans (PGs) are quantitatively the major constituents of extracellular matrices (ECM). They carry numerous crucial post-translational modifications (PTMs) that tune the resulting biomechanical properties of the corresponding tissues. The mechanisms determining these PTMs remain largely unknown, notably because available established cell lines do not recapitulate much of the complexity of the machineries involved. ATDC5 cells are a model of chondrogenesis widely used for decades, but it remains described mostly at histological and transcriptional levels. Here, we asked to what extent this model recapitulates the events of ECM synthesis and processing occurring in cartilage. Insulin-stimulated ATDC5 cells exhibit up- or down-regulation of more than one-hundred proteins, including a number of known participants in chondrogenesis and major markers thereof. However, they also lack several ECM components considered of significant, yet more subtle, function in cartilage. Still, they assemble the large PG aggrecan and type II collagen, both carrying most of their in vivo PTMs, into an ECM. Remarkably, collagen crosslinking is fully lysyl oxidase (LOX)-dependent. The ATDC5 model recapitulates critical aspects of the cartilage ECM-processing machinery and should be useful to decipher the mechanisms involved. Proteomics data are available via ProteomeXchange with identifier PXD014121. The present work provides the first proteome characterization of the ATDC5 chondrogenesis model, which has been used for decades in the field of cartilage biology. The results demonstrate the up- and down-regulation of more than one hundred proteins. Overall, specific drawbacks of the model are pointed out, that will be important to take into consideration for future studies. However, major cartilage components are massively assembled into an extracellular matrix and carry most of their post-translational modifications occurring in cartilage tissue. Unlike other available established cell lines, the ATDC5 model recapitulates major aspects of cartilage biosynthesis and should be useful in investigating the mechanisms that regulate collagen maturation events.