Some of the tryptophan catabolites produced through the kynurenine pathway (KP), and more particularly the excitotoxin quinolinic acid (QA), are likely to play a role in the pathogenesis of Alzheimer's disease (AD). We have previously shown that the KP is over activated in AD brain and that QA accumulates in amyloid plaques and within dystrophic neurons. We hypothesized that QA in pathophysiological concentrations affects tau phosphorylation. Using immunohistochemistry, we found that QA is co-localized with hyperphosphorylated tau (HPT) within cortical neurons in AD brain. We then investigated in vitro the effects of QA at various pathophysiological concentrations on tau phosphorylation in primary cultures of human neurons. Using western blot, we found that QA treatment increased the phosphorylation of tau at serine 199/202, threonine 231 and serine 396/404 in a dose dependent manner. Increased accumulation of phosphorylated tau was also confirmed by immunocytochemistry. This increase in tau phosphorylation was paralleled by a substantial decrease in the total protein phosphatase activity. A substantial decrease in PP2A expression and modest decrease in PP1 expression were observed in neuronal cultures treated with QA. These data clearly demonstrate that QA can induce tau phosphorylation at residues present in the PHF in the AD brain. To induce tau phosphorylation, QA appears to act through NMDA receptor activation similar to other agonists, glutamate and NMDA. The QA effect was abrogated by the NMDA receptor antagonist memantine. Using PCR arrays, we found that QA significantly induces 10 genes in human neurons all known to be associated with AD pathology. Of these 10 genes, 6 belong to pathways involved in tau phosphorylation and 4 of them in neuroprotection. Altogether these results indicate a likely role of QA in the AD pathology through promotion of tau phosphorylation. Understanding the mechanism of the neurotoxic effects of QA is essential in developing novel therapeutic strategies for AD.

Obesity is a risk factor for neurodegenerative disease associated with cognitive dysfunction, including Alzheimer's disease. Low-grade inflammation is common in obesity, but the mechanism between inflammation and cognitive impairment in obesity is unclear. Accumulative evidence shows that quinolinic acid (QA), a neuroinflammatory neurotoxin, is involved in the pathogenesis of neurodegenerative processes. We investigated the role of QA in obesity-induced cognitive impairment and the beneficial effect of butyrate in counteracting impairments of cognition, neural morphology, and signaling. We show that in human obesity, there was a negative relationship between serum QA levels and cognitive function and decreased cortical gray matter. Diet-induced obese mice had increased QA levels in the cortex associated with cognitive impairment. At single-cell resolution, we confirmed that QA impaired neurons, altered the dendritic spine's intracellular signal, and reduced brain-derived neurotrophic factor (BDNF) levels. Using Caenorhabditis elegans models, QA induced dopaminergic and glutamatergic neuron lesions. Importantly, the gut microbiota metabolite butyrate was able to counteract those alterations, including cognitive impairment, neuronal spine loss, and BDNF reduction in both in vivo and in vitro studies. Finally, we show that butyrate prevented QA-induced BDNF reductions by epigenetic enhancement of H3K18ac at BDNF promoters. These findings suggest that increased QA is associated with cognitive decline in obesity and that butyrate alleviates neurodegeneration.

The molecular mechanisms underlying the diverse psychiatric and neuropathological sequalae documented in subsets of athletes with concussion have not been identified. We have previously reported elevated quinolinic acid (QuinA), a neurotoxic kynurenine pathway metabolite, acutely following concussion in football players with prior concussion. Similarly, work from our group and others has shown that increased functional connectivity strength, assessed using resting state fMRI, occurs following concussion and is associated with worse concussion-related symptoms and outcome. Moreover, other work has shown that repetitive concussion may have cumulative effects on functional connectivity and is a risk factor for adverse outcomes. Understanding the molecular mechanisms underlying these cumulative effects may ultimately be important for therapeutic interventions or the development of prognostic biomarkers. Thus, in this work, we tested the hypothesis that the relationship between QuinA in serum and functional connectivity following concussion would depend on the presence of a prior concussion. Concussed football players with prior concussion (N = 21) and without prior concussion (N = 16) completed a MRI session and provided a blood sample at approximately 1 days, 8 days, 15 days, and 45 days post-injury. Matched, uninjured football players with (N = 18) and without prior concussion (N = 24) completed similar visits. The association between QuinA and global connectivity strength differed based on group (F(3, 127) = 3.46, p = 0.019); post-hoc analyses showed a positive association between QuinA and connectivity strength in concussed athletes with prior concussion (B = 16.05, SE = 5.06, p = 0.002, 95%CI[6.06, 26.03]), but no relationship in concussed athletes without prior concussion or controls. Region-specific analyses showed that this association was strongest in bilateral orbitofrontal cortices, insulae, and basal ganglia. Finally, exploratory analyses found elevated global connectivity strength in concussed athletes with prior concussion who reported depressive symptoms at the 1-day visit compared to those who did not report depressive symptoms (t(15) = 2.37, mean difference = 13.50, SE = 5.69, p = 0.032, 95%CI[1.36, 25.63], Cohen's d = 1.15.). The results highlight a potential role of kynurenine pathway (KP) metabolites in altered functional connectivity following concussion and raise the possibility that repeated concussion has a "priming" effect on KP metabolism.

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.

NAD+ biosynthesis is an attractive and promising therapeutic target for influencing health span and obesity-related phenotypes as well as tumor growth. Full and effective use of this target for therapeutic benefit requires a complete understanding of NAD+ biosynthetic pathways. Here, we report a previously unrecognized role for a conserved phosphoribosyltransferase in NAD+ biosynthesis. Because a required quinolinic acid phosphoribosyltransferase (QPRTase) is not encoded in its genome, Caenorhabditis elegans are reported to lack a de novo NAD+ biosynthetic pathway. However, all the genes of the kynurenine pathway required for quinolinic acid (QA) production from tryptophan are present. Thus, we investigated the presence of de novo NAD+ biosynthesis in this organism. By combining isotope-tracing and genetic experiments, we have demonstrated the presence of an intact de novo biosynthesis pathway for NAD+ from tryptophan via QA, highlighting the functional conservation of this important biosynthetic activity. Supplementation with kynurenine pathway intermediates also boosted NAD+ levels and partially reversed NAD+-dependent phenotypes caused by mutation of pnc-1, which encodes a nicotinamidase required for NAD+ salvage biosynthesis, demonstrating contribution of de novo synthesis to NAD+ homeostasis. By investigating candidate phosphoribosyltransferase genes in the genome, we determined that the conserved uridine monophosphate phosphoribosyltransferase (UMPS), which acts in pyrimidine biosynthesis, is required for NAD+ biosynthesis in place of the missing QPRTase. We suggest that similar underground metabolic activity of UMPS may function in other organisms. This mechanism for NAD+ biosynthesis creates novel possibilities for manipulating NAD+ biosynthetic pathways, which is key for the future of therapeutics.

The excitotoxin quinolinic acid, a by-product of the kynurenine pathway, is known to be involved in several neurological diseases including multiple sclerosis (MS). Quinolinic acid levels are elevated in experimental autoimmune encephalomyelitis rodents, the widely used animal model of MS. Our group has also found pathophysiological concentrations of quinolinic acid in MS patients. This led us to investigate the effect of quinolinic acid on oligodendrocytes; the main cell type targeted by the autoimmune response in MS. We have examined the kynurenine pathway (KP) profile of two oligodendrocyte cell lines and show that these cells have a limited threshold to catabolize exogenous quinolinic acid. We further propose and demonstrate two strategies to limit quinolinic acid gliotoxicity: 1) by neutralizing quinolinic acid's effects with anti-quinolinic acid monoclonal antibodies and 2) directly inhibiting quinolinic acid production from activated monocytic cells using specific KP enzyme inhibitors. The outcome of this study provides a new insight into therapeutic strategies for limiting quinolinic acid-induced neurodegeneration, especially in neurological disorders that target oligodendrocytes, such as MS.

One of the mechanisms of neurotoxicity is the induction of oxidative stress. There is hardly any cure for neurotoxicity in modern medicine, whereas many drugs in Ayurveda possess neuroprotective effects; however, there is no scientific validation for these drugs. Ksheerabala is an ayurvedic drug which is used to treat central nervous system disorders, arthritis, and insomnia. The aim of our study was to evaluate the effect of Ksheerabala on quinolinic acid-induced toxicity in rat brain. The optimal dose of Ksheerabala was found from a dose escalation study, wherein it was found that Ksheerabala showed maximum protection against quinolinic acid-induced neurotoxicity at a dose of 15 microL/100 g body weight/day, which was selected for further experiments. Four groups of female albino rats were maintained for 21 days as follows: 1. Control group, 2. Quinolinic acid (55 microg/100 g body weight), 3. Ksheerabala (15 microL/100 g body weight), 4. Ksheerabala (15 microL/100 g body weight) + Quinolinic acid (55 microg/100 g body weight). At the end of the experimental period, levels of lipid peroxidation products, protein carbonyls, and activities of scavenging enzymes were analyzed. The results revealed that quinolinic acid intake caused enhanced lipid and protein peroxidation as evidenced by increased levels of peroxidation products such as malondialdehyde, hydroperoxide, conjugated dienes, and protein carbonyls. On the other hand, the activities of scavenging enzymes such as catalase, superoxide dismutase (SOD), glutathione peroxidase, and glutathione reductase as well as the concentration of glutathione were reduced. On coadminstration of Ksheerabala along with quinolinic acid, the levels of all the biochemical parameters were restored to near-normal levels, indicating the protective effect of the drug. These results were reinforced by histopathological studies.

The kynurenine pathway (KP) and one of its end-products, the excitotoxin quinolinic acid (QUIN), are involved in the pathogenesis of several major neuroinflammatory brain diseases. A relevant animal model to study KP metabolism is now needed to assess whether intervention in this pathway may improve the outcome of such diseases. Humans and macaques share a very similar genetic makeup. In this study, we characterized the KP metabolism in macaque primary macrophages of three different species in comparison to human cells. We found that the KP profiles in simian macrophages were very similar to those in humans when challenged with inflammatory cytokines. Further, we found that macaque macrophages are capable of producing a pathophysiological concentration of QUIN. Our data validate the simian model as a relevant model to study the human cellular KP metabolism in the context of inflammation.

The endocannabinoid system (ECS) regulates several physiological processes in the Central Nervous System, including the modulation of neuronal excitability via activation of cannabinoid receptors (CBr). Both glutaric acid (GA) and quinolinic acid (QUIN) are endogenous metabolites that, under pathological conditions, recruit common toxic mechanisms. A synergistic effect between them has already been demonstrated, supporting potential implications for glutaric acidemia type I (GA I). Here we investigated the possible involvement of a cannabinoid component in the toxic model exerted by QUIN + GA in rat cortical slices and primary neuronal cell cultures. The effects of the CB1 receptor agonist anandamide (AEA), and the fatty acid amide hydrolase inhibitor URB597, were tested on cell viability in cortical brain slices and primary neuronal cultures exposed to QUIN, GA, or QUIN + GA. As a pre-treatment to the QUIN + GA condition, AEA prevented the loss of cell viability in both preparations. URB597 only protected in a moderate manner the cultured neuronal cells against the QUIN + GA-induced damage. The use of the CB1 receptor reverse agonist AM251 in both biological preparations prevented partially the protective effects exerted by AEA, thus suggesting a partial role of CB1 receptors in this toxic model. AEA also prevented the cell damage and apoptotic death induced by the synergic model in cell cultures. Altogether, these findings demonstrate a modulatory role of the ECS on the synergic toxic actions exerted by QUIN + GA, thus providing key information for the understanding of the pathophysiological events occurring in GA I.

Drugs and their metabolites often produce undesirable effects. These may be due to a number of mechanisms, including biotransformation by P450 enzymes which are not exclusively expressed by hepatocytes but also by endothelial cells in brain from epileptics. The possibility thus exists that the potency of systemically administered central nervous system therapeutics can be modulated by a metabolic blood-brain barrier (BBB). Surgical brain specimens and blood samples (ex vivo) were obtained from drug-resistant epileptic subjects receiving the antiepileptic drug carbamazepine prior to temporal lobectomies. An in vitro blood-brain barrier model was then established using primary cell culture derived from the same brain specimens. The pattern of carbamazepine (CBZ) metabolism was evaluated in vitro and ex vivo using high performance liquid chromatography-mass spectroscopy. Accelerated mass spectroscopy was used to identify (14)C metabolites deriving from the parent (14)C-carbamazepine. Under our experimental conditions carbamazepine levels could not be detected in drug resistant epileptic brain ex situ; low levels of carbamazepine were detected in the brain side of the in vitro BBB established with endothelial cells derived from the same patients. Four carbamazepine-derived fractions were detected in brain samples in vitro and ex vivo. HPLC-accelerated mass spectroscopy confirmed that these signals derived from (14)C-carbamazepine administered as parental drug. Carbamazepine 10, 11 epoxide (CBZ-EPO) and 10, 11-dihydro-10, 11-dihydrooxy-carbamazepine (DiOH-CBZ) were also detected in the fractions analyzed. (14)C-enriched fractions were subsequently analyzed by mass spectrometry to reveal micromolar concentrations of quinolinic acid (QA). Remarkably, the disappearance of carbamazepine-epoxide (at a rate of 5% per hour) was comparable to the rate of quinolinic acid production (3% per hour). This suggested that quinolinic acid may be a result of carbamazepine metabolism. Quinolinic acid was not detected in the brain of patients who received antiepileptic drugs other than carbamazepine prior to surgery or in brain endothelial cultures obtained from a control patient. Our data suggest that a drug resistant BBB not only impedes drug access to the brain but may also allow the formation of neurotoxic metabolites.

Tryptophan and its catabolites (TRYCATs) have been suggested to link peripheral immune system activation and central neurotransmitter abnormalities with relevance to the etio-pathophysiology of schizophrenia (SZ) and major depressive disorder (MDD). The relationship to different psychopathological dimensions within these disorders however remains to be elucidated. We thus investigated potential group differences of tryptophan, kynurenine, kynurenic acid, 3-hydroxy kynurenine and quinolinic acid in the plasma of 19 healthy controls (HC), 45 patients with SZ and 43 patients with MDD and correlated plasma proteins with the "motivation and pleasure" dimension and cognition. After correcting for the covariates age, sex, body mass index, smoking and medication, patients with MDD showed lower kynurenine and 3-hydroxy kynurenine levels compared to HC. Quinolinic acid correlated negatively with composite cognitive score in patients with SZ, indicating that more severe cognitive impairments were associated with increased plasma levels of quinolinic acid. No correlations were found in patients with MDD. These results indicate that MDD and SZ are associated with dysregulation of the kynurenine pathway. Quinolinic acid might be specifically implicated in the pathophysiology of cognitive deficits in patients with SZ. Further studies are needed to determine whether TRYCATs are causally involved in the etiology of these neuropsychiatric disorders.

. Mycobacterium tuberculosis is the single most deadly human pathogen and is responsible for nearly three million deaths every year. Recent elucidation of the mode of action of isoniazid, a frontline antimycobacterial drug, suggests that NAD metabolism is extremely critical for this microorganism. M. tuberculosis depends solely on the de novo pathway to meet its NAD demand. Quinolinic acid phosphoribosyltransferase (QAPRTase), a key enzyme in the de novo biosynthesis of NAD, provides an attractive target for designing novel antitubercular drugs.

The excitotoxin quinolinic acid (QUIN) is synthesized through the kynurenine pathway (KP) by activated monocyte lineage cells. QUIN is likely to play a role in the pathogenesis of several major neuroinflammatory diseases including Alzheimer's disease (AD). The presence of reactive astrocytes, astrogliosis, increased oxidative stress and inflammatory cytokines are important pathological hallmarks of AD. We assessed the stimulatory effects of QUIN at low physiological to high excitotoxic concentrations in comparison with the cytokines commonly associated with AD including IFN-gamma and TNF-alpha on primary human astrocytes. We found that QUIN induces IL-1beta expression, a key mediator in AD pathogenesis, in human astrocytes. We also explored the effect of QUIN on astrocyte morphology and functions. At low concentrations, QUIN treatment induced concomitantly a marked increase in glial fibrillary acid protein levels and reduction in vimentin levels compared to controls; features consistent with astrogliosis. At pathophysiological concentrations QUIN induced a switch between structural protein expressions in a dose dependent manner, increasing VIM and concomitantly decreasing GFAP expression. Glutamine synthetase (GS) activity was used as a functional metabolic test for astrocytes. We found a significant dose-dependent reduction in GS activity following QUIN treatment. All together, this study showed that QUIN is an important factor for astroglial activation, dysregulation and cell death with potential relevance to AD and other neuroinflammatory diseases.

Quinolinic acid (QA) is an N-methyl-D-aspartate receptor agonist that also promotes glutamate release and inhibits glutamate uptake by astrocytes. QA is used in experimental models of seizures studying the effects of overstimulation of the glutamatergic system. The guanine-based purines (GBPs), including the nucleoside guanosine, have been shown to modulate the glutamatergic system when administered extracellularly. GBPs were shown to inhibit the binding of glutamate and analogs, to be neuroprotective under excitotoxic conditions, as well as anticonvulsant against seizures induced by glutamatergic agents, including QA-induced seizure. In this work, we studied the electrophysiological effects of guanosine against QA-induced epileptiform activity in rats at the macroscopic cortical level, as inferred by electroencephalogram (EEG) signals recorded at the epidural surface. We found that QA disrupts a prominent basal theta (4-10 Hz) activity during peri-ictal periods and also promotes a relative increase in gamma (20-50 Hz) oscillations. Guanosine, when successfully preventing seizures, counteracted both these spectral changes. MK-801, an NMDA-antagonist used as positive control, was also able counteract the decrease in theta power; however, we observed an increase in the power of gamma oscillations in rats concurrently treated with MK-801 and QA. Given the distinct spectral signatures, these results suggest that guanosine and MK-801 prevent QA-induced seizures by different network mechanisms.

Quinolinic acid (QUIN) is an endogenous excitatory amino acid, which is elevated in brain tissues or cerebrospinal fluid (CSF) in several acute and chronic inflammatory central nervous system (CNS) diseases. The functional significance of this elevation is unknown but speculations of excitotoxicity have been raised. We have begun to address the pathologic consequences of elevated CSF QUIN by studying the effects of intracerebroventricular (i.cv) administration of QUIN on regional choline acetyltransferase (ChAT) activity, somatostatin content and glucose metabolism in the rat brain. QUIN (12 and 60 nmol) i.cv administration once a day for 7 days (total dose; 84 and 420 nmol, respectively) had minimal effect on somatostatin content and no effect on ChAT activity. In contrast, following continuous i.cv infusion of QUIN for 14 days using an osmotic minipump (480 nmol), ChAT activity dropped in the hippocampus and the striatum and somatostatin content was reduced in the frontal cortex, hippocampus, striatum and amygdala. Moreover, following the QUIN infusion, glucose utilization decreased in the basal nucleus of Meynert, frontal cortex, and portions of the basal ganglia and the limbic system. These results indicate that subchronic i.cv infusion of QUIN to rats results in selective regional neurochemical and metabolic changes distributed throughout the CNS. These results suggest target brain areas and transmitter systems which may be associated with neurologic syndromes characterized by elevated CSF QUIN levels.

Stereotaxic intrastriatal injection of the naturally occurring N-methyl-D-aspartate (NMDA) agonist quinolinic acid (QA) serves as a valuable in vivo model to study excitotoxic cell damage in the central nervous system (CNS). Although morphological changes such as neuronal loss, glial activation and remote reactions following QA injection have been described in some detail, much less is known about the molecular mechanisms mediating the accompanying glial response. Cytokines are known to play a crucial role in almost all kinds of CNS alterations. We now demonstrate that IL-6, a multifunctional glycoprotein which belongs to the family of neurokines, is expressed endogenously in the rat striatum following QA injection. Using Northern blot analysis, a massive but transient upregulation of IL-6 mRNA could be detected. This started 3 h after QA injection, reached a maximum at 6 h and disappeared within 24 h. That activated microglia are the most likely cellular source of the observed corresponding IL-6 protein expression could be concluded by comparing the immunocytochemical pattern of IL-6 expression and microglial activation. Interestingly, astrocytes initially downregulate their expression of glial fibrillary acidic protein (GFAP) in the excitotoxically injured striatum, but show a delayed increase in GFAP immunoreactivity starting in the periphery of the striatum, subsequently expanding to the core. The early transient IL-6 expression may play an important role in initiating the delayed astrocytic response following excitotoxic cell injury.

Although both quinolinic acid and 3-nitropropionic acid destroy medium sized, GABAergic, spiny projection neurons after direct perfusion of neurotoxin into the rat striatum, changes in extracellular GABA concentration in the striatum within the first 90 min reflect different toxic mechanisms in these two animal models for Huntington's disease. Since quinolinic acid acts as a potent excitotoxin, the early depolarizing response in GABAergic neurons results in an early increase in extracellular GABA activity (peak at 40 min) whereas the more indirect action of 3-nitropropionic acid on mitochondrial energy metabolism results in a delayed increase in extracellular GABA activity (peak at 60 min) with a pattern of gradual increase and decline. The localized delivery of cytotoxin provides an opportunity for kinetic comparisons of direct and indirect cytotoxic mechanisms that can be useful in developing neuroprotective treatment strategies in Huntington's disease.

Quinolinic acid (QA) is a neurotoxin and has been shown to be present at high levels in the central nervous system of patients with certain diseases, such as AIDS and meningitis. The enzyme quinolinic acid phosphoribosyltransferase (QAPRTase) provides the only route for QA metabolism and is also an essential step in de novo NAD biosynthesis. QAPRTase catalyzes the synthesis of nicotinic acid mononucleotide (NAMN) from QA and 5-phosphoribosyl-1-pyrophosphate (PRPP). The structures of several phosphoribosyltransferases (PRTases) have been reported, and all have shown a similar fold of a five-strandard beta sheet surrounded by four alpha helices. A conserved sequence motif of 13 residues is common to these 'type I' PRTases but is not observed in the QAPRTase sequence, suggestive of a different fold for this enzyme.

Decreased activity of cAMP responsive element-binding protein (CREB) is thought to contribute to the death of striatal medium spiny neurons in Huntington's disease (HD). Therefore, therapies that increase levels of activated CREB, may be effective in fighting neurodegeneration in HD. In this study, we sought to determine whether the phosphodiesterase type 10 (PDE10A) inhibitor TP10 exerts a neuroprotective effect in an excitotoxic model of HD. Rats were surgically administered with quinolinic acid into striatum and subsequently treated with TP10 daily for two or eight weeks. After 2 weeks of TP10 treatment, striatal lesion size was 52% smaller and the surviving cell number was several times higher than in the vehicle-treated group. These beneficial effects of TP10 were maintained through 8 weeks. TP10 treatment also increased significantly the levels of activated CREB in the striatal spiny neurons, which is hypothesized to be a contributing mechanism for the neuroprotective effect. Our findings suggest PDE10A inhibition as a novel neuroprotective approach to the treatment of HD and confirm the importance of phosphodiesterase inhibition in fighting the disease.

Mangiferin (MGF), a xanthonoid polyphenol, confers neuroprotection via combating oxidative stress and inflammation. The current investigation aimed to assess the neuroprotective potential of MGF on behavioral and neurochemical anomalies evoked by administration of quinolinic acid (QA) through intrastriatal injection in male Wistar rats and to reveal the associated mechanisms.