Extracellular nicotinamide adenine dinucleotide induces t cell apoptosis in vivo and in vitro

NAD+ metabolism-based immunoregulation and therapeutic potential

Nicotinamide adenine dinucleotide (NAD⁺) is a critical metabolite that acts as a cofactor in energy metabolism, and serves as a cosubstrate for non-redox NAD⁺-dependent enzymes, including sirtuins, CD38 and poly(ADP-ribose) polymerases. NAD⁺ metabolism can regulate functionality attributes of innate and adaptive immune cells and contribute to inflammatory responses. Thus, the manipulation of NAD⁺ bioavailability can reshape the courses of immunological diseases. Here, we review the basics of NAD⁺ biochemistry and its roles in the immune response, and discuss current challenges and the future translational potential of NAD⁺ research in the development of therapeutics for inflammatory diseases, such as COVID-19.

Ex Vivo Lung Perfusion with β-Nicotinamide Adenine Dinucleotide (NAD+) Improves Ischemic Lung Function

Ischemia-reperfusion injury compromises short- and long-term outcomes after lung transplantation. The scarce existing data on NAD⁺ suggest effects on hypoxia-induced vasoconstriction, on reactive oxygen species and on tampering inflammation. We exposed rat lungs to 14 h of cold ischemic storage and perfused them in a rat ex vivo lung perfusion (EVLP) system for 4 h. A control group (n = 6) was compared to groups receiving 100 µM (n = 6) or 200 µM NAD⁺ (n = 6) in the preservation solution and groups receiving 200 µM (n = 4) or 2000 µM (n = 6) NAD⁺ every 30 min in the perfusate, starting at 1 h of EVLP. Compared to the control, significant effects were only achieved in the 2000 µM NAD⁺ group. During the 4 h of EVLP, we monitored higher vascular flow, lower mean pulmonary arterial pressure and increased oxygenation capacity. Tissue inflammation estimated with the myeloperoxidase assay was lower in the 2000 µM NAD⁺ group. We observed higher levels of anti-inflammatory IL-10, higher anti-inflammatory IL-6/IL-10 ratios and lower levels of pro-inflammatory IL-12 and IL-18 as well as a trend of more anti-inflammatory IFNy in the 2000 µM NAD⁺ perfusate. In the bronchoalveolar lavage, the pro-inflammatory levels of IL-1α and IL-1β were lower in the 2000 µM NAD⁺ group. NAD⁺ administered during EVLP is a promising agent with both anti-inflammatory properties and the ability to improve ischemic lung function.

The new insight into extracellular NAD+ degradation-the contribution of CD38 and CD73 in calcific aortic valve disease

Nicotinamide adenine dinucleotide (NAD+ ) is crucial for cell energy metabolism and many signalling processes. Recently, we proved the role of ecto-enzymes in controlling adenine nucleotide-dependent pathways during calcific aortic valve disease (CAVD). This study aimed to investigate extracellular hydrolysis of NAD+ and mononucleotide nicotinamide (NMN) in aortic valves and aorta fragments of CAVD patients and on the inner aortic surface of ecto-5'-nucleotidase knockout mice (CD73-/-). Human non-stenotic valves (n = 10) actively converted NAD+ and NMN via both CD73 and NAD+ -glycohydrolase (CD38) according to our analysis with RP-HPLC and immunofluorescence. In stenotic valves (n = 50), due to reduced CD73 activity, NAD+ was degraded predominantly by CD38 and additionally by ALP and eNPP1. CAVD patients had significantly higher hydrolytic rates of NAD+ (0.81 ± 0.07 vs 0.56 ± 0.10) and NMN (1.12 ± 0.10 vs 0.71 ± 0.08 nmol/min/cm2 ) compared with controls. CD38 was also primarily engaged in human vascular NAD+ metabolism. Studies using specific ecto-enzyme inhibitors and CD73-/- mice confirmed that CD73 is not the only enzyme involved in NAD+ and NMN hydrolysis and that CD38 had a significant contribution to these pathways. Modifications of extracellular NAD+ and NMN metabolism in aortic valve cells may be particularly important in valve pathology and could be a potential therapeutic target.

Nicotinamide adenine dinucleotide metabolism in the immune response, autoimmunity and inflammageing

Metabolism is dynamically regulated to accompany immune cell function, and altered immunometabolism can result in impaired immune responses. Concomitantly, the pharmacological manipulation of metabolic processes offers an opportunity for therapeutic intervention in inflammatory disorders. The nicotinamide adenine dinucleotide (NAD⁺) is a critical metabolic intermediate that serves as enzyme cofactor in redox reactions, and is also used as a co‐substrate by many enzymes such as sirtuins, adenosine diphosphate ribose transferases and synthases. Through these activities, NAD⁺ metabolism regulates a broad spectrum of cellular functions such as energy metabolism, DNA repair, regulation of the epigenetic landscape and inflammation. Thus, the manipulation of NAD⁺ availability using pharmacological compounds such as NAD⁺ precursors can have immune‐modulatory properties in inflammation. Here, we discuss how the NAD⁺ metabolism contributes to the immune response and inflammatory conditions, with a special focus on multiple sclerosis, inflammatory bowel diseases and inflammageing.

Is nuclear sirtuin SIRT6 a master regulator of immune function?

The ability to ward off pathogens with minimal damage to the host determines the immune system's robustness. Multiple factors, including pathogen processing, identification, secretion of mediator and effector molecules, and immune cell proliferation and differentiation into various subsets, constitute the success of mounting an effective immune response. Cellular metabolism controls all of these intricate processes. Cells utilize diverse fuel sources and switch back and forth between different metabolic pathways depending on their energy needs. The three most critical metabolic pathways on which immune cells depend to meet their energy needs are oxidative metabolism, glycolysis, and glutaminolysis. Dynamic switching between these metabolic pathways is needed for optimal function of the immune cells. Moreover, switching between these metabolic pathways needs to be tightly regulated to achieve the best results. Immune cells depend on the Warburg effect for their growth, proliferation, secretory, and effector functions. Here, we hypothesize that the sirtuin, SIRT6, could be a negative regulator of the Warburg effect. We also postulate that SIRT6 could act as a master regulator of immune cell metabolism and function by regulating critical signaling pathways.

NAD+ therapy in age-related degenerative disorders: A benefit/risk analysis

Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that is present in all living cells. NAD+ acts as an important cofactor and substrate for a multitude of biological processes including energy production, DNA repair, gene expression, calcium-dependent secondary messenger signalling and immunoregulatory roles. The de novo synthesis of NAD+ is primarily dependent on the kynurenine pathway (KP), although NAD+ can also be recycled from nicotinic acid (NA), nicotinamide (NAM) and nicotinamide riboside (NR). NAD+ levels have been reported to decline during ageing and age-related diseases. Recent studies have shown that raising intracellular NAD+ levels represents a promising therapeutic strategy for age-associated degenerative diseases in general and to extend lifespan in small animal models. A systematic review of the literature available on Medline, Embase and Pubmed was undertaken to evaluate the potential health and/or longevity benefits due to increasing NAD+ levels. A total of 1545 articles were identified and 147 articles (113 preclinical and 34 clinical) met criteria for inclusion. Most studies indicated that the NAD+ precursors NAM, NR, nicotinamide mononucleotide (NMN), and to a lesser extent NAD+ and NADH had a favourable outcome on several age-related disorders associated with the accumulation of chronic oxidative stress, inflammation and impaired mitochondrial function. While these compounds presented with a limited acute toxicity profile, evidence is still quite limited and long-term human clinical trials are still nascent in the current literature. Potential risks in raising NAD+ levels in various clinical disorders using NAD+ precursors include the accumulation of putative toxic metabolites, tumorigenesis and promotion of cellular senescence. Therefore, NAD+ metabolism represents a promising target and further studies are needed to recapitulate the preclinical benefits in human clinical trials.

Increased TLR4 Expression Aggravates Sepsis by Promoting IFN-γ Expression in CD38-/- Mice

Gram-negative bacterial sepsis accounts for up to 50% worldwide sepsis that causes hospital mortality. Acute kidney injury (AKI), a common complication of Gram-negative bacterial sepsis, is caused by Toll-like receptor 4 (TLR4) activation. Lipopolysaccharide (LPS) is an endotoxin in Gram-negative bacteria and is recognized specifically by TLR4, which initiates innate immune response. Also, TLR4 signaling pathway activation is essential in response to LPS infection. CD38 is one of the well-known regulators of innate immunity, whose dysregulation contributes to sepsis. Many studies have proven that an attenuated Gram-positive bacterium induces sepsis in a CD38-blocking model. However, the pathogenesis of Gram-negative bacteria-induced sepsis in a CD38^−/− mouse model remains unclear. The aim of this study is to investigate whether kidney injury is still attenuated in a LPS-induced CD38^−/− sepsis model and identify the potential mechanism. We assess the severity of kidney injury related to proinflammatory cytokine expressions (IFN-γ, TNF-α, IL-1β, and IL-6) in WT and CD38^−/− mice. Our results showed more aggravated kidney damage in CD38^−/− mice than in WT mice, accompanied with an increase of proinflammatory cytokine expression. In addition, compared with CD38^−/−TLR4^mut mice, we found an increase of TLR4 expression and mRNA expression of these cytokines in the kidney of CD38^−/− mice, although only increased IFN-γ level was detected in the serum. Taken together, these results demonstrated that an increased TLR4 expression in CD38^−/− mice could contribute to the aggravation of AKI through boosting of the production of IFN-γ.

Role of Nicotinamide Adenine Dinucleotide and Related Precursors as Therapeutic Targets for Age-Related Degenerative Diseases: Rationale, Biochemistry, Pharmacokinetics, and Outcomes

Significance: Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that serves as an essential cofactor and substrate for a number of critical cellular processes involved in oxidative phosphorylation and ATP production, DNA repair, epigenetically modulated gene expression, intracellular calcium signaling, and immunological functions. NAD+ depletion may occur in response to either excessive DNA damage due to free radical or ultraviolet attack, resulting in significant poly(ADP-ribose) polymerase (PARP) activation and a high turnover and subsequent depletion of NAD+, and/or chronic immune activation and inflammatory cytokine production resulting in accelerated CD38 activity and decline in NAD+ levels. Recent studies have shown that enhancing NAD+ levels can profoundly reduce oxidative cell damage in catabolic tissue, including the brain. Therefore, promotion of intracellular NAD+ anabolism represents a promising therapeutic strategy for age-associated degenerative diseases in general, and is essential to the effective realization of multiple benefits of healthy sirtuin activity. The kynurenine pathway represents the de novo NAD+ synthesis pathway in mammalian cells. NAD+ can also be produced by the NAD+ salvage pathway. Recent Advances: In this review, we describe and discuss recent insights regarding the efficacy and benefits of the NAD+ precursors, nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside (NR), and nicotinamide mononucleotide (NMN), in attenuating NAD+ decline in degenerative disease states and physiological aging. Critical Issues: Results obtained in recent years have shown that NAD+ precursors can play important protective roles in several diseases. However, in some cases, these precursors may vary in their ability to enhance NAD+ synthesis via their location in the NAD+ anabolic pathway. Increased synthesis of NAD+ promotes protective cell responses, further demonstrating that NAD+ is a regulatory molecule associated with several biochemical pathways. Future Directions: In the next few years, the refinement of personalized therapy for the use of NAD+ precursors and improved detection methodologies allowing the administration of specific NAD+ precursors in the context of patients' NAD+ levels will lead to a better understanding of the therapeutic role of NAD+ precursors in human diseases.

Restoring NAD(+) Levels with NAD(+) Intermediates, the Second Law of Thermodynamics, and Aging Delay

The hypothesis regarding the role of increased nicotinamide adenine dinucleotide (NAD+) levels with reference to the fundamental concepts of aging and entropy is presented. Considering the second law of thermodynamics, NAD+ seems the appropriate candidate for reversing many aging-associated pathologies. NAD+ is presented as an essential compound that enables organisms to stay highly organized and well maintained, with a lower entropy state.

Complex role of nicotinamide adenine dinucleotide in the regulation of programmed cell death pathways

Over the past few years, a growing body of experimental observations has led to the identification of novel and alternative programs of regulated cell death. Recently, autophagic cell death and controlled forms of necrosis have emerged as major alternatives to apoptosis, the best characterized form of regulated cell demise. These recently identified, caspase-independent, forms of cell death appear to play a role in the response to several forms of stress, and their importance in different pathological conditions such as ischemia, infection and inflammation has been recognized. The functional link between cell metabolism and survival has also been the matter of recent studies. Nicotinamide adenine dinucleotide (NAD(+)) has gained particular interest due to its role in cell energetics, and as a substrate for several families of enzymes, comprising poly ADP-ribose polymerases (PARPs) and sirtuins, involved in numerous biological functions including cell survival and death. The recently uncovered diversity of cell death programs has led us to reevaluate the role of this important metabolite as a universal pro-survival factor, and to discuss the potential benefits and limitations of pharmacological approaches targeting NAD(+) metabolism.

CD8-β ADP-ribosylation affects CD8(+) T-cell function

The CD8αβ coreceptor is crucial for effective peptide: MHC-I recognition by the TCR of CD8(+) T cells. Adenosine diphosphate ribosyl transferase 2.2 (ART2.2) utilizes extracellular NAD(+) to transfer ADP-ribose to arginine residues of extracellular domains of surface proteins. Here, we show that in the presence of extracellular NAD(+) , ART2.2 caused ADP-ribosylation of CD8-β on murine CD8(+) T cells in vitro and in vivo. Treatment with NAD(+) prevented binding of anti-CD8-β mAb YTS156.7.7 but not of mAb H35-17.2, indicating that NAD(+) caused modification of certain epitopes and not a general loss of CD8-β. Loss of antibody binding was strictly dependent on ART2.2, because it was not observed on ART2-deficient T cells or in the presence of inhibitory anti-ART2.2 single-domain antibodies. ADP-ribosylation of CD8-β occurred during cell isolation, particularly when cells were isolated from CD38-deficient mice. Incubation of ART2-expressing, but not of ART2-deficient, OVA-specific CD8(+) T cells with NAD(+) interfered with binding of OVA257-264 :MHC-I tetramers. In line with this result, treatment of WT mice with NAD(+) resulted in reduced CD8(+) T-cell mediated cytotoxicity in vivo. We propose that ADP-ribosylation of CD8-β can regulate the coreceptor function of CD8 in the presence of elevated levels of extracellular NAD(+) .

Identification of a novel pathway of transforming growth factor-β1 regulation by extracellular NAD+ in mouse macrophages: in vitro and in silico studies

Extracellular β-nicotinamide adenine dinucleotide (NAD(+)) is anti-inflammatory. We hypothesized that NAD(+) would modulate the anti-inflammatory cytokine Transforming Growth Factor (TGF)-β1. Indeed, NAD(+) led to increases in both active and latent cell-associated TGF-β1 in RAW 264.7 mouse macrophages as well as in primary peritoneal macrophages isolated from both C3H/HeJ (TLR4-mutant) and C3H/HeOuJ (wild-type controls for C3H/HeJ) mice. NAD(+) acts partially via cyclic ADP-ribose (cADPR) and subsequent release of Ca(2+). Treatment of macrophages with the cADPR analog 3-deaza-cADPR or Ca(2+) ionophores recapitulated the effects of NAD(+) on TGF-β1, whereas the cADPR antagonist 8-Br-cADPR, Ca(2+) chelation, and antagonism of L-type Ca(2+) channels suppressed these effects. The time and dose effects of NAD(+) on TGF-β1 were complex and could be modeled both statistically and mathematically. Model-predicted levels of TGF-β1 protein and mRNA were largely confirmed experimentally but also suggested the presence of other mechanisms of regulation of TGF-β1 by NAD(+). Thus, in vitro and in silico evidence points to NAD(+) as a novel modulator of TGF-β1.

Review: NAD +: a modulator of immune functions

Latterly, nicotinamide adenine dinucleotide (NAD+) has emerged as a molecule with versatile functions and of enormous impact on the maintenance of cell integrity. Besides playing key roles in almost all major aspects of energy metabolism, there is mounting evidence that NAD+ and its degradation products affect various biological activities including calcium homeostasis, gene transcription, DNA repair, and intercellular communication. This review is aimed at giving a brief insight into the life cycle of NAD+ in the cell, referring to synthesis, action and degradation aspects. With respect to their immunological relevance, the importance and function of the major NAD+ metabolizing enzymes, namely CD38/CD157, ADP-ribosyltransferases (ARTs), poly-ADP-ribose-polymerases (PARPs), and sirtuins are summarized and roles of NAD+ and its main degradation product adenosine 5'-diphosphoribose (ADPR) in cell signaling are discussed. In addition, an outline of the variety of immunological processes depending on the activity of nicotinamide phosphoribosyltransferase (Nampt), the key enzyme of the salvage pathway of NAD+ synthesis, is presented. Taken together, an efficient supply of NAD+ seems to be a crucial need for a multitude of cell functions, underlining the yet only partly revealed potency of this small molecule to influence cell fate.

The ability of selected pyridinium salts to increase the cytotoxic activity of vincristine but not doxorubicin towards sensitive and multidrug resistant promyelocytic leukaemia HL60 cells

The aim of this study was to examine the effect of selected pyridinium salts, 1-methyl-3-nitropyridine chloride (MNP+Cl−) and 3,3,6,6,10-pentamethyl-3,4,6,7-tetrahydro-[1,8(2H,5H)-dion]acridine chloride (MDION+Cl−), on the activity of doxorubicin (DOX) and vincristine (VINC) towards human promyelocytic leukaemia HL60 cells as well as its multidrug resistant (MDR) sublines exhibiting two different phenotypes of MDR related to the overexpression of P-glycoprotein (HL60/VINC) or MRP1 (HL60/DOX). MNP and MDION salts were much less cytotoxic themselves (about 100-fold and 2000-fold compared with DOX and VINC, respectively) against HL60 cells but, in contrast to DOX and VINC, they conserved an important cytotoxic activity towards resistant HL60/VINC and HL60/DOX cells (resistance factor, RF = 2–4.5). It was shown that MNP+Cl− and MDION+Cl− increased the cytotoxicity of non-bioreductive antitumour agent VINC towards human promyelocytic leukaemia HL60 cells and its resistant sublines HL60/VINC and HL60/DOX. However, in the case of DOX the decrease in its cytotoxic activity towards all studied cell lines was observed in the presence of MNP+Cl− and MDION+Cl−. Presented data suggest that the bioreductive drug DOX, in contrast to VINC, could compete with pyridinium salts (MNP+Cl− and MDION+Cl−) for NADPH-dependent oxidoreductases and for undergoing cellular reductive activation. This could explain the inefficiency of these salts to increase the cytotoxic activity of DOX against examined leukaemic HL60 cell line and its MDR sublines, HL60/VINC and HL60/DOX.

Basal and inducible expression of the thiol-sensitive ART2.1 ecto-ADP-ribosyltransferase in myeloid and lymphoid leukocytes

ADP-ribosylation of cell surface proteins in mammalian cells is a post-translational modification by which ecto-ADP-ribosyltransferases (ARTs) transfer ADP-ribose from extracellular NAD to protein targets. The ART2 locus at murine chromosome 7 encompasses the tandem Art2a and Art2b genes that encode the distinct ART2.1 and ART2.2 proteins. Although both ecto-enzymes share 80% sequence identity, ART2.1 activity is uniquely regulated by an allosteric disulfide bond that is reducible in the presence of extracellular thiols, such as cysteine and glutathione, that accumulate in hypoxic and ischemic tissues. Previous studies have characterized the expression of ART2.1 and ART2.2 in murine T lymphocytes but not in other major classes of lymphoid and myeloid leukocytes. Here, we describe the expression of ART2.1 activity in a wide range of freshly isolated or tissue-cultured murine myeloid and lymphoid leukocytes. Spleen-derived macrophages, dendritic cells (DC), and B cells constitutively express ART2.1 as their predominant ART while spleen T cells express both ART2.1 and the thiol-independent ART2.2 isoform. Although bone-marrow-derived macrophages (BMDM) and dendritic cells (BMDC) constitutively express ART2.1 at low levels, it is markedly up-regulated when these cells are stimulated in vitro with IFNbeta or IFNgamma. ART2.1 expression and activity in splenic B cells is modestly up-regulated during incubation in vitro for 24 h, a condition that promotes B cell apoptosis. This increase in ART2.1 is attenuated by IL-4 (a B cell survival factor), but is not affected by IFNbeta/gamma, suggesting a possible induction of ART2.1 as an ancillary response to B cell apoptosis. In contrast, ART2.1 and ART2.2, which are highly expressed in freshly isolated splenic T cells, are markedly down-regulated when purified T cells are incubated in vitro for 12-24 h. Studies with the BW5147 mouse thymocyte line verified basal expression of ART2.1 and ART2.2, as in primary spleen T cells, and demonstrated that both isoforms can be up-regulated when T cells are maintained in the presence of IFNs. Comparison of the surface proteins which are ADP-ribosylated by ART2.1 in the different leukocyte subtypes indicated both shared and cell-specific proteins as ART2.1 substrates. The LFA-1 integrin, a major target for ART2.2 in T cells, is also ADP-ribosylated by the ART2.1 expressed in macrophages. Thus, ART2.1, in contrast to ART2.2, is expressed in a broad range of myeloid and lymphoid leukocytes. The thiol redox-sensitive nature of this ecto-enzyme suggests an involvement in purinergic signaling that occurs in the combined context of inflammation and hypoxia/ischemia.

Cyclic ADP-ribose mediates formyl methionyl leucyl phenylalanine (fMLP)-induced intracellular Ca(2+) rise and migration of human neutrophils

Although cyclic ADP-ribose (cADPR), a novel Ca(2+)-mobilizing mediator, is suggested to be involved in the functions of neutrophils in rodents, its role in human neutrophils remains unclear. The present study examined the ability of cADPR to mobilize Ca(2+) and mediate formyl methionyl leucyl phenylalanine (fMLP)-stimulated increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and migration in human neutrophils. cADPR induced Ca(2+) release from digitonin-permeabilized neutrophils, and the release was blocked by 8Br-cADPR, an antagonist of cADPR. Immunophilin ligands, FK506 and rapamycin, but not cyclosporine A, inhibited cADPR-induced Ca(2+) release. 8Br-cADPR partially reduced fMLP-induced [Ca(2+)](i) rise and abolished the rise in combination with 2APB, an IP(3)-receptor antagonist. Anti-CD38Ab and NADase that interfere with cADPR formation, reduced the fMLP-induced [Ca(2+)](i) rise. When beta-NAD(+), a substrate of ADP-ribosyl cyclase, and cADPR were added to the medium, the former gradually increased [Ca(2+)](i) and the latter potentiated the fMLP-induced [Ca(2+)](i) rise. The beta-NAD(+)-induced [Ca(2+)](i) rise in Ca(2+)-free medium was inhibited by anti-CD38Ab, 8Br-cADPR, FK506, ruthenium red, and thapsigargin. mRNAs of nucleoside transporter (NT), ENT1, ENT2, CNT, and CNT3 were expressed in neutrophils; and their inhibitors, inosine, uridine, and s-(4-nitrobenzyl)-6-thioinosine, reduced the [Ca(2+)](i) rise induced by beta-NAD(+) and fMLP. fMLP-timulated migration was inhibited by the removal of Ca(2+) from the medium or by the addition of 8Br-cADPR, anti-CD38Ab, NADase, and NT inhibitors. These results suggest that cADPR was synthesized extracellularly by CD38, transported into the cells through NTs, and then Ca(2+) was mobilized by FK506-binding protein-dependent process. This process may be involved in fMLP-induced intracellular Ca(2+) signaling and migration in human neutrophils.

NAD+ released during inflammation participates in T cell homeostasis by inducing ART2-mediated death of naive T cells in vivo

Mono ADP-ribosyltransferase 2 (ART2) is an ectoenzyme expressed on mouse T lymphocytes, which catalyze the transfer of ADP-ribose groups from NAD(+) onto several target proteins. In vitro, ADP-ribosylation by ART2 activates the P2X7 ATP receptor and is responsible for NAD(+)-induced T cell death (NICD). Yet, the origin of extracellular NAD(+) and the role of NICD in vivo remain elusive. In a model of acute inflammation induced by polyacrylamide beads, we demonstrate release of NAD(+) into exudates during the early phase of the inflammatory response. This leads to T cell depletion in the draining lymph nodes from wild-type and, more severely, from mice lacking the CD38 NAD(+) glycohydrolase, whereas no effect is observed in ART2-deficient animals. Intravenous injection of NAD(+) used to exacerbate NICD in vivo results in fast and dramatic ART2- and P2X7-dependent depletion of CD4+ and CD8+ T lymphocytes, which can affect up to 80% of peripheral T cells in CD38(-/-) mice. This affects mainly naive T cells as most cells surviving in vivo NAD+ treatment exhibit the phenotype of recently activated/memory cells. Consistently, treatment with NAD(+) abolishes primary Ab response to a T-dependent Ag in NICD-susceptible CD38(-/-) mice but has no effect on the secondary response when given several days after priming. Unexpectedly NAD+ treatment improves the response in their wild-type BALB/c counterparts. We propose that NAD(+) released during early inflammation facilitates the expansion of primed T cells, through ART2-driven death of resting cells, thus contributing to the dynamic regulation of T cell homeostasis.

Substrate specificity of soluble and membrane-associated ADP-ribosyltransferase ART2.1

ADP-ribosyltransferases (ARTs) are a family of enzymes that catalyze the covalent transfer of an ADP-ribose moiety, derived from NAD, to an amino acid of an acceptor protein, thereby altering its function. To date, little information is available on the protein target specificity of different ART family members. ART2 is a T-cell-specific transferase, attached to the cell surface by a glycosylphosphatidylinositol (GPI) anchor, and also found in serum. Here we investigated the role of ART2 localization in serum or on the cell surface, or solubilized with detergents or enzymes, on its target protein specificity. We found that detergent solubilization of cell membranes, or release of ART2 by phosphoinositide-specific phospholipase C treatment, altered the ability of ART2 to ADP-ribosylate high or low molecular weight histone proteins. Similarly, soluble recombinant ART2 (lacking the GPI anchor) showed a different histone specificity than did cell-bound ART2. When soluble ART2 was incubated with serum proteins in the presence of [32P]-labeled NAD, several serum proteins were ADP-ribosylated in a thiol-specific manner. Mass spectrometry of labeled proteins identified albumin and transferrin as ADP-ribosylated proteins in serum. Collectively, these studies reveal that the membrane or solution environment of ART2 plays a pivotal role in determining its substrate specificity.

P2X7 receptors regulate NKT cells in autoimmune hepatitis

Adenine nucleotides induce danger signals in T cells via purinergic receptors, raising the question whether they exert similar effects on innate immunity. Here we show that micromolar concentrations of nicotinamide adenine dinucleotide (NAD) induce a rapid increase of annexin V staining in NKT cells in vitro, a response that requires expression of P2X(7)Rs. Consistent with this result, treatment of mice with NAD causes a temporary decrease of NKT cells in the liver and protects from Con A- and alpha-galactosylceramide-induced hepatitis, both of which require functional NKT cells. Resistance to liver injury is associated with decreased cytokine production by NKT cells in NAD-treated mice. In contrast, when NAD is injected into Con A- or alpha-galactosylceramide-primed mice, liver injury is exacerbated and cytokine production by NKT cells is increased. This effect is caused by P2X(7)R-mediated stimulation of activated NKT cells. In agreement, mice lacking P2X(7)Rs on lymphocytes suffer reduced liver injury, and animals lacking ADP-ribosyltransferase, the enzyme that uses NAD to attach ADP-ribosyl groups to cell surfaces, are also resistant to Con A-induced hepatitis. These results prompt the conclusion that engagement of P2X(7)Rs on NKT cells inhibits naive, while stimulating activated cells, resulting in suppression or stimulation of autoimmune hepatitis.

Ecto-ADP-ribose transferases: cell-surface response to local tissue injury

Ecto-ADP-ribose transferases (ecto-ARTs) catalyze the transfer of ADP-ribose from NAD(+) to arginine residues in cell-surface proteins. Since the concentration of extracellular NAD(+) is very low under normal physiological conditions but rises significantly upon tissue injury or membrane stress, it is postulated that the main role of ecto-ARTs is to ADP-ribosylate and regulate the function of certain membrane receptors in response to elevated levels of NAD(+).

ADP-ribosylation of membrane proteins: unveiling the secrets of a crucial regulatory mechanism in mammalian cells

Many bacterial toxins kill animal cells by adenosine diphosphate (ADP)-ribosylating intracellular target proteins. Mammalian cells express toxin-related cell surface ADP-ribosyltransferases (ARTs) that transfer ADP-ribose from nicotinamide adenine dinucleotide (NAD) onto arginine residues of other membrane proteins. The association of these glycosylphosphatidylinositol (GPI)-anchored ectoenzymes with glycolipid rafts focuses them onto components of the signal transduction machinery. Exposing murine T cells to NAD, the ART substrate, induces a cascade of reactions that culminates in cell death by apoptosis. This mechanism, dubbed 'NAD-induced cell death' or NICD, is initiated when ART2 ADP-ribosylates the cytolytic P2X7 purinergic receptor, inducing formation of a cation channel, opening of a nonselective pore, shedding of CD62L from the cell surface, exposure of phosphatidylserine on the outer leaflet of the plasma membrane, breakdown of the mitochondrial membrane potential, and DNA-fragmentation. The ART substrate NAD is produced in large amounts inside the cell and can be released from damaged cells during inflammation and tissue injury. In the extracellular environment, the signaling function of NAD is terminated by NAD-degrading ectoenzymes such as CD38. We propose that ART2-catalyzed ADP-ribosylation of P2X7 represents the paradigm of a regulatory mechanism by which ART-expressing cells can sense and respond to the release of NAD from damaged cells.

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.

High sensitivity of CD4+CD25+ regulatory T cells to extracellular metabolites nicotinamide adenine dinucleotide and ATP: a role for P2X7 receptors

Although regulatory lymphocytes play an important role in the immune system, the regulation of their functions is poorly understood and remains to be elucidated. In this study we demonstrate that micromolar concentrations of the common cell metabolite NAD induce death in murine forkhead/winged helix transcription factor gene-expressing CD4+CD25+ regulatory T cells with high efficiency and within minutes. Similar, but less dramatic, effects are demonstrable with ATP and its nonhydrolysable derivative, benzoylbenzoyl-ATP. Other T cell subsets are more resistant, with CD8 cells being the least sensitive and CD4 cells expressing intermediate sensitivity. The higher sensitivity of CD4+CD25+ cells is demonstrable in vivo. Injection of NAD or benzoylbenzoyl-ATP causes preferential induction of a cell death signal in CD4+CD25+ cells. Transmission of the death signal requires functional P2X7 receptors, pointing to a role for these receptors in regulation and homeostasis of CD4+CD25+ regulatory T cells. Consistent with this, P2X7R gene-deleted mice possess increased levels of forkhead/winged helix transcription factor gene-expressing CD4+CD25+ cells.

CD38 controls ADP-ribosyltransferase-2-catalyzed ADP-ribosylation of T cell surface proteins

ADP-ribosyltransferase-2 (ART2), a GPI-anchored, toxin-related ADP-ribosylating ectoenzyme, is prominently expressed by murine T cells but not by B cells. Upon exposure of T cells to NAD, the substrate for ADP-ribosylation, ART2 catalyzes ADP-ribosylation of the P2X7 purinoceptor and other functionally important cell surface proteins. This in turn activates P2X7 and induces exposure of phosphatidylserine and shedding of CD62L. CD38, a potent ecto-NAD-glycohydrolase, is strongly expressed by most B cells but only weakly by T cells. Following incubation with NAD, CD38-deficient splenocytes exhibited lower NAD-glycohydrolase activity and stronger ADP-ribosylation of cell surface proteins than their wild-type counterparts. Depletion of CD38(high) cells from wild-type splenocytes resulted in stronger ADP-ribosylation on the remaining cells. Similarly, treatment of total splenocytes with the CD38 inhibitor nicotinamide 2'-deoxy-2'-fluoroarabinoside adenine dinucleotide increased the level of cell surface ADP-ribosylation. Furthermore, the majority of T cells isolated from CD38-deficient mice "spontaneously" exposed phosphatidylserine and lacked CD62L, most likely reflecting previous encounter with ecto-NAD. Our findings support the notion that ecto-NAD functions as a signaling molecule following its release from cells by lytic or nonlytic mechanisms. ART2 can sense and translate the local concentration of ecto-NAD into corresponding levels of ADP-ribosylated cell surface proteins, whereas CD38 controls the level of cell surface protein ADP-ribosylation by limiting the substrate availability for ART2.

P2X7 receptor-dependent and -independent T cell death is induced by nicotinamide adenine dinucleotide

Adding NAD to murine T lymphocytes inhibits their functions and induces annexin V binding. This report shows that NAD induces cell death in a subset of T cells within seconds whereas others do not die until many hours later. Low NAD concentrations (<10 microM) suffice to trigger rapid cell death, which is associated with annexin V binding and membrane pore formation, is not blocked by the caspase inhibitor Z-VADfmk, and requires functional P2X7 receptors. The slower induction of death requires higher NAD concentrations (>100 microM), is blocked by caspase inhibitor Z-VADfmk, is associated with DNA fragmentation, and does not require P2X7 receptors. T cells degrade NAD to ADP-ribose (ADPR), and adding ADPR to T cells leads to slow but not rapid cell death. NAD but not ADPR provides the substrate for ADP-ribosyltransferase (ART-2)-mediated attachment of ADP-ribosyl groups to cell surface proteins; expression of ART-2 is required for NAD to trigger rapid but not slow cell death. These results support the hypothesis that cell surface ART-2 uses NAD but not ADPR to attach ADP-ribosyl groups to the cell surface, and that these groups act as ligands for P2X7 receptors that then induce rapid cell death. Adding either NAD or ADPR also triggers a different set of mechanisms, not requiring ART-2 or P2X7 receptors that more slowly induce cell death.

Extracellular NAD+ regulates intracellular free calcium concentration in human monocytes

Ca(2+) ions play a critical role in the biochemical cascade of signal transduction pathways, leading to the activation of immune cells. In the present study, we show that the exposure of freshly isolated human monocytes to NAD(+) results in a rapid concentration-dependent elevation of [Ca(2+)](i) (intracellular free Ca(2+) concentration) caused by the influx of extracellular Ca(2+). NAD(+) derivatives containing a modified adenine or nicotinamide ring failed to trigger a Ca(2+) increase. Treating monocytes with ADPR (ADP-ribose), a major degradation product of NAD(+), also resulted in a rise in [Ca(2+)](i). Selective inhibition of CD38, an NAD-glycohydrolase that generates free ADPR from NAD(+), does not abolish the effect of NAD(+), excluding the possibility that NAD(+) might act via ADPR. The NAD(+)-induced Ca(2+) response was prevented by the prior addition of ADPR and vice versa, indicating that both compounds share some mechanisms mediating the rise in [Ca(2+)](i). NAD(+), as well as ADPR, were ineffective when applied following ATP, suggesting that ATP controls events that intersect with NAD(+) and ADPR signalling.

Comparing antigen-independent mechanisms of T cell regulation

Key features of the kinetics of T lymphocyte proliferative responses are remarkably insensitive to the nature of the antigenic stimulus. This consistency suggests the presence of an antigen-independent mechanism regulating T cell clonal expansion. Knowledge of such a mechanism could allow us to modulate T helper cell (CD4+) and cytotoxic T cell (CD8+) responses more effectively. Using a simple mathematical model of T cell proliferation and death, we investigate a variety of plausible mechanisms and compare the model predictions to experimental data from the literature. We find that irrespective of the details of the mechanism, rates of apoptosis must progressively increase to control a T cell response. If apoptosis is mediated by cell-cell contact this alone is sufficient to regulate both (CD4+) and (CD8+) T cell responses. Proliferation of both T cell subsets can also be regulated by an internal programme, by cytokine signalling, or by an APC-mediated route. To regulate (CD8+) T cells these mechanisms must change both apoptosis and division rates, and this change must occur with time not division number.

NAD-Induced T Cell Death

T cells express a toxin-related ADP-ribosylating ectoenzyme, ART2. Exposure of mature T cells to NAD, the substrate for ADP-ribosylation, induces cell death. ART2-catalyzed ADP-ribosylation activates the cytolytic P2X7 purinoceptor, causing calcium flux, pore formation, phosphatidylserine exposure, shedding of CD62L, cell shrinkage, and propidium iodide uptake. Interestingly, much lower NAD than ATP concentrations are required to activate P2X7. NAD-induced cell death (NICD) operates with endogenous sources of NAD released upon cell lysis. These findings identify P2X7 as a key effector of NICD and demonstrate that P2X7 can be activated by an endogenous ligand other than ATP. Our results delineate an alternative mechanism for inducing T cell death and set an interesting precedent for immunoregulation via crosstalk between NAD-dependent ADP-ribosyltransferases and purinoceptors.

Tryptophan and the immune response

The immune system continuously modulates the balance between responsiveness to pathogens and tolerance to non-harmful antigens. The mechanisms that mediate tolerance are not well understood, but recent findings have implicated tryptophan catabolism through the kynurenine metabolic pathway as one of many mechanisms involved. The enzymes that break down tryptophan through this pathway are found in numerous cell types, including cells of the immune system. Some of these enzymes are induced by immune activation, including the rate limiting enzyme present in macrophages and dendritic cells, indoleamine 2,3-dioxygenase (IDO). It has recently been found that inhibition of IDO can result in the rejection of allogenic fetuses, suggesting that tryptophan breakdown is necessary for maintaining aspects of immune tolerance. Two theories have been proposed to explain how tryptophan catabolism facilitates tolerance. One theory posits that tryptophan breakdown suppresses T cell proliferation by dramatically reducing the supply of this critical amino acid. The other theory postulates that the downstream metabolites of tryptophan catabolism act to suppress certain immune cells, probably by pro-apoptotic mechanisms. Reconciling these disparate views is crucial to understanding immune-related tryptophan catabolism and the roles it plays in immune tolerance. In this review we examine the issue in detail, and offer additional insight provided by studies with antibodies to quinolinate, a tryptophan catabolite which is also necessary for nicotinamide adenine dinucleotide (NAD +) production. In addition to the immunomodulatory actions of tryptophan catabolites, we discuss the possible involvement of quinolinate as a means of replenishing NAD + in leucocytes, which is depleted by oxidative stress during an immune response.

Flow cytometric and immunoblot assays for cell surface ADP-ribosylation using a monoclonal antibody specific for ethenoadenosine

NAD-dependent ADP-ribosylation is one of the posttranslational protein modifications. On mammalian cells, glycosylphosphatidylinositol-anchored cell surface ADP-ribosyltransferases (ARTs) ADP-ribosylate other cell surface proteins and thereby affect important cellular functions. Here we describe convenient flow-cytometric and immunoblot assays for monitoring ADP-ribosylation of cell surface proteins on living cells by exploiting the capacity of ARTs to utilize etheno-NAD as substrate. Etheno-ADP-ribosylation of cell surface proteins can be detected by flow cytometry with 1G4, a monoclonal antibody specific for ethenoadenosine. Labeling of cells with 1G4 is dependent on the expression of cell surface ARTs and occurs only after incubation of ART-expressing cells with etheno-NAD and not with etheno-ADP-ribose. Dose-response analyses show efficient 1G4 staining of ART-expressing cells at micromolar etheno-NAD concentrations. Half-maximal staining is obtained with 1-2 micro M etheno-NAD, saturation is reached at 5-20 micro M etheno-NAD. Immunoblot analyses confirm that ART-expressing cells incorporate ethenoadenosine covalently (i.e., SDS resistant) into several cell surface proteins. The flow-cytometric 1G4 staining assay can be used to identify subpopulations of cells expressing cell surface ART activity and to select ART(hi) cell variants. The immunoblot 1G4 staining assay can also be used to identify etheno-ADP-ribosylated target proteins. These new assays hold promise for many interesting applications in biochemistry and cell biology.

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

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

Structure of the ecto-ADP-ribosyl transferase ART2.2 from rat

The mammalian extracellular ADP-ribosyl transferases ART1 through ART5 are sequence-related to each other. Among them ART2 is involved in immuno regulation. The variant ART2.2 was expressed in the periplasm of Escherichia coli and crystallized. Its structure was determined by X-ray diffraction at 1.7A resolution in one crystal form and at slightly lower resolutions in two others. The active center was indicated by a ligated nicotinamide analogue, which also revealed a small induced-fit. The centerpiece of the chainfold of ART2.2 agrees with those of all bacterial ADP-ribosyl transferases. This correspondence and the nicotinamide position were used to model the binding structure of the whole substrate NAD(+) at ART2.2. Two of the bacterial enzymes are structurally more closely related to ART2.2 while the others are more closely related to the eukaryotic poly(ADP-ribosyl)polymerase. This splits the ADP-ribosyl transferases into two distinct subfamilies. A special feature of ART2.2 is its long N-terminal extension and two disulfide bridges that are far away from the active center. They stabilize the protein against denaturation and presumably also against shearing forces parallel with the membrane where ART2.2 is anchored.

Generation and characterization of ecto-ADP-ribosyltransferase ART2.1/ART2.2-deficient mice

This is the first study reporting the inactivation of a member of the mouse gene family of toxin-related ecto-ADP-ribosyltransferases (ARTs). Transfer of the ADP-ribose moiety from NAD onto extracellular arginine residues on T-cell membrane proteins is mediated by glycosylphosphatidylinositol-linked cell surface ARTs. Exposure of T cells to ecto-NAD blocks T-cell activation and induces T-cell apoptosis. To determine a possible role of ecto-ART2.1 and ART2.2 in these processes, we generated ART2.1/ART2.2 double-knockout mice. ART2-deficient mice were healthy and fertile and showed normal development of lymphoid organs. ART2-deficient T cells showed a dramatically reduced capacity to ADP-ribosylate cell surface proteins, indicating that most if not all ART activity on the T-cell surface can be attributed to the ART2s. Moreover, ART2-deficient T cells were completely resistant to NAD-induced apoptosis and partially resistant to NAD-mediated suppression of proliferation. These results demonstrate that the ART2 ectoenzymes are an essential component in the regulation of T-cell functions by extracellular NAD, e.g., following release of NAD upon lysis of cells in tissue injury and inflammation.

NAD-dependent inhibition of the NAD-glycohydrolase activity in A549 cells

NAD glycohydrolases are enzymes that catalyze the hydrolysis of NAD to produce ADP-ribose and nicotinamide. Regulation of these enzymes has not been fully elucidated. We have identified a NAD-glycohydrolase activity associated with the outer surface of the plasma membrane in human lung epithelial cell line A549. This activity is negatively regulated by its substrate beta-NAD but not by alpha-NAD. Partial restoration of NADase activity after incubation of the cells with arginine or histidine, known ADP-ribose acceptors, suggests that inhibition be regulated by ADP-ribosylation. A549 do not undergo to apoptosis upon NAD treatment indicating that this effect be likely mediated by a cellular component(s) lacking in epithelial cells.