Kynurenic acid inhibits intestinal hypermotility and xanthine oxidase activity during experimental colon obstruction in dogs

Tryptophan metabolism and piglet diarrhea: Where we stand and the challenges ahead

The intestinal architecture of piglets is vulnerable to disruption during weaning transition and leads to diarrhea, frequently accompanied by inflammation and metabolic disturbances (including amino acid metabolism). Tryptophan (Trp) plays an essential role in orchestrating intestinal immune tolerance through its metabolism via the kynurenine, 5-hydroxytryptamine, or indole pathways, which could be dictated by the gut microbiota either directly or indirectly. Emerging evidence suggests a strong association between piglet diarrhea and Trp metabolism. Here we aim to summarize the intricate balance of microbiota-host crosstalk by analyzing alterations in both the host and microbial pathways of Trp and discuss how Trp metabolism may affect piglet diarrhea. Overall, this review could provide valuable insights to explore effective strategies for managing piglet diarrhea and the related challenges.

Different Kynurenine Pathway Dysregulation in Systemic Sclerosis in Men and Women

Systemic sclerosis (SSc), a predominantly female-affected systemic autoimmune disease, requires tailored treatment strategies contingent on organ involvement and symptom severity. Given SSc's inflammatory nature, the involvement of the kynurenine pathway (KP) in its pathophysiology is underexplored. Our study aimed to investigate sex-related differences in KP activation among SSc patients and assess the impact of angiotensin-converting enzyme (ACE) inhibitors and estimated glomerular filtration rate (eGFR) on KP metabolite concentrations. We enrolled 48 SSc patients and 53 healthy controls, quantifying KP metabolites (tryptophan (TRP), kynurenine (KYN), and kynurenic acid (KYNA)) in serum via high-performance liquid chromatography. Separate multivariate analyses of covariance (MANCOVAs) for women and men were performed to ascertain mean differences between patients and healthy controls while correcting for age. For our secondary objective, we conducted a MANCOVA to explore disparities in ACE inhibitor users and non-users among patients, with BMI correction. Our findings revealed decreased TRP concentrations but increased KYNA/TRP ratio and KYN/TRP ratio in both male and female SSc patients compared to their respective controls. Unlike women, SSc males exhibited higher KYN concentrations and decreased KYNA/KYN ratio relative to their controls. Additionally, SSc patients using ACE inhibitors had higher serum KYNA levels than non-users. Notably, we established a significant correlation between eGFR and KYNA in SSc patients. These results indicate differential KP activation in male and female SSc patients, with males demonstrating heightened KP activation. While ACE inhibitors may influence the KP in SSc patients, further research is necessary to comprehensively understand their impact on symptoms and prognosis in the context of these KP alterations.

Distinct Metabolic Profiles of Ocular Hypertensives in Response to Hypoxia

Glaucoma is a neurodegenerative disease that affects the retinal ganglion cells (RGCs). The main risk factor is elevated intraocular pressure (IOP), but the actual cause of the disease remains unknown. Emerging evidence indicates that metabolic dysfunction plays a central role. The aim of the current study was to determine and compare the effect of universal hypoxia on the metabolomic signature in plasma samples from healthy controls (n = 10), patients with normal-tension glaucoma (NTG, n = 10), and ocular hypertension (OHT, n = 10). By subjecting humans to universal hypoxia, we aim to mimic a state in which the mitochondria in the body are universally stressed. Participants were exposed to normobaric hypoxia for two hours, followed by a 30 min recovery period in normobaric normoxia. Blood samples were collected at baseline, during hypoxia, and in recovery. Plasma samples were analyzed using a non-targeted metabolomics approach based on liquid chromatography coupled with high-resolution mass spectrometry (LC-HRMS). Multivariate analyses were conducted using principal component analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA), and univariate analysis using the Wilcoxon signed-rank test and false discovery rate (FDR) correction. Unique metabolites involved in fatty acid biosynthesis and ketone body metabolism were upregulated, while metabolites of the kynurenine pathway were downregulated in OHT patients exposed to universal hypoxia. Differential affection of metabolic pathways may explain why patients with OHT initially do not suffer or are more resilient from optic nerve degeneration. The metabolomes of NTG and OHT patients are regulated differently from control subjects and show dysregulation of metabolites important for energy production. These dysregulated processes may potentially contribute to the elevation of IOP and, ultimately, cell death of the RGCs.

A Review of the Health Benefits of Food Enriched with Kynurenic Acid

Kynurenic acid (KYNA), a metabolite of tryptophan, is an endogenous substance produced intracellularly by various human cells. In addition, KYNA can be synthesized by the gut microbiome and delivered in food. However, its content in food is very low and the total alimentary supply with food accounts for only 1-3% of daily KYNA excretion. The only known exception is chestnut honey, which has a higher KYNA content than other foods by at least two orders of magnitude. KYNA is readily absorbed from the gastrointestinal tract; it is not metabolized and is excreted mainly in urine. It possesses well-defined molecular targets, which allows the study and elucidation of KYNA's role in various pathological conditions. Following a period of fascination with KYNA's importance for the central nervous system, research into its role in the peripheral system has been expanding rapidly in recent years, bringing some exciting discoveries. KYNA does not penetrate from the peripheral circulation into the brain; hence, the following review summarizes knowledge on the peripheral consequences of KYNA administration, presents data on KYNA content in food products, in the context of its daily supply in diets, and systematizes the available pharmacokinetic data. Finally, it provides an analysis of the rationale behind enriching foods with KYNA for health-promoting effects.

Multidisciplinary analysis of Italian Alpine wildflower honey reveals criticalities, diversity and value

Wildflower honeys produced in mountain grasslands are an expression of the biodiversity of these fragile habitats. Despite its importance, the botanical origin of honey is often defined without performing formal analysis. The aim of the study was to characterize six wildflower mountain honeys produced in the Italian Alps with different analytic techniques (SPME-GC-MS, HPLC-Orbitrap, cicatrizing and antioxidant activity) alongside melissopalynological analysis and botanical definition of the production area. Even though the apiaries were in mountain grasslands rich in Alpine herbaceous species, the honey could be defined as rhododendron/raspberry unifloral or raspberry and rhododendron bifloral while the honey produced at the lowest altitude differed due to the presence of linden, heather and chestnut. The non-compliance of the honey could be due to habitat (meadows and pastures) fragmentation, but also to specific compounds involved in the plant-insect relationship, such as kynurenic acid, present in a high quantity in the sample rich in chestnut pollen. 255 volatile compounds were detected as well as some well-known markers of specific botanic essences, in particular chestnut, linden and heather, also responsible for most of the differences in aroma profiling. A high correlation between nicotinaldehyde content and percentage of raspberry pollen (r = 0.853, p < 0.05) was found. Phenolic acid and hydroxy-fatty acid were predominant in the chestnut pollen dominant honey, which presented the highest antioxidant activity and the lowest cicatrizing activity, while the flavonoid fraction was accentuated in one sample (rhododendron pollen prevalent), that was also the one with the highest effect on wound closure, although all samples had similar cicatrizing effects apart from the chestnut pollen dominant honey (lowest cicatrizing activity). Our study highlighted the difficulty of producing mountain wildflower honey and the importance of a thorough characterization of this product, also to encourage its production and valorisation.

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

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

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.

Effect of 4-week physical exercises on tryptophan, kynurenine and kynurenic acid content in human sweat

The aim of the study was the detection of TRP, kynurenine (KYN), and kynurenic acid (KYNA) in human sweat, and determining whether physical activity affects their content in this secrete. Two different methods were used simultaneously—collection of sweat by means of an absorption pad from the inter scapular region, and collection of a drop of sweat from the region of the forehead. Quantitative determinations of TRP, KYN and KYNA were performed using high performance liquid chromatography with ultraviolet and fluorescence detection. Determinations of sodium was carried out by the method of inductively coupled plasma collision/reaction cell ionization mass spectrophotometry. It was found that physical exercises evoked a decrease in the amount of KYN, and an increase in the amount of KYNA in sweat recorded on day 14, but not on day 28 of training. It appears that physical exercises result in a long-term increase in the kynurenine transaminase activity responsible for the formation of KYNA from KYN. Based on this results, it can be suggested that measurement of TRP, KYN and KYNA in sweat may have diagnostic potential and may help to establish an exercise regime appropriate for the age, gender and health status of rehabilitation patients.

Apoptosis-mediated anticancer activity in prostate cancer cells of a chestnut honey (Castanea sativa L.) quinoline-pyrrolidine gamma-lactam alkaloid

Prostate cancer (PCa) is the most common malignancy in men and represents the second leading cause of cancer deaths in Western countries. PCa is initially androgen-dependent, however, this tumor inevitably progresses as castration-resistant prostate cancer (CRPC), which represents the most aggressive phase of the pathology. In this work, in two CRPC cell lines (DU145 and PC3), we studied the in vitro inhibitory properties of the tryptophan-derived endogenous metabolite kynurenic acid (KYNA) and of the lactam form of 3–2′-pyrrilonidinyl-kynurenic acid (3-PKA-L), alkaloids usually present in combination in chestnut honey. Cytotoxicity was evaluated by 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, cell colony formation assay, and Western blot analysis of the major mediator proteins involved in apoptotic processes. In all experiments, KYNA was scarcely or not active while 3-PKA-L showed anticancer activity in the high concentration range (0.01 mM – 1 mM) from 24 to 72 h. The results obtained showed that cell death was induced by extrinsic apoptotic pathway, by cell morphological changes and reduction of cell colonies number. These novel results represent the first promising step to the accurate description of 3-PKA-L cytotoxic effect, not observed with KYNA, paving the way to the search of new anticancer agents, as well as to the better understanding of the physiopathological role of this interesting natural product.

The Beneficial Effects of Edible Kynurenic Acid from Marine Horseshoe Crab (Tachypleus tridentatus) on Obesity, Hyperlipidemia, and Gut Microbiota in High-Fat Diet-Fed Mice

The marine horseshoe crab (Tachypleus tridentatus) has been considered as food and traditional medicine for many years. Kynurenic acid (KA) was isolated from horseshoe crab in this study for the first time in the world. A previous study in 2018 reported that intraperitoneal administration of KA prevented high-fat diet- (HFD-) induced body weight gain. Now, we investigated the effects of intragastric gavage of KA on HFD mice and found that KA (5 mg/kg/day) inhibited both the body weight gain and the increase of average daily energy intake. KA reduced serum triglyceride and increased serum high-density lipoprotein cholesterol. KA inhibited HFD-induced the increases of serum low-density lipoprotein cholesterol, coronary artery risk index, and atherosclerosis index. KA also suppressed HFD-induced the increase of the ratio of Firmicutes to Bacteroidetes (two dominant gut microbial phyla). KA partially reversed HFD-induced the changes in the composition of gut microbial genera. These overall effects of KA on HFD mice were similar to that of simvastatin (positive control). But the effects of 1.25 mg/kg/day KA on HFD-caused hyperlipidemia were similar to the effects of 5 mg/kg/day simvastatin. The pattern of relative abundance in 40 key genera of gut microbiota from KA group was closer to that from the normal group than that from the simvastatin group. In addition, our in vitro results showed the potential antioxidant activity of KA, which suggests that the improvement effects of KA on HFD mice may be partially associated with antioxidant activity of KA. Our findings demonstrate the potential role of KA as a functional food ingredient for the treatment of obesity and hyperlipidemia as well as the modulation of gut microbiota.

Impact of Microbial Metabolites on Microbiota-Gut-Brain Axis in Inflammatory Bowel Disease

The complex bidirectional communication system existing between the gastrointestinal tract and the brain initially termed the "gut-brain axis" and renamed the "microbiota-gut-brain axis", considering the pivotal role of gut microbiota in sustaining local and systemic homeostasis, has a fundamental role in the pathogenesis of Inflammatory Bowel Disease (IBD). The integration of signals deriving from the host neuronal, immune, and endocrine systems with signals deriving from the microbiota may influence the development of the local inflammatory injury and impacts also more distal brain regions, underlying the psychophysiological vulnerability of IBD patients. Mood disorders and increased response to stress are frequently associated with IBD and may affect the disease recurrence and severity, thus requiring an appropriate therapeutic approach in addition to conventional anti-inflammatory treatments. This review highlights the more recent evidence suggesting that alterations of the microbiota-gut-brain bidirectional communication axis may concur to IBD pathogenesis and sustain the development of both local and CNS symptoms. The participation of the main microbial-derived metabolites, also defined as "postbiotics", such as bile acids, short-chain fatty acids, and tryptophan metabolites in the development of IBD-associated gut and brain dysfunction will be discussed. The last section covers a critical evaluation of the main clinical evidence pointing to the microbiome-based therapeutic approaches for the treatment of IBD-related gastrointestinal and neuropsychiatric symptoms.

Characterization of kynurenine pathway in patients with diarrhea-predominant irritable bowel syndrome

Our objectives are to demonstrate whether the kynurenine pathway is activated in diarrhea-type irritable bowel syndrome (IBS-D) patients, and whether the neurotoxic metabolite quinolinic acid (QUIN) is out of balance with the neuroprotective metabolite kynurenic acid (KYNA), and further explore whether this can lead to increased expression of N-methyl D-aspartate receptor 2B (NMDAR2B) in the enteric nervous system and in turn leads to intestinal symptoms and mood disorders. All enrolled healthy controls and patients accepted IBS Symptom Severity Scale (IBS-SSS) score, Self-rating Depression Scale (SDS) and Self-rating Anxiety Scale (SAS) anxiety and depression scores, and also underwent colonoscopy to collect ileum and colonic mucosa specimens. The expression of NMDAR2B in intestinal mucosa was detected by immunofluorescence, and fasting serum was collected to detect the tryptophan, kynurenine (KYN), KYNA and QUIN by high performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). Our results showed that the KYN pathway of IBS-D patients was activated. The production of QUIN and KYNA was imbalanced and resulting in an increased NMDAR2B for patients with IBS-D, which may be involved in intestinal symptoms and mood disorders of IBS-D.

Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

The ‘microbiota-gut-brain axis’ plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

The kynurenine connection: how exercise shifts muscle tryptophan metabolism and affects energy homeostasis, the immune system, and the brain

Tryptophan catabolism through the kynurenine pathway generates a variety of bioactive metabolites. Physical exercise can modulate kynurenine pathway metabolism in skeletal muscle and thus change the concentrations of select compounds in peripheral tissues and in the central nervous system. Here we review recent advances in our understanding of how exercise alters tryptophan-kynurenine metabolism in muscle and its subsequent local and distal effects. We propose that the effects of kynurenine pathway metabolites on skeletal muscle, adipose tissue, immune system, and the brain suggest that some of these compounds could qualify as exercise-induced myokines. Indeed, some of the more recently discovered biological activities for kynurenines include many of the best-known benefits of exercise: improved energy homeostasis, promotion of an anti-inflammatory environment, and neuroprotection. Finally, by considering the tissue expression of the different membrane and cytosolic receptors for kynurenines, we discuss known and potential biological activities for these tryptophan metabolites.

Kynurenic acid and cancer: facts and controversies

Kynurenic acid (KYNA) is an endogenous tryptophan metabolite exerting neuroprotective and anticonvulsant properties in the brain. However, its importance on the periphery is still not fully elucidated. KYNA is produced endogenously in various types of peripheral cells, tissues and by gastrointestinal microbiota. Furthermore, it was found in several products of daily human diet and its absorption in the digestive tract was evidenced. More recent studies were focused on the potential role of KYNA in carcinogenesis and cancer therapy; however, the results were ambiguous and the biological activity of KYNA in these processes has not been unequivocally established. This review aims to summarize the current views on the relationship between KYNA and cancer. The differences in KYNA concentration between physiological conditions and cancer, as well as KYNA production by both normal and cancer cells, will be discussed. The review also describes the effect of KYNA on cancer cell proliferation and the known potential molecular mechanisms of this activity.

Increased expression of kynurenine aminotransferases mRNA in lymphocytes of patients with inflammatory bowel disease

BACKGROUND

Complex interaction of genetic defects with environmental factors seems to play a substantial role in the pathogenesis of inflammatory bowel disease (IBD). Accumulating data implicate a potential role of disturbed tryptophan metabolism in IBD. Kynurenic acid (KYNA), a derivative of tryptophan (TRP) along the kynurenine (KYN) pathway, displays cytoprotective and immunomodulating properties, whereas 3-OH-KYN is a cytotoxic compound, generating free radicals.

METHODS

The expression of lymphocytic mRNA encoding enzymes synthesizing KYNA (KAT I-III) and serum levels of TRP and its metabolites were evaluated in 55 patients with IBD, during remission or relapse [27 patients with ulcerative colitis (UC) and 28 patients with Crohn's disease (CD)] and in 50 control individuals.

RESULTS

The increased expression of KAT1 and KAT3 mRNA characterized the entire cohorts of patients with UC and CD, as well as relapse-remission subsets. Expression of KAT2 mRNA was enhanced in patients with UC and in patients with CD in remission. In the entire cohorts of UC or CD, TRP levels were lower, whereas KYN, KYNA and 3-OH-KYN were not altered. When analysed in subsets of patients with UC and CD (active phase-remission), KYNA level was significantly lower during remission than relapse, yet not versus control. Functionally, in the whole groups of patients with UC or CD, the TRP/KYN ratio has been lower than control, whereas KYN/KYNA and KYNA/3-OH-KYN ratios were not altered. The ratio KYN/3-OH-KYN increased approximately two-fold among all patients with CD; furthermore, patients with CD with relapse, manifested a significantly higher KYNA/3-OH-KYN ratio than patients in remission.

CONCLUSION

The presented data indicate that IBD is associated with an enhanced expression of genes encoding KYNA biosynthetic enzymes in lymphocytes; however, additional mechanisms appear to influence KYNA levels. Higher metabolic conversion of serum TRP in IBD seems to be followed by the functional shift of KYN pathway towards the arm producing KYNA during exacerbation. We propose that KYNA, possibly via interaction with aryl hydrocarbon receptor or G-protein-coupled orphan receptor 35, may serve as a counter-regulatory mechanism, decreasing cytotoxicity and inflammation in IBD. Further longitudinal studies evaluating the individual dynamics of TRP and KYN pathway in patients with IBD, as well as the nature of precise mechanisms regulating KYNA synthesis, should be helpful in better understanding the processes underlying the observed changes.

Kynurenic acid as the neglected ingredient of commercial baby formulas

The global increase in resorting to artificial nutritional formulas replacing breastfeeding has been identified among the complex causes of the obesity epidemic in infants and children. One of the factors recently recognized to influence metabolism and weight gain is kynurenic acid (KYNA), an agonist of G protein-coupled receptor (GPR35). Therefore the aim of the study was to determine the concentration of KYNA in artificial nutritional formulas in comparison with its level in human breast milk and to evaluate developmental changes in rats exposed to KYNA enriched diet during the time of breastfeeding. KYNA levels were measured in milk samples from 25 heathy breast-feeding women during the first six months after labor and were compared with 21 time-adjusted nutritional formulas. Animal experiments were performed on male Wistar rats. KYNA was administered in drinking water. The content of KYNA in human milk increases more than 13 times during the time of breastfeeding while its level is significantly lower in artificial formulas. KYNA was detected in breast milk of rats and it was found that the supplementation of rat maternal diet with KYNA in drinking water results in its increase in maternal milk. By means of the immunoblotting technique, GPR35 was evidenced in the mucosa of the jejunum of 1-day-old rats and distinct morphological changes in the jejunum of 21-day-old rats fed by mothers exposed to water supplemented with KYNA were found. A significant reduction of body weight gain of rats postnatally exposed to KYNA supplementation without changes in total body surface and bone mineral density was observed. The rat offspring fed with breast milk with artificially enhanced KYNA content demonstrated a lower mass gain during the first 21 days of life, which indicates that KYNA may act as an anti-obesogen. Further studies are, therefore, warranted to investigate the mechanisms regulating KYNA secretion via breast milk, as well as the influence of breast milk KYNA on mass gain. In the context of lifelong obesity observed worldwide in children fed artificially, our results imply that insufficient amount of KYNA in baby formulas could be considered as one of the factors associated with increased mass gain.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

A complex bidirectional communication system exists between the gastrointestinal tract and the brain. Initially termed the “gut-brain axis” it is now renamed the “microbiota-gut-brain axis” considering the pivotal role of gut microbiota in maintaining local and systemic homeostasis. Different cellular and molecular pathways act along this axis and strong attention is paid to neuroactive molecules (neurotransmitters, i.e., noradrenaline, dopamine, serotonin, gamma aminobutyric acid and glutamate and metabolites, i.e., tryptophan metabolites), sustaining a possible interkingdom communication system between eukaryota and prokaryota. This review provides a description of the most up-to-date evidence on glutamate as a neurotransmitter/neuromodulator in this bidirectional communication axis. Modulation of glutamatergic receptor activity along the microbiota-gut-brain axis may influence gut (i.e., taste, visceral sensitivity and motility) and brain functions (stress response, mood and behavior) and alterations of glutamatergic transmission may participate to the pathogenesis of local and brain disorders. In this latter context, we will focus on two major gut disorders, such as irritable bowel syndrome and inflammatory bowel disease, both characterized by psychiatric co-morbidity. Research in this area opens the possibility to target glutamatergic neurotransmission, either pharmacologically or by the use of probiotics producing neuroactive molecules, as a therapeutic approach for the treatment of gastrointestinal and related psychiatric disorders.

Kynurenic Acid Protects against Thioacetamide-Induced Liver Injury in Rats

Acute liver failure (ALF) is a life-threatening disorder of liver function. Kynurenic acid (KYNA), a tryptophan metabolite formed along the kynurenine metabolic pathway, possesses anti-inflammatory and antioxidant properties. Its presence in food and its potential role in the digestive system was recently reported. The aim of this study was to define the effect of KYNA on liver failure. The Wistar rat model of thioacetamide-induced liver injury was used. Morphological and biochemical analyses as well as the measurement of KYNA content in liver and hepatoprotective herbal remedies were conducted. The significant attenuation of morphological disturbances and aspartate and alanine transaminase activities, decrease of myeloperoxidase and tumor necrosis factor-α, and elevation of interleukin-10 levels indicating the protective effect of KYNA in thioacetamide (TAA) - induced liver injury were discovered. In conclusion, the hepatoprotective role of KYNA in an animal model of liver failure was documented and the use of KYNA in the treatment of ALF was suggested.

Impact of the Gut Microbiota on Intestinal Immunity Mediated by Tryptophan Metabolism

The gut microbiota influences the health of the host, especially with regard to gut immune homeostasis and the intestinal immune response. In addition to serving as a nutrient enhancer, L-tryptophan (Trp) plays crucial roles in the balance between intestinal immune tolerance and gut microbiota maintenance. Recent discoveries have underscored that changes in the microbiota modulate the host immune system by modulating Trp metabolism. Moreover, Trp, endogenous Trp metabolites (kynurenines, serotonin, and melatonin), and bacterial Trp metabolites (indole, indolic acid, skatole, and tryptamine) have profound effects on gut microbial composition, microbial metabolism, the host's immune system, the host-microbiome interface, and host immune system-intestinal microbiota interactions. The aryl hydrocarbon receptor (AhR) mediates the regulation of intestinal immunity by Trp metabolites (as ligands of AhR), which is beneficial for immune homeostasis. Among Trp metabolites, AhR ligands consist of endogenous metabolites, including kynurenine, kynurenic acid, xanthurenic acid, and cinnabarinic acid, and bacterial metabolites, including indole, indole propionic acid, indole acetic acid, skatole, and tryptamine. Additional factors, such as aging, stress, probiotics, and diseases (spondyloarthritis, irritable bowel syndrome, inflammatory bowel disease, colorectal cancer), which are associated with variability in Trp metabolism, can influence Trp-microbiome-immune system interactions in the gut and also play roles in regulating gut immunity. This review clarifies how the gut microbiota regulates Trp metabolism and identifies the underlying molecular mechanisms of these interactions. Increased mechanistic insight into how the microbiota modulates the intestinal immune system through Trp metabolism may allow for the identification of innovative microbiota-based diagnostics, as well as appropriate nutritional supplementation of Trp to prevent or alleviate intestinal inflammation. Moreover, this review provides new insight regarding the influence of the gut microbiota on Trp metabolism. Additional comprehensive analyses of targeted Trp metabolites (including endogenous and bacterial metabolites) are essential for experimental preciseness, as the influence of the gut microbiota cannot be neglected, and may explain contradictory results in the literature.

Kynurenic Acid: The Janus-Faced Role of an Immunomodulatory Tryptophan Metabolite and Its Link to Pathological Conditions

Tryptophan metabolites are known to participate in the regulation of many cells of the immune system and are involved in various immune-mediated diseases and disorders. Kynurenic acid (KYNA) is a product of one branch of the kynurenine pathway of tryptophan metabolism. The influence of KYNA on important neurophysiological and neuropathological processes has been comprehensively documented. In recent years, the link of KYNA to the immune system, inflammation, and cancer has become more apparent. Given this connection, the anti-inflammatory and immunosuppressive functions of KYNA are of particular interest. These characteristics might allow KYNA to act as a "double-edged sword." The metabolite contributes to both the resolution of inflammation and the establishment of an immunosuppressive environment, which, for instance, allows for tumor immune escape. Our review provides a comprehensive update of the significant biological functions of KYNA and focuses on its immunomodulatory properties by signaling via G-protein-coupled receptor 35 (GPR35)- and aryl hydrocarbon receptor-mediated pathways. Furthermore, we discuss the role of KYNA-GPR35 interaction and microbiota associated KYNA metabolism for gut homeostasis.

Microbiota-related Changes in Bile Acid & Tryptophan Metabolism are Associated with Gastrointestinal Dysfunction in a Mouse Model of Autism

Autism spectrum disorder (ASD) is one of the most prevalent neurodevelopmental conditions worldwide. There is growing awareness that ASD is highly comorbid with gastrointestinal distress and altered intestinal microbiome, and that host-microbiome interactions may contribute to the disease symptoms. However, the paucity of knowledge on gut-brain axis signaling in autism constitutes an obstacle to the development of precision microbiota-based therapeutics in ASD. To this end, we explored the interactions between intestinal microbiota, gut physiology and social behavior in a BTBR T+Itpr3tf/J mouse model of ASD. Here we show that a reduction in the relative abundance of very particular bacterial taxa in the BTBR gut - namely, bile-metabolizing Bifidobacterium and Blautia species, - is associated with deficient bile acid and tryptophan metabolism in the intestine, marked gastrointestinal dysfunction, as well as impaired social interactions in BTBR mice. Together these data support the concept of targeted manipulation of the gut microbiota for reversing gastrointestinal and behavioral symptomatology in ASD, and offer specific plausible targets in this endeavor.

Kynurenic acid downregulates IL-17/1L-23 axis in vitro

Exploring the function of interleukin (IL) 17 and related cytokine interactions have been proven useful toward understanding the role of inflammation in autoimmune diseases. Production of the inflammatory cytokine IL-23 by dendritic cells (DC's) has been shown to promote IL-17 expression by Th17 cells. It is well established that Th17 cells play an important role in several autoimmune diseases including psoriasis and alopecia. Our recent investigations have suggested that Kynurenine-rich environment can shift a pro-inflammatory response to an anti-inflammatory response, as is the case in the presence of the enzyme Indoleamine 2,3 dioxygenase (IDO), the rate-limiting enzyme in tryptophan degradation and Kynurenine (Kyn) production. In this study, we sought to explore the potential role of kynurenic acid (KynA), in modulating the expression of IL-23 and IL-17 by DCs and CD4⁺ cells, respectively. The result of flow cytometry demonstrated that the frequency of IL-23-producing DCs is reduced with 100 µg/ml of KynA as compared with that of LPS-stimulated DCs. KynA (100 μg/ml) addition to activated T cells significantly decreased the level of IL-17 mRNA and frequency of IL-17⁺ T cells as compared to that of concanavalin (Con) A-activated T cells. To examine the mechanism of the suppressive role of KynA on IL-23/IL-17 in these cells, cells were treated with 3 μM G-protein-coupled receptor35 (GPCR35) inhibitor (CID), for 60 min. The result showed that the reduction of both adenylate cyclase (AC) and cyclic adenosine monophosphate (cAMP) by KynA is involved in suppression of LPS-induced IL-23p19 expression. Since GPCR35 is also detected on T cells; therefore, it is concluded that KynA plays an important role in modulating the expression of IL-23 and IL-17 in DCs and Th17 cells through inhibiting GPCR35 and downregulation of both AC and cAMP.

Oxygen metabolism and barrier regulation in the intestinal mucosa

Mucosal surfaces are lined by epithelial cells and provide an important barrier to the flux of antigens from the outside. This barrier is provided at a number of levels, including epithelial junctional complexes, mucus production, and mucosa-derived antimicrobials. Tissue metabolism is central to the maintenance of homeostasis in the mucosa. In the intestine, for example, baseline pO2 levels are uniquely low due to counter-current blood flow and the presence of large numbers of bacteria. As such, hypoxia and HIF signaling predominates normal intestinal metabolism and barrier regulation during both homeostasis and active inflammation. Contributing factors that elicit important adaptive responses within the mucosa include the transcriptional regulation of tight junction proteins, metabolic regulation of barrier components, and changes in autophagic flux. Here, we review recent literature around the topic of hypoxia and barrier function in health and during disease.

The effect of kynurenic acid on the synthesis of selected cytokines by murine splenocytes - in vitro and ex vivo studies

Kynurenic acid (KYNA), a secondary product of the kynurenine pathway of tryptophan degradation, known mainly as an endogenous neuroprotectant, shows also immunotropic properties. Some quantities of KYNA are present in food and are effectively absorbed in the gastrointestinal tract. Since the spleen is an important target of dietary immunomodulators, the aim of the study was to determine the effect of exogenous KYNA on murine splenocytes. Splenocytes isolated from adult BALB/c mice were used in the study. Firstly, the effect of increasing KYNA concentrations (0-5 mM) on the viability, and proliferative and cytokine response (interleukin 1β [IL-1β], IL-6, IL-10, tumor necrosis factor α [TNF-α]) of murine splenocytes under in vitro conditions was determined. Then, proliferative and cytokine responses were determined in cells derived from animals receiving kynurenic acid in drinking water at concentrations of 2.5, 25, or 250 mg/l for 7-14 days. Cytokine levels were measured using commercial immunoassay (ELISA) kits, and cell viability and proliferation was determined with MTT reduction assay. Exogenous KYNA was characterised by a low level of cytotoxicity towards murine splenocytes, and was well tolerated by the animals receiving it in drinking water. As expected, it exhibited anti-inflammatory action towards the activated splenocytes, under both in vitro and ex vivo conditions. Surprisingly, however, KYNA itself influenced the activity of resting, non-stimulated cells, exerting an immunostimulant effect in vitro, and an immunosuppressive effect under ex vivo conditions. The obtained results indicate not only anti-inflammatory, but also more complex, immunomodulating properties of KYNA, which require more detailed investigation.

Kynurenic Acid Content in Selected Culinary Herbs and Spices

Previous studies demonstrated that kynurenic acid (KYNA) is present in various types of food in varying concentrations. Therefore, the aim of the study was to check whether KYNA is present in culinary herbs and spices. Achieved results indicate that KYNA is present in all 19 selected culinary herbs and spices. The highest concentration of KYNA was found in basil and thyme, 14.08 and 8.87 μg/g, respectively, while the lowest content of KYNA was found in cumin and black pepper, 0.64 and 0.10 μg/g, respectively. This is the first report on the concentration of KYNA in culinary herbs and spices. The need for more detailed investigation of dietary supplementation with culinary herbs and spices containing KYNA is suggested.

Changes in plasma kynurenic acid concentration in septic shock patients undergoing continuous veno-venous haemofiltration

Kynurenic acid (KYNA) is one of the end products of tryptophan metabolism. The aim of this study was to analyse plasma KYNA concentration in septic shock patients (SSP) with acute kidney injury (AKI) undergoing continuous veno-venous haemofiltration (CVVH). Changes in KYNA content were compared to alterations in the levels of procalcitonin (PCT), C-reactive protein and lactate. Adult SSP with AKI were examined. Measurements were conducted at seven time points: before beginning CVVH and at 6, 12, 24, 48, 72 and 96 h after the beginning of CVVH. Based on clinical outcomes, the data were analysed separately for survivors and non-survivors. Twenty-seven patients were studied. CVVH was associated with reduced plasma KYNA concentration only in survivors. Plasma KYNA concentration correlated with the levels of lactate and PCT only in survivors. (1) CVVH reduced plasma KYNA concentration only in survivors; (2) lack of this reduction may predict fatal outcomes in SSP.

Effect of oral administration of kynurenic acid on the activity of the peripheral blood leukocytes in mice

Kynurenic acid (KYNA), an endogenous tryptophan metabolite, is a selective ligand of the GPR35 receptor, expressed mainly on the immune cells. In inflammatory conditions, by affecting this receptor, KYNA inhibits the synthesis of pro-inflammatory cytokines and probably protects tissues from oxidative damage. However, we lack data regarding the effect of exogenous KYNA on the activity of immune cells in healthy individuals. The objective of this study has been to determine the influence of kynurenic acid administered to mice in different doses (2.5, 25 or 250 mg/l) and for different time periods (3, 7, 14, 28 days) in drinking water, on the activity of their peripheral blood leukocytes. The determinations comprised the proliferative activity of lymphocytes (MTT assay) and the phagocytic activity as well as the respiratory burst activity of granulocytes and monocytes (Phagotest, Phagoburst). It was only the lowest KYNA dose that influenced the mitogenic response of lymphocytes, namely by increasing the proliferation of T cells. The impact on the phagocytic activity was varied with KYNA dose and administration time. However, all the KYNA doses significantly lowered the activity of oxidative burst in phagocytes, which was probably associated with its antioxidant properties. In the light of the research results, kynurenic acid may find applications as an immuno-modulating agent able to correct an excessive or insufficient response of phagocytizing cells, protecting an organism from oxidative stress.

Clinical significance of quantitative detection of serum tryptophan in patients with ulcerative colitis

AIM: To investigate the clinical significance of serum tryptophan in patients with ulcerative colitis (UC).

METHODS: Serum samples were obtained from 20 UC patients and 20 healthy volunteers. For each patient, serum samples were collected when the disease was active and inactive. The diagnosis for each patient was based on clinical symptoms, endoscopic, radiographic, and histological findings. The Montreal Classification was used for disease phenotyping. Disease activity was assessed by the modified Mayo score. Tryptophan in serum was measured by liquid chromatography tandem mass spectrometry (LC-MS/MS). Differences between different groups and the relationship between serum tryptophan and ESR/CRP in patients with active UC were analyzed.

RESULTS: Compared with patients with active UC, serum levels of tryptophan in patients with inactive UC and healthy volunteers were significantly lower (P < 0.05 for both), but there was no significant difference between patients with inactive UC and healthy volunteers. Serum tryptophan in active UC patients was inversely related to ESR (r = -0.502, P = 0.024) and CRP (r = -0.530, P = 0.016). After conventional treatment, the levels of serum tryptophan were significantly elevated (P < 0.05).

CONCLUSION: Tryptophan may play a key role in the occurrence and development of UC.

Kynurenic Acid in the digestive system-new facts, new challenges

This review provides information on the most recent findings concerning presence, origin, and role of kynurenic acid (KYNA), a tryptophan metabolite, in the digestive system. KYNA is an antagonist of both the ionotropic glutamate receptors and the alpha7 nicotinic acetylcholine receptor, as well as an agonist of G-protein coupled GPR35 receptor. Since the GPR35 receptor is mainly present in the gastrointestinal tract, researchers have concentrated on the digestive system in recent years. They have found that KYNA content increases gradually and significantly along the gastrointestinal tract. Interestingly, the concentration of KYNA in the lumen is much higher than in the wall of intestine. It has been documented that KYNA may have a positive influence on the number of pathologies in the gastrointestinal tract, in particular ulcers, colon obstruction, or colitis. Future studies might determine whether it is advisable to supplement KYNA to a human organism.

Kynurenic acid in plasma and endometrium in bitches with pyometra

Kynurenic acid (KYNA) is produced enzymatically in humans and animals from kynurenine. Reports concerning changes of kynurenine metabolism during inflammation are available in the literature. Pyometra is a pathological condition characterized by the accumulation of pus in the uterine lumen and bacterial infection. The objective of the study was to compare the serum and endometrial KYNA concentrations in healthy bitches and those with pyometra. KYNA was determined by means of high-performance liquid chromatography with fluorometric detection. The serum content of KYNA in bitches with pyometra was significantly higher than in healthy bitches. The KYNA content in the endometrium of bitches with pyometra was higher, yet the difference was not statistically significant. Our result indicates that determination of KYNA might be a marker of pyometra in bitches.

Kynurenines in the CNS: recent advances and new questions

Various pathologies of the central nervous system (CNS) are accompanied by alterations in tryptophan metabolism. The main metabolic route of tryptophan degradation is the kynurenine pathway; its metabolites are responsible for a broad spectrum of effects, including the endogenous regulation of neuronal excitability and the initiation of immune tolerance. This Review highlights the involvement of the kynurenine system in the pathology of neurodegenerative disorders, pain syndromes and autoimmune diseases through a detailed discussion of its potential implications in Huntington's disease, migraine and multiple sclerosis. The most effective preclinical drug candidates are discussed and attention is paid to currently under-investigated roles of the kynurenine pathway in the CNS, where modulation of kynurenine metabolism might be of therapeutic value.

The kynurenine system and immunoregulation

There is developing interest in the role of the kynurenines in the immune function. A considerable amount of evidence has accumulated as concerns interactions between the kynurenine pathway, cytokines and the nervous system. Indoleamine 2,3-dioxygenase (IDO) occupies a key position connecting the immune system and the kynurenine pathway. There are evidences of the immunosuppressive effect of IDO. Following the interferon (IFN)-mediated activation of antigen presenting cells, the induction of IDO and the kynurenine system exerts a counter-regulating effect, maintaining the homeostasis. Inhibition of T cell functions, activation of the regulatory T cells, and the inhibition of Natural Killer cells are among the important factors in the immunosuppressive effects of IDO and kynurenines. There is a close connection between cytokines (IFN-α, IFN-γ, TNF-α, TGF-β, IL-4 and IL-23) and the kynurenine system, and an imbalance in the TH1/TH2 cytokine profile may possibly lead to neurologic or psychiatric disorders. As the tryptophan metabolic pathway is activated by pro-inflammatory stimuli, the anti-inflammatory effect of kynurenic acid provides a further feedback mechanism in modulating the immune responses.

Kynurenines and intestinal neurotransmission: the role of N-methyl-D-aspartate receptors

Gastrointestinal neuroprotection involves the net effect of many mechanisms which protect the enteral nervous system and its cells from death, dysfunction or degeneration. Neuroprotection is also a therapeutic strategy, aimed at slowing or halting the progression of primary neuronal loss following acute or chronic diseases. The neuroprotective properties of a compound clearly have implications for an understanding of the mechanism of dysfunctions and for therapeutic approaches in a number of gastrointestinal diseases.This paper focused on the roles of glutamate and N-methyl-D-aspartate (NMDA) receptors in the intrinsic neuronal control of gastrointestinal motility; the consequences of inflammation on gastrointestinal motility changes; and the involvement of tryptophan metabolites (especially kynurenic acid) in the regulatory function of the enteral nervous system and the modulation of the inflammatory response. Common features in the mechanisms of action, illustrative evidence from animal models, and experimental neuroprotective therapies making use of the currently available possibilities are also discussed.Overall, the evidence suggests that gastrointestinal neuroprotection against inflammation and glutamate-induced neurotoxicity may be mediated synergistically through the blockade of NMDA receptors and the inhibition of neuronal nitric oxide synthase activity and xanthine oxidoreductase-dependent superoxide production. These components are likewise significant factors in the pathomechanism of gastrointestinal inflammatory diseases and inflammation-linked motility alterations. Inhibition of the enteric NMDA receptors by kynurenic acid or its analogues may provide a novel option via which to influence intestinal hypermotility and inflammatory processes simultaneously.

Different inhibitory effects of kynurenic acid and a novel kynurenic acid analogue on tumour necrosis factor-α (TNF-α) production by mononuclear cells, HMGB1 production by monocytes and HNP1-3 secretion by neutrophils

Kynurenic acid (KynA), a broad spectrum antagonist of excitatory amino acid receptors, may serve as a protective agent in neurological disorders. The potential anti-inflammatory effect of KynA in human leukocytes has not been characterized. The aim of this study was to compare the effects of KynA with those of a new analogue, 2-(2-N,N-dimethylaminoethylamine-1-carbonyl)-1H-quinolin-4-one hydrochloride on tumour necrosis factor-α (TNF-α) production and high mobility group box protein 1 (HMGB1) secretion. The effects of KynA on granulocyte activation were investigated via the secretion of human neutrophil peptide 1-3 (HNP1-3). Peripheral blood mononuclear cells and granulocytes or CD14 positive monocytes were applied as effector cells, or whole blood cultures were used. TNF-α, HMGB1 and HNP1-3 concentrations were determined by ELISA, TNF-α and HNP1-3 mRNA expressions were quantified by reverse transcription PCR. KynA attenuated the TNF-α production of human mononuclear cells activated by heat-inactivated Staphylococcus aureus, inhibiting TNF-α production at the transcription level. Furthermore, KynA diminished HMGB1 secretion by U 937 monocytic cells and by peripheral blood monocytes. KynA inhibited the HNP1-3 secretion in whole blood and in granulocyte cultures. The suppressive effect of the KynA analogue was more potent than that of an equimolar concentration KynA in TNF-α, HMGB1 and HNP1-3 inhibition. These results suggest that the new KynA analogue has a more potent immunoregulatory effect than KynA on human mononuclear cells, monocytes and granulocytes and indicate the potential benefits of further exploration of its uses in human inflammatory disease.