Implications of the kynurenine pathway and quinolinic acid in Alzheimer's disease

Metabolism of sleep and aging: Bridging the gap using metabolomics

Sleep is a conserved behavior across the evolutionary timescale. Almost all known animal species demonstrate sleep or sleep like states. Despite extensive study, the mechanistic aspects of sleep need are not very well characterized. Sleep appears to be needed to generate resources that are utilized during the active stage/wakefulness as well as clearance of waste products that accumulate during wakefulness. From a metabolic perspective, this means sleep is crucial for anabolic activities. Decrease in anabolism and build-up of harmful catabolic waste products is also a hallmark of aging processes. Through this lens, sleep and aging processes are remarkably parallel- for example behavioral studies demonstrate an interaction between sleep and aging. Changes in sleep behavior affect neurocognitive phenotypes important in aging such as learning and memory, although the underlying connections are largely unknown. Here we draw inspiration from the similar metabolic effects of sleep and aging and posit that large scale metabolic phenotyping, commonly known as metabolomics, can shed light to interleaving effects of sleep, aging and progression of diseases related to aging. In this review, data from recent sleep and aging literature using metabolomics as principal molecular phenotyping methods is collated and compared. The present data suggests that metabolic effects of aging and sleep also demonstrate similarities, particularly in lipid metabolism and amino acid metabolism. Some of these changes also overlap with metabolomic data available from clinical studies of Alzheimer's disease. Together, metabolomic technologies show promise in elucidating interleaving effects of sleep, aging and progression of aging disorders at a molecular level.

Structural Basis of Inhibitor Selectivity in Human Indoleamine 2,3-Dioxygenase 1 and Tryptophan Dioxygenase

Indoleamine 2,3-dioxygenase 1 (hIDO1) and tryptophan dioxygenase (hTDO) are two of the only three heme-based dioxygenases in humans. They have recently been identified as key cancer immunotherapeutic drug targets. While structures of hIDO1 in complex with inhibitors have been documented, so far there are no structures of hTDO-inhibitor complexes available. Here we use PF-06840003 (IPD), a hIDO1-selective inhibitor in clinical trials, as a structural probe to elucidate inhibitor-selectivity in hIDO1 versus hTDO. Spectroscopic studies show that IPD exhibits 400-fold higher inhibition activity toward hIDO1 with respect to hTDO. Crystallographic structures reveal that the binding pocket of IPD in the active site in hIDO1 is much more flexible as compared to that in hTDO, which offers a molecular explanation for the superior inhibition activity of IPD in hIDO1 with respect to hTDO. In addition to the IPD bound in the active site, a second IPD molecule was identified in an inhibitory site on the proximal side of the heme in hIDO1 and in an exosite that is ∼40 Å away from the active site in hTDO. Taken together the data provide new insights into structure-based design of mono and dual inhibitors targeting hIDO1 and/or hTDO.

Elevated quinolinic acid levels in cerebrospinal fluid in subacute sclerosing panencephalitis

Subacute sclerosing panencephalitis (SSPE) is a rare neurodegenerative disorder caused by a persistent infection with aberrant measles virus. Indoleamine-2, 3-dioxygenase (IDO) initiates the increased production of kynurenine pathway (KP) metabolites quinolinic acid (QUIN), which has an excitotoxic effect for neurons. We measured serum IDO activity and cerebrospinal fluid (CSF) levels of QUIN. The CSF QUIN levels were significantly higher in SSPE patients than in controls, and increased according as neurological disability in a patient studied. Elevation of CSF QUIN and progression of SSPE indicate a pathological role of KP metabolism in the inflammatory neurodestruction.

Trisomy 21 activates the kynurenine pathway via increased dosage of interferon receptors

Trisomy 21 (T21) causes Down syndrome (DS), affecting immune and neurological function by ill-defined mechanisms. Here we report a large metabolomics study of plasma and cerebrospinal fluid, showing in independent cohorts that people with DS produce elevated levels of kynurenine and quinolinic acid, two tryptophan catabolites with potent immunosuppressive and neurotoxic properties, respectively. Immune cells of people with DS overexpress IDO1, the rate-limiting enzyme in the kynurenine pathway (KP) and a known interferon (IFN)-stimulated gene. Furthermore, the levels of IFN-inducible cytokines positively correlate with KP dysregulation. Using metabolic tracing assays, we show that overexpression of IFN receptors encoded on chromosome 21 contribute to enhanced IFN stimulation, thereby causing IDO1 overexpression and kynurenine overproduction in cells with T21. Finally, a mouse model of DS carrying triplication of IFN receptors exhibits KP dysregulation. Together, our results reveal a mechanism by which T21 could drive immunosuppression and neurotoxicity in DS.

Ex Vivo Analysis of Tryptophan Metabolism Using 19F NMR

Tryptophan, an essential amino acid, is metabolized into a variety of small molecules capable of impacting human physiology, and aberrant tryptophan metabolism has been linked to a number of diseases. There are three principal routes by which tryptophan is degraded, and thus methods for measuring metabolic flux through these pathways can be used to understand the factors that perturb tryptophan metabolism and potentially to measure disease biomarkers. Here, we describe a method utilizing 6-fluorotryptophan as a probe for detecting tryptophan metabolites in ex vivo tissue samples via ¹⁹F nuclear magnetic resonance. As a proof of concept, we demonstrate that 6-fluorotryptophan can be used to measure changes in tryptophan metabolism resulting from antibiotic-induced changes in gut microbiota composition. Taken together, we describe a general strategy for monitoring amino acid metabolism using ¹⁹F nuclear magnetic resonance that is operationally simple and does not require chromatographic separation of metabolites.

Microorganisms, Tryptophan Metabolism, and Kynurenine Pathway: A Complex Interconnected Loop Influencing Human Health Status

The kynurenine pathway is important in cellular energy generation and limiting cellular ageing as it degrades about 90% of dietary tryptophan into the essential co-factor NAD⁺ (nicotinamide adenine dinucleotide). Prior to the production of NAD⁺, various intermediate compounds with neuroactivity (kynurenic acid, quinolinic acid) or antioxidant activity (3-hydroxykynurenine, picolinic acid) are synthesized. The kynurenine metabolites can participate in numerous neurodegenerative disorders (Alzheimer disease, amyotrophic lateral sclerosis, Huntington disease, and Parkinson disease) or other diseases such as AIDS, cancer, cardiovascular diseases, inflammation, and irritable bowel syndrome. Recently, the role of gut in affecting the emotional and cognitive centres of the brain has attracted a great deal of attention. In this review, we focus on the bidirectional communication between the gut and the brain, known as the gut-brain axis. The interaction of components of this axis, namely, the gut, its microbiota, and gut pathogens; tryptophan; the kynurenine pathway on tryptophan availability; the regulation of kynurenine metabolite concentration; and diversity and population of gut microbiota, has been considered.

High phthalate exposure increased urinary concentrations of quinolinic acid, implicated in the pathogenesis of neurological disorders: Is this a potential missing link?

BACKGROUND

Quinolinic acid (QA), a neuroactive metabolite of the Kynurenine Pathway (KP), is an excitotoxin that is implicated in the pathogenesis of many neurological disorders. KP is the main tryptophan degradation pathway. Phthalates can structurally mimic tryptophan metabolites and diets containing phthalates in rats enhanced the production and excretion of QA. However, there are no human studies that have examined the association between phthalates and QA.

OBJECTIVES

Taking advantage of different mesalamine formulations with/without dibutyl phthalate (DBP), we assessed whether DBP from mesalamine (>1000x background) altered the urinary concentrations of QA.

METHODS

Men with inflammatory bowel disease participated in a prospective crossover pilot study. 15 Men were on non-DBP mesalamine (background) at baseline crossed-over for 4 months to high-DBP mesalamine (high) (B1H-Arm) and vice versa for 15 men who were on high-DBP mesalamine at baseline (H1B-Arm). Men provided 60 urine samples (2/man). We estimated crossover and cross-sectional changes in the creatinine normalized-QA using multivariable linear mixed effect models with random intercepts.

RESULTS

At baseline, men who were on high-DBP mesalamine (H1B-Arm) had 72%, (95% confidence interval (CI): 18, 151) higher normalized-QA than men who were on background exposure and when high-DBP mesalamine was removed for four months, normalized-QA decreased with 32%, (95% CI: -45.0, -15.1). Consistently, when men in B1H-Arm were newly-exposed to high-DBP mesalamine, normalized-QA increased with 11%, (95% CI: -11, 38).

CONCLUSIONS

High-DBP exposure from mesalamine increased the urinary concentrations of QA, which was largely reversed after removal of the high-DBP exposure for four months. This novel hypothesis should warrant new promising research considering the KP and QA concentrations as a plausible mediator for the neurotoxicity possibly linked with phthalate exposures.

Role of Nicotinamide Adenine Dinucleotide and Related Precursors as Therapeutic Targets for Age-Related Degenerative Diseases: Rationale, Biochemistry, Pharmacokinetics, and Outcomes

Significance: Nicotinamide adenine dinucleotide (NAD+) is an essential pyridine nucleotide that serves as an essential cofactor and substrate for a number of critical cellular processes involved in oxidative phosphorylation and ATP production, DNA repair, epigenetically modulated gene expression, intracellular calcium signaling, and immunological functions. NAD+ depletion may occur in response to either excessive DNA damage due to free radical or ultraviolet attack, resulting in significant poly(ADP-ribose) polymerase (PARP) activation and a high turnover and subsequent depletion of NAD+, and/or chronic immune activation and inflammatory cytokine production resulting in accelerated CD38 activity and decline in NAD+ levels. Recent studies have shown that enhancing NAD+ levels can profoundly reduce oxidative cell damage in catabolic tissue, including the brain. Therefore, promotion of intracellular NAD+ anabolism represents a promising therapeutic strategy for age-associated degenerative diseases in general, and is essential to the effective realization of multiple benefits of healthy sirtuin activity. The kynurenine pathway represents the de novo NAD+ synthesis pathway in mammalian cells. NAD+ can also be produced by the NAD+ salvage pathway. Recent Advances: In this review, we describe and discuss recent insights regarding the efficacy and benefits of the NAD+ precursors, nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside (NR), and nicotinamide mononucleotide (NMN), in attenuating NAD+ decline in degenerative disease states and physiological aging. Critical Issues: Results obtained in recent years have shown that NAD+ precursors can play important protective roles in several diseases. However, in some cases, these precursors may vary in their ability to enhance NAD+ synthesis via their location in the NAD+ anabolic pathway. Increased synthesis of NAD+ promotes protective cell responses, further demonstrating that NAD+ is a regulatory molecule associated with several biochemical pathways. Future Directions: In the next few years, the refinement of personalized therapy for the use of NAD+ precursors and improved detection methodologies allowing the administration of specific NAD+ precursors in the context of patients' NAD+ levels will lead to a better understanding of the therapeutic role of NAD+ precursors in human diseases.

Phytochemical treatments target kynurenine pathway induced oxidative stress

OBJECTIVE

The objective of this paper was to link the phytochemical and metabolic research treating quinolinic acid induced oxidative stress in neurodegenerative disorders.

METHODS

Quinolinic acid, a metabolite of the kynurenine pathway of tryptophan catabolism, plays a role in the oxidative stress associated with many neurological disorders and is used to simulate disorders such as Parkinson's disease.

RESULTS

In these models, phytochemicals have been shown to reduce striatal lesion size, reduce inflammation and prevent lipid peroxidation caused by quinolinic acid.

CONCLUSION

These results suggest that phenolic compounds, a class of phytochemicals, including flavonoids and diarylheptanoids, should be further studied to develop new treatments for oxidative stress related neurological disorders.

Neuroprotective Effect of Myxobacterial Extracts on Quinolinic Acid-Induced Toxicity in Primary Human Neurons

Quinolinic acid (QUIN) is a neurotoxin, gliotoxin, and proinflammatory molecule involved in the pathogenesis of several neurological diseases. Myxobacteria have been known as a rich source of secondary metabolites with diverse structures and mode of actions. In this study, we examined the potential neuroprotective effects of myxobacterial extracts on QUIN-induced excitotoxicity in primary human neurons. For this purpose, primary cultures of human neurons were pre-incubated with myxobacterial extracts and subsequently treated with QUIN at a pathophysiological concentration of 550 nM. The results showed that some myxobacterial extracts can significantly attenuate formation of reactive oxygen species (ROS), nitric oxide (NO) production, and extracellular lactate dehydrogenase (LDH) activity of human neurons. Moreover, myxobacterial extracts were also able to reduce neuronal nitric oxide synthase (nNOS) activity. Some extracts prevented cell death by reducing the activation of poly (ADP-ribose) polymerase (PARP1) by QUIN, therefore by maintaining NAD⁺ levels. In addition, myxobacterial extracts ameliorated oxidative stress by increasing the intracellular levels of glutathione after treatment with QUIN. The results showed that extracts of Stigmatella sp. UTMC 4072 and Archangium sp. UTMC 4070 and were the most effective in reducing QUIN-induced excitotoxicity in primary human neurons. Due to their antioxidative activity, myxobacterial extracts represent an underexplored source of potential new drugs for the treatment of neurodegenerative diseases.

The Regulatory Role of NAD in Human and Animal Cells

Nicotinamide adenine dinucleotide (NAD) and its phosphorylated form NADP are the major coenzymes in the redox reactions of various essential metabolic pathways. NAD+ also serves as a substrate for several families of regulatory proteins, such as protein deacetylases (sirtuins), ADP-ribosyltransferases, and poly(ADP-ribose) polymerases, that control vital cell processes including gene expression, DNA repair, apoptosis, mitochondrial biogenesis, unfolded protein response, and many others. NAD+ is also a precursor for calcium-mobilizing secondary messengers. Proper regulation of these NAD-dependent metabolic and signaling pathways depends on how efficiently cells can maintain their NAD levels. Generally, mammalian cells regulate their NAD supply through biosynthesis from the precursors delivered with the diet: nicotinamide and nicotinic acid (vitamin B3), as well as nicotinamide riboside and nicotinic acid riboside. Administration of NAD precursors has been demonstrated to restore NAD levels in tissues (i.e., to produce beneficial therapeutic effects) in preclinical models of various diseases, such as neurodegenerative disorders, obesity, diabetes, and metabolic syndrome.

The Impact of Uremic Toxins on Cerebrovascular and Cognitive Disorders

Individuals at all stages of chronic kidney disease (CKD) have a higher risk of developing cognitive disorders and dementia. Stroke is also highly prevalent in this population and is associated with a higher risk of neurological deterioration, in-hospital mortality, and poor functional outcomes. Evidence from in vitro studies and in vivo animal experiments suggests that accumulation of uremic toxins may contribute to the pathogenesis of stroke and amplify vascular damage, leading to cognitive disorders and dementia. This review summarizes current evidence on the mechanisms by which uremic toxins may favour the occurrence of cerebrovascular diseases and neurological complications in CKD.

Quinolinic acid and glutamatergic neurodegeneration in Caenorhabditis elegans

Quinolinic acid (QUIN) is an endogenous neurotoxin that acts as an N-methyl-D-aspartate receptor (NMDAR) agonist generating a toxic cascade, which can lead to neurodegeneration. The action of QUIN in Caenorhabditis elegans and the neurotoxins that allow the study of glutamatergic system disorders have not been carefully addressed. The effects of QUIN on toxicological and behavioral parameters in VM487 and VC2623 transgenic, as well as wild-type (WT) animals were performed to evaluate whether QUIN could be used as a neurotoxin in C. elegans. QUIN reduced survival of WT worms in a dose-dependent manner. A sublethal dose of QUIN (20 mM) increased reactive oxygen species (ROS) levels in an nmr-1/NMDAR-dependent manner, activated the DAF-16/FOXO transcription factor, and increased expression of the antioxidant enzymes, superoxide dismutase-3, glutathione S-transferase-4, and heat shock protein-16.2. QUIN did not change motor behavioral parameters, but altered the sensory behavior in N2 and VM487 worms. Notably, the effect of QUIN on the sensory behavioral parameters might occur, at least in part, secondary to increased ROS. However, the touch response behavior indicates a mechanism of action that is independent of ROS generation. In addition, non-lethal doses of QUIN triggered neurodegeneration in glutamatergic neurons. Our findings indicate that C. elegans might be useful as a model for studies of QUIN as a glutamatergic neurotoxin in rodent models.

Aging exacerbates cognitive and anxiety alterations induced by an intracerebroventricular injection of amyloid-β1-42 peptide in mice

An increasing body of evidence indicates that the activation of indoleamine-2,3-dyoxigenase (IDO), a first and rate-limiting enzyme in the kynurenine (KYN) pathway, is involved in Aβ_1-42-neurotoxicity and AD pathogenesis. We have reported for the first time that brain IDO activation is related to Aβ_1-42 exposure in young mice. Because aging is characterized by a brain dyshomeostasis and because it remains the most dominant risk factor for AD, the purpose of this study was to determine whether aging is associated with a higher sensitivity to behavioural and neurochemical alterations elicited by an intracerebroventricular (i.c.v.) injection of Aβ_1-42 (400 pmol/mice), and whether KYN pathway is involved in these effects. We confirmed that aged mice displayed higher cognitive deficit in the object recognition test and higher anxiety-like behaviour in the elevated plus-maze and open field tests after the Aβ_1-42 administration. Aged mice also responded to Aβ_1-42 with a higher deficiency of brain-derived neurotrophic factor, glutathione levels and total radical-trapping antioxidant capacity, a higher IDO activity, and a higher KYN and KYN/tryptophan ratio in the prefrontal cortex and hippocampus. These effects of Aβ_1-42 were associated with a higher proinflammatory status, as measured by higher levels of interleukin-6, lower levels of interleukin-10 and higher expression of glial fibrillary acidic protein (GFAP) and allograft inflammatory factor 1 (Iba1) in the brain of aged mice. These results represent primary evidence suggesting that age-associated inflammatory signature and down-regulation of neuroprotectants in the brain render aged mice more vulnerable to Aβ_1-42-induced memory loss, anxiety symptoms and KYN pathway dysregulation.

Synergistic Toxicity of the Neurometabolites Quinolinic Acid and Homocysteine in Cortical Neurons and Astrocytes: Implications in Alzheimer's Disease

The brain of patients affected by Alzheimer's disease (AD) develops progressive neurodegeneration linked to the formation of proteins aggregates. However, their single actions cannot explain the extent of brain damage observed in this disorder, and the characterization of co-adjuvant involved in the early toxic processes evoked in AD is essential. In this line, quinolinic acid (QUIN) and homocysteine (Hcy) appear to be involved in the AD neuropathogenesis. Herein, we investigate the effects of QUIN and Hcy on early toxic events in cortical neurons and astrocytes. Exposure of primary cortical cultures to these neurometabolites for 24 h induced concentration-dependent neurotoxicity. In addition, QUIN (25 μM) and Hcy (30 μM) triggered ROS production, lipid peroxidation, diminished of Na⁺,K⁺-ATPase activity, and morphologic alterations, culminating in reduced neuronal viability by necrotic cell death. In astrocytes, QUIN (100 μM) and Hcy (30 μM) induced caspase-3-dependent apoptosis and morphologic alterations through oxidative status imbalance. To establish specific mechanisms, we preincubated cell cultures with different protective agents. The combined toxicity of QUIN and Hcy was attenuated by melatonin and Trolox in neurons and by NMDA antagonists and glutathione in astrocytes. Cellular death and morphologic alterations were prevented when co-culture was treated with metabolites, suggesting the activation of protector mechanisms dependent on soluble factors and astrocyte and neuron communication through gap junctions. These findings suggest that early damaging events involved in AD can be magnified by synergistic toxicity of the QUIN and Hcy. Therefore, this study opens new possibilities to elucidate the molecular mechanisms of neuron-astrocyte interactions and their role in neuroprotection against QUIN and Hcy.

Determination of tryptophan derivatives in kynurenine pathway in fermented foods using liquid chromatography tandem mass spectrometry

This study aimed to develop an analytical method for the determination of tryptophan and its derivatives in kynurenine pathway using tandem mass spectrometry in various fermented food products (bread, beer, red wine, white cheese, yoghurt, kefir and cocoa powder). The method entails an aqueous extraction and reversed phase chromatographic separation using pentafluorophenyl (PFP) column. It allowed quantitation of low ppb levels of tryptophan and its derivatives in different fermented food matrices. It was found that beer samples were found to contain kynurenine within the range of 28.7±0.7μg/L and 86.3±0.5μg/L. Moreover, dairy products (yoghurt, white cheese and kefir) contained kynurenine ranging from 30.3 to 763.8μg/kg d.w. Though bread samples analyzed did not contain kynurenic acid, beer and red wine samples as yeast-fermented foods were found to contain kynurenic acid. Among foods analyzed, cacao powder had the highest amounts of kynurenic acid (4486.2±165.6μg/kgd.w), which is a neuroprotective compound.

Central Nervous System Infection with Borna Disease Virus Causes Kynurenine Pathway Dysregulation and Neurotoxic Quinolinic Acid Production

Central nervous system infection of neonatal and adult rats with Borna disease virus (BDV) results in neuronal destruction and behavioral abnormalities with differential immune-mediated involvement. Neuroactive metabolites generated from the kynurenine pathway of tryptophan degradation have been implicated in several human neurodegenerative disorders. Here, we report that brain expression of key enzymes in the kynurenine pathway are significantly, but differentially, altered in neonatal and adult rats with BDV infection. Gene expression analysis of rat brains following neonatal infection showed increased expression of kynurenine amino transferase II (KATII) and kynurenine-3-monooxygenase (KMO) enzymes. Additionally, indoleamine 2,3-dioxygenase (IDO) expression was only modestly increased in a brain region- and time-dependent manner in neonatally infected rats; however, its expression was highly increased in adult infected rats. The most dramatic impact on gene expression was seen for KMO, whose activity promotes the production of neurotoxic quinolinic acid. KMO expression was persistently elevated in brain regions of both newborn and adult BDV-infected rats, with increases reaching up to 86-fold. KMO protein levels were increased in neonatally infected rats and colocalized with neurons, the primary target cells of BDV infection. Furthermore, quinolinic acid was elevated in neonatally infected rat brains. We further demonstrate increased expression of KATII and KMO, but not IDO, in vitro in BDV-infected C6 astroglioma cells. Our results suggest that BDV directly impacts the kynurenine pathway, an effect that may be exacerbated by inflammatory responses in immunocompetent hosts. Thus, experimental models of BDV infection may provide new tools for discriminating virus-mediated from immune-mediated impacts on the kynurenine pathway and their relative contribution to neurodegeneration.IMPORTANCE BDV causes persistent, noncytopathic infection in vitro yet still elicits widespread neurodegeneration of infected neurons in both immunoincompetent and immunocompetent hosts. Here, we show that BDV infection induces expression of key enzymes of the kynurenine pathway in brains of newborn and adult infected rats and cultured astroglioma cells, shunting tryptophan degradation toward the production of neurotoxic quinolinic acid. Thus, our findings newly implicate this metabolic pathway in BDV-induced neurodegeneration. Given the importance of the kynurenine pathway in a wide range of human infections and neurodegenerative and neuropsychiatric disorders, animal models of BDV infection may serve as important tools for contrasting direct viral and indirect antiviral immune-mediated impacts on kynurenine pathway dysregulation and the ensuing neurodevelopmental and neuropathological consequences.

Substrate and inhibitor specificity of kynurenine monooxygenase from Cytophaga hutchinsonii

Kynurenine monooxygenase (KMO) is a potential drug target for treatment of neurodegenerative disorders such as Huntington's and Alzheimer's diseases. We have evaluated substituted kynurenines as substrates or inhibitors of KMO from Cytophaga hutchinsonii. Kynurenines substituted with a halogen at the 5-position are excellent substrates, with values of k_cat and k_cat/K_m comparable to or higher than kynurenine. However, kynurenines substituted in the 3-position are competitive inhibitors, with K_I values lower than the K_m for kynurenine. Bromination also enhances inhibition, and 3,5-dibromokynurenine is a potent competitive inhibitor with a K_I value of 1.5μM. A pharmacophore model of KMO was developed, and predicted that 3,4-dichlorohippuric acid would be an inhibitor. The K_I for this compound was found to be 34μM, thus validating the pharmacophore model. We are using these results and our model to design more potent inhibitors of KMO.

Understanding the role of the kynurenine pathway in human breast cancer immunobiology

Breast cancer (BrCa) is the leading cause of cancer related death in women. While current diagnostic modalities provide opportunities for early medical intervention, significant proportions of breast tumours escape treatment and metastasize. Gaining increasing recognition as a factor in tumour metastasis is the local immuno-surveillance environment. Following identification of the role played by the enzyme indoleamine dioxygenase 1 (IDO1) in mediating maternal foetal tolerance, the kynurenine pathway (KP) of tryptophan metabolism has emerged as a key metabolic pathway contributing to immune escape. In inflammatory conditions activation of the KP leads to the production of several immune-modulating metabolites including kynurenine, kynurenic acid, 3-hydroxykynurenine, anthranilic acid, 3-hydroxyanthranilic acid, picolinic acid and quinolinic acid. KP over-activation was first described in BrCa patients in the early 1960s. More evidence has since emerged to suggest that the IDO1 is elevated in advanced BrCa patients and is associated with poor prognosis. Further, IDO1 positive breast tumours have a positive correlation with the density of immune suppressive Foxp3+ T regulatory cells and lymph node metastasis. The analysis of clinical microarray data in invasive BrCa compared to normal tissue showed, using two microarray databank (cBioportal and TCGA), that 86.3% and 91.4% BrCa patients have altered KP enzyme expression respectively. Collectively, these data highlight the key roles played by KP activation in BrCa, particularly in basal BrCa subtypes where expression of most KP enzymes was altered. Accordingly, the use of KP enzyme inhibitors in addition to standard chemotherapy regimens may present a viable therapeutic approach.

Progesterone Alters Kynurenine Pathway Activation in IFN-γ-Activated Macrophages - Relevance for Neuroinflammatory Diseases

We have previously demonstrated that the kynurenine pathway (KP), the major biochemical pathway for tryptophan metabolism, is dysregulated in many inflammatory disorders that are often associated with sexual dimorphisms. We aimed to identify a potential functional interaction between the KP and gonadal hormones. We have treated primary human macrophages with progesterone in the presence and absence of inflammatory cytokine interferon-gamma (interferon-γ) that is known to be a potent inducer of regulating the KP enzyme. We found that progesterone attenuates interferon-γ-induced KP activity, decreases the levels of the excitotoxin quinolinic acid, and increases the neuroprotective kynurenic acid levels. We also showed that progesterone was able to reduce the inflammatory marker neopterin. These results may shed light on the gender disparity in response to inflammation.

TDO as a therapeutic target in brain diseases

Tryptophan-2, 3-dioxygenase (TDO) is a heme-containing protein catalyzing the first reaction in the kynurenine pathway, which incorporates oxygen into the indole moiety of tryptophan and catalyzes it into kynurenine (KYN). The activation of TDO results in the depletion of tryptophan and the accumulation of kynurenine and its metabolites. These metabolites can affect the function of neurons and inhibit the proliferation of T cells. Increasing evidence demonstrates that TDO is a potential therapeutic target in the treatment of brain diseases as well as in the antitumor and transplant fields. Despite its growing popularity, there are few reviews only focusing on TDO. Hence, we herein review TDO by providing a comprehensive overview of TDO, including its biological functions as well as the evolution, structure and catalytic process of TDO. Additionally, this review will focus on the role of TDO in the pathology of three groups of brain diseases: Schizophrenia, Alzheimer's disease (AD) and Glioma. Finally, we will also provide an opinion regarding the future developmental directions of TDO in brain diseases, especially whether TDO has a potential role in other brain diseases as well as the development and applications of TDO inhibitors as treatments.

Indoleamine-2,3-dioxygenase mediates neurobehavioral alterations induced by an intracerebroventricular injection of amyloid-β1-42 peptide in mice

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterized by a progressive cognitive decline along with various neuropsychiatric symptoms, including depression and anxiety. Increasing evidence has been proposed the activation of the tryptophan-degrading indoleamine-2,3-dyoxigenase (IDO), the rate-limiting enzyme of kynurerine pathway (KP), as a pathogenic factor of amyloid-beta (Aβ)-related inflammation in AD. In the current study, the effects of an intracerebroventricular (i.c.v.) injection of Aβ1-42 peptide (400pmol/mice; 3μl/site) on the regulation of KP biomarkers (IDO activity, tryptophan and kynurerine levels) and the impact of Aβ1-42 on neurotrophic factors levels were investigated as potential mechanisms linking neuroinflammation to cognitive/emotional disturbances in mice. Our results demonstrated that Aβ1-42 induced memory impairment in the object recognition test. Aβ1-42 also induced emotional alterations, such as depressive and anxiety-like behaviors, as evaluated in the tail suspension and elevated-plus maze tests, respectively. We observed an increase in levels of proinflammatory cytokines in the Aβ1-42-treated mice, which led to an increase in IDO activity in the prefrontal cortex (PFC) and the hippocampus (HC). The IDO activation subsequently increased kynurerine production and the kynurenine/tryptophan ratio and decreased the levels of neurotrophic factors in the PFC and HC, which contributed to Aβ-associated behavioral disturbances. The inhibition of IDO activation by IDO inhibitor 1-methyltryptophan (1-MT), prevented the development of behavioral and neurochemical alterations. These data demonstrate that brain IDO activation plays a key role in mediating the memory and emotional disturbances in an experimental model based on Aβ-induced neuroinflammation.

Effects of Various Kynurenine Metabolites on Respiratory Parameters of Rat Brain, Liver and Heart Mitochondria

Previously, we demonstrated that the endogenous glutamate receptor antagonist kynurenic acid dose-dependently and significantly affected rat heart mitochondria. Now we have investigated the effects of L-tryptophan, L-kynurenine, 3-hydroxykynurenine and kynurenic, anthranilic, 3-hydroxyanthranilic, xanthurenic and quinolinic acids on respiratory parameters (ie, state 2, state 3), respiratory control index (RC) and ADP/oxygen ratio in brain, liver and heart mitochondria of adult rats. Mitochondria were incubated with glutamate/malate (5 mM) or succinate (10 mM) and in the presence of L-tryptophan metabolites (1 mM) or in the absence, as control. Kynurenic and anthranilic acids significantly reduced RC values of heart mitochondria in the presence of glutamate/malate. Xanthurenic acid significantly reduced RC values of brain mitochondria in the presence of glutamate/malate. Furthermore, 3-hydroxykynurenine and 3-hydroxyanthranilic acid decreased RC values of brain, liver and heart mitochondria using glutamate/malate. In the presence of succinate, 3-hydroxykynurenine and 3-hydroxyanthranilic acid affected RC values of brain mitochondria, whereas in liver and heart mitochondria only 3-hydroxykynurenine lowered RC values significantly. Furthermore, lowered ADP/oxygen ratios were observed in brain mitochondria in the presence of succinate with 3-hydroxykynurenine and 3-hydroxyanthranilic acid, and to a lesser extent with glutamate/malate. In addition, 3-hydroxyanthranilic acid significantly lowered the ADP/oxygen ratio in heart mitochondria exposed to glutamate/malate, while in the liver mitochondria only a mild reduction was found. Tests of the influence of L-tryptophan and its metabolites on complex I in liver mitochondria showed that only 3-hydroxykynurenine, 3-hydroxyanthranilic acid and L-kynurenine led to a significant acceleration of NADH-driven complex I activities. The data indicate that L-tryptophan metabolites had different effects on brain, liver and heart mitochondria. Alterations of L-tryptophan metabolism might have an impact on the bioenergetic activities of brain, liver and/or heart mitochondria and might be involved in the development of clinical symptoms such as cardiomyopathy, hepatopathy and dementia.

Central kynurenine pathway shift with age in women

Age is considered a dominant risk factor in the development of most neurodegenerative disorders. The kynurenine pathway, a major metabolic pathway of tryptophan is altered in the majority of neurodegenerative disorders. In this study, we have analysed CSF samples from 49 healthy women across a wide age range (0-90) for kynurenine pathway metabolites and the inflammatory marker neopterin. Our results show central tryptophan metabolism is increased with age in women, with an apparent shift towards the neurotoxin quinolinic acid. We also observed an increase in central levels of the inflammatory marker neopterin with age and a positive correlation between neopterin and kynurenine pathway activation. We conclude that, the changes that occur in the kynurenine pathway as a result of normal ageing are mechanistically linked to increased inflammatory signalling and have some explanatory potential with regard to age-associated degenerative diseases in the CNS. Management of health in ageing and (preventative) treatment would do well to look to the kynurenine pathway for potentially novel solutions. Both the inflammation marker neopterin and kynurenine pathway activity were increased with age in the CSF of female subjects. While levels of quinolinic acid (QUIN), picolinic acid (PIC), kynurenine and quinaldic acid (QA) were increased, 3-hydroxykynurenine (3HK) was decreased and 3-hydroxyanthranilic acid (3HAA) and kynurenic acid (KYNA) remained unchanged. Of particular interest is the increase in QUIN, a neuroexcitotoxin associated with neurodegeneration.

Stress-related regulation of the kynurenine pathway: Relevance to neuropsychiatric and degenerative disorders

The kynurenine pathway (KP), which is activated in times of stress and infection has been implicated in the pathophysiology of neurodegenerative and psychiatric disorders. Activation of this tryptophan metabolising pathway results in the production of neuroactive metabolites which have the potential to interfere with normal neuronal functioning which may contribute to altered neuronal transmission and the emergence of symptoms of these brain disorders. This review investigates the involvement of the KP in a range of neurological disorders, examining recent in vitro, in vivo and clinical discoveries highlights evidence to indicate that the KP is a potential therapeutic target in both neurodegenerative and stress-related neuropsychiatric disorders. Furthermore, this review identifies gaps in our knowledge with regard to this field which are yet to be examined to lead to a more comprehensive understanding of the role of KP activation in brain health and disease. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

(13)C-tryptophan breath test detects increased catabolic turnover of tryptophan along the kynurenine pathway in patients with major depressive disorder

Altered tryptophan-kynurenine (KYN) metabolism has been implicated in major depressive disorder (MDD). The L-[1-(13)C]tryptophan breath test ((13)C-TBT) is a noninvasive, stable-isotope tracer method in which exhaled (13)CO2 is attributable to tryptophan catabolism via the KYN pathway. We included 18 patients with MDD (DSM-IV) and 24 age- and sex-matched controls. (13)C-tryptophan (150 mg) was orally administered and the (13)CO2/(12)CO2 ratio in the breath was monitored for 180 min. The cumulative recovery rate during the 180-min test (CRR0-180; %), area under the Δ(13)CO2-time curve (AUC; %*min), and the maximal Δ(13)CO2 (Cmax; %) were significantly higher in patients with MDD than in the controls (p = 0.004, p = 0.008, and p = 0.002, respectively). Plasma tryptophan concentrations correlated negatively with Cmax in both the patients and controls (p = 0.020 and p = 0.034, respectively). Our results suggest that the (13)C-TBT could be a novel biomarker for detecting a subgroup of MDD with increased tryptophan-KYN metabolism.

Activation of the kynurenine pathway and increased production of the excitotoxin quinolinic acid following traumatic brain injury in humans

Abstract

During inflammation, the kynurenine pathway (KP) metabolises the essential amino acid tryptophan (TRP) potentially contributing to excitotoxicity via the release of quinolinic acid (QUIN) and 3-hydroxykynurenine (3HK). Despite the importance of excitotoxicity in the development of secondary brain damage, investigations on the KP in TBI are scarce. In this study, we comprehensively characterised changes in KP activation by measuring numerous metabolites in cerebrospinal fluid (CSF) from TBI patients and assessing the expression of key KP enzymes in brain tissue from TBI victims. Acute QUIN levels were further correlated with outcome scores to explore its prognostic value in TBI recovery.

Methods

Twenty-eight patients with severe TBI (GCS ≤ 8, three patients had initial GCS = 9–10, but rapidly deteriorated to ≤8) were recruited. CSF was collected from admission to day 5 post-injury. TRP, kynurenine (KYN), kynurenic acid (KYNA), QUIN, anthranilic acid (AA) and 3-hydroxyanthranilic acid (3HAA) were measured in CSF. The Glasgow Outcome Scale Extended (GOSE) score was assessed at 6 months post-TBI. Post-mortem brains were obtained from the Australian Neurotrauma Tissue and Fluid Bank and used in qPCR for quantitating expression of KP enzymes (indoleamine 2,3-dioxygenase-1 (IDO1), kynurenase (KYNase), kynurenine amino transferase-II (KAT-II), kynurenine 3-monooxygenase (KMO), 3-hydroxyanthranilic acid oxygenase (3HAO) and quinolinic acid phosphoribosyl transferase (QPRTase) and IDO1 immunohistochemistry.

Results

In CSF, KYN, KYNA and QUIN were elevated whereas TRP, AA and 3HAA remained unchanged. The ratios of QUIN:KYN, QUIN:KYNA, KYNA:KYN and 3HAA:AA revealed that QUIN levels were significantly higher than KYN and KYNA, supporting increased neurotoxicity. Amplified IDO1 and KYNase mRNA expression was demonstrated on post-mortem brains, and enhanced IDO1 protein coincided with overt tissue damage. QUIN levels in CSF were significantly higher in patients with unfavourable outcome and inversely correlated with GOSE scores.

Conclusion

TBI induced a striking activation of the KP pathway with sustained increase of QUIN. The exceeding production of QUIN together with increased IDO1 activation and mRNA expression in brain-injured areas suggests that TBI selectively induces a robust stimulation of the neurotoxic branch of the KP pathway. QUIN’s detrimental roles are supported by its association to adverse outcome potentially becoming an early prognostic factor post-TBI.

Changes in Cathepsin D and Beclin-1 mRNA and protein expression by the excitotoxin quinolinic acid in human astrocytes and neurons

Quinolinic acid (QUIN) is an excitotoxin that has been implicated in the pathogenesis of several neurodegenerative diseases including Alzheimer's disease (AD). While QUIN has been shown to induce neuronal and astrocytic apoptosis as well as excitotoxic cell death, other mechanisms such as autophagy remain unexplored. We investigated the role of Cathepsin D (CatD) and Beclin-1 (Bc1) in QUIN-treated primary human astrocytes and neurons. We demonstrated that the expression patterns of CatD, a lysosomal aspartic protease associated with autophagy, are increased at 24 h after QUIN treatment. However, unlike CatD, the expression patterns of Bc1, a tumour suppressor protein, are significantly reduced at 24 h after QUIN treatment in both brain cell types. Furthermore, we showed that the NMDA ion channel blockers, MK801, can attenuate QUIN-induced changes CatD and Bc1 expression in both astrocytes and neurons. Taken together, these results suggest that induction of deficits in CatD and Bc1 is a significant mechanism for QUIN toxicity in glial and neuronal cells. Maintenance of autophagy may play a crucial role in neuroprotection in the setting of AD.

Lipid peroxidation markers in children with anxiety disorders and their diagnostic implications

OBJECTIVE

Numerous factors, including genetic, neurobiological, neurochemical, and psychological factors, are thought to be involved in the development of anxiety disorders. The latest findings show that the pathophysiology of anxiety disorders might be associated with oxidative stress and lipid peroxidation; however, no studies have so far investigated lipid peroxidation markers in children with anxiety disorders. Serum levels of lipid hydroperoxide (LOOH) are a reliable marker of lipid peroxidation. Paraoxonase and arylesterase are two enzymes that protect against such peroxidation, and might also be diagnostic markers. In this study, we investigated whether there are associations between anxiety disorders and lipid peroxidation markers in children, and assessed the diagnostic performance of these markers.

METHODS

The study group consisted of 37 patients (children and adolescents) with anxiety disorders. A control group, matched for age and gender, was composed of 36 healthy subjects. Venous blood samples were collected, and LOOH levels and paraoxonase and arylesterase activity were measured.

RESULTS

LOOH levels were significantly higher in the anxiety disorders group than in the control group. There were no significant differences in paraoxonase or arylesterase activities between the patient and the control groups.

DISCUSSION

Lipid peroxidation or oxidative damage might play a role in the aetiopathogenesis of anxiety disorders. LOOH may be a potential biological marker for anxiety disorders in children.

Heme-dependent dioxygenases in tryptophan oxidation

L-Tryptophan is an essential amino acid for mammals. It is utilized not only for protein synthesis but also for the biosynthesis of serotonin and melatonin by the serotonin pathway as well as nicotinamide adenine dinucleotide by the kynurenine pathway. Although the kynurenine pathway is responsible for the catabolism of over 90% of l-tryptophan in the mammalian intracellular and extracellular pools, the scientific field was dominated in the last century by studies of the serotonin pathway, due to the physiological significance of the latter's catabolic intermediates and products. However, in the past decade, the focus gradually reversed as the link between the kynurenine pathway and various neurodegenerative disorders and immune diseases is increasingly highlighted. Notably, the first step of this pathway, which is catalyzed by heme-dependent dioxygenases, has been proven to be a potential target for immune regulation and cancer treatment. A thorough understanding of the intriguing chemistry of the heme-dependent dioxygenases may yield insight for the drug discovery of these prevalent illnesses. In this review, we survey enzymatic and mechanistic studies, initially started by Kotake and Masayama over 70 years ago, at the molecular level on the heme-dependent tryptophan dioxygenation reactions.

Kynurenines and other novel therapeutic strategies in the treatment of dementia

Dementia is a common neuropsychological disorder with an increasing incidence. The most prevalent type of dementia is Alzheimer's disease. The underlying pathophysiological features of the cognitive decline are neurodegenerative processes, a cerebrovascular dysfunction and immunological alterations. The therapeutic approaches are still limited, although intensive research is being conducted with the aim of finding neuroprotective strategies. The widely accepted cholinesterase inhibitors and glutamate antagonists did not meet expectations of preventing disease progression, and research is therefore currently focusing on novel targets. Nonsteroidal anti-inflammatory drugs, secretase inhibitors and statins are promising drug candidates for the prevention and management of different forms of dementia. The kynurenine pathway has been associated with various neurodegenerative disorders and cerebrovascular diseases. This pathway is also closely related to neuroinflammatory processes and it has been implicated in the pathomechanisms of certain kinds of dementia. Targeting the kynurenine system may be of therapeutic value in the future.

Natural inhibitors of indoleamine 3,5-dioxygenase induced by interferon-gamma in human neural stem cells

Indoleamine dioxygenase (IDO) is a heme- containing enzyme that catalyzes the oxidation of tryptophan to N-formylkynurenine, kynurenine and the downstream quinolinic acid. Though IDO is physiologically important in maintaining tissue integrity, aberrant IDO expression represses T cell function and promotes regulatory T cells (Treg) in cancer. It additionally exacerbates Alzheimer, depression, Huntington and Parkinson diseases via quinolinic acid. Inhibition of IDO has thus been recently proposed as a strategy for treating cancer and neuronal disorders. In the present study, we have developed a cell-based assay to evaluate the suppressive effect of anti-inflammatory phytochemicals on the enzyme. When stimulated by INF-γ, profound high expressions of IDO-1 mRNA as well as the protein were detected in human neural stem cells (hNSC) and verified by real-time retro-transcribed PCR and western blot analysis, respectively. The protein activity was measured by kynurenine concentration and the assay was validated by dose-responsive inhibition of IDO-1 antagonists including 1-methyltryptaphan, indomethacin and acetylsalicylic acid. Among the tested compounds, apigenin, baicalein, chrysin, and wogonin exhibit a potent repressive activity with IC(50s) comparable to that of indomethacin. The inhibition was further found to be independent of gene expression and protein translation because of the unaltered levels of mRNA and protein expression. Although curcumin displayed a potent inhibitory activity to the enzyme, it appeared to be cytotoxic to hNSCs. Morphological examination of hNSC revealed that baicalein and wogonin at the inhibitory concentrations induced neurite outgrowth. In conclusion, our data shows that certain phytochemicals with 2-phenyl-1-benzopyran-4-one backbone (flavones) attenuate significantly the IDO-1 protein activity without harming hNSCs. The inhibitory activity might have partially contributed to the anti-cancer and neuro-protective property of the compounds.

Kynurenine Pathway in Skin Cells: Implications for UV-Induced Skin Damage

The kynurenine pathway (KP) is the principle route of catabolism of the essential amino acid tryptophan, leading to the production of several neuroactive and immunoregulatory metabolites. Alterations in the KP have been implicated in various neuropsychiatric and neurodegenerative diseases, immunological disorders, and many other diseased states. Although the role of the KP in the skin has been evaluated in small niche fields, limited studies are available regarding the effect of acute ultra violet exposure and the induction of the KP in human skin-derived fibroblasts and keratinocytes. Since UV exposure can illicit an inflammatory component in skin cells, it is highly likely that the KP may be induced in these cells in response to UV exposure. It is also possible that some KP metabolites may act as pro-inflammatory and anti-inflammatory mediators, since the KP is important in immunomodulation.

NAD+ metabolism and oxidative stress: the golden nucleotide on a crown of thorns

In the twentieth century, NAD+ research generated multiple discoveries. Identification of the important role of NAD+ as a cofactor in cellular respiration and energy production was followed by discoveries of numerous NAD+ biosynthesis pathways. In recent years, NAD+ has been shown to play a unique role in DNA repair and protein deacetylation. As discussed in this review, there are close interactions between oxidative stress and immune activation, energy metabolism, and cell viability in neurodegenerative disorders and ageing. Profound interactions with regard to oxidative stress and NAD+ have been highlighted in the present work. This review emphasizes the pivotal role of NAD+ in the regulation of DNA repair, stress resistance, and cell death, suggesting that NAD+ synthesis through the kynurenine pathway and/or salvage pathway is an attractive target for therapeutic intervention in age-associated degenerative disorders. NAD+ precursors have been shown to slow down ageing and extend lifespan in yeasts, and protect severed axons from degeneration in animal models neurodegenerative diseases.

Cooperation of non-effective concentration of glutamatergic system modulators and antioxidant against oxidative stress induced by quinolinic acid

Excessive formation of reactive oxygen species (ROS) and disruption of glutamate uptake have been hypothesized as key mechanisms contributing to quinolinic acid (QA)-induced toxicity. Thus, here we investigate if the use of diphenyl diselenide (PhSe)(2), guanosine (GUO) and MK-801, alone or in combination, could protect rat brain slices from QA-induced toxicity. QA (1 mM) increased ROS formation, thiobarbituric acid reactive substances (TBARS) and decreased cell viability after 2 h of exposure. (PhSe)(2) (1 μM) protected against this ROS formation in the cortex and the striatum and also prevented decreases in cell viability induced by QA. (PhSe)(2) (5 μM) prevented ROS formation in the hippocampus. GUO (10 and 100 μM) blocked the increase in ROS formation caused by QA and MK-801 (20 and 100 μM) abolished the pro-oxidant effect of QA. When the noneffective concentrations were used in combination produced a decrease in ROS formation, mainly (PhSe)(2) + GUO and (PhSe)(2) + GUO + MK-801. These results demonstrate that this combination could be effective to avoid toxic effects caused by high concentrations of QA. Furthermore, the data obtained in the ROS formation and cellular viability assays suggest different pathways in amelioration of QA toxicity present in the neurodegenerative process.

The association of the kynurenine pathway of tryptophan metabolism with acute brain dysfunction during critical illness*

OBJECTIVES

Plasma tryptophan levels are associated with delirium in critically ill patients. Although tryptophan has been linked to the pathogenesis of other neurocognitive diseases through metabolism to neurotoxins via the kynurenine pathway, a role for kynurenine pathway activity in intensive care unit brain dysfunction (delirium and coma) remains unknown. This study examined the association between kynurenine pathway activity as determined by plasma kynurenine concentrations and kynurenine/tryptophan ratios and presence or absence of acute brain dysfunction (defined as delirium/coma-free days) in intensive care unit patients.

DESIGN, SETTING, AND PATIENTS

This was a prospective cohort study that utilized patient data and blood samples from the Maximizing Efficacy of Targeted Sedation and Reducing Neurologic Dysfunction trial, which compared sedation with dexmedetomidine vs. lorazepam in mechanically ventilated patients.

MEASUREMENTS AND MAIN RESULTS

Baseline plasma kynurenine and tryptophan concentrations were measured using high-performance liquid chromatography with or without tandem mass spectrometry. Delirium was assessed daily using the Confusion Assessment Method for the Intensive Care Unit. Linear regression examined associations between kynurenine pathway activity and delirium/coma-free days after adjusting for sedative exposure, age, and severity of illness. Among 84 patients studied, median age was 60 yrs and Acute Physiology and Chronic Health Evaluation II score was 28.5. Elevated plasma kynurenine and kynurenine/tryptophan ratio were both independently associated with significantly fewer delirium/coma-free days (i.e., fewer days without acute brain dysfunction). Specifically, patients with plasma kynurenine or kynurenine/tryptophan ratios at the 75th percentile of our population had an average of 1.8 (95% confidence interval 0.6-3.1) and 2.1 (95% confidence interval 1.0-3.2) fewer delirium/coma-free days than those patients with values at the 25th percentile (p = .006 and p < .001, respectively).

CONCLUSIONS

Increased kynurenine pathway activation, assessed by plasma kynurenine and kynurenine/tryptophan ratio, was associated with fewer days alive and without acute brain dysfunction in intensive care unit patients. Future studies are warranted to clarify this relationship and investigate potential therapeutic interventions.

Ianthellamide A, a selective kynurenine-3-hydroxylase inhibitor from the Australian marine sponge Ianthella quadrangulata

Ianthellamide A (1), a novel octopamine derivative, was isolated from the Australian marine sponge Ianthella quadrangulata. Compound 1 selectively inhibited the activity of kynurenine 3-hydroxylase with an IC(50) value of 1.5 μM. It also significantly increased the level of endogenous kynurenic acid in rat brain and hence has the potential as a neuroprotective agent in the treatment of neurodegenerative disorders.

Kynurenines: from the perspective of major psychiatric disorders

Psychiatric disorders are documented to be associated with a mild pro-inflammatory state. Pro-inflammatory mediators could activate the tryptophan breakdown and kynurenine pathway with a shift toward the neurotoxic arm where excitotoxic N-methyl-D-aspartate receptor agonist quinolinic acid is formed. An unbalanced metabolism in terms of neuroprotective and neurotoxic effects, such as reduced kynurenic acid to kynurenine ratio, has been demonstrated in the major psychiatric disorders such as unipolar depression, bipolar manic-depressive disorder and schizophrenia, and in drug-induced neuropsychiatric side effects such as interferon-α treated patients. The changes in serum or plasma are shown to be associated with central changes such as in the cerebrospinal fluid and certain brain areas. While currently available antidepressants and mood stabilizers could not efficiently improve these neurochemical changes within the same period that could induce clinical improvement, some antipsychotic treatments could reverse certain metabolic imbalances. Some of these changes were tested also in animal models. In this review the role of this unbalanced kynurenine metabolism through interactions with other neurochemicals is discussed as a major contributing pathophysiological mechanism in psychiatric disorders. Moreover, the biomarker role of kynurenine metabolites and future therapeutic opportunities are also discussed.