A novel, robust method for quantification of multiple kynurenine pathway metabolites in the cerebrospinal fluid

Effects of rumen-protected niacin on inflammatory response to repeated intramammary lipopolysaccharide challenges

Nutritional strategies that improve an animal's resilience to various challenges may improve animal health and welfare. One such nutrient is niacin which has reduced inflammation in mice, humans, and swine; however, niacin's anti-inflammatory effects have not been investigated in cattle. Our objective was to determine whether rumen-protected niacin (RPN) alters lactating dairy cows' inflammatory response to intramammary lipopolysaccharide (LPS) challenges, whether RPN resulted in any carry-over effects, and whether repeated LPS challenges result in signs of immune tolerance or innate immune training. Twenty healthy, late-lactation Holstein cows (232 ± 65 d in milk; 39 ± 5.8 kg/d of milk) were enrolled in a randomized complete block experiment which lasted 70 d. Cows received 26 g/d of RPN or no top-dress (CON) for the first 42 d of the experiment. During the final milking of d 27 and 55, cows were challenged in their rear-right mammary gland (RR) with 100 µg of LPS suspended in 5 mL of phosphate buffered saline. Milk yield, milk conductivity, and feed intake were measured daily. Milk composition was measured on d 14, 23, 24, 30, 37, 45, and 52. Blood samples were collected at 0, 8, 12, 24, 48, 72, 96, and 120 h after each LPS challenge, whereas RR quarter milk samples were collected at 0, 8, 16, 24, 48, 72, 96, 120, 144, and 168 h after each LPS challenge. Body temperature was measured continuously during each challenge with an intravaginal thermometer. Linear mixed models with repeated measures were used to analyze the results. Before LPS challenge, RPN did not affect feed intake or milk production, but it reduced SCS (1.24 ± 0.41 vs. 0.05 ± 0.45). After challenge, RPN did not affect feed intake, milk production, milk composition, SCS, body temperature, plasma glucose, or plasma insulin concentrations. Our results suggest RPN reduced peak plasma haptoglobin and lipopolysaccharide binding protein (LBP) during the 1st LPS challenge. Plasma haptoglobin tended to be less after the 2nd challenge for cows previously supplemented RPN while LBP was similar for each treatment group after the 2nd challenge. The 2nd LPS challenge resulted in decreased plasma haptoglobin compared with the 1st LPS challenge, suggestive of tolerance but it also induced a greater peak SCS than the 1st LPS challenge. Our results suggest that repeated LPS challenges promote a systemic tolerance but heightened local response to LPS-induced mastitis. Feeding RPN reduced SCS before challenge and reduced plasma acute phase proteins after challenge suggesting that RPN may reduce systemic inflammation without altering the local inflammatory responses.

A Novel RP-UHPLC-MS/MS Approach for the Determination of Tryptophan Metabolites Derivatized with 2-Bromo-4'-Nitroacetophenone

Many biologically active metabolites of the essential amino acid L-tryptophan (Trp) are associated with different neurodegenerative diseases and neurological disorders. Precise and reliable methods for their determination are needed. Variability in their physicochemical properties makes the analytical process challenging. In this case, chemical modification of analyte derivatization could come into play. Here, we introduce a novel fast reversed-phase ultra-high-performance liquid chromatography (RP-UHPLC) coupled with tandem mass spectrometry (MS/MS) method for the determination of Trp and its ten metabolites in human plasma samples after derivatization with 2-bromo-4'-nitroacetophenone (BNAP). The derivatization procedure was optimized in terms of incubation time, temperature, concentration, and volume of the derivatization reagent. Method development comprises a choice of a suitable stationary phase, mobile phase composition, and gradient elution optimization. The developed method was validated according to the ICH guidelines. Results of all validation parameters were within the acceptance criteria of the guideline, i.e., intra- and inter-day precision (expressed as relative standard deviation; RSD) were in the range of 0.5-8.2% and 2.3-7.4%, accuracy was in the range of 93.3-109.7% and 94.7-110.1%, limits of detection (LODs) were in the range of 0.15-9.43 ng/mL, coefficients of determination (R2) were higher than 0.9906, and carryovers were, in all cases, less than 8.8%. The practicability of the method was evaluated using the blue applicability grade index (BAGI) with a score of 65. Finally, the developed method was used for the analysis of Alzheimer's disease and healthy control plasma to prove its applicability. Statistical analysis revealed significant changes in picolinic acid (PA), anthranilic acid (AA), 5 hydroxyindole-3-acetic acid (5-OH IAA), and quinolinic acid (QA) concentration levels. This could serve as the basis for future studies that will be conducted with a large cohort of patients.

Critical Factors in Sample Collection and Preparation for Clinical Metabolomics of Underexplored Biological Specimens

This review article compiles critical pre-analytical factors for sample collection and extraction of eight uncommon or underexplored biological specimens (human breast milk, ocular fluids, sebum, seminal plasma, sweat, hair, saliva, and cerebrospinal fluid) under the perspective of clinical metabolomics. These samples are interesting for metabolomics studies as they reflect the status of living organisms and can be applied for diagnostic purposes and biomarker discovery. Pre-collection and collection procedures are critical, requiring protocols to be standardized to avoid contamination and bias. Such procedures must consider cleaning the collection area, sample stimulation, diet, and food and drug intake, among other factors that impact the lack of homogeneity of the sample group. Precipitation of proteins and removal of salts and cell debris are the most used sample preparation procedures. This review intends to provide a global view of the practical aspects that most impact results, serving as a starting point for the designing of metabolomic experiments.

UPLC-MS/MS method for quantitative determination of the advanced glycation endproducts Nε-(carboxymethyl)lysine and Nε-(carboxyethyl)lysine

During blood storage, red blood cells (RBCs) undergo physical, chemical, and metabolic changes that may contribute to post-transfusion complications. Due to the hyperglycemic environment of typical solutions used for RBC storage, the formation of advanced glycation endproducts (AGEs) on the stored RBCs has been implicated as a detrimental chemical change during storage. Unfortunately, there are limited studies involving quantitative determination and differentiation of carboxymethyl-lysine (CML) and carboxyethyl-lysine (CEL), two commonly formed AGEs, and no reported studies comparing these AGEs in experimental storage solutions. In this study, CML and CEL were identified and quantified on freshly drawn blood samples in two types of storage solutions, standard additive solution 1 (AS-1) and a normoglycemic version of AS-1 (AS-1N). To facilitate detection of the AGEs, a novel method was developed to reliably extract AGEs from RBCs, provide Food and Drug Administration (FDA) bioanalytical guidance criteria, and enable acceptable selectivity for these analytes. Ultra-performance liquid chromatography with tandem mass spectrometry (UPLC-MS/MS) was utilized to identify and quantify the AGEs. Results show this method is accurate, precise, has minimal interferences or matrix effects, and overcomes the issue of detecting AGE byproducts. Importantly, AGEs can be detected and quantified in both types of blood storage solutions (AS-1 and AS-1N), thereby enabling long-term (6 weeks) blood storage related studies.

Normative Data on Serum and Plasma Tryptophan and Kynurenine Concentrations from 8089 Individuals Across 120 Studies: A Systematic Review and Meta-Analysis

In this systematic review and meta-analysis, a normative dataset is generated from the published literature on the kynurenine pathway in control participants extracted from case-control and methodological validation studies. Study characteristics were mapped, and studies were evaluated in terms of analytical rigour and methodological validation. Meta-analyses of variance between types of instruments, sample matrices and metabolites were conducted. Regression analyses were applied to determine the relationship between metabolite, sample matrix, biological sex, participant age and study age. The grand mean concentrations of tryptophan in the serum and plasma were 60.52 ± 15.38 μM and 51.45 ± 10.47 μM, respectively. The grand mean concentrations of kynurenine in the serum and plasma were 1.96 ± 0.51 μM and 1.82 ± 0.54 μM, respectively. Regional differences in metabolite concentrations were observed across America, Asia, Australia, Europe and the Middle East. Of the total variance within the data, mode of detection (MOD) accounted for up to 2.96%, sample matrix up to 3.23%, and their interaction explained up to 1.53%; the latter of which was determined to be negligible. This review was intended to inform future empirical research and method development studies and successfully synthesised pilot data. The pilot data reported in this study will inform future precision medicine initiatives aimed at targeting the kynurenine pathway by improving the availability and quality of normative data.

A liquid chromatography-tandem mass spectrometry based method for the quantification of adenosine nucleotides and NAD precursors and products in various biological samples

Adenine nucleotides (AN) are ubiquitous metabolites that regulate cellular energy metabolism and modulate cell communication and inflammation. To understand how disturbances in AN balance arise and affect cellular function, robust quantification techniques for these metabolites are crucial. However, due to their hydrophilicity, simultaneous quantification of AN across various biological samples has been challenging. Here we present a hydrophilic interaction high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) based method for the quantification of 26 adenosine nucleotides and precursors as well as metabolic products of nicotinamide adenine dinucleotide (NAD) in plasma, liver, and adipose tissue samples as well as cell culture supernatants and cells. Method validation was performed with regard to linearity, accuracy, precision, matrix effects, and carryover. Finally, analysis of cell culture supernatants derived from intestinal organoids and RAW 264.7 cells illustrates that the here described method is a reliable and easy-to-use tool to quantify AN and opens up new avenues to understand the role of AN generation and breakdown for cellular functions.

Developmental Exposure to Kynurenine Affects Zebrafish and Rat Behavior

Proper nutrition and supplementation during pregnancy and breastfeeding are crucial for the development of offspring. Kynurenine (KYN) is the central metabolite of the kynurenine pathway and a direct precursor of other metabolites that possess immunoprotective or neuroactive properties, with the ultimate effect on fetal neurodevelopment. To date, no studies have evaluated the effects of KYN on early embryonic development. Thus, the aim of our study was to determine the effect of incubation of larvae with KYN in different developmental periods on the behavior of 5-day-old zebrafish. Additionally, the effects exerted by KYN administered on embryonic days 1-7 (ED 1-7) on the behavior of adult offspring of rats were elucidated. Our study revealed that the incubation with KYN induced changes in zebrafish behavior, especially when zebrafish embryos or larvae were incubated with KYN from 1 to 72 h post-fertilization (hpf) and from 49 to 72 hpf. KYN administered early during pregnancy induced subtle differences in the neurobehavioral development of adult offspring. Further research is required to understand the mechanism of these changes. The larval zebrafish model can be useful for studying disturbances in early brain development processes and their late behavioral consequences. The zebrafish-medium system may be applicable in monitoring drug metabolism in zebrafish.

Liquid chromatography-tandem mass spectrometry based simultaneous quantification of tryptophan, serotonin and kynurenine pathway metabolites in tissues and cell culture systems

BACKGROUND

Kynurenine and respective metabolites exhibit bioactivity as well as tryptophan, an essential amino acid, and the neurotransmitter serotonin. Dysregulations in the kynurenine pathway are involved in neurodegenerative/neuropsychiatric disorders and diabetes mellitus type 2 but also in cancer. Therefore, measurements of kynurenine-related metabolites will improve the general understanding for kynurenine pathway relevance in disease pathogenesis.

METHODS

Tryptophan, serotonin, picolinic acid, quinolinic acid, 3-OH-kynurenine, kynurenine, 3-OH-anthranilic acid, kynurenic acid, anthranilic acid as well as nicotinic acid and the redox cofactor NAD+ were analyzed in heterogeneous matrices by ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). After validation, the described method was applied for measurements of native metabolite concentrations in murine tissues and cellular systems including pathway-shift monitoring after treatment with the tryptophan-2,3-dioxygenase-inhibitor 680C91. In addition, the method was evaluated for its ability for integration into multi-omics approaches using a single sample metabolite extraction procedure.

RESULTS

A simple and sensitive UPLC-MS/MS method for simultaneous quantification of up to 10 kynurenine-related metabolites in four biological matrices was developed. Within a run time of 6.5 min, chromatographic separation of kynurenine-related metabolites, including the isomers nicotinic acid and picolinic acid, was achieved without derivatization. Validation parameters, including interday precision (<14.8%), mean accuracy (102.4% ± 12.9%) and linear detection ranges of more than three orders of magnitude, indicate method reliability. Depending the investigated sample matrix, the majority of metabolites were successfully detected and quantified in native murine and cell culture derived sample materials. Furthermore, the method allowed to monitor the impact of a tryptophan-2,3-dioxygenase-inhibitor on kynurenine pathway in a cellular system and is suitable for multi-assay analyses using aliquots from the same cell extract.

CONCLUSION

The described UPLC-MS/MS method provides a simple tool for the simultaneous quantification of kynurenine pathway metabolites. Due to its suitability for many physiological matrices, the method provides wide application for disease-related experimental settings.

Charting the Next Road Map for CSF Biomarkers in Alzheimer's Disease and Related Dementias

Clinical prediction of underlying pathologic substrates in people with Alzheimer's disease (AD) dementia or related dementia syndromes (ADRD) has limited accuracy. Etiologic biomarkers - including cerebrospinal fluid (CSF) levels of AD proteins and cerebral amyloid PET imaging - have greatly modernized disease-modifying clinical trials in AD, but their integration into medical practice has been slow. Beyond core CSF AD biomarkers (including beta-amyloid 1-42, total tau, and tau phosphorylated at threonine 181), novel biomarkers have been interrogated in single- and multi-centered studies with uneven rigor. Here, we review early expectations for ideal AD/ADRD biomarkers, assess these goals' future applicability, and propose study designs and performance thresholds for meeting these ideals with a focus on CSF biomarkers. We further propose three new characteristics: equity (oversampling of diverse populations in the design and testing of biomarkers), access (reasonable availability to 80% of people at risk for disease, along with pre- and post-biomarker processes), and reliability (thorough evaluation of pre-analytical and analytical factors influencing measurements and performance). Finally, we urge biomarker scientists to balance the desire and evidence for a biomarker to reflect its namesake function, indulge data- as well as theory-driven associations, re-visit the subset of rigorously measured CSF biomarkers in large datasets (such as Alzheimer's disease neuroimaging initiative), and resist the temptation to favor ease over fail-safe in the development phase. This shift from discovery to application, and from suspended disbelief to cogent ingenuity, should allow the AD/ADRD biomarker field to live up to its billing during the next phase of neurodegenerative disease research.

Advances in kynurenine analysis

Kynurenine, the first product of tryptophan degradation via the kynurenine pathway, has become one of the most frequently mentioned biomarkers in recent years. Its levels in the body indicate the state of the human physiology. Human serum and plasma are the main matrixes used to evaluate kynurenine levels and liquid chromatography is the dominant technique for its determination. However, their concentrations in blood do not always correspond to the levels in other matrixes obtained from the affected individuals. It is therefore important to decide when it is appropriate to analyse kynurenine in alternative matrices. However, liquid chromatography may not be the best option for the analysis. This review presents alternatives that can be used and summarizes the features that need to be considered prior to kynurenine determination. Possible approaches to kynurenine analysis in a variety of human matrixes, their challenges, and limitations are critically discussed.

Brain Kynurenine Pathway Metabolite Levels May Reflect Extent of Neuroinflammation in ALS, FTD and Early Onset AD

OBJECTIVES

Despite distinct clinical profiles, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) patients share a remarkable portion of pathological features, with a substantial percentage of patients displaying a mixed disease phenotype. Kynurenine metabolism seems to play a role in dementia-associated neuroinflammation and has been linked to both diseases. We aimed to explore dissimilarities in kynurenine pathway metabolites in these early onset neurodegenerative disorders in a brain-region-specific manner.

METHODS

Using liquid chromatography mass spectrometry (LC-MS/MS), kynurenine metabolite levels were determined in the brain samples of 98 healthy control subjects (n = 20) and patients with early onset Alzheimer's disease (EOAD) (n = 23), ALS (n = 20), FTD (n = 24) or a mixed FTD-ALS (n = 11) disease profile.

RESULTS

Overall, the kynurenine pathway metabolite levels were significantly lower in patients with ALS compared to FTD, EOAD and control subjects in the frontal cortex, substantia nigra, hippocampus and neostriatum. Anthranilic acid levels and kynurenine-to-tryptophan ratios were consistently lower in all investigated brain regions in ALS compared to the other diagnostic groups.

CONCLUSIONS

These results suggest that the contribution of kynurenine metabolism in neuroinflammation is lower in ALS than in FTD or EOAD and may also be traced back to differences in the age of onset between these disorders. Further research is necessary to confirm the potential of the kynurenine system as a therapeutic target in these early onset neurodegenerative disorders.

Lipopolysaccharide-induced changes in the kynurenine pathway and symptoms of sickness behavior in humans

Metabolites of the kynurenine pathway are hypothesized to be implicated in inflammation-associated depression, but there is a lack of experimental studies in humans assessing the kinetics of kynurenine metabolites in relation to experimentally-induced sickness. The aim of the present study was to assess changes in the kynurenine pathway and to explore its relation to symptoms of sickness behavior during an acute experimental immune challenge. This double-blind placebo-controlled randomized cross-over study included 22 healthy human participants (n = 21 both sessions, M_age = 23.4, SD = 3.6, nine women) who received an intravenous injection of 2.0 ng/kg lipopolysaccharide (LPS) and saline (placebo) on two different occasions in a randomized order. Blood samples (0 h, 1 h, 1.5 h, 2 h, 3 h, 4 h, 5 h, 7 h post-injection) were analyzed for kynurenine metabolites and inflammatory cytokines. The intensity of symptoms of sickness behavior was assessed using the 10-item Sickness Questionnaire at 0 h, 1.5 h, 3 h, 5 h, and 7 h post-injection. LPS induced significantly lower concentrations of plasma tryptophan (at 2 h, 4 h, 5 h, and 7 h post-injection), kynurenine (at 2 h, 3 h, 4 h, and 5 h post-injection), nicotinamide (at 4 h, 5 h, and 7 h post-injection), and higher levels for quinolinic acid at 5 h post-injection as compared to placebo. LPS did not affect kynurenic acid, 3-hydroxykynurenine, and picolinic acid. The development of the sickness symptoms was largely similar across items, with the highest levels around 1.5-3 h post-injection. Changes in plasma levels of kynurenine metabolites seem to coincide rather than precede or follow changes in subjective sickness. Exploratory analyses indicate that higher Sickness Questionnaire total scores at 1.5-5 h post-injection were correlated with lower kynurenic acid and nicotinamide levels. These results lend further support for LPS-induced changes in the kynurenine pathway, but may not, as interpreted from blood levels, causally link to LPS-induced acute symptoms of sickness behavior. Future research may consider a larger sample to further scrutinize the role of the kynurenine pathway in the sickness response.

Measurement of kynurenine pathway metabolites by tandem mass spectrometry

Objectives

Recent studies have shown that derangements in kynurenine pathway metabolite levels are associated with various pathologies such as neurodegenerative diseases, schizophrenia, depression, bipolar disorder, rheumatoid arthritis, and cancer. Therefore, reliable, accurate, fast, and multiplex measurement methods for kynurenines have become increasingly important. This study aimed to validate a new mass spectrometric method for analyzing tryptophan metabolites.

Methods

A tandem mass spectrometric method, including protein precipitation and evaporation steps, was developed to measure serum levels of tryptophan, kynurenine, kynurenic acid, 3-hydroxykynurenine, and 3-hydroxyanthranilic acid. Samples were separated using a Phenomenex Luna C18 reversed-phase column. The kynurenine pathway metabolites were detected by tandem mass spectrometry. The developed method was validated according to Clinical & Laboratory Standards Institute (CLSI) guidelines and applied to hemodialysis samples.

Results

The developed method was linear at the concentrations of 48.8 - 25,000, 0.98 - 500, 1.2-5000, 1.2-5000, and 0.98-250 ng/mL for tryptophan, kynurenic acid, kynurenine, 3-hydroxyanthranilic acid, and 3-hydroxykynurenine, respectively. The imprecisions were less than 12 %. The median serum concentrations of tryptophan, kynurenine, kynurenic acid, 3-hydroxykynurenine, and 3-hydroxyanthranilic acid were 10530, 1100, 218, 17.6, and 25.4 ng/mL in pre-dialysis blood samples, respectively. They were 4560, 664, 135, 7.4, and 12.8 ng/mL in post-dialysis blood samples, respectively.

Conclusions

A fast, simple, cost-effective, accurate, robust, and validated tandem mass spectrometric method was developed, and the method was successfully used for the quantitation of kynurenine pathway metabolite concentrations in hemodialysis patients.

NAD+ Biosynthesis Impairment and Acute Kidney Injury after Major Vascular Surgery

Acute kidney injury (AKI) is a serious complication after vascular surgery. Reduced synthesis of nicotinamide adenine dinucleotide (NAD+) from tryptophan is associated with an increased risk of AKI in critically ill patients, patients hospitalized with COVID-19, and cardiac surgery patients, and is marked by elevated urinary quinolinate and quinolinate to tryptophan ratios. We measured quinolinate concentrations in vascular surgery patients to determine if impaired NAD+ synthesis was associated with AKI in this patient population. Eight preoperative and eight postoperative vascular surgery patients who developed AKI were selected from a parent study to participate in this single-center case-control study. They were matched with controls who did not develop AKI based on age, sex, BMI, eGFR, hypertension, and diabetes. Urinary quinolinate and tryptophan concentrations were measured at anesthetic induction and on postoperative day one. Two-sided Mann-Whitney U tests were used to compare quinolinate and quinolinate to tryptophan ratios. Multivariate linear regression modeling was used to estimate the relationship between quinolinate and serum creatinine. There was no difference in preoperative or postoperative urine quinolinate concentrations or the preoperative quinolinate to tryptophan ratio between patients that did and did not develop AKI (p = 0.07, 0.50, and 0.32, respectively). However, postoperative quinolinate to tryptophan ratios were higher in AKI patients (p = 0.04). Further, after adjustment for AKI risk factors, higher preoperative quinolinate concentrations and higher postoperative quinolinate to tryptophan ratios were associated with greater postoperative creatinine increases (p = 0.04 and 0.04, respectively). These data suggest that impaired NAD+ synthesis may contribute to AKI development in vascular surgery patients.

Cognitive impairments correlate with increased central nervous system immune activation after allogeneic haematopoietic stem cell transplantation

Murine studies indicate that, after allogeneic haematopoietic stem cell transplantation (aHSCT), donor-derived macrophages replace damaged microglia and alloreactive T-cells invade the central nervous system (CNS). The clinical relevance of this is unknown. We assessed CNS immune surveillance and metabolic activity involved in neuronal survival, in relation to fatigue and cognitive dysfunction in 25 long-term survivors after aHSCT. Patients with cognitive dysfunction exhibited increased proportions of activated T-cells and CD16 + NK-cells in the cerebrospinal fluid (CSF). Immune cell activation was paralleled with reduced levels of anti-inflammatory factors involved in T-cell suppression (transforming growth factor-β, programmed death ligand-1), NK-cell regulation (poliovirus receptor, nectin-2), and macrophage and microglia activation (CD200, chemokine [C-X3-C motif] ligand-1). Additionally, the CSF mRNA expression pattern was associated with neuroinflammation and oxidative stress. Furthermore, proteomic, and transcriptomic studies demonstrated decreased levels of neuroprotective factors, and an upregulation of apoptosis pathway genes. The kynurenine pathway of tryptophan metabolism was activated in the CNS of all aHSCT patients, resulting in accumulation of neurotoxic and pro-inflammatory metabolites. Cognitive decline and fatigue are overlooked but frequent complications of aHSCT. This study links post-transplant CNS inflammation and neurotoxicity to our previously reported hypoactivation in the prefrontal cortex during cognitive testing, suggesting novel treatment targets.

Validation of a liquid chromatography coupled to tandem mass spectrometry method for simultaneous quantification of tryptophan and 10 key metabolites of the kynurenine pathway in plasma and urine: Application to a cohort of acute kidney injury patients

A sensitive and rapid liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for the simultaneous determination of tryptophan (Trp) and ten metabolites of kynurenine pathway, including kynurenine (Kyn), 3-hydroxy-kynurenine (3-HK), kynurenic acid (KA), xanthurenic acid (XA), 3-Hydroxy-anthranilic acid (3-HANA), quinolinic acid (QA), nicotinic acid mononucleotide (NaMN), picolinic acid (Pic), nicotinamide (NAM) and nicotinic acid (NA) in both plasma and urine. This LC-MS/MS method was used to predict the occurrence of acute kidney injury (AKI) in a cohort of patients with cardiac surgery under cardiopulmonary bypass (CPB). Urinary concentrations of Pic, as well as Pic to Trp and Pic to 3-HANA ratios were highly predictive of an AKI episode the week after CPB, indicating that Pic could be a predictive biomarker of AKI. Thus, monitoring the kynurenine pathway activity with this LC-MS/MS method is a clinically relevant tool to identify new biomarkers of kidney injury.

Two-day fasting affects kynurenine pathway with additional modulation of short-term whole-body cooling: a quasi-randomised crossover trial

Metabolites of the kynurenine (KYN) pathway of tryptophan (TRP) degradation have attracted interest as potential pathophysiological mediators and future diagnostic biomarkers. A greater knowledge of the pathological implications of the metabolites is associated with a need for a better understanding of how the normal behaviour and physiological activities impact their concentrations. This study aimed to investigate whether fasting (FAST) and whole-body cold-water immersion (CWI) affect KYN pathway metabolites. Thirteen young women were randomly assigned to receive the 2-d FAST with two 10-min CWI on separate days (FAST-CWI), 2-d FAST without CWI (FAST-CON), 2-d two CWI on separate days without FAST (CON-CWI) or the 2-d usual diet without CWI (CON-CON) in a randomised crossover fashion. Changes in plasma concentrations of TRP, kynurenic acid (KYNA), 3-hydroxy-kynurenine (3-HK), picolinic acid (PIC), quinolinic acid (QUIN) and nicotinamide (NAA) were determined with ultra-performance liquid chromatography-tandem mass spectrometer. FAST-CWI and FAST-CON lowered TRP concentration (P < 0·05, ηp2 = 0·24), and increased concentrations of KYNA, 3-HK and PIC (P < 0·05, ηp2 = 0·21-0·71) with no additional effects of CWI. The ratio of PIC/QUIN increased after FAST-CWI and FAST-CON trials (P < 0·05) but with a blunted effect in the FAST-CWI trial (P < 0·05) compared with the FAST-CON trials (ηp2 = 0·67). Concentrations of QUIN and NAA were unaltered. This study demonstrated that fasting for 2 d considerably impacts the concentration of several metabolites in the KYN pathway. This should be considered when discussing the potential of KYN pathway metabolites as biomarkers.

COVID-19, Oxidative Stress, and Neuroinflammation in the Depression Route

COVID-19 is associated with oxidative stress, peripheral hyper inflammation, and neuroinflammation, especially in individuals with a more severe form of the disease. Some studies provide evidence on the onset or exacerbation of major depressive disorder (MDD), among other psychiatric disorders due to COVID-19. Oxidative stress and neuroinflammation are associated conditions, especially in the more severe form of MDD and in refractoriness to available therapeutic strategies. Inflammatory cytokines in the COVID-19 hyper inflammation process can activate the hypothalamic–pituitary–adrenal (HPA) axis and the indoleamine-2,3-dioxygenase (IDO) enzyme. IDO activation can reduce tryptophan and increase toxic metabolites of the kynurenine pathway, which increases glial activation, neuroinflammation, toxicity, and neuronal death. This review surveyed a number of studies and analyzed the mechanisms of oxidative stress, inflammation, and neuroinflammation involved in COVID-19 and depression. Finally, the importance of more protocols that can help elucidate the interaction between these mechanisms underlying COVID-19 and MDD and the possible therapeutic strategies involved in the interaction of these mechanisms are highlighted.

Peripheral and central kynurenine pathway abnormalities in major depression

Considerable data relate major depressive disorder (MDD) with aberrant immune system functioning. Pro-inflammatory cytokines facilitate metabolism of tryptophan along the kynurenine pathway (KP) putatively resulting in reduced neuroprotective and increased neurotoxic KP metabolites in MDD, in addition to modulating metabolic and immune function. This central nervous system hypothesis has, however, only been tested in the periphery. Here, we measured KP-metabolite levels in both plasma and cerebrospinal fluid (CSF) of depressed patients (n = 63/36 respectively) and healthy controls (n = 48/33). Further, we assessed the relation between KP abnormalities and brain-structure volumes, as well as body mass index (BMI), an index of metabolic disturbance associated with atypical depression. Plasma levels of picolinic acid (PIC), the kynurenic/quinolinic acid ratio (KYNA/QUIN), and PIC/QUIN were lower in MDD, but QUIN levels were increased. In the CSF, we found lower PIC in MDD. Confirming previous work, MDD patients had lower hippocampal, and amygdalar volumes. Hippocampal and amygdalar volumes were correlated positively with plasma KYNA/QUIN ratio in MDD patients. BMI was increased in the MDD group relative to the control group. Moreover, BMI was inversely correlated with plasma and CSF PIC and PIC/QUIN, and positively correlated with plasma QUIN levels in MDD. Our results partially confirm previous peripheral KP findings and extend them to the CSF in MDD. We present the novel finding that abnormalities in KP metabolites are related to metabolic disturbances in depression, but the relation between KP metabolites and depression-associated brain atrophy might not be as direct as previously hypothesized.

Kynurenine Pathway Metabolites as Potential Clinical Biomarkers in Coronary Artery Disease

Coronary artery disease (CAD) is one of the leading cause of mortality worldwide. Several risk factors including unhealthy lifestyle, genetic background, obesity, diabetes, hypercholesterolemia, hypertension, smoking, age, etc. contribute to the development of coronary atherosclerosis and subsequent coronary artery disease. Inflammation plays an important role in coronary artery disease development and progression. Pro-inflammatory signals promote the degradation of tryptophan via the kynurenine pathway resulting in the formation of several immunomodulatory metabolites. An unbalanced kynurenic pathway has been implicated in the pathomechanisms of various diseases including CAD. Significant improvements in detection methods in the last decades may allow simultaneous measurement of multiple metabolites of the kynurenine pathway and such a thorough analysis of the kynurenine pathway may be a valuable tool for risk stratification and determination of CAD prognosis. Nevertheless, imbalance in the activities of different branches of the kynurenine pathway may require careful interpretation. In this review, we aim to summarize clinical evidence supporting a possible use of kynurenine pathway metabolites as clinical biomarkers in various manifestations of CAD.

Intravenous administration of LPS activates the kynurenine pathway in healthy male human subjects: a prospective placebo-controlled cross-over trial

Background

Administration of lipopolysaccharide (LPS) from Gram-negative bacteria, also known as the human endotoxemia model, is a standardized and safe model of human inflammation. Experimental studies have revealed that peripheral administration of LPS leads to induction of the kynurenine pathway followed by depressive-like behavior and cognitive dysfunction in animals. The aim of the present study is to investigate how acute intravenous LPS administration affects the kynurenine pathway in healthy male human subjects.

Methods

The present study is a prospective, single-blinded, randomized, placebo-controlled cross-over study to investigate the effects of intravenously administered LPS (Escherichia coli O113, 2 ng/kg) on tryptophan and kynurenine metabolites over 48 h and their association with interleukin-6 (IL-6) and C-reactive protein (CRP). The study included 10 healthy, non-smoking men (18–40 years) free from medication. Statistical differences in tryptophan and kynurenine metabolites as well as associations with IL-6 and CRP in LPS and placebo treated subjects were assessed with linear mixed-effects models.

Results

Systemic injection of LPS was associated with significantly lower concentrations of plasma tryptophan and kynurenine after 4 h, as well as higher concentrations of quinolinic acid (QUIN) after 48 h compared to the placebo injection. No differences were found in kynurenic acid (KYNA) or picolinic acid plasma concentrations between LPS or placebo treatment. The KYNA/kynurenine ratio peaked at 6 h post LPS injection while QUIN/kynurenine maintained significantly higher from 3 h post LPS injection until 24 h. The kynurenine/tryptophan ratio was higher at 24 h and 48 h post LPS treatment. Finally, we report an association between the kynurenine/tryptophan ratio and CRP.

Conclusions

Our findings strongly support the concept that an inflammatory challenge with LPS induces the kynurenine pathway in humans, activating both the neurotoxic (QUIN) and neuroprotective (KYNA) branch of the kynurenine pathway.

Trial registration

This study is based on a study registered at ClinicalTrials.gov, NCT03392701. Registered 21 December 2017.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02196-x.

Central levels of tryptophan metabolites in subjects with bipolar disorder

The kynurenine pathway of tryptophan degradation produces several neuroactive metabolites such as kynurenic acid (KYNA), quinolinic acid (QUIN), and picolinic acid (PIC) thought to be involved in the pathophysiology of psychosis, major depression, and suicidal behavior. Here, we analyzed cerebrospinal fluid (CSF) concentrations of tryptophan, kynurenine, KYNA, QUIN, and PIC utilizing ultra-performance liquid chromatography - tandem mass spectrometry system (UPLC-MS/MS) in persons with bipolar disorder (n = 101) and healthy controls (n = 80) to investigate if the metabolites correlated with depressive symptoms or to the history of suicidal behavior. Furthermore, we analyzed if genetic variants of the enzyme amino-β-carboxymuconate-semialdehyde-decarboxylase (ACMSD) were associated with the CSF concentrations of PIC and QUIN. We found that CSF KYNA and PIC concentrations, as well as the kynurenine/tryptophan ratio were increased in bipolar disorder compared with controls. CSF PIC concentrations were lower in subjects with a history of suicidal behavior than those without, supporting the hypothesis that low CSF PIC is a marker of vulnerability for suicidality. Bipolar subjects taking antidepressants had higher CSF concentrations of kynurenine and KYNA than subjects not given these medications. A negative association was found between a genetic variant of ACMSD and the ratio of PIC/QUIN, indicating that a polymorphism in ACMSD is associated with excess of QUIN formation at the expense of PIC. The present results confirm that the kynurenine pathway is activated in bipolar disorder, and suggest that shifting the activity of the kynurenine pathway away from QUIN production towards a production of KYNA and PIC might be a beneficial therapeutic strategy.

Differential effects on blood and cerebrospinal fluid immune protein markers and kynurenine pathway metabolites from aerobic physical exercise in healthy subjects

Mounting evidence shows that physical exercise modulates systemic inflammation. However, its effect on cerebrospinal fluid (CSF) immune-marker profiles in man are largely unknown. We here report a study on healthy subjects (n = 27, males = 12, mean age 28.7, range 22-52) allocated to either an acute exercise setting over four consecutive days, or a training intervention over 4 weeks. Paired plasma and CSF samples collected at baseline, after 7 days of exercise abstention, and the day after completion of the exercise interventions, were analyzed for protein inflammation markers using a multiplex proximity extension assay and neurotransmitters and kynurenine pathway (KP) metabolites using liquid chromatography, respectively. Routine cell counts, and albumin, immunoglobulin G and neurofilament light chain concentrations in CSF remained unchanged in both paradigms, while several inflammatory proteins became upregulated after acute exercise. However, only changes in three CSF (vascular endothelial growth factor-A, interleukin-7 and matrix metalloproteinase-10) and 12 plasma proteins reached significance levels after adjustment for multiple comparisons and exclusion of less stable proteins. Similarly, KP metabolites only changed among participants after acute exercise, while neurotransmitter levels, except for increased CSF serine, remained stable. Both in plasma and CSF changes in KP metabolites and inflammatory proteins correlated, suggesting that these processes are functionally linked. These findings suggest that acute aerobic physical exercise affects immune markers and KP metabolites systemically and in the CSF.

Quantification of Plasma Kynurenine Metabolites Following One Bout of Sprint Interval Exercise

The kynurenine pathway of tryptophan degradation produces several neuroactive metabolites suggested to be involved in a wide variety of diseases and disorders, however, technical challenges in reliably detecting these metabolites hampers cross-comparisons. The main objective of this study was to develop an accurate, robust and precise bioanalytical method for simultaneous quantification of ten plasma kynurenine metabolites. As a secondary aim, we applied this method on blood samples taken from healthy subjects conducting 1 session of sprint interval exercise (SIE). It is well accepted that physical exercise is associated with health benefits and reduces risks of psychiatric illness, diabetes, cancer and cardiovascular disease, but also influences the peripheral and central concentrations of kynurenines. In line with this, we found that in healthy old adults (n = 10; mean age 64 years), levels of kynurenine increased 1 hour (P = .03) after SIE, while kynurenic acid (KYNA) concentrations were elevated after 24 hours (P = .02). In contrast, no significant changes after exercise were seen in young adults (n = 10; mean age 24 years). In conclusion, the described method performs well in reliably detecting all the analyzed metabolites in plasma samples. Furthermore, we also detected an age-dependent effect on the degree by which a single intense training session affects kynurenine metabolite levels.