High calcium transport by Polycystin-2 (TRPP2) induces channel clustering and oscillatory currents.

The regulation by Ca of Ca-permeable ion channels represents an important mechanism in the control of cell function. Polycystin-2 (PC2, TRPP2), a member of the TRP channel family (Transient Potential Receptor), is a Ca permeable non-selective cation channel. Previous studies from our laboratory demonstrated that physiological concentrations of Ca do not regulate in vitro translated PC2 (PC2) channel activity. However, the issue as to PC2's Ca permeability and regulation remain ill-defined, in particular because Ca transport is usually observed in the presence of other ionic gradients. In this study, we assessed Ca transport by PC2 in a lipid bilayer reconstitution system in a high Ca gradient (CaCl 100 mM cis, CaCl 10 mM trans) in the presence of either 3:7 or 7:3 1-palmitoyl-2-oleoyl-choline and ethanolamine lipid mixtures. Reconstituted PC2 showed spontaneous Ca currents in both lipid mixtures, with a maximum conductance of 63 ± 13 pS (n = 19) and 105 pS ± 9.8 (n = 9), respectively. In both cases, we best fitted the experimental data with the Goldman-Hodgkin-Katz equation, observing a reversal potential (V ∼ -27 mV) consistent with strict Ca selectivity. The R742X mutated PC2 (PC2), lacking the carboxy terminal domain of the channel showed no differences with wild type PC2. Interestingly, we also observed the onset of spontaneous Ca current oscillations whenever PC2-containing samples were reconstituted in the 3:7, but not 7:3 POPC:POPE lipid mixture. The amplitude and frequency of the ionic oscillations were highly dependent on the applied voltage, the imposed Ca gradient, and the presence of high Ca, which induced PC2 channel clustering as observed by atomic force microscopy (AFM). We also used the QuB suite to kinetically model the PC2 channel Ca oscillations based on the presence of subconductance states in the channel. The encompassed evidence supports a high Ca permeability by PC2, and a novel oscillatory mechanism dependent on the presence of Ca and phospholipids that provides the first evidence for the relation between stochasticity and deterministic processes mediated by ion channels.