Quantitative proteomics at different depths in human articular cartilage reveals unique patterns of protein distribution

Abstract

The articular cartilage of synovial joints ensures friction-free mobility and attenuates mechanical impact on the joint during movement. These functions are mediated by the complex network of extracellular molecules characteristic for articular cartilage. Zonal differences in the extracellular matrix (ECM) are well recognized. However, knowledge about the precise molecular composition in the different zones remains limited. In the present study, we investigated the distribution of ECM molecules along the surface-to-bone axis, using quantitative non-targeted as well as targeted proteomics.\ In a discovery approach, iTRAQ mass spectrometry was used to identify all extractable ECM proteins in the different layers of a human lateral tibial plateau full thickness cartilage sample. A targeted MRM mass spectrometry approach was then applied to verify these findings and to extend the analysis to four medial tibial plateau samples. In the lateral tibial plateau sample, the unique distribution patterns of 70 ECM proteins were identified, revealing groups of proteins with a preferential distribution to the superficial, intermediate or deep regions of articular cartilage. The detailed analysis of selected 29 proteins confirmed these findings and revealed similar distribution patterns in the four medial tibial plateau samples. The results of this study allow, for the first time, an overview of the zonal distribution of a broad range of cartilage ECM proteins and open up further investigations of the functional roles of matrix proteins in the different zones of articular cartilage in health and disease.

Chondrocyte secretome: a source of novel insights and exploratory biomarkers of osteoarthritis

“…Cleavage of aggrecan by HTRA1 was strongly enhanced by HTRA1 agonists such as CPII, a C-terminal hexapeptide derived from the C-propeptide of procollagen IIα1 [57]. HTRA1 is preferentially localized in the deep layer of cartilage [34] and increased during chondrocyte differentiation of MSCs [26], and by TGF-β1 stimulation [24]. HTRA1 is reduced 4-fold in OA vs normal chondrocytes [35], with no difference detected between OA and normal decellularized cartilage [37].…”

“…In this part of the review, we focused on proteins identified either directly in the secretome of cartilage explants [12][13][14][15][16][17][18][19][20] or chondrocytes cultures [18,[21][22][23][24][25][26][27][28][29][30][31]), both of which are listed in Table 1. We further complete this list with proteins recently identified by proteomic analysis performed directly on fresh chondrocytes or cartilage tissue [32][33][34][35][36] (sometimes de-cellularized [37]) whereby different locations in the joint were compared, or healthy joint tissues were compared with 6 OA tissues. The characteristics of all these studies are summarized in Table 2.…”

“…These are modulated by IL-1β, but are not significantly modified with the disease, except SERPINA3. SERPINA3, also known as α1-antichymotrypsin, is mainly located in the superficial layer of the articular cartilage [34] and is decreased during OA [35].…”

“…Similar to osteopontin, osteonectin is highly secreted in deep layer chondrocytes [34], and found to be increased in the secretome of medial condylar early OA cartilage [16,25,27]. TGF-β1 decreases the production of osteonectin [24].…”

“…Gremlin-1 is therefore, potentially, a potent inhibitor of chondrocyte hypertrophy in cartilage. Chondromodulin is also important for chondrocytes stabilization, acting by inhibiting hypertrophic differentiation, and is shown to be secreted only by superficially located chondrocytes [34]. Pleiotrophin is a secreted heparin-binding peptide expressed in mesodermal and neuroectodermal cells during development, but rarely in adult tissues.…”