High Calcium Transport by Polycystin-2 (TRPP2) Induces Channel Clustering and Coordinated Oscillatory Behavior

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The regulation by Ca 2+ of Ca 2+ -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 2+ permeable non-selective cation channel. Previous studies from our laboratory demonstrated that physiological concentrations of Ca 2+ do not regulate in vitro translated PC2 (PC2 iv ) channel activity. However, the issue as to PC2’s Ca 2+ permeability and regulation remain ill-defined. In this study, we assessed Ca 2+ transport by PC2 iv, in a lipid bilayer reconstitution system in the presence of a high Ca 2+ gradient (CaCl 2 100 mM cis , CaCl 2 10 mM trans ). PC2 iv channel reconstitution was conducted in the presence of either 3:7 or 7:3 1-palmitoyl-2-oleoyl-choline (POPC) and ethanolamine (POPE) lipid mixtures. Reconstituted PC2 iv showed spontaneous Ca 2+ 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, experimental data were best fitted with the Goldman-Hodgkin-Katz equation, showing a reversal potential (V rev ~ −27 mV) consistent with strict Ca 2+ selectivity. The R742X mutated PC2 (PC2 R742X ), lacking the carboxy terminal domain of the channel showed no differences with wild type PC2. Interestingly, spontaneous Ca 2+ current oscillations were observed whenever PC2-containing samples were reconstituted in the 3:7, but not 7:3 POPC:POPE lipid mixture. The amplitude and frequency of the oscillations were highly dependent on the applied voltage, the imposed Ca 2+ gradient, and the presence of high Ca 2+ , 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 2+ oscillations based on the presence of subconductance states in the channel. The encompassed data provide new evidence to support a high Ca 2+ permeability by PC2, and a novel regulatory feedback mechanism dependent on the presence of Ca 2+ and phospholipids on its function. Statement of Significance The regulation by Ca 2+ of Ca 2+ -permeable ion channels represents an important mechanism in the control of cell function. The Transient Potential Receptor channel Polycystin-2 (TRPP2, PC2), is a Ca 2+ permeable non-selective cation channel. Ca 2+ transport by PC2 has largely been inferred by changes in reversal potential. This study provides experimental evidence on the Ca 2+ -transporting capabilities of PC2 in high Ca 2+ that is modulated by lipids and generates a novel phenomenon of oscillatory currents by channel clustering and multiple subconductance behavior. PC2 can be self-regulated by feedback mechanisms, which are independent of external regulatory proteins. This oscillatory behavior, previously unknown for a single channel species, depend on the presence of Ca 2+ interaction sites as have been postulated for the channel protein.