Quinolinic acid production by macrophages stimulated with IFN-gamma, TNF-alpha, and IFN-alpha

Kynurenine pathway and its role in neurologic, psychiatric, and inflammatory bowel diseases

Tryptophan metabolism along the kynurenine pathway is of central importance for the immune function. It prevents hyperinflammation and induces long-term immune tolerance. Accumulating evidence also demonstrates cytoprotective and immunomodulatory properties of kynurenine pathway in conditions affecting either central or peripheral nervous system as well as other conditions such as inflammatory bowel disease (IBD). Although multilevel association exists between the inflammatory bowel disease (IBD) and various neurologic (e.g., neurodegenerative) disorders, it is believed that the kynurenine pathway plays a pivotal role in the development of both IBD and neurodegenerative disorders. In this setting, there is strong evidence linking the gut-brain axis with intestinal dysfunctions including IBD which is consistent with the fact that the risk of neurodegenerative diseases is higher in IBD patients. This review aims to highlight the role of kynurenine metabolic pathway in various neurologic and psychiatric diseases as well as relationship between IBD and neurodegenerative disorders in the light of the kynurenine metabolic pathway.

Complete spectrum of adverse events associated with chimeric antigen receptor (CAR)-T cell therapies

Chimeric antigen receptor (CAR)-T cell therapies have been approved by FDA to treat relapsed or refractory hematological malignancies. However, the adverse effects of CAR-T cell therapies are complex and can be challenging to diagnose and treat. In this review, we summarize the major adverse events, including cytokine release syndrome (CRS), immune effector cell-associated neurotoxicity syndrome (ICANS), and CAR T-cell associated HLH (carHLH), and discuss their pathophysiology, symptoms, grading, and diagnosis systems, as well as management. In a future outlook, we also provide an overview of measures and modifications to CAR-T cells that are currently being explored to limit toxicity.

Profile and potential role of novel metabolite biomarkers, especially indoleacrylic acid, in pathogenesis of neuromyelitis optica spectrum disorders

Background: Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune central nervous system (CNS) inflammatory and demyelinating disorder that can lead to serious disability and mortality. Humoral fluid biomarkers with specific, convenient, and efficient profiles that could characterize and monitor disease activity or severity are very useful. We aimed to develop a sensitive and high-throughput liquid chromatography-tandem mass spectrometry (LC-MS)/MS-based analytical method for novel biomarkers finding in NMOSD patients and verified its function tentatively. Methods: Serum samples were collected from 47 NMOSD patients, 18 patients with other neurological disorders (ONDs), and 35 healthy controls (HC). Cerebrospinal fluid (CSF) samples were collected from 18 NMOSD and 17 OND patients. Three aromatic amino acids (phenylalanine, tyrosine, and tryptophan) and nine important metabolites that included phenylacetylglutamine (PAGln), indoleacrylic acid (IA), 3-indole acetic acid (IAA), 5-hydroxyindoleacetic acid (HIAA), hippuric acid (HA), I-3-carboxylic acid (I-3-CA), kynurenine (KYN), kynurenic acid (KYNA), and quinine (QUIN) were analyzed by using the liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based method. The profile of IA was further analyzed, and its function was verified in an astrocyte injury model stimulated by NMO-IgG, which represents important events in NMOSD pathogenesis. Results: In the serum, tyrosine and some of the tryptophan metabolites IA and I-3-CA decreased, and HIAA increased significantly in NMOSD patients. The CSF levels of phenylalanine and tyrosine showed a significant increase exactly during the relapse stage, and IA in the CSF was also increased markedly during the relapse and remission phases. All conversion ratios had similar profiles with their level fluctuations. In addition, the serum IA levels negatively correlated with glial fibrillary acidic protein (GFAP), and neurofilament light (NfL) levels in the serum of NMOSD patients were measured by using ultra-sensitive single-molecule arrays (Simoa). IA showed an anti-inflammatory effect in an in vitro astrocyte injury model. Conclusion: Our data suggest that essential aromatic amino acid tryptophan metabolites IA in the serum or CSF may serve as a novel promising biomarker to monitor and predict the activity and severity of NMOSD disease. Supplying or enhancing IA function can promote anti-inflammatory responses and may have therapeutic benefits.

A cross-sectional study of inflammatory markers as determinants of circulating kynurenines in the Lung Cancer Cohort Consortium

Circulating concentrations of metabolites (collectively called kynurenines) in the kynurenine pathway of tryptophan metabolism increase during inflammation, particularly in response to interferon-gamma (IFN-γ). Neopterin and the kynurenine/tryptophan ratio (KTR) are IFN-γ induced inflammatory markers, and together with C-reactive protein (CRP) and kynurenines they are associated with various diseases, but comprehensive data on the strength of associations of inflammatory markers with circulating concentrations of kynurenines are lacking. We measured circulating concentrations of neopterin, CRP, tryptophan and seven kynurenines in 5314 controls from 20 cohorts in the Lung Cancer Cohort Consortium (LC3). The associations of neopterin, KTR and CRP with kynurenines were investigated using regression models. In mixed models, one standard deviation (SD) higher KTR was associated with a 0.46 SD higher quinolinic acid (QA), and 0.31 SD higher 3-hydroxykynurenine (HK). One SD higher neopterin was associated with 0.48, 0.44, 0.36 and 0.28 SD higher KTR, QA, kynurenine and HK, respectively. KTR and neopterin respectively explained 24.1% and 16.7% of the variation in QA, and 11.4% and 7.5% of HK. CRP was only weakly associated with kynurenines in regression models. In summary, QA was the metabolite that was most strongly associated with the inflammatory markers. In general, the inflammatory markers were most strongly related to metabolites located along the tryptophan-NAD axis, which may support suggestions of increased production of NAD from tryptophan during inflammation.

The Role of Interferon-α in Neurodegenerative Diseases: A Systematic Review

BACKGROUND

Neurodegenerative diseases (NDs) impose significant financial and healthcare burden on populations all over the world. The prevalence and incidence of NDs have been observed to increase dramatically with age. Hence, the number of reported cases is projected to increase in the future, as life spans continues to rise. Despite this, there is limited effective treatment against most NDs. Interferons (IFNs), a family of cytokines, have been suggested as a promising therapeutic target for NDs, particularly IFN-α, which governs various pathological pathways in different NDs.

OBJECTIVE

This systematic review aimed to critically appraise the currently available literature on the pathological role of IFN-α in neurodegeneration/NDs.

METHODS

Three databases, Scopus, PubMed, and Ovid Medline, were utilized for the literature search.

RESULTS

A total of 77 journal articles were selected for critical evaluation, based on the inclusion and exclusion criteria. The studies selected and elucidated in this current systematic review have showed that IFN-α may play a deleterious role in neurodegenerative diseases through its strong association with the inflammatory processes resulting in mainly neurocognitive impairments. IFN-α may be displaying its neurotoxic function via various mechanisms such as abnormal calcium mineralization, activation of STAT1-dependent mechanisms, and increased quinolinic acid production.

CONCLUSION

The exact role IFN-α in these neurodegenerative diseases have yet to be determine due to a lack in more recent evidence, thereby creating a variability in the role of IFN-α. Future investigations should thus be conducted, so that the role played by IFN-α in neurodegenerative diseases could be delineated.

ROLE OF GUT MICROBIOTA IN DEPRESSION: UNDERSTANDING MOLECULAR PATHWAYS, RECENT RESEARCH, AND FUTURE DIRECTION

Gut microbiota, also known as the "second brain" in humans because of the regulatory role it has on the central nervous system via neuronal, chemical and immune pathways. It has been proven that there exists a bidirectional communication between the gut and the brain. Increasing evidence supports that this crosstalk is linked to the etiology and treatment of depression. Reports suggest that the gut microbiota control the host epigenetic machinery in depression and gut dysbiosis causes negative epigenetic modifications via mechanisms like histone acetylation, DNA methylation and non-coding RNA mediated gene inhibition. The gut microbiome can be a promising approach for the management of depression. The diet and dietary metabolites like kynurenine, tryptophan, and propionic acid also greatly influence the microbiome composition and thereby, the physiological activities. This review gives a bird-eye view on the pathological updates and currently used treatment approaches targeting the gut microbiota in depression.

The Footprint of Kynurenine Pathway in Cardiovascular Diseases

Kynurenine pathway is the main route of tryptophan metabolism and produces several metabolites with various biologic properties. It has been uncovered that several cardiovascular diseases are associated with the overactivation of kynurenine pathway and kynurenine and its metabolites have diagnostic and prognostic value in cardiovascular diseases. Furthermore, it was found that several kynurenine metabolites can differently affect cardiovascular health. For instance, preclinical studies have shown that kynurenine, xanthurenic acid and cis-WOOH decrease blood pressure; kynurenine and 3-hydroxyanthranilic acid prevent atherosclerosis; kynurenic acid supplementation and kynurenine 3-monooxygenase (KMO) inhibition improve the outcome of stroke. Indoleamine 2,3-dioxygenase (IDO) overactivity and increased kynurenine levels improve cardiac and vascular transplantation outcomes, whereas exacerbating the outcome of myocardial ischemia, post-ischemic myocardial remodeling, and abdominal aorta aneurysm. IDO inhibition and KMO inhibition are also protective against viral myocarditis. In addition, dysregulation of kynurenine pathway is observed in several conditions such as senescence, depression, diabetes, chronic kidney disease (CKD), cirrhosis, and cancer closely connected to cardiovascular dysfunction. It is worth defining the exact effect of each metabolite of kynurenine pathway on cardiovascular health. This narrative review is the first review that separately discusses the involvement of kynurenine pathway in different cardiovascular diseases and dissects the underlying molecular mechanisms.

Cerebral Glutamate Regulation and Receptor Changes in Perioperative Neuroinflammation and Cognitive Dysfunction

Glutamate is the major excitatory neurotransmitter in the central nervous system and is intricately linked to learning and memory. Its activity depends on the expression of AMPA and NMDA receptors and excitatory amino transporters on neurons and glial cells. Glutamate transporters prevent the excess accumulation of glutamate in synapses, which can lead to aberrant synaptic signaling, excitotoxicity, or cell death. Neuroinflammation can occur acutely after surgical trauma and contributes to the development of perioperative neurocognitive disorders, which are characterized by impairment in multiple cognitive domains. In this review, we aim to examine how glutamate handling and glutamatergic function are affected by neuroinflammation and their contribution to cognitive impairment. We will first summarize the current data regarding glutamate in neurotransmission, its receptors, and their regulation and trafficking. We will then examine the impact of inflammation on glutamate handling and neurotransmission, focusing on changes in glial cells and the effect of cytokines. Finally, we will discuss these changes in the context of perioperative neuroinflammation and the implications they have for perioperative neurocognitive disorders.

Ginsenoside Rb1 prevents lipopolysaccharide-induced depressive-like behavior by inhibiting inflammation and neural dysfunction and F2 elicits a novel antidepressant-like effect: A metabolite-based network pharmacology study

ETHNOPHARMACOLOGICAL RELEVANCE

Inflammatory responses are associated wieh the pathophysiology of depression. Ginsenoside Rb1 (Rb1) exerts antidepressant effect, but the relationship between its activity and inflammation remains unclear.

AIM OF THE STUDY

In this study, the antidepressant-like effect and underlying mechanisms of Rb1 were been investigated.

MATERIALS AND METHODS

The neuroinflammatory mouse model of lipopolysaccharide (LPS)-induced acute depression-like behavior was employed to detect the action of Rb1. An integrative strategy combining the identification of prototype (Rb1) and its metabolites in vivo with network pharmacology analysis was used to explore therapeutic mechanisms of these ingredients. The putative targets and signalings were experimentally validated. The antidepressant-like effect of F2, the metabolite of Rb1, was firstly evaluated.

RESULTS

Rb1 significantly ameliorated LPS-induced depressive-like behavior. Rb1 and its metabolites (Rd, F2, compound K, Rh2, Rg3, PPD) were identified and then a disease-component-target network was established. Experimental validation showed that Rb1 inhibited peripheral and hippocampal inflammation via MAPK/NF-κB signaling. In inflammatory-mediated depression state, Rb1 improved impaired glucocorticoid receptor, suppressed indoleamine 2,3-dioxygenase activity, increased 5-HT level and 5-HT_1A receptor expression. Additionally, F2 was firstly discovered to exert antidepressant-like effect, and it existed higher activity than Rb1 against depression.

CONCLUSION

The study highlighted the potential of Rb1 and F2 as healthy supplement or agent for inflammation-induced depression.

Mechanisms of cytokine release syndrome and neurotoxicity of CAR T-cell therapy and associated prevention and management strategies

Chimeric antigen receptor (CAR) T-cell therapy has yielded impressive outcomes and transformed treatment algorithms for hematological malignancies. To date, five CAR T-cell products have been approved by the US Food and Drug Administration (FDA). Nevertheless, some significant toxicities pose great challenges to the development of CAR T-cell therapy, most notably cytokine release syndrome (CRS) and immune effector cell-associated neurotoxicity syndrome (ICANS). Understanding the mechanisms underlying these toxicities and establishing prevention and treatment strategies are important. In this review, we summarize the mechanisms underlying CRS and ICANS and provide potential treatment and prevention strategies.

NAD+ Metabolism, Metabolic Stress, and Infection

Nicotinamide adenine dinucleotide (NAD⁺) is an essential metabolite with wide-ranging and significant roles in the cell. Defects in NAD⁺ metabolism have been associated with many human disorders; it is therefore an emerging therapeutic target. Moreover, NAD⁺ metabolism is perturbed during colonization by a variety of pathogens, either due to the molecular mechanisms employed by these infectious agents or by the host immune response they trigger. Three main biosynthetic pathways, including the de novo and salvage pathways, contribute to the production of NAD⁺ with a high degree of conservation from bacteria to humans. De novo biosynthesis, which begins with l-tryptophan in eukaryotes, is also known as the kynurenine pathway. Intermediates of this pathway have various beneficial and deleterious effects on cellular health in different contexts. For example, dysregulation of this pathway is linked to neurotoxicity and oxidative stress. Activation of the de novo pathway is also implicated in various infections and inflammatory signaling. Given the dynamic flexibility and multiple roles of NAD⁺ intermediates, it is important to understand the interconnections and cross-regulations of NAD⁺ precursors and associated signaling pathways to understand how cells regulate NAD⁺ homeostasis in response to various growth conditions. Although regulation of NAD⁺ homeostasis remains incompletely understood, studies in the genetically tractable budding yeast Saccharomyces cerevisiae may help provide some molecular basis for how NAD⁺ homeostasis factors contribute to the maintenance and regulation of cellular function and how they are regulated by various nutritional and stress signals. Here we present a brief overview of recent insights and discoveries made with respect to the relationship between NAD⁺ metabolism and selected human disorders and infections, with a particular focus on the de novo pathway. We also discuss how studies in budding yeast may help elucidate the regulation of NAD⁺ homeostasis.

Vortioxetine Prevents Lipopolysaccharide-Induced Memory Impairment Without Inhibiting the Initial Inflammatory Cascade

Vortioxetine is a novel multimodal antidepressant that modulates a wide range of neurotransmitters throughout the brain. Preclinical and clinical studies have shown that vortioxetine exerts positive effects on different cognitive domains and neuroprotective effects. Considering the key role of microglial cells in brain plasticity and cognition, we aimed at investigating the effects of pretreatment with vortioxetine in modulating behavioral and molecular effects induced by an immune challenge: peripheral injection of lipopolysaccharide (LPS). To this purpose, C57BL/6J male mice were first exposed to a 28-day standard diet or vortioxetine-enriched diet, which was followed by an acute immune challenge with LPS. Sickness symptoms and depressive-like behaviors (anhedonia and memory impairment) were tested 6 and 24 h after exposure to LPS, respectively. Moreover, the expressions of markers of immune activation and M1/M2 markers of microglia polarization were measured in the dorsal and ventral parts of the hippocampus. The pretreatment with vortioxetine did not affect both LPS-induced sickness behavior and anhedonia but prevented the deficit in the recognition memory induced by the immune challenge. At the transcriptional level, chronic exposure to vortioxetine did not prevent LPS-induced upregulation of proinflammatory cytokines 6 h after the immune challenge but rather seemed to potentiate the immune response to the challenge also by affecting the levels of expression of markers of microglia M1 phenotype, like cluster of differentiation (CD)14 and CD86, in an area-dependent manner. However, at the same time point, LPS injection significantly increased the expression of the M2 polarization inducer, interleukin 4, only in the hippocampus of animals chronically exposed to vortioxetine. These results demonstrate that a chronic administration of vortioxetine specifically prevents LPS-induced memory impairment, without affecting acute sickness behavior and anhedonia, and suggest that hippocampal microglia may represent a cellular target of this novel antidepressant medication. Moreover, we provide a useful model to further explore the molecular mechanisms specifically underlying cognitive impairments following an immune challenge.

Dysregulation of the gut-brain-skin axis and key overlapping inflammatory and immune mechanisms of psoriasis and depression

The occurrence, progression and recurrence of psoriasis are thought to be related to mood and psychological disorders such as depression. Psoriasis can lead to depression, and depression, in turn, exacerbates psoriasis. No specific mechanism can explain the association between psoriasis and depression. The gut-brain-skin axis has been used to explain correlations among the gut microbiota, emotional states and systemic and skin inflammation, and this axis may be associated with overlapping mechanisms between psoriasis and depression. Therefore, in the context of the gut-brain-skin axis, we systematically summarized and comparatively analysed the inflammatory and immune mechanisms of psoriasis and depression and illustrated the dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and the gut microbiota. This review provides a theoretical basis and new targets for the treatment of psoriasis and depression.

Searching for Peripheral Biomarkers in Neurodegenerative Diseases: The Tryptophan-Kynurenine Metabolic Pathway

Neurodegenerative diseases are multifactorial, initiated by a series of the causative complex which develops into a certain clinical picture. The pathogenesis and disease course vary from patient to patient. Thus, it should be likewise to the treatment. Peripheral biomarkers are to play a central role for tailoring a personalized therapeutic plan for patients who suffered from neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and multiple sclerosis, among others. Nevertheless, the use of biomarkers in clinical practice is still underappreciated and data presented in biomarker research for clinical use is still uncompelling, compared to the abundant data available for drug research and development. So is the case with kynurenines (KYNs) and the kynurenine pathway (KP) enzymes, which have been associated with a wide range of diseases including cancer, autoimmune diseases, inflammatory diseases, neurologic diseases, and psychiatric disorders. This review article discusses current knowledge of KP alterations observed in the central nervous system as well as the periphery, its involvement in pathogenesis and disease progression, and emerging evidence of roles of microbiota in the gut-brain axis, searching for practical peripheral biomarkers which ensure personalized treatment plans for neurodegenerative diseases.

Cerebral dopamine neurotrophic factor (CDNF) protects against quinolinic acid-induced toxicity in in vitro and in vivo models of Huntington's disease

Huntington’s disease (HD) is a neurodegenerative disorder with a progressive loss of medium spiny neurons in the striatum and aggregation of mutant huntingtin in the striatal and cortical neurons. Currently, there are no rational therapies for the treatment of the disease. Cerebral dopamine neurotrophic factor (CDNF) is an endoplasmic reticulum (ER) located protein with neurotrophic factor (NTF) properties, protecting and restoring the function of dopaminergic neurons in animal models of PD more effectively than other NTFs. CDNF is currently in phase I–II clinical trials on PD patients. Here we have studied whether CDNF has beneficial effects on striatal neurons in in vitro and in vivo models of HD. CDNF was able to protect striatal neurons from quinolinic acid (QA)-induced cell death in vitro via increasing the IRE1α/XBP1 signalling pathway in the ER. A single intrastriatal CDNF injection protected against the deleterious effects of QA in a rat model of HD. CDNF improved motor coordination and decreased ataxia in QA-toxin treated rats, and stimulated the neurogenesis by increasing doublecortin (DCX)-positive and NeuN-positive cells in the striatum. These results show that CDNF positively affects striatal neuron viability reduced by QA and signifies CDNF as a promising drug candidate for the treatment of HD.

Kynurenine, Tetrahydrobiopterin, and Cytokine Inflammatory Biomarkers in Individuals Affected by Diabetic Neuropathic Pain

Neuropathic pain is a common complication of diabetes with high morbidity and poor treatment outcomes. Accumulating evidence suggests the immune system is involved in the development of diabetic neuropathy, whilst neuro-immune interactions involving the kynurenine (KYN) and tetrahydrobiopterin (BH4) pathways have been linked to neuropathic pain pre-clinically and in several chronic pain conditions. Here, using a multiplex assay, we quantified serum levels of 14 cytokines in 21 participants with type 1 diabetes mellitus, 13 of which were classified as having neuropathic pain. In addition, using high performance liquid chromatography and gas chromatography-mass spectrometry, all major KYN and BH4 pathway metabolites were quantified in serum from the same cohort. Our results show increases in GM-CSF and IL-8, suggesting immune cell involvement. We demonstrated increases in two inflammatory biomarkers: neopterin and the KYN/TRP ratio, a marker of indoleamine 2,3-dioxygenase activity. Moreover, the KYN/TRP ratio positively correlated with pain intensity. Total kynurenine aminotransferase activity was also higher in the diabetic neuropathic pain group, indicating there may be increased production of the KYN metabolite, xanthurenic acid. Overall, this study supports the idea that inflammatory activation of the KYN and BH4 pathways occurs due to elevated inflammatory cytokines, which might be involved in the pathogenesis of neuropathic pain in type 1 diabetes mellitus. Further studies should be carried out to investigate the role of KYN and BH4 pathways, which could strengthen the case for therapeutically targeting them in neuropathic pain conditions.

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.

Effects of inflammation on the kynurenine pathway in schizophrenia - a systematic review

Background

In the last decade, there has been growing evidence that an interaction exists between inflammation and the kynurenine pathway in schizophrenia. Additionally, many authors found microglial activation in cases of schizophrenia due to inflammatory mechanisms related mostly to an increase of pro-inflammatory cytokines. In order to gain new insights into the pathophysiology of schizophrenia, it is important to incorporate the latest published evidence concerning inflammatory mechanisms and kynurenine metabolism. This systematic review aims to collect reliable recent findings within the last decade supporting such a theory.

Methods

A structured search of electronic databases was conducted for publications between 2008 and 2018 to identify eligible studies investigating patients with schizophrenia/psychosis and the relationship between inflammation and kynurenine pathway. Applicable studies were systematically scored using the NIH Quality Assessment Tools. Two researchers independently extracted data on diagnosis (psychosis/schizophrenia), inflammation, and kynurenine/tryptophan metabolites.

Results

Ten eligible articles were identified where seven studies assessed blood samples and three assessed cerebrospinal fluid in schizophrenic patients.

Of these articles:

Four investigated the relationship between immunoglobulins and the kynurenine pathway and found correlations between IgA-mediated responses and levels of tryptophan metabolites (i.e., kynurenine pathway).

Five examined the correlation between cytokines and kynurenine metabolites where three showed a relationship between elevated IL-6, TNF-α concentrations, and the kynurenine pathway.

Only one study discovered correlations between IL-8 and the kynurenine pathway.

Two studies showed correlations with lower concentrations of IL-4 and the kynurenine pathway.

Moreover, this systematic review did not find a significant correlation between CRP (n = 1 study), IFN-γ (n = 3 studies), and the kynurenine pathway in schizophrenia.

Interpretation

These results emphasize how different inflammatory markers can unbalance the tryptophan/kynurenine pathway in schizophrenia. Several tryptophan/kynurenine pathway metabolites are produced which can, in turn, underlie different psychotic and cognitive symptoms via neurotransmission modulation. However, due to heterogeneity and the shortage of eligible articles, they do not robustly converge to the same findings. Hence, we recommend further studies with larger sample sizes to elucidate the possible interactions between the various markers, their blood vs. CSF ratios, and their correlation with schizophrenia symptoms.

A mood state-specific interaction between kynurenine metabolism and inflammation is present in bipolar disorder

OBJECTIVES

Cytokines are thought to contribute to the pathogenesis of psychiatric symptoms by kynurenine pathway activation. Kynurenine metabolites affect neurotransmission and can cause neurotoxicity. We measured inflammatory markers in patients with bipolar disorder (BD) and studied their relation to kynurenine metabolites and mood.

METHODS

Patients with BD suffering from an acute mood episode were assigned to the depressive (n = 35) or (hypo)manic (n = 32) subgroup. Plasma levels of inflammatory markers [cytokines, C-reactive protein] and kynurenine metabolites [tryptophan (TRP), kynurenine (KYN), 3-hydroxykynurenine (3-HK), quinolinic acid (QA), kynurenic acid (KYNA)] were measured on 6 time points during 8 months follow-up. Biological marker levels in patients were compared to controls (n = 35) and correlated to scores on mood scales. Spearman correlations and linear mixed models were used for statistical analysis.

RESULTS

Twenty patients of the manic subgroup, 29 of the depressive subgroup, and 30 controls completed the study. The manic subgroup had a rapid remission of mood symptoms, but in the depressive subgroup subsyndromal symptoms persisted. No differences in inflammation were found between groups. A strong correlation between tumor necrosis factor-α and KYN, KYN/TRP, 3-HK and QA (ρ > 0.60) was specific for the manic group, but only at baseline (during mania). The depressive subgroup had a lower neuroprotective ratio (KYNA/3-HK, P = .0004) and a strong association between interferon-y and kynurenine pathway activation (P < .0001). KYNA was low in both patient groups versus controls throughout the whole follow-up (P = .0008).

CONCLUSIONS

Mania and chronic depressive symptoms in BD are accompanied by a strong interaction between inflammation and a potentially neurotoxic kynurenine metabolism.

Gut microbiota-brain axis in depression: The role of neuroinflammation

Major depressive disorder (MDD) is a psychiatric condition that affects a large number of people in the world, and the treatment existents do not work for all individuals affected. Thus, it is believed that other systems or pathways which regulate brain networks involved in mood regulation and cognition are associated with MDD pathogenesis. Studies in humans and animal models have been shown that in MDD there are increased levels of inflammatory mediators, including cytokines and chemokines in both periphery and central nervous system (CNS). In addition, microglial activation appears to be a key event that triggers changes in signaling cascades and gene expression that would be determinant for the onset of depressive symptoms. Recent researches also point out that changes in the gut microbiota would lead to a systemic inflammation that in different ways would reach the CNS modulating inflammatory pathways and especially the microglia, which could influence responses to treatments. Moreover, pre- and probiotics have shown antidepressant responses and anti-inflammatory effects. This review will focus on studies that show the relationship of inflammation with the gut microbiota-brain axis and its relation with MDD.

Nocturnal Gamma-Hydroxybutyrate Reduces Cortisol-Awakening Response and Morning Kynurenine Pathway Metabolites in Healthy Volunteers

BACKGROUND

Gamma-hydroxybutyrate (GHB; or sodium oxybate) is an endogenous GHB-/gamma-aminobutyric acid B receptor agonist. It is approved for application in narcolepsy and has been proposed for the potential treatment of Alzheimer's disease, Parkinson's disease, fibromyalgia, and depression, all of which involve neuro-immunological processes. Tryptophan catabolites (TRYCATs), the cortisol-awakening response (CAR), and brain-derived neurotrophic factor (BDNF) have been suggested as peripheral biomarkers of neuropsychiatric disorders. GHB has been shown to induce a delayed reduction of T helper and natural killer cell counts and alter basal cortisol levels, but GHB's effects on TRYCATs, CAR, and BDNF are unknown.

METHODS

Therefore, TRYCAT and BDNF serum levels, as well as CAR and the affective state (Positive and Negative Affect Schedule [PANAS]) were measured in the morning after a single nocturnal dose of GHB (50 mg/kg body weight) in 20 healthy male volunteers in a placebo-controlled, balanced, randomized, double-blind, cross-over design.

RESULTS

In the morning after nocturnal GHB administration, the TRYCATs indolelactic acid, kynurenine, kynurenic acid, 3-hydroxykynurenine, and quinolinic acid; the 3-hydroxykynurenine to kynurenic acid ratio; and the CAR were significantly reduced (P < 0.05-0.001, Benjamini-Hochberg corrected). The quinolinic acid to kynurenic acid ratio was reduced by trend. Serotonin, tryptophan, and BDNF levels, as well as PANAS scores in the morning, remained unchanged after a nocturnal GHB challenge.

CONCLUSIONS

GHB has post-acute effects on peripheral biomarkers of neuropsychiatric disorders, which might be a model to explain some of its therapeutic effects in disorders involving neuro-immunological pathologies. This study was registered at ClinicalTrials.gov as NCT02342366.

Interaction of the immune-inflammatory and the kynurenine pathways in rats resistant to antidepressant treatment in model of depression

The kynurenine pathway (KP), a major route of tryptophan catabolism, may be associated with the pathophysiology of depressive disorders. KP is responsible for ca. 99% of brain tryptophan metabolism via its degradation to kynurenine (KYN) catalyzed by indoleamine 2,3-dioxygenase (IDO). Some cytokines, such as interferon-γ (IFN-γ) and interleukin (IL)-6 are potent inducers of IDO. KYN is further converted by kynurenine aminotransferase (KAT) to the more neuroprotective kynurenic acid or by kynurenine 3-monooxygenase (KMO) to neurotoxic 3-hydroxykynurenine. The aim of the present study was to delineate whether the administration of imipramine (IMI) to rats subjected to chronic mild stress (CMS) may reverse behavioral changes induced by CMS in association with changes in immune-inflammatory markers and KP. We confirmed that the CMS procedure modeled one of the main symptoms of depression, i.e. anhedonia, and administration of IMI for 5 weeks resulted in a significant reduction in anhedonia in a majority of animals (CMS IMI-R animals), whereas 20% of animals did not respond to IMI treatment (CMS IMI-NR animals). We established that CMS procedure increased IFN-γ and IDO mRNA and decreased KAT II mRNA expression in the rat cortex. In the cortex and hippocampus, IMI treatment and non-responsiveness to IMI (in CMS IMI-NR animals) were associated with increased IL-6 mRNA expression. In the spleen, CMS increased production of IFN-γ and IL-6 proteins, while these cytokines were decreased by IMI in CMS IMI-R animals. Chronic IMI administration to CMS rats decreased IDO and KMO mRNA and protein expression and increased KAT II/KMO mRNA and protein ratio in IMI responders (CMS IMI-R) in comparison to CMS rats. In CMS IMI-NR rats, a significant increase in IDO mRNA expression and protein level in comparison with IMI responders was observed. Our findings indicate that resistance to therapeutic action of IMI could be explained by a deficiency of the inhibitory properties of IMI on IDO, KMO and KYN synthesis in the cortex. We conclude that the antidepressant activity of IMI may, at least in part, be explained by modulatory activities on the KAT II/KMO ratio in brain areas.

Tuberculosis - Depression syndemic: A public health challenge

INTRODUCTION

Depression is common in Tuberculosis (TB) and associated with adverse outcomes through pathogenic mechanisms and impaired self-care behaviours including reduced treatment adherence. Undiagnosed depression can threaten the robustness of DOTS model despite large public health investment. The Depression-Tuberculosis Syndemic requires collaborative partnership with mental health professionals.

AIM

To study the evidence base for Depression-Tuberculosis Syndemic.

METHODOLOGY

A Pubmed and Google Scholar search was conducted using the key words "Depression", "Tuberculosis" and "Syndemic" and abstracts screened for appropriateness and relevance.

RESULT

Depression-TB Syndemic is common with a bidirectional relationship. Depression is associated with higher hazard ratio and increased prevalence of TB. Depression is independently associated with higher morbidity, mortality, drug resistance, risk of TB reactivation and community TB transmission. The underlying biopsychosocial mechanism of Depression- Tuberculosis Syndemic includes biological factors like inflammatory cascade, HPA axis dysregulation and psychosocial factors like perceived stigma and treatment non-adherence.

DISCUSSION

Depression is a poor prognostic factor in TB. The National Mental Health Programme (NMHP) and National Strategic Plan (NSP) for Tuberculosis Elimination (2017-2025) work in independent verticals with no integration at policy or at ground level. This results in lack of identification and appropriate management of depression in patients with Tuberculosis despite repeated contact with health care personnel in DOTS centres. A collaborative approach for early diagnosis and management of depression in patients with Tuberculosis (Secondary Prevention) can help decrease the burden of disease and improve outcomes.

CONCLUSION

Depression-TB Syndemic requires collaborative approaches at the program level and at the point of service delivery.

Old Friends, immunoregulation, and stress resilience

There is a considerable body of evidence indicating that chronic adverse experience, especially chronic psychosocial stress/trauma, represents a major risk factor for the development of many somatic and affective disorders, including inflammatory bowel disease (IBD) and posttraumatic stress disorder (PTSD). However, the mechanisms underlying the development of chronic stress-associated disorders are still in large part unknown, and current treatment and prevention strategies lack efficacy and reliability. A greater understanding of mechanisms involved in the development and persistence of chronic stress-induced disorders may lead to novel approaches to prevention and treatment of these disorders. In this review, we provide evidence indicating that increases in immune (re-)activity and inflammation, potentially promoted by a reduced exposure to immunoregulatory microorganisms ("Old Friends") in today's modern society, may be causal factors in mediating the vulnerability to development and persistence of stress-related pathologies. Moreover, we discuss strategies to increase immunoregulatory processes and attenuate inflammation, as for instance contact with immunoregulatory Old Friends, which appears to be a promising strategy to promote stress resilience and to prevent/treat chronic stress-related disorders.

IDO, COX and iNOS have an important role in the proliferation of Neospora caninum in neuron/glia co-cultures

Central nervous system (CNS) is the main site for encystment of Neospora caninum in different animal species. In this tissue, glial cells (astrocytes and microglia) modulate responses to aggression in order to preserve homeostasis and neuronal function. Previous data showed that when primary cultures of glial cells are infected with N. caninum, they develop gliosis and the immune response is characterized by the release of TNF and IL-10, followed by the control of parasite proliferation. In order to elucidate this control, three enzymatic systems involved in parasite-versus-host interactions were observed on a model of neuron/glia co/cultures obtained from rat brains. Indoleamine 2,3-dioxygenase (IDO), induced nitric oxide synthase (iNOS) responsible for the catabolism of tryptophan and arginine, respectively, and cycloxigenase (COX) were studied comparing their modulation by respective inhibitors with the number of tachyzoites or the immune response measured by the release of IL-10 and TNF. Cells were treated with the inhibitors of iNOS (1.5 mM L-NAME), IDO (1 mM 1-methyl tryptophan), COX-1 (1 μM indomethacin) and COX-2 (1 μM nimesulide) before infection with tachyzoites of N. caninum (1:1 cell: parasite). After 72 h of infection, immunocytochemistry showed astrogliosis and a significant increase in the number and length of neurites, compared with uninfected co-cultures, while an increase of IL-10 and TNF was verified. N. caninum did not change iNOS activity, but the inhibition of the basal levels of this enzyme stimulated parasite proliferation. Additionally, a significant increase of about 40% was verified in the IDO activity, whose inhibition caused 1.2-fold increase in parasitic growth. For COX-2 activity, infection of cultures stimulated a significant increase in release of PGE₂ and its inhibition by nimesulide allowed the parasitic growth. These data indicate that iNOS, IDO and COX-2 control the proliferation of N. caninum in this in vitro model. On the other hand, the release of IL-10 by glia besides modulating the inflammation also allow the continuity of parasitism.

Crosstalk Between Inflammation and Glutamate System in Depression: Signaling Pathway and Molecular Biomarkers for Ketamine's Antidepressant Effect

Depression is a worldwide illness with a significant impact on both family and society. Conventional antidepressants are ineffective for more than 30% of patients. In such patients, who have what is called treatment-resistant depression (TRD), inflammatory biomarkers are expressed excessively in both the central nervous system (CNS) and the peripheral blood. Ketamine, a glutamate receptor antagonist, exerts a rapid and sustained therapeutic effect in patients with TRD. Thus, the investigation of the relations between inflammation and glutamate underlying depression has drawn great attention. Inflammation influences glutamate release, transmission, and metabolism, resulting in accumulated extracellular glutamate in the CNS. Downstream of the glutamate receptors, the mammalian target of rapamycin (mTOR) signaling pathway plays a key role in mediating ketamine's antidepressant effect by improving neurogenesis and plasticity. Based on the mechanism and clinical evidence of the inflammatory contribution to the pathogenesis of depression, extensive research has been devoted to inflammatory biomarkers of the clinical response of depression to ketamine. The inconsistent findings from the biomarker investigations are at least partially attributable to the heterogeneity of depression, limited sample size, and complex gene-environment interactions. Deep exploration of the clinical observations and the underlying mechanism of ketamine's antidepressant response can provide new insights into the selection of specific groups of depressed patients for ketamine treatment and to aid in monitoring the therapeutic effect during antidepressant medication. Further, targeting persistent inflammation in patients with TRD and the key molecules mediating ketamine's antidepressant effect may encourage the development of novel therapeutic strategies.

Increased quinolinic acid in peripheral mononuclear cells in Alzheimer's dementia

The role of monocytes and macrophages in the pathogenesis of neurodegenerative disorders such as Alzheimer's disease (AD) is poorly understood. Recently, we have shown that the number of CD14+ monocytes remained constant during healthy aging and in AD patients. Although only little is known about the function of activated macrophages and microglia in AD, one important mechanism involves the expression of quinolinic acid (QUIN), an endogenous N-methyl-D-aspartate glutamate receptor (NMDA-R) agonist which mediates excitotoxicity especially in the hippocampus. We used immunofluorescence stainings of PBMCs to determine the expression of quinolinic acid (QUIN) and the MHC class II molecule HLA-DR in peripheral monocytic cells in 51 healthy volunteers aged 22-87 years and 43 patients with AD at diagnosis (0 weeks) and during the course of rivastigmine treatment at 0.25 year (12 weeks), 0.5 year (30 weeks), 1 year, and 1.5 years. The number of QUIN+ HLA-DR+ cells rises in healthy persons aged 30-40 years compared to persons aged 60-70 years, indicating that this cell population increases with aging. AD patients at diagnosis had an increased frequency of QUIN+, QUIN+ HLA-DR+, and QUIN+ HLA-DR+/HLA-DR+ cells compared to aged-matched controls. These cell populations remained increased in AD for up to one year after initiation of treatment with rivastigmine; no alterations were detected in aged healthy persons. We conclude that the expression of the neurotoxic agent QUIN is increased in peripheral monocytes from AD patients. These cells could enter the brain and contribute to excitotoxicity.

The Transcriptional Regulator BpsR Controls the Growth of Bordetella bronchiseptica by Repressing Genes Involved in Nicotinic Acid Degradation

Many of the pathogenic species of the genus Bordetella have an absolute requirement for nicotinic acid (NA) for laboratory growth. These Gram-negative bacteria also harbor a gene cluster homologous to the nic cluster of Pseudomonas putida which is involved in the aerobic degradation of NA and its transcriptional control. We report here that BpsR, a negative regulator of biofilm formation and Bps polysaccharide production, controls the growth of Bordetella bronchiseptica by repressing the expression of nic genes. The severe growth defect of the ΔbpsR strain in Stainer-Scholte medium was restored by supplementation with NA, which also functioned as an inducer of nic genes at low micromolar concentrations that are usually present in animals and humans. Purified BpsR protein bound to the nic promoter region, and its DNA binding activity was inhibited by 6-hydroxynicotinic acid (6-HNA), the first metabolite of the NA degradative pathway. Reporter assays with the isogenic mutant derivative of the wild-type (WT) strain harboring deletion in nicA, which encodes a putative nicotinic acid hydroxylase responsible for conversion of NA to 6-HNA, showed that 6-HNA is the actual inducer of the nic genes in the bacterial cell. Gene expression profiling further showed that BpsR dually activated and repressed the expression of genes associated with pathogenesis, transcriptional regulation, metabolism, and other cellular processes. We discuss the implications of these findings with respect to the selection of pyridines such as NA and quinolinic acid for optimum bacterial growth depending on the ecological niche.IMPORTANCE BpsR, the previously described regulator of biofilm formation and Bps polysaccharide production, controls Bordetella bronchiseptica growth by regulating the expression of genes involved in the degradation of nicotinic acid (NA). 6-Hydroxynicotinic acid (6-HNA), the first metabolite of the NA degradation pathway prevented BpsR from binding to DNA and was the actual in vivo inducer. We hypothesize that BpsR enables Bordetella bacteria to efficiently and selectively utilize NA for their survival depending on the environment in which they reside. The results reported herein lay the foundation for future investigations of how BpsR and the alteration of its activity by NA orchestrate the control of Bordetella growth, metabolism, biofilm formation, and pathogenesis.

Clinical and Biological Correlates of Neurotoxicity Associated with CAR T-cell Therapy in Patients with B-cell Acute Lymphoblastic Leukemia

CD19-specific chimeric antigen receptor (CAR) T-cell therapy is highly effective against relapsed or refractory acute lymphoblastic leukemia (ALL), but is hindered by neurotoxicity. In 53 adult patients with ALL, we found a significant association of severe neurotoxicity with high pretreatment disease burden, higher peak CAR T-cell expansion, and early and higher elevations of proinflammatory cytokines in blood. Patients with severe neurotoxicity had evidence of blood-cerebrospinal fluid (CSF) barrier disruption correlating with neurotoxicity grade without association with CSF white blood cell count or CAR T-cell quantity in CSF. Proinflammatory cytokines were enriched in CSF during severe neurotoxicity with disproportionately high levels of IL6, IL8, MCP1, and IP10, suggesting central nervous system-specific production. Seizures, seizure-like activity, myoclonus, and neuroimaging characteristics suggested excitatory neurotoxicity, and we found elevated levels of endogenous excitatory agonists in CSF during neurotoxicity.Significance: We detail the neurologic symptoms and blood, CSF, and neuroimaging correlates of neurotoxicity associated with CD19 CAR T cells and identify neurotoxicity risk factors. Our findings implicate cellular components other than T cells and suggest novel links between systemic inflammation and characteristic neurotoxicity symptoms. Cancer Discov; 8(8); 958-71. ©2018 AACR.This article is highlighted in the In This Issue feature, p. 899.

Mechanisms of poststroke fatigue

Poststroke fatigue is a debilitating symptom and is poorly understood. Here we summarise molecular, behavioural and neurophysiological changes related to poststroke fatigue and put forward potential theories for mechanistic understanding of poststroke fatigue.

Low-grade inflammation and tryptophan-kynurenine pathway activation are associated with adverse cardiac remodeling in primary hyperparathyroidism: the EPATH trial

BACKGROUND

Primary hyperparathyroidism (pHPT) is associated with low-grade inflammation, left ventricular hypertrophy and increased cardiovascular mortality, but the association between inflammatory markers and parameters of adverse cardiac remodeling is unknown. We investigated the relationship between C-reactive protein (CRP), the essential amino acid tryptophan and its pro-inflammatory derivatives kynurenine and quinolinic acid (QUIN) with echocardiographic parameters.

METHODS

Cross-sectional baseline data from the "Eplerenone in Primary Hyperparathyroidism" trial were analyzed. Patients with any acute illness were excluded. We assessed associations between CRP, serum levels of tryptophan, kynurenine and QUIN and left ventricular mass index (LVMI), left atrial volume index (LAVI) and E/e'.

RESULTS

Among 136 subjects with pHPT (79% females), 100 (73%) had arterial hypertension and the prevalence of left ventricular hypertrophy was 52%. Multivariate linear regression analyses with LVMI, LAVI and E/e' as respective dependent variables, and C-reactive protein and tryptophan, kynurenine and QUIN as respective independent variables were performed. Analyses were adjusted for age, sex, blood pressure, parathyroid hormone, calcium and other cardiovascular risk factors. LVMI was independently associated with CRP (adjusted β-coefficient=0.193, p=0.030) and QUIN (β=0.270, p=0.007), but not kynurenine. LAVI was related with CRP (β=0.315, p<0.001), kynurenine (β=0.256, p=0.005) and QUIN (β=0.213, p=0.044). E/e' was related with kynurenine (β=0.221, p=0.022) and QUIN (β=0.292, p=0.006). Tryptophan was not associated with any of the remodeling parameters. [Correction added after online publication (22 April 2017: The sentence "Among 136 subjects with pHPT (79% females), 100 (73%) had left ventricular hypertrophy." was corrected to "Among 136 subjects with pHPT (79% females), 100 (73%) had arterial hypertension and the prevalence of left ventricular hypertrophy was 52%."] Conclusions: Cardiac remodeling is common in pHPT and is associated with low-grade inflammation and activation of the tryptophan-kynurenine pathway. The potential role of kynurenine and QUIN as cardiovascular risk factors may be further investigated in future studies.

Inhibition of the kynurenine pathway protects against reactive microglial-associated reductions in the complexity of primary cortical neurons

Brain glia possess the rate limiting enzyme indoleamine 2, 3-dioxygenase (IDO) which catalyses the conversion of tryptophan to kynurenine. Microglia also express kynurenine monooxygenase (KMO) and kynureninase (KYNU) which lead to the production of the free radical producing metabolites, 3-hydroxykynurenine and 3-hydroxyanthranillic acid respectively and subsequently production of the NMDA receptor agonist quinolinic acid. The aim of this study was to examine the effect of IFNγ-stimulated kynurenine pathway (KP) induction in microglia on neurite outgrowth and complexity, and to determine whether alterations could be abrogated using pharmacological inhibitors of the KP. BV-2 microglia were treated with IFNγ (5ng/ml) for 24h and conditioned media (CM) was placed on primary cortical neurons 3 days in vitro (DIV) for 48h. Neurons were fixed and neurite outgrowth and complexity was assessed using fluorescent immunocytochemistry followed by Sholl analysis. Results show increased mRNA expression of IDO, KMO and KYNU, and increased concentrations of tryptophan, kynurenine, and 3-hydroxykynurenine in the CM of IFNγ-stimulated BV-2 microglia. The IFNγ-stimulated BV-2 microglial CM reduced neurite outgrowth and complexity with reductions in various parameters of neurite outgrowth prevented when BV-2 microglia were pre-treated with either the IDO inhibitor, 1-methyltryptophan (1-MT) (L) (0.5mM; 30min), the KMO inhibitor, Ro 61-8048 (1μM; 30min), the synthetic glucocorticoid, dexamethasone (1μM; 2h) -which suppresses IFNγ-induced IDO - and the N-methyl-D-aspartate (NMDA) receptor antagonist, MK801 (0.1μM; 30min). Overall this study indicates that inhibition of the KP in microglia may be targeted to protect against reactive microglial-associated neuronal atrophy.

Evidence for Inflammation-Associated Depression

This chapter explores the evidence supporting inflammation-associated depression. Data to date suggest a bidirectional relationship between inflammation and depression wherein one process can drive the other. A wealth of animal and clinical studies have demonstrated an association between concentrations of pro-inflammatory cytokines - specifically interleukin (IL)-1β, IL-6, and tumor necrosis factor-α - and depressive symptoms. There is also evidence that this pro-inflammatory state is accompanied by aberrant inflammation-related processes including platelet activation factor hyperactivity, oxidative and nitrosative stress, and damage to mitochondria. These complex and interrelated mechanisms can collectively contribute to negative neurobiological outcomes that may, in part, underlie the etiopathology of depression. Mounting evidence has shown a concomitant reduction in both depressive symptoms and pro-inflammatory cytokine concentrations following treatment with pharmacological anti-inflammatory interventions. Taken together, the reviewed preclinical and clinical findings may suggest the existence of a distinct inflammatory subtype of depression in which these patients exhibit unique biochemical and clinical features and may potentially experience improved clinical outcomes with inflammation-targeted pharmacotherapy.

The kynurenine pathway and the brain: Challenges, controversies and promises

Research on the neurobiology of the kynurenine pathway has suffered years of relative obscurity because tryptophan degradation, and its involvement in both physiology and major brain diseases, was viewed almost exclusively through the lens of the well-established metabolite serotonin. With increasing recognition that kynurenine and its metabolites can affect and even control a variety of classic neurotransmitter systems directly and indirectly, interest is expanding rapidly. Moreover, kynurenine pathway metabolism itself is modulated in conditions such as infection and stress, which are known to induce major changes in well-being and behaviour, so that kynurenines may be instrumental in the etiology of psychiatric and neurological disorders. It is therefore likely that the near future will not only witness the discovery of additional physiological and pathological roles for brain kynurenines, but also ever-increasing interest in drug development based on these roles. In particular, targeting the kynurenine pathway with new specific agents may make it possible to prevent disease by appropriate pharmacological or genetic manipulations. The following overview focuses on areas of kynurenine research which are either controversial, of major potential therapeutic interest, or just beginning to receive the degree of attention which will clarify their relevance to neurobiology and medicine. It also highlights technical issues so that investigators entering the field, and new research initiatives, are not misdirected by inappropriate experimental approaches or incorrect interpretations at this time of skyrocketing interest in the subject matter. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

Kynurenic acid and psychotic symptoms and personality traits in twins with psychiatric morbidity

Increased cytokines and kynurenic acid (KYNA) levels in cerebrospinal fluid (CSF) have been reported in patients with schizophrenia and bipolar disorder. The aim of the present study was to investigate cytokines and kynurenines in the CSF of twin pairs discordant for schizophrenia or bipolar disorder and to study these CSF markers in relation to psychotic symptoms and personality traits. CSF levels of tryptophan (TRP), KYNA, quinolinic acid (QUIN), interleukin (IL)-6, IL-8 and tumor necrosis factor-alpha (TNF-α) were analyzed in 23 twins with schizophrenia or bipolar disorder, and in their not affected co-twins. Ratings of psychotic symptoms and personality traits were made using the Scales for Assessment of Negative and Positive symptoms, the Structured Clinical Interview for DSM-IV - Axis II Disorders, and the Schizotypal Personality Questionnaire - Brief. A total score for psychotic symptoms and personality traits was constructed for analysis. CSF KYNA was associated with the score for psychotic symptom and personality traits. TNF-α and IL-8 were associated, and the intra-pair differences scores of TNF-α and IL-8 were highly correlated. Intraclass correlations indicated genetic influences on CSF KYNA, TRP, IL-8 and TNF-α. The association between KYNA and psychotic symptoms further supports a role of KYNA in psychotic disorders.

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.

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.

Increased plasma levels of competing amino acids, rather than lowered plasma tryptophan levels, are associated with a non-response to treatment in major depression

Lowered plasma tryptophan (TRP) and TRP/competing amino acid (CAA) ratio may be involved in the pathophysiology of major depression (MDD). Increased cortisol and immune-inflammatory mediators in MDD may affect the availability of TRP to the brain. We investigated whether baseline or post-treatment TRP, CAAs and TRP/CAA ratio are associated with a treatment response in MDD and whether these effects may be mediated by cortisol or immune biomarkers. We included 50 medication-free MDD patients with a depressive episode (DSM diagnosis) and assessed symptom severity with the Inventory of Depressive Symptomatology (IDS) before and after treatment as usual for 12 weeks (endpoint). Plasma levels of TRP, CAAs, the ratio, cortisol, CRP and 6 selected cytokines were assayed. The primary outcome was a 50% reduction in the IDS, while the secondary was a remission of the depressive episode. In IDS non-responders, CAAs increased and the TRP/CAA ratio decreased, while in IDS responders CAAs decreased and the TRP/CAA ratio increased from baseline to endpoint. In patients who were still depressed at endpoint TRP and CAAs levels had increased from baseline, while in remitted patients no such effects were found. Increases in CAAs were inversely correlated with changes in interleukin-1 receptor antagonist levels. The results show that increased CAA levels from baseline to endpoint are associated with a non-response to treatment in MDD patients. This suggests that the mechanism underpinning the CAA-related treatment resistance may be related to changes in immune pathways. CAA levels and amino acid metabolism may be new drug targets in depression.

Repeated, Intermittent Social Defeat across the Entire Juvenile Period Resulted in Behavioral, Physiological, Hormonal, Immunological, and Neurochemical Alterations in Young Adult Male Golden Hamsters

The developing brain is vulnerable to social defeat during the juvenile period. As complements of human studies, animal models of social defeat provide a straightforward approach to investigating the functional and neurobiological consequences of social defeats. Taking advantage of agonist behavior and social defeat in male golden hamster, a set of 6 experiments was conducted to investigate the consequences at multiple levels in young adulthood resulting from repeated, intermittent social defeats or “social threats” across the entire juvenile period. Male hamsters at postnatal day 28 (P28) were randomly assigned to either the social defeat, “social threat”, or arena control group, and they correspondingly received a series of nine social interaction trials (i.e., either social defeat, “social threat”, or arena control conditions) from P33 to P66. At the behavioral level (Experiment 1), we found that repeated social defeats (but not “social threats”) significantly impacted locomotor activity in the familiar context and social interaction in the familiar/unfamiliar social contexts. At the physiological and hormonal levels (Experiments 2 and 3), repeated social defeat significantly enhanced the cortisol and norepinephrine concentrations in blood. Enlargement of the spleen was also found in the social defeat and “social threat” groups. At the immunological level (Experiment 4), the social defeat group showed lower levels of pro-inflammatory cytokines in the hypothalamus and hippocampus but higher concentration of IL-6 in the striatum compared to the other two groups. At the neurochemical level (Experiment 5), the socially defeated hamsters mainly displayed reductions of dopamine, dopamine metabolites, and 5-HT levels in the striatum and decreased level of 5-HT in the hippocampus. In Experiment 6, an increase in the spine density of hippocampal CA1 pyramidal neurons was specifically observed in the “social threat” group. Collectively, our findings indicate that repeated, intermittent social defeats throughout entire adolescence in hamsters impact their adult responses at multiple levels. Our results also suggest that the “social threat” group may serve as an appropriate control. This study further suggest that the alterations of behavioral responses and neurobiological functions in the body and brain might provide potential markers to measure the negative consequences of chronic social defeats.

Neuroinflammation at the interface of depression and cardiovascular disease: Evidence from rodent models of social stress

A large body of evidence has emerged linking stressful experiences, particularly from one's social environment, with psychiatric disorders. However, vast individual differences emerge in susceptibility to developing stress-related pathology which may be due to distinct differences in the inflammatory response to social stress. Furthermore, depression is an independent risk factor for cardiovascular disease, another inflammatory-related disease, and results in increased mortality in depressed patients. This review is focused on discussing evidence for stress exposure resulting in persistent or sensitized inflammation in one individual while this response is lacking in others. Particular focus will be directed towards reviewing the literature underlying the impact that neuroinflammation has on neurotransmitters and neuropeptides that could be involved in the pathogenesis of comorbid depression and cardiovascular disease. Finally, the theme throughout the review will be to explore the notion that stress-induced inflammation is a key player in the high rate of comorbidity between psychosocial disorders and cardiovascular disease.