Objective Calcification of cartilage with basic calcium phosphate (BCP) crystals is a common phenomenon during osteoarthritis (OA). It is directly linked to the severity of the disease and known to be associated to hypertrophic differentiation of chondrocytes. One morphogen regulating hypertrophic chondrocyte differentiation is Wnt3a.
Methods Calcification and sulfation of extracellular matrix of the cartilage was analysed over a time course from 6 to 22 weeks in mice and different OA grades of human cartilage. Wnt3a and ß-catenin was stained in human and murine cartilage. Expression of sulfation modulating enzymes (HS2St1, HS6St1) was analysed using quantitative reverse transcription PCR (RT-PCR). The influence of BCP crystals on the chondrocyte phenotype was investigated using quantitative RT-PCR for the marker genes Axin2, Sox9, Col2, MMP13, ColX and Aggrecan. Using western blot for β-catenin and pLRP6 we investigated the activation of Wnt signalling. The binding capacity of BCP for Wnt3a was analysed using immunohistochemical staining and western blot.
Results Here, we report that pericellular matrix sulfation is increased in human and murine OA. Wnt3a co-localised with heparan sulfate proteoglycans in the pericellular matrix of chondrocytes in OA cartilage, in which canonical Wnt signalling was activated. In vitro, BCP crystals physically bound to Wnt3a. Interestingly, BCP crystals were sufficient to induce canonical Wnt signalling as assessed by phosphorylation of LRP6 and stabilisation of β-catenin, and to induce a hypertrophic shift of the chondrocyte phenotype.
Conclusion Consequently, our data identify BCP crystals as a concentrating factor for Wnt3a in the pericellular matrix and an inducer of chondrocyte hypertrophy.
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Handling editor Josef S Smolen
Correction notice This article was corrected since it published Online First. The funding statement has been updated.
Contributors JB performed most experiments and wrote the manuscript, TK performed the mouse histology and stainings and the Western Blots for activation of Wnt signalling, TG performed the human Alcian/PAS stainings, JS helped writing the manuscript and discussing the data, FR provided the ttw/ttw mice, FL provided the basic calcium phosphate crystals and helped discussing the data, FDL helped discussing the data and writing of the manuscript, CHL provided the human samples and discussed the data, MB performed the safranin-orange stainings of the human samples, AH performed the Wnt3a and ß-catenin stainings of the human samples, TP helped writing the manuscript and discussed the data.
Funding This study was funded by Deutsche Forschungsgemeinschaft (Emmy Noether BE4328/5-1) and the COST Action 16115 EuroSoftCalcNet.
Competing interests None declared.
Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting or dissemination plans of this research.
Patient consent for publication Not required.
Ethics approval Ethical approval for this study was given by the Institutional Review Board (IRB) of the Medical School, Otto-von-Guericke University, Magdeburg (IRB No 23/16).
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement Data are available upon reasonable request. Protocols and statistical analyses can be made available upon request. Please contact the corresponding author (email@example.com).