Prolonged inactivation of nicotinic acid adenine dinucleotide phosphate-induced Ca2+ release mediates a spatiotemporal Ca2+ memory

NAADP-Mediated Ca2+ Signalling

The discovery of NAADP-evoked Ca²⁺ release in sea urchin eggs and then as a ubiquitous Ca²⁺ mobilizing messenger has introduced several novel paradigms to our understanding of Ca²⁺ signalling, not least in providing a link between cell stimulation and Ca²⁺ release from lysosomes and other acidic Ca²⁺ storage organelles. In addition, the hallmark concentration-response relationship of NAADP-mediated Ca²⁺ release, shaped by striking activation/desensitization mechanisms, influences its actions as an intracellular messenger. There has been recent progress in our understanding of the molecular mechanisms underlying NAADP-evoked Ca²⁺ release, such as the identification of the endo-lysosomal two-pore channel family of cation channels (TPCs) as their principal target and the identity of NAADP-binding proteins that complex with them. The NAADP/TPC signalling axis has gained recent prominence in pathophysiology for their roles in such disease processes as neurodegeneration, tumorigenesis and cellular viral entry.

Endolysosomal Ca2+ signaling in cardiovascular health and disease

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) regulates a plethora of functions in the cardiovascular (CV) system, including contraction in cardiomyocytes and vascular smooth muscle cells (VSMCs), and angiogenesis in vascular endothelial cells and endothelial colony forming cells. The sarco/endoplasmic reticulum (SR/ER) represents the largest endogenous Ca²⁺ store, which releases Ca²⁺ through ryanodine receptors (RyRs) and/or inositol-1,4,5-trisphosphate receptors (InsP₃Rs) upon extracellular stimulation. The acidic vesicles of the endolysosomal (EL) compartment represent an additional endogenous Ca²⁺ store, which is targeted by several second messengers, including nicotinic acid adenine dinucleotide phosphate (NAADP) and phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂], and may release intraluminal Ca²⁺ through multiple Ca²⁺ permeable channels, including two-pore channels 1 and 2 (TPC1-2) and Transient Receptor Potential Mucolipin 1 (TRPML1). Herein, we discuss the emerging, pathophysiological role of EL Ca²⁺ signaling in the CV system. We describe the role of cardiac TPCs in β-adrenoceptor stimulation, arrhythmia, hypertrophy, and ischemia-reperfusion injury. We then illustrate the role of EL Ca²⁺ signaling in VSMCs, where TPCs promote vasoconstriction and contribute to pulmonary artery hypertension and atherosclerosis, whereas TRPML1 sustains vasodilation and is also involved in atherosclerosis. Subsequently, we describe the mechanisms whereby endothelial TPCs promote vasodilation, contribute to neurovascular coupling in the brain and stimulate angiogenesis and vasculogenesis. Finally, we discuss about the possibility to target TPCs, which are likely to mediate CV cell infection by the Severe Acute Respiratory Disease-Coronavirus-2, with Food and Drug Administration-approved drugs to alleviate the detrimental effects of Coronavirus Disease-19 on the CV system.

NAADP Receptors

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) junctions. 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.

Pyridine Nucleotide Metabolites and Calcium Release from Intracellular Stores

Ca²⁺ signals are probably the most common intracellular signaling cellular events, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca²⁺ signals by mobilizing Ca²⁺ from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release Ca²⁺ from the endoplasmic reticulum (ER), through the activation of IP₃-gated Ca²⁺ release channels (IP₃ receptors). Subsequently, two additional Ca²⁺ mobilizing messengers were discovered, cADPR and NAADP. Both are metabolites of pyridine nucleotides, and may be produced by the same class of enzymes, ADP-ribosyl cyclases, such as CD38. Whilst cADPR mobilizes Ca²⁺ from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca²⁺ from acidic stores by a mechanism involving the activation of two pore channels (TPCs). In addition, other pyridine nucleotides have emerged as intracellular messengers. ADP-ribose and 2'-deoxy-ADPR both activate TRPM2 channels which are expressed at the plasma membrane and in lysosomes.

Light-triggered release of photocaged therapeutics - Where are we now?

The current available therapeutics face several challenges such as the development of ideal drug delivery systems towards the goal of personalized treatments for patients benefit. The application of light as an exogenous activation mechanism has shown promising outcomes, owning to the spatiotemporal confinement of the treatment in the vicinity of the diseased tissue, which offers many intriguing possibilities. Engineering therapeutics with light responsive moieties have been explored to enhance the bioavailability, and drug efficacy either in vitro or in vivo. The tailor-made character turns the so-called photocaged compounds highly desirable to reduce the side effects of drugs and, therefore, have received wide research attention. Herein, we seek to highlight the potential of photocaged compounds to obtain a clear understanding of the mechanisms behind its use in therapeutic delivery. A deep overview on the progress achieved in the design, fabrication as well as current and possible future applications in therapeutics of photocaged compounds is provided, so that novel formulations for biomedical field can be designed.

A primer of NAADP-mediated Ca(2+) signalling: From sea urchin eggs to mammalian cells

Since the discovery of the Ca(2+) mobilizing effects of the pyridine nucleotide metabolite, nicotinic acid adenine dinucleotide phosphate (NAADP), this molecule has been demonstrated to function as a Ca(2+) mobilizing intracellular messenger in a wide range of cell types. In this review, I will briefly summarize the distinct principles behind NAADP-mediated Ca(2+) signalling before going on to outline the role of this messenger in the physiology of specific cell types. Central to the discussion here is the finding that NAADP principally mobilizes Ca(2+) from acidic organelles such as lysosomes and it is this property that allows NAADP to play a unique role in intracellular Ca(2+) signalling. Lysosomes and related organelles are small Ca(2+) stores but importantly may also initiate a two-way dialogue with other Ca(2+) storage organelles to amplify Ca(2+) release, and may be strategically localized to influence localized Ca(2+) signalling microdomains. The study of NAADP signalling has created a new and fruitful focus on the lysosome and endolysosomal system as major players in calcium signalling and pathophysiology.

NAADP-sensitive two-pore channels are present and functional in gastric smooth muscle cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) has been identified as an important modulator of Ca(2+) release from the endo-lysosomal system in a variety of cells by a new and ubiquitous class of endo-lysosomal ion channels known as the two-pore channels (TPCs). However, the role of TPCs in NAADP action in smooth muscle is not known. In the present work, we investigated the effects of NAADP in gastric smooth muscle cells and its ability to release Ca(2+) by TPCs. We show that Ca(2+) signals mediated by NAADP were inhibited by disrupting Ca(2+) handling by either acidic organelles (using bafilomycin A1) or the Endoplasmic Reticulum (using thapsigargin, ryanodine or 2-APB). Transcripts for endogenous TPC1 and TPC2 were readily detected and recombinant TPCs localized to the endosomes and/or lysosomes. Overexpression of wild-type TPCs but not pore mutants enhanced NAADP-mediated cytosolic Ca(2+) signals. Desensitizing the NAADP pathway inhibited Ca(2+)-responses to extracellular stimulation with carbachol but not ATP. Taken together, these results indicate that NAADP likely induces Ca(2+) release from the endolysosomal system through TPCs which is subsequently amplified via the ER in an agonist-specific manner. Thus, we suggest a second messenger role for NAADP in smooth muscle cells.

NAADP and the two-pore channel protein 1 participate in the acrosome reaction in mammalian spermatozoa

A TPCN1 gene–deficient mouse strain is used to show that two convergent working NAADP-dependent pathways with nonoverlapping activation and self-inactivation profiles for distinct NAADP concentrations drive acrosomal exocytosis, by which TPC1 is central for the pathway activated by low-micromolar NAADP concentrations.

The functional relationship between the formation of hundreds of fusion pores during the acrosome reaction in spermatozoa and the mobilization of calcium from the acrosome has been determined only partially. Hence, the second messenger NAADP, promoting efflux of calcium from lysosome-like compartments and one of its potential molecular targets, the two-pore channel 1 (TPC1), were analyzed for its involvement in triggering the acrosome reaction using a TPCN1 gene–deficient mouse strain. The present study documents that TPC1 and NAADP-binding sites showed a colocalization at the acrosomal region and that treatment of spermatozoa with NAADP resulted in a loss of the acrosomal vesicle that showed typical properties described for TPCs: Registered responses were not detectable for its chemical analogue NADP and were blocked by the NAADP antagonist trans-Ned-19. In addition, two narrow bell-shaped dose-response curves were identified with maxima in either the nanomolar or low micromolar NAADP concentration range, where TPC1 was found to be responsible for activating the low affinity pathway. Our finding that two convergent NAADP-dependent pathways are operative in driving acrosomal exocytosis supports the concept that both NAADP-gated cascades match local NAADP concentrations with the efflux of acrosomal calcium, thereby ensuring complete fusion of the large acrosomal vesicle.

Calcium pathway machinery at fertilization in echinoderms

Calcium signaling in cells directs diverse physiological processes. The calcium waves triggered by fertilization is a highly conserved calcium signaling event essential for egg activation, and has been documented in every egg tested. This activity is one of the few highly conserved events of egg activation through the course of evolution. Echinoderm eggs, as well as many other cell types, have three main intracellular Ca(2+) mobilizing messengers - IP3, cADPR and NAADP. Both cADPR and NAADP were identified as Ca(2+) mobilizing messengers using the sea urchin egg homogenate, and this experimental system, along with the intact urchin and starfish oocyte/egg, continues to be a vital tool for investigating the mechanism of action of calcium signals. While many of the major regulatory steps of the IP3 pathway are well resolved, both cADPR and NAADP remain understudied in terms of our understanding of the fundamental process of egg activation at fertilization. Recently, NAADP has been shown to trigger Ca(2+) release from acidic vesicles, separately from the ER, and a new class of calcium channels, the two-pore channels (TPCs), was identified as the likely targets for this messenger. Moreover, it was found that both cADPR and NAADP can be synthesized by the same family of enzymes, the ADP-rybosyl cyclases (ARCs). In this context of increasing amount of information, the potential coupling and functional roles of different messengers, intracellular stores and channels in the formation of the fertilization calcium wave in echinoderms will be critically evaluated.

The Molecular Basis for Ca2+ Signalling by NAADP: Two-Pore Channels in a Complex?

NAADP is a potent Ca²⁺ mobilizing messenger in a variety of cells but its molecular mechanism of action is incompletely understood. Accumulating evidence indicates that the poorly characterized two-pore channels (TPCs) in animals are NAADP sensitive Ca²⁺-permeable channels. TPCs localize to the endo-lysosomal system but are functionally coupled to the better characterized endoplasmic reticulum Ca²⁺ channels to generate physiologically relevant complex Ca²⁺ signals. Whether TPCs directly bind NAADP is not clear. Here we discuss the idea based on recent studies that TPCs are the pore-forming subunits of a protein complex that includes tightly associated, low molecular weight NAADP-binding proteins.

Sea urchin eggs in the acid reign

Sea urchin eggs have been an indispensable model system for studying egg activation and ionic signalling for at least a century. Instrumental in the discovery of two Ca(2+)-mobilizing second messengers, cyclic ADP-ribose and NAADP, the sea urchin has revolutionized cell biology for all phyla. This review attempts to summarize what we currently know about egg acidic vesicles in the context of Ca(2+) signalling. The dynamics of Ca(2+) storage, Ca(2+) mobilization, proton fluxes and two-pore channels will be discussed.

NAADP receptors

Of the established Ca(2+) mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations. Recent studies have identified a new class of calcium release channel, the two-pore channels (TPCs), as the likely targets for NAADP. These channels are endolysosomal in localization where they mediate local Ca(2+) release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca(2+) mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca(2+) release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca(2+)-induced Ca(2+) release (CICR) channels at lysosomal-ER junctions. The third is to regulate plasma membrane excitability by the targeting of Ca(2+) release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca(2+)-activated channels. In this review, I discuss the role of NAADP-mediated Ca(2+) 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 Ca(2+) signaling.

TPC2 is a novel NAADP-sensitive Ca2+ release channel, operating as a dual sensor of luminal pH and Ca2+

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a molecule capable of initiating the release of intracellular Ca(2+) required for many essential cellular processes. Recent evidence links two-pore channels (TPCs) with NAADP-induced release of Ca(2+) from lysosome-like acidic organelles; however, there has been no direct demonstration that TPCs can act as NAADP-sensitive Ca(2+) release channels. Controversial evidence also proposes ryanodine receptors as the primary target of NAADP. We show that TPC2, the major lysosomal targeted isoform, is a cation channel with selectivity for Ca(2+) that will enable it to act as a Ca(2+) release channel in the cellular environment. NAADP opens TPC2 channels in a concentration-dependent manner, binding to high affinity activation and low affinity inhibition sites. At the core of this process is the luminal environment of the channel. The sensitivity of TPC2 to NAADP is steeply dependent on the luminal [Ca(2+)] allowing extremely low levels of NAADP to open the channel. In parallel, luminal pH controls NAADP affinity for TPC2 by switching from reversible activation of TPC2 at low pH to irreversible activation at neutral pH. Further evidence earmarking TPCs as the likely pathway for NAADP-induced intracellular Ca(2+) release is obtained from the use of Ned-19, the selective blocker of cellular NAADP-induced Ca(2+) release. Ned-19 antagonizes NAADP-activation of TPC2 in a non-competitive manner at 1 μM but potentiates NAADP activation at nanomolar concentrations. This single-channel study provides a long awaited molecular basis for the peculiar mechanistic features of NAADP signaling and a framework for understanding how NAADP can mediate key physiological events.

The calcium-mobilizing messenger nicotinic acid adenine dinucleotide phosphate participates in sperm activation by mediating the acrosome reaction

Before a sperm can fertilize an egg it must undergo a final activation step induced by the egg termed the acrosome reaction. During the acrosome reaction a lysosome-related organelle, the acrosome, fuses with the plasma membrane to release hydrolytic enzymes and expose an egg-binding protein. Because NAADP (nicotinic acid adenine dinucleotide phosphate) releases Ca²⁺ from acidic lysosome-related organelles in other cell types, we investigated a possible role for NAADP in mediating the acrosome reaction. We report that NAADP binds with high affinity to permeabilized sea urchin sperm. Moreover, we used Mn²⁺ quenching of luminal fura-2 and ⁴⁵Ca²⁺ to directly demonstrate NAADP regulation of a cation channel on the acrosome. Additionally, we show that NAADP synthesis occurs through base exchange and is driven by an increase in Ca²⁺. We propose a new model for acrosome reaction signaling in which Ca²⁺ influx initiated by egg jelly stimulates NAADP synthesis and that this NAADP acts on its receptor/channel on the acrosome to release Ca²⁺ to drive acrosomal exocytosis.

Purified TPC isoforms form NAADP receptors with distinct roles for Ca(2+) signaling and endolysosomal trafficking

Intracellular Ca²⁺ signals constitute key elements in signal transduction. Of the three major Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ release, but the subsequent amplification of this trigger Ca²⁺ by IP₃Rs is more tightly coupled for TPC2. In contrast, TPC3 expression suppressed NAADP-induced Ca²⁺ 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 Ca²⁺ storage and release via TPCs and coordinates endoplasmic reticulum Ca²⁺ release in a role that impacts on Ca²⁺ signaling in health and disease [6].

Analogues of the nicotinic acid adenine dinucleotide phosphate (NAADP) antagonist Ned-19 indicate two binding sites on the NAADP receptor

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a Ca(2+)-releasing messenger. Biological data suggest that its receptor has two binding sites: one high-affinity locking site and one low-affinity opening site. To directly address the presence and function of these putative binding sites, we synthesized and tested analogues of the NAADP antagonist Ned-19. Ned-19 itself inhibits both NAADP-mediated Ca(2+) release and NAADP binding. A fluorometry bioassay was used to assess NAADP-mediated Ca(2+) release, whereas a radioreceptor assay was used to assess binding to the NAADP receptor (only at the high-affinity site). In Ned-20, the fluorine is para rather than ortho as in Ned-19. Ned-20 does not inhibit NAADP-mediated Ca(2+) release but inhibits NAADP binding. Conversely, Ned-19.4 (a methyl ester of Ned-19) inhibits NAADP-mediated Ca(2+) release but cannot inhibit NAADP binding. Furthermore, Ned-20 prevents the self-desensitization response characteristic of NAADP in sea urchin eggs, confirming that this response is mediated by a high-affinity allosteric site to which NAADP binds in the radioreceptor assay. Collectively, these data provide the first direct evidence for two binding sites (one high- and one low-affinity) on the NAADP receptor.

Investigating cADPR and NAADP in intact and broken cell preparations

The body of literature characterizing cyclic adenosine diphosphoribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) as Ca2+-mobilizing second messengers is growing apace. However, their unique properties may, for the uninitiated, make them difficult to work with. This article reviews many of the available techniques (and associated pitfalls) for investigating these nucleotide messengers, predominantly focusing upon optical techniques using fluorescent reporters to measure Ca2+ in the cytosol as well as Ca2+ or pH within the lumen of intracellular organelles.

TRP-ML1 functions as a lysosomal NAADP-sensitive Ca2+ release channel in coronary arterial myocytes

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent intracellular Ca²⁺ signalling second messenger, but the mechanism of NAADP-induced Ca²⁺ release is still poorly understood. The present study tested the hypothesis that NAADP induces Ca²⁺ release from the lysosomal store via a TRP-ML1 (transient receptor potential-mucolipin 1)-mediated Ca²⁺ release channel in coronary arterial myocytes (CAMs). RT-PCR and Western blot analyses demonstrated that TRP-ML1 was present in CAMs, and fluorescence resonance energy transfer (FRET) detection revealed that the TRP-ML1 was closely associated with some lysosomal proteins in these CAMs. ET-1, a well-known NAADP stimulator, was found to induce a local Ca²⁺ burst from lysosomes followed by a global Ca²⁺ release. This lysosome-associated Ca²⁺ release was significantly inhibited in the TRP-ML1 siRNA pre-treated CAMs by 46.8 ± 12.6% in the local Ca²⁺ burst and 73.3 ± 14.9% in the global Ca²⁺ wave. In the reconstituted lysosomal channels from CAMs, NAADP activated Ca²⁺ release channels at concentrations of 1–1000 nM, but neither activators (1 μM IP₃, 5 μM Rya) nor blockers (100 μM 2-APB, 50 μM Rya) of sarcoplasmic reticulum (SR) Ca²⁺ release channels had effect on the channel activity. Moreover, TRP-ML1 gene silencing reduced this NAADP-sensitive Ca²⁺ release channel activity in lysosomes by 71.5 ± 18.5%. Immunoprecipitation or blockade of TRP-ML1 by anti-TRP-ML1 antibodies almost abolished NAADP-induced activation of lysosomal Ca²⁺ channels (to 14.0 ± 4.4% of control). These results for the first time provide direct evidence that an NAADP-sensitive Ca²⁺ release channel is characteristic of TRP-ML1 channels.

Fertilization and nicotinic acid adenine dinucleotide phosphate induce pH changes in acidic Ca(2+) stores in sea urchin eggs

The second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) releases Ca(2+) from the acidic Ca(2+) stores of many organisms, including those of the sea urchin egg. We investigated whether the pH within the lumen of these acidic organelles changes in response to stimuli. Fertilization activates the egg by Ca(2+) release dependent upon NAADP, and accordingly, we report that fertilization also alters organellar pH in a spatio-temporally complex manner. Upon sperm fusion, vesicles deep in the egg center slowly acidify, whereas cortical vesicles undergo a rapid alkalinization. The cortical vesicle alkalinization is independent of exocytosis and cytosolic pH but coincides with the NAADP-dependent fertilization Ca(2+) wave. Microinjection of NAADP mimicked the fertilization cortical response, suggesting that it occurred within NAADP-sensitive acidic Ca(2+) stores. Our data show that NAADP and physiological stimuli alter the pH within intracellular organelles and suggest that NAADP signals through pH as well as Ca(2+).

Time sensing by NAADP receptors

NAADP (nicotinic acid-adenine dinucleotide phosphate) is a newly described intracellular messenger molecule that mediates Ca2+ increases in a variety of cells. However, little is known of the mechanism whereby ligand binding regulates the target protein. We report in the present paper that NAADP receptors from sea urchin eggs undergo an unusual stabilization process that appears to be dependent upon the time during which receptors are exposed to their ligand. We demonstrate that receptors 'tagged' with NAADP for short periods were more readily dissociated following subsequent delipidation than those labelled for longer. Stabilization of NAADP receptors by their ligand was delayed relative to ligand association taking on the order of minutes to develop at picomolar concentrations. The stabilizing effects of NAADP did not require cytosolic factors or the continued presence of NAADP and persisted upon solubilization. NAADP receptors, however, failed to stabilize at reduced temperature. We conclude that NAADP receptors possess a simple molecular memory endowing them with the remarkable ability to detect the duration of their activation.

NAADP and InsP3 play distinct roles at fertilization in starfish oocytes

NAADP participates in the response of starfish oocytes to sperm by triggering the fertilization potential (FP) through the activation of a Ca2+ current which depolarizes the membrane to the threshold of activation of the voltage-gated Ca2+ channels. The aim of this study was to investigate whether this Ca2+ influx is linked to the onset of the concomitant InsP3-mediated Ca2+ wave by simultaneously employing Ca2+ imaging and single-electrode intracellular recording techniques. In control oocytes, the sperm-induced membrane depolarization always preceded by a few seconds the onset of the Ca2+ wave. Strikingly, the self-desensitization of NAADP receptors either abolished the Ca2+ response or resulted in abnormal oocyte activation, i.e., the membrane depolarization followed the Ca2+ wave and the oocyte was polyspermic. The inhibition of InsP3 signaling only impaired the propagation of the Ca2+ wave and shortened the FP. The duration of FP was also reduced in low-Na+ sea water. Finally, uncaged InsP3 produced a Ca2+ increase, which depolarized the membrane upon the activation of a Ca2+-sensitive cation current. These results support the hypothesis that Ca2+ entry during the NAADP-triggered FP is required for the onset of the Ca2+ wave at fertilization. The InsP3-mediated Ca2+ wave, in turn, may interact with the NAADP-evoked depolarization by activating a Ca2+-dependent Na+ entry.

Modulation of NAADP (nicotinic acid-adenine dinucleotide phosphate) receptors by K+ ions: evidence for multiple NAADP receptor conformations

NAADP (nicotinic acid-adenine dinucleotide phosphate) mediates Ca2+ release from intracellular Ca2+ stores in a wide variety of cell types. In sea urchin eggs, subthreshold concentrations of NAADP can cause full inactivation of NAADP-induced Ca2+ release, an effect that may be related to the ability of the target protein to bind its ligand in an essentially irreversible manner. In the present study, we found that K+ ions inhibit dissociation of NAADP from sea urchin egg homogenates. In low K+-containing media, an addition of excess unlabelled NAADP effectively displaced bound radioligand whereas dilution of radioligand initiated only partial dissociation. The inhibitory effects of K+ on dissociation of NAADP were concentration dependent, reversible and persisted after detergent solubilization. Lowering [K+] of the medium decreased the sensitivity of NAADP receptors for their ligand in stimulating Ca2+ release, but it did not affect inactivation of NAADP-induced Ca2+ release by subthreshold concentrations of NAADP. Our results are consistent with the observation of multiple conformations of the NAADP receptor that are readily revealed in low K+-containing media.

Sperm deliver a new second messenger: NAADP

NAADP is a highly potent mobilizer of Ca(2+), which in turn triggers Ca(2+)-induced Ca(2+) release pathways in a wide range of species. Nevertheless, NAADP is not presently classified as a second messenger because it has not been shown to increase in response to a physiological stimulus. We now report a dramatic increase in NAADP during sea urchin egg fertilization that was largely due to production in sperm upon contacting egg jelly. The NAADP bolus plays a physiological role upon delivery to the egg based on its ability to induce a cortical flash, a depolarization-induced activation of L-type Ca(2+) channels. Moreover, the sperm-induced cortical flash was eliminated in eggs desensitized to NAADP. We conclude that an NAADP increase plays a physiologically relevant role during fertilization and provides the first conclusive demonstration that NAADP is a genuine second messenger.

NAADP mobilizes Ca(2+) from reserve granules, lysosome-related organelles, in sea urchin eggs

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes Ca(2+) in many cells and species. Unlike other Ca(2+)-mobilizing messengers, NAADP mobilizes Ca(2+) from an unknown store that is not the endoplasmic reticulum, the store traditionally associated with messenger-mediated Ca(2+) signaling. Here, we demonstrate the presence of a Ca(2+) store in sea urchin eggs mobilized by NAADP that is dependent on a proton gradient maintained by an ATP-dependent vacuolar-type proton pump. Moreover, we provide pharmacological and biochemical evidence that this Ca(2+) store is the reserve granule, the functional equivalent of a lysosome in the sea urchin egg. These findings represent an unsuspected mechanism for messenger-mediated Ca(2+) release from lysosome-related organelles.

Interactions between calcium release pathways: multiple messengers and multiple stores

The discovery of cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) as Ca(2+) releasing messengers has provided additional insight into how complex Ca(2+) signalling patterns are generated. There is mounting evidence that these molecules along with the more established messenger, myo-inositol 1,4,5-trisphosphate (IP(3)), have a widespread messenger role in shaping Ca(2+) signals in many cell types. These molecules have distinct structures and act on specific Ca(2+) release mechanisms. Emerging principles are that cADPR enhances the Ca(2+) sensitivity of ryanodine receptors (RYRs) to produce prolonged Ca(2+) signals through Ca(2+)-induced Ca(2+) release (CICR), while NAADP acts on a novel Ca(2+) release mechanism to produce a local trigger Ca(2+) signal which can be amplified by CICR by recruiting other Ca(2+) release mechanisms. Whilst IP(3) and cADPR mobilise Ca(2+) from the endoplasmic reticulum (ER), recent evidence from the sea urchin egg suggests that the major NAADP-sensitive Ca(2+) stores are reserve granules, acidic lysosomal-related organelles. In this review we summarise the role of multiple Ca(2+) mobilising messengers, Ca(2+) release channels and Ca(2+) stores, and the interplay between them, in the generation of specific Ca(2+) signals. Focusing upon cADPR and NAADP, we discuss how cellular stimuli may draw upon different combinations of these messengers to produce distinct Ca(2+) signalling signatures.

Nicotinic acid adenine dinucleotide phosphate: a new intracellular second messenger?

Nicotinic acid adenine dinucleotide phosphate (NAADP) is one of the most potent stimulators of intracellular Ca2+ release known to date. The role of the NAADP system in physiological processes is being extensively investigated at the present time. Exciting new discoveries in the last 5 years suggest that the NAADP-regulated system may have a significant role in intracellular Ca2+ signaling. The NAADP receptor and its associated Ca2+ pool have been hypothesized to be important in several physiological processes including fertilization, T cell activation, and pancreatic secretion. However, whether NAADP is a new second messenger or a tool for the discovery of a new Ca2+ channel is still an unanswered question.

Coordination of Ca2+ signalling by NAADP

Nicotinic acid adenine dinucleotide phosphate (NAADP) mobilizes intracellular Ca2+ stores in several cell types. Ample evidence suggests that NAADP activates intracellular Ca2+ channels distinct from those that are sensitive to inositol trisphosphate and ryanodine/cyclic ADP-ribose. Recent studies in intact cells have demonstrated functional coupling ('channel chatter') between Ca2+ release pathways mediated by NAADP, inositol trisphosphate and cyclic ADP-ribose. Thus, NAADP is probably an important determinant in shaping cytosolic Ca2+ signals.

NAADP induces Ca2+ oscillations via a two-pool mechanism by priming IP3- and cADPR-sensitive Ca2+ stores

In sea urchin eggs, Ca2+ mobilization by nicotinic acid adenine dinucleotide phosphate (NAADP) potently self-inactivates but paradoxically induces long-term Ca2+ oscillations. We investigated whether NAADP-induced Ca2+ oscillations arise from the recruitment of other Ca2+ release pathways. NAADP, inositol trisphosphate (IP3) and cyclic ADP-ribose (cADPR) all mobilized Ca2+ from internal stores but only NAADP consistently induced Ca2+ oscillations. NAADP-induced Ca2+ oscillations were partially inhibited by heparin or 8-amino-cADPR alone, but eliminated by the presence of both, indicating a requirement for both IP3- and cADPR-dependent Ca2+ release. Thapsigargin completely blocked IP3 and cADPR responses as well as NAADP-induced Ca2+ oscillations, but only reduced the NAADP-mediated Ca2+ transient. Following NAADP-mediated release from this Ca2+ pool, the amount of Ca2+ in the Ca2+-induced Ca2+ release stores was increased. These results support a mechanism in which Ca2+ oscillations are initiated by Ca2+ release from NAADP-sensitive Ca2+ stores (pool 1) and perpetuated through cycles of Ca2+ uptake into and release from Ca2+-induced Ca2+ release stores (pool 2). These results provide the first direct evidence in support of a two-pool model for Ca2+ oscillations.