Similarities and Differences in Gating of the Two-Pore Channels TPC1 and TPC2

Abstract

Endolysosomal Ca2+ homeostasis is implicated in several diseases and controls many endolysosomal functions. A key to understanding endolysosomal Ca2+ signaling to understand the function of the newly discovered endolysosomal PI(3,5)P2-regulated two-pore channels (TPCs) and their potential activation by NAADP. Our recent work concluded that the lysosomal TPC2 function as a NAADP and PI(3,5)P2-activated channel regulated by cytoplasmic Mg2+. The properties and gating of the mostly endosomal TPC1 are not known. Recording whole-organelle currents of enlarged endosomes expressing GFP-TPC1, we discovered that TPC1 is potently activated by PI(3,5)P2 and NAADP. Moreover, PI(3,5)P2 facilitates TPC1 activation by NAADP. Unlike, TPC2, the activity of TPC1 showed poor inhibition by Mg2+. Most notably, the concentration dependence of activation of TPC1 and TPC2 by NAADP are remarkably different. While activation of TPC2 by NAADP followed normal saturation dependence with no sign of inhibition by high NAADP concentration, activation of TPC1 by NAADP followed a bell shaped dependency, resembling the NAADP-mediated Ca2+ release in intact cells. These findings provide the first direct evidence for gating of TPC1 by NAADP and indicate that endosomal, rather than lysosomal, Ca2+ release is the key trigger Ca2+ pool that sensitizes the IP3-mediated Ca2+ release from the ER. The interaction between TPC1-expressing endosomes and the ER are being examined.