Cyclic ADP-ribose: a novel Ca2+-mobilising second messenger

A network-based modeling framework reveals the core signal transduction network underlying high carbon dioxide-induced stomatal closure in guard cells

Stomata are pores on plant aerial surfaces, each bordered by a pair of guard cells. They control gas exchange vital for plant survival. Understanding how guard cells respond to environmental signals such as atmospheric carbon dioxide (CO2) levels is not only insightful to fundamental biology but also relevant to real-world issues of crop productivity under global climate change. In the past decade, multiple important signaling elements for stomatal closure induced by elevated CO2 have been identified. Yet, there is no comprehensive understanding of high CO2-induced stomatal closure. In this work, we assemble a cellular signaling network underlying high CO2-induced stomatal closure by integrating evidence from a comprehensive literature analysis. We further construct a Boolean dynamic model of the network, which allows in silico simulation of the stomatal closure response to high CO2 in wild-type Arabidopsis thaliana plants and in cases of pharmacological or genetic manipulation of network nodes. Our model has a 91% accuracy in capturing known experimental observations. We perform network-based logical analysis and reveal a feedback core of the network, which dictates cellular decisions in closure response to high CO2. Based on these analyses, we predict and experimentally confirm that applying nitric oxide (NO) induces stomatal closure in ambient CO2 and causes hypersensitivity to elevated CO2. Moreover, we predict a negative regulatory relationship between NO and the protein phosphatase ABI2 and find experimentally that NO inhibits ABI2 phosphatase activity. The experimental validation of these model predictions demonstrates the effectiveness of network-based modeling and highlights the decision-making role of the feedback core of the network in signal transduction. We further explore the model's potential in predicting targets of signaling elements not yet connected to the CO2 network. Our combination of network science, in silico model simulation, and experimental assays demonstrates an effective interdisciplinary approach to understanding system-level biology.

Sarm1 activation produces cADPR to increase intra-axonal Ca++ and promote axon degeneration in PIPN

Cancer patients frequently develop chemotherapy-induced peripheral neuropathy (CIPN), a painful and long-lasting disorder with profound somatosensory deficits. There are no effective therapies to prevent or treat this disorder. Pathologically, CIPN is characterized by a "dying-back" axonopathy that begins at intra-epidermal nerve terminals of sensory neurons and progresses in a retrograde fashion. Calcium dysregulation constitutes a critical event in CIPN, but it is not known how chemotherapies such as paclitaxel alter intra-axonal calcium and cause degeneration. Here, we demonstrate that paclitaxel triggers Sarm1-dependent cADPR production in distal axons, promoting intra-axonal calcium flux from both intracellular and extracellular calcium stores. Genetic or pharmacologic antagonists of cADPR signaling prevent paclitaxel-induced axon degeneration and allodynia symptoms, without mitigating the anti-neoplastic efficacy of paclitaxel. Our data demonstrate that cADPR is a calcium-modulating factor that promotes paclitaxel-induced axon degeneration and suggest that targeting cADPR signaling provides a potential therapeutic approach for treating paclitaxel-induced peripheral neuropathy (PIPN).

The Extracellular NADome Modulates Immune Responses

The term NADome refers to the intricate network of intracellular and extracellular enzymes that regulate the synthesis or degradation of nicotinamide adenine dinucleotide (NAD) and to the receptors that engage it. Traditionally, NAD was linked to intracellular energy production through shuffling electrons between oxidized and reduced forms. However, recent data indicate that NAD, along with its biosynthetic and degrading enzymes, has a life outside of cells, possibly linked to immuno-modulating non-enzymatic activities. Extracellular NAD can engage puriginergic receptors triggering an inflammatory response, similar - to a certain extent - to what described for adenosine triphosphate (ATP). Likewise, NAD biosynthetic and degrading enzymes have been amply reported in the extracellular space, where they possess both enzymatic and non-enzymatic functions. Modulation of these enzymes has been described in several acute and chronic conditions, including obesity, cancer, inflammatory bowel diseases and sepsis. In this review, the role of the extracellular NADome will be discussed, focusing on its proposed role in immunomodulation, together with the different strategies for its targeting and their potential therapeutic impact.

Ectonucleotidases in Blood Malignancies: A Tale of Surface Markers and Therapeutic Targets

Leukemia develops as the result of intrinsic features of the transformed cell, such as gene mutations and derived oncogenic signaling, and extrinsic factors, such as a tumor-friendly, immunosuppressed microenvironment, predominantly in the lymph nodes and the bone marrow. There, high extracellular levels of nucleotides, mainly NAD⁺ and ATP, are catabolized by different ectonucleotidases, which can be divided in two families according to substrate specificity: on one side those that metabolize NAD⁺, including CD38, CD157, and CD203a; on the other, those that convert ATP, namely CD39 (and other ENTPDases) and CD73. They generate products that modulate intracellular calcium levels and that activate purinergic receptors. They can also converge on adenosine generation with profound effects, both on leukemic cells, enhancing chemoresistance and homing, and on non-malignant immune cells, polarizing them toward tolerance. This review will first provide an overview of ectonucleotidases expression within the immune system, in physiological and pathological conditions. We will then focus on different hematological malignancies, discussing their role as disease markers and possibly pathogenic agents. Lastly, we will describe current efforts aimed at therapeutic targeting of this family of enzymes.

A new discrete dynamic model of ABA-induced stomatal closure predicts key feedback loops

Stomata, microscopic pores in leaf surfaces through which water loss and carbon dioxide uptake occur, are closed in response to drought by the phytohormone abscisic acid (ABA). This process is vital for drought tolerance and has been the topic of extensive experimental investigation in the last decades. Although a core signaling chain has been elucidated consisting of ABA binding to receptors, which alleviates negative regulation by protein phosphatases 2C (PP2Cs) of the protein kinase OPEN STOMATA 1 (OST1) and ultimately results in activation of anion channels, osmotic water loss, and stomatal closure, over 70 additional components have been identified, yet their relationships with each other and the core components are poorly elucidated. We integrated and processed hundreds of disparate observations regarding ABA signal transduction responses underlying stomatal closure into a network of 84 nodes and 156 edges and, as a result, established those relationships, including identification of a 36-node, strongly connected (feedback-rich) component as well as its in- and out-components. The network's domination by a feedback-rich component may reflect a general feature of rapid signaling events. We developed a discrete dynamic model of this network and elucidated the effects of ABA plus knockout or constitutive activity of 79 nodes on both the outcome of the system (closure) and the status of all internal nodes. The model, with more than 1024 system states, is far from fully determined by the available data, yet model results agree with existing experiments in 82 cases and disagree in only 17 cases, a validation rate of 75%. Our results reveal nodes that could be engineered to impact stomatal closure in a controlled fashion and also provide over 140 novel predictions for which experimental data are currently lacking. Noting the paucity of wet-bench data regarding combinatorial effects of ABA and internal node activation, we experimentally confirmed several predictions of the model with regard to reactive oxygen species, cytosolic Ca2+ (Ca2+c), and heterotrimeric G-protein signaling. We analyzed dynamics-determining positive and negative feedback loops, thereby elucidating the attractor (dynamic behavior) repertoire of the system and the groups of nodes that determine each attractor. Based on this analysis, we predict the likely presence of a previously unrecognized feedback mechanism dependent on Ca2+c. This mechanism would provide model agreement with 10 additional experimental observations, for a validation rate of 85%. Our research underscores the importance of feedback regulation in generating robust and adaptable biological responses. The high validation rate of our model illustrates the advantages of discrete dynamic modeling for complex, nonlinear systems common in biology.

Calcium Homeostasis and Ionic Mechanisms in Pulmonary Fibroblasts

Fibroblasts are key cellular mediators of many chronic interstitial lung diseases, including idiopathic pulmonary fibrosis, scleroderma, sarcoidosis, drug-induced interstitial lung disease, and interstitial lung disease in connective tissue disease. A great deal of effort has been expended to understand the signaling mechanisms underlying the various cellular functions of fibroblasts. Recently, it has been shown that Ca(2+) oscillations play a central role in the regulation of gene expression in human pulmonary fibroblasts. However, the mechanisms whereby cytosolic [Ca(2+)] are regulated and [Ca(2+)] oscillations transduced are both poorly understood. In this review, we present the general concepts of [Ca(2+)] homeostasis, of ionic mechanisms responsible for various Ca(2+) fluxes, and of regulation of gene expression by [Ca(2+)]. In each case, we then also summarize the original findings that pertain specifically to pulmonary fibroblasts. From these data, we propose an overall signaling cascade by which excitation of the fibroblasts triggers pulsatile release of internally sequestered Ca(2+), which, in turn, activates membrane conductances, including voltage-dependent Ca(2+) influx pathways. Collectively, these events produce recurring Ca(2+) oscillations, the frequency of which is transduced by Ca(2+)-dependent transcription factors, which, in turn, orchestrate a variety of cellular events, including proliferation, synthesis/secretion of extracellular matrix proteins, autoactivation (production of transforming growth factor-β), and transformation into myofibroblasts. That unifying hypothesis, in turn, allows us to highlight several specific cellular targets and therapeutic intervention strategies aimed at controlling unwanted pulmonary fibrosis. The relationships between Ca(2+) signaling events and the unfolded protein response and apoptosis are also explored.

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

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

Design, Synthesis, and Chemical and Biological Properties of Cyclic ADP-4-Thioribose as a Stable Equivalent of Cyclic ADP-Ribose

Here we describe the successful synthesis of cyclic ADP-4-thioribose (cADPtR, 3), designed as a stable mimic of cyclic ADP-ribose (cADPR, 1), a Ca2+-mobilizing second messenger, in which the key N1-β-thioribosyladenosine structure was stereoselectively constructed by condensation between the imidazole nucleoside derivative 8 and the 4-thioribosylamine 7 via equilibrium in 7 between the α-anomer (7α) and the β-anomer (7β) during the reaction course. cADPtR is, unlike cADPR, chemically and biologically stable, while it effectively mobilizes intracellular Ca2+ like cADPR in various biological systems, such as sea urchin homogenate, NG108-15 neuronal cells, and Jurkat T-lymphocytes. Thus, cADPtR is a stable equivalent of cADPR, which can be useful as a biological tool for investigating cADPR-mediated Ca2+-mobilizing pathways.

The pathogenic relevance of the prognostic markers CD38 and CD49d in chronic lymphocytic leukemia

The interactions of chronic lymphocytic leukemia cells with the microenvironment in secondary lymphoid tissues and the bone marrow are known to promote CLL cell survival and proliferation. CD38 and CD49d are both independent prognostic risk parameters in CLL with important roles in shaping these interactions. Both are reported to influence CLL cell trafficking between blood and lymphoid organs as well as their survival and proliferation within the lymphoid organs, thereby impacting the pathophysiology of the disease. The expression of CD38 and CD49d is associated in the majority of cases, and they exist as part of macromolecular complexes. Here, we review the current evidence for the individual and associated contributions of these molecules to CLL pathophysiology.

Nitrated cyclic GMP modulates guard cell signaling in Arabidopsis

Nitric oxide (NO) is a ubiquitous signaling molecule involved in diverse physiological processes, including plant senescence and stomatal closure. The NO and cyclic GMP (cGMP) cascade is the main NO signaling pathway in animals, but whether this pathway operates in plant cells, and the mechanisms of its action, remain unclear. Here, we assessed the possibility that the nitrated cGMP derivative 8-nitro-cGMP functions in guard cell signaling. Mass spectrometry and immunocytochemical analyses showed that abscisic acid and NO induced the synthesis of 8-nitro-cGMP in guard cells in the presence of reactive oxygen species. 8-Nitro-cGMP triggered stomatal closure, but 8-bromoguanosine 3',5'-cyclic monophosphate (8-bromo-cGMP), a membrane-permeating analog of cGMP, did not. However, in the dark, 8-bromo-cGMP induced stomatal opening but 8-nitro-cGMP did not. Thus, cGMP and its nitrated derivative play different roles in the signaling pathways that lead to stomatal opening and closure. Moreover, inhibitor and genetic studies showed that calcium, cyclic adenosine-5'-diphosphate-ribose, and SLOW ANION CHANNEL1 act downstream of 8-nitro-cGMP. This study therefore demonstrates that 8-nitro-cGMP acts as a guard cell signaling molecule and that a NO/8-nitro-cGMP signaling cascade operates in guard cells.

Plants, endosymbionts and parasites: Abscisic acid and calcium signaling

It was recently discovered that the protozoan parasite, Toxoplasma gondii produces and uses the plant hormone, abscisic acid (ABA), for communication. Following intracellular replication, ABA production influences the timing of parasite egress from the host cell. This density-dependent signal may serve to coordinate exit from the host cell in a synchronous manner by triggering calcium-dependent activation of motility. In the absence of ABA production, parasites undergo differentiation to the semidormant, tissue cyst. The pathway for ABA production in T. gondii may be derived from a relict endosymbiont, acquired by ingestion of a red algal cell. Although the parasite has lost the capacity for photosynthesis, the plant-like nature of this signaling pathway may be exploited to develop new drugs. In support of this idea, an inhibitor of ABA biosynthesis protected mice against lethal infection with T. gondii. Here, we compare the role of ABA in parasites to its activities in plants, where it is know to control development and stress responses.

Glycosides, depression and suicidal behaviour: the role of glycoside-linked proteins

Nowadays depression and suicide are two of the most important worldwide public health problems. Although their specific molecular mechanisms are still largely unknown, glycosides can play a fundamental role in their pathogenesis. These molecules act presumably through the up-regulation of plasticity-related proteins: probably they can have a presynaptic facilitatory effect, through the activation of several intracellular signaling pathways that include molecules like protein kinase A, Rap-1, cAMP, cADPR and G proteins. These proteins take part in a myriad of brain functions such as cell survival and synaptic plasticity. In depressed suicide victims, it has been found that their activity is strongly decreased, primarily in hippocampus and prefrontal cortex. These studies suggest that glycosides can regulate neuroprotection through Rap-1 and other molecules, and may play a crucial role in the pathophysiology of depression and suicide.

Catalytic properties of the retinal rod outer segment disk ADP-ribosyl cyclase

Cyclic ADP-ribose (cADPR) is a second messenger modulating intracellular calcium levels. We have previously described a cADPR-dependent calcium signaling pathway in bovine rod outer segments (ROS), where calcium ions play a pivotal role. ROS ADP-ribosyl cyclase (ADPR-cyclase) was localized in the membrane fraction. In the present work, we examined the properties of the disk ADPR-cyclase through the production of cyclic GDP-ribose from the NAD(+) analogue NGD(+). The enzyme displayed an estimated K(m) for NGD(+) of 12.5 ± 0.3 μM, a V(max) of 26.50 ± 0.70 pmol cyclic GDP-ribose synthesized/min/mg, and optimal pH of 6.5. The effect of divalent cations (Zn(2+), Cu(2+), and Ca(2+)) was also tested. Micromolar Zn(2+) and Cu(2+) inhibited the disk ADPR-cyclase activity (half maximal inhibitory concentration, IC50=1.1 and 3.6 μM, respectively). By contrast, Ca(2+) ions had no effect. Interestingly, the properties of the intracellular membrane-associated ROS disk ADPR-cyclase are more similar to those of the ADPR-cyclase found in CD38-deficient mouse brain, than to those of CD38 or CD157. The novel intracellular mammalian ADPR-cyclase would elicit Ca(2+) release from the disks at various rates in response to change in free Ca(2+) concentrations, caused by light versus dark adaptation, in fact there was no difference in disk ADPR-cyclase activity in light or dark conditions. Data suggest that disk ADPR-cyclase may be a potential target of retinal toxicity of Zn(2+) and may shed light to the role of Cu(2+) and Zn(2+) deficiency in retina.

Calcium regulation and signaling in apicomplexan parasites

Apicomplexan parasites rely on calcium-mediated signaling for a variety of vital functions including protein secretion, motility, cell invasion, and differentiation. These functions are controlled by a variety of specialized systems for uptake and release of calcium, which acts as a second messenger, and on the functions of calcium-dependent proteins. Defining these systems in parasites has been complicated by their evolutionary distance from model organisms and practical concerns in working with small, and somewhat fastidious cells. Comparative genomic analyses of Toxoplasma gondii, Plasmodium spp. and Cryptosporidium spp. reveal several interesting adaptations for calcium-related processes in parasites. Apicomplexans contain several P-type Ca2+ ATPases including an ER-type reuptake mechanism (SERCA), which is the proposed target of artemisinin. All three organisms also contain several genes related to Golgi PMR-like calcium transporters, and a Ca2+/H+ exchanger, while plasma membrane-type (PMCA) Ca2+ ATPases and voltage-dependent calcium channels are exclusively found in T. gondii. Pharmacological evidence supports the presence of IP3 and ryanodine channels for calcium-mediated release. Collectively these systems regulate calcium homeostasis and release calcium to act as a signal. Downstream responses are controlled by a family of EF-hand containing calcium binding proteins including calmodulin, and an array of centrin and caltractin-like genes. Most surprising, apicomplexans contain a diversity of calcium-dependent protein kinases (CDPK), which are commonly found in plants. Toxoplasma contains more than 20 CDPK or CDPK-like proteases, while Plasmodium and Cryptosporidium have fewer than half this number. Several of these CDPKs have been shown to play vital roles in protein secretion, invasion, and differentiation, indicating that disruption of calcium-regulated pathways may provide a novel means for selective inhibition of parasites.

The base exchange reaction of NAD+ glycohydrolase: identification of novel heterocyclic alternative substrates

ADP-ribosyl cyclase and NAD+ glycohydrolase (CD38, E.C.3.2.2.5) efficiently catalyze the exchange of the nicotinamidyl moiety of NAD+, nicotinamide adenine dinucleotide phosphate (NADP+) or nicotinamide mononucleotide (NMN+) with an alternative base. 4'-Pyridinyl drugs (amrinone, milrinone, dismerinone and pinacidil) were efficient alternative substrates (k(cat)/K(M)=0.9-10 microM(-1)s(-1)) in the exchange reaction with ADP-ribosyl cyclase. When CD38 was used as a catalyst the k(cat)/K(M) values for the exchange reaction were reduced two or more orders of magnitude (0.015-0.15 microM(-1)s(-1)). The products of this reaction were novel dinucleotides. The values of the equilibrium constants for dinucleotide formation were determined for several drugs. These enzymes also efficiently catalyze the formation of novel mononucleotides in an exchange reaction with NMN+, k(cat)/K(M)=0.05-0.4 microM(-1)s(-1). The k(cat)/K(M) values for the exchange reaction with NMN+ were generally similar (0.04-0.12 microM(-1)s(-1)) with CD38 and ADP-ribosyl cyclase as catalysts. Several novel heterocyclic alternative substrates were identified as 2-isoquinolines, 1,6-naphthyridines and tricyclic bases. The k(cat)/K(M) values for the exchange reaction with these substrates varied over five orders of magnitude and approached the limit of diffusion with 1,6-naphthyridines. The exchange reaction could be used to synthesize novel mononucleotides or to identify novel reversible inhibitors of CD38.

Evolution and function of the ADP ribosyl cyclase/CD38 gene family in physiology and pathology

The membrane proteins CD38 and CD157 belong to an evolutionarily conserved family of enzymes that play crucial roles in human physiology. Expressed in distinct patterns in most tissues, CD38 (and CD157) cleaves NAD(+) and NADP(+), generating cyclic ADP ribose (cADPR), NAADP, and ADPR. These reaction products are essential for the regulation of intracellular Ca(2+), the most ancient and universal cell signaling system. The entire family of enzymes controls complex processes, including egg fertilization, cell activation and proliferation, muscle contraction, hormone secretion, and immune responses. Over the course of evolution, the molecules have developed the ability to interact laterally and frontally with other surface proteins and have acquired receptor-like features. As detailed in this review, the loss of CD38 function is associated with impaired immune responses, metabolic disturbances, and behavioral modifications in mice. CD38 is a powerful disease marker for human leukemias and myelomas, is directly involved in the pathogenesis and outcome of human immunodeficiency virus infection and chronic lymphocytic leukemia, and controls insulin release and the development of diabetes. Here, the data concerning diseases are examined in view of potential clinical applications in diagnosis, prognosis, and therapy. The concluding remarks try to frame all of the currently available information within a unified working model that takes into account both the enzymatic and receptorial functions of the molecules.

Chemotaxis of mouse bone marrow neutrophils and dendritic cells is controlled by adp-ribose, the major product generated by the CD38 enzyme reaction

The ectoenzyme CD38 catalyzes the production of cyclic ADP-ribose (cADPR) and ADP-ribose (ADPR) from its substrate, NAD(+). Both products of the CD38 enzyme reaction play important roles in signal transduction, as cADPR regulates calcium release from intracellular stores and ADPR controls cation entry through the plasma membrane channel TRPM2. We previously demonstrated that CD38 and the cADPR generated by CD38 regulate calcium signaling in leukocytes stimulated with some, but not all, chemokines and controls leukocyte migration to inflammatory sites. However, it is not known whether the other CD38 product, ADPR, also regulates leukocyte trafficking In this study we characterize 8-bromo (8Br)-ADPR, a novel compound that specifically inhibits ADPR-activated cation influx without affecting other key calcium release and entry pathways. Using 8Br-ADPR, we demonstrate that ADPR controls calcium influx and chemotaxis in mouse neutrophils and dendritic cells activated through chemokine receptors that rely on CD38 and cADPR for activity, including mouse FPR1, CXCR4, and CCR7. Furthermore, we show that the calcium and chemotactic responses of leukocytes are not dependent on poly-ADP-ribose polymerase 1 (PARP-1), another potential source of ADPR in some leukocytes. Finally, we demonstrate that NAD(+) analogues specifically block calcium influx and migration of chemokine-stimulated neutrophils without affecting PARP-1-dependent calcium responses. Collectively, these data identify ADPR as a new and important second messenger of mouse neutrophil and dendritic cell migration, suggest that CD38, rather than PARP-1, may be an important source of ADPR in these cells, and indicate that inhibitors of ADPR-gated calcium entry, such as 8Br-ADPR, have the potential to be used as anti-inflammatory agents.

Endothelin-A and -B receptors, superoxide, and Ca2+ signaling in afferent arterioles

It is unknown if endothelin-A and -B receptors (ET(A)R and ET(B)R) activate the production of superoxide via NAD(P)H oxidase and subsequently stimulate the formation of cyclic adenine diphosphate ribose (cADPR) in afferent arterioles. Vessels were isolated from rat kidney and loaded with fura 2. Endothelin-1 (ET-1) rapidly increased cytosolic Ca(2+) concentration ([Ca(2+)](i)) by 303 nM. The superoxide dismutase mimetic tempol, the NAD(P)H oxidase inhibitor apocynin, and nicotinamide, an inhibitor of ADPR cyclase, diminished the response by approximately 60%. The ET(B)R agonist sarafotoxin 6c (S6c) increased peak [Ca(2+)](i) by 117 nM. Subsequent addition of ET-1 in the continued presence of S6c caused an additional [Ca(2+)](i) peak of 225 nM. Neither nicotinamide or 8-bromo- (8-Br) cADPR nor apocynin decreased the [Ca(2+)](i) response to S6c, but inhibited the subsequent [Ca(2+)](i) response to ET-1. The ET(B)R blockers BQ-788 and A-192621 prevented the S6c [Ca(2+)](i) peak and reduced the ET-1 response by more than one-half, suggesting an ET(B)R/ET(A)R interaction. In contrast, the ET(A)R blocker BQ-123 had no effect on the S6c [Ca(2+)](i) peak and obliterated the subsequent ET-1 response. ET-1 immediately stimulated superoxide formation (measured with TEMPO-9-AC, 68 arbitrary units) that was inhibited 95% by apocynin or diphenyl iodonium. S6c or IRL-1620 increased superoxide by 8% of that caused by subsequent ET-1 addition. We conclude that ET(A)R activation of afferent arterioles increases the formation of superoxide that accounts for approximately 60% of subsequent Ca(2+) signaling. ET(B)R activation appears to result in only minor increases in superoxide production. Nicotinamide and 8-Br-cADPR results suggest that ET-1 (and primarily ET(A)R) causes the activation of vascular smooth muscle cell-ADPR cyclase.

Modulation of store-operated Ca2+ entry by cyclic-ADP-ribose

Store-operated Ca2+ entry plays an important role in Ca2+ homeostasis in cells but the mechanisms of control of these channels are not completely understood. We describe an investigation of the role of the CD38-cyclic-ADP-ribose (cADPR)-ryanodine-channel (RyR) signaling pathway in store-operated Ca2+ entry in human smooth muscle. We observed that human myometrial cells have a functional store-operated Ca2+ entry mechanism. Furthermore, we observed the presence of transient receptor potential 1, 3, 4, 5, and 6 ion channels in human myometrial cells. Store-operated Ca2+ transient was inhibited by at least 50-70% by several inhibitors of the RyR, including ryanodine (10 microM), dantrolene (10 microM), and ruthenium red (10 microM). Furthermore, the cell permeable inhibitor of the cADPR-system, 8-Br-cADPR (100 microM), is a potent inhibitor of the store-operated entry, decreasing the store operated entry by 80%. Pre-incubation of cells with 100 microM cADPR and the hydrolysis-resistant cADPR analog 3-deaza-cADPR (50 microM), but not with ADP-ribose (ADPR) leads to a 1.6-fold increase in the store-operated Ca2+ transient. In addition, we observed that nicotinamide (1-10 mM), an inhibitor of cADPR synthesis, also leads to inhibition of the store-operated Ca2+ transient by 50-80%. Finally, we observed that the transient receptor potential channels, RyR, and CD38 can be co-immunoprecipitated, indicating that they interact in vivo. Our observations clearly implicate the CD38-cADPR-ryanodine signaling pathway in the regulation of store-operated Ca2+ entry in human smooth muscle cells.

The CD38-independent ADP-ribosyl cyclase from mouse brain synaptosomes: a comparative study of neonate and adult brain

cADPR (cADP-ribose), a metabolite of NAD+, is known to modulate intracellular calcium levels and to be involved in calcium-dependent processes, including synaptic transmission, plasticity and neuronal excitability. However, the enzyme that is responsible for producing cADPR in the cytoplasm of neural cells, and particularly at the synaptic terminals of neurons, remains unknown. In the present study, we show that endogenous concentrations of cADPR are much higher in embryonic and neonate mouse brain compared with the adult tissue. We also demonstrate, by comparing wild-type and Cd38-/- tissues, that brain cADPR content is independent of the presence of CD38 (the best characterized mammalian ADP-ribosyl cyclase) not only in adult but also in developing tissues. We show that Cd38-/- synaptosome preparations contain high ADP-ribosyl cyclase activities, which are more important in neonates than in adults, in line with the levels of endogenous cyclic nucleotide. By using an HPLC method and adapting the cycling assay developed initially to study endogenous cADPR, we accurately examined the properties of the synaptosomal ADP-ribosyl cyclase. This intracellular enzyme has an estimated K(m) for NAD+ of 21 microM, a broad optimal pH at 6.0-7.0, and the concentration of free calcium has no major effect on its cADPR production. It binds NGD+ (nicotinamide-guanine dinucleotide), which inhibits its NAD+-metabolizing activities (K(i)=24 microM), despite its incapacity to cyclize this analogue. Interestingly, it is fully inhibited by low (micromolar) concentrations of zinc. We propose that this novel mammalian ADP-ribosyl cyclase regulates the production of cADPR and therefore calcium levels within brain synaptic terminals. In addition, this enzyme might be a potential target of neurotoxic Zn2+.

Adenosine2A receptor vasodilation of rat preglomerular microvessels is mediated by EETs that activate the cAMP/PKA pathway

Dilation of rat preglomerular microvessels (PGMV) by activation of adenosine A2A receptors (A2AR) is coupled to epoxyeicosatrienoic acid (EET) release. We have investigated the commonality of this signal transduction pathway, i.e., sequential inhibition of G(salpha), adenylyl cyclase, PKA, and Ca2+-activated K+ (KCa) channel activity, to the vasoactive responses to A2AR activation by a selective A2A agonist, CGS-21680, compared with those of 11,12-EET. Male Sprague-Dawley rats were anesthetized, and microdissected arcuate arteries (110-130 microm) were cannulated and pressurized to 80 mmHg. Vessels were superfused with Krebs solution containing NG-nitro-L-arginine methyl ester (L-NAME) and indomethacin and preconstricted with phenylephrine. We assessed the effect of 3-aminobenzamide (10 microM), an inhibitor of mono-ADP-ribosyltranferases, on responses to 11,12-EET (3 nM) and CGS-21680 (10 microM) and found that both were inhibited by approximately 70% (P<0.05), whereas the response to SNP (10 microM) was unaffected. Furthermore, 11,12-EET (100 nM), like cholera toxin (100 ng/ml), stimulated ADP-ribose formation in homogenates of arcuate arteries compared with control. SQ-22536 (10 microM), an inhibitor of adenylyl cyclase activity, and myristolated PKI (14-22) amide (5 microM), an inhibitor of PKA, decreased activity of 11,12-EET and CGS-21680. Incubation of 11,12-EET (3 nM-3 microM) with PGMV resulted in an increase in cAMP levels (P<0.05). The responses to both 11,12-EET and CGS-21680 were significantly reduced by superfusion of iberiotoxin (100 nM), an inhibitor of KCa channel activity. Thus in rat PGMV activation of A2AR is coupled to EET release upstream of adenylyl cyclase activation and EETs stimulate mono-ADP-ribosyltransferase, resulting in Gsalpha protein activation.

Capsaicin stimulates the non-store-operated Ca2+ entry but inhibits the store-operated Ca2+ entry in neutrophils

Rat neutrophils express the mRNA encoding for transient receptor potential (TRP) V1. However, capsaicin-stimulated [Ca2+]i elevation occurred only at high concentrations (> or = 100 microM). This response was substantially decreased in a Ca2+-free medium. Vanilloids displayed similar patterns of Ca2+ response with the rank order of potency as follows: scutigeral>resiniferatoxin>capsazepine>capsaicin=olvanil>isovelleral. Arachidonyl dopamine (AAD), an endogenous ligand for TRPV1, failed to desensitize the subsequent capsaicin challenge. Capsaicin-induced Ca2+ response was not affected by 8-bromo-cyclic ADP-ribose (8-Br-cADPR), the ryanodine receptor blocker, but was slightly attenuated by 1-[6-[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), the inhibitor of phospholipase C-coupled processes, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF-96365), the blocker of receptor-gated and store-operated Ca2+ (SOC) channels, 2-aminoethyldiphenyl borate (2-APB), the blocker of D-myo-inositol 1,4,5-trisphospahte (IP3) receptor and Ca2+ influx, and by ruthenium red, a blocker of TRPV channels, and enhanced by the Ca2+ channels blocker, cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine (MDL-12330A) and Na+-deprivation. In addition, capsaicin had no effect on the plasma membrane Ca2+-ATPase activity or the production of nitric oxide (NO) and reactive oxygen intermediates (ROI) or on the total thiols content. Capsaicin (> or = 100 microM) inhibited the cyclopiazonic acid (CPA)-induced store-operated Ca2+ entry (SOCE). In the absence of external Ca2+, the robust Ca2+ entry after subsequent addition of Ca2+ was decreased by capsaicin in CPA-activated cells. Capsaicin alone increased the actin cytoskeleton, and also increased the actin filament content in cell activation with CPA. These results indicate that capsaicin activates a TRPV1-independent non-SOCE pathway in neutrophils. The reorganization of the actin cytoskeleton is probably involved in the capsaicin inhibition of SOCE.

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

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

Ca2+ release via ryanodine receptors and Ca2+ entry: major mechanisms in NAADP-mediated Ca2+ signaling in T-lymphocytes

Nicotinic acid adenine dinucleotide phosphate (NAADP) is a potent Ca2+ mobilizing nucleotide essentially involved in T cell activation. Using combined microinjection and single cell calcium imaging, we demonstrate that co-injection of NAADP and the D-myo-inositol 1,4,5-trisphosphate antagonist heparin did not inhibit Ca2+ mobilization. In contrast, co-injection of the ryanodine receptor antagonist ruthenium red efficiently blocked NAADP induced Ca2+ signalling. This pharmacological approach was confirmed using T cell clones stably transfected with plasmids expressing antisense mRNA targeted specifically against ryanodine receptors. NAADP induced Ca2+ signaling was strongly reduced in these clones. In addition, inhibition of Ca2+ entry by SK&F 96365 resulted in a dramatically decreased Ca2+ signal upon NAADP injection. Gd3+, a known blocker of Ca2+ release activated Ca2+ entry, only partially inhibited NAADP mediated Ca2+ signaling. These data indicate that in T cells (i) ryanodine receptor are the major intracellular Ca2+ release channels involved in NAADP induced Ca2+ signals, and that (ii) such Ca2+ release events are largely amplified by Ca2+ entry.

Endothelin-1, superoxide and adeninediphosphate ribose cyclase in shark vascular smooth muscle

In vascular smooth muscle (VSM) of Squalus acanthias, endothelin-1(ET-1) signals via the ETB receptor. In both shark and mammalian VSM, ET-1 induces a rise in cytosolic Ca2+ concentration([Ca2+]i) via activation of the inositol trisphosphate (IP3) receptor (IP3R) and subsequent release of Ca2+ from the sarcoplasmic reticulum (SR). IP3R-mediated release of SR Ca2+ causes calcium-induced calcium release (CICR) via the ryanodine receptor (RyR), which can be sensitized by cyclic adeninediphosphate ribose (cADPR). cADPR is synthesized from NAD+ by a membrane-bound bifunctional enzyme, ADPR cyclase. We have previously shown that the antagonists of the RyR, Ruthenium Red, high concentrations of ryanodine and 8-Br cADPR, diminish the[Ca2+]i response to ET-1 in shark VSM. To investigate how ET-1 might influence the activity of the ADPR cyclase, we employed inhibitors of the cyclase. To explore the possibility that ET-1-induced production of superoxide (O2.-) might activate the cyclase, we used an inhibitor of NAD(P)H oxidase (NOX), DPI and a scavenger of O2.-, TEMPOL. Anterior mesenteric artery VSM was loaded with fura-2AM to measure [Ca2+]i. In Ca2+-free shark Ringers, ET-1 increased[Ca2+]i by 104±8 nmol l-1. The VSM ADPR cyclase inhibitors, nicotinamide and Zn2+, diminished the response by 62% and 72%, respectively. Both DPI and TEMPOL reduced the response by 63%. The combination of the IP3R antagonists, 2-APB or TMB-8, with DPI or TEMPOL further reduced the response by 83%. We show for the first time that in shark VSM, inhibition of the ADPR cyclase reduces the[Ca2+]i response to ET-1 and that superoxide may be involved in the activation of the cyclase.

Angiotensin II Ca2+ signaling in rat afferent arterioles: stimulation of cyclic ADP ribose and IP3 pathways

ANG II induces a rise in cytosolic Ca(2+) ([Ca(2+)](i)) in vascular smooth muscle (VSM) cells via inositol trisphosphate receptor (IP(3)R) activation and release of Ca(2+) from the sarcoplasmic reticulum (SR). The Ca(2+) signal is augmented by calcium-induced calcium release (CICR) and by cyclic adeninediphosphate ribose (cADPR), which sensitizes the ryanodine-sensitive receptor (RyR) to Ca(2+) to further amplify CICR. cADPR is synthesized from beta-nicotinamide adenine dinucleotide (NAD(+)) by a membrane-bound bifunctional enzyme, ADPR cyclase. To investigate the possibility that ANG II activates the ADPR cyclase of afferent arterioles, we used inhibitors of the IP(3)R, RyR, and ADPR cyclase. Afferent arterioles were isolated from rat kidney with the magnetized microsphere and sieving technique and loaded with fura-2 to measure [Ca(2+)](i). In Ca(2+)-containing buffer, ANG II increased [Ca(2+)](i) by 125 +/- 10 nM. In the presence of the IP(3)R antagonists TMB-8 and 2-APB, the peak responses to ANG II were reduced by 74 and 81%, respectively. The specific antagonist of cADPR 8-Br ADPR and a high concentration of ryanodine (100 microM) inhibited the ANG II-induced increases in [Ca(2+)](i) by 75 and 69%, respectively. Nicotinamide and Zn(2+) are known inhibitors of the VSM ADPR cyclase. Nicotinamide diminished the [Ca(2+)](i) response to ANG II by 66%. In calcium-free buffer, Zn(2+) reduced the ANG II response by 68%. Simultaneous blockade of the IP(3) and cADPR pathways diminished the [Ca(2+)](i) response to ANG II by 83%. We conclude that ANG II initiates Ca(2+) mobilization from the SR in afferent arterioles via the classic IP(3)R pathway and that ANG II may lead to activation of the ADPR cyclase to form cADPR, which, via its action on the RyR, substantially augments the Ca(2+) response.

Stimulation of cellular free Ca2+ elevation and inhibition of store-operated Ca2+ entry by kazinol B in neutrophils

Kazinol B, a natural isoprenylated flavan, stimulated the [Ca(2+)](i) elevation in the presence or absence of Ca(2+) in the medium. Treatment with chymotrypsin or phorbol 12-myristate 13-acetate to shedding of L: -selectin had no effect on subsequent kazinol B-induced Ca(2+) response. Upon initial cyclopiazonic acid (CPA) treatment in the absence of external Ca(2+), the subsequent [Ca(2+)](i) rise followed by challenge with kazinol B was greatly diminished. The ryanodine receptor blockers, 8-bromo-cyclic ADP-ribose and ruthenium red did not affect kazinol B-evoked Ca(2+) release from internal stores. However, the inhibitors of sphingosine kinase, dimethylsphingosine, but not dihydrosphingosine, inhibited kazinol B-induced Ca(2+) release. Kazinol B-induced [Ca(2+)](i) rise was not affected by two nitric oxidase inhibitors, N-(3-aminomethyl)benzylacetamidine (1400W) and 7-nitroindazole, cytochalasin B and Na(+)-deprivation. This response was slightly attenuated by 2-aminoethyldiphenyl borate (2-APB), a D: -myo-inositol 1,4,5-trisphosphate (IP(3)) receptor blocker, and by genistein, a general tyrosine kinase inhibitor. However, the Ca(2+) response was greatly diminished by two actin filament reorganizers, calyculin A and jasplakinolide, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY 294002), an inhibitor of phosphoinositide 3-kinase, N-(3-aminomethyl)benzylacetamidine (SB 203580), the p38 mitogen-activated protein kinase inhibitor, 1-[6-[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U-73122), the inhibitor of phospholipase C-coupled processes, and by 0.3 mM La(3+) or Ni(2+). Kazinol B did not evoke any appreciable Ba(2+) and Sr(2+) entry into cells. The Ca(2+) entry blockers, 1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1H-imidazole (SKF-96365), but not cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine (MDL-12,330A), inhibited a kazinol B-induced [Ca(2+)](i) rise. Kazinol B had no effect on the pharmacologically isolated plasma membrane Ca(2+)-ATPase activity. In a Ca(2+)-free medium, kazinol B inhibited the subsequent Ca(2+) addition, resulting in robust entry in CPA- and formyl peptide-activated cells. Kazinol B produced a concentration-dependent reduction in the mitochondrial membrane potential. These results indicate that kazinol B stimulates Ca(2+) release from internal Ca(2+) store, probably through the sphingosine 1-phosphate and IP(3) signaling, and activates external Ca(2+) influx mainly through a non-store-operated Ca(2+) entry (non-SOCE) pathway. Inhibition of SOCE by kazinol B is probably attributable to a break in the Ca(2+) driven force of mitochondria.

Altered airway responsiveness in CD38-deficient mice

Cyclic ADP-ribose (cADPR) mobilizes calcium from intracellular stores and contributes to agonist-induced intracellular calcium elevation in airway smooth muscle (ASM). In this study we determined the functional role of CD38/cADPR signaling in the regulation of airway tone using CD38 deficient (cd38(-/-)) mice. The responsiveness to different doses of methacholine, as determined by changes in lung resistance and dynamic compliance, was significantly (P < or = 0.05) lower in cd38(-/-) mice compared with wild-type controls. To determine the mechanism responsible for the reduced responsiveness, we measured the intracellular calcium responses to contractile agonists in ASM cells. In ASM cells isolated from cd38(-/-) mice, the intracellular calcium responses to acetylcholine and endothelin-1 were significantly lower than in controls. Pretreatment of ASM cells with a cADPR antagonist resulted in attenuated intracellular calcium responses to endothelin-1 in cells isolated from wild-type mice, but not in those isolated from the cd38(-/-) mice. Very low cADPR levels and no detectable ADP-ribosyl cyclase activity were observed in lung tissue from cd38(-/-) mice, suggesting that CD38 is a critical source for cADPR synthesis. The results of the present study demonstrate that CD38/cADPR contributes to airway smooth muscle tone and responsiveness through its effects on agonist-induced elevation of intracellular calcium in ASM cells.

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

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

Channelling of substrate promiscuity of the skeletal-muscle ADP-ribosyl cyclase isoform

The novel Ca2+-mobilizing second messengers cADPr (cyclic ADP-ribose) and NAADP (nicotinic acid-adenine dinucleotide phosphate) are both synthesized by ADP-ribosyl cyclases. Using HSR (heavy sarcoplasmic reticulum) fractions from rabbit skeletal muscle, NAADP-induced Ca2+ release was observed. In the present paper, we show in HSR membranes the formation of authentic cADPr, cGDPr (cyclic GDP-ribose) and NAADP. The cyclization reaction to form cADPr and cGDPr as well as the base-exchange reaction to form NAADP were strictly dependent on pH. Although the formation of cGDPr is optimized at pH 6, the synthesis of NAADP was most pronounced at a pH below 5. A novel regulation mechanism is provided for nicotinic acid, the co-substrate for NAADP synthesis. Nicotinic acid had virtually no influence on the cyclization reaction, but increased the affinity of NADP at an acidic pH and had the opposite effect at alkaline pH. Nicotinamide, the side product of cADPr synthesis, is an inhibitor of the cyclization reaction (IC50, 0.7+/-0.1 mM) and was 30-fold more potent at suppressing the base-exchange reaction. Although the synthesis of NAADP was highly sensitive to nicotinamide inhibition, this was not via a competition with the nicotinic-acid-binding site. In contrast with the ecto-ADP-ribosyl cyclase (CD38), the cyclization and base-exchange reaction of the skeletal muscle isoform was inhibited by Cu2+ and Zn2+, while other bivalent cations such as Ca2+, Mg2+ and Mn2+ had virtually no effect. These findings allow for the prediction of a novel ADP-ribosyl cyclase isoform in skeletal muscle HSR, other than CD38. Hence the enzymic prerequisite for cADPr- and NAADP-mediated Ca2+ signalling is present.

Methodologic advancements in the study of airway smooth muscle

The study of isolated airway myocytes has provided important information relative to specific processes that regulate contraction, proliferation, and synthetic properties of airway smooth muscle (ASM). To place this information in physiological context, however, improved methods to examine airway biology in vivo are needed. Advances in genetic, biochemical, and optical methods provide unprecedented opportunities to improve our understanding of in vivo physiology and pathophysiology. This article describes 4 important methodologic advances in the study of ASM: (1) the development of transgenic mice that could be used to investigate ASM proliferation and phenotype switching during the development of hypersensitivity, and to investigate excitation-contraction coupling; (2) the use of CD38-deficient mice to confirm the role of CD38-dependent, cyclic adenosine diphosphate-ribose-mediated calcium release in airway responsiveness; (3) investigation of the role of actin filament length and p38 mitogen-activated protein kinase activity in regulating the mechanical plasticity-elasticity balance in contracted ASM; and (d) the use of bronchial biopsies to study ASM structure and phenotype in respiratory science.

Innate immunity is regulated by CD38, an ecto-enzyme with ADP-ribosyl cyclase activity

Through its production of cyclic adenosine diphosphate ribose, the ecto-enzyme CD38 regulates calcium mobilization in neutrophils responding to some, but not all, chemoattractants. This signaling defect results in reduced chemotaxis of CD38-deficient neutrophils to bacterial peptides and increased susceptibility of CD38-deficient mice to bacterial infections.

Regulation of intracellular Ca2+ stores by multiple Ca2+-releasing messengers

Although glucose-elicited insulin secretion depends on Ca(2+) entry through voltage-gated Ca(2+) channels in the surface cell membrane of the pancreatic beta-cell, there is also ample evidence for an important role of intracellular Ca(2+) stores, particularly in relation to hormone- or neurotransmitter-induced insulin secretion. There is now direct evidence for Ca(2+) entry-induced release of Ca(2+) from the endoplasmic reticulum in neurons, but with regard to glucose stimulation of beta-cells, there is conflicting evidence about the operation of such a process. This finding suggests that the sensitivity of the Ca(2+) release channels in the endoplasmic reticulum membrane varies under different conditions and therefore is regulated. Recent evidence from studies of pancreatic acinar cells has revealed combinatorial roles of multiple messengers in setting the sensitivity of the endoplasmic reticulum for Ca(2+) release. Here we focus on the possible combinatorial roles of inositol 1,4,5-trisphosphate, cyclic ADP-ribose, and nicotinic acid adenine dinucleotide phosphate in beta-cell function.

The cyclic-ADP-ribose signaling pathway in human myometrium

Human myometrial contraction plays a fundamental role in labor. Dysfunction of uterine contraction is an important cause of failure in progression of labor. The mechanisms of control of uterine contractions are not completely understood. It appears that intracellular Ca(2+) mobilization may play an important role during uterine contraction. Several mechanisms of intracellular Ca(2+) mobilization have been described. However, in human uterus only the inositol 1,4,5-trisphosphate-induced Ca(2+) release has been extensively studied to date. In view of the identification of the presence of functional ryanodine channels in myometrium, we explored the role of the endogenous regulator of the ryanodine channel cyclic-ADP-ribose in human myometrial Ca(2+) regulation. Cyclic-ADP-ribose (cADPR) is a naturally occurring nucleotide implicated in the regulation of the gating properties of the ryanodine channel, in fact cADPR may be a second messenger that activates the ryanodine receptor. Here we explore the components of the cADPR system in human myometrium. We found that human myometrium contains all the components of the cADPR pathway including (1) cADPR-activated microsomal Ca(2+) release and (2) enzymes responsible for synthesis and degradation of cADPR and, furthermore, that intracellular levels of cADPR were detected in human myometrial tissue. These data indicate that the cADPR system is present and operational in human myometrial tissue. Further research is warranted to determine the role of this new signaling molecule in uterine contraction.

Nicotinic acid-adenine dinucleotide phosphate activates the skeletal muscle ryanodine receptor

Calcium is a universal second messenger. The temporal and spatial information that is encoded in Ca(2+)-transients drives processes as diverse as neurotransmitter secretion, axonal outgrowth, immune responses and muscle contraction. Ca(2+)-release from intracellular Ca(2+) stores can be triggered by diffusible second messengers like Ins P (3), cyclic ADP-ribose or nicotinic acid-adenine dinucleotide phosphate (NAADP). A target has not yet been identified for the latter messenger. In the present study we show that nanomolar concentrations of NAADP trigger Ca(2+)-release from skeletal muscle sarcoplasmic reticulum. This was due to a direct action on the Ca(2+)-release channel/ryanodine receptor type-1, since in single channel recordings, NAADP increased the open probability of the purified channel protein. The effects of NAADP on Ca(2+)-release and open probability of the ryanodine receptor occurred over a similar concentration range (EC(50) approximately 30 nM) and were specific because (i) they were blocked by Ruthenium Red and ryanodine, (ii) the precursor of NAADP, NADP, was ineffective at equimolar concentrations, (iii) NAADP did not affect the conductance and reversal potential of the ryanodine receptor. Finally, we also detected an ADP-ribosyl cyclase activity in the sarcoplasmic reticulum fraction of skeletal muscle. This enzyme was not only capable of synthesizing cyclic GDP-ribose but also NAADP, with an activity of 0.25 nmol/mg/min. Thus, we conclude that NAADP is generated in the vicinity of type 1 ryanodine receptor and leads to activation of this ion channel.

Toxoplasma gondii microneme secretion involves intracellular Ca(2+) release from inositol 1,4,5-triphosphate (IP(3))/ryanodine-sensitive stores

Calcium-mediated microneme secretion in Toxoplasma gondii is stimulated by contact with host cells, resulting in the discharge of adhesins that mediate attachment. The intracellular source of calcium and the signaling pathway(s) triggering release have not been characterized, prompting our search for mediators of calcium signaling and microneme secretion in T. gondii. We identified two stimuli of microneme secretion, ryanodine and caffeine, which enhanced release of calcium from parasite intracellular stores. Ethanol, a previously characterized trigger of microneme secretion, stimulated an increase in parasite inositol 1,4,5-triphosphate, implying that this second messenger may mediate intracellular calcium release. Consistent with this observation, xestospongin C, an inositol 1,4,5-triphosphate receptor antagonist, inhibited microneme secretion and blocked parasite attachment and invasion of host cells. Collectively, these results suggest that T. gondii possess an intracellular calcium release channel with properties of the inositol 1,4,5-triphosphate/ryanodine receptor superfamily. Intracellular calcium channels, previously studied almost exclusively in multicellular animals, appear to also be critical to the control of parasite calcium during the initial steps of host cell entry.

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.

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

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

Assay for ADP-ribosyl cyclase by reverse-phase high-performance liquid chromatography

Cyclic ADP-ribose (cADPR), a natural metabolite of beta-NAD(+), is a second messenger for Ca(2+) signaling in T cells. As a tool for purification and identification of ADP-ribosyl cyclase(s) in T cells, a sensitive and specific enzymatic assay using 1,N(6)-etheno-NAD(+) as substrate was developed. A major problem-the sensitivity of 1,N(6)-etheno-cADPR toward the extraction medium perchloric acid-was solved by replacing the perchloric acid extraction procedure of nucleotides by a filtration step. Standard compounds for the HPLC analysis of ADP-ribosyl cyclases and NAD(+)-glycohydrolases, e.g., 1,N(6)-etheno-cADPR, 1,N(6)-etheno-ADPR, and 1,N(6)-etheno-AMP, were produced by ADP-ribosyl cyclase from Aplysia californica and dinucleotide pyrophosphatase. The assay was applied to subcellular fractions prepared from human Jurkat T cells. As a result ADP-ribosyl cyclase and NAD(+)-glycohydrolase activity could be detected and precisely quantified in different subcellular fractions indicating the presence of different isoenzymes in T cells.

Cyclic ADP-ribose production by CD38 regulates intracellular calcium release, extracellular calcium influx and chemotaxis in neutrophils and is required for bacterial clearance in vivo

Cyclic ADP-ribose is believed to be an important calcium-mobilizing second messenger in invertebrate, mammalian and plant cells. CD38, the best-characterized mammalian ADP-ribosyl cyclase, is postulated to be an important source of cyclic ADP-ribose in vivo. Using CD38-deficient mice, we demonstrate that the loss of CD38 renders mice susceptible to bacterial infections due to an inability of CD38-deficient neutrophils to directionally migrate to the site of infection. Furthermore, we show that cyclic ADP-ribose can directly induce intracellular Ca++ release in neutrophils and is required for sustained extracellular Ca++ influx in neutrophils that have been stimulated by the bacterial chemoattractant, formyl-methionyl-leucyl-phenylalanine (fMLP). Finally, we demonstrate that neutrophil chemotaxis to fMLP is dependent on Ca++ mobilization mediated by cyclic ADP-ribose. Thus, CD38 controls neutrophil chemotaxis to bacterial chemoattractants through its production of cyclic ADP-ribose, and acts as a critical regulator of inflammation and innate immune responses.

Transient tyrosine phosphorylation of human ryanodine receptor upon T cell stimulation

The ryanodine receptor of Jurkat T lymphocytes was phosphorylated on tyrosine residues upon stimulation of the cells via the T cell receptor/CD3 complex. The tyrosine phosphorylation was transient, reaching a maximum at 2 min, and rapidly declined thereafter. In co-immunoprecipitates of the ryanodine receptor, the tyrosine kinases p56(lck) and p59(fyn) were detected. However, only p59(fyn) associated with the ryanodine receptor in a stimulation-dependent fashion. Both tyrosine kinases, recombinantly expressed as glutathione S-transferase (GST) fusion proteins, phosphorylated the immunoprecipitated ryanodine receptor in vitro. In permeabilized Jurkat T cells, GST-p59(fyn), but not GST-p56(lck), GST-Grb2, or GST alone, significantly and concentration-dependently enhanced Ca(2+) release by cyclic ADP-ribose. The tyrosine kinase inhibitor PP2 specifically blocked the effect of GST-p59(fyn). This indicates that intracellular Ca(2+) release via ryanodine receptors may be modulated by tyrosine phosphorylation during T cell activation.

Selected contribution: effect of volatile anesthetics on cADP-ribose-induced Ca(2+) release system

Volatile anesthetics have multiple actions on intracellular Ca(2+) homeostasis, including activation of the ryanodine channel (RyR) and sensitization of this channel to agonists such as caffeine and ryanodine. Recently it has been described that the nucleotide cADP-ribose (cADPR) is the endogenous regulator of the RyR in many mammalian cells, and cADPR has been proposed to be a second messenger in many signaling pathways. I investigated the effect of volatile anesthetics on the cADPR signaling system, using sea urchin egg homogenates as a model of intracellular Ca(2+) stores. Ca(2+) uptake and release were monitored in sea urchin egg homogenates by using the fluo-3 fluorescence technique. Activity of the ADP-ribosyl cyclase was monitored by using a fluorometric method using nicotinamide guanine dinucleotide as a substrate. Halothane in concentrations up to 800 microM did not induce Ca(2+) release by itself in sea urchin egg homogenates. However, halothane potentiates the Ca(2+) release mediated by agonists of the ryanodine channel, such as ryanodine. Furthermore, other volatile anesthetics such as isoflurane and sevoflurane had no effect. Halothane also potentiated the activation of the ryanodine channel mediated by the endogenous nucleotide cADPR. The half-maximal concentration for cADPR-induced Ca(2+) release was decreased about three times by addition of 800 microM halothane. The reverse was also true: addition of subthreshold concentrations of cADPR sensitized the homogenates to halothane. In contrast, all the volatile anesthetics used had no effect on the activity of the enzyme that synthesizes cADPR. I propose that the complex effect of volatile anesthetics on intracellular Ca(2+) homeostasis may involve modulation of the cADPR signaling system.

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.