The Ion Channel Function of Pkd1 Revealed by a Gain-Of-Function PKD1/TRPP2 Complex

Abstract

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in either PKD1 (Polycystin-1), a polycystic kidney disease protein, or TRPP2 (polycystin-2), a transient receptor potential channel. PKD1 and TRPP2 form a receptor-ion channel complex with a 1 (PKD1): 3 (TRP2) stoichiometry. The molecular mechanism of the function of this complex, especially the role of PKD1, is largely unknown due to the lack of knowledge on the activation mechanism of this complex channel. In this study, we generated a new gain-of-function (GOF) mutant of TRPP2 after mutating L677 and N681 to alanine (named “AA”) at the lower gate. Coexpressing TRPP2_AA with PKD1 in Xenopus laevis oocytes leads to the formation of A GOF PKD1/TRPP2_AA complex channel. With this mutant, we were able to record the current of the complex and, for the first time, directly dissect the role of PKD1 in this channel. Our results show that PKD1/TRP2_AA has distinct properties from that of the homomeric TRPP2_AA channel. PKD1/TRPP2_AA channel is not blocked by extracellular divalent ions and has significantly higher Ca2+ permeability than TRPP2_AA channel. More importantly, mutations in the pore region of either PKD1 or TRPP2 alter the ion permeability of the PKD1/TRPP2_AA channel, showing that both proteins contribute to the formation of the ion-conducting pore, defining PKD1 as an ion channel subunit. Our results further show that the ion channel function of PKD1 is preserved in the last six transmembrane domains that share sequence similarity with TRPP2. These results demonstrate the ion channel function of PKD1 in the PKD1/TRPP2 complex, enriching our understanding of this channel and its role in ADPKD.