Gas chromatography/tandem mass spectrometry detection of extracellular kynurenine and related metabolites in normal and lesioned rat brain

Neuroactive Kynurenines as Pharmacological Targets: New Experimental Tools and Exciting Therapeutic Opportunities

Both preclinical and clinical studies implicate functional impairments of several neuroactive metabolites of the kynurenine pathway (KP), the major degradative cascade of the essential amino acid tryptophan in mammals, in the pathophysiology of neurologic and psychiatric diseases. A number of KP enzymes, such as tryptophan 2,3-dioxygenase (TDO2), indoleamine 2,3-dioxygenases (IDO1 and IDO2), kynurenine aminotransferases (KATs), kynurenine 3-monooxygenase (KMO), 3-hydroxyanthranilic acid oxygenase (3-HAO), and quinolinic acid phosphoribosyltransferase (QPRT), control brain KP metabolism in health and disease and are therefore increasingly considered to be promising targets for the treatment of disorders of the nervous system. Understanding the distribution, cellular expression, and regulation of KP enzymes and KP metabolites in the brain is therefore critical for the conceptualization and implementation of successful therapeutic strategies. SIGNIFICANCE STATEMENT: Studies have implicated the kynurenine pathway of tryptophan in the pathophysiology of neurologic and psychiatric diseases. Key enzymes of the kynurenine pathway regulate brain metabolism in both health and disease, making them promising targets for treating these disorders. Therefore, understanding the distribution, cellular expression, and regulation of these enzymes and metabolites in the brain is critical for developing effective therapeutic strategies. This review endeavors to describe these processes in detail.

Simultaneous separation and detection of nine kynurenine pathway metabolites by reversed-phase liquid chromatography-mass spectrometry: Quantitation of inflammation in human cerebrospinal fluid and plasma

BACKGROUND

The kynurenine pathway (KP) generates eight tryptophan (TRP) metabolites collectively called kynurenines, which have gained enormous interest in clinical research. The importance of KP for different disease states calls for developing a low-cost and high-throughput chromatography-mass spectrometry method to evaluate the potential of different kynurenines. Simultaneous separation of TRP and its eight metabolites is challenging because they have substantial polarity differences (log P = -2.5 to +1.3).

RESULTS

A low-cost, reversed-phase LC-MS/MS method based on polarity partitioning was established to simultaneously separate and quantitate all nine kynurenine pathway metabolites (KPMs) in a single run for the first time in the open literature. Based on stationary phase screening and ternary mobile phase optimization strategy, high polarity KPMs were retained while medium and low polarity KPMs were eluted in a shorter time. After method validation, we demonstrated the applicability of this LC/MS/MS method by quantitative measurement of all nine KPM in cerebrospinal fluid (CSF) and plasma among two groups of human subjects diagnosed with depression. Furthermore, we measured the differential KPMs in these two groups of low and high inflammation and correlated the results with CRP or TNF-α markers for depression.

SIGNIFICANCE

Our proposed LC-MS/MS provides a new metabolite assay that can be easily applied in various clinical applications to simultaneously quantify multiple biomarkers in KP dysfunction.

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.

Deletion of quinolinate phosphoribosyltransferase gene accelerates frailty phenotypes and neuromuscular decline with aging in a sex-specific pattern

Decline in neuromuscular function with aging is known to be a major determinant of disability and all-cause mortality in late life. Despite the importance of the problem, the neurobiology of age-associated muscle weakness is poorly understood. In a previous report, we performed untargeted metabolomics on frail older adults and discovered prominent alteration in the kynurenine pathway, the major route of dietary tryptophan degradation that produces neurotoxic intermediate metabolites. We also showed that neurotoxic kynurenine pathway metabolites are correlated with increased frailty score. For the present study, we sought to further examine the neurobiology of these neurotoxic intermediates by utilizing a mouse model that has a deletion of the quinolinate phosphoribosyltransferase (QPRT) gene, a rate-limiting step of the kynurenine pathway. QPRT-/- mice have elevated neurotoxic quinolinic acid level in the nervous system throughout their lifespan. We found that QPRT-/- mice have accelerated declines in neuromuscular function in an age- and sex-specific manner compared to control strains. In addition, the QPRT-/- mice show premature signs of frailty and body composition changes that are typical for metabolic syndrome. Our findings suggest that the kynurenine pathway may play an important role in frailty and age-associated muscle weakness.

Kynurenine Pathway Regulation at Its Critical Junctions with Fluctuation of Tryptophan

The kynurenine pathway (KP) is the primary route for the catabolism of the essential amino acid tryptophan. The central KP metabolites are neurologically active molecules or biosynthetic precursors to critical molecules, such as NAD⁺. Within this pathway are three enzymes of interest, HAO, ACMSD, and AMSDH, whose substrates and/or products can spontaneously cyclize to form side products such as quinolinic acid (QA or QUIN) and picolinic acid. Due to their unstable nature for spontaneous autocyclization, it might be expected that the levels of these side products would be dependent on tryptophan intake; however, this is not the case in healthy individuals. On top of that, the regulatory mechanisms of the KP remain unknown, even after a deeper understanding of the structure and mechanism of the enzymes that handle these unstable KP metabolic intermediates. Thus, the question arises, how do these enzymes compete with the autocyclization of their substrates, especially amidst increased tryptophan levels? Here, we propose the formation of a transient enzyme complex as a regulatory mechanism for metabolite distribution between enzymatic and non-enzymatic routes during periods of increased metabolic intake. Amid high levels of tryptophan, HAO, ACMSD, and AMSDH may bind together, forming a tunnel to shuttle the metabolites through each enzyme, consequently regulating the autocyclization of their products. Though further research is required to establish the formation of transient complexation as a solution to the regulatory mysteries of the KP, our docking model studies support this new hypothesis.

Quinolinate promotes macrophage-induced immune tolerance in glioblastoma through the NMDAR/PPARγ signaling axis

There has been considerable scientific effort dedicated to understanding the biologic consequence and therapeutic implications of aberrant tryptophan metabolism in brain tumors and neurodegenerative diseases. A majority of this work has focused on the upstream metabolism of tryptophan; however, this has resulted in limited clinical application. Using global metabolomic profiling of patient-derived brain tumors, we identify the downstream metabolism of tryptophan and accumulation of quinolinate (QA) as a metabolic node in glioblastoma and demonstrate its critical role in promoting immune tolerance. QA acts as a metabolic checkpoint in glioblastoma by inducing NMDA receptor activation and Foxo1/PPARγ signaling in macrophages, resulting in a tumor supportive phenotype. Using a genetically-engineered mouse model designed to inhibit production of QA, we identify kynureninase as a promising therapeutic target to revert the potent immune suppressive microenvironment in glioblastoma. These findings offer an opportunity to revisit the biologic consequence of this pathway as it relates to oncogenesis and neurodegenerative disease and a framework for developing immune modulatory agents to further clinical gains in these otherwise incurable diseases.

Quantitation of tryptophan and kynurenine in human plasma using 4-vinylphenylboronic acid column by capillary electrochromatography coupled with mass spectrometry

Tryptophan (TRP) is an essential amino acid catabolized mainly through the kynurenine pathway, and part of it is catabolized in the brain. The abnormal depletion of TRP and production of kynurenine (KYN) by two enzymes, tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), have been linked to various neurological diseases. The ratio of TRP/KYN in plasma is a valuable measure for IDO/TDO activity and the prognosis of disease conditions. The 4-vinylphenylboronic acid (4-VPBA) was evaluated as a novel stationary phase for OT-CEC-MS/MS. TRP, KYN, and 3-hydroxykynurenine were separated using optimum conditions of 15 mM (NH4 )2 CO3 at pH 8 as a background electrolyte and 25 kV separation voltage on a 90 cm column. The usefulness of the 4-VPBA column for simple, fast, repeatable, and sensitive CEC-ESI-MS/MS application was demonstrated for the quantitation of TRP and KYN in the plasma of healthy human subjects and neuroinflammation subjects. The plasma sample was extracted on a zirconia-based ion-exchange cartridge for simultaneous protein precipitation and phospholipid removal. The method of standard addition, in combination with the internal standards approach, was used to prepare the calibration curve to overcome matrix matching and eliminate procedural errors. The developed quantitation method was validated according to FDA guidelines for sensitivity, accuracy, precision, and extraction recovery. The measured plasma level of TRP and KYN in healthy humans is aligned with the human metabolome database for the same two metabolites.

Ratio Fluorescent Detecting of Tryptophan and Its Metabolite 5-Hydroxyindole-3-acetic Acid Relevant with Depression via Tb(III) Modified HOFs Hybrids: Further Designing Recyclable Molecular Logic Gate Connected by Back Propagation Neural Network

Exploring intelligent fluorescent materials with high reliability and precision to diagnose diseases is significant but remains a great challenge. Herein, based on coordination post-synthetic modification, a Tb³⁺ functionalized ME-PA (Tb@1) is prepared, which can emit brilliant green fluorescence through ligand-to-mental charge transfer-assisted energy transfer (LMCT-ET) process from ME-PA to Tb³⁺ ions. Tb@1 can simultaneously distinguish Tryptophan (Try) and its metabolite 5-hydroxyindole-3-acetic acid (5-HIAA), two effective indicators for depression, in ratio and colorimetric mode. And this sensor behaves the advantages of high efficiency and sensitivity, as well as excellent reusability and anti-interference. The PET process from ME to Try and 5-HIAA, and the competitive absorption between analytes and Tb@1 may be relevant to sensing mechanism. In realistic serum or urine environment, the detection limits of Tb@1 for Try and 5-HIAA are 0.0183 and 0.0149 mg L^-1 respectively. Moreover, in conjunction with back propagation neural network (BPNN), two dual-output molecular logic gates that can be calculated circularly are further designed, which realizes intelligent control of the electronic component to identify the existence of two biomarkers and judge their concentrations from fluorescence images. This work offers a novel approach to modulate logic circuits based on ML-assisted HOF fluorescent sensor, with promising application for a precise and pictorial depression diagnosis.

Metabolite profiling and identification in living cells by coupling stable isotope tracing and induced electrospray mass spectrometry

Direct observation of metabolites in living cells by mass spectrometry offers a bright future for biological studies but also suffers a severe challenge to untargeted peak assignment to tentative metabolite candidates. In this study, we developed a method combining stable isotope tracing and induced electrospray mass spectrometry for living-cells metabolite measurement and identification. By using 13C6-glucose and ammonium chloride-15N as the sole carbon and nitrogen sources for cell culture, Escherichia coli synthesized metabolites with 15N and 13C elements. Tracing the number of carbon and nitrogen atoms could offer a complementary dimension for candidate peak searching. As a result, the identification confidence of metabolites achieved a universal improvement based on carbon/nitrogen labelling and filtration.

A Fluorescence-Based Competitive Antibody Binding Assay for Kynurenine, a Potential Biomarker of Kidney Transplant Failure

Kynurenine is a tryptophan metabolite linked to several inflammatory processes including transplant failure, a significant challenge in transplant medicine. The detection of small molecules such as kynurenine, however, is often complex and time consuming. Herein, we report the successful synthesis of a fluorescently labelled kynurenine derivative, showing proper fluorescence and anti-kynurenine antibody binding behavior in a magnetic bead immunoassay (MIA). The fluorescent kynurenine–rhodamine B conjugate shows a KD-value of 5.9 µM as well as IC50 values of 4.0 µM in PBS and 10.2 µM in saliva. We thus introduce a rapid test for kynurenine as a potential biomarker for kidney transplant failure.

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.

Elucidation of the Central Serotonin Metabolism Pathway in Rhesus Macaques (Macaca mulatta) with Self-injurious Behavior

Macaques with self-injurious behavior (SIB) have been used as a model of human SIB and have previously been shown to respond to treatments targeting enhancement of central serotonin signaling, whether by supplementation with tryptophan, or by inhibiting synaptic reuptake. Decreased serotonin signaling in the brain has also been implicated in many human psychopathologies including major depression disorder. A disturbance in tryptophan metabolism that moves away from the production of serotonin and toward the production of kynurenine has been proposed as a major etiological factor of depression. We hypothesized that in macaques with SIB, central tryptophan metabolism would be shifted toward kynurenine production, leading to lower central serotonin (5-hydroxytryptamine). We analyzed tryptophan metabolites in the cerebral spinal fluid (CSF) of macaques with and without SIB to determine whether and where tryptophan metabolism is altered in affected animals as compared with behaviorally normal controls. We found that macaques with SIB had lower CSF concentrations of serotonin than did behaviorally normal macaques, and that these deficits were inversely correlated with the severity of abnormal behavior. However, our results suggest that this decrease is not due to shifting of the tryptophan metabolic pathway toward kynurenine, as concentrations of kynurenine were also low. Concentrations of IL6 were elevated, suggesting central inflammation. Determining the mechanism by which serotonin function is altered in self-injurious macaques could shed light on novel therapies for SIB and other disorders of serotonin signaling.

A single prenatal lipopolysaccharide injection has acute, but not long-lasting, effects on cerebral kynurenine pathway metabolism in mice

In rodents, a single injection of lipopolysaccharide (LPS) during gestation causes chemical and functional abnormalities in the offspring. These effects may involve changes in the kynurenine pathway (KP) of tryptophan degradation and may provide insights into the pathophysiology of psychiatric diseases. Using CD1 mice, we examined acute and long-term effects of prenatal LPS treatment on the levels of kynurenine and its neuroactive downstream products kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK) and quinolinic acid. To this end, LPS (100 μg/kg, i.p.) was administered on gestational day 15, and KP metabolites were measured 4 and 24 h later or in adulthood. After 4 h, kynurenine, KYNA and 3-HK levels were elevated in the fetal brain, 3-HK and KYNA levels were increased in the maternal plasma, and kynurenine was increased in the maternal brain, whereas no changes were seen in the placenta. These effects were less prominent after 24 h, and prenatal LPS did not affect the basal levels of KP metabolites in the forebrain of adult animals. In addition, a second LPS injection (1 mg/kg) in adulthood in the offspring of prenatally saline- and LPS-treated mice caused a similar elevation in 3-HK levels in both groups after 24 h, but the effect was significantly more pronounced in male mice. Thus, acute immune activation during pregnancy has only short-lasting effects on KP metabolism and does not cause cerebral KP metabolites to be disproportionally affected by a second immune challenge in adulthood. However, prenatal KYNA elevations still contribute to functional abnormalities in the offspring.

Improved Voltammetric Determination of Kynurenine at the Nafion Covered Glassy Carbon Electrode - Application in Samples Delivered from Human Cancer Cells

Nowadays, development of analytical methods responding to a need for rapid and accurate determination of human metabolites is highly desirable. Herein, an electrochemical method employing a Nafion-coated glassy carbon electrode (Nafion/GCE) has been developed for reliable determination of kynurenine (a key tryptophan metabolite) using a differential pulse adsorptive stripping voltammetry. To our knowledge, this is the first analytical method to allow for kynurenine determination at the Nafion-coated electrode. The methodology involves kynurenine pre-concentration in 0.1 M H₂SO₄ in the Nafion film at the potential of +0.5 V and subsequent stripping from the electrode by differential pulse voltammetry. Under optimal conditions, the sensor can detect 5 nM kynurenine (for the accumulation time of 60 seconds), but the limit of detection can be easily lowered to 0.6 nM by prolonging the accumulation time to 600 seconds. The sensor shows sensitivity of 36.25 μAμM⁻¹cm⁻² and 185.50 μAμM⁻¹cm⁻² for the accumulation time of 60 and 600 seconds, respectively. The great advantage of the proposed method is easy sensor preparation, employing drop coating method, high sensitivity, short total analysis time, and no need for sample preparation. The method was validated for linearity, precision, accuracy (using a high-performance liquid chromatography), selectivity (towards tryptophan metabolites and different amino acids), and recovery. The comprehensive microscopic and electrochemical characterization of the Nafion/GCE was also conducted with different methods including atomic force microscopy (AFM), optical profilometry, time-of-flight secondary ion mass spectrometry (TOF-SIMS), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The method has been applied with satisfactory results for determination of kynurenine concentration in a culture medium collected from the human ovarian carcinoma cells SK-OV-3 and to measure IDO enzyme activity in the cancer cell extracts.

Capillary electrochromatography-mass spectrometry of kynurenine pathway metabolites

Few articles are reported for the simultaneous separation and sensitive detection of the kynurenine pathway (KP) metabolites. This work describes a capillary electrochromatography-mass spectrometry (CEC-MS) method using acrylamido-2-methyl-1-propanesulfonic acid (AMPS) functionalized stationary phase. The AMPS column was prepared by first performing silanization of bare silica with gamma-maps, followed by polymerization with AMPS. The CEC-MS/MS methods were established for six upstream and three downstream KP metabolites. The simultaneous separation of all nine KP metabolites is achieved without derivatization for the first time in the open literature. Numerous parameters such as pH and the concentration of background electrolyte, the concentration of the polymerizable AMPS monomer, column length, field strength, and internal pressure were all tested to optimize the separation of multiple KP metabolites. A baseline separation of six upstream metabolites, namely tryptophan (TRP), kynurenine (KYN), 3-hydroxykynurenine (HKYN), kynurenic acid (KA), anthranilic acid (AA), and xanthurenic acid (XA), was possible at pH 9.25 within 26 min. Separation of six downstream and related metabolites, namely: tryptamine (TRPM), hydroxy‑tryptophan (HTRP), hydroxyindole-3 acetic acid (HIAA), 3-hydroxyanthranilic acid (3-HAA), picolinic acid (PA), and quinolinic acid (QA), was achieved at pH 9.75 in 30 min. However, the challenging simultaneous separation of all nine KP metabolites was only accomplished by increasing the column length and simultaneous application of internal pressure and voltage in 114 min. Quantitation of KP metabolites in commercial human plasma was carried out, and endogenous concentration of five KP metabolites was validated. The experimental limit of quantitation ranges from 100 to 10,000 nM (S/N = 8-832, respectively), whereas the experimental limit of detection ranges from 31 to 1000 nM (S/N = 2-16, respectively). Levels of five major KP metabolites, namely TRP, KYN, KA, AA, and QA, and their ratios in patient plasma samples previously screened for inflammatory biomarkers [C-reactive protein (CRP) and tumor necrosis factor-alpha (TNF-α)] was measured. Pairs of the level of metabolites with significant positive correlation were statistically evaluated.

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

Aim:

Kynurenine metabolites are potential modulators of psychiatric disease. We aimed to develop a highly sensitive biochemical analysis of cerebrospinal fluid (CSF) tryptophan (TRP) metabolites, to investigate the stability of metabolites and to confirm our previous findings of aberrant CSF quinolinic acid (QUIN) and picolinic acid (PIC) in suicide attempters using this method.

Methodology & results:

Ten CSF TRP metabolites were analyzed with ultraperformance LC–MS/MS. The method showed small intra- and interassay variation. Metabolites were stable following freeze–thaw cycles. A decreased CSF PIC/QUIN ratio was found in suicide attempters.

Conclusion:

The feasibility of reliably determining CSF TRP metabolites were demonstrated, including separation of the two isomers PIC and nicotinic acid (NA) and the finding of a reduced PIC/QUIN ratio replicated in suicide attempters.

Maternal immune activation in rats blunts brain cytokine and kynurenine pathway responses to a second immune challenge in early adulthood

Maternal immune activation (MIA) with the viral mimic poly I:C provides an established rodent model for studying schizophrenia (SZ) and other human neurodevelopmental disorders. Postnatal infections are additional risk factors in SZ and may cumulatively contribute to the emergence of pathophysiology. Underlying mechanisms may involve metabolites of the kynurenine pathway (KP) of tryptophan degradation, which is readily induced by inflammatory stimuli. Here we compared the expression of selected cytokines and KP enzymes, and the levels of selected KP metabolites, in the brain of MIA offspring following a second, acute immune challenge with lipopolysaccharides (LPS) on postnatal day (PND) 35 (adolescence) or PND 60 (early adulthood). Assessed in adolescence, MIA did not alter the expression of pro-inflammatory cytokines (except TNF-α) or KP metabolite levels compared to controls, but substantially reduced the expression of the anti-inflammatory cytokines IL-4 and IL-10 and influenced the expression of two of the four KP enzymes examined (IDO1 and TDO2). LPS treatment caused distinct changes in the expression of pro- and anti-inflammatory cytokines, as well as KP enzymes in MIA offspring, but had no effect on KP metabolites compared to control rats. Several of these effects were blunted in MIA offspring receiving LPS on PND 60. Notably, LPS caused a significant reduction in brain kynurenine levels in these animals. Of relevance for SZ-related hypotheses, these results indicate that MIA leads to an increasingly defective, rather than an overactive, immune regulation of cerebral KP metabolism during the postnatal period.

Alzheimer's Disease: Recent Concepts on the Relation of Mitochondrial Disturbances, Excitotoxicity, Neuroinflammation, and Kynurenines

The pathomechanism of Alzheimer's disease (AD) certainly involves mitochondrial disturbances, glutamate excitotoxicity, and neuroinflammation. The three main aspects of mitochondrial dysfunction in AD, i.e., the defects in dynamics, altered bioenergetics, and the deficient transport, act synergistically. In addition, glutamatergic neurotransmission is affected in several ways. The balance between synaptic and extrasynaptic glutamatergic transmission is shifted toward the extrasynaptic site contributing to glutamate excitotoxicity, a phenomenon augmented by increased glutamate release and decreased glutamate uptake. Neuroinflammation in AD is predominantly linked to central players of the innate immune system, with central nervous system (CNS)-resident microglia, astroglia, and perivascular macrophages having been implicated at the cellular level. Several abnormalities have been described regarding the activation of certain steps of the kynurenine (KYN) pathway of tryptophan metabolism in AD. First of all, the activation of indolamine 2,3-dioxygenase, the first and rate-limiting step of the pathway, is well-demonstrated. 3-Hydroxy-L-KYN and its metabolite, 3-hydroxy-anthranilic acid have pro-oxidant, antioxidant, and potent immunomodulatory features, giving relevance to their alterations in AD. Another metabolite, quinolinic acid, has been demonstrated to be neurotoxic, promoting glutamate excitotoxicity, reactive oxygen species production, lipid peroxidation, and microglial neuroinflammation, and its abundant presence in AD pathologies has been demonstrated. Finally, the neuroprotective metabolite, kynurenic acid, has been associated with antagonistic effects at glutamate receptors, free radical scavenging, and immunomodulation, giving rise to potential therapeutic implications. This review presents the multiple connections of KYN pathway-related alterations to three main domains of AD pathomechanism, such as mitochondrial dysfunction, excitotoxicity, and neuroinflammation, implicating possible therapeutic options.

Sensitive determination of monoamine neurotransmitters, their main metabolites and precursor amino acids in different mouse brain components by liquid chromatography-electrospray tandem mass spectrometry after selective sample clean-up

For the assessment of diets and supplements formulated for the treatment of phenylketonuria, a highly sensitive and selective method was developed and validated for the quantification of dopamine (DA), serotonin (5-HT), 3,4-dihydroxyphenylacetic acid (DOPAC), 5-hydroxyindoleacetic acid (5-HIAA), phenylalanine, tyrosine and tryptophan in mouse cerebellum, brain stem, hypothalamus, parietal cortex, anterior piriform cortex and bulbus olfactorius. Samples were extracted by deproteinization with acetonitrile, and the extracts were cleaned up by strong anion exchange and weak cation exchange applied sequentially. The substances were detected by rapid liquid chromatography tandem mass spectrometry. Matrix components were largely removed by the clean-up, resulting in low matrix effects. The lower limits of quantification for an extracted tissue mass of 100 mg were 0.3, 0.3, 0.2 and 2 ng/g for DA, 5-HT, 5-HIAA and DOPAC, respectively. The mean true extraction recoveries were 80-102%. The relative intra-laboratory reproducibility standard deviations were generally <11% at concentrations of 20-1000 ng/g for DA, 5-HT, 5-HIAA and DOPAC and 7% at concentrations of 5-50 μg/g for the amino acids. This method was successfully used in a phenylketonuria mice study including nearly 300 brain tissue samples and for small sample masses (for example, 2 mg of bulbus olfactorius).

Maternal genotype determines kynurenic acid levels in the fetal brain: Implications for the pathophysiology of schizophrenia

BACKGROUND

Several studies suggest a pathophysiologically relevant association between increased brain levels of the neuroinhibitory tryptophan metabolite kynurenic acid and cognitive dysfunctions in people with schizophrenia. Elevated kynurenic acid in schizophrenia may be secondary to a genetic alteration of kynurenine 3-monooxygenase, a pivotal enzyme in the kynurenine pathway of tryptophan degradation. In rats, prenatal exposure to kynurenine, the direct bioprecursor of kynurenic acid, induces cognitive impairments reminiscent of schizophrenia in adulthood, suggesting a developmental dimension to the link between kynurenic acid and schizophrenia.

AIM

The purpose of this study was to explore the possible impact of the maternal genotype on kynurenine pathway metabolism.

METHODS

We exposed pregnant wild-type ( Kmo^+/+ ) and heterozygous ( Kmo^+/-) mice to kynurenine (10 mg/day) during the last week of gestation and determined the levels of kynurenic acid and two other neuroactive kynurenine pathway metabolites, 3-hydroxykynurenine and quinolinic acid, in fetal brain and placenta on embryonic day 17/18.

RESULTS

Maternal kynurenine treatment raised kynurenic acid levels significantly more in the brain of heterozygous offspring of Kmo^+/- than in the brain of Kmo^+/+ offspring. Conversely, 3-hydroxykynurenine and quinolinic acid levels in the fetal brain tended to be lower in heterozygous animals derived from kynurenine-treated Kmo^+/- mice than in corresponding Kmo^+/+ offspring. Genotype-related effects on the placenta were qualitatively similar but less pronounced. Kynurenine treatment also caused a preferential elevation in cerebral kynurenic acid levels in Kmo^+/- compared to Kmo^+/+ dams.

CONCLUSIONS

The disproportionate kynurenic acid increase in the brain of Kmo^+/- animals indicates that the maternal Kmo genotype may play a key role in the pathophysiology of schizophrenia.

Chromatographic analysis of tryptophan metabolites

The kynurenine pathway generates multiple tryptophan metabolites called collectively kynurenines and leads to formation of the enzyme cofactor nicotinamide adenine dinucleotide. The first step in this pathway is tryptophan degradation, initiated by the rate-limiting enzymes indoleamine 2,3-dioxygenase, or tryptophan 2,3-dioxygenase, depending on the tissue. The balanced kynurenine metabolism, which has been a subject of multiple studies in last decades, plays an important role in several physiological and pathological conditions such as infections, autoimmunity, neurological disorders, cancer, cataracts, as well as pregnancy. Understanding the regulation of tryptophan depletion provide novel diagnostic and treatment opportunities, however it requires reliable methods for quantification of kynurenines in biological samples with complex composition (body fluids, tissues, or cells). Trace concentrations, interference of sample components, and instability of some tryptophan metabolites need to be addressed using analytical methods. The novel separation approaches and optimized extraction protocols help to overcome difficulties in analyzing kynurenines within the complex tissue material. Recent developments in chromatography coupled with mass spectrometry provide new opportunity for quantification of tryptophan and its degradation products in various biological samples. In this review, we present current accomplishments in the chromatographic methodologies proposed for detection of tryptophan metabolites and provide a guide for choosing the optimal approach.

Maternal Inflammation Results in Altered Tryptophan Metabolism in Rabbit Placenta and Fetal Brain

Maternal inflammation has been linked to neurodevelopmental and neuropsychiatric disorders such as cerebral palsy, schizophrenia, and autism. We had previously shown that intrauterine inflammation resulted in a decrease in serotonin, one of the tryptophan metabolites, and a decrease in serotonin fibers in the sensory cortex of newborns in a rabbit model of cerebral palsy. In this study, we hypothesized that maternal inflammation results in alterations in tryptophan pathway enzymes and metabolites in the placenta and fetal brain. We found that intrauterine endotoxin administration at gestational day 28 (G28) resulted in a significant upregulation of indoleamine 2,3-dioxygenase (IDO) in both the placenta and fetal brain at G29 (24 h after treatment). This endotoxin-mediated IDO induction was also associated with intense microglial activation, an increase in interferon gamma expression, and increases in kynurenine and the kynurenine pathway metabolites kynurenine acid and quinolinic acid, as well as a significant decrease in 5-hydroxyindole acetic acid (a precursor of serotonin) levels in the periventricular region of the fetal brain. These results indicate that maternal inflammation shunts tryptophan metabolism away from the serotonin to the kynurenine pathway, which may lead to excitotoxic injury along with impaired development of serotonin-mediated thalamocortical fibers in the newborn brain. These findings provide new targets for prevention and treatment of maternal inflammation-induced fetal and neonatal brain injury leading to neurodevelopmental disorders such as cerebral palsy and autism.

Distinct Patterns of Tryptophan Maintenance in Tissues during Kynurenine Pathway Activation in Simian Immunodeficiency Virus-Infected Macaques

Induction of the kynurenine pathway (KP) of tryptophan (TRP) catabolism has been proposed to contribute to T cell dysfunction during human/simian immunodeficiency virus (SIV) infection via depletion of local TRP levels and production of immunomodulatory KP metabolites. However, while changes in TRP and KP metabolites have been observed in plasma, their levels in lymphoid tissues and levels of enzymes downstream of indoleamine 2,3-dioxygenase (IDO1) have been relatively unexplored. We used our SIV-infected pigtailed macaque model to analyze longitudinal changes in KP metabolites and enzymes by gas chromatography/mass spectrometry and NanoString nCounter gene expression analysis, respectively, in spleen and blood compared to changes previously established in brain and CSF. We found that TRP levels were remarkably stable in tissue sites despite robust depletion in the circulating plasma and CSF. We also demonstrated that intracellular TRP reserves were maintained in cultured cells even in the presence of depleted extracellular TRP levels. Kynurenine (KYN), 3-hydroxykynurenine, quinolinic acid, and the KP enzymes all displayed highly divergent patterns in the sites examined, though IDO1 expression always correlated with local KYN/TRP ratios. Finally, we demonstrated by fluorescence-activated cell sorting that myeloid dendritic cells and cells of monocytic lineage were the highest producers of IDO1 in chronically infected spleens. Overall, our study reveals insights into the tissue-specific regulation of KP enzymes and metabolites and, in particular, highlights the multiple mechanisms by which cells and tissues seek to prevent TRP starvation during inflammation.

Reduced kynurenine pathway metabolism and cytokine expression in the prefrontal cortex of depressed individuals

BACKGROUND

Neuroinflammatory processes are increasingly believed to participate in the pathophysiology of a number of major psychiatric diseases, including depression. Immune activation stimulates the conversion of the amino acid tryptophan to kynurenine, leading to the formation of neuroactive metabolites, such as quinolinic acid and kynurenic acid. These compounds affect glutamatergic neurotransmission, which plays a prominent role in depressive pathology. Increased tryptophan degradation along the kynurenine pathway (KP) has been proposed to contribute to disease etiology.

METHODS

We used postmortem brain tissue from the ventrolateral prefrontal cortex (VLPFC) to assess tissue levels of tryptophan and KP metabolites, the expression of several KP enzymes and a series of cytokines as well as tissue pathology, including microglial activation. Tissue samples came from nonpsychiatric controls (n = 36) and individuals with depressive disorder not otherwise specified (DD-NOS, n = 45) who died of natural causes, homicide, accident, or suicide.

RESULTS

We found a reduction in the enzymatic conversion of tryptophan to kynurenine, determined using the kynurenine:tryptophan ratio, and reduced messenger RNA expression of the enzymes indoleamine-2,3-dioxygenase 1 and 2 and tryptophan-2,3-dioxygenase in depressed individuals irrespective of the cause of death. These findings correlated with reductions in the expression of several cytokines, including interferon-γ and tumour necrosis factor-α. Notably, quinolinic acid levels were also lower in depressed individuals than controls.

LIMITATIONS

Information on the use of antidepressants and other psychotropic medications was insufficient for statistical comparisons.

CONCLUSION

Contrary to expectations, the present results indicate that depression, in the absence of medical illness or an overt inflammatory process, is associated with compromised, rather than increased, KP metabolism in the VLPFC.

Tryptophan 2,3-dioxygenase and indoleamine 2,3-dioxygenase 1 make separate, tissue-specific contributions to basal and inflammation-induced kynurenine pathway metabolism in mice

BACKGROUND

In mammals, the majority of the essential amino acid tryptophan is degraded via the kynurenine pathway (KP). Several KP metabolites play distinct physiological roles, often linked to immune system functions, and may also be causally involved in human diseases including neurodegenerative disorders, schizophrenia and cancer. Pharmacological manipulation of the KP has therefore become an active area of drug development. To target the pathway effectively, it is important to understand how specific KP enzymes control levels of the bioactive metabolites in vivo.

METHODS

Here, we conducted a comprehensive biochemical characterization of mice with a targeted deletion of either tryptophan 2,3-dioxygenase (TDO) or indoleamine 2,3-dioxygenase (IDO), the two initial rate-limiting enzymes of the KP. These enzymes catalyze the same reaction, but differ in biochemical characteristics and expression patterns. We measured KP metabolite levels and enzyme activities and expression in several tissues in basal and immune-stimulated conditions.

RESULTS AND CONCLUSIONS

Although our study revealed several unexpected downstream effects on KP metabolism in both knockout mice, the results were essentially consistent with TDO-mediated control of basal KP metabolism and a role of IDO in phenomena involving stimulation of the immune system.

Kynurenine pathway metabolism following prenatal KMO inhibition and in Mecp2+/- mice, using liquid chromatography-tandem mass spectrometry

To quantify the full range of tryptophan metabolites along the kynurenine pathway, a liquid chromatography – tandem mass spectrometry method was developed and used to analyse brain extracts of rodents treated with the kynurenine-3-mono-oxygenase (KMO) inhibitor Ro61-8048 during pregnancy. There were significant increases in the levels of kynurenine, kynurenic acid, anthranilic acid and 3-hydroxy-kynurenine (3-HK) in the maternal brain after 5 h but not 24 h, while the embryos exhibited high levels of kynurenine, kynurenic acid and anthranilic acid after 5 h which were maintained at 24 h post-treatment. At 24 h there was also a strong trend to an increase in quinolinic acid levels (P = 0.055). No significant changes were observed in any of the other kynurenine metabolites. The results confirm the marked increase in the accumulation of some neuroactive kynurenines when KMO is inhibited, and re-emphasise the potential importance of changes in anthranilic acid. The prolonged duration of metabolite accumulation in the embryo brains indicates a trapping of compounds within the embryonic CNS independently of maternal levels. When brains were examined from young mice heterozygous for the meCP2 gene – a potential model for Rett syndrome - no differences were noted from control mice, suggesting that the proposed roles for kynurenines in autism spectrum disorder are not relevant to Rett syndrome, supporting its recognition as a distinct, independent, condition.

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Pregnant rats were treated with an inhibitor of kynurenine-3-monoxygenase.

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Levels of several kynurenine metabolites increased in the maternal and foetal brains.

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The maternal changes at 5 h disappeared by 24 h, but were maintained in embryos.

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No changes were noted in the brains of Mecp2^+/− mice.

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KMO inhibition but not Mecp2^+/− suppression alters kynurenine metabolism.

LC–MS/MS-based quantification of kynurenine metabolites, tryptophan, monoamines and neopterin in plasma, cerebrospinal fluid and brain

Aim: The kynurenine (KYN) pathway is implicated in diseases such as cancer, psychiatric, neurodegenerative and autoimmune disorders. Measurement of KYN metabolite levels will help elucidating the involvement of the KYN pathway in the disease pathology and inform drug development. Methodology: Samples of plasma, cerebrospinal fluid or brain tissue were spiked with deuterated internal standards, processed and analyzed by LC–MS/MS; analytes were chromatographically separated by gradient elution on a C18 reversed phase analytical column without derivatization. Conclusion: We established an LC–MS/MS method to measure 11 molecules, namely tryptophan, KYN, 3-OH-KYN, 3-OH-anthranilic acid, quinolinic acid, picolinic acid, kynurenic acid, xanthurenic acid, serotonin, dopamine and neopterin within 5.5 min, with sufficient sensitivity to quantify these molecules in small sample volumes of plasma, cerebrospinal fluid and brain tissue.

Role of d-amino acid oxidase in the production of kynurenine pathway metabolites from d-tryptophan in mice

The kynurenine pathway (KP), the major catabolic route of the essential amino acid l-tryptophan (l-TRP), contains several neuroactive compounds, including kynurenic acid, 3-hydroxykynurenine (3-HK), and quinolinic acid (QUIN). The role of the d-enantiomer (d-TRP) in KP metabolism has received little attention so far. d-TRP can be converted to l-TRP by d-amino acid oxidase, and the same enzyme can produce d-kynurenine, a known bioprecursor of KYNA. To analyze these complex metabolic events systematically in vivo, we injected mice with d-TRP (300 mg/kg, i.p.) and examined KP metabolism in the absence or presence of the d-amino acid oxidase inhibitor 3-methylpyrazole-5-carboxylic acid (MPC; 100 mg/kg, i.p.,). After 90 min, newly formed l-TRP was recovered in plasma, liver, forebrain, and cerebellum, and MPC prevented its neosynthesis in all tissues. In the same animals, de novo production of d-kynurenine from d-TRP was also observed, but was much higher in the periphery than in the brain. d-TRP administration raised KYNA, 3-HK, and QUIN levels in all tissues examined, and KYNA production from d-TRP was significantly reduced after pre-treatment with MPC. These results indicate that catabolic routes other than those classically ascribed to l-TRP and l-kynurenine can account for the synthesis of KYNA, 3-HK and QUINin vivo. The essential amino acid l-tryptophan is catabolized via the kynurenine pathway (KP). We explored the role of the d-enantiomer in KP metabolism in mice in vivo. We report that d-tryptophan is metabolized in both brain and periphery and converted to KP metabolites, including d-kynurenine and l-kynurenine, kynurenic acid, 3-hydroxykynurenine, and quinolinic acid. Pharmacological experiments confirm the involvement of d-amino acid oxidase in these processes. Our results indicate that this enzyme participates in the synthesis of KP metabolites from d-tryptophan.

Quinolinic acid/tryptophan ratios predict neurological disease in SIV-infected macaques and remain elevated in the brain under cART

Activation of the kynurenine pathway (KP) of tryptophan catabolism likely contributes to HIV-associated neurological disorders. However, KP activation in brain tissue during HIV infection has been understudied, and the effect of combination antiretroviral therapy (cART) on KP induction in the brain is unknown. To examine these questions, tryptophan, kynurenine, 3-hydroxykynurenine, quinolinic acid, and serotonin levels were measured longitudinally during SIV infection in the striatum and CSF from untreated and cART-treated pigtailed macaques. Messenger RNA (mRNA) levels of KP enzymes also were measured in the striatum. In untreated macaques, elevations in KP metabolites coincided with transcriptional induction of upstream enzymes in the KP. Striatal KP induction was also temporally associated-but did not directly correlate-with serotonin losses in the brain. CSF quinolinic acid/tryptophan ratios were found to be the earliest predictor of neurological disease in untreated SIV-infected macaques, outperforming other KP metabolites as well as the putative biomarkers interleukin-6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1). Finally, cART did not restore KP metabolites to control levels in the striatum despite the control of the virus, though CSF metabolite levels were normalized in most animals. Overall, these results demonstrate that cerebral KP activation is only partially resolved with cART and that CSF QUIN/TRP ratios are an early, predictive biomarker of CNS disease.

α-Ketoglutaramate: an overlooked metabolite of glutamine and a biomarker for hepatic encephalopathy and inborn errors of the urea cycle

Glutamine metabolism is generally regarded as proceeding via glutaminase-catalyzed hydrolysis to glutamate and ammonia, followed by conversion of glutamate to α-ketoglutarate catalyzed by glutamate dehydrogenase or by a glutamate-linked aminotransferase (transaminase). However, another pathway exists for the conversion of glutamine to α-ketoglutarate that is often overlooked, but is widely distributed in nature. This pathway, referred to as the glutaminase II pathway, consists of a glutamine transaminase coupled to ω-amidase. Transamination of glutamine results in formation of the corresponding α-keto acid, namely, α-ketoglutaramate (KGM). KGM is hydrolyzed by ω-amidase to α-ketoglutarate and ammonia. The net glutaminase II reaction is: L - Glutamine + α - keto acid + H2O → α - ketoglutarate + L - amino acid + ammonia. In this mini-review the biochemical importance of the glutaminase II pathway is summarized, with emphasis on the key component KGM. Forty years ago it was noted that the concentration of KGM is increased in the cerebrospinal fluid (CSF) of patients with hepatic encephalopathy (HE) and that the level of KGM in the CSF correlates well with the degree of encephalopathy. In more recent work, we have shown that KGM is markedly elevated in the urine of patients with inborn errors of the urea cycle. It is suggested that KGM may be a useful biomarker for many hyperammonemic diseases including hepatic encephalopathy, inborn errors of the urea cycle, citrin deficiency and lysinuric protein intolerance.

Impact of minocycline on cerebrospinal fluid markers of oxidative stress, neuronal injury, and inflammation in HIV-seropositive individuals with cognitive impairment

Elevated cerebrospinal fluid (CSF) levels of markers of oxidative stress, neuronal injury, and inflammation and decreased neurotransmitter levels have been reported in HIV-associated neurocognitive disorders (HAND). Minocycline may have a neuroprotective effect by inhibiting inducible nitric oxide synthase, which produces nitric oxide, a compound that induces oxygen free radical production. In A5235, "Phase II, Randomized, Placebo-Controlled, Double-Blind Study of Minocycline in the Treatment of HIV-Associated Cognitive Impairment," minocycline was not associated with cognitive improvement, but the effect on the above CSF measures was not examined previously. The objective of this study was to examine the effect of minocycline on markers of oxidative stress, neuronal injury, neurotransmitter levels, and inflammation from CSF in participants in A5235. One hundred seven HIV+ individuals received either minocycline 100 mg or placebo orally every 12 h for 24 weeks. Twenty-one HIV+ individuals received the optional lumbar punctures. Lipid and protein markers of oxidative stress (e.g., ceramides and protein carbonyls), glutamate, neurotransmitter precursors, kynurenine metabolites, neurofilament heavy chain, and inflammatory cytokines were measured in the CSF before and after treatment. The 24-week change in ceramides was larger in a beneficial direction in the minocycline group compared to the placebo group. The two groups did not differ in the 24-week changes for other markers.These results suggest that minocycline may decrease lipid markers of oxidative stress (ceramides) in individuals with HAND; however, an effect of minocycline on other CSF markers was not observed. A larger sample size is needed to further validate these results.

Evaluation of kynurenine pathway metabolism in Toxoplasma gondii-infected mice: implications for schizophrenia

Toxoplasma gondii, an intracellular protozoan parasite, is a major cause of opportunistic infectious disease affecting the brain and has been linked to an increased incidence of schizophrenia. In murine hosts, infection with T. gondii stimulates tryptophan degradation along the kynurenine pathway (KP), which contains several neuroactive metabolites, including 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN) and kynurenic acid (KYNA). As these endogenous compounds may provide a mechanistic connection between T. gondii and the pathophysiology of schizophrenia, we measured KP metabolites in both the brain and periphery of T. gondii-treated C57BL/6 mice 8 and 28 days post-infection. Infected mice showed early decreases in the levels of tryptophan in the brain and serum, but not in the liver. These reductions were associated with elevated levels of kynurenine, KYNA, 3-HK and QUIN in the brain. In quantitative terms, the most significant increases in these KP metabolites were observed in the brain at 28 days post-infection. Notably, the anti-parasitic drugs pyrimethamine and sulfadiazine, a standard treatment of toxoplasmosis, significantly reduced 3-HK and KYNA levels in the brain of infected mice when applied between 28 and 56 days post-infection. In summary, T. gondii infection, probably by activating microglia and astrocytes, enhances the production of KP metabolites in the brain. However, during the first two months after infection, the KP changes in these mice do not reliably duplicate abnormalities seen in the brain of individuals with schizophrenia.

Improved separation and detection of picolinic acid and quinolinic acid by capillary electrophoresis-mass spectrometry: application to analysis of human cerebrospinal fluid

"Quinolinic acid (QA)", a metabolite of the kynurenine pathway (KP), is implicated as a major neurological biomarker, which causes inflammatory disorders, whereas there is an increase evidence of the role of picolinic acid (PA) in neuroinflammation. Therefore, there is an urgent need to develop new clinical test for early diagnosis of neuroinflammatory disorders. A comparison is made between three different platforms such as high performance liquid chromatography-electrospray mass spectrometry (HPLC-ESI-MS/MS), nano LC-Chip/ESI-MS/MS, as well as the use of cationic (quaternary ammonium) and anionic (sulfonated) coated capillaries in capillary electrophoresis (CE)-ESI-MS/MS. The comparison revealed that CE-ESI-MS/MS method using a quaternary ammonium coated capillary is the best method for analysis of PA and QA. A simple stacking procedure by the inclusion of acetonitrile in the artificial cerebrospinal fluid (CSF) sample was employed to improve the peak shape and sensitivity of KP metabolites in CE-ESI-MS/MS. The developed CE-ESI-MS/MS assay provided high resolution, high specificity and high sensitivity with a total analysis time including sample preparation of nearly 12 min. In addition, excellent intra-day and inter-day repeatability of migration times and peak areas of the metabolites were observed with respective relative standard deviation (RSD) of less than 2.0% and 2.5%. Somewhat broader variations in repeatability for a 3 independently prepared coated capillary (total 35 runs each) with % RSD up to 3.8% and 5.8% was observed for migration time and peak areas, respectively. Artificial CSF was used as a surrogate matrix to simultaneously generate calibration curves over a concentration range of 0.02-10 μM for PA and 0.4-40 μM for QA. The method was then successfully applied to analyze PA and QA in human CSF, demonstrating the potential of this CE-ESI-MS/MS method to accurately quantitate with high specificity and sensitivity.

Targeted deletion of kynurenine 3-monooxygenase in mice: a new tool for studying kynurenine pathway metabolism in periphery and brain

Kynurenine 3-monooxygenase (KMO), a pivotal enzyme in the kynurenine pathway (KP) of tryptophan degradation, has been suggested to play a major role in physiological and pathological events involving bioactive KP metabolites. To explore this role in greater detail, we generated mice with a targeted genetic disruption of Kmo and present here the first biochemical and neurochemical characterization of these mutant animals. Kmo(-/-) mice lacked KMO activity but showed no obvious abnormalities in the activity of four additional KP enzymes tested. As expected, Kmo(-/-) mice showed substantial reductions in the levels of its enzymatic product, 3-hydroxykynurenine, in liver, brain, and plasma. Compared with wild-type animals, the levels of the downstream metabolite quinolinic acid were also greatly decreased in liver and plasma of the mutant mice but surprisingly were only slightly reduced (by ∼20%) in the brain. The levels of three other KP metabolites: kynurenine, kynurenic acid, and anthranilic acid, were substantially, but differentially, elevated in the liver, brain, and plasma of Kmo(-/-) mice, whereas the liver and brain content of the major end product of the enzymatic cascade, NAD(+), did not differ between Kmo(-/-) and wild-type animals. When assessed by in vivo microdialysis, extracellular kynurenic acid levels were found to be significantly elevated in the brains of Kmo(-/-) mice. Taken together, these results provide further evidence that KMO plays a key regulatory role in the KP and indicate that Kmo(-/-) mice will be useful for studying tissue-specific functions of individual KP metabolites in health and disease.

Adenosine triphosphate released from HIV-infected macrophages regulates glutamatergic tone and dendritic spine density on neurons

Despite wide spread use of combination antiretroviral therapy (cART) in developed countries, approximately half of HIV-infected patients will develop impairments in cognitive function. Accumulating evidence suggests that neuronal dysfunction can be precipitated by HIV-infection of macrophages by mechanisms that involve alterations in innate and adaptive immune responses. HIV-infection of macrophages is known to increase the release of soluble neurotoxins. However, the composition of products released from infected macrophages is complex and not fully known. In this study we provide evidence that ATP and other immuno-/neuromodulatory nucleotides are exported from HIV-infected macrophages and modify neuronal structure. Supernatants collected from HIV-infected macrophages (HIV/MDM) contained large amounts of ATP, ADP, AMP and small amounts of adenosine, in addition to glutamate. Dilutions of these supernatants that were sub-threshold for glutamate receptor activation evoked rapid calcium flux in neurons that were completely inhibited by the enzymatic degradation of ATP, or by blockade of calcium permeable purinergic receptors. Applications of these highly diluted HIV/MDM onto neuronal cultures increased the amount of extracellular glutamate by mechanisms dependent on purinergic receptor activation, and downregulated spine density on neurons by mechanisms dependent on purinergic and glutamate receptor activation. We conclude from these data that ATP released from HIV-infected macrophages downregulates dendritic spine density on neurons by a mechanism that involves purinergic receptor mediated modulation of glutamatergic tone. These data suggest that neuronal function may be depressed in HIV infected individuals by mechanisms that involve macrophage release of ATP that triggers secondary effects on glutamate handling.

Development of a CZE-ESI-MS assay with a sulfonated capillary for profiling picolinic acid and quinolinic acid formation in multienzyme system

This article describes the development of a reliable CZE-ESI-MS method to simultaneously separate and quantitate three specific metabolites (3-hydroxyanthranilic acid (3-HAA), quinolinic acid (QA), and picolinic acid (PA)) of the kynurenine pathway (KP) of tryptophan catabolism. Using a covalently bonded sulfonated capillary, the parameters such as pH, type of background electrolyte, type of organic solvent, nebulizer pressure as well as both negative and positive ESI-MS modes were optimized to achieve the best Rs and S/N of three KP metabolites. The developed CZE-ESI-MS assay provided high resolution of PA/QA, high specificity, a total analysis time of 10 min with satisfactory intraday and interday repeatability of migration time and peak areas. Under optimized CZE-ESI-MS conditions, the calibration curves over a concentration range of 19-300 μM for 3-HAA and QA, and 75-300 μM for PA were simultaneously generated. The method was successfully applied for the first time to profile the concentrations of initial substrate, 3-HAA, and its eventual products, PA and QA, formed in the complex multienzyme system. As the ratio of two enzymes, 3-hydroxyanthranilate 3,4-dioxygenase (HAO) and α-amino-β-carboxymuconate-ε-semialdehyde decarboxylase (ACMSD) decreases, the concentration of QA approaches essentially zero indicating that all ACMS formed by the action of HAO is consumed by ACMSD rather than its spontaneous decay to QA.

Development of a fluorescent chemosensor for the detection of kynurenine

Kynurenine, a metabolite of tryptophan, is known to contribute to cancer progression when overproduced. A method for facile fluorescent sensing of kynurenine using sensor 1 has been developed. When bound at low pH, sensor 1 undergoes a very large bathochromic shift because kynurenine extends the conjugation of the fluorophore. This unusual mechanism of activation provides a 390-fold fluorescence enhancement that is very specific to kynurenine and a wavelength of fluorescence that extends into the red.

3-Hydroxykynurenine: an intriguing molecule exerting dual actions in the central nervous system

Kynurenine pathway is gaining attention due to the many metabolic processes in which it has been involved. The tryptophan conversion into several other metabolites through this pathway provides neuronal and redox modulators useful for maintenance of major functions in the brain. However, when physiopathological conditions prevail - i.e. oxidative stress, excitotoxicity, and inflammation - preferential formation and accumulation of toxic metabolites could trigger factors for degeneration in neurological disorders. 3-Hydroxykynurenine has been largely described as one of these toxic metabolites capable of inducing oxidative damage and cell death; consequently, this metabolite has been hypothesized to play a pivotal role in different neurological and psychiatric disorders. Supporting evidence has shown altered 3-hydroxykynurenine levels in samples of patients from several disorders. In contrast, some experimental studies have provided evidence of antioxidant and scavenging properties inherent to this molecule. In this review, we explored most of literature favoring one or the other concept, in order to provide an accurate vision on the real participation of this tryptophan metabolite in both experimental paradigms and human brain pathologies. Through this collected evidence, we provide an integrative hypothesis on how 3-hydroxykynurenine is exerting its dual actions in the central nervous system and what will be the course of investigations in this field for the next years.