In a recently published study bioRxiv* Pre-print servers, researchers examined blood levels of neurotoxic metabolites in patients with mild coronavirus disease 2019 (COVID-19).
Study: Elevated levels of circulating neurotoxic metabolites in patients with mild Covid19. Photo Credit: Maker/Shutterstock
There is currently a lack of understanding of the factors driving neuropsychiatric symptoms in acute and long-term COVID-19. Studies have shown a correlation between kynurenine pathway metabolites such as quinolinic acid (QUIN) and 3-hydroxykynurenine (3-HK) and the severity of COVID-19. Therefore, they could potentially serve as biomarkers for neuropsychiatric long COVID symptoms.
Some pathogenic infections, including COVID-19 and subsequent inflammatory diseases, can alter the activity of kynurenine pathway enzymes. QUIN is a glutamatergic N-methyl-D78-aspartate (NMDA) receptor antagonist and its high concentrations induce excitotoxic neuronal death. Conversely, kynurenic acid (KYNA), an antagonist of the NMDA receptor, prevents excessive calcium influx into a cell. On the other hand, 3-HK is neurotoxic and pro-inflammatory, promotes the formation of reactive oxygen species (ROS) and accelerates endothelial cell apoptosis.
While a blood-brain barrier (BBB) partially prevents entry of QUIN and KYNA into the central nervous system (CNS), 3-HK and KYN, a metabolite derived from the kynurenine pathway-mediated tryptophan (TRP)- Metabolism arises, pass unhindered BBB. SARS-CoV-2 infection affects KYN levels by inducing the production of the pro-inflammatory cytokine interferon-gamma (IFN-ɣ), which breaks down the kynurenine pathway enzyme, indoleamine-2,3-dioxygenase (IDO), stimulated. In addition, SARS-CoV-2 infection can lead to an inflammatory response that affects the levels of some intracellular glycoproteins, such as . A decrease in IDO during pathogenic infections leads to an increase in VCAM-1 in endothelial cells of vessel walls.
About the study
In the present study, researchers collected and analyzed serum samples from 150 patients, 44 of whom tested positive for COVID-19 but had mild illness. They determined whether the neurotoxic metabolites and glycoproteins involved in inflammation were altered in these patients.
A recent study showed that the inflammatory response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), driven by monocyte-derived macrophages, was detectable in convalescent COVID-19 patients after mild infection. ICAM-1 and VCAM-1 are also involved in the inflammatory response that directs the migration of immune cells to sites of T cell-mediated immunity in tissues. Therefore, the authors also determined whether ICAM-1 and VCAM-1 levels were altered in patients with mild COVID-19 and associated with activation of the kynurenine pathway.
They used reverse-phase ultra-high performance liquid chromatography (UHPLC) in conjunction with a mass spectrometer to quantify all metabolites of the kynurenine pathway.
The researchers found that ICAM-1 and VCAM-1 blood levels were significantly elevated in patients with mild SARS-CoV-2, particularly those with hypertension. This finding links the increase in adhesion molecules of endothelial cells to metabolites of the kynurenine pathway, such as IDO.
In addition, the authors observed a higher KYN/TRP ratio in SARS-CoV-2 positive patients compared to the healthy controls and those who tested SARS-CoV-2 negative. In addition to a higher KYN/TRP ratio, hypertensive COVID-19 patients had a higher cerebrospinal fluid (CSF) QUIN/TRP ratio. The KYN/TRP ratio is an indirect measure of IDO activity, while the QUIN/TRP ratio is an early, predictive marker of central nervous system (CNS) disease.
The authors observed no differences in serum IFN-ɣ levels in samples from COVID-19 patients with mild disease. In addition to IFN-ɣ, some non-canonical signaling pathways could activate IDO. This explains the activation of the kynurenine signaling pathway observed in the current study cohort. Activation of the kynurenine pathway leads to long-lasting inflammation; Therefore, kynurenine metabolites could be potent diagnostic markers for long COVID patients.
Cihan and colleagues analyzed kynurenine pathway metabolites and inflammatory cytokines to find a positive correlation between interleukin-6 (IL-6) and its various metabolites. The present study also found a positive correlation between IL-6 and 3-HK and between IL-6 and QUIN. In addition, QUIN, 3-HK and KYN increased in patients with mild COVID-19. Therefore, the inflammation observed in COVID-19 patients could be a major contributor to neuronal damage caused by neurotoxic metabolites.
The kynurenine pathway is altered in patients with mild COVID-19 who have elevated levels of neurotoxic metabolites. Significantly increased levels of kynurenine (age and sex adjusted data, ANOVA test F: 11.195, p<0.001***), 3-hydroxykynurenine (age and sex adjusted data, ANOVA test F: 3.390, p=0.009* *) , anthranilic acid (age and sex adjusted data, ANOVA test F: 4.024, p=0.009 **) and quinolinic acid (age and sex adjusted data, ANOVA test F: 8.492, p<0.001 ***). in patients with mild COVID-19 compared to controls. When the ratio of metabolites was analyzed, significantly increased levels of KYN/TRP (age and sex adjusted data, ANOVA test F: 6.377, p<0.001***) and QUIN/TRP (age and sex adjusted data) were found , ANOVA test F: 5.837, p < 0.001 ***) and QUIN/KYNA (age- and sex-adjusted data, ANOVA test F: 2.847, p = 0.040 *) were found in patients with COVID-19. The graphs show the median with 95% of the CI. Abbreviations: IDO, indolamine-2,3-dioxygenase; TDO, tryptophan 2,3-dioxygenase; KATs, kynurenine aminotransferase; KYNA, kynurenic acid; KMO, kynurenine-3-monooxygenase; AA, anthranilic acid; 3-HK, 3-hydroxykynurenine; KYNU, kynureninase; 3-HAA, 3-hydroxyanthranilic acid; 3HAO, 3-hydroxyanthranilate oxidase; ACMSD, aminocarboxymuconate semialdehyde decarboxylase and NAD, nicotinamide adenine dinucleotide.
Taken together, the study results showed an increase in blood levels of neurotoxic metabolites of the kynurenine pathway in patients with mild COVID-19. These neurotoxic metabolites correlated with markers of inflammation and vascular injury such as VCAM-1, tumor necrosis factor (TNF)-α, IL-6 and ICAM-1. Future studies should further evaluate their potential as biomarkers of long COVID and as potential contributors to the underlying mechanisms of long COVID.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be relied upon as conclusive, guide clinical practice/health behavior, or be treated as established information.
- Elevated levels of circulating neurotoxic metabolites in patients with mild Covid19, Estibaliz Santiago-Mujika, Kevin Heinrich, Sonia George, Colt D Capan, Cameron Forton, Zachary Madaj, Amanda R Burmeister, Matthew Sims, Andrew Pospisilik, Patrik Brundin, Stewart F. Graham, Lena Brundin, bioRxiv preprint 2022, DOI: https://doi.org/10.1101/2022.06.22.497189, https://www.biorxiv.org/content/10.1101/2022.06.22.497189v1