Roles of ß-catenin signaling in phenotypic expression and proliferation of articular cartilage superficial zone cells.

The superficial zone (SFZ) of articular cartilage has unique structural and biomechanical features, is thought to promote self-renewal of articular cartilage, and is thus important for joint long-term function, but the mechanisms regulating its properties remain unclear. Previous studies revealed that Wnt/ß-catenin signaling is continuously active in SFZ, indicating that it may be essential for SFZ function. Thus, we examined whether Wnt/ß-catenin signaling regulates proliferation and phenotypic expression in SFZ cells. Using transgenic mice, we found that acute activation of Wnt/ß-catenin signaling increases SFZ thickness, Proteoglycan 4 (Prg4, also called lubricin) expression and the number of slow-cell cycle cells, whereas conditional ablation of ß-catenin causes the opposite. We developed a novel method to isolate SFZ cell-rich populations from the epiphyseal articular cartilage of neonatal mice, and found that the SFZ cells in culture exhibit a fibroblastic cytoarchitecture and higher Prg4 and Ets-related gene (Erg) expression and lower aggrecan expression compared with chondrocyte cultures. Gene array analyses indicated that SFZ cells have distinct gene expression profiles compared with underlying articular chondrocytes. Treatment of Wnt3a strongly stimulated SFZ cell proliferation and maintained strong expression of Prg4 and Erg, whereas ablation of ß-catenin strongly impaired proliferation and phenotypic expression. When the cells were transplanted into athymic mice, they formed Prg4- and aggrecan-expressing cartilaginous masses attesting to their autonomous phenotypic capacity. Ablation of ß-catenin caused a rapid loss of Prg4 gene expression and strong increases in expression of aggrecan and collagen 10, the latter being a trait of hypertrophic chondrocytes. Together, the data reveal that Wnt/ß-catenin signaling is a key regulator of SFZ cell phenotype and proliferation, and may be as important for articular cartilage long-term function.