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.     

NAADP-induced calcium release in sea urchin eggs

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent activator of Ca2+ release from intracellular stores described. It acts on a mechanism distinct from inositol trisphosphate and ryanodine receptors, the two major Ca2+ release channels characterised. NAADP-gated Ca2+ release channels do not appear to be regulated by Ca2+ and may be better suited for triggering Ca2+ signals rather than propagating them. They exhibit a remarkable pharmacology for a putative intracellular Ca2+ release channel in that they are selectively blocked by potassium and L-type Ca2+ channel antagonists. Furthermore, in contrast to microsomal Ca2+ stores expressing IP3Rs and RyRs, those sensitive to NAADP are thapsigargin-insensitive, suggesting that they may be expressed on a different part of the endoplasmic reticulum. Perhaps the most unusual feature of the NAADP-gated Ca2+ release mechanisms is its inactivation properties. Unlike the mechanisms regulated by IP3 and cADPR in sea urchin eggs which after induction of Ca2+ release appear to become refractory to subsequent activation, very low concentrations of NAADP are able to inactivate NAADP-induced Ca2+ release fully at concentrations well below those required to activate Ca2+ release. The mechanism and physiological significance of this most unusual desensitisation phenomenon are unclear. More recently, NAADP has been shown to mobilise Ca2+ in ascidian oocytes, brain microsomes and pancreatic acinar cells suggesting a more widespread role in Ca2+ signalling. A possible role for this novel Ca2+ release mechanism in sea urchin egg fertilisation is discussed.     

NAADP induces pH changes in the lumen of acidic Ca2+ stores

NAADP (nicotinic acid-adenine dinucleotide phosphate)-induced Ca2+ release has been proposed to occur selectively from acidic stores in several cell types, including sea urchin eggs. Using fluorescence measurements, we have investigated whether NAADP-induced Ca2+ release alters the pH(L) (luminal pH) within these acidic stores in egg homogenates and observed their prompt, concentration-dependent alkalinization by NAADP (but not beta-NAD+ or NADP). Like Ca2+ release, the pH(L) change was desensitized by low concentrations of NAADP suggesting it was secondary to NAADP receptor activation. Moreover, this was a direct effect of NAADP upon the acidic stores and not secondary to increases in cytosolic Ca2+ as it was not mimicked by IP3 (inositol 1,4,5-trisphosphate), cADPR (cyclic adenine diphosphoribose), ionomycin, thapsigargin or by direct addition of Ca2+, and was not blocked by EGTA. The results of the present study further support acidic stores as targets for NAADP and for the first time reveal an adjunct role for NAADP in regulating the pH(L) of intracellular organelles.     

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.     

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.     

Novel mechanism of intracellular calcium release in pituitary cells

In sea urchin eggs an enzymatic metabolite of beta-NAD+, called cyclic ADP-ribose (cADPR), is as potent and powerful a releaser of sequestered intracellular Ca2+ as is inositol 1,4,5-trisphosphate (IP3). The enzyme that synthesizes cADPR is present in several vertebrate animal tissues, but the Ca(2+)-releasing activity of cADPR has not been described in mammalian cells. We report here that incubation of beta-NAD+ with cell-free extracts of several rat tissues (including pituitary gland) generates a product which releases intracellular Ca2+ stores in permeabilized rat pituitary GH4C1 cells. This product has the biological characteristics of cADPR (it acts after depletion of the IP3 stores and after blockade of the IP3 receptor by heparin). The response is mimicked, in a concentration-dependent manner, by authentic cADPR and is desensitized by prior incubation with cADPR. We conclude that cADPR is not only synthesized by certain mammalian cells but also acts in such cells to release compartmentalized intracellular Ca2+ by a mechanism that differs from that used by IP3. Therefore, cADPR may serve, in addition to IP3, as a second messenger for intracellular Ca2+ mobilization in mammalian cells.     

Effect of luminal and extravesicular Ca2+ on NAADP binding and release properties

Nicotinic acid adenine dinucleotide phosphate (NAADP) has been shown to be a powerful Ca2+ release agent in numerous systems, including echinoderms, plants, and mammalian cells. NAADP has been shown to release Ca2+ via a separate mechanism to IP3 and ryanodine receptors, and specific binding sites have recently been characterised. However, functional studies have shown that there is a functional interplay between the NAADP-sensitive mechanism and the other two. In particular, it appears that activation of the NAADP receptor might act as a trigger to facilitate responses from IP3 and ryanodine receptors. To further characterise this interplay, we have investigated the effects of luminal and cytosolic Ca2+ on the NAADP receptor in sea urchin egg homogenates. We report that neither cytosolic nor luminal Ca2+ appears to influence NAADP binding. Conversely, emptying of stores significantly amplifies NAADP-induced fractional Ca2+-release, providing a mechanism of self-adjustment independent of store loading.     

NAADP mobilizes Ca2+ from a thapsigargin-sensitive store in the nuclear envelope by activating ryanodine receptors

Ca2+ release from the envelope of isolated pancreatic acinar nuclei could be activated by nicotinic acid adenine dinucleotide phosphate (NAADP) as well as by inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose (cADPR). Each of these agents reduced the Ca2+ concentration inside the nuclear envelope, and this was associated with a transient rise in the nucleoplasmic Ca2+ concentration. NAADP released Ca2+ from the same thapsigargin-sensitive pool as IP3. The NAADP action was specific because, for example, nicotineamide adenine dinucleotide phosphate was ineffective. The Ca2+ release was unaffected by procedures interfering with acidic organelles (bafilomycin, brefeldin, and nigericin). Ryanodine blocked the Ca2+-releasing effects of NAADP, cADPR, and caffeine, but not IP3. Ruthenium red also blocked the NAADP-elicited Ca2+ release. IP3 receptor blockade did not inhibit the Ca2+ release elicited by NAADP or cADPR. The nuclear envelope contains ryanodine and IP3 receptors that can be activated separately and independently; the ryanodine receptors by either NAADP or cADPR, and the IP3 receptors by IP3.     

NAADP+ is an agonist of the human P2Y11 purinergic receptor

Nicotinic acid adenine dinucleotide phosphate (NAADP+) is an intracellular second messenger releasing Ca2+ from intracellular stores in different cell types. In addition, it is also active in triggering [Ca2+](i) increase when applied extracellularly and various underlying mechanisms have been proposed. Here, we used hP2Y(11)-transfected 1321N1 astrocytoma cells to unequivocally establish whether extracellular NAADP+ is an agonist of the P2Y(11) receptor, as previously reported for beta-NAD+ [I. Moreschi, S. Bruzzone, R.A. Nicholas, et al., Extracellular NAD+ is an agonist of the human P2Y11 purinergic receptor in human granulocytes, J. Biol. Chem. 281 (2006) 31419-31429]. Extracellular NAADP+ triggered a concentration-dependent two-step elevation of [Ca2+](i) in 1321N1-hP2Y(11) cells, but not in wild-type 1321N1 cells, secondary to the intracellular production of IP(3), cAMP and cyclic ADP-ribose (cADPR). Specifically, the transient [Ca2+](i) rise proved to be related to IP(3) overproduction and to consequent Ca2+ mobilization, while the sustained [Ca2+](i) elevation was caused by the cAMP/ADP-ribosyl cyclase (ADPRC)/cADPR signalling cascade and by influx of extracellular Ca2+. In human granulocytes, endogenous P2Y(11) proved to be responsible for the NAADP+-induced cell activation (as demonstrated by the use of NF157, a selective and potent inhibitor of P2Y(11)), unveiling a role of NAADP+ as a pro-inflammatory cytokine. In conclusion, we provide unequivocal evidence for the activation of a member of the P2Y receptor subfamily by NAADP+.     

Two neuropeptides recruit different messenger pathways to evoke Ca2+ signals in the same cell

Bombesin and cholecystokinin (CCK) peptides act as signalling molecules in both the central nervous system and gastrointestinal tract [1-4]. It was reported recently that nicotinic acid adenine dinucleotide phosphate (NAADP) releases Ca2+ from mammalian brain microsomes [5] and triggers Ca2+ signals in pancreatic acinar cells, where it is proposed to mediate CCK-evoked Ca2+ signals [6]. Here, for the first time, we have finely resolved bombesin-induced cytosolic Ca2+ oscillations in single pancreatic acinar cells by whole-cell patch-clamp monitoring of Ca2+-dependent ionic currents [6-8]. Picomolar concentrations of bombesin and CCK evoked similar patterns of cytosolic Ca2+ oscillations, but high, desensitising, NAADP concentrations selectively inhibited CCK, but not bombesin-evoked signals. Inhibiting inositol trisphosphate (IP3) receptors with a high concentration of caffeine blocked both types of oscillations. We further tested whether NAADP is involved in Ca2+ signals triggered by activation of the low-affinity CCK receptor sites. Nanomolar concentrations of CCK evoked non-oscillatory Ca2+ signals, which were not affected by desensitising NAADP receptors. Our results suggest that Ca2+-release channels gated by the novel Ca2+-mobilising molecule NAADP are only essential in specific Ca2+-mobilising pathways, whereas the IP3 receptors are generally required for Ca2+ signals. Thus, the same cell may use different combinations of intracellular Ca2+-releasing messengers to encode different external messages.     

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP): novel regulators of Ca2+-signaling and cell function

Ca2+ ions are involved in the regulation of many diverse functions in animal and plant cells, e.g. muscle contraction, secretion of neurotransmitters, hormones and enzymes, fertilization of oocytes, and lymphocyte activation and proliferation. The intracellular Ca2+ concentration can be increased by different molecular mechanisms, such as Ca2+ influx from the extracellular space or Ca2+ release from intracellular Ca2+ stores. Release from intracellular Ca2+ stores is accomplished by the small molecular compounds D-myo-inositol 1,4,5-trisphosphate (InsP3), cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review concentrates on (i) receptor-mediated formation of cADPR by ADP-ribosyl cyclases, (ii) intracellular and extracellular effects of cADPR in a variety of cell types, and (iii) cADPR in the nucleus. Though our understanding of the role of NAADP is still unclear in many aspects, important recent findings are reviewed, e.g. Ca2+ release activity and binding studies in mammalian cell types.     

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 (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.     

Comparison of Ca2+ mobilizing activities of cyclic ADP-ribose and inositol trisphosphate.

We have previously shown that a metabolite of NAD+ generated by an enzyme present in sea urchin eggs and mammalian tissues can mobilize intracellular Ca2+ in the eggs. Structural determination established it to be a cyclized ADP-ribose, and the name cyclic ADP-ribose (cADPR) has been proposed. In this study, Ca2+ mobilizations induced by cADPR and inositol trisphosphate (IP3) in sea urchin egg homogenates were monitored with Ca2+ indicators and Ca2(+)-specific electrodes. Both methods showed that cADPR can release Ca2+ from egg homogenates. Evidence indicated that it did not act as a nonspecific Ca2(+)-ionophore or as a blocker of the microsomal Ca2(+)-transport; instead, it was likely to be operating through a specific receptor system. This was supported by its half-maximal effective concentration of 18 nM, which was 7 times lower than that of IP3. The receptor for cADPR appeared to be different from that of IP3 because heparin, an inhibitor of IP3 binding, had no effect on the cADPR action. The Ca2+ releases induced by cADPR and IP3 were not additive and had an inverse relationship, indicating overlapping stores were mobilized. Microinjection of cADPR into intact eggs induced transient intracellular Ca2+ changes and activated the cortical reaction. The in vivo effectiveness of cADPR was directly comparable with IP3 and neither required external Ca2+. In addition, both were effective in activating the eggs to undergo multiple nuclear cycles and DNA synthesis. These results suggest that cADPR could function as a second messenger in sea urchin eggs.     

Inositol 1,4,5-trisphosphate and its co-players in the concert of Ca2+ signalling--new faces in the line up

Release of Ca2+ from intracellular stores can occur by different intracellular messengers such as InsP3, cADPR and NAADP. Although in some cells messengers may operate on different stores, there are also Ca2+ stores with sensitivities for all three of these messengers. It is well documented, that InsP3- and cADPR-sensitive Ca2+ stores are involved in the activation of "store-operated Ca2+ channels" (SOCC). It has not yet been unequivocally shown, however, if Ca2+ release from stores, which respond to NAADP but not to InsP3 or cADPR, also generate signals which lead to "store-operated Ca2+ entry". Neither localization nor the mechanism of coupling to the plasma membrane of those InsP3- and cADPR-sensitive Ca2+ stores which activate SOCCs is yet clear. In this review localization and properties of InsP3-, cADPR- and NAADP-sensitive Ca2+ pools and their mutual interactions are discussed. Differential sensitivities of Ca2+ release mechanisms to InsP3, cADPR and NAADP have consequences on Ca2+ release, Ca2+ oscillations, propagation of Ca2+ waves and on activation of SOCC. Possible interaction of InsP3R and cADPR with candidates of SOCCs (TRP channels) and mechanisms involved in the regulation of SOCCs (activation-deactivation) will be elaborated.     

Determination of endogenous levels of cyclic ADP-ribose in rat tissues

Cyclic ADP-ribose (cADPR) is a potent mediator of calcium mobilization in sea urchin eggs. The cADPR synthesizing enzyme is present not only in the eggs but also in various mammalian tissue extracts. The purpose of this study was to ascertain whether cADPR is a naturally occurring nucleotide in mammalian tissues. Rat tissues were frozen and powdered in liquid N2, followed by extraction with perchloric acid at -10 degrees C. [32P]cADPR was prepared and used as a tracer. The acid extracts were chromatographed on a Mono-Q column and cADPR in the fractions were determined by its ability to release Ca2+ from egg homogenates. That the release was mediated by cADPR and not inositol trisphosphate (IP3) in the extracts was shown by the fact that the homogenates, subsequent to Ca2+ release induced by active fractions, were desensitized to authentic cADPR but not to IP3. Furthermore, the Ca2+ release activity was shown to co-elute with [32P]cADPR. The endogenous level of cADPR determined in rat liver is 3.37 +/- 0.64 pmol/mg, in heart is 1.04 +/- 0.08 pmol/mg and in brain is 2.75 +/- 0.35 pmol/mg. These results indicate cADPR is a naturally occurring nucleotide and suggest that it may be a general second messenger for mobilizing intracellular Ca2+.     

Specific binding of cyclic ADP-ribose to calcium-storing microsomes from sea urchin eggs.

Cyclic ADP-ribose (cADPR) is a metabolite of NAD+ which is as active as inositol trisphosphate (IP3) in mobilizing intracellular Ca2+ in sea urchin eggs. The enzyme responsible for synthesizing cADPR is found not only in sea urchin eggs but also in various mammalian tissue extracts, suggesting that it may be a general messenger for Ca2+ mobilization in cells. In this study I address questions of whether an intracellular receptor for cADPR exists and, if so, whether it is different from the IP3 receptor. A procedure employing nitrogen decompression was used to homogenize sea urchin eggs, and the Ca2(+)-storing microsomes were separated from mitochondria and other organelles by Percoll density centrifugation. Radioactive cADPR with high specific activity was produced by incubating [32P]NAD+ with the synthesizing enzyme and the product purified by high pressure liquid chromatography. The enzyme was membrane bound and was isolated from dog brain extracts by sucrose density gradient centrifugation. Partial purification of the enzyme was achieved by DEAE ion-exchange chromatography after solubilization with 3-[(cholamidopropyl)dimethylammonio]-1-propanesulfonate. Specific binding of 32P-labeled cADPR to a saturable site on the Ca2(+)-storing microsomes was detected by a filtration assay. Scatchard analysis indicated a binding affinity of about 17 nM and a capacity of about 25 fmol/mg protein. The binding was not affected by either NAD+ (the precursor) or ADP-ribose (the hydrolysis product) at 0.5 microM but was eliminated by 0.3 microM nonlabeled cADPR. The receptor for cADPR appeared to be different from that of IP3 since IP3 was not an effective competitor at a concentration as high as 3 microM. Similarly, heparin at a concentration that inhibits most of the IP3-induced calcium release from the microsomes did not affect the binding. The binding showed a prominent pH optimum at about 6.7. Calcium at 40 microM decreased the binding by about 50%. These dependencies of the binding on pH and Ca2+ are different from those reported for the IP3 receptor and provide further support that the intracellular receptors for cADPR and IP3 are different.     

Transformation of local Ca2+ spikes to global Ca2+ transients: the combinatorial roles of multiple Ca2+ releasing messengers

In pancreatic acinar cells, low, threshold concentrations of acetylcholine (ACh) or cholecystokinin (CCK) induce repetitive local cytosolic Ca2+ spikes in the apical pole, while higher concentrations elicit global signals. We have investigated the process that transforms local Ca2+ spikes to global Ca2+ transients, focusing on the interactions of multiple intracellular messengers. ACh-elicited local Ca2+ spikes were transformed into a global sustained Ca2+ response by cyclic ADP-ribose (cADPR) or nicotinic acid adenine dinucleotide phosphate (NAADP), whereas inositol 1,4,5-trisphosphate (IP3) had a much weaker effect. In contrast, the response elicited by a low CCK concentration was strongly potentiated by IP3, whereas cADPR and NAADP had little effect. Experiments with messenger mixtures revealed a local interaction between IP3 and NAADP and a stronger global potentiating interaction between cADPR and NAADP. NAADP strongly amplified the local Ca2+ release evoked by a cADPR/IP3 mixture eliciting a vigorous global Ca2+ response. Different combinations of Ca2+ releasing messengers can shape the spatio-temporal patterns of cytosolic Ca2+ signals. NAADP and cADPR are emerging as key messengers in the globalization of Ca2+ signals.     

PPADS is a reversible competitive antagonist of the NAADP receptor

NAADP has been shown to act as a second messenger in a wide range of systems from plants to mammalian cells. Although it had always been considered as a canonical second messenger, recent work has shown that it is also active when applied extracellularly. It has also been suggested that NAADP might have a direct action on P2 receptors, based on the action of a pharmacological agent, PPADS, on Ca2+ signals in response to extracellular NAADP. We have therefore investigated whether PPADS can act directly on the intracellular NAADP-induced Ca2+-release system in the well characterised sea urchin egg homogenate system. Indeed, PPADS, and its structural analogue PPNDS were able to compete with [32P]NAADP for the binding site and binding curves revealed that both compounds display affinities in the low micromolar range. The binding of PPADS was reversible in contrast to that of NAADP. In fluorimetric Ca2+-release experiments, PPADS was able to competitively antagonise NAADP-induced Ca2+-release with an IC50 of 20 microM, while it did not affect the other Ca2+-release channels. This is the first report of a reversible, competitive antagonist of the sea urchin NAADP receptor. Furthermore, PPADS might reveal itself as an invaluable tool to investigate NAADP signalling and is a lead compound for the synthesis of potent and specific antagonists.     

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.