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Sortilin is associated with cartilage calcification in osteoarthritis


Elisabeth Richter, Christoph Lohmann, Claudia Goettsch, Francesco Dell’Accio, Jessica Bertrand

Otto-Von-Guericke University, Magdeburg, Germany


Background: Osteoarthritis is a chronic joint disease characterized by articular cartilage calcification, loss of articular cartilage, bone changes, pain, and disability. Cartilage calcification is predominantly caused by basic calcium crystals formed due to an imbalance of pyrophosphate pathway. Sortilin is a transmembrane protein that has been shown to contribute to the calcification of vessel walls in atherosclerosis by externalizing alkaline phosphatase-containing vesicles. The aim of this study was to investigate a potential role of sortilin on osteoarthritis-mediated cartilage calcification.

Methods: Osteoarthritic cartilage from human knee joints was collected after unicondylar or total endoprothetic surgery and samples were analyzed by immunohistochemistry and quantitative RT-PCR analysis. Human chondrocytes were treated with osteogenic medium up to 21 days to induce calcification. Western Blot for sortilin and alkaline phosphatase as well as alkaline phosphatase activity assay were performed. Sections of knee joints from a murine osteoarthritis model were stained and analyzed by immunohistochemistry. Treatment of wild-type chondrocytes using osteogenic medium similar to human chondrocytes was performed.

Results: Osteoarthritic cartilage from human knee joints showed an increase of sortilin in chondrocytes compared to healthy cartilage. This observation was corroborated by an increased gene expression of sortilin in mild and moderate osteoarthritic cartilage samples. To investigate the mechanism of sortilin regulation human chondrocytes were treated with osteogenic medium to induce calcification. Sortilin protein levels and expression were increased after 7 days of stimulation whereas alkaline phosphatase levels and activity were upregulated after 21 days of stimulation. Similar observations were made in a murine osteoarthritis model. Both Sortilin and alkaline phosphatase were increased in chondrocytes. To induce calcification, chondrocytes from wild-type mice were also treated with osteogenic medium, which resulted in an increase of sortilin as well as alkaline phosphatase and alkaline phosphatase activity.

Conclusion: Sortilin and alkaline phosphatase therefore appear to have an influence on cartilage cell calcification. The causal effect of sortilin on alkaline phosphatase should be the subject of further research. Our data indicate increased sortilin expression in cartilage calcification and thus might contribute to the pathogenesis of osteoarthritis.