TPCs: Endolysosomal channels for Ca2+ mobilization from acidic organelles triggered by NAADP

Abstract

Two-pore channels (TPCs or TPCNs) are novel members of the large superfamily of voltage-gated cation channels with slightly higher sequence homology to the pore-forming subunits of voltage-gated Ca(2+) and Na(+) channels than most other members. Recent studies demonstrate that TPCs locate to endosomes and lysosomes and form Ca(2+) release channels that respond to activation by the Ca(2+) mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). With multiple endolysosomal targeted NAADP receptors now identified, important new insights into the regulation of endolysosomal function in health and disease will therefore be unveiled.

Fig. 1

Membrane topology and sequence homology of two-pore channels. (A) Putative transmembrane organization of TPCs based on hydropathy analysis and membrane orientation of voltage-gated Ca²⁺ channels and TRP channels. P loop: pore loop. The tree-like symbol indicates a glycosylation site in human TPC2. (B) Pair-wise comparison of amino acid identities of TPCs separated by different domains. N-ter, C-ter, TM, and P, indicate N-termini, C-termini, transmembrane, and pore loop, respectively. Note: the three TPCs are overall quite distant from each other; higher homology is found at TM segments 4–6.

Fig. 2

Distribution and phylogenetic relationship of TPC proteins in animals and plants. The unrooted phylogenetic tree was made for known TPC sequences from representative members of animal kingdom and two land plants using ClusterW (http://clustalw.genome.jp/) and plotted using neighbour-joining algorithm. The N- and C- termini were removed and for some sequences the loop between the two transmembrane domains were also partially removed before performing the alignment. GenBank accession numbers for the corresponding nucleotide sequences are shown in parentheses.

Fig. 3

Subcellular distribution and potential function of mammalian TPCs. Diagram depicts the locations of human TPC1 and chicken TPC3 in different endosome populations and of human TPC2 in lysosomes when stably expressed in HEK293 cells. In addition to mediating NAADP-evoked Ca²⁺ release from endolysosomes, other potential functions for TPCs expressed in the acidic organelles are indicated. SOC and ROC: store- and receptor-operated channels, TGN: trans-Golgi network (adapted from [10] with modifications).

Fig. 4

The triggering role of TPC-mediated Ca²⁺ mobilization. The flow chart shows pathways for NAADP, cADPR and IP₃ production and their effects on intracellular Ca²⁺. The drawings at the bottom depict “quantal” Ca²⁺ signals generated by TPCs (left, lighter color and higher position for higher Ca²⁺ concentrations) and global Ca²⁺ signals generated by activating IP₃Rs and/or RyRs (right), as well as their contributions to the two phases of Ca²⁺ transients evoked by NAADP (blue arrows). Red dashed lines indicate alternative pathways. The green pathway on the left shows an example of physiological functions directly regulated by the local Ca²⁺ signals. ARC, ADP-ribose cyclase, including CD38, PLC, phospholipase C, Ψ, membrane potential. I_Ca, Ca²⁺ currents through plasma membrane.