Quinolinic acid selectively induces apoptosis of human astrocytes: potential role in AIDS dementia complex

Characterizing Microglial Signaling Dynamics During Inflammation Using Single-Cell Mass Cytometry

Microglia play a critical role in maintaining central nervous system (CNS) homeostasis and display remarkable plasticity in their response to inflammatory stimuli. However, the specific signaling profiles that microglia adopt during such challenges remain incompletely understood. Traditional transcriptomic approaches provide valuable insights, but fail to capture dynamic post-translational changes. In this study, we utilized time-resolved single-cell mass cytometry (CyTOF) to measure distinct signaling pathways activated in microglia upon exposure to bacterial and viral mimetics-lipopolysaccharide (LPS) and polyinosinic-polycytidylic acid (Poly(I:C)), respectively. Furthermore, we evaluated the immunomodulatory role of astrocytes on microglial signaling in mixed cultures. Microglia or mixed cultures derived from neonatal mice were treated with LPS or Poly(I:C) for 48 h. Cultures were stained with a panel of 33 metal-conjugated antibodies targeting signaling and identity markers. High-dimensional clustering analysis was used to identify emergent signaling modules. We found that LPS treatment led to more robust early activation of pp38, pERK, pRSK, and pCREB compared to Poly(I:C). Despite these differences, both LPS and Poly(I:C) upregulated the classical reactivity markers CD40 and CD86 at later time points. Strikingly, the presence of astrocytes significantly blunted microglial responses to both stimuli, particularly dampening CD40 upregulation. Our studies demonstrate that single-cell mass cytometry effectively captures the dynamic signaling landscape of microglia under pro-inflammatory conditions. This approach may pave the way for targeted therapeutic investigations of various neuroinflammatory disorders. Moreover, our findings underscore the necessity of considering cellular context, such as astrocyte presence, in interpreting microglial behavior during inflammation.

Kynurenine pathway dysregulation as a mechanistic link between cognitive impairment and brain damage: Implications for multiple sclerosis

Cognitive impairment is a core symptom of multiple sclerosis (MS), resulting from inflammation-related brain damage and brain network dysfunction. Inflammation also causes dysregulation of the kynurenine pathway, which is the primary route of tryptophan metabolism. Kynurenine pathway dysregulation is characterised by a shift in concentrations of tryptophan catabolites, also referred to as kynurenines. Some kynurenines have neurotoxic effects that partly resemble the molecular mechanisms of MS pathophysiology underpinning brain damage and brain network dysfunction. The kynurenine pathway may therefore qualify as a mechanistic link between systemic inflammation, brain damage, and cognitive impairment in MS. This perspective article (1) provides an overview of inflammation-related kynurenine pathway dysregulation and MS-relevant neuroimmune properties of kynurenines and (2) summarises the current evidence on associations between systemic kynurenines, imaging metrics of brain structure or related markers, and cognitive performance in populations that present with kynurenine pathway dysregulation and are prone to cognitive impairment. These findings are used to (3) set a research agenda for future studies aimed at clarifying the role of the kynurenine pathway in brain damage and cognitive impairment in MS.

Importance of Modulating Kynurenic Acid Metabolism-Approaches for the Treatment of Dementia

In this article, we focus on kynurenic acid metabolism in neuropsychiatric disorders and the biochemical processes involved in memory and cognitive impairment, followed by different approaches in the fight against dementia. Kynurenic acid-a biochemical part of L-tryptophan catabolism-is synthesized from L-kynurenine by kynurenine aminotransferases. Experimental pharmacological studies have shown that elevated levels of kynurenic acid in the brain are associated with impaired learning and that lowering kynurenic acid levels can improve these symptoms. The discovery of new compounds with the ability to block kynurenine aminotransferases opens new therapeutic avenues for the treatment of memory impairment and dementia. The newly developed Helix pomatia snail model of memory can be used for the assessment of novel pharmacological approaches. Dietary supplementation with natural molecular/herbal extracts, exercise, and physical activity have significant impacts on endogenous pharmacology by reducing kynurenic acid synthesis, and these factors are likely to significantly modulate steady-state biological conditions and delay the negative consequences of aging, including the onset of pathological processes.

Activation of the kynurenine pathway identified in individuals with covert hepatic encephalopathy

BACKGROUND

HE is a neuropsychiatric complication of liver disease characterized by systemic elevation in ammonia and proinflammatory cytokines. These neurotoxins cross the blood-brain barrier and cause neuroinflammation, which can activate the kynurenine pathway (KP). This results in dysregulated production of neuroactive KP metabolites, such as quinolinic acid, which is known to cause astrocyte and neuronal death. Our aim was to compare KP activity between patients with covert HE (CHE), patients without encephalopathic cirrhosis (NHE), and healthy controls (HCs).

METHODS

This was a single-center prospective cohort study conducted between 2018 and 2021 at St Vincent's Hospital, Sydney. Overall, 13 patients with CHE, 10 patients with NHE, and 12 with HC were recruited. Patients with cirrhosis were diagnosed with CHE if they scored ≤-4 on the Psychometric Hepatic Encephalopathy Score. KP metabolite levels were quantified on plasma samples via HPLC and gas chromatography/mass spectrometry. One-way Kruskal-Wallis test was used to compare the expression levels of KP enzymes.

RESULTS

KP was highly activated in patients with cirrhosis, demonstrated by higher levels of activity in the rate-limiting enzymes, indoleamine 2,3-dioxygenase, and tryptophan-2,3-dioxygenase in both CHE (65.04±20.72, p=0.003) and patients with NHE (64.85±22.10, p=0.015) compared to HC (40.95±7.301). Higher quinolinic acid concentrations were demonstrated in CHE (3726 nM±3385, p<0.001) and patients with NHE (1788 nM±632.3, p=0.032) compared to HC (624 nM±457). KP activation was positively correlated with inflammatory marker C-reactive protein in patients with CHE (Rs=0.721, p≤0.01).

CONCLUSIONS

KP is highly activated in patients with CHE, resulting in heightened production of neurotoxic metabolites. Dysregulation of the pathway is demonstrable in patients who do not yet show clinical signs of neurocognitive impairment. Therapeutic agents that modulate KP activity may be able to alleviate symptoms of patients with CHE.

Characterizing microglial signaling dynamics during inflammation using single-cell mass cytometry

Microglia play a critical role in maintaining central nervous system (CNS) homeostasis and display remarkable plasticity in their response to inflammatory stimuli. However, the specific signaling profiles that microglia adopt during such challenges remain incompletely understood. Traditional transcriptomic approaches provide valuable insights, but fail to capture dynamic post-translational changes. In this study, we utilized time-resolved single-cell mass cytometry (CyTOF) to measure distinct signaling pathways activated in microglia upon exposure to bacterial and viral mimetics-lipopolysaccharide (LPS) and polyinosinic-polycytidylic acid (Poly(I:C)), respectively. Furthermore, we evaluated the immunomodulatory role of astrocytes on microglial signaling in mixed cultures. Microglia or mixed cultures derived from neonatal mice were treated with LPS or Poly(I:C) for 48 hrs. Cultures were stained with a panel of 33 metal-conjugated antibodies targeting signaling and identity markers. High-dimensional clustering analysis was used to identify emergent signaling modules. We found that LPS treatment led to more robust early activation of pp38, pERK, pRSK, and pCREB compared to Poly(I:C). Despite these differences, both LPS and Poly(I:C) upregulated the classical activation markers CD40 and CD86 at later time-points. Strikingly, the presence of astrocytes significantly blunted microglial responses to both stimuli, particularly dampening CD40 upregulation. Our studies demonstrate that single-cell mass cytometry effectively captures the dynamic signaling landscape of microglia under pro-inflammatory conditions. This approach may pave the way for targeted therapeutic investigations of various neuroinflammatory disorders. Moreover, our findings underscore the necessity of considering cellular context, such as astrocyte presence, in interpreting microglial behavior during inflammation.

Changes in tryptophan breakdown associated with response to multimodal treatment in depression

Background

Research on depression showed that dysregulations in tryptophan (TRP), kynurenine (KYN), and its KYN pathway metabolites are key aspects in the development and maintenance of depressive symptoms. In our previous reports, we described sex-specific changes in TRP breakdown as well as changes in KYN and KYN/TRP in association with treatment response and inflammatory and metabolic parameters. However, results of treatment effects on KYN pathway metabolites as well as how pathway changes are related to treatment response remain sparse.

Objective

We investigated potential changes of KYN and KYN pathway metabolites in association with therapeutic response of individuals with depression during a six-week multimodal psychiatric rehabilitation program.

Methods

87 participants were divided into treatment responders and non-responders (48 responders, 39 non-responders; 38 male, 49 female; M age = 51.09; SD age = 7.70) using scores of psychological questionnaires. KYN pathway metabolites serum concentrations as well as their ratios were collected using high performance liquid chromatography. Changes over time (time of admission (t1) vs. time of discharge (t2)) were calculated using repeated measure analyses of (co)variance.

Results

Non-responders exhibited higher levels of 3-Hydroxyanthralinic acid (3-HAA), nicotinic acid (NA), and 3-HAA/KYN, independently of measurement time. NA levels decreased, while 3-HAA levels increased over time in both groups, independently of treatment response. 3-HK/KYN levels decreased, while KYN levels increased in non-responders, but not in responders over time.

Discussion

The results indicate that some compounds of the KYN pathway metabolites can be altered through multimodal long-term interventions in association with treatment response. Especially the pathway degrading KYN further down to 3-HAA and 3-HK/KYN might be decisive for treatment response in depression.

A Review of the Evidence for Tryptophan and the Kynurenine Pathway as a Regulator of Stem Cell Niches in Health and Disease

Stem cells are ubiquitously found in various tissues and organs in the body, and underpin the body's ability to repair itself following injury or disease initiation, though repair can sometimes be compromised. Understanding how stem cells are produced, and functional signaling systems between different niches is critical to understanding the potential use of stem cells in regenerative medicine. In this context, this review considers kynurenine pathway (KP) metabolism in multipotent adult progenitor cells, embryonic, haematopoietic, neural, cancer, cardiac and induced pluripotent stem cells, endothelial progenitor cells, and mesenchymal stromal cells. The KP is the major enzymatic pathway for sequentially catabolising the essential amino acid tryptophan (TRP), resulting in key metabolites including kynurenine, kynurenic acid, and quinolinic acid (QUIN). QUIN metabolism transitions into the adjoining de novo pathway for nicotinamide adenine dinucleotide (NAD) production, a critical cofactor in many fundamental cellular biochemical pathways. How stem cells uptake and utilise TRP varies between different species and stem cell types, because of their expression of transporters and responses to inflammatory cytokines. Several KP metabolites are physiologically active, with either beneficial or detrimental outcomes, and evidence of this is presented relating to several stem cell types, which is important as they may exert a significant impact on surrounding differentiated cells, particularly if they metabolise or secrete metabolites differently. Interferon-gamma (IFN-γ) in mesenchymal stromal cells, for instance, highly upregulates rate-limiting enzyme indoleamine-2,3-dioxygenase (IDO-1), initiating TRP depletion and production of metabolites including kynurenine/kynurenic acid, known agonists of the Aryl hydrocarbon receptor (AhR) transcription factor. AhR transcriptionally regulates an immunosuppressive phenotype, making them attractive for regenerative therapy. We also draw attention to important gaps in knowledge for future studies, which will underpin future application for stem cell-based cellular therapies or optimising drugs which can modulate the KP in innate stem cell populations, for disease treatment.

The Role of the Kynurenine/AhR Pathway in Diseases Related to Metabolism and Cancer

The Aryl hydrocarbon receptor (AhR) is a xenobiotic and endobiotic receptor, which regulates many cellular processes from contaminant metabolism to immunomodulation. Consequently, it is also involved in pathophysiological pathways and now represents a potential therapeutical target. In this review, we will highlight the ancestral function of the protein together with an illustration of its ligand's battery, emphasizing the different responses triggered by these high diverse molecules. Among them, several members of the kynurenine pathway (one key process of tryptophan catabolism) are AhR agonists and are subsequently involved in regulatory functions. We will finally display the interplay between Tryptophan (Trp) catabolism and dysregulation in metabolic pathways drawing hypothesis on the involvement of the AhR pathway in these cancer-related processes.

The kynurenine pathway in traumatic brain injuries and concussion

Up to 10 million people per annum experience traumatic brain injury (TBI), 80-90% of which are categorized as mild. A hit to the brain can cause TBI, which can lead to secondary brain injuries within minutes to weeks after the initial injury through unknown mechanisms. However, it is assumed that neurochemical changes due to inflammation, excitotoxicity, reactive oxygen species, etc., that are triggered by TBI are associated with the emergence of secondary brain injuries. The kynurenine pathway (KP) is an important pathway that gets significantly overactivated during inflammation. Some KP metabolites such as QUIN have neurotoxic effects suggesting a possible mechanism through which TBI can cause secondary brain injury. That said, this review scrutinizes the potential association between KP and TBI. A more detailed understanding of the changes in KP metabolites during TBI is essential to prevent the onset or at least attenuate the severity of secondary brain injuries. Moreover, this information is crucial for the development of biomarker/s to probe the severity of TBI and predict the risk of secondary brain injuries. Overall, this review tries to fill the knowledge gap about the role of the KP in TBI and highlights the areas that need to be studied.

Kynurenine Pathway Metabolites as Potential Biomarkers in Chronic Pain

Chronic pain is a pressing medical and socioeconomic issue worldwide. It is debilitating for individual patients and places a major burden on society in the forms of direct medical costs and lost work productivity. Various biochemical pathways have been explored to explain the pathophysiology of chronic pain in order to identify biomarkers that can potentially serve as both evaluators of and guides for therapeutic effectiveness. The kynurenine pathway has recently been a source of interest due to its suspected role in the development and sustainment of chronic pain conditions. The kynurenine pathway is the primary pathway responsible for the metabolization of tryptophan and generates nicotinamide adenine dinucleotide (NAD+), in addition to the metabolites kynurenine (KYN), kynurenic acid (KA), and quinolinic acid (QA). Dysregulation of this pathway and changes in the ratios of these metabolites have been associated with numerous neurotoxic and inflammatory states, many of which present simultaneously with chronic pain symptoms. While further studies utilizing biomarkers to elucidate the kynurenine pathway's role in chronic pain are needed, the metabolites and receptors involved in its processes nevertheless present researchers with promising sources of novel and personalized disease-modifying treatments.

Synthesis, In Silico, In Vivo, and Ex Vivo Evaluation of a Boron-Containing Quinolinate Derivative with Presumptive Action on mGluRs

In the brain, canonical excitatory neurotransmission is mediated by L-glutamate and its ionotropic (iGluR) and metabotropic (mGluR) receptors. The wide diversity of these often limits the development of glutamatergic drugs. This is due to the arduousness of achieving selectivity with specific ligands. In the present article, encouraged by reports of bioactive organoboron compounds, a diphenylboroxazolidone derived from quinolinate (BZQuin) was evaluated. BZQuin was synthesized with a yield of 87%. Its LD50 was 174 mg/kg in male CD-1 mice, as estimated by a modified Lorke’s method. BZQuin exerted a reduced ability to cause seizures when compared against its precursor, quinolinate. The latter suggested that it does not directly stimulate the ionotropic NMDA receptors or other ionic channels. The observation that the antiglutamatergic drugs riluzole and memantine displaced the BZQuin effect left the mGluRs as their possible targets. This is in line with results from molecular-docking simulations. During these simulations, BZQuin bound only to orthosteric sites on mGluR1, mGluR2, and mGluR7, with higher affinity than quinolinate. The survival of the neurons of mice previously administered with BZQuin or quinolinate was quantified in four neuroanatomical structures of the brain. The BZQuin effect was more appreciable in brain regions with a high expression of the previously mentioned mGluRs, while both antiglutamatergic drugs exerted a neuroprotective effect against it. Together, these results suggest that BZQuin exerts a positive influence on glutamatergic neurotransmission while selectively interacting with certain mGluRs.

Fecal Microbiota Transplantation in Reducing Uremic Toxins Accumulation in Kidney Disease: Current Understanding and Future Perspectives

During the past decades, the gut microbiome emerged as a key player in kidney disease. Dysbiosis-related uremic toxins together with pro-inflammatory mediators are the main factors in a deteriorating kidney function. The toxicity of uremic compounds has been well-documented in a plethora of pathophysiological mechanisms in kidney disease, such as cardiovascular injury (CVI), metabolic dysfunction, and inflammation. Accumulating data on the detrimental effect of uremic solutes in kidney disease supported the development of many strategies to restore eubiosis. Fecal microbiota transplantation (FMT) spread as an encouraging treatment for different dysbiosis-associated disorders. In this scenario, flourishing studies indicate that fecal transplantation could represent a novel treatment to reduce the uremic toxins accumulation. Here, we present the state-of-the-art concerning the application of FMT on kidney disease to restore eubiosis and reverse the retention of uremic toxins.

The Protective Effect of IDO1 Inhibition in Aβ-Treated Neurons and APP/PS1 Mice

Alzheimer's disease (AD) is an inflammatory associated disease, in which dysregulated kynurenine pathway (KP) plays a key role. Through KP, L-tryptophan is catabolized into neurotoxic and neuroprotective metabolites. The overactivation of indolamine 2,3-dioxygenase1 (IDO1), the first rate-limiting enzyme of KP, and the abnormal accumulation of KP metabolites have been noted in AD, and blocking IDO1 has been suggested as a therapeutic strategy. However, the expression patterns of KP enzymes in AD, and whether these enzymes are related to AD pathogenesis, have not been fully studied. Herein, we examined the expression patterns of inflammatory cytokines, neurotrophic factors and KP enzymes, and the activity of IDO1 and IDO1 effector pathway AhR (aryl hydrocarbon receptor) in AD mice. We studied the effects of IDO1 inhibitors on Aβ-related neuroinflammation in rat primary neurons, mouse hippocampal neuronal cells, and APP/PS1 mice. The results further demonstrated the importance of IDO1-catalyzed KP in neuroinflammation in Alzheimer's disease.

The Role of Tryptophan Dysmetabolism and Quinolinic Acid in Depressive and Neurodegenerative Diseases

Emerging evidence suggests that neuroinflammation is involved in both depression and neurodegenerative diseases. The kynurenine pathway, generating metabolites which may play a role in pathogenesis, is one of several competing pathways of tryptophan metabolism. The present article is a narrative review of tryptophan metabolism, neuroinflammation, depression, and neurodegeneration. A disturbed tryptophan metabolism with increased activity of the kynurenine pathway and production of quinolinic acid may result in deficiencies in tryptophan and derived neurotransmitters. Quinolinic acid is an N-methyl-D-aspartate receptor agonist, and raised levels in CSF, together with increased levels of inflammatory cytokines, have been reported in mood disorders. Increased quinolinic acid has also been observed in neurodegenerative diseases, including Parkinson’s disease, Alzheimer’s disease, amyotrophic lateral sclerosis, and HIV-related cognitive decline. Oxidative stress in connection with increased indole-dioxygenase (IDO) activity and kynurenine formation may contribute to inflammatory responses and the production of cytokines. Increased formation of quinolinic acid may occur at the expense of kynurenic acid and neuroprotective picolinic acid. While awaiting ongoing research on potential pharmacological interventions on tryptophan metabolism, adequate protein intake with appropriate amounts of tryptophan and antioxidants may offer protection against oxidative stress and provide a balanced set of physiological receptor ligands.

Is Poor Lithium Response in Individuals with Bipolar Disorder Associated with Increased Degradation of Tryptophan along the Kynurenine Pathway? Results of an Exploratory Study

Bipolar disorder is associated with an inflammation-triggered elevated catabolism of tryptophan to the kynurenine pathway, which impacts psychiatric symptoms and outcomes. The data indicate that lithium exerts anti-inflammatory effects by inhibiting indoleamine-2,3-dioxygenase (IDO)-1 activity. This exploratory study aimed to investigate the tryptophan catabolism in individuals with bipolar disorder (n = 48) compared to healthy controls (n = 48), and the associations with the response to mood stabilizers such as lithium, valproate, or lamotrigine rated with the Retrospective Assessment of the Lithium Response Phenotype Scale (or the Alda scale). The results demonstrate an association of a poorer response to lithium with higher levels of kynurenine, kynurenine/tryptophan ratio as a proxy for IDO-1 activity, as well as quinolinic acid, which, overall, indicates a pro-inflammatory state with a higher degradation of tryptophan towards the neurotoxic branch. The treatment response to valproate and lamotrigine was not associated with the levels of the tryptophan metabolites. These findings support the anti-inflammatory properties of lithium. Furthermore, since quinolinic acid has neurotoxic features via the glutamatergic pathway, they also strengthen the assumption that the clinical drug response might be associated with biochemical processes. The relationship between the lithium response and the measurements of the tryptophan to the kynurenine pathway is of clinical relevance and may potentially bring advantages towards a personalized medicine approach to bipolar disorder that allows for the selection of the most effective mood-stabilizing drug.

Indoleamine 2,3-dioxygenase (IDO)-1 and IDO-2 activity and severe course of COVID-19

COVID-19 is a pandemic with high morbidity and mortality. In an autopsy cohort of COVID-19 patients, we found extensive accumulation of the tryptophan degradation products 3-hydroxy anthranilic acid and quinolinic acid in lungs, heart, and brain. This was not related to the expression of the tryptophan-catabolizing indoleamine 2,3-dioxygenase (IDO)-1, but rather to that of its isoform IDO-2, which otherwise is expressed rarely. Bioavailability of tryptophan is an absolute requirement for proper cell functioning and synthesis of hormones, whereas its degradation products can cause cell death. Markers of apoptosis and severe cellular stress were associated with IDO-2 expression in large areas of lung and heart tissue, whereas affected areas in brain were more restricted. Analyses of tissue, cerebrospinal fluid, and sequential plasma samples indicate early initiation of the kynurenine/aryl-hydrocarbon receptor/IDO-2 axis as a positive feedback loop, potentially leading to severe COVID-19 pathology. This article is protected by copyright. All rights reserved.

Neurodegeneration in Multiple Sclerosis: Symptoms of Silent Progression, Biomarkers and Neuroprotective Therapy-Kynurenines Are Important Players

Neurodegeneration is one of the driving forces behind the pathogenesis of multiple sclerosis (MS). Progression without activity, pathopsychological disturbances (cognitive impairment, depression, fatigue) and even optic neuropathy seems to be mainly routed in this mechanism. In this article, we aim to give a comprehensive review of the clinical aspects and symptomology, radiological and molecular markers and potential therapeutic targets of neurodegeneration in connection with MS. As the kynurenine pathway (KP) was evidenced to play an important role in the pathogenesis of other neurodegenerative conditions (even implied to have a causative role in some of these diseases) and more and more recent evidence suggest the same central role in the neurodegenerative processes of MS as well, we pay special attention to the KP. Metabolites of the pathway are researched as biomarkers of the disease and new, promising data arising from clinical evaluations show the possible therapeutic capability of KP metabolites as neuroprotective drugs in MS. Our conclusion is that the kynurenine pathway is a highly important route of research both for diagnostic and for therapeutic values and is expected to yield concrete results for everyday medicine in the future.

Kynurenines and Neurofilament Light Chain in Multiple Sclerosis

Multiple sclerosis is an autoimmune, demyelinating, and neurodegenerative disease of the central nervous system. In recent years, it has been proven that the kynurenine system plays a significant role in the development of several nervous system disorders, including multiple sclerosis. Kynurenine pathway metabolites have both neurotoxic and neuroprotective effects. Moreover, the enzymes of the kynurenine pathway play an important role in immunomodulation processes, among others, as well as interacting with neuronal energy balance and various redox reactions. Dysregulation of many of the enzymatic steps in kynurenine pathway and upregulated levels of these metabolites locally in the central nervous system, contribute to the progression of multiple sclerosis pathology. This process can initiate a pathogenic cascade, including microglia activation, glutamate excitotoxicity, chronic oxidative stress or accumulated mitochondrial damage in the axons, that finally disrupt the homeostasis of neurons, leads to destabilization of neuronal cell cytoskeleton, contributes to neuro-axonal damage and neurodegeneration. Neurofilaments are good biomarkers of the neuro-axonal damage and their level reliably indicates the severity of multiple sclerosis and the treatment response. There is increasing evidence that connections exist between the molecules generated in the kynurenine metabolic pathway and the change in neurofilament concentrations. Thus the alterations in the kynurenine pathway may be an important biomarker of the course of multiple sclerosis. In our present review, we report the possible relationship and connection between neurofilaments and the kynurenine system in multiple sclerosis based on the available evidences.

Papaverine, a Phosphodiesterase 10A Inhibitor, Ameliorates Quinolinic Acid-Induced Synaptotoxicity in Human Cortical Neurons

Phosphodiesterase-10A (PDE10A) hydrolyse the secondary messengers cGMP and cAMP, two molecules playing important roles in neurodevelopment and brain functions. PDE10A is associated to progression of neurodegenerative diseases like Alzheimer's, Parkinson's, Huntington's diseases, and a critical role in cognitive functions. The present study was undertaken to determine the possible neuroprotective effects and the associated mechanism of papaverine (PAP), a PDE10A isoenzyme inhibitor, against quinolinic acid (QUIN)-induced excitotoxicity using human primary cortical neurons. Cytotoxicity potential of PAP was analysed using MTS assay. Reactive oxygen species (ROS) and mitochondrial membrane potential were measured by DCF-DA and JC10 staining, respectively. Caspase 3/7 and cAMP levels were measured using ELISA kits. Effect of PAP on the CREB, BNDF and synaptic proteins such as SAP-97, synaptophysin, synapsin-I, and PSD-95 expression was analysed by Western blot. Pre-treatment with PAP increased intracellular cAMP and nicotinamide adenine dinucleotide (NAD⁺) levels, restored mitochondrial membrane potential (ΔΨm), and decreased ROS and caspase 3/7 content in QUIN exposed neurons. PAP up-regulated CREB and BDNF, and synaptic protein expression. In summary, these data indicate that PDE10A is involved in QUIN-mediated synaptotoxicity and its inhibition elicit neuroprotection by reducing the oxidative stress and protecting synaptic proteins via up-regulation of cAMP signalling cascade.

Longitudinal Magnetic Resonance Spectroscopy and Diffusion Tensor Imaging in Sheep (Ovis aries) With Quinolinic Acid Lesions of the Striatum: Time-Dependent Recovery of N-Acetylaspartate and Fractional Anisotropy

We created an excitotoxic striatal lesion model of Huntington disease (HD) in sheep, using the N-methyl-d-aspartate receptor agonist, quinolinic acid (QA). Sixteen sheep received a bolus infusion of QA (75 µL, 180 mM) or saline, first into the left and then (4 weeks later) into the right striatum. Magnetic resonance spectroscopy (MRS) and diffusion tensor imaging (DTI) of the striata were performed. Metabolite concentrations and fractional anisotropy (FA) were measured at baseline, acutely (1 week after each surgery) and chronically (5 weeks or greater after the surgeries). There was a significant decrease in the neuronal marker N-acetylaspartate (NAA) and in FA in acutely lesioned striata of the QA-lesioned sheep, followed by a recovery of NAA and FA in the chronically lesioned striata. NAA level changes indicate acute death and/or impairment of neurons immediately after surgery, with recovery of reversibly impaired neurons over time. The change in FA values of the QA-lesioned striata is consistent with acute structural disruption, followed by re-organization and glial cell infiltration with time. Our study demonstrates that MRS and DTI changes in QA-sheep are consistent with HD-like pathology shown in other model species and that the MR investigations can be performed in sheep using a clinically relevant human 3T MRI scanner.

Cyclooxygenase Inhibition Safety and Efficacy in Inflammation-Based Psychiatric Disorders

According to the World Health Organization, the major psychiatric and neurodevelopmental disorders include major depression, bipolar disorder, schizophrenia, and autism spectrum disorder. The potential role of inflammation in the onset and progression of these disorders is increasingly being studied. The use of non-steroidal anti-inflammatory drugs (NSAIDs), well-known cyclooxygenase (COX) inhibitors, combined with first-choice specific drugs have been long investigated. The adjunctive administration of COX inhibitors to classic clinical treatments seems to improve the prognosis of people who suffer from psychiatric disorders. In this review, a broad overview of the use of COX inhibitors in the treatment of inflammation-based psychiatric disorders is provided. For this purpose, a critical analysis of the use of COX inhibitors in the last ten years of clinical trials of the major psychiatric disorders was carried out.

Kynurenine Pathway in Chronic Kidney Disease: What’s Old, What’s New, and What’s Next?

Impaired kidney function and increased inflammatory process occurring in the course of Chronic Kidney Disease (CKD) contribute to the development of complex amino-acid alterations. The essential amino-acid tryptophan (TRP) undergoes extensive metabolism along several pathways, resulting in the production of many biologically active compounds. The results of many studies have shown that its metabolism via the kynurenine pathway is potently increased in the course of CKD. Metabolites of this pathway exhibit differential, sometimes opposite, roles in several biological processes. Their accumulation in the course of CKD may induce oxidative cell damage which stimulates inflammatory processes. They can also modulate the activity of numerous cellular signaling pathways through activation of the aryl hydrocarbon receptor, leading to the disruption of homeostasis of various organs. As a result, they can contribute to the development of the systemic disorders accompanying the course of chronic renal failure. This review gathers and systematizes reports concerning the knowledge connecting the kynurenine pathway metabolites to systemic disorders accompanying the development of CKD.

Kynurenines in the Pathogenesis of Multiple Sclerosis: Therapeutic Perspectives

Over the past years, an increasing amount of evidence has emerged in support of the kynurenine pathway’s (KP) pivotal role in the pathogenesis of several neurodegenerative, psychiatric, vascular and autoimmune diseases. Different neuroactive metabolites of the KP are known to exert opposite effects on neurons, some being neuroprotective (e.g., picolinic acid, kynurenic acid, and the cofactor nicotinamide adenine dinucleotide), while others are toxic to neurons (e.g., 3-hydroxykynurenine, quinolinic acid). Not only the alterations in the levels of the metabolites but also disturbances in their ratio (quinolinic acid/kynurenic acid) have been reported in several diseases. In addition to the metabolites, the enzymes participating in the KP have been unearthed to be involved in modulation of the immune system, the energetic upkeep of neurons and have been shown to influence redox processes and inflammatory cascades, revealing a sophisticated, intertwined system. This review considers various methods through which enzymes and metabolites of the kynurenine pathway influence the immune system, the roles they play in the pathogenesis of neuroinflammatory diseases based on current evidence with a focus on their involvement in multiple sclerosis, as well as therapeutic approaches.

Differences in Kynurenine Metabolism During Depressive, Manic, and Euthymic Phases of Bipolar Affective Disorder

Background & Objectives:

The kynurenine pathway is involved in inflammatory diseases. Alterations of this pathway were shown in psychiatric entities as well. The aim of this study was to determine whether specific changes in kynurenine metabolism are associated with current mood symptoms in bipolar disorder.

Methods:

Sum scores of the Hamilton Depression Scale, Beck Depression Inventory, and Young Mania Rating Scale were collected from 156 bipolar individuals to build groups of depressive, manic and euthymic subjects according to predefined cut-off scores. Severity of current mood symptoms was correlated with activities of the enzymes kynurenine 3-monooxygenase (ratio of 3-hydroxykynurenine/ kynurenine), kynurenine aminotransferase (ratio of kynurenic acid/ kynurenine) and kynureninase (ratio of 3-hydroxyanthranilic acid/ 3-hydroxykynurenine), proxied by ratios of serum concentrations.

Results:

Individuals with manic symptoms showed a shift towards higher kynurenine 3-monooxygenase activity (χ2 = 7.14, Df = 2, p = .028), compared to euthymic as well as depressed individuals. There were no differences between groups regarding activity of kynurenine aminotransferase and kynureninase. Within the group of depressed patients, Hamilton Depression Scale and kynurenine aminotransferase showed a significant negative correlation (r = -0.41, p = .036), displaying lower metabolism in the direction of kynurenic acid.

Conclusion:

Depression severity in bipolar disorder seems to be associated with a decreased synthesis of putative neuroprotective kynurenic acid. Furthermore, higher kynurenine 3-monooxygenase activity in currently manic individuals indicates an increased inflammatory state within bipolar disorder with more severe inflammation during manic episodes. The underlying pathophysiological mechanisms of the different affective episodes could represent parallel mechanisms rather than opposed processes.

IDO and Kynurenine Metabolites in Peripheral and CNS Disorders

The importance of the kynurenine pathway in normal immune system function has led to an appreciation of its possible contribution to autoimmune disorders such as rheumatoid arthritis. Indoleamine-2,3-dioxygenase (IDO) activity exerts a protective function, limiting the severity of experimental arthritis, whereas deletion or inhibition exacerbates the symptoms. Other chronic disorder with an inflammatory component, such as atherosclerosis, are also suppressed by IDO activity. It is suggested that this overall anti-inflammatory activity is mediated by a change in the relative production or activity of Th17 and regulatory T cell populations. Kynurenines may play an anti-inflammatory role also in CNS disorders such as Huntington's disease, Alzheimer's disease and multiple sclerosis, in which signs of inflammation and neurodegeneration are involved. The possibility is discussed that in Huntington's disease kynurenines interact with other anti-inflammatory molecules such as Human Lymphocyte Antigen-G which may be relevant in other disorders. Kynurenine involvement may account for the protection afforded to animals with cerebral malaria and trypanosomiasis when they are treated with an inhibitor of kynurenine-3-monoxygenase (KMO). There is some evidence that changes in IL-10 may contribute to this protection and the relationship between kynurenines and IL-10 in arthritis and other inflammatory conditions should be explored. In addition, metabolites of kynurenine downstream of KMO, such as anthranilic acid and 3-hydroxy-anthranilic acid can influence inflammation, and the ratio of these compounds is a valuable biomarker of inflammatory status although the underlying molecular mechanisms of the changes require clarification. Hence it is essential that more effort be expended to identify their sites of action as potential targets for drug development. Finally, we discuss increasing awareness of the epigenetic regulation of IDO, for example by DNA methylation, a phenomenon which may explain differences between individuals in their susceptibility to arthritis and other inflammatory disorders.

Inflammation, insulin resistance and neuroprogression in depression

Chronic low-grade inflammation has been observed in major depression and other major psychiatric disorders and has been implicated in metabolic changes that are commonly associated with these disorders. This raises the possibility that the effects of dysfunctional metabolism may facilitate changes in neuronal structure and function which contribute to neuroprogression. Such changes may have implications for the progress from major depression to dementia in the elderly patient. The purpose of this review is to examine the contribution of inflammation and hypercortisolaemia, which are frequently associated with major depression, to neurodegeneration and how they detrimentally impact on brain energy metabolism. A key factor in these adverse events is insulin insensitivity caused by pro-inflammatory cytokines in association with desensitised glucocorticoid receptors. Identifying the possible metabolic changes initiated by inflammation opens new targets to ameliorate the adverse metabolic changes. This has resulted in the identification of dietary and drug targets which are of interest in the development of a new generation of psychotropic drugs.

Kynurenine 3-Monooxygenase Inhibition during Acute Simian Immunodeficiency Virus Infection Lowers PD-1 Expression and Improves Post-Combination Antiretroviral Therapy CD4+ T Cell Counts and Body Weight

The kynurenine pathway (KP) is a key regulator of many important physiological processes and plays a harmful role in cancer, many neurologic conditions, and chronic viral infections. In HIV infection, KP activity is consistently associated with reduced CD4 T cell counts and elevated levels of T cell activation and viral load; it also independently predicts mortality and morbidity from non-AIDS events. Kynurenine 3-monooxygenase (KMO) is a therapeutically important target in the KP. Using the nonhuman primate model of SIV infection in rhesus macaques, we investigated whether KMO inhibition could slow the course of disease progression. We used a KMO inhibitor, CHDI-340246, to perturb the KP during early acute infection and followed the animals for 1 y to assess clinical outcomes and immune phenotype and function during pre-combination antiretroviral therapy acute infection and combination antiretroviral therapy-treated chronic infection. Inhibition of KMO in acute SIV infection disrupted the KP and prevented SIV-induced increases in downstream metabolites, improving clinical outcome as measured by both increased CD4+ T cell counts and body weight. KMO inhibition increased naive T cell frequency and lowered PD-1 expression in naive and memory T cell subsets. Importantly, early PD-1 expression during acute SIV infection predicted clinical outcomes of body weight and CD4+ T cell counts. Our data indicate that KMO inhibition in early acute SIV infection provides clinical benefit and suggest a rationale for testing KMO inhibition as an adjunctive treatment in SIV/HIV infection to slow the progression of the disease and improve immune reconstitution.

Memantine Is Protective against Cytotoxicity Caused by Lead and Quinolinic Acid in Cultured Rat Embryonic Hippocampal Cells

Quinolinic acid (QA) is an excitotoxic metabolite of the kynurenine pathway of tryptophan metabolism produced in response to inflammation and oxidative stress. Lead (Pb) causes oxidative stress and thus may produce neurotoxicity by increasing QA production. We investigated the in vitro cytotoxic effects of Pb and QA and the protective effects of the NMDA receptor antagonist memantine. Primary cultures of embryonic hippocampal cells from Wistar rats were treated with different concentrations of Pb, QA, and Pb + QA with and without memantine. Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). Apoptosis was analyzed by flow cytometry after Annexin-V/propidium iodide staining. The numbers of immunostained neurons (with β3-Tubulin; Tuj1) and astrocytes (with glial fibrillary acidic protein) were counted. Pb at 20 μg/dL (0.97 μM) and QA at 500 nM concentrations showed significant cytotoxic effects, as evidenced by decreased cell viability, increased apoptosis, and a decrease in the number of both astrocytes and neurons. The combination of Pb and QA showed significant synergistic apoptotic effects at lower doses. Memantine (500 nM) was largely protective against the cytotoxic effects of both Pb and QA, suggesting that Pb's and QA's cytotoxicity involves NMDA receptor activation. Whereas the neuroprotection by memantine from QA-induced toxicity has been previously reported, this is the first study reporting the protection by memantine against Pb-induced cytotoxicity in cultured hippocampal cells. Protection by memantine against these neurotoxicants in vivo needs to be investigated.

Alzheimer's Disease: Recent Concepts on the Relation of Mitochondrial Disturbances, Excitotoxicity, Neuroinflammation, and Kynurenines

The pathomechanism of Alzheimer's disease (AD) certainly involves mitochondrial disturbances, glutamate excitotoxicity, and neuroinflammation. The three main aspects of mitochondrial dysfunction in AD, i.e., the defects in dynamics, altered bioenergetics, and the deficient transport, act synergistically. In addition, glutamatergic neurotransmission is affected in several ways. The balance between synaptic and extrasynaptic glutamatergic transmission is shifted toward the extrasynaptic site contributing to glutamate excitotoxicity, a phenomenon augmented by increased glutamate release and decreased glutamate uptake. Neuroinflammation in AD is predominantly linked to central players of the innate immune system, with central nervous system (CNS)-resident microglia, astroglia, and perivascular macrophages having been implicated at the cellular level. Several abnormalities have been described regarding the activation of certain steps of the kynurenine (KYN) pathway of tryptophan metabolism in AD. First of all, the activation of indolamine 2,3-dioxygenase, the first and rate-limiting step of the pathway, is well-demonstrated. 3-Hydroxy-L-KYN and its metabolite, 3-hydroxy-anthranilic acid have pro-oxidant, antioxidant, and potent immunomodulatory features, giving relevance to their alterations in AD. Another metabolite, quinolinic acid, has been demonstrated to be neurotoxic, promoting glutamate excitotoxicity, reactive oxygen species production, lipid peroxidation, and microglial neuroinflammation, and its abundant presence in AD pathologies has been demonstrated. Finally, the neuroprotective metabolite, kynurenic acid, has been associated with antagonistic effects at glutamate receptors, free radical scavenging, and immunomodulation, giving rise to potential therapeutic implications. This review presents the multiple connections of KYN pathway-related alterations to three main domains of AD pathomechanism, such as mitochondrial dysfunction, excitotoxicity, and neuroinflammation, implicating possible therapeutic options.

IDO and TDO as a potential therapeutic target in different types of depression

Depression is highly prevalent worldwide and a leading cause of disabilty. However, the medications currently available to treat depression fail to adequately relieve depressive symptoms for a large number of patients. Research into the aberrant overactivation of the kynurenine pathway and the production of various active metabolites has brought new insight into the progression of depression. IDO and TDO are the first and rate-limiting enzymes in the kynurenine pathway and regulate the production of active metabolites. There is substantial evidence that TDO and IDO enzyme are activated during depression, and therefore, IDO and TDO inhibitors have been identified as ideal therapeutic targets for depressive disorder. Hence, this review will focus on the kynurenine branch of tryptophan metabolism and describe the role of IDO and TDO in the pathology of depression. In addition, this review will compare the relative imbalance between KYNA and neurotoxic kynurenine metabolites in different psychiatric disorders. Finally, this review is also directed toward assessing whether IDO and TDO are potential therapeutic target in depression associated with other diseases such as diabetes and/or cancer, as well as the development of potent IDO and TDO inhibitors.

Immune and Neuroprotective Effects of Physical Activity on the Brain in Depression

Physical activity-a lifestyle factor that is associated with immune function, neuroprotection, and energy metabolism-modulates the cellular and molecular processes in the brain that are vital for emotional and cognitive health, collective mechanisms that can go awry in depression. Physical activity optimizes the stress response, neurotransmitter level and function (e.g., serotonergic, noradrenergic, dopaminergic, and glutamatergic), myokine production (e.g., interleukin-6), transcription factor levels and correlates [e.g., peroxisome proliferator-activated receptor C coactivator-1α [PGC-1α], mitochondrial density, nitric oxide pathway activity, Ca2+ signaling, reactive oxygen specie production, and AMP-activated protein kinase [AMPK] activity], kynurenine metabolites, glucose regulation, astrocytic health, and growth factors (e.g., brain-derived neurotrophic factor). Dysregulation of these interrelated processes can effectuate depression, a chronic mental illness that affects millions of individuals worldwide. Although the biogenic amine model has provided some clinical utility in understanding chronic depression, a need remains to better understand the interrelated mechanisms that contribute to immune dysfunction and the means by which various therapeutics mitigate them. Fortunately, convergent evidence suggests that physical activity improves emotional and cognitive function in persons with depression, particularly in those with comorbid inflammation. Accordingly, the aims of this review are to (1) underscore the link between inflammatory correlates and depression, (2) explicate immuno-neuroendocrine foundations, (3) elucidate evidence of neurotransmitter and cytokine crosstalk in depressive pathobiology, (4) determine the immunomodulatory effects of physical activity in depression, (5) examine protocols used to effectuate the positive effects of physical activity in depression, and (6) highlight implications for clinicians and scientists. It is our contention that a deeper understanding of the mechanisms by which inflammation contributes to the pathobiology of depression will translate to novel and more effective treatments, particularly by identifying relevant patient populations that can benefit from immune-based therapies within the context of personalized medicine.

Effect of wedelolactone and gallic acid on quinolinic acid-induced neurotoxicity and impaired motor function: significance to sporadic amyotrophic lateral sclerosis

Quinolinic acid (QUIN) is a well-known neuroactive metabolite of tryptophan degradation pathway or kynurenine pathway. The QUIN is involved in the development of several toxic cascades which leads to the neuronal degeneration processes. The QUIN-induced toxicity is also responsible for the impairment of the motor function and motor learning ability. This study seeks to investigate the several mechanisms which are involved in the intrastriatal administration of QUIN-induced neurodegeneration and the neuroprotective effects of wedelolactone (WL) and gallic acid (GA) over QUIN-induced toxicity. The Wistar rats were used for the study and conducted behavioral model to evaluate the effects of WL (100 & 200 mg/kg) and GA (100 & 200 mg/kg) on impaired motor function and motor learning ability. We also assessed the effects of WL and GA on the antioxidant profile, cytotoxicity, apoptosis, excitotoxicity, inflammatory cascades, and on growth factors which helps in neurogenesis. The compounds effectively improved the motor function, motor learning memory in the rats. Similarly, enhanced the activity of Glutathione peroxidase, SOD, catalase, and declined the lipid peroxidation and nitrite production in the brain. The treatment with WL and GA lowered the activities of LDH, m-calpain, and caspase-3. The reports strongly support that both compounds are useful in the prevention of glutamate excitotoxicity induced by QUIN. The NAA, IGF-1, and VEGF levels in the brain were improved after treatment with WL and GA. The neuroprotective effects of WL and GA further proved through the anti-inflammatory effects. The compounds significantly down-regulated the expression of TNF-α, IL-6, and IL-β in the brain. Immunohistochemical analysis shows that the WL and GA reduced the expression of NF-κB. The histopathological studies for cerebellum, hippocampus, striatum, and spinal cord confirms the toxic effects of QUIN and neuroprotective effects of WL and GA. The results suggest that WL and GA could ameliorate the toxic events triggered by QUIN and might be effective in the prevention and progression of several cascades which lead to the development of sALS.

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.

HIV/HAART-associated oxidative stress is detectable by metabonomics

Chronic human immunodeficiency virus (HIV) infection, separately and in combination with highly active antiretroviral therapy (HAART) is closely associated with oxidative stress (OS). Most studies demonstrating redox imbalances in HIV-infected individuals have done so using conventional biochemical methodologies. The limited simultaneous detection of multiple OS markers within one sample is a major drawback of these methodologies and can be addressed through the use of metabonomics. HIV-metabonomic studies utilizing biofluids from HAART cohorts as the investigative source, are on the increase. Data from many of these studies identified metabolic markers indicative of HIV-induced OS, usually as an outcome of an untargeted metabonomics study. Untargeted studies cast a wide net for any and all detectable metabolites in complex mixtures. Given the prevalence of OS during HIV infection and antiviral treatment, it is perhaps not surprising that indicators of this malady would become evident during metabolite identification. At times, targeted studies for specific (non-OS) metabolites would also yield OS markers as an outcome. This review examines the findings of these studies by first providing the necessary background information on OS and the main ways in which free radicals/reactive oxygen species (ROS) produced during OS, cause biomolecular damage. This is followed by information on the biomarkers which come about as a result of free radical damage and the techniques used for assaying these stress indicators. The established links between elevated ROS and lowered antioxidants during HIV infection and the subsequent use of HAART is then presented followed by a review of the OS markers detected in HIV metabonomic studies to date. We identify gaps in HIV/HAART-associated OS research and finally suggest how these research gaps can be addressed through metabonomic analysis, specifically targeting the multiple markers of HIV-induced OS.

Inflammation and depression: a causal or coincidental link to the pathophysiology?

This review summarises the evidence that chronic low grade inflammation triggers changes that contribute to the mental and physical ill health of patients with major depression. Inflammation, and the activation of the hypothalamic pituitary axis by stress, are normal components of the stress response but when stress is prolonged and the endocrine and immune system become chronic resulting in the activation of the peripheral macrophages, the central microglia and hypercortisolemia, the neuronal networks are damaged and become dysfunctional. The proinflammatory cytokines, in addition to activating the hypothalamic-pituitary-adrenal axis and thereby increasing cortisol synthesis, also activate the tryptophan-kynurenine pathway. This results in the synthesis of the neurotoxic N-methyl-d-aspartate (NMDA) glutamate agonist quinolinic acid and 3-hydroxykynurenine thereby enhancing oxidative stress and contributes to neurodegeneration which characterise major depression particularly in late life.While antidepressants attenuate some of the endocrine and immune changes caused by inflammation, not all therapeutically effective antidepressants do so. This suggests that drugs which specifically target the immune, endocrine and neurotransmitter systems may be more effective antidepressants.The preliminary clinical evidence that some non-steroidal anti-inflammatory drugs, such as the cyclooxygenase 2 inhibitor celecoxib, can enhance the response to standard antidepressant treatment is therefore considered and a critical assessment made of the possible limitations of such an approach to novel antidepressant development.

Neurotransmitter signalling via NMDA receptors leads to decreased T helper type 1-like and enhanced T helper type 2-like immune balance in humans

Given the pivotal roles that CD4⁺ T cell imbalance plays in human immune disorders, much interest centres on better understanding influences that regulate human helper T-cell subset dominance in vivo. Here, using primary CD4⁺ T cells and short-term T helper type 1 (Th1) and Th2-like lines, we investigated roles and mechanisms by which neurotransmitter receptors may influence human type 1 versus type 2 immunity. We hypothesized that N-methyl-d-aspartate receptors (NMDA-R), which play key roles in memory and learning, can also regulate human CD4⁺ T cell function through induction of excitotoxicity. Fresh primary CD4⁺ T cells from healthy donors express functional NMDA-R that are strongly up-regulated upon T cell receptor (TCR) mediated activation. Synthetic and physiological NMDA-R agonists elicited Ca²⁺ flux and led to marked inhibition of type 1 but not type 2 or interleukin-10 cytokine responses. Among CD4⁺ lines, NMDA and quinolinic acid preferentially reduced cytokine production, Ca²⁺ flux, proliferation and survival of Th1-like cells through increased induction of cell death whereas Th2-like cells were largely spared. Collectively, the findings demonstrate that (i) NMDA-R is rapidly up-regulated upon CD4⁺ T cell activation in humans and (ii) Th1 versus Th2 cell functions such as proliferation, cytokine production and cell survival are differentially affected by NMDA-R agonists. Differential cytokine production and proliferative capacity of Th1 versus Th2 cells is attributable in part to increased physiological cell death among fully committed Th1 versus Th2 cells, leading to increased Th2-like dominance. Hence, excitotoxicity, beyond its roles in neuronal plasticity, may contribute to ongoing modulation of human T cell responses.

Chromatographic analysis of tryptophan metabolites

The kynurenine pathway generates multiple tryptophan metabolites called collectively kynurenines and leads to formation of the enzyme cofactor nicotinamide adenine dinucleotide. The first step in this pathway is tryptophan degradation, initiated by the rate-limiting enzymes indoleamine 2,3-dioxygenase, or tryptophan 2,3-dioxygenase, depending on the tissue. The balanced kynurenine metabolism, which has been a subject of multiple studies in last decades, plays an important role in several physiological and pathological conditions such as infections, autoimmunity, neurological disorders, cancer, cataracts, as well as pregnancy. Understanding the regulation of tryptophan depletion provide novel diagnostic and treatment opportunities, however it requires reliable methods for quantification of kynurenines in biological samples with complex composition (body fluids, tissues, or cells). Trace concentrations, interference of sample components, and instability of some tryptophan metabolites need to be addressed using analytical methods. The novel separation approaches and optimized extraction protocols help to overcome difficulties in analyzing kynurenines within the complex tissue material. Recent developments in chromatography coupled with mass spectrometry provide new opportunity for quantification of tryptophan and its degradation products in various biological samples. In this review, we present current accomplishments in the chromatographic methodologies proposed for detection of tryptophan metabolites and provide a guide for choosing the optimal approach.

Does Vitamin C Influence Neurodegenerative Diseases and Psychiatric Disorders?

Vitamin C (Vit C) is considered to be a vital antioxidant molecule in the brain. Intracellular Vit C helps maintain integrity and function of several processes in the central nervous system (CNS), including neuronal maturation and differentiation, myelin formation, synthesis of catecholamine, modulation of neurotransmission and antioxidant protection. The importance of Vit C for CNS function has been proven by the fact that targeted deletion of the sodium-vitamin C co-transporter in mice results in widespread cerebral hemorrhage and death on post-natal day one. Since neurological diseases are characterized by increased free radical generation and the highest concentrations of Vit C in the body are found in the brain and neuroendocrine tissues, it is suggested that Vit C may change the course of neurological diseases and display potential therapeutic roles. The aim of this review is to update the current state of knowledge of the role of vitamin C on neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis and amyotrophic sclerosis, as well as psychiatric disorders including depression, anxiety and schizophrenia. The particular attention is attributed to understanding of the mechanisms underlying possible therapeutic properties of ascorbic acid in the presented disorders.

Neuroinflammation in liver disease: sessional talks from ISHEN

At the recent ISHEN ('International Symposium of Hepatic Encephalopathy & Nitrogen Metabolism') conference in London, a whole session was dedicated to our increasing awareness of the importance of inflammation in the brain - termed 'neuroinflammation', in the development of Hepatic Encephalopathy (HE) - the neurological manifestations of advanced liver disease. In this review our ISHEN speakers further discuss the content of their sessional presentations and more broadly we discuss our understanding of the role of neuroinflammation in HE pathogenesis.

Kynurenine pathway metabolism following prenatal KMO inhibition and in Mecp2+/- mice, using liquid chromatography-tandem mass spectrometry

To quantify the full range of tryptophan metabolites along the kynurenine pathway, a liquid chromatography – tandem mass spectrometry method was developed and used to analyse brain extracts of rodents treated with the kynurenine-3-mono-oxygenase (KMO) inhibitor Ro61-8048 during pregnancy. There were significant increases in the levels of kynurenine, kynurenic acid, anthranilic acid and 3-hydroxy-kynurenine (3-HK) in the maternal brain after 5 h but not 24 h, while the embryos exhibited high levels of kynurenine, kynurenic acid and anthranilic acid after 5 h which were maintained at 24 h post-treatment. At 24 h there was also a strong trend to an increase in quinolinic acid levels (P = 0.055). No significant changes were observed in any of the other kynurenine metabolites. The results confirm the marked increase in the accumulation of some neuroactive kynurenines when KMO is inhibited, and re-emphasise the potential importance of changes in anthranilic acid. The prolonged duration of metabolite accumulation in the embryo brains indicates a trapping of compounds within the embryonic CNS independently of maternal levels. When brains were examined from young mice heterozygous for the meCP2 gene – a potential model for Rett syndrome - no differences were noted from control mice, suggesting that the proposed roles for kynurenines in autism spectrum disorder are not relevant to Rett syndrome, supporting its recognition as a distinct, independent, condition.

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Pregnant rats were treated with an inhibitor of kynurenine-3-monoxygenase.

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Levels of several kynurenine metabolites increased in the maternal and foetal brains.

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The maternal changes at 5 h disappeared by 24 h, but were maintained in embryos.

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No changes were noted in the brains of Mecp2^+/− mice.

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KMO inhibition but not Mecp2^+/− suppression alters kynurenine metabolism.

Characterization of the Kynurenine Pathway in CD8+ Human Primary Monocyte-Derived Dendritic Cells

The kynurenine (KYN) pathway (KP) is a major degradative pathway of the amino acid, L-tryptophan (TRP), that ultimately leads to the anabolism of the essential pyridine nucleotide, nicotinamide adenine dinucleotide. TRP catabolism results in the production of several important metabolites, including the major immune tolerance-inducing metabolite KYN, and the neurotoxin and excitotoxin quinolinic acid. Dendritic cells (DCs) have been shown to mediate immunoregulatory roles that mediated by TRP catabolism. However, characterization of the KP in human DCs has so far only been partly delineated. It is critical to understand which KP enzymes are expressed and which KP metabolites are produced to be able to understand their regulatory effects on the immune response. In this study, we characterized the KP in human monocyte-derived DCs (MDDCs) in comparison with the human primary macrophages using RT-PCR, high-pressure gas chromatography, mass spectrometry, and immunocytochemistry. Our results show that the KP is entirely expressed in human MDDC. Following activation of the KP using interferon gamma, MDDCs can mediate apoptosis of T _h cells in vitro. Understanding the molecular mechanisms regulating KP metabolism in MDDCs may provide renewed insight for the development of novel therapeutics aimed at modulating immunological effects and peripheral tolerance.

Interference of α-Synuclein Uptake by Monomeric β-Amyloid1-40 and Potential Core Acting Site of the Interference

Increasing evidence suggests an important role of alpha-synuclein (α-Syn) in the pathogenesis of Parkinson's disease (PD). The inter-neuronal spread of α-Syn via exocytosis and endocytosis has been proposed as an explanation for the neuropathological findings of PD in sub-clinical and clinical phases. Therefore, interfering the uptake of α-Syn by neurons may be an important step in slowing or modifying the propagation of the disease. The purposes of our study were to investigate if the uptake of α-Syn fibrils can be specifically interfered with monomeric β-Amyloid1-40 (Aβ40) and to characterise the core acting site of interference. Using a radioisotope-labelled uptake assay, we found an 80 % uptake reduction of α-Syn fibrils in neurons interfered with monomeric Aβ40, but not β-Amyloid1-42 (Aβ42) as compared to controls. This finding was further confirmed by enzyme-linked immunosorbent assay (ELISA) with α-Syn uptake reduced from about 80 % (Aβ42) to about 20 % (Aβ40) relative to controls. To define the region of Aβ40 peptide capable of the interference, we explored shorter peptides with less amino acid residues from both the C-terminus and N-terminus. We found that the interference effect was preserved if amino acid residue was trimmed to position 11 (from N-terminus) and 36 (from C-terminus), but dropped off significantly if residues were trimmed beyond these positions. We therefore deduced that the "core acting site" lies between amino acid residue positions 12-36. These findings suggest α-Syn uptake can be interfered with monomeric Aβ40 and that the core acting site of interference might lie between amino acid residue positions 12-36.

Structural Insights into the Quaternary Catalytic Mechanism of Hexameric Human Quinolinate Phosphoribosyltransferase, a Key Enzyme in de novo NAD Biosynthesis

Quinolinate phosphoribosyltransferase (QPRT) catalyses the production of nicotinic acid mononucleotide, a precursor of de novo biosynthesis of the ubiquitous coenzyme nicotinamide adenine dinucleotide. QPRT is also essential for maintaining the homeostasis of quinolinic acid in the brain, a possible neurotoxin causing various neurodegenerative diseases. Although QPRT has been extensively analysed, the molecular basis of the reaction catalysed by human QPRT remains unclear. Here, we present the crystal structures of hexameric human QPRT in the apo form and its complexes with reactant or product. We found that the interaction between dimeric subunits was dramatically altered during the reaction process by conformational changes of two flexible loops in the active site at the dimer-dimer interface. In addition, the N-terminal short helix α1 was identified as a critical hexamer stabilizer. The structural features, size distribution, heat aggregation and ITC studies of the full-length enzyme and the enzyme lacking helix α1 strongly suggest that human QPRT acts as a hexamer for cooperative reactant binding via three dimeric subunits and maintaining stability. Based on our comparison of human QPRT structures in the apo and complex forms, we propose a drug design strategy targeting malignant glioma.

Quinolinic Acid Responses during Interferon-α-Induced Depressive Symptomatology in Patients with Chronic Hepatitis C Infection - A Novel Aspect for Depression and Inflammatory Hypothesis

BACKGROUND

The aim of this exploratory study is to gain for the first time a more comprehensive picture of the impact of changes of quinolinic acid concentrations on depressive symptomatology during and after IFN-α therapy.

METHODS

The quinolinic acid concentrations of 35 HCV patients are examined in a prospective survey over the entire period of IFN-α treatment as well as three months later at six different times (baseline, one, three, six and nine months after the beginning of IFN-α treatment, and after the end of treatment).

RESULTS

During IFN-α treatment Hamilton Depression Rating Scale scores rise significantly. At the same time there is greater activity of indoleamine 2,3-dioxygenase, with a resulting increase in plasma kynurenine concentrations. Compared to baseline values quinolinic acid concentrations increase significantly during therapy, reflecting an increased neurotoxic challenge. In addition, patients with higher scores in the Hamilton Depression Rating Scale at six and nine months after starting therapy show significantly higher levels of quinolinic acid concentration.

CONCLUSIONS

The increase of quinolinic acid during IFN-α therapy might contribute to depressive symptomatology through the neurotoxic challenge caused by quinolinic acid. Subsequently, our exploratory study results support the inflammatory hypothesis of depression. The awareness of relevant risk factors of IFN-α treatment-induced depression is essential to develop preventative treatment strategies.

Kynurenines and Multiple Sclerosis: The Dialogue between the Immune System and the Central Nervous System

Multiple sclerosis is an inflammatory disease of the central nervous system, in which axonal transection takes place in parallel with acute inflammation to various, individual extents. The importance of the kynurenine pathway in the physiological functions and pathological processes of the nervous system has been extensively investigated, but it has additionally been implicated as having a regulatory function in the immune system. Alterations in the kynurenine pathway have been described in both preclinical and clinical investigations of multiple sclerosis. These observations led to the identification of potential therapeutic targets in multiple sclerosis, such as synthetic tryptophan analogs, endogenous tryptophan metabolites (e.g., cinnabarinic acid), structural analogs (laquinimod, teriflunomid, leflunomid and tranilast), indoleamine-2,3-dioxygenase inhibitors (1MT and berberine) and kynurenine-3-monooxygenase inhibitors (nicotinylalanine and Ro 61-8048). The kynurenine pathway is a promising novel target via which to influence the immune system and to achieve neuroprotection, and further research is therefore needed with the aim of developing novel drugs for the treatment of multiple sclerosis and other autoimmune diseases.

Expression of the Kynurenine Pathway in Human Peripheral Blood Mononuclear Cells: Implications for Inflammatory and Neurodegenerative Disease

The kynurenine pathway is a fundamental mechanism of immunosuppression and peripheral tolerance. It is increasingly recognized as playing a major role in the pathogenesis of a wide variety of inflammatory, neurodegenerative and malignant disorders. However, the temporal dynamics of kynurenine pathway activation and metabolite production in human immune cells is currently unknown. Here we report the novel use of flow cytometry, combined with ultra high-performance liquid chromatography and gas chromatography-mass spectrometry, to sensitively quantify the intracellular expression of three key kynurenine pathway enzymes and the main kynurenine pathway metabolites in a time-course study. This is the first study to show that up-regulation of indoleamine 2,3-dioxygenase (IDO-1), kynurenine 3-monoxygenase (KMO) and quinolinate phosphoribosyltransferase (QPRT) is lacking in lymphocytes treated with interferon gamma. In contrast, peripheral monocytes showed a significant elevation of kynurenine pathway enzymes and metabolites when treated with interferon gamma. Expression of IDO-1, KMO and QPRT correlated significantly with activation of the kynurenine pathway (kynurenine:tryptophan ratio), quinolinic acid concentration and production of the monocyte derived, pro-inflammatory immune response marker: neopterin. Our results also describe an original and sensitive methodological approach to quantify kynurenine pathway enzyme expression in cells. This has revealed further insights into the potential role of these enzymes in disease processes.

Inhibition of Cholinesterases and Some Pro-Oxidant induced Oxidative Stress in Rats Brain by Two Tomato (Lycopersicon Esculentum) Varieties

This study sought to investigate the effects of two tomato varieties [Lycopersicon esculentum Mill. var. esculentum (ESC) and Lycopersicon esculentum Mill. var. cerasiforme (CER)] on acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activities in vitro. Phenolics content, carotenoids characterisation, inhibition of Fe²⁺ and quinolinic acid-induced malondialdehyde (MDA) production in rats brain homogenate and NO^* scavenging abilities were also assesed in addition to the AChE and BChE inhibition assays. There was no significant difference in the AChE inhibitory ability of the samples, while CER had significantly higher BChE inhibitory activity. Furthermore, the tomatoes inhibited Fe²⁺ and quinolinic acid-induced MDA production and further exhibited antioxidant activities through their NO^* scavenging abilities. There was no significant difference in the phenolic content of the samples, while significantly high amounts of lycopene were detected in the tomatoes. The cholinesterase-inhibition and antioxidant properties of the “tomatoes” could make them good dietary means for the management of neurodegenerative disorders.