WNT16 in Chondrocyte Biology and Lineage Determination

Abstract

Background. During development WNT16 is a specific marker of the superficial zone of the developing articular cartilage. In adult joints WNT16 is no longer expressed, but it becomes rapidly upregulated in the articular cartilage following injury and in osteoarthritis. Unpublished data from our laboratory show that WNT16 deficient mice are more susceptible to instability-induced osteoarthritis. The superficial zone of the articular cartilage contains chondrogenic progenitor cells which are essential for the long-term maintenance of cartilage homeostasis. These cells express WNT16 and also specifically express Lubricin. Lubricin, in adulthood, is essential for joint lubrication and in its absence, mice develop spontaneous osteoarthritis. Results. Exogenous WNT16 dose-dependently increased Lubricin expression in wnt16-/- superficial zone cells and primary bovine chondrocytes. WNT16 treatment also dose-dependently modulated Axin2, a transcriptional target of the canonical Wnt pathway. At low concentration Wnt16 downregulated axin2 expression, while higher concentrations caused upregulation compared to control. WNT16 also caused a dose-dependent phosphorylation of c-Jun transcription factor. In keeping with my data in chondrocytes, in xenopus laevis experiments, WNT16 had a limited capacity to induce features of axis duplication, but could efficiently inhibit axis duplication induced by WNT8. Importantly, both DKK1 and TCS (inhibitors of the canonical Wnt pathway and of the JNK pathways, respectively) prevented the WNT16-induced Lubricin upregulation, thereby demonstrating that modulation of the gene by WNT16 requires both the canonical Wnt pathway and the JNK pathways. WNT16 supported the phenotype of superficial zone cells by enhancing Lubricin expression and by preventing their full chondrocytic maturation: extracellular matrix production was increased in wnt16-/- superficial zone cells and chondrocyte specific marker were lost upon WNT16 stimulation in these cells as well as in bovine primary chondrocytes. Conclusion. WNT16 is a weak activator of the canonical Wnt pathway which supports lubricin expression and the phenotype of the cartilage superficial zone cells.