Reconstituted human TPC1 is a proton-permeable ion channel and is activated by NAADP or Ca²⁺

NAADP potently triggers Ca²⁺ release from acidic lysosomal and endolysosomal Ca²⁺stores. Human two-pore channels (TPC1 and TPC2), which are located on
these stores, are involved in this process, but there is controversy over whether TPC1 and TPC2 constitute the Ca²⁺ release channels. We therefore examined the single-channel properties of human TPC1 after reconstitution into bilayers of controlled composition. We found that TPC1 was permeable not only to Ca²⁺ but also to monovalent cations and that permeability to protons was the highest (relative permeability sequence: H⁺ >> K⁺ > Na⁺ ≥ Ca²⁺). NAADP or Ca²⁺
activated TPC1, and the presence of one of these ligands was required for channel activation. The endolysosome-located lipid phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂] had no effect on TPC1 open probability but significantly increased the relative permeability of Na⁺ to Ca²⁺ and of H⁺ to Ca²⁺.
Furthermore, our data showed that, although both TPC1 and TPC2 are
stimulated by NAADP, these channels differ in ion selectivity and modulation by Ca²⁺ and pH. We propose that NAADP triggers H⁺ release from lysosomes and endolysomes through activation of TPC1, but that the Ca²⁺-releasing ability of TPC1 will depend on the ionic composition of the acidic stores and may be influenced by other regulators that affect TPC1 ion permeation.