Development of a highly sensitive chemiluminescence immunoassay for quantification of aggrecanase-generated ARGS aggrecan fragments in serum

Soluble Proteoglycans and Proteoglycan Fragments as Biomarkers of Pathological Extracellular Matrix Remodeling

Proteoglycans and their proteolytic fragments diffuse into biological fluids such as plasma, serum, urine, or synovial fluid, where they can be detected by antibodies or mass-spectrometry. Neopeptides generated by the proteolysis of proteoglycans are recognized by specific neoepitope antibodies and can act as a proxy for the activity of certain proteases. Proteoglycan and proteoglycan fragments can be potentially used as prognostic, diagnostic, or theragnostic biomarkers for several diseases characterized by dysregulated extracellular matrix remodeling such as osteoarthritis, rheumatoid arthritis, atherosclerosis, thoracic aortic aneurysms, central nervous system disorders, viral infections, and cancer. Here, we review the main mechanisms accounting for the presence of soluble proteoglycans and their fragments in biological fluids, their potential application as diagnostic, prognostic, or theragnostic biomarkers, and highlight challenges and opportunities ahead of their clinical translation.

Biological variation of human aggrecan ARGS neoepitope in synovial fluid and serum in early-stage knee osteoarthritis and after knee injury

Objective

To determine biological variation of the aggrecanase-generated aggrecan ARGS neoepitope in serum (sARGS) and synovial fluid (sfARGS) within and between patients with osteoarthritis (OA) or anterior cruciate ligament (ACL) injury.

Design

Matched samples of serum and synovial fluid were available, as parts of clinical trials, from i) 16 subjects with early-stage OA on 8 occasions over 1 year, and ii) 120 subjects with acute ACL injury with samples available from at least 2 of 6 visits over 5 years. We used an in-house immunoassay to quantify ARGS and one-way ANOVA for statistical analyses.

Results

Variability in ARGS was higher in synovial fluid than in serum in both patient groups. Subjects with OA had the lowest variability both within and between patients and showed no variation over time in the degree of variability or in the cross-sectional mean, neither in serum nor in synovial fluid. After ACL injury, the concentration and the variability of ARGS was highest immediately after injury, with a subsequent decline both in concentration and in variability with time. In both patient groups there was a positive correlation between sfARGS and sARGS both within and between individuals (correlation coefficients between 0.16 and 0.20).

Conclusions

The biological variation of ARGS is lower in serum than in synovial fluid, and lower in OA than after ACL injury. Serum ARGS is a measure of the total release of ARGS aggrecan from the whole body and a poor reflection of the release of ARGS aggrecan within the affected joint.

Soluble biological markers in osteoarthritis

In recent years, markers research has focused on the structural components of cartilage matrix. Specifically, a second generation of degradation markers has been developed against type II collagen neoepitopes generated by specific enzymes. A particular effort has been made to measure the degradation of minor collagens III and X of the cartilage matrix. However, because clinical data, including longitudinal controlled studies, are very scarce, it remains unclear whether they will be useful as an alternative to or in combination with current more established collagen biological markers to assess patients with osteoarthritis (OA). In addition, new approaches using high-throughput technologies allowed to detect new types of markers and improve the knowledge about the metabolic changes linked to OA. The relative advances coming from phenotype research are a first attempt to classify the heterogeneity of OA, and several markers could improve the phenotype characterization. These phenotypes could improve the selection of patients in clinical trials limiting the size of the studies by selecting patients with OA characteristics corresponding to the metabolic pathway targeted by the molecules evaluated. In addition, the inclusion of rapid progressors only in clinical trials would facilitate the demonstration of efficacy of the investigative drug to reduce joint degradation. The combination of selective biochemical markers appears as a promising and cost-effective approach to fulfill this unmet clinical need. Among the various potential roles of biomarkers in OA, their ability to monitor drug efficacy is probably one of the most important, in association with clinical and imaging parameters. Biochemical markers have the unique property to detect changes in joint tissue metabolism within a few weeks.