Kynurenine pathway metabolism and neuroinflammatory disease

Nady Braidy, Ross Grant

1 Centre for Healthy Brain Ageing, School of Psychiatry, University of New South Wales, Faculty of Medicine, Sydney, Australia

2 School of Medical Sciences, University of New South Wales, Faculty of Medicine, Sydney, Australia; Australasian Research Institute, Sydney Adventist Hospital, Sydney, Australia

Kynurenine pathway metabolism and neuroinflammatory disease

Nady Braidy1,＊, Ross Grant2

1 Centre for Healthy Brain Ageing, School of Psychiatry, University of New South Wales, Faculty of Medicine, Sydney, Australia

2 School of Medical Sciences, University of New South Wales, Faculty of Medicine, Sydney, Australia; Australasian Research Institute, Sydney Adventist Hospital, Sydney, Australia

How to cite this article:Braidy N, Grant R (2017) Kynurenine pathway metabolism and neuroinflammatory disease. Neural Regen Res 12(1):39-42.

Open access statement:is is an open access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License, which allows others to remix, tweak, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Immune‐mediated activation of tryptophan (TRYP) catabolism via the kynurenine pathway (KP) is a con‐sistent fi nding in all in fl ammatory disorders. Several studies by our group and others have examined the neurotoxic potential of neuroreactive TRYP metabolites, including quinolinic acid (QUIN) in neuroin fl am‐matory neurological disorders, including Alzheimer’s disease (AD), multiple sclerosis, amylotropic lateral sclerosis (ALS), and AIDS related dementia complex (ADC). Our current work aims to determine whether there is any bene fi t to the a ff ected individuals in enhancing the catabolism of TRYPviathe KP during an immune response. Under physiological conditions, QUIN is metabolized to the essential pyridine nucle‐otide, nicotinamide adenine dinucleotide (NAD+), which represents an important metabolic cofactor and electron transporter. NAD+also serves as a substrate for the DNA ‘nick sensor’ and putative nuclear repair enzyme, poly(ADP‐ribose) polymerase (PARP). Free radical initiated DNA damage, PARP activation and NAD+depletion may contribute to brain dysfunction and cell death in neuroin fl ammatory disease.

kynurenine pathway; tryptophan; NAD+; PARP; in fl ammation

Accepted: 2017-01-16

TRYP is the least abundant amino acid in mammalian or‐ganisms, and accounts for only 1–1.5% of the total protein amino acid content. However, it was not until the late 1970s and 1980s that KP generated considerable interest among neuroscientists, since it was discovered that QUIN and ky‐nurenine acid (KYNA), (two metabolites of the KP) exhibit‐ed signi fi cant and opposing actions on neuronal cells. QUIN is a selective agonist at the N‐methyl D‐aspartate (NMDA) site of the excitatory NMDA (glutamate subtype) receptor, while KYNA is an antagonist at both the NMDA and glycine site of this ionotropic receptor (Guillemin, 2012). Activation of the NMDA receptor has been shown to permeate cells to Ca2+, Na+and K+ions. Increased intracellular Ca2+in fl ux has been shown to activate several secondary messenger sig‐nalling pathways leading to synaptic alterations. Moreover, increased intracellular Ca2+influx due to excessive NMDA receptor activation can induce excitotoxicity and neuronal cell death in several neurodegenerative diseases (Braidy et al., 2010).

In humans, CNS QUIN levels are increased in several neu‐rological disorders including AD, depression, epilepsy, au‐tism, schizophrenia, and patients are more susceptible to sui‐cide risk (Brundin et al., 2016).is has led to the hypothesis that the increase in CNS levels of QUIN is pivotal to the pathogenesis of these disorders through its mode of action at the NMDA receptor. QUIN may also induce toxicity to brain cellsviaexogenous free radical production. On the contrary, reduced levels of KYNA have been reported in neurological disorders such as Huntington’s disease (HD), and AD. It has been suggested that the development of in fl ammatory medi‐ated neuropathology is correlated to changes in the ratio of KYNA to QUIN rather than QUIN levels alone.

Given the extent that the KP in fl uences neuronal function, identifying the site of TRYP metabolite production during CNS in fl ammation is bene fi cial to gain a sound understand‐ing of in fl ammatory mediated neuropathology.ese metab‐olites may have originated either from upregulated TRYP ca‐tabolism within the CNS during in fl ammation, or may cross the blood‐brain barrier aer being systemically produced. It has been reported that CNS levels of KP metabolites increase independently of their systemic concentration during neu‐roin fl ammation, thus suggesting that KP metabolites can be present at upregulated levels locally within the CNS.

De novoNAD+Synthesisviathe Kynurenine Pathway

QUIN is converted to nicotinic acid mononucleotide (NaMN) by the enzyme quinolinic acid phosphoribosyltransferase (QPRT) in the presence of Mg2+. Further trans‐formation leading to the synthesis of the parent molecule of the pyridine nucleotide, NAD+, appears to be nuclear, mitochondrial, and golgi speci fi c, by nicotinamide mono‐nucleotide adenyl transferase (NMNAT1, 2, and 3) in the presence of ATP to produce desamido‐NAD+. In the pres‐ence of glutamine desamido‐NAD is amidated to the par‐ent pyridine nucleotide, NAD+, as the fi nal product of the KP In addition to itsde novosynthesis for TRYP, NAD+can also be synthesised from either one of three routes: (1) nicotinic acid (NA), which is then converted to NAD+viathe three‐step Preiss‐Handler pathway; (2) the enzyme nicotinamide phosphoribosyl transferase (NAMPT) con‐verts NM to nicotinamide mononucleotide (NMN) and then to NAD+by the action of NMNAT1, 2, and 3 in the presence of ATP, or (3) phosphorylation of nicotinamide riboside (NR) to NMN by NR kinases (NRKs) (Ratajczak et al., 2016) (Figure 1A and B). In spite of the potential for NAD+production from vitamins, thede novosynthesis of NAD+from TRYP appears to be more important than NAD+production from vitamins under normal physiolog‐ical conditions.

Source ofryptophan Catabolism During Neuroin fl ammation

IFN‐γ is the primary activating factor of macrophage/mi‐croglial/dendritic cells in the CNS and elsewhere, increas‐ing their antimicrobial activity through the modulation and upregulation of a variety of activities including, enhanced production of reactive oxygen species (ROS), increased ni‐tric oxide synthase activity, upregulation of MHC antigens, secretion of cytokines such as interleukin (IL)‐1β, IL‐6, tumour necrosis factor‐α (TNF‐α), platelet activating fac‐tor (PAF), macrophage chemotactic protein (MCP‐1), and secretion of other biologically active proteins such as com‐plement pathway components. Indoleamine 2,3 dioxygen‐ase (IDO) is the primary enzyme of the KP, and is potently induced by IFN‐γ in both astrocytes and in fl ammatory cells leading to a marked increase in KP metabolites in these cells. IFN‐γ activated microglial/macrophages/dendritic cells will readily catabolize TRYP through induction of IDO, producing signi fi cant amounts of metabolic neurore‐active intermediates such as KYN, anthranilic acid (AA), 3‐hydroxykynurenine (3‐HK), 3‐hydroxyanthranilic acid (3‐HAA), and QUIN.e KP also leads to the production of the metal chelating agent, picolinic acid (PIC) (Schwarcz and Stone, 2016).

Astrocytes also readily degrade TRYP in response to IFN‐γ treatment through the induction of IDO. KYN ap‐pears to be the main metabolite found in the supernatants of IFN‐γ treated astroglial cells. However, significant but trace amounts of AA from an astrocytoma cell line, and QUIN, from human foetal brain cultures have also been re‐ported (Guillemin et al., 2000).

It has been suggested that TRYP catabolism increased in some cells to decrease TRYP concentrations in the microen‐vironment thereby reducing the availability of this essential amino acid for microbial metabolism. However, this hy‐pothesis has been challenged by others. One of the products of TRYP catabolism, 3‐HAA, can function as an effective antioxidant, and potential nitric oxide synthase inhibitor. Production of this metabolite may therefore serve to reduce non‐speci fi c oxidative damage at the site of neuroin fl amma‐tion induced by activated mononuclear phagocytes, and may explain the increased TRYP catabolism under these condi‐tions (Krause et al., 2011). However, increased secretion of KYN and QUIN under these circumstances has not been con fi rmed. Moreover, IFN‐γ induced IDO TRYP catabolism has been shown to increase cellular NAD+concentrations in an astroglioma cell line (Grant et al., 1999).

Despite the consolidated role of the classic KP metab‐olites (e.g., KYN, KYNA, 3‐HAA, and QUIN) during im‐mune responses, it has recently emerged that cinnabarinic acid (CA) is crucial for immune system functions (Hira‐matsu et al., 2008; Fazio et al., 2014). It has been shown that CA can be produced by stimulated human PBMCs. CA can also be producedvianon‐enzymatic reactions under oxida‐tive conditions. In in fl ammatory cells such as neutrophils, co‐expression of IDO and other enzymes involved in the formation of free radicals may divert the KP away from the production of QUIN, and instead towards CA over PIC or QUIN (Lowe et al., 2014). Therefore, concomitant imple‐mentation of NAD+levels could be enhanced by increased CA amounts.

Poly(ADP‐ribose) polymerase and NAD+depletion: Dou‐ble strand DNA breaks caused by excessive oxidative dam‐age activate the PARP enzyme. Neuronal cells exposed to pathological concentrations of the excitotoxic neurotrans‐mitter, glutamate show both an increase in intracellular oxidative stress and PARP activity. PARP is a protein mod‐ifying nucleotide polymerising enzyme found abundant in the nucleus, with approximately one molecule of enzyme per 1,000 base pairs. PARP, along with DNA dependentprotein kinases appears to play an important role in main‐taining genomic integrity (Massudi et al., 2012). However, the precise physiological roles of PARP are not completely understood.

Figure 1e nicotinamide adenine dinucleotide (NAD+) metabolome.

As a ‘DNA nick sensor’, PARP rapidly binds to DNA strand breaks and is activated. Activated PARP uses up NAD+(and NADP+), exclusively as substrate to poly(ADP‐ribosylate) itself and a number of nuclear pro‐teins that are involved in the repair of DNA, releasing nic‐otinamide as a by‐product. Recent evidence suggests that polyADP‐ribosylation of histones or transcription factors may also be involved in nuclear receptor signalling (Morales et al., 2014) (Figure 1C).

A significant decrease in intracellular NAD+levels has been reported in the brain and a variety of cell types as a result of DNA strand breaks and PARP activation following exposure to free radical generators, excitotoxins, infec‐tions, AD, and during in fl ammation or ageing (Abeti and Duchen, 2012; Braidy et al., 2014). Increased PARP activity resulting in decreased NAD+has been shown to decrease ATP and neurotransmitter levels in the brain, as well as cell lysis and death. Inhibition of PARP activity following oxidant injury has been shown to preserve NAD+and ATP levels, prevent cell lysis, although damage to the DNA was not prevented. Additionally, at low levels of oxidant, PARP–/–cells survived better than PARP+/+cells, suggesting that loss of NAD+may be a cause of cell death. Elevated levels of free radicals, pro oxidants, and excitotoxins have been reportedin in fl ammatory brain disorders, and in most cases, DNA damage has been observed. This suggests that NAD+de‐pletion through PARP activation may play a crucial role in CNS dysfunction and pathology under these conditions (Massudi et al., 2012).

Conclusion

Immune activation of macrophage/microglial/dendritic cells results in a marked increase in their generation of ROS. Elevated extracellular fl uid levels of these free rad‐icals have been implicated in the cause of tissue damage during inflammation in several disorders. Damage to surrounding tissue, including astrocytes and neurons in the CNS, may occur in response to inflammation in the CNS. As well, the non‐discriminating nature of these free radicals may induce damage to the activated macrophage itself. At least one study has reported an increase in PARP activity in IFN‐γ activated macrophages, suggesting that DNA damage has occurred with a corresponding increase in the rate of NAD+catabolism.

Acknowledgments:Nady Braidy is the recipient of an NHMRC Postdoctoral Fellowship at the University of New South Wales.

Author contributions:NB and RG conceptualized the research and the review of the topic, reviewed the manuscript and cleared for submission. NB undertook literature search and prepared the draof the manuscript under supervision of RG.

Con fl icts of interest:None declared.

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Nady Braidy, Ph.D., n.braidy@unsw.edu.au.

10.4103/1673-5374.198971

＊< class="emphasis_italic">Correspondence to: Nady Braidy, Ph.D., n.braidy@unsw.edu.au.