Kynurenine Pathway of Tryptophan Metabolism: Regulatory and Functional Aspects

Establishment of a UPLC-MS method for quantitative analysis of tryptophan-kynurenine metabolism in IBS-D model rats

Background and Aims Abnormalities in tryptophan (TRP) metabolism induce abdominal pain and intestinal motility disorders. The study of TRP metabolism in diarrhea-predominant-irritable bowel syndrome (IBS-D) is important for the prevention, diagnosis, and treatment of this disease. In this study, a rapid and reliable ultra performance liquid chromatography-mass spectrometry (UPLC-MS) method was established to quantify tryptophan-kynurenine (TRP-Kyn) metabolism in the colon of a rat model with IBS-D. Methods The proteins were precipitated by methanol, chromatographically separated on a Welch Ultimate® Polar RP column with a gradient elution for 12 min, and detected by high-resolution tandem mass spectrometry. Pure water were used as an alternative mechanism for standard calibration, and the stable structural analog 2-Cl-Phe was used as an internal standard. Results Within a certain range, the r of TRP, kynurenine (Kyn) and quinolinic acid (QA), kynurenic acid (KA) are greater than 0.99, were found to be accurate and precise. The metabolism of TRP was significantly up-regulated along the Kyn pathway in the IBS-D model rats and normalized after treatment with pivacurium bromide. Conclusion This study investigates the mechanisms of IBS-D gastrointestinal dysfunction from the perspective of colonic TRP metabolism, and also provides new directions for the diagnosis and therapeutic approach of this disease.

Baseline Inflammation but not Exercise Modality Impacts Exercise-induced Kynurenine Pathway Modulation in Persons With Multiple Sclerosis: Secondary Results From a Randomized Controlled Trial

Background

The kynurenine pathway (KP) is an important hub in neuroimmune crosstalk that is dysregulated in persons with multiple sclerosis (pwMS) and modulated by exercise in a modality-specific manner.

Objectives

To compare changes in the KP metabolite profile of pwMS (1) following combined treatments including either high-intensity interval training (HIIT) or moderate-intensity continuous training (MICT) during a 3-week multimodal rehabilitation, (2) to evaluate exercise response in relation to baseline systemic inflammation, and (3) to investigate associations of kynurenines with physical capacity and clinical outcomes.

Methods

For this secondary analysis of a randomized controlled trial, serum concentrations of kynurenines at baseline and after 3 weeks were determined using targeted metabolomics (LC-MS/MS). Exercise-induced changes in the KP metabolite profile according to treatment and baseline systemic inflammation (neutrophil-to-lymphocyte ratio (NLR) <3.12 versus ⩾3.12) were investigated using covariance analyses.

Results

Regardless of treatment, concentrations of tryptophan and most kynurenines decreased over time. Quinolinic acid concentration increased (p < .001). Participants with low and high NLR revealed differential exercise-induced changes in concentrations of kynurenines and NLR. The systemic inflammation markers neopterin (p = .015) and NLR (p < .001) decreased in the whole group and in participants with high NLR, respectively.

Conclusions

Combined treatments including HIIT or MICT do not differentially modulate the KP metabolite profile, with both reducing concentrations of most kynurenines. Baseline systemic inflammation may impact exercise-induced changes in the KP metabolite profile and anti-inflammatory effects of exercise in pwMS.

Trial registration

clinicaltrials.gov (identifier: NCT04356248).

Association between gut microbiota, plasma metabolites, and ovarian cancer: A Mendelian randomization study

Numerous studies have demonstrated a correlation between alterations in gut microbiota (GM) and levels of body metabolites in ovarian cancer (OC). However, the specific causal relationships underlying these associations remain unclear. This study utilized summary statistics of GM from the MiBioGen consortium, along with an unprecedented dataset comprising 1091 blood metabolites and 309 metabolite ratios from the UK Biobank, in conjunction with OC data from the FinnGen Consortium R9 release. We conducted bidirectional Mendelian randomization (MR) analyses to investigate the causal relationships between GM and OC. Additionally, a two-step MR approach was employed to identify potential mediating metabolites. Our analysis revealed significant associations between 6 specific microbiota taxa and OC. Furthermore, we identified several plasma metabolites that act as mediators of the association between GM and OC. In the two-step MR analysis, we observed a negative correlation between 4-methoxyphenol sulfate and pregnenetriol disulfate levels with OC. The genus Lachnospiraceae UCG008 potentially increases the risk of OC by decreasing 4-methoxyphenol sulfate levels, while the genus Howardella may elevate the risk of OC by reducing pregnenetriol disulfate levels, with mediation proportions of 22.35% and 4.23%, respectively. Additionally, levels of dilinoleoyl-GPE (18:2/18:2) and N-acetylkynurenine (2) were positively correlated with OC. The inhibitory effect of the genus Ruminococcus 1 on OC may be mediated through 1,2-dilinoleoyl-GPE (18:2/18:2) and N-acetylkynurenine (2), with mediation proportions of 10.15% and 11.32%, respectively. Our findings highlight the complex relationship among GM, plasma metabolites, and OC. The identified associations and mediation effects offer valuable insights into potential therapeutic approaches targeting GM for the management of OC.

Characterization of a fish-specific immunoglobulin-like domain-containing protein (Igldcp) in zebrafish (Danio rerio) induced after nodavirus infection

One of the most highly induced genes in zebrafish (Danio rerio) larvae after infection with the nodavirus red-spotted grouper nervous necrosis virus (RGNNV) was a member of the immunoglobulin superfamily (IgSF), which has remained uncharacterized and erroneously annotated in zebrafish and other fish species as galectin 17 (lgals17). We characterized this gene and named it immunoglobulin (Ig)-like domain-containing protein (igldcp), a new member of the IgSF that does not possess orthologs in mammals. Igldcp expression is induced by viral infection and it belongs to the group of interferon-stimulated genes (ISGs). In vitro overexpression of igldcp decreased RGNNV replication, whereas in vivo knockdown of this gene had the opposite effect, resulting in increased larval mortality. RNA-Seq analyses of larvae overexpressing igldcp in the absence or presence of infection with RGNNV showed that the main processes affected by Igldcp could be directly involved in the regulation of various cellular processes associated with the modulation of the immune system.

Amino acid metabolic reprogramming in the tumor microenvironment and its implication for cancer therapy

Amino acids are essential building blocks for proteins, crucial energy sources for cell survival, and key signaling molecules supporting the resistant growth of tumor cells. In tumor cells, amino acid metabolic reprogramming is characterized by the enhanced uptake of amino acids as well as their aberrant synthesis, breakdown, and transport, leading to immune evasion and malignant progression of tumor cells. This article reviews the altered amino acid metabolism in tumor cells and its impact on tumor microenvironment, and also provides an overview of the current clinical applications of amino acid metabolism. Innovative drugs targeting amino acid metabolism hold great promise for precision and personalized cancer therapy.

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

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

True Digestibility of Tryptophan in Plant and Animal Protein

BACKGROUND

Protein quality, evaluated using Digestible Indispensable Amino Acid Score (DIAAS), requires ileal digestibility values of individual indispensable amino acids (IAAs) in each protein. However, true tryptophan (Trp) digestibility has rarely been quantified in humans.

OBJECTIVE

To measure the true Trp digestibility and DIAAS of 2H-intrinsically labeled plant and animal protein sources in humans, using the dual isotope tracer technique.

METHODS

The true Trp digestibility of 2H intrinsically labeled plant proteins such as whole mung bean (n = 6) and dehulled mung bean (n = 6), chickpea (n = 5), and yellow pea (n = 5), and protein from animal source foods such as egg white (n = 6), whole egg (n = 6), chicken meat (n = 6), and goat milk (n = 7) was determined against the known digestibility of U-13C spirulina whole cell protein as reference, except for goat milk protein that was measured against free crystalline 13C-Trp as reference. Banked samples from earlier studies conducted to determine true IAA digestibility of different protein sources were used for the analysis. DIAAS was calculated for each test protein using digestibility corrected IAA scores (mg IAA/g of protein) in comparison with the IAA requirement score for adults.

RESULTS

The true Trp digestibility of whole mung bean, dehulled mung bean, chickpea, yellow pea, egg white, whole egg, chicken meat, and goat milk were 67.6 ± 3.7%, 74.5 ± 4.4%, 72.6 ± 2.3%, 72.5 ± 2.2%, 89.7 ± 2.5%, 91.4 ± 2.6%, 95.9 ± 2.2%, and 92.8 ± 2.9%, respectively. The true Trp digestibility of plant protein sources was significantly lower than that of animal protein sources (P < 0.05). Trp was not a limiting IAA in all the tested proteins.

CONCLUSION

The true Trp digestibility determined in this study ranged from 67.6 ± 3.7% to 95.9 ± 2.2% for whole mung bean and chicken meat, respectively, and adds to the database of individual true IAA digestibility of different protein sources.

TRIAL REGISTRATION NUMBER

This study was registered in Clinical Trials Registry of India (CTRI) with registration numbers CTRI/2017/11/010468, CTRI/2020/04/024512, and CTRI/2018/03/012265.

The impact of maternal vulnerability on stress biomarkers and first-trimester growth: the Rotterdam Periconceptional Cohort (Predict Study)

STUDY QUESTION

Is the degree of maternal vulnerability positively associated with stress biomarkers (stress hormones, C-reactive protein, tryptophan metabolites, and one-carbon metabolites), and does long-term exposure to stress hormones reduce first-trimester growth?

SUMMARY ANSWER

The maternal vulnerability risk score is positively associated with concentrations of hair cortisol and cortisone and negatively with tryptophan, while higher hair cortisol concentrations are associated with reduced first-trimester growth without mediation of tryptophan.

WHAT IS KNOWN ALREADY

A high degree of maternal vulnerability during the periconception period is associated with impaired first-trimester growth and pregnancy complications, with consequences for long-term health of the child and future life course. However, due to the challenges of early identification of vulnerable women, the uptake of periconception care is low in this target group.

STUDY DESIGN, SIZE, DURATION

Between June 2022 and June 2023, this study was conducted in a sub-cohort of 160 pregnant women participating in the Rotterdam Periconceptional Cohort (Predict Study), an ongoing prospective tertiary hospital-based cohort.

PARTICIPANTS/MATERIALS, SETTING, METHODS

One hundred and thirty-two women with ongoing pregnancies and available stress biomarker data were included in the analysis. Data on periconceptional social, lifestyle, and medical risk factors were collected via self-administered questionnaires, and these factors were used for the development of a composite maternal vulnerability risk score. Stress biomarkers, including stress hormones (hair cortisol and cortisone) and inflammatory and oxidative stress biomarkers (C-reactive protein, total homocysteine, and tryptophan metabolites) were determined in the first trimester of pregnancy. First-trimester growth was assessed by crown-rump length (CRL) and embryonic volume (EV) measurements at 7, 9, and 11 weeks gestation by making use of an artificial intelligence algorithm and virtual reality techniques using 3D ultrasound data sets. The associations between the maternal vulnerability risk score and stress biomarkers were identified using linear regression models, and between stress hormones and CRL- and EV-trajectories using mixed models. A mediation analysis was performed to assess the contribution of tryptophan. All associations were adjusted for potential confounders, which were identified using a data-driven approach. Several sensitivity analyses were performed to check the robustness of the findings.

MAIN RESULTS AND THE ROLE OF CHANCE

The maternal vulnerability risk score was positively associated with concentrations of hair cortisol and cortisone (pg/mg) (β = 0.366, 95% CI = 0.010-0.722; β = 0.897, 95% CI = 0.102-1.691, respectively), and negatively with tryptophan concentrations (µmol/L) (β = -1.637, 95% CI = -2.693 to -0.582). No associations revealed for C-reactive protein and total homocysteine. Higher hair cortisol concentrations were associated with reduced EV-trajectories (3√EV: β = -0.010, 95% CI = -0.017 to -0.002), while no associations were found with CRL-trajectories. Mediation by tryptophan was not shown.

LIMITATIONS, REASONS FOR CAUTION

Residual confounding cannot be ruled out, and the external validity may be limited due to the study's single-center observational design in a tertiary hospital.

WIDER IMPLICATIONS OF THE FINDINGS

There is mounting evidence that a high degree of maternal vulnerability negatively affects maternal and perinatal health, and that of the future life course. The results of our study emphasize the need to identify highly vulnerable women as early as possible, at least before conception. Our findings suggest that the chronic stress response and alterations of the maternal tryptophan metabolism are involved in maternal vulnerability, affecting first-trimester growth, with potential impact on the long-term health of the offspring.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by the Departments of Obstetrics and Gynecology and Clinical Chemistry of the Erasmus MC, University Medical Center, Rotterdam, the Netherlands, and the Junior Award granted by the De Snoo-van 't Hoogerhuijs Foundation in March 2022. There are no conflicts of interest.

TRIAL REGISTRATION NUMBER

N/A.

Serum metabolite signature of the modified Mediterranean-DASH intervention for neurodegenerative delay (MIND) diet

INTRODUCTION

There is a lack of biomarkers of clinically important diets, such as the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet.

OBJECTIVES

Our study explored serum metabolites associated with adherence to the MIND diet.

METHODS

In 3,908 Atherosclerosis Risk in Communities (ARIC) study participants, we calculated a modified MIND diet score based on a 66-item self-reported food frequency questionnaire (FFQ). The modified score did not include berries and olive oil, as these items were not assessed in the FFQ. We used multivariable linear regression models in 2 subgroups of ARIC study participants and meta-analyzed results using fixed effects regression to identify significant metabolites after Bonferroni correction. We also examined associations between these metabolites and food components of the modified MIND diet. C-statistics evaluated the prediction of high modified MIND diet adherence using significant metabolites beyond participant characteristics.

RESULTS

Of 360 metabolites analyzed, 27 metabolites (15 positive, 12 negative) were significantly associated with the modified MIND diet score (lipids, n = 13; amino acids, n = 5; xenobiotics, n = 3; cofactors and vitamins, n = 3; carbohydrates n = 2; nucleotide n = 1). The top 4 metabolites that improved the prediction of high dietary adherence to the modified MIND diet were 7-methylxanthine, theobromine, docosahexaenoate (DHA), and 3-carboxy-4-methyl-5-propyl-2-furanpropanoate (CMPF).

CONCLUSION

Twenty-seven metabolomic markers were correlated with the modified MIND diet. The biomarkers, if further validated, could be useful to objectively assess adherence to the MIND diet.

Sirtuin 1 regulates the phenotype and functions of dendritic cells through Ido1 pathway in obesity

Sirtuin 1 (SIRT1) is a class III histone deacetylase (HDAC3) that plays a crucial role in regulating the activation and differentiation of dendritic cells (DCs) as well as controlling the polarization and activation of T cells. Obesity, a chronic inflammatory condition, is characterized by the activation of immune cells in various tissues. We hypothesized that SIRT1 might influence the phenotype and functions of DCs through the Ido1 pathway, ultimately leading to the polarization towards pro-inflammatory T cells in obesity. In our study, we observed that SIRT1 activity was reduced in bone marrow-derived DCs (BMDCs) from obese animals. These BMDCs exhibited elevated oxidative phosphorylation (OXPHOS) and increased extracellular acidification rates (ECAR), along with enhanced expression of class II MHC, CD86, and CD40, and elevated secretion of IL-12p40, while the production of TGF-β was reduced. The kynurenine pathway activity was decreased in BMDCs from obese animals, particularly under SIRT1 inhibition. SIRT1 positively regulated the expression of Ido1 in DCs in a PPARγ-dependent manner. To support these findings, ATAC-seq analysis revealed that BMDCs from obese mice had differentially regulated open chromatin regions compared to those from lean mice, with reduced chromatin accessibility at the Sirt1 genomic locus in BMDCs from obese WT mice. Gene Ontology (GO) enrichment analysis indicated that BMDCs from obese animals had disrupted metabolic pathways, including those related to GTPase activity and insulin response. Differential expression analysis showed reduced levels of Pparg and Sirt1 in BMDCs from obese mice, which was challenged and confirmed using BMDCs from mice with conditional knockout of Sirt1 in dendritic cells (SIRT1∆). This study highlights that SIRT1 controls the metabolism and functions of DCs through modulation of the kynurenine pathway, with significant implications for obesity-related inflammation.

Animal and plant protein intake association with mental health, tryptophan metabolites pathways, and gut microbiota in healthy women: a cross-sectional study

Mental health is affected by tryptophane (TRP) metabolism regulation. Diet-influenced gut microbiome regulates TRP metabolism. Thus, the present study aimed to explore the relationship between type of dietary protein intake, gut microbiota, TRP metabolites homeostasis, and mental well-being in healthy women. 91 healthy females aged 18-50 were recruited based on the study protocol. Validate and reliable questionnaires assessed dietary intake and mental health. Biochemical tests and gut microbiota composition were analyzed following the manufacturer's instructions for each enzyme-linked immune sorbent assay (ELISA) kit and Real-time quantitative polymerase chain reaction (qPCR) methods respectively. Regression methods were used to estimate the considered associations. The results show that in the fully adjusted model, plant protein consumption was partially inversely associated with depression risk (OR = 0.27; 95% CI: 0.06, 1.09; P = 0.06). Higher dietary animal protein intake was marginally associated with psychological distress (OR = 2.59; 95% CI: 0.91, 7.34; P = 0.07). KYN to serotonin ratio was inversely associated with animal protein consumption (ß = 1.10; 95% CI: -0.13, 2.33; P = 0.07). Firmicutes/Bacteriodetes ratio (β = -1.27 × 103, SE = 5.99 × 102, P = 0.03) was lower in the top tertile of plant protein. A partially negative correlation was found between dietary animal protein and Prevotella abundance (β = -9.20 × 1018, SE = 5.04 × 1018, P = 0.06). Overall, significant inverse associations were found between a diet high in plant protein with mental disorders, KYN levels, and Firmicutes to Bacteroidetes ratio while adhering to higher animal protein could predispose women to psychological stress.

Proteomics Analysis of Renal Cell Line Caki-2 with AFMID Overexpression and Potential Biomarker Discovery in Urine

Aromatic caninurine formamase (AFMID) is an enzyme involved in the tryptophan pathway, metabolizing N-formylkynurenine to kynurenine. AFMID had been found significantly downregulated in clear cell renal cell carcinoma (ccRCC) in both tissue and urine samples. Although ccRCC is characterized by a typical Warburg-like phenotype, mitochondrial dysfunction, and elevated fat deposition, it is unknown whether AFMID plays a role in tumorigenesis and the development of ccRCC. In the present study, AFMID overexpression had inhibitory effects for ccRCC cells, decreasing the rate of cell proliferation. Quantitative proteomics showed that AFMID overexpression altered cellular signaling pathways involved in cell growth and cellular metabolism pathways, including lipid metabolism and inositol phosphate metabolism. Further urine proteomic analysis indicated that cellular function dysfunction with AFMID overexpression could be reflected in the urine. The activity of predicted upregulators DDX58, TREX1, TGFB1, SMARCA4, and TNF in ccRCC cells and urine showed opposing change trends. Potential urinary biomarkers were tentatively discovered and further validated using an independent cohort. The protein panel of APOC3, UMOD, and CILP achieved an AUC value of 0.862 for the training cohort and 0.883 for the validation cohort. The present study is of significance in terms of highlighting various aspects of pathway changes associated with AFMID enzymes, discovering potential specific biomarkers for potential patient diagnosis, and therapeutic targeting.

Mediation-adjusted multivariable Mendelian randomisation study identified novel metabolites related to mental health

BACKGROUND

From the pathway perspective, metabolites have the potential to improve knowledge about the aetiology of psychiatric diseases. Previous studies suggested a link between specific blood metabolites and mental disorders, but some Mendelian randomisation (MR) studies in particular are insufficient for various reasons.

OBJECTIVE

This study focused on bias assessment due to interdependencies between metabolites and psychiatric mediation effects.

METHODS

In a multistep framework containing network and multivariable MR, direct effects of 21 mutually adjusted metabolites on 8 psychiatric disorders were estimated based on summary statistics of genome-wide association studies from multiple resources. Robust inverse-variance weighted models were used in primary analyses. Several sensitivity analyses were performed to assess different patterns of pleiotropy and weak instrument bias. Estimates for the same phenotypes from different resources were pooled using fixed effect meta-analysis models.

FINDINGS

After adjusting for mediation effects, genetically predicted metabolite levels of six metabolites of lipid, amino acid and cofactors pathways were directly associated with overall six mental disorders (attention-deficit/hyperactivity disorder, bipolar disorder, anorexia nervosa, depression, post-traumatic stress disorder and schizophrenia). Point estimates ranged from -0.45 (95% CI -0.67; -0.24, p=1.0×104) to 1.78 (95% CI 0.85; 2.71, p=0.006). No associations were found with anxiety and suicide attempt.

CONCLUSIONS

This study provides insights into new metabolic pathways that seems to be causally related to certain mental disorders.

CLINICAL IMPLICATIONS

Further studies are needed to investigate whether the identified associations are effects of the metabolites itself or the biochemical pathway regulating the metabolites.

Immunomodulatory and Anti-Inflammatory Effects of Vitamin A and Tryptophan on Monocyte-Derived Dendritic Cells Stimulated with Gliadin in Celiac Disease Patients

Celiac Disease (CeD) is an autoimmune disorder with various symptoms upon gluten exposure. Dendritic cells (DCs) play a crucial role in gliadin-induced inflammation. Vitamin A (retinol; Ret) and its metabolite, retinoic acid (RA), along with tryptophan (Trp) and its metabolite, kynurenic acid (KYNA), are known to influence the immune function of DCs and enhance their tolerogenicity. This research aims to assess the impact of gliadin on DC maturation and the potential of vitamin A and tryptophan to induce immune tolerance in DCs. The monocyte cells obtained from peripheral blood mononuclear cells (PBMCs) of celiac disease patients were differentiated into DCs in the absence or presence of Ret, RA, Trp, KYNA, and then stimulated with peptic and tryptic (PT) digested of gliadin. We used flow cytometry to analyze CD11c, CD14, HLA-DR, CD83, CD86, and CD103 expression. ELISA was carried out to measure TGF-β, IL-10, IL-12, and TNF-α levels. qRT-PCR was used to assess the mRNA expression of retinaldehyde dehydrogenase 2 (RALDH2) and integrin αE (CD103). The mRNA and protein levels of Indoleamine 2, 3-dioxygenase (IDO) was analyzed by qRT-PCR and Western blot assays, respectively. Our findings demonstrate that PT-gliadin enhances the expression of maturation markers, i.e. CD83, CD86 and HLA-DR and promote the secretion of TNF-α and IL-12 in DCs. Interestingly, vitamin A, tryptophan, and their metabolites increase the expression of CD103, while limiting the expression of HLA-DR, CD83, and CD86. They also enhance RALDH2 and IDO expression and promote the secretion of TGF-β and IL-10, while limiting IL-12 and TNF-α secretion. These findings suggest that vitamin A and tryptophan have beneficial effects on PT-gliadin-stimulated DCs, highlighting their potential as therapeutic agents for celiac disease. However, further research is needed to fully understand their underlying mechanisms of action in these cells.

The role of nonesterified fatty acids in cancer biology: Focus on tryptophan and related metabolism

Plasma nonesterified fatty acids (NEFA) are elevated in cancer, because of decreased albumin levels and of fatty acid oxidation, and increased fatty acid synthesis and lipolysis. Albumin depletion and NEFA elevation maximally release albumin-bound tryptophan (Trp) and increase its flux down the kynurenine pathway, leading to increased production of proinflammatory kynurenine metabolites, which tumors use to undermine T-cell function and achieve immune escape. Activation of the aryl hydrocarbon receptor by kynurenic acid promotes extrahepatic Trp degradation by indoleamine 2,3-dioxygenase and leads to upregulation of poly (ADP-ribose) polymerase, activation of which and also of SIRT1 (silent mating type information regulation 2 homolog 1) could lead to depletion of NAD+ and ATP, resulting in cell death. NEFA also modulate heme synthesis and degradation, changes in which impact homocysteine metabolism and production of reduced glutathione and hydrogen sulphide. The significance of the interactions between heme and homocysteine metabolism in cancer biology has received little attention. Targeting Trp disposition in cancer to prevent the NEFA effects is suggested.

Global 13C tracing and metabolic flux analysis of intact human liver tissue ex vivo

Liver metabolism is central to human physiology and influences the pathogenesis of common metabolic diseases. Yet, our understanding of human liver metabolism remains incomplete, with much of current knowledge based on animal or cell culture models that do not fully recapitulate human physiology. Here, we perform in-depth measurement of metabolism in intact human liver tissue ex vivo using global 13C tracing, non-targeted mass spectrometry and model-based metabolic flux analysis. Isotope tracing allowed qualitative assessment of a wide range of metabolic pathways within a single experiment, confirming well-known features of liver metabolism but also revealing unexpected metabolic activities such as de novo creatine synthesis and branched-chain amino acid transamination, where human liver appears to differ from rodent models. Glucose production ex vivo correlated with donor plasma glucose, suggesting that cultured liver tissue retains individual metabolic phenotypes, and could be suppressed by postprandial levels of nutrients and insulin, and also by pharmacological inhibition of glycogen utilization. Isotope tracing ex vivo allows measuring human liver metabolism with great depth and resolution in an experimentally tractable system.

Tryptophan Metabolism in Alzheimer's Disease with the Involvement of Microglia and Astrocyte Crosstalk and Gut-Brain Axis

Alzheimer's disease (AD) is an age-dependent neurodegenerative disease characterized by extracellular Amyloid Aβ peptide (Aβ) deposition and intracellular Tau protein aggregation. Glia, especially microglia and astrocytes are core participants during the progression of AD and these cells are the mediators of Aβ clearance and degradation. The microbiota-gut-brain axis (MGBA) is a complex interactive network between the gut and brain involved in neurodegeneration. MGBA affects the function of glia in the central nervous system (CNS), and microbial metabolites regulate the communication between astrocytes and microglia; however, whether such communication is part of AD pathophysiology remains unknown. One of the potential links in bilateral gut-brain communication is tryptophan (Trp) metabolism. The microbiota-originated Trp and its metabolites enter the CNS to control microglial activation, and the activated microglia subsequently affect astrocyte functions. The present review highlights the role of MGBA in AD pathology, especially the roles of Trp per se and its metabolism as a part of the gut microbiota and brain communications. We (i) discuss the roles of Trp derivatives in microglia-astrocyte crosstalk from a bioinformatics perspective, (ii) describe the role of glia polarization in the microglia-astrocyte crosstalk and AD pathology, and (iii) summarize the potential of Trp metabolism as a therapeutic target. Finally, we review the role of Trp in AD from the perspective of the gut-brain axis and microglia, as well as astrocyte crosstalk, to inspire the discovery of novel AD therapeutics.

The roles of the kynurenine pathway in COVID-19 neuropathogenesis

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the highly contagious respiratory disease Corona Virus Disease 2019 (COVID-19) that may lead to various neurological and psychological disorders that can be acute, lasting days to weeks or months and possibly longer. The latter is known as long-COVID or more recently post-acute sequelae of COVID (PASC). During acute COVID-19 infection, a strong inflammatory response, known as the cytokine storm, occurs in some patients. The levels of interferon-γ (IFN-γ), interferon-β (IFN-β), interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-α) are particularly increased. These cytokines are known to activate the enzyme indoleamine 2,3-dioxygenase 1 (IDO-1), catalysing the first step of tryptophan (Trp) catabolism through the kynurenine pathway (KP) leading to the production of several neurotoxic and immunosuppressive metabolites. There is already data showing elevation in KP metabolites both acutely and in PASC, especially regarding cognitive impairment. Thus, it is likely that KP involvement is significant in SARS-CoV-2 pathogenesis especially neurologically.

Effects of Different Protein Sources on Amino Acid Absorption and Plasma Appearance of Tryptophan, Large Neutral Amino Acids, and Tryptophan Metabolites in Pigs

BACKGROUND

Absorption of tryptophan (TRP) across the gut epithelium is potentially modulated by competing large neutral amino acids (LNAAs), which could affect the appearance of TRP and its metabolites in the bloodstream.

OBJECTIVES

This study aimed to determine, in a growing pig model of an adult human, the absorption of TRP and other LNAAs from the gastrointestinal tract, and plasma appearance of TRP, LNAAs, and TRP metabolites, in response to dietary proteins varying in TRP content.

METHODS

Pigs were adapted for 7 d to each of 4 diets that differed in their protein source and TRP content: 1) alpha-lactalbumin (AL; 9.95 mg TRP/g diet DM), 2) whey protein (6.59 mg TRP/g), 3) casein (3.73 mg TRP/g), or 4) zein (0.14 mg TRP/g). On day 8, pigs were euthanised after a 12-h fast (baseline), or 1, 2, 3, 4, or 6 h after they received a test meal consisting of 45 g protein, or a protein-free meal (n = 6 pigs at each time in each meal group). Tryptophan and LNAA absorption from the small intestine, and appearance of TRP, LNAAs, and TRP metabolites (melatonin, serotonin, kynurenine pathway metabolites), in the portal vein and systemic circulation, were determined.

RESULTS

AL intake resulted in sustained elevated plasma TRP concentrations after an overnight fast. The amount of TRP absorbed was dose-dependently related to protein TRP content (P = 0.028), with fastest rates for pigs fed AL (371 mg/h). Portal and systemic plasma TRP, TRP/LNAA, and the TRP metabolites were highest (P ≤ 0.05) after AL intake, and remained above baseline levels for ∼4 h postprandially. Absorption rates of TRP correlated with postprandial plasma TRP and TRP metabolites (P ≤ 0.05).

CONCLUSIONS

In adult humans, postprandial plasma TRP and TRP metabolite concentrations can likely be modulated by the TRP content of the meal.

Characterization of Circulating Protein Profiles in Individuals with Prader-Willi Syndrome and Individuals with Non-Syndromic Obesity

Background: Prader-Willi syndrome (PWS) is a rare genetic disorder characterized by distinctive physical, cognitive, and behavioral manifestations, coupled with profound alterations in appetite regulation, leading to severe obesity and metabolic dysregulation. These clinical features arise from disruptions in neurodevelopment and neuroendocrine regulation, yet the molecular intricacies of PWS remain incompletely understood. Methods: This study aimed to comprehensively profile circulating neuromodulatory factors in the serum of 53 subjects with PWS and 34 patients with non-syndromic obesity, utilizing a proximity extension assay with the Olink Target 96 neuro-exploratory and neurology panels. The ANOVA p-values were adjusted for multiple testing using the Benjamani-Hochberg method. Protein-protein interaction networks were generated in STRING V.12. Corrplots were calculated with R4.2.2 by using the Hmisc, Performance Analytics, and Corrplot packages Results: Our investigation explored the potential genetic underpinnings of the circulating protein signature observed in PWS, revealing intricate connections between genes in the PWS critical region and the identified circulating proteins associated with impaired oxytocin, NAD metabolism, and sex-related neuromuscular impairment involving, CD38, KYNU, NPM1, NMNAT1, WFIKKN1, and GDF-8/MSTN. The downregulation of CD38 in individuals with PWS (p < 0.01) indicates dysregulation of oxytocin release, implicating pathways associated with NAD metabolism in which KYNU and NMNAT1 are involved and significantly downregulated in PWS (p < 0.01 and p < 0.05, respectively). Sex-related differences in the circulatory levels of WFIKKN1 and GDF-8/MSTN (p < 0.05) were also observed. Conclusions: This study highlights potential circulating protein biomarkers associated with impaired oxytocin, NAD metabolism, and sex-related neuromuscular impairment in PWS individuals with potential clinical implications.

NAD+ homeostasis and its role in exercise adaptation: A comprehensive review

Nicotinamide adenine dinucleotide (NAD+) is a crucial coenzyme involved in catalyzing cellular redox reactions and serving as a substrate for NAD+-dependent enzymes. It plays a vital role in maintaining tissue homeostasis and promoting healthy aging. Exercise, a well-established and cost-effective method for enhancing health, can influence various pathways related to NAD+ metabolism. Strategies such as supplementing NAD+ precursors, modulating NAD+ synthesis enzymes, or inhibiting enzymes that consume NAD+ can help restore NAD+ balance and improve exercise performance. Various overlapping signaling pathways are known to play a crucial role in the beneficial effects of both NAD+ and exercise on enhancing health and slowing aging process. Studies indicate that a combined strategy of exercise and NAD+ supplementation could synergistically enhance athletic capacity. This review provides an overview of current research on the interactions between exercise and the NAD+ network, underscoring the significance of NAD+ homeostasis in exercise performance. It also offers insights into enhancing exercise capacity and improving aging-related diseases through the optimal use of exercise interventions and NAD+ supplementation methods.

Molecular Insights into the Interaction of Tryptophan Metabolites with the Human Aryl Hydrocarbon Receptor in Silico: Tryptophan as Antagonist and no Direct Involvement of Kynurenine

BACKGROUND

A direct link between the tryptophan (Trp) metabolite kynurenine (Kyn) and the aryl hydrocarbon receptor (AhR) is not supported by metabolic considerations and by studies demonstrating the failure of Kyn concentrations of up to 100 μM to activate the receptor in cell culture systems using the proxy system of cytochrome P-450-dependent metabolism. The Kyn metabolite kynurenic acid (KA) activates the AhR and may mediate the Kyn link. Recent studies demonstrated down regulation and antagonism of activation of the AhR by Trp. We have addressed the link between Kyn and the AhR by looking at their direct molecular interaction in silico.

METHODS

Molecular docking of Kyn, KA, Trp and a range of Trp metabolites to the crystal structure of the human AhR was performed under appropriate docking conditions.

RESULTS

Trp and 30 of its metabolites docked to the AhR to various degrees, whereas Kyn and 3-hydroxykynurenine did not. The strongest docking was observed with the Trp metabolite and photooxidation product 6-Formylindolo[3,2-b]carbazole (FICZ), cinnabarinic acid, 5-hydroxytryptophan, N-acetyl serotonin and indol-3-yllactic acid. Strong docking was also observed with other 5-hydroxyindoles.

CONCLUSIONS

We propose that the Kyn-AhR link is mediated by KA. The strong docking of Trp and its recently reported down regulation of the receptor suggest that Trp is an AhR antagonist and may thus play important roles in body homeostasis beyond known properties or simply being the precursor of biologically active metabolites. Differences in AhR activation reported in the literature are discussed.

Circulating tryptophan-kynurenine pathway metabolites are associated with all-cause mortality among patients with stage I-III colorectal cancer

Alterations within the tryptophan-kynurenine metabolic pathway have been linked to the etiology of colorectal cancer (CRC), but the relevance of this pathway for prognostic outcomes in CRC patients needs further elucidation. Therefore, we investigated associations between circulating concentrations of tryptophan-kynurenine pathway metabolites and all-cause mortality among CRC patients. This study utilizes data from 2102 stage I-III CRC patients participating in six prospective cohorts involved in the international FOCUS Consortium. Preoperative circulating concentrations of tryptophan, kynurenine, kynurenic acid (KA), 3-hydroxykynurenine (HK), xanthurenic acid (XA), 3-hydroxyanthranilic acid (HAA), anthranilic acid (AA), picolinic acid (PA), and quinolinic acid (QA) were measured by liquid chromatography-tandem mass spectrometry. Using Cox proportional hazards regression, we examined associations of above-mentioned metabolites with all-cause mortality, adjusted for potential confounders. During a median follow-up of 3.2 years (interquartile range: 2.2-4.9), 290 patients (13.8%) deceased. Higher blood concentrations of tryptophan, XA, and PA were associated with a lower risk of all-cause mortality (per doubling in concentrations: tryptophan: HR = 0.56; 95%CI:0.41,0.76, XA: HR = 0.74; 95%CI:0.64,0.85, PA: HR = 0.76; 95%CI:0.64,0.92), while higher concentrations of HK and QA were associated with an increased risk of death (per doubling in concentrations: HK: HR = 1.80; 95%CI:1.47,2.21, QA: HR = 1.31; 95%CI:1.05,1.63). A higher kynurenine-to-tryptophan ratio, a marker of cell-mediated immune activation, was associated with an increased risk of death (per doubling: HR = 2.07; 95%CI:1.52,2.83). In conclusion, tryptophan-kynurenine pathway metabolites may be prognostic markers of survival in CRC patients.

mRNA-delivery of IDO1 suppresses T cell-mediated autoimmunity

Indoleamine-2,3-dioxygenase (IDO)1 degrades tryptophan, obtained through dietary intake, into immunoregulatory metabolites of the kynurenine pathway. Deficiency or blockade of IDO1 results in the enhancement of autoimmune severity in rodent models and increased susceptibility to developing autoimmunity in humans. Despite this, therapeutic modalities that leverage IDO1 for the treatment of autoimmunity remain limited. Here, we use messenger (m)RNA formulated in lipid nanoparticles (LNPs) to deliver a human IDO1 variant containing the myristoylation site of Src to anchor the protein to the inner face of the plasma membrane. This membrane-anchored IDO1 has increased protein production, leading to increased metabolite changes, and ultimately ameliorates disease in three models of T cell-mediated autoimmunity: experimental autoimmune encephalomyelitis (EAE), rat collagen-induced arthritis (CIA), and acute graft-versus-host disease (aGVHD). The efficacy of IDO1 is correlated with hepatic expression and systemic tryptophan depletion. Thus, the delivery of membrane-anchored IDO1 by mRNA suppresses the immune response in several well-characterized models of autoimmunity.

Integrated analyses reveal IDO1 as a prognostic biomarker coexpressed with PD-1 on tumor-associated macrophages in esophageal squamous cell carcinoma

Background

Inhibition of indolamine-2,3-dioxygenase 1 (IDO1) has been proposed as a promising strategy for cancer immunotherapy; however, it has failed in clinical trials. Macrophages in the tumor microenvironment (TME) contribute to immune escape and serve as potential therapeutic targets. This study investigated the expression pattern of IDO1 in TME and its impact on prognosis and therapeutic response of patients with esophageal squamous cell carcinoma (ESCC).

Methods

RNA sequencing data from 95 patients with ESCC from The Cancer Genome Atlas (TCGA) database were used to explore the prognostic value of IDO1. Bioinformatics tools were used to estimate scores for stromal and immune cells in tumour tissues, abundance of eight immune cell types in TME, and sensitivity of chemotherapeutic drugs and immune checkpoint (IC) blockage. The results were validated using digitalized immunohistochemistry and multiplexed immunofluorescence in ESCC tissue samples obtained from our clinical center.

Results

TCGA and validation data suggested that high expression of IDO1 was associated with poor patient survival, and IDO1 was an independent prognostic factor. IDO1 expression positively correlated with macrophages in TME and PDCD1 within diverse IC genes. Single-cell RNA sequencing data analysis and multiplexed immunofluorescence verified the coexpression of IDO1 and PD-1 in tumor-associated macrophages (TAMs). Patients with high IDO1 expression showed increased sensitivity to various chemotherapeutic drugs, while were more likely to resist IC blockage.

Conclusion

This study identifies IDO1 as an independent prognostic indicator of OS in patients with ESCC, reveals a compelling connection of IDO1, PD-1, and TAMs, and explores the sensitivity of patients with high IDO1 expression to chemotherapeutic drugs and their resistance to IC blockade. These findings open new avenues for potential targets in ESCC immunotherapy.

Suppression of certain intestinal microbiota metabolites may lead to gestational diabetes in mice fed a high-fat diet

Background

We aim to establish a gestational diabetes mellitus (GDM) mouse model with mice fed with a high-fat diet (HFD) in comparison with pregnant mice with normal blood glucose levels to investigate the role of intestinal microbiota in the development of HFD-induced GDM.

Methods

We divided healthy 6-week-old female C57BL mice into an HFD-induced GDM group and a normal diet group. Their bacterial flora and metabolites in intestinal fecal exosomes were co-analyzed using 16 s multi-region sequencing and compared.

Findings

Alpha (α) diversity was lower within the model group compared to the control group. Beta (β) diversity was significantly different between the two groups. The relative abundances of Lactobacillus, Actinomyces, Rothia, and Bacteroidetes were significantly different between the two groups. Fermentation and nitrate consumption were significantly higher in the GDM group. Multiple bacteria were associated with glycerophosphocholine, S-methyl-5'-thioadenosine, quinolinate, galactinol, deoxyadenosine, DL-arginine, and 2-oxoadenic acid.

Interpretation

Imbalances in the production of Lactobacillus, Bacteroidetes, Actinomyces, and Rothia and their related metabolites may lead to metabolic disturbances in GDM. These indicators may be used to assess changes affecting the intestinal microbiota during pregnancy and thus help modulate diet and alter blood glucose.

Roles of the tumor microenvironment in the resistance to programmed cell death protein 1 inhibitors in patients with gastric cancer

Despite the continuous developments and advancements in the treatment of gastric cancer (GC), which is one of the most prevalent types of cancer in China, the overall survival is still poor for most patients with advanced GC. In recent years, with the progress in tumor immunology research, attention has shifted toward immunotherapy as a therapeutic approach for GC. Programmed cell death protein 1 (PD-1) inhibitors, as novel immunosuppressive medications, have been widely utilized in the treatment of GC. However, many patients are still resistant to PD-1 inhibitors and experience recurrence in the advanced stages of PD-1 immunotherapy. To reduce the occurrence of drug resistance and recurrence in GC patients receiving PD-1 immunotherapy, to maximize the clinical activity of immunosuppressive drugs, and to elicit a lasting immune response, it is essential to research the tumor microenvironment mechanisms leading to PD-1 inhibitor resistance in GC patients. This article reviews the progress in studying the factors influencing the resistance to PD-1 inhibitors in the GC tumor microenvironment, aiming to provide insights and a basis for reducing resistance to PD-1 inhibitors for GC patients in the future.

The aryl hydrocarbon receptor and FOS mediate cytotoxicity induced by Acinetobacter baumannii

Acinetobacter baumannii is a pathogenic and multidrug-resistant Gram-negative bacterium that causes severe nosocomial infections. To better understand the mechanism of pathogenesis, we compare the proteomes of uninfected and infected human cells, revealing that transcription factor FOS is the host protein most strongly induced by A. baumannii infection. Pharmacological inhibition of FOS reduces the cytotoxicity of A. baumannii in cell-based models, and similar results are also observed in a mouse infection model. A. baumannii outer membrane vesicles (OMVs) are shown to activate the aryl hydrocarbon receptor (AHR) of host cells by inducing the host enzyme tryptophan-2,3-dioxygenase (TDO), producing the ligand kynurenine, which binds AHR. Following ligand binding, AHR is a direct transcriptional activator of the FOS gene. We propose that A. baumannii infection impacts the host tryptophan metabolism and promotes AHR- and FOS-mediated cytotoxicity of infected cells.

Deciphering the microbial map and its implications in the therapeutics of neurodegenerative disorder

Every facet of biological anthropology, including development, ageing, diseases, and even health maintenance, is influenced by gut microbiota's significant genetic and metabolic capabilities. With current advancements in sequencing technology and with new culture-independent approaches, researchers can surpass older correlative studies and develop mechanism-based studies on microbiome-host interactions. The microbiota-gut-brain axis (MGBA) regulates glial functioning, making it a possible target for the improvement of development and advancement of treatments for neurodegenerative diseases (NDDs). The gut-brain axis (GBA) is accountable for the reciprocal communication between the gastrointestinal and central nervous system, which plays an essential role in the regulation of physiological processes like controlling hunger, metabolism, and various gastrointestinal functions. Lately, studies have discovered the function of the gut microbiome for brain health-different microbiota through different pathways such as immunological, neurological and metabolic pathways. Additionally, we review the involvement of the neurotransmitters and the gut hormones related to gut microbiota. We also explore the MGBA in neurodegenerative disorders by focusing on metabolites. Further, targeting the blood-brain barrier (BBB), intestinal barrier, meninges, and peripheral immune system is investigated. Lastly, we discuss the therapeutics approach and evaluate the pre-clinical and clinical trial data regarding using prebiotics, probiotics, paraprobiotics, fecal microbiota transplantation, personalised medicine, and natural food bioactive in NDDs. A comprehensive study of the GBA will felicitate the creation of efficient therapeutic approaches for treating different NDDs.

Tryptophan metabolism as a 'reflex' feature of neuroimmune communication: Sensor and effector functions for the indoleamine-2, 3-dioxygenase kynurenine pathway

Although the central nervous system (CNS) and immune system were regarded as independent entities, it is now clear that immune system cells can influence the CNS, and neuroglial activity influences the immune system. Despite the many clinical implications for this 'neuroimmune interface', its detailed operation at the molecular level remains unclear. This narrative review focuses on the metabolism of tryptophan along the kynurenine pathway, since its products have critical actions in both the nervous and immune systems, placing it in a unique position to influence neuroimmune communication. In particular, since the kynurenine pathway is activated by pro-inflammatory mediators, it is proposed that physical and psychological stressors are the stimuli of an organismal protective reflex, with kynurenine metabolites as the effector arm co-ordinating protective neural and immune system responses. After a brief review of the neuroimmune interface, the general perception of tryptophan metabolism along the kynurenine pathway is expanded to emphasize this environmentally driven perspective. The initial enzymes in the kynurenine pathway include indoleamine-2,3-dioxygenase (IDO1), which is induced by tissue damage, inflammatory mediators or microbial products, and tryptophan-2,3-dioxygenase (TDO), which is induced by stress-induced glucocorticoids. In the immune system, kynurenic acid modulates leucocyte differentiation, inflammatory balance and immune tolerance by activating aryl hydrocarbon receptors and modulates pain via the GPR35 protein. In the CNS, quinolinic acid activates N-methyl-D-aspartate (NMDA)-sensitive glutamate receptors, whereas kynurenic acid is an antagonist: the balance between glutamate, quinolinic acid and kynurenic acid is a significant regulator of CNS function and plasticity. The concept of kynurenine and its metabolites as mediators of a reflex coordinated protection against stress helps to understand the variety and breadth of their activity. It should also help to understand the pathological origin of some psychiatric and neurodegenerative diseases involving the immune system and CNS, facilitating the development of new pharmacological strategies for treatment.

Exploring the kynurenine pathway in mild traumatic brain injury: A longitudinal study

A potential source of novel biomarkers for mTBI is the kynurenine pathway (KP), a metabolic pathway of tryptophan (Trp), that is up-regulated by neuroinflammation and stress. Considering that metabolites of the KP (kynurenines) are implicated in various neuropsychiatric diseases, exploration of this pathway could potentially bridge the gap between physiological and psychological factors in the recovery process after mTBI. This study, therefore, set out to characterize the KP after mTBI and to examine associations with long-term outcome. Patients were prospectively recruited at the emergency department (ED), and blood samples were obtained in the acute phase (<24 h; N = 256) and at 1-month follow-up (N = 146). A comparison group of healthy controls (HC; N = 32) was studied at both timepoints. Trp, kynurenines, and interleukin (IL)-6 and IL-10 were quantified in plasma. Clinical outcome was measured at six months post-injury. Trp, xanthurenic acid (XA), and picolinic acid (PA) were significantly reduced in patients with mTBI relative to HC, corrected for age and sex. For Trp (d = -0.57 vs. d = -0.29) and XA (d = -0.98 vs. d = -0.32), larger effects sizes were observed during the acute phase compared to one-month follow-up, while for PA (d = -0.49 vs. d = -0.52) effect sizes remained consistent. Findings for other kynurenines (e.g., kynurenine, kynurenic acid, and quinolinic acid) were non-significant after correction for multiple testing. Within the mTBI group, lower acute Trp levels were significantly related to incomplete functional recovery and higher depression scores at 6 months post-injury. No significant relationships were found for Trp, XA, and PA with IL-6 or IL-10 concentrations. In conclusion, our findings indicate that perturbations of the plasma KP in the hyperacute phase of mTBI and 1 month later are limited to the precursor Trp, and glutamate system modulating kynurenines XA and PA. Correlations between acute reductions of Trp and unfavorable outcomes may suggest a potential substrate for pharmacological intervention.

Elucidating the pivotal role of TSPO in porphyrin-related cellular processes, in Bacillus cereus

A structural homolog of the mammalian TSPO has been identified in the human pathogen Bacillus cereus. BcTSPO, in its recombinant form, has previously been shown to bind and degrade porphyrins. In this study, we generated a ΔtspO mutant strain in B. cereus ATCC 14579 and assessed the impact of the absence of BcTSPO on cellular proteomics and physiological characteristics. The proteomic analysis revealed correlations between the lack of BcTSPO and the observed growth defects, increased oxygen consumption, ATP deficiency, heightened tryptophan catabolism, reduced motility, and impaired biofilm formation in the ΔtspO mutant strain. Our results also suggested that BcTSPO plays a crucial role in regulating intracellular levels of metabolites from the coproporphyrin-dependent branch of the heme biosynthetic pathway. This regulation potentially underlies alterations in the metabolic landscape, emphasizing the pivotal role of BcTSPO in B. cereus aerobic metabolism. Notably, our study unveils, for the first time, the involvement of TSPO in tryptophan metabolism. These findings underscore the multifaceted role of TSPO, not only in metabolic pathways but also potentially in the microorganism's virulence mechanisms.

Investigating the Role of Serotonin Levels in Cognitive Impairments Associated with Long COVID-19

This study aimed to investigate the activation of the inflammation process, triggered as an immune response to combat the invasion by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). SARS-CoV-2 is a highly transmissible and pathogenic coronavirus that emerged in late 2019 and has caused a pandemic of acute respiratory disease, named 'coronavirus disease 2019' (COVID-19). Several mechanisms contribute to the reduction in serotonin levels, such as the impaired absorption of dietary tryptophan, hindered serotonin transport via platelets, and increased activity of an enzyme responsible for breaking down serotonin. Individuals seeking treatment for long COVID-19 had lower serotonin levels in their blood than those who had fully recovered from the infection. Furthermore, patients with long COVID-19 also had reduced tryptophan levels. The potential benefits of dietary supplementation with tryptophan or the use of selective serotonin reuptake inhibitors (SSRIs) to improve cognitive impairments and depressive and anxiety disorders in long-term COVID-19 patients. The findings support the immune response's pivotal role in modulating serotonin levels and further highlight the intricate connection between the immune system and neurotransmitter regulation.

Homeostatic regulation of NAD(H) and NADP(H) in cells

Nicotinamide adenine dinucleotide (NAD+)/reduced NAD+ (NADH) and nicotinamide adenine dinucleotide phosphate (NADP+)/reduced NADP+ (NADPH) are essential metabolites involved in multiple metabolic pathways and cellular processes. NAD+ and NADH redox couple plays a vital role in catabolic redox reactions, while NADPH is crucial for cellular anabolism and antioxidant responses. Maintaining NAD(H) and NADP(H) homeostasis is crucial for normal physiological activity and is tightly regulated through various mechanisms, such as biosynthesis, consumption, recycling, and conversion between NAD(H) and NADP(H). The conversions between NAD(H) and NADP(H) are controlled by NAD kinases (NADKs) and NADP(H) phosphatases [specifically, metazoan SpoT homolog-1 (MESH1) and nocturnin (NOCT)]. NADKs facilitate the synthesis of NADP+ from NAD+, while MESH1 and NOCT convert NADP(H) into NAD(H). In this review, we summarize the physiological roles of NAD(H) and NADP(H) and discuss the regulatory mechanisms governing NAD(H) and NADP(H) homeostasis in three key aspects: the transcriptional and posttranslational regulation of NADKs, the role of MESH1 and NOCT in maintaining NAD(H) and NADP(H) homeostasis, and the influence of the circadian clock on NAD(H) and NADP(H) homeostasis. In conclusion, NADKs, MESH1, and NOCT are integral to various cellular processes, regulating NAD(H) and NADP(H) homeostasis. Dysregulation of these enzymes results in various human diseases, such as cancers and metabolic disorders. Hence, strategies aiming to restore NAD(H) and NADP(H) homeostasis hold promise as novel therapeutic approaches for these diseases.

"Inflamed" depression: A review of the interactions between depression and inflammation and current anti-inflammatory strategies for depression

Depression is a common mental disorder, the effective treatment of which remains a challenging issue worldwide. The clinical pathogenesis of depression has been deeply explored, leading to the formulation of various pathogenic hypotheses. Among these, the monoamine neurotransmitter hypothesis holds a prominent position, yet it has significant limitations as more than one-third of patients do not respond to conventional treatments targeting monoamine transmission disturbances. Over the past few decades, a growing body of research has highlighted the link between inflammation and depression as a potential key factor in the pathophysiology of depression. In this review, we first summarize the relationship between inflammation and depression, with a focus on the pathophysiological changes mediated by inflammation in depression. The mechanisms linking inflammation to depression as well as multiple anti-inflammatory strategies are also discussed, and their efficacy and safety are assessed. This review broadens the perspective on specific aspects of using anti-inflammatory strategies for treating depression, laying the groundwork for advancing precision medicine for individuals suffering from "inflamed" depression.

Mechanisms of Tolerance Induction in Liver Transplantation: Lessons Learned from Fetomaternal Tolerance, Autoimmunity and Tumor Immunity

Since the first published report of experimental kidney transplantation in dogs in 1902, there were many experimental and clinical trials of organ transplantation, with many sacrifices. After the establishment of the surgical technique and the discovery of immunosuppressive drugs, transplantation became the definitive treatment strategy for patients with terminal organ failure. However, this is not a common therapy method due to the difficulty of solving the fundamental issues behind organ transplantation, including the shortage of donor graft, potential risks of transplant surgery and economic capability. The pre- and post-transplant management of recipients is another critical issue that may affect transplant outcome. Most liver transplant recipients experience post-transplant complications, including infection, acute/chronic rejection, metabolic syndrome and the recurrence of hepatocellular carcinoma. Therefore, the early prediction and diagnosis of these complications may improve overall and disease-free survival. Furthermore, how to induce operational tolerance is the key to achieving the ultimate goal of transplantation. In this review, we focus on liver transplantation, which is known to achieve operational tolerance in some circumstances, and the mechanical similarities and differences between liver transplant immunology and fetomaternal tolerance, autoimmunity or tumor immunity are discussed.

Amino acid metabolism in kidney health and disease

Amino acids form peptides and proteins and are therefore considered the main building blocks of life. The kidney has an important but under-appreciated role in the synthesis, degradation, filtration, reabsorption and excretion of amino acids, acting to retain useful metabolites while excreting potentially harmful and waste products from amino acid metabolism. A complex network of kidney transporters and enzymes guides these processes and moderates the competing concentrations of various metabolites and amino acid products. Kidney amino acid metabolism contributes to gluconeogenesis, nitrogen clearance, acid-base metabolism and provision of fuel for tricarboxylic acid cycle and urea cycle intermediates, and is thus a central hub for homeostasis. Conversely, kidney disease affects the levels and metabolism of a variety of amino acids. Here, we review the metabolic role of the kidney in amino acid metabolism and describe how different diseases of the kidney lead to aberrations in amino acid metabolism. Improved understanding of the metabolic and communication routes that are affected by disease could provide new mechanistic insights into the pathogenesis of kidney diseases and potentially enable targeted dietary or pharmacological interventions.

The Biology and Biochemistry of Kynurenic Acid, a Potential Nutraceutical with Multiple Biological Effects

Kynurenic acid (KYNA) is an antioxidant degradation product of tryptophan that has been shown to have a variety of cytoprotective, neuroprotective and neuronal signalling properties. However, mammalian transporters and receptors display micromolar binding constants; these are consistent with its typically micromolar tissue concentrations but far above its serum/plasma concentration (normally tens of nanomolar), suggesting large gaps in our knowledge of its transport and mechanisms of action, in that the main influx transporters characterized to date are equilibrative, not concentrative. In addition, it is a substrate of a known anion efflux pump (ABCC4), whose in vivo activity is largely unknown. Exogeneous addition of L-tryptophan or L-kynurenine leads to the production of KYNA but also to that of many other co-metabolites (including some such as 3-hydroxy-L-kynurenine and quinolinic acid that may be toxic). With the exception of chestnut honey, KYNA exists at relatively low levels in natural foodstuffs. However, its bioavailability is reasonable, and as the terminal element of an irreversible reaction of most tryptophan degradation pathways, it might be added exogenously without disturbing upstream metabolism significantly. Many examples, which we review, show that it has valuable bioactivity. Given the above, we review its potential utility as a nutraceutical, finding it significantly worthy of further study and development.

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

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

Kynurenine pathway in type 2 diabetes: Role of metformin

The Kynurenine pathway (KP) which is involved in the synthesis of nicotinamide adenine dinucleotide (NAD) from tryptophan (Trp) is intricate in the development of insulin resistance (IR) and type 2 diabetes (T2D). Inflammatory reactions in response to cardiometabolic disorders can induce the development of IR through the augmentation of KP. However, kynurenine (KYN), a precursor of kynurenic acid (KA) is increased following physical exercise and involved in the reduction of IR. Consequently, KP metabolites KA and KYN have anti-diabetogenic effects while other metabolites have diabetogenic effects. KP modulators, either inhibitors or activators, affect glucose homeostasis and insulin sensitivity in T2D in a bidirectional way, either protective or detrimental, that is not related to the KP effect. However, metformin through inhibition of inflammatory signaling pathways can reduce the activation of KP in T2D. These findings indicated a strong controversy regarding the role of KP in T2D. Therefore, the objectives of this mini review were to clarify how KP induces the development of IR and T2D. In addition, this review aimed to find the mechanistic role of antidiabetic drug metformin on the KP, and how KP modulators affect the pathogenesis of T2D.

Chronic infusion of the tryptophan metabolite kynurenine increases mean arterial pressure in male Sprague-Dawley rats

Chronic kidney disease is the loss of renal function that can occur from aging or through a myriad of other disease states. Rising serum concentrations of kynurenine, a tryptophan metabolite, have been shown to correlate with increasing severity of chronic kidney disease. This study used chronic intravenous infusion in conscious male Sprague-Dawley rats to test the hypothesis that kynurenine can induce renal damage and promote alterations in blood pressure, heart rate, and decreased renal function. We found that kynurenine infusion increased mean arterial pressure, increased the maximum and minimum range of heart rate, decreased glomerular filtration rate, and induced kidney damage in a dose-dependent manner. This study shows that kynurenine infusion can promote kidney disease in healthy, young rats, implying that the increase in kynurenine levels associated with chronic kidney disease may establish a feed-forward mechanism that exacerbates the loss of renal function.NEW & NOTEWORTHY In humans, an elevated serum concentration of kynurenine has long been associated with negative outcomes in various disease states as well as in aging. However, it has been unknown whether these increased kynurenine levels are mediating the disorders or simply associated with them. This study shows that chronically infusing kynurenine can contribute to the development of hypertension and kidney impairment. The mechanism of this action remains to be determined in future studies.

Urine metabolite changes after cardiac surgery predict acute kidney injury

Background

Acute kidney injury (AKI) is a serious complication in patients undergoing cardiac surgery, with the underlying mechanism remaining elusive and a lack of specific biomarkers for cardiac surgery-associated AKI (CS-AKI).

Methods

We performed an untargeted metabolomics analysis of urine samples procured from a cohort of patients with or without AKI at 6 and 24 h following cardiac surgery. Based on the differential urinary metabolites discovered, we further examined the expressions of the key metabolic enzymes that regulate these metabolites in kidney during AKI using a mouse model of ischemia-reperfusion injury (IRI) and in hypoxia-treated tubular epithelial cells (TECs).

Results

The urine metabolomic profiles in AKI patients were significantly different from those in non-AKI patients, including upregulation of tryptophan metabolism- and aerobic glycolysis-related metabolites, such as l-tryptophan and d-glucose-1-phosphate, and downregulation of fatty acid oxidation (FAO) and tricarboxylic acid (TCA) cycle-related metabolites. Spearman correlation analysis showed that serum creatinine was positively correlated with urinary l-tryptophan and indole, which had high accuracy for predicting AKI. In animal experiments, we demonstrated that the expression of rate-limiting enzymes in glycolysis, such as hexokinase II (HK2), was significantly upregulated during renal IRI. However, the TCA cycle-related key enzyme citrate synthase was significantly downregulated after IRI. In vitro, hypoxia induced downregulation of citrate synthase in TECs. In addition, FAO-related gene peroxisome proliferator-activated receptor alpha (PPARα) was remarkably downregulated in kidney during renal IRI.

Conclusion

This study presents urinary metabolites related to CS-AKI, indicating the rewiring of the metabolism in kidney during AKI, identifying potential AKI biomarkers.

Exploring the effect of prolonged fasting on kynurenine pathway metabolites and stress markers in healthy male individuals

BACKGROUND/OBJECTIVES

Prolonged fasting triggers a stress response within the human body. Our objective was to investigate the impact of prolonged fasting, in conjunction with stress, on kynurenine pathway metabolites.

SUBJECTS/METHODS

Healthy males were divided into fasting group (zero-calorie-restriction) for 6 days (FAST, n = 14), and control group (CON, n = 10). Blood and saliva samples were collected at baseline, Day 2, Day 4, Day 6 during fasting period, and 1 week after resuming regular diet. Plasma levels of kynurenine pathway metabolites were measured using ultra-performance liquid chromatography-mass spectrometry (UPLC-MS/MS). Plasma and salivary samples were analyzed for stress markers.

RESULTS

A pronounced activation of the kynurenine pathway in individuals on FAST trial was revealed. Concentrations of picolinic acid (PIC), kynurenic acid (KYNA) and 3-hydroxykynurenine (3-HK) were significantly increased, with peak levels observed on Day 6 (P < 0.0001). Conversely, concentrations of tryptophan (TRP) and quinolinic acid (QUIN) decreased (P < 0.0001), while kynurenine (KYN) and nicotinamide (NAM) levels remained stable. Cortisol and noradrenaline concentrations remained unchanged. However, adrenaline levels significantly increased on Day 4 within FAST compared to CON (P = 0.005). Notably, all deviations in kynurenine pathway metabolite levels returned to baseline values upon resuming regular diet following the 6-day fasting regimen, even when weight and BMI parameters were not restored.

CONCLUSIONS

Extended fasting over 6 days induces the kynurenine pathway and has minimal effects on stress markers. Restoration of metabolite concentrations upon regular feeding implies rapid adaptation of the kynurenine pathway synthetic enzymes to maintain homeostasis when faced with perturbations.

Essential oils ameliorate the intestinal damages induced by nonylphenol exposure by modulating tryptophan metabolism and activating aryl hydrocarbon receptor via gut microbiota regulation

Nonylphenol (NP) is a ubiquitous endocrine disruptor that persists in the environment and can significantly contribute to serious health hazards, particularly intestinal barrier injury. Plant essential oils (EOs) have recently gained widespread interest due to their potential for improving intestinal health. However, the precise mechanism and protective effects of EOs ameliorating the intestinal damages induced by NP exposure remain unclear. To clarify the potential mechanism and protective impact of EOs against intestinal injury induced by NP, a total of 144 one-day-old male ducks were randomly allocated to four groups: CON (basal diet), EO (basal diet + 200 mg/kg EOs), NP (basal diet + 40 mg/kg NP), and NPEO (basal diet + 200 mg/kg EOs + 40 mg/kg NP). The data revealed that NP exposure significantly damaged intestinal barrier, as evidenced by a reduction in the levels of tight junction gene expression and an increase in intestinal permeability. Additionally, it disturbed gut microbiota, as well as interfered with tryptophan (Trp) metabolism. The NP-induced disorder of Trp metabolism restrained the activation of aryl hydrocarbon receptor (AhR) and resulted in decreased the expression levels of CYP1A1, IL-22, and STAT3 genes, which were alleviated after treatment with EOs. Taken together, NP exposure resulted in impairment of the intestinal barrier function, disruption of gut microbiota, and disturbances in Trp metabolism. Dietary EOs supplementation alleviated the intestinal barrier injury induced by NP through the Trp/AhR/IL-22 signaling pathway.

Alcohol- and Low-Iron Induced Changes in Antioxidant and Energy Metabolism Associated with Protein Lys Acetylation

Understanding the role of iron in ethanol-derived hepatic stress could help elucidate the efficacy of dietary or clinical interventions designed to minimize liver damage from chronic alcohol consumption. We hypothesized that normal levels of iron are involved in ethanol-derived liver damage and reduced dietary iron intake would lower the damage caused by ethanol. We used a pair-fed mouse model utilizing basal Lieber-DeCarli liquid diets for 22 weeks to test this hypothesis. In our mouse model, chronic ethanol exposure led to mild hepatic stress possibly characteristic of early-stage alcoholic liver disease, seen as increases in liver-to-body weight ratios. Dietary iron restriction caused a slight decrease in non-heme iron and ferritin (FeRL) expression while it increased transferrin receptor 1 (TfR1) expression without changing ferroportin 1 (FPN1) expression. It also elevated protein lysine acetylation to a more significant level than in ethanol-fed mice under normal dietary iron conditions. Interestingly, iron restriction led to an additional reduction in nicotinamide adenine dinucleotide (NAD+) and NADH levels. Consistent with this observation, the major mitochondrial NAD+-dependent deacetylase, NAD-dependent deacetylase sirtuin-3 (SIRT3), expression was significantly reduced causing increased protein lysine acetylation in ethanol-fed mice at normal and low-iron conditions. In addition, the detection of superoxide dismutase 1 and 2 levels (SOD1 and SOD2) and oxidative phosphorylation (OXPHOS) complex activities allowed us to evaluate the changes in antioxidant and energy metabolism regulated by ethanol consumption at normal and low-iron conditions. We observed that the ethanol-fed mice had mild liver damage associated with reduced energy and antioxidant metabolism. On the other hand, iron restriction may exacerbate certain activities of ethanol further, such as increased protein lysine acetylation and reduced antioxidant metabolism. This metabolic change may prove a barrier to the effectiveness of dietary reduction of iron intake as a preventative measure in chronic alcohol consumption.

Are Women with Polycystic Ovary Syndrome at Increased Risk of Alzheimer Disease? Lessons from Insulin Resistance, Tryptophan and Gonadotropin Disturbances and Their Link with Amyloid-Beta Aggregation

Alzheimer disease, the leading cause of dementia, and polycystic ovary syndrome, one of the most prevalent female endocrine disorders, appear to be unrelated conditions. However, studies show that both disease entities have common risk factors, and the amount of certain protein marker of neurodegeneration is increased in PCOS. Reports on the pathomechanism of both diseases point to the possibility of common denominators linking them. Dysregulation of the kynurenine pathway, insulin resistance, and impairment of the hypothalamic-pituitary-gonadal axis, which are correlated with amyloid-beta aggregation are these common areas. This article discusses the relationship between Alzheimer disease and polycystic ovary syndrome, with a particular focus on the role of disorders of tryptophan metabolism in both conditions. Based on a review of the available literature, we concluded that systemic changes occurring in PCOS influence the increased risk of neurodegeneration.

Risk of Fat Mass- and Obesity-Associated Gene-Dependent Obesogenic Programming by Formula Feeding Compared to Breastfeeding

It is the purpose of this review to compare differences in postnatal epigenetic programming at the level of DNA and RNA methylation and later obesity risk between infants receiving artificial formula feeding (FF) in contrast to natural breastfeeding (BF). FF bears the risk of aberrant epigenetic programming at the level of DNA methylation and enhances the expression of the RNA demethylase fat mass- and obesity-associated gene (FTO), pointing to further deviations in the RNA methylome. Based on a literature search through Web of Science, Google Scholar, and PubMed databases concerning the dietary and epigenetic factors influencing FTO gene and FTO protein expression and FTO activity, FTO's impact on postnatal adipogenic programming was investigated. Accumulated translational evidence underscores that total protein intake as well as tryptophan, kynurenine, branched-chain amino acids, milk exosomal miRNAs, NADP, and NADPH are crucial regulators modifying FTO gene expression and FTO activity. Increased FTO-mTORC1-S6K1 signaling may epigenetically suppress the WNT/β-catenin pathway, enhancing adipocyte precursor cell proliferation and adipogenesis. Formula-induced FTO-dependent alterations of the N6-methyladenosine (m6A) RNA methylome may represent novel unfavorable molecular events in the postnatal development of adipogenesis and obesity, necessitating further investigations. BF provides physiological epigenetic DNA and RNA regulation, a compelling reason to rely on BF.

Sex-specific associations of kynurenic acid with neopterin in Alzheimer's disease

BACKGROUND

Sex differences in neuroinflammation could contribute to women's increased risk of Alzheimer's disease (AD), providing rationale for exploring sex-specific AD biomarkers. In AD, dysregulation of the kynurenine pathway (KP) contributes to neuroinflammation and there is some evidence of sex differences in KP metabolism. However, the sex-specific associations between KP metabolism and biomarkers of AD and neuroinflammation need to be explored further.

METHODS

Here we investigate sex differences in cerebrospinal fluid concentrations of seven KP metabolites and sex-specific associations with established AD biomarkers and neopterin, an indicator of neuroinflammation. This study included 311 patients with symptomatic AD and 105 age-matched cognitively unimpaired (CU) controls, followed for up to 5 years.

RESULTS

We found sex differences in KP metabolites in the AD group, with higher levels of most metabolites in men, while there were no sex differences in the CU group. In line with this, more KP metabolites were significantly altered in AD men compared to CU men, and there was a trend in the same direction in AD women. Furthermore, we found sex-specific associations between kynurenic acid and the kynurenic acid/quinolinic acid ratio with neopterin, but no sex differences in the associations between KP metabolites and clinical progression.

DISCUSSION

In our cohort, sex differences in KP metabolites were restricted to AD patients. Our results suggest that dysregulation of the KP due to increased inflammation could contribute to higher AD risk in women.

Dendritic Cell - Fibroblast Crosstalk via TLR9 and AHR Signaling Drives Lung Fibrogenesis

Idiopathic pulmonary fibrosis (IPF) is characterized by progressive scarring and loss of lung function. With limited treatment options, patients succumb to the disease within 2-5 years. The molecular pathogenesis of IPF regarding the immunologic changes that occur is poorly understood. We characterize a role for non-canonical aryl-hydrocarbon receptor signaling (ncAHR) in dendritic cells (DCs) that leads to production of IL-6 and IL-17, promoting fibrosis. TLR9 signaling in myofibroblasts is shown to regulate production of TDO2 which converts tryptophan into the endogenous AHR ligand kynurenine. Mice with augmented ncAHR signaling were created by crossing floxed AHR exon-2 deletion mice (AHR Δex2 ) with mice harboring a CD11c-Cre. Bleomycin was used to study fibrotic pathogenesis. Isolated CD11c+ cells and primary fibroblasts were treated ex-vivo with relevant TLR agonists and AHR modulating compounds to study how AHR signaling influenced inflammatory cytokine production. Human datasets were also interrogated. Inhibition of all AHR signaling rescued fibrosis, however, AHR Δex2 mice treated with bleomycin developed more fibrosis and DCs from these mice were hyperinflammatory and profibrotic upon adoptive transfer. Treatment of fibrotic fibroblasts with TLR9 agonist increased expression of TDO2. Study of human samples corroborate the relevance of these findings in IPF patients. We also, for the first time, identify that AHR exon-2 floxed mice retain capacity for ncAHR signaling.

Interstitial Foci Expression of Indoleamine 2,3-Dioxygenase 1: A Potential Biomarker for Kidney Transplant Rejection

(1) Background: Kidney transplantation is the best therapy for patients with end-stage renal disease, but the risk of rejection complicates it. Indoleamine 2,3-dioxygenase 1 (IDO1), an enzyme involved in immune response modulation, has been suggested to play a role in transplant immunological injury. The aim of the study was to explore the expression of IDO1 in the interstitial foci of transplanted kidneys and its potential association with rejection episodes. (2) Methods: This retrospective study analysed kidney transplant biopsies from 121 patients, focusing on IDO1 expression in interstitial foci. Immunohistochemistry was used to detect IDO1, and patients were categorised based on IDO1 presence (IDO1-IF positive or negative). The incidence of rejection was compared between these groups. (3) Results: Patients with IDO1 expression in interstitial foci (IDO1-IF(+)) exhibited higher incidences of rejection 46/80 (57.5%) vs. 10/41 (24.34%) patients compared to IDO1-IF(-) patients, which was statistically significant with p = 0.0005. The analysis of antibody-mediated rejection showed that IDO1-IF(+) patients developed AMR at 12/80 (15%), while only 1 IDO1-IF(-) negative patient did (2,44%), with p = 0.035. T-cell-mediated rejection was also more common in IDO1-IF(+) patients 43/80 (53.75%) than in IDO1-IF(-) patients 7/41 (17.07%), with p = 0.0001. (4) Conclusions: IDO1 expression in interstitial foci of renal transplant biopsies is associated with a higher incidence of rejection, suggesting that IDO1 could serve as a potential biomarker for transplant rejection. These findings highlight the importance of IDO1 in immune regulation and its potential utility in improving the management of kidney transplant recipients.