The Cytoplasmic C-terminal Fragment of Polycystin-1 Regulates a Ca 2+-permeable Cation Channel

Abstract

The cytoplasmic C-terminal portion of the polycystin-1 polypeptide (PKD1(1-226)) regulates several important cell signaling pathways, and its deletion suffices to cause autosomal dominant polycystic kidney disease. However, a functional link between PKD1 and the ion transport processes required to drive renal cyst enlargement has remained elusive. We report here that expression at the Xenopus oocyte surface of a transmembrane fusion protein encoding the C-terminal portion of the PKD1 cytoplasmic tail, PKD1(115-226), but not the N-terminal portion, induced a large, Ca2+-permeable cation current, which shifted oocyte reversal potential (Erev) by +33 mV. Whole cell currents were sensitive to inhibition by La3+, Gd3+, and Zn2+, and partially inhibited by SKF96365 and amiloride. Currents were not activated by bath hypertonicity, but were inhibited by acid pH. Outside-out patches pulled from PKD1(115-226)-expressing oocytes exhibited a 5.1-fold increased NPo of endogenous 20-picosiemens cation channels of linear conductance. PKD1(115-226)-injected oocytes also exhibited elevated NPo of unitary calcium currents in outside-out and cell-attached patches, and elevated calcium permeability documented by fluorescence ratio and 45Ca2+ flux experiments. Both Ca2+ conductance and influx were inhibited by La3+. Mutation of candidate phosphorylation sites within PKD1(115-226) abolished the cation current. We conclude that the C-terminal cytoplasmic tail of PKD1 up-regulates inward current that includes a major contribution from Ca2+-permeable nonspecific cation channels. Dysregulation of these or similar channels in autosomal dominant polycystic kidney disease may contribute to cyst formation or expansion.