Phosphorylation-dependent Akt-Inversin interaction at the basal body of primary cilia

A primary cilium is a microtubule‐based sensory organelle that plays an important role in human development and disease. However, regulation of Akt in cilia and its role in ciliary development has not been demonstrated. Using yeast two‐hybrid screening, we demonstrate that Inversin (INVS) interacts with Akt. Mutation in the INVS gene causes nephronophthisis type II (NPHP2), an autosomal recessive chronic tubulointerstitial nephropathy. Co‐immunoprecipitation assays show that Akt interacts with INVS via the C‐terminus. In vitro kinase assays demonstrate that Akt phosphorylates INVS at amino acids 864–866 that are required not only for Akt interaction, but also for INVS dimerization. Co‐localization of INVS and phosphorylated form of Akt at the basal body is augmented by PDGF‐AA. Akt‐null MEF cells as well as siRNA‐mediated inhibition of Akt attenuated ciliary growth, which was reversed by Akt reintroduction. Mutant phosphodead‐ or NPHP2‐related truncated INVS, which lack Akt phosphorylation sites, suppress cell growth and exhibit distorted lumen formation and misalignment of spindle axis during cell division. Further studies will be required for elucidating functional interactions of Akt–INVS at the primary cilia for identifying the molecular mechanisms underlying NPHP2. Synopsis Cilia are sensory organelles that play important roles in human kidney development and disease. Mutation of the cilial protein Inversin (INVS) causes autosomal recessive chronic nephropathy (nephronophthisis type II; NPHP2), and accordingly, INVS has been found to associate with nephrocystin 3 (NPHP3) and microtubule cytoskeleton. Here, the kinase Akt is shown to regulate cilia physiology and renal integrity by interaction with the INVS at the basal body. Akt directly interacts with and phosphorylates INVS. INVS conserved amino acids 864–866 are required for phosphorylation by Akt, as well as INVS homodimerization. INVS and p‐Akt co‐localize at the ciliary base centrosome, and recruitment of INVS is stimulated by PDGFRα/Akt signaling. MDCK cells expressing a mutant INVS that lacks the phosphorylation site 864–866 exhibit impaired ciliogenesis and misalignment of spindle axis during cell division, leading to suppressed cell growth and distorted formation of tubular lumina. Graphical Abstract Akt–Inversin interplay exemplifies a new Akt role in ciliary physiology, ensuring spindle axis integrity during cell division and kidney development.