PI3K/Akt signaling regulates ciliogenesis initiation via Rab11-effector vesicular trafficking switch

Authors

• V Walia
• C Insinna (Kettenhofen)
• Q Lu
• S Specht
• CJ Westlake

Abstract

Defects in primary cilium formation and signaling are associated with a growing list of genetic diseases. A critical step in the initiation of ciliogenesis requires pre-ciliary vesicle trafficking to the mother centriole. Previously, we showed that serum starvation induced rapid association of Rabin8, a Rab8 GEF, with Rab11 pre-ciliary vesicles, a step needed to initiate ciliogenesis. We identified lysophosphatidic acid (LPA) and TGF-b as novel inhibitors of ciliogenesis. Interestingly, only LPA blocked Rabin8 pre-ciliary trafficking suggesting that LPA functions specifically at the ciliogenesis initiation step. We show that the LPAR1 receptor, but not LPAR2-5, functions in the inhibition of pre-ciliary trafficking and ciliogenesis. Using chemical inhibitors against LPAR1 downstream pathways, we determined that inhibition of PI3K/Akt signaling induced Rabin8 pre-ciliary trafficking and promoted ciliogenesis. While Rabin8 was found to be an Akt substrate, there was no observed effect of this phosphorylation on Rab11 binding or ciliogenesis. We next theorized that Akt-phosphorylation regulates Rab11-Rabin8 interaction indirectly via a Rab-effector switch. RNAi studies for Rab11-effectors demonstrated that WDR44/Rabphillin-11 ablation promotes Rabin8 centrosomal trafficking in the presence of serum. We mapped Akt sites on WDR44 within the Rab11 binding domain and showed that phosphorylation status of WDR44 coincided with Rab11-Rabin8 interaction upon serum starvation. Importantly, a phospho-mimetic WDR44 mutant showed stronger binding to Rab11 and inhibited ciliation upon serum starvation. Together these findings uncover a novel role for a growth factor signaling network mediated by Akt kinase and its downstream substrate, WDR44, in the regulation of pre-ciliary vesicle trafficking and ciliogenesis initiation.

Scientific Focus Area: Developmental Biology

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