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Tryptophan Metabolism and Gut-Brain Homeostasis.
Roth, W, Zadeh, K, Vekariya, R, Ge, Y, Mohamadzadeh, M
International journal of molecular sciences. 2021;(6)
Abstract
Tryptophan is an essential amino acid critical for protein synthesis in humans that has emerged as a key player in the microbiota-gut-brain axis. It is the only precursor for the neurotransmitter serotonin, which is vital for the processing of emotional regulation, hunger, sleep, and pain, as well as colonic motility and secretory activity in the gut. Tryptophan catabolites from the kynurenine degradation pathway also modulate neural activity and are active in the systemic inflammatory cascade. Additionally, tryptophan and its metabolites support the development of the central and enteric nervous systems. Accordingly, dysregulation of tryptophan metabolites plays a central role in the pathogenesis of many neurologic and psychiatric disorders. Gut microbes influence tryptophan metabolism directly and indirectly, with corresponding changes in behavior and cognition. The gut microbiome has thus garnered much attention as a therapeutic target for both neurologic and psychiatric disorders where tryptophan and its metabolites play a prominent role. In this review, we will touch upon some of these features and their involvement in health and disease.
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Tryptophan metabolism in brain tumors - IDO and beyond.
Platten, M, Friedrich, M, Wainwright, DA, Panitz, V, Opitz, CA
Current opinion in immunology. 2021;:57-66
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Abstract
Metabolism of the essential amino acid tryptophan is a key metabolic pathway that restricts antitumor immunity and is a drug development target for cancer immunotherapy. Tryptophan metabolism is active in brain tumors including gliomas and promotes a malignant phenotype and contributes to the immunosuppressive tumor microenvironment. In recent years, improved understanding of the regulation and downstream function of tryptophan metabolism has been significantly expanded beyond the initial in vitro observation that the enzyme indoleamine-2,3-dioxygenase 1 (IDO1) promotes the depletion of intracellular tryptophan. Here, we revisit the specific roles of tryptophan metabolites in regulating brain functioning and neuronal integrity as well as in the context of brain tumors. This review summarizes recent developments in identifying key regulators, as well as the cellular and molecular effects of tryptophan metabolism with a particular focus on potential therapeutic targets in glioma.
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Targeting Dietary and Microbial Tryptophan-Indole Metabolism as Therapeutic Approaches to Colon Cancer.
Wyatt, M, Greathouse, KL
Nutrients. 2021;(4)
Abstract
Tryptophan metabolism, via the kynurenine (Kyn) pathway, and microbial transformation of tryptophan to indolic compounds are fundamental for host health; both of which are altered in colon carcinogenesis. Alterations in tryptophan metabolism begin early in colon carcinogenesis as an adaptive mechanism for the tumor to escape immune surveillance and metastasize. The microbial community is a key part of the tumor microenvironment and influences cancer initiation, promotion and treatment response. A growing awareness of the impact of the microbiome on tryptophan (Trp) metabolism in the context of carcinogenesis has prompted this review. We first compare the different metabolic pathways of Trp under normal cellular physiology to colon carcinogenesis, in both the host cells and the microbiome. Second, we review how the microbiome, specifically indoles, influence host tryptophan pathways under normal and oncogenic metabolism. We conclude by proposing several dietary, microbial and drug therapeutic modalities that can be utilized in combination to abrogate tumorigenesis.
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Tryptophan: A Unique Role in the Critically Ill.
Kanova, M, Kohout, P
International journal of molecular sciences. 2021;(21)
Abstract
Tryptophan is an essential amino acid whose metabolites play key roles in diverse physiological processes. Due to low reserves in the body, especially under various catabolic conditions, tryptophan deficiency manifests itself rapidly, and both the serotonin and kynurenine pathways of metabolism are clinically significant in critically ill patients. In this review, we highlight these pathways as sources of serotonin and melatonin, which then regulate neurotransmission, influence circadian rhythm, cognitive functions, and the development of delirium. Kynurenines serve important signaling functions in inter-organ communication and modulate endogenous inflammation. Increased plasma kynurenine levels and kynurenine-tryptophan ratios are early indicators for the development of sepsis. They also influence the regulation of skeletal muscle mass and thereby the development of polyneuromyopathy in critically ill patients. The modulation of tryptophan metabolism could help prevent and treat age-related disease with low grade chronic inflammation as well as post intensive care syndrome in all its varied manifestations: cognitive decline (including delirium or dementia), physical impairment (catabolism, protein breakdown, loss of muscle mass and tone), and mental impairment (depression, anxiety or post-traumatic stress disorder).
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Tryptophan: From Diet to Cardiovascular Diseases.
Melhem, NJ, Taleb, S
International journal of molecular sciences. 2021;(18)
Abstract
Cardiovascular disease (CVD) is one of the major causes of mortality worldwide. Inflammation is the underlying common mechanism involved in CVD. It has been recently related to amino acid metabolism, which acts as a critical regulator of innate and adaptive immune responses. Among different metabolites that have emerged as important regulators of immune and inflammatory responses, tryptophan (Trp) metabolites have been shown to play a pivotal role in CVD. Here, we provide an overview of the fundamental aspects of Trp metabolism and the interplay between the dysregulation of the main actors involved in Trp metabolism such as indoleamine 2, 3-dioxygenase 1 (IDO) and CVD, including atherosclerosis and myocardial infarction. IDO has a prominent and complex role. Its activity, impacting on several biological pathways, complicates our understanding of its function, particularly in CVD, where it is still under debate. The discrepancy of the observed IDO effects could be potentially explained by its specific cell and tissue contribution, encouraging further investigations regarding the role of this enzyme. Thus, improving our understanding of the function of Trp as well as its derived metabolites will help to move one step closer towards tailored therapies aiming to treat CVD.
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Tryptophan Metabolism and Related Pathways in Psychoneuroimmunology: The Impact of Nutrition and Lifestyle.
Gostner, JM, Geisler, S, Stonig, M, Mair, L, Sperner-Unterweger, B, Fuchs, D
Neuropsychobiology. 2020;(1):89-99
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Abstract
In the past, accelerated tryptophan breakdown was considered to be a feature of clinical conditions, such as infection, inflammation, and malignant disease. More recently, however, the focus has changed to include the additional modulation of tryptophan metabolism by changes in nutrition and microbiota composition. The regulation of tryptophan concentration is critical for the maintenance of systemic homeostasis because it integrates essential pathways involved in nutrient sensing, metabolic stress response, and immunity. In addition to tryptophan being important as a precursor for the synthesis of the neurotransmitter serotonin, several catabolites along the kynurenine axis are neuroactive. This emphasizes the importance of the immunometabolic fate of this amino acid for processes relevant to neuropsychiatric symptoms. In humans, besides hepatic catabolism, there is usually a strong relationship between immune activation-associated tryptophan breakdown and increased levels of biomarkers, such as neopterin, which has particular relevance for both acute and chronic diseases. A shift towards neopterin synthesis during oxidative stress may indicate a corresponding decrease in tetrahydrobiopterin, a cofactor of several mono-oxygenases, providing a further link between tryptophan metabolism and serotonergic and catecholaminergic neurotransmission. The psychoneuroimmunological consequences of tryptophan metabolism and the susceptibility of this pathway to modulation by a variety of nutritional and lifestyle-related factors have important implications for the development of both diagnostic and treatment options.
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Nutritional Therapy to Modulate Tryptophan Metabolism and Aryl Hydrocarbon-Receptor Signaling Activation in Human Diseases.
Ghiboub, M, Verburgt, CM, Sovran, B, Benninga, MA, de Jonge, WJ, Van Limbergen, JE
Nutrients. 2020;(9)
Abstract
The aryl hydrocarbon receptor (AhR) is a nuclear protein which, upon association with certain endogenous and exogenous ligands, translocates into the nucleus, binds DNA and regulates gene expression. Tryptophan (Trp) metabolites are one of the most important endogenous AhR ligands. The intestinal microbiota is a critical player in human intestinal homeostasis. Many of its effects are mediated by an assembly of metabolites, including Trp metabolites. In the intestine, Trp is metabolized by three main routes, leading to kynurenine, serotonin, and indole derivative synthesis under the direct or indirect involvement of the microbiota. Disturbance in Trp metabolism and/or AhR activation is strongly associated with multiple gastrointestinal, neurological and metabolic disorders, suggesting Trp metabolites/AhR signaling modulation as an interesting therapeutic perspective. In this review, we describe the most recent advances concerning Trp metabolism and AhR signaling in human health and disease, with a focus on nutrition as a potential therapy to modulate Trp metabolites acting on AhR. A better understanding of the complex balance between these pathways in human health and disease will yield therapeutic opportunities.
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Gut Microbiota Regulation of Tryptophan Metabolism in Health and Disease.
Agus, A, Planchais, J, Sokol, H
Cell host & microbe. 2018;(6):716-724
Abstract
The gut microbiota is a crucial actor in human physiology. Many of these effects are mediated by metabolites that are either produced by the microbes or derived from the transformation of environmental or host molecules. Among the array of metabolites at the interface between these microorganisms and the host is the essential aromatic amino acid tryptophan (Trp). In the gut, the three major Trp metabolism pathways leading to serotonin (5-hydroxytryptamine), kynurenine (Kyn), and indole derivatives are under the direct or indirect control of the microbiota. In this review, we gather the most recent advances concerning the central role of Trp metabolism in microbiota-host crosstalk in health and disease. Deciphering the complex equilibrium between these pathways will facilitate a better understanding of the pathogenesis of human diseases and open therapeutic opportunities.
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Review of the endothelial pathogenic mechanism of TIE2-related venous malformation.
Du, Z, Zheng, J, Zhang, Z, Wang, Y
Journal of vascular surgery. Venous and lymphatic disorders. 2017;(5):740-748
Abstract
BACKGROUND Venous malformation (VM) is a type of disease involving vascular morphogenesis in humans. Clinically, VM can be sporadic or inherited. TIE2, also known as TEK or HYK, is a member of the receptor tyrosine kinase subfamily and is highly conserved among species. In 1996, an arginine-to-tryptophan substitution at position 849 (R849W) in TIE2 was found to induce hereditary VM. Additional alterations in TIE2 involved in the pathogenesis of inherited or sporadic VM have since been reported. METHODS The relevant key literature was selectively reviewed, including case reports, reviews, research studies, and meta-analyses. RESULTS TIE2 can be thought of as the basis for VM, with a potential role in determining locations, through intracorporal endothelium-specific distribution and expression from the embryonic phase. It has a sophisticated protein structure, and various point mutations destroy its function and physiologic processes by obviously different activation mechanisms, of which some inhibit dephosphorylation and others maintain phosphorylation. Extracellularly, whereas angiopoietins (ANGs) are ligands of TIE2, the chaotic balance between ANG1 and ANG2 in VM is related to their effects on switching between the cell-cell/cell-extracellular matrix contact conditions and vascular quiescence/angiogenesis state, resulting in corrupted contacts. Intracellularly, among diverse cellular pathways, phosphatidylinositol 4,5-bisphosphate 3-kinase/AKT serine-threonine kinase, mitogen-activated protein kinase, and Dok-related protein are irreplaceable keys underlying changes in endothelial morphology and behavioral biology in VM. For example, R849W and L914F (a leucine-to-phenylalanine substitution at position 914), the most important and frequent TIE2 mutations associated with VM, share similar phenotypes but differ with respect to signaling pathways, heredity, and triggering factors. CONCLUSIONS Based on this comprehensive analysis, we propose an avalanche theory, in which mutant TIE2 is a trigger and pathogenic core, the intercellular network is a tool, altered extracellular matrix and contacts are the final foothold, and fragile contacts are the result. Precise classification according to TIE2 mutation type in VM, especially the mutation site, is important for future targeted therapies.
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Influence of Tryptophan and Serotonin on Mood and Cognition with a Possible Role of the Gut-Brain Axis.
Jenkins, TA, Nguyen, JC, Polglaze, KE, Bertrand, PP
Nutrients. 2016;(1)
Abstract
The serotonergic system forms a diffuse network within the central nervous system and plays a significant role in the regulation of mood and cognition. Manipulation of tryptophan levels, acutely or chronically, by depletion or supplementation, is an experimental procedure for modifying peripheral and central serotonin levels. These studies have allowed us to establish the role of serotonin in higher order brain function in both preclinical and clinical situations and have precipitated the finding that low brain serotonin levels are associated with poor memory and depressed mood. The gut-brain axis is a bi-directional system between the brain and gastrointestinal tract, linking emotional and cognitive centres of the brain with peripheral functioning of the digestive tract. An influence of gut microbiota on behaviour is becoming increasingly evident, as is the extension to tryptophan and serotonin, producing a possibility that alterations in the gut may be important in the pathophysiology of human central nervous system disorders. In this review we will discuss the effect of manipulating tryptophan on mood and cognition, and discuss a possible influence of the gut-brain axis.