The overall pattern of aggrecan fragments in human OA synovial fluid and cartilage supports an important role for aggrecanase in aggrecan degradation. However, the fragment patterns and their differential distribution between cartilage and synovial fluid are consistent with the existence of at least two proteolytic pathways for aggrecan degradation in human OA, generating both (342)FFGV- and (374)ARGS-fragments.

Objective. To prospectively monitor levels of proinflammatory cytokines and aggrecan ARGS neoepitope in synovial fluid and serum as well as levels of C-terminal crosslinking telopeptide of type II collagen (CTX-II) and N-terminal crosslinking telopeptide of type I collagen (NTX-I) in urine after acute anterior cruciate ligament (ACL) rupture.Methods. Synovial fluid, serum, and urine were collected from 121 adults on 6 occasions over 5 years after acute ACL injury. Reference samples were obtained from subjects without knee injury. Concentrations of interleukin-6 (IL-6), IL-8, IL-10, interferon-g (IFNg), tumor necrosis factor (TNF), aggrecan ARGS neoepitope, CTX-II, and NTX-I were measured by enzymelinked immunosorbent assay.Results. Shortly after ACL injury, cytokine concentrations in synovial fluid were elevated 6-fold (TNF) to 1,050-fold (IL-6) compared to reference levels, while concentrations of aggrecan ARGS neoepitope in synovial fluid and serum and CTX-II in urine were elevated 1.4-fold to 8-fold. Thereafter, concentrations of cytokines and aggrecan ARGS neoepitope in synovial fluid decreased with different half-lives (in years: IL-6 0.9, IL-8 2.2, IL-10 2.3, IFNg 3.1, TNF 3.6, aggrecan ARGS neoepitope 4.0). After 5 years, the TNF concentration in synovial fluid remained higher than the reference level. There was a correlation between the concentrations of aggrecan ARGS neoepitope in synovial fluid and serum (r s 5 0.36). Concentrations of aggrecan ARGS neoepitope in synovial fluid and of CTX-II and NTX-I in urine were correlated with concentrations of cytokines in synovial fluid (r s 5 0.41-0.49 and r s 5 0.21-0.31, respectively).Conclusion. Acute ACL injury induced highly increased levels of inflammatory cytokines in the joint, and these were associated with proteolysis of aggrecan and type II collagen. Cytokine levels remained increased up to 5 years after injury, indicative of extended local inflammation in the joint.

Our results suggest that an acute knee injury is associated with an instant local biochemical response to the trauma, which may affect cartilage and bone as well as the inflammatory activity.

Osteochondritis dissecans is a disorder in which fragments of articular cartilage and subchondral bone dislodge from the joint surface. We analyzed a five-generation family in which affected members had autosomal-dominant familial osteochondritis dissecans. A genome-wide linkage analysis identified aggrecan (ACAN) as a prime candidate gene for the disorder. Sequence analysis of ACAN revealed heterozygosity for a missense mutation (c.6907G > A) in affected individuals, resulting in a p.V2303M amino acid substitution in the aggrecan G3 domain C-type lectin, which mediates interactions with other proteins in the cartilage extracellular matrix. Binding studies with recombinant mutated and wild-type G3 proteins showed loss of fibulin-1, fibulin-2, and tenascin-R interactions for the V2303M protein. Mass spectrometric analyses of aggrecan purified from patient cartilage verified that V2303M aggrecan is produced and present in the tissue. Our results provide a molecular mechanism for the etiology of familial osteochondritis dissecans and show the importance of the aggrecan C-type lectin interactions for cartilage function in vivo.

Introduction Aggrecanase cleavage at the 392 Glu-393 Ala bond in the interglobular domain (IGD) of aggrecan, releasing Nterminal 393 ARGS fragments, is an early key event in arthritis and joint injuries. Here, we use a quantitative immunoassay of aggrecan ARGS neoepitope fragments in human synovial fluid to determine if this cleavage-site specific method better identifies joint pathology than previously available less specific aggrecan assays.