β1 Integrin Deficiency Results in Multiple Abnormalities of the Knee Joint

Abstract

The lack of ␤1 integrins on chondrocytes leads to severe chondrodysplasia associated with high mortality rate around birth. To assess the impact of ␤1 integrin-mediated cell-matrix interactions on the function of adult knee joints, we conditionally deleted the ␤1 integrin gene in early limb mesenchyme using the Prx1-cre transgene. Mutant mice developed short limbed dwarfism and had joint defects due to ␤1 integrin deficiency in articular regions. The articular cartilage (AC) was structurally disorganized, accompanied by accelerated terminal differentiation, altered shape, and disrupted actin cytoskeleton of the chondrocytes. Defects in chondrocyte proliferation, cytokinesis, and survival resulted in hypocellularity. However, no significant differences in cartilage erosion, in the expression of matrix-degrading proteases, or in the exposure of aggrecan and collagen II cleavage neoepitopes were observed between control and mutant AC. We found no evidence for disturbed activation of MAPKs (ERK1/2, p38, and JNK) in vivo. Furthermore, fibronectin fragment-stimulated ERK activation and MMP-13 expression were indistinguishable in control and mutant femoral head explants. The mutant synovium was hyperplastic and frequently underwent chondrogenic differentiation. ␤1-null synoviocytes showed increased proliferation and phospho-focal adhesion kinase expression. Taken together, deletion of ␤1 integrins in the limb bud results in multiple abnormalities of the knee joints; however, it does not accelerate AC destruction, perturb cartilage metabolism, or influence intracellular MAPK signaling pathways. Chondrocytes of the articular cartilage (AC)2 secrete a unique set of extracellular matrix (ECM) molecules that assemble into interactive associates composed of collagens, proteoglycans (PGs), and non-collagenous glycoproteins (1). The fibrillar collagen meshwork supplies cartilage with its tensile strength, whereas the hydrated glycosaminoglycan (GAG) chains of PGs (mainly aggrecan) generate an osmotic swelling pressure that resists compressive forces. In diarthrodial joints, the molecular composition and the physical properties of the cartilage are principal determinants for the shock-absorbing function of articular surfaces upon mechanical loading. During the development of osteoarthritis (OA), an imbalance between anabolic and catabolic processes increases the proteolysis of PGs and collagens (2, 3), which eventually leads to the mechanical weakening of the AC and culminates in its progressive destruction. Physiological and pathological remodeling of the AC ECM is primarily attributed to the activities of matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinase with thrombospondin-like repeat (ADAMTS) proteases (4, 5) and is controlled by the communication between the cells and their environment.An increasing amount of evidence suggests that interactions between chondrocytes and the ECM through the integrin family of heterodimeric (␣␤) transmembrane receptors play a central role in cartilage function (6). Int...

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