Versatile HaloTag Resource Illuminates PKD2 Localization and Turnover

Background: Polycystin-2 (PC2), encoded by PKD2, is a cation channel essential for renal physiology, and its dysfunction is a major cause of autosomal dominant polycystic kidney disease. However, limited tools to visualize endogenous PC2 has hindered efforts to define its subcellular distribution and molecular functions. To address this, we developed a HaloTag knock-in mouse model and temperature sensitive renal epithelial cell lines to enable precise detection and dynamics of PC2 in live and fixed renal epithelial cells and in tissues. Methods: Using CRISPR/Cas9, we engineered a Pkd2 HaloTag knock-in mouse (Pkd2-C-Halo) and derived tsSV40-immortalized Pkd2-C-Halo renal epithelial lines. We optimized HaloTag labeling for in vivo and in vitro applications, demonstrated its use in confocal and live cell microscopy, pulse-chase assays, and in vivo imaging of Pkd2 localization after renal injury and in a disease-relevant Tulp3R383W/R383W mutant background. Results: Pkd2-C-Halo homozygote mice were viable, fertile, and phenotypically normal, confirming Pkd2-C-Halo protein function. Pkd2-C-Halo was readily detected by western blotting and localized to ER, mitochondria, and the primary cilium. Labeling in tissues in vivo occurred within ~30 minutes and plateaued by 3 hours at doses ≥2 nmol and labeling occurred in as little as 30 minutes in cells with ligand concentrations ≥25 nM. Pulse-chase analysis showed Pkd2 turnover ~24 hours in vivo in the cilium of the choroid plexus and ~8 hours in cultured cells. Despite renal injury accelerating cyst formation, unilateral ureteral obstruction did not alter Pkd2-C-Halo expression or distribution. Homozygous Tulp3(R383W) mice that correspond to a patient with hepatorenal cystic disease lacked ciliary localized Pkd2-C-Halo in the kidney tubule epithelium. Conclusion: The Pkd2-C-Halo mouse and cell lines are valuable resources for visualizing analyzing endogenous Pkd2. These resources facilitated studies of Pkd2 protein localization, dynamics, complex formation, and turnover during homeostasis and disease states, following renal injury, and in genetic perturbations. Funding: NIDDK Support, Private Foundation Support