Novel mechanism of intracellular calcium release in pituitary cells

The role of dietary niacin intake and the adenosine-5'-diphosphate-ribosyl cyclase enzyme CD38 in spatial learning ability: is cyclic adenosine diphosphate ribose the link between diet and behaviour?

The pyridine nucleotide NAD+ is derived from dietary niacin and serves as the substrate for the synthesis of cyclic ADP-ribose (cADPR), an intracellular Ca signalling molecule that plays an important role in synaptic plasticity in the hippocampus, a region of the brain involved in spatial learning. cADPR is formed in part via the activity of the ADP-ribosyl cyclase enzyme CD38, which is widespread throughout the brain. In the present review, current evidence of the relationship between dietary niacin and behaviour is presented following investigations of the effect of niacin deficiency, pharmacological nicotinamide supplementation and CD38 gene deletion on brain nucleotides and spatial learning ability in mice and rats. In young male rats, both niacin deficiency and nicotinamide supplementation significantly altered brain NAD+ and cADPR, both of which were inversely correlated with spatial learning ability. These results were consistent across three different models of niacin deficiency (pair feeding, partially restricted feeding and niacin recovery). Similar changes in spatial learning ability were observed in Cd38- / - mice, which also showed decreases in brain cADPR. These findings suggest an inverse relationship between spatial learning ability, dietary niacin intake and cADPR, although a direct link between cADPR and spatial learning ability is still missing. Dietary niacin may therefore play a role in the molecular events regulating learning performance, and further investigations of niacin intake, CD38 and cADPR may help identify potential molecular targets for clinical intervention to enhance learning and prevent or reverse cognitive decline.

Transformation and actions of extracellular NADP(+) in the rat liver

The possible actions and transformation of extracellular NADP(+) in the rat liver have not yet been studied. Considering the various effects of its analogue NAD(+) in the liver, however, effects of NADP(+) can equally be expected. In the present work, this question was approached in the isolated perfused rat liver to get a preliminary picture of the action of extracellular NADP(+) in this organ. NADP(+) (100 microM) produced transient increases in the portal perfusion pressure. Glucose release (glycogenolysis) and lactate production from endogenous glycogen were transiently increased in antegrade and retrograde perfusion. Oxygen uptake was stimulated after a transient inhibition in antegrade perfusion, which was practically absent in retrograde perfusion. Pyruvate production was transiently inhibited. In the absence of Ca(2+), all of these effects were no longer observed. Bromophenacyl bromide, an inhibitor of eicosanoid synthesis, almost abolished all effects. Suramin, a non-specific purinergic P2(YX) antagonist, also inhibited the action of NADP(+). Single pass transformation of 75 microM NADP(+) was equal to 92%. Besides nicotinamide, at least two additional transformation products were detected: 2'-phospho-ADP-ribose and a non-identified component, the former being more important (67% of the transformed NADP(+)). Nicotinic acid adenine dinucleotide phosphate (NAADP) was not found in the outflowing perfusate. It was concluded that NADP(+), like NAD(+), acts on perfusion pressure and glycogen catabolism in the liver mainly via eicosanoid synthesis mediated by purinergic P2(YX) receptors.

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.

Transformation products of extracellular NAD(+) in the rat liver: kinetics of formation and metabolic action

The perfused rat liver responds in several ways to NAD(+) infusion (20-100 microM). Increases in portal perfusion pressure and glycogenolysis and transient inhibition of oxygen consumption and gluconeogenesis are some of the effects that were observed. Extracellular NAD(+) is also extensively transformed in the liver. The purpose of the present work was to determine the main products of extracellular NAD(+) transformation under various conditions and to investigate the possible contribution of these products for the metabolic effects of the parent compound. The experiments were done with the isolated perfused rat liver. The NAD(+) transformation was monitored by HPLC. Confirming previous findings, the single-pass transformation of 100 microM NAD(+) ranged between 75% at 1.5 min after starting infusion to 95% at 8 min. The most important products of single-pass NAD(+) transformation appearing in the outflowing perfusate were nicotinamide, ADP-ribose, uric acid, and inosine. The relative proportions of these products presented some variations with the time after initiation of NAD(+) infusion and the perfusion conditions, but ADP-ribose was always more abundant than uric acid and inosine. Cyclic ADP-ribose (cADP-ribose) as well as adenosine were not detected in the outflowing perfusate. The metabolic effects of ADP-ribose were essentially those already described for NAD(+). These effects were sensitive to suramin (P2(XY) purinergic receptor antagonist) and insensitive to 3,7-dimethyl-1-(2-propargyl)-xanthine (A2 purinergic receptor antagonist). Inosine, a known purinergic A3 agonist, was also active on metabolism, but uric acid and nicotinamide were inactive. It was concluded that the metabolic and hemodynamic effects of extracellular NAD(+) are caused mainly by interactions with purinergic receptors with a highly significant participation of its main transformation product ADP-ribose.

Nicotinamide 2-fluoroadenine dinucleotide unmasks the NAD+ glycohydrolase activity of Aplysia californica adenosine 5'-diphosphate ribosyl cyclase

ADP-ribosyl cyclases catalyze the transformation of nicotinamide adenine dinucleotide (NAD+) into the calcium-mobilizing nucleotide second messenger cyclic adenosine diphosphoribose (cADP-ribose) by adenine N1-cyclization onto the C-1' ' position of NAD+. The invertebrate Aplysia californica ADP-ribosyl cyclase is unusual among this family of enzymes by acting exclusively as a cyclase, whereas the other members, such as CD38 and CD157, also act as NAD+ glycohydrolases, following a partitioning kinetic mechanism. To explore the intramolecular cyclization reaction, the novel nicotinamide 2-fluoroadenine dinucleotide (2-fluoro-NAD+) was designed as a sterically very close analogue to the natural substrate NAD+, with only an electronic perturbation at the critical N1 position of the adenine base designed to impede the cyclization reaction. 2-Fluoro-NAD+ was synthesized in high yield via Lewis acid catalyzed activation of the phosphoromorpholidate derivative of 2-fluoroadenosine 5'-monophosphate and coupling with nicotinamide 5'-monophosphate. With 2-fluoro-NAD+ as substrate, A. californica ADP-ribosyl cyclase exhibited exclusively a NAD+ glycohydrolase activity, catalyzing its hydrolytic transformation into 2-fluoro-ADP-ribose, albeit at a rate ca. 100-fold slower than for the cyclization of NAD+ and also, in the presence of methanol, into its methanolysis product beta-1' '-O-methyl 2-fluoro-ADP-ribose with a preference for methanolysis over hydrolysis of ca. 100:1. CD38 likely converted 2-fluoro-NAD+ exclusively into the same product. We conclude that A. californica ADP-ribosyl cyclase can indeed be classified as a multifunctional enzyme that also exhibits a classical NAD+ glycohydrolase function. This alternative pathway that remains, however, kinetically cryptic when using NAD+ as substrate can be unmasked with a dinucleotide analogue whose conversion into the cyclic derivative is blocked. 2-Fluoro-NAD+ is therefore a useful molecular tool allowing dissection of the kinetic scheme for this enzyme.

CD38 is critical for social behaviour by regulating oxytocin secretion

CD38, a transmembrane glycoprotein with ADP-ribosyl cyclase activity, catalyses the formation of Ca2+ signalling molecules, but its role in the neuroendocrine system is unknown. Here we show that adult CD38 knockout (CD38-/-) female and male mice show marked defects in maternal nurturing and social behaviour, respectively, with higher locomotor activity. Consistently, the plasma level of oxytocin (OT), but not vasopressin, was strongly decreased in CD38-/- mice. Replacement of OT by subcutaneous injection or lentiviral-vector-mediated delivery of human CD38 in the hypothalamus rescued social memory and maternal care in CD38-/- mice. Depolarization-induced OT secretion and Ca2+ elevation in oxytocinergic neurohypophysial axon terminals were disrupted in CD38-/- mice; this was mimicked by CD38 metabolite antagonists in CD38+/+ mice. These results reveal that CD38 has a key role in neuropeptide release, thereby critically regulating maternal and social behaviours, and may be an element in neurodevelopmental disorders.

Liver parenchyma heterogeneity in the response to extracellular NAD+

The perfused rat liver responds intensely to NAD+ infusion (20-100 microM). Increases in portal perfusion pressure and glycogenolysis and transient inhibition of oxygen consumption are some of the effects that were observed. The aim of the present work was to investigate the distribution of the response to extracellular NAD+ along the hepatic acinus. The bivascularly perfused rat liver was used. Various combinations of perfusion directions (antegrade and retrograde) and infusion routes (portal vein, hepatic vein and hepatic artery) were used in order to supply NAD+ to different regions of the liver parenchyma, also taking advantage of the fact that its extracellular transformation generates steep concentration gradients. Oxygen uptake was stimulated by NAD+ in retrograde perfusion (irrespective of the infusion route) and transiently inhibited in antegrade perfusion. This indicates that the signal causing oxygen uptake inhibition is generated in the periportal area. The signal responsible for oxygen uptake stimulation is homogenously distributed. Stimulation of glucose release was more intense when NAD+ was infused into the portal vein or into the hepatic artery, indicating that stimulation of glycogenolysis predominates in the periportal area. The increases in perfusion pressure were more pronounced when the periportal area was supplied with NAD+ suggesting that the vasoconstrictive elements responding to NAD+ predominate in this region. The response to extracellular NAD+ is thus unequally distributed in the liver. As a paracrine agent, NAD+ is likely to be released locally. It can be concluded that its effects will be different depending on the area where it is released.

Vascular physiology of a Ca2+ mobilizing second messenger - cyclic ADP-ribose

Cyclic ADP-ribose (cADPR) is a novel Ca(2+) mobilizing second messenger, which is capable of inducing Ca(2+) release from the sarcoplasmic reticulum (SR) via activation of ryanodine receptors (RyR) in vascular cells. This signaling nucleotide has also been reported to participate in generation or modulation of intracellular Ca(2+) sparks, Ca(2+) waves or oscillations, Ca(2+)- induced Ca(2+) release (CICR) and spontaneous transient outward currents (STOCs) in vascular smooth muscle cells (VSMCs). With respect to the role of cADPR-mediated signaling in mediation of vascular responses to different stimuli, there is accumulating evidence showing that cADPR is importantly involved in the Ca(2+) response of vascular endothelial cells (ECs) and VSMCs to various chemical factors such as vasoactive agonists acetylcholine, oxotremorine, endothelin, and physical stimuli such as stretch, electrical depolarization and sheer stress. This cADPR-RyR-mediated Ca(2+) signaling is now recognized as a fundamental mechanism regulating vascular function. Here we reviewed the literature regarding this cADPR signaling pathway in vascular cells with a major focus on the production of cADPR and its physiological roles in the control of vascular tone and vasomotor response. We also summarized some publish results that unveil the underlying mechanisms mediating the actions of cADPR in vascular cells. Given the importance of Ca(2+) in the regulation of vascular function, the results summarized in this brief review will provide new insights into vascular physiology and circulatory regulation.

The action of extracellular NAD+ on gluconeogenesis in the perfused rat liver

In the rat liver NAD+ infusion produces increases in portal perfusion pressure and glycogenolysis and transient inhibition of oxygen consumption. The aim of the present work was to investigate the possible action of this agent on gluconeogenesis using lactate as a gluconeogenic precursor. Hemoglobin-free rat liver perfusion in antegrade and retrograde modes was used with enzymatic determination of glucose production and polarographic assay of oxygen uptake. NAD+ infusion into the portal vein (antegrade perfusion) produced a concentration-dependent (25-100 microM) transient inhibition of oxygen uptake and gluconeogenesis. For both parameters inhibition was followed by stimulation. NAD+ infusion into the hepatic vein (retrograde perfusion) produced only transient stimulations. During Ca2+-free perfusion the action of NAD+ was restricted to small transient stimulations. Inhibitors of eicosanoid synthesis with different specificities (indo-methacin, nordihydroguaiaretic acid, bromophenacyl bromide) either inhibited or changed the action of NAD+. The action of NAD+ on gluconeogenesis is probably mediated by eicosanoids synthesized in non-parenchymal cells. As in the fed state, in the fasted condition extracellular NAD+ is also able to exert two opposite effects, inhibition and stimulation. Since inhibition did not manifest significantly in retrograde perfusion it is likely that the generating signal is located in pre-sinusoidal regions.

Ca release induced by cyclic adenosine diphosphoribose (cADPr) in sea urchin egg homogenates: mechanisms of release and heterogeneity of the Ca compartments

A rapid superfusion system measuring the amounts, kinetics, and Ca dependencies of released 45Ca, was used to examine the effects of ryanodine (RY), caffeine (CF), and cyclic ADP ribose (cADPr) on sea urchin egg homogenates. The RY-sensitive compartment had more than twice the Ca release capacity of the CF-sensitive or cADPr-sensitive compartment. cADPr-stimulated 45Ca release required calcium with half-maximal activation at approximately 0.2 to 0.6 microM [Ca2+]. K(1/2) for cADPr activation was approximately 100 nM, and in spite of the Ca requirement for cADPr-stimulated release, the cADPr affinity was not affected by [Ca2+]. Peak 45Ca release rate with cADPr (3 microM) was greater than with CF (20 mM), yet the release amounts were similar and both were [Ca2+]-dependent. When activated with CF and cADPr simultaneously, 45Ca release was large and, no longer [Ca2+]-dependent. Mg competitively inhibited the Ca activation site(s), yet did not inhibit the activation with CF-plus-cADPr. Pre-release of 45Ca by cADPr with low (approximately 0.1 microM) [Ca2+] right-shifted the [Ca2+] dependence of the remaining cADPr-response. These data suggest that (a) only a portion of RY-sensitive compartments empty when stimulated with cADPr or CF, (b) Ca and cADPr act on non-interacting sites, and (c) cADPr-sensitive compartments represent a heterogeneous population with different [Ca2+] dependencies.

Role of the second-messenger cyclic-adenosine 5'-diphosphate-ribose on adrenocorticotropin secretion from pituitary cells

We examined the role of the second-messenger cyclic-ADP-ribose (cADPR) on the regulation of ACTH secretion using AtT20 corticotroph tumor cell line. We found that the cADPR antagonist, 8-Br-cADPR, substantially diminished the secretion of ACTH induced by CRH and potassium in these cells, whereas xestospongin C, an inositol 1,4,5-triphosphate receptor antagonist, had no effect. In addition, the cADPR agonist, 3-deaza-cADPR, augmented ACTH secretion. The presence of the components of the cADPR system, namely ryanodine receptor, CD38, and cADPR itself, was determined in AtT20 cells. Furthermore, we observed that antagonists of the ryanodine channel and cADPR system can decrease the potassium-induced Ca2+ transients in these cells. These results suggest that cADPR is a second messenger in pituitary cells and regulates ACTH secretion by a mechanism dependent on activation of the ryanodine channel by extracellular Ca2+.

Enhanced production and action of cyclic ADP-ribose during oxidative stress in small bovine coronary arterial smooth muscle

Recent studies in our lab and by others have indicated that cyclic ADP-ribose (cADPR) as a novel second messenger is importantly involved in vasomotor response in various vascular beds. However, the mechanism regulating cADPR production and actions remains poorly understood. The present study determined whether changes in redox status influence the production and action of cADPR in coronary arterial smooth muscle cells (CASMCs) and thereby alters vascular tone in these arteries. HPLC analyses demonstrated that xanthine (X, 40 microM)/xanthine oxidase (XO, 0.1 U/ml), a superoxide-generating system, increased the ADP-ribosyl cyclase activity by 59% in freshly isolated bovine CASMCs. However, hydrogen peroxide (H2O2, 1-100 microM) had no significant effect on ADP-ribosyl cyclase activity. In these CASMCs, X/XO produced a rapid increase in [Ca2+]i (Delta[Ca2+]i=201 nM), which was significantly attenuated by a cADPR antagonist, 8-Br-cADPR. Both inhibition of cADPR production by nicotinamide (Nicot) and blockade of Ca2+-induced Ca2+ release (CICR) by tetracaine (TC) and ryanodine (Rya) significantly reduced X/XO-induced rapid Ca2+ responses. In isolated, perfused, and pressurized small bovine coronary arteries, X at 2.5-80 microM with a fixed XO level produced a concentration-dependent vasoconstriction with a maximal decrease in arterial diameter of 45%. This X/XO-induced vasoconstriction was significantly attenuated by 8-Br-cADPR, Nicot, TC, or Rya. We conclude that superoxide activates cADPR production, and thereby mobilizes intracellular Ca2+ from the SR and produces vasoconstriction in coronary arteries.

Cyclic ADP-ribose, a putative Ca2+-mobilizing second messenger, operates in submucosal gland acinar cells

Cyclic ADP-ribose (cADPR), a putative Ca(2+)-mobilizing second messenger, has been reported to operate in several mammalian cells. To investigate whether cADPR is involved in electrolyte secretion from airway glands, we used a patch-clamp technique, the measurement of microsomal Ca(2+) release, quantification of cellular cADPR, and RT-PCR for CD38 mRNA in human and feline tracheal glands. cADPR (>6 microM), infused into the cell via the patch pipette, caused ionic currents dependent on cellular Ca(2+). Infusions of lower concentrations (2-4 microM) of cADPR or inositol 1,4,5-trisphosphate (IP(3)) alone were without effect on the baseline current, but a combined application of cADPR and IP(3) mimicked the cellular response to low concentrations of acetylcholine (ACh). Microsomes derived from the isolated glands released Ca(2+) in response to both IP(3) and cADPR. cADPR released Ca(2+) from microsomes desensitized to IP(3) or those treated with heparin. The mRNA for CD38, an enzyme protein involved in cADPR metabolism, was detected in human tissues, including tracheal glands, and the cellular content of cADPR was increased with physiologically relevant concentrations of ACh. We conclude that cADPR, in concert with IP(3), operates in airway gland acinar cells to mobilize Ca(2+), resulting in Cl(-) secretion.

High-performance liquid chromatographic technique for detection of a fluorescent analogue of ADP-ribose in isolated blood vessel preparations

Analysis of endogenous nucleotides in biologic media is hampered by rapid degradation and low final concentrations that are difficult to detect. A reversed-phase high-performance liquid chromatographic (HPLC) technique is described that efficiently detects a stable fluorescence derivative of adenosine 5'-diphosphoribose (ADPR), 1,N6-etheno-ADPR (epsilon-ADPR), at low femtomolar concentration range in vascular tissue superfusates. epsilon-ADPR was formed by the reaction of ADPR with chloroacetaldehyde at 80 degrees C and pH 4.0. Gradient elution with 0.1 M KH2PO4 (pH 6.0), increasing methanol (0-35% over 18 min), and a 25-cm by 4.5-mm (5 microm) silica ODS-AM column were employed. epsilon-ADPR was detected by fluorescence at an excitation wavelength of 230 nm and an emission wavelength of 410 nm. The detection sensitivity for epsilon-ADPR was approximately 10 fmol. Linearity of the HPLC detection method was demonstrated in the range from 0.0125 to 1 pmol epsilon-ADPR. The method was validated in terms of within-day and between-day reproducibility of retention times and peak areas of standard nucleotide. Matrix-assisted laser desorption/ionization mass spectrometry measurements confirmed the presence of an etheno ring after reaction of ADPR with chloroacetaldehyde. The method was applied to quantitate the overflow of ADPR upon electrical field stimulation (8 Hz, 0.3 ms, 15 V, 1-2 min) of both canine and guinea-pig isolated mesenteric artery segments.

Estrogen increases CD38 gene expression and leads to differential regulation of adenosine diphosphate (ADP)-ribosyl cyclase and cyclic ADP-ribose hydrolase activities in rat myometrium

Hormones influence uterine contractility through their effects on intracellular calcium. The regulation of intracellular calcium in uterine smooth muscle is achieved by several mechanisms and includes mobilization from intracellular stores by inositol 1,4,5-trisphosphate and ryanodine-sensitive channels. Cyclic ADP-ribose (cADPR), a metabolite of NAD(+), is known to mediate calcium release through ryanodine receptor channels. A cell surface glycoprotein, CD38, catalyzes the synthesis and breakdown of cADPR and thus possesses bifunctional enzymatic activity. The regulation of cADPR synthesis by ADP-ribosyl cyclase (cyclase) or degradation by cADP-ribose hydrolase (hydrolase) by hormones in the myometrium is poorly understood. We investigated the effects of estradiol-17 beta on CD38 expression and the synthesis and degradation of cADPR in myometrial smooth muscle obtained from ovariectomized rats. CD38 expression was studied by reverse transcription polymerase chain reaction and Western blot analyses. In uterine microsomal fractions, cyclase and hydrolase activities were measured using nicotinamide guanine dinucleotide and [(32)P]cADPR as substrates, respectively. Microsomal proteins subfractionated by SDS-PAGE and gel filtration were used to determine the fractions containing cyclase and hydrolase activities. The results demonstrate that cyclase and hydrolase activities are associated with a single protein fraction, similar to CD38 in uteri from both ovariectomized and estradiol-treated rats, and estradiol-17 beta causes 1) increased CD38 mRNA and protein expression and 2) significantly enhanced cyclase but not hydrolase activity. The differential regulation of CD38 by estradiol-17 beta, resulting in increased cADPR synthesis, would have profound effects on calcium regulation and myometrial contractility.

NMR studies and semi-empirical energy calculations for cyclic ADP-ribose

A possible pH-dependent conformational switch was investigated for cyclic ADP-ribose. NMR signals for the exchangeable protons were observed in H2O at low temperature, but there was no direct evidence for the protonation of N-3 at neutral pH that has previously been postulated. MNDO calculations indicated that pH dependent 31P chemical shift changes are attributable to protonation of the phosphate adjacent to the N-1 of adenine, and not due to trans-annular hydrogen bonding with a protonated N-3.

Cyclic ADP-ribose as a second messenger revisited from a new aspect of signal transduction from receptors to ADP-ribosyl cyclase

Cyclic ADP-ribose (cADPR), an endogenous modulator of ryanodine receptor Ca(2+)-releasing channels, is found in various tissues. Cytosolic injection of cADPR induces an elevation of intracellular Ca(2+) concentrations or potentiates Ca(2+) increases. cADPR facilitates neurotransmitter or insulin release and modifies ionic currents. cADPR is synthesized by ADP-ribosyl cyclase and is metabolized by cADPR hydrolase. ADP-ribosyl cyclase activity is up-regulated by nitric oxide/cyclic GMP-dependent phosphorylation or receptor stimulation via G-proteins within membranes. These findings suggest that cADPR is a second messenger in cellular Ca(2+) signaling. However, many intriguing issues remain to be addressed before this identity is confirmed.

Caffeine stores and dopamine differentially require Ca(2+) channels in goldfish somatotropes

The regulation of growth hormone (GH) secretion by intracellular Ca(2+) stores was studied in dissociated goldfish somatotropes. We characterized a caffeine-activated intracellular store that had been shown to mediate GH release in response to gonadotropin-releasing hormone. The peak response of caffeine stimulation was reduced by approximately 28% by 100 microM ryanodine in a use-dependent manner suggesting that the first 10 min of GH release is partially mediated by a caffeine-activated ryanodine receptor. The temporal sensitivities of caffeine- and dopamine-evoked GH release to blockade of Cd(2+)-sensitive Ca(2+) channels were compared. We demonstrated that the initial phase of dopamine-evoked release was dependent on Ca(2+) channels, whereas the initial phase of caffeine-evoked release was sensitive only to pretreatment blockade. This would suggest that the maintenance of one class of caffeine-activated intracellular stores requires entry of Ca(2+) through Cd(2+)-sensitive Ca(2+) channels. This differential temporal requirement for Ca(2+) channels in Ca(2+) signaling may be a mechanism to segregate intracellular signaling pathways of multiple neuroendocrine regulators in the teleost pituitary.

Cyclic ADP-ribose as a potential second messenger for neuronal Ca2+ signaling

Cyclic ADP-ribose (cADPR), a known endogenous modulator of ryanodine receptor Ca2+ releasing channels, is found in the nervous system. Injection of cADPR into neuronal cells primarily induces a transient elevation of intracellular Ca2+ concentration ([Ca2+]i), and/or secondarily potentiates [Ca2+]i increases that are the result of depolarization-induced Ca2+ influx. Acetylcholine release from cholinergic neurons is facilitated by cADPR. cADPR modifies K+ currents or elicits Ca2+-dependent inward currents. cADPR is synthesized by both membrane-bound and cytosolic forms of ADP-ribosyl cyclase in neuronal cells. cADPR hydrolase activity is weak in the membrane fraction, but high in the cytoplasm. Cytosolic ADP-ribosyl cyclase activity is upregulated by nitric oxide/cyclic GMP-dependent phosphorylation. Stimulation of muscarinic and beta-adrenergic receptors activates membrane-bound ADP-ribosyl cyclase via G proteins within membranes of neuronal tumor cells and cortical astrocytes. These findings strongly suggest that cADPR is a second messenger in Ca2+ signaling in the nervous system, although many intriguing issues remain to be addressed before this identity is confirmed.

Nitric oxide inhibits Ca(2+) mobilization through cADP-ribose signaling in coronary arterial smooth muscle cells

The present study was designed to determine whether the cADP-ribose-mediated Ca(2+) signaling is involved in the inhibitory effect of nitric oxide (NO) on intracellular Ca(2+) mobilization. With the use of fluorescent microscopic spectrometry, cADP-ribose-induced Ca(2+) release from sarcoplasmic reticulum (SR) of bovine coronary arterial smooth muscle cells (CASMCs) was determined. In the alpha-toxin-permeabilized primary cultures of CASMCs, cADP-ribose (5 microM) produced a rapid Ca(2+) release, which was completely blocked by pretreatment of cells with the cADP-ribose antagonist 8-bromo-cADP-ribose (8-Br-cADPR). In intact fura 2-loaded CASMCs, 80 mM KCl was added to depolarize the cells and increase intracellular Ca(2+) concentration ([Ca(2+)](i)). Sodium nitroprusside (SNP), an NO donor, produced a concentration-dependent inhibition of the KCl-induced increase in [Ca(2+)](i), but it had no effect on the U-46619-induced increase in [Ca(2+)](i). In the presence of 8-Br-cADPR (100 microM) and ryanodine (10 microM), the inhibitory effect of SNP was markedly attenuated. HPLC analyses showed that CASMCs expressed the ADP-ribosyl cyclase activity, and SNP (1-100 microM) significantly reduced the ADP-ribosyl cyclase activity in a concentration-dependent manner. The effect of SNP was completely blocked by addition of 10 microM oxygenated hemoglobin. We conclude that ADP-ribosyl cyclase is present in CASMCs, and NO may decrease [Ca(2+)](i) by inhibition of cADP-ribose-induced Ca(2+) mobilization.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

Membrane-bound form of ADP-ribosyl cyclase in rat cortical astrocytes in culture

ADP-ribosyl cyclase activities in cultured rat astrocytes were examined by using TLC for separation of enzymatic products. A relatively high rate of [3H]cyclic ADP-ribose production converted from [3H]NAD+ by ADP-ribosyl cyclase (2.015+/-0.554 nmol/min/mg of protein) was detected in the crude membrane fraction of astrocytes, which contained approximately 50% of the total cyclase activity in astrocytes. The formation rate of [3H]ADP-ribose from cyclic ADP-ribose by cyclic ADP-ribose hydrolase and/or from NAD+ by NAD glycohydrolase was low and enriched in the cytosolic fraction. Although NAD+ in the extracellular medium was metabolized to cyclic ADP-ribose by incubating cultures of intact astrocytes, the presence of Triton X-100 in the medium for permeabilizing cells increased cyclic ADP-ribose production three times as much. Isoproterenol and GTP increased [3H]cyclic ADP-ribose formation in crude membrane-associated cyclase activity. This isoproterenol-induced stimulation of membrane-associated ADP-ribosyl cyclase activity was confirmed by cyclic GDP-ribose formation fluorometrically. This stimulatory action was blocked by prior treatment of cells with cholera toxin but not with pertussis toxin. These results suggest that ADP-ribosyl cyclase in astrocytes has both extracellular and intracellular actions and that signals of beta-adrenergic stimulation are transduced to membrane-bound ADP-ribosyl cyclase via G proteins within cell surface membranes of astrocytes.

Characterization of a plasma membrane calcium oscillator in rat pituitary somatotrophs

In excitable cells, oscillations in intracellular free calcium concentrations ([Ca(2+)](i)) can arise from action-potential-driven Ca(2+) influx, and such signals can have either a localized or global form, depending on the coupling of voltage-gated Ca(2+) influx to intracellular Ca(2+) release pathway. Here we show that rat pituitary somatotrophs generate spontaneous [Ca(2+)](i) oscillations, which rise from fluctuations in the influx of external Ca(2+) and propagate within the cytoplasm and nucleus. The addition of caffeine and ryanodine, modulators of ryanodine-receptor channels, and the depletion of intracellular Ca(2+) stores by thapsigargin and ionomycin did not affect the global nature of spontaneous [Ca(2+)](i) signals. Bay K 8644, an L-type Ca(2+) channel agonist, initiated [Ca(2+)](i) signaling in quiescent cells, increased the amplitude of [Ca(2+)](i) spikes in spontaneously active cells, and stimulated growth hormone secretion in perifused pituitary cells. Nifedipine, a blocker of L-type Ca(2+) channels, decreased the amplitude of spikes and basal growth hormone secretion, whereas Ni(2+), a blocker of T-type Ca(2+) channels, abolished spontaneous [Ca(2+)](i) oscillations. Spiking was also abolished by the removal of extracellular Na(+) and by the addition of 10 mM Ca(2+), Mg(2+), or Sr(2+), the blockers of cyclic nucleotide-gated channels. Reverse transcriptase-polymerase chain reaction and Southern blot analyses indicated the expression of mRNAs for these channels in mixed pituitary cells and purified somatotrophs. Growth hormone-releasing hormone, an agonist that stimulated cAMP and cGMP productions in a dose-dependent manner, initiated spiking in quiescent cells and increased the frequency of spiking in spontaneously active cells. These results indicate that in somatotrophs a cyclic nucleotide-controlled plasma membrane Ca(2+) oscillator is capable of generating global Ca(2+) signals spontaneously and in response to agonist stimulation. The Ca(2+)-signaling activity of this oscillator is dependent on voltage-gated Ca(2+) influx but not on Ca(2+) release from intracellular stores.

Induction of hippocampal LTD requires nitric-oxide-stimulated PKG activity and Ca2+ release from cyclic ADP-ribose-sensitive stores

Long-term depression (LTD) of synaptic transmission can be induced by several mechanisms, one thought to involve Ca2+-dependent activation of postsynaptic nitric oxide (NO) synthase and subsequent diffusion of NO to the presynaptic terminal. We used the stable NO donor S-nitroso-N-acetylpenicillamine (SNAP) to study the NO-dependent form of LTD at Schaffer collateral-CA1 synapses in vitro. SNAP (100 microM) enhanced the induction of LTD via a cascade that was blocked by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonopentanoic acid (50 microM), NO guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (10 microM), and the PKG inhibitor KT5823 (1 microM). We further show that LTD induced by low-frequency stimulation in the absence of SNAP also is blocked by KT5823 or Rp-8-(4-chlorophenylthio)-guanosine 3',5'-cyclic monophosphorothioate (10 microM), cyclic guanosine 3',5' monophosphate-dependent protein kinase (PKG) inhibitors with different mechanisms of action. Furthermore SNAP-facilitated LTD was blocked when release from intracellular calcium stores was inhibited by ryanodine (10 microM). Finally, two cell-permeant antagonists of the cyclic ADP-ribose binding site on ryanodine receptors also were able to block the induction of LTD. These results support a cascade for induction of homosynaptic, NO-dependent LTD involving activation of guanylyl cyclase, production of guanosine 3',5' cyclic monophosphate and subsequent PKG activation. This process has an additional requirement for release of Ca2+ from ryanodine-sensitive stores, perhaps dependent on the second-messenger cyclic ADP ribose.

Imaging of intracellular calcium stores in single permeabilized lens cells

Intracellular Ca2+ stores in permeabilized sheep lens cells were imaged with mag-fura 2 to characterize their distribution and sensitivity to Ca2+-releasing agents. Inositol 1,4,5-trisphosphate (IP3) or cyclic ADP-ribose (cADPR) released Ca2+ from intracellular Ca2+ stores that were maintained by an ATP-dependent Ca2+ pump. The IP3 antagonist heparin inhibited IP3- but not cADPR-mediated Ca2+ release, whereas the cADPR antagonist 8-amino-cADPR inhibited cADPR- but not IP3-mediated Ca2+ release, indicating that IP3 and cADPR were operating through separate mechanisms. A Ca2+ store sensitive to IP3, cADPR, and thapsigargin appeared to be distributed throughout all intracellular regions. In some cells a Ca2+ store insensitive to IP3, cADPR, thapsigargin, and 2,4-dinitrophenol, but not ionomycin, was present in a juxtanuclear region. We conclude that lens cells contain intracellular Ca2+ stores that are sensitive to IP3, cADPR, and thapsigargin, as well as a Ca2+ store that appears insensitive to all these agents.

Involvement of cytosolic NAD+ glycohydrolase in cyclic ADP-ribose metabolism

The NAD+ glycohydrolase homogeneously purified from bovine brain cytosol was found to catalyze the synthesis and hydrolysis of cyclic ADP-ribose (cADPR). Although the formation of cADPR from NAD+ does not exceed about 2% of the reaction products, the cyclase activity is clearly evidenced by its conversion of NGD+ to cyclic GDP-ribose (cGDPR), which cannot be hydrolyzed to GDPR. Importantly, a steep increase in cADPR hydrolytic activity was observed at cADPR concentrations above 60 microM, which could be reproduced on a Hill curve with a Hill coefficient of 2. Thus, the allosteric binding of cADPR to the NAD+ glycohydrolase (E) molecule promotes the hydrolysis of cADPR. These results suggest that NAD+ hydrolysis to ADPR and nicotinamide catalyzed by the NAD+ glycohydrolase occurs through the formation of a cADPR. E. cADP-ribosyl complex. The low production of cADPR by NAD+ glycohydrolase compared with invertebrate ADP-ribosyl cyclase is believed to be attributable to the fast hydrolysis of cADPR by the allosteric effect of cADPR bound to the same enzyme that produces it.

Cyclic ADP-ribose induces Ca2+ release from caffeine-insensitive Ca2+ pools in canine salivary gland cells

Cyclic ADP-ribose (cADPR), a novel putative messenger of the ryanodine receptor, was examined regarding its ability to mobilize Ca2+ from intracellular Ca2+ stores in isolated cells of parotid and submandibular glands of the dog. cADPR induced a rapid and transient Ca2+ release in the digitonin-permeabilized cells of salivary glands. cADPR-induced Ca2+ release was inhibited by ryanodine receptor antagonists ruthenium red, ryanodine, benzocaine, and imperatoxin inhibitor but not by the inositol 1,4,5-trisphosphate (IP3)-receptor antagonist heparin. Thapsigargin, at a concentration of 3 to 30 microM, inhibited IP3-induced Ca2+ release, while higher concentrations were required to inhibit cADPR-induced Ca2+ release. Cross-potentiation was observed between cADPR and ryanodine or SrCl2, suggesting that cADPR sensitizes the Ca2+-induced Ca2+ release mechanism. Cyclic AMP plays a stimulatory role on cADPR- and IP3-induced Ca2+ release in digitonin-permeabilized cells. Calmodulin also potentiated cADPR-induced Ca2+ release, but inhibited IP3-induced Ca2+ release. Acetylcholine and ryanodine caused the rise in intracellular free Ca2+ concentration ([Ca2+]i) in intact submandibular and parotid cells. Caffeine did not produce any increase in Ca2+ release or [Ca2+]i rise in any preparation. ADP-ribosyl cyclase activity was found in the centrifuged particulate fractions of the salivary glands. These results suggest that cADPR serves as an endogenous modulator of Ca2+ release from Ca2+ pools through a caffeine-insensitive ryanodine receptor channel, which are different from IP3-sensitive pools in canine salivary gland cells. This system is positively regulated by cyclic AMP and calmodulin.

Regulation of KCa-channel activity by cyclic ADP-ribose and ADP-ribose in coronary arterial smooth muscle

The enzymatic pathway responsible for the production and metabolism of cyclic ADP-ribose (cADP-R) in small bovine coronary arteries was characterized, and the role of cADP-R and ADP-ribose (ADP-R) in the regulation of the activity of large-conductance Ca2+-activated K+ (KCa) channels was determined in vascular smooth muscle cells (SMC) prepared from these vessels. We found that cADP-R and ADP-R were produced when the coronary arterial homogenates were incubated with 1 mM beta-NAD. The time course of the enzyme reactions showed that the maximal conversion rate (1.37 +/- 0.03 nmol . min-1 . mg protein-1) of beta-NAD to cADP-R was reached after 3 min of incubation. As incubation time was prolonged, the production of ADP-R was increased to a maximal rate of 3.66 +/- 0.03 nmol . min-1 . mg protein-1, whereas cADP-R production decreased. Incubation of the homogenate with cADP-R produced a time-dependent increase in the synthesis of ADP-R. Comparison of coronary arterial microsomes with cytosols shows that the production of both cADP-R and ADP-R in microsomes was significantly greater. In excised inside-out membrane patches of single coronary SMC, the KCa channels were activated when beta-NAD, the precursor for both cADP-R and ADP-R, was applied to the internal surface. This effect of beta-NAD may be associated with the production of ADP-R, because the KCa-channel activity was increased by ADP-R in a concentration-dependent manner. The open-state probability of the KCa channels increased from a control level of 0.08 +/- 0.03 to 0.17 +/- 0.05 even at the lowest ADP-R concentration (0.1 microM) studied. However, cADP-R reduced the KCa-channel activity, and the threshold concentration of cADP-R that decreased the average channel activity of the KCa channels was 1 microM. These results provide evidence that cADP-R is produced and metabolized in the coronary arterial smooth muscle and that a cADP-R/ADP-R pathway participates in the control of the KCa-channel activity in vascular SMC.

IP3 receptor blockade fails to prevent intracellular Ca2+ release by ET-1 and alpha-thrombin

The effect of inositol 1,4,5-trisphosphate (IP3) receptor blockade on platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), endothelin-1 (ET-1), or alpha-thrombin receptor-mediated intracellular Ca2+ (Ca2+i) release was examined using fura 2 microspectrofluorometry in single Chinese hamster ovary cells and myoblasts. Blockade of the IP3 receptor was achieved by microinjection of heparin or monoclonal antibody (MAb) 18A10 into the IP3 type 1 receptor. Heparin completely inhibited Ca2+i release after flash photolysis with caged IP3 and after exposure to PDGF and FGF. In contrast, heparin failed to block Ca2+i release after alpha-thrombin and ET-1. After application of ligand, IP3 levels were five- to sevenfold higher for alpha-thrombin than for ET-1 or PDGF. IP3 levels after PDGF and ET-1 were comparable. Similar to heparin, MAb 18A10 blocked Ca2+i release after PDGF but failed to block Ca2+i release after ET-1 or alpha-thrombin. These data suggest that the mechanisms of Ca2+i release by tyrosine kinase and certain 7-transmembrane receptors may differ. Although both receptor types use the IP3-signaling system, the ET-1 and alpha-thrombin receptors may have a second, alternative mechanism for activating CA2+i release.

Cyclic ADP-ribose and calcium-induced calcium release regulate neurotransmitter release at a cholinergic synapse of Aplysia

1. Presynaptic injection of cyclic ADP-ribose (cADPR), a modulator of the ryanodine receptor, increased the postsynaptic response evoked by a presynaptic spike at an identified cholinergic synapse in the buccal ganglion of Aplysia californica. 2. The statistical analysis of long duration postsynaptic responses evoked by square depolarizations of the voltage-clamped presynaptic neurone showed that the number of evoked acetylcholine (ACh) quanta released was increased following cADPR injection. 3. Overloading the presynaptic neurone with cADPR led to a transient increase of ACh release followed by a depression. 4. cADPR injections did not modify the presynaptic Ca2+ current triggering ACh release. 5. Ca2+ imaging with the fluorescent dye rhod-2 showed that cADPR injection rapidly increased the free intracellular Ca2+ concentration indicating that the effects of cADPR on ACh release might be related to Ca2+ release from intracellular stores. 6. Ryanodine and 8-amino-cADPR, a specific antagonist of cADPR, decreased ACh release. 7. ADP-ribosyl cyclase, which cyclizes NAD+ into cADPR, was present in the presynaptic neurone as shown by reverse transcriptase-polymerase chain reaction experiments. 8. Application of NAD+, the substrate of ADP-ribosyl cyclase, increased ACh release and this effect was prevented by both ryanodine and 8-amino-cADPR. 9. These results support the view that Ca(2+)-induced Ca2+ release might be involved in the build-up of the Ca2+ concentration which triggers ACh release, and thus that cADPR might have a role in transmitter release modulation.

Cyclic GMP-dependent and -independent effects on the synthesis of the calcium messengers cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) have been shown to mobilize intracellular Ca2+ stores by totally independent mechanisms, which are pharmacologically distinct from that activated by inositol trisphosphate. Although cADPR and NAADP are structurally and functionally different, they can be synthesized by a single enzyme having ADP-ribosyl cyclase activity. In this study, three different assays were used to measure the metabolism of cADPR in sea urchin egg homogenates including a radioimmunoassay, a Ca2+ release assay, and a thin layer chromatographic assay. Soluble and membrane-bound ADP-ribosyl cyclases were identified and both cyclized NAD to produce cADPR. The soluble cyclase was half-maximally stimulated by 5.3 microM cGMP, but not by cAMP, while the membrane-bound form was independent of cGMP. The two forms of the cyclase were also different in the pH dependence of utilizing nicotinamide guanine dinucleotide (NGD), a guanine analog of NAD, as substrate, indicating they are two separate enzymes. The stimulatory effect of cGMP required ATP or ATPgammaS (adenosine 5'-O-(3-thiotriphosphate)) and a cGMP-dependent kinase activity was shown to be present in the soluble fraction. The degradation of cADPR to ADP-ribose was catalyzed by cADPR hydrolase, which was found to be predominantly associated with membranes. Similar to the membrane-bound cyclase, the cADPR hydrolase activity was also independent of cGMP. Both the soluble and membrane fractions also catalyzed the synthesis of NAADP through exchanging the nicotinamide group of NADP with nicotinic acid (NA). The base-exchange activity was independent of cGMP and the half-maximal concentrations of NADP and NA needed were about 0.2 mM and 10 mM, respectively. The exchange reaction showed a preference for acidic pH, contrasting with the neutral pH optimum of the cyclase activities. The complex metabolic pathways characterized in this study indicate that there may be a multitude of regulatory mechanisms for controlling the endogenous concentrations of cADPR and NAADP.

Calcium signaling by cyclic ADP-ribose and NAADP. A decade of exploration

Ca2+ mobilization as a signaling mechanism has been placed on center stage with the discovery of the first Ca2+ messenger, inositol trisphosphate (IP3). This article focuses on two new Ca2+ release activators, which mobilize internal Ca2+ stores via mechanisms totally independent of IP3. They are cyclic ADP-ribose (cADPR) and nicotinic acid dinucleotide phosphate (NAADP), metabolites derived respectively from NAD and NADP. Major advances in the past decade in the understanding of these two novel signaling mechanisms are chronologically summarized.

Stimulation of cyclic ADP-ribose synthesis by acetylcholine and its role in catecholamine release in bovine adrenal chromaffin cells

Cyclic ADP-ribose (cADPR) is suggested to be a novel messenger of ryanodine receptors in various cellular systems. However, the regulation of its synthesis in response to cell stimulation and its functional roles are still unclear. We examined the physiological relevance of cADPR to the messenger role in stimulation-secretion coupling in cultured bovine adrenal chromaffin cells. Sensitization of Ca2+-induced Ca2+ release (CICR) and stimulation of catecholamine release by cADPR in permeabilized cells were demonstrated along with the contribution of CICR to intracellular Ca2+ dynamics and secretory response during stimulation of intact chromaffin cells. ADP-ribosyl cyclase was activated in the membrane preparation from chromaffin cells stimulated with acetylcholine (ACh), excess KCl depolarization, and 8-bromo-cyclic-AMP. ACh-induced activation of ADP-ribosyl cyclase was dependent on the influx of Ca2+ into cells and on the activation of cyclic AMP-dependent protein kinase. These and previous findings that ACh activates adenylate cyclase by Ca2+ influx in chromaffin cells suggested that ACh induces activation of ADP-ribosyl cyclase through Ca2+ influx and cyclic AMP-mediated pathways. These results provide evidence that the synthesis of cADPR is regulated by cell stimulation, and the cADPR/CICR pathway forms a significant signal transduction for secretion.

Roles for adenosine ribose hydroxyl groups in cyclic adenosine 5'-diphosphate ribose-mediated Ca2+ release

Cyclic adenosine diphosphate ribose (cADPR) is a naturally occurring and potent Ca2+-mobilizing agent. Structural analogues are currently required as pharmacological tools for the investigation of this topical molecule, but modifications to date have concentrated primarily upon the purine ring. Two novel dehydroxylated analogues of cADPR have now been prepared from chemically synthesized nicotinamide adenine dinucleotide (NAD+) precursors modified in the ribose moiety linked to adenine. ADP-ribosyl cyclase of Aplysia californica catalyzed the conversion of 2'A-deoxy-NAD+ and 3'A-deoxy-NAD+ into the corresponding 2'A-deoxy-cADPR and 3'A-deoxy-cADPR analogues, respectively. These analogues were used to assess the effect of 2'- and 3'-hydroxyl group deletion in the adenosine ribose moiety of cADPR on the Ca2+-releasing potential of cADPR. These compounds were found to have comparatively markedly different activities as agonists for Ca2+ mobilization in sea urchin egg homogenate. 2'A-Deoxy-cADPR is similar to cADPR, whereas 3'A-deoxy-cADPR is at least 100-fold less potent, indicating that the 3'A-hydroxyl group, but not the 2'A-hydroxyl group, is essential for calcium releasing activity. EC50 values recorded were 32 nM, 58 nM, and 5 microM for cADPR, 2'A-deoxy-cADPR, and 3'A-deoxy-cADPR, respectively. Moreover, 200 nM 2'A-deoxy-cADPR was required to desensitize the cADPR-sensitive Ca2+ channel to a subsequent addition of 100 nM cADPR, but 20 microM 3'A-deoxy-cADPR was required to produce the same desensitizing effect. This is in accordance with the 100-fold lower potency exhibited by the latter analogue. To further investigate the importance of the 3'-hydroxyl group, we have also synthesized 3'A-O-methyl-cADPR, in which the 3'-hydroxyl group of adenosine has been methylated and its ability potentially to donate a hydrogen atom in a hydrogen bond has been removed. Although inactive in releasing Ca2+, 3'A-O-methyl-cADPR inhibited cADPR-induced Ca2+ release in a dose-dependent manner with an approximate IC50 value of 5 microM, whereas 3'-O-methyladenosine had no effect. This further supports the requirement of a 3'-OH group for Ca2+ releasing activity. In addition, however, it suggests that this group may not be crucial for ligand-receptor recognition. Thus, replacement of the hydrogen atom of the hydroxyl with a methyl group effects a change of activity from an agonist to an antagonist of cADPR-induced Ca2+ release. Two other analogues with modifications in the 2' and/or 3' positions, 3'-cADPR phosphate and 2',3'-cyclic-cADPR phosphate, were synthesized and tested for their Ca2+-mobilizing activity in sea urchin egg homogenates. Both analogues were inactive with respect to both agonistic and antagonistic activities on the cADPR-sensitive Ca2+ release mechanism. These are the first steps toward a wider structure-activity relationship for cADPR, and this is the first study to implicate a crucial role for the adenosine ribose hydroxyl groups of cADPR in the biological activity of this cyclic nucleotide. Additionally, this is the first report of a cADPR receptor antagonist that is not modified at the 8-position of the purine ring.

Sarcoplasmic reticulum-associated and protein kinase C-regulated ADP-ribosyl cyclase in cardiac muscle

Two types of ADP-ribosyl cyclase activity were distinguished in dog and rat cardiac muscles by measuring the enzymatic conversion of NGD (as an NAD analog) into the fluorescent product cyclic GDP-ribose in cardiac muscle subcellular fractions. Both types of activity were confined to membrane fractions isolated from microsomes by sucrose gradient centrifugation. One of the activities co-purified with fractions that were enriched in sarcolemma (SLM), as evidenced by immunodetection of the dihydropyridine receptor, while the other activity was found to co-precipitate with the sarcoplasmic reticulum (SR), that was identified on the basis of its immuno-staining with a ryanodine receptor monoclonal antibody. In certain aspects, the plasma membrane-bound ADP-ribosyl cyclase activity resembled the characteristics of CD38 or CD38-like proteins: it was sensitive to thiols and lectins and was recognized by a monoclonal anti CD38 antibody. The SR enzyme had apparently distinct properties, as it was insensitive to both thiols and lectins and was not recognized by the CD38 antibody. In addition, the SR-associated ADP-ribosyl cyclase was inhibited by endogenous protein kinase C (PKC)-dependent phosphorylation in both dog and rat cardiac SR. The PKC-modulated SR ADP-ribosyl cyclase we describe here might be a principal component of the signal transduction machinery that is responsible for regulation of the intracellular levels of cADPR.

Ca2+ entry induced by cyclic ADP-ribose in intact T-lymphocytes

Cyclic ADP-ribose (cADPr) is a potent Ca2+-mobilizing natural compound (Lee, H. C., Walseth, T. F., Bratt, G. T., Hayes, R. N., and Clapper, D. L. (1989) J. Biol. Chem. 264, 1608-1615) which has been shown to release Ca2+ from an intracellular store of permeabilized T-lymphocytes (Guse, A. H., Silva, C. P., Emmrich, F., Ashamu, G., Potter, B. V. L., and Mayr, G. W. (1995) J. Immunol. 155, 3353-3359). Microinjection of cADPr into intact single T lymphocytes dose dependently induced repetitive but irregular Ca2+ spikes which were almost completely dependent on the presence of extracellular Ca2+. The Ca2+ spikes induced by cADPr could be blocked either by co-injection of cADPr with the specific antagonist 8-NH2-cADPr, by omission of Ca2+ from the medium, or by superfusion of the cells with Zn2+ or SK-F 96365. Ratiometric digital Ca2+ imaging revealed that single Ca2+ spikes were initiated at several sites ("hot spots") close to the plasma membrane. These hot spots then rapidly formed a circular zone of high Ca2+ concentration below the plasma membrane which subsequently propagated like a closing optical diaphragm into the center of the cell. Taken together these data indicate a role for cADPr in Ca2+ entry in T-lymphocytes.

Lysine 129 of CD38 (ADP-ribosyl cyclase/cyclic ADP-ribose hydrolase) participates in the binding of ATP to inhibit the cyclic ADP-ribose hydrolase

CD38 catalyzes not only the formation of cyclic ADP-ribose (cADPR) from NAD+ but also the hydrolysis of cADPR to ADP-ribose (ADPR), and ATP inhibits the hydrolysis (Takasawa, S., Tohgo, A., Noguchi, N., Koguma, T., Nata, K., Sugimoto, T., Yonekura, H., and Okamoto, H. (1993) J. Biol. Chem. 268, 26052-26054). In the present study, using purified recombinant CD38, we showed that the cADPR hydrolase activity of CD38 was inhibited by ATP in a competitive manner with cADPR. To identify the binding site for ATP and/or cADPR, we labeled the purified CD38 with FSBA. Sequence analysis of the lysylendopeptidase-digested fragment of the labeled CD38 indicated that the FSBA-labeled residue was Lys-129. We introduced site-directed mutations to change the Lys-129 of CD38 to Ala and to Arg. Neither mutant was labeled with FSBA nor catalyzed the hydrolysis of cADPR to ADPR. Furthermore, the mutants did not bind cADPR, whereas they still used NAD+ as a substrate to form cADPR and ADPR. These results indicate that Lys-129 of CD38 participates in cADPR binding and that ATP competes with cADPR for the binding site, resulting in the inhibition of the cADPR hydrolase activity of CD38.

Cyclic ADP-ribose binds to FK506-binding protein 12.6 to release Ca2+ from islet microsomes

Cyclic ADP-ribose (cADPR) is a second messenger for Ca2+ mobilization via the ryanodine receptor (RyR) from islet microsomes for insulin secretion (Takasawa, S., Nata, K., Yonekura, H., and Okamoto, H. (1993) Science 259, 370-373). In the present study, FK506, an immunosuppressant that prolongs allograft survival, as well as cADPR were found to induce the release of Ca2+ from islet microsomes. After islet microsomes were treated with FK506, the Ca2+ release by cADPR from microsomes was reduced. cADPR as well as FK506 bound to FK506-binding protein 12.6 (FKBP12.6), which we also found occurs naturally in islet microsomes. When islet microsomes were treated with cADPR, FKBP12.6 dissociated from the microsomes and moved to the supernatant, releasing Ca2+ from the intracellular stores. The microsomes that were then devoid of FKBP12.6 did not show Ca2+ release by cADPR. These results strongly suggest that cADPR may be the ligand for FKBP12.6 in islet RyR and that the binding of cADPR to FKBP12.6 frees the RyR from FKBP12.6, causing it to release Ca2+.

7-Deaza cyclic adenosine 5'-diphosphate ribose: first example of a Ca(2+)-mobilizing partial agonist related to cyclic adenosine 5'-diphosphate ribose

BACKGROUND

Cyclic adenosine 5'-diphosphate ribose (cADPR), a naturally occurring metabolite of nicotinamide adenine dinucleotide (NAD+), mobilizes Ca2+ from non-mitochondrial stores in a variety of mammalian and invertebrate tissues. It has been shown that cADPR activates ryanodine-sensitive Ca(2+)-release channels, working independently of inositol 1,4,5-trisphosphate (IP3) to mobilize intracellular Ca2+ stores. In some systems, cADPR has been shown to be more potent than IP3. The chemo-enzymatic synthesis of structurally modified analogues of cADPR can provide pharmacological tools for probing this new Ca(2+)-signaling pathway. In this work, we describe the synthesis and evaluation of a structural mimic of cADPR with different Ca(2+)-releasing properties.

RESULTS

7-Deaza cyclic adenosine 5'-diphosphate ribose (7-deaza cADPR), a novel cADPR analogue modified in the purine ring, was synthesized and its ability to release Ca2+ from non-mitochondrial pools in homogenates made from sea urchin eggs was investigated. 7-Deaza cADPR was more effective in releasing Ca2+ than cADPR, but it only released approximately 66% of the Ca2+ released by a maximal concentration of cADPR. It was also more resistant to hydrolysis than cADPR. If we administered increasing concentrations of 7-deaza cADPR at the same time as a maximal concentration of cADPR, the induction of Ca2+ release by cADPR was antagonized.

CONCLUSIONS

7-Deaza cADPR has a Ca(2+)-release profile consistent with that of a partial agonist, and it is the first reported example of such a compound to act at the cADPR receptor. The imidazole ring of cADPR is clearly important in stimulating the Ca(2+)-release machinery, and the present results demonstrate that structural modification of a site other than position 8 of the purine ring can affect the efficacy of Ca2+ release. 7-Deaza cADPR represents a significant step forwards in designing modulators of the cADPR signaling pathway.

ADP-ribosylation of serum proteins: elevated levels in neoplastic cases due to altered NAD/ADP-ribose metabolism

ADP-ribosylation of human serum proteins was studied in various groups of disorders. In most of these groups, the extent of ADP-ribosylation did not show a divergence from the group of normal controls. Neoplastic diseases revealed, however, a unique group, with more than fivefold increases in ADP-ribosylation levels over the other groups. Blood samples with high levels of ADP-ribosylation revealed, in general, increased serum NAD glycohydrolase activities and low levels of serum NAD.

ADP-ribosylation of serum proteins: evaluation as a potential tumor marker

Serum samples from cancer patients revealed elevated levels of in vitro ADP-ribosylation through non-enzymic binding of ADP-ribose to free acceptor sites on serum proteins. Low concentrations of serum ADP-ribose caused by high NAD glycohydrolase activity together with elevated rates of ADP-ribose transport into erythrocytes appeared to account for under-saturation of the acceptor sites on serum proteins. ADP-ribosylation of serum proteins was assessed as an indicator of cancer disease, and an attempt was made to determine the correlation of ADP-ribosylation levels with carcinoembryonic antigen (CEA) values. Based on positive test results for all tumor patients and negative test results for all healthy controls, sensitivity and specificity of ADP-ribosylation as a tumor indicator were estimated as 67% and 95%, respectively. A close correlation appeared to exist with CEA (r = 0.67; P < 0.001). Similarly, the changes in the levels of ADP-ribosylation correlated with the changes in the levels of CEA during the clinical course (r = 0.58; P < 0.05).