Role of Ciliary Protein Intraflagellar Transport Protein 88 in the Regulation of Cartilage Thickness and Osteoarthritis Development in Mice
Authors: Coveney Clarissa R. , Zhu Linyi , Miotla‐Zarebska Jadwiga , Stott Bryony , Parisi Ida , Batchelor Vicky , Duarte Claudia , Chang Emer , McSorley Eleanor , Vincent Tonia L. , Wann Angus K. T. 
Published: June 2021

ObjectiveMechanical and biologic cues drive cellular signaling in cartilage development, health, and disease. Primary cilia proteins, which are implicated in the transduction of biologic and physiochemical signals, control cartilage formation during skeletal development. This study was undertaken to assess the influence of the ciliary protein intraflagellar transport protein 88 (IFT88) on postnatal cartilage from mice with conditional knockout of theIft88gene (Ift88‐KO).MethodsIft88fl/flandaggrecanCreERT2mice were crossed to create a strain of cartilage‐specificIft88‐KO mice (aggrecanCreERT2;Ift88fl/fl). In theseIft88‐KO mice andIft88fl/flcontrol mice, tibial articular cartilage thickness was assessed by histomorphometry, and the integrity of the cartilage was assessed using Osteoarthritis Research Society International (OARSI) damage scores, from adolescence through adulthood. In situ mechanisms of cartilage damage were investigated in the microdissected cartilage sections using immunohistochemistry, RNAScope analysis, and quantitative polymerase chain reaction. Osteoarthritis (OA) was induced inaggrecanCreERT2;Ift88fl/flmice andIft88fl/flcontrol mice using surgical destabilization of the medial meniscus (DMM). Following tamoxifen injection and DMM surgery, the mice were given free access to exercise on a wheel.ResultsDeletion ofIft88resulted in progressive reduction in the thickness of the medial tibial cartilage in adolescent mice, as well as marked atrophy of the cartilage in mice during adulthood. InaggrecanCreERT2;Ift88fl/flmice at age 34 weeks, the median thickness of the medial tibial cartilage was 89.42 μm (95% confidence interval [95% CI] 84.00–93.49), whereas inIft88fl/flcontrols at the same age, the median cartilage thickness was 104.00 μm (95% CI 100.30–110.50;P < 0.0001). At all time points, the median thickness of the calcified cartilage was reduced. In some mice, atrophy of the medial tibial cartilage was associated with complete, spontaneous degradation of the cartilage. Following DMM,aggrecanCreERT2;Ift88fl/flmice were found to have increased OARSI scores of cartilage damage. In articular cartilage from maturing mice, atrophy was not associated with obvious increases in aggrecanase‐mediated destruction or chondrocyte hypertrophy. Of the 44 candidate genes analyzed, onlyTcf7l2expression levels correlated withIft88expression levels in the microdissected cartilage. However, RNAScope analysis revealed that increased hedgehog (Hh) signaling (as indicated by increased expression ofGli1) was associated with the reductions inIft88expression in the tibial cartilage fromIft88‐deficient mice. Wheel exercise restored both the articular cartilage thickness and levels of Hh signaling in these mice.ConclusionOur results in a mouse model of OA demonstrate that IFT88 performs a chondroprotective role in articular cartilage by controlling the calcification of cartilage via maintenance of a threshold of Hh signaling during physiologic loading.