Defects in polycystin-2, a ubiquitous transmembrane glycoprotein of unknown function, is a major cause of autosomal dominant polycystic kidney disease (ADPKD), whose manifestation entails the development of fluid-filled cysts in target organs. Here, we demonstrate that polycystin-2 is present in term human syncytiotrophoblast, where it behaves as a nonselective cation channel. Lipid bilayer reconstitution of polycystin-2-positive human syncytiotrophoblast apical membranes displayed a nonselective cation channel with multiple subconductance states, and a high perm-selectivity to Ca2+. This channel was inhibited by anti-polycystin-2 antibody, Ca2+, La3+, Gd3+, and the diuretic amiloride. Channel function by polycystin-2 was confirmed by patch-clamping experiments of polycystin-2 heterologously infected Sf9 insect cells. Further, purified insect cell-derived recombinant polycystin-2 and in vitro translated human polycystin-2 had similar ion channel activity. The polycystin-2 channel may be associated with fluid accumulation and/or ion transport regulation in target epithelia, including placenta. Dysregulation of this channel provides a mechanism for the onset and progression of ADPKD.

Polycystin-2 (PC2, TRPP2), the gene product of PKD2, whose mutations cause autosomal dominant polycystic kidney disease (ADPKD), belongs to the superfamily of TRP channels. PC2 is a non-selective cation channel, with multiple subconductance states. In this report, we explored structural and functional properties of PC2 and whether the conductance substates represent monomeric contributions to the channel complex. A kinetic analysis of spontaneous channel currents of PC2 showed that four intrinsic, non-stochastic subconductance states, which followed a staircase behavior, were both pH- and voltage-dependent. To confirm the oligomeric contributions to PC2 channel function, heteromeric PC2/TRPC1 channel complexes were also functionally assessed by single channel current analysis. Low pH inhibited the PC2 currents in PC2 homomeric complexes, but failed to affect PC2 currents in PC2/TRPC1 heteromeric complexes. Amiloride, in contrast, abolished PC2 currents in both the homomeric PC2 complexes and the heteromeric PC2/TRPC1 complexes, thus PC2/TRPC1 complexes have distinct functional properties from the homomeric complexes. The topological features of the homomeric PC2-, TRPC1- and heteromeric PC2/TRPC1 channel complexes, assessed by atomic force microscopy, were consistent with structural tetramers. TRPC1 homomeric channels had different average diameter and protruding height when compared with the PC2 homomers. The contribution of individual monomers to the PC2/TRPC1 hetero-complexes was easily distinguishable. The data support tetrameric models of both the PC2 and TRPC1 channels, where the overall conductance of a particular channel will depend on the contribution of the various functional monomers in the complex.

Autosomal dominant polycystic kidney disease (AD-PKD) is a prevalent genetic disorder largely caused by mutations in the PKD1 and PKD2 genes that encode the transmembrane proteins polycystin-1 and -2, respectively. Both proteins appear to be involved in the regulation of cell growth and maturation, but the precise mechanisms are not yet well defined. Polycystin-2 has recently been shown to function as a Ca 2؉ -permeable, non-selective cation channel. Polycystin-2 interacts through its cytoplasmic carboxyl-terminal region with a coiled-coil motif in the cytoplasmic tail of polycystin-1 (P1CC). The functional consequences of this interaction on its channel activity, however, are unknown. In this report, we show that P1CC enhanced the channel activity of polycystin-2. R742X, a disease-causing polycystin-2 mutant lacking the polycystin-1 interacting region, fails to respond to P1CC. Also, P1CC containing a diseasecausing mutation in its coiled-coil motif loses its stimulatory effect on wild-type polycystin-2 channel activity. The modulation of polycystin-2 channel activity by polycystin-1 may be important for the various biological processes mediated by this molecular complex.ADPKD 1 is a common genetic disorder caused by mutations in either one of the two genes, PKD1 and PKD2, that encode polycystin-1 and -2, respectively (1). Polycystin-1 is an 11-membranespanning desmosome-associated protein (2, 3) that may be involved in the regulation of cell growth (4). Polycystin-2 is a six-span membrane protein with homology to voltage-dependent (5) and transient receptor potential (TRP) channel proteins (6). Recently, we and others demonstrated that polycystin-2 indeed functions as a Ca 2ϩ -permeable nonselective cation channel (7-9).Mutations in either PKD1 or PKD2 cause nearly identical clinical manifestations, suggesting that these two proteins either interact directly or are components of a common signaling pathway (reviewed in Ref. 1). Polycystin-1 and -2 interact with each other through their carboxyl-terminal cytoplasmic tails both in vitro (10, 11) and in vivo (12). This interaction has been implicated in various cellular processes, including the activation of Jak and the consequent regulation of cell growth (4), the activation of whole-cell cation-permeable currents (13), and the regulation of G-protein signaling (14). The functional consequences of polycystin-1 interaction on polycystin-2 channel activity, however, have not been determined. Here, we demonstrate that binding of polycystin-2 to the coiled-coil-containing segment of the polycystin-1 carboxyl-tail (P1CC) increased and stabilized polycystin-2 channel function. In contrast, a single point mutation in P1CC, Q4215P, abolished the regulatory role on wild-type polycystin-2 channel function. Furthermore, the polycystin-2 truncation mutant R742X, an active channel (15) missing most of its cytoplasmic tail (including the polycystin-1 binding segment), was not regulated by P1CC. MATERIALS AND METHODSPlasmid Constructs-The PKD2 baculovirus expression construct p...

Polycystin-2, the product of the human PKD2 gene, whose mutations cause autosomal dominant polycystic kidney disease, is a large conductance, Ca 2؉ -permeable non-selective cation channel. Polycystin-2 is functionally expressed in the apical membrane of the human syncytiotrophoblast, where it may play a role in the control of fetal electrolyte homeostasis. Little is known, however, about the mechanisms that regulate polycystin-2 channel function. In this study, the role of pH in the regulation of polycystin-2 was assessed by ion channel reconstitution of both apical membranes of human syncytiotrophoblast and the purified FLAG-