Background/Purpose: Articular cartilage consists of a single type of cells called chondrocytes, which comprise 2-5% of total tissue mass and an extracellular matrix (ECM) mainly composed of water, proteoglycans and collagens. The tissue function is dependent on the molecular composition of this ECM. Articular cartilage is characterized by a zonal architecture with unique cell phenotypes and ECM properties in the superficial, middle and deep zone. In osteoarthritis (OA), cartilage is thinned, eventually completely degraded, resulting in joint dysfunction and pain. OA is also associated with hypertrophic bone changes with osteophyte formation, subchondral bone remodeling, and, chronic inflammation of the synovial membrane. The final common pathway of cartilage destruction results from a failure of chondrocytes to maintain a homeostatic balance between matrix synthesis and degradation. There is a considerable interest pointed in the characterization of new cartilage specific OA biomarkers for diagnosis and disease progression studies in OA. The aim of this work was to identify and localize proteins in normal cartilage, and compare them to the osteoarthritic tissue in a quantitative way.

Methods: Cartilage samples were obtained from OA patients undergoing joint replacement and normal donors without history of joint disease (n=3). For the localization studies, independent normal cartilage samples (n=3) were sectioned into three layers (superficial, middle and deep). Cartilage proteins were extracted, 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 identified 220 different proteins in normal articular cartilage. An increased abundance of type VI collagen and small proteoglycans (mimecan, lumican or PRG4) was detected in the superficial layer of cartilage. Several proteins involved in cell adhesion processes were also increased in this layer (gelsolin, vitronectin, tenascins). The middle 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 281 proteins: 23 were increased in the pathologic tissue (including aggrecan, COMP, complement factors or thrombospondin 1), whereas 36 were decreased in OA cartilage, such as type I, II and VI collagens, proteoglycans (biglycan, PRG4), tenascins or actin.

Conclusion: In summary, more than 300 different human articular cartilage proteins have been mapped according to their presence in the three different tissue layers. 59 of them were altered in OA cartilage when compared to normal tissue. This information would be of high relevance in the search of tissue-specific OA biomarkers.

---

« Back to 2012 ACR/ARHP Annual Meeting

ACR Meeting Abstracts - https://acrabstracts.org/abstract/zone-specific-protein-profiles-in-human-cartilage-unraveled-by-a-quantitative-proteomic-approach/