The Kynurenine Pathway and Kynurenine 3-Monooxygenase Inhibitors

Revolutionizing our understanding of Parkinson's disease: Dr. Heinz Reichmann's pioneering research and future research direction

Millions of individuals around the world are afflicted with Parkinson's disease (PD), a prevalent and incapacitating neurodegenerative disorder. Dr. Reichmann, a distinguished professor and neurologist, has made substantial advancements in the domain of PD research, encompassing both fundamental scientific investigations and practical applications. His research has illuminated the etiology and treatment of PD, as well as the function of energy metabolism and premotor symptoms. As a precursor to a number of neurotransmitters and neuromodulators that are implicated in the pathophysiology of PD, he has also investigated the application of tryptophan (Trp) derivatives in the disease. His principal findings and insights are summarized and synthesized in this narrative review article, which also emphasizes the challenges and implications for future PD research. This narrative review aims to identify and analyze the key contributions of Reichmann to the field of PD research, with the ultimate goal of informing future research directions in the domain. By examining Reichmann's work, the study seeks to provide a comprehensive understanding of his major contributions and how they can be applied to advance the diagnosis and treatment of PD. This paper also explores the potential intersection of Reichmann's findings with emerging avenues, such as the investigation of Trp and its metabolites, particularly kynurenines, which could lead to new insights and potential therapeutic strategies for managing neurodegenerative disorders like PD.

Depression, stress, and tryptophan metabolism through the kynurenine pathway: treatment strategies from the perspective of Chinese herbal medicine

The pathogenesis of depression is complex, involving abnormalities in tryptophan (TRP) metabolism through the kynurenine pathway (KP). Moreover, depression is closely related to the hypothalamic-pituitary-adrenal (HPA) axis, the gut-brain axis, neuroinflammation, and stress. These factors collectively influence the multidimensional pathological mechanisms of depression. TRP, a fundamental amino acid, serves as a precursor for neuroactive metabolites vital to physiological functions. Central to TRP metabolism is the KP, and the imbalance between neurotoxic and neuroprotective metabolites is closely related to the onset and progression of depression. Therefore, maintaining the balance of KP metabolites is important. In this review, we have investigated the role of the KP in depression and explored the complexity of KP dysregulation and its therapeutic importance. Here, we highlight how a deeper understanding of the KP and its regulation can pave the way for new treatment strategies. Specifically, we have summarized the latest advances in elucidating the key mechanisms of rate-limiting enzyme inhibitors, providing insights into their potential therapeutic efficacy. In addition, we have explored the emerging field of Chinese herbal medicine, discussing its potential to regulate KP metabolites and alleviate depressive symptoms, thereby expanding the treatment options for depression. Our findings emphasize the multifaceted nature of depression and the necessity of interdisciplinary research to fully utilize KP regulation and Chinese herbal medicine as strategies to advance the treatment of depression.

Usefulness of the 3-hydroxykynurenine/kynurenic acid ratio as a diagnostic biomarker for diffuse larger B-cell lymphoma

OBJECTIVES

Reports have shown that the kynurenine pathway, one of the pathways by which tryptophan is metabolized, is activated in patients with diffuse large B-cell lymphoma (DLBCL). Activation of the kynurenine pathway triggers the production of various metabolites, such as kynurenine (Kyn), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranilic acid (3-HAA), kynurenic acid (KA), and anthranilic acid (AA), which contribute to immune tolerance. The current study aimed to investigate the changes in metabolites of kynurenine pathway in DLBCL patients and evaluate their performance predicting DLBCL.

METHODS

Changes in metabolites of kynurenine pathway were examined using high-performance liquid chromatography in 35 DLBCL patients (age 61.2 ± 13.5 years) and 44 healthy controls (age 58.5 ± 12.5 years).

RESULTS

DLBCL patients had significantly higher levels of 3-HK, AA, and 3-HAA but lower levels of tryptophan (Trp) and KA compared to healthy controls. Given that the ratio of each metabolite represents the change in the Kyn pathway, the 3-HK/KA ratio was examined. Notably, DLBCL patients had a significantly higher 3-HK/KA ratio compared to healthy controls. In DLBCL, the area under the receiver operative characteristic (ROC) curve for 3-HK/KA (0.999) was higher than that for lactate dehydrogenase (0.885) and comparable to that for soluble interleukin-2 receptor (sIL-2R) (0.997). Based on ROC curve analysis, the 3-HK/KA ratio was found to be useful biomarker for the diagnosis of DLBCL.

CONCLUSION

Our results suggest that the 3-HK/KA ratio is a clinically useful biomarker of DLBCL. Moreover, its combination with existing markers, such as sIL-2R, can improve its effectiveness of diagnosing DLBCL.

Level of selected exponents of the kynurenine pathway in patients diagnosed with depression and selected cancers

Altered immune system activity is one of the common pathomechanisms of depressive disorders and cancer. The aim of this study is to evaluate level of selected elements of the kynurenine pathway in groups of depressed and oncological patients. The study included 156 individuals, aged 19-65 years (M = 43.46, SD = 13.99), divided into three groups, namely depressive disorders (DD), oncology patients (OG), and a comparison group of healthy subjects (CG). A sociodemographic questionnaire and the Hamilton Depression Rating Scale (HDRS) were used in the study to assess the intensity of depressive symptoms. Level of TDO2, L-KYN, HK, AA and QA was significantly higher in patients from OG and DD groups than in the comparison group. TDO2 level in the OG group was positively correlated with the severity of depressive symptoms. When the OG and DD groups were analyzed together, level of TDO2, 3-HKYN, AA, QA correlated positively with the severity of depressive symptoms. Thus, kynurenine pathway might play an integral role in the pathogenesis of depression.

Molecular mechanisms and therapeutic significance of Tryptophan Metabolism and signaling in cancer

Tryptophan (Trp) metabolism involves three primary pathways: the kynurenine (Kyn) pathway (KP), the 5-hydroxytryptamine (serotonin, 5-HT) pathway, and the indole pathway. Under normal physiological conditions, Trp metabolism plays crucial roles in regulating inflammation, immunity, and neuronal function. Key rate-limiting enzymes such as indoleamine-2,3-dioxygenase (IDO), Trp-2,3-dioxygenase (TDO), and kynurenine monooxygenase (KMO) drive these metabolic processes. Imbalances in Trp metabolism are linked to various cancers and often correlate with poor prognosis and adverse clinical characteristics. Dysregulated Trp metabolism fosters tumor growth and immune evasion primarily by creating an immunosuppressive tumor microenvironment (TME). Activation of the KP results in the production of immunosuppressive metabolites like Kyn, which modulate immune responses and promote oncogenesis mainly through interaction with the aryl hydrocarbon receptor (AHR). Targeting Trp metabolism therapeutically has shown significant potential, especially with the development of small-molecule inhibitors for IDO1, TDO, and other key enzymes. These inhibitors disrupt the immunosuppressive signals within the TME, potentially restoring effective anti-tumor immune responses. Recently, IDO1 inhibitors have been tested in clinical trials, showing the potential to enhance the effects of existing cancer therapies. However, mixed results in later-stage trials underscore the need for a deeper understanding of Trp metabolism and its complex role in cancer. Recent advancements have also explored combining Trp metabolism inhibitors with other treatments, such as immune checkpoint inhibitors, chemotherapy, and radiotherapy, to enhance therapeutic efficacy and overcome resistance mechanisms. This review summarizes the current understanding of Trp metabolism and signaling in cancer, detailing the oncogenic mechanisms and clinical significance of dysregulated Trp metabolism. Additionally, it provides insights into the challenges in developing Trp-targeted therapies and future research directions aimed at optimizing these therapeutic strategies and improving patient outcomes.

Integrating network pharmacology and bioinformatics to explore the mechanism of Xiaojian Zhongtang in treating major depressive disorder: An observational study

Major depressive disorder (MDD) is a common mental illness. The traditional Chinese medicine compound Xiaojian Zhongtang (XJZT) has a good therapeutic effect on MDD, but the specific mechanism is not clear. The aim of this study is to explore the molecular mechanism of XJZT in the treatment of MDD through network pharmacology and bioinformatics. The traditional Chinese medicine system pharmacology database was used to screen the chemical components and targets of XJZT, while the online Mendelian inheritance in man, DisGeNET, Genecards, and therapeutic target database databases were used to collect MDD targets and identify the intersection targets of XJZT and MDD. A "drugs-components-targets" network was constructed using the Cytoscape platform, and the STRING was used for protein-protein interaction analysis of intersecting targets. Gene Ontology and Kyoto encyclopedia of genes and genomes analysis of intersecting targets was performed using the DAVID database. Obtain serum and brain transcriptome datasets of MDD from the gene expression omnibus database, and perform differentially expressed genes, weighted gene co-expression network analysis, gene set enrichment analysis, and receiver operating characteristic analysis. A total of 127 chemical components and 767 targets were obtained from XJZT, among which quercetin, kaempferol, and maltose are the core chemical components, and 1728 MDD targets were screened out, with 77 intersecting targets between XJZT and MDD. These targets mainly involve AGE-RAGE signaling pathway in diabetic complexes, epidermal growth factor receptor tyrosine kinase inhibitor resistance, and HIF-1 signaling pathway, and these core targets have strong binding activity with core components. In addition, 1166 differentially expressed genes were identified in the MDD serum transcriptome dataset, and weighted gene co-expression network analysis identified the most relevant gene modules (1269 genes), among which RAC-alpha serine/threonine-protein kinase (AKT1), D(4) dopamine receptor (DRD4), and kynurenine 3-monooxygenase (KMO) were target genes for the treatment of MDD with XJZT, these 3 genes are mainly related to the ubiquitin-mediated proteolysis, arachidonic acid (AA) metabolism, and Huntington disease pathways, and the expression of AKT1, DRD4, and KMO was also found in the MDD brain transcriptome dataset, which is significantly correlated with the occurrence of MDD. We have identified 3 key targets for XJZT treatment of MDD, including AKT1, KMO, and DRD4, and they can be regulated by the key components of XJZT, including quercetin, maltose, and kaempferol. This provides valuable insights for the early clinical diagnosis and development of therapeutic drugs for MDD.

The Kynurenine Pathway in Gut Permeability and Inflammation

The gut-brain axis (GBA) is a crucial communication network linking the gastrointestinal (GI) tract and the central nervous system (CNS). The gut microbiota significantly influences metabolic, immune, and neural functions by generating a diverse array of bioactive compounds that modulate brain function and maintain homeostasis. A pivotal mechanism in this communication is the kynurenine pathway, which metabolises tryptophan into various derivatives, including neuroactive and neurotoxic compounds. Alterations in gut microbiota composition can increase gut permeability, triggering inflammation and neuroinflammation, and contributing to neuropsychiatric disorders. This review elucidates the mechanisms by which changes in gut permeability may lead to systemic inflammation and neuroinflammation, with a focus on the kynurenine pathway. We explore how probiotics can modulate the kynurenine pathway and reduce neuroinflammation, highlighting their potential as therapeutic interventions for neuropsychiatric disorders. The review integrates experimental data, discusses the balance between neurotoxic and neuroprotective kynurenine metabolites, and examines the role of probiotics in regulating inflammation, cognitive development, and gut-brain axis functions. The insights provided aim to guide future research and therapeutic strategies for mitigating GI complaints and their neurological consequences.

Xanthurenic acid: A role in brain intercellular signaling

Xanthurenic acid (XA) raises a growing multidisciplinary interest based upon its oxidizing properties, its ability to complex certain metal ions, and its detoxifier capacity of 3-hydroxykynurenine (3-HK), its brain precursor. However, little is still known about the role and mechanisms of action of XA in the central nervous system (CNS). Therefore, many research groups have recently investigated XA and its central functions extensively. The present paper critically reviews and discusses all major data related to XA properties and neuronal activities to contribute to the improvement of the current knowledge on XA's central roles and mechanisms of action. In particular, our data showed the existence of a specific G-protein-coupled receptor (GPCR) for XA localized exclusively in brain neurons exhibiting Ca2+-dependent dendritic release and specific electrophysiological responses. XA properties and central activities suggest a role for this compound in brain intercellular signaling. Indeed, XA stimulates cerebral dopamine (DA) release contrary to its structural analog, kynurenic acid (KYNA). Thus, KYNA/XA ratio could be fundamental in the regulation of brain glutamate and DA release. Cerebral XA may also represent an homeostatic signal between the periphery and several brain regions where XA accumulates easily after peripheral administration. Therefore, XA status in certain psychoses or neurodegenerative diseases seems to be reinforced by its brain-specific properties in balance with its formation and peripheral inputs.

The Complex World of Kynurenic Acid: Reflections on Biological Issues and Therapeutic Strategy

It has been unequivocally established that kynurenic acid has a number of actions in a variety of cells and tissues, raising, in principle, the possibility of targeting its generation, metabolism or sites of action to manipulate those effects to a beneficial therapeutic end. However, many basic aspects of the biology of kynurenic acid remain unclear, potentially leading to some confusion and misinterpretations of data. They include questions of the source, generation, targets, enzyme expression, endogenous concentrations and sites of action. This essay is intended to raise and discuss many of these aspects as a source of reference for more balanced discussion. Those issues are followed by examples of situations in which modulating and correcting kynurenic acid production or activity could bring significant therapeutic benefit, including neurological and psychiatric conditions, inflammatory diseases and cell protection. More information is required to obtain a clear overall view of the pharmacological environment relevant to kynurenic acid, especially with respect to the active concentrations of kynurenine metabolites in vivo and changed levels in disease. The data and ideas presented here should permit a greater confidence in appreciating the sites of action and interaction of kynurenic acid under different local conditions and pathologies, enhancing our understanding of kynurenic acid itself and the many clinical conditions in which manipulating its pharmacology could be of clinical value.

Eicosanoid signaling in neuroinflammation associated with Alzheimer's disease

Alzheimer's disease (AD) is a prevalent neurodegenerative condition affecting a substantial portion of the global population. It is marked by a complex interplay of factors, including the accumulation of amyloid plaques and tau tangles within the brain, leading to neuroinflammation and neuronal damage. Recent studies have underscored the role of free lipids and their derivatives in the initiation and progression of AD. Eicosanoids, metabolites of polyunsaturated fatty acids like arachidonic acid (AA), emerge as key players in this scenario. Remarkably, eicosanoids can either promote or inhibit the development of AD, and this multifaceted role is determined by how eicosanoid signaling influences the immune responses within the brain. However, the precise molecular mechanisms dictating the dual role of eicosanoids in AD remain elusive. In this comprehensive review, we explore the intricate involvement of eicosanoids in neuronal function and dysfunction. Furthermore, we assess the therapeutic potential of targeting eicosanoid signaling pathways as a viable strategy for mitigating or halting the progression of AD.

Kynurenines as a Novel Target for the Treatment of Inflammatory Disorders

This review discusses the potential of targeting the kynurenine pathway (KP) in the treatment of inflammatory diseases. The KP, responsible for the catabolism of the amino acid tryptophan (TRP), produces metabolites that regulate various physiological processes, including inflammation, cell cycle, and neurotransmission. These metabolites, although necessary to maintain immune balance, may accumulate excessively during inflammation, leading to systemic disorders. Key KP enzymes such as indoleamine 2,3-dioxygenase 1 (IDO1), indoleamine 2,3-dioxygenase 2 (IDO2), tryptophan 2,3-dioxygenase (TDO), and kynurenine 3-monooxygenase (KMO) have been considered promising therapeutic targets. It was highlighted that both inhibition and activation of these enzymes may be beneficial, depending on the specific inflammatory disorder. Several inflammatory conditions, including autoimmune diseases, for which modulation of KP activity holds therapeutic promise, have been described in detail. Preclinical studies suggest that this modulation may be an effective treatment strategy for diseases for which treatment options are currently limited. Taken together, this review highlights the importance of further research on the clinical application of KP enzyme modulation in the development of new therapeutic strategies for inflammatory diseases.

Kynurenines and Inflammation: A Remarkable Axis for Multiple Sclerosis Treatment

Multiple sclerosis (MS) is a chronic inflammatory autoimmune neurological disease characterized by the recurrent appearance of demyelinating lesions and progressive disability. Currently, there are multiple disease-modifying treatments, however, there is a significant need to develop new therapeutic targets, especially for the progressive forms of the disease. This review article provides an overview of the most recent studies aimed at understanding the inflammatory processes that are activated in response to the accumulation of kynurenine pathway (KP) metabolites, which exacerbate an imbalance between immune system cells (e.g., Th1, Th2, and T reg) and promote the release of pro-inflammatory interleukins that modulate different mechanisms: membrane-receptors function; nuclear factors expression; and cellular signals. Together, these alterations trigger cell death mechanisms in brain cells and promote neuron loss and axon demyelination. This hypothesis could represent a remarkable approach for disease-modifying therapies for MS. Here, we also provide a perspective on the repositioning of some already approved drugs involved in other signaling pathways, which could represent new therapeutic strategies for MS treatment.

Identification and experimental validation of KMO as a critical immune-associated mitochondrial gene in unstable atherosclerotic plaque

BACKGROUND

The heightened risk of cardiovascular and cerebrovascular events is associated with the increased instability of atherosclerotic plaques. However, the lack of effective diagnostic biomarkers has impeded the assessment of plaque instability currently. This study was aimed to investigate and identify hub genes associated with unstable plaques through the integration of various bioinformatics tools, providing novel insights into the detection and treatment of this condition.

METHODS

Weighted Gene Co-expression Network Analysis (WGCNA) combined with two machine learning methods were used to identify hub genes strongly associated with plaque instability. The cell-type identification by estimating relative subsets of RNA transcripts (CIBERSORT) method was utilized to assess immune cell infiltration patterns in atherosclerosis patients. Additionally, Gene Set Variation Analysis (GSVA) was conducted to investigate the potential biological functions, pathways, and mechanisms of hub genes associated with unstable plaques. To further validate the diagnostic efficiency and expression of the hub genes, immunohistochemistry (IHC), quantitative real-time polymerase chain reaction (RT-qPCR), and enzyme-linked immunosorbent assay (ELISA) were performed on collected human carotid plaque and blood samples. Immunofluorescence co-staining was also utilized to confirm the association between hub genes and immune cells, as well as their colocalization with mitochondria.

RESULTS

The CIBERSORT analysis demonstrated a significant decrease in the infiltration of CD8 T cells and an obvious increase in the infiltration of M0 macrophages in patients with atherosclerosis. Subsequently, two highly relevant modules (blue and green) strongly associated with atherosclerotic plaque instability were identified. Through intersection with mitochondria-related genes, 50 crucial genes were identified. Further analysis employing least absolute shrinkage and selection operator (LASSO) logistic regression and support vector machine recursive feature elimination (SVM-RFE) algorithms revealed six hub genes significantly associated with plaque instability. Among them, NT5DC3, ACADL, SLC25A4, ALDH1B1, and MAOB exhibited positive correlations with CD8 T cells and negative correlations with M0 macrophages, while kynurenine 3-monooxygenas (KMO) demonstrated a positive correlation with M0 macrophages and a negative correlation with CD8 T cells. IHC and RT-qPCR analyses of human carotid plaque samples, as well as ELISA analyses of blood samples, revealed significant upregulation of KMO and MAOB expression, along with decreased ALDH1B1 expression, in both stable and unstable samples compared to the control samples. However, among the three key genes mentioned above, only KMO showed a significant increase in expression in unstable plaque samples compared to stable plaque samples. Furthermore, the expression patterns of KMO in human carotid unstable plaque tissues and cultured mouse macrophage cell lines were assessed using immunofluorescence co-staining techniques. Finally, lentivirus-mediated KMO silencing was successfully transduced into the aortas of high-fat-fed ApoE-/- mice, with results indicating that KMO silencing attenuated plaque formation and promoted plaque stability in ApoE-/- mice.

CONCLUSIONS

The results suggest that KMO, a mitochondria-targeted gene associated with macrophage cells, holds promise as a valuable diagnostic biomarker for assessing the instability of atherosclerotic plaques.

The role of the kynurenine pathway in cardiovascular disease

The kynurenine pathway (KP) serves as the primary route for tryptophan metabolism in most mammalian organisms, with its downstream metabolites actively involved in various physiological and pathological processes. Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) serve as the initial and pivotal enzymes of the KP, with IDO playing important and intricate roles in cardiovascular diseases. Multiple metabolites of KP have been observed to exhibit elevated concentrations in plasma across various cardiovascular diseases, such as atherosclerosis, hypertension, and acute myocardial infarction. Multiple studies have indicated that kynurenine (KYN) may serve as a potential biomarker for several adverse cardiovascular events. Furthermore, Kynurenine and its downstream metabolites have complex roles in inflammation, exhibiting both inhibitory and stimulatory effects on inflammatory responses under different conditions. In atherosclerosis, upregulation of IDO stimulates KYN production, mediating aromatic hydrocarbon receptor (AhR)-induced exacerbation of vascular inflammation and promotion of foam cell formation. Conversely, in arterial calcification, this mediation alleviates osteogenic differentiation of vascular smooth muscle cells. Additionally, in cardiac remodeling, KYN-mediated AhR activation exacerbates pathological left ventricular hypertrophy and fibrosis. Interventions targeting components of the KP, such as IDO inhibitors, 3-hydroxyanthranilic acid, and anthranilic acid, demonstrate cardiovascular protective effects. This review outlines the mechanistic roles of KP in coronary atherosclerosis, arterial calcification, and myocardial diseases, highlighting the potential diagnostic, prognostic, and therapeutic value of KP in cardiovascular diseases, thus providing novel insights for the development and application of related drugs in future research.

The influence of the model pesticides parathion and paraoxon on human cytochrome P450 and associated oxygenases in HepaRG cells

INTRODUCTION

Intentional and unintentional organophosphorus pesticide exposure is a public health concern. Organothiophosphate compounds require metabolic bioactivation by the cytochrome P450 system to their corresponding oxon analogues to act as potent inhibitors of acetylcholinesterase. It is known that interactions between cytochrome P450 and pesticides include the inhibition of major xenobiotic metabolizing cytochrome P450 enzymes and changes on the genetic level.

METHODS

In this in vitro study, the influence of the pesticides parathion and paraoxon on human cytochrome P450 and associated oxygenases was investigated with a metabolically competent cell line (HepaRG cells). First, the viability of the cells after exposure to parathion and paraoxon was evaluated. The inhibitory effect of both pesticides on cytochrome P450 3A4, which is a pivotal enzyme in the metabolism of xenobiotics, was examined by determining the dose-response curve. Changes on the transcription level of 92 oxygenase associated genes, including those for important cytochrome P450 enzymes, were evaluated.

RESULTS

The exposure of HepaRG cells to parathion and paraoxon at concentrations up to 100 µM resulted in a viability of 100 per cent. After exposure for 24 hours, pronounced inhibition of cytochrome P450 3A4 enzyme activity was shown, indicating 50 per cent effective concentrations of 1.2 µM (parathion) and 2.1 µM (paraoxon). The results revealed that cytochrome P450 involved in parathion metabolism were significantly upregulated.

DISCUSSION

Relevant changes of the cytochrome P450 3A4 enzyme activity and significant alteration of genes associated with cytochrome P450 suggest an interference of pesticide exposure with numerous metabolic processes. The major limitations of the work involve the use of a single pesticide and the in vitro model as surrogate to human hepatocytes.

CONCLUSION

The data of this study might be of relevance after survival of acute, life-threatening intoxications with organophosphorus compounds, particularly for the co-administration of drugs, which are metabolized by the affected cytochrome P450.

Mechanism of simulated lunar dust-induced lung injury in rats based on transcriptomics

Lunar dust particles are an environmental threat to lunar astronauts, and inhalation of lunar dust can cause lung damage. The current study explored the mechanism of lunar dust simulant (CLDS-i) inducing inflammatory pulmonary injury. Wistar rats were exposed to CLDS-i for 4 h/d and 7d/week for 4 weeks. Pathological results showed that a large number of inflammatory cells gathered and infiltrated in the lung tissues of the simulated lunar dust group, and the alveolar structures were destroyed. Transcriptome analysis confirmed that CLDS-i was mainly involved in the regulation of activation and differentiation of immune inflammatory cells, activated signaling pathways related to inflammatory diseases, and promoted the occurrence and development of inflammatory injury in the lung. Combined with metabolomics analysis, the results of joint analysis of omics were found that the genes Kmo, Kynu, Nos3, Arg1 and Adh7 were involved in the regulation of amino acid metabolism in rat lung tissues, and these genes might be the key targets for the treatment of amino acid metabolic diseases. In addition, the imbalance of amino acid metabolism might be related to the activation of nuclear factor kappaB (NF-κB) signaling pathway. The results of quantitative real-time polymerase chain reaction and Western blot further confirmed that CLDS-i may promote the occurrence and development of lung inflammation and lead to abnormal amino acid metabolism by activating the B cell activation factor (BAFF)/ B cell activation factor receptor (BAFFR)-mediated NF-κB signaling pathway.

Chronic kidney disease may evoke anxiety by altering CRH expression in the amygdala and tryptophan metabolism in rats

Chronic kidney disease (CKD) is associated with anxiety; however, its exact mechanism is not well understood. Therefore, the aim of the present study was to assess the effect of moderate CKD on anxiety in rats. 5/6 nephrectomy was performed in male Wistar rats. 7 weeks after, anxiety-like behavior was assessed by elevated plus maze (EPM), open field (OF), and marble burying (MB) tests. At weeks 8 and 9, urinalysis was performed, and blood and amygdala samples were collected, respectively. In the amygdala, the gene expression of Avp and the gene and protein expression of Crh, Crhr1, and Crhr2 were analyzed. Furthermore, the plasma concentration of corticosterone, uremic toxins, and tryptophan metabolites was measured by UHPLC-MS/MS. Laboratory tests confirmed the development of CKD. In the CKD group, the closed arm time increased; the central time and the total number of entries decreased in the EPM. There was a reduction in rearing, central distance and time in the OF, and fewer interactions with marbles were detected during MB. CKD evoked an upregulation of gene expression of Crh, Crhr1, and Crhr2, but not Avp, in the amygdala. However, there was no alteration in protein expression. In the CKD group, plasma concentrations of p-cresyl-sulfate, indoxyl-sulfate, kynurenine, kynurenic acid, 3-hydroxykynurenine, anthranilic acid, xanthurenic acid, 5-hydroxyindoleacetic acid, picolinic acid, and quinolinic acid increased. However, the levels of tryptophan, tryptamine, 5-hydroxytryptophan, serotonin, and tyrosine decreased. In conclusion, moderate CKD evoked anxiety-like behavior that might be mediated by the accumulation of uremic toxins and metabolites of the kynurenine pathway, but the contribution of the amygdalar CRH system to the development of anxiety seems to be negligible at this stage.

The Kynurenine Pathway in Attention-Deficit/Hyperactivity Disorder: A Systematic Review and Meta-Analysis of Blood Concentrations of Tryptophan and Its Catabolites

Preliminary evidence shows that the kynurenine pathway (KP) may be altered in attention-deficit/hyperactivity disorder (ADHD). We thus conducted a systematic review and meta-analysis exploring the peripheral blood concentrations of tryptophan catabolites (TRYCATs) in people with ADHD. We searched the main electronic databases up to 7th December 2023. Standardised mean differences (SMDs) with 95% confidence intervals (95%CIs) were used to compare TRYCAT concentrations between participants with ADHD and healthy controls (HCs). We included eight studies. Random-effects meta-analyses found higher kynurenine (SMD = 0.56; 95%CI: 0.04 to 1.08; p = 0.033; I2 = 90.3%) and lower kynurenic acid (SMD = -0.33; 95%CI: -0.49 to -0.17; p < 0.001; I2 = 0%) concentrations in people with ADHD compared to HCs. Additional analyses on drug-free children with ADHD showed higher tryptophan (SMD = 0.31; 95%CI: 0.11 to 0.50; p = 0.002; I2 = 0%) and kynurenine (SMD = 0.74; 95%CI: 0.30 to 1.17; p < 0.001; I2 = 76.5%), as well as lower kynurenic acid (SMD = -0.37; 95%CI: -0.59 to -0.15; p < 0.001; I2 = 0%) blood levels, as compared to HCs. Despite some limitations, our work provides preliminary evidence on KP alterations in ADHD that may suggest decreased neuroprotection. Further research is needed to clarify the role of the KP in ADHD.

Developmental and Nutritional Dynamics of Malpighian Tubule Autofluorescence in the Asian Tiger Mosquito Aedes albopictus

Malpighian tubules (MTs) are arthropod excretory organs crucial for the osmoregulation, detoxification and excretion of xenobiotics and metabolic wastes, which include tryptophan degradation products along the kynurenine (KYN) pathway. Specifically, the toxic intermediate 3-hydroxy kynurenine (3-HK) is metabolized through transamination to xanthurenic acid or in the synthesis of ommochrome pigments. Early investigations in Drosophila larval fat bodies revealed an intracellular autofluorescence (AF) that depended on tryptophan administration. Subsequent observations documented AF changes in the MTs of Drosophila eye-color mutants genetically affecting the conversion of tryptophan to KYN or 3-HK and the intracellular availability of zinc ions. In the present study, the AF properties of the MTs in the Asian tiger mosquito, Aedes albopictus, were characterized in different stages of the insect's life cycle, tryptophan-administered larvae and blood-fed adult females. Confocal imaging and microspectroscopy showed AF changes in the distribution of intracellular, brilliant granules and in the emission spectral shape and amplitude between the proximal and distal segments of MTs across the different samples. The findings suggest AF can serve as a promising marker for investigating the functional status of MTs in response to metabolic alterations, contributing to the use of MTs as a potential research model in biomedicine.

Effects of Fenofibrate and Gemfibrozil on Kynurenic Acid Production in Rat Kidneys In Vitro: Old Drugs, New Properties

Kidney dysfunction significantly increases the cardiovascular risk, even in cases of minor functional declines. Hypertriglyceridemia is the most common lipid abnormality reported in patients with kidney disorders. PPAR-α (peroxisome proliferator-activated receptor-α) agonists called fibrates are the main agents used to lower triglyceride levels. Kynurenic acid (KYNA) is a tryptophan (Trp) derivative directly formed from L-kynurenine (L-KYN) by kynurenine aminotransferases (KATs). KYNA is classified as a uremic toxin, the level of which is correlated with kidney function impairments and lipid abnormalities. The aim of this study was to analyze the effect of the most commonly used triglyceride-lowering drugs, fenofibrate and gemfibrozil, on KYNA production and KAT activity in rat kidneys in vitro. The influence of fenofibrate and gemfibrozil on KYNA formation and KAT activity was tested in rat kidney homogenates in vitro. Fenofibrate and gemfibrozil at 100 µM-1 mM significantly inhibited KYNA synthesis in rat kidney homogenates. Both fibrates directly affected the KAT I and KAT II isoenzyme activities in a dose-dependent manner at similar concentrations. The presented results reveal the novel mechanism of action of fibrates in the kidneys and suggest their potential role in kidney function protection beyond the well-known anti-hyperlipidemic effect.

Analysis of the Circulating Metabolome of Patients with Cutaneous, Mucosal and Uveal Melanoma Reveals Distinct Metabolic Profiles with Implications for Response to Immunotherapy

Cutaneous melanoma (CM) patients respond better to immune checkpoint inhibitors (ICI) than mucosal and uveal melanoma patients (MM/UM). Aiming to explore these differences and understand the distinct response to ICI, we evaluated the serum metabolome of advanced CM, MM, and UM patients. Levels of 115 metabolites were analyzed in samples collected before ICI, using a targeted metabolomics platform. In our analysis, molecules involved in the tryptophan-kynurenine axis distinguished UM/MM from CM. UM/MM patients had higher levels of 3-hydroxykynurenine (3-HKyn), whilst patients with CM were found to have higher levels of kynurenic acid (KA). The KA/3-HKyn ratio was significantly higher in CM versus the other subtypes. UM, the most ICI-resistant subtype, was also associated with higher levels of sphingomyelin-d18:1/22:1 and the polyamine spermine (SPM). Overall survival was prolonged in a cohort of CM patients with lower SPM levels, suggesting there are also conserved metabolic factors promoting ICI resistance across melanoma subtypes. Our study revealed a distinct metabolomic profile between the most resistant melanoma subtypes, UM and MM, compared to CM. Alterations within the kynurenine pathway, polyamine metabolism, and sphingolipid metabolic pathway may contribute to the poor response to ICI. Understanding the different metabolomic profiles introduces opportunities for novel therapies with potential synergic activity to ICI, to improve responses of UM/MM.

Modulation of T cells by tryptophan metabolites in the kynurenine pathway

Lymphocytes maturing in the thymus (T cells) are key factors in adaptive immunity and the regulation of inflammation. The kynurenine pathway of tryptophan metabolism includes several enzymes and compounds that can modulate T cell function, but manipulating these pharmacologically has not achieved the expected therapeutic activity for the treatment of autoimmune disorders and cancer. With increasing knowledge of other pathways interacting with kynurenines, the expansion of screening methods, and the application of virtual techniques to understanding enzyme structures and mechanisms, details of interactions between kynurenines and other pathways are being revealed. This review surveys some of these alternative approaches to influence T cell function indirectly via the kynurenine pathway and summarizes the most recent work on the development of compounds acting directly on the kynurenine pathway.

Modulation of immunity by tryptophan microbial metabolites

Tryptophan (Trp) is an essential amino acid that can be metabolized via endogenous and exogenous pathways, including the Kynurenine Pathway, the 5-Hydroxyindole Pathway (also the Serotonin pathway), and the Microbial pathway. Of these, the Microbial Trp metabolic pathways in the gut have recently been extensively studied for their production of bioactive molecules. The gut microbiota plays an important role in host metabolism and immunity, and microbial Trp metabolites can influence the development and progression of various diseases, including inflammatory, cardiovascular diseases, neurological diseases, metabolic diseases, and cancer, by mediating the body's immunity. This review briefly outlines the crosstalk between gut microorganisms and Trp metabolism in the body, starting from the three metabolic pathways of Trp. The mechanisms by which microbial Trp metabolites act on organism immunity are summarized, and the potential implications for disease prevention and treatment are highlighted.

Kynurenine Pathway in Psoriasis-a Promising Link?

Psoriasis is a common dermatosis which affects the patient's skin and general well-being because of its link to diseases such as depression, kidney disease and metabolic syndrome. Pathogenesis remains unknown; however, genetic, environmental and immunological factors seem to play a role in the development of the disease. Due to a lack of complete understanding of the psoriasis pathology, effective treatment is yet to be developed. The kynurenine pathway is one of the ways amino acid tryptophan is metabolised. In comorbidities typical for psoriasis such as chronic kidney disease, depression and atherosclerotic alterations in the activation of the kynurenine pathway were observed, which were mainly characterised by higher activity compared to that in healthy individuals. However, the kynurenine pathway has not been thoroughly studied among patients with psoriasis even though increased levels of L-kynurenine, one of the enzymes in the kynurenine pathway, were found in psoriatic skin lesions. Given the unknown pathogenesis of the disease, this finding seems to be a potential new field of study and shows a possible link between psoriasis and its comorbidities that could also lead to novel effective treatment for this chronic condition.

Intensity of Depression Symptoms Is Negatively Associated with Catalase Activity in Master Athletes

BACKGROUND

This study examined associations between scores of depression (DEPs), thiobarbituric acid-reactive substances (TBARS), superoxide dismutase (SOD), and catalase activity (CAT) in master athletes and untrained controls.

METHODS

Participants were master sprinters (MS, n = 24; 50.31 ± 6.34 year), endurance runners (ER, n = 11; 51.35 ± 9.12 year), untrained middle-aged (CO, n = 13; 47.21 ± 8.61 year), and young untrained (YU, n = 15; 23.70 ± 4.02 year). CAT, SOD, and TBARS were measured in plasma using commercial kits. DEPs were measured by the Beck Depression Inventory-II. An ANOVA, Kruskal-Wallis, Pearson's, and Spearman's correlations were applied, with a significance level of p ≤ 0.05.

RESULTS

The CATs of MS and YU [760.4 U·μL 1 ± 170.1 U·μL 1 and 729.9 U·μL 1 ± 186.9 U·μL 1] were higher than CO and ER. The SOD levels in the YU and ER [84.20 U·mL^-1 ± 8.52 U·mL^-1 and 78.24 U·mL^-1 ± 6.59 U·mL^-1 (p < 0.0001)] were higher than CO and MS. The TBARS in CO [11.97 nmol·L^-1 ± 2.35 nmol·L^-1 (p < 0.0001)] was higher than in YU, MS and ER. MS had lower DEPs compared to the YU [3.60 ± 3.66 vs. 12.27 ± 9.27 (p = 0.0002)]. A negative correlation was found between CAT and DEPs for master athletes [r = -0.3921 (p = 0.0240)] and a weak correlation [r = -0.3694 (p = 0.0344)] was found between DEPs and the CAT/TBARS ratio.

CONCLUSIONS

In conclusion, the training model of master sprinters may be an effective strategy for increasing CAT and reducing DEPs.

The Role of Tryptophan Metabolism in Alzheimer's Disease

The need to identify new potentially druggable biochemical mechanisms for Alzheimer's disease (AD) is an ongoing priority. The therapeutic limitations of amyloid-based approaches are further motivating this search. Amino acid metabolism, particularly tryptophan metabolism, has the potential to emerge as a leading candidate and an alternative exploitable biomolecular target. Multiple avenues support this contention. Tryptophan (trp) and its associated metabolites are able to inhibit various enzymes participating in the biosynthesis of β-amyloid, and one metabolite, 3-hydroxyanthranilate, is able to directly inhibit neurotoxic β-amyloid oligomerization; however, whilst certain trp metabolites are neuroprotectant, other metabolites, such as quinolinic acid, are directly toxic to neurons and may themselves contribute to AD progression. Trp metabolites also have the ability to influence microglia and associated cytokines in order to modulate the neuroinflammatory and neuroimmune factors which trigger pro-inflammatory cytotoxicity in AD. Finally, trp and various metabolites, including melatonin, are regulators of sleep, with disorders of sleep being an important risk factor for the development of AD. Thus, the involvement of trp biochemistry in AD is multifactorial and offers a plethora of druggable targets in the continuing quest for AD therapeutics.

The Role of Inflammation in the Pathophysiology of Depression and Suicidal Behavior: Implications for Treatment

Depression and suicidal behavior are 2 complex psychiatric conditions of significant public health concerns due to their debilitating nature. The need to enhance contemporary treatments and preventative approaches for these illnesses not only calls for distillation of current views on their pathogenesis but also provides an impetus for further elucidation of their novel etiological determinants. In this regard, inflammation has recently been recognized as a potentially important contributor to the development of depression and suicidal behavior. This review highlights key evidence that supports the presence of dysregulated neurometabolic and immunologic signaling and abnormal interaction with microbial species as putative etiological hallmarks of inflammation in depression as well as their contribution to the development of suicidal behavior. Furthermore, therapeutic insights addressing candidate mechanisms of pathological inflammation in these disorders are proposed.

Gram-Scale Preparation of Cannflavin A from Hemp (Cannabis sativa L.) and Its Inhibitory Effect on Tryptophan Catabolism Enzyme Kynurenine-3-Monooxygenase

Inhibitors targeting kynurenine-3-monooxygenase (KMO), an enzyme in the neurotoxic kynurenine pathway (KP), are potential therapeutics for KP metabolites-mediated neuroinflammatory and neurodegenerative disorders. Although phytochemicals from Cannabis (C. sativa L.) have been reported to show modulating effects on enzymes involved in the KP metabolism, the inhibitory effects of C. sativa compounds, including phytocannabinoids and non-phytocannabinoids (i.e., cannflavin A; CFA), on KMO remain unknown. Herein, CFA (purified from hemp aerial material at a gram-scale) and a series of phytocannabinoids were evaluated for their anti-KMO activity. CFA showed the most active inhibitory effect on KMO, which was comparable to the positive control Ro 61-8048 (IC50 = 29.4 vs. 5.1 μM, respectively). Furthermore, a molecular docking study depicted the molecular interactions between CFA and the KMO protein and a biophysical binding assay with surface plasmon resonance (SPR) technique revealed that CFA bound to the protein with a binding affinity of 4.1×10−5 M. A competitive SPR binding analysis suggested that CFA and Ro 61-8048 bind to the KMO protein in a competitive manner. Our findings show that C. sativa derived phytochemicals, including CFA, are potential KMO inhibitors, which provides insight into the development of therapeutics targeting the KP and its related pathological conditions.

Cytokine changes in cerebrospinal fluid following vascular surgery on the thoracic aorta

There is growing evidence that surgery can drive an inflammatory response in the brain. However, the mechanisms behind this response are incompletely understood. Here, we investigate the hypotheses that 1. Cerebrospinal fluid (CSF) cytokines increase after vascular surgery and 2. That these changes in CSF cytokines are interrelated. Patients undergoing either open or endovascular elective surgery of the thoracic aorta were invited to participate in this study. Cerebrospinal fluid samples were taken before surgery and on the first post-operative day. These were analysed for the presence of ten cytokines by immunoassay to examine for post-operative changes in cytokine levels. After surgery, there were significant increases in six out of the ten measured CSF cytokines (IL-1β, 2, 6, 8, 10 and 13). This included changes in both putative pro-inflammatory (IL-1β, 6 and 8) and putative anti-inflammatory (IL-2, 10 and 13) cytokines. The greatest increases occurred in IL-6 and IL-8, which showed a 63-fold and a 31-fold increase respectively. There was strong intercorrelation between CSF cytokines after the operation. Following surgery on the thoracic aorta, there was a marked increase in CSF cytokines, consistent with a potential role in neuroinflammation. The ten measured cytokines showed intercorrelation after the operation, indicating that a balance between multiple pro- and anti-inflammatory cytokines may be present.

The Tryptophan Catabolite or Kynurenine Pathway in Alzheimer's Disease: A Systematic Review and Meta-Analysis

BACKGROUND

Alzheimer's disease (AD), which is characterized by progressive brain dysfunction and memory loss, is one of the most significant global health concerns for older adults. Neuroinflammation and increased oxidative stress contribute to the pathophysiology of AD, thereby presumably inducing tryptophan (TRP) degradation through the TRP catabolite (TRYCAT) pathway.

OBJECTIVE

To delineate the activity of the TRYCAT pathway along with levels of TRP and tryptophan catabolites (TRYCATs) in AD patients.

METHODS

We used PubMed, Google Scholar, Web of Science, and SciFinder during the month of January 2022 to gather the pertinent publications. We found 19 eligible articles which involved 738 patients and 665 healthy controls.

RESULTS

Our results revealed a significant difference (p = 0.008) in the kynurenine (KYN)/TRP ratio (standardized mean difference, SMD = 0.216, 95% confidence interval, CI: 0.057; 0.376), and a significant decrease in TRP in AD patients (SMD = -0.520, 95% CI: -0.738; -0.302, p < 0.0001). Moreover, we also found a significant increase in the central nervous system (CNS), brain, and cerebrospinal fluid kynurenic acid (KA)/KYN ratio but not in peripheral blood, as well as a significant decrease in plasma KA and xanthurenic acid in the CNS and blood.

CONCLUSION

AD is characterized by TRP depletion but not by an overactivity of the TRYCAT pathway. IDO-induced production of neurotoxic TRYCATs is not a key factor in the pathophysiology of AD.

Tryptophan Metabolism in Central Nervous System Diseases: Pathophysiology and Potential Therapeutic Strategies

The metabolism of L-tryptophan (TRP) regulates homeostasis, immunity, and neuronal function. Altered TRP metabolism has been implicated in the pathophysiology of various diseases of the central nervous system. TRP is metabolized through two main pathways, the kynurenine pathway and the methoxyindole pathway. First, TRP is metabolized to kynurenine, then kynurenic acid, quinolinic acid, anthranilic acid, 3-hydroxykynurenine, and finally 3-hydroxyanthranilic acid along the kynurenine pathway. Second, TRP is metabolized to serotonin and melatonin along the methoxyindole pathway. In this review, we summarize the biological properties of key metabolites and their pathogenic functions in 12 disorders of the central nervous system: schizophrenia, bipolar disorder, major depressive disorder, spinal cord injury, traumatic brain injury, ischemic stroke, intracerebral hemorrhage, multiple sclerosis, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and Huntington's disease. Furthermore, we summarize preclinical and clinical studies, mainly since 2015, that investigated the metabolic pathway of TRP, focusing on changes in biomarkers of these neurologic disorders, their pathogenic implications, and potential therapeutic strategies targeting this metabolic pathway. This critical, comprehensive, and up-to-date review helps identify promising directions for future preclinical, clinical, and translational research on neuropsychiatric disorders.