NAADP receptors are present and functional in the heart

NAADP-Dependent TPC Current

Two-pore channels, TPC1 and TPC2, are Ca²⁺- and Na⁺-permeable cation channels expressed on the membranes of endosomes and lysosomes in nearly all mammalian cells. These channels have been implicated in Ca²⁺ signaling initiated from the endolysosomes, vesicular trafficking, cellular metabolism, macropinocytosis, and viral infection. Although TPCs have been shown to mediate Ca²⁺ release from acidic organelles in response to NAADP (nicotinic acid adenine dinucleotide phosphate), the most potent Ca²⁺ mobilizing messenger, questions remain whether NAADP is a direct ligand of these channels. In whole-endolysosomal patch clamp recordings, it has been difficult to detect NAADP-evoked currents in vacuoles that expressed TPC1 or TPC2, while PI(3,5)P₂ (phosphatidylinositol 3,5-bisphosphate) activated a highly Na⁺-selective current under the same experimental configuration. In this chapter, we summarize recent progress in this area and provide our observations on NAADP-elicited TPC2 currents from enlarged endolysosomes as well as their possible relationships with the currents evoked by PI(3,5)P₂. We propose that TPCs are channels dually regulated by PI(3,5)P₂ and NAADP in an interdependent manner and the two endogenous ligands may have both distinguished and cooperative roles.

NAADP-binding proteins find their identity

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a second messenger that releases Ca²⁺ from endosomes and lysosomes by activating ion channels called two-pore channels (TPCs). However, no NAADP-binding site has been identified on TPCs. Rather, NAADP activates TPCs indirectly by engaging NAADP-binding proteins (NAADP-BPs) that form part of the TPC complex. After a decade of searching, two different NAADP-BPs were recently identified: Jupiter microtubule associated homolog 2 (JPT2) and like-Sm protein 12 (LSM12). These discoveries bridge the gap between NAADP generation and NAADP activation of TPCs, providing new opportunity to understand and manipulate the NAADP-signaling pathway. The unmasking of these NAADP-BPs will catalyze future studies to define the molecular choreography of NAADP action.

A modified density gradient proteomic-based method to analyze endolysosomal proteins in cardiac tissue

The importance of lysosomes in cardiac physiology and pathology is well established, and evidence for roles in calcium signaling is emerging. We describe a label-free proteomics method suitable for small cardiac tissue biopsies based on density-separated fractionation, which allows study of endolysosomal (EL) proteins. Density gradient fractions corresponding to tissue lysate; sarcoplasmic reticulum (SR), mitochondria (Mito) (1.3 g/mL); and EL with negligible contamination from SR or Mito (1.04 g/mL) were analyzed using Western blot, enzyme activity assay, and liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis (adapted discontinuous Percoll and sucrose differential density gradient). Kyoto Encyclopedia of Genes and Genomes, Reactome, Panther, and Gene Ontology pathway analysis showed good coverage of RAB proteins and lysosomal cathepsins (including cardiac-specific cathepsin D) in the purified EL fraction. Significant EL proteins recovered included catalytic activity proteins. We thus present a comprehensive protocol and data set of guinea pig atrial EL organelle proteomics using techniques also applicable for non-cardiac tissue.

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.

Calcium Signaling in Cardiomyocyte Function

Rhythmic increases in intracellular Ca²⁺ concentration underlie the contractile function of the heart. These heart muscle-wide changes in intracellular Ca²⁺ are induced and coordinated by electrical depolarization of the cardiomyocyte sarcolemma by the action potential. Originating at the sinoatrial node, conduction of this electrical signal throughout the heart ensures synchronization of individual myocytes into an effective cardiac pump. Ca²⁺ signaling pathways also regulate gene expression and cardiomyocyte growth during development and in pathology. These fundamental roles of Ca²⁺ in the heart are illustrated by the prevalence of altered Ca²⁺ homeostasis in cardiovascular diseases. Indeed, heart failure (an inability of the heart to support hemodynamic needs), rhythmic disturbances, and inappropriate cardiac growth all share an involvement of altered Ca²⁺ handling. The prevalence of these pathologies, contributing to a third of all deaths in the developed world as well as to substantial morbidity makes understanding the mechanisms of Ca²⁺ handling and dysregulation in cardiomyocytes of great importance.

Calcium Signaling in the Heart

The aim of this chapter is to discuss evidence concerning the many roles of calcium ions, Ca²⁺, in cell signaling pathways that control heart function. Before considering details of these signaling pathways, the control of contraction in ventricular muscle by Ca²⁺ transients accompanying cardiac action potentials is first summarized, together with a discussion of how myocytes from the atrial and pacemaker regions of the heart diverge from this basic scheme. Cell signaling pathways regulate the size and timing of the Ca²⁺ transients in the different heart regions to influence function. The simplest Ca²⁺ signaling elements involve enzymes that are regulated by cytosolic Ca²⁺. Particularly important examples to be discussed are those that are stimulated by Ca²⁺, including Ca²⁺-calmodulin-dependent kinase (CaMKII), Ca²⁺ stimulated adenylyl cyclases, Ca²⁺ stimulated phosphatase and NO synthases. Another major aspect of Ca²⁺ signaling in the heart concerns actions of the Ca²⁺ mobilizing agents, inositol trisphosphate (IP₃), cADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate, (NAADP). Evidence concerning roles of these Ca²⁺ mobilizing agents in different regions of the heart is discussed in detail. The focus of the review will be on short term regulation of Ca²⁺ transients and contractile function, although it is recognized that Ca²⁺ regulation of gene expression has important long term functional consequences which will also be briefly discussed.

5-Azido-8-ethynyl-NAADP: A bifunctional, clickable photoaffinity probe for the identification of NAADP receptors

Nicotinic acid adenine dinucleotide phosphate is an evolutionarily conserved second messenger, which mobilizes Ca²⁺ from acidic stores. The molecular identity of the NAADP receptor has yet to be defined. In pursuit of isolating and identifying NAADP-binding proteins, we synthesized and characterized a bifunctional probe that incorporates both a photoactivatable crosslinking azido moiety at the 5-position of the nicotinic ring and a 'clickable' ethynyl moiety to the 8-adenosyl position in NAADP. Microinjection of this 5N₃-8-ethynyl-NAADP into cultured U2OS cells induced robust Ca²⁺ responses. Higher concentrations of 5N₃-8-ethynyl were required to elicit Ca²⁺ release or displace ³²P-NAADP in radioligand binding experiments in sea urchin egg homogenates. In human cell extracts, incubation of ³²P-5N₃-8-ethynyl-NAADP followed by UV irradiation resulted in selective labeling of 23 kDa and 35 kDa proteins and photolabeling of these proteins was prevented when incubated in the presence of unlabeled NAADP. Compared to the monofunctional ³²P-5N₃-NAADP, the clickable ³²P-5N₃-8-ethynyl-NAADP demonstrated less labeling of the 23 kDa and 35 kDa proteins (~3-fold) but provided an opportunity for further enrichment through the 'clickable' ethynyl moiety. No proteins were specifically labeled by ³²P-5N₃-8-ethynyl-NAADP in sea urchin egg homogenate. These experiments demonstrate that 5N₃-8-ethynyl-NAADP is biologically active and selectively labels putative NAADP-binding proteins in mammalian systems, evidencing a 'bifunctional' probe with utility for isolating NAADP-binding proteins.

Two-pore channels and disease

Two-pore channels (TPCs) are Ca²⁺-permeable endo-lysosomal ion channels subject to multi-modal regulation. They mediate their physiological effects through releasing Ca²⁺ from acidic organelles in response to cues such as the second messenger, NAADP. Here, we review emerging evidence linking TPCs to disease. We discuss how perturbing both local and global Ca²⁺ changes mediated by TPCs through chemical and/or molecular manipulations can induce or reverse disease phenotypes. We cover evidence from models of Parkinson's disease, non-alcoholic fatty liver disease, Ebola infection, cancer, cardiac dysfunction and diabetes. A need for more drugs targeting TPCs is identified.

High resolution structural evidence suggests the Sarcoplasmic Reticulum forms microdomains with Acidic Stores (lysosomes) in the heart

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP) stimulates calcium release from acidic stores such as lysosomes and is a highly potent calcium-mobilising second messenger. NAADP plays an important role in calcium signalling in the heart under basal conditions and following β-adrenergic stress. Nevertheless, the spatial interaction of acidic stores with other parts of the calcium signalling apparatus in cardiac myocytes is unknown. We present evidence that lysosomes are intimately associated with the sarcoplasmic reticulum (SR) in ventricular myocytes; a median separation of 20 nm in 2D electron microscopy and 3.3 nm in 3D electron tomography indicates a genuine signalling microdomain between these organelles. Fourier analysis of immunolabelled lysosomes suggests a sarcomeric pattern (dominant wavelength 1.80 μm). Furthermore, we show that lysosomes form close associations with mitochondria (median separation 6.2 nm in 3D studies) which may provide a basis for the recently-discovered role of NAADP in reperfusion-induced cell death. The trigger hypothesis for NAADP action proposes that calcium release from acidic stores subsequently acts to enhance calcium release from the SR. This work provides structural evidence in cardiac myocytes to indicate the formation of microdomains between acidic and SR calcium stores, supporting emerging interpretations of NAADP physiology and pharmacology in heart.

Inhibition of NAADP signalling on reperfusion protects the heart by preventing lethal calcium oscillations via two-pore channel 1 and opening of the mitochondrial permeability transition pore

Aims

In the heart, a period of ischaemia followed by reperfusion evokes powerful cytosolic Ca²⁺ oscillations that can cause lethal cell injury. These signals represent attractive cardioprotective targets, but the underlying mechanisms of genesis are ill-defined. Here, we investigated the role of the second messenger nicotinic acid adenine dinucleotide phosphate (NAADP), which is known in several cell types to induce Ca²⁺ oscillations that initiate from acidic stores such as lysosomes, likely via two-pore channels (TPCs, TPC1 and 2).

Methods and results

An NAADP antagonist called Ned-K was developed by rational design based on a previously existing scaffold. Ned-K suppressed Ca²⁺ oscillations and dramatically protected cardiomyocytes from cell death in vitro after ischaemia and reoxygenation, preventing opening of the mitochondrial permeability transition pore. Ned-K profoundly decreased infarct size in mice in vivo. Transgenic mice lacking the endo-lysosomal TPC1 were also protected from injury.

Conclusion

NAADP signalling plays a major role in reperfusion-induced cell death and represents a potent pathway for protection against reperfusion injury.

Two-Pore Channels: Lessons from Mutant Mouse Models

Recent interest in two-pore channels (TPCs) has resulted in a variety of studies dealing with the functional role and mechanism of action of these endo-lysosomal proteins in diverse physiological processes. With the availability of mouse lines harbouring mutant alleles for Tpcnl and/or Tpcn2 genes, several studies have made use of them to validate, consolidate and discover new roles for these channels not only at the cellular level but, importantly, also at the level of the whole organism. The different mutant mouse lines that have been used were derived from distinct genetic manipulation strategies, with the aim of knocking out expression of TPC proteins. However, the expression of different residual TPC sequences predicted to occur in these mutant mouse lines, together with the varied degree to which the effects on Tpcn expression have been studied, makes it important to assess the true knockout status of some of the lines. In this review we summarize these Tpcn mutant mouse lines with regard to their predicted effect on Tpcn expression and the extent to which they have been characterized. Additionally, we discuss how results derived from studies using these Tpcn mutant mouse lines have consolidated previously proposed roles for TPCs, such as mediators of NAADP signalling, endo-lysosomal functions, and pancreatic β cell physiology. We will also review how they have been instrumental in the assignment of new physiological roles for these cation channels in processes such as membrane electrical excitability, neoangiogenesis, viral infection and brown adipose tissue and heart function, revealing, in some cases, a specific contribution of a particular TPC isoform.

Nicotinic Acid Adenine Dinucleotide Phosphate Analogues Substituted on the Nicotinic Acid and Adenine Ribosides. Effects on ReceptorMediated Ca²⁺ Release

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a Ca(2+) releasing intracellular second messenger in both mammals and echinoderms. We report that large functionalized substituents introduced at the nicotinic acid 5-position are recognized by the sea urchin receptor, albeit with a 20-500-fold loss in agonist potency. 5-(3-Azidopropyl)-NAADP was shown to release Ca(2+) with an EC50 of 31 μM and to compete with NAADP for receptor binding with an IC50 of 56 nM. Attachment of charged groups to the nicotinic acid of NAADP is associated with loss of activity, suggesting that the nicotinate riboside moiety is recognized as a neutral zwitterion. Substituents (Br- and N3-) can be introduced at the 8-adenosyl position of NAADP while preserving high potency and agonist efficacy and an NAADP derivative substituted at both the 5-position of the nicotinic acid and at the 8-adenosyl position was also recognized although the agonist potency was significantly reduced.

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.

NAD derived second messengers: Role in spontaneous diastolic Ca(2+) transients in murine cardiac myocytes

Strong β-adrenergic stimulation induced spontaneous diastolic Ca(2+) transients (SCTs) in electrically paced murine cardiac myocytes [28]. To obtain further insights into the underlying mechanism, we developed a method for a simultaneous analysis, in which the free luminal Ca(2+) concentration in the sarcoplasmic reticulum (SR) ([Ca(2+)]SR) and the free cytosolic Ca(2+) concentration ([Ca(2+)]i) were measured in parallel in the same cell. Each spontaneous diastolic Ca(2+) transient was exactly mirrored by a decrease of [Ca(2+)]SR. Since antagonism of the Ca(2+) mobilizing second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) was shown to block SCTs in single cardiac myocytes [28], we analyzed the effect of the novel ADP-ribosyl cyclase inhibitor SAN4825 on both cytosolic and intra-luminal Ca(2+) transients upon strong β-adrenergic stimulation. A strong antagonist effect of SAN4825 on SCTs at low micromolar concentrations was observed. Our results suggest that the underlying mechanism of spontaneous diastolic Ca(2+) transients observed upon strong β-adrenergic stimulation is sensitization of type 2 ryanodine receptor by the Ca(2+) releasing activity of the products of ADP-ribosyl cyclase activity.

Activity of nicotinic acid substituted nicotinic acid adenine dinucleotide phosphate (NAADP) analogs in a human cell line: difference in specificity between human and sea urchin NAADP receptors

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca2+ mobilizing second messenger that has been identified. We have previously shown that NAADP analogs substituted at the 5-position of nicotinic acid were recognized by the sea urchin receptor at low concentration, whereas the 4- substituted analogs were not as potent. However, to date the structure-activity relationship (SAR) of these analogs has not been addressed in mammalian systems. Thus, we asked whether these structurally modified analogs behave similarly in an NAADP-responsive mammalian cell line (SKBR3) using microinjection and single cell fluorescent imaging methods. Novel "caged" 4- and 5-substituted NAADP analogs that were activated inside the cell by flash photolysis resulted in Ca2+ mobilizing activity in SKBR3 cells in a concentration dependent manner, but with reduced effectiveness compared to unmodified NAADP. The SAR in mammalian SKBR3 cells was quite different from that of sea urchin and may suggest that there are differences between NAADP receptors in different species or tissues. Importantly, these data indicate that modifications at the 4- and 5-position of the nicotinic acid ring may lead to the development of functional photoaffinity labels that could be used for receptor localization and isolation in mammalian systems.

Nicotinic acid adenine dinucleotide phosphate (NAADP)-mediated calcium signaling and arrhythmias in the heart evoked by β-adrenergic stimulation

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent Ca(2+)-releasing second messenger known to date. Here, we report a new role for NAADP in arrhythmogenic Ca(2+) release in cardiac myocytes evoked by β-adrenergic stimulation. Infusion of NAADP into intact cardiac myocytes induced global Ca(2+) signals sensitive to inhibitors of both acidic Ca(2+) stores and ryanodine receptors and to NAADP antagonist BZ194. Furthermore, in electrically paced cardiac myocytes BZ194 blocked spontaneous diastolic Ca(2+) transients caused by high concentrations of the β-adrenergic agonist isoproterenol. Ca(2+) transients were recorded both as increases of the free cytosolic Ca(2+) concentration and as decreases of the sarcoplasmic luminal Ca(2+) concentration. Importantly, NAADP antagonist BZ194 largely ameliorated isoproterenol-induced arrhythmias in awake mice. We provide strong evidence that NAADP-mediated modulation of couplon activity plays a role for triggering spontaneous diastolic Ca(2+) transients in isolated cardiac myocytes and arrhythmias in the intact animal. Thus, NAADP signaling appears an attractive novel target for antiarrhythmic therapy.

β-Adrenergic receptor signaling increases NAADP and cADPR levels in the heart

Evidence suggests that β-Adrenergic receptor signaling increases heart rate and force through not just cyclic AMP but also the Ca(2+)-releasing second messengers NAADP (nicotinic acid adenine dinucleotide phosphate) and cADPR (cyclic ADP-ribose). Nevertheless, proof of the physiological relevance of these messengers requires direct measurements of their levels in response to receptor stimulation. Here we report that in intact Langendorff-perfused hearts β-adrenergic stimulation increased both messengers, with NAADP being transient and cADPR being sustained. Both NAADP and cADPR have physiological and therefore pathological relevance by providing alternative drug targets in the β-adrenergic receptor signaling pathway.

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.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

NAADP regulates human platelet function

Platelets play a vital role in maintaining haemostasis. Human platelet activation depends on Ca2+ release, leading to cell activation, granule secretion and aggregation. NAADP (nicotinic acid-adenine dinucleotide phosphate) is a Ca2+-releasing second messenger that acts on acidic Ca2+ stores and is used by a number of mammalian systems. In human platelets, NAADP has been shown to release Ca2+ in permeabilized human platelets and contribute to thrombin-mediated platelet activation. In the present study, we have further characterized NAADP-mediated Ca2+ release in human platelets in response to both thrombin and the GPVI (glycoprotein VI)-specific agonist CRP (collagen-related peptide). Using a radioligand-binding assay, we reveal an NAADP-binding site in human platelets, indicative of a platelet NAADP receptor. We also found that NAADP releases loaded 45Ca2+ from intracellular stores and that total platelet Ca2+ release is inhibited by the proton ionophore nigericin. Ned-19, a novel cell-permeant NAADP receptor antagonist, competes for the NAADP-binding site in platelets and can inhibit both thrombin- and CRP-induced Ca2+ release in human platelets. Ned-19 has an inhibitory effect on platelet aggregation, secretion and spreading. In addition, Ned-19 extends the clotting time in whole-blood samples. We conclude that NAADP plays an important role in human platelet function. Furthermore, the development of Ned-19 as an NAADP receptor antagonist provides a potential avenue for platelet-targeted therapy and the regulation of thrombosis.

Photoaffinity labeling of nicotinic acid adenine dinucleotide phosphate (NAADP) targets in mammalian cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) is an agonist-generated second messenger that releases Ca(2+) from intracellular acidic Ca(2+) stores. Recent evidence has identified the two-pore channels (TPCs) within the endolysosomal system as NAADP-regulated Ca(2+) channels that release organellar Ca(2+) in response to NAADP. However, little is known about the mechanism coupling NAADP binding to calcium release. To identify the NAADP binding site, we employed a photoaffinity labeling method using a radioactive photoprobe based on 5-azido-NAADP ([(32)P-5N(3)]NAADP) that exhibits high affinity binding to NAADP receptors. In several systems that are widely used for studying NAADP-evoked Ca(2+) signaling, including sea urchin eggs, human cell lines (HEK293, SKBR3), and mouse pancreas, 5N(3)-NAADP selectively labeled low molecular weight sites that exhibited the diagnostic pharmacology of NAADP-sensitive Ca(2+) release. Surprisingly, we were unable to demonstrate labeling of endogenous, or overexpressed, TPCs. Furthermore, labeling of high affinity NAADP binding sites was preserved in pancreatic samples from TPC1 and TPC2 knock-out mice. These photolabeling data suggest that an accessory component within a larger TPC complex is responsible for binding NAADP that is unique from the core channel itself. This observation necessitates critical evaluation of current models of NAADP-triggered activation of the TPC family.

NAADP influences excitation-contraction coupling by releasing calcium from lysosomes in atrial myocytes

In atrial myocytes, the sarcoplasmic reticulum (SR) has an essential role in regulating the force of contraction as a consequence of its involvement in excitation-contraction coupling (ECC). Nicotinic acid adenine dinucleotide phosphate (NAADP) is a Ca(2+) mobilizing messenger that acts to release Ca(2+) from an acidic store in mammalian cells. The photorelease of NAADP in atrial myocytes increased Ca(2+) transient amplitude with no effect on accompanying action potentials or the L-type Ca(2+) current. NAADP-AM, a cell permeant form of NAADP, increased Ca(2+) spark amplitude and frequency. The effect on Ca(2+) spark frequency could be prevented by bafilomycin A1, a vacuolar H(+)-ATPase inhibitor, or by disruption of lysosomes by GPN. Bafilomycin prevented staining of acidic stores with LysoTracker red by increasing lysosomal pH. NAADP-AM also produced an increase in the lysosomal pH, as detected by a reduction in LysoSensor green fluorescence. These effects of NAADP were associated with an increase in the amount of caffeine-releasable Ca(2+) in the SR and may be regulated by β-adrenoceptor stimulation with isoprenaline. These observations are consistent with a role for NAADP in regulating ECC in atrial myocytes by releasing Ca(2+) from an acidic store, which enhances SR Ca(2+) release by increasing SR load.

Nicotinic acid adenine dinucleotide phosphate analogues containing substituted nicotinic acid: effect of modification on Ca(2+) release

Analogues of nicotinic acid adenine dinucleotide phosphate (NAADP) with substitution at either the 4- or the 5-position position of the nicotinic acid moiety have been synthesized from NADP enzymatically using Aplysia californica ADP-ribosyl cyclase or mammalian NAD glycohydrolase. Substitution at the 4-position of the nicotinic acid resulted in the loss of agonist potency for release of Ca(2+)-ions from sea urchin egg homogenates and in potency for competition ligand binding assays using [(32)P]NAADP. In contrast, several 5-substituted NAADP derivatives showed high potency for binding and full agonist activity for Ca(2+) release. 5-Azido-NAADP was shown to release calcium from sea urchin egg homogenates at low concentration and to compete with [(32)P]NAADP in a competition ligand binding assay with an IC(50) of 18 nM, indicating that this compound might be a potential photoprobe useful for specific labeling and identification of the NAADP receptor.

From eggs to hearts: what is the link between cyclic ADP-ribose and ryanodine receptors?

It was first proposed that cyclic ADP-ribose (cADPR) could activate ryanodine receptors (RyR) in 1991. Following a subsequent report that cADPR could activate cardiac RyR (RyR2) reconstituted into artificial membranes and stimulate Ca(2+) -release from isolated cardiac SR, there has been a steadily mounting stockpile of publications proclaiming the physiological and pathophysiological importance of cADPR in the cardiovascular system. It was only 2 years earlier, in 1989, that cADPR was first identified as the active metabolite of nicotinamide adenine dinucleotide (NAD), responsible for triggering the release of Ca(2+) from crude homogenates of sea urchin eggs. Twenty years later, can we boast of being any closer to unraveling the mechanisms by which cADPR modulates intracellular Ca(2+) -release? This review sets out to examine the mechanisms underlying the effects of cADPR and ask whether cADPR is an important signaling molecule in the heart.

NAADP mobilizes calcium from acidic organelles through two-pore channels

Ca(2+) mobilization from intracellular stores represents an important cell signalling process that is regulated, in mammalian cells, by inositol-1,4,5-trisphosphate (InsP(3)), cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). InsP(3) and cyclic ADP ribose cause the release of Ca(2+) from sarcoplasmic/endoplasmic reticulum stores by the activation of InsP(3) and ryanodine receptors (InsP(3)Rs and RyRs). In contrast, the nature of the intracellular stores targeted by NAADP and the molecular identity of the NAADP receptors remain controversial, although evidence indicates that NAADP mobilizes Ca(2+) from lysosome-related acidic compartments. Here we show that two-pore channels (TPCs) comprise a family of NAADP receptors, with human TPC1 (also known as TPCN1) and chicken TPC3 (TPCN3) being expressed on endosomal membranes, and human TPC2 (TPCN2) on lysosomal membranes when expressed in HEK293 cells. Membranes enriched with TPC2 show high affinity NAADP binding, and TPC2 underpins NAADP-induced Ca(2+) release from lysosome-related stores that is subsequently amplified by Ca(2+)-induced Ca(2+) release by InsP(3)Rs. Responses to NAADP were abolished by disrupting the lysosomal proton gradient and by ablating TPC2 expression, but were only attenuated by depleting endoplasmic reticulum Ca(2+) stores or by blocking InsP(3)Rs. Thus, TPCs form NAADP receptors that release Ca(2+) from acidic organelles, which can trigger further Ca(2+) signals via sarcoplasmic/endoplasmic reticulum. TPCs therefore provide new insights into the regulation and organization of Ca(2+) signals in animal cells, and will advance our understanding of the physiological role of NAADP.

Nicotinic acid adenine dinucleotide phosphate (NAADP) and Ca2+ mobilization

Many physiological processes are controlled by a great diversity of Ca2+ signals that depend on Ca2+ entry into the cell and/or Ca2+ release from internal Ca2+ stores. Ca2+ mobilization from intracellular stores is gated by a family of messengers including inositol-1,4,5-trisphosphate (InsP3), cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). There is increasing evidence for a novel intracellular Ca2+ release channel that may be targeted by NAADP and that displays properties distinctly different from the well-characterized InsP3 and ryanodine receptors. These channels appear to localize on a wider range of intracellular organelles, including the acidic Ca2+ stores. Activation of the NAADP-sensitive Ca2+ channels evokes complex changes in cytoplasmic Ca2+ levels by means of channel chatter with other intracellular Ca2+ channels. The recent demonstration of changes in intracellular NAADP levels in response to physiologically relevant extracellular stimuli highlights the significance of NAADP as an important regulator of intracellular Ca2+ signaling.

NAADP-induced Ca(2+ signaling in response to endothelin is via the receptor subtype B and requires the integrity of lipid rafts/caveolae

We have investigated the role of NAADP-mediated Ca(2+) mobilization in endothelin (ET) signaling via endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) in rat peritubular smooth muscle cells. Microinjection and extracellular application of NAADP were both able to elicit Ca(2+) release which was blocked by inhibitory concentrations of NAADP, by impairing Ca(2+) uptake in acidic stores with bafilomycin, and by thapsigargin. Ca(2+) release in response to selective ETB stimulation was abolished by inhibition of NAADP signaling through the same strategies, while these treatments only partially impaired ETA-dependent Ca(2+) signaling, showing that transduction of the ETB signal is dependent on NAADP. In addition, we show that lipid rafts/caveolae contain ETA, ETB, and NAADP/cADPR generating enzyme CD38 and that stimulation of ETB receptors results in increased CD38 activity; interestingly, ETB- (but not ETA-) mediated Ca(2+) responses were antagonized by disruption of lipid rafts/caveolae with methyl-beta-cyclodextrin. These data demonstrate a primary role of NAADP in ETB-mediated Ca(2+) signaling and strongly suggest a novel role of lipid rafts/caveolae in triggering ET-induced NAADP signaling.

NAADP receptors

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a recently described Ca2+ mobilizing messenger. First described in the sea urchin egg, it has been shown to mobilize Ca2+ from intracellular stores. It is a remarkably potent molecule, and recent reports show that its cellular levels change in response to a variety of agonists confirming its role as a Ca2+ mobilizing messenger. In many cases NAADP interacts with other Ca2+ mobilizing messengers such as inositol 1,4,5 trisphosphate (IP3 and cyclic adenosine diphosphate ribose (cADPR) in shaping cytosolic Ca2+ signals. What is not clear is the molecular nature of the NAADP-sensitive Ca2+ release mechanism and its sub-cellular localization. In this review we focus on the recent progress made in sea urchin eggs, which indicates that NAADP activates a novel Ca2+ release channel distinct from the relatively well-characterized IP3 and ryanodine receptors. Furthermore, in the sea urchin egg, the NAADP-sensitive store appears to be separate from the endoplasmic reticulum (ER) and is most likely an acidic store. These findings have also been reinforced by similar findings by some in mammalian cells. Finally, we discuss ongoing strategies to characterise NAADP-binding proteins which will greatly enhance our understanding of NAADP-mediated Ca2+ signalling, and lead to the development of more selective tools to probe the role of this messenger.

Calcium signaling in airway smooth muscle

Contractility of airway smooth muscle requires elevation of intracellular calcium concentration. Under resting conditions, airway smooth muscle cells maintain a relatively low intracellular calcium concentration, and activation of the surface receptors by contractile agonists results in an elevation of intracellular calcium, culminating in contraction of the cell. The pattern of elevation of intracellular calcium brought about by agonists is a dynamic process and involves the coordinated activities of ion channels located in the plasma membrane and the sarcoplasmic reticulum. Among the signaling molecules involved in this dynamic calcium regulation in airway smooth muscle cells are inositol 1,4,5-trisphosphate and cyclic ADP-ribose, which mobilize calcium from the sarcoplasmic reticulum by acting via the inositol 1,4,5-trisphosphate and ryanodine receptors, respectively. In addition, calcium influx from the extracellular space is critical for the repletion of the intracellular calcium stores during activation of the cells by agonists. Calcium influx can occur via voltage- and receptor-gated channels in the plasma membrane, as well as by influx that is triggered by depletion of the intracellular stores (i.e., store-operated calcium entry mechanism). Transient receptor potential proteins appear to mediate the calcium influx via receptor- and store-operated channels. Recent studies have shown that proinflammatory cytokines regulate the expression and activity of the pathways involved in intracellular calcium regulation, thereby contributing to airway smooth muscle cell hyperresponsiveness. In this review, we will discuss the specific roles of cyclic ADP-ribose/ryanodine receptor channels and transient receptor potential channels in the regulation of intracellular calcium in airway smooth muscle cells.

NAADP controls cross-talk between distinct Ca2+ stores in the heart

In cardiac muscle the sarcoplasmic reticulum (SR) plays a key role in the control of contraction, releasing Ca(2+) in response to Ca(2+) influx across the sarcolemma via voltage-gated Ca(2+) channels. Here we report evidence for an additional distinct Ca(2+) store and for actions of nicotinic acid adenine dinucleotide phosphate (NAADP) to mobilize Ca(2+) from this store, leading in turn to enhanced Ca(2+) loading of the SR. Photoreleased NAADP increased Ca(2+) transients accompanying stimulated action potentials in ventricular myocytes. The effects were prevented by bafilomycin A (an H(+)-ATPase inhibitor acting on acidic Ca(2+) stores), by desensitizing concentrations of NAADP, and by ryanodine and thapsigargin to suppress SR function. Bafilomycin A also suppressed staining of acidic stores with Lysotracker Red without affecting SR integrity. Cytosolic application of NAADP by means of its membrane permeant acetoxymethyl ester increased myocyte contraction and the frequency and amplitude of Ca(2+) sparks, and these effects were inhibited by bafilomycin A. Effects of NAADP were associated with an increase in SR Ca(2+) load and appeared to be regulated by beta-adrenoreceptor stimulation. The observations are consistent with a novel role for NAADP in cardiac muscle mediated by Ca(2+) release from bafilomycin-sensitive acidic stores, which in turn enhances SR Ca(2+) release by increasing SR Ca(2+) load.

Chemo-enzymatic synthesis and biological evaluation of photolabile nicotinic acid adenine dinuclotide phosphate (NAADP+)

A chemo-enzymatic synthesis of novel caged NAADP+ without the formation of multiple cage compounds has been achieved. The biological activity of the caged NAADP+ was demonstrated by its fast uncaging in intact sea-urchin eggs.

Cell-permeant small-molecule modulators of NAADP-mediated Ca2+ release

Nicotinic acid adenine dinucleotide phosphate (NAADP, 1) is the most potent intracellular Ca2+ mobilizing agent in important mammalian cells and tissues, yet the identity of the NAADP receptor is elusive. Significantly, the coenzyme NADP is completely inactive in this respect. Current studies are restricted by the paucity of any chemical probes beyond NAADP itself, and importantly, none is cell permeant. We report simple nicotinic acid-derived pyridinium analogs as low molecular weight compounds that (1) inhibit Ca2+ release via the NAADP receptor (IC50 approximately 15 microM - 1 mM), (2) compete with NAADP binding, (3) cross the cell membrane of sea urchin eggs to inhibit NAADP-evoked Ca2+ release, and (4) selectively ablate NAADP-dependent Ca2+ oscillations induced by the external gastric peptide hormone agonist cholecystokinin (CCK) in murine pancreatic acinar cells.

Second messenger function of nicotinic acid adenine dinucleotide phosphate revealed by an improved enzymatic cycling assay

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent activator of Ca2+ release from intracellular stores known today. Although recent reports have suggested an important function of NAADP in human T lymphocytes, direct evidence for receptor-induced formation of NAADP is yet missing in these cells. Thus, we developed a highly sensitive and specific enzyme assay capable of quantifying low fmol amounts of NAADP. In unstimulated T cells, the NAADP concentration amounted to 4.4 +/- 1.6 nm (0.055 +/- 0.028 pmol/mg of protein). Stimulation of the cells via the T cell receptor/CD3 complex resulted in biphasic elevation kinetics of cellular NAADP levels and was characterized by a bell-shaped concentration-response curve for NAADP. In contrast, the NAADP concentration was elevated neither upon activation of the ADP-ribose/TRPM2 channel Ca2+ signaling system nor by an increase of the intracellular Ca2+ concentration upon thapsigargin stimulation. T cell receptor/CD3 complex-mediated NAADP formation was dependent on the activity of tyrosine kinases because genistein completely blocked NAADP elevation. Thus, we propose a regulated formation of NAADP upon specific stimulation of the T cell receptor/CD3 complex, suggesting a function of NAADP as a Ca2+-mobilizing second messenger during T cell activation.

Ca2+ release triggered by NAADP in hepatocyte microsomes

NAADP (nicotinic acid-adenine dinucleotide phosphate) is fast emerging as a new intracellular Ca2+-mobilizing messenger. NAADP induces Ca2+ release by a mechanism that is distinct from IP3 (inositol 1,4,5-trisphosphate)- and cADPR (cADP-ribose)-induced Ca2+ release. In the present study, we demonstrated that micromolar concentrations of NAADP trigger Ca2+ release from rat hepatocyte microsomes. Cross-desensitization to IP3 and cADPR by NAADP did not occur in liver microsomes. We report that non-activating concentrations of NAADP can fully inactivate the NAADP-sensitive Ca2+-release mechanism in hepatocyte microsomes. The ability of thapsigargin to block the NAADP-sensitive Ca2+ release is not observed in sea-urchin eggs or in intact mammalian cells. In contrast with the Ca2+ release induced by IP3 and cADPR, the Ca2+ release induced by NAADP was completely independent of the free extravesicular Ca2+ concentration and pH (in the range 6.4-7.8). The NAADP-elicited Ca2+ release cannot be blocked by the inhibitors of the IP3 receptors and the ryanodine receptor. On the other hand, verapamil and diltiazem do inhibit the NAADP- (but not IP3- or cADPR-) induced Ca2+ release.

A transport mechanism for NAADP in a rat basophilic cell line

NAADP is a second messenger that releases Ca2+ from intracellular stores. Surprisingly, it has been recently shown that extracellular application of NAADP is capable of inducing intracellular Ca2+ release. This is particularly important since the only mammalian enzymes known to catalyze the synthesis of this second messenger are located extracellularly. In the present manuscript, we have investigated whether mammalian cells possess a transport system capable of transporting this highly charged molecule into cells. Indeed, in RBL-2H3 cells, a rat basophilic cell line, and in SK-N-BE cells, a neuroblastoma cell line, [32P]NAADP is efficiently transported inside cells. NAADP transport is Na+ and Ca2+ dependent, is partially blocked by dipyridamole, but is unaffected by nitrobenzylthioinosine. RBL-2H3 cells also transport [32P]cADPR, but the differences in the pharmacological profile suggest that NAADP transport proceeds by a novel mechanism. Lastly, extracellular application of NAADP, but not NADP, induced a raise in intracellular Ca2+. This is the first demonstration that NAADP is transported into cells and highlights the possibility that, alongside a second messenger, NAADP might also act as an autocrine/paracrine signal.

Calcium signalling by nicotinic acid adenine dinucleotide phosphate (NAADP)

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a recently described Ca2+ mobilizing messenger, and probably the most potent. We briefly review its unique properties as a Ca2+ mobilizing agent. We present arguments for its action in targeting acidic calcium stores rather than the endoplasmic reticulum. Finally, we discuss possible biosynthetic pathways for NAADP in cells and candidates for its target Ca2+ release channel, which has eluded identification so far.

NAADP triggers the fertilization potential in starfish oocytes

In invertebrates oocytes or eggs, the fertilization or activation potential establishes the fast electrical block to polyspermy and, in some species, provides the Ca2+ influx which contributes to the following intracellular Ca2+ wave. In echinoderms, the molecule triggering the activation potential is still unknown. The aim of this study was to assess whether nicotinic acid-adenine dinucleotide phosphate (NAADP) elicited the fertilization potential in starfish oocytes. The changes in membrane potential induced by the sperm were measured in oocytes held at a low resting potential, so that the Ca2+-action potential was inactivated and only the initial slower depolarization caused by the sperm could be studied. Decreasing extracellular Na+ concentration did not prevent the onset of the fertilization potential, while removal of external Ca2+ abolished it. The pre-incubation with SK&F 96365 and verapamil and the pre-injection of BAPTA inhibited the fertilization potential, while the injection of heparin only reduced its duration. The biophysical and pharmacological properties of the sperm-elicited depolarization were similar to those displayed by the NAADP-activated Ca2+-mediated current recently described in starfish oocytes. Indeed, the desensitization of NAADP-receptors prevented the onset of the fertilization potential. Taken together, these data suggest that NAADP could trigger the fertilization potential in starfish oocytes.

Emerging role of cyclic ADP-ribose (cADPR) in smooth muscle

Cyclic adenosine diphosphate ribose (cADPR) is a naturally occurring cyclic nucleotide and represents a novel class of endogenous Ca(2+) messengers implicated in the regulation of the gating properties of ryanodine receptors (RyRs). This action of cADPR occurs independently from the inositol-1,4,5-trisphosphate (IP(3)) receptor. The regulation of intracellular Ca(2+) release is a fundamental element of cellular Ca(2+) homeostasis since a number of smooth muscle functions (tone, proliferation, apoptosis, and gene expression) are modulated by intracellular Ca(2+) concentration ([Ca(2+)](i)). There has been a surge in the efforts aimed at understanding the mechanisms of cADPR-mediated Ca(2+) mobilization and its impact on smooth muscle function. This review summarizes the proposed roles of cADPR in the regulation of smooth muscle tone.

The NAADP receptor: commentary on Billington et al

NAADP is a recently described calcium-mobilizing messenger. First discovered as a potent calcium-releasing molecule in sea urchin eggs, its actions have now been reported in several mammalian cell types. In the sea urchin egg, NAADP-sensitive calcium release channels appear distinct from inositol trisphosphate or ryanodine receptors, and are mainly localized to acidic compartments. In this study, Billington et al. extend the pharmacology of the putative NAADP receptor utilizing molecules unrelated to NAADP itself. This work may provide an important step in developing selective NAADP receptor modulators that will help define the role of NAADP in cell signalling.

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.

NAADP activates a Ca2+ current that is dependent on F-actin cytoskeleton

Nicotinic acid adenine dinucleotide phosphate (NAADP) is involved in the Ca2+ response observed at fertilization in several species, including starfish. In this study, we have employed Ca2+ imaging and the single-electrode voltage-clamp technique to investigate whether the NAADP-mediated Ca2+ entry discovered in our laboratory in starfish oocytes was underlain by a membrane current and whether the response to NAADP required an intact cytoskeleton. Uncaging of preinjected NAADP evoked a cortical Ca2+ flash that was followed by the spreading of the wave to the remainder of the cell. No Ca2+ increase was detected in Ca2+-free sea water. Under voltage-clamp conditions, the photoliberation of NAADP activated an inward rectifying membrane current, which reversed at potentials more positive than +50 mV and was abolished by removal of Ca2+ but not of Na+. The current was affected by preincubation with verapamil, SKF 96356, and thapsigargin but not by preinjection of heparin, 8-NH2- cyclic ADP-ribose, or both antagonists. The membrane current and the Ca2+ wave were inhibited by latrunculin-A and jasplakinolide, which depolymerize and stabilize actin cytoskeleton, respectively. These data offer the first demonstration that NAADP initiates a Ca2+ sweep by activating a Ca2+-permeable membrane current that requires an intact F-actin cytoskeleton as other Ca2+-permeable currents, such as ICRAC and IARC.

Ryanodine receptor type I and nicotinic acid adenine dinucleotide phosphate receptors mediate Ca2+ release from insulin-containing vesicles in living pancreatic beta-cells (MIN6)

We have demonstrated recently (Mitchell, K. J., Pinton, P., Varadi, A., Tacchetti, C., Ainscow, E. K., Pozzan, T., Rizzuto, R., and Rutter, G. A. (2001) J. Cell Biol. 155, 41-51) that ryanodine receptors (RyR) are present on insulin-containing secretory vesicles. Here we show that pancreatic islets and derived beta-cell lines express type I and II, but not type III, RyRs. Purified by subcellular fractionation and membrane immuno-isolation, dense core secretory vesicles were found to possess a similar level of type I RyR immunoreactivity as Golgi/endoplasmic reticulum (ER) membranes but substantially less RyR II than the latter. Monitored in cells expressing appropriately targeted aequorins, dantrolene, an inhibitor of RyR I channels, elevated free Ca(2+) concentrations in the secretory vesicle compartment from 40.1 +/- 6.7 to 90.4 +/- 14.8 microm (n = 4, p < 0.01), while having no effect on ER Ca(2+) concentrations. Furthermore, nicotinic acid adenine dinucleotide phosphate (NAADP), a novel Ca(2+)-mobilizing agent, decreased dense core secretory vesicle but not ER free Ca(2+) concentrations in permeabilized MIN6 beta-cells, and flash photolysis of caged NAADP released Ca(2+) from a thapsigargin-insensitive Ca(2+) store in single MIN6 cells. Because dantrolene strongly inhibited glucose-stimulated insulin secretion (from 3.07 +/- 0.51-fold stimulation to no significant glucose effect; n = 3, p < 0.01), we conclude that RyR I-mediated Ca(2+)-induced Ca(2+) release from secretory vesicles, possibly potentiated by NAADP, is essential for the activation of insulin secretion.

NAADP: a new second messenger for glucose-induced Ca2+ responses in clonal pancreatic beta cells

Important questions remain concerning how elevated blood glucose levels are coupled to insulin secretion from pancreatic beta cells and how this process is impaired in type 2 diabetes. Glucose uptake and metabolism in beta cells cause the intracellular Ca(2+) concentration ([Ca(2+)](i)) to increase to a degree necessary and sufficient for triggering insulin release. Although both Ca(2+) influx and Ca(2+) release from internal stores are critical, the roles of inositol 1,4,5-trisphosphate (IP(3)) and cyclic adenosine dinucleotide phosphate ribose (cADPR) in regulating the latter have proven equivocal. Here we show that glucose also increases [Ca(2+)](i) via the novel Ca(2+)-mobilizing agent nicotinic acid adenine dinucleotide phosphate (NAADP) in the insulin-secreting beta-cell line MIN6. NAADP binds to specific, high-affinity membrane binding sites and at low concentrations elicits robust Ca(2+) responses in intact cells. Higher concentrations of NAADP inactivate NAADP receptors and attenuate the glucose-induced Ca(2+) increases. Importantly, glucose stimulation increases endogenous NAADP levels, providing strong evidence for recruitment of this pathway. In conclusion, our results support a model in which NAADP mediates glucose-induced Ca(2+) signaling in pancreatic beta cells and are the first demonstration in mammalian cells of the presence of endogenous NAADP levels that can be regulated by a physiological stimulus.

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.

Nicotinic acid adenine dinucleotide phosphate mediates Ca2+ signals and contraction in arterial smooth muscle via a two-pool mechanism

Previous studies of arterial smooth muscle have shown that inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose mobilize Ca2+ from the sarcoplasmic reticulum. In contrast, little is known about Ca2+ mobilization by nicotinic acid adenine dinucleotide phosphate, a pyridine nucleotide derived from beta-NADP+. We show here that intracellular dialysis of nicotinic acid adenine dinucleotide phosphate (NAADP) induces spatially restricted "bursts" of Ca2+ release that initiate a global Ca2+ wave and contraction in pulmonary artery smooth muscle cells. Depletion of sarcoplasmic reticulum Ca2+ stores with thapsigargin and inhibition of ryanodine receptors with ryanodine, respectively, block the global Ca2+ waves by NAADP. Under these conditions, however, localized Ca2+ bursts are still observed. In contrast, xestospongin C, an IP3 receptor antagonist, had no effect on Ca2+ signals by NAADP. We propose that NAADP mobilizes Ca2+ via a 2-pool mechanism, and that initial Ca2+ bursts are amplified by subsequent sarcoplasmic reticulum Ca2+ release via ryanodine receptors but not via IP3 receptors.

Regulation of intracellular Ca2+ stores by multiple Ca2+-releasing messengers

Although glucose-elicited insulin secretion depends on Ca(2+) entry through voltage-gated Ca(2+) channels in the surface cell membrane of the pancreatic beta-cell, there is also ample evidence for an important role of intracellular Ca(2+) stores, particularly in relation to hormone- or neurotransmitter-induced insulin secretion. There is now direct evidence for Ca(2+) entry-induced release of Ca(2+) from the endoplasmic reticulum in neurons, but with regard to glucose stimulation of beta-cells, there is conflicting evidence about the operation of such a process. This finding suggests that the sensitivity of the Ca(2+) release channels in the endoplasmic reticulum membrane varies under different conditions and therefore is regulated. Recent evidence from studies of pancreatic acinar cells has revealed combinatorial roles of multiple messengers in setting the sensitivity of the endoplasmic reticulum for Ca(2+) release. Here we focus on the possible combinatorial roles of inositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in beta-cell function.

Solubilization of receptors for the novel Ca2+-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca(2+) mobilizing agent in a variety of broken and intact cell preparations. In sea urchin egg homogenates, NAADP releases Ca(2+) independently of inositol trisphosphate or ryanodine receptor activation. Little, however, is known concerning the molecular target for NAADP. Here we report for the first time solubilization of NAADP receptors from sea urchin egg homogenates. Supernatant fractions, prepared following Triton X-100 treatment, bound [(32)P]NAADP with similar affinity and selectivity as membrane preparations. Furthermore, the unusual non-dissociating nature of NAADP binding to its receptor was preserved upon solubilization. NAADP receptors could also be released into supernatant fractions upon detergent treatment of membranes prelabeled with [(32)P]NAADP. Tagged receptors prepared in this way, were readily resolved by native gel electrophoresis as a single protein target. Gel filtration and sucrose density gradient centrifugation analysis indicates that NAADP receptors are substantially smaller than inositol trisphosphate or ryanodine receptors, providing further biochemical evidence that NAADP activates a novel intracellular Ca(2+) release channel.

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.