Aggrecanase Cleavage in Juvenile Idiopathic Arthritis Patients Is Minimally Detected in the Aggrecan Interglobular Domain but Robust at the Aggrecan C-Terminus

OBJECTIVE: To understand aggrecan degradation in juvenile idiopathic arthritis (JIA), the pattern and abundance of aggrecan fragments in synovial fluid aspirates from JIA patients were analysed and compared with aggrecan fragments in synovial fluids from patients with other arthritides, juvenile knee injury and a knee-healthy reference group. METHODS: The concentration of sulphated glycosaminoglycans in synovial fluid was measured by the Alcian blue precipitation assay. Aggrecan fragments were purified by dissociative CsCl density gradient centrifugation, deglycosylated and analysed by Western blot using antibodies specific for either aggrecanase-derived ARGS, SELE and KEEE neoepitopes, or the aggrecan G3-domain. RESULTS: The concentration of sulphated glycosaminoglycans in JIA synovial fluids was significantly lower compared with the levels in fluids from OA (P<0.001), juvenile knee injury (P=0.006) and knee-healthy reference (P=0.022) groups. Western blot analysis detected KEEE, SELE, and G3 fragments generated by aggrecanase cleavage in the chondroitin sulphate-rich region of JIA aggrecan. The pattern of JIA aggrecan fragments was not identical to that in synovial fluids pooled from OA patients, although there were notable similarities. Surprisingly, aggrecanase-derived ARGS fragments were barely detectable in the JIA synovial fluids, in marked contrast to the levels of ARGS fragments in OA synovial fluids. CONCLUSIONS: Aggrecanases appear to cleave minimally in the interglobular domain of aggrecan in JIA patients despite robust levels of cleavage in aggrecan's chondroitin-sulphate rich region. The results suggest that unlike other arthritides, aggrecanase cleavage in the aggrecan interglobular domain might not be a major pathogenic event in JIA. © 2012 American College of Rheumatology

Evidence for lysosomal exocytosis and release of aggrecan-degrading hydrolases from hypertrophic chondrocytes, in vitro and in vivo

The abundant proteoglycan, aggrecan, is resorbed from growth plate cartilage during endochondral bone ossification, yet mice with genetically-ablated aggrecan-degrading activity have no defects in bone formation. To account for this apparent anomaly, we propose that lysosomal hydrolases degrade extracellular, hyaluronan-bound aggrecan aggregates in growth plate cartilage, and that lysosomal hydrolases are released from hypertrophic chondrocytes into growth plate cartilage via Ca2+-dependent lysosomal exocytosis. In this study we confirm that hypertrophic chondrocytes release hydrolases via lysosomal exocytosis in vitro and we show in vivo evidence for lysosomal exocytosis in hypertrophic chondrocytes during skeletal development. We show that lysosome-associated membrane protein 1 (LAMP1) is detected at the cell surface following in vitro treatment of epiphyseal chondrocytes with the calcium ionophore, ionomycin. Furthermore, we show that in addition to the lysosomal exocytosis markers, cathepsin D and beta-hexosaminidase, ionomycin induces release of aggrecan-and hyaluronan-degrading activity from cultured epiphyseal chondrocytes. We identify VAMP-8 and VAMP7 as v-SNARE proteins with potential roles in lysosomal exocytosis in hypertrophic chondrocytes, based on their colocalisation with LAMP1 at the cell surface in secondary ossification centers in mouse tibiae. We propose that resorbing growth plate cartilage involves release of destructive hydrolases from hypertrophic chondrocytes, via lysosomal exocytosis. (C) 2012. Published by The Company of Biologists Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial Share Alike License (http://creativecommons.org/licenses/by-nc-sa/3.0)

Matrix metalloproteinases are not essential for aggrecan turnover during normal skeletal growth and development

The growth plate is a transitional region of cartilage and highly diversified chondrocytes that controls long bone formation. The composition of growth plate cartilage changes markedly from the epiphysis to the metaphysis, notably with the loss of type II collagen, concomitant with an increase in MMP-13; type X collagen; and the C-propeptide of type II collagen. In contrast, the fate of aggrecan in the growth plate is not clear: there is biosynthesis and loss of aggrecan from hypertrophic cartilage, but the mechanism of loss is unknown. All matrix metalloproteinases (MMPs) cleave aggrecan between amino acids N(341) and F(342) in the proteinase-sensitive interglobular domain (IGD), and MMPs in the growth plate are thought to have a role in aggrecanolysis. We have generated mice with aggrecan resistant to proteolysis by MMPs in the IGD and found that the mice develop normally with no skeletal deformities. The mutant mice do not accumulate aggrecan, and there is no significant compensatory proteolysis occurring at alternate sites in the IGD. Our studies reveal that MMP cleavage in this key region is not a predominant mechanism for removing aggrecan from growth plate cartilage

Matrix metalloproteinases 19 and 20 cleave aggrecan and cartilage oligomeric matrix protein (COMP)

Matrix metalloproteinase (MMP)-19 and MMP-20 (enamelysin) are two recently discovered members of the MMP family. These enzymes are involved in the degradation of the various components of the extracellular matrix (ECM) during development, haemostasis and pathological conditions. Whereas MMP-19 mRNA is found widely expressed in body tissues, including the synovium of normal and rheumatoid arthritic patients, MMP-20 expression is restricted to the enamel organ. In this study we investigated the ability of MMP-19 and MMP-20 to cleave two of the macromolecules characterising the cartilage ECM, namely aggrecan and the cartilage oligomeric matrix protein (COMP). Both MMPs hydrolysed aggrecan efficiently at the well-described MMP cleavage site between residues Asn341 and Phe342, as shown by Western blotting using neo-epitope antibodies. Furthermore, the two enzymes cleaved COMP in a distinctive manner, generating a major proteolytic product of 60 kDa. Our results suggest that MMP-19 may participate in the degradation of aggrecan and COMP in arthritic disease, whereas MMP-20, due to its unique expression pattern, may primarily be involved in the turnover of these molecules during tooth development. Copyright (C) 2000 Federation of European Biochemical Societies