Quinolinic acid: a pathogen in seizure disorders?

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.

NAD(+) metabolism: Bioenergetics, signaling and manipulation for therapy

We survey the historical development of scientific knowledge surrounding Vitamin B3, and describe the active metabolite forms of Vitamin B3, the pyridine dinucleotides NAD+ and NADP+ which are essential to cellular processes of energy metabolism, cell protection and biosynthesis. The study of NAD+ has become reinvigorated by new understandings that dynamics within NAD+ metabolism trigger major signaling processes coupled to effectors (sirtuins, PARPs, and CD38) that reprogram cellular metabolism using NAD+ as an effector substrate. Cellular adaptations include stimulation of mitochondrial biogenesis, a process fundamental to adjusting cellular and tissue physiology to reduced nutrient availability and/or increased energy demand. Several mammalian metabolic pathways converge to NAD+, including tryptophan-derived de novo pathways, nicotinamide salvage pathways, nicotinic acid salvage and nucleoside salvage pathways incorporating nicotinamide riboside and nicotinic acid riboside. Key discoveries highlight a therapeutic potential for targeting NAD+ biosynthetic pathways for treatment of human diseases. A recent emergence of understanding that NAD+ homeostasis is vulnerable to aging and disease processes has stimulated testing to determine if replenishment or augmentation of cellular or tissue NAD+ can have ameliorative effects on aging or disease phenotypes. This experimental approach has provided several proofs of concept successes demonstrating that replenishment or augmentation of NAD+ concentrations can provide ameliorative or curative benefits. Thus NAD+ metabolic pathways can provide key biomarkers and parameters for assessing and modulating organism health.

Vitamin B3, the nicotinamide adenine dinucleotides and aging

Organism aging is a process of time and maturation culminating in senescence and death. The molecular details that define and determine aging have been intensely investigated. It has become appreciated that the process is partly an accumulation of random yet inevitable changes, but it can be strongly affected by genes that alter lifespan. In this review, we consider how NAD(+) metabolism plays important roles in the random patterns of aging, and also in the more programmatic aspects. The derivatives of NAD(+), such as reduced and oxidized forms of NAD(P)(+), play important roles in maintaining and regulating cellular redox state, Ca(2+) stores, DNA damage and repair, stress responses, cell cycle timing and lipid and energy metabolism. NAD(+) is also a substrate for signaling enzymes like the sirtuins and poly-ADP-ribosylpolymerases, members of a broad family of protein deacetylases and ADP-ribosyltransferases that regulate fundamental cellular processes such as transcription, recombination, cell division, proliferation, genome maintenance, apoptosis, stress resistance and senescence. NAD(+)-dependent enzymes are increasingly appreciated to regulate the timing of changes that lead to aging phenotypes. We consider how metabolism, specifically connected with Vitamin B3 and the nicotinamide adenine dinucleotides and their derivatives, occupies a central place in the aging processes of mammals.

Nicotinic acid, nicotinamide, and nicotinamide riboside: a molecular evaluation of NAD+ precursor vitamins in human nutrition

Although baseline requirements for nicotinamide adenine dinucleotide (NAD+) synthesis can be met either with dietary tryptophan or with less than 20 mg of daily niacin, which consists of nicotinic acid and/or nicotinamide, there is growing evidence that substantially greater rates of NAD+ synthesis may be beneficial to protect against neurological degeneration, Candida glabrata infection, and possibly to enhance reverse cholesterol transport. The distinct and tissue-specific biosynthetic and/or ligand activities of tryptophan, nicotinic acid, nicotinamide, and the newly identified NAD+ precursor, nicotinamide riboside, reviewed herein, are responsible for vitamin-specific effects and side effects. Because current data suggest that nicotinamide riboside may be the only vitamin precursor that supports neuronal NAD+ synthesis, we present prospects for human nicotinamide riboside supplementation and propose areas for future research.

Identification and Expression of α Cdna Encoding Human 2-Amino-3- Carboxymuconate-6-Semialdehyde Decarboxylase (Acmsd): A Key Enzyme for the Tryptophan-Niacine Pathway and Quinolinate Hypothesis

Quinolinate (quinolinic acid) is a potent endogenous excitotoxin of neuronal cells. Elevation of quinolinate levels in the brain has been implicated in the pathogenesis of various neurodegenerative disorders, the so-called "quinolinate hypothesis." Quinolinate is non-enzymatically derived from 2-amino-3-carboxymuconate-6-semialdehyde (ACMS). 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase (ACMSD) is the only known enzyme which can process ACMS to a benign catabolite and thus prevent the accumulation of quinolinate from ACMS. ACMSD seems to be regulated by nutritional and hormonal signals, but its molecular mechanism has, to date, been largely unknown. Utilizing partial amino acid sequences obtained from highly purified porcine kidney ACMSD, a cDNA encoding human ACMSD was cloned and characterized. The cDNA encodes a unique open reading frame of 336 amino acids and displays little homology to any known enzymes or motifs in mammalian databases, suggesting that ACMSD may contain a new kind of protein fold. Real-time PCR-based quantification of ACMSD revealed very low but significant levels of the expression in the brain. Brain ACMSD messages was down- and up-regulated in response to low protein diet and streptozocin-induced diabetes, respectively. Expression of QPRT, another enzyme which catabolizes quinolinate, was also found in the brain. This suggests that a pathway does exist by which the levels of quinolinate in the brain are regulated. In this report, we address the molecular basis underlying quinolinate metabolism and the regulation of ACMSD expression.

Tryptophan metabolism through the kynurenine pathway in rat brain and liver slices

We hypothesized that hyperbaric oxygen (HBO) enhances tryptophan (TRP) flux through the kynurenine (KYN) pathway because oxygen is a substrate for four pathway enzymes. Our objective was to compare the biosynthesis of KYN pathway intermediates by rat brain and liver slices with air or HBO as the gas phase. One-millimeter thick liver and brain slices were obtained from male Sprague-Dawley rats and incubated individually in chambers containing Hanks'-HEPES- buffer with (3)H-TRP (30 Ci/mmol) for 2 h (37 degrees C) in either room air or oxygen (1.2 or 5.2 atmospheres absolute [ATA] oxygen). After incubation, tissue was snap-frozen and analyzed for protein content while medium was extracted for high-performance liquid chromatography analysis. Radiolabeled nicotinamide adenine dinucleotide (NAD) was produced by brain and liver; liver (with air as the gas phase) also produced quinolinic acid (QA). HBO at 1.2 and 5.2 ATA caused increased QA and NAD from liver slices. HBO did not affect KYN metabolism in brain slices, although there was decreased production of NAD during high oxygen. We conclude that rat brain and liver contain the complete KYN pathway and that HBO enhances KYN flux in liver tissue.

Characterization and functional expression of the cDNA encoding human brain quinolinate phosphoribosyltransferase

To elucidate the molecular basis underlying quinolinate metabolism, the cDNA encoding human brain QPRTase was cloned.

Effects of oxygen on 3-hydroxyanthranilate oxidase of the kynurenine pathway

Iron containing 3-Hydroxyanthranilate oxidase (3HAO) converts 3-hydroxyanthranilate (3HAA) and dioxygen into a precursor which spontaneously converts to quinolinic acid (QA). 3HAO participates in de novo biosynthesis of NAD in mammalian kidney and liver, and it is present in low concentrations in brain where its function is controversial. However, QA increases in spinal fluid and is associated with convulsions in AIDS dementia, Huntington's disease, and CNS inflammation. QA is a known N-methyl, D-aspartate receptor agonist and excitotoxin that causes convulsions when injected into the brain. Hyperbaric oxygen (HBO) also causes convulsions and we investigated the interrelationships among the stimulating and toxic effects of oxygen and the role of iron in vitro using rat liver enzyme which is reported to be identical to brain enzyme and is more abundant. 3HAO requires dioxygen as a substrate but it was inactivated approximately 40% by 5.2 atm HBO in vitro in 15 min. The apparent Km was 2.6 x 10(-4) M for oxygen and 5 x 10(-5) M for 3HAA, and these values did not change for enzyme that was half-inactivated by HBO oxygen. Thus, oxygen-inactivation appears to be all-or-none for individual enzyme molecules. Freshly prepared enzyme was activated about 3-fold by incubation with acidic iron. Iron-staining of 3HAO, separated by gel electrophoresis after partial purification by FPLC, showed that loss of iron and loss of enzyme activity during HBO exposure were correlated. The apparent oxygen Km of 3HAO is far higher than the oxygen concentration in brain cells. Thus, 3HAO is capable of being stimulated initially in animals breathing HBO, and subsequently of being inactivated with potential significance for brain QA and convulsions.

Characterization and functional expression of the cDNA encoding human brain quinolinate phosphoribosyltransferase

Mammalian quinolinate phosphoribosyltransferase (QPRTase) (EC 2.4.2.19) is a key enzyme in catabolism of quinolinate, an intermediate in the tryptophan-nicotinamide adenine dinucleotide (NAD) pathway. Quinolinate acts as a most potent endogenous exitotoxin to neurons. Elevation of quinolinate levels in the brain has been linked to the pathogenesis of neurodegenerative disorders. As the first step to elucidate molecular basis underlying the quinolinate metabolism, the cDNA encoding human brain QPRTase was cloned and characterized. Utilizing partial amino acid sequences obtained from highly purified porcine kidney QPRTase, a human isolog was obtained from a human brain cDNA library. The cDNA encodes a open reading frame of 297 amino acids, and shares 30 to 40% identity with those of bacterial QPRTases. To confirm that the cDNA clone encodes human QPRTase, its functional expression was studied in a bacterial host. Introduction of the human cDNA into a QPRTase defective (nadC) E. coli strain brought about an abrupt increase in QPRTase activity and allowed the cells to grow in the absence of nicotinic acid. It is concluded that the cloned cDNA encodes human QPRTase which is functional beyond the phylogenic boundary.

Kynurenine pathway metabolites in cerebrospinal fluid and serum in complex partial seizures

The kynurenine pathway metabolites, quinolinic acid (QUIN) and L-kynurenine are convulsants, whereas kynurenic acid (KYNA) is an antagonist of excitatory amino acid receptors. Imbalances in the concentrations of these metabolites have been implicated in the etiology of human seizure disorders. In the present study, L-kynurenine and QUIN concentrations in both cerebrospinal fluid (CSF) and serum were reduced in patients with intractable complex partial seizures (CPS) in both the postictal period (15-75 min after a seizure) and the interictal period (absence of seizure for > 24 h) as compared with neurologically normal control subjects. Linear regression analyses and analysis of covariance showed that the reductions in serum QUIN and L-kynurenine were correlated to blood antiepileptic medication. L-Tryptophan (L-TRP) levels also tended to be lower in both CSF and serum of the seizure patients. CSF KYNA and serum 3-hydroxykynurenine concentrations were not affected in seizure patients, whereas serum levels of KYNA were reduced. 3-Hydroxykynurenine was not detected in the CSF of either control or seizure patients. The results do not support a role for a generalized reduction in KYNA concentrations or an increased ratio of QUIN:KYNA, or increases in CSF L-kynurenine in initiation and maintenance of intractable CPS humans.

Distribution of quinolinic acid phosphoribosyltransferase in the human hippocampal formation and parahippocampal gyrus

The morphological distribution of quinolinic acid phosphoribosyltransferase (QPRT), the degradative enzyme of the endogenous excitotoxin quinolinic acid, was studied in the human hippocampal formation and parahippocampal gyrus by immunohistochemical techniques. In seven neurologically normal human brains obtained at autopsy, QPRT-immunoreactivity (QPRT-i) was found in both glial cells and neurons. Glial cells exhibiting QPRT-immunoreactivity morphological features of astrocytes, were observed in all hippocampal subfields. The polymorphic layer of the dentate gyrus contained the highest density of QPRT-i glial cells. Numerous QPRT-i glial cells were also found along both sides of the fused hippocampal fissure and in the white matter including the alveus of Ammon's horn, whereas only a few were observed in the granule cell layer and the stratum pyramidale. Neurons containing QPRT-i were found mainly in the subiculum and in the strata oriens and pyramidale of CA1. They were mostly small and polymorphic or fusiform, thus indicating that they may belong to a subpopulation of interneurons. Moderate numbers of QPRT-i glial cells and neurons were also observed throughout layers II-VI of parahippocampal cortex. The localization of QPRT-i in selected glial cells and neurons suggests that in the regions examined these cellular elements might play specific roles in the regulation of quinolinic acid function.

Quinolinic-phosphoribosyl transferase activity is decreased in epileptic human brain tissue

The presence of the excitotoxic and convulsant agent quinolinic acid (QUIN) in human brain has led to the hypothesis that an increase of this tryptophan metabolite could serve as an endogenous epileptogen. A possible mechanism for a pathological accumulation of QUIN being a deficiency in its degradation, we have measured the activity of quinolinic-phosphoribosyl transferase (QPRTase) (its first degradative enzyme) in stereo-EEG identified biopsies of human brain tissue. A specific reduction of QPRTase activity was observed in tissue primarily involved in the epileptic discharge compared to values from postmortem human brain tissue with no neurological disorders or nonpathological tissue from epileptic brains. A more severe decrease was noticed in the frontal and temporal cortices as compared to the amygdala or Ammon's horn. We suggest that this local deficit may contribute to the establishment or maintenance of an epileptic focus.