Quinolinic acid: neurotoxin or oxidative stress modulator?

Microglia Sing the Prelude of Neuroinflammation-Associated Depression

Major depressive disorder (MDD) is a psychiatric condition characterized by sadness and anhedonia and is closely linked to chronic low-grade neuroinflammation, which is primarily induced by microglia. Nonetheless, the mechanisms by which microglia elicit depressive symptoms remain uncertain. This review focuses on the mechanism linking microglia and depression encompassing the breakdown of the blood-brain barrier, the hypothalamic-pituitary-adrenal axis, the gut-brain axis, the vagus and sympathetic nervous systems, and the susceptibility influenced by epigenetic modifications on microglia. These pathways may lead to the alterations of microglia in cytokine levels, as well as increased oxidative stress. Simultaneously, many antidepressant treatments can alter the immune phenotype of microglia, while anti-inflammatory treatments can also have antidepressant effects. This framework linking microglia, neuroinflammation, and depression could serve as a reference for targeting microglia to treat depression.

Does the kynurenine pathway play a pathogenic role in autism spectrum disorder?

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in communication, sociability, and repetitive/stereotyped behavior. The etiology of autism is diverse, with genetic susceptibility playing an important role alongside environmental insults and conditions. Human and preclinical studies have shown that ASD is commonly accompanied by inflammation, and inhibition of the inflammatory response can ameliorate, or prevent the phenotype in preclinical studies. The kynurenine pathway, responsible for tryptophan metabolism, is upregulated by inflammation. Hence, this metabolic route has drawn the attention of investigators across different disciplines such as cancer, immunology, and neuroscience. Over the past decade, studies have identified evidence that the kynurenine pathway is also altered in autism spectrum disorders. In this mini review, we will explore the current status quo of the link between the kynurenine pathway and ASD, shedding light on the compelling but still preliminary evidence of this relationship.

The Role of Amino Acids in Non-Enzymatic Antioxidant Mechanisms in Cancer: A Review

Currently, the antioxidant properties of amino acids and their role in the physicochemical processes accompanying oxidative stress in cancer remain unclear. Cancer cells are known to extensively uptake amino acids, which are used as an energy source, antioxidant precursors that reduce oxidative stress in cancer, and as regulators of inhibiting or inducing tumor cell-associated gene expression. This review examines nine amino acids (Cys, His, Phe, Met, Trp, Tyr, Pro, Arg, Lys), which play a key role in the non-enzymatic oxidative process in various cancers. Conventionally, these amino acids can be divided into two groups, in one of which the activity increases (Cys, Phe, Met, Pro, Arg, Lys) in cancer, and in the other, it decreases (His, Trp, Tyr). The review examines changes in the metabolism of nine amino acids in eleven types of oncology. We have identified the main nonspecific mechanisms of changes in the metabolic activity of amino acids, and described direct and indirect effects on the redox homeostasis of cells. In the future, this will help to understand better the nature of life of a cancer cell and identify therapeutic targets more effectively.

Effect of Quinolinic Acid on Behavior, Morphology, and Expression of Inflammatory/oxidative Status in Rats' Striatum: Is Coenzyme Q10 a Good Protector?

Quinolinic acid (QUIN) is a toxic compound with pro-oxidant, pro-inflammatory, and pro-apoptotic actions found at high levels in the central nervous system (CNS) in several pathological conditions. Due to the toxicity of QUIN, it is important to evaluate strategies to protect against the damage caused by this metabolite in the brain. In this context, coenzyme Q10 (CoQ10) is a provitamin present in the mitochondria with a protective role in cells through several mechanisms of action. Based on these, the present study was aimed at evaluating the possible neuroprotective role of CoQ10 against damage caused by QUIN in the striatum of young Wistar rats. Twenty-one-day-old rats underwent a 10-day pretreatment with CoQ10 or saline (control) intraperitoneal injections and on the 30th day of life received QUIN intrastriatal or saline (control) administration. The animals were submitted to behavior tests or euthanized, and the striatum was dissected to neurochemical studies. Results showed that CoQ10 was able to prevent behavioral changes (the open field, object recognition, and pole test tasks) and neurochemical parameters (alteration in the gene expression of IL-1β, IL-6, SOD, and GPx, as well as in the immunocontent of cytoplasmic Nrf2 and nuclear p-Nf-κβ) caused by QUIN. These findings demonstrate the promising therapeutic effects of CoQ10 against QUIN toxicity.

Altered Tryptophan-Kynurenine Pathway in Delirium: A Review of the Current Literature

Delirium, a common form of acute brain dysfunction, is associated with increased morbidity and mortality, especially in older patients. The underlying pathophysiology of delirium is not clearly understood, but acute systemic inflammation is known to drive delirium in cases of acute illnesses, such as sepsis, trauma, and surgery. Based on psychomotor presentations, delirium has three main subtypes, such as hypoactive, hyperactive, and mixed subtype. There are similarities in the initial presentation of delirium with depression and dementia, especially in the hypoactive subtype. Hence, patients with hypoactive delirium are frequently misdiagnosed. The altered kynurenine pathway (KP) is a promising molecular pathway implicated in the pathogenesis of delirium. The KP is highly regulated in the immune system and influences neurological functions. The activation of indoleamine 2,3-dioxygenase, and specific KP neuroactive metabolites, such as quinolinic acid and kynurenic acid, could play a role in the event of delirium. Here, we collectively describe the roles of the KP and speculate on its relevance in delirium.

An integrated cytokine and kynurenine network as the basis of neuroimmune communication

Two of the molecular families closely associated with mediating communication between the brain and immune system are cytokines and the kynurenine metabolites of tryptophan. Both groups regulate neuron and glial activity in the central nervous system (CNS) and leukocyte function in the immune system, although neither group alone completely explains neuroimmune function, disease occurrence or severity. This essay suggests that the two families perform complementary functions generating an integrated network. The kynurenine pathway determines overall neuronal excitability and plasticity by modulating glutamate receptors and GPR35 activity across the CNS, and regulates general features of immune cell status, surveillance and tolerance which often involves the Aryl Hydrocarbon Receptor (AHR). Equally, cytokines and chemokines define and regulate specific populations of neurons, glia or immune system leukocytes, generating more specific responses within restricted CNS regions or leukocyte populations. In addition, as there is a much larger variety of these compounds, their homing properties enable the superimposition of dynamic variations of cell activity upon local, spatially limited, cell populations. This would in principle allow the targeting of potential treatments to restricted regions of the CNS. The proposed synergistic interface of 'tonic' kynurenine pathway affecting baseline activity and the superimposed 'phasic' cytokine system would constitute an integrated network explaining some features of neuroimmune communication. The concept would broaden the scope for the development of new treatments for disorders involving both the CNS and immune systems, with safer and more effective agents targeted to specific CNS regions.

The protective effect of roflumilast and ibuprofen on testicular ischemia reperfusion injury: An experimental study

BACKGROUND

The aim of the present study is to investigate the efficiency of roflumilast and ibuprofen in an experimental rat testicular ischemia reperfusion injury model in the light of histological and biochemical data.

METHODS

A total of 32 prepubertal male rats were randomly divided into four groups as G1: Control Group (testicular torsion/detorsion + saline (0.9% of 2 ml) was applied). G2: Sham Group only right scrotal incision was performed; G3: Ibuprofen Group (tes-ticular torsion/detorsion + ibuprofen administration); and G4 Roflumilast Group (testicular torsion/detorsion + roflumilast adminis-tration). Oxidative markers such as malondialdehyde (MDA), myeloperoxidase (MPO), total sulfhydryl (TSH), and nitrite (NO) levels as well as histopathological changes were analyzed.

RESULTS

Tissue MPO, MDA, and NO levels were significantly higher and TSH levels significantly lower in control group compared to sham group (p<0.001). The histopathologic scores of drug groups (Groups 3 and 4) were significantly lower than group 1 (p<0.001). In comparison of Group 3 and Group 4 with each other, the mean values of MPO and MDA were statistically significantly lower in Group 4 (p<0.001). A higher mean value of TSH was found in Group 3 without statistically significance (p=0.32). There was also an insignificant decrease in mean NO values of Group 3 compared to Group 4 (p=0.44). In comparison of drug groups, Group 4 had statistically insignificant better scores.

CONCLUSION

Our results indicate that administrating ibuprofen and roflumilast reduced testicular ischemia reperfusion injury in rat testis torsion model. In comparison, roflumilast is found to be more beneficial.

Plumbagin Alleviates Intracerebroventricular-Quinolinic Acid Induced Depression-like Behavior and Memory Deficits in Wistar Rats

Plumbagin, a hydroxy-1,4-naphthoquinone, confers neuroprotection via antioxidant and anti-inflammatory properties. The present study aimed to assess the effect of plumbagin on behavioral and memory deficits induced by intrahippocampal administration of Quinolinic acid (QA) in male Wistar rats and reveal the associated mechanisms. QA (300 nM/4 μL in Normal saline) was administered i.c.v. in the hippocampus. QA administration caused depression-like behavior (forced swim test and tail suspension tests), anxiety-like behavior (open field test and elevated plus maze), and elevated anhedonia behavior (sucrose preference test). Furthermore, oxidative–nitrosative stress (increased nitrite content and lipid peroxidation with reduction of GSH), inflammation (increased IL-1β), cholinergic dysfunction, and mitochondrial complex (I, II, and IV) dysfunction were observed in the hippocampus region of QA-treated rats as compared to normal controls. Plumbagin (10 and 20 mg/kg; p.o.) treatment for 21 days significantly ameliorated behavioral and memory deficits in QA-administered rats. Moreover, plumbagin treatment restored the GSH level and reduced the MDA and nitrite level in the hippocampus. Furthermore, QA-induced cholinergic dysfunction and mitochondrial impairment were found to be ameliorated by plumbagin treatment. In conclusion, our results suggested that plumbagin offers a neuroprotective potential that could serve as a promising pharmacological approach to mitigate neurobehavioral changes associated with neurodegeneration.

Vitamin C and E Treatment Blocks Changes in Kynurenine Metabolism Triggered by Three Weeks of Sprint Interval Training in Recreationally Active Elderly Humans

The kynurenine pathway (KP) is gaining attention in several clinical fields. Recent studies show that physical exercise offers a therapeutic way to improve ratios of neurotoxic to neuroprotective KP metabolites. Antioxidant supplementation can blunt beneficial responses to physical exercise. We here studied the effects of endurance training in the form of sprint interval training (SIT; three sessions of 4–6 × 30 s cycling sprints per week for three weeks) in elderly (~65 years) men exposed to either placebo (n = 9) or the antioxidants vitamin C (1 g/day) and E (235 mg/day) (n = 11). Blood samples and muscle biopsies were taken under resting conditions in association with the first (untrained state) and last (trained state) SIT sessions. In the placebo group, the blood plasma level of the neurotoxic quinolinic acid was lower (~30%) and the neuroprotective kynurenic acid to quinolinic acid ratio was higher (~50%) in the trained than in the untrained state. Moreover, muscle biopsies showed a training-induced increase in kynurenine aminotransferase (KAT) III in the placebo group. All these training effects were absent in the vitamin-treated group. In conclusion, KP metabolism was shifted towards neuroprotection after three weeks of SIT in elderly men and this shift was blocked by antioxidant treatment.

The Pah-R261Q mouse reveals oxidative stress associated with amyloid-like hepatic aggregation of mutant phenylalanine hydroxylase

Phenylketonuria (PKU) is caused by autosomal recessive variants in phenylalanine hydroxylase (PAH), leading to systemic accumulation of L-phenylalanine (L-Phe) that may reach neurotoxic levels. A homozygous Pah-R261Q mouse, with a highly prevalent misfolding variant in humans, reveals the expected hepatic PAH activity decrease, systemic L-Phe increase, L-tyrosine and L-tryptophan decrease, and tetrahydrobiopterin-responsive hyperphenylalaninemia. Pah-R261Q mice also present unexpected traits, including altered lipid metabolism, reduction of liver tetrahydrobiopterin content, and a metabolic profile indicative of oxidative stress. Pah-R261Q hepatic tissue exhibits large ubiquitin-positive, amyloid-like oligomeric aggregates of mutant PAH that colocalize with selective autophagy markers. Together, these findings reveal that PKU, customarily considered a loss-of-function disorder, can also have toxic gain-of-function contribution from protein misfolding and aggregation. The proteostasis defect and concomitant oxidative stress may explain the prevalence of comorbid conditions in adult PKU patients, placing this mouse model in an advantageous position for the discovery of mutation-specific biomarkers and therapies.

Roflumilast, a cAMP-Specific Phosphodiesterase-4 Inhibitor, Reduces Oxidative Stress and Improves Synapse Functions in Human Cortical Neurons Exposed to the Excitotoxin Quinolinic Acid

The overexpression of phosphodiesterase 4 (PDE4) enzymes is reported in several neurodegenerative diseases. PDE4 depletes cyclic 3'-5' adenosine monophosphate (cAMP) and, in turn, cAMP response element-binding protein (CREB) and brain-derived neurotrophic factor (BDNF), the key players in cognitive function. The present study was undertaken to investigate the mechanism behind the protective effects of roflumilast (ROF), a cAMP-specific PDE4 inhibitor, against quinolinic acid (QUIN)-induced neurotoxicity using human primary cortical neurons. Cytotoxicity was analyzed using an MTS assay. Reactive oxygen species (ROS) and mitochondrial membrane potential were measured by DCF-DA and JC-10 staining, respectively. Caspase 3/7 activity was measured using an ApoTox-Glo Triplex assay kit. cAMP was measured using an ELISA kit. The protein expression of CREB, BDNF, SAP-97, synaptophysin, synapsin-I, and PSD-95 was analyzed by the Western blotting technique. QUIN exposure down-regulated CREB, BDNF, and synaptic protein expression in neurons. Pretreatment with ROF increased the intracellular cAMP, mitochondrial membrane potential, and nicotinamide adenine dinucleotide (NAD⁺) content and decreased the ROS and caspase 3/7 levels in QUIN-exposed neurons. ROF up-regulated the expression of synapse proteins SAP-97, synaptophysin, synapsin-I, PSD-95, and CREB and BDNF, which indicates its potential role in memory. This study suggests for the first time that QUIN causes pre- and postsynaptic protein damage. We further demonstrate the restorative effects of ROF on the mitochondrial membrane potential and antiapoptotic properties in human neurons. These data encourage further investigations to reposition ROF in neurodegenerative diseases and their associated cognitive deficits.

The antidepressant effects of asperosaponin VI are mediated by the suppression of microglial activation and reduction of TLR4/NF-κB-induced IDO expression

AIM

Indoleamine 2,3-dioxygenase 1 (IDO) is responsible for the progression of the kynurenine pathway, which has been implicated in the pathophysiology of inflammation-induced depression. It has been reported that asperosaponin VI (ASA VI) could play a neuroprotective role through anti-inflammatory and antioxidant. In this study, we examined the antidepressant effect of ASA VI in lipopolysaccharide (LPS)-treated mice and further explored its molecular mechanism by looking into the microglial kynurenine pathway.

METHODS

To generate the model, LPS (0.83 mg/kg) was administered intraperitoneally to mice. The mice received ASA VI (10 mg/kg, 20 mg/kg, 40 mg/kg, and 80 mg/kg, i.p.) 30 min before LPS injection. Depressive-like behaviors were evaluated based on the duration of immobility in the forced swim test. Microglial activation and inflammatory cytokines were detected by immunohistochemistry, real-time PCR, and ELISA. The TLR4/NF-κB signaling pathway and the expression of IDO, GluA2, and CamKIIβ were also measured by western blotting.

RESULTS

ASA VI exhibited significant antidepressant activity in the presence of LPS on immobility and latency times in the forced swim test. The LPS-induced activation of microglia and inflammatory response were inhibited by ASA VI, which showed a dose-dependent pattern. TLR4/NF-κB signaling pathway also was suppressed by ASA VI in the hippocampus and prefrontal cortex of LPS-treated mice. Furthermore, ASA VI inhibited the increase in IDO protein expression and normalized the aberrant glutamate transmission in the hippocampus and prefrontal cortex caused by LPS administration.

CONCLUSION

Our results propose a promising antidepressant effect for ASA VI possibly through the downregulation of IDO expression and normalization of the aberrant glutamate transmission. This remedying effect of ASA VI could be attributed to suppress microglia-mediated neuroinflammatory response via inhibiting the TLR4/NF-κB signaling pathway.

Intrastriatal Quinolinic Acid Administration Impairs Redox Homeostasis and Induces Inflammatory Changes: Prevention by Kynurenic Acid

Kynurenic acid (KYNA) and quinolinic acid (QUIN) are metabolites formed in the degradation of tryptophan (Trp). QUIN is a selective NMDA receptor antagonist and may exert neurotoxic effects, whereas KYNA is an agonist of glutamatergic and cholinergic receptors and presents antioxidant properties. KYNA/QUIN ratio is decreased in several central nervous system disorders, but the mechanisms involved are not well elucidated. In the present study, we try to determine the neuroprotective capacity of KYNA on the QUIN effects in redox homeostasis changes (H₂DCF oxidation, superoxide dismutase/catalase (SOD/CAT) ratio, glutathione peroxidase (GPx) activity, sulfhydryl content, and nitrite levels), as well as on inflammatory parameters (levels of TNF-α, IL-1β, and IL-6). KYNA and QUIN effects on the activities of Na⁺,K⁺-ATPase and acetylcholinesterase (AChE) were also evaluated. Thirty-day-old male Wistar rats underwent stereotactic surgery and received intrastriatal injections as follows: group 1-control (PBS-injected), group 2-KYNA (100 μM), group 3-QUIN (150 nM), and group 4-KYNA + QUIN (KYNA-injected followed QUIN-injected). Results demonstrated that the KYNA administration was able to prevent the increase in reactive oxygen species, SOD/CAT ratio, and pro-inflammatory cytokines (IL-1β and IL-6) and the decrease in GPx activity, sulfhydryl content, and nitrite levels caused by QUIN. KYNA was also able to partially prevent the decrease in Na⁺,K⁺-ATPase activity and the increase in AChE activity caused by QUIN. This study may help in the elucidation of neuroprotective effects of KYNA against oxidative and inflammatory insults caused by QUIN in the striatum of young male Wistar rats.

Kynurenine pathway, NAD+ synthesis, and mitochondrial function: Targeting tryptophan metabolism to promote longevity and healthspan

Aging is characterized by a progressive decline in the normal physiological functions of an organism, ultimately leading to mortality. Nicotinamide adenine dinucleotide (NAD⁺) is an essential cofactor that plays a critical role in mitochondrial energy production as well as many enzymatic redox reactions. Age-associated decline in NAD⁺ is implicated as a driving factor in several categories of age-associated disease, including metabolic and neurodegenerative disease, as well as deficiency in the mechanisms of cellular defense against oxidative stress. The kynurenine metabolic pathway is the sole de novo NAD⁺ biosynthetic pathway, generating NAD⁺ from ingested tryptophan. Altered kynurenine pathway activity is associated with both aging and a variety of age-associated diseases. Kynurenine pathway interventions can extend lifespan in both fruit flies and nematodes, and altered NAD⁺ metabolism represents one potential mediating mechanism. Recent studies demonstrate that supplementation with NAD⁺ or NAD⁺-precursors increase longevity and promote healthy aging in fruit flies, nematodes, and mice. NAD⁺ levels and the intrinsic relationship to mitochondrial function have been widely studied in the context of aging. Mitochondrial function and dynamics have both been implicated in longevity determination in a range of organisms from yeast to humans, at least in part due to their intimate link to regulating an organism's cellular energy economy and capacity to resist oxidative stress. Recent findings support the idea that complex communication between the mitochondria and the nucleus orchestrates a series of events and stress responses involving mitophagy, mitochondrial number, mitochondrial unfolded protein response (UPR^mt), and mitochondria fission and fusion events. In this review, we discuss how mitochondrial morphological changes and dynamics operate during aging, and how altered metabolism of tryptophan to NAD⁺ through the kynurenine pathway interacts with these processes.

Coordination Complex Formation and Redox Properties of Kynurenic and Xanthurenic Acid Can Affect Brain Tissue Homeodynamics

Reactive oxygen species (ROS) are known for their participation in various physiological and pathological processes in organisms, including ageing or degeneration. Kynurenine pathway metabolites, such as kynurenic (KYNA) or xanthurenic (XA) acid, can affect neurodegenerative diseases due to their ROS scavenging and Fe ion coordination complex formation but insights are still incomplete. Therefore, we investigated the formation and antioxidant capabilities of KYNA– and XA–Fe complexes by nano-electrospray−mass spectrometry, differential pulse voltammetry, deoxyribose degradation and Fe^II autoxidation assays. XA formed coordination complexes with Fe^II or Fe^III ions and was an effective antioxidant. By contrast, only Fe^II–KYNA complexes could be detected. Moreover, KYNA showed no antioxidant effects in the FeCl₃/ascorbic acid deoxyribose degradation assay variant and only negligible activities in the Fe^II autoxidation assay. Coordination complexes of Fe ions with KYNA probably stabilize KYNA in its keto tautomer form. Nevertheless, both KYNA and XA exhibited sufficient antioxidant activities in some of the employed assay variants. The results provide evidence that both have the potential to alleviate neurodegenerative diseases by helping to maintain tissue redox homeodynamics.

Galantamine-Memantine Combination as an Antioxidant Treatment for Schizophrenia

PURPOSE OF REVIEW

The objective of this article is to highlight the potential role of the galantamine-memantine combination as a novel antioxidant treatment for schizophrenia.

RECENT FINDINGS

In addition to the well-known mechanisms of action of galantamine and memantine, these medications also have antioxidant activity. Furthermore, an interplay exists between oxidative stress, inflammation (redox-inflammatory hypothesis), and kynurenine pathway metabolites. Also, there is an interaction between brain-derived neurotrophic factor and oxidative stress in schizophrenia. Oxidative stress may be associated with positive, cognitive, and negative symptoms and impairments in white matter integrity in schizophrenia. The antipsychotic-galantamine-memantine combination may provide a novel strategy in schizophrenia to treat positive, cognitive, and negative symptoms.

SUMMARY

A "single antioxidant" may be inadequate to counteract the complex cascade of oxidative stress. The galantamine-memantine combination as "double antioxidants" is promising. Hence, randomized controlled trials are warranted with the antipsychotic-galantamine-memantine combination with oxidative stress and antioxidant biomarkers in schizophrenia.

The antidepressant effects of GM-CSF are mediated by the reduction of TLR4/NF-ĸB-induced IDO expression

BACKGROUND

Indoleamine 2, 3-dioxygenase 1 (IDO) is responsible for the progression of the kynurenine pathway. This pathway has been implicated in the pathophysiology of inflammation-induced depression in which conventional antidepressants are not effective. It has been reported that granulocyte-macrophage stimulating factor (GM-CSF) could interfere with the induction of IDO in septic patients. We hypothesized that GM-CSF could exert antidepressant effects through IDO downregulation in a model for acute inflammation-induced depression.

METHODS

To produce the model, lipopolysaccharide (LPS) (0.83 mg/kg) was administered intraperitoneally to mice. It has been well documented that LPS mediates IDO overexpression through TLR4/NF-ĸB signaling. In the treatment group, mice received GM-CSF (30 μg/kg, i.p.) thirty minutes prior to LPS injection. A validated selective serotonin reuptake inhibitor, fluoxetine (30 mg/kg i.p.), was also administered to an experimental group 30 min prior to LPS. Depressive-like behaviors were evaluated based on the duration of immobility in the forced swim test. To confirm that GM-CSF interferes with IDO induction in LPS treated mice, real-time PCR was used to quantify IDO mRNA expression. Furthermore, in order to study whether GM-CSF inhibits the TLR4/NF-ĸB signaling pathway, we measured levels ofpNF-ĸB and TLR4 by western blotting.

RESULTS

GM-CSF demonstrated significant antidepressant activity in the presence of LPS on immobility (p < .001) and latency (p = .010) times in the forced swim test. In contrast, fluoxetine did not show any antidepressant activity on either immobility (p = .918) or latency (p = .566) times. Furthermore, GM-CSF inhibited the increase in IDO mRNA (p = .032) and protein (p = .016) expression as a result of LPS administration. A similar trend was observed for TLR4 (p = .042) and pNF-ĸB (p = .026) expression as both proteins showed reduced expression levels in the GM-CSF-pretreated group compared to the untreated (LPS) group.

CONCLUSION

Our results propose a promising antidepressant effect for GM-CSF possibly through the downregulation of IDO expression. This remedying effect of GM-CSF could be attributed to decreased amounts of TLR4 and active NF-ĸB in the treated mice.

Quinolinic Acid and Nuclear Factor Erythroid 2-Related Factor 2 in Depression: Role in Neuroprogression

Depression is an incapacitating neuropsychiatric disorder. The serotonergic system in the brain plays an important role in the pathophysiology of depression. However, due to delayed and/or poor performance of selective serotonin reuptake inhibitors in treating depressive symptoms, the role of the serotonergic system in depression has been recently questioned further. Evidence from recent studies suggests that increased inflammation and oxidative stress may play significant roles in the pathophysiology of depression. The consequences of these factors can lead to the neuroprogression of depression, involving neurodegeneration, astrocytic apoptosis, reduced neurogenesis, reduced plasticity (neuronal and synaptic), and enhanced immunoreactivity. Specifically, increased proinflammatory cytokine levels have been shown to activate the kynurenine pathway, which causes increased production of quinolinic acid (QA, an N-Methyl-D-aspartate agonist) and decreases the synthesis of serotonin. QA exerts many deleterious effects on the brain via mechanisms including N-methyl-D-aspartate excitotoxicity, increased oxidative stress, astrocyte degeneration, and neuronal apoptosis. QA may also act directly as a pro-oxidant. Additionally, the nuclear translocation of antioxidant defense factors, such as nuclear factor (erythroid-derived 2)-like 2 (Nrf2), is downregulated in depression. Hence, in the present review, we discuss the role of QA in increasing oxidative stress in depression by modulating the nuclear translocation of nuclear factor (erythroid-derived 2)-like 2 and thus affecting the synthesis of antioxidant enzymes.

Antioxidant Properties and the Formation of Iron Coordination Complexes of 8-Hydroxyquinoline

Background: The alkaloid 8-hydroxyquinoline (8HQ) is well-known for various biological activities, including antioxidant effects and especially for the formation of coordination complexes with various transition metals, such as iron, amongst others. Therefore, 8HQ was extensively explored as a promising antineurodegenerative agent. However, other authors noted pro-oxidant effects of 8HQ. Here, we explore the pro- and antioxidant properties of 8HQ, especially in context of coordination complexes with iron (II) and iron (III). Methods: Nano-electrospray−mass spectrometry, differential pulse voltammetry, deoxyribose degradation, iron (II) autoxidation, and brine shrimp mortality assays were used. Results: 8HQ formed a complex mixture of coordination complexes with iron (II) and iron (III). Furthermore, 8HQ showed antioxidant effects but no pro-oxidant ones. In the brine shrimp mortality assay, 8HQ demonstrated toxicity that decreased in the presence of iron (III). Conclusions: 8HQ is a potent antioxidant whose effects depend not only on the formation of the coordination complexes with iron ions, but surely on the scavenging activities due to the redox properties of the 8-hydroxyl group. No pro-oxidant effects were observed in the set of the used assays.

Rusty Microglia: Trainers of Innate Immunity in Alzheimer's Disease

Alzheimer's disease, the most common form of dementia, is marked by progressive cognitive and functional impairment believed to reflect synaptic and neuronal loss. Recent preclinical data suggests that lipopolysaccharide (LPS)-activated microglia may contribute to the elimination of viable neurons and synapses by promoting a neurotoxic astrocytic phenotype, defined as A1. The innate immune cells, including microglia and astrocytes, can either facilitate or inhibit neuroinflammation in response to peripherally applied inflammatory stimuli, such as LPS. Depending on previous antigen encounters, these cells can assume activated (trained) or silenced (tolerized) phenotypes, augmenting or lowering inflammation. Iron, reactive oxygen species (ROS), and LPS, the cell wall component of gram-negative bacteria, are microglial activators, but only the latter can trigger immune tolerization. In Alzheimer's disease, tolerization may be impaired as elevated LPS levels, reported in this condition, fail to lower neuroinflammation. Iron is closely linked to immunity as it plays a key role in immune cells proliferation and maturation, but it is also indispensable to pathogens and malignancies which compete for its capture. Danger signals, including LPS, induce intracellular iron sequestration in innate immune cells to withhold it from pathogens. However, excess cytosolic iron increases the risk of inflammasomes' activation, microglial training and neuroinflammation. Moreover, it was suggested that free iron can awaken the dormant central nervous system (CNS) LPS-shedding microbes, engendering prolonged neuroinflammation that may override immune tolerization, triggering autoimmunity. In this review, we focus on iron-related innate immune pathology in Alzheimer's disease and discuss potential immunotherapeutic agents for microglial de-escalation along with possible delivery vehicles for these compounds.

An Expanded Neuroimmunomodulation Axis: sCD83-Indoleamine 2,3-Dioxygenase-Kynurenine Pathway and Updates of Kynurenine Pathway in Neurologic Diseases

Many neurologic diseases are related to autoimmune dysfunction and a variety of molecules or reaction pathways are involved in the regulation of immune function of the nervous system. Soluble CD83 (sCD83) is the soluble form of CD83, a specific marker of mature dendritic cell, which has recently been shown to have an immunomodulatory effect. Indoleamine 2,3-dioxygenase (IDO; corresponding enzyme intrahepatic, tryptophan 2,3-dioxygenase, TDO), a rate-limiting enzyme of extrahepatic tryptophan kynurenine pathway (KP) participates in the immunoregulation through a variety of mechanisms solely or with the synergy of sCD83, and the imbalances of metabolites of KP were associated with immune dysfunction. With the complement of sCD83 to IDO-KP, a previously known immunomodulatory axis, this review focused on an expanded neuroimmunomodulation axis: sCD83-IDO-KP and its involvement in nervous system diseases.

Understanding the role of the kynurenine pathway in human breast cancer immunobiology

Breast cancer (BrCa) is the leading cause of cancer related death in women. While current diagnostic modalities provide opportunities for early medical intervention, significant proportions of breast tumours escape treatment and metastasize. Gaining increasing recognition as a factor in tumour metastasis is the local immuno-surveillance environment. Following identification of the role played by the enzyme indoleamine dioxygenase 1 (IDO1) in mediating maternal foetal tolerance, the kynurenine pathway (KP) of tryptophan metabolism has emerged as a key metabolic pathway contributing to immune escape. In inflammatory conditions activation of the KP leads to the production of several immune-modulating metabolites including kynurenine, kynurenic acid, 3-hydroxykynurenine, anthranilic acid, 3-hydroxyanthranilic acid, picolinic acid and quinolinic acid. KP over-activation was first described in BrCa patients in the early 1960s. More evidence has since emerged to suggest that the IDO1 is elevated in advanced BrCa patients and is associated with poor prognosis. Further, IDO1 positive breast tumours have a positive correlation with the density of immune suppressive Foxp3+ T regulatory cells and lymph node metastasis. The analysis of clinical microarray data in invasive BrCa compared to normal tissue showed, using two microarray databank (cBioportal and TCGA), that 86.3% and 91.4% BrCa patients have altered KP enzyme expression respectively. Collectively, these data highlight the key roles played by KP activation in BrCa, particularly in basal BrCa subtypes where expression of most KP enzymes was altered. Accordingly, the use of KP enzyme inhibitors in addition to standard chemotherapy regimens may present a viable therapeutic approach.

Antioxidant Properties of Kynurenines: Density Functional Theory Calculations

Kynurenines, the main products of tryptophan catabolism, possess both prooxidant and anioxidant effects. Having multiple neuroactive properties, kynurenines are implicated in the development of neurological and cognitive disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases. Autoxidation of 3-hydroxykynurenine (3HOK) and its derivatives, 3-hydroxyanthranilic acid (3HAA) and xanthommatin (XAN), leads to the hyperproduction of reactive oxygen species (ROS) which damage cell structures. At the same time, 3HOK and 3HAA have been shown to be powerful ROS scavengers. Their ability to quench free radicals is believed to result from the presence of the aromatic hydroxyl group which is able to easily abstract an electron and H-atom. In this study, the redox properties for kynurenines and several natural and synthetic antioxidants have been calculated at different levels of density functional theory in the gas phase and water solution. Hydroxyl bond dissociation enthalpy (BDE) and ionization potential (IP) for 3HOK and 3HAA appear to be lower than for xanthurenic acid (XAA), several phenolic antioxidants, and ascorbic acid. BDE and IP for the compounds with aromatic hydroxyl group are lower than for their precursors without hydroxyl group. The reaction rate for H donation to *O-atom of phenoxyl radical (Ph-O*) and methyl peroxy radical (Met-OO*) decreases in the following rankings: 3HOK ~ 3HAA > XAA_OXO > XAA_ENOL. The enthalpy absolute value for Met-OO* addition to the aromatic ring of the antioxidant radical increases in the following rankings: 3HAA* < 3HOK* < XAA_OXO* < XAA_ENOL*. Thus, the high free radical scavenging activity of 3HAA and 3HOK can be explained by the easiness of H-atom abstraction and transfer to O-atom of the free radical, rather than by Met-OO* addition to the kynurenine radical.

Kynurenines, the tryptophan metabolites with multiple biological activities, regulate the production of reactive oxygen species (ROS) during several neurodegenerative diseases. Many experiments show that kynurenines can be both prooxidants and antioxidants depending on their concentration, mode of action, and cell redox potential. However, there is lack of computational studies of kynurenines properties which could help us better understand the biophysical mechanism of their antioxidant activity. We performed the computations of kynurenines' hydrogen and electron donating power, both in the gas phase and in water solution. We found that aromatic hydroxyl group facilitates hydrogen and electron abstraction by kynurenines, in agreement with experimental data and computations earlier performed for phenolic antioxidants. We revealed the correlations of kynurenines' antioxidant power with their electronic structure, charge, and surroundings. We also found that 3-hydroxykynurenine and 3-hydroxyanthranilic acid can fastly quench free radicals by hydrogen atom donation. Hence both of them are potent antioxidants. The therapeutic strategy may be to inhibit their oxidative dimerization leading to ROS production.

Current Evidence for a Role of the Kynurenine Pathway of Tryptophan Metabolism in Multiple Sclerosis

The kynurenine pathway (KP) is the major metabolic pathway of the essential amino acid tryptophan (TRP). Stimulation by inflammatory molecules, such as interferon-γ (IFN-γ), is the trigger for induction of the KP, driving a complex cascade of production of both neuroprotective and neurotoxic metabolites, and in turn, regulation of the immune response and responses of brain cells to the KP metabolites. Consequently, substantial evidence has accumulated over the past couple of decades that dysregulation of the KP and the production of neurotoxic metabolites are associated with many neuroinflammatory and neurodegenerative diseases, including Parkinson’s disease, AIDS-related dementia, motor neurone disease, schizophrenia, Huntington’s disease, and brain cancers. In the past decade, evidence of the link between the KP and multiple sclerosis (MS) has rapidly grown and has implicated the KP in MS pathogenesis. KP enzymes, indoleamine 2,3-dioxygenase (IDO-1) and tryptophan dioxygenase (highest expression in hepatic cells), are the principal enzymes triggering activation of the KP to produce kynurenine from TRP. This is in preference to other routes such as serotonin and melatonin production. In neurological disease, degradation of the blood–brain barrier, even if transient, allows the entry of blood monocytes into the brain parenchyma. Similar to microglia and macrophages, these cells are highly responsive to IFN-γ, which upregulates the expression of enzymes, including IDO-1, producing neurotoxic KP metabolites such as quinolinic acid. These metabolites circulate systemically or are released locally in the brain and can contribute to the excitotoxic death of oligodendrocytes and neurons in neurological disease principally by virtue of their agonist activity at N-methyl-d-aspartic acid receptors. The latest evidence is presented and discussed. The enzymes that control the checkpoints in the KP represent an attractive therapeutic target, and consequently several KP inhibitors are currently in clinical trials for other neurological diseases, and hence may make suitable candidates for MS patients. Underpinning these drug discovery endeavors, in recent years, several advances have been made in how KP metabolites are assayed in various biological fluids, and tremendous advancements have been made in how specimens are imaged to determine disease progression and involvement of various cell types and molecules in MS.

Iron chelation and redox chemistry of anthranilic acid and 3-hydroxyanthranilic acid: A comparison of two structurally related kynurenine pathway metabolites to obtain improved insights into their potential role in neurological disease development

Anthranilic acid (ANA) and 3-hydroxyanthranilic acid (3-HANA) are kynurenine pathway intermediates of the tryptophan metabolism. A hitherto unemployed method combination, differential pulse voltammetry, mass spectrometry (nano-ESI-MS), deoxyribose degradation and iron(II) autoxidation assays has been employed for studying of their redox chemistry and their interactions with iron(II) and iron(III) ions. Both acids inhibited the Fenton reaction by iron chelation and ROS scavenging in the deoxyribose degradation assay. In the iron(II) autoxidation assay, anthranilic acid showed antioxidant effects, whereas 3-hydroxyanthranilic acid exhibited apparent pro-oxidant activity. The differential pulse voltammograms of free metabolites and their iron(II) coordination complexes reflected these properties. Nano-ESI-MS confirmed ANA and 3-HANA as efficient iron(II) chelators, both of which form coordination complexes of ligand:iron(II) ratio 1:1, 2:1, and 3:1. In addition, nano-ESI-MS analyses of the oxidation effects by hydroxyl radical attack identified 3-HANA as strikingly more susceptible than ANA. 3-HANA susceptibility to oxidation may explain its decreased concentrations in the reaction mixture. The presented observations can add to explaining why 3-HANA levels decrease in patients with some neurological and other diseases which can often associated with elevated concentrations of ROS.

Effects of endogenous neurotoxin quinolinic acid on reactive oxygen species production by Fenton reaction catalyzed by iron or copper

The tryptophan metabolite, quinolinic (2,3-pyridinedicarboxylic) acid, is known as an endogenous neurotoxin. Quinolinic acid can form coordination complexes with iron or copper. The effects of quinolinic acid on reactive oxygen species production in the presence of iron or copper were explored by a combination of chemical assays, classical site-specific and ascorbic acid-free variants of the deoxyribose degradation assay, and mass spectrometry (ESI–MS). Quinolinic acid showed evident antioxidant activity in chemical assays, but the effect was more pronounced in the presence of copper as transition metal catalyst than in presence of iron. Nano-ESI–MS confirmed the ability of quinolinic acid to form coordination complexes with iron(II) or copper(II) and quinolinic acid stability against oxidative attack by hydroxyl radicals. The results illustrate a highly milieu-dependent quinolinic acid chemistry when it enters reactions as competitive ligand.

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Quinolinic acid is considered as a neurotoxin but it can also act as an antioxidant.

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MS proves quinolinic acid's ability to form coordination complexes with Fe or Cu.

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Quinolinic acid showed a relative robustness when under oxidative attack.

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Quinolinic acid can protect cells suffering from the elevated oxidative stress.

New considerations on hormetic response against oxidative stress

In order to survive living organisms have developed multiple mechanisms to deal with tough environmental conditions. Hormesis is defined as a process in which exposure to a low dose of a chemical agent or environmental factor that is damaging at higher doses induces an adaptive beneficial effect on the cell or organism. In this paper, we examine several ideas that might be taken into consideration before using hormesis as a therapeutic tool to improve health and life span, and hopefully will open the discussion for new and interesting debates regard hormesis. The first one is to understand that the same stressor or inductor can activate different pathways in a parallel or dual response, which might lead to diverse outcomes. Another idea is related to the mechanisms involved in activating Nrf2, which might be different and have diverse hormetic effects.Last, we discuss mild oxidative stress in association to low-grade chronic inflammation as a stimulating avenue to be explored and the unexpected effects proposed by the obesity paradox theory. All the previous might help to clarify the reasons why centenarians are able to reach the extreme limits of human life span, which could probably be related to the way they deal with homeostasis maintenance, providing an opportunity for hormesis to intervene significantly.

Effects of selected dietary secondary metabolites on reactive oxygen species production caused by iron(II) autoxidation

Iron is an essential co-factor for many enzymes that catalyze electron transfer reactions. It is well known that so-called “poorly liganded” iron can increase ROS concentrations and trigger oxidative stress that is capable of initiating apoptosis. Conversely, controlled ROS production has been recognized as an integral part of cellular signaling. Elevated ROS concentrations are associated with aging, inflammatory and degenerative diseases. Anti-aging properties have been attributed especially to antioxidant phenolic plant metabolites that represent food additives in our diet. Consequently, this study explores the effects of flavonoids (quercetin and rutin), several phenolic acids (caffeic, chlorogenic, and protocatechuic acid), and the alkaloid caffeine on iron(II) autoxidation and ROS production in comparison to the standard antioxidants ascorbic acid and Trolox. The iron(II) autoxidation assay was carried out in pH 6.0 (plant apoplast and inflamed human tissue) and 7.4 (cell cytoplasm and human blood plasma). The obtained results accentuate phenolic acids as the more specific antioxidants compared to ascorbic acid and Trolox. Flavonoid redox chemistry depends more on the chemical milieu, specifically on pH. In vivo, the presence of iron cannot be ruled out and “wrongly” or “poorly” complexed iron has been pointed out as causative agent of various age-related diseases.