Cleavage of N-terminus of Polycystin-1 Increases Calcium Permeability of Polycystin-1/Polycystin-2 Receptor Channel Complexes

Background: Mutations on PKD1 and PKD2 encoding PC1 and PC2, respectively, cause autosomal-dominant polycystic kidney disease. It is postulated that PC1 and PC2 form receptor channel complexes. PC1 undergoes N-terminal autocleavage to expose a hidden “stalk” region hypothesized acting as “tethered agonist”. Current studies of PC2 channel focus on patch-clamp recording on the primary cilium and gain-of-function (GOF) PC2 in Xenopus oocytes. Difficulty to record functional wildtype (WT) PC2 limits the progress on receptor-channel complex hypothesis. Methods: Xenopus oocytes expressing PC2 ± PC1 or PC1 deletion mutants were recorded by two-electrode voltage-clamp. Results: Longer expression of WT PC2 in oocytes produced functional channels with similar order of cation selectivity as in primary cilia. PC1 and PC2 formed complexes with unique biophysical properties but same order of cation selectivity as PC2 homomers. Biophysical properties of PC1 and GOF PC2 differ from WT PC1/PC2 heteromers. Deleting PC1 N-terminus to expose the stalk increased calcium permeability of PC1/PC2 heteromers that required the presence of stalk region and intracellular C-terminus. Extracellular application of synthetic stalk peptide increased calcium permeation on stalkless PC1/PC2. Pull-down assays showed Wnt9B interacted with the leucine-rich repeat (LRR) at the N-terminus of PC1. Application of Wnt9B increased calcium permeability of PC1/PC2, but not heteromers containing cleavage resistant mutant PC1. Preincubation with LRR fusion protein prevented the effect of Wnt9B from increasing calcium permeability of the PC1/PC2 complexes. Conclusion: Xenopus oocytes is a good experimental system for studying wildtype polycystin function. PC1 and PC2 form heteromeric receptor-channel complexes that differ from complexes formed by PC1 and GOF-PC2. Wnt9B interacts with LRR in PC1 N-terminus to expose stalk domain of PC1 and activates heteromeric channel including increased calcium permeability. Future studies will include structure-function of the complexes, identification of additional ligands, and further elucidation of regulatory mechanism. Our study provides proof-of-principle for new therapeutic strategies for PKD1 N-terminal mutations by direct targeting downstream regions such as TOP domain. Funding: NIDDK Support