An uncanonical CDK6 activity inhibits cilia function by suppressing axoneme polyglutamylation

Tubulin polyglutamylation is a post-translational modification that occurs primarily along the axoneme of cilia. Defective axoneme polyglutamylation impairs cilia function and has been correlated with ciliopathies, including Joubert Syndrome (JBTS). However, the precise mechanisms regulating proper axoneme polyglutamylation remain vague. Here, we show that Cyclin-Dependent Kinase 6 (CDK6), but not its paralog CDK4, localizes to cilia base and suppresses axoneme polyglutamylation by phosphorylating RAB11-interacting protein FIP5 at site S641, a critical regulator of cilia import of glutamylases. S641 phosphorylation disrupts the ciliary recruitment of FIP5 by impairing its association with RAB11, thereby reducing the ciliary import of glutamylases. Interestingly, significant upregulation of CDK6 and defective axoneme polyglutamylation were observed in Autosomal dominant polycystic kidney disease (ADPKD) cells. Encouragingly, the FDA-approved CDK4/6 inhibitor Abemaciclib can effectively restore cilia function in JBTS and ADPKD cells with defective glutamylation and suppresses renal cystogenesis in an ex vivo ADPKD model. In summary, our study elucidates regulatory mechanisms governing axoneme polyglutamylation and suggests developing CDK6-specific inhibitors could be a promising therapeutic strategy to enhance cilia function in ciliopathy patients.