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

From Descriptive to Functional Genomics of Leukemias Focusing on Genome Engineering Techniques

Genome engineering makes the precise manipulation of DNA sequences possible in a cell. Therefore, it is essential for understanding gene function. Meganucleases were the start of genome engineering, and it continued with the discovery of Zinc finger nucleases (ZFNs), followed by Transcription activator-like effector nucleases (TALENs). They can generate double-strand breaks at a desired target site in the genome, and therefore can be used to knock in mutations or knock out genes in the same way. Years later, genome engineering was transformed by the discovery of clustered regularly interspaced short palindromic repeats (CRISPR). Implementation of CRISPR systems involves recognition guided by RNA and the precise cleaving of DNA molecules. This property proves its utility in epigenetics and genome engineering. CRISPR has been and is being continuously successfully used to model mutations in leukemic cell lines and control gene expression. Furthermore, it is used to identify targets and discover drugs for immune therapies. The descriptive and functional genomics of leukemias is discussed in this study, with an emphasis on genome engineering methods. The CRISPR/Cas9 system's challenges, viewpoints, limits, and solutions are also explored.

CD38 in Neurodegeneration and Neuroinflammation

Neurodegenerative diseases are characterized by neuronal degeneration as well as neuroinflammation. While CD38 is strongly expressed in brain cells including neurons, astrocytes as well as microglial cells, the role played by CD38 in neurodegeneration and neuroinflammation remains elusive. Yet, CD38 expression increases as a consequence of aging which is otherwise the primary risk associated with neurodegenerative diseases, and several experimental data demonstrated that CD38 knockout mice are protected from neurodegenerative and neuroinflammatory insults. Moreover, nicotinamide adenine dinucleotide, whose levels are tightly controlled by CD38, is a recognized and potent neuroprotective agent, and NAD supplementation was found to be beneficial against neurodegenerative diseases. The aims of this review are to summarize the physiological role played by CD38 in the brain, present the arguments indicating the involvement of CD38 in neurodegeneration and neuroinflammation, and to discuss these observations in light of CD38 complex biology.

Nicotinamide Adenine Dinucleotide Metabolism and Neurodegeneration

SIGNIFICANCE

Nicotinamide adenine dinucleotide (NAD+) participates in redox reactions and NAD+-dependent signaling processes, which involve the cleavage of NAD+ coupled to posttranslational modifications of proteins or the production of second messengers. Either as a primary cause or as a secondary component of the pathogenic process, mitochondrial dysfunction and oxidative stress are prominent features of several neurodegenerative diseases. Activation of NAD+-dependent signaling pathways has a major effect in the capacity of the cell to modulate mitochondrial function and counteract the deleterious effects of increased oxidative stress. Recent Advances: Progress in the understanding of the biological functions and compartmentalization of NAD+-synthesizing and NAD+-consuming enzymes have led to the emergence of NAD+ metabolism as a major therapeutic target for age-related diseases.

CRITICAL ISSUES

Three distinct families of enzymes consume NAD+ as substrate: poly(ADP-ribose) polymerases (PARPs), ADP-ribosyl cyclases (CD38/CD157) and sirtuins. Two main strategies to increase NAD+ availability have arisen. These strategies are based on the utilization of NAD+ intermediates/precursors or the inhibition of the NAD+-consuming enzymes, PARPs and CD38. An increase in endogenous sirtuin activity seems to mediate the protective effect that enhancing NAD+ availability confers in several models of neurodegeneration and age-related diseases.

FUTURE DIRECTIONS

A growing body of evidence suggests the beneficial role of enhancing NAD+ availability in models of neurodegeneration. The challenge ahead is to establish the value and safety of the long-term use of these strategies for the treatment of neurodegenerative diseases. Antioxid. Redox Signal. 28, 1652-1668.

Specific cyclic ADP-ribose phosphohydrolase obtained by mutagenic engineering of Mn2+-dependent ADP-ribose/CDP-alcohol diphosphatase

Cyclic ADP-ribose (cADPR) is a messenger for Ca²⁺ mobilization. Its turnover is believed to occur by glycohydrolysis to ADP-ribose. However, ADP-ribose/CDP-alcohol diphosphatase (ADPRibase-Mn) acts as cADPR phosphohydrolase with much lower efficiency than on its major substrates. Recently, we showed that mutagenesis of human ADPRibase-Mn at Phe³⁷, Leu¹⁹⁶ and Cys²⁵³ alters its specificity: the best substrate of the mutant F37A + L196F + C253A is cADPR by a short difference, Cys²⁵³ mutation being essential for cADPR preference. Its proximity to the 'northern' ribose of cADPR in docking models indicates Cys²⁵³ is a steric constraint for cADPR positioning. Aiming to obtain a specific cADPR phosphohydrolase, new mutations were tested at Asp²⁵⁰, Val²⁵², Cys²⁵³ and Thr²⁷⁹, all near the 'northern' ribose. First, the mutant F37A + L196F + C253G, with a smaller residue 253 (Ala > Gly), showed increased cADPR specificity. Then, the mutant F37A + L196F + V252A + C253G, with another residue made smaller (Val > Ala), displayed the desired specificity, with cADPR k_cat/K_M ≈20-200-fold larger than for any other substrate. When tested in nucleotide mixtures, cADPR was exhausted while others remained unaltered. We suggest that the specific cADPR phosphohydrolase, by cell or organism transgenesis, or the designed mutations, by genome editing, provide opportunities to study the effect of cADPR depletion on the many systems where it intervenes.

Cyclic ADP-ribose as an endogenous inhibitor of the mTOR pathway downstream of dopamine receptors in the mouse striatum

The role of cyclic ADP-ribose (cADPR) as a second messenger and modulator of the mTOR pathway downstream of dopamine (DA) receptors and/or CD38 was re-examined in the mouse. ADP-ribosyl activity was low in the membranes of neonates, but DA stimulated it via both D1- and D2-like receptors. ADP-ribosyl cyclase activity increased significantly during development in association with increased expression of CD38. The cADPR binding proteins, FKBP12 and FKBP12.6, were expressed in the adult mouse striatum. The ratio of phosphorylated to non-phosphorylated S6 kinase (S6K) in whole mouse striatum homogenates decreased after incubation of adult mouse striatum with extracellular cADPR for 5 min. This effect of cADPR was much weaker in MPTP-treated Parkinson's disease model mice. The inhibitory effects of cADPR and rapamycin were identical. These data suggest that cADPR is an endogenous inhibitor of the mTOR signaling pathway downstream of DA receptors in the mouse striatum and that cADPR plays a certain role in the brain in psychiatric and neurodegenerative diseases.

NAD(+) Metabolism and the Control of Energy Homeostasis: A Balancing Act between Mitochondria and the Nucleus

NAD(+) has emerged as a vital cofactor that can rewire metabolism, activate sirtuins, and maintain mitochondrial fitness through mechanisms such as the mitochondrial unfolded protein response. This improved understanding of NAD(+) metabolism revived interest in NAD(+)-boosting strategies to manage a wide spectrum of diseases, ranging from diabetes to cancer. In this review, we summarize how NAD(+) metabolism links energy status with adaptive cellular and organismal responses and how this knowledge can be therapeutically exploited.

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.

NAD derived second messengers: Role in spontaneous diastolic Ca(2+) transients in murine cardiac myocytes

Strong β-adrenergic stimulation induced spontaneous diastolic Ca(2+) transients (SCTs) in electrically paced murine cardiac myocytes [28]. To obtain further insights into the underlying mechanism, we developed a method for a simultaneous analysis, in which the free luminal Ca(2+) concentration in the sarcoplasmic reticulum (SR) ([Ca(2+)]SR) and the free cytosolic Ca(2+) concentration ([Ca(2+)]i) were measured in parallel in the same cell. Each spontaneous diastolic Ca(2+) transient was exactly mirrored by a decrease of [Ca(2+)]SR. Since antagonism of the Ca(2+) mobilizing second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) was shown to block SCTs in single cardiac myocytes [28], we analyzed the effect of the novel ADP-ribosyl cyclase inhibitor SAN4825 on both cytosolic and intra-luminal Ca(2+) transients upon strong β-adrenergic stimulation. A strong antagonist effect of SAN4825 on SCTs at low micromolar concentrations was observed. Our results suggest that the underlying mechanism of spontaneous diastolic Ca(2+) transients observed upon strong β-adrenergic stimulation is sensitization of type 2 ryanodine receptor by the Ca(2+) releasing activity of the products of ADP-ribosyl cyclase activity.

Selective inhibitors of cardiac ADPR cyclase as novel anti-arrhythmic compounds

ADP-ribosyl cyclases (ADPRCs) catalyse the conversion of nicotinamide adenine dinucleotide to cyclic adenosine diphosphoribose (cADPR) which is a second messenger involved in Ca(2+) mobilisation from intracellular stores. Via its interaction with the ryanodine receptor Ca(2+) channel in the heart, cADPR may exert arrhythmogenic activity. To test this hypothesis, we have studied the effect of novel cardiac ADPRC inhibitors in vitro and in vivo in models of ventricular arrhythmias. Using a high-throughput screening approach on cardiac sarcoplasmic reticulum membranes isolated from pig and rat and nicotinamide hypoxanthine dinuleotide as a surrogate substrate, we have identified potent and selective inhibitors of an intracellular, membrane-bound cardiac ADPRC that are different from the two known mammalian ADPRCs, CD38 and CD157/Bst1. We show that two structurally distinct cardiac ADPRC inhibitors, SAN2589 and SAN4825, prevent the formation of spontaneous action potentials in guinea pig papillary muscle in vitro and that compound SAN4825 is active in vivo in delaying ventricular fibrillation and cardiac arrest in a guinea pig model of Ca(2+) overload-induced arrhythmia. Inhibition of cardiac ADPRC prevents Ca(2+) overload-induced spontaneous depolarizations and ventricular fibrillation and may thus provide a novel therapeutic principle for the treatment of cardiac arrhythmias.

CD38: a NAADP degrading enzyme

The role of the multifunctional enzyme CD38 in formation of the Ca(2+)-mobilizing second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) was investigated. Gene silencing of CD38 did neither inhibit NAADP synthesis in intact Jurkat T cells nor in thymus or spleen obtained from CD38 knock out mice. In vitro, both NAADP formation by base-exchange and degradation to 2-phospho adenosine diphosphoribose were efficiently decreased. Thus in vivo CD38 appears to be a NAADP degrading rather than a NAADP forming enzyme, perhaps avoiding desensitizing NAADP levels in intact cells.

Ca2+ signaling tools acquired from prostasomes are required for progesterone-induced sperm motility

Progesterone-induced calcium ion (Ca2+) signals in the neck region of sperm play a pivotal role in promoting sperm motility. Here, we show that a long-lasting Ca2+ signal required for sperm motility in response to progesterone depends on their pH-dependent fusion with prostasomes, which are small vesicles secreted by the prostate. We found that prostasome fusion led to the transfer of progesterone receptors, cyclic adenosine diphosphoribose (cADPR)-synthesizing enzymes, ryanodine receptors (RyRs), and other Ca2+ signaling tools from prostasomes to the sperm neck. Progesterone-induced sperm motility relied on cADPR-mediated Ca2+ mobilization through RyR located on acidic Ca2+ stores, followed by Ca2+ entry through store-operated channels. Treatment of prostasome-fused sperm with a cADPR antagonist or fusion with prostasomes in which type 2 RyR was depleted resulted in low fertilization rates, reduced sperm motility, or both. Thus, we conclude that sperm motility depends on the acquisition of Ca2+ signaling tools from prostasomes.

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.

A single residue in a novel ADP-ribosyl cyclase controls production of the calcium-mobilizing messengers cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate

Cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate are ubiquitous calcium-mobilizing messengers produced by the same family of multifunctional enzymes, the ADP-ribosyl cyclases. Not all ADP-ribosyl cyclases have been identified, and how production of different messengers is achieved is incompletely understood. Here, we report the cloning and characterization of a novel ADP-ribosyl cyclase (SpARC4) from the sea urchin, a key model organism for the study of calcium-signaling pathways. Like several other members of the ADP-ribosyl cyclase superfamily, SpARC4 is a glycoprotein targeted to the plasma membrane via a glycosylphosphatidylinositol anchor. However, unlike most other members, SpARC4 shows a remarkable preference for producing cyclic ADP-ribose over nicotinic acid adenine dinucleotide phosphate. Mutation of a single residue (tyrosine 142) within a noncanonical active site reversed this striking preference. Our data highlight further diversification of this unusual enzyme family, provide mechanistic insight into multifunctionality, and suggest that different ADP-ribosyl cyclases are fine-tuned to produce specific calcium-mobilizing messengers.

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

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

Ca(2+) signaling occurs via second messenger release from intraorganelle synthesis sites

Cyclic ADP-ribose is an important Ca(2+)-mobilizing cytosolic messenger synthesized from beta-NAD(+) by ADP-ribosyl cyclases (ARCs). However, the focus upon ectocellular mammalian ARCs (CD38 and CD157) has led to confusion as to how extracellular enzymes generate intracellular messengers in response to stimuli. We have cloned and characterized three ARCs in the sea urchin egg and found that endogenous ARCbeta and ARCgamma are intracellular and located within the lumen of acidic, exocytotic vesicles, where they are optimally active. Intraorganelle ARCs are shielded from cytosolic substrate and targets by the organelle membrane, but this barrier is circumvented by nucleotide transport. We show that a beta-NAD(+) transporter provides ARC substrate that is converted luminally to cADPR, which, in turn, is shuttled out to the cytosol via a separate cADPR transporter. Moreover, nucleotide transport is integral to ARC activity physiologically because three transport inhibitors all inhibited the fertilization-induced Ca(2+) wave that is dependent upon cADPR. This represents a novel signaling mechanism whereby an extracellular stimulus increases the concentration of a second messenger by promoting messenger transport from intraorganelle synthesis sites to the cytosol.

NAADP: a universal Ca2+ trigger

Cells possess multiple calcium ion (Ca2+) stores and multiple messenger molecules to mobilize them. These include d-myo-inositol 1,4,5-trisphosphate (IP(3)), cyclic adenosine diphosphoribose (cADPR), and the most recently identified Ca2+-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP), which acts on a wide spectrum of cells, from plant cells to mammalian cells. Accumulating evidence indicates that NAADP targets both acidic (lysosome-like) Ca2+ stores and endoplasmic reticular stores. Recent studies in invertebrate and mammalian cells suggest that NAADP provides an initiating Ca2+ signal, which is amplified by cADPR- or IP(3)-dependent mechanisms (or both) through Ca2+-induced Ca2+ release. Diverse stimuli activate a rapid rise of endogenous NAADP concentration, resulting in severalfold increases of NAADP over basal values within seconds. The enzyme CD38 can catalyze both the synthesis and hydrolysis of NAADP, making it ideal for effecting the rapid metabolism of NAADP. The crystal structure of CD38 and the structures of its various substrate complexes have now been determined, clarifying the mechanism of its multifunctional catalysis. We anticipate that these advances will lead to the unmasking of all the key components of the Ca2+ signaling pathway mediated by NAADP.

Nicotinic acid adenine dinucleotide phosphate (NAADP) and Ca2+ mobilization

Many physiological processes are controlled by a great diversity of Ca2+ signals that depend on Ca2+ entry into the cell and/or Ca2+ release from internal Ca2+ stores. Ca2+ mobilization from intracellular stores is gated by a family of messengers including inositol-1,4,5-trisphosphate (InsP3), cyclic ADP-ribose (cADPR), and nicotinic acid adenine dinucleotide phosphate (NAADP). There is increasing evidence for a novel intracellular Ca2+ release channel that may be targeted by NAADP and that displays properties distinctly different from the well-characterized InsP3 and ryanodine receptors. These channels appear to localize on a wider range of intracellular organelles, including the acidic Ca2+ stores. Activation of the NAADP-sensitive Ca2+ channels evokes complex changes in cytoplasmic Ca2+ levels by means of channel chatter with other intracellular Ca2+ channels. The recent demonstration of changes in intracellular NAADP levels in response to physiologically relevant extracellular stimuli highlights the significance of NAADP as an important regulator of intracellular Ca2+ signaling.

Molecular characterization of a novel cell surface ADP-ribosyl cyclase from the sea urchin

The sea urchin is an extensively used model system for the study of calcium signalling by the messenger molecules NAADP and cyclic ADP-ribose. Both are synthesized by ADP-ribosyl cyclases but our molecular understanding of these enzymes in the sea urchin is limited. We have recently reported the cloning of an extended family of sea urchin ADP-ribosyl cyclases and shown that one of these enzymes (SpARC1) is active within the endoplasmic reticulum lumen. These studies suggest that production of messengers is compartmentalized. Here we characterize the properties of SpARC2. SpARC2 catalyzed both NAADP and cyclic ADP-ribose production. Unusually, the NAD surrogate, NGD was a poor substrate. In contrast to SpARC1, heterologously expressed SpARC2 localized to the plasma membrane via a glycosylphosphatidylinositol (GPI)-anchor. Transcripts for SpARC2 were readily detectable in sea urchin eggs and a majority of the endogenous membrane bound activity was found to be GPI-anchored. Our data reveal striking differences in the properties of sea urchin ADP-ribosyl cyclases and provide further evidence that messenger production may occur outside of the cytosol.

Inhibition of ADP-ribosyl cyclase attenuates angiotensin II-induced cardiac hypertrophy

AIMS

Here, we report the discovery of a small molecule inhibitor, 2,2'-dihydroxyazobenzene (DAB), of ADP ribosyl cyclase (ADPR-cyclase) and showed that this inhibitor attenuated angiotensin (Ang) II-induced hypertrophic responses.

METHODS

and results The intracellular concentration of free Ca(2+) [Ca(2+)](i) in adult rat cardiomyocytes was measured by using a confocal microscope. Cardiac hypertrophy was induced by the two-kidney one-clip (2K1C) method. Hypertrophy was determined by de novo protein synthesis, cell volume, echocardiography, nuclear translocation of nuclear factor of activated T-cells, and transforming growth factor-beta1 protein expression. Treatment of cardiomyocytes with Ang II generated a biphasic [Ca(2+)](i) increase that included an initial Ca(2+)peak and sustained Ca(2+) rise via inositol trisphosphate and cyclic ADP-ribose (cADPR) formation, respectively. A cADPR antagonistic analogue, 8-Br-cADPR, and an ADPR-cyclase inhibitor, DAB, blocked the sustained Ca(2+) signal, but not the initial Ca(2+) rise. Furthermore, DAB significantly inhibited Ang II-mediated cADPR formation and hypertrophic responses in vitro. Echocardiography and histological examination revealed significant cardiac hypertrophy in 2K1C rats that was potently inhibited by treatment with DAB. In addition, the hypertrophic responses induced by Ang II in vitro were significantly increased by 2K1C, and DAB treatment reversed these hypertrophic responses to the levels of sham Control.

CONCLUSION

ADPR-cyclase is an important mediator of cardiac hypertrophy, and inhibition of ADPR-cyclase by DAB may provide a new therapeutic strategy for cardiac diseases.

Dual role of CD38 in microglial activation and activation-induced cell death

Microglia, the resident immune cells of the CNS, are normally quiescent but become activated after infection or injury. Their properties then change, and they promote both repair and damage processes. The extent of microglial activation is regulated, in part, by activation-induced cell death (AICD). Although many apoptotic aspects of the microglial AICD mechanism have been elucidated, little is known about the connection between the activation step and the death process. Using mouse primary microglial cultures, we show that the ectoenzyme CD38, via its calcium-mobilizing metabolite cyclic-ADP-ribose (cADPR), helps promote microglial activation and AICD induced by LPS plus IFN-gamma (LPS/IFN-gamma), suggesting that CD38 links the two processes. Accordingly, CD38 expression and activity, as well as the intracellular calcium concentration ([Ca2+]i) in the primary microglia were increased by LPS/IFN-gamma treatment. Moreover, CD38 deficiency or treatment with cADPR antagonists conferred partial resistance to LPS/IFN-gamma-induced AICD and also reduced [Ca2+]i. Microglial activation, indicated by induced expression of NO synthase-2 mRNA and production of NO, secretion and mRNA expression of TNF-alpha and IL-12 p40, and expression of IL-6 mRNA, was attenuated by CD38 deficiency or cADPR-antagonist treatment. The observed effects of CD38 on microglial activation are probably mediated via a cADPR-dependent increase in [Ca2+]i and the effect on AICD by regulation of NO production. Our results thus suggest that CD38 significantly affects regulation of the amount and function of activated microglia, with important consequences for injury and repair processes in the brain.

Regulation of nuclear Ca2+ signaling by translocation of the Ca2+ messenger synthesizing enzyme ADP-ribosyl cyclase during neuronal depolarization

In neurons, voltage-gated Ca(2+) channels and nuclear Ca(2+) signaling play important roles, such as in the regulation of gene expression. However, the link between electrical activity and biochemical cascade activation involved in the generation of the nuclear Ca(2+) signaling is poorly understood. Here we show that depolarization of Aplysia neurons induces the translocation of ADP-ribosyl cyclase, a Ca(2+) messenger synthesizing enzyme, from the cytosol into the nucleus. The translocation is dependent on Ca(2+) influx mainly through the voltage-dependent L-type Ca(2+) channels. We report also that specific nucleoplasmic Ca(2+) signals can be induced by three different calcium messengers, cyclic ADP-ribose, nicotinic acid adenine dinucleotide phosphate (NAADP), both produced by the ADP-ribosyl cyclase, and inositol 1,4,5-trisphosphate (IP(3)). Moreover, our pharmacological data show that NAADP acts on its own receptor, which cooperates with the IP(3) and the ryanodine receptors to generate nucleoplasmic Ca(2+) oscillations. We propose a new model where voltage-dependent L-type Ca(2+) channel-induced nuclear translocation of the cytosolic cyclase is a crucial step in the fine tuning of nuclear Ca(2+) signals in neurons.

A novel signaling pathway of ADP-ribosyl cyclase activation by angiotensin II in adult rat cardiomyocytes

ADP-ribosyl cyclase (ADPR-cyclase) produces a Ca(2+)-mobilizing second messenger, cADP-ribose (cADPR), from NAD(+). In this study, we investigated the molecular basis of ADPR-cyclase activation in the ANG II signaling pathway and cellular responses in adult rat cardiomyocytes. The results showed that ANG II generated biphasic intracellular Ca(2+) concentration increases that include a rapid transient Ca(2+) elevation via inositol trisphosphate (IP(3)) receptor and sustained Ca(2+) rise via the activation of L-type Ca(2+) channel and opening of ryanodine receptor. ANG II-induced sustained Ca(2+) rise was blocked by a cADPR antagonistic analog, 8-bromo-cADPR, indicating that sustained Ca(2+) rise is mediated by cADPR. Supporting the notion, ADPR-cyclase activity and cADPR production by ANG II were increased in a time-dependent manner. Application of pharmacological inhibitors and immunological analyses revealed that cADPR formation was activated by sequential activation of Src, phosphatidylinositol 3-kinase (PI 3-kinase)/protein kinase B (Akt), phospholipase C (PLC)-gamma1, and IP(3)-mediated Ca(2+) signal. Inhibitors of these signaling molecules not only completely abolished the ANG II-induced Ca(2+) signals but also inhibited cADPR formation. Application of the cADPR antagonist and inhibitors of upstream signaling molecules of ADPR-cyclase inhibited ANG II-stimulated hypertrophic responses, which include nuclear translocation of Ca(2+)/calcineurin-dependent nuclear factor of activated T cells 3, protein expression of transforming growth factor-beta1, and incorporation of [(3)H]leucine in cardiomyocytes. Taken together, these findings suggest that activation of ADPR-cyclase by ANG II entails a novel signaling pathway involving sequential activation of Src, PI 3-kinase/Akt, and PLC-gamma1/IP(3) and that the activation of ADPR-cyclase can lead to cardiac hypertrophy.

Back from the dormant stage: second messenger cyclic ADP-ribose essential for Toxoplasma gondii pathogenicity

Cyclic adenosine diphosphoribose (cADPR) is an endogenous Ca2+-mobilizing second messenger found in cells of animals, plants, and protozoans. It is formed by a specific class of enzymes, the ADP-ribosyl cyclases. cADPR stimulates Ca2+ release by means of ryanodine receptors located in the sarcoplasmic and endoplasmic reticulum. Recently, a role for cADPR has been demonstrated in the obligate intracellular protozoan pathogen Toxoplasma gondii. In T. gondii, stress conditions evoked synthesis of the plant hormone abscisic acid by the apicoplast, a remnant organelle of an algal endosymbiont of T. gondii. Abscisic acid in turn activated formation of cADPR within T. gondii, resulting in Ca2+ release and secretion of proteins involved in egress of T. gondii from its host cell. Evidence for a synthetic pathway of plant origin was obtained with the ABA synthesis inhibitor fluridone, which antagonized cellular egress and induced differentiation of long-lived semidormant cystic forms of T. gondii. Moreover, fluridone protected mice from toxoplasmosis.

Molecular mechanism of ADP-ribosyl cyclase activation in angiotensin II signaling in murine mesangial cells

ADP-ribosyl cyclase (ADPR-cyclase) produces a Ca(2+)-mobilizing second messenger cyclic ADP-ribose (cADPR) from NAD(+). In this study, we investigated the molecular basis of ADPR-cyclase activation and the following cellular events in angiotensin II (ANG II) signaling in mouse mesangial cells (MMCs). Treatment of MMCs with ANG II induced an increase in intracellular Ca(2+) concentrations through a transient Ca(2+) release via an inositol 1,4,5-trisphosphate receptor and a sustained Ca(2+) influx via L-type Ca(2+) channels. The sustained Ca(2+) signal, but not the transient Ca(2+) signal, was blocked by a cADPR antagonistic analog, 8-bromo-cADPR (8-Br-cADPR), and an ADPR-cyclase inhibitor, 4,4'-dihydroxyazobenzene (DHAB). In support of the results, ANG II stimulated cADPR production in a time-dependent manner, and DHAB inhibited ANG II-induced cADPR production. Application of pharmacological inhibitors revealed that activation of ADPR-cyclase by ANG II involved ANG II type 1 receptor, phosphoinositide 3-kinase, protein tyrosine kinase, and phospolipase C-gamma1. Moreover, DHAB as well as 8-Br-cADPR abrogated ANG II-mediated Akt phosphorylation, nuclear translocation of nuclear factor of activated T cell, and uptake of [(3)H]thymidine and [(3)H]leucine in MMCs. These results demonstrate that ADPR-cyclase in MMCs plays a pivotal role in ANG II signaling for cell proliferation and protein synthesis.

Nucleoplasmic reticulum is not essential in nuclear calcium signalling mediated by cyclic ADPribose in primary neurons

Nuclear calcium regulation is essential for controlling nuclear processes such as gene expression. Recent studies, mostly performed on immortalized or transformed cell lines, reported the presence of a nucleoplasmic reticulum (NR). It has been suggested that NR acts as a storage organelle having an important role in nuclear Ca2+ signalling. However, whether NR is present and necessary in primary neurons for generation of nuclear Ca2+ signalling has never been investigated. Here, we show, by confocal microscopy and by electronic microscopy, that nuclei in intact neurons or isolated nuclei are not endowed with NR. Finally, our experiments performed on isolated nuclei from Aplysia giant neurons show that the nuclear envelope acts as a functional Ca2+ store which can be mobilized by the second messenger cyclic ADPribose to elicit a nucleoplasmic Ca2+ elevation. Our study provides evidence that nuclear Ca2+ signals can be independent of the presence of NR in neurons.

Calcium signaling in airway smooth muscle

Contractility of airway smooth muscle requires elevation of intracellular calcium concentration. Under resting conditions, airway smooth muscle cells maintain a relatively low intracellular calcium concentration, and activation of the surface receptors by contractile agonists results in an elevation of intracellular calcium, culminating in contraction of the cell. The pattern of elevation of intracellular calcium brought about by agonists is a dynamic process and involves the coordinated activities of ion channels located in the plasma membrane and the sarcoplasmic reticulum. Among the signaling molecules involved in this dynamic calcium regulation in airway smooth muscle cells are inositol 1,4,5-trisphosphate and cyclic ADP-ribose, which mobilize calcium from the sarcoplasmic reticulum by acting via the inositol 1,4,5-trisphosphate and ryanodine receptors, respectively. In addition, calcium influx from the extracellular space is critical for the repletion of the intracellular calcium stores during activation of the cells by agonists. Calcium influx can occur via voltage- and receptor-gated channels in the plasma membrane, as well as by influx that is triggered by depletion of the intracellular stores (i.e., store-operated calcium entry mechanism). Transient receptor potential proteins appear to mediate the calcium influx via receptor- and store-operated channels. Recent studies have shown that proinflammatory cytokines regulate the expression and activity of the pathways involved in intracellular calcium regulation, thereby contributing to airway smooth muscle cell hyperresponsiveness. In this review, we will discuss the specific roles of cyclic ADP-ribose/ryanodine receptor channels and transient receptor potential channels in the regulation of intracellular calcium in airway smooth muscle cells.

Molecular characterization of a novel intracellular ADP-ribosyl cyclase

Background

ADP-ribosyl cyclases are remarkable enzymes capable of catalyzing multiple reactions including the synthesis of the novel and potent intracellular calcium mobilizing messengers, cyclic ADP-ribose and NAADP. Not all ADP-ribosyl cyclases however have been characterized at the molecular level. Moreover, those that have are located predominately at the outer cell surface and thus away from their cytosolic substrates.

Methodology/Principal Findings

Here we report the molecular cloning of a novel expanded family of ADP-ribosyl cyclases from the sea urchin, an extensively used model organism for the study of inositol trisphosphate-independent calcium mobilization. We provide evidence that one of the isoforms (SpARC1) is a soluble protein that is targeted exclusively to the endoplasmic reticulum lumen when heterologously expressed. Catalytic activity of the recombinant protein was readily demonstrable in crude cell homogenates, even under conditions where luminal continuity was maintained.

Conclusions/Significance

Our data reveal a new intracellular location for ADP-ribosyl cyclases and suggest that production of calcium mobilizing messengers may be compartmentalized.

Regulation of calcium signalling by adenine-based second messengers

cADPR [cyclic ADPR (ADP-ribose)], NAADP (nicotinic acid-adenine dinucleotide phosphate) and ADPR belong to the family of adenine-containing second messengers. They are metabolically related and are all involved in the regulation of cellular Ca(2+) homoeostasis. Activation of specific plasma membrane receptors is connected to cADPR formation in many cell types and tissues. In contrast receptor-mediated formation of NAADP and ADPR has been shown only in a few selected cellular systems. The intracellular Ca(2+) channel triggered by cADPR is the RyR (ryanodine receptor); in the case of NAADP, both activation of RyR and a novel Ca(2+) channel have been proposed. In contrast, ADPR opens the non-specific cation channel TRPM2 [TRP (transient receptor potential) melastatin 2] that belongs to the TRP family of ion channels.

Structure and enzymatic functions of human CD38

CD38 is a novel multifunctional protein that serves not only as an antigen but also as an enzyme. It catalyzes the metabolism of cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate, two structurally and functionally distinct Ca(2+) messengers targeting, respectively, the endoplasmic reticulum and lysosomal Ca(2+) stores. The protein has recently been crystallized and its three-dimensional structure solved to a resolution of 1.9 A. The crystal structure of a binary complex reveals critical interactions between residues at the active site and a bound substrate, providing mechanistic insights to its novel multi-functional catalysis. This article reviews the current advances in the understanding of the structural determinants that control the multiple enzymatic reactions catalyzed by CD38.

Structure and enzymatic functions of human CD38

CD38 is a novel multifunctional protein that serves not only as an antigen but also as an enzyme. It catalyzes the metabolism of cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate, two structurally and functionally distinct Ca(2+) messengers targeting, respectively, the endoplasmic reticulum and lysosomal Ca(2+) stores. The protein has recently been crystallized and its three-dimensional structure solved to a resolution of 1.9 A. The crystal structure of a binary complex reveals critical interactions between residues at the active site and a bound substrate, providing mechanistic insights to its novel multi-functional catalysis. This article reviews the current advances in the understanding of the structural determinants that control the multiple enzymatic reactions catalyzed by CD38.

The power to reduce: pyridine nucleotides--small molecules with a multitude of functions

The pyridine nucleotides NAD and NADP play vital roles in metabolic conversions as signal transducers and in cellular defence systems. Both coenzymes participate as electron carriers in energy transduction and biosynthetic processes. Their oxidized forms, NAD+ and NADP+, have been identified as important elements of regulatory pathways. In particular, NAD+ serves as a substrate for ADP-ribosylation reactions and for the Sir2 family of NAD+-dependent protein deacetylases as well as a precursor of the calcium mobilizing molecule cADPr (cyclic ADP-ribose). The conversions of NADP+ into the 2'-phosphorylated form of cADPr or to its nicotinic acid derivative, NAADP, also result in the formation of potent intracellular calcium-signalling agents. Perhaps, the most critical function of NADP is in the maintenance of a pool of reducing equivalents which is essential to counteract oxidative damage and for other detoxifying reactions. It is well known that the NADPH/NADP+ ratio is usually kept high, in favour of the reduced form. Research within the past few years has revealed important insights into how the NADPH pool is generated and maintained in different subcellular compartments. Moreover, tremendous progress in the molecular characterization of NAD kinases has established these enzymes as vital factors for cell survival. In the present review, we summarize recent advances in the understanding of the biosynthesis and signalling functions of NAD(P) and highlight the new insights into the molecular mechanisms of NADPH generation and their roles in cell physiology.

Integrin ligands mobilize Ca2+ from ryanodine receptor-gated stores and lysosome-related acidic organelles in pulmonary arterial smooth muscle cells

Extracellular matrix (ECM) protein receptors, or integrins, participate in vascular remodeling and the systemic myogenic response. Synthetic ligands and ECM fragments regulate the vascular smooth muscle cell contractile state by altering intracellular Ca2+ levels ([Ca2+]i). Information on the Ca2+ effect of integrins in vascular smooth muscle cells is limited, but nonexistent in pulmonary arterial smooth muscle cells (PASMCs). We therefore characterized integrin expression in endothelium-denuded pulmonary arteries, and explored [Ca2+]i mobilization pathways induced by soluble ligands in rat PASMCs. Reverse transcriptase-PCR showed mRNA expression of integrins alpha1, alpha2, alpha3, alpha4, alpha5, alpha7, alpha8, alpha(v), beta1, beta3, and beta4, and immunoblots of alpha5, alpha(v), beta1, and beta3 confirmed protein expression. Exposure of PASMCs to integrin-binding peptides (0.5 mM) containing the arginine-glycine-aspartate (RGD) motif elicited [Ca2+]i responses with an order of potency of GRGDNP > GRGDSP > GRGDTP = cyclo-RGD. Pharmacological analysis revealed that the GRGDSP-induced Ca2+ response was unrelated to Ca2+ influx and the inositol triphosphate receptor-gated Ca2+ store, but partially blocked by ryanodine or inhibition of lysosome-related acidic organelles with bafilomycin A1. Simultaneous inhibition of both pathways was necessary to abolish the response. GRGDSP treatment increased cyclic ADP-ribose, the endogenous activator of ryanodine receptors, by 70%. GRGDSP also rapidly reduced Lysotracker Red accumulation, confirming direct modulation of acidic organelles. These data are the first demonstration of integrin-mediated Ca2+ regulation in PASMCs. The presence of an array of integrins, and activation of ryanodine-sensitive Ca2+ stores and lysosome-like organelles by GRGDSP suggest important roles for integrin-dependent Ca2+ signaling in regulating PASMC function.

The CD38/CD157 mammalian gene family: An evolutionary paradigm for other leukocyte surface enzymes

Human CD38 is the mammalian prototype of a family of phylogenetically conserved proteins which share structural similarities and enzymatic activities involved in the production of an intracellular second messenger with calcium mobilizing effects. Engagement of CD38 by agonistic monoclonal antibodies and the CD31 ligand initiates a cytoplasmic signaling cascade involving tyrosine phosphorylation of the proto-oncogene c-cbl and of the extracellular regulated kinase 1 of 2 complex. Further requirements for signal transduction include a privileged localization within the cholesterol-rich areas of the plasma membrane and physical association with specialized surface receptors. CD38-mediated signals are crucial in heterotypic cell adhesion and migration as well as in the activation of proliferation/survival programs by normal and neoplastic cells. Here we review the most recent literature on this complex topic and attempt to formulate a single model reconciling the enzymatic and receptorial activities of CD38.

Regulation of intracellular levels of NAD: a novel role for CD38

Nicotinamide adenine dinucleotide (NAD) plays key roles in many cellular functions. In addition to its well-known role in energy metabolism, NAD also plays a role in signal transduction, ageing, and cellular injury. NAD is also involved in many signal transduction pathways. Therefore, it is imperative to understand the mechanisms that control intracellular NAD levels. However, to date, the mechanisms that regulate intracellular levels of NAD have not been completely elucidated. CD38 is a multifunctional enzyme ubiquitously distributed in mammalian tissues. CD38 has been implicated as the enzyme responsible for the synthesis of the second messengers. However, its major enzymatic activity is the hydrolysis of NAD, in fact, CD38 will generate one molecule of cADPR for every 100 molecules of NAD hydrolyzed. To date, the role of CD38 as a modulator of levels of NAD has not been explored. We postulated that CD38 is the major NADase in mammalian cells and that it regulates intracellular NAD levels. In the current studies we examined the NADase activities and NAD levels in a variety of tissues from both wild-type and CD38 deficient mice. In accordance with our hypothesis, we found that tissue levels of NAD in CD38 deficient mice are 10- to 20-fold higher than in wild-type animals. In addition, NADase activity in the plasma membrane, mitochondria, sarcoplasmic reticulum, and nuclei is essentially absent in most tissues from CD38 deficient mice. These data support the novel concept that CD38 is a major regulator of cellular NAD levels. These findings have implications for understanding the mechanisms that regulate intracellular NAD levels and its role in energy homeostasis, signal transduction, and ageing.

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+.

Production of NAADP and its role in Ca2+ mobilization associated with lysosomes in coronary arterial myocytes

The present study was designed to determine the production of nicotinic acid adenine dinucleotide phosphate (NAADP) and its role associated with lysosomes in mediating endothelin-1 (ET-1)-induced vasoconstriction in coronary arteries. HPLC assay showed that NAADP was produced in coronary arterial smooth muscle cells (CASMCs) via endogenous ADP-ribosyl cyclase. Fluorescence microscopic analysis of intracellular Ca2+ concentration ([Ca2+]i) in CASMCs revealed that exogenous 100 nM NAADP increased [Ca2+]i by 711 +/- 47 nM. Lipid bilayer experiments, however, demonstrated that NAADP did not directly activate ryanodine (Rya) receptor Ca2+ release channels on the sarcoplasmic reticulum. In CASMCs pretreated with 100 nM bafilomycin A1 (Baf), an inhibitor of lysosomal Ca2+ release and vacuolar proton pump function, NAADP-induced [Ca2+]i increase was significantly abolished. Moreover, ET-1 significantly increased NAADP formation in CASMCs and resulted in the rise of [Ca2+]i in these cells with a large increase in global Ca2+ level of 1,815 +/- 84 nM. Interestingly, before this large Ca2+ increase, a small Ca2+ spike with an increase in [Ca2+]i of 529 +/- 32 nM was observed. In the presence of Baf (100 nM), this ET-1-induced two-phase [Ca2+]i response was completely abolished, whereas Rya (50 microM) only markedly blocked the ET-1-induced large global Ca2+ increase. Functional studies showed that 100 nM Baf significantly attenuated ET-1-induced maximal constriction from 82.26 +/- 4.42% to 51.80 +/- 4.36%. Our results suggest that a lysosome-mediated Ca2+ regulatory mechanism via NAADP contributes to ET-1-induced Ca2+ mobilization in CASMCs and consequent vasoconstriction of coronary arteries.

The Ca2+-releasing messenger NAADP, a new player in the nervous system

Many physiological processes are controlled by a great diversity of Ca2+ signals. Within cell, Ca2+ signals depend upon Ca2+ entry and/or Ca2+ release from internal Ca2+ stores. The control of Ca2+-store mobilization is ensured by a family of messengers comprising inositol 1,4,5 trisphosphate, cyclic ADP-ribose and nicotinic acid adenine dinucleotide phosphate (NAADP). From recent works, new concepts have emerged where activation of the cells by outside stimuli, acting at the plasma membrane, results in the synthesis of multiple Ca2+-releasing messengers which may interact and shape complex Ca2+ signals in the cytosol as well as in the nucleus. This contribution will cover the most recent advances on NAADP signalling with some emphasis on neurons.

Acetylcholine stimulates cyclic ADP-ribose formation via M1 muscarinic receptors in rat superior cervical ganglion

The role of cyclic ADP-ribose (cADPR) as the downstream signal of neuronal muscarinic acetylcholine receptors (mAChRs) and the enzyme responsible for its synthesis, ADP-ribosyl cyclase, were examined in the rat superior cervical ganglion (SCG). Application of acetylcholine or other mAChR agonists increased the ADP-ribosyl cyclase activity by about 250-300% in crude membrane fractions from the SCG of 14-day-old rats. This effect was inhibited by atropine or by the M1-mAChR antagonist, pirenzepine, and was mimicked by GTP. These results indicate that the M1 mAChRs couple to the membrane-bound form of ADP-ribosyl cyclase and suggest that cADPR is a second messenger of M1 mAChR signaling in nervous tissue.

Second messenger function and the structure-activity relationship of cyclic adenosine diphosphoribose (cADPR)

Cyclic ADP-ribose (cADPR) is a Ca2+ mobilizing second messenger found in various cell types, tissues and organisms. Receptor-mediated formation of cADPR may proceed via transmembrane shuttling of the substrate NAD and involvement of the ectoenzyme CD38, or via so far unidentified ADP-ribosyl cyclases located within the cytosol or in internal membranes. cADPR activates intracellular Ca2+ release via type 2 and 3 ryanodine receptors. The exact molecular mechanism, however, remains to be elucidated. Possibilities are the direct binding of cADPR to the ryanodine receptor or binding via a separate cADPR binding protein. In addition to Ca2+ release, cADPR also evokes Ca2+ entry. The underlying mechanism(s) may comprise activation of capacitative Ca2+ entry and/or activation of the cation channel TRPM2 in conjunction with adenosine diphosphoribose. The development of novel cADPR analogues revealed new insights into the structure-activity relationship. Substitution of either the northern ribose or both the northern and southern ribose resulted in much simpler molecules, which still retained significant biological activity.

ADP-ribosyl cyclase couples to cyclic AMP signaling in the cardiomyocytes

ADP-ribosyl cyclase (ADPR-cyclase) produces a Ca(2+)-mobilizing second messenger cyclic ADP-ribose (cADPR) from beta-NAD(+). In this study, we examined the molecular basis of which beta-adrenergic receptor (betaAR) stimulation induces cADPR formation and characterized cardiac ADPR-cyclase. The results revealed that isoproterenol-mediated increase of [Ca(2+)](i) in rat cardiomyocytes was blocked by pretreatment with a cADPR antagonistic derivative 8-Br-cADPR, a PKA inhibitor H89 or high concentration of ryanodine. Moreover, incubation of ventricular lysates with isoproterenol, forskolin or cAMP resulted in activation of ADPR-cyclase that was inhibited by pretreatment with H89. Supporting the observations, the cADPR antagonist and H89 blocked 8-CPT-cAMP, a cell-permeant cAMP analog-induced increase in [Ca(2+)](i) but not cGMP-mediated increase. Characterization of partially purified cardiac ADPR-cyclase showed a molecular mass of approximately 42 kDa and no cross-activity with CD38 antibodies, and the enzyme activity was inhibited by Zn(2+) but not dithiothreitol. Microinjection of the enzyme into rat cardiomyocytes increased the level of [Ca(2+)](i) in a concentration-dependent manner. The enzyme-mediated increase of [Ca(2+)](i) was blocked by the cADPR antagonist. These findings suggest that betaAR-mediated regulation of [Ca(2+)](i) in rat cardiomyocytes is primed by activation of cardiac ADPR-cyclase via cAMP/PKA signaling and that cardiac ADPR-cyclase differs from CD38 in biochemical and immunological properties.

CD38/cyclic ADP-ribose signaling: role in the regulation of calcium homeostasis in airway smooth muscle

The contractility of airway smooth muscle cells is dependent on dynamic changes in the concentration of intracellular calcium. Signaling molecules such as inositol 1,4,5-trisphosphate and cyclic ADP-ribose play pivotal roles in the control of intracellular calcium concentration. Alterations in the processes involved in the regulation of intracellular calcium concentration contribute to the pathogenesis of airway diseases such as asthma. Recent studies have identified cyclic ADP-ribose as a calcium-mobilizing second messenger in airway smooth muscle cells, and modulation of the pathway involved in its metabolism results in altered calcium homeostasis and may contribute to airway hyperresponsiveness. In this review, we describe the basic mechanisms underlying the dynamics of calcium regulation and the role of CD38/cADPR, a novel pathway, in the context of airway smooth muscle function and its contribution to airway diseases such as asthma.

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.