Background Osteoarthritis (OA) is a slowly progressive degenerative disease characterized by degradation of the matrix and cell death, resulting in a gradual loss of articular cartilage integrity. Despite its high prevalence, disease-modifying therapies are not available and this is in part related limited sensitivity of current methods for the assessment of disease progression.

Objectives To identify and localize proteins in normal cartilage, and compare them to the OA tissue using quantitative proteomic techniques.

Methods Cartilage samples were obtained from OA patients undergoing joint replacement and normal donors without history of joint disease. For the localization studies, independent normal cartilage samples (n=3) were sectioned into three layers (superficial, intermediate and deep). Cartilage proteins were extracted with Urea 6M, 2% SDS using a mixer mill. Then, proteins were quantified, digested with trypsin and differentially labelled with iTRAQ isobaric tags. The peptide mixture was separated by two-dimensional LC coupled to MALDI-TOF/TOF mass spectrometry. Identification and relative quantification of the proteins were performed using ProteinPilot 3.0 software.

Results We could identify more than 260 different proteins in articular cartilage. 47% of them were intracellular, and the rest were ECM components. When comparing between tissue layers, we found an increased abundance of type VI collagen and small proteoglycans (mimecan, lumican or PRG4) in the superficial layer. Several proteins involved in cell adhesion processes were also increased in this layer (gelsolin, vitronectin, tenascins). The intermediate layer was characterized by a high presence of type II, V, IX and XXVIII collagens, cartilage intermediate layer proteins (CILPs), COMP, vitrin and decorin. Finally, the deep layer exhibited an increased abundance of type I and XI collagens, aggrecan and bone-related proteins (bone sialoprotein 2, osteomodulin, and bone morphogenetic protein 3). Comparison of this normal cartilage proteome with that from OA tissue led to the identification of 23 proteins increased in the pathologic tissue, including aggrecan, COMP, complement factors or thrombospondin 1. We could also identify 36 proteins that were decreased in OA cartilage, such as type I, II and VI collagens, proteoglycans (biglycan, PRG4), tenascins or actin.

Conclusions In summary, we have mapped more than 250 different human normal articular cartilage proteins according to their presence in the three different tissue layers, and identified 59 different proteins that are altered in OA cartilage when compared to normal tissue. This information would be of high relevance in the search of tissue-specific OA biomarkers.

Disclosure of Interest None Declared