Long term cyclic compression of articular cartilage alters extracellular matrix turnover

Abstract

Purpose: Osteoarthritis (OA) is a disease characterized by progressive degradation of articular cartilage extracellular matrix (ECM), Mechanical factors play a large role in the initiation and progression of OA, where mechanical compression at physiologic levels of cartilage is shown to have chondroprotective effects, activating anabolic processes and stimulating matrix synthesis. Previous studies suggest that mechanical loading of cartilage increases aggrecan and type II collagen gene expression, and reduce matrix metalloprotease formation. Biomarkers specifically targeting aggrecan and type II collagen turnover, provides a unique opportunity to study the dynamic effects of mechanical loading on cartilage ECM turnover. A better understanding of these processes might aid the development of novel DMOADs. The aim of this study was to investigate the effects of long-term cyclic compression on the cartilage ECM turnover, through measurement of specific cartilage turnover biomarkers, using a bovine articular cartilage ex vivo model. Methods: Full depth bovine cartilage explants (BEX) were cultured for 2 weeks. The explants were treated 3 times a week with either OSM [10 ng/mL] and TNF-a [2 ng/mL] (O+T), or TGF-B1 [50 ng/mL]. Untreated samples were included as negative controls (w/o). For each condition two groups were established; an unloaded group and a group compressed 3 times a week. Compression was applied using Electroforce 5500® (TA Instruments), in a sine wave with a maximum load per cycle of 1 MPa, at 1 Hz frequency for 1200 cycles. Metabolic activity was measured once a week using AlamarBlue. Biomarkers released to the supernatant were assessed using the following well-described ELISAs: type II collagen was measured by ProC2, MMP-degradation of type II collagen was measured by C2M, and aggrecanase-mediated degradation of aggrecan was measured by AGNx1. Sulfated glycosaminoglycans (GAGs) released to media was measured using a 1.9-dimethylmethylene blue (DMB) assay. Results: Compression of BEX significantly decreased the MMP-degradation of type II collagen measured by C2M at day 14 with 48% and 42% compared to the unloaded samples in the untreated and the O-T treated groups respectively (Figure 1c). The compressed samples release similar levels of AGNx1 and ProC2 to the unloaded explants (Figure 1b+d). The metabolic activity similarly appears to be unaltered by compression (Figure 1a). Compression significantly increased the GAG release in the w/o group by 195% and by 152% in the TGF-B group. Conclusions: Compression of cartilage, and compared to standard cultures, resulted in significant effect on cartilage tissue turnover and metabolic profile. Compression reduced the degradation of type II collagen, which indicates an inhibition of the involved catabolic mechanisms. Furthermore, type II collagen formation was unaltered, which in combination with the reduced degradation may result in an elevated deposition of collagen in the explants. While the AGNx1 release was unchanged by compression, the total amount of GAG was increased in the supernatants, suggesting an increase in proteoglycan turnover in the explants. In conclusion, compression of cartilage resulted in altered cartilage turnover, which was quantified by molecular cartilage turnover biomarkers. This approach may be essential for translation science, selecting and validating the best approaches to regenerate cartilage or block degradation.