The ciliary protein IFT88 controls post-natal cartilage thickness and influences development of osteoarthritis.

Mechanical forces are known to drive cellular signalling programmes in cartilage development, health, and disease. Proteins of the primary cilium, implicated in mechanoregulation, control cartilage formation during skeletal development, but their role in post-natal cartilage is unknown. Ift88fl/fl and AggrecanCreERT2 mice were crossed to create a cartilage specific inducible knockout mouse AggrecanCreERT2;Ift88fl/fl. Tibial articular cartilage thickness was assessed, through adolescence and adulthood, by histomorphometry and integrity by OARSI score. In situ cell biology was investigated by immunohistochemistry (IHC) and qPCR of micro-dissected cartilage. OA was induced by destabilisation of the medial meniscus (DMM). Some mice were provided with exercise wheels in their cage.Deletion of IFT88 resulted in a reduction in medial articular cartilage thickness (atrophy) during adolescence from 102.57m, 95% CI [94.30, 119.80] in control (Ift88fl/fl) to 87.36m 95% CI [81.35, 90.97] in AggrecanCreERT2;Ift88fl/fl by 8-weeks p<0.01, and adulthood (104.00m, 95% CI [100.30, 110.50] in Ift88fl/fl to 89.42m 95% CI [84.00, 93.49] in AggrecanCreERT2;Ift88fl/fl, 34-weeks, p<0.0001) through a reduction in calcified cartilage. Thinning in adulthood was associated with spontaneous cartilage degradation. Following DMM, AggrecanCreERT2;Ift88fl/fl mice had increased OA (OARSI scores at 12 weeks Ift88fl/fl = 22.08 +/- 9.30, and AggrecanCreERT2;Ift88fl/fl = 29.83 +/- 7.69). Atrophy was not associated with aggrecanase-mediated destruction or chondrocyte hypertrophy. Ift88 expression positively correlated with Tcf7l2 and connective tissue growth factor. Cartilage thickness was restored in AggrecanCreERT2;Ift88fl/fl by voluntary wheel exercise.