Kynurenine Pathway of Tryptophan Metabolism in Neuropsychiatric Disorders: Pathophysiologic and Therapeutic Considerations

Targeted metabolomics suggests a probable role of the FTO gene in the kynurenine pathway in prediabetes

Background

Genome-wide association studies have identified the alpha-ketoglutarate dependent dioxygenase gene (FTO) as the first susceptibility gene of obesity. In the present study, we utilized targeted metabolomics in an attempt to further elucidate mechanisms underlying the action of the FTO gene.

Methods

This study was part of a health survey of employees of the Electricity Generating Authority of Thailand (n = 79, 10 female and 69 male). Targeted metabolomics was performed by liquid chromatography-mass spectrometry using Biocrates AbsoluteIDQ-p180 kit. Genotyping of FTO rs9939609 was performed by real-time PCR (TaqMan™ MGB probes).

Results

Using OPLS-DA variable importance in projection (VIP), tryptophan was found to be among the metabolites with the 10 highest VIP scores. Pearson's correlation analysis showed that kynurenine and tryptophan were positively correlated only in subjects with the rs9939609 A allele (n = 32, r = 0.56, p < 0.001) and the correlation coefficients were significantly higher in subjects having the A allele than in those without the A allele (p < 0.05). Moreover, the kynurenine/tryptophan ratio was significantly associated with the presence of the A allele, independently of body mass index and sex.

Conclusions

The FTO gene is likely to influences the conversion of tryptophan to kynurenine.

Neuromolecular Etiology of Bipolar Disorder: Possible Therapeutic Targets of Mood Stabilizers

Bipolar disorder is a mental illness that causes extreme mood swings and has a chronic course. However, the mechanism by which mood episodes with completely opposite characteristics appear repeatedly, or a mixture of symptoms appears, in patients with bipolar disorder remains unknown. Therefore, mood stabilizers are indicated only for single mood episodes, such as manic episodes and depressive episodes, and no true mood-stabilizing drugs effective for treating both manic and depressive episodes currently exist. Therefore, in this review, therapeutic targets that facilitate the development of mood stabilizers were examined by reviewing the current understanding of the neuromolecular etiology of bipolar disorder.

Elevated serum levels of kynurenine pathway metabolites in patients with Behçet disease

Behçet disease (BD) is an inflammatory, multisystemic vasculitis of unknown etiopathogenesis. However, innate and adaptive immune system involvement and immune-mediated networks play a vital role in the inflammatory cascade. Indoleamine 2,3-dioxygenase 1 (IDO1) is activated in chronic inflammatory states and catalyzes the first and rate-limiting step of tryptophan (TRP) metabolism along the kynurenine pathway (KP). The study aimed to measure KP metabolites levels in patients with BD and investigate the relationship between disease activity and clinical findings with these metabolites. The study included 120 patients with BD and 120 healthy volunteers. Serum TRP, kynurenine (KYN), kynurenic acid (KYNA), 3-hydroxyanthranilic acid (3HAA), 3-hydroxykynurenine (3HK), and quinolinic acid (QUIN) levels were measured with the tandem mass spectrometric method. Demographic data, clinical manifestations, and disease activity score (BDCAF) were recorded. Serum KYN, KYNA, 3HK, 3HAA, QUIN levels, and KYN/TRP ratio were higher (p < 0.05) in patients with BD compared to the control group, while TRP levels were lower (p < 0.05). KYN/TRP ratio and QUIN levels were significantly higher in the presence of neuro-Behçet, while serum KYN levels were significantly higher in the presence of arthritis (p < 0.05). In addition, serum QUIN levels were significantly higher in the presence of thrombosis (p < 0.05). BDCAF score positively correlated with KYN/TRP ratio. Our findings showed that serum KP metabolite levels were elevated in patients with BD, and there is a relationship between these metabolites with disease activity, clinical findings, and inflammatory burden.

Alteration in cerebral blood flow, kynurenines with respect to mood profile in freshly recruited armed forces personnel

The present study is conducted to understand the association of mood profile with the kynurenine pathway (KP) metabolites, and cerebral hemodynamics in freshly recruited central armed forces personnel. Profile of Mood States questionnaire was utilized to assess mood profile, and Total Mood Disturbance (TMD) score was calculated. Transcranial Doppler was used to record blood flow velocity bilaterally of the middle cerebral artery. Chromatographic profile of the kynurenine metabolites was obtained in serum. Further, personnel were stratified according to sociodemographic variables (gender, age and diet) to observe the changes in their KP metabolic status. An activation of the kynurenic acid branch of the KP and the reduction in the mean blood flow velocity, and an increase in Gosling pulsatility index (PI) were observed in females having high TMD score. On gender comparative analysis, kynurenine metabolites of quinolinic acid branch and serotonin were significantly high in males. In males, with increase in age, a significant increase in the quinolinic acid branch of the KP was observed. Furthermore, a significant difference in level metabolites of the KP among the vegetarian and non-vegetarian groups was also observed. In conclusion we observed that increased TMD score was associated with cerebral hypoperfusion and higher vascular resistance along with activation of the KP. Our findings highlighted the importance of multi-facet brain function to showcase the close interaction of various dimensionalities and true picture of the assessee.

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.

Bacterial neurotoxic metabolites in multiple sclerosis cerebrospinal fluid and plasma

The identification of intestinal dysbiosis in patients with neurological and psychiatric disorders has highlighted the importance of gut-brain communication, and yet the question regarding the identity of the components responsible for this cross-talk remains open. We previously reported that relapsing remitting multiple sclerosis patients treated with dimethyl fumarate have a prominent depletion of the gut microbiota, thereby suggesting that studying the composition of plasma and CSF samples from these patients may help to identify microbially derived metabolites. We used a functional xenogeneic assay consisting of cultured rat neurons exposed to CSF samples collected from multiple sclerosis patients before and after dimethyl fumarate treatment to assess neurotoxicity and then conducted a metabolomic analysis of plasma and CSF samples to identify metabolites with differential abundance. A weighted correlation network analysis allowed us to identify groups of metabolites, present in plasma and CSF samples, whose abundance correlated with the neurotoxic potential of the CSF. This analysis identified the presence of phenol and indole group metabolites of bacterial origin (e.g. p-cresol sulphate, indoxyl sulphate and N-phenylacetylglutamine) as potentially neurotoxic and decreased by treatment. Chronic exposure of cultured neurons to these metabolites impaired their firing rate and induced axonal damage, independent from mitochondrial dysfunction and oxidative stress, thereby identifying a novel pathway of neurotoxicity. Clinical, radiological and cognitive test metrics were also collected in treated patients at follow-up visits. Improved MRI metrics, disability and cognition were only detected in dimethyl fumarate-treated relapsing remitting multiple sclerosis patients. The levels of the identified metabolites of bacterial origin (p-cresol sulphate, indoxyl sulphate and N-phenylacetylglutamine) were inversely correlated to MRI measurements of cortical volume and directly correlated to the levels of neurofilament light chain, an established biomarker of neurodegeneration. Our data suggest that phenol and indole derivatives from the catabolism of tryptophan and phenylalanine are microbially derived metabolites, which may mediate gut-brain communication and induce neurotoxicity in multiple sclerosis.

Understanding the Role of the Gut Microbiome in Brain Development and Its Association With Neurodevelopmental Psychiatric Disorders

The gut microbiome has a tremendous influence on human physiology, including the nervous system. During fetal development, the initial colonization of the microbiome coincides with the development of the nervous system in a timely, coordinated manner. Emerging studies suggest an active involvement of the microbiome and its metabolic by-products in regulating early brain development. However, any disruption during this early developmental process can negatively impact brain functionality, leading to a range of neurodevelopment and neuropsychiatric disorders (NPD). In this review, we summarize recent evidence as to how the gut microbiome can influence the process of early human brain development and its association with major neurodevelopmental psychiatric disorders such as autism spectrum disorders, attention-deficit hyperactivity disorder, and schizophrenia. Further, we discuss how gut microbiome alterations can also play a role in inducing drug resistance in the affected individuals. We propose a model that establishes a direct link of microbiome dysbiosis with the exacerbated inflammatory state, leading to functional brain deficits associated with NPD. Based on the existing research, we discuss a framework whereby early diet intervention can boost mental wellness in the affected subjects and call for further research for a better understanding of mechanisms that govern the gut-brain axis may lead to novel approaches to the study of the pathophysiology and treatment of neuropsychiatric disorders.

Role of indoleamine 2,3-dioxygenase 1 (IDO1) and kynurenine pathway in the regulation of the aging process

Indoleamine 2,3-dioxygenase 1 (IDO1) is activated in chronic inflammatory states, e.g., in the aging process and age-related diseases. IDO1 enzyme catabolizes L-tryptophan (L-Trp) into kynurenine (KYN) thus stimulating the KYN pathway. The depletion of L-Trp inhibits the proliferation of immune cells in inflamed tissues and it also reduces serotonin synthesis predisposing to psychiatric disorders. Interestingly, IDO1 protein contains two immunoreceptor tyrosine-based inhibitory motifs (ITIM) which trigger suppressive signaling through the binding of PI3K p110 and SHP-1 proteins. This immunosuppressive activity is not dependent on the catalytic activity of IDO1. KYN and its metabolite, kynurenic acid (KYNA), are potent activators of the aryl hydrocarbon receptor (AhR) which can enhance immunosuppression. IDO1-KYN-AhR signaling counteracts excessive pro-inflammatory responses in acute inflammation but in chronic inflammatory states it has many harmful effects. A chronic low-grade inflammation is associated with the aging process, a state called inflammaging. There is substantial evidence that the activation of the IDO1-KYN-AhR pathway robustly increases with the aging process. The activation of IDO1-KYN-AhR signaling does not only suppress the functions of effector immune cells, probably promoting immunosenescence, but it also impairs autophagy, induces cellular senescence, and remodels the extracellular matrix as well as enhancing the development of osteoporosis and vascular diseases. I will review the function of IDO1-KYN-AhR signaling and discuss its activation with aging as an enhancer of the aging process.

Indoleamine dioxygenase and tryptophan dioxygenase activities are regulated through GAPDH- and Hsp90-dependent control of their heme levels

Indoleamine-2, 3-dioxygenase (IDO1) and Tryptophan-2, 3-dioxygense (TDO) are heme-containing dioxygenases that catalyze the conversion of tryptophan to N-formyl-kynurenine and thus enable generation of l-kynurenine and related metabolites that govern the immune response and broadly impact human biology. Given that TDO and IDO1 activities are directly proportional to their heme contents, it is important to understand their heme delivery and insertion processes. Early studies established that TDO and IDO1 heme levels are sub-saturating in vivo and subject to change but did not identify the cellular mechanisms that provide their heme or enable dynamic changes in their heme contents. We investigated the potential involvement of GAPDH and chaperone Hsp90, based on our previous studies linking these proteins to intracellular heme allocation. We studied heme delivery and insertion into IDO1 and TDO expressed in both normal and heme-deficient HEK293T cells and into IDO1 naturally expressed in HeLa cells in response to IFN-γ, and also investigated the interactions of TDO and IDO1 with GAPDH and Hsp90 in cells and among their purified forms. We found that GAPDH delivered both mitochondrially-generated and exogenous heme to apo-IDO1 and apo-TDO in cells, potentially through a direct interaction with either enzyme. In contrast, we found Hsp90 interacted with apo-IDO1 but not with apo-TDO, and was only needed to drive heme insertion into apo-IDO1. By uncovering the cellular processes that allocate heme to IDO1 and TDO, our study provides new insight on how their activities and l-kynurenine production may be controlled in health and disease.

Seizure modulation by the gut microbiota and tryptophan-kynurenine metabolism in an animal model of infantile spasms

BACKGROUND

The infantile spasms syndrome is an early-onset epileptic encephalopathy presenting in the first 2 years of life, often with severe developmental consequences. The role of the gut microbiota and metabolism in infantile spasms remains unexplored.

METHODS

Employing a brain injury neonatal rat model of infantile spasms intractable to anticonvulsant medication treatments, we determined how the ketogenic diet and antibiotics affected specific microbial communities and the resultant circulating factors that confer spasms protection in the infantile spasms model. To confirm a role of kynurenine metabolism pathway in spasms protection, indoleamine 2,3-dioxygenase 1 was pharmacologically inhibited and comprehensive metabolomics was applied.

FINDINGS

We show that antibiotics reduced spasms and improved the effectiveness of the ketogenic diet when given in combination. Examination of the gut microbiota and metabolomics showed the downregulation of indoleamine 2,3-dioxygenase 1 and upregulation of hippocampal kynurenic acid, a metabolite with antiepileptic effects. To further test the involvement of indoleamine 2,3-dioxygenase 1, a specific antagonist 1-methyltryptophan and minocycline, an antibiotic and inhibitor of kynurenine formation from tryptophan, were administered, respectively. Both treatments were effective in reducing spasms and elevating hippocampal kynurenic acid. A fecal microbiota transplant experiment was then performed to examine the contribution of the gut microbiota on spasm mitigation. Transplant of feces of ketogenic diet animals into normal diet animals was effective in reducing spasms.

INTERPRETATION

These results highlight the importance of tryptophan-kynurenine metabolism in infantile spasms and provide evidence for new-targeted therapies such as indoleamine 2,3-dioxygenase 1 inhibition or microbiota manipulation to promote kynurenic acid production as a strategy to reduce spasms in infantile spasms.

FUNDING

This study was funded by the Alberta Children's Hospital Research Institute and the Owerko Centre.

Genetic variations related to inflammation in suicidal ideation and behavior: A systematic review

BACKGROUND/OBJECTIVES

Immune-inflammatory changes have been found in all types of suicidal ideation and behavior (SIB), independently of associated mental disorders. Since several Single Nucleotide Polymorphisms (SNPs) affect the function of inflammation-related genes, we searched the literature for genetic variations potentially altering inflammatory processes in SIB.

METHODS

We included studies that looked for associations between SIB and SNPs in genes related to inflammatory processes. Case reports, literature reviews, and animal studies were excluded. Articles were retrieved from PubMed and PsycINFO databases, Google Scholar and GreySource Index until September 17th, 2022. Quality was assessed using Q-Genie.

RESULTS

We analyzed 32 studies. SIB has been associated with eighteen SNPs located in genes encoding for interleukin-8 (rs4073), C-reactive protein (rs1130864), tumor necrosis factor α (rs1800629, rs361525, and rs1099724), tumor necrosis factor receptor 2 (rs1061622), transforming growth factor β-1 (rs1982073), acid phosphatase 1 (rs7419262, rs300774), interleukin-10 (rs1800896), interferon γ (rs2430561), amino-carboxy muconate semialdehyde decarboxylase (rs2121337), interleukin 7 (rs10448044, rs10448042), macrophage migration inhibitory factor (rs755622), interleukin 1-α (rs1800587), and interleukin 1-β (rs1143634 and rs16944. A genome-wide association study reported one association at the threshold of significance with the rs300774 SNP, located in the 2p25 region containing ACP1 gene.

DISCUSSION

The studies included were methodologically and clinically diverse and of moderate quality. Their findings suggest that some inflammation-related SNPs could increase the likelihood of SIB but the evidence to date is insufficient. Further research using gene-gene (GxG) and gene-environment (GxE) approaches is warranted.

SYSTEMATIC REVIEW REGISTRATION

[https://www.crd.york.ac.uk], identifier [CRD42022296310].

[Cinnabarinic acid as a potential prognostic marker of schizophrenia]

OBJECTIVE

To develop a method for determining cinnabarinic acid (CA) and its immediate precursor 3-hydroxyanthranylic acid (3HAA) in blood plasma and to study their concentrations in patients with schizophrenia before and after treatment.

MATERIAL AND METHODS

The study was carried out on a sample of 23 female patients with an attack-like progredient schizophrenia (F20.01). The levels of CA and 3HAA in blood plasma were measured using liquid chromatography with tandem mass spectrometry.

RESULTS

We found an inverse statistically significant correlation of the sum of CA and 3HAA concentrations before treatment with the total PANSS score after treatment (R=-0.50; p<0.05). There was also an inverse correlation of the CA concentration of before treatment with the total PANSS score after treatment (R=-0.41, p=0.052), statistically significant at the trend level (0.05<p<0.1).

CONCLUSION

CA concentrations in human blood were determined for the first time. The obtained correlations with clinical data show the potential prognostic significance of determining the levels of CA and 3HAA in blood plasma and increase the relevance of further studies.

Kynurenine pathway in kidney diseases

Kidney diseases have become one of the most common health care problems. Due to a growing number of advanced aged patients with concomitant disorders the prevalence of these diseases will increase over the coming decades. Despite available laboratory tests, accurate and rapid diagnosis of renal dysfunction has yet to be realized, and prognosis is uncertain. Moreover, data on diagnostic and prognostic markers in kidney diseases are lacking. The kynurenine (KYN) pathway is one of the routes of tryptophan (Trp) degradation, with biologically active substances presenting ambiguous properties. The KYN pathway is known to be highly dependent on immunological system activity. As the kidneys are one of the main organs involved in the formation, degradation and excretion of Trp end products, pathologies involving the kidneys result in KYN pathway activity disturbances. This review aims to summarize changes in the KYN pathway observed in the most common kidney disease, chronic kidney disease (CKD), with a special focus on diabetic kidney disease, acute kidney injury (AKI), glomerulonephritis and kidney graft function monitoring. Additionally, the importance of KYN pathway activity in kidney cancer pathogenesis is discussed, as are available pharmacological agents affecting KYN pathway activity in the kidney. Despite limited clinical data, the KYN pathway appears to be a promising target in the diagnosis and prognosis of kidney diseases. Modulation of KYN pathway activity by pharmacological agents should be considered in the treatment of kidney diseases.

Combining Metabolomics and Interpretable Machine Learning to Reveal Plasma Metabolic Profiling and Biological Correlates of Alcohol-Dependent Inpatients: What About Tryptophan Metabolism Regulation?

Alcohol dependence (AD) is a condition of alcohol use disorder in which the drinkers frequently develop emotional symptoms associated with a continuous alcohol intake. AD characterized by metabolic disturbances can be quantitatively analyzed by metabolomics to identify the alterations in metabolic pathways. This study aimed to: i) compare the plasma metabolic profiling between healthy and AD-diagnosed individuals to reveal the altered metabolic profiles in AD, and ii) identify potential biological correlates of alcohol-dependent inpatients based on metabolomics and interpretable machine learning. Plasma samples were obtained from healthy (n = 42) and AD-diagnosed individuals (n = 43). The plasma metabolic differences between them were investigated using liquid chromatography-tandem mass spectrometry (AB SCIEX^® QTRAP 4500 system) in different electrospray ionization modes with scheduled multiple reaction monitoring scans. In total, 59 and 52 compounds were semi-quantitatively measured in positive and negative ionization modes, respectively. In addition, 39 metabolites were identified as important variables to contribute to the classifications using an orthogonal partial least squares-discriminant analysis (OPLS-DA) (VIP > 1) and also significantly different between healthy and AD-diagnosed individuals using univariate analysis (p-value < 0.05 and false discovery rate < 0.05). Among the identified metabolites, indole-3-carboxylic acid, quinolinic acid, hydroxy-tryptophan, and serotonin were involved in the tryptophan metabolism along the indole, kynurenine, and serotonin pathways. Metabolic pathway analysis revealed significant changes or imbalances in alanine, aspartate, glutamate metabolism, which was possibly the main altered pathway related to AD. Tryptophan metabolism interactively influenced other metabolic pathways, such as nicotinate and nicotinamide metabolism. Furthermore, among the OPLS-DA-identified metabolites, normetanephrine and ascorbic acid were demonstrated as suitable biological correlates of AD inpatients from our model using an interpretable, supervised decision tree classifier algorithm. These findings indicate that the discriminatory metabolic profiles between healthy and AD-diagnosed individuals may benefit researchers in illustrating the underlying molecular mechanisms of AD. This study also highlights the approach of combining metabolomics and interpretable machine learning as a valuable tool to uncover potential biological correlates. Future studies should focus on the global analysis of the possible roles of these differential metabolites and disordered metabolic pathways in the pathophysiology of AD.

Tryptophan 2,3-dioxygenase, a novel therapeutic target for Parkinson's disease

INTRODUCTION

Alterations in the activity of tryptophan 2,3-dioxygenase (TDO) cause imbalances in the levels of serotonin and other neuroactive metabolites which can contribute to motor, psychiatric, gastrointestinal, and other dysfunctions often seen in Parkinson's disease (PD). TDO is a key enzyme of tryptophan metabolism at the entry of the kynurenine pathway (KP) which moderates production of neuroactive compounds primarily outside the central nervous system (CNS). Recent data from experimental models indicate that TDO modulation could have beneficial effects on PD symptoms not targeted by traditional dopamine substitution therapies.

AREAS COVERED

Based on data available in PubMed and ClinicalTrials databases up until 1 August 2021, we summarize current knowledge of KP alterations in relation to PD. We overview effects of TDO inhibition in preclinical models of neurodegeneration and discuss findings of the impact of enzyme inhibition on motor, memory and gastrointestinal dysfunctions, and neuronal cell loss.

EXPERT OPINION

TDO inhibition potentially alleviates motor and non-motor dysfunctions of PD. However, data suggesting harmful effects of long-term TDO inhibition raise concerns. To exploit possibilities of TDO inhibitory treatment, development of further selective TDO inhibitor compounds with good bioavailability features and models adequately replicating PD symptoms of systemic origin should be prioritized.

A comprehensive analysis of TDO2 expression in immune cells and characterization of immune cell phenotype in TDO2 knockout mice

Tryptophan 2,3-dioxygenase (TDO2) was an initial rate-limiting enzyme of the kynurenine (Kyn) pathway in tryptophan (Trp) metabolism. We undertook this study to determine a comprehensive analysis of TDO2 expression in immune cells and assess the characterization of immune cell phenotype in TDO2 knockout mice. The expression of TDO2 in various tissues of DBA/1 mice was detected by quantitative real-time PCR (qPCR) and immunohistochemistry. Both flow cytometry and immunofluorescence were used to analyze the expression of TDO2 in immune cells. Furthermore, TDO2 knockout (KO) mice were generated by CRISPR/Cas9 technology to detect immune cell phenotype. TDO2 protein level in liver was tested by western blot. High-performance liquid chromatography was used to detect the level of Trp and Kyn. Flow cytometry was used to test the proportions of splenic lymphocyte subsets in wild-type (WT) and TDO2 KO mice. We found that TDO2 was expressed in various tissues and immune cells, and TDO2 staining was mainly observed in the cytoplasm of cells. There was no difference in the development of immune cells between TDO2 KO mice and WT mice, including T cells, B cells, memory B cells, plasma cells, dendritic cells, and natural killer cells. Interestingly, the reduced M1/M2 ratio was observed in the peritoneal macrophages of TDO2 KO mice. Taken together, these findings enriched the known expression profile of TDO2, especially its expression in immune cells. Our study suggested that TDO2-mediated Trp-Kyn metabolism pathway might be involved in the immune response.

A Potential Interface between the Kynurenine Pathway and Autonomic Imbalance in Schizophrenia

Schizophrenia is a neuropsychiatric disorder characterized by various symptoms including autonomic imbalance. These disturbances involve almost all autonomic functions and might contribute to poor medication compliance, worsened quality of life and increased mortality. Therefore, it has a great importance to find a potential therapeutic solution to improve the autonomic disturbances. The altered level of kynurenines (e.g., kynurenic acid), as tryptophan metabolites, is almost the most consistently found biochemical abnormality in schizophrenia. Kynurenic acid influences different types of receptors, most of them involved in the pathophysiology of schizophrenia. Only few data suggest that kynurenines might have effects on multiple autonomic functions. Publications so far have discussed the implication of kynurenines and the alteration of the autonomic nervous system in schizophrenia independently from each other. Thus, the coupling between them has not yet been addressed in schizophrenia, although their direct common points, potential interfaces indicate the consideration of their interaction. The present review gathers autonomic disturbances, the impaired kynurenine pathway in schizophrenia, and the effects of kynurenine pathway on autonomic functions. In the last part of the review, the potential interaction between the two systems in schizophrenia, and the possible therapeutic options are discussed.

Immune Influencers in Action: Metabolites and Enzymes of the Tryptophan-Kynurenine Metabolic Pathway

The tryptophan (TRP)-kynurenine (KYN) metabolic pathway is a main player of TRP metabolism through which more than 95% of TRP is catabolized. The pathway is activated by acute and chronic immune responses leading to a wide range of illnesses including cancer, immune diseases, neurodegenerative diseases and psychiatric disorders. The presence of positive feedback loops facilitates amplifying the immune responses vice versa. The TRP-KYN pathway synthesizes multifarious metabolites including oxidants, antioxidants, neurotoxins, neuroprotectants and immunomodulators. The immunomodulators are known to facilitate the immune system towards a tolerogenic state, resulting in chronic low-grade inflammation (LGI) that is commonly present in obesity, poor nutrition, exposer to chemicals or allergens, prodromal stage of various illnesses and chronic diseases. KYN, kynurenic acid, xanthurenic acid and cinnabarinic acid are aryl hydrocarbon receptor ligands that serve as immunomodulators. Furthermore, TRP-KYN pathway enzymes are known to be activated by the stress hormone cortisol and inflammatory cytokines, and genotypic variants were observed to contribute to inflammation and thus various diseases. The tryptophan 2,3-dioxygenase, the indoleamine 2,3-dioxygenases and the kynurenine-3-monooxygenase are main enzymes in the pathway. This review article discusses the TRP-KYN pathway with special emphasis on its interaction with the immune system and the tolerogenic shift towards chronic LGI and overviews the major symptoms, pro- and anti-inflammatory cytokines and toxic and protective KYNs to explore the linkage between chronic LGI, KYNs, and major psychiatric disorders, including depressive disorder, bipolar disorder, substance use disorder, post-traumatic stress disorder, schizophrenia and autism spectrum disorder.

Some CSF Kynurenine Pathway Intermediates Associated with Disease Evolution in Amyotrophic Lateral Sclerosis

The aim of this study was to evaluate the kynurenine pathway (KP) and amino acids profile, using mass spectrometry, in the cerebrospinal fluid (CSF) of 42 amyotrophic lateral sclerosis (ALS) patients at the diagnosis and 40 controls to detect early disorders of these pathways. Diagnostic and predictive ability (based on weight loss, forced vital capacity, ALS Functional Rating Scale-Revised evolution over 12 months, and survival time) of these metabolites were evaluated using univariate followed by supervised multivariate analysis. The multivariate model between ALS and controls was not significant but highlighted some KP metabolites (kynurenine (KYN), kynurenic acid (KYNA), 3-Hydroxynurenine (3-HK)/KYNA ratio), and amino acids (Lysine, asparagine) as involved in the discrimination between groups (accuracy 62%). It revealed a probable KP impairment toward neurotoxicity in ALS patients and in bulbar forms. Regarding the prognostic effect of metabolites, 12 were commonly discriminant for at least 3 of 4 disease evolution criteria. This investigation was crucial as it did not show significant changes in CSF concentrations of amino acids and KP intermediates in early ALS evolution. However, trends of KP modifications suggest further exploration. The unclear kinetics of neuroinflammation linked to KP support the interest in exploring these pathways during disease evolution through a longitudinal strategy.

Addiction and the kynurenine pathway: A new dancing couple?

Drug use poses a serious threat to health systems throughout the world and the number of consumers rises relentlessly every year. The kynurenine pathway, main pathway of tryptophan degradation, has drawn interest in this field due to its relationship with addictive behaviour. Recently it has been confirmed that modulation of kynurenine metabolism at certain stages of the pathway can reduce, prevent or abolish drug seeking-like behaviours in studies with several different drugs. In this review, we present an up-to-date summary of the evidences of a relationship between drug use and the kynurenine pathway, both the alterations of the pathway due to drug use as well as modulation of the pathway as a potential approach to treat drug addiction. The review discusses ethanol, nicotine, cannabis, amphetamines, cocaine and opioids and new prospects in the drug research field are proposed.

Involvement of Kynurenine Metabolism in Bipolar Disorder: An Updated Review

Bipolar disorder (BD) is a severe affective disorder, mainly characterized by alternative depressive and manic or hypomanic episodes, yet the pathogenesis of BD has not been fully elucidated. Recent researches have implicated the altered kynurenine (KYN) metabolism involved in the neurobiology of BD. Excessive activation of the immune system also occurs in patients with BD, which further accelerates the KYN pathway for tryptophan metabolism. Changes of the KYN metabolites have effects on neuronal receptors and are involved in neuroendocrine transmissions. Interactions between KYN metabolism and the immune system may contribute to the neuropathogenesis of BD. Various studies have shown that alterations of the KYN metabolites were associated with mood, psychotic symptoms, and cognitive functions in patients with BD. In this review, we briefly introduce the KYN pathway and describe the immune dysregulation in BD as well as their interactions. We then focus on the research advances on the KYN metabolism in BD, which hold promise for identifying novel treatment targets in patients stricken with this disorder.