Organic Cation Permeation through the Channel Formed by Polycystin-2

Abstract

Polycystin-2 (PC2), a member of the transient receptor potential family of ion channels (TRPP2), forms a calcium-permeable cation channel. Mutations in PC2 lead to polycystic kidney disease. From the primary sequence and by analogy with other channels in this family, PC2 is modeled to have six transmembrane domains. However, most of the structural features of PC2, such as how large the channel is and how many subunits make up the pore of the channel, are unknown. In this study, we estimated the pore size of PC2 from the permeation properties of the channel. Organic cations of increasing size were used as current carriers through the PC2 channel after PC2 was incorporated into lipid bilayers. We found that dimethylamine, triethylamine, tetraethylammonium, tetrabutylammonium, tetrapropylammonium, and tetrapentylammonium were permeable through the PC2 channel. The slope conductance of the PC2 channel decreased as the ionic diameter of the organic cation increased. For each organic cation tested, the currents were inhibited by gadolinium and anti-PC2 antibody. Using the dimensions of the largest permeant cation, the minimum pore diameter of the PC2 channel was estimated to be at least 11 Å. The large pore size suggests that the primary state of this channel found in vivo is closed to avoid rundown of cation gradients across the plasma membrane and excessive calcium leak from endoplasmic reticulum stores.

Function and regulation of TRPP2 at the plasma membrane

“…However, the exact role of Ca 2ϩ influx in the mating process of C. reinhardtii is not immediately clear, as most of the evidence is indirect and is based on the use of general cation channel blockers such as lidocaine, La 3ϩ , and Cd 2ϩ (27,81). In this regard, while mammalian TRPP2 has been shown to be blocked by trivalents such as La 3ϩ (17,26,45) or Gd 3ϩ (2,19,26), it would be interesting to test whether all these inhibitors used to block mating could inhibit CrPKD2 channel activity in vitro. The most consistent data in terms of Ca 2ϩ fluxes during the mating process are data showing Ca 2ϩ efflux rather than influx (7,27).…”

“…Mammalian TRP channels have been grouped into seven categories (TRPC, TRPM, TRPP, TRPV, TRPML, TRPN, and TRPA1) based on primary sequence homology (54). All TRP channels span the plasma membrane six times with their NH 2 and COOH termini located in the cytoplasm. The region responsible for forming the ionic pore is believed to be between transmembrane segments five and six.…”

Fibrocystin/Polyductin Modulates Renal Tubular Formation by Regulating Polycystin-2 Expression and Function

“…We first tested whether Pkd2-induced whole-cell current could be detected under published conditions 37,38 and then investigated whether the recorded channel-like current is Pkd2-specific. First, pri- BASIC RESEARCH www.jasn.org mary cultured fibroblasts derived from E13.5 WT and Pkd2 Ϫ/Ϫ embryos were tested to determine whether whole-cell current densities were dysregulated by the lack of Pkd2.…”

Triptolide is a traditional Chinese medicine-derived inhibitor of polycystic kidney disease

“…Mutations in the gene product of PKD1, polycystin-1 (PC1), account for Ϸ85% of all cases of ADPKD, with the remaining 15% being attributed to the transient receptor potential (TRP) Ca 2ϩ channel polycystin-2 (PC2) (2,3), the PKD2 gene product (4). ADPKD cyst cells are characterized by somatic loss of the normal allele of the respective disease gene, resulting in increased cellular proliferation and, ultimately, the formation and growth of fluid-filled cysts.…”