Indoleamine 2,3 dioxygenase and quinolinic acid immunoreactivity in Alzheimer's disease hippocampus

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.

Tryptophan Metabolism in Alzheimer's Disease with the Involvement of Microglia and Astrocyte Crosstalk and Gut-Brain Axis

Alzheimer's disease (AD) is an age-dependent neurodegenerative disease characterized by extracellular Amyloid Aβ peptide (Aβ) deposition and intracellular Tau protein aggregation. Glia, especially microglia and astrocytes are core participants during the progression of AD and these cells are the mediators of Aβ clearance and degradation. The microbiota-gut-brain axis (MGBA) is a complex interactive network between the gut and brain involved in neurodegeneration. MGBA affects the function of glia in the central nervous system (CNS), and microbial metabolites regulate the communication between astrocytes and microglia; however, whether such communication is part of AD pathophysiology remains unknown. One of the potential links in bilateral gut-brain communication is tryptophan (Trp) metabolism. The microbiota-originated Trp and its metabolites enter the CNS to control microglial activation, and the activated microglia subsequently affect astrocyte functions. The present review highlights the role of MGBA in AD pathology, especially the roles of Trp per se and its metabolism as a part of the gut microbiota and brain communications. We (i) discuss the roles of Trp derivatives in microglia-astrocyte crosstalk from a bioinformatics perspective, (ii) describe the role of glia polarization in the microglia-astrocyte crosstalk and AD pathology, and (iii) summarize the potential of Trp metabolism as a therapeutic target. Finally, we review the role of Trp in AD from the perspective of the gut-brain axis and microglia, as well as astrocyte crosstalk, to inspire the discovery of novel AD therapeutics.

The Kynurenine Pathway, Aryl Hydrocarbon Receptor, and Alzheimer's Disease

Alzheimer's disease (AD) is the leading cause of dementia, mainly affecting elderly individuals. AD is characterized by β-amyloid plaques, abnormal tau tangles, neuronal loss, and metabolic disruptions. Recent studies have revealed the involvement of the kynurenine (KP) pathway and the aryl hydrocarbon receptor (AhR) in AD development. The KP pathway metabolizes tryptophan to produce neuroactive substances like kynurenine, kynurenic acid, and quinolinic acid. In AD, high levels of kynurenine and the neurotoxic quinolinic acid are associated with increased neuroinflammation and excitotoxicity; conversely, reduced levels of kynurenic acid, which acts as a glutamate receptor antagonist, compromise neuroprotection. Research has indicated elevated KP metabolites and enzymes in the hippocampus of AD patients and other tissues such as blood, cerebrospinal fluid, and urine. However, the finding that KP metabolites are AD biomarkers in blood, cerebrospinal fluid, and urine has been controversial. This controversy, stemming from the lack of consideration of the specific stage of AD, details of the patient's treatment, cognitive deficits, and psychiatric comorbidities, underscores the need for more comprehensive research. AhR, a ligand-activated transcription factor, regulates immune response, oxidative stress, and xenobiotic metabolism. Various ligands, including tryptophan metabolites, can activate it. Some studies suggest that AhR activation contributes to AD, while others propose that it provides neuroprotection. This discrepancy may be explained by the specific ligands that activate AhR, highlighting the complex relationship between the KP pathway, AhR activation, and AD, where the same pathway can produce both neuroprotective and harmful effects.

Restoring hippocampal glucose metabolism rescues cognition across Alzheimer's disease pathologies

Impaired cerebral glucose metabolism is a pathologic feature of Alzheimer's disease (AD), with recent proteomic studies highlighting disrupted glial metabolism in AD. We report that inhibition of indoleamine-2,3-dioxygenase 1 (IDO1), which metabolizes tryptophan to kynurenine (KYN), rescues hippocampal memory function in mouse preclinical models of AD by restoring astrocyte metabolism. Activation of astrocytic IDO1 by amyloid β and tau oligomers increases KYN and suppresses glycolysis in an aryl hydrocarbon receptor-dependent manner. In amyloid and tau models, IDO1 inhibition improves hippocampal glucose metabolism and rescues hippocampal long-term potentiation in a monocarboxylate transporter-dependent manner. In astrocytic and neuronal cocultures from AD subjects, IDO1 inhibition improved astrocytic production of lactate and uptake by neurons. Thus, IDO1 inhibitors presently developed for cancer might be repurposed for treatment of AD.

Are Women with Polycystic Ovary Syndrome at Increased Risk of Alzheimer Disease? Lessons from Insulin Resistance, Tryptophan and Gonadotropin Disturbances and Their Link with Amyloid-Beta Aggregation

Alzheimer disease, the leading cause of dementia, and polycystic ovary syndrome, one of the most prevalent female endocrine disorders, appear to be unrelated conditions. However, studies show that both disease entities have common risk factors, and the amount of certain protein marker of neurodegeneration is increased in PCOS. Reports on the pathomechanism of both diseases point to the possibility of common denominators linking them. Dysregulation of the kynurenine pathway, insulin resistance, and impairment of the hypothalamic-pituitary-gonadal axis, which are correlated with amyloid-beta aggregation are these common areas. This article discusses the relationship between Alzheimer disease and polycystic ovary syndrome, with a particular focus on the role of disorders of tryptophan metabolism in both conditions. Based on a review of the available literature, we concluded that systemic changes occurring in PCOS influence the increased risk of neurodegeneration.

Role of Kynurenine and Its Derivatives in the Neuroimmune System

In recent years, there has been a growing realization of intricate interactions between the nervous and immune systems, characterized by shared humoral factors and receptors. This interplay forms the basis of the neuroimmune system, the understanding of which will provide insights into the pathogenesis of neurological diseases, in which the involvement of the immune system has been overlooked. Kynurenine and its derivatives derived from tryptophan have long been implicated in the pathogenesis of various neurological diseases. Recent studies have revealed their close association not only with neurological disorders but also with sepsis-related deaths. This review provides an overview of the biochemistry of kynurenine and its derivatives, followed by a discussion of their role via the modulation of the neuroimmune system in various diseases.

Restoring hippocampal glucose metabolism rescues cognition across Alzheimer's disease pathologies

Impaired cerebral glucose metabolism is a pathologic feature of Alzheimer Disease (AD), and recent proteomic studies highlight a disruption of glial carbohydrate metabolism with disease progression. Here, we report that inhibition of indoleamine-2,3-dioxygenase 1 (IDO1), which metabolizes tryptophan to kynurenine (KYN) in the first step of the kynurenine pathway, rescues hippocampal memory function and plasticity in preclinical models of amyloid and tau pathology by restoring astrocytic metabolic support of neurons. Activation of IDO1 in astrocytes by amyloid-beta 42 and tau oligomers, two major pathological effectors in AD, increases KYN and suppresses glycolysis in an AhR-dependent manner. Conversely, pharmacological IDO1 inhibition restores glycolysis and lactate production. In amyloid-producing APP Swe -PS1 ΔE9 and 5XFAD mice and in tau-producing P301S mice, IDO1 inhibition restores spatial memory and improves hippocampal glucose metabolism by metabolomic and MALDI-MS analyses. IDO1 blockade also rescues hippocampal long-term potentiation (LTP) in a monocarboxylate transporter (MCT)-dependent manner, suggesting that IDO1 activity disrupts astrocytic metabolic support of neurons. Indeed, in vitro mass-labeling of human astrocytes demonstrates that IDO1 regulates astrocyte generation of lactate that is then taken up by human neurons. In co-cultures of astrocytes and neurons derived from AD subjects, deficient astrocyte lactate transfer to neurons was corrected by IDO1 inhibition, resulting in improved neuronal glucose metabolism. Thus, IDO1 activity disrupts astrocytic metabolic support of neurons across both amyloid and tau pathologies and in a model of AD iPSC-derived neurons. These findings also suggest that IDO1 inhibitors developed for adjunctive therapy in cancer could be repurposed for treatment of amyloid- and tau-mediated neurodegenerative diseases.

Neuroimmune modulation by tryptophan derivatives in neurological and inflammatory disorders

The nervous and immune systems are highly developed, and each performs specialized physiological functions. However, they work together, and their dysfunction is associated with various diseases. Specialized molecules, such as neurotransmitters, cytokines, and more general metabolites, are essential for the appropriate regulation of both systems. Tryptophan, an essential amino acid, is converted into functional molecules such as serotonin and kynurenine, both of which play important roles in the nervous and immune systems. The role of kynurenine metabolites in neurodegenerative and psychiatric diseases has recently received particular attention. Recently, we found that hyperactivity of the kynurenine pathway is a critical risk factor for septic shock. In this review, we first outline neuroimmune interactions and tryptophan derivatives and then summarized the changes in tryptophan metabolism in neurological disorders. Finally, we discuss the potential of tryptophan derivatives as therapeutic targets for neuroimmune disorders.

3-Hydroxyanthranilic Acid Delays Paralysis in Caenorhabditis elegans Models of Amyloid-Beta and Polyglutamine Proteotoxicity

Age is the primary risk factor for neurodegenerative diseases such as Alzheimer's and Huntington's disease. Alzheimer's disease is the most common form of dementia and a leading cause of death in the elderly population of the United States. No effective treatments for these diseases currently exist. Identifying effective treatments for Alzheimer's, Huntington's, and other neurodegenerative diseases is a major current focus of national scientific resources, and there is a critical need for novel therapeutic strategies. Here, we investigate the potential for targeting the kynurenine pathway metabolite 3-hydroxyanthranilic acid (3HAA) using Caenorhabditis elegans expressing amyloid-beta or a polyglutamine peptide in body wall muscle, modeling the proteotoxicity in Alzheimer's and Huntington's disease, respectively. We show that knocking down the enzyme that degrades 3HAA, 3HAA dioxygenase (HAAO), delays the age-associated paralysis in both models. This effect on paralysis was independent of the protein aggregation in the polyglutamine model. We also show that the mechanism of protection against proteotoxicity from HAAO knockdown is mimicked by 3HAA supplementation, supporting elevated 3HAA as the mediating event linking HAAO knockdown to delayed paralysis. This work demonstrates the potential for 3HAA as a targeted therapeutic in neurodegenerative disease, though the mechanism is yet to be explored.

A Review of the Evidence for Tryptophan and the Kynurenine Pathway as a Regulator of Stem Cell Niches in Health and Disease

Stem cells are ubiquitously found in various tissues and organs in the body, and underpin the body's ability to repair itself following injury or disease initiation, though repair can sometimes be compromised. Understanding how stem cells are produced, and functional signaling systems between different niches is critical to understanding the potential use of stem cells in regenerative medicine. In this context, this review considers kynurenine pathway (KP) metabolism in multipotent adult progenitor cells, embryonic, haematopoietic, neural, cancer, cardiac and induced pluripotent stem cells, endothelial progenitor cells, and mesenchymal stromal cells. The KP is the major enzymatic pathway for sequentially catabolising the essential amino acid tryptophan (TRP), resulting in key metabolites including kynurenine, kynurenic acid, and quinolinic acid (QUIN). QUIN metabolism transitions into the adjoining de novo pathway for nicotinamide adenine dinucleotide (NAD) production, a critical cofactor in many fundamental cellular biochemical pathways. How stem cells uptake and utilise TRP varies between different species and stem cell types, because of their expression of transporters and responses to inflammatory cytokines. Several KP metabolites are physiologically active, with either beneficial or detrimental outcomes, and evidence of this is presented relating to several stem cell types, which is important as they may exert a significant impact on surrounding differentiated cells, particularly if they metabolise or secrete metabolites differently. Interferon-gamma (IFN-γ) in mesenchymal stromal cells, for instance, highly upregulates rate-limiting enzyme indoleamine-2,3-dioxygenase (IDO-1), initiating TRP depletion and production of metabolites including kynurenine/kynurenic acid, known agonists of the Aryl hydrocarbon receptor (AhR) transcription factor. AhR transcriptionally regulates an immunosuppressive phenotype, making them attractive for regenerative therapy. We also draw attention to important gaps in knowledge for future studies, which will underpin future application for stem cell-based cellular therapies or optimising drugs which can modulate the KP in innate stem cell populations, for disease treatment.

MM165 - A Small Hybrid Molecule Modulates the Kynurenine Pathway and Attenuates Lipopolysaccharide-Induced Memory Deficits and Inflammation

Cognitive dysfunctions are now recognized as core symptoms of various psychiatric disorders e.g., major depressive disorder. Sustained immune activation may leads to cognitive dysfunctions. Proinflammatory cytokines shunt the metabolism of tryptophan towards kynurenine and quinolinic acid may accumulate at toxic concentrations. This acid triggers an increase in neuronal nitric oxide synthase function and promotes oxidative stress. The searching for small molecules that can regulate tryptophan metabolites produced in the kynurenic pathway has become an important goal in developing treatments for various central nervous system diseases with an inflammatory component. Previously we have identified a small hybrid molecule - MM165 which significantly reduces depressive-like symptoms caused by inflammation induced by lipopolysaccharide administration. In the present study, we investigated whether this compound would mitigate cognitive deficits induced by lipopolysaccharide administration and whether treatment with it would affect the plasma or brain levels of quinolinic acid and kynurenic acid. Neuroinflammation was induced in rats by administering lipopolysaccharide at a dose of 0.5 mg/kg body weight for 10 days. We conducted two tests: novel object recognition and object location, to assess the effect on memory impairment in animals previously treated with lipopolysaccharide. In plasma collected from rats, the concentrations of C-reactive protein and tumor necrosis factor alfa were determined. The concentrations of kynurenic acid and quinolinic acid were determined in plasma and homogenates obtained from the cerebral cortex of rats. Interleukin 6 in the cerebral cortex of rats was determined. Additionally, the body and spleen mass and spontaneous activity were measured in rats. Our study shows that MM165 may mitigate cognitive deficits induced by inflammation after administration of lipopolysaccharide and alter the concentrations of tryptophan metabolites in the brain. Compounds exhibiting a mechanism of action analogous to that of MM165 may serve as foundational structures for the development of a new class of antidepressants.

Total Synthesis of Racemic Benzomalvin E, a Quinazolinone Isolated from Pencilium sp. FN070315 and Exploration to the Direct Synthesis of (E)-Benzomalvin B

We present the first total synthesis of (±) benzomalvin E, featuring a quinazolino moiety with a 6-6-6-7-fused tetracyclic skeleton containing three nitrogen atoms. The key transformation involves Cu-catalyzed intramolecular C-N arylation of quinazolinone, leading to a sclerotigenin analogue that undergoes nucleophilic addition with benzaldehyde, enabling the synthesis of (±) benzomalvin E in six linear steps with a 33% overall yield. The (±) benzomalvin E's structure was validated by 2-D NMR and single crystal XRD analysis and was further transformed into (E)-benzomalvin B.

Gut Microbiota Dysbiosis and Inflammation Dysfunction in Late-Life Depression: An Observational Cross-Sectional Analysis

Purpose

There are some challenges to diagnosis in the context of similar diagnostic criteria for late-life depression (LLD) and adult depression due to cognitive impairment and other clinical manifestations. The association between gut microbiota and inflammation remains unclear in LLD. We analyzed gut microbiota characteristics and serum inflammatory cytokines in individuals with LLD to explore the combined role of these two factors in potential biomarkers of LLD.

Methods

This was an observational cross-sectional study. Fecal samples and peripheral blood from 29 patients and 33 sex- and age-matched healthy controls (HCs) were collected to detect gut microbiota and 12 inflammatory factors. We analyzed differences in diversity and composition of gut microbiota and evaluated relations among gut microbiota, inflammatory factors, and neuropsychological scales. We extracted potential biomarkers using receiver-operating characteristic curve analysis to predict LLD utilizing the combination of the microbiota and inflammatory cytokines.

Results

Elevated systemic inflammatory cytokine levels and gut microbiota dysbiosis were found in LLD patients. Relative abundance of Verrucomicrobia at the phylum level and Megamonas, Citrobacter, and Akkermansia at the genus level among LLD patients was lower than HCs. Abundance of Coprococcus, Lachnobacterium, Oscillospira, and Sutterella was higher in LLD patients. Notably, IL6, IFNγ, Verrucomicrobia, and Akkermansia levels were correlated with depression severity. Our study identified IL6, Akkermansia, and Sutterella as predictors of LLD, and their combination achieved an area under the curve of 0.962 in distinguishing LLD patients from HCs.

Conclusion

This research offers evidence of changes within gut microbiota and systemic inflammation in LLD. These findings possibly help elucidate functions of gut microbiota and systemic inflammation in LLD development and offer fresh ideas on biomarkers for clinical practise in the context of LLD.

Gut Microbiome in Alzheimer's Disease: from Mice to Humans

Alzheimer's disease (AD) is the most prevalent type of dementia, but its etiopathogenesis is not yet fully understood. Recent preclinical studies and clinical evidence indicate that changes in the gut microbiome could potentially play a role in the accumulation of amyloid beta. However, the relationship between gut dysbiosis and AD is still elusive. In this review, the potential impact of the gut microbiome on AD development and progression is discussed. Pre-clinical and clinical literature exploring changes in gut microbiome composition is assessed, which can contribute to AD pathology including increased amyloid beta deposition and cognitive impairment. The gut-brain axis and the potential involvement of metabolites produced by the gut microbiome in AD are also highlighted. Furthermore, the potential of antibiotics, prebiotics, probiotics, fecal microbiota transplantation, and dietary interventions as complementary therapies for the management of AD is summarized. This review provides valuable insights into potential therapeutic strategies to modulate the gut microbiome in AD.

Stability Studies of Kynurenine Pathway Metabolites in Blood Components Define Optimal Blood Processing Conditions

The kynurenine pathway (KP) is the main pathway of tryptophan (TRP) metabolism that generates energy for multiple cellular processes. The activity of this pathway has been shown to be dysregulated in multiple human diseases. The resultant modulation of metabolites has been suggested to comprise biomarkers to track disease progression or could identify new therapeutic targets. While metabolite changes can be measured readily in blood, there is limited knowledge on the effect of blood matrices and sample processing time may have on the stability of KP metabolites. Understanding the stability of KP metabolites in blood is integral to obtaining accurate KP data to correlate with clinical pathology. Hence, the aim of this study was to assess the concentration of KP metabolites in matched whole blood, plasma and serum. The impact of pre-analytical sample processing time in the various blood matrices was also analysed. Serum and plasma had the higher concentration of KP metabolites compared to whole blood. Furthermore, concentrations of KP metabolites declined when the collected blood was processed after 24 hours storage at 4°C. Our study shows that that type of blood matrix and the time to processing have an impact on the stability of the KP metabolites. Serum or plasma are the preferred choice of matrix and the isolation of these matrices from whole blood is best performed immediately after collection for optimal analytical KP data.

Dietary Kynurenine Pathway Metabolites-Source, Fate, and Chromatographic Determinations

Tryptophan metabolism plays an essential role in human health. In mammals, about 95% of dietary tryptophan is metabolized through the kynurenine pathway, which is associated with the development of several pathologies, including neurodegeneration. Some of the kynurenine pathway metabolites are agonists of the aryl hydrocarbon receptor involved in metabolic functions, inflammation, and carcinogenesis. Thus, their origins, fates, and roles are of widespread interest. Except for being produced endogenously, these metabolites can originate from exogenous sources (e.g., food) and undergo absorption in the digestive tract. Recently, a special focus on exogenous sources of tryptophan metabolites was observed. This overview summarizes current knowledge about the occurrence of the kynurenine pathway metabolites (kynurenines) in food and the analytical method utilized for their determination in different food matrices. Special attention was paid to sample preparation and chromatographic analysis, which has proven to be a core technique for the detection and quantification of kynurenines. A discussion of the fate and role of dietary kynurenines has also been addressed. This review will, hopefully, guide further studies on the impact of dietary kynurenines on human health.

The human microglia responsome: a resource to better understand microglia states in health and disease

Microglia, the immune cells of the brain, are increasingly implicated in neurodegenerative disorders through genetic studies. However, how genetic risk factors for these diseases are related to microglial gene expression, microglial function, and ultimately disease, is still largely unknown. Microglia change rapidly in response to alterations in their cellular environment, which is regulated through changes in transcriptional programs, which are as yet poorly understood. Here, we compared the effects of a set of inflammatory and restorative stimuli (lipopolysaccharide, interferon-gamma, resiquimod, tumor necrosis factor-alpha, adenosine triphosphate, dexamethasone, and interleukin-4) on human microglial cells from 67 different donors (N = 398 samples) at the gene and transcript level. We show that microglia from different anatomical brain regions show distinct responses to inflammatory stimuli. We observed a greater overlap between human stimulated microglia and human monocytes than with mouse microglia. We define specific microglial signatures across conditions which are highly relevant for a wide range of biological functions and complex human diseases. Finally, we used our stimulation signatures to interpret associations from Alzheimer's disease (AD) genetic studies and microglia by integrating our inflammatory gene expression profiles with common genetic variants to map cis -expression QTLs (eQTLs). Together, we provide the most comprehensive transcriptomic database of the human microglia responsome.

Highlights

RNA-sequencing of 398 human microglial samples exposed to six different triggers.Microglia from different anatomical regions show distinct stimulation responses.Responses in human microglia show a greater overlap with human monocytes than murine microglia.Mapping of response Quantitative Trait Loci identifies interactions between genotype and effect of stimulation on gene expression.Our atlas provides a reference map for interpreting microglia signatures in health and disease.

Platelets bridging the gap between gut dysbiosis and neuroinflammation in stress-linked disorders: A narrative review

In this narrative review, we examine the association between gut dysbiosis, neuroinflammation, and stress-linked disorders, including depression, anxiety, and post-traumatic stress disorder (PTSD), and investigate whether tryptophan (TRP) metabolism and platelets play a role in this association. The mechanisms underlying the aetiology of stress-linked disorders are complex and not yet completely understood. However, a potential link between chronic inflammation and these disorders may potentially be found in TRP metabolism and platelets. By critically analysing existing literature on platelets, the gut microbiome, and stress-linked disorders, we hope to elicit the role of platelets in mediating the effects on serotonin (5-HT) levels and neuroinflammation. We have included studies specifically investigating platelets and TRP metabolism in relation to inflammation, neuroinflammation and neuropsychiatric disorders. Alteration in microbial composition due to stress could contribute to increased intestinal permeability, facilitating the translocation of microbial products, and triggering the release of pro-inflammatory cytokines. This causes platelets to become hyperactive and secrete 5-HT into the plasma. Increased levels of pro-inflammatory cytokines may also lead to increased permeability of the blood-brain barrier (BBB), allowing inflammatory mediators entry into the brain, affecting the balance of TRP metabolism products, such as 5-HT, kynurenic acid (KYNA), and quinolinic acid (QUIN). These alterations may contribute to neuroinflammation and possible neurological damage. Furthermore, platelets can cross the compromised BBB and interact with astrocytes and neurons, leading to the secretion of 5-HT and pro-inflammatory factors, exacerbating inflammatory conditions in the brain. The mechanisms underlying neuroinflammation resulting from peripheral inflammation are still unclear, but the connection between the brain and gut through the bloodstream could be significant. Identifying peripheral biomarkers and mechanisms in the plasma that reflect neuroinflammation may be important. This review serves as a foundation for further research on the association between the gut microbiome, blood microbiome, and neuropsychiatric disorders. The integration of these findings with protein and metabolite markers in the blood may expand our understanding of the subject.

Tryptophan metabolism in health and disease

Tryptophan (Trp) metabolism primarily involves the kynurenine, 5-hydroxytryptamine, and indole pathways. A variety of bioactive compounds produced via Trp metabolism can regulate various physiological functions, including inflammation, metabolism, immune responses, and neurological function. Emerging evidence supports an intimate relationship between Trp metabolism disorder and diseases. The levels or ratios of Trp metabolites are significantly associated with many clinical features. Additionally, studies have shown that disease progression can be controlled by modulating Trp metabolism. Indoleamine-2,3-dioxygenase, Trp-2,3-dioxygenase, kynurenine-3-monooxygenase, and Trp hydroxylase are the rate-limiting enzymes that are critical for Trp metabolism. These key regulatory enzymes can be targeted for treating several diseases, including tumors. These findings provide novel insights into the treatment of diseases. In this review, we have summarized the recent research progress on the role of Trp metabolites in health and disease along with their clinical applications.

Correlations between kynurenines in plasma and CSF, and their relation to markers of Alzheimer's disease pathology

INTRODUCTION

Altered levels of kynurenines in blood and cerebrospinal fluid (CSF) have been reported in Alzheimer's disease (AD). However, it is still largely unknown whether peripheral kynurenine concentrations resemble those found in CSF and how they relate to AD pathology. We therefore studied correlations between kynurenines in plasma and CSF and their associations with CSF amyloid-beta (Aβ1-42) and tau levels in patients from the memory clinic spanning the whole cognitive spectrum.

METHODS

The Biobank Alzheimer Center Limburg study is a prospective cohort study of consecutive patients referred to the memory clinic of the Alzheimer Center Limburg. Plasma and CSF concentrations of tryptophan (TRP), eight kynurenines and neopterin from 138 patients were determined by means of LC-MS/MS. Additionally, CSF Aβ1-42, total-tau (t-tau) and phosphorylated tau (p-tau) concentrations were determined using commercially available single-parameter ELISA methods. Partial correlations were used to analyze cross-sectional associations between kynurenines in plasma and CSF and their relation to AD related CSF-biomarkers adjusted for age, sex, educational level, and kidney function.

RESULTS

Moderate to strong correlations were observed between plasma and CSF levels for quinolinic acid (QA; r = 0.63), TRP (r = 0.47), anthranilic acid (r = 0.59), picolinic acid (r = 0.55), and the kynurenine (KYN)/TRP ratio (KTR; r = 0.55; all p < 0.0001), while other kynurenines correlated only weakly with their corresponding CSF values. No correlations were found between plasma and CSF levels of KA/QA. Several kynurenines were also weakly correlated with Aβ1-42, t-tau or p-tau. Plasma levels of KA/QA were negatively correlated with Aβ1-42 (r = -0.21, p < 0.05). Plasma levels of TRP were negatively correlated with t-tau (r = -0.19) and levels of KYN with p-tau (r = -0.18; both p < 0.05). CSF levels of KYN (r = 0.20, p < 0.05), KA (r = 0.23, p < 0.01), and KTR (r = 0.18, p < 0.05) were positively correlated with Aβ1-42. Finally, TRP and KYN were negatively (r = -0.22 and r = -0.18, respectively), and neopterin positively (r = 0.19) correlated with p-tau (all p < 0.05).

CONCLUSIONS

Plasma concentrations of TRP, KP metabolites, KTR, and neopterin all significantly correlated positively with their corresponding CSF concentrations, but many correlations were weak. Additionally, our results suggest a relation between higher kynurenine levels and lower AD pathology load. These results need verification in future studies and require more research into (shared) underlying mechanisms.

Host-Microbiome Interactions: Tryptophan Metabolism and Aromatic Hydrocarbon Receptors after Traumatic Brain Injury

Traumatic brain injury refers to the damage caused to intracranial tissues by an external force acting on the head, leading to both immediate and prolonged harmful effects. Neuroinflammatory responses play a critical role in exacerbating the primary injury during the acute and chronic phases of TBI. Research has demonstrated that numerous neuroinflammatory responses are mediated through the "microbiota-gut-brain axis," which signifies the functional connection between the gut microbiota and the brain. The aryl hydrocarbon receptor (AhR) plays a vital role in facilitating communication between the host and microbiota through recognizing specific ligands produced directly or indirectly by the microbiota. Tryptophan (trp), an indispensable amino acid in animals and humans, represents one of the key endogenous ligands for AhR. The metabolites of trp have significant effects on the functioning of the central nervous system (CNS) through activating AHR signalling, thereby establishing bidirectional communication between the gut microbiota and the brain. These interactions are mediated through immune, metabolic, and neural signalling mechanisms. In this review, we emphasize the co-metabolism of tryptophan in the gut microbiota and the signalling pathway mediated by AHR following TBI. Furthermore, we discuss the impact of these mechanisms on the underlying processes involved in traumatic brain injury, while also addressing potential future targets for intervention.

Indoleamine 2,3-Dioxygenase as a Therapeutic Target for Alzheimer's Disease and Geriatric Depression

Neuroimmune-triggered neuroinflammation of the central nervous system is emerging as an important aetiopathogenic factor for multiple neurological disorders, including depression, dementia, Alzheimer's disease, multiple sclerosis and others. Tryptophan metabolism via the kynurenic pathway, which is initiated by the indoleamine-2,3-dioxygenase (IDO-1) enzyme, is a key regulator of the neuroimmune system and its associated neuroinflammatory effects. As discussed in this review, targeting the production of immunopathic and potentially neurotoxic kynurenine metabolites by inhibitory downregulation of IDO-1 may prove a viable target against inflammation-induced neurological conditions, particularly depression and dementia.

The Missing Piece? A Case for Microglia's Prominent Role in the Therapeutic Action of Anesthetics, Ketamine, and Psychedelics

There is much excitement surrounding recent research of promising, mechanistically novel psychotherapeutics - psychedelic, anesthetic, and dissociative agents - as they have demonstrated surprising efficacy in treating central nervous system (CNS) disorders, such as mood disorders and addiction. However, the mechanisms by which these drugs provide such profound psychological benefits are still to be fully elucidated. Microglia, the CNS's resident innate immune cells, are emerging as a cellular target for psychiatric disorders because of their critical role in regulating neuroplasticity and the inflammatory environment of the brain. The following paper is a review of recent literature surrounding these neuropharmacological therapies and their demonstrated or hypothesized interactions with microglia. Through investigating the mechanism of action of psychedelics, such as psilocybin and lysergic acid diethylamide, ketamine, and propofol, we demonstrate a largely under-investigated role for microglia in much of the emerging research surrounding these pharmacological agents. Among others, we detail sigma-1 receptors, serotonergic and γ-aminobutyric acid signalling, and tryptophan metabolism as pathways through which these agents modulate microglial phagocytic activity and inflammatory mediator release, inducing their therapeutic effects. The current review includes a discussion on future directions in the field of microglial pharmacology and covers bidirectional implications of microglia and these novel pharmacological agents in aging and age-related disease, glial cell heterogeneity, and state-of-the-art methodologies in microglial research.

Butyrate ameliorates quinolinic acid-induced cognitive decline in obesity models

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 Role of Tryptophan Metabolism in Alzheimer's Disease

The need to identify new potentially druggable biochemical mechanisms for Alzheimer's disease (AD) is an ongoing priority. The therapeutic limitations of amyloid-based approaches are further motivating this search. Amino acid metabolism, particularly tryptophan metabolism, has the potential to emerge as a leading candidate and an alternative exploitable biomolecular target. Multiple avenues support this contention. Tryptophan (trp) and its associated metabolites are able to inhibit various enzymes participating in the biosynthesis of β-amyloid, and one metabolite, 3-hydroxyanthranilate, is able to directly inhibit neurotoxic β-amyloid oligomerization; however, whilst certain trp metabolites are neuroprotectant, other metabolites, such as quinolinic acid, are directly toxic to neurons and may themselves contribute to AD progression. Trp metabolites also have the ability to influence microglia and associated cytokines in order to modulate the neuroinflammatory and neuroimmune factors which trigger pro-inflammatory cytotoxicity in AD. Finally, trp and various metabolites, including melatonin, are regulators of sleep, with disorders of sleep being an important risk factor for the development of AD. Thus, the involvement of trp biochemistry in AD is multifactorial and offers a plethora of druggable targets in the continuing quest for AD therapeutics.

The Protective Effect of IDO1 Inhibition in Aβ-Treated Neurons and APP/PS1 Mice

Alzheimer's disease (AD) is an inflammatory associated disease, in which dysregulated kynurenine pathway (KP) plays a key role. Through KP, L-tryptophan is catabolized into neurotoxic and neuroprotective metabolites. The overactivation of indolamine 2,3-dioxygenase1 (IDO1), the first rate-limiting enzyme of KP, and the abnormal accumulation of KP metabolites have been noted in AD, and blocking IDO1 has been suggested as a therapeutic strategy. However, the expression patterns of KP enzymes in AD, and whether these enzymes are related to AD pathogenesis, have not been fully studied. Herein, we examined the expression patterns of inflammatory cytokines, neurotrophic factors and KP enzymes, and the activity of IDO1 and IDO1 effector pathway AhR (aryl hydrocarbon receptor) in AD mice. We studied the effects of IDO1 inhibitors on Aβ-related neuroinflammation in rat primary neurons, mouse hippocampal neuronal cells, and APP/PS1 mice. The results further demonstrated the importance of IDO1-catalyzed KP in neuroinflammation in Alzheimer's disease.

Development and Optimization of a Target Engagement Model of Brain IDO Inhibition for Alzheimer's Disease

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO1) inhibition is a promising target as an Alzheimer's disease (AD) Disease-modifying therapy capable of downregulating immunopathic neuroinflammatory processes.

METHODS

To aid in the development of IDO inhibitors as potential AD therapeutics, we optimized a lipopolysaccharide (LPS) based mouse model of brain IDO1 inhibition by examining the dosedependent and time-course of the brain kynurenine:tryptophan (K:T) ratio to LPS via intraperitoneal dosing.

RESULTS

We determined the optimal LPS dose to increase IDO1 activity in the brain, and the ideal time point to quantify the brain K:T ratio after LPS administration. We then used a brain penetrant tool compound, EOS200271, to validate the model, determine the optimal dosing profile and found that a complete rescue of the K:T ratio was possible with the tool compound.

CONCLUSION

This LPS-based model of IDO1 target engagement is a useful tool that can be used in the development of brain penetrant IDO1 inhibitors for AD. A limitation of the present study is the lack of quantification of potential clinically relevant biomarkers in this model, which could be addressed in future studies.

Tryptophan Metabolism and Neurodegeneration: Longitudinal Associations of Kynurenine Pathway Metabolites with Cognitive Performance and Plasma Alzheimer's Disease and Related Dementias Biomarkers in the Duke Physical Performance Across the LifeSpan Study

BACKGROUND

The kynurenine pathway (KP) comprises a family of tryptophan-derived metabolites that some studies have reported are associated with poorer cognitive performance and an increased risk of Alzheimer's disease and related dementias (ADRD).

OBJECTIVE

The objective of this study was to determine the associations of plasma KP metabolites (kynurenine [KYN], kynurenic acid [KA], and tryptophan [TRP]) with a panel of plasma ADRD biomarkers (Aβ42/ β40 ratio, pTau-181, glial fibrillary acidic protein [GFAP], and neurofilament light [NfL]) and cognitive performance in a subset of older adults drawn from the Duke Physical Performance Across the LifeSpan (PALS) study.

METHODS

The Montreal Cognitive Assessment (MoCA) was used to assess cognitive performance. We used multivariate multiple regression to evaluate associations of the KYN/TRP and KA/KYN ratios with MoCA score and plasma ADRD biomarkers at baseline and over two years (n = 301; Age = 74.8±8.7).

RESULTS

Over two years, an increasing KYN/TRP ratio was associated with increasing plasma concentrations of plasma p-Tau181 (β= 6.151; 95% CI [0.29, 12.01]; p = 0.040), GFAP (β= 11.12; 95% CI [1.73, 20.51]; p = 0.020), and NfL (β= 11.13; 95% CI [2.745, 19.52]; p = 0.009), but not MoCA score or the Aβ42/Aβ40 ratio. There were no significant associations of KA/KYN with MoCA score or plasma ADRD biomarkers.

CONCLUSION

Our findings provide evidence that greater concentrations of KP metabolites are associated longitudinally over two years with greater biomarker evidence of neurofibrillary tau pathology (pTau-181), neuroinflammation (GFAP), and neurodegeneration (NfL), suggesting that dysregulated KP metabolism may play a role in ADRD pathogenesis.

Importance of NAD+ Anabolism in Metabolic, Cardiovascular and Neurodegenerative Disorders

The role of nicotinamide adenine dinucleotide (NAD+) in ageing has emerged as a critical factor in understanding links to a wide range of chronic diseases. Depletion of NAD+, a central redox cofactor and substrate of numerous metabolic enzymes, has been detected in many major age-related diseases. However, the mechanisms behind age-associated NAD+ decline remains poorly understood. Despite limited conclusive evidence, supplements aimed at increasing NAD+ levels are becoming increasingly popular. This review provides renewed insights regarding the clinical utility and benefits of NAD+ precursors, namely nicotinamide (NAM), nicotinic acid (NA), nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), in attenuating NAD+ decline and phenotypic characterization of age-related disorders, including metabolic, cardiovascular and neurodegenerative diseases. While it is anticipated that NAD+ precursors can play beneficial protective roles in several conditions, they vary in their ability to promote NAD+ anabolism with differing adverse effects. Careful evaluation of the role of NAD⁺, whether friend or foe in ageing, should be considered.

The Involvement of Kynurenine Pathway in Neurodegenerative Diseases

BACKGROUND

A growing body of evidence has shown the involvement of the kynurenine pathway (KP), the primary route of tryptophan (TRP) catabolism, in the pathophysiology of neuropsychiatric disorders.

OBJECTIVE

The study aims to provide a comprehensive and critical overview of the clinical evidence on the KP involvement in the pathophysiology of Alzheimer's disease (AD) and Parkinson's disease (PD), discussing therapeutic opportunities.

METHODS

We searched for studies investigating KP metabolites in human subjects with AD and/or PD.

RESULTS

Postmortem studies showed altered levels of KP metabolites in the brain of AD and PD patients compared with controls. Cross-sectional studies have reported associations between peripheral levels (serum or plasma) of KP metabolites and cognitive function in these patients, but the results are not always concordant.

CONCLUSION

Given the emerging evidence of the involvement of KP in the pathophysiology of neuropsychiatric/ neurodegenerative diseases and promising results from preclinical pharmacological studies, a better understanding of the KP involvement in AD and PD is warranted. Future longitudinal studies are needed to define the direction of the observed associations and specific therapeutic targets within the KP.

Effect of apigenin on tryptophan metabolic key enzymes expression in lipopolysaccharide-induced microglial cells and its mechanism

[Aims] Flavonoid apigenin (API) has a wide range of biological functions, particularly anti-inflammation. Indoleamine 2,3-dioxygenase (IDO) and 2-Amino-3-carboxymuconate-6-semialdehyde decarboxylase (ACMSD) are important tryptophan metabolic enzymes that play pivotal roles in the production of toxic metabolite quinolinic acid. However, the relationship between inflammation and ACMSD remains unclear. The present study investigated the relationship between inflammation and tryptophan metabolic key enzymes. Similarly, the anti-inflammatory effect of API on important tryptophan metabolic enzymes was examined in lipopolysaccharide (LPS)-treated microglial cells. [Main methods] MG6 cells were exposed to LPS with or without API treatment for 24-48 h. IDO and ACMSD mRNA expression and production of inflammatory mediators were analyzed. Activation of inflammatory signaling pathways, such as mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB), was also examined to study the mechanism of API in the inflammatory state. [Key findings] LPS suppressed ACMSD expression and enhanced IDO expression. However, API elevated ACMSD mRNA expression and suppressed IDO mRNA expression in LPS-treated MG6 cells. Furthermore, API suppressed interleukin-6 and nitric oxide production, whereas overproduction of inflammatory mediators enhanced IDO expression and assisted tryptophan degradation. API also inhibited activation of extracellular signal-regulated kinase (Erk) and jun N-terminal kinase (JNK) MAPK, and degradation of IκBα. [Significance] These results indicate alteration of ACMSD expression under inflammatory conditions. Moreover, API recovers expression of tryptophan metabolic key enzymes, which may be mediated by inhibition of proinflammatory mediator production via inactivation of Erk, JNK MAPK, and NF-κB pathways in LPS-stimulated microglial cells.

Mitochondrial Impairment: A Common Motif in Neuropsychiatric Presentation? The Link to the Tryptophan-Kynurenine Metabolic System

Nearly half a century has passed since the discovery of cytoplasmic inheritance of human chloramphenicol resistance. The inheritance was then revealed to take place maternally by mitochondrial DNA (mtDNA). Later, a number of mutations in mtDNA were identified as a cause of severe inheritable metabolic diseases with neurological manifestation, and the impairment of mitochondrial functions has been probed in the pathogenesis of a wide range of illnesses including neurodegenerative diseases. Recently, a growing number of preclinical studies have revealed that animal behaviors are influenced by the impairment of mitochondrial functions and possibly by the loss of mitochondrial stress resilience. Indeed, as high as 54% of patients with one of the most common primary mitochondrial diseases, mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) syndrome, present psychiatric symptoms including cognitive impairment, mood disorder, anxiety, and psychosis. Mitochondria are multifunctional organelles which produce cellular energy and play a major role in other cellular functions including homeostasis, cellular signaling, and gene expression, among others. Mitochondrial functions are observed to be compromised and to become less resilient under continuous stress. Meanwhile, stress and inflammation have been linked to the activation of the tryptophan (Trp)-kynurenine (KYN) metabolic system, which observably contributes to the development of pathological conditions including neurological and psychiatric disorders. This review discusses the functions of mitochondria and the Trp-KYN system, the interaction of the Trp-KYN system with mitochondria, and the current understanding of the involvement of mitochondria and the Trp-KYN system in preclinical and clinical studies of major neurological and psychiatric diseases.

Microbial-derived metabolites as a risk factor of age-related cognitive decline and dementia

A consequence of our progressively ageing global population is the increasing prevalence of worldwide age-related cognitive decline and dementia. In the absence of effective therapeutic interventions, identifying risk factors associated with cognitive decline becomes increasingly vital. Novel perspectives suggest that a dynamic bidirectional communication system between the gut, its microbiome, and the central nervous system, commonly referred to as the microbiota-gut-brain axis, may be a contributing factor for cognitive health and disease. However, the exact mechanisms remain undefined. Microbial-derived metabolites produced in the gut can cross the intestinal epithelial barrier, enter systemic circulation and trigger physiological responses both directly and indirectly affecting the central nervous system and its functions. Dysregulation of this system (i.e., dysbiosis) can modulate cytotoxic metabolite production, promote neuroinflammation and negatively impact cognition. In this review, we explore critical connections between microbial-derived metabolites (secondary bile acids, trimethylamine-N-oxide (TMAO), tryptophan derivatives and others) and their influence upon cognitive function and neurodegenerative disorders, with a particular interest in their less-explored role as risk factors of cognitive decline.

A Review of Metabolomic Profiling in Rheumatoid Arthritis: Bringing New Insights in Disease Pathogenesis, Treatment and Comorbidities

Metabolomic analysis provides a wealth of information that can be predictive of distinctive phenotypes of pathogenic processes and has been applied to better understand disease development. Rheumatoid arthritis (RA) is an autoimmune disease with the establishment of chronic synovial inflammation that affects joints and peripheral tissues such as skeletal muscle and bone. There is a lack of useful disease biomarkers to track disease activity, drug response and follow-up in RA. In this review, we describe potential metabolic biomarkers that might be helpful in the study of RA pathogenesis, drug response and risk of comorbidities. TMAO (choline and trimethylamine oxide) and TCA (tricarboxylic acid) cycle products have been suggested to modulate metabolic profiles during the early stages of RA and are present systemically, which is a relevant characteristic for biomarkers. Moreover, the analysis of lipids such as cholesterol, FFAs and PUFAs may provide important information before disease onset to predict disease activity and treatment response. Regarding therapeutics, TNF inhibitors may increase the levels of tryptophan, valine, lysine, creatinine and alanine, whereas JAK/STAT inhibitors may modulate exclusively fatty acids. These observations indicate that different disease modifying antirheumatic drugs have specific metabolic profiles and can reveal differences between responders and non-responders. In terms of comorbidities, physical impairment represented by higher fatigue scores and muscle wasting has been associated with an increase in urea cycle, FFAs, tocopherols and BCAAs. In conclusion, synovial fluid, blood and urine samples from RA patients seem to provide critical information about the metabolic profile related to drug response, disease activity and comorbidities.

Kynurenine Metabolism and Alzheimer's Disease: The Potential Targets and Approaches

L-tryptophan, an essential amino acid, regulates protein homeostasis and plays a role in neurotransmitter-mediated physiological events. It also influences age-associated neurological alterations and neurodegenerative changes. The metabolism of tryptophan is carried majorly through the kynurenine route, leading to the production of several pharmacologically active enzymes, substrates, and metabolites. These metabolites and enzymes influence a variety of physiological and pathological outcomes of the majority of systems, including endocrine, haemopoietic, gastrointestinal, immunomodulatory, inflammatory, bioenergetic metabolism, and neuronal functions. An extensive literature review of PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was carried out to understand the nature of the extensive work done on the kynurenine metabolites that influence cellular redox potential, immunoregulatory mechanisms, inflammatory pathways, cell survival channels, and cellular communication in close association with several neurodegenerative changes. The imbalanced state of kynurenine pathways has found a close association to several pathological disorders, including HIV infections, cancer, autoimmune disorders, neurodegenerative and neurological disorders including Parkinson's disease, epilepsy and has found special attention in Alzheimer's disease (AD). Kynurenine pathway (KP) is intricately linked to AD pathogenesis owing to the influence of kynurenine metabolites on excitotoxic neurotransmission, oxidative stress, uptake of neurotransmitters, and modulation of neuroinflammation, amyloid aggregation, microtubule disruption, and their ability to induce a state of dysbiosis. Pharmacological modulation of KP pathways has shown encouraging results, indicating that it may be a viable and explorable target for the therapy of AD.

Kynurenine Pathway Metabolites as Biomarkers in Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that deteriorates cognitive function. Patients with AD generally exhibit neuroinflammation, elevated beta-amyloid (Aβ), tau phosphorylation (p-tau), and other pathological changes in the brain. The kynurenine pathway (KP) and several of its metabolites, especially quinolinic acid (QA), are considered to be involved in the neuropathogenesis of AD. The important metabolites and key enzymes show significant importance in neuroinflammation and AD. Meanwhile, the discovery of changed levels of KP metabolites in patients with AD suggests that KP metabolites may have a prominent role in the pathogenesis of AD. Further, some KP metabolites exhibit other effects on the brain, such as oxidative stress regulation and neurotoxicity. Both analogs of the neuroprotective and antineuroinflammation metabolites and small molecule enzyme inhibitors preventing the formation of neurotoxic and neuroinflammation compounds may have potential therapeutic significance. This review focused on the KP metabolites through the relationship of neuroinflammation in AD, significant KP metabolites, and associated molecular mechanisms as well as the utility of these metabolites as biomarkers and therapeutic targets for AD. The objective is to provide references to find biomarkers and therapeutic targets for patients with AD.

Kynurenic acid in neurodegenerative disorders-unique neuroprotection or double-edged sword?

AIMS

The family of kynurenine pathway (KP) metabolites includes compounds produced along two arms of the path and acting in clearly opposite ways. The equilibrium between neurotoxic kynurenines, such as 3-hydroxykynurenine (3-HK) or quinolinic acid (QUIN), and neuroprotective kynurenic acid (KYNA) profoundly impacts the function and survival of neurons. This comprehensive review summarizes accumulated evidence on the role of KYNA in Alzheimer's, Parkinson's and Huntington's diseases, and discusses future directions of potential pharmacological manipulations aimed to modulate brain KYNA.

DISCUSSION

The synthesis of specific KP metabolites is tightly regulated and may considerably vary under physiological and pathological conditions. Experimental data consistently imply that shift of the KP to neurotoxic branch producing 3-HK and QUIN formation, with a relative or absolute deficiency of KYNA, is an important factor contributing to neurodegeneration. Targeting specific brain regions to maintain adequate KYNA levels seems vital; however, it requires the development of precise pharmacological tools, allowing to avoid the potential cognitive adverse effects.

CONCLUSIONS

Boosting KYNA levels, through interference with the KP enzymes or through application of prodrugs/analogs with high bioavailability and potency, is a promising clinical approach. The use of KYNA, alone or in combination with other compounds precisely influencing specific populations of neurons, is awaiting to become a significant therapy for neurodegenerative disorders.

Increased Kynurenine Indicates a Fatal Course of COVID-19

(1) Background: An inefficient immune response accompanied by an overwhelming inflammatory reaction is involved in severe courses of COVID-19. Kynurenine (KYN) has important immune-modulatory functions and may contribute to a failure in controlling SARS-CoV-2. The present study aims to explore biomarkers that hint at a fatal outcome of COVID-19 early on. (2) Methods: We established a cohort of 148 hospitalized COVID-19 patients for this study. Thirty-one patients died due to a severe COVID-19 course, and 117 recovered within 90 days. We built a biobank by collecting left-over material from these patients whenever blood arrived at the central laboratory of our University hospital for analysis of routine markers. The scientific laboratory analysis comprised KYN, Tryptophan (TRP), KYN/TRP ratio, ferritin, interleukin-6 (IL-6), C-reactive protein (CRP), creatinine, N-terminal pro-natriuretic peptide (NTproBNP), troponin T (TnT), fibrinogen, D-Dimer, prothrombin time (PT), activated partial thromboplastin time (aPTT), antithrombin (AT), protein C, protein S, factor XIII, lupus aPTT, angiotensin-2, vitamin D metabolites, and telomeres in all COVID-19 patients. Basic clinical characteristics and anteceding diseases including cardiovascular, oncologic, renal, hypertension, pulmonary, metabolic (diabetes, obesity) were recorded in a database together with the laboratory data. (3) Results: At the time of diagnosis of SARS-CoV-2 infection those patients who deceased within 90 days afterwards due to COVID-19, had a significantly higher age, higher KYN, KYN/TRP ratio, ferritin, creatinine, and NTproBNP values than SARS-CoV-2 patients who survived COVID-19 along the same time span. In a Kaplan-Meier analysis the variables age, KYN, ferritin, D-Dimer, TnT, NTproBNP, and creatinine showed a significant influence on survival time. Gender, however, showed no influence. In a combined Cox regression analysis KYN had the highest hazard ratio (1.188, 95% CI: 1.071-1.319) followed by age (1.041, 95% CI: 1.011-1.073). In a ROC analysis, KYN values above the cut off limit of 4.82 nmol/l (as specified by Youden index) had a sensitivity of 82% (95% CI: 66-95%) and a specificity of 72% (95% CI: 65-82%) to predict COVID-19 related death within 90 days observation time. (4) Conclusions: Kynurenine is a promising blood biomarker to predict an increased risk of mortality in SARS-CoV-2 infected people already at the time of the first positive SARS-CoV-2 verification detected in these persons.

Hypoxia and the Kynurenine Pathway: Implications and Therapeutic Prospects in Alzheimer's Disease

Neurodegenerative diseases (NDs) like Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, Parkinson's disease, and Huntington's disease predominantly pose a significant socioeconomic burden. Characterized by progressive neural dysfunction coupled with motor or intellectual impairment, the pathogenesis of ND may result from contributions of certain environmental and molecular factors. One such condition is hypoxia, characterized by reduced organ/tissue exposure to oxygen. Reduced oxygen supply often occurs during the pathogenesis of ND and the aging process. Despite the well-established relationship between these two conditions (i.e., hypoxia and ND), the underlying molecular events or mechanisms connecting hypoxia to ND remain ill-defined. However, the relatedness may stem from the protective or deleterious effects of the transcription factor, hypoxia-inducible factor 1-alpha (HIF-1α). The upregulation of HIF-1α occurs in the pathogenesis of most NDs. The dual function of HIF-1α in acting as a "killer factor" or a "protective factor" depends on the prevailing local cellular condition. The kynurenine pathway is a metabolic pathway involved in the oxidative breakdown of tryptophan. It is essential in neurotransmission and immune function and, like hypoxia, associated with ND. Thus, a good understanding of factors, including hypoxia (i.e., the biochemical implication of HIF-1α) and kynurenine pathway activation in NDs, focusing on Alzheimer's disease could prove beneficial to new therapeutic approaches for this disease, thus the aim of this review.

Dietary Regulation of Gut-Brain Axis in Alzheimer's Disease: Importance of Microbiota Metabolites

Alzheimer's disease (AD) is a neurodegenerative disease that impacts 45 million people worldwide and is ranked as the 6th top cause of death among all adults by the Centers for Disease Control and Prevention. While genetics is an important risk factor for the development of AD, environment and lifestyle are also contributing risk factors. One such environmental factor is diet, which has emerged as a key influencer of AD development/progression as well as cognition. Diets containing large quantities of saturated/trans-fats, refined carbohydrates, limited intake of fiber, and alcohol are associated with cognitive dysfunction while conversely diets low in saturated/trans-fats (i.e., bad fats), high mono/polyunsaturated fats (i.e., good fats), high in fiber and polyphenols are associated with better cognitive function and memory in both humans and animal models. Mechanistically, this could be the direct consequence of dietary components (lipids, vitamins, polyphenols) on the brain, but other mechanisms are also likely to be important. Diet is considered to be the single greatest factor influencing the intestinal microbiome. Diet robustly influences the types and function of micro-organisms (called microbiota) that reside in the gastrointestinal tract. Availability of different types of nutrients (from the diet) will favor or disfavor the abundance and function of certain groups of microbiota. Microbiota are highly metabolically active and produce many metabolites and other factors that can affect the brain including cognition and the development and clinical progression of AD. This review summarizes data to support a model in which microbiota metabolites influence brain function and AD.

Exploring the roles of tryptophan metabolism in MS beyond neuroinflammation and neurodegeneration: A paradigm shift to neuropsychiatric symptoms

The metabolism of tryptophan through the kynurenine pathway (KP) has been increasingly recognised in contributing to disease progression in the autoimmune and inflammatory disease multiple sclerosis (MS). In this review, the roles of inflammation and the KP are recontextualised to better understand the aetiology of the neuropsychiatric symptoms (depression, postpartum depression, suicidality, fatigue and cognitive dysfunction) in MS. These symptoms will be discussed in the context of cytokine-induced sickness behaviours, KP activation and levels of neurotoxicity and neuroprotection in MS. In particular, there will be emphasis on how neuropsychiatric symptoms in MS occur against the shared background of inflammation and KP dysregulation. The discourse of this review aims to promote future research in elucidating KP mechanisms in MS that would inevitably lead to more targeted treatment options for neuropsychiatric symptoms and disease progression.

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Research on tryptophan metabolism and neuroinflammation on neurodegeneration in multiple sclerosis (MS) is mounting.

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This review reframes the roles of neuroinflammation and tryptophan metabolism dysregulation on mental health issues in MS.

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The impact of neuroinflammation and tryptophan metabolism on depression, suicidality, fatigue, and cognitive impairment in MS are discussed.

Research Paper5,6,7 trihydroxy flavone armoured neurodegeneration caused by Quinolinic acid induced huntington's like disease in rat striatum - reinstating the level of brain neurotrophins with special reference to cognitive-socio behaviour, biochemical and histopathological aspects

Huntington Disease (HD), a predominant Neurodegenerative Disorder which might be induced by endogenous neurotoxin called Quinolinic Acid (QA), an N-methyl-D aspartate receptor (NMDAR) agonist, the bilaterally intrastriatal administration (200 nm/2 μL of saline) offers rise to the toxic events like neuronal death, neuroinflammation by inflicting excitotoxicity and oxidative stress in the striatum of male Wistar rats by exhibiting the behavioural changes which was accessed by rotarod, open field analysis. In this study, the neuropharmacological effect of Baicalein (BC) against QA induced HD was evaluated. Baicalein (BC), scientifically 5,6,7 trihydroxy flavone present naturally in the edible plants like Scutellaria baicalensis and Oroxylum indicum possess a better neuroprotective effect in the dosage of 10 mg/kg and 30 mg/kg intraperitoneally in the striatum of HD induced rats. This study proved that BC is efficient to revive the level of enzymatic & non-enzymatic antioxidants and mitochondrial complexes by decreasing the number of inflammatory mediators such as MDA, protein carbonyls and Nitric Oxide at the significance of P < 0.01 and restores the amount of BDNF and GDNF thereby preventing the neurophysiological changes which were analysed by haematoxylin & eosin staining. Thus finally, the protective effect of Baicalein displays the up-gradation of psychological and behavioural changes induced by QA.

New Approaches to Profile the Microbiome for Treatment of Neurodegenerative Disease

Progressive neurodegenerative diseases represent some of the largest growing treatment challenges for public health in modern society. These diseases mainly progress due to aging and are driven by microglial surveillance and activation in response to changes occurring in the aging brain. The lack of efficacious treatment options for Alzheimer's disease (AD), as the focus of this review, and other neurodegenerative disorders has encouraged new approaches to address neuroinflammation for potential treatments. Here we will focus on the increasing evidence that dysbiosis of the gut microbiome is characterized by inflammation that may carry over to the central nervous system and into the brain. Neuroinflammation is the common thread associated with neurodegenerative diseases, but it is yet unknown at what point and how innate immune function turns pathogenic for an individual. This review will address extensive efforts to identify constituents of the gut microbiome and their neuroactive metabolites as a peripheral path to treatment. This approach is still in its infancy in substantive clinical trials and requires thorough human studies to elucidate the metabolic microbiome profile to design appropriate treatment strategies for early stages of neurodegenerative disease. We view that in order to address neurodegenerative mechanisms of the gut, microbiome and metabolite profiles must be determined to pre-screen AD subjects prior to the design of specific, chronic titrations of gut microbiota with low-dose antibiotics. This represents an exciting treatment strategy designed to balance inflammatory microglial involvement in disease progression with an individual's manifestation of AD as influenced by a coercive inflammatory gut.

Systematic review of human post-mortem immunohistochemical studies and bioinformatics analyses unveil the complexity of astrocyte reaction in Alzheimer's disease

AIMS

Reactive astrocytes in Alzheimer's disease (AD) have traditionally been demonstrated by increased glial fibrillary acidic protein (GFAP) immunoreactivity; however, astrocyte reaction is a complex and heterogeneous phenomenon involving multiple astrocyte functions beyond cytoskeletal remodelling. To better understand astrocyte reaction in AD, we conducted a systematic review of astrocyte immunohistochemical studies in post-mortem AD brains followed by bioinformatics analyses on the extracted reactive astrocyte markers.

METHODS

NCBI PubMed, APA PsycInfo and WoS-SCIE databases were interrogated for original English research articles with the search terms 'Alzheimer's disease' AND 'astrocytes.' Bioinformatics analyses included protein-protein interaction network analysis, pathway enrichment, and transcription factor enrichment, as well as comparison with public human -omics datasets.

RESULTS

A total of 306 articles meeting eligibility criteria rendered 196 proteins, most of which were reported to be upregulated in AD vs control brains. Besides cytoskeletal remodelling (e.g., GFAP), bioinformatics analyses revealed a wide range of functional alterations including neuroinflammation (e.g., IL6, MAPK1/3/8 and TNF), oxidative stress and antioxidant defence (e.g., MT1A/2A, NFE2L2, NOS1/2/3, PRDX6 and SOD1/2), lipid metabolism (e.g., APOE, CLU and LRP1), proteostasis (e.g., cathepsins, CRYAB and HSPB1/2/6/8), extracellular matrix organisation (e.g., CD44, MMP1/3 and SERPINA3), and neurotransmission (e.g., CHRNA7, GABA, GLUL, GRM5, MAOB and SLC1A2), among others. CTCF and ESR1 emerged as potential transcription factors driving these changes. Comparison with published -omics datasets validated our results, demonstrating a significant overlap with reported transcriptomic and proteomic changes in AD brains and/or CSF.

CONCLUSIONS

Our systematic review of the neuropathological literature reveals the complexity of AD reactive astrogliosis. We have shared these findings as an online resource available at www.astrocyteatlas.org.

NAD+ homeostasis in human health and disease

Depletion of nicotinamide adenine dinucleotide (NAD+ ), a central redox cofactor and the substrate of key metabolic enzymes, is the causative factor of a number of inherited and acquired diseases in humans. Primary deficiencies of NAD+ homeostasis are the result of impaired biosynthesis, while secondary deficiencies can arise due to other factors affecting NAD+ homeostasis, such as increased NAD+ consumption or dietary deficiency of its vitamin B3 precursors. NAD+ depletion can manifest in a wide variety of pathological phenotypes, ranging from rare inherited defects, characterized by congenital malformations, retinal degeneration, and/or encephalopathy, to more common multifactorial, often age-related, diseases. Here, we discuss NAD+ biochemistry and metabolism and provide an overview of the etiology and pathological consequences of alterations of the NAD+ metabolism in humans. Finally, we discuss the state of the art of the potential therapeutic implications of NAD+ repletion for boosting health as well as treating rare and common diseases, and the possibilities to achieve this by means of the different NAD+ -enhancing agents.

Neuroinflammation and the Kynurenine Pathway in CNS Disease: Molecular Mechanisms and Therapeutic Implications

Diseases of the central nervous system (CNS) remain a significant health, social and economic problem around the globe. The development of therapeutic strategies for CNS conditions has suffered due to a poor understanding of the underlying pathologies that manifest them. Understanding common etiological origins at the cellular and molecular level is essential to enhance the development of efficacious and targeted treatment options. Over the years, neuroinflammation has been posited as a common link between multiple neurological, neurodegenerative and neuropsychiatric disorders. Processes that precipitate neuroinflammatory conditions including genetics, infections, physical injury and psychosocial factors, like stress and trauma, closely link dysregulation in kynurenine pathway (KP) of tryptophan metabolism as a possible pathophysiological factor that 'fuel the fire' in CNS diseases. In this study, we aim to review emerging evidence that provide mechanistic insights between different CNS disorders, neuroinflammation and the KP. We provide a thorough overview of the different branches of the KP pertinent to CNS disease pathology that have therapeutic implications for the development of selected and efficacious treatment strategies.

Genetic Analysis of Tryptophan Metabolism Genes in Sporadic Amyotrophic Lateral Sclerosis

The essential amino acid tryptophan (TRP) is the initiating metabolite of the kynurenine pathway (KP), which can be upregulated by inflammatory conditions in cells. Neuroinflammation-triggered activation of the KP and excessive production of the KP metabolite quinolinic acid are common features of multiple neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). In addition to its role in the KP, genes involved in TRP metabolism, including its incorporation into proteins, and synthesis of the neurotransmitter serotonin, have also been genetically and functionally linked to these diseases. ALS is a late onset neurodegenerative disease that is classified as familial or sporadic, depending on the presence or absence of a family history of the disease. Heritability estimates support a genetic basis for all ALS, including the sporadic form of the disease. However, the genetic basis of sporadic ALS (SALS) is complex, with the presence of multiple gene variants acting to increase disease susceptibility and is further complicated by interaction with potential environmental factors. We aimed to determine the genetic contribution of 18 genes involved in TRP metabolism, including protein synthesis, serotonin synthesis and the KP, by interrogating whole-genome sequencing data from 614 Australian sporadic ALS cases. Five genes in the KP (AFMID, CCBL1, GOT2, KYNU, HAAO) were found to have either novel protein-altering variants, and/or a burden of rare protein-altering variants in SALS cases compared to controls. Four genes involved in TRP metabolism for protein synthesis (WARS) and serotonin synthesis (TPH1, TPH2, MAOA) were also found to carry novel variants and/or gene burden. These variants may represent ALS risk factors that act to alter the KP and lead to neuroinflammation. These findings provide further evidence for the role of TRP metabolism, the KP and neuroinflammation in ALS disease pathobiology.