RESUMEN

The balance between synthesis and degradation of the cartilage extracellular matrix is severely altered in osteoarthritis, where degradation predominates. One reason for this imbalance is believed to be due to the ligation of the α5ß1 integrin, the classic fibronectin (FN) receptor, with soluble FN fragments instead of insoluble FN fibrils, which induces matrix metalloproteinase (MMP) expression. Our objective was to determine whether the lack of α5ß1-FN binding influences cartilage morphogenesis in vivo and whether non-ligated α5ß1 protects or aggravates the course of osteoarthritis in mice. We engineered mice (Col2a-Cre;Fn1RGE/fl), whose chondrocytes express an α5ß1 binding-deficient FN, by substituting the aspartic acid of the RGD cell-binding motif with a glutamic acid (FN-RGE). At an age of 5 months the knee joints were stressed either by forced exercise (moderate mechanical load) or by partially resecting the meniscus followed by forced exercise (high mechanical load). Sections of femoral articular knees were analysed by Safranin-O staining and by immunofluorescence to determine tissue morphology, extracellular matrix proteins and matrix metalloproteinase expression. The articular cartilage from untrained control and Col2a-Cre;Fn1RGE/fl mice was normal, while the exposure to high mechanical load induced osteoarthritis characterized by proteoglycan and collagen type II loss. In the Col2a-Cre;Fn1RGE/fl articular cartilage osteoarthritis progressed significantly faster than in wild type mice. Mechanistically, we observed increased expression of MMP-13 and MMP-3 metalloproteinases in FN-RGE expressing articular cartilage, which severely affected matrix remodelling. Our results underscore the critical role of FN-α5ß1 adhesion as ECM sensor in circumstances of articular cartilage regeneration.

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RESUMEN

BACKGROUND: Osteoarthritis (OA) is a chronic degenerative disease of the articular cartilage, and its diagnosis is based on symptoms and radiological signs that are only present in the late stages of the disease. Due to the limitations in diagnosing OA before the onset of symptoms, such as pain, little is known about the molecular mechanisms involved in the pathogenesis of OA. Experimental OA models are often used to study the kinetics of the progression of this disease. In this report, we conducted a proteomic study of osteoarthritic cartilage during the early stages of OA using an experimental rat model. RESULTS: Ten proteins that are differentially expressed under early OA conditions were identified by 2-DE and MALDI-TOF/MS. These proteins mediated many processes, such as glycolysis and energy production (Nme2 and Pnp), cartilage matrix (Col2a1), transcription and protein synthesis (Eef1a1 and DJ-1), signal transduction (CaM and Pebp1), transport (Alb and Hba1), and latexin (Lxn). In addition, changes in Lxn expression in early OA were observed and validated by western blot and immunofluorescence analysis. CONCLUSIONS: The proteins that we identified indicate that energy metabolism, cartilage matrix remodelling, and protective cellular mechanisms are associated with early OA. In addition, latexin expression during the early stages of OA could be implicated in cartilage repair.

RESUMEN

We identify changes in the expression and localization of α5, α4, and α2 integrins during osteoarthritis (OA) pathogenesis in a rat experimental model. The changes were concomitant with variations in the extracellular matrix (ECM) content and the increase of metalloproteinases (MMPs) activity during OA pathogenesis, which were analyzed by immunofluorescence and Western blot assays. Our results showed an increased expression of α5 and α2 integrins at OA late stages, which was co-related with changes in the ECM content, as a consequence of the MMPs activity. In addition, this is the first report that has shown the presence of α4 integrin since OA early stages, which was co-related with the loss of proteoglycans and clusters formation. However, at late OA stages, the increased expression of α4 integrin in the middle and deep zones of the cartilage was also co-related with the abnormal endochondral ossification of the cartilage through its interaction with osteopontin. Finally, we conclude that ECM-chondrocytes interaction through specific cell receptors is essential to maintain the cartilage homeostasis. However, due to integrins cell signaling is ligand-dependent; changes in the ECM contents could induce activation of either anabolic or catabolic processes, which limits the reparative capacity of chondrocytes, favoring OA severity.

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