Divergent kynurenine metabolite profiles and neuroproliferative mechanisms in long COVID-19 patients with and without depression

Introduction

The kynurenine pathway (KP), a key mediator of neuroinflammation, is dysregulated in both long COVID-19 and major depressive disorder [1], [2], suggesting shared immunometabolic mechanisms underlying their co-occurrence. We previously demonstrated that serum from acute COVID-19 patients with neuropsychiatric symptoms impairs neurogenesis and increases apoptosis in human hippocampal progenitor cells [3]. However, whether these serum-induced neurotoxic effects persist in long COVID-19, a condition characterised by ongoing immune dysregulation, and whether comorbid depression further exacerbates this, remains unclear. In this study, we aim to examine KP metabolite profiles in serum and cell supernatants, and their relationship to neurogenic markers in long COVID-19 patients with and without depression.

Methods

Fifty-nine long COVID-19 patients (depressed, n=30; non-depressed, n=29), stratified by the Patient Health Questionnaire-9 (PHQ-9; depressed≥10, non-depressed <10), were recruited. Patients with co-morbid neurological or psychiatric diagnoses and symptoms (i.e, psychosis) were excluded. Long COVID was defined as symptoms persisting beyond 12 weeks following a confirmed SARS-CoV-2 infection. Hippocampal progenitor cells (HPC0A07/03C) were treated with 1% serum under proliferative (2-day) and differentiative (7-day) conditions. Kynurenine metabolites, 3-hydroxyanthranilic acid (HANA), 3-hydroxykynurenine (3HK), anthranilic acid (ANA), kynurenine (KYN), and xanthurenic acid (XANA), were quantified in serum and cell supernatant using liquid chromatography-triple quadrupole mass spectrometry. Neurogenic markers, Ki67 (proliferation), doublecortin (DCX; neuroblasts), and microtubule-associated protein 2 (MAP2; neuronal maturation), were assessed by immunocytochemistry. Group differences were analysed using independent samples t-tests, and Pearson correlations were used to examine relationships between metabolite profiles and neurogenic markers.

Results

Across both patient groups, KP metabolite profiles differed between serum and cell supernatant, indicating active metabolic remodelling at the cellular level. Serum profiling revealed significantly elevated KYN (p<0.01), 3HK (p<0.01), 3HANA (p<0.01), and XANA (p<0.01), alongside reduced ANA (p<0.01) in depressed patients compared with non-depressed patients. Immunostaining demonstrated increased Ki67 intensity in depressed patients (p<0.05) and no changes in DCX and MAP2, indicating enhanced proliferation without neuronal differentiation or maturation. Cell supernatant from the proliferative condition exposed to serum from depressed patients showed decreased KYN (p<0.05) and 3HANA (p<0.01) but increased ANA (p<0.01), with 3HK and XANA being unaltered. Ki67 intensity did not correlate with serum metabolites in either group. However, Ki67 intensity positively correlated with supernatant ANA (p<0.01) in depressed patients, and with supernatant 3HANA in non-depressed patients (p<0.01).

Conclusions

Collectively, these findings reveal a dichotomous KP immunometabolic signature in long COVID-19 depression, in which elevated neurotoxic metabolites in serum contrast with a neuroprotective metabolite profile in cell supernatant, coinciding with enhanced hippocampal progenitor cell proliferation. Notably, this proliferative response occurred without neuronal differentiation, suggesting that while cells may counteract neurotoxic insult, the downstream neurogenic potential remains impaired. Serum KYN, 3HK, and XANA, therefore, warrant further investigation as candidate biomarkers for depression comorbidity in long COVID-19, with implications for patient stratification and the identification of therapeutic targets within the KP.

References

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2. Paul ER, Schwieler L, Erhardt S, Boda S, Trepci A, Kämpe R, Asratian A, Holm L, Yngve A, Dantzer R, Heilig M. Peripheral and central kynurenine pathway abnormalities in major depression. Brain, behavior, and immunity. 2022 Mar 1;101:136-45.
3. Borsini A, Merrick B, Edgeworth J, Mandal G, Srivastava DP, Vernon AC, Nebbia G, Thuret S, Pariante CM. Neurogenesis is disrupted in human hippocampal progenitor cells upon exposure to serum samples from hospitalized COVID-19 patients with neurological symptoms. Molecular psychiatry. 2022 Dec;27(12):5049-61.