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

Molecular bases of catalysis and ADP-ribose preference of human Mn2+-dependent ADP-ribose/CDP-alcohol diphosphatase and conversion by mutagenesis to a preferential cyclic ADP-ribose phosphohydrolase

Among metallo-dependent phosphatases, ADP-ribose/CDP-alcohol diphosphatases form a protein family (ADPRibase-Mn-like) mainly restricted, in eukaryotes, to vertebrates and plants, with preferential expression, at least in rodents, in immune cells. Rat and zebrafish ADPRibase-Mn, the only biochemically studied, are phosphohydrolases of ADP-ribose and, somewhat less efficiently, of CDP-alcohols and 2´,3´-cAMP. Furthermore, the rat but not the zebrafish enzyme displays a unique phosphohydrolytic activity on cyclic ADP-ribose. The molecular basis of such specificity is unknown. Human ADPRibase-Mn showed similar activities, including cyclic ADP-ribose phosphohydrolase, which seems thus common to mammalian ADPRibase-Mn. Substrate docking on a homology model of human ADPRibase-Mn suggested possible interactions of ADP-ribose with seven residues located, with one exception (Cys253), either within the metallo-dependent phosphatases signature (Gln27, Asn110, His111), or in unique structural regions of the ADPRibase-Mn family: s2s3 (Phe37 and Arg43) and h7h8 (Phe210), around the active site entrance. Mutants were constructed, and kinetic parameters for ADP-ribose, CDP-choline, 2´,3´-cAMP and cyclic ADP-ribose were determined. Phe37 was needed for ADP-ribose preference without catalytic effect, as indicated by the increased ADP-ribose Km and unchanged kcat of F37A-ADPRibase-Mn, while the Km values for the other substrates were little affected. Arg43 was essential for catalysis as indicated by the drastic efficiency loss shown by R43A-ADPRibase-Mn. Unexpectedly, Cys253 was hindering for cADPR phosphohydrolase, as indicated by the specific tenfold gain of efficiency of C253A-ADPRibase-Mn with cyclic ADP-ribose. This allowed the design of a triple mutant (F37A+L196F+C253A) for which cyclic ADP-ribose was the best substrate, with a catalytic efficiency of 3.5´104 M-1s-1 versus 4´103 M-1s-1 of the wild type.

Calcium mobilizing second messengers derived from NAD

Nicotinamide adenine dinucleotide (NAD) has been known since a long period of time as co-factor of oxidoreductases. However, in the past couple of decades further roles have been assigned to NAD. Here, metabolism of NAD to the Ca²⁺ mobilizing second messengers cyclic adenosine diphosphoribose, nicotinic acid adenine dinucleotide phosphate and adenosine diphosphoribose is reviewed. Moreover, the mechanisms of Ca²⁺ mobilization by these adenine nucleotides and their putative target Ca²⁺ channels, ryanodine receptors and transient receptor potential channels are discussed. This article is part of a Special Issue entitled: Cofactor-dependent proteins: evolution, chemical diversity and bio-applications.

Nucleoplasmic calcium signaling and cell proliferation: calcium signaling in the nucleus

Calcium (Ca²⁺) is an essential signal transduction element involved in the regulation of several cellular activities and it is required at various key stages of the cell cycle. Intracellular Ca²⁺ is crucial for the orderly cell cycle progression and plays a vital role in the regulation of cell proliferation. Recently, it was demonstrated by in vitro and in vivo studies that nucleoplasmic Ca²⁺ regulates cell growth. Even though the mechanism by which nuclear Ca²⁺ regulates cell proliferation is not completely understood, there are reports demonstrating that activation of tyrosine kinase receptors (RTKs) leads to translocation of RTKs to the nucleus to generate localized nuclear Ca²⁺ signaling which are believed to modulate cell proliferation. Moreover, nuclear Ca²⁺ regulates the expression of genes involved in cell growth. This review will describe the nuclear Ca²⁺ signaling machinery and its role in cell proliferation. Additionally, the potential role of nuclear Ca²⁺ as a target in cancer therapy will be discussed.

The complex contribution of NOS interneurons in the physiology of cerebrovascular regulation

Following the discovery of the vasorelaxant properties of nitric oxide (NO) by Furchgott and Ignarro, the finding by Bredt and coll. of a constitutively expressed NO synthase in neurons (nNOS) led to the presumption that neuronal NO may control cerebrovascular functions. Consequently, numerous studies have sought to determine whether neuraly-derived NO is involved in the regulation of cerebral blood flow (CBF). Anatomically, axons, dendrites, or somata of NO neurons have been found to contact the basement membrane of blood vessels or perivascular astrocytes in all segments of the cortical microcirculation. Functionally, various experimental approaches support a role of neuronal NO in the maintenance of resting CBF as well as in the vascular response to neuronal activity. Since decades, it has been assumed that neuronal NO simply diffuses to the local blood vessels and produce vasodilation through a cGMP-PKG dependent mechanism. However, NO is not the sole mediator of vasodilation in the cerebral microcirculation and is known to interact with a myriad of signaling pathways also involved in vascular control. In addition, cerebrovascular regulation is the result of a complex orchestration between all components of the neurovascular unit (i.e., neuronal, glial, and vascular cells) also known to produce NO. In this review article, the role of NO interneuron in the regulation of cortical microcirculation will be discussed in the context of the neurovascular unit.

Synthesis and use of cell-permeant cyclic ADP-ribose

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

Immunohistochemical study on the expression of calcium binding proteins (calbindin-D28k, calretinin, and parvalbumin) in the cerebral cortex and in the hippocampal region of nNOS knock-out(-/-) mice

Nitric oxide (NO) modulates the activities of various channels and receptors to participate in the regulation of neuronal intracellular Ca(2+) levels. Ca(2+) binding protein (CaBP) expression may also be altered by NO. Accordingly, we examined expression changes in calbindin-D28k, calretinin, and parvalbumin in the cerebral cortex and hippocampal region of neuronal NO synthase knockout(-/-) (nNOS(-/-)) mice using immunohistochemistry. For the first time, we demonstrate that the expression of CaBPs is specifically altered in the cerebral cortex and hippocampal region of nNOS(-/-) mice and that their expression changed according to neuronal type. As changes in CaBP expression can influence temporal and spatial intracellular Ca(2+) levels, it appears that NO may be involved in various functions, such as modulating neuronal Ca(2+) homeostasis, regulating synaptic transmission, and neuroprotection, by influencing the expression of CaBPs. Therefore, these results suggest another mechanism by which NO participates in the regulation of neuronal Ca(2+) homeostasis. However, the exact mechanisms of this regulation and its functional significance require further investigation.

Mitochondrial dysfunction and nicotinamide dinucleotide catabolism as mechanisms of cell death and promising targets for neuroprotection

Both acute and chronic neurodegenerative diseases are frequently associated with mitochondrial dysfunction as an essential component of mechanisms leading to brain damage. Although loss of mitochondrial functions resulting from prolonged activation of the mitochondrial permeability transition (MPT) pore has been shown to play a significant role in perturbation of cellular bioenergetics and in cell death, the detailed mechanisms are still elusive. Enzymatic reactions linked to glycolysis, the tricarboxylic acid cycle, and mitochondrial respiration are dependent on the reduced or oxidized form of nicotinamide dinucleotide [NAD(H)] as a cofactor. Loss of mitochondrial NAD(+) resulting from MPT pore opening, although transient, allows detrimental depletion of mitochondrial and cellular NAD(+) pools by activated NAD(+) glycohydrolases. Poly(ADP-ribose) polymerase (PARP) is considered to be a major NAD(+) degrading enzyme, particularly under conditions of extensive DNA damage. We propose that CD38, a main cellular NAD(+) level regulator, can significantly contribute to NAD(+) catabolism. We discuss NAD(+) catabolic and NAD(+) synthesis pathways and their role in different strategies to prevent cellular NAD(+) degradation in brain, particularly following an ischemic insult. These therapeutic approaches are based on utilizing endogenous intermediates of NAD(+) metabolism that feed into the NAD(+) salvage pathway and also inhibit CD38 activity.

Immunohistochemical study on the expression of calcium binding proteins (calbindin-D28k, calretinin, and parvalbumin) in the cerebellum of the nNOS knock-out(-/-) mice

Nitric Oxide (NO) actively participates in the regulation of neuronal intracellular Ca²⁺ levels by modulating the activity of various channels and receptors. To test the possibility that modulation of Ca²⁺ buffer protein expression level by NO participates in this regulatory effect, we examined expression of calbindin-D28k, calretinin, and parvalbumin in the cerebellum of neuronal NO synthase knock-out (nNOS^(-/-)) mice using immunohistochemistry. We observed that in the cerebellar cortex of the nNOS^(-/-) mice, expression of calbindin-D28k and parvalbumin were significantly increased while expression of calretinin was significantly decreased. These results suggest another mechanism by which NO can participate in the regulation of Ca²⁺ homeostasis.

Maturation and long-term hypoxia alters Ca2+-induced Ca2+ release in sheep cerebrovascular sympathetic neurons

The contribution of sympathetic nerves arising from the superior cervical ganglia (SCG) toward the growth and function of cerebral blood vessels is pertinent throughout maturation as well as in response to cardiovascular stress imposed by high-altitude long-term hypoxia (LTH). The function of SCG sympathetic neurons is dependent on intracellular Ca2+ concentration ([Ca2+]i) signaling, which is strongly influenced by a process known as Ca(2+)-induced Ca2+ release (CICR) from the smooth endoplasmic reticulum (SER). In this study, we used the sheep SCG neuronal model to test the hypotheses that maturation decreases CICR and high-altitude LTH depresses CICR in fetal SCG neurons but not in those of the adult. We found that the contribution of CICR to electric field stimulation (EFS)-evoked [Ca2+]i transients was greatest in SCG cells from normoxic fetuses and was abolished by LTH. The decline in CICR was associated with a reduction in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) function in fetal SCG cells during LTH, reducing SER Ca2+ levels below the threshold needed for the coupling of Ca2+ influx and CICR. With respect to the maturation from the fetus to adult, the decrease in CICR may reflect both a reduction in the levels of ryanodine receptor isoforms 2 and 3 and SERCA function. In response to LTH and in contrast to the fetus, CICR function in adult SCG cells is maintained and may reflect alterations in other mechanisms that modulate the CICR process. As CICR is instrumental in the function of sympathetic neurons within the cerebrovasculature, the loss of this signaling mechanism in the fetus may have consequences for the adaptation to LTH in terms of fetal susceptibility to vascular insults.

CD38/cADPR/Ca2+ pathway promotes cell proliferation and delays nerve growth factor-induced differentiation in PC12 cells

Intracellular Ca(2+) mobilization plays an important role in a wide variety of cellular processes, and multiple second messengers are responsible for mediating intracellular Ca(2+) changes. Here we explored the role of one endogenous Ca(2+)-mobilizing nucleotide, cyclic adenosine diphosphoribose (cADPR), in the proliferation and differentiation of neurosecretory PC12 cells. We found that cADPR induced Ca(2+) release in PC12 cells and that CD38 is the main ADP-ribosyl cyclase responsible for the acetylcholine (ACh)-induced cADPR production in PC12 cells. In addition, the CD38/cADPR signaling pathway is shown to be required for the ACh-induced Ca(2+) increase and cell proliferation. Inhibition of the pathway, on the other hand, accelerated nerve growth factor (NGF)-induced neuronal differentiation in PC12 cells. Conversely, overexpression of CD38 increased cell proliferation but delayed NGF-induced differentiation. Our data indicate that cADPR plays a dichotomic role in regulating proliferation and neuronal differentiation of PC12 cells.

CD38 as a regulator of cellular NAD: a novel potential pharmacological target for metabolic conditions

CD38 is a multifunctional enzyme that uses nicotinamide adenine dinucleotide (NAD) as a substrate to generate second messengers. Recently, CD38 was also identified as one of the main cellular NADases in mammalian tissues and appears to regulate cellular levels of NAD in multiple tissues and cells. Due to the emerging role of NAD as a key molecule in multiple signaling pathways, and metabolic conditions it is imperative to determine the cellular mechanisms that regulate the synthesis and degradation of this nucleotide. In fact, recently it has been shown that NAD participates in multiple physiological processes such as insulin secretion, control of energy metabolism, neuronal and cardiac cell survival, airway constriction, asthma, aging and longevity. The discovery of CD38 as the main cellular NADase in mammalian tissues, and the characterization of its role on the control of cellular NAD levels indicate that CD38 may serve as a pharmacological target for multiple conditions.

Advancing age alters the contribution of calcium release from smooth endoplasmic reticulum stores in superior cervical ganglion cells

In superior cervical ganglion (SCG) neurons calcium-induced calcium release (CICR), mediated by ryanodine receptors (RyRs), contributes to stimulation-evoked intracellular calcium ([Ca²⁺]_i) transients. Hypothesis: The contribution of CICR to electrical field stimulation (EFS)–evoked [Ca²⁺]_i transients in SCG cells declines with senescence and may be partially recovered in the presence of caffeine. We measured EFS-evoked [Ca²⁺]_i transients in isolated fura-2–loaded SCG cells from Fischer-344 rats aged 6, 12, and 24 months with either the RyR antagonist ryanodine to block the contribution of CICR to [Ca²⁺]_i transients or caffeine to sensitize CICR to EFS. EFS-evoked [Ca²⁺]_i transients increased from 6 to 12 months and declined at 24 months and ryanodine decreased [Ca²⁺]_i transients in SCG cells from 6- and 12-month-old animals only. Caffeine significantly increased EFS-evoked [Ca²⁺]_i transients in all age groups. These data suggest that CICR declines with senescence and residual CICR function may be reclaimed in senescent cells with caffeine.

Nitric oxide regulates growth cone filopodial dynamics via ryanodine receptor-mediated calcium release

Nitric oxide (NO) is a gaseous intercellular messenger involved in numerous processes during development, including wiring of the nervous system. Neuronal growth cones are responsible for establishing the correct connectivity in the nervous system, but how NO might affect neuronal pathfinding is not fully understood. We have demonstrated in a previous study that local application of a NO donor, NOC-7, via micropipette onto individual growth cones from Helisoma trivolvis B5 neurons results in an increase in filopodial length, a decrease in filopodial number and an increase in the intracellular calcium concentration ([Ca(2+)](i)). Moreover, these NO-induced effects were demonstrated to be mediated via an intracellular cascade involving soluble guanylyl cyclase, protein kinase G (PKG) and cyclic adenosine diphosphate ribose (cADPR). We now demonstrate that the increase in the [Ca(2+)](i) that results from local NO application is mediated via release from ryanodine receptor (RyR)-sensitive intracellular stores. We also show that PKG and RyRs are localized within growth cones and microinjection of cADPR mimics the effects of NO, providing further support that the NO-induced effects are mediated via cADPR. Lastly, we provide evidence that calcium influx across the plasma membrane is a necessary component of the NO-induced calcium increase; however, this calcium influx is secondary to the RyR-induced calcium release from intracellular stores. This study details a signalling pathway by which NO can cause changes in growth cone morphology and thus provides a mechanism by which NO could affect neuronal wiring by acting locally on individual growth cones during the pathfinding process.

Cyclic ADP-ribose as a universal calcium signal molecule in the nervous system

beta-NAD(+) is as abundant as ATP in neuronal cells. beta-NAD(+) functions not only as a coenzyme but also as a substrate. beta-NAD(+)-utilizing enzymes are involved in signal transduction. We focus on ADP-ribosyl cyclase/CD38 which synthesizes cyclic ADP-ribose (cADPR), a universal Ca(2+) mobilizer from intracellular stores, from beta-NAD(+). cADPR acts through activation/modulation of ryanodine receptor Ca(2+) releasing Ca(2+) channels. cADPR synthesis in neuronal cells is stimulated or modulated via different pathways and various factors. Subtype-specific coupling of various neurotransmitter receptors with ADP-ribosyl cyclase confirms the involvement of the enzyme in signal transduction in neurons and glial cells. Moreover, cADPR/CD38 is critical in oxytocin release from the hypothalamic cell dendrites and nerve terminals in the posterior pituitary. Therefore, it is possible that pharmacological manipulation of intracellular cADPR levels through ADP-ribosyl cyclase activity or synthetic cADPR analogues may provide new therapeutic opportunities for treatment of neurodevelopmental disorders.

Enhancement of Ca2+-induced Ca2+ release by cyclic ADP-ribose in frog motor nerve terminals

Ca2+-induced Ca2+ release (CICR) occurs via activation of ryanodine receptors (RyRs) in frog motor nerve terminals after RyRs are primed for activation by repetitive Ca2+ entries, thereby contributing to synaptic plasticity. To clarify how the mechanism of CICR becomes activable by repetitive Ca2+ entries, we studied effects of a RyR modulator, cyclic ADP-ribose (cADPr), on CICR by Ca2+ imaging techniques. Use-dependent binding of fluorescent ryanodine and its blockade by ryanodine revealed the existence of RyRs in the terminals. Repetition of tetani applied to the nerve produced repetitive rises in intracellular Ca2+ ([Ca2+]i) in the terminals. The amplitude of each rise slowly waxed and waned during the course of the stimulation. These slow rises and decays were blocked by ryanodine, indicating the priming, activation and inactivation of CICR. Uncaging of caged-cADPr loaded in the terminals increased the amplitude of short tetanus-induced rises in [Ca2+]i and the amplitude, time to peak and half decay time of the slow waxing and waning rises in [Ca2+]i evoked by repetitive tetani. A cADPr blocker, 8-amino-cADPr, loaded in the terminals decreased the slow waxing and waning component of rises and blocked all the actions of exogenous cADPr. It is concluded that cADPr enhances the priming and activation of CICR. The four-state model for RyRs suggests that cADPr inhibits the inactivation of CICR and increases the activation efficacy of RyR.

New perspectives on the mechanisms through which nitric oxide may affect learning and memory processes

Nitric oxide (NO) has been well established as a molecule necessary for memory consolidation. Interestingly, the majority of research has focused on only a single mechanism through which NO acts, namely the up-regulation of guanylate cyclase (GC). However, since NO and NO-derived reactive nitrogen species are capable of interacting with a broad array of enzymes, ion channels and receptors, a singular focus on GC appears short-sighted. Although NO inhibits the action of a number of molecules there are four, in addition to GC, which are up-regulated by the direct presence of NO, or NO-derived radicals, and implicated in memory processing. They are: cyclic nucleotide-gated channels; large conductance calcium-activated potassium channels; ryanodine receptor calcium release (RyR) channels; and the enzyme mono(ADP-ribosyl) transferase. This review presents evidence that not only are these four molecules worthy of investigation as GC-independent mechanisms through which NO may act, but that behavioural evidence already exists suggesting a relationship between NO and the RyR channel.

CCL5 evokes calcium signals in microglia through a kinase-, phosphoinositide-, and nucleotide-dependent mechanism

Microglia, the resident macrophages of the CNS, are responsible for the innate immune response in the brain and participate in the pathogenesis of certain neurodegenerative disorders. Chemokines initiate activation and migration of microglia. The beta-chemokine CCL5 induces an elevation in intracellular calcium concentration ([Ca(2+)](i)) in human microglia. Here, we examined the signal transduction pathway linking activation of chemokine receptor CCR5 to an elevation in [Ca(2+)](i) in cultured microglia by using pharmacological approaches in combination with Fura-2-based digital imaging. The CCL5-induced response required Janus kinase (Jak) activity and the stimulation of an inhibitory G protein. Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores. Activation of both the kinase and the lipase pathways was required for eliciting the Ca(2+) response. However, the majority of the [Ca(2+)](i) increase was derived from sources activated by NAD metabolites. Cyclic ADP-ribose (cADPR) evoked Ca(2+) release from intracellular stores, and ADPR evoked Ca(2+) influx via a nimodipine-sensitive channel. Thus, a multistep cascade couples CCR5 activation to Ca(2+) increases in human microglia. Because changes in [Ca(2+)](i) affect chemotaxis, secretion, and gene expression, pharmacologic modulation of this pathway may alter inflammatory and degenerative processes in the CNS.

Cytotoxic effects and apoptotic signalling mechanisms of the sesquiterpenoid euplotin C, a secondary metabolite of the marine ciliate Euplotes crassus, in tumour cells

Most antitumour agents with cytotoxic properties induce apoptosis. The lipophilic compound euplotin C, isolated from the ciliate Euplotes crassus, is toxic to a number of different opportunistic or pathogenic microorganisms, although its mechanism of action is currently unknown. We report here that euplotin C is a powerful cytotoxic and pro-apoptotic agent in mouse AtT-20 and rat PC12 tumour-derived cell lines. In addition, we provide evidence that euplotin C treatment results in rapid activation of ryanodine receptors, depletion of Ca2+ stores in the endoplasmic reticulum (ER), the release of cytochrome c from the mitochondria, activation of caspase-12, and activation of caspase-3, leading to apoptosis. Intracellular Ca2+ overload is an early event which induces apoptosis and is parallelled by ER stress and the release of cytochrome c, whereas caspase-12 may be activated by euplotin C at a later stage in the apoptosis pathway. These events, either independently or concomitantly, lead to the activation of the caspase-3 and its downstream effectors, triggering the cell to undergo apoptosis. These results demonstrate that euplotin C may be considered for the design of cytotoxic and pro-apoptotic new drugs.

Agonist-induced cyclic ADP ribose production in airway smooth muscle

Cyclic ADP-ribose (cADPR) triggers sarcoplasmic reticulum (SR) Ca(2+) release in airway smooth muscle (ASM). SR Ca(2+) release is an important component of the intracellular Ca(2+) ([Ca(2+)](i)) response of ASM to agonists. Whether cADPR is endogenously produced in ASM during agonist stimulation has not been established. In this study, cADPR production was examined in acutely dissociated porcine ASM cells. ACh stimulation (> or = 1 microM) significantly increased cADPR levels, peaking between 30s and 1 min. This effect was inhibited by M(2) and M(3) muscarinic receptor antagonists. Histamine ((> or = 5 microM) increased cADPR levels to a greater extent than ACh, while diphenhydramine blocked histamine-induced cADPR elevation. Both bradykinin (100 nM) and endothelin-1 (100 nM) also increased cADPR levels to a greater extent than ACh or histamine. These results indicate that in porcine ASM, certain agonists acting via receptors increase cADPR levels. Furthermore, the extent of cADPR responses to agonist varies, possibly reflecting differences in G-protein coupling.

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.

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

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

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.

Nitric oxide donors induce calcium-mobilisation from internal stores but do not stimulate catecholamine secretion by bovine chromaffin cells in resting conditions

The potential role of nitric oxide (NO) donors and peroxynitrites on both basal catecholamine (CA) secretion and modulation of calcium levels has been investigated in primary cultures of bovine chromaffin cells. NO donors did not modulate catecholamine secretion, while peroxynitrites induced a time dose-dependent increase in basal CA secretion. Two facts may explain the lack of these compounds on basal CA secretion. NO donors induce, on the one hand, an increase in intracellular calcium levels by depletion of internal IP3-stores from endoplasmic reticulum. On the other hand, a small calcium influx through N-type voltage-dependent calcium channels (VDCC), which seem not to be coupled to exocytosis of adrenaline and noradrenaline in chromaffin cells. Both effects, calcium-mobilisation from internal stores and calcium entry through N-type VDCC are mediated by cGMP synthesis. In contrast, peroxynitrites induce an increase in basal CA secretion by both a decrease of intracellular catecholamine content and a toxic effect on cellular membrane. All these results, taken together, could explain contradictory results in the literature on the role of NO on basal catecholamine secretion and on modulation of intracellular calcium in chromaffin cells.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

CD38 gene disruption inhibits the contraction induced by alpha-adrenoceptor stimulation in mouse aorta

CD38 is an ectoenzyme with ADP-ribosyl cyclase and hydrolase activities, which synthesizes cyclic ADP-ribose from NAD and hydrolyzes cyclic ADP-ribose to ADP-ribose. It has been shown that cyclic ADP-ribose is a potent Ca(2+) mobilizing messenger in many cells. To know the physiological role of cyclic ADP-ribose in vascular smooth muscle, we examined the effects of various agonists in the aorta isolated from CD38 knockout (CD38(-/-)) mouse. Western blot analysis showed that CD38 protein was detected in the aorta isolated from wild-type (CD38(+/+)) mouse, but not from CD38(-/-) mouse. In the aortae isolated from both CD38(+/+) and CD38(-/-) mice, KCl, phenylephrine and norepinephrine induced concentration-dependent contraction. KCl produced similar concentration-dependent responses in the aortae from both CD38(+/+) and CD38(-/-) mice. Maximum force of contraction induced by KCl (65 mM) was same in the size. Phenylephrine- and norepinephrine-induced contractions were, however, significantly smaller in the aortae from CD38(-/-) mice than in those from CD38(+/+) mice. 5-Hydroxytryptamine, endothelin-1, caffeine and thapsigargin-induced contractions were not significantly different in these two aortae. These results suggest that CD38 gene disruption inhibits alpha-adrenoceptor-induced vascular contractions and cyclic ADP-ribose-mediated signal transduction system is committed in these responses.

Downregulation of nitric oxide formation by cytosolic phospholipase A2-released arachidonic acid

Exposure of PC12 cells to A23187 or thapsigargin caused a concentration-dependent release of arachidonic acid (AA) mediated by cytosolic phospholipase A2 (PLA2). Under the same conditions, however, analysis of nitric oxide (NO) formation revealed that activation of NO synthase (NOS) is best described by a bell-shaped curve. Reduced detection of NO observed at increasing A23187 or thapsigargin concentrations was not due to formation of peroxynitrite or to activation of NO-consuming processes, but rather to AA-dependent inhibition of NOS activity. Furthermore, NO formation observed under optimal conditions for NOS activity was suppressed by AA as well as by the PLA2 activator melittin. Finally, the effects of AA were not the consequence of direct enzyme inhibition, because this lipid messenger failed to inhibit formation of NO by purified neuronal NOS, but were mediated by an AA-dependent signaling and not by downstream products of the cyclooxygenase and lipoxygenase pathways. In conclusion, the present study underscores a novel mechanism whereby endogenous, or exogenous, AA promotes inhibition of NOS activity. Because AA is generated in response to various agonists acting on membrane receptors and extensively released in inflammatory conditions, these findings have important physiopathological implications.

Non-genomic modulation of dopamine release by bisphenol-A in PC12 cells

An endocrine disruptor chemical, bisphenol-A (BPA), is reported to have several short-term actions in various tissues and/or cells; however, the mechanisms of these actions have not been fully elucidated. We investigated short-term actions evoked by BPA in pheochromocytoma PC12 cells. BPA elicited dopamine release in PC12 cells in a dose-dependent manner. A selective N-type calcium channel antagonist (omega-conotoxin GVIA) and a ryanodine receptor blocker (ryanodine) inhibited the BPA-induced dopamine release. The expression of ryanodine receptor mRNA was detected by RT-PCR in PC12 cells. Subsequently, in order to prove whether membrane receptors participate in BPA-evoked dopamine release, a guanine nucleotide-binding protein inhibitor [guanosine 5'-(beta-thio) diphosphate], cyclic AMP antagonist (Rp-cAMPS) or protein kinase A inhibitor (H7 or H89) was added to PC12 cells prior to BPA-treatment. All of these agents suppressed BPA-evoked dopamine release, indicating that multiple signaling pathways may be involved in BPA-evoked dopamine release in PC12 cells. In conclusion, we demonstrated that BPA induced dopamine release in a non-genomic manner through guanine nucleotide-binding protein and N-type calcium channels. These findings illustrate a novel function of BPA and suggest that exposure to BPA influences the function of dopaminergic neurons.

FKBP12.6 and cADPR regulation of Ca2+ release in smooth muscle cells

Intracellular Ca2+ release through ryanodine receptors (RyRs) plays important roles in smooth muscle excitation-contraction coupling, but the underlying regulatory mechanisms are poorly understood. Here we show that FK506 binding protein of 12.6 kDa (FKBP12.6) associates with and regulates type 2 RyRs (RyR2) in tracheal smooth muscle. FKBP12.6 binds to RyR2 but not other RyR or inositol 1,4,5-trisphosphate receptors, and FKBP12, known to bind to and modulate skeletal RyRs, does not associate with RyR2. When dialyzed into tracheal myocytes, cyclic ADP-ribose (cADPR) alters spontaneous Ca2+ release at lower concentrations and produces macroscopic Ca2+ release at higher concentrations; neurotransmitter-evoked Ca2+ release is also augmented by cADPR. These actions are mediated through FKBP12.6 because they are inhibited by molar excess of recombinant FKBP12.6 and are not observed in myocytes from FKBP12.6-knockout mice. We also report that force development in FKBP12.6-null mice, observed as a decrease in the concentration/tension relationship of isolated trachealis segments, is impaired. Taken together, these findings point to an important role of the FKBP12.6/RyR2 complex in stochastic (spontaneous) and receptor-mediated Ca2+ release in smooth muscle.

Evidence for an intracellular ADP-ribosyl cyclase/NAD+-glycohydrolase in brain from CD38-deficient mice

Cyclic ADP-ribose, a metabolite of NAD+, is known to modulate intracellular calcium levels and signaling in various cell types, including neural cells. The enzymes responsible for producing cyclic ADP-ribose in the cytoplasm of mammalian cells remain unknown; however, two mammalian enzymes that are capable of producing cyclic ADP-ribose extracellularly have been identified, CD38 and CD157. The present study investigated whether an ADP-ribosyl cyclase/NAD+-glycohydrolase independent of CD38 is present in brain tissue. To address this question, NAD+ metabolizing activities were accurately examined in developing and adult Cd38-/- mouse brain protein extracts and cells. Low ADP-ribosyl cyclase and NAD+-glycohydrolase activities (in the range of pmol of product formed/mg of protein/min) were detected in Cd38-/- brain at all developmental stages studied. Both activities were found to be associated with cell membranes. The activities were significantly higher in Triton X-100-treated neural cells compared with intact cells, suggesting an intracellular location of the novel cyclase. The cyclase and glycohydrolase activities were optimal at pH 6.0 and were inhibited by zinc, properties which are distinct from those of CD157. Both activities were enhanced by guanosine 5'-O-(3-thiotriphosphate), a result suggesting that the novel enzyme may be regulated by a G protein-dependent mechanism. Altogether our results indicate the presence of an intracellular membrane-bound ADP-ribosyl cyclase/NAD+-glycohydrolase distinct from CD38 and from CD157 in mouse brain. This novel enzyme, which is more active in the developing brain than in the adult tissue, may play an important role in cyclic ADP-ribose-mediated calcium signaling during brain development as well as in adult tissue.

Chronic hypoxia alters the function of NOS nerves in cerebral arteries of near-term fetal and adult sheep

In addition to adrenergic innervation, cerebral arteries also contain neuronal nitric oxide synthase (nNOS)-expressing nerves that augment adrenergic nerve function. We examined the impact of development and chronic high-altitude hypoxia (3,820 m) on nNOS nerve function in near-term fetal and adult sheep middle cerebral arteries (MCA). Electrical stimulation-evoked release of norepinephrine (NE) was measured with HPLC and electrochemical detection, whereas nitric oxide (NO) release was measured by chemiluminescence. An inhibitor of NO synthase, N(omega)-nitro-l-arginine methyl ester (l-NAME), significantly inhibited stimulation-evoked NE release in MCA from normoxic fetal and adult sheep with no effect in MCA from hypoxic animals. Addition of the NO donor S-nitroso-N-acetyl-dl-penicillamine fully reversed the effect of l-NAME in MCA from normoxic animals with no effect in MCA from hypoxic animals. Electrical stimulation caused a significant increase in NO release in MCA from normoxic animals, an effect that was blocked by the neurotoxin tetrodotoxin, whereas there was no increase in NO release in MCA from hypoxic animals. Relative abundance of nNOS as measured by Western blot analysis was similar in normoxic fetal and adult MCA. However, after hypoxic acclimitization, nNOS levels dramatically declined in both fetal and adult MCA. These data suggest that the function of nNOS nerves declines during chronic high-altitude hypoxia, a functional change that may be related to a decline in nNOS protein levels.

Nitric oxide signalling in salivary glands

Nitric oxide (NO) plays multiple roles in both intracellular and extracellular signalling mechanisms with implications for health and disease. This review focuses on the role of NO signalling in salivary secretion. Attention will be paid primarily to endogenous NO production in acinar cells resulting from specific receptor stimulation and to NO-regulated Ca2+ homeostasis. Due to the fact that NO readily crosses membranes by simple diffusion, endogenous NO may play a physiological role in processes as diverse as modifying the secretory output, controlling blood supply to the gland, modulating transmitter output from nerve endings, participating in the host defence barrier, and affecting growth and differentiation of surrounding tissue. Furthermore, the role of NO in the pathogenesis of human oral diseases will be considered.

Functional relation between caffeine- and muscarine-sensitive Ca2+ stores and no Ca2+ releasing action of cyclic adenosine diphosphate-ribose in guinea-pig adrenal chromaffin cells

In voltage-clamped guinea-pig chromaffin cells, muscarine (50 microM) or caffeine (30 mM) produced a transient intracellular Ca(2+) concentration ([Ca(2+)](i)) increase, catecholamine release and an outward K(+) current mediated through Ca(2+) released from internal Ca(2+) stores at a holding potential of -40 mV. Caffeine followed by muscarine failed to evoke these responses, while muscarine followed by caffeine was effective in producing about 30% of [Ca(2+)](i) increase and catecholamine secretion. In cells dialyzed with inositol 1,4,5-trisphosphate (IP(3)), caffeine failed to produce the [Ca(2+)](i) increase. Intracellular application of cyclic adenosine 5'-diphosphate-ribose (cADP-ribose) or 8-bromo cADP-ribose exerted no effect on the resting [Ca(2+)](i) and the caffeine-induced [Ca(2+)](i) increase. These results suggest that IP(3)-sensitive stores are functionally divided into two subpopulations, sensitive and insensitive to caffeine, and it is unlikely that cADP-ribose plays a role as a Ca(2+) releaser in guinea-pig adrenal chromaffin cells.

Activation of the cation channel long transient receptor potential channel 2 (LTRPC2) by hydrogen peroxide. A splice variant reveals a mode of activation independent of ADP-ribose

LTRPC2 is a cation channel recently reported to be activated by adenosine diphosphate-ribose (ADP-ribose) and NAD. Since ADP-ribose can be formed from NAD and NAD is elevated during oxidative stress, we studied whole cell currents and increases in the intercellular free calcium concentration ([Ca(2+)](i)) in long transient receptor potential channel 2 (LTRPC2)-transfected HEK 293 cells after stimulation with hydrogen peroxide (H(2)O(2)). Cation currents carried by monovalent cations and Ca(2+) were induced by H(2)O(2) (5 mm in the bath solution) as well as by intracellular ADP-ribose (0.3 mm in the pipette solution) but not by NAD (1 mm). H(2)O(2)-induced currents developed slowly after a characteristic delay of 3-6 min and receded after wash-out of H(2)O(2). [Ca(2+)](i) was rapidly increased by H(2)O(2) in LTRPC2-transfected cells as well as in control cells; however, in LTRPC2-transfected cells, H(2)O(2) evoked a second delayed rise in [Ca(2+)](i). A splice variant of LTRPC2 with a deletion in the C terminus (amino acids 1292-1325) was identified in neutrophil granulocytes. This variant was stimulated by H(2)O(2) as the wild type. However, it did not respond to ADP-ribose. We conclude that activation of LTRPC2 by H(2)O(2) is independent of ADP-ribose and that LTRPC2 may mediate the influx of Na(+) and Ca(2+) during oxidative stress, such as the respiratory burst in granulocytes.

Nitric oxide inhibits neuroendocrine Ca(V)1 L-channel gating via cGMP-dependent protein kinase in cell-attached patches of bovine chromaffin cells

Nitric oxide (NO) regulates the release of catecholamines from the adrenal medulla but the molecular targets of its action are not yet well identified. Here we show that the NO donor sodium nitroprusside (SNP, 200 microM) causes a marked depression of the single Ca(V)1 L-channel activity in cell-attached patches of bovine chromaffin cells. SNP action was complete within 3-5 min of cell superfusion. In multichannel patches the open probability (NP(o)) decreased by approximately 60 % between 0 and +20 mV. Averaged currents over a number of traces were proportionally reduced and showed no drastic changes to their time course. In single-channel patches the open probability (P(o)) at +10 mV decreased by the same amount as that of multichannel patches (approximately 61 %). Such a reduction was mainly associated with an increased probability of null sweeps and a prolongation of mean shut times, while first latency, mean open time and single-channel conductance were not significantly affected. Addition of the NO scavenger carboxy-PTIO or cell treatment with the guanylate cyclase inhibitor ODQ prevented the SNP-induced inhibition. 8-Bromo-cyclicGMP (8-Br-cGMP; 400 microM) mimicked the action of the NO donor and the protein kinase G blocker KT-5823 prevented this effect. The depressive action of SNP was preserved after blocking the cAMP-dependent up-regulatory pathway with the protein kinase A inhibitor H89. Similarly, the inhibitory action of 8-Br-cGMP proceeded regardless of the elevation of cAMP levels, suggesting that cGMP/PKG and cAMP/PKA act independently on L-channel gating. The inhibitory action of 8-Br-cGMP was also independent of the G protein-induced inhibition of L-channels mediated by purinergic and opiodergic autoreceptors. Since Ca(2+) channels contribute critically to both the local production of NO and catecholamine release, the NO/PKG-mediated inhibition of neuroendocrine L-channels described here may represent an important autocrine signalling mechanism for controlling the rate of neurotransmitter release from adrenal glands.

Ca(2+)-dependent and -independent Cl(-) secretion stimulated by the nitric oxide donor, GEA 3162, in rat colonic epithelium

The lipophilic nitric oxide-liberating drug, 1,2,3,4-oxatriazolium,5-amino-3-(3,4-dichlorophenyl)-chloride (GEA 3162), concentration-dependently induced a Cl(-) secretion in rat colon. At a low concentration (5 x 10(-5) M), the action was Ca(2+)-dependent, whereas at a high concentration (5 x 10(-4) M), the response was independent from extracellular Ca(2+). Fura-2 experiments at isolated colonic crypts revealed that GEA 3162 induced an increase of the cytoplasmic Ca(2+) concentration due to an influx of extracellular Ca(2+), probably mediated by an activation of a nonselective cation conductance as demonstrated by whole-cell patch-clamp studies. After depolarization of the basolateral membrane, GEA 3162 (5 x 10(-4) M) stimulated a current, which was suppressed by glibenclamide but was resistant against blockade of protein kinases by staurosporine, suggesting an activation of apical Cl(-) channels directly by the nitric oxide (NO) donor. After permeabilizing the apical membrane with the ionophore, nystatin, GEA 3162 (5 x 10(-4) M) activated basolateral K(+) conductances and the Na(+)-K(+)-ATPase. Thus, the lipophilic NO donor GEA 3162 stimulates a Cl(-) secretion in a Ca(2+)-dependent and -independent manner.

The type II inositol 1,4,5-trisphosphate receptor can trigger Ca2+ waves in rat hepatocytes

BACKGROUND & AIMS

Ca2+ regulates cell functions through signaling patterns such as Ca2+ oscillations and Ca2+ waves. The type I inositol 1,4,5-trisphosphate receptor is thought to support Ca2+ oscillations, whereas the type III inositol 1,4,5-trisphosphate receptor is thought to initiate Ca2+ waves. The role of the type II inositol 1,4,5-trisphosphate receptor is less clear, because it behaves like the type III inositol 1,4,5-trisphosphate receptor at the single-channel level but can support Ca2+ oscillations in intact cells. Because the type II inositol 1,4,5-trisphosphate receptor is the predominant isoform in liver, we examined whether this isoform can trigger Ca2+ waves in hepatocytes.

METHODS

The expression and distribution of inositol 1,4,5-trisphosphate receptor isoforms was examined in rat liver by immunoblot and confocal immunofluorescence. The effects of inositol 1,4,5-trisphosphate on Ca2+ signaling were examined in isolated rat hepatocyte couplets by using flash photolysis and time-lapse confocal microscopy.

RESULTS

The type II inositol 1,4,5-trisphosphate receptor was concentrated near the canalicular pole in hepatocytes, whereas the type I inositol 1,4,5-trisphosphate receptor was found elsewhere. Stimulation of hepatocytes with vasopressin or directly with inositol 1,4,5-trisphosphate induced Ca2+ waves that began in the canalicular region and then spread to the rest of the cell. Inositol 1,4,5-Trisphosphate-induced Ca2+ signals also increased more rapidly in the canalicular region. Hepatocytes did not express the ryanodine receptor, and cyclic adenosine diphosphate-ribose had no effect on Ca2+ signaling in these cells.

CONCLUSIONS

The type II inositol 1,4,5-trisphosphate receptor establishes a pericanalicular trigger zone from which Ca2+ waves originate in hepatocytes.

Ca2+ waves require sequential activation of inositol trisphosphate receptors and ryanodine receptors in pancreatic acini

BACKGROUND & AIMS

The inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) and the ryanodine receptor (RyR) are the principal Ca2+-release channels in cells and are believed to serve distinct roles in cytosolic Ca2+ (Ca(i)2+) signaling. This study investigated whether these receptors instead can release Ca2+ in a coordinated fashion.

METHODS

Apical and basolateral Ca(i)2+ signals were monitored in rat pancreatic acinar cells by time-lapse confocal microscopy. Caged forms of second messengers were microinjected into individual cells and then photoreleased in a controlled fashion by either UV or 2-photon flash photolysis.

RESULTS

InsP3 increased Ca(i)2+ primarily in the apical region of pancreatic acinar cells, whereas the RyR agonist cyclic adenosine diphosphate ribose (cADPR) increased Ca(i)2+ primarily in the basolateral region. Apical-to-basal Ca(i)2+ waves were induced by acetylcholine and initiation of these waves was blocked by the InsP3R inhibitor heparin, whereas propagation into the basolateral region was inhibited by the cADPR inhibitor 8-amino-cADPR. To examine integration of apical and basolateral Ca(i)2+ signals, Ca2+ was selectively released either apically or basolaterally using 2-photon flash photolysis. Ca(i)2+ increases were transient and localized in unstimulated cells. More complex Ca(i)2+ signaling patterns, including polarized Ca(i)2+ waves, were observed when Ca2+ was photoreleased in cells stimulated with subthreshold concentrations of acetylcholine.

CONCLUSIONS

Polarized Ca(i)2+ waves are induced in acinar cells by serial activation of apical InsP3Rs and then basolateral RyRs, and subcellular release of Ca2+ coordinates the actions of these 2 types of Ca2+ channels. This subcellular integration of Ca2+-release channels shows a new level of complexity in the formation of Ca(i)2+ waves.

A novel cycling assay for cellular cADP-ribose with nanomolar sensitivity

cADP-ribose (cADPR) is a novel cyclic nucleotide derived from NAD(+) that has now been established as a general Ca(2+) messenger in a wide variety of cells. Despite the obvious importance of monitoring its cellular levels under various physiological conditions, its measurement has been technically difficult and requires specialized reagents. In this study a widely applicable high-sensitivity assay for cADPR is described. ADP-ribosyl cyclase normally catalyses the synthesis of cADPR from NAD(+), but the reaction can be reversed in the presence of high concentrations of nicotinamide, producing NAD(+) from cADPR stoichiometrically. The resultant NAD(+) can then be coupled to a cycling assay involving alcohol dehydrogenase and diaphorase. Each time NAD(+) cycles through these coupled reactions, a molecule of highly fluorescent resorufin is generated. The reaction can be conducted for hours, resulting in more than a thousand-fold amplification of cADPR. Concentrations of cADPR in the nanomolar range can be measured routinely. The unique ability of ADP-ribosyl cyclase to catalyse the reverse reaction provides the required specificity. Using this assay, it is demonstrated that cADPR is present in all tissues tested and that the levels measured are directly comparable with those obtained using a radioimmunoassay. All the necessary reagents are widely available and the assay can be performed using a multiwell fluorescence plate reader, providing a high-throughput method for monitoring cADPR levels. This assay should be valuable in elucidating the messenger role of cADPR in cells.

Rapid increase of NO release in plant cell cultures induced by cytokinin

4,5-Diaminofluorescein, a fluorescence indicator for NO, was applied to detect the release of NO from plant cells. NO production was increased within 3 min when plant cell cultures (Arabidopsis, parsley, and tobacco) were treated by cytokinin and was dose-dependent and signal-specific in that other plant hormones and inactive cytokinin analog were not effective in stimulating of NO release. The response was quenched by addition of 2-(aminoethyl)-2-thiopseudourea, an inhibitor of the animal NO synthase, and by addition of an NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-1-oxy-3-oxide. These results imply that NO may act in cytokinin signal transduction.

Increase of cGMP, cADP-ribose and inositol 1,4,5-trisphosphate preceding Ca2+ transients in fertilization of sea urchin eggs

Transient increases, or oscillations, of cytoplasmic free Ca2+ concentration, [Ca2+]i, occur during fertilization of animal egg cells. In sea urchin eggs, the increased Ca2+ is derived from intracellular stores, but the principal signaling and release system involved has not yet been agreed upon. Possible candidates are the inositol 1,4,5-trisphosphate receptor/channel (IP3R) and the ryanodine receptor/channel (RyR) which is activated by cGMP or cyclic ADP-ribose (cADPR). Thus, it seemed that direct measurements of the likely second messenger candidates during sea urchin fertilization would be essential to an understanding of the Ca2+ signaling pathway. We therefore measured the cGMP, cADPR and inositol 1,4,5-trisphosphate (IP3) contents of sea urchin eggs during the early stages of fertilization and compared these with the [Ca2+]i rise in the presence or absence of an inhibitor against soluble guanylate cyclase. We obtained three major experimental results: (1) cytosolic cGMP levels began to rise first, followed by cADPR and IP3 levels, all almost doubling before the explosive increase of [Ca2+]i; (2) most of the rise in IP3 occurred after the Ca2+ peak; IP3 production could also be induced by the artificial elevation of [Ca2+]i, suggesting the large increase in IP3 is a consequence, rather than a cause, of the Ca2+ transient; (3) the measured increase in cGMP was produced by the soluble guanylate cyclase of eggs, and inhibition of soluble guanylate cyclase of eggs diminished the production of both cADPR and IP3 and the [Ca2+]i increase without the delay of Ca2+ transients. Taken together, these results suggest that the RyR pathway involving cGMP and cADPR is not solely responsible for the initiating event, but contributes to the Ca2+ transients by stimulating IP3 production during fertilization of sea urchin eggs.

Different effects of tert-butylhydroperoxide-induced peroxynitrite-dependent and -independent DNA single-strand breakage on PC12 cell poly(ADP-ribose) polymerase activity

The short-chain lipid hydroperoxide analogue tert-butylhydroperoxide induces peroxynitrite-dependent and -independent DNA single strand breakage in PC12 cells. U937 cells that do not express constitutive nitric oxide synthase respond to tert-butylhydroperoxide treatment with peroxynitrite-independent DNA cleavage. Under experimental conditions leading to equivalent strand break frequencies, the analysis of poly(ADP-ribose) polymerase activity showed an increase in PC12 cells but not in U937 cells. The enhanced poly(ADP-ribose) polymerase activity observed in PC12 cells was paralleled by a significant decline in NAD+ content and both events were prevented by treatments suppressing formation of peroxynitrite. Although DNA breaks were rejoined at similar rates in the two cell lines, an inhibitor of poly(ADP-ribose) polymerase delayed DNA repair in PC12 cells but had hardly any effect in U937 cells. The results obtained using the latter cell type were confirmed with an additional cell line (Chinese hamster ovary cells) that does not express nitric oxide synthase. Collectively, our data suggest that tert-butylhydroperoxide-induced peroxynitrite-independent DNA strand scission is far less effective than the DNA cleavage generated by endogenous peroxynitrite in stimulating the activity of poly(ADP-ribose) polymerase.

Nitric oxide inhibition of cAMP synthesis in parotid acini: regulation of type 5/6 adenylyl cyclase

The nitric oxide (NO) donor, GEA 3162, inhibited isoproterenol-induced cyclic AMP (cAMP) accumulation in a concentration- and time-dependent manner in mouse parotid acini; SIN-1 mimicked these effects. Inhibition of stimulated cAMP accumulation was independent of phosphodiesterase activity. GEA 3162 also inhibited forskolin-induced cAMP accumulation. Removal of extracellular Ca(2+), addition of La(3+), or the calmodulin (CaM) inhibitor, calmidazolium, did not prevent the NO-mediated response, and addition of the soluble guanylyl inhibitor, ODQ, did not reverse GEA 3162-induced inhibition of cAMP accumulation. GEA 3162 also inhibited adenylyl cyclase in vitro independently of Ca(2+)/CaM. Further studies revealed that the NO synthase (NOS) inhibitor, 7-nitroindazole (7-NI), reduced significantly thapsigargin-induced Ca(2+) release and capacitative Ca(2+) entry and reversed thapsigargin inhibition of the AC Type 5/6 isoform (AC5/6). Data suggest that NO produced endogenously has dual effects on cAMP accumulation in mouse parotid acini, an inhibitory effect on AC activity and a modulatory effect on capacitative Ca(2+) entry resulting in AC5/6 inhibition.

Signaling and gene expression in the neuron-glia unit during brain function and dysfunction: Holger Hydén in memoriam

Holger Hydén demonstrated almost 40 years ago that learning changes the base composition of nuclear RNA, i.e. induces an alteration in gene expression. An equally revolutionary observation at that time was that a base change occurred in both neurons and glia. From these findings, Holger Hydén concluded that establishment of memory is correlated with protein synthesis, and he demonstrated de novo synthesis of several high-molecular protein species after learning. Moreover, the protein, S-100, which is mainly found in glial cells, was increased during learning, and antibodies towards this protein inhibited memory consolidation. S-100 belongs to a family of Ca(2+)-binding proteins, and Holger Hydén at an early point realized the huge importance of Ca(2+) in brain function. He established that glial cells show more marked and earlier changes in RNA composition in Parkinson's disease than neurons. Holger Hydén also had the vision and courage to suggest that "mental diseases could as well be thought to depend upon a disturbance of processes in glia cells as in the nerve cells", and he showed that antidepressant drugs cause profound changes in glial RNA. The importance of Holger Hydén's findings and visions can only now be fully appreciated. His visionary concepts of the involvement of glia in neurological and mental illness, of learning being associated with changes in gene expression, and of the functional importance of Ca(2+)-binding proteins and Ca(2+) are presently being confirmed and expanded by others. This review briefly summarizes highlights of Holger Hydén's work in these areas, followed by a discussion of recent research, confirming his findings and expanding his visions. This includes strong evidence that glial dysfunction is involved in the development of Parkinson's disease, that drugs effective in mood disorders alter gene expression and exert profound effects on astrocytes, and that neuronal-astrocytic interactions in glutamate signaling, NO synthesis, Ca(2+) signaling, beta-adrenergic activity, second messenger production, protein kinase activities, and transcription factor phosphorylation control the highly programmed events that carry the memory trace through the initial, signal-mediated short-term and intermediate memory stages to protein synthesis-dependent long-term memory.

tert-Butylhydroperoxide induces peroxynitrite-dependent mitochondrial permeability transition leading PC12 cells to necrosis

A short-term exposure of PC12 cells to tert-butylhydroperoxide, followed by recovery in fresh culture medium, causes cell death and the extent of this response progressively increases during the 120 min of post-treatment incubation. Morphological and biochemical analyses of these cells revealed that the mode of cell death was necrosis. Cell killing induced by the hydroperoxide seems to be in part mediated by peroxynitrite because the lethal response was markedly and similarly reduced by the nitric oxide synthase inhibitor N omega-nitro-L-arginine methylester and by scavengers of nitric oxide or peroxynitrite. This peroxynitrite-dependent mechanism of cytotoxicity was blunted by antioxidants and inhibitors of mitochondrial permeability transition and the onset of cell death was preceded by mitochondrial depolarization and loss of cellular ATP. We conclude that tert-butylhydroperoxide promotes peroxynitrite-dependent PC12 cell necrosis causally linked to peroxidation of membrane lipids and mitochondrial permeability transition.

Specific Ca2+ signaling evoked by cholecystokinin and acetylcholine: the roles of NAADP, cADPR, and IP3

In order to control cell functions, hormones and neurotransmitters generate an amazing diversity of Ca2+ signals such as local and global Ca2+ elevations and also Ca2+ oscillations. In pancreatic acinar cells, cholecystokinin (CCK) stimulates secretion of digestive enzyme and promotes cell growth, whereas acetylcholine (ACh) essentially triggers enzyme secretion. Pancreatic acinar cells are a classic model for the study of CCK- and ACh-evoked specific Ca2+ signals. In addition to inositol 1,4,5 trisphosphate (IP3), recent studies have shown that cyclic ADPribose (cADPr) and nicotinic acid adenine dinucleotide phosphate (NAADP) release Ca2+ in pancreatic acinar cells. Moreover, it has also been shown that both ACh and CCK trigger Ca2+ spikes by co-activation of IP3 and ryanodine receptors but by different means. ACh uses IP3 and Ca2+, whereas CCK uses cADPr and NAADP. In addition, CCK activates phospholipase A2 and D. The concept emerging from these studies is that agonist-specific Ca2+ signals in a single target cell are generated by combination of different intracellular messengers.

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.

Acetylcholine-evoked calcium mobilization and ion channel activation in human labial gland acinar cells from patients with primary Sjögren's syndrome

Recent evidence has indicated that the salivary gland dysfunction associated with Sjögren's syndrome (SjS) is not necessarily due to immune-mediated destruction of acinar tissue. SjS sufferers may possess substantial reserves of acinar tissue but nevertheless be incapable of maintaining salivary flow rates in the normal range. We have investigated the ability of isolated labial gland acinar cells from SjS patients to fluid secrete by measuring agonist-evoked changes in intracellular Ca(2+) ([Ca(2+)](i)) using fura-2 microfluorimetry and activation of K(+) and Cl(-) channels using the patch-clamp whole cell technique. We can confirm that stimulation with a super-maximal dose of acetylcholine (ACh) increased [Ca(2+)]i equally in both control acinar cells and those derived from SjS patients. However, at submaximal concentrations, the dose-response curve for ACh was shifted to the right by approximately one order of magnitude in acinar cells from SjS patients compared to control acinar cells. Patch-clamp measurements consistent with the presence of Ca(2+)-activated K(+) and Cl(-) conductances were obtained from both control acinar cells and those obtained from SjS patients. Dose-dependent activation of the ion channels by acetylcholine was also right-shifted in acinar cells from SjS patients compared to control cells. Our data show that labial gland acinar cells from SjS patients were capable of responding to agonist stimulation by mobilizing [Ca(2+)](i) and activating K(+) and Cl(-) channels consistent with the requirements of fluid secretion. However, the persistent loss of sensitivity to ACh observed in from SjS patients may account for the lack of saliva production observed in these patients in vivo.

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.

Role of Ins(1,4,5)P3, cADP-ribose and nicotinic acid-adenine dinucleotide phosphate in Ca2+ signalling in mouse submandibular acinar cells

cADP-ribose (cADPr) and nicotinic acid-adenine dinucleotide phosphate (NAADP) are two putative second messengers; they were first shown to stimulate Ca(2+) mobilization in sea urchin eggs. We have used the patch-clamp whole-cell technique to determine the role of cADPr and NAADP in relation to that of Ins(1,4,5)P(3) in mouse submandibular acinar cells by measuring agonist-evoked and second-messenger-evoked changes in Ca(2+)-dependent K(+) and Cl(-) currents. Both Ins(1,4,5)P(3) and cADPr were capable of reproducing the full range of responses normally seen in response to stimulation with acetylcholine (ACh). Low concentrations of agonist (10-20 nM ACh) or second messenger [1-10 microM Ins(1,4,5)P(3) or cADPr] elicited a sporadic transient activation of the Ca(2+)-dependent currents; mid-range concentrations [50-500 nM ACh, 50 microM Ins(1,4,5)P(3) or 50-100 microM cADPr] elicited high-frequency (approx. 2 Hz) trains of current spikes; and high concentrations [more than 500 nM ACh, more than 50 microM Ins(1,4,5)P(3) or more than 100 microM cADPr] gave rise to a sustained current response. The response to ACh was inhibited by antagonists of both the Ins(1,4,5)P(3) receptor [Ins(1,4,5)P(3)R] and the ryanodine receptor (RyR) but could be completely blocked only by an Ins(1,4,5)P(3)R antagonist (heparin). NAADP (50 nM to 100 microM) did not itself activate the Ca(2+)-dependent ion currents, nor did it inhibit the activation of these currents by ACh. These results show that, in these cells, both Ins(1,4,5)P(3)R and RyR are involved in the propagation of the Ca(2+) signal stimulated by ACh and that cADPr can function as an endogenous regulator of RyR. Furthermore, although NAADP might have a role in hormone-stimulated secretion in pancreatic acinar cells, it does not contribute to ACh-evoked secretion in submandibular acinar cells.