Distinct galectin profile in inflammation-mediated cartilage damage

Purpose: Galectins (Gal) are a family of carbohydrate binding proteins that serve as regulators of fundamental numerous biological processes events like proliferation, cell adhesion and inflammation. Recently, specific galectins including Gal-1, -3, and -8 were shown to be overexpressed in human osteoarthritic cartilage and to trigger a gene signature in chondrocytes in vitro that is associated with inflammatory processes and matrix degradation. To date, however, the role of galectins in cartilage breakdown has not been elucidated using in vivo models, limiting mechanistic insights into their precise function. Thus, the present study aims to comprehensively investigate galectin profiles (including Gal-1, -2, -3, -4, -7, -8 and 9) in a mouse model of TNF-driven inflammatory cartilage damage. Methods: Knee, ankle and wrist joints from 10-week old human tumor necrosis factor transgenic mice (hTNFtg, Tg197 strain), an established chronic inflammatory, erosive arthritis model, and from age-matched wild-type (wt) littermates were isolated for histological analysis and RNA isolation. Immunohistochemical staining for Gal-1, -2,-3,-4, -7, -8 and 9 was performed in paraffin-embedded tissue section from knee joints. Areas of galectin expression in femoral and tibial cartilage were quantitatively assessed using TissueQuest software. Safranin-O (SafO) and Toluidine-blue (TB) stained sections were histologically evaluated for cartilage damage indicated by proteoglycan loss and cartilage erosion. Cartilage damage was semi-quantitatively assessed by using a modified OARSI score and quantitatively assessed by using the Osteomeasure software. Linkage between galectin-stained areas and cartilage damage was calculated by Spearman correlation analysis. mRNA expression of galectins, pro-inflammatory cytokines and matrix-degrading enzymes was determined by RT-qPCR from RNA isolated from front paws. Results: By investigating SafO and TB stained knee sections, we observed femoral and tibial cartilage damage in hTNFtg mice upon progressive TNF-driven joint inflammation. Cartilage damage was indicated by complete proteoglycan loss, partial cartilage erosions of the deeper, calcified zone and invasion of the inflammatory synovial pannus tissue. Immunohistochemical stainings revealed an increased expression of Gal-1, -4, -7 and -8, but not of Gal-2, -3 and -9 in cartilage from hTNFtg mice compared to healthy cartilage from wt animals. Interestingly, these findings were observed particularly in tibial rather than femoral cartilage. Whereas most galectins were found to be exclusively expressed in chondrocytes, Gal-1, -4 and -7 overexpression was also apparent in cartilage matrix of hTNFtg mice. Furthermore, correlation analysis revealed a significant linkage between increased galectin-1, -4, -7, -8 expression and progression of cartilage erosion. In contrast, Gal-9 was found to be downregulated upon damage. Conclusions: Under TNF-driven inflammatory conditions, a distinct pattern of glycan-binding galectins is notable in articular cartilage. The change in galectin pattern is strongly linked to the progression of cartilage erosion, which suggests a functional role of galectins in the pathophysiology of inflammatory cartilage damage.