Lactobacillus reuteri tryptophan metabolism promotes host susceptibility to CNS autoimmunity

Abstract

Background Dysregulation of gut microbiota-associated tryptophan metabolism has been observed in patients with multiple sclerosis. However, defining direct mechanistic links between this apparent metabolic rewiring and individual constituents of the gut microbiota remains challenging. We and others have previously shown that colonization with the gut commensal and putative probiotic species, Lactobacillus reuteri, unexpectedly enhances host susceptibility to experimental autoimmune encephalomyelitis (EAE), a murine model of multiple sclerosis. To identify underlying mechanisms, we characterized the genome of commensal L. reuteri isolates, coupled with in vitro and in vivo metabolomic profiling, modulation of dietary substrates, and gut microbiota manipulation. Results The enzymes necessary to metabolize dietary tryptophan into immunomodulatory indole derivatives were enriched in the L. reuteri genomes, including araT, fldH, and amiE. Moreover, metabolite profiling of L. reuteri monocultures and serum of L. reuteri-colonized mice revealed a depletion of kynurenines and production of a wide array of known and novel tryptophan-derived aryl hydrocarbon receptor (AhR) agonists and antagonists, including indole acetate, indole-3-glyoxylic acid, tryptamine, p-cresol, and diverse imidazole derivatives. Functionally, dietary tryptophan was required for L. reuteri-dependent EAE exacerbation, while depletion of dietary tryptophan suppressed disease activity and inflammatory T cell responses in the CNS. Mechanistically, L. reuteri tryptophan-derived metabolites activated the AhR and enhanced T cell production of IL-17. Conclusions Our data suggests that tryptophan metabolism by gut commensals, such as the putative probiotic species L. reuteri, can unexpectedly enhance autoimmunity, inducing broad shifts in the metabolome and immunological repertoire.

Uncovering the specificity and predictability of tryptophan metabolism in lactic acid bacteria with genomics and metabolomics

“…This suggests that there might be multiple metabolic pathways for IAld. There is still no experimental proof explaining the precise chemical changes and corresponding catalytic enzyme system in the process of IAA conversion to IAld, even though many studies believe that IAA is the precursor to IAld ( Montgomery et al., 2022 ). The upstream products of IAld and its corresponding metabolic pathway still need experimental proof.…”

“…To produce the corresponding metabolites, a complete metabolic enzyme system is required (Dodd et al, 2017). Previous studies have found a small amount of gene evidence for tryptophan metabolism-related enzymes encoded in LAB, but only ArAT has been experimentally confirmed at present (Zelante et al, 2013;Cervantes-Barragan et al, 2017;Montgomery et al, 2022). It is widely present in L. reuteri and has numerous gene copies.…”

“…Tryptophan will be converted to ILA by fldH or LDH after being catabolized to indole-3-pyruvic acid (IPYA) by ArAT (Dodd et al, 2017;Laursen et al, 2021). The evidence of fldH was first found in Clostridium, and the previous labeling of fldH in Lactobacillus was mostly replaced by LDH (Montgomery et al, 2022), which was enriched in LAB, especially in Limosillactobacillus (Figure 2). LDH and fldH originate from the same EC number (1.1.1.-), and the protein sequence homology was ~40%.…”

Change in the Gut Microbiome and Immunity by Lacticaseibacillusrhamnosus Probio-M9

“…Although previous studies supported a beneficial effect of probiotics in enhancing immunity against the common cold, which can reduce the incidence ( 30 ), duration ( 31 ), and symptoms ( 32 ) of the common cold ( 28 , 33 , 34 ), some reported that the effects of the probiotics on the immune system and gastrointestinal symptoms in nonpatients are limited ( 29 , 35 ). One approach by which probiotics impact the host immune system is through the microbial metabolism that arises from intestinal microbiota catabolism ( 36 – 39 ): e.g., functional metagenomic studies have identified the associations between host proinflammatory cytokines and microbial tryptophan and palmitoleic acid metabolic pathways ( 37 , 38 , 40 – 42 ). Moreover, previous studies have illustrated that the gut microbiota and metabolites activate the host immune system, thereby increasing the expression of endocrine peptides and promoting host metabolic homeostasis ( 37 , 43 , 44 ).…”

Neuromicrobiology, an emerging neurometabolic facet of the gut microbiome?

“…The same authors highlighted that indole could contribute to adapting bacterial cells to nutrient-poor environments where amino acid catabolism is an important energy source ( Walters and Sperandio, 2006 ). A recent study showed that Lactobacillus reuteri isolated from murine gut microbiomes metabolize host dietary tryptophan into indole derivatives, kynurenines, and cresol and imidazoles, which may be involved in the regulation of CNS autoimmunity ( Montgomery et al, 2022 ).…”