β-Nicotinamide Adenine Dinucleotide (β-NAD) Inhibits ATP-Dependent IL-1β Release from Human Monocytic Cells

Immunological insights into hypertension: unraveling triggers and potential therapeutic avenues

Hypertension remains the leading cause of morbidity and mortality worldwide. Despite its prevalence, the development of novel antihypertensive therapies has only recently accelerated, with novel agents not yet commercialized, leaving a substantial proportion of individuals resistant to existing treatments. The intricate pathophysiology of hypertension is now understood to involve chronic low-grade inflammation, which places the immune system in the spotlight as a potential target for new therapeutics. This review explores the factors that initiate and sustain an immune response in hypertension, offering insights into potential targets for new treatments. Several factors contribute to immune activation in hypertension, including diet and damage-associated molecular pattern (DAMP) generation. Diets rich in fat or sodium can promote inflammation by inducing intestinal barrier dysfunction and triggering salt-sensitive receptors in T cells and dendritic cells. DAMPs, such as extracellular adenosine triphosphate and heat-shock protein 70, are released during episodes of increased blood pressure, contributing to immune cell activation and inflammation. Unconventional innate-like γδ T cells contribute to initiating and maintaining an immune response through their potential involvement in antigen presentation and regulating cytokine-mediated responses. Immunologic memory, sustained through the formation of effector memory T cells after exposure to hypertensive insults, likely contributes to maintaining an immune response in hypertension. When exposed to hypertensive insults, these memory cells are rapidly activated and contribute to elevated blood pressure and end-organ damage. Evidence from human hypertension, although limited, supports the relevance of distinct immune pathways in hypertension, and highlights the potential of targeted immune interventions in human hypertension. Diet and acute bouts of high blood pressure result in the release of dietary triggers, neoantigens, and damage-associated molecular patterns (DAMPs), which promote immune system activation. Elements such as lipopolysaccharides (LPS), sodium, heat-shock protein (HSP)70, extracellular adenosine triphosphate (eATP), and growth arrest-specific 6 (GAS6) promote activation of innate immune cells such as dendritic cells (DCs) and monocytes (Mo) through their respective receptors (toll-like receptor [TLR]4, amiloride-sensitive epithelial sodium channel [ENaC], TLR2/4, P2X7 receptor [P2RX7], and Axl) leading to costimulatory molecule expression and interleukin (IL)-1β and IL-23 production. The neoantigens HSP70 and isolevuglandins (IsoLGs) are presented to T cells by DCs and possibly γδ T cells, triggering T cell activation, IL-17 and interferon (IFN)-γ production, and the formation of T effector memory (TEM) cells in the kidney, perivascular adipose tissue, bone marrow, and spleen. Exposure of TEM cells to their cognate antigen or previous activating stimuli causes these cells rapid expansion and activation. Cumulatively, this inflammatory state contributes to hypertension and end-organ damage. The figure was created using images from smart.servier.com and is licensed under a Creative Commons Attribution 4.0 license (CC BY 4.0).

Impact of IDO activation and alterations in the kynurenine pathway on hyperserotonemia, NAD+ production, and AhR activation in autism spectrum disorder

Hyperserotonemia is the most replicated biochemical anomaly associated with autism spectrum disorder (ASD) and has been reported in 35-46% of individuals with ASD. Serotonin is synthesised from the essential amino acid tryptophan (TRP). However, the main catabolic route of TRP is the kynurenine pathway (KP), which competes with serotonin synthesis when indoleamine dioxygenase (IDO) is activated. Using the same cohort of individuals with ASD, we used to report extensive studies of the serotonin/melatonin pathway, and found increased kynurenine (KYN), suggesting IDO activation in 58.7% of individuals with ASD (159/271), supported by a strong negative correlation between KYN/TRP ratio and miR-153-3p plasma levels, which negatively regulates IDO. IDO activation was associated with normoserotonemia, suggesting that IDO activation could mask hyperserotonemia which meant that hyperserotonemia, if not masked by IDO activation, could be present in ~94% of individuals with ASD. We also identified several KP alterations, independent of IDO status. We observed a decrease in the activity of 3-hydroxyanthranilate dioxygenase which translated into the accumulation of the aryl hydrocarbon receptor (AhR) selective ligand cinnabarinic acid, itself strongly positively correlated with the AhR target stanniocalcin 2. We also found a deficit in NAD+ production, the end-product of the KP, which was strongly correlated with plasma levels of oxytocin used as a stereotypical neuropeptide, indicating that regulated neuropeptide secretion could be limiting. These results strongly suggest that individuals with ASD exhibit low-grade chronic inflammation that is mediated in most cases by chronic AhR activation that could be associated with the highly prevalent gastrointestinal disorders observed in ASD, and explained IDO activation in ~58% of the cases. Taken together, these results extend biochemical anomalies of TRP catabolism to KP and posit TRP catabolism as a possible major component of ASD pathophysiology.

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.

Signaling of nicotinic acetylcholine receptors in mononuclear phagocytes

Nicotinic acetylcholine receptors are not only expressed by the nervous system and at the neuro-muscular junction but also by mononuclear phagocytes, which belong to the innate immune system. Mononuclear phagocyte is an umbrella term for monocytes, macrophages, and dendritic cells. These cells play pivotal roles in host defense against infection but also in numerous often debilitating diseases that are characterized by exuberant inflammation. Nicotinic acetylcholine receptors of the neuronal type dominate in these cells, and their stimulation is mainly associated with anti-inflammatory effects. Although the cholinergic modulation of mononuclear phagocytes is of eminent clinical relevance for the prevention and treatment of inflammatory diseases and neuropathic pain, we are only beginning to understand the underlying mechanisms on the molecular level. The purpose of this review is to report and critically discuss the current knowledge on signal transduction mechanisms elicited by nicotinic acetylcholine receptors in mononuclear phagocytes.

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.

Biological Properties of Vitamins of the B-Complex, Part 1: Vitamins B1, B2, B3, and B5

This review summarizes the current knowledge on essential vitamins B₁, B₂, B₃, and B₅. These B-complex vitamins must be taken from diet, with the exception of vitamin B₃, that can also be synthetized from amino acid tryptophan. All of these vitamins are water soluble, which determines their main properties, namely: they are partly lost when food is washed or boiled since they migrate to the water; the requirement of membrane transporters for their permeation into the cells; and their safety since any excess is rapidly eliminated via the kidney. The therapeutic use of B-complex vitamins is mostly limited to hypovitaminoses or similar conditions, but, as they are generally very safe, they have also been examined in other pathological conditions. Nicotinic acid, a form of vitamin B₃, is the only exception because it is a known hypolipidemic agent in gram doses. The article also sums up: (i) the current methods for detection of the vitamins of the B-complex in biological fluids; (ii) the food and other sources of these vitamins including the effect of common processing and storage methods on their content; and (iii) their physiological function.

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.

Does Cholinergic Stimulation Affect the P2X7 Receptor-Mediated Dye Uptake in Mast Cells and Macrophages?

Background: Extracellular ATP is a powerful trigger of neuroinflammation by activating immune cells via P2X7 receptors. Acetylcholine and nicotinic agonists inhibit ATP-triggered proinflammatory cytokines via the so-called “cholinergic anti-inflammatory pathway” (CAP). However, it remains unclear as to what stage of ATP-induced signaling cholinergic agents provide this anti-inflammatory effect. Using the specific property of P2X7 receptor to open a pathway permeable to large molecules, associated with activation of inflammasome, we studied the action of cholinergic agents on this key event in CAP activation.

Methods: Freshly isolated mouse peritoneal mast cells and primary human macrophages were used. To assess P2X7 channel opening, the permeability to the fluorescent dye YO-PRO1 or ethidium bromide (EtBr) was measured by flow cytometry. Expression of nicotinic receptors was probed in macrophages with the fluorescently labeled α-bungarotoxin or with patch-clamp recordings.

Results: ATP opened P2X7 ion channels in mast cells and macrophages permeable to YO-PRO1 or EtBr, respectively. This stimulatory effect in mast cells was inhibited by the specific P2X7 antagonist A839977 confirming that YO-PRO1 uptake was mediated via ATP-gated P2X7 ion channels. Cholinergic agents also slightly induced dye uptake to mast cells but not in macrophages, which expressed functional α7 nicotinic receptors. However, both in mast cells and in macrophages, acetylcholine and nicotine failed to inhibit the stimulatory effect of ATP on dye uptake.

Conclusion: These data suggest that in immune cells, cholinergic agents do not act on P2X7 receptor-coupled large pore formation but can mediate the anti-inflammatory effect underlying CAP downstream of ATP-driven signaling.

Does heart surgery change the capacity of α1-antitrypsin to inhibit the ATP-induced release of monocytic interleukin-1β? A preliminary study

Heart surgery involving cardiopulmonary bypass induces systemic inflammation that is, at least in part, caused by extracellular ATP originating from damaged cells and by proteases secreted by activated neutrophils. The anti-protease α1-antitrypsin (AAT) forms complexes with several proteases including neutrophil elastase, resulting in a mutual loss of activity. We demonstrated recently that AAT inhibits the ATP-induced release of the pro-inflammatory cytokine interleukin-1β by human monocytes by a mechanism involving activation of metabotropic functions at nicotinic acetylcholine receptors. Interleukin-1β importantly contributes to the pathogenesis of sterile inflammatory response syndrome. Thus, AAT might function as an endogenous safeguard against life-threatening systemic inflammation. In this preliminary study, we test the hypothesis that during cardiopulmonary bypass, AAT is inactivated as an anti- protease and as an inhibitor of ATP-induced interleukin-1β release. AAT was affinity-purified from the blood plasma of patients before, during and after surgery. Lipopolysaccharide-primed human monocytic U937 cells were stimulated with ATP in the presence or absence of patient AAT to test for its inhibitory effect on interleukin-1β release. Anti-protease activity was investigated via complex formation with neutrophil elastase. The capacity of patient AAT to inhibit the ATP-induced release of interleukin-1β might be slightly reduced in response to heart surgery and complex formation of patient AAT with neutrophil elastase was unimpaired. We conclude that surgery involving cardiopulmonary bypass does not markedly reduce the anti-inflammatory and the anti-protease activity of AAT. The question if AAT augmentation therapy during heart surgery is suited to attenuate postoperative inflammation warrants further studies in vivo.

Exogenous nicotinamide adenine dinucleotide administration alleviates ischemia/reperfusion-induced oxidative injury in isolated rat hearts via Sirt5-SDH-succinate pathway

The metabolic disorder of succinate in myocardial tissue during ischemia-reperfusion can lead to the myocardial oxidative injury. The activation of succinate dehydrogenase (SDH) plays a vital role in the process. Silent information regulator 5 (Sirt5), a nicotinamide adenine dinucleotide (NAD)-dependent desuccinylase, desuccinylates and inactivates SDH thus exerting a protective effect on the myocardium. This research was designed to investigate whether exogenous NAD protects the myocardium from the ischemia-reperfusion-induced oxidative injury through regulating Sirt5-SDH pathway and succinate metabolism. We first found that myocardial total NAD level was remarkably increased with NAD treatment (10 mg/kg) for 14 days. NAD administration significantly decreased the lactate dehydrogenase (LDH) level in coronary leakage, decreased the malondialdehyde (MDA) level and increased the reduced glutathione/oxidized glutathione disulfide ratio (GSH/GSSG) in myocardial tissue. In addition, NAD treatment effectively attenuated the depression of cardiac function in the isolated rat hearts after ischemia-reperfusion. Furthermore, we found that exogenous NAD attenuated the succinate accumulation during ischemia and decreased its depleting rate during reperfusion. We also found that NAD administration had no obvious effects on myocardial Sirt5 and SDH-a expressions. However, the results of immunofluorescence showed that Sirt5 and SDH-a interacted in ischemia-reperfused myocardium. Utilizing co-immunoprecipitation method, we found that NAD administration promoted the Sirt5 and SDH-a interaction and decreased the succinylation level of SDH-a. These results implied that exogenous NAD administration promoted Sirt5-mediated SDH-a desuccinylation and decreased the activity of SDH-a, which attenuated the succinate accumulation during ischemia and its depleting rate during reperfusion and finally alleviated reactive oxygen species generation.

SLPI Inhibits ATP-Mediated Maturation of IL-1β in Human Monocytic Leukocytes: A Novel Function of an Old Player

Interleukin-1β (IL-1β) is a potent, pro-inflammatory cytokine of the innate immune system that plays an essential role in host defense against infection. However, elevated circulating levels of IL-1β can cause life-threatening systemic inflammation. Hence, mechanisms controlling IL-1β maturation and release are of outstanding clinical interest. Secretory leukocyte protease inhibitor (SLPI), in addition to its well-described anti-protease function, controls the expression of several pro-inflammatory cytokines on the transcriptional level. In the present study, we tested the potential involvement of SLPI in the control of ATP-induced, inflammasome-dependent IL-1β maturation and release. We demonstrated that SLPI dose-dependently inhibits the ATP-mediated inflammasome activation and IL-1β release in human monocytic cells, without affecting the induction of pro-IL-1β mRNA by LPS. In contrast, the ATP-independent IL-1β release induced by the pore forming bacterial toxin nigericin is not impaired, and SLPI does not directly modulate the ion channel function of the human P2X₇ receptor heterologously expressed in Xenopus laevis oocytes. In human monocytic U937 cells, however, SLPI efficiently inhibits ATP-induced ion-currents. Using specific inhibitors and siRNA, we demonstrate that SLPI activates the calcium-independent phospholipase A2β (iPLA2β) and leads to the release of a low molecular mass factor that mediates the inhibition of IL-1β release. Signaling involves nicotinic acetylcholine receptor subunits α7, α9, α10, and Src kinase activation and results in an inhibition of ATP-induced caspase-1 activation. In conclusion, we propose a novel anti-inflammatory mechanism induced by SLPI, which inhibits the ATP-dependent maturation and secretion of IL-1β. This novel signaling pathway might lead to development of therapies that are urgently needed for the prevention and treatment of systemic inflammation.

Adenine nucleotides as paracrine mediators and intracellular second messengers in immunity and inflammation

Adenine nucleotides (AdNs) play important roles in immunity and inflammation. Extracellular AdNs, such as adenosine triphosphate (ATP) or nicotinamide adenine dinucleotide (NAD) and their metabolites, act as paracrine messengers by fine-tuning both pro- and anti-inflammatory processes. Moreover, intracellular AdNs derived from ATP or NAD play important roles in many cells of the immune system, including T lymphocytes, macrophages, neutrophils and others. These intracellular AdNs are signaling molecules that transduce incoming signals into meaningful cellular responses, e.g. activation of immune responses against pathogens.

Conopeptides [V11L;V16D]ArIB and RgIA4: Powerful Tools for the Identification of Novel Nicotinic Acetylcholine Receptors in Monocytes

Venomous marine snails of the genus Conus employ small peptides to capture prey, mainly osteichthyes, mollusks, and worms. A subset of these peptides known as α-conotoxins, are antagonists of nicotinic acetylcholine receptors (nAChRs). These disulfide-rich peptides provide a large number of evolutionarily refined templates that can be used to develop conopeptides that are highly selective for the various nAChR subtypes. Two such conopeptides, namely [V11L;V16D]ArIB and RgIA4, have been engineered to selectively target mammalian α7^∗ and α9^∗ nAChRs, respectively, and have been used to study the functional roles of these subtypes in immune cells. Unlike in neurons and cochlear hair cells, where α7^∗ and α9^∗ nAChRs, respectively, function as ligand-gated ion channels, in immune cells ligand-evoked ion currents have not been demonstrated. Instead, different metabotropic functions of α7^∗ and α9^∗ nAChRs have been described in monocytic cells including the inhibition of ATP-induced ion currents, inflammasome activation, and interleukin-1β (IL-1β) release. In addition to conventional nAChR agonists, diverse compounds containing a phosphocholine group inhibit monocytic IL-1β release and include dipalmitoyl phosphatidylcholine, palmitoyl lysophosphatidylcholine, glycerophosphocholine, phosphocholine, phosphocholine-decorated lipooligosaccharides from Haemophilus influenzae, synthetic phosphocholine-modified bovine serum albumin, and the phosphocholine-binding C-reactive protein. In monocytic cells, the effects of [V11L;V16D]ArIB and RgIA4 suggested that activation of nAChRs containing α9, α7, and/or α10 subunits inhibits ATP-induced IL-1β release. These results have been corroborated utilizing gene-deficient mice and small interfering RNA. Targeted re-engineering of native α-conotoxins has resulted in excellent tools for nAChR research as well as potential therapeutics. ^∗indicates possible presence of additional subunits.