CARTILAGE ABNORMALITIES ASSOCIATED WITH CHONDROCYTIC PRIMARY CILIA DEFECTS

Abstract

Introduction: Primary cilia are found on virtually every mammalian cell; however, functions of primary cilia have not been extensively studied in chondrocytes. Interestingly, defects in the primary cilium result in skeletal defects such as polydactyly in Bardet-Biedl Syndrome (Bbs), a ciliary disorder that also results in obesity and retinopathy.

Wild-type mice and mutant mice of the ciliary proteins Bbs1, Bbs2, and Bbs6 were evaluated for histological and biochemical differences in chondrocytes from articular cartilage. The aim was to examine cartilage abnormalities related to ciliary defects in Bbs mutant mice.

Methods: Using immunofluorescence microscopy, chondrocytic cilia were visualized from load-bearing joints. Knee joints were then embedded in paraffin, stained, and serially sectioned. Articular cartilage was analyzed microscopically to evaluate histological differences between wild-type and mutant mice. Separately, chondrocytes were expanded in cell culture and implanted in solid agarose plugs that were sectioned over two weeks to quantify differences between mouse strains.

Results: Significant differences in ciliary morphology were not identified between mouse strains. However, histological analysis revealed that Bbs mutant mice had significantly lower articular joint thickness (p< .05) and lower proteogly-can content saturation (p< .05) than wild-type. Moreover, there were significant cell distribution differences between mouse strains (p< .05), indicating that mutant cartilage had changes consistent with early osteoarthritis. In cell culture, the fraction of ciliated cells in Bbs mutant cultures was significantly lower than in wild-type cultures (p< .05).

Discussion/Conclusion: These data indicate that Bbs gene function plays a role in normal cartilage maintenance and suggest that the chondrocytic primary cilium contributes significantly to articular cartilage biochemistry.