7-Deaza-8-bromo-cyclic ADP-ribose, the first membrane-permeant, hydrolysis-resistant cyclic ADP-ribose antagonist

cADPR Does Not Activate TRPM2

cADPR is a second messenger that releases Ca²⁺ from intracellular stores via the ryanodine receptor. Over more than 15 years, it has been controversially discussed whether cADPR also contributes to the activation of the nucleotide-gated cation channel TRPM2. While some groups have observed activation of TRPM2 by cADPR alone or in synergy with ADPR, sometimes only at 37 °C, others have argued that this is due to the contamination of cADPR by ADPR. The identification of a novel nucleotide-binding site in the N-terminus of TRPM2 that binds ADPR in a horseshoe-like conformation resembling cADPR as well as the cADPR antagonist 8-Br-cADPR, and another report that demonstrates activation of TRPM2 by binding of cADPR to the NUDT9H domain raised the question again and led us to revisit the topic. Here we show that (i) the N-terminal MHR1/2 domain and the C-terminal NUDT9H domain are required for activation of human TRPM2 by ADPR and 2'-deoxy-ADPR (2dADPR), (ii) that pure cADPR does not activate TRPM2 under a variety of conditions that have previously been shown to result in channel activation, (iii) the cADPR antagonist 8-Br-cADPR also inhibits activation of TRPM2 by ADPR, and (iv) cADPR does not bind to the MHR1/2 domain of TRPM2 while ADPR does.

On a Magical Mystery Tour with 8-Bromo-Cyclic ADP-Ribose: From All-or-None Block to Nanojunctions and the Cell-Wide Web

A plethora of cellular functions are controlled by calcium signals, that are greatly coordinated by calcium release from intracellular stores, the principal component of which is the sarco/endooplasmic reticulum (S/ER). In 1997 it was generally accepted that activation of various G protein-coupled receptors facilitated inositol-1,4,5-trisphosphate (IP3) production, activation of IP3 receptors and thus calcium release from S/ER. Adding to this, it was evident that S/ER resident ryanodine receptors (RyRs) could support two opposing cellular functions by delivering either highly localised calcium signals, such as calcium sparks, or by carrying propagating, global calcium waves. Coincidentally, it was reported that RyRs in mammalian cardiac myocytes might be regulated by a novel calcium mobilising messenger, cyclic adenosine diphosphate-ribose (cADPR), that had recently been discovered by HC Lee in sea urchin eggs. A reputedly selective and competitive cADPR antagonist, 8-bromo-cADPR, had been developed and was made available to us. We used 8-bromo-cADPR to further explore our observation that S/ER calcium release via RyRs could mediate two opposing functions, namely pulmonary artery dilation and constriction, in a manner seemingly independent of IP3Rs or calcium influx pathways. Importantly, the work of others had shown that, unlike skeletal and cardiac muscles, smooth muscles might express all three RyR subtypes. If this were the case in our experimental system and cADPR played a role, then 8-bromo-cADPR would surely block one of the opposing RyR-dependent functions identified, or the other, but certainly not both. The latter seemingly implausible scenario was confirmed. How could this be, do cells hold multiple, segregated SR stores that incorporate different RyR subtypes in receipt of spatially segregated signals carried by cADPR? The pharmacological profile of 8-bromo-cADPR action supported not only this, but also indicated that intracellular calcium signals were delivered across intracellular junctions formed by the S/ER. Not just one, at least two. This article retraces the steps along this journey, from the curious pharmacological profile of 8-bromo-cADPR to the discovery of the cell-wide web, a diverse network of cytoplasmic nanocourses demarcated by S/ER nanojunctions, which direct site-specific calcium flux and may thus coordinate the full panoply of cellular processes.

25 Years of Collaboration with A Genius: Deciphering Adenine Nucleotide Ca2+ Mobilizing Second Messengers Together with Professor Barry Potter

Ca²⁺-mobilizing adenine nucleotide second messengers cyclic adenosine diphosphoribose, (cADPR), nicotinic acid adenine dinucleotide phosphate (NAADP), adenosine diphosphoribose (ADPR), and 2'deoxy-ADPR were discovered since the late 1980s. They either release Ca²⁺ from endogenous Ca²⁺ stores, e.g., endoplasmic reticulum or acidic organelles, or evoke Ca²⁺ entry by directly activating a Ca²⁺ channel in the plasma membrane. For 25 years, Professor Barry Potter has been one of the major medicinal chemists in this topical area, designing and contributing numerous analogues to develop structure-activity relationships (SAR) as a basis for tool development in biochemistry and cell biology and for lead development in proof-of-concept studies in disease models. With this review, I wish to acknowledge our 25-year-long collaboration on Ca²⁺-mobilizing adenine nucleotide second messengers as a major part of Professor Potter's scientific lifetime achievements on the occasion of his retirement in 2020.

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.

Probing Ca2+ release mechanisms using sea urchin egg homogenates

Sea urchin eggs have been extensively used to study Ca²⁺ release through intracellular Ca²⁺-permeable channels. Their amenability to homogenization yields a robust, cell-free preparation that was central to establishing the Ca²⁺ mobilizing actions of cyclic ADP-ribose and NAADP. Egg homogenates have continued to provide insight into the basic properties and pharmacology of intracellular Ca²⁺ release channels. In this chapter, we describe methods for the preparation of egg homogenates and monitoring Ca²⁺ release using fluorimetry and radiotracer flux.

Divergent TCR-Initiated Calcium Signals Govern Recruitment versus Activation of Human Alloreactive Effector Memory T Cells by Endothelial Cells

Early human allograft rejection can be initiated when circulating human host versus graft Ag-specific CD8 and CD4 effector memory T cells directly recognize MHC class I and II, respectively, expressed on the luminal surface by endothelium lining graft blood vessels. TCR engagement triggers both graft entry (TCR-driven transendothelial migration or TEM) and production of proinflammatory cytokines. Both TCR-driven TEM and cytokine expression are known to depend on T cell enzymes, myosin L chain kinase, and calcineurin, respectively, that are activated by cytoplasmic calcium and calmodulin, but whether the sources of calcium that control these enzymes are the same or different is unknown. Using superantigen or anti-CD3 Ab presented by cultured human dermal microvascular cells to freshly isolated peripheral blood human effector memory T cells under conditions of flow (models of alloantigen recognition in a vascularized graft), we tested the effects of pharmacological inhibitors of TCR-activated calcium signaling pathways on TCR-driven TEM and cytokine expression. We report that extracellular calcium entry via CRAC channels is the dominant contributor to cytokine expression, but paradoxically these same inhibitors potentiate TEM. Instead, calcium entry via TRPV1, L-Type Ca_v, and pannexin-1/P2X receptors appear to control TCR-driven TEM. These data reveal new therapeutic targets for immunosuppression.

CD38 in the pathogenesis of allergic airway disease: Potential therapeutic targets

CD38 is an ectoenzyme that catalyzes the conversion of β-nicotinamide adenine dinucleotide (β-NAD) to cyclic adenosine diphosphoribose (cADPR) and adenosine diphosphoribose (ADPR) and NADP to nicotinic acid adenine dinucleotide phosphate (NAADP) and adenosine diphosphoribose-2'-phosphate (ADPR-P). The metabolites of NAD and NADP have roles in calcium signaling in different cell types including airway smooth muscle (ASM) cells. In ASM cells, inflammatory cytokines augment CD38 expression and to a greater magnitude in cells from asthmatics, indicating a greater capacity for the generation of cADPR and ADPR in ASM from asthmatics. CD38 deficient mice develop attenuated airway responsiveness to inhaled methacholine following allergen sensitization and challenge compared to wild-type mice indicating its potential role in asthma. Regulation of CD38 expression in ASM cells is achieved by mitogen activated protein kinases, specific isoforms of PI3 kinases, the transcription factors NF-κB and AP-1, and post-transcriptionally by microRNAs. This review will focus on the role of CD38 in intracellular calcium regulation in ASM, contribution to airway inflammation and airway hyperresponsiveness in mouse models of allergic airway inflammation, the transcriptional and post-transcriptional mechanisms of regulation of expression, and outline approaches to inhibit its expression and activity.

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

Synthesis of 7-Deaza-cyclic Adenosine-5'-diphosphate-carbocyclic-ribose and Its 7-Bromo Derivative as Intracellular Ca(2+)-Mobilizing Agents

Cyclic ADP-carbocyclic-ribose (cADPcR, 3) is a biologically and chemically stable equivalent of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. We became interested in the biological activity of the 7-deaza analogues of cADPcR, i.e., 7-deaza-cADPcR (7) and its 7-bromo derivative, i.e., 7-deaza-7-Br-cADPcR (8), because 7-deazaadenosine is an efficient bioisostere of adenosine. The synthesis of 7 and 8 required us to construct the key N1-carbocyclic-ribosyl-7-deazaadenosine structure. Therefore, we developed a general method for preparing N1-substituted 7-deazaadenosines by condensing a 2,3-disubstituted pyrrole nucleoside with amines. Using this method, we prepared the N1-carbocyclic ribosyl 7-deazaadenosine derivative 10a, from which we then synthesized the target 7-deaza-cADPcR (7) via an Ag(+)-promoted intramolecular condensation to construct the 18-membered pyrophosphate ring structure. The corresponding 7-bromo derivative 8, which was the first analogue of cADPR with a substitution at the 7-position, was similarly synthesized. Biological evaluation for Ca(2+)-mobilizing activity in the sea urchin egg homogenate system indicated that 7-deaza-cADPcR (7) and 7-deaza-7-Br-cADPcR (8) acted as a full agonist and a partial agonist, respectively.

Cyclic adenosine 5'-diphosphoribose (cADPR) mimics used as molecular probes in cell signaling

Cyclic adenosine 5'-diphosphate ribose (cADPR) is a second messenger in the Ca(2+) signaling pathway. To elucidate its molecular mechanism in calcium release, a series of cADPR analogues with modification on ribose, nucleobase, and pyrophosphate have been investigated. Among them, the analogue with the modification of the northern ribose by ether linkage substitution (cIDPRE) exhibits membrane-permeate Ca(2+) agonistic activity in intact HeLa cells, human T cells, mouse cardiac myocytes and neurosecretory PC12 cell lines; thus, cIDPRE and coumarin-caged cIDPRE are valuable probes to investigate the cADPR-mediated Ca(2+) signal pathway.

Identification of a novel pathway of transforming growth factor-β1 regulation by extracellular NAD+ in mouse macrophages: in vitro and in silico studies

Extracellular β-nicotinamide adenine dinucleotide (NAD(+)) is anti-inflammatory. We hypothesized that NAD(+) would modulate the anti-inflammatory cytokine Transforming Growth Factor (TGF)-β1. Indeed, NAD(+) led to increases in both active and latent cell-associated TGF-β1 in RAW 264.7 mouse macrophages as well as in primary peritoneal macrophages isolated from both C3H/HeJ (TLR4-mutant) and C3H/HeOuJ (wild-type controls for C3H/HeJ) mice. NAD(+) acts partially via cyclic ADP-ribose (cADPR) and subsequent release of Ca(2+). Treatment of macrophages with the cADPR analog 3-deaza-cADPR or Ca(2+) ionophores recapitulated the effects of NAD(+) on TGF-β1, whereas the cADPR antagonist 8-Br-cADPR, Ca(2+) chelation, and antagonism of L-type Ca(2+) channels suppressed these effects. The time and dose effects of NAD(+) on TGF-β1 were complex and could be modeled both statistically and mathematically. Model-predicted levels of TGF-β1 protein and mRNA were largely confirmed experimentally but also suggested the presence of other mechanisms of regulation of TGF-β1 by NAD(+). Thus, in vitro and in silico evidence points to NAD(+) as a novel modulator of TGF-β1.

PLCγ, G-protein of the Gαq type and cADPr pathway are associated to trigger the fertilization Ca2+ signal in the sea urchin egg

In all species, fertilization triggers in the egg a rapid and transient increase of intracellular free calcium (Cai), but how this signal is generated following sperm and egg interaction has not been clearly characterised yet. In sea urchin, a signalling pathway involving tyrosine kinase and PLCγ has been proposed to be at the origin of the fertilization Cai signal. We report here that injection of src homology-2 (SH2) domains of the sea urchin PLCγ inhibits in a competitive manner the endogenous PLCγ, alters both the amplitude and duration of the fertilization Cai wave, but does not abrogate it. Our results suggest that PLCγ acts in conjunction with a cADPr pathway and G-proteins of the Gαq type to trigger the fertilization Cai wave, and reinforce a crucial role for PLCγ at mitosis and cytokinesis.

Aberrant cyclization affords a C-6 modified cyclic adenosine 5'-diphosphoribose analogue with biological activity in Jurkat T cells

Two nicotinamide adenine dinucleotide (NAD(+)) analogues modified at the 6 position of the purine ring were synthesized, and their substrate properties toward Aplysia californica ADP-ribosyl cyclase were investigated. 6-N-Methyl NAD(+) (6-N-methyl nicotinamide adenosine 5'-dinucleotide 10) hydrolyzes to give the linear 6-N-methyl ADPR (adenosine 5'-diphosphoribose, 11), whereas 6-thio NHD(+) (nicotinamide 6-mercaptopurine 5'-dinucleotide, 17) generates a cyclic dinucleotide. Surprisingly, NMR correlation spectra confirm this compound to be the N1 cyclic product 6-thio N1-cIDPR (6-thio cyclic inosine 5'-diphosphoribose, 3), although the corresponding 6-oxo analogue is well-known to cyclize at N7. In Jurkat T cells, unlike the parent cyclic inosine 5'-diphosphoribose N1-cIDPR 2, 6-thio N1-cIDPR antagonizes both cADPR- and N1-cIDPR-induced Ca(2+) release but possesses weak agonist activity at higher concentration. 3 is thus identified as the first C-6 modified cADPR (cyclic adenosine 5'-diphosphoribose) analogue antagonist; it represents the first example of a fluorescent N1-cyclized cADPR analogue and is a new pharmacological tool for intervention in the cADPR pathway of cellular signaling.

Synthesis and use of cell-permeant cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a second messenger that acts on ryanodine receptors to mobilize Ca(2+). cADPR has a net negative charge at physiological pH making it not passively membrane permeant thereby requiring it to be injected, electroporated or loaded via liposomes. Such membrane impermeance of other charged intracellular messengers (including cyclic AMP, inositol 1,4,5-trisphosphate and nicotinic acid adenine dinucleotide phosphate) and fluorescent dyes (including fura-2 and fluorescein) has been overcome by synthesizing masked analogs (prodrugs), which are passively permeant and hydrolyzed to the parent compound inside cells. We now report the synthesis and biological activity of acetoxymethyl (AM) and butoxymethyl (BM) analogs of cADPR. Extracellular addition of cADPR-AM or cADPR-BM to neuronal cells in primary culture or PC12 neuroblastoma cells induced increases in cytosolic Ca(2+). Pre-incubation of PC12 cells with thapsigargin, ryanodine or caffeine eliminated the response to cADPR-AM, whereas the response still occurred in the absence of extracellular Ca(2+). Combined, these data demonstrate that masked cADPR analogs are cell-permeant and biologically active. We hope these cell-permeant tools will facilitate cADPR research and reveal its diverse physiological functions.

Pyridine nucleotide metabolites and calcium release from intracellular stores

Ca(2+) signals are probably the most common intracellular signaling elements, controlling an extensive range of responses in virtually all cells. Many cellular stimuli, often acting at cell surface receptors, evoke Ca(2+) signals by mobilizing Ca(2+) from intracellular stores. Inositol trisphosphate (IP₃) was the first messenger shown to link events at the plasma membrane to release of Ca(2+) from the endoplasmic reticulum (ER), through activation of IP₃-gated Ca(2+) release channels (IP₃ receptors). Subsequently, two additional Ca(2+) 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 Ca(2+) from the ER by activation of ryanodine receptors (RyRs), NAADP releases Ca(2+) from acidic stores by a mechanism involving the activation of two pore channels (TPCs).

A novel membrane-permeant cADPR antagonist modified in the pyrophosphate bridge

A concise method for the formation of cyclopyrophosphate of cIDPRE as well as sulfur and selenium-substituted pyrophosphate cIDPRE analogues (P(1)(S)-cIDPRE, P(1)(Se)-cIDPRE, P(2)(S)-cIDPRE and P(2)(Se)-cIDPRE) was reported and one of the P(S)-diastereoisomers, P(1)(S)-cIDPRE-1, is a novel membrane-permeant cADPR antagonist.

Synthesis of cyclic adenosine 5'-diphosphate ribose analogues: a C2'endo/syn "southern" ribose conformation underlies activity at the sea urchin cADPR receptor

Novel 8-substituted base and sugar-modified analogues of the Ca(2+) mobilizing second messenger cyclic adenosine 5'-diphosphate ribose (cADPR) were synthesized using a chemoenzymatic approach and evaluated for activity in sea urchin egg homogenate (SUH) and in Jurkat T-lymphocytes; conformational analysis investigated by (1)H NMR spectroscopy revealed that a C2'endo/syn conformation of the "southern" ribose is crucial for agonist or antagonist activity at the SUH-, but not at the T cell-cADPR receptor.

Role of CD38, a cyclic ADP-ribosylcyclase, in morphine antinociception and tolerance

Our previous studies have demonstrated that an increase in intracellular levels of Ca(2+) in neurons is an important component of both the antinociception produced by morphine and morphine's tolerance. The present study tested the hypothesis that the Ca(2+) signaling second messenger, cyclic ADP-ribose (cADPR), derived from CD38 activation participates in morphine antinociception and tolerance. We first showed that morphine's antinociceptive potency was increased by the intracerebroventricular injection of CD38 substrate beta-NAD(+) in mice. Furthermore, morphine tolerance was reversed by intracerebroventricular administration of each of three different inhibitors of the CD38-cADPR-ryanodine receptor Ca(2+) signaling pathway. These inhibitors were the ADP-ribosylcyclase inhibitor nicotinamide, cADPR analog 8-bromo-cADPR, and a large dose of ryanodine (>50 muM) that blocks the ryanodine receptor. In CD38 gene knockout [CD38(-/-)] mice, the antinociceptive action of morphine was found to be less potent compared with wild-type (WT) mice, as measured by tail-flick response, hypothermia assay, and observations of straub tail. However, there was no difference in locomotor activation between CD38(-/-) and WT animals. It was also found that less tolerance to morphine developed in CD38(-/-) mice compared with WT animals. These results indicate that cADRP-ryanodine receptor Ca(2+) signaling associated with CD38 plays an important role in morphine tolerance.

2'-deoxy cyclic adenosine 5'-diphosphate ribose derivatives: importance of the 2'-hydroxyl motif for the antagonistic activity of 8-substituted cADPR derivatives

The structural features needed for antagonism at the cyclic ADP-ribose (cADPR) receptor are unclear. Chemoenzymatic syntheses of novel 8-substituted 2'-deoxy-cADPR analogues, including 8-bromo-2'-deoxy-cADPR 7, 8-amino-2'-deoxy-cADPR 8, 8- O-methyl-2'-deoxy-cADPR 9, 8-phenyl-2'-deoxy-cADPR 10 and its ribose counterpart 8-phenyl-cADPR 5 are reported, including improved syntheses of established antagonists 8-amino-cADPR 2 and 8-bromo-cADPR 3. Aplysia californica ADP-ribosyl cyclase tolerates even the bulky 8-phenyl-nicotinamide adenine 5'-dinucleotide as a substrate. Structure-activity relationships of 8-substituted cADPR analogues in both Jurkat T-lymphocytes and sea urchin egg homogenate (SUH) were investigated. 2'-OH Deletion decreased antagonistic activity (at least for the 8-amino series), showing it to be an important motif. Some 8-substituted 2'-deoxy analogues showed agonist activity at higher concentrations, among which 8-bromo-2'-deoxy-cADPR 7 was, unexpectedly, a weak but almost full agonist in SUH and was membrane-permeant in whole eggs. Classical antagonists 2 and 3 also showed previously unobserved agonist activity at higher concentrations in both systems. The 2'-OH group, without effect on the Ca (2+)-mobilizing ability of cADPR itself, is an important motif for the antagonistic activities of 8-substituted cADPR analogues.

Chemoenzymatic synthesis of 7-deaza cyclic adenosine 5'-diphosphate ribose analogues, membrane-permeant modulators of intracellular calcium release

An optimized synthetic route to 7-deaza-8-bromo-cyclic adenosine 5'-diphosphate ribose (7-deaza-8-bromo-cADPR 3), an established cell-permeant, hydrolysis-resistant cyclic adenosine 5'-diphosphate ribose (cADPR) antagonist, is presented. Using NMR analysis, we found that 3 adopted a C-2' endo conformation in the N9-linked ribose and a syn conformation about the N9-glycosyl linkage, which are similar to that of cADPR. The synthetic route was also employed to produce 7-deaza-2'-deoxy-cADPR 4, a potential cell-permeant cADPR analogue. 3 and 4 were more stable to chemical hydrolysis, consistent with the observation that 7-deaza-cADPR analogues are more stable than their parent adenosine derivatives. 3 was also found to be stable to enzyme-mediated hydrolysis using CD38 ectoenzyme.

Structural determinants for N1/N7 cyclization of nicotinamide hypoxanthine 5'-dinucleotide (NHD+) derivatives by ADP-ribosyl cyclase from aplysia californica: Ca2+-mobilizing activity of 8-substituted cyclic inosine 5'-diphosphoribose analogues in T-lymphocytes

A series of nicotinamide hypoxanthine 5'-dinucleotide (NHD+) analogues modified at C-8 (2-5) and 7-deaza-NHD+ were synthesized, and cyclization in the presence of Aplysia ADP-ribosyl cyclase was studied. All 8-substituted NHD+ analogues were converted into their N1-cyclic forms by the enzyme, while in contrast, 7-deaza-NHD+ 17 was hydrolyzed into 7-deazainosine 5'-diphosphoribose (7-deaza-IDPR) 25. Correlations are made showing that the conformation of the NHD+ substrate is the key to successful cyclization. The pharmacological activities of these novel cIDPR derivatives were evaluated in both permeabilized and intact Jurkat T-lymphocytes. The results show that in permeabilized cells both 8-iodo 1g and 8-N3-N1-cIDPR 1d have an activity comparable to that of cADPR, while 8-iodo 1g and 8-phenyl-N1-cIDPR 1c have a small but significant effect in intact cells and can therefore be regarded as membrane-permeant; thus, cIDPR derivatives are emerging as important novel biological tools to study cADPR-mediated Ca2+ release in T-cells.

Cellular effects and metabolic stability of N1-cyclic inosine diphosphoribose and its derivatives

BACKGROUND AND PURPOSE

Recently, a number of mimics of the second messenger cyclic ADP-ribose (cADPR) with replacement of adenosine by inosine were introduced. In addition, various alterations in the molecule ranging from substitutions at C8 of the base up to full replacement of the ribose moieties still retained biological activity. However, nothing is known about the metabolic stability and cellular effects of these novel analogues.

EXPERIMENTAL APPROACH

cADPR and the inosine-based analogues were incubated with CD38, ADP-ribosyl cyclase and NAD-glycohydrolase and metabolism was analysed by RP-HPLC. Furthermore, the effect of the analogues on cytokine expression and proliferation was investigated in primary T-lymphocytes and T-lymphoma cells.

KEY RESULTS

Incubation of cADPR with CD38 resulted in degradation to adenosine diphosphoribose. ADP-ribosyl cyclase weakly catabolised cADPR whereas NAD-glycohydrolase showed no such activity. In contrast, N1-cyclic inosine 5'-diphosphoribose (N1-cIDPR) was not hydrolyzed by CD38. Three additional N1-cIDPR analogues showed a similar stability. Proliferation of Jurkat T-lymphoma cells was inhibited by N1-cIDPR, N1-[(phosphoryl-O-ethoxy)-methyl]-N9-[(phosphoryl-O-ethoxy)-methyl]-hypoxanthine-cyclic pyrophosphate (N1-cIDP-DE) and N1-ethoxymethyl-cIDPR (N1-cIDPRE). In contrast, in primary T cells neither proliferation nor cytokine expression was affected by these compounds.

CONCLUSIONS AND IMPLICATIONS

The metabolic stability of N1-cIDPR and its analogues provides an advantage for the development of novel pharmaceutical compounds interfering with cADPR mediated Ca2+ signalling pathways. The differential effects of N1-cIDPR and N1-cIDPRE on proliferation and cytokine expression in primary T cells versus T-lymphoma cells may constitute a starting point for novel anti-tumor drugs.

AMP-activated protein kinase underpins hypoxic pulmonary vasoconstriction and carotid body excitation by hypoxia in mammals

In order to maintain tissue partial pressure of oxygen (P(O(2))) within physiological limits, vital homeostatic mechanisms monitor O(2) supply and respond to a fall in P(O(2)) by altering respiratory and circulatory function, and the capacity of the blood to transport O(2). Two systems that are key to this process in the acute phase are the pulmonary arteries and the carotid bodies. Hypoxic pulmonary vasoconstriction is driven by mechanisms intrinsic to the pulmonary arterial smooth muscle and endothelial cells, and aids ventilation-perfusion matching in the lung by diverting blood flow from areas with an O(2) deficit to those that are rich in O(2). By contrast, a fall in arterial P(O(2)) precipitates excitation-secretion coupling in carotid body type I cells, increases sensory afferent discharge from the carotid body and thereby elicits corrective changes in breathing patterns via the brainstem. There is a general consensus that hypoxia inhibits mitochondrial oxidative phosphorylation in these O(2)-sensing cells over a range of P(O(2)) values that has no such effect on other cell types. However, the question remains as to the identity of the mechanism that underpins hypoxia-response coupling in O(2)-sensing cells. Here, I lay out the case in support of a primary role for AMP-activated protein kinase in mediating chemotransduction by hypoxia.

AMP-activated protein kinase and the regulation of Ca2+ signalling in O2-sensing cells

All cells respond to metabolic stress. However, a variety of specialized cells, commonly referred to as O2-sensing cells, are acutely sensitive to relatively small changes in PO2. Within a variety of organisms such O2-sensing cells have evolved as vital homeostatic mechanisms that monitor O2 supply and alter respiratory and circulatory function, as well as the capacity of the blood to transport O2. Thereby, arterial PO2 may be maintained within physiological limits. In mammals, for example, two key tissues that contribute to this process are the pulmonary arteries and the carotid bodies. Constriction of pulmonary arteries by hypoxia optimizes ventilation-perfusion matching in the lung, whilst carotid body excitation by hypoxia initiates corrective changes in breathing patterns via increased sensory afferent discharge to the brain stem. Despite extensive investigation, the precise mechanism(s) by which hypoxia mediates these responses has remained elusive. It is clear, however, that hypoxia inhibits mitochondrial function in O2-sensing cells over a range of PO2 that has no such effect on other cell types. This raised the possibility that AMP-activated protein kinase might function to couple mitochondrial oxidative phosphorylation to Ca2+ signalling mechanisms in O2-sensing cells and thereby underpin pulmonary artery constriction and carotid body excitation by hypoxia. Our recent investigations have provided significant evidence in support of this view.

IL-4 inhibits calcium transients in bovine trachealis cells by a ryanodine receptor-dependent mechanism

IL-4 and IL-13 have important roles in the pathogenesis of asthma. A novel finding was that brief exposure of airway smooth muscle cells to IL-4 inhibited carbachol-stimulated calcium transients. We hypothesized that IL-4 inhibits transients by decreasing calcium store content and tested this by measuring the effects of IL-4 on transients induced by a nonspecific ionophore. Bovine trachealis cells were loaded with fura 2-AM, and cytosolic calcium concentrations ([Ca2+]i) were measured in single cells by digital microscopy. Stimulation (S1) with carbachol (10 microM) caused rapid, transient increases in [Ca2+]i to 1299 +/- 355 nM (n=5). After recovery of calcium stores, stimulation (S2) of the same cells with ionomycin (10 microM), in the absence of extracellular calcium, also increased [Ca2+]i to give S2/S1 ratio of 1.03 +/- 0.29. However, after 20 min of IL-4 (50 ng/ml), but not IL-13, ionomycin transients were decreased to 0.50 +/- 0.16 (S2/S1, P=0.02, n=6). IL-4 did not inhibit transients with ryanodine receptor calcium release channels (RyR) blocked by ryanodine (200 microM) (S2/S1=1.01+/-0.11) but still did in the presence of 8-bromo cyclic ADP-ribose, an antagonist of cyclic ADP-ribose (cADPR) signaling at RyR (S2/S1=0.48+/-0.13). Together, findings suggest that IL-4 decreases intracellular calcium stores by mechanisms dependent on RyR, but not on cADPR signaling.

Evidence that the cADPR signalling pathway controls calcium-mediated microneme secretion in Toxoplasma gondii

The protozoan parasite Toxoplasma gondii relies on calcium-mediated exocytosis to secrete adhesins on to its surface where they can engage host cell receptors. Increases in intracellular calcium occur in response to Ins(1,4,5)P3 and caffeine, an agonist of ryanodine-responsive calcium-release channels. We examined lysates and microsomes of T. gondii and detected evidence of cADPR (cyclic ADP ribose) cyclase and hydrolase activities, the two enzymes that control the second messenger cADPR, which causes calcium release from RyR (ryanodine receptor). We also detected endogenous levels of cADPR in extracts of T. gondii. Furthermore, T. gondii microsomes that were loaded with 45Ca2+ released calcium when treated with cADPR, and the RyR antagonists 8-bromo-cADPR and Ruthenium Red blocked this response. Although T. gondii microsomes also responded to Ins(1,4,5)P3, the inhibition profiles of these calcium-release channels were mutually exclusive. The RyR antagonists 8-bromo-cADPR and dantrolene inhibited protein secretion and motility in live parasites. These results indicate that RyR calcium-release channels that respond to the second-messenger cADPR play an important role in regulating intracellular Ca2+, and hence host cell invasion, in protozoan parasites.

Pyridine nucleotides and calcium signalling in arterial smooth muscle: from cell physiology to pharmacology

It is generally accepted that the mobilisation of intracellular Ca2+ stores plays a pivotal role in the regulation of arterial smooth muscle function, paradoxically during both contraction and relaxation. However, the spatiotemporal pattern of different Ca2+ signals that elicit such responses may also contribute to the regulation of, for example, differential gene expression. These findings, among others, demonstrate the importance of discrete spatiotemporal Ca2+ signalling patterns and the mechanisms that underpin them. Of fundamental importance in this respect is the realisation that different Ca2+ storing organelles may be selected by the discrete or coordinated actions of multiple Ca2+ mobilising messengers. When considering such messengers, it is generally accepted that sarcoplasmic reticulum (SR) stores may be mobilised by the ubiquitous messenger inositol 1,4,5 trisphosphate. However, relatively little attention has been paid to the role of Ca2+ mobilising pyridine nucleotides in arterial smooth muscle, namely, cyclic adenosine diphosphate-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). This review will therefore focus on these novel mechanisms of calcium signalling and their likely therapeutic potential.

Hypoxia stimulates Ca2+ release from intracellular stores in astrocytes via cyclic ADP ribose-mediated activation of ryanodine receptors

The ability of O(2) levels to regulate Ca(2+) signalling in non-excitable cells is poorly understood, yet crucial to our understanding of Ca(2+)-dependent cell functions in physiological and pathological situations. Here, we demonstrate that hypoxia mobilizes Ca(2+) from an intracellular pool in primary cultures of cortical astrocytes. This pool can also be mobilized by bradykinin, which acts via phospholipase C and inositol trisphosphate production. By contrast, hypoxic Ca(2+) mobilization utilizes ryanodine receptors, which appear to be either present on the same intracellular pool, or on a separate but functionally coupled pool. Hypoxic activation of ryanodine receptors requires formation of cyclic ADP ribose, since hypoxic Ca(2+) mobilization was fully prevented by nicotinamide (which inhibits ADP ribosyl cyclase) or by 8-Br-cADP ribose, an antagonist of cyclic ADP ribose. Our results demonstrate for the first time the involvement of cyclic ADP ribose in hypoxic modulation of Ca(2+) signalling in the central nervous system, and suggest that this modulator of ryanodine receptors may play a key role in the function of astrocytes under conditions of fluctuating O(2) levels.

Amplification of depolarization-induced and ryanodine-sensitive cytosolic Ca2+ elevation by synthetic carbocyclic analogs of cyclic ADP-ribose and their antagonistic effects in NG108-15 neuronal cells

We synthesized analogs modified in the ribose unit (ribose linked to N1 of adenine) of cyclic ADP-ribose (cADPR), a Ca2+-mobilizing second messenger. The biological activities of these analogs were determined in NG108-15 neuroblastoma x glioma hybrid cells that were pre-loaded with fura-2 acetoxymethylester and subjected to whole-cell patch-clamp. Application of the hydrolysis-resistant cyclic ADP-carbocyclic-ribose (cADPcR) through patch pipettes potentiated elevation of the cytoplasmic free Ca2+ concentration ([Ca2+]i) at the depolarized membrane potential. The increase in [Ca2+]i evoked upon sustained membrane depolarization was significantly larger in cADPcR-infused cells than in non-infused cells and its degree was equivalent to or significantly greater than that induced by cADPR or beta-NAD+. 8-Chloro-cADPcR and two inosine congeners (cyclic IDP-carbocyclic-ribose and 8-bromo-cyclic IDP-carbocyclic-ribose) did not induce effects similar to those of cADPcR or cADPR. Instead, 8-chloro-cADPcR together with cADPR or cADPcR caused inhibition of the depolarization-induced [Ca2+]i increase as compared with either cADPR or cADPcR alone. These results demonstrated that our cADPR analogs have agonistic or antagonistic effects on the depolarization-induced [Ca2+]i increase and suggested the presence of functional reciprocal coupling between ryanodine receptors and voltage-activated Ca2+ channels via cADPR in mammalian neuronal cells.

Synthesis of stable and cell-type selective analogues of cyclic ADP-ribose, a Ca(2+)-mobilizing second messenger. Structure--activity relationship of the N1-ribose moiety

We previously developed cyclic ADP-carbocyclic ribose (cADPcR, 2) as a stable mimic of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. A series of the N1-ribose modified cADPcR analogues, designed as novel stable mimics of cADPR, which were the 2"-deoxy analogue 3, the 3"-deoxy analogue 4, the 3"-deoxy-2"-O-(methoxymethyl) analogue 5, the 3"-O-methyl analogue 6, the 2",3"-dideoxy analogue 7, and the 2",3"-dideoxydidehydro analogue 8, were successfully synthesized using the key intramolecular condensation reaction with phenylthiophosphate-type substrates. We investigated the conformations of these analogues and of cADPR and found that steric repulsion between both the adenine and N9-ribose moieties and between the adenine and N1-ribose moieties was a determinant of the conformation. The Ca(2+)-mobilizing effects were evaluated systematically using three different biological systems, i.e., sea urchin eggs, NG108-15 neuronal cells, and Jurkat T-lymphocytes. The relative potency of Ca(2+)-mobilization by these cADPR analogues varies depending on the cell-type used: e.g., 3"-deoxy-cADPcR (4) > cADPcR (2) > cADPR (1) in sea urchin eggs; cADPR (1) >> cADPcR (2) approximately 3"-deoxy-cADPcR (4) in T-cells; and cADPcR (2) > cADPR (1) > 3"-deoxy-cADPcR (4) in neuronal cells, respectively. These indicated that the target proteins and/or the mechanism of action of cADPR in sea urchin eggs, T-cells, and neuronal cells are different. Thus, this study represents an entry to cell-type selective cADPR analogues, which can be used as biological tools and/or novel drug leads.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Hypoxic modulation of Ca2+ signaling in human venous endothelial cells. Multiple roles for reactive oxygen species

The effects of hypoxia (pO2 approximately 25 mm Hg) on Ca2+ signaling stimulated by extracellular ATP in human saphenous vein endothelial cells were investigated using fluorimetric recordings from Fura-2 loaded cells. In the absence of extracellular Ca2+, ATP-evoked rises of cytosolic Ca2+ concentration ([Ca2+]i) because of mobilization from the endoplasmic reticulum (ER). These responses were reduced by prior exposure to hypoxia but potentiated during hypoxia. Hypoxia itself liberated Ca2+ from the ER, but unlike the effects of ATP this effect was not inhibited by blockade of the inositol trisphosphate receptor. By contrast, ryanodine blocked the effects of hypoxia but not those of ATP. Antioxidants abolished the effects of hypoxia but potentiated the effects of ATP. Inhibition of NADPH oxidase also augmented ATP-evoked responses but was without effect on hypoxia-evoked rises of [Ca2+]i. However, either uncoupling mitochondrial electron transport or inhibiting complex I markedly suppressed the actions of hypoxia yet exerted only small inhibitory effects on ATP-evoked rises of [Ca2+]i. Both hypoxia and ATP were able to activate capacitative Ca2+ entry. Our results indicate that hypoxia regulates intracellular Ca2+ signaling via two distinct pathways. First, it modulates agonist-evoked liberation of Ca2+ from the ER primarily through regulation of reactive oxygen species generation from NADPH oxidase. Second, it liberates Ca2+ from the ER via ryanodine receptors, an effect requiring mitochondrial reactive oxygen species generation. These findings suggest that local O2 tension is a major determinant of Ca2+ signaling in the vascular endothelium, a finding that is likely to be of both physiological and pathophysiological importance.

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.

Aplysia californica mediated cyclisation of novel 3'-modified NAD+ analogues: a role for hydrogen bonding in the recognition of cyclic adenosine 5'-diphosphate ribose

Cyclic ADP-ribose mobilizes intracellular Ca2+ in a variety of cells. To elucidate the nature of the interaction between the C3' substituent of cADP-ribose and the cADPR receptor, three analogues of NAD+ modified in the adenosine ribase (xyloNAD+ 3'F-xyloNAD+ and 3'F-NAD+ were chemically synthesised from D-xylose and adenine starting materials. 3'F-NAD+ was readily converted to cyclic 3'F-ADP ribose by the action of the cyclase enzyme derived from the mollusc Aplysia californica. XyloNAD+ and 3'F-xyloNAD+ were cyclised only reluctantly and in poor yield to afford unstable cyclic products. Biological evaluation of cyclic 3'F-ADP ribose for calcium release in sea urchin egg homogenate gave an EC(50) of 1.5+/-0.5 microM. This high value suggests that the ability of the C3' substituent to donate a hydrogen bond is crucial for agonism.

Convergent synthesis and unexpected Ca(2+)-mobilizing activity of 8-substituted analogues of cyclic ADP-carbocyclic-ribose, a stable mimic of the Ca(2+)-mobilizing second messenger cyclic ADP-ribose

Cyclic ADP-carbocyclic-ribose (cADPcR, 2) is a biologically and chemically stable equivalent of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. In this study, a series of 8-substituted analogues of cADPcR, namely the 8-chloro analogue 6 (8-Cl-cADPcR), the 8-azido analogue 7 (8-N(3)-cADPcR), the 8-amino analogue 8 (8-NH(2)-cADPcR), and the 8-phenylthio analogue 9 (8-SPh-cADPcR), were designed as effective pharmacological tools for studies on cADPR-modulated Ca(2+) signaling pathways. These target compounds were synthesized by a convergent route via 8-Cl-cADPcR bisacetonide (14) as the common intermediate, in which a method for forming the intramolecular pyrophosphate linkage by activation of the phenylthiophosphate type substrate 15 with AgNO(3) to produce 14 was used as the key step. The carbocyclic analogues were tested for activity in the sea urchin egg homogenate system. Compounds were assessed for their calcium-mobilizing effects and their ability to cross-desensitize with calcium release induced by a normally maximal concentration of cADPR, as well as cADPR antagonism of cADPR-evoked calcium release. While cADPcR was 3-4 times more potent than cADPR, the 8-substituted analogues were less efficacious, with 8-SPh-cADPcR largely acting as a competitive antagonist. Most surprisingly, given that 8-N(3)-cADPR and 8-NH(2)-cADPR are known as potent antagonists, 8-N(3)-cADPcR and 8-NH(2)-cADPcR were full agonists, but ca. 80 and 2 times less potent than cADPR, respectively. These data contribute to developing structure-activity relationships for the interaction of cADPR with its receptor.

Intracellular calcium signaling through the cADPR pathway is agonist specific in porcine airway smooth muscle

Cyclic ADP-ribose (cADPR) induces intracellular Ca2+ ([Ca2+]i) release in airway smooth muscle, and the cADPR antagonist, 8-amino-cADPR, abolishes [Ca2+]i oscillations elicited by acetylcholine (ACh), suggesting that cADPR is involved during muscarinic receptor activation. Whether the cADPR signaling pathway is common to agonists acting through different G protein-coupled receptors is not known. Using digital video imaging of Fura2-AM loaded porcine airway smooth muscle cells, we examined the effects of the membrane-permeant cADPR antagonist, 8-bromo-cADPR (8Br-cADPR), on the [Ca2+]i responses to ACh, histamine and endothelin-1 (ET-1). In cells preincubated with 100 microM 8Br-cADPR, the [Ca2+]i responses to ACh and ET-1 were significantly attenuated, whereas responses to histamine were not, suggesting agonist specificity of cADPR signaling. The effects of 8Br-cADPR were concentration dependent. We further examined whether muscarinic receptor subtypes specifically couple to this pathway, because in porcine airway smooth muscle cells, ACh activates both M2 and M3 muscarinic receptors coupled to Gai and Gaq, respectively. Methoctramine, an M2-selective antagonist, attenuated the [Ca2+]i responses to Ach, and there was no further attenuation by 8Br-cADPR. In airway smooth muscle, the CD38/cADPR signaling pathway is involved in [Ca2+]i responses to contractile agonists in an agonist-specific manner.

Knock-down of the type 3 ryanodine receptor impairs sustained Ca2+ signaling via the T cell receptor/CD3 complex

In Jurkat T cells, the type 3 ryanodine receptor (RyR) was knocked-down by stable integration of plasmid expressing type 3 ryanodine receptor antisense RNA. Stable integration of the antisense plasmid in individual clones was demonstrated by PCR of genomic DNA, expression of antisense RNA by reverse transcriptase PCR, and efficiently reduced expression of type 3 ryanodine receptor protein by Western blot. Selected clones were successfully used to analyze T cell receptor/CD3 complex-mediated Ca(2+) signaling. Reduced expression of the type 3 RyR resulted in (i) significantly decreased Ca(2+) signaling in the sustained phase and (ii) in permeabilized cells in a significantly impaired response toward cyclic ADP-ribose but not to d-myo-inositol 1,4,5-trisphosphate. For the first time, the role of the type 3 RyR in sustained Ca(2+) signaling was directly visualized by confocal Ca(2+) imaging as a significant contribution to the number and the magnitude of subcellular Ca(2+) signals. These data suggest that the type 3 ryanodine receptor is essential in the sustained Ca(2+) response in T cells.

Syntheses and calcium-mobilizing evaluations of N1-glycosyl-substituted stable mimics of cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is not only a potent endogenous calcium modulator but also a second messenger. However, studies on the mechanism of cADPR action were limited due to its instability and lack of available structural modifications in the N1-glyosyl unit of cADPR. In the present work, a series of N1-glycosyl mimics with different configurational glycosyls or an ether strand were designed and synthesized mimicking the furanose ring. S(N)2 substitutions were carried out between the protected inosine and glycosyl triflates to form the N1-glycosylinosine derivatives, accompanied with some O6-glycosyl-substituted as side products. The intramolecular cyclization was followed the strategy described by Matsuda et al. It was found that the 8-unsubstituted substrate could also be used to construct the intramolecular cyclic pyrophosphate. The activities of N1-glycosyl-substituted cADPR mimics were evaluated by induced Ca2+ release in rat brain microsomes and HeLa cells. It was found that the configuration of the N1-glycosyl moiety in cADPR is not a critical structural factor for retaining the activity of mobilizing Ca2+ release. More interestingly, the N1-acyclic analogue 6 exhibited strong activity by inducing Ca2+ release in both rat brain microsomes and HeLa cells. It constitutes a useful tool for further studies.

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.

Mechanism-based inhibitors of CD38: a mammalian cyclic ADP-ribose synthetase

The soluble domain of human CD38 catalyzes the conversion of NAD(+) to cyclic ADP-ribose and to ADP-ribose via a common covalent intermediate [Sauve, A. A., Deng, H. T., Angelletti, R. H., and Schramm, V. L. (2000) J. Am. Chem. Soc. 122, 7855-7859]. Here we establish that mechanism-based inhibitors can be produced by chemical stabilization of this intermediate. The compounds nicotinamide 2'-deoxyriboside (1), 5-methylnicotinamide 2'-deoxyriboside (2), and pyridyl 2'-deoxyriboside (3) were synthesized and evaluated as inhibitors for human CD38. The nicotinamide derivatives 1 and 2 were inhibitors of the enzyme as determined by competitive behavior in CD38-catalyzed conversion of nicotinamide guanine dinucleotide (NGD(+)) to cyclic GDP-ribose. The K(i) values for competitive inhibition were 1.2 and 4.0 microM for 1 and 2, respectively. Slow-onset characteristics of reaction progress curves indicated a second higher affinity state of these two inhibitors. Inhibitor off-rates were slow with rate constants k(off) of 1.5 x 10(-5) s(-1) for 1 and 2.5 x 10(-5) s(-1) for 2. Apparent dissociation constants K(i(total)) for 1 and 2 were calculated to be 4.5 and 12.5 nM, respectively. The similar values for k(off) are consistent with the hydrolysis of common enzymatic intermediates formed by the reaction of 1 and 2 with the enzyme. Both form covalently attached deoxyribose groups to the catalytic site nucleophile. Chemical evidence for this intermediate is the ability of nicotinamide to rescue enzyme activity after inactivation by either 1 or 2. A covalent intermediate is also indicated by the ability of CD38 to catalyze base exchange, as observed by conversion of 2 to 1 in the presence of nicotinamide. The deoxynucleosides 1 and 2 demonstrate that the chemical determinants for mechanism-based inhibition of CD38 can be satisfied by nucleosides that lack the 5'-phosphate, the adenylate group, and the 2'-hydroxyl moiety. In addition, these compounds reveal the mechanism of CD38 catalysis to proceed by the formation of a covalent intermediate during normal catalytic turnover with faster substrates. The covalent 2'-deoxynucleoside inactivators of CD38 are powerful inhibitors by acting as good substrates for formation of the covalent intermediate but are poor leaving groups from the intermediate complex because hydrolytic assistance of the 2'-hydroxyl group is lacking. The removal of the adenylate nucleophile required for the cyclization reaction provides slow hydrolysis as the only exit from the covalent complex.

Novel hydrolysis-resistant analogues of cyclic ADP-ribose: modification of the "northern" ribose and calcium release activity

Three novel analogues modified in the "northern" ribose (ribose linked to N1 of adenine) of the Ca(2+) mobilizing second messenger cyclic adenosine diphosphoribose, termed 2"-NH(2)-cyclic adenosine diphosphoribose, cyclic adenosine diphospho-carbocyclic-ribose, and 8-NH(2)-cyclic adenosine diphospho-carbocyclic-ribose, were synthesized (chemoenzymatically and by total synthesis) and spectroscopically characterized, and the pK(a) values for the 6-amino/imino transition were determined in two cases. The biological activity of these analogues was determined in permeabilized human Jurkat T-lymphocytes. 2"-NH(2)-cyclic adenosine diphosphoribose mediated Ca(2+) release was slightly more potent than that of the endogenous cyclic adenosine diphosphoribose in terms of the concentration-reponse relationship. Both compounds released Ca(2+) from the same intracellular Ca(2+) pool. In addition, the control compound 2"-NH(2)-adenosine diphosphoribose was almost without effect. In contrast, only at much higher concentrations (> or =50 microM) did the "northern" carbocyclic analogue, cyclic adenosine diphospho-carbocyclic-ribose, significantly release Ca(2+) from permeabilized T cells, whereas the previously reported "southern" carbocyclic analogue, cyclic aristeromycin diphosphoribose, was slightly more active than the endogenous cyclic adenosine diphosphoribose. Likewise, 8-NH(2)-cyclic adenosine diphospho-carbocyclic-ribose, expected to antagonize Ca(2+) release as demonstrated previously for 8-NH(2)-cyclic adenosine diphosphoribose, did not inhibit cyclic adenosine diphosphoribose mediated Ca(2+) release. This indicates that the 2"-NH(2)-group substitutes well for the 2"-OH-group it replaces; it may be oriented toward the outside of the putative cyclic adenosine diphosphoribose receptor binding domain and/or it can potentially also engage in H bonding interactions with residues of that domain. In sharp contrast to this, replacement of the endocyclic furanose oxygen atom by CH(2) in a carbocyclic system obviously interferes with a crucial element of interaction between cyclic adenosine diphosphoribose and its receptor in T-lymphocytes.

Mechanisms involved in alpha6beta1-integrin-mediated Ca(2+) signalling

Contact of Jurkat T-lymphocytes with the extracellular matrix (ECM) protein laminin resulted in long-lasting alpha6beta1-integrin-mediated Ca(2+) signalling. Both Ca(2+) release from thapsigargin-sensitive Ca(2+) stores and capacitative Ca(2+) entry via Ca(2+) channels sensitive to SKF 96365 constitute important parts of this process. Inhibition of alpha6beta1-integrin-mediated Ca(2+) signalling by (1) the src kinase inhibitor PP2, (2) the PLC inhibitor U73122, and (3) the cyclic adenosine diphosphoribose (cADPR) antagonist 7-deaza-8-Br-cADPR indicate the involvement of src tyrosine kinases and the Ca(2+)-releasing second messengers D-myo-inositol 1,4,5-trisphosphate (InsP3) and cADPR.

Total synthesis of cyclic ADP-carbocyclic-ribose, a stable mimic of Ca2+-mobilizing second messenger cyclic ADP-ribose

The synthesis of cyclic ADP-carbocyclic-ribose (cADPcR, 4) designed as a stable mimic of cyclic ADP-ribose (cADPR, 1), a Ca2+-mobilizing second messenger, was achieved using as the key step a condensation reaction with the phenylthiophosphate-type substrate 14 to form an intramolecular pyrophosphate linkage. The N-1-carbocyclic-ribosyladenosine derivative 16 was prepared via the condensation between the imidazole nucleoside derivative 17, prepared from AICA-riboside (19), and the readily available optically active carbocyclic amine 18. Compound 16 was then converted to the corresponding 5' '-phosphoryl-5'-phenylthiophosphate derivatives 14. Treatment of 14 with AgNO3 in the presence of molecular sieves (3 A) in pyridine at room temperature gave the desired cyclization product 32 in 93% yield, and subsequent acidic treatment provided the target cADPcR (4). This represents a general method for synthesizing biologically important cyclic nucleotides of this type. 1H NMR analysis of cADPcR suggested that its conformation in aqueous medium is similar to that of cADPR. cADPcR, unlike cADPR, was stable under neutral and acidic conditions, where under basic conditions, it formed the Dimroth-rearranged N6-cyclized product 34. cADPcR was also stable in rat brain membrane homogenate which has cADPR degradation activity. Furthermore, cADPcR was resistant to the hydrolysis by CD38 cADPR hydrolase, while cADPR was rapidly hydrolyzed under the same conditions. When cADPcR was injected into sea urchin eggs, it caused a significant release of Ca2+ in the cells, an effect considerably stronger than that of cADPR. Thus, cADPcR was identified as a stable mimic of cADPR.

Cyclic ADP-ribose is the primary trigger for hypoxic pulmonary vasoconstriction in the rat lung in situ

Hypoxic pulmonary vasoconstriction (HPV) is unique to pulmonary arteries, and it aids ventilation/perfusion matching. However, in diseases such as emphysema, HPV can promote hypoxic pulmonary hypertension. We recently showed that hypoxia constricts pulmonary arteries in part by increasing cyclic ADP-ribose (cADPR) accumulation in the smooth muscle and, thereby, Ca(2+) release by ryanodine receptors. We now report on the role of cADPR in HPV in isolated rat pulmonary arteries and in the rat lung in situ. In isolated pulmonary arteries, the membrane-permeant cADPR antagonist, 8-bromo-cADPR, blocked sustained HPV by blocking Ca(2+) release from smooth muscle ryanodine-sensitive stores in the sarcoplasmic reticulum. Most importantly, we showed that 8-bromo-cADPR blocks HPV induced by alveolar hypoxia in the ventilated rat lung in situ. Inhibition of HPV was achieved without affecting (1) constriction by membrane depolarization and voltage-gated Ca(2+) influx, (2) the release (by hypoxia) of an endothelium-derived vasoconstrictor, or (3) endothelium-dependent vasoconstriction. Our findings suggest that HPV is both triggered and maintained by cADPR in the rat lung in situ.

Physiological functions of cyclic ADP-ribose and NAADP as calcium messengers

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are two Ca(2+) messengers derived from NAD and NADP, respectively. Although NAADP is a linear molecule, structurally distinct from the cyclic cADPR, it is synthesized by similar enzymes, ADP-ribosyl cyclase and its homolog, CD38. The crystal structure of the cyclase has been solved and its active site identified. These two novel nucleotides have now been shown to be involved in a wide range of cellular functions including: cell cycle regulation in Euglena, a protist; gene expression in plants; and in animal systems, from fertilization to neurotransmitter release and long-term depression in brain. A battery of pharmacological reagents have been developed, providing valuable tools for elucidating the physiological functions of these two novel Ca(2+) messengers. This article reviews these recent results and explores the implications of the existence of multiple Ca(2+) messengers and Ca(2+) stores in cells.

Paracrine roles of NAD+ and cyclic ADP-ribose in increasing intracellular calcium and enhancing cell proliferation of 3T3 fibroblasts

CD38 is a bifunctional ectoenzyme synthesizing from NAD(+) (ADP-ribosyl cyclase) and degrading (hydrolase) cyclic ADP-ribose (cADPR), a powerful universal calcium mobilizer from intracellular stores. Recently, hexameric connexin 43 (Cx43) hemichannels have been shown to release cytosolic NAD(+) from isolated murine fibroblasts (Bruzzone, S., Guida, L., Zocchi, E., Franco, L. and De Flora, A. (2001) FASEB J. 15, 10-12), making this dinucleotide available to the ectocellular active site of CD38. Here we investigated transwell co-cultures of CD38(+) (transfected) and CD38(-) 3T3 cells in order to establish the role of extracellular NAD(+) and cADPR on [Ca(2+)](i) levels and on proliferation of the CD38(-) target cells. CD38(+), but not CD38(-), feeder cells induced a [Ca(2+)](i) increase in the CD38(-) target cells which was comparable to that observed with extracellular cADPR alone and inhibitable by NAD(+)-glycohydrolase or by the cADPR antagonist 8-NH(2)-cADPR. Addition of recombinant ADP-ribosyl cyclase to the medium of CD38(-) feeders induced sustained [Ca(2+)](i) increases in CD38(-) target cells. Co-culture on CD38(+) feeders enhanced the proliferation of CD38(-) target cells over control values and significantly shortened the S phase of cell cycle. These results demonstrate a paracrine process based on Cx43-mediated release of NAD(+), its CD38-catalyzed conversion to extracellular cADPR, and influx of this nucleotide into responsive cells to increase [Ca(2+)](i) and stimulate cell proliferation.

Adp-ribosyl cyclase and cyclic ADP-ribose hydrolase act as a redox sensor. a primary role for cyclic ADP-ribose in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction is unique to pulmonary arteries and serves to match lung perfusion to ventilation. However, in disease states this process can promote hypoxic pulmonary hypertension. Hypoxic pulmonary vasoconstriction is associated with increased NADH levels in pulmonary artery smooth muscle and with intracellular Ca(2+) release from ryanodine-sensitive stores. Because cyclic ADP-ribose (cADPR) regulates ryanodine receptors and is synthesized from beta-NAD(+), we investigated the regulation by beta-NADH of cADPR synthesis and metabolism and the role of cADPR in hypoxic pulmonary vasoconstriction. Significantly higher rates of cADPR synthesis occurred in smooth muscle homogenates of pulmonary arteries, compared with homogenates of systemic arteries. When the beta-NAD(+):beta-NADH ratio was reduced, the net amount of cADPR accumulated increased. This was due, at least in part, to the inhibition of cADPR hydrolase by beta-NADH. Furthermore, hypoxia induced a 10-fold increase in cADPR levels in pulmonary artery smooth muscle, and a membrane-permeant cADPR antagonist, 8-bromo-cADPR, abolished hypoxic pulmonary vasoconstriction in pulmonary artery rings. We propose that the cellular redox state may be coupled via an increase in beta-NADH levels to enhanced cADPR synthesis, activation of ryanodine receptors, and sarcoplasmic reticulum Ca(2+) release. This redox-sensing pathway may offer new therapeutic targets for hypoxic pulmonary hypertension.

An efficient synthesis of cyclic IDP- and cyclic 8-bromo-IDP-carbocyclic-riboses using a modified Hata condensation method to form an intramolecular pyrophosphate linkage as a key step. An entry to a general method for the chemical synthesis of cyclic ADP-ribose analogues

An efficient synthesis of cyclic IDP-carbocyclic-ribose (3) and its 8-bromo derivative 6, as stable mimics of cyclic ADP-ribose, was achieved, and a condensation reaction with phenylthiophosphate-type substrate 15 or 16 to form an intramolecular pyrophosphate linkage was a key step. N-1-Carbocyclic-ribosylinosine derivative 28 and the corresponding 8-bromo congener 24 were prepared via condensation between N-1-(2,4-dinitrophenyl)inosine derivative 17 and a known optically active carbocyclic amine 18. Compounds 24 and 28 were then converted to the corresponding 5"-phosphoryl-5'-phenylthiophosphate derivatives 15 and 16, respectively, which were substrates for the condensation reaction to form an intramolecular pyrophosphate linkage. Treatment of 8-bromo substrate 15 with I2 or AgNO3 in the presence of molecular sieves 3A (MS 3A) in pyridine at room temperature gave the desired cyclic product 12 quantitatively, while the yield was quite low without MS. The similar reaction of 8-unsubstituted substrate 16 gave the corresponding cyclized product 32 in 81% yield. Acidic treatment of these cyclic pyrophosphates 12 and 32 readily gave the targets 6 and 3, respectively. This result suggests that the construction of N-1-substituted hypoxanthine nucleoside structures from N-1-(2,4-dinitrophenyl)inosine derivatives and the intramolecular condensation by activation of the phenylthiophosphate group with I2 or AgNO3/MS 3A combine to provide a very efficient route for the synthesis of analogues of cyclic ADP-ribose such as 3 and 6. Thus, this may be an entry to a general method for synthesizing biologically important cyclic nucleotides of this type.

Cyclic ADP ribose as a calcium-mobilizing messenger

This Perspective by Galione and Churchill is one in a series on intracellular calcium release mechanisms. The authors review the evidence for cyclic adenosine diphosphate ribose (cADPR) being a second messenger involved in regulating intracellular calcium. In addition, the physiological stimuli and responses mediated by cADPR are discussed. The Perspective is accompanied by a movie showing a calcium wave triggered by cADPR.