Article

Three metabolites are possible candidates for “toxic” gut-brain communication in patients with multiple sclerosis

Aug 14, 2023 | Neuroscience

Recent studies supported the hypothesis that microbial dysbiosis has a significant role in the pathogenesis of several neurological disorders. The gut microbiota composition in patients with multiple sclerosis (MS) differs from healthy controls. It was suggested that certain metabolites or bacterial toxins produced in the gut may enter the bloodstream and subsequently be transferred to the cerebrospinal fluid (CSF), where they may exert a toxic effect and influence a disease progression. In this study, the authors from the United States conducted targeted metabolomic analysis of plasma and CSF samples obtained from patients diagnosed with relapsing-remitting MS (RRMS) before and after they were treated with dimethyl fumarate (DMF) and healthy controls. The results demonstrated the presence of a potentially neurotoxic group of metabolites (indoxyl sulfate, p-cresol sulfate, and N-phenylacetylglutamine). According to the researchers, these three metabolites are microbially derived and represent possible candidates for ‘toxic’ gut-brain communication in patients with MS.

About the study

The authors analyzed the metabolites in the CSF samples of patients diagnosed with relapsing-remitting MS (RRMS) before and after they were treated with dimethyl fumarate (DMF). MS diagnosis was established according to McDonald’s criteria. The exclusion criteria were current smoking status and treatment with steroids within 30 days from enrollment.

Previous studies have shown that DMF has various immunomodulatory, epigenetic, and gut microbiome-related effects. The same research group demonstrated before that DMF significantly affected the gut microbiota composition.

The first cohort included 16 patients with RRMS, 17 patients with secondary progressive MS (SPMS), and 20 healthy controls. The second cohort included 11 patients with RRMS  treated with DMF and 8 patients with RRMS treated with ocrelizumab. 

Participants underwent lumbar puncture and venipuncture at baseline and six months after the treatment. The magnetic resonance imaging (MRI) scans were performed at baseline and during follow-up visits. The enzyme-linked immunosorbent assay was used to measure the CSF level of a neurodegeneration biomarker neurofilament light chain (NFL). To assess neurotoxicity, cultured hippocampal and cortical neurons isolated from Sprague–Dawley rat embryos were exposed to CSF samples obtained from MS patients, before and after DMF treatment. 

Results

The cultured hippocampal and cortical rat neurons exposed to CSF samples obtained from patients with RRMS or SPMS showed signs of axonal damage. The CSF samples of patients with RRMS treated with DMF exhibited reduced neurotoxicity, whereas CSF samples from patients with SPMS did not show this effect. CSF samples from healthy individuals did not exhibit any effect on cultured hippocampal and cortical rat neurons.

An in-depth analysis of the metabolites in plasma and CSF samples of patients diagnosed with RRMS revealed a specific group of three metabolites from the catabolism of two essential amino acids, tryptophan and phenylalanine. The ‘red module’ group included indoxyl sulfate (IS), an indole-derivative from the tryptophan metabolism, and two phenol-derivatives from the tyrosine and phenylalanine metabolism, p-cresol sulfate (pCS), and N-phenylacetylglutamine (PAG). The relative concentrations of these three metabolites highly correlated in plasma and CSF, and their concentration as a ‘group’ was higher in the CSF of RRMS patients than in healthy controls. The authors pointed out that phenol and indole derivatives after being produced by microbial species in the gut, enter the bloodstream and reach the CNS via CSF. In this way, they come into contact and interact with neurons in the superficial cortical layers.

Chronic exposure of cultured hippocampal and cortical rat neurons to increasing concentrations of three “red module” metabolites resulted in a dose-dependent neurotoxic effect, axonal damage, neuronal dysfunction, and reduction of spontaneous neuronal activity (average firing rate, number of spikes per second, and number of network bursts). The electrophysiological testing of cultured neurons showed significant and distinct effects of each metabolite from the “red module”, but also their synergistic effect on the mean firing rate and the number of spikes per second.

Cultured hippocampal and cortical rat neurons treated with increasing concentrations of “red module” metabolites did not differ in terms of mitochondrial function, oxygen consumption rate or respiratory capacity, compared to neurons that were not treated. According to the authors, the neurotoxic effect observed in cultured neurons treated with increasing concentrations of “red module” metabolites was independent of mitochondrial dysfunction and oxidative stress.

Treatment of MS patients with DMF  affected a specific group of “red module” metabolites. The pCS, IS and PAG concentrations significantly decreased in CSF and plasma obtained from patients diagnosed with RRMS and treated with DMF for 6 months. The relative levels of NFL, a neurodegeneration biomarker, correlated with reduced levels of pCS, IS, and PAG in CSF and plasma of RRMS patients treated with DMF. In contrast, the levels of the same metabolites remained unchanged in age-related RRMS patients treated with anti-CD20 therapy with properties independent of gut alterations.

After 12 months of DMF treatment, the ‘red module’ metabolite levels in the CSF samples from RRMS patients showed an inverse correlation with magnetic resonance imaging (MRI) metrics of cortical volume. There was no correlation with the volume of deep gray matter. Since the superficial brain layers are in closer contact with CSF and are exposed to neurotoxic microbial metabolites, the deeper structures of the gray matter are more protected from the CSF composition.

The authors noted that the healthy gut microbiota under physiological conditions processes tryptophan, to generate serotonin or metabolites such as kynurenate (which can be further converted into nicotinamide). In pathological conditions, the levels of serotonin and kynurenic acid decrease, as tryptophan is converted into an excess of indole-derivatives, such as indole acetate and indoxyl sulphate.

Conclusion

This study demonstrated the presence of a group of metabolites (indoxyl sulfate, p-cresol sulfate, and N-phenylacetylglutamine) as possible candidates for ‘toxic’ gut-brain communication in patients with MS. This study supports a pathological ‘metabolic shunt’ from healthy to toxic catabolites in patients diagnosed with RRMS. 

The authors noted that an imbalance between bacteria with beneficial effects and bacteria with detrimental effects, as detected in patients with MS, may lead to the depletion of neuroprotective metabolites and the potential accumulation of neurotoxic compounds. They emphasized that future studies are needed to accurately decipher the selective contribution and precise mechanism of these metabolites’ action.

This article was published in Brain.

Journal Reference

Ntranos A et al. Bacterial neurotoxic metabolites in multiple sclerosis cerebrospinal fluid and plasma. Brain 2022: 145; 569–583.  https://doi.org/10.1093/brain/awab320

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