Involvement of P2X receptors in the NAD(+)-induced rise in [Ca (2+)] (i) in human monocytes

Redox signaling and oxidative stress in systemic acquired resistance

Plants fully depend on their immune systems to defend against pathogens. Upon pathogen attack, plants not only activate immune responses at the infection site but also trigger a defense mechanism known as systemic acquired resistance (SAR) in distal systemic tissues to prevent subsequent infections by a broad-spectrum of pathogens. SAR is induced by mobile signals produced at the infection site. Accumulating evidence suggests that reactive oxygen species (ROS) play a central role in SAR signaling. ROS burst at the infection site is one of the earliest cellular responses following pathogen infection and can spread to systemic tissues through membrane-associated NADPH oxidase-dependent relay production of ROS. It is well known that ROS ignite redox signaling and, when in excess, cause oxidative stress, damaging cellular components. In this review, we summarize current knowledge on redox regulation of several SAR signaling components. We discuss the ROS amplification loop in systemic tissues involving multiple SAR mobile signals. Moreover, we highlight the essential role of oxidative stress in generating SAR signals including azelaic acid and extracellular NAD(P) [eNAD(P)]. Finally, we propose that eNAD(P) is a damage-associated molecular pattern serving as a converging point of SAR mobile signals in systemic tissues.

Methods for studying P2X4 receptor ion channels in immune cells

The P2X4 receptor is a trimeric ligand-gated ion channel activated by adenosine 5'-triphosphate (ATP). P2X4 is present in immune cells with emerging roles in inflammation and immunity, and related disorders. This review aims to provide an overview of the methods commonly used to study P2X4 in immune cells, focusing on those methods used to assess P2RX4 gene expression, the presence of the P2X4 protein, and P2X4 ion channel activity in these cells from humans, dogs, mice and rats. P2RX4 gene expression in immune cells is commonly assessed using semi-quantitative and quantitative reverse-transcriptase-PCR. The presence of P2X4 protein in immune cells is mainly assessed using anti-P2X4 polyclonal antibodies with immunoblotting or immunochemistry, but the use of these antibodies, as well as monoclonal antibodies and nanobodies to detect P2X4 with flow cytometry is increasing. Notably, use of an anti-P2X4 monoclonal antibody and flow cytometry has revealed that P2X4 is present on immune cells with a rank order of expression in eosinophils, then neutrophils and monocytes, then basophils and B cells, and finally T cells. P2X4 ion channel activity has been assessed mainly by Ca2+ flux assays using the cell permeable Ca2+-sensitive dyes Fura-2 and Fluo-4 with fluorescence microscopy, spectrophotometry, or flow cytometry. However, other methods including electrophysiology, and fluorescence assays measuring Na+ flux (using sodium green tetra-acetate) and dye uptake (using YO-PRO-12+) have been applied. Collectively, these methods have demonstrated the presence of functional P2X4 in monocytes and macrophages, microglia, eosinophils, mast cells and CD4+ T cells, with other evidence suggestive of functional P2X4 in dendritic cells, neutrophils, B cells and CD8+ T cells.

Sirtuin 6 Regulates the Activation of the ATP/Purinergic Axis in Endothelial Cells

Sirtuin 6 (SIRT6) is a member of the mammalian NAD+-dependent deac(et)ylase sirtuin family. SIRT6’s anti-inflammatory roles are emerging increasingly often in different diseases and cell types, including endothelial cells. In this study, the role of SIRT6 in pro-inflammatory conditions was investigated by engineering human umbilical vein endothelial cells to overexpress SIRT6 (SIRT6+ HUVECs). Our results showed that SIRT6 overexpression affected the levels of adhesion molecules and sustained megakaryocyte proliferation and proplatelet formation. Interestingly, the pro-inflammatory activation of the ATP/purinergic axis was reduced in SIRT6+ HUVECs. Specifically, the TNFα-induced release of ATP in the extracellular space and the increase in pannexin-1 hemichannel expression, which mediates ATP efflux, were hampered in SIRT6+ cells. Instead, NAD+ release and Connexin43 expression were not modified by SIRT6 levels. Moreover, the Ca2+ influx in response to ATP and the expression of the purinergic receptor P2X7 were decreased in SIRT6+ HUVECs. Contrary to extracellular ATP, extracellular NAD+ did not evoke pro-inflammatory responses in HUVECs. Instead, NAD+ administration reduced endothelial cell proliferation and motility and counteracted the TNFα-induced angiogenesis. Altogether, our data reinforce the view of SIRT6 activation as an anti-inflammatory approach in vascular endothelium.

Physiologic roles of P2 receptors in leukocytes

Since their discovery in the 1970s, purinergic receptors have been shown to play key roles in a wide variety of biologic systems and cell types. In the immune system, purinergic receptors participate in innate immunity and in the modulation of the adaptive immune response. In particular, P2 receptors, which respond to extracellular nucleotides, are widely expressed on leukocytes, causing the release of cytokines and chemokines and the formation of inflammatory mediators, and inducing phagocytosis, degranulation, and cell death. The activity of these receptors is regulated by ectonucleotidases-expressed in these same cell types-which regulate the availability of nucleotides in the extracellular environment. In this article, we review the characteristics of the main purinergic receptor subtypes present in the immune system, focusing on the P2 family. In addition, we describe the physiologic roles of the P2 receptors already identified in leukocytes and how they can positively or negatively modulate the development of infectious diseases, inflammation, and pain.

The Extracellular NADome Modulates Immune Responses

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

Sex-related differences in human plasma NAD+/NADH levels depend on age

Nicotinamide adenine dinucleotide (NAD) is a coenzyme in metabolic reactions and cosubstrate in signaling pathways of cells. While the intracellular function of NAD is well described, much less is known about its importance as an extracellular molecule. Moreover, there is only little information about the concentration of extracellular NAD and the ratio between its oxidized (NAD+) and reduced (NADH) form in humans. Therefore, our study aimed at the analysis of total NAD and NAD+/NADH ratio in human plasma depending on sex and age. First, an enzymatic assay was established for detecting NAD+ and NADH in human plasma samples. Then, plasma NAD was analyzed in 205 probands without severe diseases (91 men, 114 women) being 18-83 years old. The total plasma NAD concentration was determined with median 1.34 µM (0.44-2.88 µM) without difference between men and women. Although the amounts of NAD+ and NADH were nearly balanced, women had higher plasma NAD+/NADH ratios than men (median 1.33 vs. 1.09, P<0.001). The sex-related difference in the plasma NAD+/NADH ratio reduces with increasing age, an effect that was more obvious for two parameters of the biological age (skin autofluorescence, brachial-femoral pulse wave velocity (PWV)) than for the chronological age. However, plasma values for total NAD and NAD+/NADH ratio did not generally alter with increasing age. In conclusion, human plasma contains low micromolar concentrations of total NAD with higher NAD+/NADH redox ratios in adult but not older women compared with same-aged men.

Perception of Damaged Self in Plants

Plants use specific receptor proteins on the cell surface to detect host-derived danger signals released in response to attacks by pathogens or herbivores and activate immune responses against them.

Exogenous Nicotinamide Adenine Dinucleotide Induces Resistance to Citrus Canker in Citrus

Nicotinamide adenine dinucleotide (NAD) is a universal electron carrier that participates in important intracellular metabolic reactions and signaling events. Interestingly, emerging evidence in animals indicates that cellular NAD can be actively or passively released into the extracellular space, where it is processed or perceived by ectoenzymes or cell-surface receptors. We have recently shown in Arabidopsis thaliana that exogenous NAD induces defense responses, that pathogen infection leads to release of NAD into the extracellular space at concentrations sufficient for defense activation, and that depletion of extracellular NAD (eNAD) by transgenic expression of the human NAD-hydrolyzing ectoenzyme CD38 inhibits plant immunity. We therefore hypothesize that, during plant-microbe interactions, NAD is released from dead or dying cells into the extracellular space where it interacts with adjacent naïve cells' surface receptors, which in turn activate downstream immune signaling. However, it is currently unknown whether eNAD signaling is unique to Arabidopsis or the Brassicaceae family. In this study, we treated citrus plants with exogenous NAD+ and tested NAD+-induced transcriptional changes and disease resistance. Our results show that NAD+ induces profound transcriptome changes and strong resistance to citrus canker, a serious citrus disease caused by the bacterial pathogen Xanthomonas citri subsp. citri (Xcc). Furthermore, NAD+-induced resistance persists in new flushes emerging after removal of the tissues previously treated with NAD+. Finally, NAD+ treatment primes citrus tissues, resulting in a faster and stronger induction of multiple salicylic acid pathway genes upon subsequent Xcc infection. Taken together, these results indicate that exogenous NAD+ is able to induce immune responses in citrus and suggest that eNAD may also be an elicitor in this woody plant species.

Coupling of P2Y receptors to Ca2+ mobilization in mesenchymal stromal cells from the human adipose tissue

The purinergic transduction was examined in mesenchymal stromal cells (MSCs) from the human adipose tissue, and several nucleotides, including ATP, UTP, and ADP, were found to mobilize cytosolic Ca²⁺. Transcripts for multiple purinoreceptors were detected in MSC preparations, including A₁, A_2A, A_2B, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₃, P2Y₁₄, P2X₂, P2X₄, and P2X₇. Cellular responses to nucleotides were insignificantly sensitive to bath Ca²⁺, pointing at a minor contribution of Ca²⁺ entry, and were suppressed by U73122 and 2-APB, implicating the phosphoinositide cascade in coupling P2Y receptors to Ca²⁺ release. While individual cells were sensitive to several P2Y agonists, responsiveness to a given nucleotide varied from cell to cell, suggesting that particular MSCs could employ different sets of purinoreceptors. Caged Ca²⁺ stimulated Ca²⁺-induced Ca²⁺ release (CICR) that was mediated largely by IP₃ receptors, and resultant Ca²⁺ transients were similar to nucleotide responses by magnitude and kinetics. A variety of findings hinted at CICR to be a universal mechanism that finalizes Ca²⁺ signaling initiated by agonists in MSCs. Individual MSCs responded to nucleotides in an all-or-nothing manner. Presumably just CICR provided invariant Ca²⁺ responses observed in MSCs at different nucleotide concentrations. The effects of isoform specific agonists and antagonists suggested that both P2Y₁ and P2Y₁₃ were obligatory for ADP responses, while P2Y₄ and P2Y₁₁ served as primary UTP and ATP receptors, respectively. Extracellular NAD⁺ stimulated Ca²⁺ signaling in each ATP-responsive MSC by involving P2Y₁₁. The overall data indicate that extracellular nucleotides and NAD⁺ can serve as autocrine/paracrine factors regulating MSC functions.

Extracellular pyridine nucleotides as immune elicitors in arabidopsis

The pyridine nucleotides nicotinamide adenine dinucleotide (NAD) and NAD phosphate (NADP) are coenzymes that function in both metabolic reactions and intracellular signaling. Emerging evidence from animal research indicates that NAD(P) also acts in the extracellular space (ECS). We have shown in the model plant Arabidopsis that (1) exogenous NAD(P) induces immune responses, (2) pathogen infection causes leakage of intracellular NAD(P) into the extracellular fluid at concentrations sufficient to induce immune responses, and (3) removal of extracellular NAD(P) [eNAD(P)] by expressing the human NAD(P)-metabolizing ectoenzyme CD38 partially compromises systemic acquired resistance. Based on these results, we hypothesize that eNAD(P) is a novel damage-associated molecular pattern (DAMP) in plants; during plant-microbe interaction, intracellular NAD(P) is released from dead or dying cells into the ECS where it interacts with the adjacent healthy cells' surface receptors/targets, which in turn activate downstream specific immune signaling pathways. Our recent identification of LecRK-I.8, a lectin receptor kinase, as the first cell surface NAD⁺-binding receptor has provided compelling evidence for this hypothesis. Further identification of cell surface eNAD(P) receptors/targets and their downstream signaling components in Arabidopsis as well as determination of the generality of eNAD(P) signaling in crops will help establish eNAD(P) as a conserved DAMP in plants.

A lectin receptor kinase as a potential sensor for extracellular nicotinamide adenine dinucleotide in Arabidopsis thaliana

Nicotinamide adenine dinucleotide (NAD+) participates in intracellular and extracellular signaling events unrelated to metabolism. In animals, purinergic receptors are required for extracellular NAD+ (eNAD+) to evoke biological responses, indicating that eNAD+ may be sensed by cell-surface receptors. However, the identity of eNAD+-binding receptors still remains elusive. Here, we identify a lectin receptor kinase (LecRK), LecRK-I.8, as a potential eNAD+ receptor in Arabidopsis. The extracellular lectin domain of LecRK-I.8 binds NAD+ with a dissociation constant of 436.5 ± 104.8 nM, although much higher concentrations are needed to trigger in vivo responses. Mutations in LecRK-I.8 inhibit NAD+-induced immune responses, whereas overexpression of LecRK-I.8 enhances the Arabidopsis response to NAD+. Furthermore, LecRK-I.8 is required for basal resistance against bacterial pathogens, substantiating a role for eNAD+ in plant immunity. Our results demonstrate that lectin receptors can potentially function as eNAD+-binding receptors and provide direct evidence for eNAD+ being an endogenous signaling molecule in plants.

Effects of exogenous nicotinamide adenine dinucleotide (NAD+) in the rat heart are mediated by P2 purine receptors

Background

Recently, NAD+ has been considered as an essential factor, participating in nerve control of physiological functions and intercellular communication. NAD+ also has been supposed as endogenous activator of P1 and P2 purinoreceptors. Effects of extracellular NAD+ remain poorly investigated in cardiac tissue. This study aims to investigate the effects of extracellular NAD+ in different types of supraventricular and ventricular working myocardium from rat and their potential mechanisms.

Methods

The standard technique of sharp microelectrode action potential recording in cardiac multicellular preparations was used to study the effects of NAD+.

Results

Extracellular NAD+ induced significant changes in bioelectrical activity of left auricle (LA), right auricle (RA), pulmonary veins (PV) and right ventricular wall (RV) myocardial preparations. 10–100 μM NAD+ produced two opposite effects in LA and RA – quickly developing and transient prolongation of action potentials (AP) and delayed sustained AP shortening, which follows the initial positive effect. In PV and RV only AP shortening was observed in response to NAD+ application. In PV preparations AP shortening induced by NAD+ may be considered as a potential proarrhythmic effect. Revealed cardiotropic effects of NAD+ are likely to be mediated by P2 purine receptors, since P1 blocker DPCPX failed to affect them and P2 antagonist suramin abolished NAD + −induced alterations of electrical activity. P2X receptors may be responsible for NAD + −induced short-lasting AP prolongation, while P2Y receptors mediate persistent AP shortening. The latter effect is partially removed by PLC inhibitor U73122 showing the potential involvement of phosphoinositide signaling pathway in mediation of NAD+ cardiotropic effects.

Conclusions

Extracellular NAD+ is supposed to be a novel regulator of cardiac electrical activity. P2 receptors represent the main target of NAD+ at least in the rat heart.

ATP releases ATP or other nucleotides from human peripheral blood leukocytes through purinergic P2 receptors

AIMS

Almost every eukaryotic cell releases ATP under certain conditions. The idea that ATP induces the release of ATP has been scantly investigated.

METHODS

We explored this possibility by assessing the rate of exogenous ATP breakdown (measured by phosphates production) by human peripheral blood leukocytes. The role of P2Y and P2X receptors was evaluated pharmacologically, by patch clamp, or by flow cytometry.

KEY FINDINGS

In mononuclear and/or polymorphonuclear cells, ATP increased phosphates formation in a time- and concentration-dependent manner. Uncoupling of P2Y receptors with N-ethylmaleimide and antagonism of P2Y and P2X receptors through suramin reduced phosphate formation after 500μM ATP, suggesting that part of the phosphate production was due to activation of P2 receptors, with subsequent release of ATP or other nucleotides. Similar results were obtained with UTP and ATPγS. Gadolinium (connexins inhibitor) also significantly reduced the ATP-induced phosphate production. Blockade of P2X receptors with SKF 96365 or NF023 did not modify the phosphate production. In monocytes, 500μM ATP induced inward currents suggestive of P2X1 activation, but higher concentrations (1-5mM) induced inward currents suggestive of P2X7 activation. We discarded a role of adenosine in the ATP-evoked nucleotides release. Flow cytometry identified that almost all mononuclear and polymorphonuclear cells expressed P2Y1,2,4,6,11 receptors.

SIGNIFICANCE

500μM ATP induced the release of ATP or other nucleotides through activation of P2Y2,4,6,11 receptors in human leukocytes, and probably via P2X receptors at higher concentrations. This ATP-induced nucleotides release constitutes a potential mechanism leading to amplification of ATP signaling.

CD38 mediates the intracellular ATP levels and cell survival of C6 glioma cells

CD38 is a multifunctional enzyme that can act as a NADase, generate cyclic adenosine diphosphate-ribose (cADPR) - a key Ca²⁺-mobilizing second messenger - and transport cADPR into cells. There have only been a small number of studies on the functions of CD38 in the central nervous system. In this study, we applied CD38 small interfering RNA (siRNA) to determine the effects of decreased CD38 on the intracellular ATP levels and survival of C6 glioma cells. Our study showed that both CD38 siRNA and 8-bromo-cADPR - a ryanodine receptor antagonist - can lead to a significant increase in the intracellular ATP levels of C6 glioma cells. We further found that the siRNA-induced CD38 reductions can induce decreases in the number of surviving cells. Our study has also suggested that P2X receptors contribute toward the CD38 siRNA-induced decreases in the cell survival. In summary, our study has provided novel evidence suggesting that CD38 mediates both the intracellular ATP levels and the survival of C6 glioma cells, suggesting that CD38 may become a therapeutic target for gliomas.

The purinergic neurotransmitter revisited: a single substance or multiple players?

The past half century has witnessed tremendous advances in our understanding of extracellular purinergic signaling pathways. Purinergic neurotransmission, in particular, has emerged as a key contributor in the efficient control mechanisms in the nervous system. The identity of the purine neurotransmitter, however, remains controversial. Identifying it is difficult because purines are present in all cell types, have a large variety of cell sources, and are released via numerous pathways. Moreover, studies on purinergic neurotransmission have relied heavily on indirect measurements of integrated postjunctional responses that do not provide direct information for neurotransmitter identity. This paper discusses experimental support for adenosine 5'-triphosphate (ATP) as a neurotransmitter and recent evidence for possible contribution of other purines, in addition to or instead of ATP, in chemical neurotransmission in the peripheral, enteric and central nervous systems. Sites of release and action of purines in model systems such as vas deferens, blood vessels, urinary bladder and chromaffin cells are discussed. This is preceded by a brief discussion of studies demonstrating storage of purines in synaptic vesicles. We examine recent evidence for cell type targets (e.g., smooth muscle cells, interstitial cells, neurons and glia) for purine neurotransmitters in different systems. This is followed by brief discussion of mechanisms of terminating the action of purine neurotransmitters, including extracellular nucleotide hydrolysis and possible salvage and reuptake in the cell. The significance of direct neurotransmitter release measurements is highlighted. Possibilities for involvement of multiple purines (e.g., ATP, ADP, NAD(+), ADP-ribose, adenosine, and diadenosine polyphosphates) in neurotransmission are considered throughout.

Effects of NAD at purine receptors in isolated blood vessels

Nicotinamide adenine dinucleotide (NAD) belongs to the family of naturally occurring adenine dinucleotides, best known for their various intracellular roles. However, there is evidence that they can also be released from cells to act as novel extracellular signalling molecules. Relatively little is known about the extracellular actions of NAD, especially in the cardiovascular system. The present study investigated the actions of NAD in the rat thoracic aorta, porcine coronary artery and porcine mesenteric arteries, mounted in organ baths for isometric tension recording. In the rat thoracic aorta and porcine coronary artery, NAD caused endothelium-independent concentration-dependent vasorelaxations which were unaffected by palmitoylCoA, a P2Y1 receptor antagonist, but which were blocked by CGS15943, a non-selective adenosine receptor antagonist. In the porcine coronary artery, NAD-evoked relaxations were abolished by SCH58261, a selective A2A receptor antagonist. In the rat thoracic aorta, NAD-evoked relaxations were attenuated by A2A receptor antagonism with SCH58261 but were unaffected by an A2B receptor antagonist, MRS1754. In contrast, in the porcine mesenteric artery, NAD-evoked endothelium-independent contractions, which were unaffected by a P2 receptor antagonist, suramin, or by NF449, a P2X1 receptor antagonist, but were attenuated following P2X receptor desensitisation with αβ-meATP. In conclusion, the present results show that NAD can alter vascular tone through actions at purine receptors in three different arteries from two species; its molecular targets differ according to the type of blood vessel.

Glibenclamide decreases ATP-induced intracellular calcium transient elevation via inhibiting reactive oxygen species and mitochondrial activity in macrophages

Increasing evidence has revealed that glibenclamide has a wide range of anti-inflammatory effects. However, it is unclear whether glibenclamide can affect the resting and adenosine triphosphate (ATP)-induced intracellular calcium ([Ca²⁺]_i) handling in Raw 264.7 macrophages. In the present study, [Ca²⁺]_i transient, reactive oxygen species (ROS) and mitochondrial activity were measured by the high-speed TILLvisION digital imaging system using the indicators of Fura 2-am, DCFDA and rhodamine-123, respectively. We found that glibenclamide, pinacidil and other unselective K⁺ channel blockers had no effect on the resting [Ca²⁺]_i of Raw 264.7 cells. Extracellular ATP (100 µM) induced [Ca²⁺]_i transient elevation independent of extracellular Ca²⁺. The transient elevation was inhibited by an ROS scavenger (tiron) and mitochondria inhibitor (rotenone). Glibenclamide and 5-hydroxydecanoate (5-HD) also decreased ATP-induced [Ca²⁺]_i transient elevation, but pinacidil and other unselective K⁺ channel blockers had no effect. Glibenclamide also decreased the peak of [Ca²⁺]_i transient induced by extracellular thapsigargin (Tg, 1 µM). Furthermore, glibenclamide decreased intracellular ROS and mitochondrial activity. When pretreated with tiron and rotenone, glibenclamide could not decrease ATP, and Tg induced maximal [Ca²⁺]_i transient further. We conclude that glibenclamide may inhibit ATP-induced [Ca²⁺]_i transient elevation by blocking mitochondria K_ATP channels, resulting in decreased ROS generation and mitochondrial activity in Raw 264.7 macrophages.

Protective role of acidic pH-activated chloride channel in severe acidosis-induced contraction from the aorta of spontaneously hypertensive rats

Severe acidic pH-activated chloride channel (I_Cl,acid) has been found in various mammalian cells. In the present study, we investigate whether this channel participates in reactions of the thoracic aorta to severe acidosis and whether it plays a role in hypertension. We measured isometric contraction in thoracic aorta rings from spontaneously hypertensive rats (SHRs) and normotensive Wistar rats. Severe acidosis induced contractions of both endothelium-intact and -denuded thoracic aorta rings. In Wistar rats, contractions did not differ at pH 6.4, 5.4 and 4.4. However, in SHRs, contractions were higher at pH 5.4 or 4.4 than pH 6.4, with no difference between contractions at pH 5.4 and 4.4. Nifedipine, I_Cl,acid blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and 4,4′-diisothiocyanatostilbene-2, 2′-disulfonic acid (DIDS) inhibited severe acidosis-induced contraction of aortas at different pH levels. When blocking I_Cl,acid, the remnant contraction was greater at pH 4.4 than pH 5.4 and 6.4 for both SHRs and Wistar rats. With nifedipine, the remnant contraction was greatly reduced at pH 4.4 as compared with at pH 6.4 and 5.4. With NPPB or DIDS, the ratio of remnant contractions at pH 4.4 and 5.4 (R_4.4/5.4) was lower for SHRs than Wistar rats (all <1). However, with nifedipine, the R_4.4/5.4 was higher for SHRs than Wistar rats (both >1). Furthermore, patch clamp recordings of I_Cl,acid and intracellular Ca²⁺ measurements in smooth muscle cells confirmed these findings. I_Cl,acid may protect arteries against excess vasoconstriction under extremely acidic extracellular conditions. This protective effect may be decreased in hypertension.

P2X receptor-dependent erythrocyte damage by α-hemolysin from Escherichia coli triggers phagocytosis by THP-1 cells

The pore-forming exotoxin α-hemolysin from E.coli causes a significant volume reduction of human erythrocytes that precedes the ultimate swelling and lysis. This shrinkage results from activation of Ca²⁺-sensitive K⁺ (K_Ca3.1) and Cl⁻ channels (TMEM16A) and reduced functions of either of these channels potentiate the HlyA-induced hemolysis. This means that Ca²⁺-dependent activation of K_Ca3.1 and TMEM16A protects the cells against early hemolysis. Simultaneous to the HlyA-induced shrinkage, the erythrocytes show increased exposure of phosphatidylserine (PS) in the outer plasma membrane leaflet, which is known to be a keen trigger for phagocytosis. We hypothesize that exposure to HlyA elicits removal of the damaged erythrocytes by phagocytic cells. Cultured THP-1 cells as a model for erythrocytal phagocytosis was verified by a variety of methods, including live cell imaging. We consistently found the HlyA to very potently trigger phagocytosis of erythrocytes by THP-1 cells. The HlyA-induced phagocytosis was prevented by inhibition of K_Ca3.1, which is known to reduce PS-exposure in human erythrocytes subjected to both ionomycin and HlyA. Moreover, we show that P2X receptor inhibition, which prevents the cell damages caused by HlyA, also reduced that HlyA-induced PS-exposure and phagocytosis. Based on these results, we propose that erythrocytes, damaged by HlyA-insertion, are effectively cleared from the blood stream. This mechanism will potentially reduce the risk of intravascular hemolysis.

Intracellular β-nicotinamide adenine dinucleotide inhibits the skeletal muscle ClC-1 chloride channel

ClC-1 is the dominant sarcolemmal chloride channel and plays an important role in regulating membrane excitability that is underscored by ClC-1 mutations in congenital myotonia. Here we show that the coenzyme β-nicotinamide adenine dinucleotide (NAD), an important metabolic regulator, robustly inhibits ClC-1 when included in the pipette solution in whole cell patch clamp experiments and when transiently applied to inside-out patches. The oxidized (NAD(+)) form of the coenzyme was more efficacious than the reduced (NADH) form, and inhibition by both was greatly enhanced by acidification. Molecular modeling, based on the structural coordinates of the homologous ClC-5 and CmClC proteins and in silico docking, suggest that NAD(+) binds with the adenine base deep in a cleft formed by ClC-1 intracellular cystathionine β-synthase domains, and the nicotinamide base interacts with the membrane-embedded channel domain. Consistent with predictions from the models, mutation of residues in cystathionine β-synthase and channel domains either attenuated (G200R, T636A, H847A) or abrogated (L848A) the effect of NAD(+). In addition, the myotonic mutations G200R and Y261C abolished potentiation of NAD(+) inhibition at low pH. Our results identify a new biological role for NAD and suggest that the main physiological relevance may be the exquisite sensitivity to intracellular pH that NAD(+) inhibition imparts to ClC-1 gating. These findings are consistent with the reduction of sarcolemmal chloride conductance that occurs upon acidification of skeletal muscle and suggest a previously unexplored mechanism in the pathophysiology of myotonia.

Extracellular ATP induces P2X7-dependent nicotinamide phosphoribosyltransferase release in LPS-activated human monocytes

Nicotinamide phosphoribosyltransferase (NAMPT), an enzyme involved in NAD biosynthesis, has recently been identified as a novel mediator of innate immunity. In the present study, we report that treatment of LPS-primed monocytes with ATP greatly enhanced the secretion of NAMPT in a time- and concentration-dependent manner without displaying any cytotoxic effect. NAMPT release was suppressed by pretreatment with the P2X(7) receptor (P2X(7)R) inhibitors oxidized ATP (oxATP) and KN-62, indicating the engagement of P2X(7)Rs. Furthermore, P2X(7)R was found to be involved in mediating cell permeability caused by the addition of ATP. To define a role of endogenous ATP in NAMPT secretion, LPS-primed monocytes were incubated in the presence of oxATP and KN-62, as well as the ATP-hydrolyzing enzymes apyrase and hexokinase. With the exception of oxATP, neither substance led to a decrease in NAMPT release, suggesting that autocrine/paracrine ATP is unlikely to be responsible for the LPS-induced release of NAMPT. In conclusion, the enhanced release of NAMPT by extracellular ATP described here indicates the requirement of a second stimulus for the efficient secretion of NAMPT. This mode of secretion, which also applies to IL-1β, might represent a general mechanism for the release of leaderless secretory proteins at locally restricted sites.

Regulation of human mesenchymal stem cell functions by an autocrine loop involving NAD+ release and P2Y11-mediated signaling

In several cell types, a regulated efflux of NAD(+) across Connexin 43 hemichannels (Cx43 HC) can occur, and extracellular NAD(+) (NAD(+)(e)) affects cell-specific functions. We studied the capability of bone marrow-derived human mesenchymal stem cells (MSC) to release intracellular NAD(+) through Cx43 HC. NAD(+) efflux, quantified by a sensitive enzymatic cycling assay, was significantly upregulated by low extracellular Ca(2+) (5-6-fold), by shear stress (13-fold), and by inflammatory conditions (3.1- and 2.5-fold in cells incubated with lipopolysaccharide (LPS) or at 39°C, respectively), as compared with untreated cells, whereas it was downregulated in Cx43-siRNA-transfected MSC (by 53%) and by cell-to-cell contact (by 45%). Further, we show that NAD(+)(e) activates the purinergic receptor P2Y(11) and a cyclic adenosin monophosphate (cAMP)/cyclic ADP-ribose/[Ca(2+)](i) signaling cascade, involving the opening, unique to MSC, of L-type Ca(2+) channels. Extracellular NAD(+) enhanced nuclear translocation of cAMP/Ca(2+)-dependent transcription factors. Moreover, NAD(+), either extracellularly added or autocrinally released, resulted in stimulation of MSC functions, including proliferation, migration, release of prostaglandin E(2) and cytokines, and downregulation of T lymphocyte proliferation compared with controls. No detectable modifications of MSC markers and of adipocyte or osteocyte differentiation were induced by NAD(+)(e). Controls included Cx43-siRNA transfected and/or NAD(+)-glycohydrolase-treated MSC (autocrine effects), and NAD(+)-untreated or P2Y(11)-siRNA-transfected MSC (exogenous NAD(+)). These findings suggest a potential beneficial role of NAD(+)(e) in modulating MSC functions relevant to MSC-based cell therapies.

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.

Extracellular NAD(+) induces a rise in [Ca(2+)](i) in activated human monocytes via engagement of P2Y(1) and P2Y(11) receptors

Extracellular nicotinamide adenine dinucleotide (NAD(+)) is known to increase the intracellular calcium concentration [Ca(2+)](i) in different cell types and by various mechanisms. Here we show that NAD(+) triggers a transient rise in [Ca(2+)](i) in human monocytes activated with lipopolysaccharide (LPS), which is caused by a release of Ca(2+) from IP(3)-responsive intracellular stores and an influx of extracellular Ca(2+). By the use of P2 receptor-selective agonists and antagonists we demonstrate that P2 receptors play a role in the NAD(+)-induced calcium response in activated monocytes. Of the two subclasses of P2 receptors (P2X and P2Y) the P2Y receptors were considered the most likely candidates, since they share calcium signaling properties with NAD(+). The identification of P2Y(1) and P2Y(11) as receptor subtypes responsible for the NAD(+)-triggered increase in [Ca(2+)](i) was supported by several lines of evidence. First, specific P2Y(1) and P2Y(11) receptor antagonists inhibited the NAD(+)-induced increase in [Ca(2+)](i). Second, NAD(+) was shown to potently induce calcium signals in cells transfected with either subtype, whereas untransfected cells were unresponsive. Third, NAD(+) caused an increase in [cAMP](i), prevented by the P2Y(11) receptor-specific antagonist NF157.