Cilia-Dependent Cyst Growth Linked to Tryptophan Metabolism in PKD

Background: Research has shown that concomitant genetic removal of cilia in autosomal dominant PKD (ADPKD) mouse models suppresses cyst growth. These data indicate the presence of a “cilia-dependent cyst growth” factor which has yet to be identified. Mitochondrial/metabolic defects are also strongly linked to PKD, with increased levels of the tryptophan catabolites kynurenine and kynurenic acid associated with increased cystogenesis in PKD mouse models and patients. We hypothesize that primary cilia are necessary for tryptophan catabolism, with cilia loss inhibiting the formation of kynurenine and kynurenic acid. Methods: We utilized Madine Darby Canine Kidney (MDCK) Exoc5 mutant, Ift88 knockout, and WT 9-12 (ADPKD) cell lines in addition to mouse models of ADPKD with cilia loss via Ift88 or Exoc5 knockout (KO). We tested mitochondrial function via Seahorse Assay, Resipher, ROS and ATP synthase staining. We measured changes in the kynurenine pathway via qPCR, immunohistochemistry, HPLC-MS, and broad metabolomic screening. IDO inhibitors were tested on WT 9-12 cells. Results: Cells with cilia loss showed significantly diminished respiratory capacity and greater amounts of ROS compared with controls. Tissues with cilia loss had less ATP synthase compared with controls. Metabolomics screening showed significantly elevated intracellular tryptophan in cilia loss cell lines compared to controls. Concomitantly, kynurenine was significantly lower in these cells. Catabolism of tryptophan to kynurenine in kidney cells occurs via indoleamine 2,3-dioxygenases (IDO1/IDO2) and cell lines with cilia loss had significantly less IDO1 and IDO2 transcripts compared to controls. Immunohistochemistry of Ift88 and Exoc5 KO kidneys showed significantly greater tryptophan staining in cells with cilia loss compared to controls. Kynurenic acid was significantly more abundant in Pkd1RC/RC mouse kidneys compared to controls. ADPKD mouse kidneys with cilia loss had significantly less kynurenic acid compared to controls. The IDO inhibitor linrodostat significantly decreased oxygen consumption rate in WT 9-12 cells. Conclusion: Tryptophan catabolism via the kynurenine pathway is a critical link between primary cilia and metabolism leading to cilia-dependent cyst growth, and IDO1/IDO2 inhibitors, therefore, could be novel therapeutic agents for the treatment of PKD. Funding: Veterans Affairs Support - Veterans Affairs Support, Veterans Affairs Support