Downloaded from http://cshperspectives.cshlp.org/ on October 3, 2021 - Published by Cold Spring Harbor Laboratory Press NAADP Receptors Antony Galione Department of Pharmacology, University of Oxford, Oxford OX1 3QT, United Kingdom Correspondence: [email protected] Of the established Ca2+-mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations, and its receptors undergo dramatic desensitization. Recent studies have identified a new class of calcium-release channel, the two-pore channels (TPCs), as the likely targets for NAADP regulation, even though the effect may be indirect. These channels localized at endolysosomes, where they mediate local Ca2+ release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca2+ mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca2+ release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca2+-induced Ca2+-release (CICR) channels at lysosomal–endoplasmic reticulum (ER) junc- tions. The third is to regulate plasma membrane excitability by the targeting of Ca2+ release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca2+-activated channels. In this review, I discuss the role of nicotinic acid adenine nucleotide diphosphate (NAADP)-mediated Ca2+ release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca2+ signaling. alcium is the most evolutionarily ubiquitous 1998). Many cell surface receptors are linked to Cof intracellular signals and controls cellular signaling pathways that lead to the mobilization mechanisms as diverse as cellular motility, of calcium from intracellular storage organelles membrane fusion, ion channel function, enzyme through the activation of specificCa2+-release activity, and gene expression (Berridge et al. channels (Clapham 2007). Three major small 2003). Free cytoplasmic calcium levels are kept molecule intracellular messengers have been es- under tight control by pumps, exchangers, and tablished to link cell stimulation with organellar 2+ buffering mechanisms including storage by or- Ca release: inositol trisphosphate (IP3), cyclic ganelles (Pozzan et al. 1994). Ca2+ signals may be adenosine diphosphate ribose (cADPR), and elicited when these mechanisms are transiently nicotinic acid adenine nucleotide diphosphate overwhelmed by the opening of calcium-perme- (NAADP) (Bootman et al. 2002). In addition, able channels at the plasma membrane or there have been reports that sphingosine 1 phos- in membranes of calcium-storing organelles. phate may activate a novel Ca2+-release mecha- Chronic activation of such channels may lead nism (Mao et al. 1996; Schnurbus et al. 2002; to cell death, for example, through the activation Cavalli et al. 2003), whereas leukotriene B4 of apoptotic signaling cascades (Berridge et al. may activate, and arachidonic acid may inhib- Editors: Geert Bultynck, Martin D. Bootman, Michael J. Berridge, and Grace E. Stutzmann Additional Perspectives on Calcium Signaling available at www.cshperspectives.org Copyright © 2019 Cold Spring Harbor Laboratory Press; all rights reserved Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a035071 1 Downloaded from http://cshperspectives.cshlp.org/ on October 3, 2021 - Published by Cold Spring Harbor Laboratory Press A. Galione it ryanodine receptors (Striggow and Ehrlich HO OH NH2 N 1997). N O PP N N + DISCOVERY OF NAADP AS A N O 2+ Ca -MOBILIZING MOLECULE HO HO P NAADP was discovered as a contaminant of O commercial batches of β-NADP+ by Lee and colleagues while investigating the effects of var- Lysosome Cytosol ious pyridine nucleotides on calcium release TPC2 Ca2+ from sea urchin egg homogenates (Clapper Ca2+ et al. 1987). The rationale for this was that at NAADP fertilization, in sea urchin eggs, dramatic changes in pyridine nucleotide levels occur (Epel 1964) with a similar time course to the Figure 1. Structure and function of nicotinic acid ad- generation of the calcium wave. Egg homoge- enine nucleotide diphosphate (NAADP). NAADP differs from β-NADP in that that the base nicotinic nates can be simply prepared from sea urchin acid is substituted for nicotinamide (upper panel). eggs and are remarkably stable, even after freez- NAADP, unlike NADP, is a potent Ca2+-mobilizing ing. They sequester calcium, and robustly release agent and activates two-pore channels in the mem- it when challenged with messengers and drugs branes of lysosomes (lower panel). (Morgan and Galione 2008). Three distinct cal- cium-release mechanisms were demonstrated. at pico- or low nanomolar concentrations. A These were the early days of IP , the founding 3 growing number of cellular stimuli and cell sur- Ca2+-mobilizing messenger that was demon- face receptors have been found to be coupled to strated to link cell membrane receptors with increases in NAADP levels, confirming its role as Ca2+ mobilization (Streb et al. 1983). Soon after- an intracellular messenger (Churchill et al. 2003; ward, IP was shown to activate sea urchin eggs 3 Masgrau et al. 2003; Rutter 2003; Yamasaki et al. (Whitaker and Irvine 1984) and to release calci- 2005; Galione 2006; Gasser et al. 2006; Kim et al. um from sea urchin egg homogenate microsom- 2008; Pandey et al. 2009; Rah et al. 2010; Barceló- al stores (Clapperand Lee 1985). In addition, two Torns et al. 2011; Esposito et al. 2011; Lewis et al. pyridine nucleotide metabolites were found to 2012; Park et al. 2015). Mediation of calcium release Ca2+ from different subcellular nonmito- signaling by NAADP has been implicated by chondrial fractions from egg homogenate: an two approaches: inhibition of agonist-evoked enzyme-activated metabolite related to NAD+, calcium signals by prior self-inactivation of subsequently identified as cyclic adenosine di- the NAADP receptor (Cancela et al. 1999) or nucleotide phosphate (cADPR) (Lee et al. NAADP receptor pharmacological blockers 1989), and alkaline-treated NADP, later shown (Naylor et al. 2009; Zhang et al. 2018) and mea- to be NAADP (Fig. 1; Lee and Aarhus 1995). A surements of cellular NAADP levels in response key feature of each Ca2+-mobilizing mechanism to stimuli. Measurements of NAADP have been is their display of homologous desensitization performed using either a radioreceptor assay, (i.e., saturating, but nonoverlapping Ca2+ release based on the high-affinity NAADP-binding pro- in response to IP , cADPR, or NAADP), under- 3 tein of sea urchin eggs (Churchill et al. 2003; scoring the independence of each of the three Churamani et al. 2004; Lewis et al. 2007), or mechanisms in this broken cell system. by using a cycling assay of coupled enzyme reactions resulting in fluorescent resorufin NAADP AS A Ca2+-MOBILIZING production (Graeff and Lee 2002). Although MESSENGER some receptors appear to couple to NAADP NAADP is the most potent of Ca2+-mobilizing production selectively, increasingly it is be- messengers described, being typically efficacious coming apparent that receptors couple to 2 Advanced Online Article. Cite this article as Cold Spring Harb Perspect Biol doi: 10.1101/cshperspect.a035071 Downloaded from http://cshperspectives.cshlp.org/ on October 3, 2021 - Published by Cold Spring Harbor Laboratory Press NAADP Receptors multiple Ca2+-mobilizing messengers (Cancela organelles. The initial evidence for this came et al. 2002; Aley et al. 2010) and this may be the from the study of sea urchin eggs and was sub- norm. sequently extended to mammalian cells. NAADP may be synthesized from NADP by ADP-ribosyl cyclases, which are multifunctional Sea Urchin Eggs enzymes that also cyclize NAD to cADPR (Ma- lavasi et al. 2008). CD38 is a transmembrane The initial report of NAADP-evoked Ca2+ re- ectoenzyme, but is also associated with intracel- lease using alkaline-activated NADP demon- lular compartments, and can catalyze the pro- strated that the responsive subcellular fraction duction of both cADPR and NAADP. It has been in egg homogenates was largely separate from reported that the CD38 molecule may exist in the microsomal/ER fraction sensitive to IP3 two orientations with respect to its catalytic do- and cADPR (Clapper et al. 1987). Abrogation main: cytosolic (type 3) or the more common of Ca2+ storage by the ER by the SERCA inhib- luminal/extracellular (type 2) (Liu et al. 2017). itor thapsigargin, while inhibiting Ca2+ release The predominant mammalian ADP-ribosyl cy- by either IP3 or cADPR, only partially reduced clase is CD38 (Ferrero et al. 2014), and there are Ca2+ release evoked by NAADP in both sea several studies demonstrating the requirement urchin egg homogenates (Genazzani and Gali- of CD38 for NAADP synthesis in different cell one 1996) and intact eggs (Churchill and Ga- types (Lee 2011; Lee et al. 2015; Lin et al. 2017). lione 2001a). Visualization of two separate However, this is not seen in all cases (Soares et al. Ca2+ stores was observed in elegant sea urchin 2007), and CD38 has also been proposed to me- egg stratification studies (Lee and Aarhus 2000). tabolize NAADP (Graeff et al. 2006; Schmid Stratification of intact eggs by centrifugation re- et al. 2011) in addition to phosphatases (Ber- sults in the formation of elongated structures