Kynurenine-3-monooxygenase: A new direction for the treatment in different diseases

The Complex World of Kynurenic Acid: Reflections on Biological Issues and Therapeutic Strategy

It has been unequivocally established that kynurenic acid has a number of actions in a variety of cells and tissues, raising, in principle, the possibility of targeting its generation, metabolism or sites of action to manipulate those effects to a beneficial therapeutic end. However, many basic aspects of the biology of kynurenic acid remain unclear, potentially leading to some confusion and misinterpretations of data. They include questions of the source, generation, targets, enzyme expression, endogenous concentrations and sites of action. This essay is intended to raise and discuss many of these aspects as a source of reference for more balanced discussion. Those issues are followed by examples of situations in which modulating and correcting kynurenic acid production or activity could bring significant therapeutic benefit, including neurological and psychiatric conditions, inflammatory diseases and cell protection. More information is required to obtain a clear overall view of the pharmacological environment relevant to kynurenic acid, especially with respect to the active concentrations of kynurenine metabolites in vivo and changed levels in disease. The data and ideas presented here should permit a greater confidence in appreciating the sites of action and interaction of kynurenic acid under different local conditions and pathologies, enhancing our understanding of kynurenic acid itself and the many clinical conditions in which manipulating its pharmacology could be of clinical value.

Kynurenines as a Novel Target for the Treatment of Inflammatory Disorders

This review discusses the potential of targeting the kynurenine pathway (KP) in the treatment of inflammatory diseases. The KP, responsible for the catabolism of the amino acid tryptophan (TRP), produces metabolites that regulate various physiological processes, including inflammation, cell cycle, and neurotransmission. These metabolites, although necessary to maintain immune balance, may accumulate excessively during inflammation, leading to systemic disorders. Key KP enzymes such as indoleamine 2,3-dioxygenase 1 (IDO1), indoleamine 2,3-dioxygenase 2 (IDO2), tryptophan 2,3-dioxygenase (TDO), and kynurenine 3-monooxygenase (KMO) have been considered promising therapeutic targets. It was highlighted that both inhibition and activation of these enzymes may be beneficial, depending on the specific inflammatory disorder. Several inflammatory conditions, including autoimmune diseases, for which modulation of KP activity holds therapeutic promise, have been described in detail. Preclinical studies suggest that this modulation may be an effective treatment strategy for diseases for which treatment options are currently limited. Taken together, this review highlights the importance of further research on the clinical application of KP enzyme modulation in the development of new therapeutic strategies for inflammatory diseases.

Protocatechuic acid prevents isoproterenol-induced heart failure in mice by downregulating kynurenine-3-monooxygenase

Protocatechuic acid (3,4-dihydroxybenzoic acid) prevents oxidative stress, inflammation and cardiac hypertrophy. This study aimed to investigate the therapeutic effects of protocatechuic acid in an isoproterenol-induced heart failure mouse model and to identify the underlying mechanisms. To establish the heart failure model, C57BL/6NTac mice were given high-dose isoproterenol (80 mg/kg body weight) for 14 days. Echocardiography revealed that protocatechuic acid reversed the isoproterenol-induced downregulation of fractional shortening and ejection fraction. Protocatechuic acid attenuated cardiac hypertrophy as evidenced by the decreased heart-weight-to-body-weight ratio and the expression of Nppb. RNA sequencing analysis identified kynurenine-3-monooxygenase (Kmo) as a potential target of protocatechuic acid. Protocatechuic acid treatment or transfection with short-interfering RNA against Kmo ameliorated transforming growth factor β1-induced upregulation of Kmo, Col1a1, Col1a2 and Fn1 in vivo or in neonatal rat cardiac fibroblasts. Kmo knockdown attenuated the isoproterenol-induced increase in cardiomyocyte size, as well as Nppb and Col1a1 expression in H9c2 cells or primary neonatal rat cardiomyocytes. Moreover, protocatechuic acid attenuated Kmo overexpression-induced increases in Nppb mRNA levels. Protocatechuic acid or Kmo knockdown decreased isoproterenol-induced ROS generation in vivo and in vitro. Thus, protocatechuic acid prevents heart failure by downregulating Kmo. Therefore, protocatechuic acid and Kmo constitute a potential novel therapeutic agent and target, respectively, against heart failure.

Gram-Scale Preparation of Cannflavin A from Hemp (Cannabis sativa L.) and Its Inhibitory Effect on Tryptophan Catabolism Enzyme Kynurenine-3-Monooxygenase

Inhibitors targeting kynurenine-3-monooxygenase (KMO), an enzyme in the neurotoxic kynurenine pathway (KP), are potential therapeutics for KP metabolites-mediated neuroinflammatory and neurodegenerative disorders. Although phytochemicals from Cannabis (C. sativa L.) have been reported to show modulating effects on enzymes involved in the KP metabolism, the inhibitory effects of C. sativa compounds, including phytocannabinoids and non-phytocannabinoids (i.e., cannflavin A; CFA), on KMO remain unknown. Herein, CFA (purified from hemp aerial material at a gram-scale) and a series of phytocannabinoids were evaluated for their anti-KMO activity. CFA showed the most active inhibitory effect on KMO, which was comparable to the positive control Ro 61-8048 (IC50 = 29.4 vs. 5.1 μM, respectively). Furthermore, a molecular docking study depicted the molecular interactions between CFA and the KMO protein and a biophysical binding assay with surface plasmon resonance (SPR) technique revealed that CFA bound to the protein with a binding affinity of 4.1×10−5 M. A competitive SPR binding analysis suggested that CFA and Ro 61-8048 bind to the KMO protein in a competitive manner. Our findings show that C. sativa derived phytochemicals, including CFA, are potential KMO inhibitors, which provides insight into the development of therapeutics targeting the KP and its related pathological conditions.

Prophylactic and Therapeutic Effect of Kynurenine for Experimental Autoimmune Encephalomyelitis (EAE) Disease

Background

The essential amino acid, tryptophan, is predominantly metabolised through the kynurenine pathway (KP) to generate kynurenine, an aryl-hydrocarbon receptor (AhR) pro-ligand that exerts its effects in a ligand-dependent manner. Interaction between kynurenine and the AhR is an effector mechanism of immunosuppression. We previously found that the KP is involved in multiple sclerosis (MS) disease progression. We postulated that AhR activation by kynurenine might be neuroprotective by encouraging differentiation of Tregs. In this study, we assess both the prophylactic and therapeutic efficiency of kynurenine on disease severity and progression in mice with experimental autoimmune encephalomyelitis (EAE), an MS model.

Methods

Myelin oligodendrocyte glycoprotein induced EAE mice (n = 6 per group) were treated with 200 mg/kg L-kynurenine once daily for 10 days beginning on either day 1 of EAE induction (prophylactic) or once they demonstrated motor weakness (therapeutic). Clinical disease severity measured by disease score, time on rotarod, and body weight.

Results

The prophylactic kynurenine treatment significantly (P < .0001) prevented the development of a more severe disease course with mice demonstrating diminished relapse rate and improved clinical and behavioural outcomes. However, therapeutic kynurenine did not significantly (P = .4463) decrease the clinical signs until 36 days following induction of disease; after 36 days, it also significantly (P = .0479) reduced disease relapse. Mean body weight measurements only correlated with time on rotarod (r = -.6410; P = .0007) but not clinical scores (r = .1925; P = .3674).

Conclusions

Kynurenine ameliorates EAE disease progression prophylactically and reduces relapses therapeutically. Further investigations are needed to elucidate the molecular mechanism explaining the therapeutic role of kynurenine for MS.

An Emerging Cross-Species Marker for Organismal Health: Tryptophan-Kynurenine Pathway

Tryptophan (TRP) is an essential dietary amino acid that, unless otherwise committed to protein synthesis, undergoes metabolism via the Tryptophan-Kynurenine (TRP-KYN) pathway in vertebrate organisms. TRP and its metabolites have key roles in diverse physiological processes including cell growth and maintenance, immunity, disease states and the coordination of adaptive responses to environmental and dietary cues. Changes in TRP metabolism can alter the availability of TRP for protein and serotonin biosynthesis as well as alter levels of the immune-active KYN pathway metabolites. There is now considerable evidence which has shown that the TRP-KYN pathway can be influenced by various stressors including glucocorticoids (marker of chronic stress), infection, inflammation and oxidative stress, and environmental toxicants. While there is little known regarding the role of TRP metabolism following exposure to environmental contaminants, there is evidence of linkages between chemically induced metabolic perturbations and altered TRP enzymes and KYN metabolites. Moreover, the TRP-KYN pathway is conserved across vertebrate species and can be influenced by exposure to xenobiotics, therefore, understanding how this pathway is regulated may have broader implications for environmental and wildlife toxicology. The goal of this narrative review is to (1) identify key pathways affecting Trp-Kyn metabolism in vertebrates and (2) highlight consequences of altered tryptophan metabolism in mammals, birds, amphibians, and fish. We discuss current literature available across species, highlight gaps in the current state of knowledge, and further postulate that the kynurenine to tryptophan ratio can be used as a novel biomarker for assessing organismal and, more broadly, ecosystem health.

Neuroinflammation in Huntington's Disease: A Starring Role for Astrocyte and Microglia

Huntington's disease (HD) is a neurodegenerative genetic disorder caused by a CAG repeat expansion in the huntingtin gene. HD causes motor, cognitive, and behavioral dysfunction. Since no existing treatment affects the course of this disease, new treatments are needed. Inflammation is frequently observed in HD patients before symptom onset. Neuroinflammation, characterized by the presence of reactive microglia, astrocytes and inflammatory factors within the brain, is also detected early. However, in comparison to other neurodegenerative diseases, the role of neuroinflammation in HD is much less known. Work has been dedicated to altered microglial and astrocytic functions in the context of HD, but less attention has been given to glial participation in neuroinflammation. This review describes evidence of inflammation in HD patients and animal models. It also discusses recent knowledge on neuroinflammation in HD, highlighting astrocyte and microglia involvement in the disease and considering anti-inflammatory therapeutic approaches.

Kynurenine-3-monooxygenase (KMO) broadly inhibits viral infections via triggering NMDAR/Ca2+ influx and CaMKII/ IRF3-mediated IFN-β production

Tryptophan (Trp) metabolism through the kynurenine pathway (KP) is well known to play a critical function in cancer, autoimmune and neurodegenerative diseases. However, its role in host-pathogen interactions has not been characterized yet. Herein, we identified that kynurenine-3-monooxygenase (KMO), a key rate-limiting enzyme in the KP, and quinolinic acid (QUIN), a key enzymatic product of KMO enzyme, exerted a novel antiviral function against a broad range of viruses. Mechanistically, QUIN induced the production of type I interferon (IFN-I) via activating the N-methyl-d-aspartate receptor (NMDAR) and Ca²⁺ influx to activate Calcium/calmodulin-dependent protein kinase II (CaMKII)/interferon regulatory factor 3 (IRF3). Importantly, QUIN treatment effectively inhibited viral infections and alleviated disease progression in mice. Furthermore, kmo^-/- mice were vulnerable to pathogenic viral challenge with severe clinical symptoms. Collectively, our results demonstrated that KMO and its enzymatic product QUIN were potential therapeutics against emerging pathogenic viruses.

The outbreaks of emerging infectious diseases have become a severe challenge worldwide, and therefore it is a public health priority to explore novel broad-spectrum antiviral agents with various mechanisms. This study reported that kynurenine-3-monooxygenase (KMO), a key rate-limiting enzyme during tryptophan metabolism, showed promise as a novel broad-spectrum antiviral factor against emerging pathogenic viruses. We further found that quinolinic acid (QUIN), an enzymatic product of KMO, could also act as a novel broad-spectrum antiviral agent. We then systematically studied the underlying mechanisms and broadly antiviral function of KMO and QUIN in vitro and in vivo. Our data highlight the importance of exploring novel antiviral targets from the key enzymes and their metabolites in tryptophan metabolism.

A review of chromatographic methods for bioactive tryptophan metabolites, kynurenine, kynurenic acid, quinolinic acid, and others, in biological fluids

sKynurenine (KYN) is synthesized from an essential amino acid, tryptophan by tryptophan 2,3-dioxygenase or indoleamine 2,3-dioxygenase via N-formyl- KYN in vivo. Subsequently, KYN acts as a precursor of some neuroactive metabolites such as kynurenic acid, quinolinic acid, and an important enzyme co-factor, nicotine adenine dinucleotide. These metabolites of tryptophan are a part of the "kynurenine pathway." In addition, KYN functions as an endogenous ligand for the aryl hydrocarbon receptor, which acts as a transcription factor. The levels of tryptophan metabolites are important for the assessment of the stage of neurological disorders, and hence, have garnered significant interest for clinical diagnosis. In this review, the detection of kynurenine, kynurenic acid, quinolinic acid, and other tryptophan metabolites performed via chromatographic methods such as HPLC using UV absorbance, fluorescence, and chromatographic-mass spectrometric detection is summarized.

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.

Tryptophan: From Diet to Cardiovascular Diseases

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.

Pharmacokinetics and metabolic disposition of a potent and selective kynurenine monooxygenase inhibitor, CHDI-340246, in laboratory animals

The disposition of a novel kynurenine monooxygenase inhibitor, CHDI-340246, was investigated in vitro and in animals.In vitro, there was minimal metabolic turnover of CHDI-340246 in all species. The protein binding was higher in human plasma (99.7%) relative to other species.In all species, blood clearance was low (<20% of liver blood flow) and volume of distribution was small (<0.5 L/kg). The terminal half-life was longer in monkeys (9 hr) than in mice, rats, or dogs (1-2 hr). CHDI-340246 was orally bioavailable (>60%) in all species.In rats, [¹⁴C]CHDI-340246 showed wide distribution of radioactivity in all tissues except brain and testes. In rats, the parent drug was the major circulating moiety with minor amounts of a sulphate conjugate of an O-dealkylated metabolite. The elimination occurred via the urinary route and to a lesser extent by biliary route, but mostly as metabolites. In cynomolgus monkeys, the parent drug predominated in plasma with only trace amounts of metabolites detected.Acyl glucuronide conjugate of CHDI-340246 was not detected in plasma of rats or monkeys.Overall, the ADME profile of CHDI-340246 was favourable in rats and monkeys for potential evaluation of KMO inhibition in humans.

Toward a neuroprotective shift: Eight weeks of high intensity interval training reduces the neurotoxic kynurenine activity concurrently to impulsivity in emotionally impulsive humans - A randomized controlled trial

BACKGROUND

Previous findings suggest that impulsivity is related to chronic low-grade inflammation. Inflammation is known to trigger the kynurenine pathway to a pathological level in various impulsivity-related disorders. Nonetheless, murine models and recent human studies have shown that physical exercise, in particular High Intensity Interval Training (HIIT), could counterbalance the negative effects of inflammation on the kynurenine pathway.

AIM

This study evaluates the effects of eight weeks of HIIT versus an active control group on impulsivity levels and accompanying alterations of inflammatory-mediated changes of the kynurenine pathway in a sample of emotionally impulsive humans.

METHODS

Participants were randomly allocated to either HIIT or stretching conditions (three trainings per week for eight weeks). Fitness level was evaluated via VO₂peak values at the beginning at end of the intervention. Kynurenine metabolites, pro-inflammatory cytokines, and impulsivity levels were evaluated at T0, T4, and T8 weeks. Statistical analyses were performed using mixed models.

RESULTS

Fifty-three participants were included in the modified Intention To Treat analysis (45 finished the intervention). The HIIT group (n = 28) largely increased the aerobic fitness of its participants and produced physiological changes while the stretching group (n = 25) did not. HIIT reduced interleukin 6 levels (small to moderate interaction) and reduced the activity of the neurotoxic branch of the kynurenine pathway (small to moderate interaction for KYNA/QA and KYN/QA) after eight weeks of training while the active control did not change. Both interventions were effective to decrease emotion-related impulsivity, however only the HIIT group decreased participants' emotion-unrelated levels. Changes in emotion-related and -unrelated impulsivity were moderately correlated to changes in KYNA/KYN.

CONCLUSION

This study demonstrated that HIIT was able to switch the kynurenine pathway from its neurotoxic branch to its neuroprotective one. This shift was associated with a decrease in impulsivity. Based on these findings, future work may consider investigating more intensively the effect of HIIT on impulsivity-related disorders.