Featured Articles

2024 March Highlight

‘You are what you eat’: 3-hydroxykynurenine and 3-hydroxyanthranilic acid supplementation is associated with a shortened lifespan.

Dr. Amanda R Burmeister, 28 March 2024

Aging-related diseases have been associated with kynurenine pathway dysregulation, which we touched on in issue 3 of our newsletter. Research has shown that with age there are increases in quinolinic acid (QUIN), suggesting that the kynurenine pathway (KP) plays a role in aging-associated diseases. The KP impacts bioenergetic activities through the reduction of nicotinamide adenine dinucleotide (NAD+) levels and ATP production, as well as impacting mitochondria function. Interestingly, longer lifespans have been correlated to suppressed kynurenine activity.

Previous studies have focused on NAD+ levels and how increasing NAD+ through supplementation can impact longevity. Here, Dr. Mariann Gabrawy and colleagues combined KP metabolites with NAD+ precursor supplementation to see the impact on the aging process in Drosophila. In young Drosophila (1 week) the potentially neurotoxic metabolite 3-hydroxykynrenine (3-HK) impacted their physical performance and was associated with an increased failure rate. Feeding 3-hydroxyanthranilic acid (3-HAA) decreased the climbing speed and distance covered in 15s, as well as increased the failure rate of older flies (weeks 5 and 7), which is surprising given that this metabolite has been associated with having an anti-inflammatory effect. Additionally, both 3-HAA and 3-HK supplementation decreased the average lifespan of flies. Suggesting that chronic increases of 3-HAA may have a detrimental effect.

Combining alpha-methyl tryptophan (α-MT) and NAD+ precursors (NAM and NR) significantly improved performance as well as reduced the failure rates of older flies. Additionally, α-MT +NR was associated with increases in dendritic branching within the ventral nerve cord. The combination supplementation also led to significant reductions in kynurenine and 3-HK levels, as well as the kynurenine/tryptophan ratio compared to controls. Furthermore, flies given α-MT, NAM, NR, and combination treatments had an increased lifespan compared to controls, and the longest life span was observed in flies given α-MT + NAM. This research demonstrates that inhibiting the KP and increasing NAD+ levels through supplementation positively affects the aging process in Drosophila. However, this study used life-long alterations, and more research into how this combination therapy affects longevity in aged individuals is warranted.

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2024 February Highlight

Could the kynurenine pathway underlie the disrupted glutamatergic transmission observed in attention-deficit/hyperactivity disorder?

Dr. Amanda R Burmeister, 28 February 2024

Globally attention-deficit/hyperactivity disorder (ADHD) affects around 6% of children and around 5% of adults. While the exact cause of ADHD is unknown, a combination of factors including environmental and genetic are thought to play a role in this neurodevelopmental disorder. Like other psychiatric diseases, such as depression, individuals with ADHD have elevated inflammatory markers such as interleukin-6. Additionally, research has highlighted dysregulation of kynurenine pathway (KP) metabolite levels suggesting that KP activity may drive pathogenesis in ADHD. Therefore, in a recent meta-analysis study, Dr Daniele Cavaleri and colleagues looked at differences in the KP metabolite levels between ADHD patients and healthy controls. 

A total of nearly 1,200 individuals were included in this meta-analysis, covering eight studies that focused on tryptophan and its metabolites. Participants with ADHD had significantly higher levels of kynurenine and lower levels of kynurenic acid (KYNA) compared with healthy controls, including individuals who were undergoing treatment. Tryptophan was significantly increased in the ADHD participants who were untreated however, this increase was not observed in individuals receiving treatment. Other KP metabolite levels investigated (3HK, AA, and 3HAA) had high variation between studies and therefore were not correlated with ADHD.

Disrupted glutamatergic transmission has also been reported in ADHD patients and there is evidence of N-methyl-D-aspartate (NMDA) receptor dysfunction. KYNA is a neuroprotective metabolite known to be an NMDA antagonist. Due to the significantly lower levels of KYNA, it is plausible that the loss of KYNA plays a role in ADHD pathogenesis. However, more research into the KP metabolite differences in ADHD needs to be investigated, especially in regard to the neurotoxic metabolite quinolinic acid (QA) given that KYNA and QA have opposite effects on the NMDA receptor. 

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2024 January Highlight

Can kynurenine pathway metabolites be used as biomarkers of drug-induced lung disease?

Dr. Amanda R Burmeister, 24 January 2024

Various therapeutics can lead to drug-induced interstitial lung disease (DIILD) either via direct toxicity or from secondary effects due to immune activation. Mortality rates in DIILD have been reported as upwards of 50% and individuals who recover often have long-term effects, including lung fibrosis. Currently, patients are diagnosed by high-resolution computed tomography scans however, there is a need for a non-invasive method of diagnosis that can differentiate individuals with DILD from other pulmonary diseases while limiting radiation exposure.

Dr. Yuchen Sun and colleagues sought to find new biomarkers that are specific to DIILD. In their metabolomic screening, they identified five kynurenine-derived peaks that were significantly increased in DIILD patients compared to recovered patients. Due to their initial observations, the researchers looked further into the kynurenine pathway. Quinolinic acid, kynurenine, and the kynurenine/tryptophan ratio were increased in the serum of DIILD patients but not in individuals who had recovered. Furthermore, using these markers distinguished DIILD from other pneumonia/lung diseases. Suggesting that these KP metabolites can be used to detect DIILD. These findings need to be confirmed in larger cohorts and different demographics.

Since immune activation can lead to DIILD they looked further into the molecular mechanisms that may underlie the kynurenine pathway activation. Consistent with other studies, they found that IDO1 expression is induced by inflammatory mediators, such as interferon-γ, both in macrophages and lung endothelial cells. Kynurenine and quinolinic acid were increased in macrophages following interferon stimulation but only kynurenine was increased in lung endothelial cells. Highlighting the need for further research into the cell types and mechanisms involved in kynurenine pathway dysregulation in DIILD.

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2023 December Highlight

Exploring Longevity: Unveiling the Impact of Kynurenine Pathway Modulation Beyond NAD

Dr. Edwin Lim, 31 December 2023

The role of tryptophan metabolism in aging and longevity has been an ongoing and intriguing topic. One end product of tryptophan metabolism, NAD, has been a major focus in the study of aging and longevity. However, recent years have brought forth emerging evidence suggesting that other metabolites of the kynurenine pathway (KP) may also play a crucial role in aging and longevity. For example, metabolites of the KP have been linked to use as predictors of biological age, frailty (see April 2023 highlight), and Alzheimer’s Disease (see June 2022 highlight). These clinical findings underscore a strong motivation to delve deeper into understanding how alterations to tryptophan metabolism impact the aging process. Indeed, previous studies have already demonstrated that modifying the KP can affect aging. Notably, restricting the activities of TDO-2, KYNU-1, and ACSD-1 has been shown to promote longevity. In this issue, let’s explore another KP enzyme and its intermediate, HAAO and 3HAA, as a potential target for extending the lifespan of C. elegans and mice. The implications of these findings may extend to humans, providing valuable insights into the intricate relationship between tryptophan metabolism and longevity.

Dang and Castro-Portuguez et al. (2023) demonstrated that the 3HAA-producing enzyme, HAAO, promotes aging. Inhibiting HAAO function genetically results in lifespan extension in C. elegans. Importantly, their study revealed that interference with HAAO function had minimal impact on the essential development and survival of C. elegans. Conversely, exogenous supplementation of 3HAA at a level (1μM) comparable to the condition where HAAO function is disrupted also conferred lifespan extension in C. elegans. Moreover, treatment with a 3HAA analog (4CL-3HAA) yielded similar findings. Notably, the researchers established that 3HAA exhibits antioxidant activity. These findings were validated in mammals using a mouse model, which showed similar outcomes.

These findings contribute to existing research suggesting that modulation of the kynurenine pathway (KP) may hold the key to slowing aging. Previously, it was believed that longevity was primarily linked to the KP as the metabolic determinant of NAD production, and it was NAD that had a direct effect on aging. However, it is becoming increasingly clear that, aside from NAD, various targets within the KP are directly responsible for modulating the aging process. Given that NAD is a relatively unstable molecule and is present in very low amounts in biofluids, these findings present a significant opportunity for biomarker discovery in aging, in addition to identifying therapeutic targets.

While the knockdown of HAAO and 3HAA supplementation extends lifespan with minimal disruption to normal function in C. elegans and mouse models, the results from human studies are less favorable. A study published in the New England Journal of Medicine demonstrated that two families with loss-of-function in HAAO and increased 3HAA experienced congenital malformations. When replicated in a mouse model to mimic the same pathogenic gene variant, similar results were observed. Clearly, some disparities need to be resolved before the findings by Dang and Castro-Portuguez et al. can be translated into human studies.

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2023 November Highlight

Could kynurenine pathway alteration underlie the success of selective serotonin reuptake inhibitors (SSRIs) in the treatment of long COVID?

Dr. Amanda R Burmeister, 29 November 2023

The COVID-19 pandemic has had lasting effects, and many individuals are still suffering from post-COVID syndrome (PCS). Patients suffering from PCS can have a wide range of symptoms including fatigue, shortness of breath, heart palpitations, “brain fog”, headache, depression, and/or anxiety. PCS can affect individuals for months or years, and it remains to be seen if some individuals will have life-long effects from COVID-19. Therefore, it is important to understand the underlying pathophysiology of PCS and identify potential therapeutic targets to lessen or resolve the symptoms of PCS.

PCS has marked similarities with chronic fatigue syndrome (listen to the keynote about KP and chronic fatigue syndrome in our latest webinar 5), which drew the attention of Rus and colleagues to look at the hypothalamic-pituitary-adrenal (HPA) axis function and inflammation in PCS patients. Notably, in a previous publication, they determined a similar reduction in the HPA axis function, with reduced glucocorticoid levels in PCS individuals. In this recent publication, they investigate the impact that selective serotonin reuptake inhibitors (SSRIs) have on PCS. This study used self-reported questionnaires from patients diagnosed with PCS before and after SSRI treatment. SSRI treatment decreased PCS symptoms/severity in nearly 64% of the patients. However, the effectiveness of the SSRI treatment fluctuates depending on the symptom, and in this small study they saw the most improvement with brain fog and sensory overload. This study demonstrates that SSRI treatment may improve PCS overall, although this study lacked a control group, so the researchers were unable to rule out a placebo effect.

In our first newsletter (July 2021), we emphasized the dysregulation of tryptophan metabolism in COVID-19 patients and since then many publications have further confirmed KP dysregulation in both those with COVID-19 and PCS. While this study did not specifically look at the kynurenine pathway (KP) metabolite levels, it did highlight the connection between SSRIs, PCS, and the KP. Since both the serotonin and kynurenine pathways stem from tryptophan they can influence each other, suggesting that SSRIs impact the KP in individuals with PCS. Additional research is needed to identify the underlying mechanism of action of SSRIs in individuals with PCS.

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2023 October Highlight

Serotonin depletion and Long COVID.

Miss. Lorraine Tan, 30 October 2023

ISTRY has been covering the advancements in knowledge around tryptophan metabolism and COVID-19 since our newsletter in 2021. We have provided ongoing updates in our Featured Article series (from August 2022 and June/July 2023) which have highlighted the duality of tryptophan catabolites; quinolinic acid (QA) has been implicated in long-covid, and picolinic acid (PA) has broad-spectrum antiviral properties in in-vivo models. There is now new evidence from Wong et al. 2023, from the University of Pennsylvania, Philadelphia, that implicates peripheral serotonin deficiency in the post-viral sequelae of “long COVID”.  Metabolomic analysis in long COVID patients, acute COVID patients, and fully recovered COVID patients revealed evidence of serotonin depletion in both acute and post-acute phases of COVID-19, with the latter being predictive of the development of long COVID symptoms. These results were further corroborated in separate long COVID cohort samples and experimental animal models.

Wong et al. also used mice models of viral infection to elucidate the mechanisms of serotonin depletion, which was protracted in long COVID patients, and primarily driven by type 1 interferon-mediated inflammation. The researchers clarified that the presence of the virus persists in the gastrointestinal tract of long COVID patients, and their mice model demonstrated that this reduced gut gene expression for the absorption of serotonin precursor, tryptophan. Interestingly, both serotonin and tryptophan levels could be normalised via supplementation in mice diets. 

Other mechanisms of serotonin depletion in the context of interferon-mediated viral inflammation included reduced peripheral serotonin storage in platelets due to thrombocytopenia and increased serotonin metabolism. This manifested clinically in mice, as hippocampal-dependent memory impairment, and was linked to reduced vagus nerve sensory neuronal activity.   

This study highlights (i) aberrant tryptophan catabolism in the context of type 1 interferon inflammation; (ii) the role of dietary supplementation (i.e., tryptophan and serotonin) in remediating symptoms of long COVID, and (iii) the need for further exploration of the relationship between gut microbiome changes in the context of viral-driven inflammation, peripheral tryptophan catabolites, and brain function.

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2023 September Highlight

Sex- and suicide-specific changes in kynurenic acid in major depressive disorder.

Dr. Samara J Brown, 27 September 2023

Major depressive disorder (MDD) is one of the most prominent psychiatric disorders, as well as one of the most disabling. In some cases, MDD can lead to suicide which is a major global problem. Current antidepressant treatments targeting serotonin and norepinephrine brain signalling pathways are not effective in all cases of depression and they have significant limitations as it can take weeks to develop beneficial mood-enhancing effects. The limited understanding of MDD pathophysiology at a molecular level has been a major setback in the development of more effective treatment targets. Evidence suggests that alterations in the kynurenine pathway contribute to the aetiology of MDD. However, there is limited information on whether the kynurenine pathway is altered specifically in the brain in MDD.

Dr Samara Brown and colleagues recently investigated the gene expression of the kynurenine pathway enzymes and relevant neuroinflammatory markers in addition to the kynurenine pathway metabolites. Overall, the study reported interesting sex- and suicide-specific alterations in the kynurenine pathway. In the MDD cohort overall, there was evidence of increased neuroinflammation. Subgroup analysis found that female MDD subjects had significantly decreased KYNA and a trend decrease in the KYNA/QUIN ratio compared to female controls, however, there was no change specific to males with MDD. In addition, the study also reported that MDD subjects who died by suicide had significantly decreased KYNA in comparison to controls and MDD subjects who did not die by suicide, while subjects who did not die by suicide had increased KYAT2 mRNA. These findings are consistent with evidence from the CSF implicating reduced KYNA following suicide attempts. However, not all studies have reported reduced CSF KYNA following suicide attempts or MDD. Overall, these findings suggest that treatments aimed at upregulation of the KYNA arm in the brain may be favourable for female MDD sufferers and might also assist in managing suicidal behaviour.

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2023 August Highlight

A glimpse into the kynurenine pathway activation of specific maternal-fetal tissues.

Dr. Amanda R Burmeister, 28 August 2023

While dysregulation of the kynurenine pathway (KP) has been associated with pathogenesis, a complete loss of the KP can also have detrimental effects. For example, inhibiting indoleamine 2,3-dioxygenase (IDO)-1 during pregnancy results in fetal loss in a mouse model, which is likely driven by the loss of maternal immune tolerance. Furthermore, KP metabolites may also play a role in fetal development, kynurenic acid has been shown to be involved in neural plasticity and brain development.

Since maternal kynurenine levels can impact the developing fetus, Dr. Bruno Pedraz-Petrozzi and colleagues recently investigated tryptophan and KP metabolite levels in the placenta, umbilical cord, and fetal membrane using an LC-MS/MS method. There was a high kynurenine/tryptophan ratio in the placenta compared to the umbilical cord and fetal membrane, which is not surprising as other studies indicate high levels of IDO expression within the placenta. Kynurenine levels are significantly lower in the fetal membrane, whereas kynurenic acid (KYNA) levels are significantly higher than in the placenta. Suggesting that KYNA plays a role in fetal development. Since samples were taken at delivery it is important to remember that local levels of KP metabolites may fluctuate throughout pregnancy. Further research is required to understand how these levels are affected at different gestational stages, as well as by different maternal diseases, such as during peripartum depression. 

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2023 June/July Highlight

Picolinic acid as an antiviral agent against enveloped viruses – implication for SARS-CoV-2.

Dr. Edwin Lim, 27 July 2023

Apart from its structural role that confers metal chelating properties, picolinic acid has been known to play vital roles against neurotoxicity, osteoporosis, and depression. Over the years, picolinic acid has shown to exhibit antiviral activity. The recent pandemic has placed the kynurenine pathway in the spotlight, as we previously covered details in our Newsletter issue 1, and more recently, it has been implicated in long COVID, as described in our featured article (see 2022 August issue below). Now, we have further evidence that picolinic acid may play an important role against COVID.

Narayan et al showed that picolinic acid is a broad-spectrum antiviral agent that specifically targets enveloped viruses, including SARS-CoV-2 and influenza A viruses. Picolinic acid exerts its effect by blocking cellular membrane fusion, thereby inhibiting viral entry into the host cell. In contrast, picolinic acid is ineffective against non-enveloped viruses. Preclinical data showed promising therapeutic effects of exogenous administration of picolinic acid at a non-toxic dosage to attenuate SARS-CoV-2 infection in an animal model.

Piecing this together, neuropathy and depression have been observed in (long) COVID, which is associated with increases in the Kyn/Trp ratio and quinolinic acid levels. Considering the enzyme ACMSD that governs the switch between quinolinic and picolinic acids production, it is not hard to envisage that a therapeutic strategy to increase picolinic acid production would have multiple beneficial effects in neuroprotection, promoting bone health, and acting as an anti-depressant.  As is demonstrated here picolinic acid therapy may also act as an antiviral for certain viruses and would be especially beneficial in the geriatric population, being the most vulnerable group of COVID.

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2023 May Highlight

Could kynurenine pathway activation be used to identify individuals with psychotic disorders?

Dr. Amanda R Burmeister, 29 May 2023

Psychotic disorders are mental illnesses with psychosis, which includes schizophrenia. It can often be difficult to pinpoint the underlying disorder due to symptom overlap and being confounded by the patient’s delusional mindset. In order to identify at-risk patients, researchers have been investigating potential biomarkers. This work has primarily been focused on blood biomarkers, as these samples are easily obtained. However, it is appreciated that the changes in the blood biochemistry do not always reflect the changes in the brain biochemistry. Therefore, it is important to identify changes in the brain biochemistry of individuals with psychotic disorders, as this may be driving disease pathogenesis or could be used clinically for diagnosis or targeted therapeutics.

Dr. Troels Boldt Rømer and colleagues recently published a meta-analysis of perturbations in the cerebral spinal fluid (CSF) composition of patients with psychotic disorders with the goal of compiling a comprehensive list of potential biomarkers that can be used to diagnose patients. In this study data from 145 studies were included, which is an impressive number of studies, but as the authors point out, for many potential biomarkers there was a lack of overlap to make conclusive results. However, of the biomarkers that had ample representation, inflammatory cytokines, kynurenine (KYN), and kynurenic acid (KYNA) levels were elevated in the CSF of patients compared to healthy controls. Additionally, this study highlights findings of noradrenergic disturbances and dysfunction of the blood-brain barrier, as evident by increased noradrenaline levels as well as albumin and IgG CSF to serum ratios in psychotic disorder patients.

It is interesting to note that this study labeled KYNA as “proinflammatory” which contradicts with what the current literature knows about KYNA in terms of immune modulation. Although KYNA can increase production of the pro-inflammatory cytokine IL-6 via Aryl hydrocarbon Receptor, at physiological relevant concentration during inflammation, several studies showed that KYNA can mediate immunosuppression (i.e. anti-inflammation) by at least 2 mechanisms: 1) TNF-stimulated gene 6; and 2) G Protein-coupled Receptor 35. Read more about the role of KYNA in immune modulation here. Whether KYNA plays a pro- or anti- inflammatory role in psychosis warrants further investigation.

Regardless of the authors conclusions on KYNA, the authors do bring up valid limitations that we should consider when designing our experiments. First, identifying the best population of controls especially in CSF studies where obtaining samples from healthy controls may be limited. Second, ideally would be to try to optimize a standard analysis method, but at the very least detailed reporting of the range of detection and samples outside of that range should be included in the results. While research has started to point to potential biomarkers, there is a clear need for further research in order to concretely identify biomarkers that can be used to identify individuals suffering from psychosis.

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2023 April Highlight

New insight into how the neurotoxic metabolite quinolinic acid plays a role in neuromuscular decline in frailty.

Dr. Amanda R Burmeister, 26 April 2023

Frailty is a state in geriatrics where individuals have age-associated decline and increased vulnerability to multiple physiological systems. Individuals with neuromuscular function decline with associated sarcopenia have a greater incidence of hip fractures, increasing their risk of death. Recent studies have shown kynurenine pathway alterations in other age-related diseases including neurodegenerative disorders. Therefore, it is not surprising that frail adults also have alterations in KP metabolite levels including increased 3-hydroxykynurenine (3-HK) and quinolinic acid (QUIN) (Westbrook et al., 2020). You can learn more about the kynurenine pathway and frailty from one of our previous keynote speakers, Prof Gustavo Duque in here (members only). 

Since QUIN has been shown to be a neurotoxic metabolite, Dr. Tae Chung and colleagues wanted to further investigate its role in frailty. This follow-up study used an aging quinolinate phosphoribosyl transferase (QPRT) knock-out mouse model. QPRT null mice have increased QUIN levels in the brain compared to wild type mice making this model ideal for testing the impact that QUIN plays in neuromuscular decline. Here the authors show that QUIN is also elevated in the spinal cord of these mice. Aged male QPRT knock out animals had increased frailty scores compared to control animals which was not observed in females. Forelimb grip strength was significantly reduced in the aged QPRT null male mice. Additionally, neuromuscular denervation was shown in QRPT null animals, starting around 11-13 months of age. Taken together, motor neurons chronically exposed to QUIN may lead to degeneration and play a role in neuromuscular decline. Further research is needed to understand the overall impact that QUIN and the activity of the KP have on frailty.

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2023 March Highlight

Defining a new role for quinolinic acid as an immune modulator in brain cancer.

Dr. Edwin Lim, 23 March 2023

The tryptophan research community is well aware that the kynurenine pathway contributes to the pathophysiology of neurodegeneration and cancer immunology. In the case of neurodegeneration, the chronic activation of the pathway leading to the production of neuroactive metabolites such as quinolinic acid (QA) has been known to propagate excitotoxicity across several neurodegenerative diseases. In the field of cancer immunology, the Indoleamine 2,3-dioxygenase-1-kynurenine-aryl hydrocarbon receptor (IDO1-kyn-AhR) axis has been a significant target for cancer immunotherapy. The paper highlighted in this feature article focuses on the NMDA agonist QA in cancer immunology, revealing a new role for QA in immuno-oncology.

Kesarwani et al. demonstrated that the kynurenine pathway (KP) is altered in glioblastoma (GBM), leading to the accumulation of QA. The source of increased QA was found to be M2 macrophages, a protumorigenic phenotype, that had a symbiotic relationship with the tumour cells. This is because GBM cells can produce kynurenine from tryptophan but not QA, and M2 macrophages alone are unable to catabolize tryptophan to kynurenine. As a result, they require kynurenine from tumour cells for the production of QA by the M2 macrophages.

In addition, Kesarwani et al. demonstrated that QA can be immunosuppressive by priming macrophages towards an M2-like phenotype. Furthermore, although QA does not affect M1 polarization, it can alter the functionality of M1 macrophages and microglia to mimic M2-like phenotypes, gaining the ability to suppress T-cell proliferation when exposed to QA. The authors further identified the underlying mechanism of macrophage plasticity modulation by QA through NMDAR1 activation, which is present in M2 macrophages but not unpolarized macrophages. This activation leads to Foxo1 phosphorylation and increased PPARγ expression, promoting M2 macrophage polarization. This mechanism differs from the well-known IDO-kyn-AhR-mediated immunosuppression.

Translating this finding into a therapeutic approach, the team then went on to demonstrate that targeting KYNU leads to fewer M2 macrophages, thereby promoting anti-tumor activity. The new role of QA as a promoter of M2 macrophage polarization not only brings a new immunotherapeutic target to the cancer field, highlighting the importance of the kynurenine pathway, but also raises questions about the role of QA in the context of neuroinflammation and whether it may be beneficial rather than detrimental.

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2023 February Highlight

Central and peripheral kynurenine pathway activation is associated with an animal model of inflammatory bowel disease.

Dr. Amanda R Burmeister, 23 February 2023

Inflammatory bowel disease (IBD) affects nearly 7 million individuals worldwide and includes cases of Chrohn’s Disease and ulcerative colitis. This chronic inflammatory disorder affects the gastrointestinal tract. While the exact cause of the disease remains to be elucidated, there seems to be environmental risk factors, such as gut microbiome alterations, that are associated with IBD. Alterations in the gut-microbiome have also been linked to psychiatric diseases via the gut-brain axis. Not surprisingly, depression and anxiety are often reported in patients with IBD.

Dr. Li-Ping Zhao and colleagues looked into the central and peripheral kynurenine pathway (KP) alterations in a mouse model of colitis and recently published their findings in Frontiers in Immunology.  In this study, the researchers used dextran sulfate sodium (DSS) to induce colitis in mice. DSS-induced colitis was associated with decreased serum tryptophan levels and increased serum kynurenine (KYN), suggesting increased activation of the KP. Interestingly, KYN was also significantly increased in cerebral cortex tissue of animals with DSS-induced colitis. Activation of the KP was likely indolamine-2,3-dioxygenase (IDO-1) driven, as IDO-1 mRNA levels were increased in both the liver and brain following DSS-treatment. Furthermore, IDO-1 is significantly increased in peripheral blood mononuclear cells and colon tissue samples taken from ulcerative colitis patients. DSS-induced colitis was also associated with phenotypic changes in astrocytes and a decrease in serum kynurenic acid levels however, there were no significant changes observed in the microglial phenotypes. Taken together, these data suggests that colitis has a central impact and may lead to increased neurotoxicity. Additionally, the researchers investigated the microbial alterations following DSS-induced colitis. Not surprisingly, colitis was associated with intestinal flora changes, with increased abundance of seven bacterial species and decreased abundance of one compared to control animals. Interestingly, the altered gut microbiota was associated with increased tryptophan metabolism. This study demonstrates that gut microbiota and tryptophan metabolism play a role in IBD and may contribute to the psychiatric comorbidities of IBD.

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2023 January Highlight

Minocycline alters both central and peripheral kynurenine pathway metabolite levels in mice: Could this antibiotic offer similar effects in humans?

Dr. Amanda R Burmeister, 18 January 2023

It is becoming appreciated that the kynurenine pathway (KP) plays a role in the pathophysiology and/or progression of numerous neurological diseases. In June 2022 we highlighted a study that demonstrated that KP metabolites may serve as biomarkers of disease progression in Alzheimer’s patients (M. Cespedes et al., 2022). As such, observed KP alterations in Alzheimer’s disease (AD) may serve as a potential therapeutic target.

Dr. Dan Cheng and colleagues published a study on how minocycline affected neuroinflammation and the KP in Brain Behavior and Immunity. The chronic unpredictable mild stress (CUMS) model was used in this study to induce a cognitive decline in mice. This model also leads to anxiety and depression, which are commonly observed in AD patients (R. Botto et al., 2022). Minocycline treatment decreased depression- and anxiety-like behavior as well as improved cognitive ability in CUMS mice. Inflammatory cytokines were significantly elevated in the hippocampus following CUMS and when treated with the high dose of minocycline this increase was significantly reduced. It is well known that inflammatory cytokines can impact KP activation through their interaction with KP enzymes such as indoleamine-2,3-dioxygenase (IDO). Therefore, the authors investigated the KP enzyme expression in the plasma and prefrontal cortex. Indeed, there were reductions observed following minocycline treatment demonstrating an overall reduction in the activation of the KP. Kynurenine, 3-HK, and quinolinic acid levels were significantly decreased in the serum and the prefrontal cortex following treatment. Interestingly, minocycline therapy also induced interleukin 10 (IL-10) and kynurenic acid (KYNA), an anti-inflammatory cytokine and neuroprotective metabolite, while reducing both pro-inflammatory cytokines and neurotoxic KP metabolites. These data demonstrate that minocycline impacts the KP and observed improvements in cognition, depression, and anxiety may be driven by alterations in the KP. Further research is required to understand the impact minocycline would have on psychiatric and neurodegenerative diseases.

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2022 December Highlight

Increased activation of the kynurenine pathway contributes to the pathophysiology of polycystic kidney disease.

Dr. Amanda R Burmeister, 20 December 2022

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited genetic disease that is estimated to affect around 1 in 400 – 1,000 people (NIDDK, NIH). Most people with ADPKD can be attributed to defects in either PKD1, or to lesser extent, PKD2 genes. Of interest, metabolites of the kynurenine pathway (KP) have been associated with the cause of chronic kidney disease (Grams et al., 2017).

Nguyen, Kleczko and colleagues studied the role of the KP underlying polycystic kidney disease (PKD) model, published in JCI insight. In this study, the authors investigated KP activation in a PKD mouse model (C57Bl/6J PKD1 RC/RC) over 3, 6 and 9 months. They showed increased IDO1 expression in both the animal model and ADPKD cell lines. To examine the effect of IDO1 on the disease progression, they demonstrate that both genetic loss and pharmacological inhibition (1-MT) of IDO function resulted in reduction of PKD severity and changes in the cystic immune microenvironment which promotes an anti-cystogenic composition. Interestingly, a recent study (Pires et al., 2022) showed dysregulated KP activation in autosomal recessive PKD although their study pinpointed TDO rather than IDO1. Collectively, these studies indicate the detrimental role of KP in PKD and presents novel therapeutic targets for PKD.

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2022 November Highlight

Can we heal a broken heart by targeting kynurenine?

Dr. Amanda R Burmeister, 22 November 2022

Over 17 million yearly fatalities are attributed to cardiovascular diseases and the majority are due to myocardial infarctions (MI). Having just one MI event increases the yearly mortality risk by 5% for the remaining lifetime. Adult cardiac myocytes have a limited capacity to regenerate and instead, scarring occurs at the site of damage. Whereas in neonates, damaged cardiac myocytes regenerate, healing the damaged tissue. Understanding the difference between adult and neonatal cardiomyocytes regenerative mechanisms could help promote tissue repair following MI.

Previous research has demonstrated that MI induces an inflammatory response to stimulate regeneration. It is well known that the kynurenine pathway (KP) and the inflammatory response influence each other via aryl hydrocarbon receptor (AHR) activity. In this study, Dr. Donghong Zhang and associates delineate the role of the KP in neonatal cardiac regeneration, published in Nature Communications. Following cardiac apical resection, there was a rapid but transient increase in indolamine 2,3 dioxygenase-1 (IDO1), which resulted in increased kynurenine (KYN) levels and decreased tryptophan. Furthermore, there was less fibrosis (i.e. better recovery) following apical resection in IDO1-intact (wild-type control) compared to IDO1-KO mice. Cardiomyocytes isolated from IDO1-KO mice exhibited increased inflammatory pathways signaling with an attenuated proliferation of cardiomyocytes. These data demonstrate that KYN generated by IDO1 activity promotes regeneration following cardiac damage. KYN colocalized with the AHR in areas of regeneration following apical resection. The binding of KYN to the AHR was associated with increased angiogenesis. Taken together, IDO1-derived KYN signaling plays a crucial role in neonatal cardiac angiogenesis. More research is required to determine whether KYN-targeted therapeutics promote regeneration in adult cardiomyocytes.

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2022 October Highlight

Dendritic cell-mediated kynurenine pathway activation contributes to neuropathic pain.

Dr. Amanda R Burmeister, 18 October 2022

Neuropathic pain can occur due to trauma to the central or peripheral nervous system. However, neuropathy is also associated with diseases such as diabetes, accounting for nearly 30% of cases. The underlying cause of neuropathic pain is diverse and for some patients, it remains to be elucidated. Due to the widespread symptoms and causes of neuropathy, effective treatment has been challenging.

Depression is common in individuals suffering from chronic pain and the kynurenine pathway (KP) has been shown to be altered in both of these states. In a recent study by Dr. Alexandre Maganin and colleagues, they investigated the potential role that KP plays in neuropathy pathogenesis. Indoleamine 2,3-dioxygenase-1 (IDO1) inhibition reduced pain in the spared nerve injury model. Interestingly, expression of IDO1 in the spinal cord after injury was observed in hematopoietic cells and not in resident spinal cord cells. Specifically, dendritic cells (DCs) were shown to be the source of IDO1. Following nerve injury, DCs infiltrate the lesion site and in DC-depleted mice, the increase of IDO1 was not observed. DCs play a potential role in the development of neuropathic pain via KP activation. Direct injection of KP metabolites demonstrated that 3-hydroxykynurenine (3-Hk) and Quinolinic acid (QA) drive neuropathic pain. Taken together, KP activation contributes to neuropathic pain and pharmacological inhibition of the KP may alleviate neuropathic pain, however, further research is warranted.

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2022 September Highlight

Grey matter loss is associated with increased plasma kynurenic acid in schizophrenic patients.

Dr. Amanda R Burmeister, 21 September 2022

Schizophrenia (SCZ) is a chronic mental disorder that affects around 1 in 300 people. The exact cause of SCZ remains to be elucidated; however, it is likely a combination of events that contributes to the pathogenesis of this disease. Some genetic risk factors for SCZ are pro-inflammatory factors. In fact, research has demonstrated an increased inflammatory state in individuals with SCZ. Additionally, schizophrenia has been associated with a reduction in gray matter volume.

Inflammatory states can alter kynurenine pathway (KP) activity. Therefore, in a recent study published by Dr. Sumiao Zhou and colleagues in Frontiers in Psychiatry, they investigated KP metabolite levels and gray matter volume. In this study, 41 SCZ patients and 60 controls were recruited from Guangzhou, China. Plasma kynurenic acid levels were significantly increased in SCZ patients, as well as the kynurenic acid/ kynurenine ratio. Interestingly, these KP metabolites were negatively correlated with gray matter volume in schizophrenia patients. High kynurenine levels were correlated with decreased gray matter volume in the left anterior cingulate gyrus which has been previously associated with psychosis (Morgan et al., 2018). Low gray matter volume in bilateral insula was negatively correlated with kynurenine/tryptophan ratio. This relationship between gray matter volume and KP metabolites should be further investigated in a larger cohort study.

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2022 August Highlight

Does targeting the Kynurenine pathway hold promise to abolishing the neurological deficits in COVID?

Dr. Edwin Lim, 18 August 2022

Over a year ago, in our first issue Newsletter, we highlighted the importance of the kynurenine pathway (KP) in COVID-19 and asked if KP alteration in people with COVID-19 may lead to neurodegeneration (see our Newsletter issue 1 to recap). Fast track to the present, some interesting insights into that question have emerged and are covered in this month’s feature article.

Giron et al focus on a subset of individuals who experience persistent, recurrent, or new symptoms following COVID-19 acute infection, commonly referred to as long COVID (see WHO) in comparison to those without long COVID. They examined the symptoms, quality of life, and overall health status together with plasma markers of inflammation and metabolites using untargeted metabolomics.

Given the known KP alterations in people with SARS-CoV-2 infection, not surprisingly, this study found that those with long COVID have higher quinolinic acid (QA) levels compared to those without long COVID. Individuals with long COVID who presented with neuropathy showed the highest Kyn/Trp ratio compared to those without neuropathy and long COVID. Interestingly, the long COVID group with neuropathy also showed a higher QA/TRP ratio when compared to those without long COVID. Further, these KP ratios had a moderately strong correlation with several pro-inflammatory markers. The study also extends their findings to evidence of fungal (translocation) marker and NF-kB signaling, which we know are associated with alteration in the KP activity.

Taken together, this suggests that inflammation may drive the activation of the KP, altering the overproduction of QA to drive excitotoxicity-induced neurodegeneration. Moreover, the association between higher QA/TRP ratio and poorer overall health scores in the long COVID group implies the potential role of targeting glutamatergic modulation for a better outcome for long COVID.

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2022 July Highlight

Spotlight on kynureninase as an immunotherapeutic target in cancer.

Dr. Edwin Lim, 15 July 2022

Exactly a year ago in our featured article (see Jun/July 2021), we highlighted the importance of IDO-1 interaction with AhR and PD-1 in overcoming immune tolerance within the tumour microenvironment. This has implications for improving the efficacy of future immunotherapies in cancer or at least in ovarian cancer. However, limiting IDO-1 activity may not inhibit kynurenine (KYN) production completely since there are other enzymes, such as TDO-2 that also convert tryptophan to KYN. Further, small-molecule inhibitors via systemic administration can have a short half-life, lack localized effect, and may result in unexpected adverse effects.

To overcome these issues, B. Wang et al. focused on the use of kynureninase (KYNU) instead of IDO-1 as a better immunotherapeutic strategy to overcome immune tolerance within the tumour microenvironment. They demonstrated this using cell-line-derived xenograft mouse models with site-specific hydrogel consisting of KYNU on both breast and melanoma cell lines. They also investigated the synergistic effect of combining KYNU with doxorubicin (Dox), a chemotherapy drug.

They showed that KYNU encapsulated in the hydrogel results in the sustained activity of KYN degradation at the tumour site. This leads to the greatest penetration of immune cells into the tumour microenvironment when used in combination with Dox compared to either a single treatment of KYNU or Dox. This combinatorial therapy can retard tumour growth, induce systemic anti-tumour immunity and limit tumour relapse. It is apparently clear that targeting the kynurenine pathway holds high hopes in cancer treatment.

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2022 June Highlight

Kynurenine pathway alterations may be able to predict cognitive decline prior to Alzheimer’s disease progression.

Dr. Amanda R Burmeister, 19 June 2022

Alzheimer’s disease (AD) is the most common form of dementia worldwide affecting nearly 40 million individuals. While the exact pathophysiology of AD remains to be elucidated it is known that the risk of AD increases with aging. It is appreciated that aging results in changes that can include neuronal atrophy, increased neuroinflammation, and vascular damage. However, it is unknown what factors exacerbate the natural aging process driving disease progression.

Kynurenine metabolites have been shown to be either neuroprotective or neurotoxic, as such the pathway has been investigated in AD. A recent study published by Dr. Marcela Cespedes et al. in Neurobiology of Disease looked at systemic changes in the kynurenine pathway (KP) with the hopes of identifying biomarkers of dementia. This study used samples from nearly 300 individuals who participated in a longitudinal study of aging in Australia. Individuals were grouped into three cohorts; Baseline (AD diagnosis at enrollment), Progressor (AD at a follow-up), and Non-progressors (no AD diagnosis at follow-up). Progressors had increased levels of 3-hydroxyanthranilic acid (3-HAA) and the 3-HAA/anthranilic acid (AA) ratio at baseline. Notably, increased 3-HAA/AA ratio levels at baseline were a strong predictor for progressors (Odds Ratio 35.3). Progressors also had low levels of picolinic acid (PIC) and quinolinic acid (QUIN)/kynurenic acid ratio at baseline as compared to non-progressors. However, this finding was not associated with cognitive decline. This study confirms that the KP metabolites are altered in AD and might predict which individuals will progress. Early detection of AD is critical so that treatment can start prior to irreversible damage occurs. We will be diving further into the KP and aging in our upcoming newsletter (issue 3). Stay tuned!

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2022 May Highlight

Microbial indole signaling through AhR facilitates learning and memory behaviors in honeybees.

Dr. Amanda R Burmeister, 20 May 2022

Germ-free mice have offered a model to investigate the roles that the gut microbiota play during development and disease. It is appreciated that the commensal microbes help to educate the immune system and without their presence the immune system is under developed. Furthermore, gut microbes contribute to the hosts metabolism. For example, microbes generate indolic compounds from tryptophan and these microbial-derived metabolites have been shown to be either protective or risk-factors for type 2 diabetes, as was shown in our feature article in September 2021. With the gut-brain axis these metabolites can also impact the brain.

As such, a recent study published by Dr. Zijing Zhang et al. in Nature Communications investigated the impact that the gut microbiota has on leaning and memory behaviors in honeybees. An olfactory learning test was used to determine the impact gut microbes have on memory. Antibiotic-treated honeybees had a reduction in the number of successful tests however, microbe-free bees were unable to pass the test suggesting that the gut microbiome affects learning and memory. This was further supported by striking transcriptional changes between the groups. Microbe-free bees had reduced expression of proteins which are key for olfaction. Interestingly, tryptophan metabolism was both up- and downregulated with the absence of gut microbes. There was an overall significant decrease in the levels of tryptophan and indole-3-acrylic acid as well as in the kynurenic acid/kynurenine ratio in microbe-free bees. Since there were alterations in tryptophan metabolism with the loss of the microbiota, the researchers looked at the genomes of  honeybee specific bacterial species to indentify potential tryptophan metabolizers. They identified that Lactobacillus apis express ArATs which suggests it can breakdown tryptophan. When microbe-free bees were recolonized with Lactobacillus there was in increase in indole derivatives and their memory was improved in the presence of tryptophan. Importantly, indole activation of the aryl hydrocarbon receptor (AhR) was shown to play a role in learning and memory behaviors. Taken together, these data demonstrates that the gut-brain axis plays a role in learning and memory via tryptophan metabolism.

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2022 April Highlight

Underlying therapeutic effects of lithium target inflammation and reduce kynurenine pathway activity in microglia.

Dr. Amanda R Burmeister, 15 April 2022

Many pharmacological treatments for psychiatric disorders are thought to target serotonin, either its neurotransmission or bioavailability. Lithium has been used since the mid-19^th century to treat psychiatric disorders however, its underlying mechanisms have yet to be fully elucidated. Lithium treatment has been shown to increase serotonin neurotransmission in animal studies. Since tryptophan underpins both serotonin and the kynurenine pathway it is critical to understand how therapeutics impact the balance of these biological pathways.

A recent study published by Dr. Ria Göttert et al. in Glia investigated the impact lithium treatment has on the inflammation-induced kynurenine pathway. Microglia cells stimulated with the inflammatory mediator interferon-γ expressed higher levels of mRNA encoding IDO1 compared to unstimulated cells. Interestingly, when treated with lithium IDO1 mRNA was significantly reduced in stimulated cells resulting in less activation of the kynurenine pathway (kynurenine/tryptophan ratio). Furthermore, inflammatory induction of STAT1 (Signal transducer and activator of transcription 1) and STAT3 was significantly reduced following lithium treatment. An enzyme, glycogen synthase kinase (GSK)3β, has been demonstrated to play a key role during inflammatory responses and lithium has been shown to inhibit this enzyme. Here the researchers show that lithium treatment of microglia induces the phosphorylation of GSK3β blocking STAT1 and STAT3 activation which results in increases in the anti-inflammatory cytokine, interleukin 10. Psychiatric disorders have been associated with inflammation; therefore, part of lithium’s underlying mechanism may target and reduce inflammatory pathways as well as increase the overall bioavailability of tryptophan as is suggested by these data.

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2022 March Highlight

New mechanistic insight into cancer immunobiology by tryptophan depletion.

Dr. Edwin Lim, 17 Mar 2022

In our recent newsletter (issue 2), we talked about the importance of tryptophan (Trp) starvation in cancer immunobiology, owing much of its initial discovery as a physiological phenomenon in maternal-fetal rejection. We now know that Trp depletion via IDO upregulation leads to IDO-AhR mediated suppression of immune activity. From an immune cell perspective, this mechanism is highly undesirable but presents a new insight that perhaps targeting immune cells to overcome Trp depletion may improve cancer prognosis. Recently, published in Nature, Abhijeet Pataskar and colleagues present a new mechanistic function of Trp depletion – tryptophan-to-phenylalanine (Trp-to-Phe) ‘substitutants.’

What is Trp-to-Phe substitutant? Pataskar and colleagues showed that depleting Trp via interferon-gamma treatment can result in codon reassignment of Trp to other amino acids. They further showed that the action of codon reassignment led to a preferential increase in Phe as substitutants driven by tryptophanyl-tRNA synthetase (WRS). Thus, the name Trp-to-Phe substitutants. You may want to gain more insight into the mechanism here.

Why is this mechanism important? By identifying this new phenomenon, the study showed that substitutants mediated by Trp depletion can lead to potent T cell recognition, thereby destroying cancer cells. Moreover, the study also showed that Trp-to-Phe substitutants were highly abundant in multiple cancer types. Hence, apart from being a physiological process in allograft rejection, Trp depletion in the context of cancer may be a physiological process to hinder cancer formation, which is beneficial to the host.

What is the significance to the Trp research community? On one hand, Trp depletion-mediated immunotolerance used by cancer cells to overcome immune cells is detrimental to the host. Yet, on the other hand, Trp-depletion-mediated Trp-to-Phe substitutants can have an opposing immune effect that is beneficial to the host. Further study leveraging on both mechanisms targeting cancer is warranted.

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2022 February/March Highlight

Neurons express KMO and could be the driver of 3-Hydroxykynurenine production.

Dr. Amanda R Burmeister, Feb (17 Mar) 2022 – delayed due to release of Newsletter Issue 2 in Feb.

It is appreciated that metabolites generated by the metabolism of tryptophan via the kynurenine pathway (KP) play a role in neurobiological diseases and processes (R Schwarcz and TW Stone, 2017). However, there is a need to better understand the cellular expression of key enzymes of the KP, the ability of metabolites to cross the blood-brain barrier, and the biological effects of these metabolites in the brain.

A recent study published by Dr. Korrapati V. Sathyasaikumar et al. in Antioxidants investigated the expression of kynurenine 3-monooxygenase (KMO) in the brain of mice. Previously microglia were considered to be the predominate cell type that expresses KMO. In this study researchers chemically depleted microglia through a PLX5622 infused diet. First, researchers confirmed the loss/reduction of microglia by investigating microglia specific markers and noted 89%-97% decreases across all five markers. Interestingly, KMO expression and activity remained unchanged in the PLX5622 fed mice suggesting that another cell type could contribute to the generation of 3-hydroxykynurenine (3-HK) within the brain. To investigate the expression of KMO further, the researchers isolated astrocytes, neurons, and microglia from murine brain tissue and investigated KMO mRNA and protein expression. Microglia and neurons, but not astrocytes, express KMO at the level of mRNA and protein. This finding is important since the proportion of neurons outweighs that of microglia within the brain. In fact, this study attributed neurons with nearly 30% of the proportion of cells isolated and microglia making up for 15%.  Furthermore, they demonstrated that KMO activity was significantly elevated in neurons compared with microglia. Taken together, neurons likely play a bigger role in the generation of 3-HK than once was perceived. Further studies should look into region- and neuron-specific expression of KMO, as well as looking at other cell types such as oligodendrocytes.

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2022 January Highlight

Laboratory diets can alter kynurenine and tryptophan levels, a potential experimental pitfall.

Dr. Amanda R Burmeister, 20 Jan 2022

Tryptophan metabolism has been demonstrated to play a role in many disease states and changes in the kynurenine pathway metabolites are being linked to depression, cancer, and neurodegenerative disorders. Like other metabolic pathways, diet can affect the observed levels of downstream metabolites. Therefore, it is critical to understand how common laboratory diets can alter levels of these metabolites in experimental animals.

A recent study published by Dr. Yuhei Yajima et al. in the International Journal of Tryptophan Research investigated how common laboratory diets can alter kynurenine metabolite levels. In this study three experimental laboratory diets (MF, AIN-93G, and AIN-93M) were fed to male C57BL/6J mice. Mice fed the MF diet had greater food and water intake compared to those on either of the AIN diets however, there was no statistical differences in weight gain. Interestingly, mice on the MF diet had significantly more plasma kynurenine. Furthermore, there were marked metabolic changes in brain tissue. MF mice had significantly lower levels of tryptophan in the hippocampus and striatum and lower kynurenine levels in the hippocampus as compared to both AIN diets. The researchers next investigated amino acid levels in the plasma and found that several including asparagine, glutamine, serine, and lysine were significantly lower and ornithine was significantly higher in the MF fed animals. Not surprisingly, PCA analysis of urinary metabolites demonstrated that MF animals were separated from both AIN diet groups which clustered together. This study highlights the importance of choosing an appropriate diet for laboratory animals and an understanding that metabolite levels can be affected by feed choice, thereby potentially confounding study outcome. Further studies should look into metabolite differences in other mouse strains, laboratory feeds, as well as in females.

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2021 December Highlight

Host tryptophan metabolism plays a key role in plant health.

Miss Lorraine Tan, 22 December 2021

In humans, changes in the gut-microbiome can have a significant impact on peripheral immune responses. Gut-microbiota imbalances are associated with health conditions such as irritable bowel syndrome, type 2 diabetes and eczema. There is emerging research suggesting that tryptophan metabolism may be linked to gut-microbiota interactions that maintain healthy immune responses in humans. Correspondingly, plants have evolved to possess a complex innate immune system characterized by “multikingdom microbial communities”. Plant roots are colonized by a diverse range of symbiotic microbes such as bacteria and fungi and together, they maintain host-microbe homeostasis and plant health. However, this can be compromised under pathological conditions, causing dysbiosis. A recent study published by Dr Katarzyna Wolinska et. al in Proceedings of the National Academy of Sciences has highlighted that tryptophan derivatives may play a role in the immune systems of plants, promoting survival under pathological conditions.

The authors studied Arabidopsis thaliana (A. Thaliana), known as the thale cress, as a model organism to study microbiota diversity in plant roots. Their initial experiments found that immunocompromised plants demonstrated dysbiosis in the bacterial and fungal composition of their roots, however, they could not explain the variability in plant growth and performance in these plants. In subsequent experiments, they discovered that the aggressiveness of fungal colonization in A. Thaliana roots is related to plant performance and growth. A high fungal load puts the plant at greater risk of pathogenesis than root-derived bacteria. Importantly, their results indicated that host (plant) TRP metabolism plays an integral role in driving microbe-mediated plant growth promotion as TRP derivates and bacterial root commensals are key factors in limiting fungal growth in A. thaliana roots. As such, the fungal composition and maintenance of fungal-host homeostasis in A. thaliana roots is directly related to plant health and can promote microbiota-induced plant growth, and TRP-derived metabolites can maintain optimal fungal-load in plant roots, thereby upholding the innate immunity system in A thaliana. Overall, it seems that TRP metabolism plays a role in both human and plant immune systems via host-microbiota interactions.

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2021 November Highlight

Evidence that tryptophan metabolism plays a role in anxiety via the gut-brain axis.

Dr. Amanda R Burmeister, 22 November 2021

Anxiety disorders affect around 4% of the world’s population and is one of the most common mental illnesses. Recent studies have shown that the gut-brain axis plays an integral part in psychiatric health. In fecal transplant models, mice exhibit anxiety-like behaviors when receiving fecal microbiota from depressed individuals. This highlights the contribution that microbes play in human diseases.

A recent study published by Dr. Christopher R Brydges et al. in Scientific Reports investigated how microbe tryptophan metabolism affects depression and anxiety levels. Indole levels are significantly associated with age, height, and weight (indole-3-lactic acid (ILA) and the ILA/Indoxyl sulfate (IS) ratio are positively associated; and the indole acetic acid (IAA)/ILA ratio is negatively associated). Furthermore, the authors noted sex differences with significantly higher levels in males, specifically IAA, ILA, and the ILA/ indole-3-propionic acid (IPA) and ILA/IS ratios. There were noted treatment-specific changes in indole levels. Unfortunately, there were no baseline measurements that predicted treatment outcomes in this population, although individuals who remitted following medication treatment had a significant increase in IPA as well as a decrease in the ILA/IPA ratio. IS was significantly associated with depression and anxiety scores whereas ILA/IS and IPA/IS ratios were negatively correlated with depression and anxiety. The researchers next investigated brain connectivity and the correlation with IS. There was a significant correlation between IS levels and the functional connectivity of the Subcallosal Cingulate Cortex-right premotor region. Connectivity differences in this brain region have been shown to be associated with remission and treatment failure in Major depression disorder patients (Dunlop et al., 2017).

Taken together, this study adds to our understanding that crosstalk between microbes and their host can have an impact on overall human health and importantly on psychiatric health via alterations in tryptophan metabolism. Therefore, understanding dysbiosis and how microbial- and host-derived tryptophan metabolites impact human health and treatment success is critical for many disease/disorders including anxiety and depression.

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2021 October Highlight

Alcohol use disorder and alcohol detoxification are associated with a neurotoxic shift in kynurenine pathway metabolites

Dr. Amanda R Burmeister, 15 October 2021

In 2017, alcohol use accounted for 2.84 million deaths, and it is estimated that nearly 1.5% of the population has alcohol use disorder (AUD) worldwide. Risk factors include binge drinking, adolescent alcohol use, family history of AUD, and psychiatric conditions including PTSD and ADHD. Treatment typically includes detoxification, counseling, and medical intervention.  AUD is associated with significant changes in the gut microbiota which could alter the overall metabolic profile and impact the brain via the gut-brain axis.    

A recent study published by Dr. Sophie Leclercq et al. in Translational Psychiatry investigated alterations in the levels of kynurenine pathway metabolites and investigated their correlation with gut microbiota and levels of microbe-derived fatty acids in AUD patients before and after hospitalization for alcohol detoxification. Patients had significantly lower levels of plasma 5-hydroxytryptophan, kynurenic acid (KYNA), anthranilic acid, 3-hydroxy-kynurenine, and xanthurenic acid compared to healthy matched controls. Additionally, there were significant elevations in plasma quinolinic acid (QUIN), picolinic acid, and kynurenine in patients, which were further elevated following the 3-week detox period. Furthermore, the KYNA to QUIN ratio was significantly lower in patients demonstrating an overall potential neurotoxic effect. However, it remains to be elucidated as to how much peripheral QUIN and KYNA effect the brain specifically. KYNA levels after detoxification were negatively correlated with alcohol cravings highlighting the potential therapeutic aspect for this NMDA receptor antagonist. In a previous study, Dr. Leclercq and colleagues demonstrated AUD patients had decreased abundance of several bacteria including Faecalibacterium (Leclercq et al, 2014). In the present study, they investigated the association of kynurenine metabolites with bacterial species. There were 22 identified bacterial taxa that associated with tryptophan and multiple kynurenine metabolites. Of these, QUIN was negatively associated with Faecalibacterium and positively associated with Akkermansia whereas KYNA was positively associated with Prevotella. Interestingly, Faecalibacterium prausnitzii has been demonstrated as one of the main bacterial species that is altered in gut dysbiosis (as discussed in detail by Lopez-Siles et al, 2017). This bacterium has anti-inflammatory properties and can metabolize tryptophan. Here, the authors demonstrated that F. prausnitzii was negatively correlated with KYN, QUIN, and importantly, there was a positive association with the neuroprotective ratio (KYNA/QUIN). Taken together this study demonstrates that F. prausnitzii might play a critical role in regulating the kynurenine pathway potentially via its anti-inflammatory properties.

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2021 September Highlight

Microbial-derived indolepropionate confers protection against type 2 diabetes

Dr. Amanda R Burmeister, 14 September 2021

Nearly 500 million people worldwide have diabetes and around 90% of all cases are type 2 diabetes (T2D). Physical inactivity, obesity, and poor diet are risk factors for T2D. Interestingly, obesity and insulin resistance has been associated with the kynurenine pathway. Suggesting downstream metabolites may play a role in T2D. It is becoming increasing clear that changes in the microbiome can contribute to many diseases. In mice lacking IDO, microbial breakdown of tryptophan is observed, likely due to the increased availability of tryptophan. Therefore, the competition between host and microbiota for tryptophan might underpin the risk for diseases via shifts in downstream metabolites.    

A recent study published by Dr. Qibin Qi et al. in Gut investigated host- and microbial-derived metabolites generated by the breakdown of tryptophan and the risk for T2D. T2D risk was associated with four host-derived metabolites (kynurenine, kynurenate, xanthurenate, and quinolinate) and one microbial-derived metabolite (indoleactate). Indolepropionate, a microbial-derived metabolite, has been associated with a reduced risk of T2D in other studies, and the authors of this study suggest that changes in the level of this metabolite underlies T2D. Not all bacterial species generate indole derivates however, 21 bacterial genera were associated with indolepropionate levels and of those, all but 5 were also associated dietary fiber intake. These bacteria were mostly Firmicutes, but also included some Actinobacteria and Bacteroides. These findings demonstrate the importance of diet and highlights why a poor diet (usually consisting of low fiber intake) is a risk factor for T2D. Furthermore, genetics plays a role in metabolism. Here, the authors show genetic variants that are associated with tryptophan metabolites, not surprisingly many were in genes involved in the metabolism or transport of TRP. Taken together, serum tryptophan metabolite levels may be used to identify individuals with an increased risk of T2D and it seems that targeting the microbiome through dietary changes has the potential to circumvent that risk.

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2021 August Highlight

Kidney transplant success dependent on the protective role of KMO, 3HK, and 3HAA

Dr. Amanda R Burmeister, 19 August 2021

Kidney failure effects two million people worldwide. Currently, the best treatment option is transplantation however there is a severe shortage of donated kidneys. Of individuals who receive a kidney, around 4% reject the new organ. Failed transplantation is the result of tubular epithelial cell (TEC) injury by the host immune system. Over 20 years ago, the role of IDO was made known to the world when Dr. David Munn and colleagues showed that allogenic rejection is prevented by indoleamine 2, 3-dioxygenase (IDO), published in Science (Munn et al. 1998). Now there is more evidence that downstream catabolites of TRP play a role in the success of kidney transplants.

A recent study published by Dr. Randi Lassiter et al. in Frontiers in Immunology investigated the effects of the kynurenine (KYN) pathway on the outcome of transplantation.  In human kidney tissues, kynurenine 3-monooxygenase (KMO) is highly expressed in TEC. In this research the authors used a swine model and compared allograft, where kidneys were transplanted from one individual to another, to autograft, where the kidney was removed from an individual and retransplanted. Rejections were associated with diminished expression of KMO suggesting a protective role whereas, IDO expression did not correlate to allograft rejection. Based on the authors previous work (Wang et al. 2018), TEC were treated with a combination of IFNγ, TNFα, and IL1β. This cytokine challenge caused increased IDO protein expression while KMO expression was decreased. Furthermore, TEC had increased apoptosis as determined by increased Caspase 3 and 8 expression, and reduced expression of Bcl2 and Bcl-xL. Downstream metabolites 3-hydroxykynurenine (3HK) and 3-hydroxyanthranilic acid (3HAA) were shown to help protect TEC from injury and inhibited T cell proliferation following cytokine stimulation. Other research has shown that 3HAA is immunosuppressive and protective (Iken et al. 2012 and Gargaro et al. 2019) Taken together, KMO expression and the subsequent 3HAA and 3HK levels contribute to the overall success of kidney transplantation. Potential future therapies could include 3HAA and 3HK administration, although more research is warranted since 3HK has been shown to be neurotoxic.

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2021 June/July Highlight

Crosstalk between IDO-1, AHR, and PD-1 in the tumor microenvironment of ovarian cancer

Dr. Amanda R Burmeister, 19 July 2021

Epithelial ovarian cancer is the most common type of ovarian cancer and has a high degree of mortality. Chemotherapy in conjunction with surgery is used to treat ovarian cancers however, there has been a high degree of chemotherapy-resistant tumors leading to a decreased survival rating. Therefore, understanding and overcoming immune tolerance within the tumor microenvironment (TME) is critical for the success of future therapeutics.  

A recent study published by Dr. Adaobi Amobi-McCloud et al. in Frontiers in Immunology investigated indoleamine 2,3-dioxygenase (IDO-1) and the kynurenine (KYN) pathway as a potential immunosuppressive target. The study showed that ovarian cancer patients with low IDO-1 expression and high levels of tumor-infiltrating effector T lymphocytes (TIL) had significant increases in their median survival. They examined the roles of IDO-1 in TME immune suppression by exploring the differential expression of IDO-1 between host (IDO-1 KO mice) and tumor (overexpressed IDO-1 ovarian cell line). They found that increased IDO-1 expression in the ovarian cell line, not only alters TRP metabolism, but affects other cellular metabolic pathways. The resulting increase in KYN production facilitated significant upregulation in the cell surface expression of the T-cell inhibitory, PD-1, receptor on host’s CD8+ T-cells. Furthermore, the authors found that KYN-treated CD8+ T-cells had increased chromatin accessibility to other immune-inhibitory receptor genes, Pdcd1 and Lag3. The authors also found AHR binding sites upstream from the PD-1 gene promoter region on CD8+ T-cells, implying a potential crosstalk between IDO-1, AHR, and PD-1.  Importantly, this study demonstrates that KYN-mediated PD-1 activity on CD8+ T-cells requires activation of AHR to drive immunosuppression in the TME. Therefore, targeting both IDO-1 and AHR may offer a better immunotherapeutic strategy against ovarian cancer.  

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2021 May Highlight

High-Intensity Interval Training alters KP metabolites, shifting to neuroprotection in people with MS.

Dr. Amanda R Burmeister, 24 May 2021

Multiple sclerosis (MS) is a chronic autoimmune disease leading to the demyelination of nerve cells in the central nervous system. Nearly 3 million people worldwide suffer from this debilitating disease. Although there is no cure for MS, recent studies have shown that exercise regimens have a positive effect on both motor and cognitive function in people with MS.

The recent study published by Niklas Joisten et al. in Neurology Neuroimmunology & Neuroinflammation is a secondary analysis (Joisten et al, 2019) that assessed how aerobic exercise influences plasma levels of kynurenine pathway (KP) metabolites, neurofilament light chain (pNfL), and interleukin 6 (IL-6).  Participants were randomly assigned to either the high-intensity interval training (HIIT) or the standard exercise training program (moderate continuous training (MCT) cohort for 3-weeks. Joisten et al. reported that pNfL levels decreased significantly following the HIIT exercise regimen. Decreases in pNfL could signify increased neuroprotection since other studies have shown higher levels correlate with inflammation and decreased motor function in people with MS. KP metabolite changes consisted of decreases in both tryptophan (TRP) and kynurenine (KYN) for both exercise programs. Importantly, there was a significant increase in plasma kynurenic acid levels resulting in a decrease in the quinolinic acid/kynurenic acid (QA/KA) ratio in the HIIT individuals. These changes in the KP are also indicative of neuroprotection, demonstrating the intensity-dependent effects of exercise on people with MS. Substantial research has demonstrated the link between inflammation and KP activation. Likewise, in the current study the activation of the kynurenine pathway correlated with IL-6 levels, specifically there were positive correlations between QA, KYN, and the KYN/TRP ratio with IL-6. This study (along with Joisten et al, 2020 and Zimmer et al, 2018) suggests that changing the therapeutic exercise program for people with MS from MCT to HIIT may improve and limit the disease progression. However, more research is warranted to determine whether these effects would be sustained following long-term HIIT intervention. Furthermore, it is tempting to speculate that therapeutic exercise may be alter KP metabolites in other disease states such as amyotrophic lateral sclerosis.

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2021 April Highlight

Sex hormones may contribute to inflammation and kynurenine pathway dysregulation during peripartum depression

Dr. Amanda R Burmeister, 22 April 2021

Peripartum depression (PPD) is a mood disorder with symptom onset during pregnancy and up to four weeks after childbirth and is reported in 15-20% of pregnant women worldwide.  Depressive symptoms can range from mild mood changes to severe depression with suicidal behaviors. After delivery the most common cause of death in mothers is suicide, highlighting the severity of PPD.

The recent study published by Qiong Sha et al. in the Journal of Affective Disorders expands on previous research that looked at inflammation and kynurenine pathway dysregulation in women with PPD that experienced severe and suicidal depression. In the most recent study, the authors investigated associations between sex hormones, the kynurenine pathway and inflammation in the postpartum. Sha et al. reported that estrogen is positively correlated with the pro-inflammatory cytokine IL-6 and negatively correlated with serum kynurenine and picolinic acid levels. In their earlier study (Achtyes et al, 2019), depression was associated with increased IL-6 and IL-8 therefore suggesting that this hormone may contribute to PPD. However, they also found that progesterone was negatively correlated with the pro-inflammatory cytokine IL-1β, quinolinic acid, and nicotinamide. Their findings were consistent with previous studies that showed progesterone had anti-inflammatory effects. Importantly, this study also found that higher levels of estrogen and progesterone in the post-partum are linked to depression symptom severity.

Due to the dramatic changes in sex hormone levels in pregnant women, Dr. Qiong Sha hypothesizes that sex hormones may contribute to the gender disparity when it comes to depression. Dr. Sha thinks that investigating the effects of hormonal changes on mental health in other life stages such as during menopause is warranted. Currently, the Dr. Lena Brundin laboratory and collaborators are looking further into inflammation and kynurenine metabolite changes in individuals suffering from suicidal ideation with hopes of identify a potential biomarker that can be used to pinpoint depressed individuals who may be more at risk of suicide with the goal of providing optimal care.