Diversity of two-pore channels and the accessory NAADP receptors in intracellular Ca2+ signaling

Abstract

Intracellular Ca²⁺ signaling via changes or oscillation in cytosolic Ca²⁺ concentration controls almost every aspect of cellular function and physiological processes, such as gene transcription, cell motility and proliferation, muscle contraction, and learning and memory. Two-pore channels (TPCs) are a class of eukaryotic cation channels involved in intracellular Ca²⁺ signaling, likely present in a multitude of organisms from unicellular organisms to mammals. Accumulated evidence indicates that TPCs play a critical role in Ca²⁺ mobilization from intracellular stores mediated by the second messenger molecule, nicotinic acid adenine dinucleotide phosphate (NAADP). In recent years, significant progress has been made regarding our understanding of the structures and function of TPCs, including Cryo-EM structure determination of mammalian TPCs and characterization of a plastid TPC in a single-celled parasite.. The recent identification of Lsm12 and JPT2 as NAADP-binding proteins provides a new molecular basis for understanding NAADP-evoked Ca²⁺ signaling. In this review, we summarize basic structural and functional aspects of TPCs and highlight the most recent studies on the newly discovered TPC in a parasitic protozoan and the NAADP-binding proteins LSM12 and JPT2 as new key players in NAADP signaling.

Keywords: Calcium mobilization; Endolysosome; JPT2; Lsm12; Lysosome; NAADP; Two-pore channels.

Fig. 1.. Diversity and molecular basis of TPCs.

(A) Phylogeny tree of TPCs from different species. Amino acid seuqneces of TPC paralogues and orthologues from same and different species were retrived from NCBI protein database and then aligned and analyzed with methods of ClustalW and Minimum-Evolution Tree using program MEGA (v. 11) [66]. (B) Structure of open-state human TPC2 in complex with PI(3,5)P₂ (pdb: 6NQ0) [37]. (C) Amino acid sequence alignment of selected TPCs of different species. Most amino acids are shaded collectively in colors according to their physicochemical properties (hydrophobic in yellow, polar in green, positively charged in blue and negatively charged in red) and special amino acids (glycine, histidine, tyrosine, and cysteine) are each shaded in other different colors. Residues discussed to be important for channel gating are shown in stick mode (sidechain only) in (B) and labeled according to human TPC2 sequence on top in (C).

Fig. 2.. A hypothetic model of NAADP-induced Ca²⁺ mobilization signaling pathways.

Lsm12 as an NAADP receptor interactes with TPCs and mediates NAADP-evoked and TPC-dependent Ca²⁺ release from the endolysosome that subsequently triggers a larger scale Ca²⁺ release from endoplasmic reticulum (ER) through ryanodine receptors and/or IP3 receptors via a mechanism of CICR. The possibility of limited Ca²⁺ permeability through TPCs is indicated with a gray arrow. JPT2 as another NAADP receptor interacts with ryanodine receptor 1 and likely mediates Ca²⁺ release direclty from endoplasmic reticulum without the involvement of endolysosome system. The uncertainty on the role of JTP2 in TPC-dependent NAADP signaling is indicated by a question mark.