Kynurenines: Tryptophan's metabolites in exercise, inflammation, and mental health

Inflammation-Related Marker Profiling of Dietary Patterns and All-cause Mortality in the Melbourne Collaborative Cohort Study

BACKGROUND

Nutritional epidemiology research using self-reported dietary intake is prone to measurement error. Objective methods are being explored to overcome this limitation.

OBJECTIVES

We aimed to examine 1) the association between plasma markers related to inflammation and derive marker scores for dietary patterns [Mediterranean dietary score (MDS), energy-adjusted Dietary Inflammatory Index (E-DIITM), Alternative Healthy Eating Index 2010 (AHEI)] and 2) the associations of these marker scores with mortality.

METHODS

Weighted marker scores were derived from the cross-sectional association between 30 plasma markers and each dietary score (assessed using food-frequency questionnaires) using linear regression for 770 participants in the Melbourne Collaborative Cohort Study (aged 50-82 y). Prospective associations between marker scores and mortality (n = 249 deaths) were assessed using Cox regression (median follow-up: 14.4 y).

RESULTS

The MDS, E-DII, and AHEI were associated (P < 0.05) with 9, 14, and 11 plasma markers, respectively. Healthier diets (higher MDS and AHEI, and lower anti-inflammatory, E-DII) were associated with lower concentrations of kynurenines, neopterin, IFN-γ, cytokines, and C-reactive protein. Five of 6 markers common to the 3 dietary scores were components of the kynurenine pathway. The 3 dietary-based marker scores were highly correlated (Spearman ρ: -0.74, -0.82, and 0.93). Inverse associations (for 1-SD increment) were observed with all-cause mortality for the MDS marker score (HR: 0.84; 95% CI: 0.72-0.98) and the AHEI marker score (HR: 0.76; 95% CI: 0.66-0.89), whereas a positive association was observed with the E-DII marker score (HR: 1.18; 95% CI: 1.01-1.39). The same magnitude of effect was not observed for the respective dietary patterns.

CONCLUSIONS

Markers involved in inflammation-related processes are associated with dietary quality, including a substantial overlap between markers associated with the MDS, the E-DII, and the AHEI, especially kynurenines. Unfavorable marker scores, reflecting poorer-quality diets, were associated with increased mortality.

The association between plasma tryptophan catabolites and depression: The role of symptom profiles and inflammation

BACKGROUND

Tryptophan catabolites ("TRYCATs") produced by the kynurenine pathway (KP) may play a role in depression pathophysiology. Studies comparing TRYCATs levels in depressed subjects and controls provided mixed findings. We examined the association of TRYCATs levels with 1) the presence of Major Depressive Disorder (MDD), 2) depressive symptom profiles and 3) inflammatory markers.

METHODS

The sample from the Netherlands Study of Depression and Anxiety included participants with current (n = 1100) or remitted (n = 753) MDD DSM-IV diagnosis and healthy controls (n = 642). Plasma levels of tryptophan (TRP), kynurenine (KYN), kynurenic acid (KynA), quinolinic acid (QA), C-reactive protein (CRP), interleukin-6 (IL-6) and tumor necrosis factor (TNF) were measured. Atypical/energy-related symptom (AES), melancholic symptom (MS) and anxious-distress symptom (ADS) profiles were derived from questionnaires.

RESULTS

After adjustment for age, sex, education, smoking status, alcohol consumption and chronic diseases, no significant differences in TRYCATs were found comparing MDD cases versus controls. The MS profile was associated (q < 0.05) with lower KynA (β = -0.05), while AES was associated with higher KYN (β = 0.05), QA (β = 0.06) and TRP (β = 0.06). Inflammatory markers were associated with higher KYN (CRP β = 0.12, IL-6 β = 0.08, TNF β = 0.10) and QA (CRP β = 0.21, IL-6 β = 0.12, TNF β = 0.18). Significant differences against controls emerged after selecting MDD cases with high (top 30%) CRP (KYN d = 0.20, QA d = 0.33) and high TNF (KYN d = 0.24; QA d = 0.39).

CONCLUSIONS

TRYCATs levels were related to specific clinical and biological features, such as atypical symptoms or a proinflammatory status. Modulation of KP may potentially benefit a specific subset of depressed patients. Clinical studies should focus on patients with clear evidence of KP dysregulations.

Altered tryptophan absorption and metabolism could underlie long-term symptoms in survivors of coronavirus disease 2019 (COVID-19)

The global pandemic of COVID-19 has been lasting for more than one year and there is little known about the long-term health effects of the disease. Long-COVID is a new term that is used to describe the enduring symptoms of COVID-19 survivors. Huang et al. reported that fatigue, muscle weakness, sleep disturbances, anxiety, and depression were the most common complaints in COVID-19 survivors after 6 months of the infection. A recent meta-analysis showed that 80% of COVID-19 survivors have developed at least one long-term symptom and the most common five were fatigue, headache, attention deficit disorder, hair loss, and dyspnea. In this paper, we discuss the hypothesis that altered tryptophan absorption and metabolism could be the main contributor to the long-term symptoms in COVID-19 survivors.

Gut Microbiota-Mediated Modulation of Cancer Progression and Therapy Efficacy

The role of gut microbiota in the development of various tumors has been a rising topic of public interest, and in recent years, many studies have reported a close relationship between microbial groups and tumor development. Gut microbiota play a role in host metabolism, and the positive and negative alterations of these microbiota have an effect on tumor treatment. The microbiota directly promote, eliminate, and coordinate the efficacy of chemotherapy drugs and the toxicity of adjuvant drugs, and enhance the ability of patients to respond to tumors in adjuvant immunotherapy. In this review, we outline the significance of gut microbiota in tumor development, reveal its impacts on chemotherapy and immunotherapy, and discover various potential mechanisms whereby they influence tumor treatment. This review demonstrates the importance of intestinal microbiota-related research for clinical tumor treatment and provides additional strategy for clinical assistance in cancer treatment.

Clinical Relevance of Serum Kyn/Trp Ratio and Basal and IFNγ-Upregulated IDO1 Expression in Peripheral Monocytes in Early Stage Melanoma

Immune escape is an early phenomenon in cancer development/progression. Indoleamine 2,3-dioxygenase 1 (IDO1) is a normal endogenous mechanism of acquired peripheral immune tolerance and may therefore be tumor-promoting. This study investigated the clinical relevance of IDO1 expression by immune cells in the lymph nodes and blood and of the serum kynurenine/tryptophan (Kyn/Trp) ratio in 65 systemic treatment naïve stage I-III melanoma patients. Blood samples were collected within the first year of diagnosis. Patients had a median follow-up of 61 months. High basal IDO1 expression in peripheral monocytes and low IFNγ-induced IDO1 upregulation correlated with worse outcome independent from disease stage. Interestingly studied factors were not interrelated. During follow-up, the risk of relapse was 9% (2/22) in the subgroup with high IFNγ-induced IDO1 upregulation in monocytes. In contrast, if IDO1 upregulation was low, relapse occurred in 30% (3/10) of patients with low basal IDO1 expression in monocytes and in 61.5% (8/13) in the subgroup with high basal IDO1 expression in monocytes (Log-Rank test, p=0.008). This study reveals some immune features in the blood of early stage melanoma that may be of relevance for disease outcome. These may offer a target for sub-stratification and early intervention.

Artificial Sweeteners: History and New Concepts on Inflammation

Since the introduction of artificial sweeteners (AS) to the North American market in the 1950s, a growing number of epidemiological and animal studies have suggested that AS may induce changes in gut bacteria and gut wall immune reactivity, which could negatively affect individuals with or susceptible to chronic inflammatory conditions such as inflammatory bowel disease (IBD), a disorder that has been growing exponentially in westernized countries. This review summarizes the history of current FDA-approved AS and their chemical composition, metabolism, and bacterial utilization, and provides a scoping overview of the disease mechanisms associated with the induction or prevention of inflammation in IBD. We provide a general outlook on areas that have been both largely and scarcely studied, emerging concepts using silica, and describe the effects of AS on acute and chronic forms of intestinal inflammation.

High levels of Tryptophan reduce cell wall or membrane stress tolerance in Saccharomyces cerevisiae

Tryptophan (Trp) is a proteinogenic aromatic amino acid; however, high levels of Trp are toxic in animals and yeast with unknown mechanisms. Previously, we suggested that aromatic aminotransferase Aro9 is important for excess Trp degradation. Besides, Schroeder and Ikui showed that aro9Δ is sensitive to membrane stress by sodium dodecyl sulfate. Therefore, Trp accumulation may reduce the cell wall or membrane (CW/M) stress tolerance through participation of cell wall integrity (CWI) pathway, which detects and responds to CW/M perturbations. In this study, we found that yeast mutants of the CWI mitogen-activated protein kinase cascade were susceptible to excess Trp. Also, the Trp degradation deficient mutant aro8Δ aro9Δ cells, in which Trp accumulation was confirmed, were sensitive to several CW/M stresses. These results indicated that accumulation of Trp is adverse for the CW/M stress resistance and may disturb appropriate signal transduction responding to the stress.

Metabolic drift in the aging nervous system is reflected in human cerebrospinal fluid

Chronic diseases affecting the central nervous system (CNS) like Alzheimer's or Parkinson's disease typically develop with advanced chronological age. Yet, aging at the metabolic level has been explored only sporadically in humans using biofluids in close proximity to the CNS such as the cerebrospinal fluid (CSF). We have used an untargeted liquid chromatography high-resolution mass spectrometry (LC-HRMS) based metabolomics approach to measure the levels of metabolites in the CSF of non-neurological control subjects in the age of 20 up to 74. Using a random forest-based feature selection strategy, we extracted 69 features that were strongly related to age (p_age < 0.001, r_age = 0.762, R²_{Boruta age} = 0.764). Combining an in-house library of known substances with in silico chemical classification and functional semantic annotation we successfully assigned putative annotations to 59 out of the 69 CSF metabolites. We found alterations in metabolites related to the Cytochrome P450 system, perturbations in the tryptophan and kynurenine pathways, metabolites associated with cellular energy (NAD+, ADP), mitochondrial and ribosomal metabolisms, neurological dysfunction, and an increase of adverse microbial metabolites. Taken together our results point at a key role for metabolites found in CSF related to the Cytochrome P450 system as most often associated with metabolic aging.

Psychological Stresses in Children Trigger Cytokine- and Kynurenine Metabolite-Mediated Abdominal Pain and Proinflammatory Changes

Recurrent abdominal pain (RAP) is a common medically unexplained symptom among children worldwide. However, the biological mechanisms behind the development of functional and behavioral symptoms and changes in blood markers have not been well explored. This study aimed to assess changes in the concentrations of inflammatory markers, including cytokines and tryptophan catabolites, in the serum of children with RAP compared to those with subclinical infections. Children with RAP but without organic diseases were included, and those with asymptomatic intestinal parasitic infections were used as a subclinical infection cohort. Blood samples were collected and used to measure the cytokine profile using Multiplex Immunoassay and tryptophan catabolites using high performance liquid chromatography. Children with RAP showed significantly higher concentrations of serum tumor necrotic factor-α, p<0.05, but lower concentrations of IL-10, p<0.001, IL-6, p<0.001 and brain-derived neurotrophic factors (BDNF) p<0.01. In addition, a significant increase in the metabolite of the kynurenine pathway, 3-hydroxyanthranilic acid (3-HAA) p<0.01, a significant decrease in the concentrations of anthranilic acid (AA) p<0.001, together with an increased ratio of serum 3-HAA to AA (3-HAA/AA) p<0.001, was found in this cohort. These findings indicate the significant activation of the immune system and presence of inflammation in children with RAP than those with subclinical parasitic infections. Moreover, children with RAP tested with the Strengths and Difficulties Questionnaire (SDQ), displayed high psychological problems though these SDQ scores were not statistically associated with measured cytokines and kynurenine metabolites. We however could hypothesize that the pro-inflammatory state together with concomitant low concentrations of BDNF in those children with RAP could play a role in psychological stress and experiencing medically unexplained symptoms.

Kynurenine Pathway of Tryptophan Metabolism in Migraine and Functional Gastrointestinal Disorders

Migraine, the leading cause of disability in the population aged below 50, is associated with functional gastrointestinal (GI) disorders (FGIDs) such as functional nausea, cyclic vomiting syndrome, and irritable bowel syndrome (IBS). Conversely, changes in intestinal GI transit may cause diarrhea or constipation and are a component of the autonomic symptoms associated with pre- and post-dorsal phases of migraine attack. These mutual relationships provoke a question on a common trigger in migraine and FGIDs. The kynurenine (l-kyn) pathway (KP) is the major route for l-tryptophan (l-Trp) metabolism and transforms l-Trp into several neuroactive compounds. Changes in KP were reported in both migraine and FGIDs. Migraine was largely untreatable, but several drugs approved lately by the FDA, including monoclonal antibodies for calcitonin gene-related peptide (CGRP) and its receptor, create a hope for a breakthrough in migraine treatment. Derivatives of l-kyn were efficient in pain relief with a mechanism including CGRP inhibition. KP products are important ligands to the aryl hydrocarbon receptor (AhR), whose activation is implicated in the pathogenesis of GI and migraine. Toll-like receptors (TLRs) may play a role in migraine and IBS pathogeneses, and KP metabolites detected downstream of TLR activation may be an IBS marker. The TLR4 signaling was observed in initiating and maintaining migraine-like behavior through myeloid differentiation primary response gene 88 (MyD88) in the mouse. The aim of this review is to justify the view that KP modulation may provide common triggers for migraine and FGIDs with the involvement of TLR, AhR, and MyD88 activation.

Vitamin B6 in Health and Disease

Vitamin B6 is a fascinating molecule involved in the vast majority of changes in the human body because it is a coenzyme involved in over 150 biochemical reactions. It is active in the metabolism of carbohydrates, lipids, amino acids, and nucleic acids, and participates in cellular signaling. It is an antioxidant and a compound with the ability to lower the advanced glycation end products (AGE) level. In this review, we briefly summarize its involvement in biochemical pathways and consider whether its deficiency may be associated with various diseases such as diabetes, heart disease, cancer, or the prognosis of COVID-19.

The Kynurenine Pathway-New Linkage between Innate and Adaptive Immunity in Autoimmune Endocrinopathies

The kynurenine pathway (KP) is highly regulated in the immune system, where it promotes immunosuppression in response to infection or inflammation. Indoleamine 2,3-dioxygenase 1 (IDO1), the main enzyme of KP, has a broad spectrum of activity on immune cells regulation, controlling the balance between stimulation and suppression of the immune system at sites of local inflammation, relevant to a wide range of autoimmune and inflammatory diseases. Various autoimmune diseases, among them endocrinopathies, have been identified to date, but despite significant progress in their diagnosis and treatment, they are still associated with significant complications, morbidity, and mortality. The precise cellular and molecular mechanisms leading to the onset and development of autoimmune disease remain poorly clarified so far. In breaking of tolerance, the cells of the innate immunity provide a decisive microenvironment that regulates immune cells' differentiation, leading to activation of adaptive immunity. The current review provided a comprehensive presentation of the known role of IDO1 and KP activation in the regulation of the innate and adaptive arms of the immune system. Significant attention has been paid to the immunoregulatory role of IDO1 in the most prevalent, organ-specific autoimmune endocrinopathies-type 1 diabetes mellitus (T1DM) and autoimmune thyroiditis.

Analysis of the gut microbiome in dogs and cats

The gut microbiome is an important immune and metabolic organ. Intestinal bacteria produce various metabolites that influence the health of the intestine and other organ systems, including kidney, brain, and heart. Changes in the microbiome in diseased states are termed dysbiosis. The concept of dysbiosis is constantly evolving and includes changes in microbiome diversity and/or structure and functional changes (eg, altered production of bacterial metabolites). Molecular tools are now the standard for microbiome analysis. Sequencing of microbial genes provides information about the bacteria present and their functional potential but lacks standardization and analytical validation of methods and consistency in the reporting of results. This makes it difficult to compare results across studies or for individual clinical patients. The Dysbiosis Index (DI) is a validated quantitative PCR assay for canine fecal samples that measures the abundance of seven important bacterial taxa and summarizes the results as one single number. Reference intervals are established for dogs, and the DI can be used to assess the microbiome in clinical patients over time and in response to therapy (eg, fecal microbiota transplantation). In situ hybridization or immunohistochemistry allows the identification of mucosa‐adherent and intracellular bacteria in animals with intestinal disease, especially granulomatous colitis. Future directions include the measurement of bacterial metabolites in feces or serum as markers for the appropriate function of the microbiome. This article summarizes different approaches to the analysis of gut microbiota and how they might be applicable to research studies and clinical practice in dogs and cats.

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

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

The Endocannabinoid System: A Bridge between Alzheimer's Disease and Gut Microbiota

Alzheimer's disease (AD) is a neurodegenerative disease that progresses from mild cognitive impairment to severe dementia over time. The main clinical hallmarks of the disease (e.g., beta-amyloid plaques and neurofibrillary tangles) begin during preclinical AD when cognitive deficits are not yet apparent. Hence, a more profound understanding of AD pathogenesis is needed to develop new therapeutic strategies. In this context, the endocannabinoid (eCB) system and the gut microbiome are increasingly emerging as important players in maintaining the general homeostasis and the health status of the host. However, their interaction has come to light just recently with gut microbiota regulating the eCB tone at both receptor and enzyme levels in intestinal and adipose tissues. Importantly, eCB system and gut microbiome, have been suggested to play a role in AD in both animal and human studies. Therefore, the microbiome gut-brain axis and the eCB system are potential common denominators in the AD physiopathology. Hence, the aim of this review is to provide a general overview on the role of both the eCB system and the microbiome gut-brain axis in AD and to suggest possible mechanisms that underlie the potential interplay of these two systems.

Tryptophan-derived serotonin-kynurenine balance in immune activation and intestinal inflammation

Endogenous tryptophan metabolism pathways lead to the production of serotonin (5‐hydroxytryptamine; 5‐HT), kynurenine, and several downstream metabolites which are involved in a multitude of immunological functions in both health and disease states. Ingested tryptophan is largely shunted to the kynurenine pathway (95%) while only minor portions (1%–2%) are sequestered for 5‐HT production. Though often associated with the functioning of the central nervous system, significant production of 5‐HT, kynurenine and their downstream metabolites takes place within the gut. Accumulating evidence suggests that these metabolites have essential roles in regulating immune cell function, intestinal inflammation, as well as in altering the production and suppression of inflammatory cytokines. In addition, both 5‐HT and kynurenine have a considerable influence on gut microbiota suggesting that these metabolites impact host physiology both directly and indirectly via compositional changes. It is also now evident that complex interactions exist between the two pathways to maintain gut homeostasis. Alterations in 5‐HT and kynurenine are implicated in the pathogenesis of many gastrointestinal dysfunctions, including inflammatory bowel disease. Thus, these pathways present numerous potential therapeutic targets, manipulation of which may aid those suffering from gastrointestinal disorders. This review aims to update both the role of 5‐HT and kynurenine in immune regulation and intestinal inflammation, and analyze the current knowledge of the relationship and interactions between 5‐HT and kynurenine pathways.

Exercise as a multi-modal disease-modifying medicine in systemic sclerosis: An introduction by The Global Fellowship on Rehabilitation and Exercise in Systemic Sclerosis (G-FoRSS)

Systemic sclerosis (SSc) is a heterogeneous multisystem autoimmune disease whereby its main pathological drivers of disability and damage are vascular injury, inflammatory cell infiltration, and fibrosis. These mechanisms result in diffuse and diverse impairments arising from ischemic circulatory dysfunction leading to painful skin ulceration and calcinosis, neurovascular aberrations hindering gastrointestinal (GI) motility, progressive painful, incapacitating or immobilizing effects of inflammatory and fibrotic effects on the lungs, skin, articular and periarticular structures, and muscle. SSc-related impairments impede routine activities of daily living (ADLs) and disrupt three critical life areas: work, family, social/leisure, and also impact on psychological well-being. Physical activity and exercise are globally recommended; however, for connective tissue diseases, this guidance carries greater impact on inflammatory disease manifestations, recovery, and cardiovascular health. Exercise, through myogenic and vascular phenomena, naturally targets key pathogenic drivers by downregulating multiple inflammatory and fibrotic pathways in serum and tissue, while increasing circulation and vascular repair. G-FoRSS, The Global Fellowship on Rehabilitation and Exercise in Systemic Sclerosis recognizes the scientific basis of and advocates for education and research of exercise as a systemic and targeted SSc disease-modifying treatment. An overview of biophysiological mechanisms of physical activity and exercise are herein imparted for patients, clinicians, and researchers, and applied to SSc disease mechanisms, manifestations, and impairment. A preliminary guidance on exercise in SSc, a research agenda, and the current state of research and outcome measures are set forth.

Next Generation Microbiome Research: Identification of Keystone Species in the Metabolic Regulation of Host-Gut Microbiota Interplay

The community of the diverse microorganisms residing in the gastrointestinal tract, known as the gut microbiota, is exceedingly being studied for its impact on health and disease. This community plays a major role in nutrient metabolism, maintenance of the intestinal epithelial barrier but also in local and systemic immunomodulation. A dysbiosis of the gut microbiota, characterized by an unbalanced microbial ecology, often leads to a loss of essential functions that may be associated with proinflammatory conditions. Specifically, some key microbes that are depleted in dysbiotic ecosystems, called keystone species, carry unique functions that are essential for the balance of the microbiota. In this review, we discuss current understanding of reported keystone species and their proposed functions in health. We also elaborate on current and future bioinformatics tools needed to identify missing functions in the gut carried by keystone species. We propose that the identification of such keystone species functions is a major step for the understanding of microbiome dynamics in disease and toward the development of microbiome-based therapeutics.

Redox imbalance links COVID-19 and myalgic encephalomyelitis/chronic fatigue syndrome

Although most patients recover from acute COVID-19, some experience postacute sequelae of severe acute respiratory syndrome coronavirus 2 infection (PASC). One subgroup of PASC is a syndrome called "long COVID-19," reminiscent of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). ME/CFS is a debilitating condition, often triggered by viral and bacterial infections, leading to years-long debilitating symptoms including profound fatigue, postexertional malaise, unrefreshing sleep, cognitive deficits, and orthostatic intolerance. Some are skeptical that either ME/CFS or long COVID-19 involves underlying biological abnormalities. However, in this review, we summarize the evidence that people with acute COVID-19 and with ME/CFS have biological abnormalities including redox imbalance, systemic inflammation and neuroinflammation, an impaired ability to generate adenosine triphosphate, and a general hypometabolic state. These phenomena have not yet been well studied in people with long COVID-19, and each of them has been reported in other diseases as well, particularly neurological diseases. We also examine the bidirectional relationship between redox imbalance, inflammation, energy metabolic deficits, and a hypometabolic state. We speculate as to what may be causing these abnormalities. Thus, understanding the molecular underpinnings of both PASC and ME/CFS may lead to the development of novel therapeutics.

Hypoxic preconditioning protects against ischemic kidney injury through the IDO1/kynurenine pathway

Prolonged cellular hypoxia leads to energetic failure and death. However, sublethal hypoxia can trigger an adaptive response called hypoxic preconditioning. While prolyl-hydroxylase (PHD) enzymes and hypoxia-inducible factors (HIFs) have been identified as key elements of oxygen-sensing machinery, the mechanisms by which hypoxic preconditioning protects against insults remain unclear. Here, we perform serum metabolomic profiling to assess alterations induced by two potent cytoprotective approaches, hypoxic preconditioning and pharmacologic PHD inhibition. We discover that both approaches increase serum kynurenine levels and enhance kynurenine biotransformation, leading to preservation of NAD+ in the post-ischemic kidney. Furthermore, we show that indoleamine 2,3-dioxygenase 1 (Ido1) deficiency abolishes the systemic increase of kynurenine and the subsequent renoprotection generated by hypoxic preconditioning and PHD inhibition. Importantly, exogenous administration of kynurenine restores the hypoxic preconditioning in the context of Ido1 deficiency. Collectively, our findings demonstrate a critical role of the IDO1-kynurenine axis in mediating hypoxic preconditioning.

Investigating novel biomarkers of immune activation and modulation in the context of sedentary behaviour: a multicentre prospective ischemic stroke cohort study

Background

Sedentary behaviour is associated with disease, but the molecular mechanisms are not understood. Valid biomarkers with predictive and explanatory properties are required. Therefore, we have investigated traditional and novel biomarkers of inflammation and immune modulation and their association to objectively measured sedentary behaviour in an ischemic stroke population.

Methods

Patients admitted to hospital with acute ischemic stroke were included in the multicentre Norwegian Cognitive Impairment After Stroke (Nor-COAST) study (n = 815). For this sub-study (n = 257), sedentary behaviour was registered 3 months after stroke using position transition data from the body-worn sensor, ActivPal®. Blood samples were analysed for high sensitive C-reactive protein (hsCRP), the cytokines interleukin-6 (IL-6) and 10 (IL-10), neopterin, tryptophan (Trp), kynurenine (kyn), kynurenic acid (KA), and three B6 vitamers, pyridoxal 5′-phosphate (PLP), pyridoxal (PL), and pyridoxic acid (PA). The kynurenine/tryptophan ratio (KTR) and the pyridoxic acid ratio index (PAr = PA: PL + PLP) were calculated.

Results

Of the 815 patients included in the main study, 700 attended the three-month follow-up, and 257 fulfilled the inclusion criteria for this study. Sedentary time was significantly associated with levels of hsCRP, IL-6, neopterin, PAr-index, and KA adjusted for age, sex, waist circumference, and creatinine. In a fully adjusted model including all the significant biomarkers except hsCRP (because of missing values), sedentary time was independently positively associated with the PAr-index and negatively with KA. We did not find an association between sedentary behaviour, IL-10, and KTR.

Conclusions

The PAr-index is known to capture several modes of inflammation and has previously shown predictive abilities for future stroke. This novel result indicates that the PAr-index could be a useful biomarker in future studies on sedentary behaviour and disease progression. KA is an important modulator of inflammation, and this finding opens new and exciting pathways to understand the hazards of sedentary behaviour.

Trial registration

The study was registered at Clinicaltrials.gov (NCT02650531). First posted 08/01/2016.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12883-021-02343-0.

Bupi Yishen formula attenuates kidney injury in 5/6 nephrectomized rats via the tryptophan-kynurenic acid-aryl hydrocarbon receptor pathway

BACKGROUND

Bupi Yishen Formula (BYF), a patent traditional Chinese medicine (TCM) formulation, has been used in the clinical treatment of chronic kidney disease (CKD). However, the mechanism of action of BYF has not been fully elucidated.

METHOD

To investigate the variation in the metabolic profile in response to BYF treatment in a rat model of 5/6 nephrectomy (Nx), rats in the treatment groups received low- or high-dose BYF. At the end of the study, serum and kidney samples were collected for biochemical, pathological, and western blotting analysis. Metabolic changes in serum were analyzed by liquid chromatography-tandem mass spectrometry.

RESULTS

The results showed that BYF treatment could reduce kidney injury, inhibit inflammation and improve renal function in a dose-dependent manner. In total, 405 and 195 metabolites were identified in negative and positive ion modes, respectively. Metabolic pathway enrichment analysis of differential metabolites based on the Kyoto Encyclopedia of Genes and Genomes database identified 35 metabolic pathways, 3 of which were related to tryptophan metabolism. High-dose BYF reduced the level of kynurenic acid (KA) by more than 50%, while increasing melatonin 25-fold and indole-3-acetic acid twofold. Expression levels of aryl hydrocarbon receptor (AhR), Cyp1A1, and CyP1B1 were significantly reduced in the kidney tissue of rats with high-dose BYF, compared to 5/6 Nx rats.

CONCLUSION

BYF has a reno-protective effect against 5/6 Nx-induced CKD, which may be mediated via inhibition of the tryptophan-KA-AhR pathway.

Gut microbial biomarkers for the treatment response in first-episode, drug-naïve schizophrenia: a 24-week follow-up study

Preclinical studies have shown that the gut microbiota can play a role in schizophrenia (SCH) pathogenesis via the gut-brain axis. However, its role in the antipsychotic treatment response is unclear. Here, we present a 24-week follow-up study to identify gut microbial biomarkers for SCH diagnosis and treatment response, using a sample of 107 first-episode, drug-naïve SCH patients, and 107 healthy controls (HCs). We collected biological samples at baseline (all participants) and follow-up time points after risperidone treatment (SCH patients). Treatment response was assessed using the Positive and Negative Symptoms Scale total (PANSS-T) score. False discovery rate was used to correct for multiple testing. We found that SCH patients showed lower α-diversity (the Shannon and Simpson's indices) compared to HCs at baseline (p = 1.21 × 10-9, 1.23 × 10-8, respectively). We also found a significant difference in β-diversity between SCH patients and HCs (p = 0.001). At baseline, using microbes that showed different abundance between patients and controls as predictors, a prediction model can distinguish patients from HCs with an area under the curve (AUC) of 0.867. In SCH patients, after 24 weeks of risperidone treatment, we observed an increase of α-diversity toward the basal level of HCs. At the genus level, we observed decreased abundance of Lachnoclostridium (p = 0.019) and increased abundance Romboutsia (p = 0.067). Moreover, the treatment response in SCH patients was significantly associated with the basal levels of Lachnoclostridium and Romboutsia (p = 0.005 and 0.006, respectively). Our results suggest that SCH patients may present characteristic microbiota, and certain microbiota biomarkers may predict treatment response in this patient population.

The tryptophan biosynthetic pathway is essential for Mycobacterium tuberculosis to cause disease

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is the most significant cause of death from a single infectious agent worldwide. Antibiotic-resistant strains of M. tuberculosis represent a threat to effective treatment, and the long duration, toxicity and complexity of current chemotherapy for antibiotic-resistant disease presents a need for new therapeutic approaches with novel modes of action. M. tuberculosis is an intracellular pathogen that must survive phagocytosis by macrophages, dendritic cells or neutrophils to establish an infection. The tryptophan biosynthetic pathway is required for bacterial survival in the phagosome, presenting a target for new classes of antitubercular compound. The enzymes responsible for the six catalytic steps that produce tryptophan from chorismate have all been characterised in M. tuberculosis, and inhibitors have been described for some of the steps. The innate immune system depletes cellular tryptophan in response to infection in order to inhibit microbial growth, and this effect is likely to be important for the efficacy of tryptophan biosynthesis inhibitors as new antibiotics. Allosteric inhibitors of both the first and final enzymes in the pathway have proven effective, including by a metabolite produced by the gut biota, raising the intriguing possibility that the modulation of tryptophan biosynthesis may be a natural inter-bacterial competition strategy.

Tryptophan and Its Metabolites in Lung Cancer: Basic Functions and Clinical Significance

Lung cancer is the most lethal malignancy worldwide. Recently, it has been recognized that metabolic reprogramming is a complex and multifaceted factor, contributing to the process of lung cancer. Tryptophan (Try) is an essential amino acid, and Try and its metabolites can regulate the progression of lung cancer. Here, we review the pleiotropic functions of the Try metabolic pathway, its metabolites, and key enzymes in the pathogenic process of lung cancer, including modulating the tumor environment, promoting immune suppression, and drug resistance. We summarize the recent advance in therapeutic drugs targeting the Try metabolism and kynurenine pathway and their clinical trials.

Effects of Tryptophan Supplementation and Exercise on the Fate of Kynurenine Metabolites in Mice and Humans

The kynurenine pathway of tryptophan (TRP) degradation (KP) generates metabolites with effects on metabolism, immunity, and mental health. Endurance exercise training can change KP metabolites by changing the levels of KP enzymes in skeletal muscle. This leads to a metabolite pattern that favors energy expenditure and an anti-inflammatory immune cell profile and reduces neurotoxic metabolites. Here, we aimed to understand if TRP supplementation in untrained vs. trained subjects affects KP metabolite levels and biological effects. Our data show that chronic TRP supplementation in mice increases all KP metabolites in circulation, and that exercise reduces the neurotoxic branch of the pathway. However, in addition to increasing wheel running, we did not observe other effects of TRP supplementation on training adaptations, energy metabolism or behavior in mice. A similar increase in KP metabolites was seen in trained vs. untrained human volunteers that took a TRP drink while performing a bout of aerobic exercise. With this acute TRP administration, TRP and KYN were higher in the trained vs. the untrained group. Considering the many biological effects of the KP, which can lead to beneficial or deleterious effects to health, our data encourage future studies of the crosstalk between TRP supplementation and physical exercise.

The kynurenine pathway in major depressive disorder, bipolar disorder, and schizophrenia: a meta-analysis of 101 studies

The importance of tryptophan as a precursor for neuroactive compounds has long been acknowledged. The metabolism of tryptophan along the kynurenine pathway and its involvement in mental disorders is an emerging area in psychiatry. We performed a meta-analysis to examine the differences in kynurenine metabolites in major depressive disorder (MDD), bipolar disorder (BD), and schizophrenia (SZ). Electronic databases were searched for studies that assessed metabolites involved in the kynurenine pathway (tryptophan, kynurenine, kynurenic acid, quinolinic acid, 3-hydroxykynurenine, and their associate ratios) in people with MDD, SZ, or BD, compared to controls. We computed the difference in metabolite concentrations between people with MDD, BD, or SZ, and controls, presented as Hedges' g with 95% confidence intervals. A total of 101 studies with 10,912 participants were included. Tryptophan and kynurenine are decreased across MDD, BD, and SZ; kynurenic acid and the kynurenic acid to quinolinic acid ratio are decreased in mood disorders (i.e., MDD and BD), whereas kynurenic acid is not altered in SZ; kynurenic acid to 3-hydroxykynurenine ratio is decreased in MDD but not SZ. Kynurenic acid to kynurenine ratio is decreased in MDD and SZ, and the kynurenine to tryptophan ratio is increased in MDD and SZ. Our results suggest that there is a shift in the tryptophan metabolism from serotonin to the kynurenine pathway, across these psychiatric disorders. In addition, a differential pattern exists between mood disorders and SZ, with a preferential metabolism of kynurenine to the potentially neurotoxic quinolinic acid instead of the neuroprotective kynurenic acid in mood disorders but not in SZ.

Aberrant NAD+ metabolism underlies Zika virus-induced microcephaly

Zika virus (ZIKV) infection during pregnancy can cause microcephaly in newborns, yet the underlying mechanisms remain largely unexplored. Here, we reveal extensive and large-scale metabolic reprogramming events in ZIKV-infected mouse brains by performing a multi-omics study comprising transcriptomics, proteomics, phosphoproteomics and metabolomics approaches. Our proteomics and metabolomics analyses uncover dramatic alteration of nicotinamide adenine dinucleotide (NAD⁺)-related metabolic pathways, including oxidative phosphorylation, TCA cycle and tryptophan metabolism. Phosphoproteomics analysis indicates that MAPK and cyclic GMP-protein kinase G signaling may be associated with ZIKV-induced microcephaly. Notably, we demonstrate the utility of our rich multi-omics datasets with follow-up in vivo experiments, which confirm that boosting NAD⁺ by NAD⁺ or nicotinamide riboside supplementation alleviates cell death and increases cortex thickness in ZIKV-infected mouse brains. Nicotinamide riboside supplementation increases the brain and body weight as well as improves the survival in ZIKV-infected mice. Our study provides a comprehensive resource of biological data to support future investigations of ZIKV-induced microcephaly and demonstrates that metabolic alterations can be potentially exploited for developing therapeutic strategies.

Association between urinary per- and poly-fluoroalkyl substances and COVID-19 susceptibility

BACKGROUND AND OBJECTIVE

The growing impact of the COVID-19 pandemic has heightened the urgency of identifying individuals most at risk of infection. Per- and poly-fluoroalkyl substances (PFASs) are manufactured fluorinated chemicals widely used in many industrial and household products. The objective of this case-control study was to assess the association between PFASs exposure and COVID-19 susceptibility and to elucidate the metabolic dysregulation associated with PFASs exposure in COVID-19 patients.

METHODS

Total 160 subjects (80 COVID-19 patients and 80 symptom-free controls) were recruited from Shanxi and Shandong provinces, two regions heavily polluted by PFASs in China. Twelve common PFASs were quantified in both urine and serum. Urine metabolome profiling was performed by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS).

RESULTS

In unadjusted models, the risk of COVID-19 infection was positively associated with urinary levels of perfluorooctanesulfonic acid (PFOS) (Odds ratio: 2.29 [95% CI: 1.52-3.22]), perfluorooctanoic acid (PFOA) (2.91, [1.95-4.83], and total PFASs (∑ (12) PFASs) (3.31, [2.05-4.65]). After controlling for age, sex, body mass index (BMI), comorbidities, and urine albumin-to-creatinine ratio (UACR), the associations remained statistically significant (Adjusted odds ratio of 1.94 [95% CI: 1.39-2.96] for PFOS, 2.73 [1.71-4.55] for PFOA, and 2.82 [1.97-3.51] for ∑ (12) PFASs). Urine metabolome-PFASs association analysis revealed that 59% of PFASs-associated urinary endogenous metabolites in COVID-19 patients were identified to be produced or largely regulated by mitochondrial function. In addition, the increase of PFASs exposure was associated with the accumulation of key metabolites in kynurenine metabolism, which are involved in immune responses (Combined β coefficient of 0.60 [95% CI: 0.25-0.95, P = 0.001]). Moreover, alternations in PFASs-associated metabolites in mitochondrial and kynurenine metabolism were also correlated with clinical lab biomarkers for mitochondrial function (serum growth/differentiation factor-15) and immune activity (lymphocyte percentage), respectively.

CONCLUSION

Elevated exposure to PFASs was independently associated with an increased risk of COVID-19 infection. PFASs-associated metabolites were implicated in mitochondrial function and immune activity. Larger studies are needed to confirm our findings and further understand the underlying mechanisms of PFASs exposure in the pathogenesis of SARS-CoV2 infection.

The genie in the bottle-magnified calcium signaling in dorsolateral prefrontal cortex

Neurons in the association cortices are particularly vulnerable in cognitive disorders such as schizophrenia and Alzheimer's disease, while those in primary visual cortex remain relatively resilient. This review proposes that the special molecular mechanisms needed for higher cognitive operations confer vulnerability to dysfunction, atrophy, and neurodegeneration when regulation is lost due to genetic and/or environmental insults. Accumulating data suggest that higher cortical circuits rely on magnified levels of calcium (from NMDAR, calcium channels, and/or internal release from the smooth endoplasmic reticulum) near the postsynaptic density to promote the persistent firing needed to maintain, manipulate, and store information without "bottom-up" sensory stimulation. For example, dendritic spines in the primate dorsolateral prefrontal cortex (dlPFC) express the molecular machinery for feedforward, cAMP-PKA-calcium signaling. PKA can drive internal calcium release and promote calcium flow through NMDAR and calcium channels, while in turn, calcium activates adenylyl cyclases to produce more cAMP-PKA signaling. Excessive levels of cAMP-calcium signaling can have a number of detrimental effects: for example, opening nearby K+ channels to weaken synaptic efficacy and reduce neuronal firing, and over a longer timeframe, driving calcium overload of mitochondria to induce inflammation and dendritic atrophy. Thus, calcium-cAMP signaling must be tightly regulated, e.g., by agents that catabolize cAMP or inhibit its production (PDE4, mGluR3), and by proteins that bind calcium in the cytosol (calbindin). Many genetic or inflammatory insults early in life weaken the regulation of calcium-cAMP signaling and are associated with increased risk of schizophrenia (e.g., GRM3). Age-related loss of regulatory proteins which result in elevated calcium-cAMP signaling over a long lifespan can additionally drive tau phosphorylation, amyloid pathology, and neurodegeneration, especially when protective calcium binding proteins are lost from the cytosol. Thus, the "genie" we need for our remarkable cognitive abilities may make us vulnerable to cognitive disorders when we lose essential regulation.

GCH1 induces immunosuppression through metabolic reprogramming and IDO1 upregulation in triple-negative breast cancer

Purpose

Regulatory T cells (Tregs) heavily infiltrate triple-negative breast cancer (TNBC), and their accumulation is affected by the metabolic reprogramming in cancer cells. In the present study, we sought to identify cancer cell-intrinsic metabolic modulators correlating with Tregs infiltration in TNBC.

Experimental design

Using the RNA-sequencing data from our institute (n=360) and the Molecular Taxonomy of Breast Cancer International Consortium TNBC cohort (n=320), we calculated the abundance of Tregs in each sample and evaluated the correlation between gene expression levels and Tregs infiltration. Then, in vivo and in vitro experiments were performed to verify the correlation and explore the underlying mechanism.

Results

We revealed that GTP cyclohydrolase 1 (GCH1) expression was positively correlated with Tregs infiltration and high GCH1 expression was associated with reduced overall survival in TNBC. In vivo and in vitro experiments showed that GCH1 increased Tregs infiltration, decreased apoptosis, and elevated the programmed cell death-1 (PD-1)-positive fraction. Metabolomics analysis indicated that GCH1 overexpression reprogrammed tryptophan metabolism, resulting in L-5-hydroxytryptophan (5-HTP) accumulation in the cytoplasm accompanied by kynurenine accumulation and tryptophan reduction in the supernatant. Subsequently, aryl hydrocarbon receptor, activated by 5-HTP, bound to the promoter of indoleamine 2,3-dioxygenase 1 (IDO1) and thus enhanced the transcription of IDO1. Furthermore, the inhibition of GCH1 by 2,4-diamino-6-hydroxypyrimidine (DAHP) decreased IDO1 expression, attenuated tumor growth, and enhanced the tumor response to PD-1 blockade immunotherapy.

Conclusions

Tumor-cell-intrinsic GCH1 induced immunosuppression through metabolic reprogramming and IDO1 upregulation in TNBC. Inhibition of GCH1 by DAHP serves as a potential immunometabolic strategy in TNBC.

BDNF Impact on Biological Markers of Depression-Role of Physical Exercise and Training

Depression is the most common and devastating psychiatric disorder in the world. Its symptoms, especially during the pandemic, are observed in all age groups. Exercise training (ET) is well known as a non-pharmacological strategy to alleviate clinical depression. The brain-derived neurotrophic factor (BDNF) is one of the biological factors whose expression and secretion are intensified in response to ET. BDNF is also secreted by contracted skeletal muscle that likely exerts para-, auto- and endocrine effects, supporting the crosstalk between skeletal muscle and other distant organs/tissues, such as the nervous system. This finding suggests that they communicate and work together to induce improvements on mood, cognition, and learning processes as BDNF is the main player in the neurogenesis, growth, and survival of neurons. Therefore, BDNF has been recognized as a therapeutic factor in clinical depression, especially in response to ET. The underlying mechanisms through which ET impacts depression are varied. The aim of this review was to provide information of the biological markers of depression such as monoamines, tryptophan, endocannabinoids, markers of inflammatory processes (oxidative stress and cytokines) stress and sex hormones and their relationship to BDNF. In addition, we reviewed the effects of ET on BNDF expression and how it impacts depression as well as the potential mechanisms mediating this process, providing a better understanding of underlying ET-related mechanisms in depression.

UVB Radiation and Selected Tryptophan-Derived AhR Ligands-Potential Biological Interactions in Melanoma Cells

Excessive UV exposure is considered the major environmental factor in melanoma progression. Human skin is constantly exposed to selected tryptophan-derived aryl hydrocarbon receptor (AhR) ligands, including kynurenine (KYN) and kynurenic acid (KYNA), as they are endogenously produced and present in various tissues and body fluids. Importantly, recent studies confirmed the biological activity of KYN and KYNA toward melanoma cells in vitro. Thus, in this study, the potential biological interactions between UVB and tryptophan metabolites KYN and KYNA were studied in melanoma A375, SK-MEL-3, and RPMI-7951 cells. It was shown that UVB enhanced the antiproliferative activity of KYN and KYNA in melanoma cells. Importantly, selected tryptophan-derived AhR ligands did not affect the invasiveness of A375 and RPMI-7951 cells; however, the stimulatory effect was observed in SK-MEL-3 cells exposed to UVB. Thus, the effect of tryptophan metabolites on metabolic activity, cell cycle regulation, and cell death in SK-MEL-3 cells exposed to UVB was assessed. In conclusion, taking into account that both UVB radiation and tryptophan-derived AhR ligands may have a crucial effect on skin cancer formation and progression, these results may have a significant impact, revealing the potential biological interactions in melanoma cells in vitro.

Feeding state greatly modulates the effect of xenobiotics on gut microbiome metabolism: A case study of tetracycline

The human gut microbiome is crucial in modulating host health mostly through bacterial metabolites. Chemical exposure is typical external stress which alters its composition and functionality. To date, very few studies have investigated the effect of feeding state on chemical-induced gut microbial metabolic dysregulations. Here, we set up an in vitro human gut microbiome and incorporated a metabolomics approach to investigate the effect of tetracycline (TET) at multiple doses (i.e., 10, 1, and 0.01 mg/L) on gut microbiome under the fed and fasted states. Overall, the metabolome was highly responsive at the fed state with 62 metabolites dysregulated while only 14 were altered at the fasted state under 10 mg/L (clinical TET dose). As expected, nutrient consumption was significantly inhibited under clinical TET dose at the fed state accumulating nutrients such as glutamate and leucine. Interestingly, at the fed state, TET could increase the synthesis of indole and phenyl derivatives including indole-3-aldehyde and hydrocinnamate, while inhibiting indoxyl, tryptamine, and vitamin B production, all of which have host health implications. Furthermore, metabolites like indoxyl and xanthurenic acid were still responsive at 0.01 mg/L (dietary TET dose). Collectively, results demonstrated that the feeding state greatly modulates the chemical-induced gut microbial metabolic alterations.

Kynurenine metabolism and metabolic syndrome in patients with schizophrenia

Accumulating evidence indicates that a dysregulated kynurenine (KYN) pathway (KP) metabolism may play an important role in the pathogenesis of both schizophrenia and metabolic syndrome (MS). However, the underlying mechanisms remain poorly understood. Here, we aimed to evaluate the potential roles of KP in the pathogenesis of MS in schizophrenia. A total of 160 schizophrenia patients and 70 healthy controls were enrolled in this study. KP metabolites were quantified, and MS scores were calculated, for comparisons between patients and controls. Associations among the indices were explored in both groups. Multiple linear regression analyses were performed to investigate the effects of KP metabolites on MS factors. We observed a significantly higher MS score, lower levels of all KP metabolites, and higher nicotinamide adenine dinucleotide (NAD⁺)/quinolinic acid (QUNA) in patients than in controls (all p < 0.01). Partial correlation analyses revealed that, in the patient group, QUNA and QUNA/KYN correlated positively with MS score (r = 0.24 and 0.27, respectively, both p < 0.025), and NAD⁺/QUNA correlated negatively with MS score (r = -0.25, p = 0.002). Results of multiple regression analyses showed significant QUNA × group interactions in the model representing QUNA effects on MS score (β = 0.25) and a significant QUNA/KYN × group interaction in the model representing QUNA/KYN effects on MS score (β = 0.23) (both p < 0.001). Among all factors contributing to MS in schizophrenia, an interactive effect of schizophrenia itself and dysregulated KP plays a contributory role. Conceivably, modulation of the KP could theoretically lead to treating schizophrenia and MS simultaneously.

The kynurenine pathway in bipolar disorder: a meta-analysis on the peripheral blood levels of tryptophan and related metabolites

Growing evidence suggests that a dysregulation of the kynurenine pathway (KP) occurs in bipolar disorder (BD). This systematic review and meta-analysis aimed at assessing the possible differences in peripheral blood levels of KP metabolites between individuals with BD and healthy controls. We searched Medline, Embase, and PsycInfo electronic databases for articles indexed up to February 2020. We included any observational study comparing the peripheral blood levels of at least one KP metabolite between adults with BD and healthy controls. Random-effects meta-analyses were carried out generating pooled standardized mean differences (SMDs). Heterogeneity between studies was estimated using the I² index. Meta-regression and sensitivity analyses were conducted. Sixteen studies met inclusion criteria and were included in our study. Meta-analyses showed that individuals with BD have lower peripheral blood levels of tryptophan (SMD = -0.29), kynurenine (SMD = -0.28), kynurenic acid (SMD = -0.30), and xanthurenic acid (SMD = -0.55), along with lower kynurenic acid to kynurenine (SMD = -0.60) and kynurenic acid to quinolinic acid (SMD = -0.37) ratios, than healthy controls. Individuals with a manic episode showed the greatest reductions in tryptophan levels (SMD = -0.51), whereas kynurenic acid levels were more reduced among subjects in a depressive phase (SMD = -0.70). Meta-regression and sensitivity analyses confirmed our results. The findings of the present meta-analysis support the hypothesis of an abnormality of the KP in BD. Considering the partial inconsistency of the findings and the small-to-medium magnitude of the estimated effect sizes, additional research assessing possible mediators or confounders is needed.

Wide-Targeted Metabolome Analysis Identifies Potential Biomarkers for Prognosis Prediction of Epithelial Ovarian Cancer

Epithelial ovarian cancer (EOC) is a fatal gynecologic cancer, and its poor prognosis is mainly due to delayed diagnosis. Therefore, biomarker identification and prognosis prediction are crucial in EOC. Altered cell metabolism is a characteristic feature of cancers, and metabolomics reflects an individual’s current phenotype. In particular, plasma metabolome analyses can be useful for biomarker identification. In this study, we analyzed 624 metabolites, including uremic toxins (UTx) in plasma derived from 80 patients with EOC using ultra-high-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Compared with the healthy control, we detected 77 significantly increased metabolites and 114 significantly decreased metabolites in EOC patients. Especially, decreased concentrations of lysophosphatidylcholines and phosphatidylcholines and increased concentrations of triglycerides were observed, indicating a metabolic profile characteristic of EOC patients. After calculating the parameters of each metabolic index, we found that higher ratios of kynurenine to tryptophan correlates with worse prognosis in EOC patients. Kynurenine, one of the UTx, can affect the prognosis of EOC. Our results demonstrated that plasma metabolome analysis is useful not only for the diagnosis of EOC, but also for predicting prognosis with the variation of UTx and evaluating response to chemotherapy.

Abnormal Tryptophan Metabolism in HIV and Mycobacterium tuberculosis Infection

Host metabolism has recently gained more attention for its roles in physiological functions and pathologic conditions. Of these, metabolic tryptophan disorders generate a pattern of abnormal metabolites that are implicated in various diseases. Here, we briefly highlight the recent advances regarding abnormal tryptophan metabolism in HIV and Mycobacterium tuberculosis infection and discuss its potential impact on immune regulation, disease progression, and neurological disorders. Finally, we also discuss the potential for metabolic tryptophan interventions toward these infectious diseases.

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.

The Footprint of Kynurenine Pathway in Neurodegeneration: Janus-Faced Role in Parkinson's Disorder and Therapeutic Implications

Progressive degeneration of neurons and aggravation of dopaminergic neurons in the substantia nigra pars compacta results in the loss of dopamine in the brain of Parkinson's disease (PD) patients. Numerous therapies, exhibiting transient efficacy have been developed; however, they are mostly accompanied by side effects and limited reliability, therefore instigating the need to develop novel optimistic treatment targets. Significant therapeutic targets have been identified, namely: chaperones, protein Abelson, glucocerebrosidase-1, calcium, neuromelanin, ubiquitin-proteasome system, neuroinflammation, mitochondrial dysfunction, and the kynurenine pathway (KP). The role of KP and its metabolites and enzymes in PD, namely quinolinic acid (QUIN), kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranillic acid (3-HAA), kunurenine-3-monooxygenase (KMO), etc. has been reported. The neurotoxic QUIN, N-methyl-D-aspartate (NMDA) receptor agonist, and neuroprotective KYNA-which antagonizes QUIN actions-primarily justify the Janus-faced role of KP in PD. Moreover, KP has been reported to play a biomarker role in PD detection. Therefore, the authors detail the neurotoxic, neuroprotective, and immunomodulatory neuroactive components, alongside the upstream and downstream metabolic pathways of KP, forming a basis for a therapeutic paradigm of the disease while recognizing KP as a potential biomarker in PD, thus facilitating the development of a suitable target in PD management.

Neuron-Glia-Immune Triad and Cortico-Limbic System in Pathology of Pain

Pain is an unpleasant sensation that alerts one to the presence of obnoxious stimuli or sensations. These stimuli are transferred by sensory neurons to the dorsal root ganglia-spinal cord and finally to the brain. Glial cells in the peripheral nervous system, astrocytes in the brain, dorsal root ganglia, and immune cells all contribute to the development, maintenance, and resolution of pain. Both innate and adaptive immune responses modulate pain perception and behavior. Neutrophils, microglial, and T cell activation, essential components of the innate and adaptive immune responses, can play both excitatory and inhibitory roles and are involved in the transition from acute to chronic pain. Immune responses may also exacerbate pain perception by modulating the function of the cortical-limbic brain regions involved in behavioral and emotional responses. The link between an emotional state and pain perception is larger than what is widely acknowledged. In positive psychological states, perception of pain along with other somatic symptoms decreases, whereas in negative psychological states, these symptoms may worsen. Sex differences in mechanisms of pain perception are not well studied. In this review, we highlight what is known, controversies, and the gaps in this field.

Reliable chromatographic assay for measuring of indoleamine 2,3-dioxygenase 1 (IDO1) activity in human cancer cells

The kynurenine pathway is the major tryptophan degradation routes generating bioactive compounds important in physiology and diseases. Depending on cell type it is initiated enzymatically by tryptophan-2,3-dioxygenase (TDO) or indoleamine-2,3-dioxygenase 1 and 2 (IDO1 and IDO2) to yield N-formylkynurenine as the precursor of further metabolites. Herein, we describe an accurate high-pressure liquid chromatography coupled with a diode array detector (HPLC-DAD) method to serve for IDO1 activity determination in human cancer cells cultured in vitro. Enzymatic activity was expressed as the rate of ʟ-kynurenine generation by 1 mg of proteins obtained from cancer cells. Our approach shows the limit of detection and limit of quantification at 12.9 and 43.0 nM Kyn, respectively. Applicability of this method was demonstrated in different cells (ovarian and breast cancer)exposed to various conditions and has successfully passed the validation process. This approach presents a useful model to study the role of kynurenine pathway in cancer biology.

Tryptophan-kynurenine metabolism: a link between the gut and brain for depression in inflammatory bowel disease

Inflammatory bowel disease (IBD), which mainly includes ulcerative colitis (UC) and Crohn's disease (CD), is a group of chronic bowel diseases that are characterized by abdominal pain, diarrhea, and bloody stools. IBD is strongly associated with depression, and its patients have a higher incidence of depression than the general population. Depression also adversely affects the quality of life and disease prognosis of patients with IBD. The tryptophan-kynurenine metabolic pathway degrades more than 90% of tryptophan (TRP) throughout the body, with indoleamine 2,3-dioxygenase (IDO), the key metabolic enzyme, being activated in the inflammatory environment. A series of metabolites of the pathway are neurologically active, among which kynerunic acid (KYNA) and quinolinic acid (QUIN) are molecules of great interest in recent studies on the mechanisms of inflammation-induced depression. In this review, the relationship between depression in IBD and the tryptophan-kynurenine metabolic pathway is overviewed in the light of recent publications.

Effects of stress associated with academic examination on the kynurenine pathway profile in healthy students

The effects of stress on the neuroendocrine, central nervous and immune systems are extremely complex. The kynurenine pathway (KP) of the tryptophan metabolism is recognised as a cross-link between the neuroendocrine- and immune systems. However, the effects of acute stress from everyday life on KP activation have not yet been studied. This study aims to investigate changes in the levels of the KP neuroactive metabolites and cytokines in response to stress triggered by academic examinations. Ninety-two healthy first year medical students benevolently participated in the study. Parameters were measured pre- examination, which is considered to be a high-stress period, and post-examination, as a low-stress period. Stress induced by academic examinations significantly increases the perceived stress scores (p<0.001), serum cortisol levels (p<0.001) and brain-derived neurotrophic factor (BDNF) levels (p<0.01). It decreased IL-10 levels (p<0.05) but had no effect on IL-6 and TNF-alpha levels. Only the KP neuroactive metabolite, 3-hydroxykynurenine (3-HK) significantly increased (p<0.01) in the post-examination period. In addition, the stress scores positively correlated with the levels of cortisol (r² = 0.297, p<0.01) at post examination. Acute stress triggered by academic examinations increases cortisol and BDNF production and suppresses the anti-inflammatory cytokine, IL-10, but did not increase significantly the levels of other pro-inflammatory cytokines, tryptophan, kynurenine and downstream KP metabolites. The concomitant increased levels of BDNF under the duress of acute examination stress appear to limit the levels pro-inflammatory markers, which may attenuate the action of cortisol and the neuroinflammatory branch of the KP.

The changes in kynurenine metabolites induced by rTMS in treatment-resistant depression: A pilot study

BACKGROUND

Repetitive transcranial magnetic stimulation (rTMS) is a noninvasive brain stimulation technique that is considered a valuable and promising technique for improving depressive symptoms in treatment-resistant depression (TRD). However, the exact mechanism by which rTMS ameliorates depressive symptoms remains to be clarified.

OBJECTIVE

The aim of the present study was to analyzed the changes in metabolites of patients with TRD in the rTMS treatment, especially focusing on the kynurenine (KYN) pathway.

METHODS

Thirteen participants with TRD were enrolled in a high-frequency (10 Hz) rTMS study. Cognitive function, depressive symptoms and the concentration of plasma tryptophan (TRP) metabolites were measured at baseline and at the endpoint of rTMS treatment.

RESULTS

rTMS treatment significantly improved depressive symptom scores and some subscales of cognitive dysfunction. The present study has demonstrated that rTMS treatment significantly increased plasma TRP levels and significantly decreased plasma serotonin levels, while plasma KYN and kynurenic acid level as well as KYN/TRP ratio remained unchanged.

CONCLUSIONS

This is the first metabolomic study of patients with TRD undergoing rTMS treatment. To validate the present results, it is necessary to increase the number of cases including controls, use a sample of cerebrospinal fluid, and measure blood concentration over time in the course of rTMS treatment.

Effect of You-Gui-Wan on House Dust Mite-Induced Mouse Allergic Asthma via Regulating Amino Acid Metabolic Disorder and Gut Dysbiosis

Chinese herbal remedies have long been used for enhancing immunity and treating asthma. However, the evidence-based efficacy remains to be supported. This study aimed to explore the potential bio-signatures in allergic asthma and the effect of You-Gui-Wan (YGW), a traditional Chinese herbal prescription, on dust mite-induced mouse allergic asthma. Extract of Dermatophagoides pteronyssinus (Der p), a dust mite, was intratracheally administered to induce allergic asthma in mice. Serum metabolomic and 16S rRNA-based microbiome profiling were used to analyze untargeted metabolites with levels significantly changed and gut microbiota composition, respectively. Results indicated that 10 metabolites (acetylcarnitine, carnitine, hypoxanthine, tryptophan, phenylalanine, norleucine, isoleucine, betaine, methionine, and valine), mainly associated with branched-chain amino acid (BCAA) metabolism, aromatic amino acid (AAA) biosynthesis, and phenylalanine metabolism were markedly elevated after Der p treatment. YGW administration reversed the levels for 7 of the 10 identified metabolites, chiefly affecting BCAA metabolism. On 16S DNA sequencing, disordered Der p-induced gut microbiota was significantly alleviated by YGW. Multiple correlation analysis showed a good correlation between gut microbiota composition and levels of selected metabolites. Our study showed YGW administration effectively alleviated BCAA metabolic disorder and improved gut dysbiosis. This study provides support for YGW administration with benefits for allergic asthma.

Chronic restraint stress induced changes in colonic homeostasis-related indexes and tryptophan-kynurenine metabolism in rats

Chronic stressors represented risk factors for the etiology or exacerbation of several gastrointestinal diseases. The goal of the present study was to examine whether chronic restraint stress (CRS) could initiate and aggravate colonic inflammation, integrity damage and metabolic disturbance of rats. Firstly, increased inflammatory cytokines (interferon-γ (IFN-γ), tumor necrosis factor-α(TNF-α) and interleukin-10(IL-10)) and decreased tight junction (TJ) proteins (occludin and zonula occludins-1 (ZO-1)) in rat colon were observed. Secondly, untargeted metabolomics based on ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass (UPLC-Q-TOF/MS) revealed that TRP metabolism was the most prominently affected. Thirdly, quantification of TRP and its metabolites via prominence ultrafast liquid chromatography coupled with a QTRAP 5500 mass (UFLC-QTRAP-5500/MS) showed that TRP, kynurenine (KYN), kynurenic acid (KA) and 3-hydroxykynurenine (3-HK) were significantly increased. At the same time, 5-hydroxytryptamine (5-HT) was unchanged and 5-hydroxyindolacetic acid (5-HIAA) was significantly decreased in the colon of CRS rats. Besides, TRP metabolic enzyme changes were with the same trends as the corresponding metabolites. Thus, our data showed that CRS could initiate colonic inflammation, integrity damage and colonic metabolism disturbance, especially TRP-KYN metabolism pathway of rats, which may provide an experimental background for future research on stress-related gastrointestinal dysfunction. SIGNIFICANCE: Chronic exposure to psychological stress could induce metabolic imbalance of the body, and stressful life events were intimately correlated with frequent relapses in patients with intestinal disorders. The present study showed that chronic restraint stress (CRS) could initiate and aggravate colonic inflammation, integrity damage and metabolic disturbance, especially tryptophan-kynurenine metabolism of rats. Tryptophan-kynurenine pathway may be involved in the initiation and development of diseases induced by chronic stress. This research may shed light on future research on stress-related gastrointestinal dysfunction.

Oral Administration of Brain Protein Combined With Probiotics Induces Immune Tolerance Through the Tryptophan Pathway

Excessive inflammation leads to secondary immune damage after traumatic brain injury (TBI). The intestinal mucosa is a key component of immune tolerance due to gut-brain axis regulation, but the curative effect is not optimal. Intestinal dysfunction impairs the establishment of immune tolerance in patients with TBI. Therefore, we orally administered brain protein (BP) combined with probiotics to induce immune tolerance and explored the mechanism by which the homeostasis of the microbiota contributes to the enhancement of curative effects by BPs. Herein, we demonstrated that patients with TBI and surgical brain injury (SBI) models of rats had obvious dysbiosis. Notably, the intestinal barrier, proinflammatory cytokines, and activation of microglia demonstrated that excessive inflammatory damage was better controlled in the combined group (oral administration of BP combined with probiotics) than in the BP group (oral administration of BP). Fundamentally, tandem mass tag (TMT)-based quantitative proteomics analysis revealed that BP and probiotics preferentially affect Try-related pathways. A series of experiments further confirmed that Indoleamine 2,3 dioxygenase (IDO)/Kynurenine (Kyn)/Aryl hydrocarbon receptor (AhR) expression was high in the BP group, while Tryptophan hydroxylase 1(TpH1)/5-hydroxytryptamine (5-HT) only changed in the combined group. This study suggests that probiotics can enhance the efficacy of oral BP-induced immune tolerance through the Try pathway.