Cyclic ADP-ribose-induced Ca2+ release from rat brain microsomes

Synthesis and biological evaluation of novel photo-clickable adenosine and cyclic ADP-ribose analogs: 8-N3-2'-O-propargyladenosine and 8-N3-2'-O-propargyl-cADPR

A photo-clickable analog of adenosine was devised and synthesized in which the photoactive functional group (8-azidoadenosine) and the click moiety (2'-O-propargyl-ether) were compactly combined within the structure of the adenosine nucleoside itself. We synthesized 8-N3-2'-O-propargyl adenosine in four steps starting from adenosine. This photo-clickable adenosine was 5'-phosphorylated and coupled to nicotinamide mononucleotide to form the NAD analog 8-N3-2'-O-propargyl-NAD. This NAD analog was recognized by Aplysia californica ADP-ribosyl cyclase and enzymatically cyclized producing 8-N3-2'-O-propargyl cyclic ADP-ribose. Photo-clickable cyclic-ADP-ribose analog was envisioned as a probe to label cyclic ADP-ribose binding proteins. The monofunctional 8-N3-cADPR has previously been shown to be an antagonist of cADPR-induced calcium release [T.F. Walseth et. al., J. Biol. Chem (1993) 268, 26686-26691]. 2'-O-propargyl-cADPR was recognized as an agonist which elicited Ca2+ release when added at low concentration to sea urchin egg homogenates. The bifunctional 8-N3-2'-O-propargyl cyclic ADP-ribose did not elicit Ca2+ release at low concentration or impact cyclic ADP-ribose mediated Ca2+ release either when added to sea urchin egg homogenates or when microinjected into cultured human U2OS cells. The photo-clickable adenosine will none-the-less be a useful scaffold for synthesizing photo-clickable probes for identifying proteins that interact with a variety of adenosine nucleotides.

From insulin synthesis to secretion: Alternative splicing of type 2 ryanodine receptor gene is essential for insulin secretion in pancreatic β cells

Increases in the intracellular Ca²⁺ concentration in pancreatic islets, resulting from the Ca²⁺ mobilization from the intracellular source through the ryanodine receptor, are essential for insulin secretion by glucose. Cyclic ADP-ribose, a potent Ca²⁺ mobilizing second messenger synthesized from NAD⁺ by CD38, regulates the opening of ryanodine receptor. A novel ryanodine receptor mRNA (the islet-type ryanodine receptor) was found to be generated from the type 2 ryanodine receptor gene by the alternative splicing of exons 4 and 75. The islet-type ryanodine receptor mRNA is expressed in a variety of tissues such as pancreatic islets, cerebrum, cerebellum, and other neuro-endocrine cells, whereas the authentic type 2 ryanodine receptor mRNA (the heart-type ryanodine receptor) was found to be generated using GG/AG splicing of intron 75 and is expressed in the heart and the blood vessel. The islet-type ryanodine receptor caused a greater increase in the Ca²⁺ release by caffeine when expressed in HEK293 cells pre-treated with cyclic ADP-ribose, suggesting that the novel ryanodine receptor is an intracellular target for the CD38-cyclic ADP-ribose signal system in mammalian cells and that the tissue-specific alternative splicing of type 2 ryanodine receptor mRNA plays an important role in the functioning of the cyclic ADP-ribose-sensitive Ca²⁺ release.

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.

Effects of FK506 on ca release channels (review)

Tacrolimus (FK506), which was isolated from the fermentation broth of Streptomyces tsukubaensis No. 9993, has an immunosuppressive effect. In T-lymphocytes, FK506 binds to the intracellular receptor, a 12-kDa FK506-binding protein (FKBP12). The FK506-FKBP12 complex binds to the phosphatase calcineurin (CN) and inhibits the activity of CN. By inhibition of the activity of CN, dephosphorylation of a nuclear factor of activated T-cells (NFAT) is inhibited, and translocation of the NFAT to the nucleus is suppressed. Thereby, the production of T-cell-derived mediators such as interleukin 2 (IL-2) is inhibited, and the proliferation of cytotoxic T-cells is suppressed. In muscle cells, FKBP12 and FKBP12.6 are associated with ryanodine-sensitive Ca²⁺ release channels (ryanodine receptors: RyRs) on the skeletal and cardiac muscle sarcoplasmic reticulum (SR), respectively. FK506 modulates the RyR by dissociating FKBP12 or FKBP12.6 from the RyR complex. FKBP12 is also associated with inositol 1,4,5-trisphosphate (IP₃)-sensitive Ca²⁺ release channels (IP₃ receptors: IP₃Rs) on the endoplasmic reticulum (ER) of non-muscle cells. The IP₃R-FKBP12 complex binds to CN, which dephosphorylates the protein kinase C (PKC) phosphorylation site on the receptor. When FKBP12 is dissociated from the IP₃R complex by FK506, CN is also dissociated from the IP₃R. Thereby, the IP₃R is phosphorylated by PKC, and the receptor is modulated. Recently, it was found that FK506 itself induces Ca²⁺ release through RyRs in some tissues.

Mechanism of nitric oxide action on inhibitory GABAergic signaling within the nucleus tractus solitarii

The cellular mechanisms mediating nitric oxide (NO) modulation of the inhibitory transmission in the nucleus tractus solitarii (NTS) remain unclear, even though this could be extremely important for various physiological and pathological processes. Specifically, in the NTS NO-evoked glutamate and gamma-aminobutyric acid (GABA) release might contribute to pathological hypertension. In cultured rat brainstem slices, NTS GABAergic neurons were targeted using an adenoviral vector to express enhanced green fluorescent protein and studied with a combination of patch clamp and confocal microscopy. Low nanomolar concentrations of NO increased intracellular Ca2+ concentration ([Ca2+]i) in somata, dendrites, and putative axons of GABAergic neurons, with axons being the most sensitive compartment. This effect was cGMP mediated and not related to depolarization or indirect presynaptic effects on glutamatergic transmission. Blockade of the cyclic adenosine diphosphate ribose (cADPR)/ryanodine-sensitive stores but not the inositol triphosphate-sensitive stores, inhibited NO effect. Since cADPR/ryanodine-sensitive stores are implicated in the Ca2+-induced Ca2+ release, NO can be expected to potentiate GABA release. In support of this notion, a cADPR antagonist abolished the NO-induced potentiation of GABAergic inhibitory postsynaptic potentials in the NTS. Thus, the NO-cGMP-cADPR-Ca2+ pathway, previously described in sea urchin eggs, also operates in mammalian GABAergic neurons. Potentiation of GABA release by NO may have implications for numerous brain functions.

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.

Elucidation of the ryanodine-sensitive Ca2+ release mechanism of rat pancreatic acinar cells: modulation by cyclic ADP-ribose and FK506

The effects of cyclic ADP-ribose (cADPR) and the immunosuppressant drug FK506 on microsomal Ca2+ release through a ryanodine-sensitive mechanism were investigated in rat pancreatic acinar cells. After a steady state of 45Ca2+ uptake into the microsomal vesicles, ryanodine or caffeine was added. Preincubation of the vesicles with cADPR (0.5 microM) shifted the dose-response curve of ryanodine- or caffeine-induced 45Ca2+ release from the vesicles to the left. Preincubation with cADPR shifted the dose-response curve of the FK506-induced 45Ca2+ release upward. Preincubation with FK506 (3 microM) shifted the dose-response curve of the ryanodine- or caffeine-induced 45Ca2+ release to the left by the same extent as that in the case of cADPR. FK506 shifted the dose-response curve of the cADPR-induced 45Ca2+ release upward. The presence of both cADPR and FK506 enhanced the ryanodine (30 microM)- or caffeine (10 mM)-induced 45Ca2+ release by the same extent as that in the case of cADPR alone or FK506 alone. These results indicate that cADPR and FK506 modulate the ryanodine-sensitive Ca2+ release mechanism of rat pancreatic acinar cells by increasing the ryanodine or caffeine sensitivity to the mechanism. In addition, there is a possibility that the mechanisms of modulation by cADPR and FK506 are the same.

Calmodulin dissociation mediates desensitization of the cADPR-induced Ca2+ release mechanism

Ryanodine receptor (RyR) activation by cyclic ADP-ribose (cADPR) is followed by homologous desensitization. Though poorly understood, this "switching off" process has provided a key experimental tool for determining the pathway through which cADPR mediates Ca(2+) release. Moreover, desensitization is likely to play an important role in shaping the complexities of Ca(2+) signaling involving cADPR, for example, localized release events and propagated waves. Using the sea urchin egg, we unmask a role of calmodulin, a component of the RyR complex and a key cofactor for cADPR activity, during RyR/cADPR desensitization. Recovery from desensitization in calmodulin-depleted purified endoplasmic reticulum (microsomes) is severely impaired compared to that in crude egg homogenates. An active, soluble factor, identified as calmodulin, is required to restore the capacity of microsomes to recover from desensitization. Calmodulin mediates recovery in a manner that tightly parallels its time course of association with the RyR. Conversely, direct measurement of calmodulin binding to microsomes reveals a loss of specific binding during cADPR, but not IP(3), desensitization. Our results support a mechanism in which cycles of calmodulin dissociation and reassociation to an endoplasmic reticulum protein, most likely the RyR itself, mediate RyR/cADPR desensitization and resensitization, respectively.

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.

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.

Role of ADP-ribose in 11,12-EET-induced activation of K(Ca) channels in coronary arterial smooth muscle cells

We recently reported that cADP-ribose (cADPR) and ADP-ribose (ADPR) play an important role in the regulation of the Ca(2+)-activated K(+) (K(Ca)) channel activity in coronary arterial smooth muscle cells (CASMCs). The present study determined whether these novel signaling nucleotides participate in 11,12-epoxyeicosatrienoic acid (11,12-EET)-induced activation of the K(Ca) channels in CASMCs. HPLC analysis has shown that 11,12-EET increased the production of ADPR but not the formation of cADPR. The increase in ADPR production was due to activation of NAD glycohydrolase as measured by a conversion rate of NAD into ADPR. The maximal conversion rate of NAD into ADPR in coronary homogenate was increased from 2.5 +/- 0.2 to 3.4 +/- 0.3 nmol*(-1) *mg protein(-1) by 11,12-EET. The regioisomers of 8,9-EET, 11,12-EET, and 14,15-EET also significantly increased ADPR production from NAD. Western blot analysis and immunoprecipitation demonstrated the presence of NAD glycohydrolase, which mediated 11,12-EET-activated production of ADPR. In cell-attached patches, 11,12-EET (100 nM) increases K(Ca) channel activity by 5.6-fold. The NAD glycohydrolase inhibitor cibacron blue 3GA (3GA, 100 microM) significantly attenuated 11,12-EET-induced increase in the K(Ca) channel activity in CASMCs. However, 3GA had no effect on the K(Ca) channels activity in inside-out patches. 11,12-EET produced a concentration-dependent relaxation of precontracted coronary arteries. This 11,12-EET-induced vasodilation was substantially attenuated by 3GA (30 microM) with maximal inhibition of 57%. These results indicate that 11,12-EET stimulates the production of ADPR and that intracellular ADPR is an important signaling molecule mediating 11,12-EET-induced activation of the K(Ca) channels in CASMCs and consequently results in vasodilation of coronary artery.

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.

cADP-ribose activates reconstituted ryanodine receptors from coronary arterial smooth muscle

The present study was designed to test the hypothesis that cADP-ribose (cADPR) increases Ca(2+) release through activation of ryanodine receptors (RYR) on the sarcoplasmic reticulum (SR) in coronary arterial smooth muscle cells (CASMCs). We reconstituted RYR from the SR of CASMCs into planar lipid bilayers and examined the effect of cADPR on the activity of these Ca(2+) release channels. In a symmetrical cesium methanesulfonate configuration, a 245 pS Cs(+) current was recorded. This current was characterized by the formation of a subconductance and increase in the open probability (NP(o)) of the channels in the presence of ryanodine (0.01-1 microM) and imperatoxin A (100 nM). A high concentration of ryanodine (50 microM) and ruthenium red (40-80 microM) substantially inhibited the activity of RYR/Ca(2+) release channels. Caffeine (0.5-5 mM) markedly increased the NP(o) of these Ca(2+) release channels of the SR, but D-myo-inositol 1,4,5-trisphospate and heparin were without effect. Cyclic ADPR significantly increased the NP(o) of these Ca(2+) release channels of SR in a concentration-dependent manner. Addition of cADPR (0.01 microM) into the cis bath solution produced a 2.9-fold increase in the NP(o) of these RYR/Ca(2+) release channels. An eightfold increase in the NP(o) of the RYR/Ca(2+) release channels (0.0056 +/- 0.001 vs. 0.048 +/- 0.017) was observed at a concentration of cADPR of 1 microM. The effect of cADPR was completely abolished by ryanodine (50 microM). In the presence of cADPR, Ca(2+)-induced activation of these channels was markedly enhanced. These results provide evidence that cADPR activates RYR/Ca(2+) release channels on the SR of CASMCs. It is concluded that cADPR stimulates Ca(2+) release through the activation of RYRs on the SR of these smooth mucle cells.

Adenosine diphosphate ribose dilates bovine coronary small arteries through apyrase- and 5'-nucleotidase-mediated metabolism

Cyclic adenosine diphosphate ribose and adenosine diphosphate ribose (ADPR) play an important role in the regulation of intracellular Ca(2+) release and K(+) channel activity in the coronary arterial smooth muscle. The role of these signaling nucleotides in the control of vascular tone has yet to be determined. The present study was designed to determine whether ADPR produces vasodilation in coronary arteries and to explore the mechanism of action of ADPR. ADPR (10-60 micromol/l) was found to produce endothelium-independent relaxation in a concentration-dependent manner in isolated and pressurized small bovine coronary arteries. The ADPR-induced vasodilation was substantially attenuated by adenosine deaminase (0.2 U/ml), and the P(1) purinoceptor antagonist 8-(p-sulfophenyl)theophylline (50 micromol/l), with maximal inhibitions of 60 and 80%, respectively. When the coronary arterial homogenates were incubated with ADPR, the production of adenosine and 5'-AMP was detected. The adenosine production was blocked by the 5'-nucleotidase inhibitor, alpha,beta-methylene adenosine 5'-diphosphate (MADP, 1 mmol/l), which was accompanied by a corresponding accumulation of 5'-AMP. This 5'-AMP accumulation was substantially inhibited by the apyrase inhibitor sodium azide (10 mmol/l). Moreover, ADPR was hydrolyzed into 5'-AMP by purified apyrase. In agreement with their inhibitory effect on the adenosine production, MADP and sodium azide significantly attenuated the vasodilator response to ADPR. The metabolism of ADPR to adenosine was only detected in cultured coronary arterial smooth muscle cells but not in endothelial cells. We concluded that ADPR produces vasodilation in small coronary arteries and that the action of ADPR is associated with the adenosine production via an apyrase- and 5'-nucleotidase-mediated metabolism.

Subcellular localization of cyclic ADP-ribosyl cyclase and cyclic ADP-ribose hydrolase activities in porcine airway smooth muscle

Recent studies have provided evidence for a role of cyclic ADP-ribose (cADPR) in the regulation of intracellular calcium in smooth muscles of the intestine, blood vessels and airways. We investigated the presence and subcellular localization of ADP-ribosyl cyclase, the enzyme that catalyzes the conversion of beta-NAD(+) to cADPR, and cADPR hydrolase, the enzyme that degrades cADPR to ADPR, in tracheal smooth muscle (TSM). Sucrose density fractionation of TSM crude membranes provided evidence that ADP-ribosyl cyclase and cADPR hydrolase activities were associated with a fraction enriched in 5'-nucleotidase activity, a plasma membrane marker enzyme, but not in a fraction enriched in either sarcoplasmic endoplasmic reticulum calcium ATPase or ryanodine receptor channels, both sarcoplasmic reticulum markers. The ADP-ribosyl cyclase and cADPR hydrolase activities comigrated at a molecular weight of approximately 40 kDa on SDS-PAGE. This comigration was confirmed by gel filtration chromatography. Investigation of kinetics yielded K(m) values of 30.4+/-1.5 and 695. 3+/-171.2 microM and V(max) values of 330.4+/-90 and 102.8+/-17.1 nmol/mg/h for ADP-ribosyl cyclase and cADPR hydrolase, respectively. These results suggest a possible role for cADPR as an endogenous modulator of [Ca(2+)](i) in porcine TSM cells.

Formation and actions of cyclic ADP-ribose in renal microvessels

Recent studies indicated that cyclic ADP-ribose (cADPR) serves as a second messenger for intracellular Ca(2+) mobilization in a variety of mammalian cells. However, the metabolism and actions of cADPR in the renal vasculature are poorly understood. In the present study, we characterized the enzymatic pathway of the production and metabolism of cADPR along the renal vascular tree and determined the role of cADPR in the control of intracellular [Ca(2+)] and vascular tone. The high performance liquid chromatographic analyses showed that cADPR was produced and hydrolyzed along the renal vasculature. The maximal conversion rate of nicotinamide guanine dinucleotide (NGD) into cyclic GDP-ribose (that represents ADP-ribosyl cyclase activity for cADPR formation) was 8.69 +/- 2.39 nmol/min/mg protein in bulk-dissected intrarenal preglomerular vessels (n = 7) and 4.35 +/- 0.13, 2.23 +/- 0.27, 2.40 +/- 0.19, and 0.31 +/- 0.02 nmol/min/mg protein, respectively, in microdissected arcuate arteries (n = 6), interlobular arteries (n = 6), afferent arterioles (n = 7), and vasa recta (n = 10). The activity of cADPR hydrolase was also detected in the renal vasculature. Using the fluorescence microscopic spectrometry, cADPR was found to produce a large rapid Ca(2+) release from beta-escin-permeabilized renal arterial smooth muscle cells (SMCs). In isolated, perfused, and pressurized small renal arteries, cADPR produced a concentration-dependent vasoconstriction when added into the bath solution. The vasoconstrictor effect of cADPR was completely blocked by tetracaine, a Ca(2+)-induced Ca(2+) release (CICR) inhibitor. These results suggest that an enzymatic pathway for cADPR production and metabolism is present along the renal vasculature and that cADPR may importantly contribute to the control of renal vascular tone through CICR.

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.

ADP-Ribosyl cyclase in rat salivary glands

Both the Ca(2+)-releasing mechanism induced by cyclic ADP-ribose (cADPR) and the ADP-ribosyl cyclase (ADPRC) activity that converts NAD(+) to cADPR were observed in a variety of cell types. We studied the ADPRC activity in rat major salivary glands that include parotid gland (PG), submandiblar gland (SMG), and sublingual gland (SLG). The enzyme activity responsible for cADPR synthesis was detected by spectrofluorometric assay using NGD(+) as a substrate. The enzyme activities in SLG, SMG, and PG were about 400, 30, and 40 nmol/min/g tissue, respectively, in 5-week-old rats. The highest value was observed in SLG and this value was higher than those in other tissues; e.g., spleen (200 nmol/min/g tissue). The enzyme activity in SLG increased gradually after birth and showed a maximum value at 3 weeks. On the other hand, the enzyme activities almost did not change in both PG and SMG between 0 and 9 weeks. In spite of the high ADPRC activity in SLG, we could not detect the cADPR-induced Ca(2+)-release from SLG microsomes. These results suggest that the ADPRC in SLG does not function through Ca(2+)-release observed in various tissues.

Metabotropic glutamate receptor activation and intracellular cyclic ADP-ribose release Ca2+ from the same store in cultured DRG neurones

The whole cell patch clamp technique has been used to record Ca(2+)-activated cation and chloride conductances evoked by release of Ca2+ from intracellular stores of cultured neonatal dorsal root ganglion neurones. The aim of this study was to investigate metabotropic glutamate receptor (mGluR) mechanisms and evaluate a possible role for cyclic ADP-ribose as an intracellular signalling molecule. Glutamate and the metabotropic glutamate receptor agonist (1S, 3R)-ACPD-evoked transient depolarizations, Ca(2+)-activated inward currents and rises in intracellular Ca2+. The (1S, 3R)-ACPD-activated currents were insensitive to InsP3 signalling inhibitors, heparin and pentosan polysulphate. Intracellular application of ryanodine alone activated currents in this study and proved a difficult tool to use as a potential inhibitor of cyclic ADP-ribose-mediated responses. However, intracellular dantrolene did attenuate both (1S, 3R)-ACPD and cyclic ADP-ribose responses. Intracellular photo-release of cGMP and cyclic ADP-ribose mimicked the responses to mGluR receptor activation. Intracellular application of nicotinamide and W7 inhibited the responses to photo-released cGMP but did not prevent responses to mGluR activation. The cyclic ADP-ribose receptor antagonist 8-amino cyclic ADP-ribose attenuated responses to (1S, 3R)-ACPD, cGMP and cyclic ADP-ribose, but some Ca(2+)-activated inward currents were still observed in the presence of this antagonist. In conclusion, mGluR receptor activation, cGMP and cyclic ADP-ribose release Ca2+ from intracellular stores. Some evidence suggests that pharmacologically related pathways are involved.

Nicotinic acid adenine dinucleotide phosphate triggers Ca2+ release from brain microsomes

Mobilization of Ca2+ from intracellular stores is an important mechanism for generating cytoplasmic Ca2+ signals [1]. Two families of intracellular Ca(2+)-release channels - the inositol-1,4, 5-trisphosphate (IP3) receptors and the ryanodine receptors (RyRs) - have been described in mammalian tissues [2]. Recently, nicotinic acid adenine dinucleotide phosphate (NAADP), a molecule derived from NADP+, has been shown to trigger Ca2+ release from intracellular stores in invertebrate eggs [3] [4] [5] [6] and pancreatic acinar cells [7]. The nature of NAADP-induced Ca2+ release is unknown but it is clearly distinct from the IP3- and cyclic ADP ribose (cADPR)-sensitive mechanisms in eggs (reviewed in [8] [9]). Furthermore, mammalian cells can synthesize and degrade NAADP, suggesting that NAADP-induced Ca2+ release may be widespread and thus contribute to the complexity of Ca2+ signalling [10] [11]. Here, we show for the first time that NAADP evokes Ca2+ release from rat brain microsomes by a mechanism that is distinct from those sensitive to IP3 or cADPR, and has a remarkably similar pharmacology to the action of NAADP in sea urchin eggs [12]. Membranes prepared from the same rat brain tissues are able to support the synthesis and degradation of NAADP. We therefore suggest that NAADP-mediated Ca2+ signalling could play an important role in neuronal Ca2+ signalling.

Cyclic ADP-ribose-dependent Ca2+ release is modulated by free [Ca2+] in the scallop sarcoplasmic reticulum

Cyclic ADP-ribose (cADPR) elicits calcium-induced calcium release (CICR) in a variety of cell types. We studied the effect of cADPR on Ca2+ release in muscle cells by incubating SR vesicles from scallop (Pecten jacobaeus) adductor muscle in the presence of the Ca2+ tracer fluo-3. Exposure of SR to cADPR (20 microM) produced Ca2+ release, which was a function of free [Ca2+] in a range between about 150 and 1000 nM, indicating an involvement of ryanodine-sensitive Ca2+ channels. This Ca2+ release was not significantly enhanced by calmodulin (7 micrograms/ml), but it was enhanced by equimolar addition of noncyclic ADPR. Also, the Ca2+ release elicited by cADPR/ADPR was a function of free [Ca2+] in a range between about 150 and 3000 nM, over which Ca2+ was inhibitory. cADPR self-inactivation was observed at low free [Ca2+] (about 150 nM), but it tended to disappear upon [Ca2+] elevation (about 250 nM). Caffeine or ryanodine induced a Ca2+ release which was ruthenium red (2.5 microM) sensitive at low [Ca2+]. However, the Ca2+ release induced by either ryanodine or cADPR was no longer ruthenium red sensitive when free [Ca2+] was increased. Based on these data, a model is proposed for Ca2+ signaling in muscle cells, where a steady-state cADPR level would trigger Ca2+ release when free [Ca2+] does reach a threshold slightly above its resting level, hence producing cascade RyR recruitment along SR cisternae from initial Ca2+ signaling sites.

Role of the ryanodine receptor in ischemic brain damage--localized reduction of ryanodine receptor binding during ischemia in hippocampus CA1

1. The ryanodine receptor has recently been shown to play a pivotal role in the regulation of intracellular Ca2+ concentration via Ca(2+)-induced Ca2+ release (CICR). Effects of ischemia on CICR in the brain tissue, however, remain largely unknown since only a few reports have been published on this subject. In this paper we report on work in this area by our group and review related progress in this field. 2. We examined alterations of ryanodine receptor binding and local cerebral blood flow (LCBF) at 15 min, 30 min, and 2 hr after occlusion of the right common carotid artery in the gerbil brain. A quantitative autoradiographic method permitted simultaneous measurement of these parameters in the same brain. The LCBF was significantly reduced in most of the cerebral regions on the occluded side during each time period of ischemia. In contrast, only in the hippocampus CA1 on the occluded side was a significant reduction in ryanodine binding found at 15 min, 30 min and 2 hr after the occlusion. 3. These findings suggest that suppression of ryanodine binding in the hippocampus CA1 may be attributable to a regionally specific perturbation of CICR and that this perturbation may be closely associated with the pathophysiological mechanism that leads to be selective ischemic vulnerability of this region. 4. Other recent studies have also reported an important role for ryanodine receptors in neuronal injury such as the delayed neuronal death in the hippocampus CA1. These data suggest that derangement of CICR is likely to be involved in acute neuronal necrosis as well as in delayed neuronal death in ischemia. 5. Further studies on clarifying the role of CICR in ischemic brain damage are needed in order to develop new therapeutic strategies for stroke patients.

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.

Neuronal calcium stores

Neuronal calcium stores associated with specialized intracellular organelles, such as endoplasmic reticulum and mitochondria, dynamically participate in generation of cytoplasmic calcium signals which accompany neuronal activity. They fulfil a dual role in neuronal Ca2+ homeostasis being involved in both buffering the excess of Ca2+ entering the cytoplasm through plasmalemmal channels and providing an intracellular source for Ca2+. Increase of Ca2+ content within the stores regulates the availability and magnitude of intracellular calcium release, thereby providing a mechanism which couples the neuronal activity with functional state of intracellular Ca2+ stores. Apart of 'classical' calcium stores (endoplasmic reticulum and mitochondria) other organelles (e.g. nuclear envelope and neurotransmitter vesicles) may potentially act as a functional Ca2+ storage compartments. Calcium ions released from internal stores participate in many neuronal functions, and might be primarily involved in regulation of various aspects of neuronal plasticity.

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.

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.

Muscarinic receptor-mediated dual regulation of ADP-ribosyl cyclase in NG108-15 neuronal cell membranes

Cyclic ADP-ribose (cADP-ribose) is an endogenous modulator of ryanodine-sensitive Ca2+ release channels. An unsolved question is whether or not cADP-ribose mediates intracellular signals from hormone or neurotransmitter receptors. The first step in this study was to develop a TLC method to measure ADP-ribosyl cyclase, by which conversion of [3H]NAD+ to [3H]cADP-ribose was confirmed in COS-7 cells overexpressing human CD38. A membrane fraction of NG108-15 neuroblastoma x glioma hybrid cells possessed ADP-ribosyl cyclase activity measured by TLC. Carbamylcholine increased this activity by 2.6-fold in NG108-15 cells overexpressing m1 or m3 muscarinic acetylcholine receptors (mAChRs), but inhibited it by 30-52% in cells expressing m2 and/or m4 mAChRs. Both of these effects were mimicked by GTP. Pretreatment of cells with cholera toxin blocked the activation, whereas pertussis toxin blocked the inhibition. Application of carbamylcholine caused significant decreases in NAD+ concentrations in untreated m1-transformed NG108-15 cells, but an increase in cholera toxin-treated cells. These results suggest that mAChRs couple to ADP-ribosyl cyclase within cell membranes via trimeric G proteins and can thereby control cellular function by regulating cADP-ribose formation.

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.

Ectocellular CD38-catalyzed synthesis and intracellular Ca(2+)-mobilizing activity of cyclic ADP-ribose

CD38 is a type-II transmembrane glycoprotein occurring in several hematopoietic and mature blood cells as well as in other cell types, including neurons. Although classified as an orphan receptor, CD38 is also a bifunctional ectoenzyme that catalyzes both the conversion of NAD+ to nicotinamide and cyclic ADP-ribose (cADPR), via an ADP-ribosyl cyclase reaction, and also the hydrolysis of cADPR to ADP-ribose (hydrolase). Major unresolved questions concern the correlation between receptor and catalytic properties of CD38, and also the apparent contradiction between ectocellular generation and intracellular Ca(2+)-mobilizing activity of cADPR. Results are presented that provide some explanations to this topological paradox in two different cell types. In cultured rat cerebellar granule neurons, extracellular cADPR (either generated by CD38 or directly added) elicited an enhanced intracellular Ca(2+)-response to KCl-induced depolarization, a process that can be qualified as a Ca(2+)-induced Ca2+ release (CICR) mechanism. On the other hand, in the CD38+ human Namalwa B lymphoid cells, NAD+ (and thiol compounds as well) induced a two-step process of self-aggregation followed by endocytosis of CD38, which resulted in a shift of cADPR metabolism from the cell surface to the cytosol. Both distinctive types of cellular responses to extracellular NAD+ seem to be suitable to elicit changes in the intracellular Ca2+ homeostasis.

Inositol 1,4,5-trisphosphate and cyclic ADP-ribose mobilize Ca2+ in a protist, Euglena gracilis

Inositol 1,4,5-trisphosphate (InsP3) and cyclic ADP-ribose (cADPR) released Ca2+ from microsome fraction prepared from Euglena gracilis in dose-dependent manners. Caffeine, which also induced Ca2+ release from the microsomes, caused desensitization of the Ca2+ response to cADPR, although the Ca2+ response to InsP3 was not affected by caffeine. Further, ruthenium red inhibited the Ca2+ release induced by cADPR, but not by InsP3. These results suggest that cADPR functions as an endogenous messenger to activate a caffeine-sensitive, Ca(2+)-release mechanism, whereas InsP3 induces Ca2+ release by a distinct mechanism in E. gracilis.

Ryanodine receptor expression is associated with intracellular Ca2+ release in rat parotid acinar cells

The ryanodine receptor mediates intracellular Ca2+ mobilization in muscle and nerve, but its physiological role in nonexcitable cells is less well defined. Like adenosine 3',5'-cyclic monophosphate and inositol 1,4,5-trisphosphate, cyclic ADP-ribose (0.3-5 microM) and ADP (1-25 microM) produced a concentration-dependent rise in cytosolic Ca2+ in permeabilized rat parotid acinar cells. Adenosine and AMP were less effective. Ryanodine markedly depressed the Ca2+-mobilizing action of the adenine nucleotides and forskolin in permeabilized cells and was likewise effective in depressing the action of forskolin in intact cells. Cyclic ADP-ribose-evoked Ca2+ release was enhanced by calmodulin and depressed by W-7, a calmodulin inhibitor. A fluorescently labeled ligand, 4,4-difluoro-1,3,5,7-tetramethyl-4-bora-3,4-diaza-s-indac ene-3-propionic acid-glycyl ryanodine, was synthesized to detect the expression and distribution of ryanodine receptors. In addition, ryanodine receptor expression was detected in rat parotid cells with a sequence highly homologous to a rat skeletal muscle type 1 and a novel brain type 1 ryanodine receptor. These findings demonstrate the presence of a ryanodine-sensitive intracellular Ca2+ store in rat parotid cells that shares many of the characteristics of stores in muscle and nerve and may mediate Ca2+-induced Ca2+ release or a modified form of this process.

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.

Ortho-substituted polychlorinated biphenyls alter microsomal calcium transport by direct interaction with ryanodine receptors of mammalian brain

A stringent structure-activity relationship among polychlorinated biphenyls (PCBs) possessing two or more ortho-chlorine substituents is observed for activation of ryanodine receptors in mammalian brain, revealing an arylhydrocarbon receptor-independent mechanism through which non-coplanar PCBs disrupt neuronal Ca2+ signaling. Of the congeners assayed, non-coplanar PCB 95 exhibits the highest potency (EC50 = 12-24 microM) toward activating high affinity [3H]ryanodine-binding in rat hippocampus, cerebellum, and cerebral cortex. Coplanar PCB 66 and PCB 126 have no ryanodine receptor activity in all brain regions examined. PCB 95 enhances [3H]ryanodine-binding affinity and capacity by significantly altering modulation by Ca2+ and Mg2+, thereby stabilizing a high affinity conformation of the ryanodine receptor. Ca2+ transport measurements using cortical microsomes reveal that PCB 95 discriminates between inositol 1,4,5-trisphosphate- and ryanodine-sensitive stores. PCB 95 selectively mobilizes Ca2+ from ryanodine-sensitive stores in a dose-dependent manner (EC50 = 3.5 microM) and is completely inhibited by ryanodine receptor blockers, whereas coplanar PCBs are inactive. These data demonstrate that ortho-substituted PCBs disrupt Ca2+ transport in central neurons by direct interaction with ryanodine receptors, showing high selectivity and specificity. Alteration of Ca2+ signaling mediated by ryanodine receptors in specific regions of the central nervous system may account, at least in part, for the significant impact of these agents toward neurodevelopment and neuroplasticity in mammals.

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.

Transient synaptic potentiation in the visual cortex. II. Developmental regulation

In our previous study, pairing-induced transient synaptic potentiation in supragranular layers of the visual cortex was described in mature guinea pigs. In the present study, the development of this type of synaptic plasticity and the underlying cellular mechanisms that mediate it were evaluated in animals from postnatal day (PND) 5 to 180. Potentiation is more reliably evoked in younger animals (likelihood: 75%, PND 5-30; 51%, PND > or = 34), and the magnitude of the effect is greater (+40 +/- 3%, mean +/- SE, PND 5-30; +26 +/- 3%, PND > or = 34). Similar to data obtained from the mature animals, visual cortical transient synaptic potentiation in the immature cortex occurs at excitatory synaptic sites directly activated by the stimulation, and activation by local recurrent cortical circuits is not necessary for the induction of this potentiation. This is demonstrated by 1) experiments in which action potential output from the paired neuron was blocked by Lidocaine, N-ethyl bromide quaternary salt applied into the neuron (5 of 5), and 2) experiments in which the contribution to the compound postsynaptic potential by inhibitory synapses was eliminated by selective, intracellular blockade by gamma-aminobutyric acid-mediated inhibitory postsynaptic potentials only onto the recorded neuron (7 of 11). Thus these perturbations do not reduce the likelihood or magnitude of this synaptic potentiation. In contrast to the N-methyl-D-aspartate (NMDA) receptor dependence for induction of this synaptic potentiation in the cortex of mature animals, in the young animals' cortices (PND 11-27) potentiation is readily induced during blockade of NMDA receptors (72%, 13 of 18, did not different from control: 75%, 40 of 53). Thus the NMDA receptor becomes functionally linked to a synaptic potentiation cascade during development, replacing another 2-amino-5-phosphonovaleric acid (APV)-insensitive potentiation process in the neonatal cortex. Postsynaptic intracellular calcium has a critical role in the induction of this form of synaptic potentiation in all ages studied. Synaptic potentiation was prevented (8 of 11 cases) or was replaced by synaptic depression (3 of 11 cells) in experiments in which postsynaptic calcium levels were reduced by intracellular application of 1,2-bis-2-aminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA) in the cortex of young (PND 7-14) animals, or in which the extracellular calcium concentrations was lowered. Inhibition of postsynaptic calcium-induced calcium release blocked synaptic potentiation (4 of 4 cells). Prolonged superfusion (3 h) of the nitric oxide synthase inhibitor L-nitro-arginine (LNA) did not significantly affect the likelihood (in LNA, 81%; 13 of 16 cells), or the magnitude (+38 +/- 7% increase in LNA vs. +40 +/- 3% in control cases) of potentiation, in contrast to its effects in the mature cortex.

Ca2+ stores in the chick embryo retina cells

The Ca2+ stores of digitonin permeabilized chick embryo retina cells in culture were characterized, by using the fluorescence of Fluo-3 potassium salt to follow continuously the free [Ca2+] in the medium. After ATP dependent Ca2+ accumulation, the Ca2+ release was induced by several agents; 10 microM cyclic-ADP-ribose (cADPR), 40 microM Ins (1,4,5)P3 10 microM thapsigargin (Th), 25 microM ionomycin (Ion), 15 microM CCCP together with 4.5 micrograms/ml oligomycin (CCCP/Olig), 50 microM arachidonic acid (AA). Neither Ins(1,4,5)P3 nor cADPR were able to mobilize Ca2+ from internal stores in these cells, but Th and AA were effective in releasing Ca2+. Four major Ca2+ stores in chick embryo retina cells were distinguished: i) the thapsigargin sensitive Ca2+ store, most likely the ER; ii) the Ca2+ store sensitive to oligomycin and CCCP, most likely the mitochondrial Ca2+ store, iii) an AA sensitive Ca2+ store, which is distinct from the previous two; and, iv) the Ca2+ store only sensitive to ionomycin. The capacities of these different Ca2+ stores of the chick embryo retina cells, relative to the total intracellular stores, are: 63.3%, 14.1%, 8.2%, for the ER, the mitochondrial and for the AA sensitive Ca2+ stores, respectively.

Extracellular synthesis of cADP-ribose from nicotinamide-adenine dinucleotide by rat cortical astrocytes in culture

cADPR is an endogenous calcium-mobilizing agent that in vertebrates is synthesized from nicotinamide-adenine dinucleotide (NAD) by bifunctional enzymes with ADP-ribosyl cyclase and cADPR hydrolase activity. ADP-ribosyl cyclase and cADPR hydrolase activity have been reported in the brain, but the cellular localization of these activities has not been determined previously. In the present study, selective culturing techniques were employed to localize ADP-ribosyl cyclase activity and cADPR hydrolase activity to astrocytes or neurons in cultures derived from rat embryonic cerebral cortex. ADP-ribosyl cyclase activity was determined by incubating cultures with 1 mM NAD in the extracellular medium for 60 min at 37 degrees C and measuring formation of cADPR by bioassay and by HPLC. Astrocyte cultures and mixed cultures of astrocytes and neurons had mean specific activities of 0.84 +/- 0.06 and 0.9 +/- 0.18 nmol cADPR produced/mg protein/hr, respectively. No detectable ADP-ribosyl cyclase activity was found in neuron-enriched/ astrocyte-poor cultures. cADPR hydrolase activity was detectable by incubating cultures with 300 microM cADPR for 60 min at 37 degrees C and assaying loss of cADPR or accumulation of ADPR. The demonstration of extracellular ADP-ribosyl cyclase and cADPR hydrolase activities associated with astrocytes may have important implications for the role of extracellular cADPR in signal transduction and in intercellular communication in the nervous system.

Pharmacology of Ca2+ release from red beet microsomes suggests the presence of ryanodine receptor homologs in higher plants

Cyclic ADP-ribose (cADPR) is known to release Ca2+ from plant vacuoles, implying that this NAD+ metabolite may possess a second messenger role in plants. The degree to which the plant cADPR-gated Ca2+ release mechanism resembles cADPR action in animals has been evaluated. cADPR-elicited Ca2+ release from red beet microsomes was inhibited by 1 mM procaine but insensitive to heparin. Furthermore, pre-release of Ca2+ from red beet vesicles by either 5 mM caffeine or micromolar levels of ryanodine precluded further Ca2+ mobilisation by cADPR. Thus, this study argues strongly for conservation between the plant and animal cADPR-elicited Ca2+ release mechanisms.

Adenine nucleotide diphosphates: emerging second messengers acting via intracellular Ca2+ release

Release of Ca2+ from intracellular stores is a widespread mechanism in regulation of cell function. Two hitherto unknown adenine diphosphonucleotides were recently identified, which trigger Ca2+ release from intracellular stores via channels that are distinct from the well-known receptor/channel controlled by inositol 1,4,5,-trisphosphate (IP3): cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Here we review synthesis of cADPR from beta-NAD, its hydrolysis to adenosine diphosphoribose (noncyclic) by cADPR glycohydrolase, as well as our knowledge about the metabolism of NAADP. The Ca2+ release triggered by cADPR, NAADP, or IP3 can be distinguished by the action of inhibitors and by desensitization studies. Evidence now emerges that cADPR synthesis from beta-NAD can be stimulated, at least in some cell types by all-trans-retinoic acid as a first messenger. We then review the properties of cADPR and NAADP as potential second messengers in the intracrine regulation of cell functions. Although their exact role in signaling sequences is not yet known, cADPR and NAADP are likely to play important intracellular regulatory functions, as extensively documented for the process of egg fertilization.

Antipsychotic drugs block IP3-dependent Ca(2+)-release from rat brain microsomes

Cellular Ca(2+)-dysregulation has been proposed as an important mechanism in certain diseases such as bipolar affective disorder (BPAD) and malignant hyperthermia. Recently it has been found that in BPAD, the plasma membrane Ca(2+)-channel blockers verapamil and nimodipine are useful substitutes in Li(+)-treatable patients. We have investigated the effects of these drugs and the antipsychotic drugs (clozapine, fluspirilene, and haloperidol) on IP3-induced Ca(2+)-release from Ca(2+)-loaded rat brain microsomes. In the presence of either the Ca(2+)-channel blockers or the neuroleptic drugs, Ca(2+)-release was blocked in a dose-dependent fashion. For the neuroleptics, the EC50 ranged from 22 microM for fluspirilene to 145 microM for haloperidol. The EC50 for nimodipine was 160 microM and 450 microM for verapamil. Carbamazapine and valproic acid, anticonvulsants recently used for treating BPAD, were relatively ineffective, as were various haloperidol metabolites. The research described in this paper establishes for the first time that antipsychotic drugs, as well as certain Ca(2+)-channel blockers, directly block the IP3-induced Ca(2+)-release in a rat brain microsome assay.

Alteration of ryanodine receptor in the hippocampus CA1 after hemispheric cerebral ischemia

Alterations in ryanodine binding and local cerebral blood flow (LCBF) were examined at 30 minutes and 2 hours post-ischemia in the gerbil brain in order to evaluate the influence of cerebral ischemia on the intracellular channels of Ca2+-induced Ca2+ release (CICR). Severe hemispheric cerebral ischemia was induced by occluding the right common carotid artery. LCBF was measured at the end of the experiment using [14C]iodoantipyrine method, and the ryanodine binding was evaluated in vitro using [3H]ryanodine as a specific ligand for CICR channels. An autoradiographic method developed in our laboratory enabled us to determine both parameters within the same brain. A group of gerbils who underwent a sham procedure served as controls. LCBF was found to be significantly reduced in most of the cerebral regions on the occluded side at both 30 minutes as well as 2 hours post-ischemia. In contrast, a significant reduction in ryanodine binding was noted only in the hippocampus CA1 on the occluded side at 30 minutes and 2 hours after the occlusion. These findings suggest that regionally specific changes of CICR may be the cause of decreased ryanodine binding in the hippocampus CA1, and that these changes may be related to the pathophysiological mechanisms that cause this region to be particularly vulnerable to ischemia.

The type 2 ryanodine receptor of neurosecretory PC12 cells is activated by cyclic ADP-ribose. Role of the nitric oxide/cGMP pathway

Of two neurosecretory PC12 cell clones that respond to NO donors and 8-bromo-cGMP with similar increases in cADP-ribose and that possess molecularly similar Ca2+ stores, only one (clone 16A) expresses the type 2 ryanodine receptor, whereas the other (clone 27) is devoid of ryanodine receptors. In PC12-16A cells, activation of the NO/cGMP pathway induced slow [Ca2+]i responses, sustained by release from Ca2+ stores. In contrast, PC12-27 cells were insensitive to NO donors. Likewise, in PC12-16A cells preincubated with NO donors, Ca2+ stores were partially depleted, as revealed by a test with thapsigargin, whereas those in clone 27 were unchanged. The NO-induced Ca2+ release was increased synergistically by caffeine, and the corresponding store depletion was magnified by ryanodine. The specificity for the NO/cGMP pathway was confirmed by the effects of two blockers of cGMP-dependent protein kinase I, while the role of cADP-ribose was demonstrated by the effects of its antagonist, 8-amino-cADP-ribose, administered to permeabilized cells. These results demonstrate in neurosecretory cells a ryanodine receptor activation pathway similar to that known in sea urchin oocytes. The signaling events described here could be of great physiological importance, especially in the nervous system.

Cyclic ADP-ribose does not regulate sarcoplasmic reticulum Ca2+ release in intact cardiac myocytes

Cyclic ADP-ribose (cADPR), an intracellular second messenger known to mobilize Ca2+ in sea urchin eggs, has been implicated in modulating Ca2+ release in a variety of mammalian tissues. On the basis of studies of isolated cardiac sarcoplasmic reticulum (SR) vesicles and single SR Ca2+ release channels, cADPR has also been proposed to be a modulator of SR Ca2+ release in heart. In the present study, we directly examined the ability of cADPR to trigger SR Ca2+ release and to modulate Ca(2+)-induced Ca2+ release (CICR) in intact rat ventricular myocytes. Voltage-clamped myocytes were dialyzed with up to 100 mumol/L caged cADPR and 0.6 mumol/L calmodulin along with the Ca(2+)-sensitive dye fluo 3. A step increase in the cADPR concentration induced by flash photolysis of caged cADPR neither directly triggered SR Ca2+ release nor modulated CICR in intact myocytes. In contrast, under similar conditions, extracellular application of caffeine (1 to 2.5 mmol/L) onto myocytes produced both effects. Under equivalent conditions, flash photolysis of caged cADPR-loaded sea urchin eggs resulted in large Ca2+ transients. Further, the sustained presence of high cytosolic concentrations of either cADPR or its antagonist, 8-amino-cADPR, was ineffective in altering normal CICR in myocytes. These findings indicate that cADPR does not regulate SR Ca2+ release in intact cardiac myocytes.