Two-pore Channels for Calcium Mobilization from Acidic Organelles and Cell Signaling by NAADP

Abstract

Two-pore channels (TPCs) are novel members of the superfamily of voltage-gated ion channels. Their predicted structures indicate 2-fold symmetry with a total of 12 putative transmembrane (TM) α-helices. Sequence homology and membrane topology analyses suggest that TPCs may represent evolutionary intermediates from the single domain 6-TM architecture K+ and non-selective cation channels to the four-repeat 24-TM structure of voltage-gated Ca2+ and Na+ channels. Three genes (TPCN1-3) exist in most vertebrates but their functions remain elusive. We now show that TPC1 and TPC3 are expressed on the membrane of different endosome populations while TPC2 is expressed on the membrane of lysosomes. We provide functional data showing that TPC2 is a target of nicotinic acid adenine dinucleotide phosphate (NAADP), a potent Ca2+ mobilizing messenger that evokes Ca2+ release from acidic organelles rather than from the sarco/endoplasmic reticulum. Thus, microsomal membranes enriched with TPC2 exhibit similar high affinity binding with NAADP as native NAADP receptors. In response to NAADP, cells overexpressing TPC2 exhibit enhanced intracellular Ca2+ release and more efficient coupling to IP3 receptors to evoke global Ca2+ transients. These effects were blocked by disrupting lysosomal H+ gradient or RNAi-mediated silencing of TPC2 expression. Our findings provide for the first time a molecular basis for further detailed characterization of the regulatory mechanisms and physiological functions of NAADP-mediated signaling and, in addition, suggest a general role for TPCs in Ca2+ mobilization, Ca2+ homeostasis, and Ca2+ signaling from endosomal/lysosomal compartments of vertebrate cells, which are known to be important for diverse functions in many physiological systems.