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

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.