Astrocytes as Pharmacological Targets in the Treatment of Schizophrenia

Adrenalectomy exacerbates stress-induced impairment in fear discrimination: A causal role for kynurenic acid?

Impaired activity of the hypothalamic-pituitary axis and reduced blood levels of glucocorticoids (GCs) are signature features of stress-related maladies. Recent evidence suggests a possible role of the tryptophan metabolite kynurenic acid (KYNA) in this context. Here we investigated possible causal relationships in adult male rats, using stress-induced fear discrimination as a translationally relevant behavioral outcome measure. One week following adrenalectomy (ADX) or sham surgery, animals were for 2 h either physically restrained or exposed to a predator odor, which caused a much milder stress response. Extracellular KYNA levels were determined before, during and after stress by in vivo microdialysis in the prefrontal cortex. Separate cohorts underwent a fear discrimination procedure starting immediately after stress termination. Different auditory conditioned stimuli (CS) were either paired with a foot shock (CS+) or non-reinforced (CS-). One week later, fear was assessed by re-exposing the animals to each CS. Separate groups of rats were treated with the KYNA synthesis inhibitor BFF-816 prior to stress initiation to test a causal role of KYNA in fear discrimination. Restraint stress raised extracellular KYNA levels by ∼85 % in ADX rats for several hours, and these animals were unable to discriminate between CS+ and CS-. Both effects were prevented by BFF-816 and were not observed after exposure to predator odor or in sham-operated rats. These findings suggest that a causal connection exists between adrenal function, stress-induced KYNA increases, and behavioral deficits. Pharmacological inhibition of KYNA synthesis may therefore be an attractive, novel option for the treatment of stress-related disorders.

Identification of blood metabolites associated with risk of Alzheimer's disease by integrating genomics and metabolomics data

Specific metabolites have been reported to be potentially associated with Alzheimer's disease (AD) risk. However, the comprehensive understanding of roles of metabolite biomarkers in AD etiology remains elusive. We performed a large AD metabolome-wide association study (MWAS) by developing blood metabolite genetic prediction models. We evaluated associations between genetically predicted levels of metabolites and AD risk in 39,106 clinically diagnosed AD cases, 46,828 proxy AD and related dementia (proxy-ADD) cases, and 401,577 controls. We further conducted analyses to determine microbiome features associated with the detected metabolites and characterize associations between predicted microbiome feature levels and AD risk. We identified fourteen metabolites showing an association with AD risk. Five microbiome features were further identified to be potentially related to associations of five of the metabolites. Our study provides new insights into the etiology of AD that involves blood metabolites and gut microbiome, which warrants further investigation.

Beyond NMDA Receptors: Homeostasis at the Glutamate Tripartite Synapse and Its Contributions to Cognitive Dysfunction in Schizophrenia

Cognitive deficits are core symptoms of schizophrenia but remain poorly addressed by dopamine-based antipsychotic medications. Glutamate abnormalities are implicated in schizophrenia-related cognitive deficits. While the role of the NMDA receptor has been extensively studied, less attention was given to other components that control glutamate homeostasis. Glutamate dynamics at the tripartite synapse include presynaptic and postsynaptic components and are tightly regulated by neuron-astrocyte crosstalk. Here, we delineate the role of glutamate homeostasis at the tripartite synapse in schizophrenia-related cognitive dysfunction. We focus on cognitive domains that can be readily measured in humans and rodents, i.e., working memory, recognition memory, cognitive flexibility, and response inhibition. We describe tasks used to measure cognitive function in these domains in humans and rodents, and the relevance of glutamate alterations in these domains. Next, we delve into glutamate tripartite synaptic components and summarize findings that implicate the relevance of these components to specific cognitive domains. These collective findings indicate that neuron-astrocyte crosstalk at the tripartite synapse is essential for cognition, and that pre- and postsynaptic components play a critical role in maintaining glutamate homeostasis and cognitive well-being. The contribution of these components to cognitive function should be considered in order to better understand the role played by glutamate signaling in cognition and develop efficient pharmacological treatment avenues for schizophrenia treatment-resistant symptoms.

Emerging effects of tryptophan pathway metabolites and intestinal microbiota on metabolism and intestinal function

The metabolism of dietary tryptophan occurs locally in the gut primarily via host enzymes, with ~ 5% metabolized by gut microbes. Three major tryptophan metabolic pathways are serotonin (beyond the scope of this review), indole, kynurenine and related derivatives. We introduce the gut microbiome, dietary tryptophan and the potential interplay of host and bacterial enzymes in tryptophan metabolism. Examples of bacterial transformation to indole and its derivative indole-3 propionic acid demonstrate associations with human metabolic disease and gut permeability, although causality remains to be determined. This review will focus on less well-known data, suggestive of local generation and functional significance in the gut, where kynurenine is converted to kynurenic acid and xanthurenic acid via enzymatic action present in both host and bacteria. Our functional data demonstrate a limited effect on intestinal epithelial cell monolayer permeability and on healthy mouse ileum. Other data suggest a modulatory effect on the microbiome, potentially in pathophysiology. Supportive of this, we found that the expression of mRNA for three kynurenine pathway enzymes were increased in colon from high-fat-fed mice, suggesting that this host pathway is perturbed in metabolic disease. These data, along with that from bacterial genomic analysis and germ-free mice, confirms expression and functional machinery of enzymes in this pathway. Therefore, the host and microbiota may play a significant dual role in either the production or regulation of these kynurenine metabolites which, in turn, can influence both host and microbiome, especially in the context of obesity and intestinal permeability.

On the Antioxidant Properties of L-Kynurenine: An Efficient ROS Scavenger and Enhancer of Rat Brain Antioxidant Defense

L-kynurenine (L-KYN) is an endogenous metabolite, that has been used as a neuroprotective strategy in experimental models. The protective effects of L-KYN have been attributed mainly to kynurenic acid (KYNA). However, considering that L-KYN is prone to oxidation, this redox property may play a substantial role in its protective effects. The aim of this work was to characterize the potential impact of the redox properties of L-KYN, in both synthetic and biological systems. First, we determined whether L-KYN scavenges reactive oxygen species (ROS) and prevents DNA and protein oxidative degradation in synthetic systems. The effect of L-KYN and KYNA (0.1–100 µM) on redox markers (ROS production, lipoperoxidation and cellular function) was compared in rat brain homogenates when exposed to FeSO₄ (10 µM). Then, the effect of L-KYN administration (75 mg/kg/day for 5 days) on the GSH content and the enzymatic activity of glutathione reductase (GR) and glutathione peroxidase (GPx) was determined in rat brain tissue. Finally, brain homogenates from rats pretreated with L-KYN were exposed to pro-oxidants and oxidative markers were evaluated. The results show that L-KYN is an efficient scavenger of ^●OH and ONOO⁻, but not O₂^●– or H₂O₂ and that it prevents DNA and protein oxidative degradation in synthetic systems. L-KYN diminishes the oxidative effect induced by FeSO₄ on brain homogenates at lower concentrations (1 µM) when compared to KYNA (100 µM). Furthermore, the sub-chronic administration of L-KYN increased the GSH content and the activity of both GR and GPx, and also prevented the oxidative damage induced by the ex vivo exposure to pro-oxidants. Altogether, these findings strongly suggest that L-KYN can be considered as a potential endogenous antioxidant.

The Footprint of Kynurenine Pathway in Neurodegeneration: Janus-Faced Role in Parkinson's Disorder and Therapeutic Implications

Progressive degeneration of neurons and aggravation of dopaminergic neurons in the substantia nigra pars compacta results in the loss of dopamine in the brain of Parkinson's disease (PD) patients. Numerous therapies, exhibiting transient efficacy have been developed; however, they are mostly accompanied by side effects and limited reliability, therefore instigating the need to develop novel optimistic treatment targets. Significant therapeutic targets have been identified, namely: chaperones, protein Abelson, glucocerebrosidase-1, calcium, neuromelanin, ubiquitin-proteasome system, neuroinflammation, mitochondrial dysfunction, and the kynurenine pathway (KP). The role of KP and its metabolites and enzymes in PD, namely quinolinic acid (QUIN), kynurenic acid (KYNA), 3-hydroxykynurenine (3-HK), 3-hydroxyanthranillic acid (3-HAA), kunurenine-3-monooxygenase (KMO), etc. has been reported. The neurotoxic QUIN, N-methyl-D-aspartate (NMDA) receptor agonist, and neuroprotective KYNA-which antagonizes QUIN actions-primarily justify the Janus-faced role of KP in PD. Moreover, KP has been reported to play a biomarker role in PD detection. Therefore, the authors detail the neurotoxic, neuroprotective, and immunomodulatory neuroactive components, alongside the upstream and downstream metabolic pathways of KP, forming a basis for a therapeutic paradigm of the disease while recognizing KP as a potential biomarker in PD, thus facilitating the development of a suitable target in PD management.

Subchronic N-acetylcysteine Treatment Decreases Brain Kynurenic Acid Levels and Improves Cognitive Performance in Mice

The tryptophan (Trp) metabolite kynurenic acid (KYNA) is an α7-nicotinic and N-methyl-d-aspartate receptor antagonist. Elevated brain KYNA levels are commonly seen in psychiatric disorders and neurodegenerative diseases and may be related to cognitive impairments. Recently, we showed that N-acetylcysteine (NAC) inhibits kynurenine aminotransferase II (KAT II), KYNA's key biosynthetic enzyme, and reduces KYNA neosynthesis in rats in vivo. In this study, we examined if repeated systemic administration of NAC influences brain KYNA and cognitive performance in mice. Animals received NAC (100 mg/kg, i.p.) daily for 7 days. Redox markers, KYNA levels, and KAT II activity were determined in the brain. We also assessed the effect of repeated NAC treatment on Trp catabolism using brain tissue slices ex vivo. Finally, learning and memory was evaluated with and without an acute challenge with KYNA's bioprecursor L-kynurenine (Kyn; 100 mg/kg). Subchronic NAC administration protected against an acute pro-oxidant challenge, decreased KYNA levels, and lowered KAT II activity and improved memory both under basal conditions and after acute Kyn treatment. In tissue slices from these mice, KYNA neosynthesis from Trp or Kyn was reduced. Together, our data indicate that prolonged treatment with NAC may enhance memory at least in part by reducing brain KYNA levels.

Alpha7 nicotinic-N-methyl-D-aspartate hypothesis in the treatment of schizophrenia and beyond

Development of novel treatments for positive, cognitive, and negative symptoms continue to be a high-priority area of schizophrenia research and a major unmet clinical need. Given that all randomized controlled trials (RCTs) conducted to date failed with one add-on medication/mechanism of action, future RCTs with the same approach are not warranted. Even if the field develops a medication for cognition, others are still needed to treat negative and positive symptoms. Therefore, fixing one domain does not completely solve the problem. Also, targeting the cholinergic system, glutamatergic system, and cholinergic plus alpha7 nicotinic and N-methyl-D-aspartate (NMDA) receptors failed independently. Hence, targeting other less important pathophysiological mechanisms/targets is unlikely to be successful. Meta-analyses of RCTs targeting major pathophysiological mechanisms have found some efficacy signal in schizophrenia; thus, combination treatments with different mechanisms of action may enhance the efficacy signal. The objective of this article is to highlight the importance of conducting RCTs with novel combination treatments in schizophrenia to develop antischizophrenia treatments. Positive RCTs with novel combination treatments that target the alpha7 nicotinic and NMDA receptors simultaneously may lead to a disease-modifying therapeutic armamentarium in schizophrenia. Novel combination treatments that concurrently improve the three domains of psychopathology and several prognostic and theranostic biomarkers may facilitate therapeutic discovery in schizophrenia.

Stress-induced impairment in fear discrimination is causally related to increased kynurenic acid formation in the prefrontal cortex

RATIONALE

Stress is related to cognitive impairments which are observed in most major brain diseases. Prior studies showed that the brain concentration of the tryptophan metabolite kynurenic acid (KYNA) is modulated by stress, and that changes in cerebral KYNA levels impact cognition. However, the link between these phenomena has not been tested directly so far.

OBJECTIVES

To investigate a possible causal relationship between acute stress, KYNA, and fear discrimination.

METHODS

Adult rats were exposed to one of three acute stressors-predator odor, restraint, or inescapable foot shocks (ISS)-and KYNA in the prefrontal cortex was measured using microdialysis. Corticosterone was analyzed in a subset of rats. Another cohort underwent a fear discrimination procedure immediately after experiencing stress. Different auditory conditioned stimuli (CSs) were either paired with foot shock (CS+) or were non-reinforced (CS-). One week later, fear was assessed by re-exposing rats to each CS. Finally, to test whether stress-induced changes in KYNA causally impacted fear discrimination, a group of rats that received ISS were pre-treated with the selective KYNA synthesis inhibitor PF-04859989.

RESULTS

ISS caused the greatest increase in circulating corticosterone levels and raised extracellular KYNA levels by ~ 85%. The two other stressors affected KYNA much less (< 25% increase). Moreover, only rats that received ISS were unable to discriminate between CS+ and CS-. PF-04859989 abolished the stress-induced KYNA increase and also prevented the impairment in fear discrimination in animals that experienced ISS.

CONCLUSIONS

These findings demonstrate a causal connection between stress-induced KYNA increases and cognitive deficits. Pharmacological manipulation of KYNA synthesis therefore offers a novel approach to modulate cognitive processes in stress-related disorders.

Discovery of sulfonamides and 9-oxo-2,8-diazaspiro[5,5]undecane-2-carboxamides as human kynurenine aminotransferase 2 (KAT2) inhibitors

Human kynurenine aminotransferase 2 (KAT2) inhibitors could be potentially used to treat the cognitive deficits associated with bipolar disease and schizophrenia. Although, there has been active drug research activity by several industrial and academic groups in developing KAT2 inhibitors over the years, no such compound has proceeded to the clinics. Here, we report two different chemical series of reversible KAT2 inhibitors with sub-micromolar activities. The first series was identified by a high-throughput screening of a diverse random library and the second one by structure-based virtual screening. Two novel crystal structures of KAT2 complexed with different reversible inhibitors were also deposited to the Protein databank which could be useful for future drug discovery efforts.

Kynurenines and the Endocannabinoid System in Schizophrenia: Common Points and Potential Interactions

Schizophrenia, which affects around 1% of the world’s population, has been described as a complex set of symptoms triggered by multiple factors. However, the exact background mechanisms remain to be explored, whereas therapeutic agents with excellent effectivity and safety profiles have yet to be developed. Kynurenines and the endocannabinoid system (ECS) play significant roles in both the development and manifestation of schizophrenia, which have been extensively studied and reviewed previously. Accordingly, kynurenines and the ECS share multiple features and mechanisms in schizophrenia, which have yet to be reviewed. Thus, the present study focuses on the main common points and potential interactions between kynurenines and the ECS in schizophrenia, which include (i) the regulation of glutamatergic/dopaminergic/γ-aminobutyric acidergic neurotransmission, (ii) their presence in astrocytes, and (iii) their role in inflammatory mechanisms. Additionally, promising pharmaceutical approaches involving the kynurenine pathway and the ECS will be reviewed herein.

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.

Inhibition of kynurenine aminotransferase II attenuates hippocampus-dependent memory deficit in adult rats treated prenatally with kynurenine

A combination of genetic and environmental factors contributes to schizophrenia (SZ), a catastrophic psychiatric disorder with a hypothesized neurodevelopmental origin. Increases in the brain levels of the tryptophan metabolite kynurenic acid (KYNA), an endogenous antagonist of α7 nicotinic acetylcholine and NMDA receptors, have been implicated specifically in the cognitive deficits seen in persons with SZ. Here we evaluated this role of KYNA by adding the KYNA precursor kynurenine (100 mg/day) to chow fed to pregnant rat dams from embryonic day (ED) 15 to ED 22 (control: ECon; kynurenine treated: EKyn). Upon termination of the treatment, all rats received normal rodent chow until the animals were evaluated in adulthood (postnatal days 56-85). EKyn treatment resulted in increased extracellular KYNA and reduced extracellular glutamate in the hippocampus, measured by in vivo microdialysis, and caused impairments in hippocampus-dependent learning in adult rats. Acute administration of BFF816, a systemically active inhibitor of kynurenine aminotransferase II (KAT II), the major KYNA-synthesizing enzyme in the brain, normalized neurochemistry and prevented contextual memory deficits in adult EKyn animals. Collectively, these results demonstrate that acute inhibition of KYNA neosynthesis can overcome cognitive impairments that arise as a consequence of elevated brain KYNA in early brain development.

Sex Differences in Hippocampal Memory and Kynurenic Acid Formation Following Acute Sleep Deprivation in Rats

Inadequate sleep is a prevalent problem within our society that can result in cognitive dysfunction. Elevations in kynurenic acid (KYNA), a metabolite of the kynurenine pathway (KP) of tryptophan degradation known to impact cognition, in the brain may constitute a molecular link between sleep loss and cognitive impairment. To test this hypothesis, we investigated the impact of 6 hours of sleep deprivation on memory and KP metabolism (brain and plasma) in male and female rats. Sleep-deprived males were impaired in a contextual memory paradigm, and both sexes were impaired in a recognition memory paradigm. After sleep deprivation, hippocampal KYNA levels increased significantly only in males. The response in hippocampal KYNA levels to sleep loss was suppressed in gonadectomized males, delineating a role of circulating gonadal hormones. Circulating corticosterone, which has previously been linked to KP metabolism, correlated negatively with hippocampal KYNA in sleep-deprived females, however the relationship was not significant in male animals. Taken together, our study introduces striking sex differences in brain KYNA formation and circulating corticosterone in response to sleep deprivation. Relating these findings to sex differences in cognitive outcomes after sleep deprivation may further advance the development of novel therapeutic agents to overcome sleep loss-induced cognitive dysfunction.

Acute Kynurenine Challenge Disrupts Sleep-Wake Architecture and Impairs Contextual Memory in Adult Rats

Study Objectives

Tryptophan metabolism via the kynurenine pathway may represent a key molecular link between sleep loss and cognitive dysfunction. Modest increases in the kynurenine pathway metabolite kynurenic acid (KYNA), which acts as an antagonist at N-methyl-d-aspartate and α7 nicotinic acetylcholine receptors in the brain, result in cognitive impairments. As glutamatergic and cholinergic neurotransmissions are critically involved in modulation of sleep, our current experiments tested the hypothesis that elevated KYNA adversely impacts sleep quality.

Methods

Adult male Wistar rats were treated with vehicle (saline) and kynurenine (25, 50, 100, and 250 mg/kg), the direct bioprecursor of KYNA, intraperitoneally at zeitgeber time (ZT) 0 to rapidly increase brain KYNA. Levels of KYNA in the brainstem, cortex, and hippocampus were determined at ZT 0, ZT 2, and ZT 4, respectively. Analyses of vigilance state-related parameters categorized as wake, rapid eye movement (REM), and non-REM (NREM) as well as spectra power analysis during NREM and REM were assessed during the light phase. Separate animals were tested in the passive avoidance paradigm, testing contextual memory.

Results

When KYNA levels were elevated in the brain, total REM duration was reduced and total wake duration was increased. REM and wake architecture, assessed as number of vigilance state bouts and average duration of each bout, and theta power during REM were significantly impacted. Kynurenine challenge impaired performance in the hippocampal-dependent contextual memory task.

Conclusions

Our results introduce kynurenine pathway metabolism and formation of KYNA as a novel molecular target contributing to sleep disruptions and cognitive impairments.

Glial and tissue-specific regulation of Kynurenine Pathway dioxygenases by acute stress of mice

Stressors activate the hypothalamic-pituitary-adrenal (HPA) axis and immune system eliciting changes in cognitive function, mood and anxiety. An important link between stress and altered behavior is stimulation of the Kynurenine Pathway which generates neuroactive and immunomodulatory kynurenines. Tryptophan entry into this pathway is controlled by rate-limiting indoleamine/tryptophan 2,3-dioxygenases (DOs: Ido1, Ido2, Tdo2). Although implicated as mediating changes in behavior, detecting stress-induced DO expression has proven inconsistent. Thus, C57BL/6J mice were used to characterize DO expression in brain-regions, astrocytes and microglia to characterize restraint-stress-induced DO expression. Stress increased kynurenine in brain and plasma, demonstrating increased DO activity. Of three Ido1 transcripts, only Ido1-v1 expression was increased by stress and within astrocytes, not microglia, indicating transcript- and glial-specificity. Stress increased Ido1-v1 only in frontal cortex and hypothalamus, indicating brain-region specificity. Of eight Ido2 transcripts, Ido2-v3 expression was increased by stress, again only within astrocytes. Likewise, stress increased Tdo2-FL expression in astrocytes, not microglia. Interestingly, Ido2 and Tdo2 transcripts were not correspondingly induced in Ido1-knockout (Ido1KO) mice, suggesting that Ido1 is necessary for the central DO response to acute stress. Unlike acute inflammatory models resulting in DO induction within microglia, only astrocyte DO expression was increased by acute restraint-stress, defining their unique role during stress-dependent activation of the Kynurenine Pathway.