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

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.

TNF-alpha induced CD38 expression in human airway smooth muscle cells: role of MAP kinases and transcription factors NF-kappaB and AP-1

In human airway smooth muscle (HASM) cells, the expression of CD38, which synthesizes the calcium-mobilizing molecule cyclic ADP-ribose, is augmented by TNF-alpha, a cytokine implicated in asthma. We determined the role of mitogen-activated protein kinase (MAPK) in the activation of NF-kappaB and AP-1 in the regulation of CD38 expression in HASM cells. In HASM cells exposed to TNF-alpha (40 ng/ml), the inhibitors of extracellular signal-regulated kinase (ERK), p38, or c-Jun NH(2)-terminal kinase (JNK) decreased CD38 expression and ADP-ribosyl cyclase activity. Transfection of HASM cells with a dominant negative MEK decreased while a wild-type ERK increased TNF-alpha-induced CD38 expression. Electrophoretic mobility shift assays (EMSAs) were performed using nuclear proteins and consensus sequences to detect the effect of the MAPKs on NF-kappaB and AP-1 activation. EMSAs confirmed the role of p38 and JNK in mediating NF-kappaB and AP-1 activation. Transfection of a dominant negative c-Jun decreased TNF-alpha-induced CD38 expression indicating involvement of AP-1. Stability of TNF-alpha-induced CD38 transcripts were determined in the presence of MAPK inhibitors after arresting the transcription with actinomycin D. Transcript stability decreased in the presence of ERK and p38 MAPK, but not the JNK, inhibitors. These results indicate that regulation of CD38 expression through p38 and JNK MAPKs involves NF-kappaB and AP-1 activation, and ERK and p38 MAPKs also regulate expression posttranscriptionally through message stability.

Association of CD38 with Nonmuscle Myosin Heavy Chain IIA and Lck Is Essential for the Internalization and Activation of CD38

Activation of CD38 in lymphokine-activated killer (LAK) cells involves interleukin-8 (IL8)-mediated protein kinase G (PKG) activation and results in an increase in the sustained intracellular Ca(2+) concentration ([Ca(2+)](i)), cADP-ribose, and LAK cell migration. However, direct phosphorylation or activation of CD38 by PKG has not been observed in vitro. In this study, we examined the molecular mechanism of PKG-mediated activation of CD38. Nonmuscle myosin heavy chain IIA (MHCIIA) was identified as a CD38-associated protein upon IL8 stimulation. The IL8-induced association of MHCIIA with CD38 was dependent on PKG-mediated phosphorylation of MHCIIA. Supporting these observations, IL8- or cell-permeable cGMP analog-induced formation of cADP-ribose, increase in [Ca(2+)](i), and migration of LAK cells were inhibited by treatment with the MHCIIA inhibitor blebbistatin. Binding studies using purified proteins revealed that the association of MHCIIA with CD38 occurred through Lck, a tyrosine kinase. Moreover, these three molecules co-immunoprecipitated upon IL8 stimulation of LAK cells. IL8 treatment of LAK cells resulted in internalization of CD38, which co-localized with MHCIIA and Lck, and blebbistatin blocked internalization of CD38. These findings demonstrate that the association of phospho-MHCIIA with Lck and CD38 is a critical step in the internalization and activation of CD38.

Overexpression of human CD38/ADP-ribosyl cyclase enhances acetylcholine-induced Ca2+ signalling in rodent NG108-15 neuroblastoma cells

The role of cyclic ADP-ribose (cADPR) and its synthetic enzyme, CD38, as a downstream signal of muscarinic acetylcholine receptors (mAChRs) was examined in neuroblastoma cells expressing M1 mAChRs (NGM1). NGM1 cells were further transformed with both wild-type and mutant (C119K/C201E) human CD38. The dual transformed cells exhibited higher cADPR formation than ADPR production and elevated intracellular free Ca(2+) concentrations ([Ca(2+)](i)) in response to ACh. These phenotypes were analyzed in detail in a representative CD38 clone. The intracellular cADPR concentration by ACh application was significantly increased by CD38 overexpression. Digital image analysis by a confocal microscopy revealed that topographical distribution of the sites of Ca(2+) release was unchanged between control and overexpressed cells. These results indicate that cADPR is an intracellular messenger of Ca(2+) signalling, suggesting that CD38 can contribute to mAChR-cADPR signalling.

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.

Endogenous ADP-ribose enables calcium-regulated cation currents through TRPM2 channels in neutrophil granulocytes

TRPM2 (transient receptor potential melastatin 2) is a Ca2+-permeable cation channel gated by ADPR (ADP-ribose) from the cytosolic side. To test whether endogenous concentrations of intracellular ADPR are sufficient for TRPM2 gating in neutrophil granulocytes, we devised an HPLC method to determine ADPR contents in HClO4 cell extracts. The reversed-phase ion-pair HPLC method with an Mg2+-containing isocratic eluent allows baseline resolution of one ADPR peak. Intracellular ADPR concentrations were approx. 5 muM in granulocytes and not significantly altered by stimulation with the chemoattractant peptide fMLP (N-formylmethionyl-leucylphenylalanine). We furthermore determined intracellular concentrations of cADPR (cyclic ADPR) with a cyclase assay involving enzymatic conversion of cADPR into NAD+ and fluorimetric determination of NAD+. Intracellular cADPR concentrations were approx. 0.2 microM and not altered by fMLP. In patch-clamp experiments, ADPR (0.1-100 microM) was dialysed into granulocytes to analyse its effects on whole-cell currents characteristic for TRPM2, in the presence of a low (<10 nM) or a high (1 microM) intracellular Ca2+ concentration. TRPM2 currents were significantly larger at high than at low [Ca2+] (e.g. -225+/-27.1 versus -7+/-2.0 pA/pF at 5 muM ADPR), but no currents at all were observed in the absence of ADPR (ADPR concentration < or =0.3 microM). cADPR (0.1, 0.3 and 10 microM) was without effect even in the presence of subthreshold ADPR (0.1 microM). We conclude that ADPR enables an effective regulation of TRPM2 by cytosolic Ca2+. Thus ADPR and Ca2+ in concert behave as a messenger system for agonist-induced influx of Ca2+ through TRPM2 in granulocytes.

Regulation of SIRT 1 mediated NAD dependent deacetylation: a novel role for the multifunctional enzyme CD38

The SIRT 1 enzyme is a NAD dependent deacetylase implicated in ageing, cell protection, and energy metabolism in mammalian cells. How the endogenous activity of SIRT 1 is modulated is not known. The enzyme CD38 is a multifunctional enzyme capable of synthesis of the second messenger, cADPR, NAADP, and ADPR. However, the major enzymatic activity of CD38 is the hydrolysis of NAD. Of particular interest is the fact that CD38 is present on the inner nuclear membrane. Here, we investigate the modulation of the SIRT 1 activity by CD38. We propose that by modulating availability of NAD to the SIRT1 enzyme, CD38 may regulate SIRT1 enzymatic activity. We observed that in CD38 knockout mice, tissue levels of NAD are significantly increased. We also observed that incubation of purified recombinant SIRT1 enzyme with CD38 or nuclear extracts of wild-type mice led to a significant inhibition of its activity. In contrast, incubation of SIRT1 with cellular extract from CD38 knockout mice was without effect. Furthermore, the endogenous activity of SIRT1 was several time higher in nuclear extracts from CD38 knockout mice when compared to wild-type nuclear extracts. Finally, the in vivo deacetylation of the SIRT1 substrate P53 is increased in CD38 knockout mice tissue. Our data support the novel concept that nuclear CD38 is a major regulator of cellular/nuclear NAD level, and SIRT1 activity. These findings have strong implications for understanding the basic mechanisms that modulate intracellular NAD levels, energy homeostasis, as well as ageing and cellular protection modulated by the SIRT enzymes.

Bradykinin activates ADP-ribosyl cyclase in neuroblastoma cells: intracellular concentration decrease in NAD and increase in cyclic ADP-ribose

ADP-ribosyl cyclase activity in the crude membrane fraction of neuroblastomaxglioma NGPM1-27 hybrid cells was measured by monitoring [(3)H] cyclic ADP-ribose (cADPR) formation from [(3)H] NAD(+). Bradykinin (BK) at 100nM increased ADP-ribosyl cyclase activity by about 2.5-fold. Application of 300nM BK to living NGPM1-27 cells decreased NAD(+) to 78% of the prestimulation level at 30s. In contrast, intracellular cADPR concentrations were increased by 2-3-fold during the period from 30 to 120s after the same treatment. Our results suggest that cADPR is one of the second messengers downstream of B(2) BK receptors.

Agonist-induced cyclic ADP ribose production in airway smooth muscle

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

NAADP as a second messenger: neither CD38 nor base-exchange reaction are necessary for in vivo generation of NAADP in myometrial cells

Nicotinic acid adenine dinucleotide phosphate (NAADP) has recently been shown to act as a second messenger controlling intracellular Ca(2+) responses in mammalian cells. Many questions remain regarding this signaling pathway, including the role of the ryanodine receptor (RyR) in NAADP-induced Ca(2+) transients. Furthermore, the exact metabolic pathway responsible for the synthesis of NAADP in vivo has not been determined. Here, we demonstrate that the NAADP mediated Ca(2+) release system is present in human myometrial cells. We also demonstrate that human myometrial cells use the NAADP second messenger system to generate intracellular Ca(2+) transients in response to histamine. It has been proposed in the past that the NAADP system in mammalian cells is dependent on the presence of functional RyRs. Here, we observed that the histamine-induced Ca(2+) transients are dependent on both the NAADP and inositol 1,4,5-trisphosphate signaling pathways but are independent of RyRs. The enzyme CD38 has been shown to catalyze the synthesis of NAADP in vitro by the base-exchange reaction. Furthermore, it has been proposed that this enzyme is responsible for the intracellular generation of NAADP in vivo. Using CD38 knockout mice, we observed that both the basal and histamine stimulated levels of NAADP are independent of CD38 and the base-exchange reaction. Our group is the first to demonstrate that NAADP is a second messenger for histamine-elicited Ca(2+) transients in human myometrial cells. Furthermore, the NAADP mediated mechanism in mammalian cells can be independent of RyRs and CD38. Our data provides novel insights into the understanding of the mechanism of action and metabolism of this new second messenger system.

Decreased cADPR and increased NAD+ in the Cd38-/- mouse

CD38 is a type II glycoprotein that catalyzes the formation of cyclic ADP-ribose (cADPR), an intracellular calcium signalling molecule, from nicotinamide adenine dinucleotide (NAD(+)). Using a modified version of the fluorimetric cycling assay for cADPR which reduces between-subject variability, we report significant decreases in brain and lung cADPR, which although similar to previously published values, showed much less individual variation. The reduced variation within each group suggests that the range of cADPR is narrower than previously thought, and that the regulatory mechanisms controlling these levels are more finely tuned. We also report significant increases in brain, lung, and kidney NAD(+) in the Cd38(-/-) mouse, and provide the first experimental demonstration of the proximate relationship between CD38 and NAD(+).

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.

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.

NAADP+ synthesis from cADPRP and nicotinic acid by ADP-ribosyl cyclases

ADP-ribosyl cyclases (ADPRCs) are present from lower Metazoa to mammals and synthesize the Ca2+-active (di)nucleotides cyclic ADP-ribose (cADPR), NAADP+, and ADP-ribose (ADPR), involved in the regulation of important cellular functions. NAADP+ can be synthesized by ADPRCs from NADP+ through a base-exchange reaction, which substitutes nicotinamide for nicotinic acid (NA). Here we demonstrate that ADPRCs from both lower and higher Metazoa (including human CD38) can also synthesize NAADP+ starting from 2'-phospho-cyclic ADP-ribose (cADPRP) and NA. Comparison, on the two substrates cADPRP and NADP+, of the relative rates of the reactions introducing NA and hydrolyzing/cyclizing the substrate, respectively, indicates that with all ADPRCs tested cADPRP is preferentially transformed into NAADP+, while NADP+ is preferentially cyclized or hydrolyzed to cADPRP/2'-phospho-ADP-ribose. cADPRP was detectable in retinoic acid-differentiated, CD38+ HL-60 cells, but not in undifferentiated, CD38- cells. These results suggest that cADPRP may be a NAADP+ precursor in ADPRC+ cells.

Second messenger function of nicotinic acid adenine dinucleotide phosphate revealed by an improved enzymatic cycling assay

Nicotinic acid adenine dinucleotide phosphate (NAADP) is the most potent activator of Ca2+ release from intracellular stores known today. Although recent reports have suggested an important function of NAADP in human T lymphocytes, direct evidence for receptor-induced formation of NAADP is yet missing in these cells. Thus, we developed a highly sensitive and specific enzyme assay capable of quantifying low fmol amounts of NAADP. In unstimulated T cells, the NAADP concentration amounted to 4.4 +/- 1.6 nm (0.055 +/- 0.028 pmol/mg of protein). Stimulation of the cells via the T cell receptor/CD3 complex resulted in biphasic elevation kinetics of cellular NAADP levels and was characterized by a bell-shaped concentration-response curve for NAADP. In contrast, the NAADP concentration was elevated neither upon activation of the ADP-ribose/TRPM2 channel Ca2+ signaling system nor by an increase of the intracellular Ca2+ concentration upon thapsigargin stimulation. T cell receptor/CD3 complex-mediated NAADP formation was dependent on the activity of tyrosine kinases because genistein completely blocked NAADP elevation. Thus, we propose a regulated formation of NAADP upon specific stimulation of the T cell receptor/CD3 complex, suggesting a function of NAADP as a Ca2+-mobilizing second messenger during T cell activation.

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+.

Regulation of CD 38 expression and function by steroid hormones in myometrium

CD 38, a 45-kDa transmembrane glycoprotein, is expressed ubiquitously in many cell types, including the myometrial smooth muscle cells. CD 38 is a bifunctional protein, and has both ADP-ribosyl cyclase (cyclase) and cyclic ADP-ribose (cADPR)-hydrolase (hydrolase) activities. The cyclase converts beta-NAD to cADPR, a calcium mobilizing second messenger involved in fertilization, insulin secretion, and muscle contraction. CD 38 expression in smooth muscle is regulated by cytokines, by the steroid hormones estrogen and progesterone, and during gestation in the rat. Estrogen increases CD 38 expression, which is associated with increased cyclase, but not hydrolase, activity, indicating a differential post-translational regulation. Progesterone attenuates estrogen-induced effects on CD 38 expression and activities. This will have implications for increased calcium mobilization and contractility of the myometrium during parturition.

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.

Altered CD38 expression in thioacetamide-induced rat model of liver cirrhosis

BACKGROUND

Cirrhosis is a gradually developing, chronic disease which involves the whole liver. Here, we have shown that CD38 undergoes altered expression upon thioacetamide-induced cirrhosis in rats. CD38 is a type II transmembrane glycoprotein that exhibits ADP-ribosyl cyclase and cADPR hydrolase activities. In this study, the gene and protein expressions of CD38 were investigated in a thioacetamide-induced rat model of cirrhosis.

METHODS

CD38 expression was studied by using real-time RT-PCR, immunohistochemistry, and immunoblotting. cADPR content in liver was measured using cycling assay.

RESULTS

There was a significant increase in CD38 mRNA and protein expressions as well as ADP-ribosyl cyclase activity in cirrhotic liver compared to the control liver. cADPR level was found to be modestly but significantly augmented in cirrhotic liver.

CONCLUSIONS

These results raised the possibility that altered CD38 expression and a concomitant elevation of the enzymatic activity as well as cADPR may be involved in the pathogenesis of liver cirrhosis.

Identification of the mitochondrial NAD+ transporter in Saccharomyces cerevisiae

The mitochondrial carriers are a family of transport proteins that shuttle metabolites, nucleotides, and cofactors across the inner mitochondrial membrane. In Saccharomyces cerevisiae, NAD+ is synthesized outside the mitochondria and must be imported across the permeability barrier of the inner mitochondrial membrane. However, no protein responsible for this transport activity has ever been isolated or identified. In this report, the identification and functional characterization of the mitochondrial NAD+ carrier protein (Ndt1p) is described. The NDT1 gene was overexpressed in bacteria. The purified protein was reconstituted into liposomes, and its transport properties and kinetic parameters were characterized. It transported NAD+ and, to a lesser extent, (d)AMP and (d)GMP but virtually not alpha-NAD+, NADH, NADP+, or NADPH. Transport was saturable with an apparent Km of 0.38 mM for NAD+. The Ndt1p-GFP was found to be targeted to mitochondria. Consistently with Ndt1p localization and its function as a NAD+ transporter, cells lacking NDT1 had reduced levels of NAD+ and NADH in their mitochondria and reduced activity of mitochondrial NAD+-requiring enzymes. Similar results were also found in the mitochondria of cells lacking NDT2 that encodes a protein (Ndt2p) displaying 70% homology with Ndt1p. The delta ndt1 delta ndt2 double mutant exhibited lower mitochondrial NAD+ and NADH levels than the single deletants and a more pronounced delay in growth on nonfermentable carbon sources. The main role of Ndt1p and Ndt2p is to import NAD+ into mitochondria by unidirectional transport or by exchange with intramitochondrially generated (d)AMP and (d)GMP.

Cyclic ADP ribose-mediated Ca2+ signaling in mediating endothelial nitric oxide production in bovine coronary arteries

The present study tested the hypothesis that cyclic ADP ribose (cADPR) serves as a novel second messenger to mediate intracellular Ca2+ mobilization in coronary arterial endothelial cells (CAECs) and thereby contributes to endothelium-dependent vasodilation. In isolated and perfused small bovine coronary arteries, bradykinin (BK)-induced concentration-dependent vasodilation was significantly attenuated by 8-bromo-cADPR (a cell-permeable cADPR antagonist), ryanodine (an antagonist of ryanodine receptors), or nicotinamide (an ADP-ribosyl cyclase inhibitor). By in situ simultaneously fluorescent monitoring, Ca2+ transient and nitric oxide (NO) levels in the intact coronary arterial endothelium preparation, 8-bromo-cADPR (30 microM), ryanodine (50 microM), and nicotinamide (6 mM) substantially attenuated BK (1 microM)-induced increase in intracellular [Ca2+] by 78%, 80%, and 74%, respectively, whereas these compounds significantly blocked BK-induced NO increase by about 80%, and inositol 1,4,5-trisphosphate receptor blockade with 2-aminethoxydiphenyl borate (50 microM) only blunted BK-induced Ca2+-NO signaling by about 30%. With the use of cADPR-cycling assay, it was found that inhibition of ADP-ribosyl cyclase by nicotinamide substantially blocked BK-induced intracellular cADPR production. Furthermore, HPLC analysis showed that the conversion rate of beta-nicotinamide guanine dinucleotide into cyclic GDP ribose dramatically increased by stimulation with BK, which was blockable by nicotinamide. However, U-73122, a phospholipase C inhibitor, had no effect on this BK-induced increase in ADP-ribosyl cyclase activity for cADPR production. In conclusion, these results suggest that cADPR importantly contributes to BK- and A-23187-induced NO production and vasodilator response in coronary arteries through its Ca2+ signaling mechanism in CAECs.

Production of calcium-mobilizing metabolites by a novel member of the ADP-ribosyl cyclase family expressed in Schistosoma mansoni

ADP-ribosyl cyclases are structurally conserved enzymes that are best known for catalyzing the production of the calcium-mobilizing metabolite, cyclic adenosine diphosphate ribose (cADPR), from nicotinamide adenine dinucleotide (NAD(+)). However, these enzymes also produce adenosine diphosphate ribose (ADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP(+)), both of which have been shown to modulate calcium mobilization in vitro. We have now characterized a new member of the cyclase family from Schistosoma mansoni, a member of the Platyhelminthes phylum. We show that the novel NAD(P)(+) catabolizing enzyme (NACE) expressed by schistosomes is structurally most closely related to the cyclases cloned from Aplysia but also shows significant homology with the mammalian cyclases, CD38 and CD157. NACE expression is developmentally regulated in schistosomes, and the GPI-anchored protein is localized to the outer tegument of the adult schistosome. Importantly, NACE, like all members of the cyclase family, is a multifunctional enzyme and catalyzes NAD(+) glycohydrolase and base-exchange reactions to produce ADPR and NAADP(+). However, despite being competent to generate a cyclic product from NGD(+), a nonphysiologic surrogate substrate, NACE is so far the only enzyme in the cyclase family that is unable to produce significant amounts of cADPR (<0.02% of reaction products) using NAD(+) as the substrate. This suggests that the other calcium-mobilizing metabolites produced by NACE may be more important for calcium signaling in schistosomes. Alternatively, the function of NACE may be to catabolize extracellular NAD(+) to prevent its use by host enzymes that utilize this source of NAD(+) to facilitate immune responses.

T-cell suppression mediated by mesenchymal stem cells is deficient in patients with severe aplastic anemia

OBJECTIVE

To compare the suppressive effect of mesenchymal stem cells (MSC), derived from normal individuals or severe aplastic anemia patients (SAA), on T-cell activation.

PATIENTS AND METHODS

We studied bone marrow MSC from 19 healthy donors and 23 SAA patients in different phases of the disease: at diagnosis (n = 3), following immunosuppressive therapy (IS) (n = 16), or after a bone marrow transplant (BMT) (n = 4). MSC were tested for T-cell suppression in the following assays: mixed lymphocyte reaction (MLR), phytohemaglutinin (PHA)-primed cultures, activation surface markers, gamma-IFN production, hematopoietic colony formation (CFC), production of cyclic ADP-ribose (cADPR).

RESULTS

The abnormalities of SAA MSC included: 1) significantly lower suppression of T-cell proliferation induced by alloantigens (p = 0.009) or PHA (p = 0.006); 2) impaired capacity to suppress CD38 expression on PHA-primed T cells (p = 0.001); 3) impaired ability to suppress gamma-IFN production in PHA cultures, resulting in an 11-fold higher gamma-IFN concentration; 4) no preventive effect on T cell-mediated inhibition of CFC; and 5) significantly reduced (p = 0.009) production of cADPR, a universal calcium mobilizer. MSC-mediated suppression of PHA-induced T-cell proliferation was restored to control levels in 3 of 4 patients post-BMT.

CONCLUSION

The ability of MSC to downregulate T-cell priming, proliferation, and cytokine release is deficient in patients with SAA, persists indefinitely after immunosuppressive therapy, but seems to be restored after BMT. Whether these abnormalities are relevant to the pathogenesis of aplastic anemia remains to be determined.

ADP-ribosyl cyclases generate two unusual adenine homodinucleotides with cytotoxic activity on mammalian cells

ADP-ribosyl cyclases are ubiquitous enzymes responsible for synthesis from NAD(+) of the intracellular calcium-releasing signal molecules cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP(+)). Here, we show that cyclases from lower and higher Metazoa also synthesize three adenylic dinucleotides from cADPR and adenine: diadenosine diphosphate and two isomers thereof. These dinucleotides are present and metabolized in mammalian cells and affect intracellular calcium and cell proliferation. The diadenosine diphosphate isomers are naturally occurring nucleotides containing an N-glycosidic bond different from the usual C1'-N9. The identification of these members of the family of NAD(+)-derived, calcium-active nucleotides opens new areas of investigation into their functional cooperation with cADPR and NAADP(+) and into their involvement in the physiology and pathology of calcium-controlled cell functions.

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

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

Emerging role of cyclic ADP-ribose (cADPR) in smooth muscle

Cyclic adenosine diphosphate ribose (cADPR) is a naturally occurring cyclic nucleotide and represents a novel class of endogenous Ca(2+) messengers implicated in the regulation of the gating properties of ryanodine receptors (RyRs). This action of cADPR occurs independently from the inositol-1,4,5-trisphosphate (IP(3)) receptor. The regulation of intracellular Ca(2+) release is a fundamental element of cellular Ca(2+) homeostasis since a number of smooth muscle functions (tone, proliferation, apoptosis, and gene expression) are modulated by intracellular Ca(2+) concentration ([Ca(2+)](i)). There has been a surge in the efforts aimed at understanding the mechanisms of cADPR-mediated Ca(2+) mobilization and its impact on smooth muscle function. This review summarizes the proposed roles of cADPR in the regulation of smooth muscle tone.

Concentrative uptake of cyclic ADP-ribose generated by BST-1+ stroma stimulates proliferation of human hematopoietic progenitors

Cyclic ADP-ribose (cADPR) is an intracellular calcium mobilizer generated from NAD(+) by the ADP-ribosyl cyclases CD38 and BST-1. cADPR, both exogenously added and paracrinally produced by a CD38(+) feeder layer, has recently been demonstrated to stimulate the in vitro proliferation of human hemopoietic progenitors (HP) and also the in vivo expansion of hemopoietic stem cells. The low density of BST-1 expression on bone marrow (BM) stromal cells and the low specific activity of the enzyme made it unclear whether cADPR generation by a BST-1(+) stroma could stimulate HP proliferation in the BM microenvironment. We developed and characterized two BST-1(+) stromal cell lines, expressing an ectocellular cyclase activity similar to that of BST-1(+) human mesenchymal stem cells, the precursors of BM stromal cells. Long term co-culture of cord blood-derived HP over these BST-1(+) feeders determined their expansion. Influx of paracrinally generated cADPR into clonogenic HP was mediated by a concentrative, nitrobenzylthioinosine- and dipyridamole-inhibitable nucleoside transporter, this providing a possible explanation to the effectiveness of the hormone-like concentrations of the cyclic nucleotide measured in the medium conditioned by BST-1(+) feeders. These results suggest that the BST-1-catalyzed generation of extracellular cADPR, followed by the concentrative uptake of the cyclic nucleotide by HP, may be physiologically relevant in normal hemopoiesis.

Activation of CD38 by interleukin-8 signaling regulates intracellular Ca2+ level and motility of lymphokine-activated killer cells

CD38 is an ADP-ribosyl cyclase, producing a potent Ca(2+) mobilizer cyclic ADP-ribose (cADPR). In this study, we have investigated a role of CD38 and its regulation through interleukin-8 (IL8) signaling in lymphokine-activated killer (LAK) cells. Incubation of LAK cells with IL8 resulted in an increase of cellular cADPR level and a rapid rise of intracellular Ca(2+) concentration ([Ca(2+)](i)), which was sustained for a long period of time (>10 min). Preincubation of an antagonistic cADPR analog, 8-Br-cADPR (8-bromo-cyclic adenosine diphosphate ribose), abolished the sustained Ca(2+) signal only but not the initial Ca(2+) rise. An inositol 1,4,5-trisphosphate (IP(3)) receptor antagonist blocked both Ca(2+) signals. Interestingly, the sustained Ca(2+) rise was not observed in the absence of extracellular Ca(2+). Functional CD38-null (CD38(-)) LAK cells showed the initial rapid increase of [Ca(2+)](i) but not the sustained Ca(2+) rise in response to IL8 treatment. An increase of cellular cADPR level by cGMP analog, 8-pCPT-cGMP (8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphate), but not cAMP analog or phorbol 12-myristate 13-acetate was observed. IL8 treatment resulted in the increase of cGMP level that was inhibited by the IP(3) receptor blocker but not a protein kinase C inhibitor. cGMP-mediated Ca(2+) rise was blocked by 8-Br-cADPR. In addition, IL8-mediated LAK cell migration was inhibited by 8-Br-cADPR and a protein kinase G inhibitor. Consistent with these observations, IL8-induced migration of CD38(-) LAK cells was not observed. However, direct application of cADPR or 8-pCPT-cGMP stimulated migration of CD38(-) cells. These results demonstrate that CD38 is stimulated by sequential activation of IL8 receptor, IP(3)-mediated Ca(2+) rise, and cGMP/protein kinase G and that CD38 plays an essential role in IL8-induced migration of LAK cells.

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.

Release of beta-nicotinamide adenine dinucleotide upon stimulation of postganglionic nerve terminals in blood vessels and urinary bladder

Chemical signaling in autonomic neuromuscular transmission involves agents that function as neurotransmitters and/or neuromodulators. Using high performance liquid chromatography techniques with fluorescence and electrochemical detection we observed that, in addition to ATP and norepinephrine (NE), electrical field stimulation (EFS, 4-16 Hz, 0.1-0.3 ms, 15 V, 60-120 s) of isolated vascular and non-vascular preparations co-releases a previously unidentified compound with apparent nucleotide or nucleoside structure. Extensive screening of more than 25 nucleotides and nucleosides followed by detailed peak identification revealed that beta-nicotinamide adenine dinucleotide (beta-NAD) is released in tissue superfusates upon EFS of canine mesenteric artery (CMA), canine urinary bladder, and murine urinary bladder in the amounts of 7.1 +/- 0.7, 26.5 +/- 4.5, and 15.1 +/- 3.2 fmol/mg of tissue, respectively. Smaller amounts of the beta-NAD metabolites cyclic adenosine 5'-diphosphoribose (cADPR) and ADPR were also present in the superfusates collected during EFS of CMA (2.5 +/- 0.9 and 5.8 +/- 0.8 fmol/mg of tissue, respectively), canine urinary bladder (1.8 +/- 0.5 and 9.0 +/- 6.0 fmol/mg of tissue, respectively), and murine urinary bladder (1.4 +/- 0.1 and 6.2 +/- 2.4 fmol/mg of tissue, respectively). The three nucleotides were also detected in the samples collected before EFS (0.2-1.6 fmol/mg of tissue). Exogenous beta-NAD, cADPR, and ADPR (all 100 nm) reduced the release of NE in CMA at 16 Hz from 27.8 +/- 6.0 fmol/mg of tissue to 15.5 +/- 5.0, 12 +/- 3.0, and 10.0 +/- 4.0 fmol/mg of tissue, respectively. In conclusion, we detected constitutive and nerve-evoked overflow of beta-NAD, cADPR, and ADPR in vascular and non-vascular smooth muscles, beta-NAD being the prevailing compound. These substances modulate the release of NE, implicating novel nucleotide mechanisms of autonomic nervous system control of smooth muscle.

Abscisic acid signaling through cyclic ADP-ribose in hydroid regeneration

Cyclic ADP-ribose (cADPR) is an intracellular calcium (Ca(2+)(i)) mobilizer involved in fundamental cell functions from protists to higher plants and mammals. Biochemical similarities between the drought-signaling cascade in plants and the temperature-sensing pathway in marine sponges suggest an ancient evolutionary origin of a signaling cascade involving the phytohormone abscisic acid (ABA), cADPR, and Ca(2+)(i). In Eudendrium racemosum (Hydrozoa, Cnidaria), exogenously added ABA stimulated ADP-ribosyl cyclase activity via a protein kinase A (PKA)-mediated phosphorylation and increased regeneration in the dark to levels observed under light conditions. Light stimulated endogenous ABA synthesis, which was conversely inhibited by the inhibitor of plant ABA synthesis Fluridone. The signal cascade of light-induced regeneration uncovered in E. racemosum: light --> increasing ABA --> PKA --> cyclase activation --> increasing [cADPR](i) --> increasing [Ca(2+)](i) --> regeneration is the first report of a complete signaling pathway in Eumetazoa involving a phytohormone.

ABA activates ADPR cyclase and cADPR induces a subset of ABA-responsive genes in Arabidopsis

Cyclic ADP-ribose (cADPR) was previously shown to activate transient expression of two abscisic acid (ABA)-responsive genes in tomato cells. Here, we show that the activity of the enzyme responsible for cADPR synthesis, ADP-ribosyl (ADPR) cyclase, is rapidly induced by ABA in both wild-type (WT) and abi1-1 mutant Arabidopsis plants in the absence of protein synthesis. Furthermore, in transgenic Arabidopsis plants, induced expression of the Aplysia ADPR cyclase gene resulted in an increase in ADPR cyclase activity and cADPR levels, as well as elevated expression of ABA-responsive genes KIN2, RD22, RD29a, and COR47 (although to a lesser extent than after ABA induction). Genome-wide profiling indicated that about 28% of all ABA-responsive genes in Arabidopsis are similarly up- and downregulated by cADPR and contributed to the identification of new ABA-responsive genes. Our results suggest that activation of ADPR cyclase is an early ABA-signaling event partially insensitive to the abi1-1 mutation and that an increase in cADPR plays an important role in downstream molecular and physiological ABA responses.

Glutamate-mediated overexpression of CD38 in astrocytes cultured with neurones

Recently, a new system of astrocyte-neurone glutamatergic signalling has been identified. It is started in astrocytes by ectocellular, CD38-catalysed conversion of NAD(+) to the calcium mobilizer cyclic ADP-ribose (cADPR). This is then pumped by CD38 itself into the cytosol where the resulting free intracellular Ca(2+) concentration [Ca(2+)](i) transients elicit an increased release of glutamate, which can induce an enhanced Ca(2+) response in neighbouring neurones. Here, we demonstrate that co-culture of either cortical or hippocampal astrocytes with neurones results in a significant overexpression of astrocyte CD38 both on the plasma membrane and intracellularly. The causal role of neurone-released glutamate in inducing overexpression of astrocyte CD38 is demonstrated by two observations: first, in the absence of neurones, induction of CD38 in pure astrocyte cultures can be obtained with glutamate and second, it can be prevented in co-cultures by glutamate receptor antagonists. The neuronal glutamate-mediated effect of neurones on astrocyte CD38 expression is paralleled by increased intracellular cADPR and [Ca(2+)](i) levels, both findings indicating functionality of overexpressed CD38. These results reveal a new neurone-to-astrocyte glutamatergic signalling based on the CD38/cADPR system, which affects the [Ca(2+)](i) in both cell types, adding further complexity to the bi-directional patterns of communication between astrocytes and neurones.

Concentrative influx of functionally active cyclic ADP-ribose in dimethyl sulfoxide-differentiated HL-60 cells

Native human HL-60 cells do not express CD38, a multifunctional ectoenzyme, which generates cyclic ADP-ribose (cADPR), a potent calcium mobilizer. However, when HL-60 cells are induced to differentiate to granulocytes by treatment with retinoic acid (RA), they express CD38 and accumulate cADPR. Both processes play a causal role in RA-induced differentiation. Other granulocyte differentiation-inducers, including dimethyl sulfoxide (Me(2)SO), fail to induce CD38 expression. We investigated whether treatment of HL-60 cells with Me(2)SO involves any changes in the cADPR/intracellular calcium ([Ca(2+)](i)) signaling system and, specifically, whether Me(2)SO affects those nucleoside transporters (NT) (both equilibrative (ENT) and concentrative (CNT)) that mediate influx of extracellular cADPR. Semiquantitative polymerase chain reaction analysis of transcripts, binding of [(3)H]nitrobenzylthioinosine (NBMPR) to intact cells, and influx experiments of extracellular cADPR (with selective inhibitors of NT as NBMPR or in specific conditions) were performed in native and Me(2)SO-differentiated HL-60 cells. The native cells showed uptake of cADPR across ENT2, whereas influx of cADPR into the Me(2)SO-differentiated cells occurred mostly by concentrative processes mediated by CNT3 and by an NBMPR-inhibitable concentrative NT designated cs-csg. Me(2)SO-differentiated, but not native HL-60 cells, accumulated cADPR and showed increased [Ca(2+)](i) levels when grown in a transwell co-culture setting over CD38-transfected 3T3 fibroblasts where nanomolar cADPR concentrations are present in the medium. NBMPR inhibited both responses of Me(2)SO-induced cells. Thus, concentrative influx of extracellular cADPR across CNT3 and cs-csg NT could substitute in the absence of CD38 in eliciting cADPR-dependent [Ca(2+)](i) increases in granulocyte-differentiated HL-60 cells, as well as in other CD38(-) cells.

Changes in CD38 expression and ADP-ribosyl cyclase activity in rat myometrium during pregnancy: influence of sex steroid hormones

Cyclic ADP-ribose (cADPR), synthesized by CD38, regulates intracellular calcium in uterine smooth muscle. CD38 is a transmembrane protein that has both ADP-ribosyl cyclase and cADPR hydrolase enzyme activities involved in cADPR metabolism. CD38 expression and its enzyme activities in uterine smooth muscle are regulated by estrogen. In the present study, we examined CD38 expression, its enzyme activities, and cADPR levels in myometrium obtained from rats at 14-17 days of gestation (preterm) and at parturition (term). CD38 expression, ADP-ribosyl cyclase activity, and cADPR levels were higher in uterine tissues obtained from term rats compared with that of preterm rats, while activity of cADPR hydrolase did not significantly change. In an effort to address whether changes in estrogen: progesterone ratio that occur during pregnancy account for the observed effects on CD38 expression and function, we determined the effect of different doses of progesterone in the presence of estrogen on CD38 expression and its enzyme activities in uterine smooth muscle obtained from ovariectomized rats. In myometrium obtained from ovariectomized rats, estrogen administration caused increased CD38 protein expression and ADP-ribosyl cyclase activity. The estrogen-induced increases in CD38 expression and ADP-ribosyl cyclase activity were inhibited by simultaneous administration of 10 or 20 mg of progesterone. These results indicate that the estrogen:progesterone ratio determines CD38 expression and ADP-ribosyl cyclase activity. These changes in CD38/cADPR pathway may contribute to increased uterine motility and onset of labor.

Modulation of calcium signaling by interleukin-13 in human airway smooth muscle: role of CD38/cyclic adenosine diphosphate ribose pathway

CD38/cyclic adenosine diphosphate ribose (cADPR) signaling plays an important role in the regulation of intracellular calcium responses to agonists in a variety of cells, including airway smooth muscle (ASM) cells. The present study was aimed at determining the effect of interleukin (IL)-13, a cytokine implicated in the pathogenesis of asthma, on CD38/cADPR signaling and to ascertain the contribution of CD38/cADPR signaling to IL-13-induced airway hyperresponsiveness. Human ASM cells maintained in culture were exposed to 50 ng/ml IL-13 for 22 h and levels of CD38 expression and intracellular calcium responses to agonists were measured. Treatment of human ASM cells with IL-13 resulted in increased CD38 expression as determined by real-time polymerase chain reaction, Western blot analysis, and indirect immunofluorescence. Increased CD38 expression was reflected as increased ADP-ribosyl cyclase activity in the ASM cell membranes. The net intracellular calcium responses to bradykinin, thrombin, and histamine were significantly (P < or = 0.05) higher in cells treated with IL-13 compared with controls. Furthermore, 8-bromo-cADPR, a cADPR antagonist, attenuated IL-13-induced augmented intracellular calcium responses to agonists in human ASM cells. These findings indicate that the CD38/cADPR-dependent pathway has a major role in IL-13-induced modulation of calcium signaling in human ASM.

Spatio-temporal propagation of Ca2+ signals by cyclic ADP-ribose in 3T3 cells stimulated via purinergic P2Y receptors

The role of cyclic ADP-ribose in the amplification of subcellular and global Ca2+ signaling upon stimulation of P2Y purinergic receptors was studied in 3T3 fibroblasts. Either (1) 3T3 fibroblasts (CD38- cells), (2) 3T3 fibroblasts preloaded by incubation with extracellular cyclic ADP-ribose (cADPR), (3) 3T3 fibroblasts microinjected with ryanodine, or (4) 3T3 fibroblasts transfected to express the ADP-ribosyl cyclase CD38 (CD38+ cells) were used. Both preincubation with cADPR and CD38 expression resulted in comparable intracellular amounts of cyclic ADP-ribose (42.3 +/- 5.2 and 50.5 +/- 8.0 pmol/mg protein). P2Y receptor stimulation of CD38- cells yielded a small increase of intracellular Ca2+ concentration and a much higher Ca2+ signal in CD38-transfected cells, in cADPR-preloaded cells, or in cells microinjected with ryanodine. Confocal Ca2+ imaging revealed that stimulation of ryanodine receptors by cADPR or ryanodine amplified localized pacemaker Ca2+ signals with properties resembling Ca2+ quarks and triggered the propagation of such localized signals from the plasma membrane toward the internal environment, thereby initiating a global Ca2+ wave.

Cyclic ADP-ribose is a second messenger in the lipopolysaccharide-stimulated proliferation of human peripheral blood mononuclear cells

Cyclic ADP-ribose (cADPR), a universal calcium mobilizer from intracellular stores, was recently demonstrated to stimulate proliferation of various cell types. The role of cADPR in a specific process of monocyte- and plasma-mediated activation of T-lymphocytes by lipopolysaccharide (LPS) was addressed using human mononuclear cells from peripheral blood (PBMCs). Incubation of PBMCs with 0.1 microg/ml of LPS for 24 h provided a doubling in the intracellular levels of cADPR as compared with unstimulated PBMCs. The cADPR increase was abolished either by prior removal of monocytes or by pre-incubating a whole PBMC population with a monoclonal antibody against the monocyte marker CD14. The increased concentrations of intracellular cADPR elicited by LPS stimulation were paralleled by significant increases in NAD+ levels and in the activities of ectocellular and membrane-bound fractions of ADP-ribosyl cyclase/cADPR hydrolase activities. A cytosolic ADP-ribosyl cyclase was also detectable in PBMCs and its activity was comparably enhanced by LPS stimulation. This soluble cyclase is distinguished from the membrane-bound cyclase by both substrate and inhibitor sensitivities. LPS-stimulated PBMCs showed 2-3-fold increases of intracellular calcium ([Ca2+]i), and these changes were prevented completely by the cADPR antagonist 8-Br-cADPR and by ryanodine. Both compounds, and the cyclase inhibitor nicotinamide, significantly inhibited the T-lymphocyte proliferation induced by LPS in PBMCs. These results demonstrate that cADPR plays a role of second messenger in the adaptive immune recognition process of LPS-stimulated proliferation of PBMCs.

The human immunodeficiency virus-1 protein Tat and its discrete fragments evoke selective release of acetylcholine from human and rat cerebrocortical terminals through species-specific mechanisms

The effect of the human immunodeficiency virus-1 protein Tat was investigated on neurotransmitter release from human and rat cortical nerve endings. Tat failed to affect the release of several neurotransmitters, such as glutamate, GABA, norepinephrine, and others, but it evoked the release of [3H]ACh via increase of cytosolic [Ca2+]. In human nerve terminals, the Tat effect partly depends on Ca2+ entry through voltage-sensitive Ca2+ channels, because Cd2+ halved the Tat-evoked release. Activation of group I metabotropic glutamate receptors (mGluR) and mobilization of Ca2+ from IP3-sensitive intraterminal stores are also involved, because the Tat effect was prevented by mGluR antagonists 2-methyl-6-(phenylethynyl)pyridine hydrochloride and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester and by the IP3 receptor antagonists heparin and xestospongin C. Furthermore, the group I selective mGlu agonist (RS)-3,5-dihydroxyphenylglycine enhanced [3H]ACh release. In rat nerve terminals, the Tat-evoked release neither depends on external Ca2+ ions entry nor on IP3-mediated mechanisms. Tat seems to cause mobilization of Ca2+ from ryanodine-sensitive internal stores because its effect was prevented by both 8-bromo-cyclic adenosine diphosphate-ribose and dantrolene. The Tat-evoked release from human synaptosomes was mimicked by the peptide sequences Tat 32-62, Tat 49-86, and Tat 41-60. In contrast, the Tat 49-86 and Tat 61-80 fragments, but not the Tat 32-62 fragment, were active in rat synaptosomes. In conclusion, Tat elicits Ca2+-dependent [3H]ACh release by species-specific intraterminal mechanisms by binding via discrete amino acid sequences to different receptive sites on human and rat cholinergic terminals.

The human immunodeficiency virus-1 protein Tat and its discrete fragments evoke selective release of acetylcholine from human and rat cerebrocortical terminals through species-specific mechanisms

The effect of the human immunodeficiency virus-1 protein Tat was investigated on neurotransmitter release from human and rat cortical nerve endings. Tat failed to affect the release of several neurotransmitters, such as glutamate, GABA, norepinephrine, and others, but it evoked the release of [3H]ACh via increase of cytosolic [Ca2+]. In human nerve terminals, the Tat effect partly depends on Ca2+ entry through voltage-sensitive Ca2+ channels, because Cd2+ halved the Tat-evoked release. Activation of group I metabotropic glutamate receptors (mGluR) and mobilization of Ca2+ from IP3-sensitive intraterminal stores are also involved, because the Tat effect was prevented by mGluR antagonists 2-methyl-6-(phenylethynyl)pyridine hydrochloride and 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester and by the IP3 receptor antagonists heparin and xestospongin C. Furthermore, the group I selective mGlu agonist (RS)-3,5-dihydroxyphenylglycine enhanced [3H]ACh release. In rat nerve terminals, the Tat-evoked release neither depends on external Ca2+ ions entry nor on IP3-mediated mechanisms. Tat seems to cause mobilization of Ca2+ from ryanodine-sensitive internal stores because its effect was prevented by both 8-bromo-cyclic adenosine diphosphate-ribose and dantrolene. The Tat-evoked release from human synaptosomes was mimicked by the peptide sequences Tat 32-62, Tat 49-86, and Tat 41-60. In contrast, the Tat 49-86 and Tat 61-80 fragments, but not the Tat 32-62 fragment, were active in rat synaptosomes. In conclusion, Tat elicits Ca2+-dependent [3H]ACh release by species-specific intraterminal mechanisms by binding via discrete amino acid sequences to different receptive sites on human and rat cholinergic terminals.

ABA- and cADPR-mediated effects on respiration and filtration downstream of the temperature-signaling cascade in sponges

Recently, the thermosensing pathway in sponges (Porifera) was elucidated. The thermosensor triggering this cascade is a heat-activated cation channel, with the phytohormone abscisic acid (ABA), cyclic ADP-ribose (cADPR) and calcium acting as intracellular messengers, similarly to the drought-stress signaling cascade in higher plants. Here, we investigated the functional effects downstream of the temperature-signaling pathway in Axinella polypoides (Porifera, Demonspongiae). Short-term stimulation followed by long-term depression of amino acid incorporation, oxygen consumption and water filtration were observed after exposure of the sponge to a brief heat stress or to micromolar ABA. These effects could be prevented by the targeted interruption of the signaling pathway either at the level of the cation channel thermosensor or at the level of the cADPR-induced intracellular calcium increase. Moreover, release of cyclase activity into the sea water and generation of extracellular cADPR were observed following brief heat stress. Intact sponge cells were sensitive to extracellular cADPR and addition of purified cyclase increased sponge respiration similarly to heat stress. This is the first observation of functional effects exerted on Metazoa by the phytohormone ABA: conservation of the ABA/cADPR stress-signaling cascade points to its early evolution in a common precursor of modern Metazoa and Metaphyta. The functional effects induced by extracellular cyclase/cADPR suggest an evolutionary origin of cADPR as an ancient stress hormone in Porifera.

Evidence for a causal role of CD38 expression in granulocytic differentiation of human HL-60 cells

Granulocytic differentiation of human HL-60 cells can be induced by retinoic acid and is accompanied by a massive expression of CD38, a multi-functional enzyme responsible for metabolizing cyclic ADP-ribose (cADPR), a Ca(2+) messenger. Immunofluorescence staining showed that CD38 was expressed not only on the surface of intact HL-60 cells but also intracellularly, which was revealed after permeabilization with Triton. Concomitant with CD38 expression was the accumulation of cADPR, and both time courses preceded the onset of differentiation, suggesting a causal role for CD38. Consistently, treatment of HL-60 cells with a permeant inhibitor of CD38, nicotinamide, inhibited both the CD38 activity and differentiation. More specific blockage of CD38 expression was achieved by using morpholino antisense oligonucleotides targeting its mRNA, which produced a corresponding inhibition of differentiation as well. Similar inhibitory effects were observed when CD38 expression was reduced by the RNA interference technique targeting two separate regions of the coding sequence of CD38. Further support came from transfecting HL-60 cells with a Tet-On expression vector containing a full-length CD38. Subsequent treatments with doxycycline induced both CD38 expression and differentiation in the absence of retinoic acid. These results provide the first evidence that CD38 expression and the consequential accumulation of cADPR play a causal role in mediating cellular differentiation.

Equilibrative and concentrative nucleoside transporters mediate influx of extracellular cyclic ADP-ribose into 3T3 murine fibroblasts

In mammals cyclic ADP-ribose (cADPR), a universal calcium mobilizer from intracellular stores, is generated from NAD(+) at the outer cell surface by the multifunctional ectoenzyme CD38 and by related ADP-ribosyl cyclases. Recently, influx of extracellular cADPR has been observed in 3T3 murine fibroblasts, where it elicits Ca(2+)-mediated enhancement of proliferation. Here we addressed the nature and the properties of cADPR influx into CD38(-) 3T3 cells, which showed pleiotropic mechanisms of both equilibrative and concentrative transport. Based on selective inhibitors or experimental conditions (e.g. abrogation of Na(+)-dependent active symport processes and transient transfection experiments) and on reverse transcriptase-polymerase chain reaction analysis of transcripts in 3T3 fibroblasts and comparatively in HeLa cells, we identified cADPR-transporting activities with specific nucleoside transporters (NT), both equilibrative (ENT2) and concentrative (CNT2 and a nitrobenzylthioinosine (NBMPR)-inhibitable NT). A reciprocal inhibition relationship was observed between inosine and cADPR fluxes across these NT species. Concentrative (but not equilibrative) transport of nanomolar extracellular cADPR took place in CD38(-) 3T3 cells co-cultured for 48 h in transwells on feeders of CD38-transfected, cADPR-generating 3T3 fibroblasts. These results suggest possible, hitherto unrecognized, correlations between ectocellular metabolism of nucleotides/nucleosides and cADPR-mediated regulation of intracellular calcium homeostasis.

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.

A novel cycling assay for nicotinic acid-adenine dinucleotide phosphate with nanomolar sensitivity

Nicotinic acid-adenine dinucleotide phosphate (NAADP) is a novel nucleotide derived from NADP that has now been shown to be active in releasing Ca(2+) from intracellular stores in a wide variety of cells ranging from plant to human. Despite the obvious importance of monitoring its cellular levels under various physiological conditions, no assay has been reported for NAADP to date. In the present study, a widely applicable assay for NAADP with high sensitivity is described. NAADP was first dephosphorylated to nicotinic acid-adenine dinucleotide by treatment with alkaline phosphatase. The conversion was shown to be stoichiometric. NMN-adenylyltransferase was then used to convert nicotinic acid-adenine dinucleotide into NAD in the presence of high concentrations of NMN. The resultant NAD was amplified by a cycling assay involving alcohol dehydrogenase and diaphorase. Each time NAD cycled through these coupled reactions, a molecule of highly fluorescent resorufin was generated. The reaction could be performed for hours, resulting in more than a 1000-fold amplification. Concentrations of NAADP over the 10-20 nM range could be routinely measured. This novel cycling assay was combined with an enzymic treatment to provide the necessary specificity for the assay. NAADP was found to be resistant to NADase and apyrase. Pretreatment of samples with a combination of the hydrolytic enzymes completely eliminated the interference from common nucleotides. The versatility of the cycling assay can also be extended to measure nicotinic acid, which is a substrate in the synthesis of NAADP catalysed by ADP-ribosyl cyclase, over the micromolar range. All the necessary reagents for the cycling assay are widely available and it can be performed using a multi-well fluorescence plate reader, providing a high-throughput method. This is the first assay reported for NAADP and nicotinic acid, which should be valuable in elucidating the messenger functions of NAADP.

The ryanodine receptor calcium channel of beta-cells: molecular regulation and physiological significance

The list of Ca(2+) channels involved in stimulus-secretion coupling in beta-cells is increasing. In this respect the roles of the voltage-gated Ca(2+) channels and IP(3) receptors are well accepted. There is a lack of consensus about the significance of a third group of Ca(2+) channels called ryanodine (RY) receptors. These are large conduits located on Ca(2+) storage organelle. Ca(2+) gates these channels in a concentration- and time-dependent manner. Activation of these channels by Ca(2+) leads to fast release of Ca(2+) from the stores, a process called Ca(2+)-induced Ca(2+) release (CICR). A substantial body of evidence confirms that beta-cells have RY receptors. CICR by RY receptors amplifies Ca(2+) signals. Some properties of RY receptors ensure that this amplification process is engaged in a context-dependent manner. Several endogenous molecules and processes that modulate RY receptors determine the appropriate context. Among these are several glycolytic intermediates, long-chain acyl CoA, ATP, cAMP, cADPR, NO, and high luminal Ca(2+) concentration, and all of these have been shown to sensitize RY receptors to the trigger action of Ca(2+). RY receptors, thus, detect co-incident signals and integrate them. These Ca(2+) channels are targets for the action of cAMP-linked incretin hormones that stimulate glucose-dependent insulin secretion. In beta-cells some RY receptors are located on the secretory vesicles. Thus, despite their low abundance, RY receptors are emerging as distinct players in beta-cell function by virtue of their large conductance, strategic locations, and their ability to amplify Ca(2+) signals in a context-dependent manner.