Harnessing and delivering microbial metabolites as therapeutics via advanced pharmaceutical approaches

Microbial metabolites have emerged as key players in the interplay between diet, the gut microbiome, and host health. Two major classes, short-chain fatty acids (SCFAs) and tryptophan (Trp) metabolites, are recognized to regulate inflammatory, immune, and metabolic responses within the host. Given that many human diseases are associated with dysbiosis of the gut microbiome and consequent reductions in microbial metabolite production, the administration of these metabolites represents a direct, multi-targeted treatment. While a multitude of preclinical studies showcase the therapeutic potential of both SCFAs and Trp metabolites, they often rely on high doses and frequent dosing regimens to achieve systemic effects, thereby constraining their clinical applicability. To address these limitations, a variety of pharmaceutical formulations approaches that enable targeted, delayed, and/or sustained microbial metabolite delivery have been developed. These approaches, including enteric encapsulations, esterification to dietary fiber, prodrugs, and nanoformulations, pave the way for the next generation of microbial metabolite-based therapeutics. In this review, we first provide an overview of the roles of microbial metabolites in maintaining host homeostasis and outline how compromised metabolite production contributes to the pathogenesis of inflammatory, metabolic, autoimmune, allergic, infectious, and cancerous diseases. Additionally, we explore the therapeutic potential of metabolites in these disease contexts. Then, we provide a comprehensive and up-to-date review of the pharmaceutical strategies that have been employed to enhance the therapeutic efficacy of microbial metabolites, with a focus on SCFAs and Trp metabolites.

Indole-3-acetic acid ameliorates dextran sulfate sodium-induced colitis via the ERK signaling pathway

Microbiota-derived catabolism of nutrients is closely related to ulcerative colitis (UC). The level of indole-3-acetic acid (IAA), a microbiota-dependent metabolite of tryptophan, was decreased significantly in the feces of UC patients. Thus supplementation with IAA could be a potential therapeutic method for ameliorating colitis. In this work, the protective effect of supplementation with IAA on dextran sulfate sodium (DSS)-induced colitis was evaluated, and the underlying mechanism was elucidated. The results indicated that the administration of IAA significantly relieved DSS-induced weight loss, reduced the disease activity index (DAI), restored colon length, alleviated intestinal injury, and improved the intestinal tight junction barrier. Furthermore, IAA inhibited intestinal inflammation by reducing the expression of proinflammatory cytokines and promoting the production of IL-10 and TGF-β1. In addition, the ERK signaling pathway is an important mediator of various physiological processes including inflammatory responses and is closely associated with the expression of IL-10. Notably, IAA treatment induced the activation of extracellular signal-regulated kinase (ERK), which is involved in the progression of colitis, while the ERK inhibitor U0126 attenuated the beneficial effects of IAA. In summary, IAA could attenuate the clinical symptoms of colitis, and the ERK signaling pathway was involved in the underlying mechanism. Supplementation with IAA could be a potential option for preventing or ameliorating UC.

Existing and Future Strategies to Manipulate the Gut Microbiota With Diet as a Potential Adjuvant Treatment for Psychiatric Disorders

Nutrition and diet quality play key roles in preventing and slowing cognitive decline and have been linked to multiple brain disorders. This review compiles available evidence from preclinical studies and clinical trials on the impact of nutrition and interventions regarding major psychiatric conditions and some neurological disorders. We emphasize the potential role of diet-related microbiome alterations in these effects and highlight commonalities between various brain disorders related to the microbiome. Despite numerous studies shedding light on these findings, there are still gaps in our understanding due to the limited availability of definitive human trial data firmly establishing a causal link between a specific diet and microbially mediated brain functions and symptoms. The positive impact of certain diets on the microbiome and cognitive function is frequently ascribed with the anti-inflammatory effects of certain microbial metabolites or a reduction of proinflammatory microbial products. We also critically review recent research on pro- and prebiotics and nondietary interventions, particularly fecal microbiota transplantation. The recent focus on diet in relation to brain disorders could lead to improved treatment outcomes with combined dietary, pharmacological, and behavioral interventions.

Association between gut microbiome-related metabolites and symptomatic hand osteoarthritis in two independent cohorts

BACKGROUND

Since gut microbiome dysbiosis can cause inflammatory disorders by affecting host metabolism, we postulate that the gut microbiome and related metabolites could play a role in hand osteoarthritis. We characterised gut microbiome-related metabolites in people with symptomatic hand osteoarthritis (SHOA) in two independent cohorts.

METHODS

Using data collected from a large-sample community-based observational study (discovery cohort), we assessed the relations of the microbial function and plasma key metabolites related to altered microbial function with SHOA. Finally, we verified the relations of plasma metabolites to SHOA in an independent observational study (validation cohort).

FINDINGS

In the discovery cohort (n = 1359), compared to those without SHOA, participants with SHOA had significantly altered microbial functions related to tryptophan metabolism (Q = 0.025). Therefore we measured the plasma tryptophan metabolites and found that participants with SHOA had higher levels of 5-hydroxyindoleacetic acid (odds ratio [OR] = 1.25, 95% confidence interval [CI]: 1.09-1.42) and 5-hydroxytryptophol (OR = 1.13, 95% CI: 1.04-1.23), but lower levels of indole-3-lactic acid (ILA) (OR = 0.85, 95% CI: 0.72-1.00), skatole (OR = 0.93, 95% CI: 0.88-0.99) and 3-hydroxyanthranilic acid (OR = 0.90, 95% CI: 0.85-0.96). Findings from the validation cohort (n = 142) verified that lower levels of ILA were related to SHOA (OR = 0.70, 95% CI: 0.53-0.92).

INTERPRETATION

Alterations of the microbial function of tryptophan biosynthesis and tryptophan metabolites, especially lower levels of ILA, are associated with SHOA. These findings suggest the role of the microbiome and tryptophan metabolites in developing of SHOA and may contribute to future translational opportunities.

FUNDING

National Key Research and Development Plan and National Natural Science Foundation of China.

Common alterations in plasma free amino acid profiles and gut microbiota-derived tryptophan metabolites of five types of cancer patients

Amino acids not only play a vital role in the synthesis of biological molecules such as proteins in cancer malignant cells, they are also essential metabolites for immune cell activation and antitumor effects in the tumor microenvironment. The abnormal changes in amino acid metabolism are closely related to the occurrence and development of tumors and immunity. Intestinal microorganisms play an essential role in amino acid metabolism, and tryptophan and its intestinal microbial metabolites are typical representatives. However, it is known that the cyclic amino acid profile is affected by specific cancer types, so relevant studies mainly focus on one type of cancer and rarely study different cancer forms at the same time. The objective of this study was to examine the PFAA profile of five cancer patients and the characteristics of tryptophan intestinal microbial metabolites to determine whether there are general amino acid changes across tumors. Plasma samples were collected from esophageal (n = 53), lung (n = 73), colorectal (n = 94), gastric (n = 55), breast cancer (n = 25), and healthy control (HC) (n = 139) subjects. PFAA profile and tryptophan metabolites were measured, and their perioperative changes were examined using high-performance liquid chromatography. Univariate analysis revealed significant differences between cancer patients and HC. Furthermore, multivariate analysis discriminated cancer patients from HC. Regression diagnosis models were established for each cancer group using differential amino acids from univariate analysis. Receiver-operating characteristic analysis was applied to evaluate these diagnosis models. Finally, GABA, arginine, tryptophan, taurine, glutamic acid, and melatonin showed common alterations across all types of cancer patients. Metabolic pathway analysis shows that the most significant enrichment pathways were tryptophan, arginine, and proline metabolism. This study provides evidence that common alterations of the metabolites mentioned above suggest their role in the pathogenesis of each cancer patient. It was suggested that multivariate models based on PFAA profiles and tryptophan metabolites might be applicable in the screening of cancer patients.

Whey protein isolate attenuates depression-like behavior developed in a mouse model of breast tumor

Increasing evidence indicates that tryptophan (Trp) metabolism disturbance controls hippocampal 5-hydroxytryptamine (5-HT) and thereby affecting depression-like behavior, in which the gut microbiota (GM) might be involved. This study investigated the effect of Trp-rich whey protein isolate (WPI) on depressive-like behavior in 4T1 tumor-bearing mice. Female BALB/c mice were subcutaneously inoculated with murine 4T1 mammary carcinoma cells and received 2 g/kg of WPI by gavage daily for 21 days. The results showed that WPI exerted no significant effects on tumor weight and volume, but abrogated tumor-induced depression-like behavior, as evidenced by remarkably increased time and distance in the center of the open-field test, decreased immobility time in the tail suspension test, increased time and number of entries to the open arms in the elevated plus maze and sucrose preference. Moreover, WPI promoted the hippocampal Trp, 5-hydroxytryptophan (5-HTP), 5-HT, 5-hydroxyindoleacetic acid (5-HIAA) levels and inhibited kynurenine (Kyn) through up-regulating serotonin transporter (SERT) and down-regulating indoleamine 2, 3-dioxygenase (IDO). WPI showed an enriched microbial diversity indicated by increased Shannon index and decreased Simpson index, reduced the abundances of Proteobacteria, Rikenellaceae_RC9_gut_group, Alloprevotella and Prevotellaceae_UCG-001, and increased the abundance of unclassified_k__norank_d__Bacteria in tumor-bearing mice (P < 0.05). At level 3, WPI enhanced the function of microbial gene related to Trp metabolism in the KEGG pathways (P < 0.05). Our results suggest that WPI exhibits a potent antidepressant-like effect via the regulation of hippocampal Trp metabolism and alteration of GM composition and function, and it may be an effective prevention for cancer-related depression.

Aberrant gut microbiota and fecal metabolites in patients with coal-burning endemic fluorosis in Guizhou, China

Chronic exposure to excessive environmental fluoride has caused fluorosis to become a major public health problem worldwide. Although studies on stress pathways, signaling pathways, and apoptosis induced by fluoride have provided an in-depth understanding of the mechanism of this disease, its exact pathogenesis remains unclear. We hypothesized that the human gut microbiota and metabolome are associated with the pathogenesis of this disease. To get further insight into the profiles of intestinal microbiota and metabolome in coal-burning-induced endemic fluorosis patients, we conducted 16S rRNA sequencing of the intestinal microbial DNA and carried out non-targeted metabolomics of fecal samples from 32 patients with skeletal fluorosis and 33 matched healthy controls in Guizhou, China. We found that the gut microbiota of coal-burning endemic fluorosis patients displayed significant differences in composition, diversity, and abundance compared with healthy controls. This was characterized by an increase in the relative abundance of Verrucomicrobiota, Desulfobacterota, Nitrospirota, Crenarchaeota, Chloroflexi, Myxococcota, Acidobacteriota, Proteobacteria, and unidentified_Bacteria, and a significant decrease in the relative abundance of Firmicutes and Bacteroidetes at the phylum level. Additionally, at the genus level, the relative abundance of some beneficial bacteria, such as Bacteroides, Megamonas, Bifidobacterium, and Faecalibacterium, was significantly reduced. We also demonstrated that, at the genus level, some gut microbial markers, including Anaeromyxobacter, MND1, oc32, Haliangium, and Adurb.Bin063_1, showed potential for identifying coal-burning endemic fluorosis. Moreover, non-targeted metabolomics and correlation analysis revealed the changes in the metabolome, particularly the gut microbiota-derived tryptophan metabolites such as tryptamine, 5-hydroxyindoleacetic acid, and indoleacetaldehyde. Our results indicated that excessive fluoride might cause xenobiotic-mediated dysbiosis of human gut microbiota and metabolic disorders. These findings suggest that the alterations in gut microbiota and metabolome play vital roles in regulating disease susceptibility and multi-organ damage after excessive fluoride exposure.

Pectin modulates intestinal immunity in a pig model via regulating the gut microbiota-derived tryptophan metabolite-AhR-IL22 pathway

BACKGROUND

Pectin is a heteropolysaccharide that acts as an intestinal immunomodulator, promoting intestinal development and regulating intestinal flora in the gut. However, the relevant mechanisms remain obscure. In this study, pigs were fed a corn-soybean meal-based diet supplemented with either 5% microcrystalline cellulose (MCC) or 5% pectin for 3 weeks, to investigate the metabolites and anti-inflammatory properties of the jejunum.

RESULT

The results showed that dietary pectin supplementation improved intestinal integrity (Claudin-1, Occludin) and inflammatory response [interleukin (IL)-10], and the expression of proinflammatory cytokines (IL-1β, IL-6, IL-8, TNF-α) was down-regulated in the jejunum. Moreover, pectin supplementation altered the jejunal microbiome and tryptophan-related metabolites in piglets. Pectin specifically increased the abundance of Lactococcus, Enterococcus, and the microbiota-derived metabolites (skatole (ST), 3-indoleacetic acid (IAA), 3-indolepropionic acid (IPA), 5-hydroxyindole-3-acetic acid (HIAA), and tryptamine (Tpm)), which activated the aryl hydrocarbon receptor (AhR) pathway. AhR activation modulates IL-22 and its downstream pathways. Correlation analysis revealed the potential relationship between metabolites and intestinal morphology, intestinal gene expression, and cytokine levels.

CONCLUSION

In conclusion, these results indicated that pectin inhibits the inflammatory response by enhancing the AhR-IL22-signal transducer and activator of transcription 3 signaling pathway, which is activated through tryptophan metabolites.

Shenling Baizhu San ameliorates ulcerative colitis by regulating the gut microbiota and its tryptophan metabolites: A complementary medicine to mesalamine

ETHNOPHARMACOLOGICAL RELEVANCE

Shenling Baizhu San (SBS) is commonly employed to improve gastrointestinal dysfunction in patients with ulcerative colitis (UC) in China. SBS combined with mesalamine has been demonstrated to result in improve its curative effects without increasing any adverse reactions, but the underlying mechanism remains unclarified.

AIM OF THE STUDY

Our study aimed to illuminate the potential therapeutic effects and mechanisms of SBS, which is a medicine complementary to mesalamine, in the treatment of UC.

MATERIALS AND METHODS

A prospective cohort study was conducted to evaluate the efficacy of SBS as a complementary medicine to mesalamine for patients with UC (n = 48). The patients in the control group (n = 24) were given mesalamine alone, whereas those in the experimental group were administered mesalamine combined with SBS. The therapeutic outcome was assessed at 8 weeks. The structures of the gut microbiota (GMB) were characterized by 16S rRNA sequencing, and the microbial tryptophan metabolites were analyzed by UPLC-MS/MS to investigate the mechanism through which SBS achieves its effects.

RESULTS

Our results showed that the combination of SBS and mesalamine could significantly improve the clinical signs of UC by achieving mucosal healing and relieving colon damage. Interestingly, the combination of SBS and mesalamine could alter the GMB structures and increase the microbial levels of tryptophan metabolites, including indole-3-propionic acid and indole-3-acetic acid.

CONCLUSION

SBS combined with mesalamine is effective in improving the clinical and endoscopic outcomes of patients with UC. SBS, as a complementary therapy to conventional treatment, alleviates UC via the GMB-tryptophan metabolite axis.

Identification of a protective Bacteroides strain of alcoholic liver disease and its synergistic effect with pectin

The depletion of Bacteroides in the gut is closely correlated with the progression of alcoholic liver disease (ALD). This study aimed to identify Bacteroides strains with protective effects against ALD and evaluate the synergistic effects of Bacteroides and pectin in this disease. Mice were fed Lieber-DeCarli alcohol diet to establish an experimental ALD model and pre-treated with 4 Bacteroides strains. The severity of the liver injury, hepatic steatosis, and inflammation was evaluated through histological and biochemical assays. We found that Bacteroides fragilis ATCC25285 had the best protective effects against ALD strains by alleviating both ethanol-induced liver injury and steatosis. B. fragilis ATCC25285 could counteract inflammatory reactions in ALD by producing short-chain fat acids (SCFAs) and enhancing the intestinal barrier. In the subsequent experiment, the synbiotic combination of B. fragilis ATCC25285 and pectin was evaluated and the underlying mechanisms were investigated by metabolomic and microbiome analyses. The combination elicited superior anti-ALD effects than the individual agents used alone. The synergistic effects of B. fragilis ATCC25285 and pectin were driven by modulating gut microbiota, improving tryptophan metabolism, and regulating intestinal immune function. Based on our findings, the combination of B. fragilis ATCC25285 and pectin can be considered a potential treatment for ALD. KEY POINTS: • B. fragilis ATCC25285 was identified as a protective Bacteroides strain against ALD. • The synbiotic combination of B. fragilis and pectin has better anti-ALD effects. • The synbiotic combination modulates gut microbiota and tryptophan metabolism.

Host/microbiota interactions-derived tryptophan metabolites modulate oxidative stress and inflammation via aryl hydrocarbon receptor signaling

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that induces the expression of a broad range of downstream genes such as cytochromes P450 enzymes and cyclooxygenase-2. Recent research focuses are shifting from AhR activation induced by xenobiotics to its response patterns to physiological ligands that expand our understanding of how endogenous metabolites as ligands to modulate AhR signaling pathway under homeostasis and pathological conditions. With increasing interest in AhR and its endogenous ligands, it would seem advisable to summarize a variety of endogenous ligands especially host/gut microbiota-derived tryptophan metabolites. Mounting evidence has indicated that AhR play a critical role in the regulation of redox homeostasis and immune responses. In this review, we outline the canonical and non-canonical AhR signalling pathway that is mediated by host/gut microbiota-derived tryptophan metabolites. Through several typical endogenous AhR ligands, we investigated the molecular mechanisms of AhR-induced oxidative stress and inflammation in the pathological milieu, including diabetes, diabetic kidney disease and end-stage renal disease. Finally, we summarize and emphasize the limitations and breakthrough of endogenous AhR ligands from host/microbial tryptophan catabolites. This review might provide novel diagnostic and prognostic approach for refractory human diseases and establish new therapeutic strategies for AhR activation.

Migraine signaling pathways: amino acid metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a common, debilitating disorder for which attacks typically result in a throbbing, pulsating headache. Although much is known about migraine, its complexity renders understanding the complete etiology currently out of reach. However, two important facts are clear, the brain and the metabolism of the migraineur differ from that of the non-migraineur. This review centers on the altered amino acid metabolism in migraineurs and how it helps define the pathology of migraine.

Exposure to combustion derived particulate matter exacerbates influenza infection in neonatal mice by inhibiting IL22 production

Background

Particulate matter (PM) containing environmentally persistent free radicals (EPFRs) are formed during various combustion processes, including the thermal remediation of hazardous wastes. Exposure to PM adversely affects respiratory health in infants and is associated with increased morbidity and mortality due to acute lower respiratory tract infections. We previously reported that early-life exposure to PM damages the lung epithelium and suppresses immune responses to influenza virus (Flu) infection, thereby enhancing Flu severity. Interleukin 22 (IL22) is important in resolving lung injury following Flu infection. In the current study, we determined the effects of PM exposure on pulmonary IL22 responses using our neonatal mouse model of Flu infection.

Results

Exposure to PM resulted in an immediate (0.5–1-day post-exposure; dpe) increase in IL22 expression in the lungs of C57BL/6 neonatal mice; however, this IL22 expression was not maintained and failed to increase with either continued exposure to PM or subsequent Flu infection of PM-exposed mice. This contrasts with increased IL22 expression in age-matched mice exposed to vehicle and Flu infected. Activation of the aryl hydrocarbon receptor (AhR), which mediates the induction and release of IL22 from immune cells, was also transiently increased with PM exposure. The microbiome plays a major role in maintaining epithelial integrity and immune responses by producing various metabolites that act as ligands for AhR. Exposure to PM induced lung microbiota dysbiosis and altered the levels of indole, a microbial metabolite. Treatment with recombinant IL22 or indole-3-carboxaldehyde (I3A) prevented PM associated lung injury. In addition, I3A treatment also protected against increased mortality in Flu-infected mice exposed to PMs.

Conclusions

Together, these data suggest that exposure to PMs results in failure to sustain IL22 levels and an inability to induce IL22 upon Flu infection. Insufficient levels of IL22 may be responsible for aberrant epithelial repair and immune responses, leading to increased Flu severity in areas of high PM.

Gut microbiota-derived tryptophan metabolism mediates renal fibrosis by aryl hydrocarbon receptor signaling activation

The gut microbiota has a crucial effect on regulating the intestinal mucosal immunity and maintaining intestinal homeostasis both in health and in disease state. Many effects are mediated by gut microbiota-derived metabolites and tryptophan, an essential aromatic amino acid, is considered important among many metabolites in the crosstalk between gut microbiota and the host. Kynurenine, serotonin, and indole derivatives are derived from the three major tryptophan metabolism pathways modulated by gut microbiota directly or indirectly. Aryl hydrocarbon receptor (AHR) is a cytoplasmic ligand-activated transcription factor involved in multiple cellular processes. Tryptophan metabolites as ligands can activate AHR signaling in various diseases such as inflammation, oxidative stress injury, cancer, aging-related diseases, cardiovascular diseases (CVD), and chronic kidney diseases (CKD). Accumulated uremic toxins in the body fluids of CKD patients activate AHR and affect disease progression. In this review, we will elucidate the relationship between gut microbiota-derived uremic toxins by tryptophan metabolism and AHR activation in CKD and its complications. This review will provide therapeutic avenues for targeting CKD and concurrently present challenges and opportunities for designing new therapeutic strategies against renal fibrosis.

Effect of diet and intestinal AhR expression on fecal microbiome and metabolomic profiles

BACKGROUND

Diet, loss of aryl hydrocarbon receptor (AhR) expression and their modification of the gut microbiota community composition and its metabolites affect the development of colorectal cancer (CRC). However, the concordance between fecal microbiota composition and the fecal metabolome is poorly understood. Mice with specific AhR deletion (AhRKO) in intestinal epithelial cell and their wild-type littermates were fed a low-fat diet or a high-fat diet. Shifts in the fecal microbiome and metabolome associated with diet and loss of AhR expression were assessed. Microbiome and metabolome data were integrated to identify specific microbial taxa that contributed to the observed metabolite shifts.

RESULTS

Our analysis shows that diet has a more pronounced effect on mouse fecal microbiota composition than the impact of the loss of AhR. In contrast, metabolomic analysis showed that the loss of AhR in intestinal epithelial cells had a more pronounced effect on metabolite profile compared to diet. Integration analysis of microbiome and metabolome identified unclassified Clostridiales, unclassified Desulfovibrionaceae, and Akkermansia as key contributors to the synthesis and/or utilization of tryptophan metabolites.

CONCLUSIONS

Akkermansia are likely to contribute to the synthesis and/or degradation of tryptophan metabolites. Our study highlights the use of multi-omic analysis to investigate the relationship between the microbiome and metabolome and identifies possible taxa that can be targeted to manipulate the microbiome for CRC treatment.

Host-microbiome interactions: the aryl hydrocarbon receptor as a critical node in tryptophan metabolites to brain signaling

Tryptophan (Trp) is not only a nutrient enhancer but also has systemic effects. Trp metabolites signaling through the well-known aryl hydrocarbon receptor (AhR) constitute the interface of microbiome-gut-brain axis. However, the pathway through which Trp metabolites affect central nervous system (CNS) function have not been fully elucidated. AhR participates in a broad variety of physiological and pathological processes that also highly relevant to intestinal homeostasis and CNS diseases. Via the AhR-dependent mechanism, Trp metabolites connect bidirectional signaling between the gut microbiome and the brain, mediated via immune, metabolic, and neural (vagal) signaling mechanisms, with downstream effects on behavior and CNS function. These findings shed light on the complex Trp regulation of microbiome-gut-brain axis and add another facet to our understanding that dietary Trp is expected to be a promising noninvasive approach for alleviating systemic diseases.

From IBS to ME - The dysbiotic march hypothesis

Irritable bowel syndrome (IBS) is often associated with other unexplained complaints like chronic fatigue syndrome (CFS), fibromyalgia and myalgic encephalopathy (ME). The pathogenesis of the relationship is unknown. Intestinal dysbiosis may be a common abnormality, but based on 1100 consecutive IBS patients examined over a nine years period, we hypothesize that the development of the disease, often from IBS to ME, actually manifests a "dysbiotic march". In analogy with "the atopic march" in allergic diseases, we suggest "a dysbiotic march" in IBS; initiated by extensive use of antibiotics during childhood, often before school age. Various abdominal complaints including IBS may develop soon thereafter, while systemic symptom like CFS, fibromyalgia and ME may appear years later.

Application of the optimized and validated LC-MS method for simultaneous quantification of tryptophan metabolites in culture medium from cancer cells

Kynurenine pathway is the main route of tryptophan degradation generating a number of immunoregulatory compounds. Some conditions like oxidative stress, inflammatory factors might enhance tryptophan degradation. Process is active in several cells including fibroblasts, cancer cells, and immune cells, therefore it is intensively studied in context of cancer microenvironment. The validated and standardized methodology for kynurenine quantification is crucial for reliable comparison of results obtained in different studies. This paper concerns an approach for simultaneous quantification of four major tryptophan metabolites of the kynurenine pathway (kynurenine, 3-hydroxykynurenine, xanthurenic acid, 3-hydroxyanthranilic acid) in cell culture supernatants by liquid chromatography coupled with single quadrupole mass spectrometer. During development of the novel method, the principal component analysis was used to select the best mobile phase and to ensure the optimal conditions for simultaneous quantification of metabolites. The analysis involves simple protein precipitation with acidified methanol and 3-nitrotyrosine as an internal standard. The obtained limits of detection and quantification in cell culture medium were in the range of 3.31-10.80 nmol/L and 9.60-19.50 nmol/L, respectively. At the validation step, other method parameters (linearity, precision, accuracy, recovery, matrix effects) were also evaluated and satisfactory results were obtained for all target compounds. The method was applied to study tryptophan metabolites by determination of kynurenines in cell culture medium from two different human cancer cell lines (MDA-MD-231 and SK-OV-3) in context of exposure to glycation products.

Skatole regulates intestinal epithelial cellular functions through activating aryl hydrocarbon receptors and p38

Intestinal bacteria produce skatole (3-methylindole) from tryptophan in dietary proteins and ingesting large quantities of animal protein is associated with increased fecal skatole concentrations. Although possibly associated with disrupted intestinal homeostasis, the influence of skatole on intestinal epithelial cellular function has not been characterized in detail. The present study aimed to determine whether skatole induces intestinal epithelial cell (IEC) dysfunction. We found that skatole dose-dependently caused IEC death and time-dependently induced IEC apoptosis. Since skatole directly interacts with aryl hydrocarbon receptors (AhR), we investigated whether these receptors influence the skatole-induced death of IEC. In addition to increased AhR transcriptional activity induced by skatole, the AhR antagonist CH223191 partially suppressed of skatole-induced IEC death. Extracellular signal-related kinase (ERK), p38 and c-Jun N-terminal kinase (JNK) are mitogen-activated protein kinases (MAPK) induced by skatole. None of them were repressed by CH223191, whereas the p38 inhibitor SB203580 promoted skatole-induced IEC death. These findings together indicated that skatole induces both AhR-dependent activation pathways and the AhR-independent activation of p38, consequently regulating the amount of IEC death. Accumulating evidence indicates that consuming large amounts of animal protein is associated with the pathogenesis and progression of inflammatory bowel diseases (IBD). Thus, intestinal skatole production induced by large amounts of dietary animal protein might be associated via IEC death with intestinal pathologies such as IBD.

Analysis of Tryptophan and Its Metabolites by High-Performance Liquid Chromatography

Tryptophan is a nutritionally essential amino acid for both humans and animals. Besides acting as a building block for protein synthesis, tryptophan (Trp) and its metabolites are crucial for maintaining neurological function, immunity, and homeostasis in the body. To uncover the regulatory role of Trp and its metabolites in cell nutrition, metabolism and physiology, various analytical methods, including high-performance liquid chromatography (HPLC), have been developed to determine key Trp metabolites. Here we describe a rapid and sensitive method for the simultaneous analysis of Trp and its metabolites along with other amino acids by HPLC involving in-line pre-column derivatization with o-phthaldialdehyde (OPA) and dual-channel fluorescence detection. OPA reacts very rapidly (within 1 min) with Trp, 5-hydroxytryptophan, 5-hydroxytryptamine, and tryptamine at room temperature (e.g., 20-25 °C) in an autosampler. Their derivatives are immediately injected into the HPLC column without the need for extraction. Trp metabolites that cannot react with OPA but are fluorescent can be detected by setting the excitation and emission wavelengths of the fluorescence detector in another detection channel. The autosampler is programmed to mix Trp and its metabolites with OPA for 1 min to generate highly fluorescent derivatives for HPLC separation and detection (Channel A, excitation = 270 nm and emission = 350 nm; Channel B, excitation = 340 nm and emission = 450 nm). The detection limit for Trp and its metabolites is 30 pmol/mL or 150 fmol/injection. The total time for chromatographic separation (including column regeneration) is 55 min for each sample. Our HPLC method can be used for the analysis of amino acids (including Trp) in alkaline protein hydrolysates and of Trp and its metabolites in biological samples.

Diabesity and mood disorders: Multiple links through the microbiota-gut-brain axis

The global prevalence of diabesity is on the rise, and the clinical, social and economic health burden arising from this epidemic is aggravated by a significant co-morbidity of diabesity with neuropsychiatric disease, particularly depression. Importantly, not only is the prevalence of mood disorders elevated in patients with type 2 diabetes, depressed patients are also more prone to develop diabetes. This reciprocal relationship calls for a molecular and systemic analysis of diabesity-brain interactions to guide preventive and therapeutic strategies. The analysis we are presenting in this review is modelled on the microbiota-gut-brain axis, which provides the brain with information from the gut not only via the nervous system, but also via a continuous stream of microbial, endocrine, metabolic and immune messages. This communication network offers important clues as to how obesity and diabetes could target the brain to provoke neuropsychiatric disease. There is emerging evidence that the gut microbiota is orchestrating a multiplicity of bodily functions that are intimately related to the immune, metabolic and nervous systems and that gut dysbiosis spoils the homeostasis between these systems. In our article we highlight two groups of molecular links that seem to have a significant bearing on the impact of diabesity on the brain. On the one hand, we focus on microbiota-related metabolites such as short-chain fatty acids, tryptophan metabolites, immune stimulants and endocannabinoids that are likely to play a mediator role. On the other hand, we discuss signalling molecules that operate primarily in the brain, specifically neuropeptide Y, brain-derived neurotrophic factor and γ-amino butyric acid, that are disturbed by microbial factors, obesity and diabetes and are relevant to mental illness. Finally, we address the usefulness of diet-related interventions to suspend the deleterious relationship between diabesity and mood disorders.

Human α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD): a structural and mechanistic unveiling

Human α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase determines the fate of tryptophan metabolites in the kynurenine pathway by controlling the quinolinate levels for de novo nicotinamide adenine dinucleotide biosynthesis. The unstable nature of its substrate has made gaining insight into its reaction mechanism difficult. Our electron paramagnetic resonance (EPR) spectroscopic study on the Cu-substituted human enzyme suggests that the native substrate does not directly ligate to the metal ion. Substrate binding did not result in a change of either the hyperfine structure or the super-hyperfine structure of the EPR spectrum. We also determined the crystal structure of the human enzyme in its native catalytically active state (at 1.99 Å resolution), a substrate analogue-bound form (2.50 Å resolution), and a selected active site mutant form with one of the putative substrate binding residues altered (2.32 Å resolution). These structures illustrate that each asymmetric unit contains three pairs of dimers. Consistent with the EPR findings, the ligand-bound complex structure shows that the substrate analogue does not directly coordinate to the metal ion but is bound to the active site by two arginine residues through noncovalent interactions.

Scavenging of reactive oxygen species by tryptophan metabolites helps Pseudomonas aeruginosa escape neutrophil killing

Pseudomonas aeruginosa is responsible for persistent infections in cystic fibrosis patients, suggesting an ability to circumvent innate immune defenses. This bacterium uses the kynurenine pathway to catabolize tryptophan. Interestingly, many host cells also produce kynurenine, which is known to control immune system homeostasis. We showed that most strains of P. aeruginosa isolated from cystic fibrosis patients produce a high level of kynurenine. Moreover, a strong transcriptional activation of kynA (the first gene involved in the kynurenine pathway) was observed upon contact with immune cells and particularly with neutrophils. In addition, using coculture of human neutrophils with various strains of P. aeruginosa producing no (ΔkynA) or a high level of kynurenine (ΔkynU or ΔkynA pkynA), we demonstrated that kynurenine promotes bacterial survival. In addition, increasing the amount kynurenine inhibits reactive oxygen species production by activated neutrophils, as evaluated by chemiluminescence with luminol or isoluminol or SOD-sensitive cytochrome c reduction assay. This inhibition is due neither to a phagocytosis defect nor to direct NADPH oxidase inhibition. Indeed, kynurenine has no effect on oxygen consumption by neutrophils activated by PMA or opsonized zymosan. Using in vitro reactive oxygen species-producing systems, we showed that kynurenine scavenges hydrogen peroxide and, to a lesser extent, superoxide. Kynurenine׳s scavenging effect occurs mainly intracellularly after bacterial stimulation, probably in the phagosome. In conclusion, the kynurenine pathway allows P. aeruginosa to circumvent the innate immune response by scavenging neutrophil reactive oxygen species production.

Concurrent quantification of tryptophan and its major metabolites

An imbalance in tryptophan (TRP) metabolites is associated with several neurological and inflammatory disorders. Therefore, analytical methods allowing for simultaneous quantification of TRP and its major metabolites would be highly desirable, and may be valuable as potential biomarkers. We have developed a HPLC method for concurrent quantitative determination of tryptophan, serotonin, 5-hydroxyindoleacetic acid, kynurenine, and kynurenic acid in tissue and fluids. The method utilizes the intrinsic spectroscopic properties of TRP and its metabolites that enable UV absorbance and fluorescence detection by HPLC, without additional labeling. The origin of the peaks related to analytes of interest was confirmed by UV-Vis spectral patterns using a PDA detector and mass spectrometry. The developed methods were validated in rabbit fetal brain and amniotic fluid at gestational day 29. Results are in excellent agreement with those reported in the literature for the same regions. This method allows for rapid quantification of tryptophan and four of its major metabolites concurrently. A change in the relative ratios of these metabolites can provide important insights in predicting the presence and progression of neuroinflammation in disorders such as cerebral palsy, autism, multiple sclerosis, Alzheimer disease, and schizophrenia.