Purified TPC Isoforms Form NAADP Receptors with Distinct Roles for Ca2+ Signaling and Endolysosomal Trafficking

Intracellular Ca 2+ signals constitute key elements in signal transduction. Of the three major Ca 2+ mobilizing messengers described, the most potent, nicotinic acid adenine dinucleotide phosphate (NAADP) is the least well understood in terms of its molecular targets [ 1 ]. Recently, we showed that heterologous expression of two-pore channel (TPC) proteins enhances NAADP-induced Ca 2+ release, whereas the NAADP response was abolished in pancreatic beta cells from Tpcn2 gene knockout mice [ 2 ]. However, whether TPCs constitute native NAADP receptors is unclear. Here we show that immunopurified endogenous TPC complexes possess the hallmark properties ascribed to NAADP receptors, including nanomolar ligand affinity [ 3–5 ]. Our study also reveals important functional differences between the three TPC isoforms. Thus, TPC1 and TPC2 both mediate NAADP-induced Ca 2+ release, but the subsequent amplification of this trigger Ca 2+ by IP 3 Rs is more tightly coupled for TPC2. In contrast, TPC3 expression suppressed NAADP-induced Ca 2+ release. Finally, increased TPC expression has dramatic and contrasting effects on endolysosomal structures and dynamics, implicating a role for NAADP in the regulation of vesicular trafficking. We propose that NAADP regulates endolysosomal Ca 2+ storage and release via TPCs and coordinates endoplasmic reticulum Ca 2+ release in a role that impacts on Ca 2+ signaling in health and disease [ 6 ]. ► Endogenous TPC protein complex has hallmark properties of native NAADP receptors ► TPCs differentially localize to subcellular sites of NAADP-induced Ca 2+ release ► Different TPC isoforms show differential roles in NAADP-evoked Ca 2+ release ► Altered TPC expression has dramatic, differential effects on endolysosomal function