Regulation of kynurenic acid levels in the developing rat brain

Neuroactive Kynurenines as Pharmacological Targets: New Experimental Tools and Exciting Therapeutic Opportunities

Both preclinical and clinical studies implicate functional impairments of several neuroactive metabolites of the kynurenine pathway (KP), the major degradative cascade of the essential amino acid tryptophan in mammals, in the pathophysiology of neurologic and psychiatric diseases. A number of KP enzymes, such as tryptophan 2,3-dioxygenase (TDO2), indoleamine 2,3-dioxygenases (IDO1 and IDO2), kynurenine aminotransferases (KATs), kynurenine 3-monooxygenase (KMO), 3-hydroxyanthranilic acid oxygenase (3-HAO), and quinolinic acid phosphoribosyltransferase (QPRT), control brain KP metabolism in health and disease and are therefore increasingly considered to be promising targets for the treatment of disorders of the nervous system. Understanding the distribution, cellular expression, and regulation of KP enzymes and KP metabolites in the brain is therefore critical for the conceptualization and implementation of successful therapeutic strategies. SIGNIFICANCE STATEMENT: Studies have implicated the kynurenine pathway of tryptophan in the pathophysiology of neurologic and psychiatric diseases. Key enzymes of the kynurenine pathway regulate brain metabolism in both health and disease, making them promising targets for treating these disorders. Therefore, understanding the distribution, cellular expression, and regulation of these enzymes and metabolites in the brain is critical for developing effective therapeutic strategies. This review endeavors to describe these processes in detail.

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

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

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

The kynurenine pathway (KP) of tryptophan degradation contains several neuroactive metabolites that may influence brain function in health and disease. Mounting focus has been dedicated to investigating the role of these metabolites during neurodevelopment and elucidating their involvement in the pathophysiology of psychiatric disorders with a developmental component, such as schizophrenia. In this review, we describe the changes in KP metabolism in the brain from gestation until adulthood and illustrate how environmental and genetic factors affect the KP during development. With a particular focus on kynurenic acid, the antagonist of α7 nicotinic acetylcholine (α7nACh) and N-methyl-d-aspartate (NMDA) receptors, both implicated in modulating brain development, we review animal models designed to ascertain the role of perinatal KP elevation on long-lasting biochemical, neuropathological, and behavioral deficits later in life. We present new data demonstrating that combining perinatal choline-supplementation, to potentially increase activation of α7nACh receptors during development, with embryonic kynurenine manipulation is effective in attenuating cognitive impairments in adult rat offspring. With these findings in mind, we conclude the review by discussing the advancement of therapeutic interventions that would target not only symptoms, but potentially the root cause of central nervous system diseases that manifest from a perinatal KP insult. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

The Kynurenine Pathway in the Acute and Chronic Phases of Cerebral Ischemia

Kynurenines are a wide range of catabolites which derive from tryptophan through the "Kynurenine Pathway" (KP). In addition to its peripheral role, increasing evidence shows a role of the KP in the central nervous system (CNS), mediating both physiological and pathological functions. Indeed, an imbalance in this route has been associated with several neurodegenerative disorders such as Alzheimer´s and Huntington´s diseases. Altered KP catabolism has also been described during both acute and chronic phases of stroke; however the contribution of the KP to the pathophysiology of acute ischemic damage and of post-stroke disorders during the chronic phase including depression and vascular dementia, and the exact mechanisms implicated in the regulation of the KP after stroke are not well established yet. A better understanding of the regulation and activity of the KP after stroke could provide new pharmacological tools in both acute and chronic phases of stroke. In this review, we will make an overview of CNS modulation by the KP. We will detail the KP contribution in the ischemic damage, how the unbalance of the KP might trigger an alteration of the cognitive function after stroke as well as potential targets for the development of new drugs.

Expression of kynurenine aminotransferases in the rat retina during development

The study investigates the cellular expression of kynurenine aminotransferases (KAT I and II) in the rat retina during development. At P1 (the day of birth) and P7 (the 7th day after birth), KAT I expression was observed in the inner plexiform layer (IPL), the fiber layer (FL), and in vertically running processes in the ganglion cell layer (GCL) (but not in the cell bodies). At P14 (the 14th day after birth) a strong KAT I immunoreactivity was observed in Müller cell endfeet. KAT II was expressed in the IPL, the FL, and in cells in the GCL at P1 and P7. From P14 on, KAT II expression in the IPL decreased. Double labeling revealed that KAT I was expressed in Müller cell endfeet, whilst KAT II both on retinal ganglion cells (RGC) and Müller cell endfeet. In conclusion, KAT I and II are present in the rat retina during development. The heterogeneity of the KAT developmental profiles possibly reflects a neuromodulatory role in the retinal differentiation.

Ontogenic changes of kynurenine aminotransferase I activity and its expression in the chicken retina

Kynurenine aminotransferases are key enzymes for the synthesis of kynurenic acid (KYNA), an endogenous glutamate receptor antagonist. The study described here examined ontogenic changes of kynurenine aminotransferase I (KAT I) activity and its expression in the chicken retina. KAT I activity measured on embryonic day 16 (E16) was significantly higher than at all other stages (E12, P0 and P7). Double labeling with antibodies against glutamine synthetase showed that on P7 KAT I was expressed in Müller cell endfeet and their processes in the inner retina. Since KAT I activity is high in the late embryonic stages, it is conceivable that it plays a neuromodulatory role in the retina during the late phase of embryogenesis.

Kynurenine pathway metabolism in human astrocytes: a paradox for neuronal protection

There is good evidence that the kynurenine pathway (KP) and one of its products, quinolinic acid (QUIN), play a role in the pathogenesis of neurological diseases, in particular AIDS dementia complex. Although QUIN has been shown to be produced in neurotoxic concentrations by macrophages and microglia, the role of astrocytes in QUIN production is controversial. Using cytokine-stimulated cultures of human astrocytes, we assayed key enzymes and products of the KP. We found that human astrocytes lack kynurenine hydroxylase so that large amounts of kynurenine and the QUIN antagonist kynurenic acid were produced. However, the amounts of QUIN that were synthesized were subsequently completely degraded. We then showed that kynurenine in concentrations comparable with those produced by astrocytes led to significant production of QUIN by macrophages. These results suggest that astrocytes alone are neuroprotective by minimizing QUIN production and maximizing synthesis of kynurenic acid. However, it is likely that, in the presence of macrophages and/or microglia, astrocytes become indirectly neurotoxic by the production of large concentrations of kynurenine that can be secondarily metabolized by neighbouring or infiltrating monocytic cells to form the neurotoxin QUIN.