Elevated kynurenine pathway metabolism during neurodevelopment: Implications for brain and behavior

Prenatal Stress Impacts Foetal Neurodevelopment: Temporal Windows of Gestational Vulnerability

Prenatal maternal stressors ranging in severity from everyday occurrences/hassles to the experience of traumatic events negatively impact neurodevelopment, increasing the risk for the onset of psychopathology in the offspring. Notably, the timing of prenatal stress exposure plays a critical role in determining the nature and severity of subsequent neurodevelopmental outcomes. In this review, we evaluate the empirical evidence regarding temporal windows of heightened vulnerability to prenatal stress with respect to motor, cognitive, language, and behavioural development in both human and animal studies. We also explore potential temporal windows whereby several mechanisms may mediate prenatal stress-induced neurodevelopmental effects, namely, excessive hypothalamic-pituitary-adrenal axis activity, altered serotonin signalling and sympathetic-adrenal-medullary system, changes in placental function, immune system dysregulation, and alterations of the gut microbiota. While broadly defined developmental windows are apparent for specific psychopathological outcomes, inconsistencies arise when more complex cognitive and behavioural outcomes are considered. Novel approaches to track molecular markers reflective of the underlying aetiologies throughout gestation to identify tractable biomolecular signatures corresponding to critical vulnerability periods are urgently required.

Prenatal MAM exposure raises kynurenic acid levels in the prefrontal cortex of adult rats

BACKGROUND

Elevated brain levels of kynurenic acid (KYNA), a metabolite in the kynurenine pathway, are associated with cognitive dysfunctions, which are nowadays often considered as fundamental characteristics of several psychopathologies; however, the role of KYNA in mental illnesses, such as schizophrenia, is not fully elucidated. This study aimed to assess KYNA levels in the prefrontal cortex (PFC) of rats prenatally treated with methylazoxymethanol (MAM) acetate, i.e., a well-validated neurodevelopmental animal model of schizophrenia. The effects of an early pharmacological modulation of the endogenous cannabinoid system were also evaluated.

METHODS

Pregnant Sprague-Dawley rats were treated with MAM (22 mg/kg, ip) or its vehicle at gestational day 17. Male offspring were treated with the cannabinoid CB1 receptor antagonist/inverse agonist AM251 (0.5 mg/kg/day, ip) or with the typical antipsychotic haloperidol (0.6 mg/kg/day, ip) from postnatal day (PND) 19 to PND39. The locomotor activity and cognitive performance were assessed in the novel object recognition test and the open field test in adulthood. KYNA levels in the PFC of prenatally MAM-treated rats were also assessed.

RESULTS

A significant cognitive impairment was observed in prenatally MAM-treated rats (p < 0.01), which was associated with enhanced PFC KYNA levels (p < 0.05). The peripubertal AM251, but not haloperidol, treatment ameliorated the cognitive deficit (p < 0.05), by normalizing the PFC KYNA content in MAM rats.

CONCLUSIONS

The present findings suggest that the cognitive deficit observed in MAM rats may be related to enhanced PFC KYNA levels which could be, in turn, mediated by the activation of cannabinoid CB1 receptor. These results further support the modulation of brain KYNA levels as a potential therapeutic strategy to ameliorate the cognitive dysfunctions in schizophrenia.

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.

Maternal Inflammation with Elevated Kynurenine Metabolites Is Related to the Risk of Abnormal Brain Development and Behavioral Changes in Autism Spectrum Disorder

Several studies show that genetic and environmental factors contribute to the onset and progression of neurodevelopmental disorders. Maternal immune activation (MIA) during gestation is considered one of the major environmental factors driving this process. The kynurenine pathway (KP) is a major route of the essential amino acid L-tryptophan (Trp) catabolism in mammalian cells. Activation of the KP following neuro-inflammation can generate various endogenous neuroactive metabolites that may impact brain functions and behaviors. Additionally, neurotoxic metabolites and excitotoxicity cause long-term changes in the trophic support, glutamatergic system, and synaptic function following KP activation. Therefore, investigating the role of KP metabolites during neurodevelopment will likely promote further understanding of additional pathophysiology of neurodevelopmental disorders, including autism spectrum disorder (ASD). In this review, we describe the changes in KP metabolism in the brain during pregnancy and represent how maternal inflammation and genetic factors influence the KP during development. We overview the patients with ASD clinical data and animal models designed to verify the role of perinatal KP elevation in long-lasting biochemical, neuropathological, and behavioral deficits later in life. Our review will help shed light on new therapeutic strategies and interventions targeting the KP for neurodevelopmental disorders.

Kynurenine aminotransferase II inhibition promotes sleep and rescues impairments induced by neurodevelopmental insult

Dysregulated sleep is commonly reported in individuals with neuropsychiatric disorders, including schizophrenia (SCZ) and bipolar disorder (BPD). Physiology and pathogenesis of these disorders points to aberrant metabolism, during neurodevelopment and adulthood, of tryptophan via the kynurenine pathway (KP). Kynurenic acid (KYNA), a neuroactive KP metabolite derived from its precursor kynurenine by kynurenine aminotransferase II (KAT II), is increased in the brains of individuals with SCZ and BPD. We hypothesize that elevated KYNA, an inhibitor of glutamatergic and cholinergic neurotransmission, contributes to sleep dysfunction. Employing the embryonic kynurenine (EKyn) paradigm to elevate fetal brain KYNA, we presently examined pharmacological inhibition of KAT II to reduce KYNA in adulthood to improve sleep quality. Pregnant Wistar rats were fed either kynurenine (100 mg/day)(EKyn) or control (ECon) diet from embryonic day (ED) 15 to ED 22. Adult male (N = 24) and female (N = 23) offspring were implanted with devices to record electroencephalogram (EEG) and electromyogram (EMG) telemetrically for sleep-wake data acquisition. Each subject was treated with either vehicle or PF-04859989 (30 mg/kg, s.c.), an irreversible KAT II inhibitor, at zeitgeber time (ZT) 0 or ZT 12. KAT II inhibitor improved sleep architecture maintaining entrainment of the light-dark cycle; ZT 0 treatment with PF-04859989 induced transient improvements in rapid eye movement (REM) and non-REM (NREM) sleep during the immediate light phase, while the impact of ZT 12 treatment was delayed until the subsequent light phase. PF-04859989 administration at ZT 0 enhanced NREM delta spectral power and reduced activity and body temperature. In conclusion, reducing de novo KYNA production alleviated sleep disturbances and increased sleep quality in EKyn, while also improving sleep outcomes in ECon offspring. Our findings place attention on KAT II inhibition as a novel mechanistic approach to treating disrupted sleep behavior with potential translational implications for patients with neurodevelopmental and neuropsychiatric disorders.

Kynurenine pathway metabolites modulated the comorbidity of IBD and depressive symptoms through the immune response

Inflammatory bowel disease (IBD), including Crohn's disease and ulcerative colitis, is defined as chronic inflammation in the gastrointestinal tract. Notably, more than 20% of people with IBD experience depressive symptoms. Understanding the immunological mechanism of chronic intestinal inflammation on cognitive behavior has become a key research focus. Previous studies have shown that a dysregulated immune response contributes to chronic inflammation and depressive symptoms. The tolerant phenotype exhibited by immune cells regulates the course of chronic inflammation in distinct ways. In addition, neuroglia, such as microglia and astrocytes specific to the brain, are also influenced by deregulated inflammation to mediate the development of depressive symptoms. The kynurenine pathway (KP), a significant tryptophan metabolic pathway, transforms tryptophan into a series of KP metabolites that modulate chronic inflammation and depressive symptoms. In particular, indoleamine 2,3-dioxygenase 1 (IDO1), a rate-limiting enzyme in the KP, is activated by chronic inflammation and leads to the production of kynurenine. In addition, disruption of the brain-gut axis induced by IBD allows kynurenine to cross the blood-brain barrier (BBB) and form a series of neuroactive kynurenine metabolites in glial cells. Among them, quinolinic acid continuously accumulates in the brain, indicating depression. Thus, KP metabolites are critical for driving the comorbidity of IBD and depressive symptoms. In this review, the pathological mechanism of KP metabolite-mediated chronic intestinal inflammation and depressive symptoms by regulating the immune response is summarized according to the latest reports.

Enhancing the resolution of behavioral measures: Key observations during a forty year career in behavioral neuroscience

This manuscript reviews several key observations from the research program of Professor John P. Bruno that are believed to have significantly advanced our understanding of the brain's mediation of behavior. This review focuses on findings within several important research areas in behavioral neuroscience, including a) age-dependent neurobehavioral plasticity following brain damage; b) the role of the cortical cholinergic system in attentional processing and cognitive flexibility; and c) the design and validation of animal models of cognitive deficits in schizophrenia. In selecting these observations, emphasis was given to examples in which the heuristic potency was increased by maximizing the resolution and microanalysis of behavioral assays in the same fashion as one typically refines neuronal manipulations. Professor Bruno served the International Behavioral Neuroscience Society (IBNS) as an IBNS Fellow (1995-present) and President of the IBNS (2001-02).

Kynurenine pathway and autism spectrum phenotypes: an investigation among adults with autism spectrum disorder and their first-degree relatives

BACKGROUND

Increasing literature highlighted alterations of tryptophan (TRP) metabolism and kynurenine (KYN) pathway in children with autism spectrum disorder (ASD). However, no study specifically focused on adult samples. Meanwhile, several authors stressed the relevance of investigating neurobiological correlates of adult forms of ASD and of those subthreshold ASD manifestations frequently found in relatives of ASD probands, known as broad autism phenotype (BAP). This work aimed to evaluate circulating levels of TRP and metabolites of KYN pathway in a sample of ASD adults, their first-degree relatives and controls (CTLs), investigating also the correlations between biochemical variables' levels and ASD symptoms.

METHODS

A sample of ASD adults, together with a group of first-degree relatives (BAP group) and unrelated CTLs were assessed by means of psychometric scales. Circulating levels of TRP, KYN, quinolinic acid (QA), and kynurenic acid (KYNA) were assessed in all subjects.

RESULTS

ASD patients reported significantly higher total scores than the other groups on all psychometric scales. BAP subjects scored significantly higher than CTLs. ASD patients reported significantly lower TRP levels than BAP and CTL groups. Moreover, significantly lower levels of KYNA were reported in both ASD and BAP groups than in CTLs. Specific patterns of associations were found between autism symptoms and biochemical variables.

CONCLUSIONS

Our findings confirm in adult samples the presence of altered TRP metabolism through KYN pathway. The intermediate alterations reported among relatives of ASD patients further stress the presence of a continuum between subthreshold and full-threshold ASD phenotypes also from a biochemical perspective.

Modelling the neurodevelopmental pathogenesis in neuropsychiatric disorders. Bioactive kynurenines and their analogues as neuroprotective agents-in celebration of 80th birthday of Professor Peter Riederer

Following introduction of the monoamine oxidase type B inhibitor selegiline for the treatment of Parkinson's disease (PD), discovery of the action mechanism of Alzheimer's disease-modifying agent memantine, the role of iron in PD, and the loss of electron transport chain complex I in PD, and development of the concept of clinical neuroprotection, Peter Riederer launched one of the most challenging research project neurodevelopmental aspects of neuropsychiatric disorders. The neurodevelopmental theory holds that a disruption of normal brain development in utero or during early life underlies the subsequent emergence of neuropsychiatric symptoms during later life. Indeed, the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition and the International Classification of Diseases, 11th Revision categorize autism spectrum disorder and attention deficit hyperactivity disorder in neurodevelopmental disorders (NDDs). More and more evidence, especially from preclinical studies, is revealing that neurodevelopmental pathology is not limited to the diagnostic class above, but also contributes to the development of other psychiatric disorders such as schizophrenia, bipolar disorder, and obsessive-compulsive disorder as well as neurodegenerative diseases such as PD and Huntington's disease. Preclinical animal research is taking a lead in understanding the pathomechanisms of NDDs, searching for novel targets, and developing new neuroprotective agents against NDDs. This narrative review discusses emerging evidence of the neurodevelopmental etiology of neuropsychiatric disorders, recent advances in modelling neurodevelopmental pathogenesis, potential strategies of clinical neuroprotection using novel kynurenine metabolites and analogues, and future research direction for NDDs.

Unexpected content of kynurenine in mother's milk and infant formulas

Mother's milk is widely recommended as complete food for the offspring in earliest postnatal time. However, the knowledge about detailed composition and the physiological role of bioactive components of breast milk is incomplete. Therefore, the aim of our study was to determine the content of kynurenine (KYN) in human breast milk during lactation and to explore the effects exerted by intragastric KYN administration from birth to weaning on physical and psychomotor development of adult rats. We found that KYN is consistently present in human milk and its content gradually increased from day 4 to 28 after delivery and that it is present in commercial baby formulas in amounts noticeably exceeding its physiological range. Animal studies showed that KYN supplementation resulted in a marked elevation of absorptive surface of rat intestine and in enhanced expression of both, aryl hydrocarbon receptor and G protein-coupled receptor 35 in the intestinal tissue in rats. Moreover, we discovered that KYN administration from birth to weaning resulted in neurobehavioral changes in adult rats. Therefore, we postulate that further research is required to thoroughly understand the function of KYN in early developmental stages of mammals and to ensure the safety of its presence in baby food products.

Bifidobacterium longum CCFM1077 Ameliorated Neurotransmitter Disorder and Neuroinflammation Closely Linked to Regulation in the Kynurenine Pathway of Autistic-like Rats

The kynurenine pathway (KP) is abnormal in autistic patients and model animals. According to studies on the brain–gut axis, probiotics can help ameliorate the metabolic abnormalities of the KP in patients and model animals with neurological diseases. This study was aimed at evaluating the ability of Bifidobacterium longum (B. longum) CCFM077 to enhance the gut microbiome and KP metabolism and regulate the neurotransmitter levels and neuroinflammation of autistic rats. The KP metabolism of autistic rats was significantly disordered and significantly related to the regulation of neurotransmitter (excitation and inhibition) and neuroglia states. B. longum CCFM1077 could effectively alleviate autistic-like behaviours (repetitive stereotyped behaviour, learning and memory ability, and despair mood) and regulate the KP metabolism in the periphery system (gut and blood) and brain. In particular, B. longum CCFM1077 could significant regulate the quinolinic acid (QUIN) level in the brain and markedly regulate glutamic acid (Glu) and Glu/γ-aminobutyric acid (GABA) levels in the brain while alleviating microglia activity in the cerebellum. Through a correlation analysis, the QUIN level in the brain was strongly related with autistic-like behaviours and neurotransmitter levels (GABA and Glu). The QUIN level may thus be a potential therapeutic marker for treating autism through the intestinal and neural pathways.

Dynamics of the infant gut microbiota in the first 18 months of life: the impact of maternal HIV infection and breastfeeding

BACKGROUND

Access to antiretroviral therapy (ART) during pregnancy and breastfeeding for mothers with HIV has resulted in fewer children acquiring HIV peri- and postnatally, resulting in an increase in the number of children who are exposed to the virus but are not infected (HEU). HEU infants have an increased likelihood of childhood infections and adverse growth outcomes, as well as increased mortality compared to their HIV-unexposed (HUU) peers. We explored potential differences in the gut microbiota in a cohort of 272 Nigerian infants born to HIV-positive and negative mothers in this study during the first 18 months of life.

RESULTS

The taxonomic composition of the maternal vaginal and gut microbiota showed no significant differences based on HIV status, and the composition of the infant gut microbiota at birth was similar between HUU and HEU. Longitudinal taxonomic composition of the infant gut microbiota and weight-for-age z-scores (WAZ) differed depending on access to breast milk. HEU infants displayed overall lower WAZ than HUU infants at all time points. We observed a significantly lower relative abundance of Bifidobacterium in HEU infants at 6 months postpartum. Breast milk composition also differed by time point and HIV infection status. The antiretroviral therapy drugs, lamivudine and nevirapine, as well as kynurenine, were significantly more abundant in the breast milk of mothers with HIV. Levels of tiglyl carnitine (C5) were significantly lower in the breast milk of mothers without HIV. ART drugs in the breast milk of mothers with HIV were associated with a lower relative abundance of Bifidobacterium longum.

CONCLUSIONS

Maternal HIV infection was associated with adverse growth outcomes of HEU infants in this study, and these differences persist from birth through at least 18 months, which is a critical window for the development of the immune and central nervous systems. We observed that the relative abundance of Bifidobacterium spp. was significantly lower in the gut microbiota of all HEU infants over the first 6 months postpartum, even if HEU infants were receiving breast milk. Breastfeeding was of benefit in our HEU infant cohort in the first weeks postpartum; however, ART drug metabolites in breast milk were associated with a lower abundance of Bifidobacterium. Video abstract.

Cellular Localization of Kynurenine 3-Monooxygenase in the Brain: Challenging the Dogma

Kynurenine 3-monooxygenase (KMO), a key player in the kynurenine pathway (KP) of tryptophan degradation, regulates the synthesis of the neuroactive metabolites 3-hydroxykynurenine (3-HK) and kynurenic acid (KYNA). KMO activity has been implicated in several major brain diseases including Huntington’s disease (HD) and schizophrenia. In the brain, KMO is widely believed to be predominantly localized in microglial cells, but verification in vivo has not been provided so far. Here, we examined KP metabolism in the brain after depleting microglial cells pharmacologically with the colony stimulating factor 1 receptor inhibitor PLX5622. Young adult mice were fed PLX5622 for 21 days and were euthanized either on the next day or after receiving normal chow for an additional 21 days. Expression of microglial marker genes was dramatically reduced on day 22 but had fully recovered by day 43. In both groups, PLX5622 treatment failed to affect Kmo expression, KMO activity or tissue levels of 3-HK and KYNA in the brain. In a parallel experiment, PLX5622 treatment also did not reduce KMO activity, 3-HK and KYNA in the brain of R6/2 mice (a model of HD with activated microglia). Finally, using freshly isolated mouse cells ex vivo, we found KMO only in microglia and neurons but not in astrocytes. Taken together, these data unexpectedly revealed that neurons contain a large proportion of functional KMO in the adult mouse brain under both physiological and pathological conditions.

Characterization of the temporal, cell-specific and interferon-inducible patterns of indoleamine 2,3 dioxygenase 1 (IDO1) expression in the human placenta across gestation

INTRODUCTION

The human placenta performs multiple functions necessary for successful pregnancy, but the metabolic pathways and molecular mechanisms responsible for regulating placental development and functions remain incompletely understood. Catabolism of the essential amino acid tryptophan has numerous critical roles in normal physiology, including inflammation. The kynurenine pathway, which accounts for ∼90% of tryptophan breakdown, is mediated by indoleamine 2,3 dioxygenase 1 (IDO1) in the placenta. In pregnant mice, alterations of IDO1 activity or expression result in fetal resorption and a preeclampsia-like phenotype. Decreased IDO1 expression at the maternal-fetal interface has also been linked to preeclampsia, in utero growth restriction and recurrent miscarriage in humans. These collective observations suggest essential role(s) for IDO1 in maintaining healthy pregnancy. Despite these important roles, the precise temporal, cell-specific and inflammatory cytokine-mediated patterns of IDO1 expression in the human placenta have not been thoroughly characterized across gestation.

METHODS

Western blot and whole mount immunofluorescence (WMIF) were utilized to characterize and quantify basal and interferon (IFN)-inducible IDO1 expression in 1st trimester (7-13 weeks), 2nd trimester (14-22 weeks) and term (39-41 weeks) placental villi.

RESULTS

IDO1 expression is activated in the human placenta between the 13th and 14th weeks of pregnancy, increases through the 2nd trimester and remains elevated at term. Constitutive IDO1 expression is restricted to placental endothelial cells. Interestingly, different types of IFNs have distinct effects on IDO1 expression in the human placenta.

DISCUSSION

Our collective results are consistent with potential role(s) for IDO1 in the regulation of vascular functions in placental villi.

Pro/Antioxidant State as a Potential Biomarker of Schizophrenia

To allow better diagnosis and management of psychiatric illnesses, the use of easily accessible biomarkers are proposed. Therefore, recognition of some diseases by a set of related pathogenesis biomarkers is a promising approach. The study aims to assess the usefulness of examining oxidative stress (OS) in schizophrenia as a potential biomarker of illness using the commonly used data mining decision tree method. The study group was comprised of 147 participants: 98 patients with schizophrenia (SZ group), and the control group (n = 49; HC). The patients with schizophrenia were divided into two groups: first-episode schizophrenia (n = 49; FS) and chronic schizophrenia (n = 49; CS). The assessment included the following biomarkers in sera of patients: catalase (CAT), glutathione peroxidase (GPx), superoxide dismutase-1 (SOD-1), glutathione reductase (GR), reduced glutathione (GSH), total antioxidant capacity (TAC), ferric reducing ability of plasma (FRAP), advanced glycation end products (AGEs), advanced oxidation protein products (AOPP), dityrosine (DITYR), kynurenine (KYN), N-formylkynurenine (NFK), tryptophan (TRY), total oxidant status (TOS), nitric oxide (NO) and total protein. Maximum accuracy (89.36%) for distinguishing SZ from HC was attained with TOS and GPx (cut-off points: 392.70 and 15.33). For differentiating between FS and CS, the most promising were KYN, AOPP, TAC and NO (100%; cut-off points: 721.20, 0.55, 64.76 and 2.59). To distinguish FS from HC, maximum accuracy was found for GSH and TOS (100%; cut-off points: 859.96 and 0.31), and in order to distinguish CS from HC, the most promising were GSH and TOS (100%; cut-off points: 0.26 and 343.28). Using redox biomarkers would be the most promising approach for discriminating patients with schizophrenia from healthy individuals and, in the future, could be used as an add-on marker to diagnose and/or respond to treatment.

Methylphenidate ameliorates the homeostatic balance between levels of kynurenines in ADHD children

The kynurenine pathway of tryptophan metabolism has been involved in ADHD We quantified basal levels and daily fluctuations of tryptophan and several kynurenine metabolites, as well as their changes after treatment with methylphenidate (MPH). A total of 179 children were recruited, grouped into ADHD (n = 130) and healthy controls (CG,n = 49). Blood samples were drawn at 20:00 and 09:00 h and only in the ADHD group after 4.63±2.3 months of treatment. Nocturnal urine was collected between both draws. Factorial analysis (Stata12.0) was performed with Groups, Time, Hour of Day and Depressive Symptoms (DS) as factors. MPH significantly increased plasma Kynurenic acid (2.4 ± 1.03/2.78±1.3 ng/mL; baseline/post-treatment, morning; z = 1.96,p<0.05) and Xanthurenic acid (2.39±0.95/2.88±1.19 ng/mL; baseline/post, morning; z = 2.7,p<0.007) levels, both with higher values in the evening. In DS+ patients, MPH caused a pronounced decrease in evening Anthranilic acid [3.08±5.02/ 1.82±1.46 ng/mL, z = 2.68,p = 0.0074] until matching values to other subgroups. In urine, MPH decreased the excretion of both Nicotinamide and Quinolinic acids, but only in the DS- subgroup. The kynurenine pathway may participate in the highly clinical favorable response to MPH. The observed changes could be considered as protective (i.e. increased plasma kynurenic acid vs. decreased quinolinic acid excretion) based on the knowledge of its physiological homeostatic functions.

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.

The role of the gut microbiome in the development of schizophrenia

Schizophrenia is a heterogeneous neurodevelopmental disorder involving the convergence of a complex and dynamic bidirectional interaction of genetic expression and the accumulation of prenatal and postnatal environmental risk factors. The development of the neural circuitry underlying social, cognitive and emotional domains requires precise regulation from molecular signalling pathways, especially during critical periods or "windows", when the brain is particularly sensitive to the influence of environmental input signalling. Many of the brain regions involved, and the molecular substrates sub-serving these domains are responsive to life-long microbiota-gut-brain (MGB) axis signalling. This intricate microbial signalling system communicates with the brain via the vagus nerve, immune system, enteric nervous system, enteroendocrine signalling and production of microbial metabolites, such as short-chain fatty acids. Preclinical data has demonstrated that MGB axis signalling influences neurotransmission, neurogenesis, myelination, dendrite formation and blood brain barrier development, and modulates cognitive function and behaviour patterns, such as, social interaction, stress management and locomotor activity. Furthermore, preliminary clinical studies suggest altered gut microbiota profiles in schizophrenia. Unravelling MGB axis signalling in the context of an evolving dimensional framework in schizophrenia may provide a more complete understanding of the neurobiological architecture of this complex condition and offers the possibility of translational interventions.

DNA Methylation of PGC-1α Is Associated With Elevated mtDNA Copy Number and Altered Urinary Metabolites in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a complex disorder that is underpinned by numerous dysregulated biological pathways, including pathways that affect mitochondrial function. Epigenetic mechanisms contribute to this dysregulation and DNA methylation is an important factor in the etiology of ASD. We measured DNA methylation of peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1α), as well as five genes involved in regulating mitochondrial homeostasis to examine mitochondrial dysfunction in an ASD cohort of South African children. Using targeted Next Generation bisulfite sequencing, we found differential methylation (p < 0.05) at six key genes converging on mitochondrial biogenesis, fission and fusion in ASD, namely PGC-1α, STOML2, MFN2, FIS1, OPA1, and GABPA. PGC-1α, the transcriptional regulator of biogenesis, was significantly hypermethylated at eight CpG sites in the gene promoter, one of which contained a putative binding site for CAMP response binding element 1 (CREB1) (p = 1 × 10^–6). Mitochondrial DNA (mtDNA) copy number, a marker of mitochondrial function, was elevated (p = 0.002) in ASD compared to controls and correlated significantly with DNA methylation at the PGC-1α promoter and there was a positive correlation between methylation at PGC-1α CpG#1 and mtDNA copy number (Spearman’s r = 0.2, n = 49, p = 0.04) in ASD. Furthermore, DNA methylation at PGC-1α CpG#1 and mtDNA copy number correlated significantly (p < 0.05) with levels of urinary organic acids associated with mitochondrial dysfunction, oxidative stress, and neuroendocrinology. Our data show differential methylation in ASD at six key genes converging on PGC-1α-dependent regulation of mitochondrial biogenesis and function. We demonstrate that methylation at the PGC-1α promoter is associated with elevated mtDNA copy number and metabolomic evidence of mitochondrial dysfunction in ASD. This highlights an unexplored role for DNA methylation in regulating specific pathways involved in mitochondrial biogenesis, fission and fusion contributing to mitochondrial dysfunction in ASD.

The Effects of Maternal Interleukin-17A on Social Behavior, Cognitive Function, and Depression-Like Behavior in Mice with Altered Kynurenine Metabolites

Viral infection and chronic maternal inflammation during pregnancy are correlated with a higher prevalence of autism spectrum disorder (ASD). However, the pathoetiology of ASD is not fully understood; moreover, the key molecules that can cross the placenta following maternal inflammation and contribute to the development of ASD have not been identified. Recently, the pro-inflammatory cytokine, interleukin-17A (IL-17A) was identified as a potential mediator of these effects. To investigate the impact of maternal IL-17A on offspring, C57BL/6J dams were injected with IL-17A-expressing plasmids via the tail vein on embryonic day 12.5 (E12.5), and maternal IL-17A was expressed continuously throughout pregnancy. By adulthood, IL-17A-injected offspring exhibited behavioral abnormalities, including social and cognitive defects. Additionally, maternal IL-17A promoted metabolism of the essential amino acid tryptophan, which produces several neuroactive compounds and may affect fetal neurodevelopment. We observed significantly increased levels of kynurenine in maternal serum and fetal plasma. Thus, we investigated the effects of high maternal concentration of kynurenine on offspring by continuously administering mouse dams with kynurenine from E12.5 during gestation. Obviously, maternal kynurenine administration rapidly increased kynurenine levels in the fetal plasma and brain, pointing to the ability of kynurenine to cross the placenta and change the KP metabolites which are affected as neuroactive compounds in the fetal brain. Notably, the offspring of kynurenine-injected mice exhibited behavioral abnormalities similar to those observed in offspring of IL-17A-conditioned mice. Several tryptophan metabolites were significantly altered in the prefrontal cortex of the IL-17A-conditioned and kynurenine-injected adult mice, but not in the hippocampus. Even though we cannot exclude the possibility or other molecules being related to ASD pathogenesis and the presence of a much lower degree of pathway activation, our results suggest that increased kynurenine following maternal inflammation may be a key factor in changing the balance of KP metabolites in fetal brain during neuronal development and represents a therapeutic target for inflammation-induced ASD-like phenotypes.

Prenatal inflammation as a link between placental expression signature of tryptophan metabolism and preterm birth

Spontaneous preterm birth is a serious medical condition responsible for substantial perinatal morbidity and mortality. Its phenotypic characteristics, preterm labor with intact membranes (PTL) and preterm premature rupture of the membranes (PPROM), are associated with significantly increased risks of neurological and behavioral alterations in childhood and later life. Recognizing the inflammatory milieu associated with PTL and PPROM, here, we examined expression signatures of placental tryptophan metabolism, an important pathway in prenatal brain development and immunotolerance. The study was performed in a well-characterized clinical cohort of healthy term pregnancies (n = 39) and 167 preterm deliveries (PTL, n = 38 and PPROM, n = 129). Within the preterm group, we then investigated potential mechanistic links between differential placental tryptophan pathway expression, preterm birth and both intra-amniotic markers (such as amniotic fluid interleukin-6) and maternal inflammatory markers (such as maternal serum C-reactive protein and white blood cell count). We show that preterm birth is associated with significant changes in placental tryptophan metabolism. Multifactorial analysis revealed similarities in expression patterns associated with multiple phenotypes of preterm delivery. Subsequent correlation computations and mediation analyses identified links between intra-amniotic and maternal inflammatory markers and placental serotonin and kynurenine pathways of tryptophan catabolism. Collectively, the findings suggest that a hostile inflammatory environment associated with preterm delivery underlies the mechanisms affecting placental endocrine/transport functions and may contribute to disruption of developmental programming of the fetal brain.

Prenatal Kynurenine Elevation Elicits Sex-Dependent Changes in Sleep and Arousal During Adulthood: Implications for Psychotic Disorders

Dysregulation of the kynurenine pathway (KP) of tryptophan catabolism has been implicated in psychotic disorders, including schizophrenia and bipolar disorder. Kynurenic acid (KYNA) is a KP metabolite synthesized by kynurenine aminotransferases (KATs) from its biological precursor kynurenine and acts as an endogenous antagonist of N-methyl-D-aspartate and α7-nicotinic acetylcholine receptors. Elevated KYNA levels found in postmortem brain tissue and cerebrospinal fluid of patients are hypothesized to play a key role in the etiology of cognitive symptoms observed in psychotic disorders. Sleep plays an important role in memory consolidation, and sleep disturbances are common among patients. Yet, little is known about the effect of altered KP metabolism on sleep-wake behavior. We presently utilized a well-established experimental paradigm of embryonic kynurenine (EKyn) exposure wherein pregnant dams are fed a diet laced with kynurenine the last week of gestation and hypothesized disrupted sleep-wake behavior in adult offspring. We examined sleep behavior in adult male and female offspring using electroencephalogram and electromyogram telemetry and determined sex differences in sleep and arousal in EKyn offspring. EKyn males displayed reduced rapid eye movement sleep, while female EKyn offspring were hyperaroused compared to controls. We determined that EKyn males maintain elevated brain KYNA levels, while KYNA levels were unchanged in EKyn females, yet the activity levels of KAT I and KAT II were reduced. Our findings indicate that elevated prenatal kynurenine exposure elicits sex-specific changes in sleep-wake behavior, arousal, and KP metabolism.

The Kynurenine Pathway is Differentially Activated in Children with Lyme Disease and Tick-Borne Encephalitis

In children, tick-borne encephalitis and neuroborreliosis are common infections affecting the central nervous system. As inflammatory pathways including cytokine expression are activated in these children and appear to be of importance for outcome, we hypothesized that induction of the kynurenine pathway may be part of the pathophysiological mechanism. Inflammatory biomarkers were analyzed in cerebrospinal fluid from 22 children with tick-borne encephalitis (TBE), 34 children with neuroborreliosis (NB) and 6 children with no central nervous system infection. Cerebrospinal fluid levels of kynurenine and kynurenic acid were increased in children with neuroborreliosis compared to the comparison group. A correlation was seen between expression of several cerebrospinal fluid cytokines and levels of kynurenine and kynurenic acid in children with neuroborreliosis but not in children with tick-borne encephalitis. These findings demonstrate a strong induction of the kynurenine pathway in children with neuroborreliosis which differs from that seen in children with tick-borne encephalitis. The importance of brain kynurenic acid (KYNA) in both immune modulation and neurotransmission raises the possibility that abnormal levels of the compound in neuroborreliosis might be of importance for the pathophysiology of the disease. Drugs targeting the enzymes of this pathway may open the venue for novel therapeutic interventions.

Improvement in the Prediction of Neonatal Hypoxic-Ischemic Encephalopathy with the Integration of Umbilical Cord Metabolites and Current Clinical Makers

OBJECTIVE

To validate our previously identified candidate metabolites, and to assess the ability of these metabolites to predict hypoxic-ischemic encephalopathy (HIE) both individually and combined with clinical data.

STUDY DESIGN

Term neonates with signs of perinatal asphyxia, with and without HIE, and matched controls were recruited prospectively at birth from 2 large maternity units. Umbilical cord blood was collected for later batch metabolomic analysis by mass spectroscopy along with clinical details. The optimum selection of clinical and metabolites features with the ability to predict the development of HIE was determined using logistic regression modelling and machine learning techniques. Outcome of HIE was determined by clinical Sarnat grading and confirmed by electroencephalogram grade at 24 hours.

RESULTS

Fifteen of 27 candidate metabolites showed significant alteration in infants with perinatal asphyxia or HIE when compared with matched controls. Metabolomic data predicted the development of HIE with an area under the curve of 0.67 (95% CI, 0.62-0.71). Lactic acid and alanine were the primary metabolite predictors for the development of HIE, and when combined with clinical data, gave an area under the curve of 0.96 (95% CI, 0.92-0.95).

CONCLUSIONS

By combining clinical and metabolic data, accurate identification of infants who will develop HIE is possible shortly after birth, allowing early initiation of therapeutic hypothermia.

Dysregulation of kynurenine pathway and potential dynamic changes of kynurenine in schizophrenia: A systematic review and meta-analysis

The kynurenine (KYN) pathway is postulated to play various roles in immune system dysregulation of schizophrenia (SCZ). We conducted a meta-analysis to explore the association between six key metabolites of KYN pathway (i.e., tryptophan (TRP), KYN, quinolinic acid (QUIN), and kynurenic acid (KYNA)) and SCZ. Priori Bonferroni adjustments were conducted for multiple comparisons. In total, 42 studies that examined the relationship between the metabolites in KYN pathway mentioned above and SCZ in 4217 participants and nine studies that examined alterations of these metabolites after antipsychotic treatments were included. The results demonstrate that (1) subjects with prescribed medication had significantly higher KYN levels when compared to controls; (2) higher KYN levels in cerebrospinal fluid (CSF), lower plasma KYN levels and higher CSF KYNA levels were associated with SCZ; (3) the KYN levels were higher in subjects with SCZ after antipsychotic treatments when compared with baseline. The evidence provides valuable insight of the potential underlying involvement of the KYN pathway in the pathogenesis of SCZ.

Identifying a biological signature of prenatal maternal stress

Psychological stress affects maternal gastrointestinal (GI) permeability, leading to low-grade inflammation, which can negatively affect fetal development. We investigated a panel of circulating markers as a biological signature of this stress exposure in pregnant women with and without the stress-related GI disorder irritable bowel syndrome (IBS). Markers of GI permeability and inflammation were measured in plasma from healthy and IBS cohorts of women at 15 and 20 weeks’ gestation. Biomarkers were evaluated with respect to their degree of association to levels of stress, anxiety, and depression as indicated by responses from the Perceived Stress Scale, State-Trait Anxiety Inventory, and Edinburgh Postnatal Depression Scale. High levels of stress were associated with elevations of soluble CD14, lipopolysaccharide binding protein (LBP), and tumor necrosis factor–α, while anxiety was associated with elevated concentrations of C-reactive protein (CRP) in otherwise healthy pregnancies. Prenatal depression was associated with higher levels of soluble CD14, LBP, and CRP in the healthy cohort. High levels of prenatal anxiety and depression were also associated with lower concentrations of tryptophan and kynurenine, respectively, in the IBS cohort. These markers may represent a core maternal biological signature of active prenatal stress, which can be used to inform intervention strategies via stress reduction techniques or other lifestyle approaches. Such interventions may need to be tailored to reflect underlying GI conditions, such as IBS.

Prefrontal α7nAChR Signaling Differentially Modulates Afferent Drive and Trace Fear Conditioning Behavior in Adolescent and Adult Rats

Increased level of kynurenic acid is thought to contribute to the development of cognitive deficits in schizophrenia through an α7nAChR-mediated mechanism in the prefrontal cortex (PFC). However, it remains unclear to what extent disruption of PFC α7nAChR signaling impacts afferent transmission and its modulation of behavior. Using male rats, we found that PFC infusion of methyllycaconitine (MLA; α7nAChR antagonist) shifts ventral hippocampal-induced local field potential (LFP) suppression to LFP facilitation, an effect only observed in adults. Hippocampal stimulation can also elicit a GluN2B-mediated LFP potentiation (when PFC GABA_AR is blocked) that is insensitive to MLA. Conversely, PFC infusion of MLA diminished the gain of amygdalar transmission, which is already enabled by postnatal day (P)30. Behaviorally, the impact of prefrontal MLA on trace fear-conditioning and extinction was also age related. While freezing behavior during conditioning was reduced by MLA only in adults, it elicited opposite effects in adolescent and adult rats during extinction as revealed by the level of reduced and increased freezing response, respectively. We next asked whether the late-adolescent onset of α7nAChR modulation of hippocampal inputs contributes to the age-dependent effect of MLA during extinction. Data revealed that the increased freezing behavior elicited by MLA in adult rats could be driven by a dysregulation of the GluN2B transmission in the PFC. Collectively, these results indicate that distinct neural circuits are recruited during the extinction of trace fear memory in adolescents and adults, likely because of the late-adolescent maturation of the ventral hippocampal-PFC functional connectivity and its modulation by α7nAChR signaling.SIGNIFICANCE STATEMENT Abnormal elevation of the astrocyte-derived metabolite kynurenic acid in the prefrontal cortex (PFC) is thought to impair cognitive functions in schizophrenia through an α7nAChR-mediated mechanism. Here, we found that prefrontal α7nAChR signaling is recruited to control the gain of hippocampal and amygdalar afferent transmission in an input-specific, age-related manner during the adolescent transition to adulthood. Behaviorally, prefrontal α7nAChR modulation of trace fear memory was also age-related, likely because of the late-adolescent maturation of the ventral hippocampal pathway and its recruitment of PFC GABAergic transmission enabled by local α7nAChR signaling. Collectively, these results reveal that distinct α7nAChR-sensitive neural circuits contribute to regulate behavior responses in adolescents and adults, particularly those requiring proper integration of hippocampal and amygdalar inputs by the PFC.

Transfer of uremic solutes across the human term placenta: An ex vivo study in the dual-side perfused cotyledon

INTRODUCTION

An increasing number of women becomes pregnant while suffering from chronic kidney disease (CKD). As a result of decreased renal function, uremic solutes circulate at high levels in the maternal circulation. This study aimed to acquire more knowledge about the placental transfer of uremic solutes across the human placenta.

METHODS

Placental transfer was studied in healthy term placentas, via the ex vivo dual-side human cotyledon perfusion technique (closed-closed set-up for both maternal and fetal circulations). Uremic solute concentrations in maternal and fetal perfusates were measured via LC-MS/MS over 180 min of perfusion.

RESULTS

We found that the studied compounds demonstrated different degrees of placental transfer. Fetal-to-maternal perfusate ratios at t = 180 min were for anthranilic acid 1.00 ± 0.02, indole-3-acetic acid 0.47 ± 0.08, hippuric acid 0.36 ± 0.18, l-arabinitol 0.33 ± 0.04, indoxyl sulfate 0.33 ± 0.11, neopterin 0.28 ± 0.14 and kynurenic acid 0.13 ± 0.03. All uremic solutes studied also emerged in the perfusates when cotyledons were perfused in the absence of uremic solute concentrations added to the maternal reservoir. For kynurenin these concentrations were so high, it complicated the calculation of a transfer ratio for the exogenously administered compound.

DISCUSSION

After 180 min of exposure the extent of placental transfer differs substantially for the solutes studied, reflecting different transfer rates. Future studies should investigate to what extent specific uremic solutes reach the fetal circulation in vivo and how they may interfere with organ function and development of the unborn child.

Nutritional Support of Neurodevelopment and Cognitive Function in Infants and Young Children-An Update and Novel Insights

Proper nutrition is crucial for normal brain and neurocognitive development. Failure to optimize neurodevelopment early in life can have profound long-term implications for both mental health and quality of life. Although the first 1000 days of life represent the most critical period of neurodevelopment, the central and peripheral nervous systems continue to develop and change throughout life. All this time, development and functioning depend on many factors, including adequate nutrition. In this review, we outline the role of nutrients in cognitive, emotional, and neural development in infants and young children with special attention to the emerging roles of polar lipids and high quality (available) protein. Furthermore, we discuss the dynamic nature of the gut-brain axis and the importance of microbial diversity in relation to a variety of outcomes, including brain maturation/function and behavior are discussed. Finally, the promising therapeutic potential of psychobiotics to modify gut microbial ecology in order to improve mental well-being is presented. Here, we show that the individual contribution of nutrients, their interaction with other micro- and macronutrients and the way in which they are organized in the food matrix are of crucial importance for normal neurocognitive development.

Alpha7 nicotinic-N-methyl-D-aspartate hypothesis in the treatment of schizophrenia and beyond

Development of novel treatments for positive, cognitive, and negative symptoms continue to be a high-priority area of schizophrenia research and a major unmet clinical need. Given that all randomized controlled trials (RCTs) conducted to date failed with one add-on medication/mechanism of action, future RCTs with the same approach are not warranted. Even if the field develops a medication for cognition, others are still needed to treat negative and positive symptoms. Therefore, fixing one domain does not completely solve the problem. Also, targeting the cholinergic system, glutamatergic system, and cholinergic plus alpha7 nicotinic and N-methyl-D-aspartate (NMDA) receptors failed independently. Hence, targeting other less important pathophysiological mechanisms/targets is unlikely to be successful. Meta-analyses of RCTs targeting major pathophysiological mechanisms have found some efficacy signal in schizophrenia; thus, combination treatments with different mechanisms of action may enhance the efficacy signal. The objective of this article is to highlight the importance of conducting RCTs with novel combination treatments in schizophrenia to develop antischizophrenia treatments. Positive RCTs with novel combination treatments that target the alpha7 nicotinic and NMDA receptors simultaneously may lead to a disease-modifying therapeutic armamentarium in schizophrenia. Novel combination treatments that concurrently improve the three domains of psychopathology and several prognostic and theranostic biomarkers may facilitate therapeutic discovery in schizophrenia.

Analyzing the Potential Biological Determinants of Autism Spectrum Disorder: From Neuroinflammation to the Kynurenine Pathway

Autism Spectrum Disorder (ASD) etiopathogenesis is still unclear and no effective preventive and treatment measures have been identified. Research has focused on the potential role of neuroinflammation and the Kynurenine pathway; here we review the nature of these interactions. Pre-natal or neonatal infections would induce microglial activation, with secondary consequences on behavior, cognition and neurotransmitter networks. Peripherally, higher levels of pro-inflammatory cytokines and anti-brain antibodies have been identified. Increased frequency of autoimmune diseases, allergies, and recurring infections have been demonstrated both in autistic patients and in their relatives. Genetic studies have also identified some important polymorphisms in chromosome loci related to the human leukocyte antigen (HLA) system. The persistence of immune-inflammatory deregulation would lead to mitochondrial dysfunction and oxidative stress, creating a self-sustaining cytotoxic loop. Chronic inflammation activates the Kynurenine pathway with an increase in neurotoxic metabolites and excitotoxicity, causing long-term changes in the glutamatergic system, trophic support and synaptic function. Furthermore, overactivation of the Kynurenine branch induces depletion of melatonin and serotonin, worsening ASD symptoms. Thus, in genetically predisposed subjects, aberrant neurodevelopment may derive from a complex interplay between inflammatory processes, mitochondrial dysfunction, oxidative stress and Kynurenine pathway overexpression. To validate this hypothesis a new translational research approach is necessary.

Seasonality of plasma tryptophan and kynurenine in pregnant mothers with a history of seasonal affective disorder: Vulnerability or adaptation?

Objectives: Maternal-foetal tryptophan metabolism plays multiple roles in neurodevelopment and immunomodulation across pregnancy. Tryptophan and the immune system are both influenced by the seasons of the year. We thus compared tryptophan and kynurenine levels in subgroups of pregnant women defined by maternal seasonality and season-of-conception (SoC).Methods: Maternal plasma samples taken at 9-15 and 23-29 weeks of pregnancy were analysed in 47 women with historical full or sub-syndromal Seasonal Affective Disorder (SAD) and 144 pregnant controls. Repeated measure ANCOVAs compared tryptophan and kynurenine levels in the two study groups over the two pregnancy sampling times, using SoC as a moderator.Results: Significant differences in both plasma tryptophan and kynurenine were found across the eight subgroups defined by maternal seasonality and SoC. These results were independent of the state of depression.Conclusions: Pregnant women with a history of full or sub-syndromal SAD exhibited a different pattern of plasma tryptophan and kynurenine across the seasons compared to control mothers, independent of current mood state. Follow-up of the children will determine the implications of these findings for neurodevelopment and psychiatric risk. Maternal seasonality and SoC may be important considerations when studying tryptophan and its metabolites in human pregnancy and foetal brain development.

Glutamate Attenuates the Survival Property of IGFR through NR2B Containing N-Methyl-D-aspartate Receptors in Cortical Neurons

Glutamate-induced neurotoxicity is involved in various neuronal diseases, such as Alzheimer's disease. We have previously reported that glutamate attenuated the survival signaling of insulin-like growth factor-1 (IGF-1) by N-methyl-D-aspartate receptors (NMDARs) in cultured cortical neurons, which is viewed as a novel mechanism of glutamate-induced neurotoxicity. However, the phosphorylation sites of IGF-1 receptor (IGF-1R) affected by glutamate remain to be elucidated, and importantly, which subtype of NMDARs plays a major role in attenuating the prosurvival effect of IGF-1 is still unknown. In the present study, glutamate was found to attenuate the tyrosine phosphorylation of the IGF-1R and the prosurvival effect of IGF-1 in primary cultured cortical neurons. NMDAR inhibitors, MK801 and AP-5, blocked the inhibitory effect of glutamate on the phosphorylation of IGF-1R and increased cell survival, while DNQX, LY341495, and CPCCOEt had no effect. Interestingly, we found that glutamate decreased the phosphorylation of tyrosine residues 1131, 1135/1136, 1250/1251, and 1316, while it had no effect on tyrosine 950 in cortical neurons. Moreover, using specific antagonists and siRNA to downregulate individual NMDAR subunits, we found that the activation of NR2B-containing NMDARs was essential for glutamate to inhibit IGF-1 signaling. These findings indicate that the glutamate-induced attenuation of IGF-1 signaling is mediated by NR2B-containing NMDARs. Our study also proposes a novel mechanism of altering neurotrophic factor signaling by the activation of NMDARs.

Exposure to elevated embryonic kynurenine in rats: Sex-dependent learning and memory impairments in adult offspring

Distinct abnormalities in kynurenine pathway (KP) metabolism have been reported in various psychiatric disorders, including schizophrenia (SZ). Kynurenic acid (KYNA), a neuroactive metabolite of the KP, is elevated in individuals diagnosed with SZ and has been linked to cognitive impairments seen in the disorder. To further understand the role of KYNA in SZ etiology, we developed a prenatal insult model where kynurenine (100 mg/day) is fed to pregnant Wistar rats from embryonic day (ED) 15 to ED 22. As sex differences in the prevalence and severity of SZ have been observed, we presently investigated the impact of prenatal kynurenine exposure on KP metabolism and spatial learning and memory in male and female offspring. Specifically, brain tissue and plasma from offspring (control: ECon; kynurenine-treated: EKyn) in prepuberty (postnatal day (PD) 21), adolescence (PD 32-35), and adulthood (PD 56-85) were collected. Separate cohorts of adult offspring were tested in the Barnes maze to assess hippocampus- and prefrontal cortex-mediated learning and memory. Plasma tryptophan, kynurenine, and KYNA were unchanged between ECon and EKyn offspring across all three ages. Hippocampal and frontal cortex KYNA were elevated in male EKyn offspring only in adulthood, compared to ECon, while brain KYNA levels were unchanged in adult females. Male EKyn offspring were significantly impaired during acquisition of the Barnes maze and during reversal learning in the task. In female EKyn offspring, learning and memory remained relatively intact. Taken together, our data demonstrate that exposure to elevated kynurenine during the last week of gestation results in intriguing sex differences and further support the EKyn model as an attractive tool to study the pathophysiology of schizophrenia.

Stress-induced impairment in fear discrimination is causally related to increased kynurenic acid formation in the prefrontal cortex

RATIONALE

Stress is related to cognitive impairments which are observed in most major brain diseases. Prior studies showed that the brain concentration of the tryptophan metabolite kynurenic acid (KYNA) is modulated by stress, and that changes in cerebral KYNA levels impact cognition. However, the link between these phenomena has not been tested directly so far.

OBJECTIVES

To investigate a possible causal relationship between acute stress, KYNA, and fear discrimination.

METHODS

Adult rats were exposed to one of three acute stressors-predator odor, restraint, or inescapable foot shocks (ISS)-and KYNA in the prefrontal cortex was measured using microdialysis. Corticosterone was analyzed in a subset of rats. Another cohort underwent a fear discrimination procedure immediately after experiencing stress. Different auditory conditioned stimuli (CSs) were either paired with foot shock (CS+) or were non-reinforced (CS-). One week later, fear was assessed by re-exposing rats to each CS. Finally, to test whether stress-induced changes in KYNA causally impacted fear discrimination, a group of rats that received ISS were pre-treated with the selective KYNA synthesis inhibitor PF-04859989.

RESULTS

ISS caused the greatest increase in circulating corticosterone levels and raised extracellular KYNA levels by ~ 85%. The two other stressors affected KYNA much less (< 25% increase). Moreover, only rats that received ISS were unable to discriminate between CS+ and CS-. PF-04859989 abolished the stress-induced KYNA increase and also prevented the impairment in fear discrimination in animals that experienced ISS.

CONCLUSIONS

These findings demonstrate a causal connection between stress-induced KYNA increases and cognitive deficits. Pharmacological manipulation of KYNA synthesis therefore offers a novel approach to modulate cognitive processes in stress-related disorders.

Prenatal THC exposure raises kynurenic acid levels in the prefrontal cortex of adult rats

Cannabis remains one of the most widely used illicit drugs during pregnancy. The main psychoactive component of marijuana (Δ⁹-tetrahydrocannabinol, THC) is correlated with untoward physiological effects in the offspring. Neurobehavioral and cognitive impairments have been reported in longitudinal studies on children and adolescents prenatally exposed to marijuana, and a link to psychiatric disorders has been proposed. Interestingly, the deleterious effects of prenatal cannabis use are similar to those observed in adult rats prenatally exposed to (L)-kynurenine, the direct bioprecursor of the neuroactive metabolite kynurenic acid (KYNA). We therefore investigated whether alterations in KYNA levels in the rat brain might play a role in the long-term consequences of prenatal cannabinoid exposure. Pregnant Wistar rats were treated daily with THC [5 mg/kg, p.o.] from gestational day (GD)5 through GD20. Using in vivo microdialysis in the medial prefrontal cortex, adult animals were then used to determine the extracellular levels of KYNA and glutamate. Compared to controls, extracellular basal KYNA levels were higher, and basal glutamate levels were lower, in prenatally THC-exposed rats. These rats also showed abnormal short-term memory. Following an additional acute challenge with a low dose of kynurenine (5 mg/kg i.p.) in adulthood, the increase in extracellular KYNA levels in the mPFC was more pronounced in in prenatally THC-exposed rats. These effects could be causally related to the cognitive dysfunction seen in prenatally THC-exposed rats. In the translational realm, these experiments raise the prospect of prevention of KYNA neosynthesis as a promising novel approach to combat some of the detrimental long-term effects of prenatal cannabis use.

Kynurenic Acid Levels are Increased in the CSF of Alzheimer's Disease Patients

Kynurenic acid (KYNA) is a product of the tryptophan (TRP) metabolism via the kynurenine pathway (KP). This pathway is activated in neurodegenerative disorders, such as Alzheimer´s disease (AD). KYNA is primarily produced by astrocytes and is considered neuroprotective. Thus, altered KYNA levels may suggest an inflammatory response. Very recently, significant increases in KYNA levels were reported in cerebrospinal fluid (CSF) from AD patients compared with normal controls. In this study, we assessed the accuracy of KYNA in CSF for the classification of patients with AD, cognitively healthy controls, and patients with a variety of other neurodegenerative diseases, including frontotemporal dementia (FTD), amyotrophic lateral sclerosis (ALS), and progressive supranuclear palsy (PSP). Averaged KYNA concentration in CSF was higher in patients with AD when compared with healthy subjects and with all the other differentially diagnosed groups. There were no significant differences in KYNA levels in CSF between any other neurodegenerative groups and controls. These results suggest a specific increase in KYNA concentration in CSF from AD patients not seen in other neurodegenerative diseases.

Indole Tryptophan Metabolism and Cytokine S100B in Children with Attention-Deficit/Hyperactivity Disorder: Daily Fluctuations, Responses to Methylphenidate, and Interrelationship with Depressive Symptomatology

Background: Indole tryptophan metabolites (ITMs), mainly produced at the gastrointestinal level, participate in bidirectional gut-brain communication and have been implicated in neuropsychiatric pathologies, including attention-deficit/hyperactivity disorder (ADHD). Method: A total of 179 children, 5-14 years of age, including a healthy control group (CG, n = 49), and 107 patients with ADHD participated in the study. The ADHD group was further subdivided into predominantly attention deficit (PAD) and predominantly hyperactive impulsive (PHI) subgroups. Blood samples were drawn at 20:00 and 09:00 hours, and urine was collected between blood draws, at baseline and after 4.63 ± 2.3 months of methylphenidate treatment in the ADHD group. Levels and daily fluctuations of ITM were measured by tandem mass spectrometer, and S100B (as a glial inflammatory marker) by enzyme-linked immunosorbent assay. Factorial analysis of variance (Stata 12.0) was performed with groups/subgroups, time (baseline/after treatment), hour of day (morning/evening), and presence of depressive symptoms (DS; no/yes) as factors. Results: Tryptamine and indoleacetic acid (IAA) showed no differences between the CG and ADHD groups. Tryptamine exhibited higher evening values (p < 0.0001) in both groups. No changes were associated with methylphenidate or DS. At baseline, in comparison with the rest of study sample, PHI with DS+ group showed among them much greater morning than evening IAA (p < 0.0001), with treatment causing a 50% decrease (p = 0.002). Concerning indolepropionic acid (IPA) MPH was associated with a morning IPA decrease and restored the daily profile observed in the CG. S100B protein showed greater morning than evening concentrations (p = 0.001) in both groups. Conclusion: Variations in ITM may reflect changes associated with the presence of DS, including improvement, among ADHD patients.

IDO and Kynurenine Metabolites in Peripheral and CNS Disorders

The importance of the kynurenine pathway in normal immune system function has led to an appreciation of its possible contribution to autoimmune disorders such as rheumatoid arthritis. Indoleamine-2,3-dioxygenase (IDO) activity exerts a protective function, limiting the severity of experimental arthritis, whereas deletion or inhibition exacerbates the symptoms. Other chronic disorder with an inflammatory component, such as atherosclerosis, are also suppressed by IDO activity. It is suggested that this overall anti-inflammatory activity is mediated by a change in the relative production or activity of Th17 and regulatory T cell populations. Kynurenines may play an anti-inflammatory role also in CNS disorders such as Huntington's disease, Alzheimer's disease and multiple sclerosis, in which signs of inflammation and neurodegeneration are involved. The possibility is discussed that in Huntington's disease kynurenines interact with other anti-inflammatory molecules such as Human Lymphocyte Antigen-G which may be relevant in other disorders. Kynurenine involvement may account for the protection afforded to animals with cerebral malaria and trypanosomiasis when they are treated with an inhibitor of kynurenine-3-monoxygenase (KMO). There is some evidence that changes in IL-10 may contribute to this protection and the relationship between kynurenines and IL-10 in arthritis and other inflammatory conditions should be explored. In addition, metabolites of kynurenine downstream of KMO, such as anthranilic acid and 3-hydroxy-anthranilic acid can influence inflammation, and the ratio of these compounds is a valuable biomarker of inflammatory status although the underlying molecular mechanisms of the changes require clarification. Hence it is essential that more effort be expended to identify their sites of action as potential targets for drug development. Finally, we discuss increasing awareness of the epigenetic regulation of IDO, for example by DNA methylation, a phenomenon which may explain differences between individuals in their susceptibility to arthritis and other inflammatory disorders.

Kynurenine Pathway as a New Target of Cognitive Impairment Induced by Lead Toxicity During the Lactation

The immature brain is especially vulnerable to lead (Pb²⁺) toxicity, which is considered an environmental neurotoxin. Pb²⁺ exposure during development compromises the cognitive and behavioral attributes which persist even later in adulthood, but the mechanisms involved in this effect are still unknown. On the other hand, the kynurenine pathway metabolites are modulators of different receptors and neurotransmitters related to cognition; specifically, high kynurenic acid levels has been involved with cognitive impairment, including deficits in spatial working memory and attention process. The aim of this study was to evaluate the relationship between the neurocognitive impairment induced by Pb²⁺ toxicity and the kynurenine pathway. The dams were divided in control group and Pb²⁺ group, which were given tap water or 500 ppm of lead acetate in drinking water ad libitum, respectively, from 0 to 23 postnatal day (PND). The poison was withdrawn, and tap water was given until 60 PND of the progeny. The locomotor activity in open field, redox environment, cellular function, kynurenic acid (KYNA) and 3-hydroxykynurenine (3-HK) levels as well as kynurenine aminotransferase (KAT) and kynurenine monooxygenase (KMO) activities were evaluated at both 23 and 60 PND. Additionally, learning and memory through buried food location test and expression of KAT and KMO, and cellular damage were evaluated at 60 PND. Pb²⁺ group showed redox environment alterations, cellular dysfunction and KYNA and 3-HK levels increased. No changes were observed in KAT activity. KMO activity increased at 23 PND and decreased at 60 PND. No changes in KAT and KMO expression in control and Pb²⁺ group were observed, however the number of positive cells expressing KMO and KAT increased in relation to control, which correlated with the loss of neuronal population. Cognitive impairment was observed in Pb²⁺ group which was correlated with KYNA levels. These results suggest that the increase in KYNA levels could be a mechanism by which Pb²⁺ induces cognitive impairment in adult mice, hence the modulation of kynurenine pathway represents a potential target to improve behavioural alterations produced by this environmental toxin.

Neuroinflammation and glial cell activation in mental disorders

Mental disorders (MDs) are highly prevalent and potentially debilitating complex disorders which causes remain elusive. Looking into deeper aspects of etiology or pathophysiology underlying these diseases would be highly beneficial, as the scarce knowledge in mechanistic and molecular pathways certainly represents an important limitation. Association between MDs and inflammation/neuroinflammation has been widely discussed and accepted by many, as high levels of pro-inflammatory cytokines were reported in patients with several MDs, such as schizophrenia (SCZ), bipolar disorder (BD) and major depression disorder (MDD), among others. Correlation of pro-inflammatory markers with symptoms intensity was also reported. However, the mechanisms underlying the inflammatory dysfunctions observed in MDs are not fully understood yet. In this context, microglial dysfunction has recently emerged as a possible pivotal player, as during the neuroinflammatory response, microglia can be over-activated, and excessive production of pro-inflammatory cytokines, which can modify the kynurenine and glutamate signaling, is reported. Moreover, microglial activation also results in increased astrocyte activity and consequent glutamate release, which are both toxic to the Central Nervous System (CNS). Also, as a result of increased microglial activation in MDs, products of the kynurenine pathway were shown to be changed, influencing then the dopaminergic, serotonergic, and glutamatergic signaling pathways. Therefore, in the present review, we aim to discuss how neuroinflammation impacts on glutamate and kynurenine signaling pathways, and how they can consequently influence the monoaminergic signaling. The consequent association with MDs main symptoms is also discussed. As such, this work aims to contribute to the field by providing insights into these alternative pathways and by shedding light on potential targets that could improve the strategies for pharmacological intervention and/or treatment protocols to combat the main pharmacologically unmatched symptoms of MDs, as the SCZ.

Possible Protective Role of Melatonin in Pediatric Infectious Diseases and Neurodevelopmental Pathologies

Melatonin, produced in every cell that possesses mitochondria, acts as an endogenous free radical scavenger, and improves energetic metabolism and immune function, by complex molecular crosstalk with other intracellular compounds. There is greatly increasing evidence regarding beneficial effects of acute and chronic administration of high melatonin doses, in infectious, developmental, and degenerative pathologies, as an endothelial cell and every cell protectant.

IDO activation, inflammation and musculoskeletal disease

The IDO/kynurenine pathway is now established as a major regulator of immune system function. The initial enzyme, indoleamine 2,3-dioxygenase (IDO1) is induced by IFNγ, while tryptophan-2,3-dioxygenase (TDO) is induced by corticosteroids. The pathway is therefore positioned to mediate the effects of systemic inflammation or stress-induced steroids on tissue function and its expression increases with age. Disorders of the musculoskeletal system are a common feature of ageing and many of these conditions are characterized by an inflammatory state. In inflammatory arthritis and related disorders, kynurenine protects against the development of disease, while inhibition or deletion of IDO1 increases its severity. The long-term regulation of autoimmune disorders may be influenced by the epigenetic modulation of kynurenine pathway genes, with recent data suggesting that methylation of IDO may be involved. Osteoporosis is also associated with abnormalities of the kynurenine pathway, reflected in an inversion of the ratio between blood levels of the metabolites anthranilic acid and 3-hydroxy-anthranilic acid. This review discusses evidence to date on the role of the IDO/kynurenine pathway and the highly prevalent age-related disorders of osteoporosis and rheumatoid arthritis and identifies key areas that require further research.

Acute sleep deprivation during pregnancy in rats: Rapid elevation of placental and fetal inflammation and kynurenic acid

The kynurenine pathway (KP) is the dominant pathway for tryptophan degradation in the mammalian body and emerging evidence suggests that acute episodes of sleep deprivation (SD) disrupt tryptophan metabolism via the KP. Increases in the neuroactive KP metabolite kynurenic acid (KYNA) during pregnancy may lead to a higher risk for disrupted neurodevelopment in the offspring. As pregnancy is a critical period during which several factors, including sleep disruptions, could disrupt the fetal environment, we presently explored the relationship between maternal SD and KP metabolism and immune pathways in maternal, placenta, and fetal tissues. Pregnant Wistar rat dams were sleep deprived by gentle handling for 5 h from zeitgeber time (ZT) 0 to ZT 5. Experimental cohorts included: i) controls, ii) one session of SD on embryonic day (ED) 18 or iii) three sessions of SD occurring daily on ED 16, ED 17 and ED 18. Maternal (plasma, brain), placental and fetal (plasma, brain) tissues were collected immediately after the last session of SD or after 24 h of recovery from SD. Respective controls were euthanized at ZT 5 on ED 18 or ED 19. Maternal plasma corticosterone and fetal brain KYNA were significantly elevated only after one session of SD on ED 18. Importantly, maternal plasma corticosterone levels correlated significantly with fetal brain KYNA levels. In addition, placental levels of the proinflammatory cytokines interleukin-1β (IL-1β) and interleukin-6 (IL-6) were increased following maternal SD, suggesting a relationship between placental immune response to SD and fetal brain KYNA accumulation. Collectively, our results demonstrate that sleep loss during the last week of gestation can adversely impact maternal stress, placental immune function, and fetal brain KYNA levels. We introduce KYNA as a novel molecular target influenced by sleep loss during pregnancy.

•

Prenatal sleep deprivation influences kynurenine pathway metabolism in utero.

•

Fetal brain kynurenic acid (KYNA) is elevated after maternal sleep deprivation.

•

Maternal plasma corticosterone is increased after sleep deprivation.

•

Prenatal sleep deprivation induces placental and fetal brain cytokines.

•

These data support an interplay with stress, in utero inflammation, and KYNA.

Comparative psychopharmacology of autism and psychotic-affective disorders suggests new targets for treatment

The first treatments showing effectiveness for some psychiatric disorders, such as lithium for bipolar disorder and chlorpromazine for schizophrenia, were discovered by accident. Currently, psychiatric drug design is seen as a scientific enterprise, limited though it remains by the complexity of brain development and function. Relatively few novel and effective drugs have, however, been developed for many years. The purpose of this article is to demonstrate how evolutionary biology can provide a useful framework for psychiatric drug development. The framework is based on a diametrical nature of autism, compared with psychotic-affective disorders (mainly schizophrenia, bipolar disorder and depression). This paradigm follows from two inferences: (i) risks and phenotypes of human psychiatric disorders derive from phenotypes that have evolved along the human lineage and (ii) biological variation is bidirectional (e.g. higher vs lower, faster vs slower, etc.), such that dysregulation of psychological traits varies in two opposite ways. In this context, the author review the evidence salient to the hypothesis that autism and psychotic-affective disorders represent diametrical disorders in terms of current, proposed and potential psychopharmacological treatments. Studies of brain-derived neurotrophic factor, the PI3K pathway, the NMDA receptor, kynurenic acid metabolism, agmatine metabolism, levels of the endocannabinoid anandamide, antidepressants, anticonvulsants, antipsychotics, and other treatments, demonstrate evidence of diametric effects in autism spectrum disorders and phenotypes compared with psychotic-affective disorders and phenotypes. These findings yield insights into treatment mechanisms and the development of new pharmacological therapies, as well as providing an explanation for the longstanding puzzle of antagonism between epilepsy and psychosis. Lay Summary: Consideration of autism and schizophrenia as caused by opposite alterations to brain development and function leads to novel suggestions for pharmacological treatments.

A Review on the Receptor-ligand Molecular Interactions in the Nicotinic Receptor Signaling Systems

Nicotine is regarded as the main active addictive ingredient in tobacco products driving continued tobacco abuse behavior (smoking) to the addiction behavior, whereas nicotinic acetylcholine receptors (nAChR) is the crucial effective apparatus or molecular effector of nicotine and acetylcholine and other similar ligands. Many nAChR subunits have been revealed to bind to either neurotransmitters or exogenous ligands, such as nicotine and acetylcholine, being involved in the nicotinic receptor signal transduction. Therefore, the nicotinic receptor signalling molecules and the receptor-ligand molecular interactions between nAChRs and their ligands are universally regarded as crucial mediators of cellular functions and drug targets in medical treatment and clinical diagnosis. Given numerous endeavours have been made in defining the roles of nAChRs in response to nicotine and other addictive drugs, this review focuses on studies and reports in recent years on the receptor-ligand interactions between nAChR receptors and ligands, including lipid-nAChR and protein-nAChR molecular interactions, relevant signal transduction pathways and their molecular mechanisms in the nicotinic receptor signalling systems. All the references were carefully retrieved from the PubMed database by searching key words "nicotine", "acetylcholine", "nicotinic acetylcholine receptor(s)", "nAChR*", "protein and nAChR", "lipid and nAChR", "smok*" and "tobacco". All the relevant referred papers and reports retrieved were fully reviewed for manual inspection. This effort intend to get a quick insight and understanding of the nicotinic receptor signalling and their molecular interactions mechanisms. Understanding the cellular receptor-ligand interactions and molecular mechanisms between nAChRs and ligands will lead to a better translational and therapeutic operations and outcomes for the prevention and treatment of nicotine addiction and other chronic drug addictions in the brain's reward circuitry.

Assessment of Prenatal Kynurenine Metabolism Using Tissue Slices: Focus on the Neosynthesis of Kynurenic Acid in Mice

Several lines of evidence support the hypothesis that abnormally elevated brain levels of kynurenic acid (KYNA), a metabolite of the kynurenine pathway (KP) of tryptophan degradation, play a pathophysiologically significant role in schizophrenia and other major neurodevelopmental disorders. Studies in experimental animal models suggest that KP impairments in these diseases may originate already in utero since prenatal administration of KYNA's bioprecursor, kynurenine, leads to biochemical and structural abnormalities as well as distinct cognitive impairments in adulthood. As KP metabolism during pregnancy is still insufficiently understood, we designed this study to examine the de novo synthesis of KYNA and 3-hydroxykynurenine (3-HK), an alternative biologically active product of kynurenine degradation, in tissue slices obtained from pregnant mice on gestational day (GD) 18. Fetal brain and liver, placenta, and maternal brain and liver were collected, and the tissues were incubated in vitroin the absence or presence of micromolar concentrations of kynurenine. KYNA and 3-HK were measured in the extracellular milieu. Basal and newly produced KYNA was detected in all cases. As KYNA formation exceeded 3-HK production by 2-3 orders of magnitude in the placenta and maternal brain, and as very little 3-HK neosynthesis was detectable in fetal brain tissue, detailed follow-up experiments focused on KYNA only. The fetal brain produced 3-4 times more KYNA than the maternal brain and placenta, though less than the maternal and fetal liver. No significant differences were observed when using tissues obtained on GD 14 and GD 18. Pharmacological inhibition of KYNA's main biosynthetic enzymes, kynurenine aminotransferase (KAT) I and KAT II, revealed qualitative and quantitative differences between the tissues, with a preferential role of KAT I in the fetal and maternal brain and of KAT II in the fetal and maternal liver. Findings using tissue slices from KAT II knockout mice confirmed these conclusions. Together, these results clarify the dynamics of KP metabolism during pregnancy and provide the basis for the conceptualization of interventions aimed at manipulating cerebral KP function in the prenatal period.

Metabolism of Oxalate in Humans: A Potential Role Kynurenine Aminotransferase/Glutamine Transaminase/Cysteine Conjugate Beta-lyase Plays in Hyperoxaluria

Hyperoxaluria, excessive urinary oxalate excretion, is a significant health problem worldwide. Disrupted oxalate metabolism has been implicated in hyperoxaluria and accordingly, an enzymatic disturbance in oxalate biosynthesis can result in the primary hyperoxaluria. Alanine glyoxylate aminotransferase-1 and glyoxylate reductase, the enzymes involving glyoxylate (precursor for oxalate) metabolism, have been related to primary hyperoxalurias. Some studies suggest that other enzymes such as glycolate oxidase and alanine glyoxylate aminotransferase-2 might be associated with primary hyperoxaluria as well, but evidence of a definitive link is not strong between the clinical cases and gene mutations. There are still some idiopathic hyperoxalurias, which require a further study for the etiologies. Some aminotransferases, particularly kynurenine aminotransferases, can convert glyoxylate to glycine. Based on biochemical and structural characteristics, expression level, subcellular localization of some aminotransferases, a number of them appear able to catalyze the transamination of glyoxylate to glycine more efficiently than alanine glyoxylate aminotransferase-1. The aim of this minireview is to explore other undermining causes of primary hyperoxaluria and stimulate research toward achieving a comprehensive understanding of underlying mechanisms leading to the disease. Herein, we reviewed all aminotransferases in the liver for their functions in glyoxylate metabolism. Particularly, kynurenine aminotransferase-I and III were carefully discussed regarding their biochemical and structural characteristics, cellular localization, and enzyme inhibition. Kynurenine aminotransferase-III is, so far, the most efficient putative mitochondrial enzyme to transaminate glyoxylate to glycine in mammalian livers, might be an interesting enzyme to look over in hyperoxaluria etiology of primary hyperoxaluria and should be carefully investigated for its involvement in oxalate metabolism.