3-Hydroxykynurenine, an endogenous oxidative stress generator, causes neuronal cell death with apoptotic features and region selectivity

Neurobiological mechanisms in the kynurenine pathway and major depressive disorder

Major depressive disorder (MDD) is a prevalent psychiatric disorder that has damage to people's quality of life. Tryptophan is the precursor to serotonin, a critical neurotransmitter in mood modulation. In mammals, most free tryptophan is degraded by the kynurenine pathway (KP), resulting in a range of metabolites involved in inflammation, immune response, and neurotransmission. The imbalance between quinolinic acid (QA), a toxic metabolite, and kynurenic acid (KynA), a protective metabolite, is a relevant phenomenon involved in the pathophysiology of MDD. Proinflammatory cytokines increase the activity of the enzyme indoleamine 2,3-dioxygenase (IDO), leading to the degradation of tryptophan in the KP and an increase in the release of QA. IDO activates proinflammatory genes, potentiating neuroinflammation and deregulating other physiological mechanisms related to chronic stress and MDD. This review highlights the physiological mechanisms involved with stress and MDD, which are underlying an imbalance of the KP and discuss potential therapeutic targets.

The venom of Habrobracon hebetor induces alterations in host metabolism

The ability of parasitic wasps to manipulate a host's metabolism is under active investigation. Components of venom play a major role in this process. In the present work, we studied the effect of the venom of the ectoparasitic wasp Habrobracon hebetor on the metabolism of the greater wax moth host (Galleria mellonella). We identified and quantified 45 metabolites in the lymph (cell-free hemolymph) of wax moth larvae on the second day after H. hebetor venom injection, using NMR spectroscopy and liquid chromatography coupled with mass spectrometry. These metabolites included 22 amino acids, nine products of lipid metabolism (sugars, amines and alcohols) and four metabolic intermediates related to nitrogenous bases, nucleotides and nucleosides. An analysis of the larvae metabolome suggested that the venom causes suppression of the tricarboxylic acid cycle, an increase in the number of free amino acids in the lymph, an increase in the concentration of trehalose in the lymph simultaneously with a decrease in the amount of glucose, and destructive processes in the fat body tissue. Thus, this parasitoid venom not only immobilizes the prey but also modulates its metabolism, thereby providing optimal conditions for the development of larvae.

Potential Plausible Role of Stem Cell for Treating Depressive Disorder: a Retrospective Review

Depression poses a significant threat to global physical and mental health, impacting around 3.8% of the population with a rising incidence. Current treatment options primarily involve medication and psychological support, yet their effectiveness remains limited, contributing to high relapse rates. There is an urgent need for innovative and more efficacious treatment modalities. Stem cell therapy, a promising avenue in regenerative medicine for a spectrum of neurodegenerative conditions, has recently garnered attention for its potential application in depression. While much of this work remains preclinical, it has demonstrated considerable promise. Identified mechanisms underlying the antidepressant effects of stem cell therapy encompass the stimulation of neurotrophic factors, immune function modulation, and augmented monoamine levels. Nonetheless, these pathways and other undiscovered mechanisms necessitate further investigation. Depression fundamentally manifests as a neurodegenerative disorder. Given stem cell therapy's success in addressing a range of neurodegenerative pathologies, it opens the door to explore its application in depression treatment. This exploration may include repairing damaged nerves directly or indirectly and inhibiting neurotoxicity. Nevertheless, significant challenges must be overcome before stem cell therapies can be applied clinically. Successful resolution of these issues will ultimately determine the feasibility of incorporating stem cell therapies into the clinical landscape. This narrative review provides insights into the progress of research, potential avenues for exploration, and the prevailing challenges in the implementation of stem cell therapy for treatment of depression.

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.

3-Hydroxykynurenine targets kainate receptors to promote defense against infection

Bacterial infection involves a complex interaction between the pathogen and host where the outcome of infection is not solely determined by pathogen eradication. To identify small molecules that promote host survival by altering the host-pathogen dynamic, we conducted an in vivo chemical screen using zebrafish embryos and found that treatment with 3-hydroxykynurenine (3-HK) protects from lethal bacterial infection. 3-HK, a metabolite produced through host tryptophan metabolism, has no direct antibacterial activity but enhances host survival by restricting bacterial expansion in macrophages through a systemic mechanism that targets kainate-sensitive glutamate receptors. These findings reveal a new pathway by which tryptophan metabolism and kainate-sensitive glutamate receptors function and interact to modulate immunity, with important implications for the coordination between the immune and nervous systems in pathological conditions.

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.

Dual treatment with kynurenine pathway inhibitors and NAD+ precursors synergistically extends life span in Drosophila

Tryptophan catabolism is highly conserved and generates important bioactive metabolites, including kynurenines, and in some animals, NAD+. Aging and inflammation are associated with increased levels of kynurenine pathway (KP) metabolites and depleted NAD+, factors which are implicated as contributors to frailty and morbidity. Contrastingly, KP suppression and NAD+ supplementation are associated with increased life span in some animals. Here, we used DGRP_229 Drosophila to elucidate the effects of KP elevation, KP suppression, and NAD+ supplementation on physical performance and survivorship. Flies were chronically fed kynurenines, KP inhibitors, NAD+ precursors, or a combination of KP inhibitors with NAD+ precursors. Flies with elevated kynurenines had reduced climbing speed, endurance, and life span. Treatment with a combination of KP inhibitors and NAD+ precursors preserved physical function and synergistically increased maximum life span. We conclude that KP flux can regulate health span and life span in Drosophila and that targeting KP and NAD+ metabolism can synergistically increase life span.

Indoleamine 2,3-dioxygenase 2 deficiency associates with autism-like behavior via dopaminergic neuronal dysfunction

Indoleamine 2,3-dioxygenase 2 (IDO2) is an enzyme of the tryptophan-kynurenine pathway that is constitutively expressed in the brain. To provide insight into the physiological role of IDO2 in the brain, behavioral and neurochemical analyses in IDO2 knockout (KO) mice were performed. IDO2 KO mice showed stereotyped behavior, restricted interest and social deficits, traits that are associated with behavioral endophenotypes of autism spectrum disorder (ASD). IDO2 was colocalized immunohistochemically with tyrosine-hydroxylase-positive cells in dopaminergic neurons. In the striatum and amygdala of IDO2 KO mice, decreased dopamine turnover was associated with increased α-synuclein level. Correspondingly, levels of downstream dopamine D1 receptor signaling molecules such as brain-derived neurotrophic factor and c-Fos positive proteins were decreased. Furthermore, decreased abundance of ramified-type microglia resulted in increased dendritic spine density in the striatum of IDO2 KO mice. Both chemogenetic activation of dopaminergic neurons and treatment with methylphenidate, a dopamine reuptake inhibitor, ameliorated the ASD-like behavior of IDO2 KO mice. Sequencing analysis of exon regions in IDO2 from 309 ASD samples identified a rare canonical splice site variant in one ASD case. These results suggest that the IDO2 gene is, at least in part, a factor closely related to the development of psychiatric disorders.

Using an Interpretable Amino Acid-Based Machine Learning Method to Enhance the Diagnosis of Major Depressive Disorder

Background: Major depressive disorder (MDD) is a leading cause of disability worldwide. At present, however, there are no established biomarkers that have been validated for diagnosing and treating MDD. This study sought to assess the diagnostic and predictive potential of the differences in serum amino acid concentration levels between MDD patients and healthy controls (HCs), integrating them into interpretable machine learning models. Methods: In total, 70 MDD patients and 70 HCs matched in age, gender, and ethnicity were recruited for the study. Serum amino acid profiling was conducted by means of chromatography-mass spectrometry. A total of 21 metabolites were analysed, with 17 from a preset amino acid panel and the remaining 4 from a preset kynurenine panel. Logistic regression was applied to differentiate MDD patients from HCs. Results: The best-performing model utilised both feature selection and hyperparameter optimisation and yielded a moderate area under the receiver operating curve (AUC) classification value of 0.76 on the testing data. The top five metabolites identified as potential biomarkers for MDD were 3-hydroxy-kynurenine, valine, kynurenine, glutamic acid, and xanthurenic acid. Conclusions: Our study highlights the potential of using an interpretable machine learning analysis model based on amino acids to aid and increase the diagnostic accuracy of MDD in clinical practice.

Minocycline as a potential anxiolytic drug: systematic review and meta-analysis of evidence in murine models

Minocycline is a tetracycline antibiotic with off-label use as an anti-inflammatory drug. Because it can cross the blood-brain barrier, minocycline has been proposed as an alternative treatment for psychiatric disorders, in which inflammation plays an important role. However, its beneficial effects on anxiety disorders are unclear. Therefore, we performed a systematic review and meta-analysis to evaluate the efficacy of minocycline as an anxiolytic drug in preclinical models. We performed a PubMed search according to the PRISMA guidelines and PICOS strategy. The risk of bias was evaluated using the SYRCLE tool. We included studies that determined the efficacy of minocycline in animal models of anxiety that may involve exposures (e.g. stressors, immunomodulators, injury). Data extracted included treatment effect, dose range, route of administration, and potential mechanisms for the anxiolytic effect. Meta-analysis of twenty studies showed that minocycline reduced anxiety-like behavior in rodents previously exposed to stress or immunostimulants but not in exposure-naïve animals. This effect was not associated with the dose administered or treatment duration. The mechanism for the anxiolytic activity of minocycline may depend on its anti-inflammatory effects in the brain regions involving anxiety. These suggest that minocycline could be repurposed as a treatment for anxiety and related disorders and warrants further evaluation.

The Influence of Kynurenine Metabolites on Neurodegenerative Pathologies

As the kynurenine pathway's links to inflammation, the immune system, and neurological disorders became more apparent, it attracted more and more attention. It is the main pathway through which the liver breaks down Tryptophan and the initial step in the creation of nicotinamide adenine dinucleotide (NAD+) in mammals. Immune system activation and the buildup of potentially neurotoxic substances can result from the dysregulation or overactivation of this pathway. Therefore, it is not shocking that kynurenines have been linked to neurological conditions (Depression, Parkinson's, Alzheimer's, Huntington's Disease, Schizophrenia, and cognitive deficits) in relation to inflammation. Nevertheless, preclinical research has demonstrated that kynurenines are essential components of the behavioral analogs of depression and schizophrenia-like cognitive deficits in addition to mediators associated with neurological pathologies due to their neuromodulatory qualities. Neurodegenerative diseases have been extensively associated with neuroactive metabolites of the kynurenine pathway (KP) of tryptophan breakdown. In addition to being a necessary amino acid for protein synthesis, Tryptophan is also transformed into the important neurotransmitters tryptamine and serotonin in higher eukaryotes. In this article, a summary of the KP, its function in neurodegeneration, and the approaches being used currently to target the route therapeutically are discussed.

On the benefits of the tryptophan metabolite 3-hydroxyanthranilic acid in Caenorhabditis elegans and mouse aging

Tryptophan metabolism through the kynurenine pathway influences molecular processes critical to healthy aging including immune signaling, redox homeostasis, and energy production. Aberrant kynurenine metabolism occurs during normal aging and is implicated in many age-associated pathologies including chronic inflammation, atherosclerosis, neurodegeneration, and cancer. We and others previously identified three kynurenine pathway genes-tdo-2, kynu-1, and acsd-1-for which decreasing expression extends lifespan in invertebrates. Here we report that knockdown of haao-1, a fourth gene encoding the enzyme 3-hydroxyanthranilic acid (3HAA) dioxygenase (HAAO), extends lifespan by ~30% and delays age-associated health decline in Caenorhabditis elegans. Lifespan extension is mediated by increased physiological levels of the HAAO substrate 3HAA. 3HAA increases oxidative stress resistance and activates the Nrf2/SKN-1 oxidative stress response. In pilot studies, female Haao knockout mice or aging wild type male mice fed 3HAA supplemented diet were also long-lived. HAAO and 3HAA represent potential therapeutic targets for aging and age-associated disease.

Kynurenine pathway and its role in neurologic, psychiatric, and inflammatory bowel diseases

Tryptophan metabolism along the kynurenine pathway is of central importance for the immune function. It prevents hyperinflammation and induces long-term immune tolerance. Accumulating evidence also demonstrates cytoprotective and immunomodulatory properties of kynurenine pathway in conditions affecting either central or peripheral nervous system as well as other conditions such as inflammatory bowel disease (IBD). Although multilevel association exists between the inflammatory bowel disease (IBD) and various neurologic (e.g., neurodegenerative) disorders, it is believed that the kynurenine pathway plays a pivotal role in the development of both IBD and neurodegenerative disorders. In this setting, there is strong evidence linking the gut-brain axis with intestinal dysfunctions including IBD which is consistent with the fact that the risk of neurodegenerative diseases is higher in IBD patients. This review aims to highlight the role of kynurenine metabolic pathway in various neurologic and psychiatric diseases as well as relationship between IBD and neurodegenerative disorders in the light of the kynurenine metabolic pathway.

The Role of Tryptophan Metabolism in the Occurrence and Progression of Acute and Chronic Kidney Diseases

Acute kidney injury (AKI) and chronic kidney disease (CKD) are common kidney diseases in clinics with high morbidity and mortality, but their pathogenesis is intricate. Tryptophan (Trp) is a fundamental amino acid for humans, and its metabolism produces various bioactive substances involved in the pathophysiology of AKI and CKD. Metabolomic studies manifest that Trp metabolites like kynurenine (KYN), 5-hydroxyindoleacetic acid (5-HIAA), and indoxyl sulfate (IS) increase in AKI or CKD and act as biomarkers that facilitate the early identification of diseases. Meanwhile, KYN and IS act as ligands to exacerbate kidney damage by activating aryl hydrocarbon receptor (AhR) signal transduction. The reduction of renal function can cause the accumulation of Trp metabolites which in turn accelerate the progression of AKI or CKD. Besides, gut dysbiosis induces the expansion of Enterobacteriaceae family to produce excessive IS, which cannot be excreted due to the deterioration of renal function. The application of Trp metabolism as a target in AKI and CKD will also be elaborated. Thus, this study aims to elucidate Trp metabolism in the development of AKI and CKD, and explores the relative treatment strategies by targeting Trp from the perspective of metabolomics to provide a reference for their diagnosis and prevention.

The Tryptophan-Kynurenine Metabolic System Is Suppressed in Cuprizone-Induced Model of Demyelination Simulating Progressive Multiple Sclerosis

Progressive multiple sclerosis (MS) is a chronic disease with a unique pattern, which is histologically classified into the subpial type 3 lesions in the autopsy. The lesion is also homologous to that of cuprizone (CPZ) toxin-induced animal models of demyelination. Aberration of the tryptophan (TRP)-kynurenine (KYN) metabolic system has been observed in patients with MS; nevertheless, the KYN metabolite profile of progressive MS remains inconclusive. In this study, C57Bl/6J male mice were treated with 0.2% CPZ toxin for 5 weeks and then underwent 4 weeks of recovery. We measured the levels of serotonin, TRP, and KYN metabolites in the plasma and the brain samples of mice at weeks 1, 3, and 5 of demyelination, and at weeks 7 and 9 of remyelination periods by ultra-high-performance liquid chromatography with tandem mass spectrometry (UHPLC-MS/MS) after body weight measurement and immunohistochemical analysis to confirm the development of demyelination. The UHPLC-MS/MS measurements demonstrated a significant reduction of kynurenic acid, 3-hydoxykynurenine (3-HK), and xanthurenic acid in the plasma and a significant reduction of 3-HK, and anthranilic acid in the brain samples at week 5. Here, we show the profile of KYN metabolites in the CPZ-induced mouse model of demyelination. Thus, the KYN metabolite profile potentially serves as a biomarker of progressive MS and thus opens a new path toward planning personalized treatment, which is frequently obscured with immunologic components in MS deterioration.

Ubiquitylome study reveals the regulatory effect of α-lipoic acid on ubiquitination of key proteins in tryptophan metabolism pathway of pig liver

The decline in antioxidant defenses make it easily for human and animals to suffer from liver damage and diseases induced by oxidative stress, causing enormous losses to human health and livestock production. As one of the canonical protein post-translational modifications (PTMs), ubiquitination is widely involved in cell proliferation, apoptosis and damage/repair response, and is proven to be involved in the ability of mammals to resist oxidative stress. To explore whether α-lipoic acid (LA), a safe and efficient antioxidant, plays a role in regulating liver antioxidant status by PTMs, proteins in livers of pigs fed with LA were analyzed at the level of proteome and ubiquitylome. Based on proteome-wide enrichment of ubiquitination, a total of 7274 proteins were identified and 5326 were quantified, we also identified 1564 ubiquitination sites in 580 ubiquitinated proteins, among which there were 136 differentially ubiquitinated sites in 103 differentially ubiquitinated proteins upon LA. Further bioinformatics analysis showed that these differential proteins were mainly enriched in tryptophan metabolic pathway, and accompanied by significantly improvement of liver antioxidant capacity. We revealed the regulatory effect of LA on ubiquitination of kynurenine 3-monooxygenase (KMO) and other key proteins in tryptophan metabolism pathway of pig liver for the first time.

Fecal Microbiota Transplantation in Reducing Uremic Toxins Accumulation in Kidney Disease: Current Understanding and Future Perspectives

During the past decades, the gut microbiome emerged as a key player in kidney disease. Dysbiosis-related uremic toxins together with pro-inflammatory mediators are the main factors in a deteriorating kidney function. The toxicity of uremic compounds has been well-documented in a plethora of pathophysiological mechanisms in kidney disease, such as cardiovascular injury (CVI), metabolic dysfunction, and inflammation. Accumulating data on the detrimental effect of uremic solutes in kidney disease supported the development of many strategies to restore eubiosis. Fecal microbiota transplantation (FMT) spread as an encouraging treatment for different dysbiosis-associated disorders. In this scenario, flourishing studies indicate that fecal transplantation could represent a novel treatment to reduce the uremic toxins accumulation. Here, we present the state-of-the-art concerning the application of FMT on kidney disease to restore eubiosis and reverse the retention of uremic toxins.

The Utility of Amino Acid Metabolites in the Diagnosis of Major Depressive Disorder and Correlations with Depression Severity

Major depressive disorder (MDD) is a highly prevalent and disabling condition with a high disease burden. There are currently no validated biomarkers for the diagnosis and treatment of MDD. This study assessed serum amino acid metabolite changes between MDD patients and healthy controls (HCs) and their association with disease severity and diagnostic utility. In total, 70 MDD patients and 70 HCs matched in age, gender, and ethnicity were recruited for the study. For amino acid profiling, serum samples were analysed and quantified by liquid chromatography-mass spectrometry (LC-MS). Receiver-operating characteristic (ROC) curves were used to classify putative candidate biomarkers. MDD patients had significantly higher serum levels of glutamic acid, aspartic acid and glycine but lower levels of 3-Hydroxykynurenine; glutamic acid and phenylalanine levels also correlated with depression severity. Combining these four metabolites allowed for accurate discrimination of MDD patients and HCs, with 65.7% of depressed patients and 62.9% of HCs correctly classified. Glutamic acid, aspartic acid, glycine and 3-Hydroxykynurenine may serve as potential diagnostic biomarkers, whereas glutamic acid and phenylalanine may be markers for depression severity. To elucidate the association between these indicators and clinical features, it is necessary to conduct additional studies with larger sample sizes that involve a spectrum of depressive symptomatology.

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.

Folic Acid and Vitamin B12 Prevent Deleterious Effects of Rotenone on Object Novelty Recognition Memory and Kynu Expression in an Animal Model of Parkinson's Disease

Parkinson's disease (PD) is characterized by a range of motor signs, but cognitive dysfunction is also observed. Supplementation with folic acid and vitamin B12 is expected to prevent cognitive impairment. To test this in PD, we promoted a lesion within the substantia nigra pars compacta of rats using the neurotoxin rotenone. In the sequence, the animals were supplemented with folic acid and vitamin B12 for 14 consecutive days and subjected to the object recognition test. We observed an impairment in object recognition memory after rotenone administration, which was prevented by supplementation (p < 0.01). Supplementation may adjust gene expression through efficient DNA methylation. To verify this, we measured the expression and methylation of the kynureninase gene (Kynu), whose product metabolizes neurotoxic metabolites often accumulated in PD as kynurenine. Supplementation prevented the decrease in Kynu expression induced by rotenone in the substantia nigra (p < 0.05), corroborating the behavioral data. No differences were observed concerning the methylation analysis of two CpG sites in the Kynu promoter. Instead, we suggest that folic acid and vitamin B12 increased global DNA methylation, reduced the expression of Kynu inhibitors, maintained Kynu-dependent pathway homeostasis, and prevented the memory impairment induced by rotenone. Our study raises the possibility of adjuvant therapy for PD with folic acid and vitamin B12.

Forgetful, sad and old: Do vascular cognitive impairment and depression share a common pre-disease network and how is it impacted by ageing?

Vascular cognitive impairment (VCI) and depression frequently coexist in geriatric populations and reciprocally increase disease risks. We assert that a shared pre-disease state of the psycho-immune-neuroendocrine (PINE) network model mechanistically explains bidirectional associations between VCI and depression. Five pathophysiological sub-networks are identified that are shared by VCI and depression: neuroinflammation, kynurenine pathway imbalance, hypothalamic-pituitary-adrenal (HPA) axis overactivity, impaired neurotrophic support and cerebrovascular dysfunction. These do not act independently, and their complex interactions necessitate a systems biology approach to better define disease pathogenesis. The PINE network is already established in the context of non-communicable diseases (NCDs) such as depression, hypertension, atherosclerosis, coronary heart disease and type 2 diabetes mellitus. We build on previous literature to specifically explore mechanistic links between MDD and VCI in the context of PINE pathways and discuss key mechanistic commonalities linking these comorbid conditions and identify a common pre-disease state which precedes transition to VCI and MDD. We expand the model to incorporate bidirectional interactions with biological ageing. Diathesis factors for both VCI and depression feed into this network and the culmination of shared mechanisms (on an ageing substrate) lead to a critical network transition to one or both disease states. A common pre-disease state underlying VCI and depression can provide clinicians a unique opportunity for early risk assessment and intervention in disease development. Establishing the mechanistic elements and systems biology of this network can reveal early warning or predictive biomarkers together with novel therapeutic targets. Integrative studies are recommended to elucidate the dynamic networked biology of VCI and depression over time.

An integrated cytokine and kynurenine network as the basis of neuroimmune communication

Two of the molecular families closely associated with mediating communication between the brain and immune system are cytokines and the kynurenine metabolites of tryptophan. Both groups regulate neuron and glial activity in the central nervous system (CNS) and leukocyte function in the immune system, although neither group alone completely explains neuroimmune function, disease occurrence or severity. This essay suggests that the two families perform complementary functions generating an integrated network. The kynurenine pathway determines overall neuronal excitability and plasticity by modulating glutamate receptors and GPR35 activity across the CNS, and regulates general features of immune cell status, surveillance and tolerance which often involves the Aryl Hydrocarbon Receptor (AHR). Equally, cytokines and chemokines define and regulate specific populations of neurons, glia or immune system leukocytes, generating more specific responses within restricted CNS regions or leukocyte populations. In addition, as there is a much larger variety of these compounds, their homing properties enable the superimposition of dynamic variations of cell activity upon local, spatially limited, cell populations. This would in principle allow the targeting of potential treatments to restricted regions of the CNS. The proposed synergistic interface of 'tonic' kynurenine pathway affecting baseline activity and the superimposed 'phasic' cytokine system would constitute an integrated network explaining some features of neuroimmune communication. The concept would broaden the scope for the development of new treatments for disorders involving both the CNS and immune systems, with safer and more effective agents targeted to specific CNS regions.

Exploration for biomarkers of postpartum depression based on metabolomics: A systematic review

BACKGROUND

Postpartum depression (PPD) is the most frequent psychiatric complication during the postnatal period and its mechanisms are not fully understood. Metabolomics, can quantitatively measure metabolites in a high-throughput method, and thus uncover the underlying pathophysiology of disease.

OBJECTIVES

In this study, we reviewed metabolomics studies conducted on PPD, aiming to understand the changes of metabolites in PPD patients and analyze the potential application of metabolomics in PPD prediction and diagnosis.

METHODS

Relevant articles were searched in PubMed, Google scholar, and Web of Science databases from January 2011 to July 2022. The metabolites involved were systematically examined and compared. MetaboAnalyst online software was applied to analyze metabolic pathways.

RESULTS

A total of 14 papers were included in this study. There were several highly reported metabolites, such as kynurenine, kynurenic acid, and eicosapentaenoic acid. Dysregulation of metabolic pathways involved amino acids metabolism, fatty acids metabolism, and steroids metabolism.

LIMITATIONS

The included studies are relatively inadequate, and further work is needed.

CONCLUSIONS

This study summarized significant metabolic alterations that provided clues for the prediction, diagnosis, and pathogenesis of PPD.

The pathophysiology of major depressive disorder through the lens of systems biology: Network analysis of the psycho-immune-neuroendocrine physiome

BACKGROUND/AIMS

The psycho-immune-neuroendocrine (PINE) network is a predominantly physiological (metabolomic) model constructed from the literature, inter-linking multiple biological processes associated with major depressive disorder (MDD), thereby integrating putative mechanistic pathways for MDD into a single network.

MATERIAL AND METHODS

Previously published metabolomic pathways for the PINE network based on literature searches conducted in 1991-2021 were used to construct an edge table summarizing all physiological pathways in pairs of origin nodes and target nodes. The Gephi software program was used to calculate network metrics from the edge table, including total degree and centrality measures, to ascertain key network nodes and construct a directed network graph.

RESULTS

An edge table and directional network graph of physiological relationships in the PINE network is presented. The network has properties consistent with complex biological systems, with analysis yielding key network nodes comprising pro-inflammatory cytokines (TNF- α, IL6 and IL1), glucocorticoids and corticotropin releasing hormone (CRH). These may represent central structural and regulatory elements in the context of MDD.

CONCLUSION

The identified hubs have a high degree of connection and are known to play roles in the progression from health to MDD. These nodes represent strategic targets for therapeutic intervention or prevention. Future work is required to build a weighted and dynamic simulation of the network PINE.

cd1 Mutation in Drosophila Affects Phenoxazinone Synthase Catalytic Site and Impairs Long-Term Memory

Being involved in development of Huntington's, Parkinson's and Alzheimer's diseases, kynurenine pathway (KP) of tryptophan metabolism plays a significant role in modulation of neuropathology. Accumulation of a prooxidant 3-hydroxykynurenine (3-HOK) leads to oxidative stress and neuronal cell apoptosis. Drosophila mutant cardinal (cd¹) with 3-HOK excess shows age-dependent neurodegeneration and short-term memory impairments, thereby presenting a model for senile dementia. Although cd gene for phenoxazinone synthase (PHS) catalyzing 3-HOK dimerization has been presumed to harbor the cd¹ mutation, its molecular nature remained obscure. Using next generation sequencing, we have shown that the cd gene in cd¹ carries a long deletion leading to PHS active site destruction. Contrary to the wild type Canton-S (CS), cd¹ males showed defective long-term memory (LTM) in conditioned courtship suppression paradigm (CCSP) at days 5-29 after eclosion. The number of dopaminergic neurons (DAN) regulating fly locomotor activity showed an age-dependent tendency to decrease in cd¹ relative to CS. Thus, in accordance with the concept "from the gene to behavior" proclaimed by S. Benzer, we have shown that the aberrant PHS sequence in cd¹ provokes drastic LTM impairments and DAN alterations.

Is SARS-CoV-2 a Risk Factor of Bipolar Disorder?-A Narrative Review

For 2.5 years we have been facing the coronavirus disease (COVID-19) and its health, social and economic effects. One of its known consequences is the development of neuropsychiatric diseases such as anxiety and depression. However, reports of manic episodes related to COVID-19 have emerged. Mania is an integral part of the debilitating illness-bipolar disorder (BD). Due to its devastating effects, it is therefore important to establish whether SARS-CoV-2 infection is a causative agent of this severe mental disorder. In this narrative review, we discuss the similarities between the disorders caused by SARS-CoV-2 and those found in patients with BD, and we also try to answer the question of whether SARS-CoV-2 infection may be a risk factor for the development of this affective disorder. Our observation shows that disorders in COVID-19 showing the greatest similarity to those in BD are cytokine disorders, tryptophan metabolism, sleep disorders and structural changes in the central nervous system (CNS). These changes, especially intensified in severe infections, may be a trigger for the development of BD in particularly vulnerable people, e.g., with family history, or cause an acute episode in patients with a pre-existing BD.

Dynamic changes in metabolites of the kynurenine pathway in Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease: A systematic Review and meta-analysis

Background

Tryptophan (TRP) is an essential amino acid that must be provided in the diet. The kynurenine pathway (KP) is the main route of TRP catabolism into nicotinamide adenosine dinucleotide (NAD⁺), and metabolites of this pathway may have protective or degenerative effects on the nervous system. Thus, the KP may be involved in neurodegenerative diseases.

Objectives

The purpose of this systematic review and meta-analysis is to assess the changes in KP metabolites such as TRP, kynurenine (KYN), kynurenic acid (KYNA), Anthranilic acid (AA), 3-hydroxykynurenine (3-HK), 5-Hydroxyindoleacetic acid (5-HIAA), and 3-Hydroxyanthranilic acid (3-HANA) in Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD) patients compared to the control group.

Methods

We conducted a literature search using PubMed/Medline, Scopus, Google Scholar, Web of Science, and EMBASE electronic databases to find articles published up to 2022. Studies measuring TRP, KYN, KYNA, AA, 3-HK, 5-HIAA, 3-HANA in AD, PD, or HD patients and controls were identified. Standardized mean differences (SMDs) were used to determine the differences in the levels of the KP metabolites between the two groups.

Results

A total of 30 studies compromising 689 patients and 774 controls were included in our meta-analysis. Our results showed that the blood levels of TRP was significantly lower in the AD (SMD=-0.68, 95% CI=-0.97 to -0.40, p=0.000, I2 = 41.8%, k=8, n=382), PD (SMD=-0.77, 95% CI=-1.24 to -0.30, p=0.001, I2 = 74.9%, k=4, n=352), and HD (SMD=-0.90, 95% CI=-1.71 to -0.10, p=0.028, I2 = 91.0%, k=5, n=369) patients compared to the controls. Moreover, the CSF levels of 3-HK in AD patients (p=0.020) and the blood levels of KYN in HD patients (p=0.020) were lower compared with controls.

Conclusion

Overall, the findings of this meta-analysis support the hypothesis that the alterations in the KP may be involved in the pathogenesis of AD, PD, and HD. However, additional research is needed to show whether other KP metabolites also vary in AD, PD, and HD patients. So, the metabolites of KP can be used for better diagnosing these diseases.

Impact of amino acids on microglial activation and CNS remyelination

Amino acids and their derivatives function as building blocks as well as signaling molecules to modulate various cellular processes in living organisms. In mice, amino acids accumulate in demyelinated lesions and return to basal levels during remyelination. Studies have found that amino acids and their metabolites modulate immune activity in the central nervous system (CNS) and influence oligodendrocyte differentiation and remyelination efficiency. In this review, we discuss current studies on amino acid metabolism in the context of CNS remyelination. By understanding the mechanisms of amino acid signaling and metabolism in demyelinated lesions, we may deepen our understanding of compartmentalized CNS inflammation in demyelinating disease like multiple sclerosis (MS) and provide evidence to develop novel pharmacological therapies targeting amino acid metabolism to prevent disease worsening.

Host-microbiota interactions: The aryl hydrocarbon receptor in the acute and chronic phases of cerebral ischemia

The relationship between gut microbiota and brain function has been studied intensively in recent years, and gut microbiota has been linked to a couple of neurological disorders including stroke. There are multiple studies linking gut microbiota to stroke in the “microbiota-gut-brain” axis. The aryl hydrocarbon receptor (AHR) is an important mediator of acute ischemic damage and can result in subsequent neuroinflammation. AHR can affect these responses by sensing microbiota metabolites especially tryptophan metabolites and is engaged in the regulation of acute ischemic brain injury and chronic neuroinflammation after stroke. As an important regulator in the “microbiota-gut-brain” axis, AHR has the potential to be used as a new therapeutic target for ischemic stroke treatment. In this review, we discuss the research progress on AHR regarding its role in ischemic stroke and prospects to be used as a therapeutic target for ischemic stroke treatment, aiming to provide a potential direction for the development of new treatments for ischemic stroke.

Biochemical Evolution of a Potent Target of Mosquito Larvicide, 3-Hydroxykynurenine Transaminase

A specific mosquito enzyme, 3-hydroxykynurenine transaminase (HKT), is involved in the processing of toxic metabolic intermediates of the tryptophan metabolic pathway. The HKT enzymatic product, xanthurenic acid, is required for Plasmodium spp. development in the mosquito vectors. Therefore, an inhibitor of HKT may not only be a mosquitocide but also a malaria-transmission blocker. In this work, we present a study investigating the evolution of HKT, which is a lineage-specific duplication of an alanine glyoxylate aminotransferases (AGT) in mosquitoes. Synteny analyses, together with the phylogenetic history of the AGT family, suggests that HKT and the mosquito AGTs are paralogous that were formed via a duplication event in their common ancestor. Furthermore, 41 amino acid sites with significant evidence of positive selection were identified, which could be responsible for biochemical and functional evolution and the stability of conformational stabilization. To get a deeper understanding of the evolution of ligands’ capacity and the ligand-binding mechanism of HKT, the sequence and the 3D homology model of the common ancestor of HKT and AGT in mosquitoes, ancestral mosquito AGT (AncMosqAGT), were inferred and built. The homology model along with 3-hydroxykynurenine, kynurenine, and alanine were used in docking experiments to predict the binding capacity and ligand-binding mode of the new substrates related to toxic metabolites detoxification. Our study provides evidence for the dramatic biochemical evolution of the key detoxifying enzyme and provides potential sites that could hinder the detoxification function, which may be used in mosquito larvicide and design.

Involvement of kynurenine pathway and N-methyl-d-aspartate receptors in the antidepressant-like effect of vilazodone in the tail suspension test in mice

The present study evaluated the antidepressant-like effects of vilazodone using the tail suspension test in mice. We also investigated the contribution of kynurenine pathway and N-methyl-d-aspartate receptors to this effect. For this purpose, we pretreated animals with sub-effective doses of L-kynurenine, 3-hydroxykynurenine, or quinolinic acid. We then assessed the immobility time, an indicative measure of depressive-like behavior, in the tail suspension test. We also evaluated the possible effects of sub-effective doses of vilazodone combined with sub-effective doses of ketamine (N-methyl-d-aspartate receptor antagonist) in a separate group. Vilazodone (3mg/kg, intraperitoneal) significantly reduced immobility time in the tail suspension test. L-kynurenine (1.7 mg/kg, intraperitoneal), 3-hydroxykynurenine (10 mg/kg, intraperitoneal), and quinolinic acid (3 nmol/site, intracerebroventricular) significantly increased the immobility time in the tail suspension test. The antidepressant-like effects of vilazodone (3mg/kg, intraperitoneal) were inhibited by pre-treatment with non-effective doses of L-kynurenine (0.83 mg/kg, intraperitoneal), 3-hydroxykynurenine (3.33 mg/kg, intraperitoneal), or quinolinic acid (1 nmol/site, intracerebroventricular). Pretreatment of mice with sub-effective doses of ketamine (1 mg/kg, intraperitoneal) optimized the action of a sub-effective dose of vilazodone (0.3mg/kg, intraperitoneal) and reduced the immobility time in the tail suspension test. None of the drugs used in this study induced any changes in locomotor activity in the open field test. The results showed that vilazodone induced an antidepressant-like effect in the tail suspension test, which may be mediated through an interaction with the kynurenine pathway and N-methyl-d-aspartate receptors.

The tryptophan catabolite or kynurenine pathway in COVID-19 and critical COVID-19: a systematic review and meta-analysis

BACKGROUND

Coronavirus disease 2019 (COVID-19) is accompanied by activated immune-inflammatory pathways and oxidative stress, which both induce indoleamine-2,3-dioxygenase (IDO), a key enzyme of the tryptophan (TRP) catabolite (TRYCAT) pathway. The aim of this study was to systematically review and meta-analyze the status of the TRYCAT pathway, including the levels of TRP and kynurenine (KYN) and the activity of IDO, as measured by the ratio of KYN/TRP.

METHODS

This systematic review searched PubMed, Google Scholar, and Web of Sciences and included 14 articles that compared TRP and tryptophan catabolites (TRYCATs) in COVID-19 patients versus non-COVID-19 controls, as well as severe/critical versus mild/moderate COVID-19. The analysis was done on a total of 1269 people, including 794 COVID-19 patients and 475 controls.

RESULTS

The results show a significant (p < 0.0001) increase in the KYN/TRP ratio (standardized mean difference, SMD = 1.099, 95% confidence interval, CI: 0.714; 1.484) and KYN (SMD = 1.123, 95% CI: 0.730; 1.516) and significantly lower TRP (SMD = - 1.002, 95%CI: - 1.738; - 0.266) in COVID-19 versus controls. The KYN/TRP ratio (SMD = 0.945, 95%CI: 0.629; 1.262) and KYN (SMD = 0.806, 95%CI: 0.462; 1.149) were also significantly (p < 0.0001) higher and TRP lower (SMD = - 0.909, 95% CI: - 1.569; - 0.249) in severe/critical versus mild/moderate COVID-19. No significant difference was detected in kynurenic acid (KA) and the KA/KYN ratio between COVID-19 patients and controls.

CONCLUSIONS

Our results indicate increased activity of the IDO enzyme in COVID-19 and severe/critical patients. The TRYCAT pathway is implicated in the pathophysiology and progression of COVID-19 and may signal a worsening outcome of the disease.

The Tryptophan Catabolite or Kynurenine Pathway in Alzheimer's Disease: A Systematic Review and Meta-Analysis

BACKGROUND

Alzheimer's disease (AD), which is characterized by progressive brain dysfunction and memory loss, is one of the most significant global health concerns for older adults. Neuroinflammation and increased oxidative stress contribute to the pathophysiology of AD, thereby presumably inducing tryptophan (TRP) degradation through the TRP catabolite (TRYCAT) pathway.

OBJECTIVE

To delineate the activity of the TRYCAT pathway along with levels of TRP and tryptophan catabolites (TRYCATs) in AD patients.

METHODS

We used PubMed, Google Scholar, Web of Science, and SciFinder during the month of January 2022 to gather the pertinent publications. We found 19 eligible articles which involved 738 patients and 665 healthy controls.

RESULTS

Our results revealed a significant difference (p = 0.008) in the kynurenine (KYN)/TRP ratio (standardized mean difference, SMD = 0.216, 95% confidence interval, CI: 0.057; 0.376), and a significant decrease in TRP in AD patients (SMD = -0.520, 95% CI: -0.738; -0.302, p < 0.0001). Moreover, we also found a significant increase in the central nervous system (CNS), brain, and cerebrospinal fluid kynurenic acid (KA)/KYN ratio but not in peripheral blood, as well as a significant decrease in plasma KA and xanthurenic acid in the CNS and blood.

CONCLUSION

AD is characterized by TRP depletion but not by an overactivity of the TRYCAT pathway. IDO-induced production of neurotoxic TRYCATs is not a key factor in the pathophysiology of AD.

The Kynurenine Pathway in Obese Middle-Aged Women with Normoglycemia and Type 2 Diabetes

We examined the relationships of tryptophan (Trp) and the metabolites of the kynurenine pathway (KP) to the occurrence of type 2 diabetes (T2D) and metabolic risk factors in obese middle-aged women. The study included 128 obese women divided into two subgroups: a normoglycemic group (NG, n = 65) and a T2D group (n = 63). The concentrations of serum tryptophan (Trp), kynurenine (Kyn), 3-hydroxykynurenine (3HKyn), quinolinic acid (QA), and kynurenic acid (Kyna) were analyzed using ultra-high-performance liquid chromatography coupled with electrospray ionization/triple quadrupole mass spectrometry. Blood biochemical parameters and anthropometric parameters were measured. The women with T2D had significantly higher Trp, Kyna, Kyna/QA ratio, and Kyna/3HKyn ratio values than the NG women. Logistic regression analysis showed that the concentrations of Trp and Kyna and the values of the Kyna/3HKyn ratio were most strongly associated with T2D occurrence, even after controlling for confounding factors. The model with Trp level and Kyna/3HKyn ratio accounted for 20% of the variation in the presence of T2D. We also showed a different pattern of correlations between kynurenines and metabolic factors in the NG and T2D women, which was mostly reflected in the stronger relationship between BMI and KP metabolites in the NG obese women. An increase in Trp and Kyna levels with an accompanying increase in Kyna/3HKyn ratio value is associated with the occurrence of T2D in obese middle-aged women.

Unexpected content of kynurenine in mother's milk and infant formulas

Mother's milk is widely recommended as complete food for the offspring in earliest postnatal time. However, the knowledge about detailed composition and the physiological role of bioactive components of breast milk is incomplete. Therefore, the aim of our study was to determine the content of kynurenine (KYN) in human breast milk during lactation and to explore the effects exerted by intragastric KYN administration from birth to weaning on physical and psychomotor development of adult rats. We found that KYN is consistently present in human milk and its content gradually increased from day 4 to 28 after delivery and that it is present in commercial baby formulas in amounts noticeably exceeding its physiological range. Animal studies showed that KYN supplementation resulted in a marked elevation of absorptive surface of rat intestine and in enhanced expression of both, aryl hydrocarbon receptor and G protein-coupled receptor 35 in the intestinal tissue in rats. Moreover, we discovered that KYN administration from birth to weaning resulted in neurobehavioral changes in adult rats. Therefore, we postulate that further research is required to thoroughly understand the function of KYN in early developmental stages of mammals and to ensure the safety of its presence in baby food products.

Role of indoleamine 2,3-dioxygenase 1 (IDO1) and kynurenine pathway in the regulation of the aging process

Indoleamine 2,3-dioxygenase 1 (IDO1) is activated in chronic inflammatory states, e.g., in the aging process and age-related diseases. IDO1 enzyme catabolizes L-tryptophan (L-Trp) into kynurenine (KYN) thus stimulating the KYN pathway. The depletion of L-Trp inhibits the proliferation of immune cells in inflamed tissues and it also reduces serotonin synthesis predisposing to psychiatric disorders. Interestingly, IDO1 protein contains two immunoreceptor tyrosine-based inhibitory motifs (ITIM) which trigger suppressive signaling through the binding of PI3K p110 and SHP-1 proteins. This immunosuppressive activity is not dependent on the catalytic activity of IDO1. KYN and its metabolite, kynurenic acid (KYNA), are potent activators of the aryl hydrocarbon receptor (AhR) which can enhance immunosuppression. IDO1-KYN-AhR signaling counteracts excessive pro-inflammatory responses in acute inflammation but in chronic inflammatory states it has many harmful effects. A chronic low-grade inflammation is associated with the aging process, a state called inflammaging. There is substantial evidence that the activation of the IDO1-KYN-AhR pathway robustly increases with the aging process. The activation of IDO1-KYN-AhR signaling does not only suppress the functions of effector immune cells, probably promoting immunosenescence, but it also impairs autophagy, induces cellular senescence, and remodels the extracellular matrix as well as enhancing the development of osteoporosis and vascular diseases. I will review the function of IDO1-KYN-AhR signaling and discuss its activation with aging as an enhancer of the aging process.

Metabolic Reprogramming, Gut Dysbiosis, and Nutrition Intervention in Canine Heart Disease

This review provides a state-of-the-art overview on recent advances in systems biology in canine cardiac disease, with a focus on our current understanding of bioenergetics and amino acid metabolism in myxomatous mitral valve disease (MMVD). Cross-species comparison is drawn to highlight the similarities between human and canine heart diseases. The adult mammalian heart exhibits a remarkable metabolic flexibility and shifts its energy substrate preference according to different physiological and pathological conditions. The failing heart suffers up to 40% ATP deficit and is compared to an engine running out of fuel. Bioenergetics and metabolic readaptations are among the major research topics in cardiac research today. Myocardial energy metabolism consists of three interconnected components: substrate utilization, oxidative phosphorylation, and ATP transport and utilization. Any disruption or uncoupling of these processes can result in deranged energy metabolism leading to heart failure (HF). The review describes the changes occurring in each of the three components of energy metabolism in MMVD and HF. It also provides an overview on the changes in circulating and myocardial glutathione, taurine, carnitines, branched-chain amino acid catabolism and tryptophan metabolic pathways. In addition, the review summarizes the potential role of the gut microbiome in MMVD and HF. As our knowledge and understanding in these molecular and metabolic processes increase, it becomes possible to use nutrition to address these changes and to slow the progression of the common heart diseases in dogs.

Activation of the Kynurenine Pathway and Production of Inflammatory Cytokines by Astrocytes and Microglia Infected With Neospora caninum

In the central nervous system, astrocytes and microglia contribute to homeostasis, regulating the immune response to infectious agents. Neospora caninum is an obligate intracellular protozoan that infects different animal species and it is encysted in their nervous tissue while triggering an immune response modulated by glia. This study aimed to evaluate the infection of primary cultures of rat glial cells by N. caninum through the catabolites of tryptophan, the expression of inflammatory mediators and the integrity of neural tissue. Infection with this coccidium resulted in morphological and functional changes, particularly astrogliosis and microgliosis, and increased the expression of the inflammatory mediators TNF, IL1β, IL-10, and arginase, as well as mRNA for CCL5 and CCL2, molecules involved in the CNS chemotaxis. The infection with N. caninum in glial cells also triggered the activation of the tryptophan pathway, characterized by increased kynurenine 2,3 monooxygenase (KMO) mRNA expression, and by the production of the excitotoxin quinolinic acid (QUIN). Moreover, glia-neuron co-cultures, when exposed to the secretome derived from N. caninum infected glial cells, presented greater neurons distribution and formation of neurite extensions, associated to morphological changes in astrocytes compatible with neuro-preservation. Considering that the tryptophan catabolism is associated to immune response, these findings suggest that glial activation in N. caninum infection should be responsible for modulating the inflammatory status in an attempt to restore the nervous system homeostasis, since excessive inflammatory response can cause irreversible damage to tissue preservation.

3-Hydroxykynurenine as a Potential Ligand for Hsp70 Proteins and Its Effects on Drosophila Memory After Heat Shock

Kynurenine products of tryptophan metabolism are modifiers of the nervous activity and oxidative processes in mammals and invertebrates. 3-Hydroxykynurenine (3HOK) in moderate concentrations is a lipid peroxidation inhibitor. However, its accumulation and oxidative auto-dimerization lead to oxidative stress development manifested in age-related neurodegenerative diseases (NDD) and neurological disorders provoked by acute stress. Different forms of stress, the mostly studied being heat shock response, rely on functioning of heat shock proteins of the Hsp70 superfamily. Since kynurenines are called "kids of stress," we performed computational estimation of affinity of 3HOK and other kynurenines binding to predicted ATP site of Drosophila melanogaster Hsp cognate 71 protein (Dhsp71) using AutoDock Vina. The binding energy of 3HOK dimer is - 9.4 kcal/mol; its orientation within the active site is close to that of ATP. This might be a new mechanism of producing a competitive inhibitor of Hsp70 chaperones that decreases organism ability to adapt to heat shock. We also showed that the Drosophila cardinal (cd¹) mutant with 3HOK excess, serving as a model for Huntington's disease (HD), manifests severe defects of short-term memory after heat shock applied either in adults or at the prepupal stage.

Small Molecule Regulators Targeting NAD+ Biosynthetic Enzymes

Nicotinamide adenine dinucleotide (NAD+) is a key player in many metabolic pathways as an activated carrier of electrons. In addition to being the cofactor for redox reactions, NAD+ also serves as the substrate for various enzymatic transformations such as adenylation and ADP-ribosylation. Maintaining cellular NAD+ homeostasis has been suggested as an effective anti-aging strategy. Given the importance of NAD+ in regulating a broad spectrum of cellular events, small molecules targeting NAD+ metabolism have been pursued as therapeutic interventions for the treatment of mitochondrial disorders and agerelated diseases. In this article, small molecule regulators of NAD+ biosynthetic enzymes will be reviewed. The focus will be given to the discovery and development of these molecules, the mechanism of action as well as their therapeutic potentials.

Probiotic Releasing Angiotensin (1-7) in a Drosophila Model of Alzheimer's Disease Produces Sex-Specific Effects on Cognitive Function

BACKGROUND

While extensive research on the brain has failed to identify effective therapies, using probiotics to target the gut microbiome has shown therapeutic potential in Alzheimer's disease (AD). Genetically modified probiotics (GMP) are a promising strategy to deliver key therapeutic peptides with high efficacy and tissue specificity. Angiotensin (Ang)-(1-7) levels inversely correlate to AD severity, but its administration is challenging. Our group has successfully established a GMP-based method of Ang-(1-7) delivery.

OBJECTIVE

Since Drosophila represents an excellent model to study the effect of probiotics on complex disorders in a high throughput manner, we tested whether oral supplementation with Lactobacillus paracasei releasing Ang-(1-7) (LP-A) delays memory loss in a Drosophila AD model.

METHODS

Flies overexpressing the human amyloid-β protein precursor and its β-site cleaving enzyme in neurons were randomized to receive four 24-h doses of Lactobacillus paracasei alone (LP), LP-A or sucrose over 14 days. Memory was assessed via an aversive phototaxic suppression assay.

RESULTS

Optimal dilution,1:2, was determined based on palatability. LP-A improved memory in trained AD males but worsened cognition in AD females. LP-supplementation experiments confirmed that Ang-(1-7) conferred additional cognitive benefits in males and was responsible for the deleterious cognitive effects in females. Sex-specific differences in the levels of angiotensin peptides and differential activation of the kynurenine pathway of tryptophan metabolism in response to supplementation may underlie this male-only therapeutic response.

CONCLUSION

In summary, LP-A ameliorated the memory deficits of a Drosophila AD model, but effects were sex-specific. Dosage optimization may be required to address this differential response.

Associations between Melatonin, Neuroinflammation, and Brain Alterations in Depression

Pro-inflammatory systemic conditions that can cause neuroinflammation and subsequent alterations in brain regions involved in emotional regulation have been suggested as an underlying mechanism for the pathophysiology of major depressive disorder (MDD). A prominent feature of MDD is disruption of circadian rhythms, of which melatonin is considered a key moderator, and alterations in the melatonin system have been implicated in MDD. Melatonin is involved in immune system regulation and has been shown to possess anti-inflammatory properties in inflammatory conditions, through both immunological and non-immunological actions. Melatonin has been suggested as a highly cytoprotective and neuroprotective substance and shown to stimulate all stages of neuroplasticity in animal models. The ability of melatonin to suppress inflammatory responses through immunological and non-immunological actions, thus influencing neuroinflammation and neurotoxicity, along with subsequent alterations in brain regions that are implicated in depression, can be demonstrated by the antidepressant-like effects of melatonin. Further studies that investigate the associations between melatonin, immune markers, and alterations in the brain structure and function in patients with depression could identify potential MDD biomarkers.

Recent advances in clinical trials targeting the kynurenine pathway

The kynurenine pathway (KP) is the major catabolic pathway for the essential amino acid tryptophan leading to the production of nicotinamide adenine dinucleotide. In inflammatory conditions, the activation of the KP leads to the production of several bioactive metabolites including kynurenine, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, kynurenic acid and quinolinic acid. These metabolites can have redox and immune suppressive activity, be neurotoxic or neuroprotective. While the activity of the pathway is tightly regulated under normal physiological condition, it can be upregulated by immunological activation and inflammation. The dysregulation of the KP has been implicated in wide range of neurological diseases and psychiatric disorders. In this review, we discuss the mechanisms involved in KP-mediated neurotoxicity and immune suppression, and its role in diseases of our expertise including cancer, chronic pain and multiple sclerosis. We also provide updates on the clinical trials evaluating the efficacy of KP inhibitors and/or analogues in each respective disease.

Predicting lupus membranous nephritis using reduced picolinic acid to tryptophan ratio as a urinary biomarker

The current gold standard for classifying lupus nephritis (LN) progression is a renal biopsy, which is an invasive procedure. Undergoing a series of biopsies for monitoring disease progression and treatments is unlikely suitable for patients with LN. Thus, there is an urgent need for non-invasive alternative biomarkers that can facilitate LN class diagnosis. Such biomarkers will be very useful in guiding intervention strategies to mitigate or treat patients with LN. Urine samples were collected from two independent cohorts. Patients with LN were classified into proliferative (class III/IV) and membranous (class V) by kidney histopathology. Metabolomics was performed to identify potential metabolites, which could be specific for the classification of membranous LN. The ratio of picolinic acid (Pic) to tryptophan (Trp) ([Pic/Trp] ratio) was found to be a promising candidate for LN diagnostic and membranous classification. It has high potential as an alternative biomarker for the non-invasive diagnosis of LN.

The Kynurenine Pathway as a Potential Target for Neuropathic Pain Therapy Design: From Basic Research to Clinical Perspectives

Accumulating evidence suggests the key role of the kynurenine pathway (KP) of the tryptophan metabolism in the pathogenesis of several diseases. Despite extensive research aimed at clarifying the mechanisms underlying the development and maintenance of neuropathic pain, the roles of KP metabolites in this process are still not fully known. Although the function of the peripheral KP has been known for several years, it has only recently been acknowledged that its metabolites within the central nervous system have remarkable consequences related to physiology and behavior. Both the products and metabolites of the KP are involved in the pathogenesis of pain conditions. Apart from the neuroactive properties of kynurenines, the KP regulates several neurotransmitter systems in direct or indirect ways. Some neuroactive metabolites are known to have neuroprotective properties (kynurenic acid, nicotinamide adenine dinucleotide cofactor), while others are toxic (3-hydroxykynurenine, quinolinic acid). Numerous animal models show that modulation of the KP may turn out to be a viable target for the treatment of diseases. Importantly, some compounds that affect KP enzymes are currently described to possess analgesic properties. Additionally, kynurenine metabolites may be useful for assessing response to therapy or as biomarkers in therapeutic monitoring. The following review describes the molecular site of action and changes in the levels of metabolites of the kynurenine pathway in the pathogenesis of various conditions, with a particular emphasis on their involvement in neuropathy. Moreover, the potential clinical implications of KP modulation in chronic pain therapy as well as the directions of new research initiatives are discussed.

Does the type of exercise affect tryptophan catabolism in horses?

Tryptophan (Trp) is an essential amino acid which metabolises via the kynurenine pathway to generate a number of bioactive substances referred to as kynurenines. Among those are 3-hydroxykynurenine (3-HKyn) and quinolinic acid, which are neurotoxic, as well as kynurenic acid (Kyna) and xanthurenic acid (XA), which, similarly to nicotinamide (NAm), show neuroprotective and anti-depressive effects. Routine exercise is known to modulate Trp metabolism in skeletal muscle and is thus believed to reduce the risk of depressive states in humans and laboratory animals. Analogously, it was hypothesised that exercise can influence Trp metabolism in horses as well. The aim of this study was to evaluate the influence of two different types of exercise on Trp metabolism in horses of the same breed. A total of 32 purebred Arabian horses were involved in the study. The 22 three-year-old racehorses were subjected to short-time intense exercise. Ten other horses were made to perform endurance competitions at a distance of 80 km. Blood samples were collected at rest and following the end of the exercise period. Plasma concentrations of Trp, kynurenine (Kyn), Kyna, 3-HKyn, XA and NAm were determined using Ultra-High Performance Liquid Chromatography-Electrospray Ionisation-Tandem Mass Spectrometry. Short-time intense exercise led to an increase in plasma concentrations of Kyn, Kyna and XA. The endurance effort induced an increase in Kyna and a decrease in Trp and NAm levels. Both types of exercise, short-time intensive exercise and endurance exercise induced an increase in Trp metabolites, especially Kyna, and did not induce an increase in Trp level. Thus, from a pathophysiological perspective of the kynurenine pathway's influence on mental state, both types of exercise induced beneficial effects in horses.