# Tryptophan: A gut microbiota-derived metabolites regulating inflammation

Lucie Etienne-Mesmin, Benoit Chassaing, Andrew T Gewirtz

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Lucie Etienne-Mesmin, Benoit Chassaing, Andrew T Gewirtz. Tryptophan: A gut microbiota-derived metabolites regulating inflammation. World Journal of Gastrointestinal Pharmacology and Therapeutics, 2017, 8 (1), pp.7. 10.4292/wjgpt.v8.i1.7. hal-01658590

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# Tryptophan: A gut microbiota-derived metabolites regulating inflammation

Lucie Etienne-Mesmin, Benoit Chassaing, Andrew T Gewirtz

Lucie Etienne-Mesmin, Benoit Chassaing, Andrew T Gewirtz, Center for Inflammation, Immunity, and Infection, Institute for Biomedical Sciences, Georgia State University, Atlanta, GA 30303, United States

**Author contributions:** Etienne-Mesmin L wrote the manuscript; Chassaing B and Gewirtz AT critically revised the manuscript.

Supported by National Institutes of Health (NIH), Nos. DK099071 and DK083890; the Career Development Award from the Crohn's and Colitis Foundation of America (CCFA).

**Conflict-of-interest statement:** No potential conflicts of interest relevant to this article were reported.

**Open-Access:** This article is an open-access article which was selected by an in-house editor and fully peer-reviewed by external reviewers. It is distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/

**Manuscript source:** Invited manuscript

Correspondence to: Lucie Etienne-Mesmin, PhD, Center for Inflammation, Immunity, and Infection, Institute for Biomedical Sciences, Georgia State University, 100 Piedmont Ave SE, Atlanta, GA 30303, United States. letiennemesmin@gsu.edu
Telephone: +1-404-4133589
Fax: +1-404-4133580

Received: August 12, 2016
Peer-review started: August 12, 2016
First decision: September 16, 2016
Revised: September 22, 2016
Accepted: October 22, 2016
Article in press: October 24, 2016
Published online: February 6, 2017

## Abstract

Inflammatory bowel diseases (IBD), which comprise

Crohn's disease and ulcerative colitis, are chronic intestinal disorders with an increased prevalence and incidence over the last decade in many different regions over the world. The etiology of IBD is still not well defined, but evidence suggest that it results from perturbation of the homeostasis between the intestinal microbiota and the mucosal immune system, with the involvement of both genetic and environmental factors. Genome wide association studies, which involve large-scale genome-wide screening of potential polymorphism, have identified several mutations associated with IBD. Among them, *Card9*, a gene encoding an adapter molecule involved in innate immune response to fungi (*via* type C-lectin sensing) through the activation of IL-22 signaling pathway, has been identified as one IBD susceptible genes. Dietary compounds, which represent a source of energy and metabolites for gut bacteria, are also appreciated to be important actors in the etiology of IBD, for example by altering gut microbiota composition and by regulating the generation of short chain fatty acids. A noteworthy study published in the June 2016 issue of *Nature Medicine* by Lamas and colleagues investigates the interaction between *Card9* and the gut microbiota in the generation of the microbiota-derived tryptophan metabolite. This study highlights the role of tryptophan in dampening intestinal inflammation in susceptible hosts.

**Key words:** Intestinal inflammation; Tryptophan; Microbiota

© The Author(s) 2017. Published by Baishideng Publishing Group Inc. All rights reserved.

**Core tip:** A noteworthy article published in Nature Medicine by Lamas and colleagues highlights the role of tryptophan, a microbiota-derived metabolite, in reducing inflammation in the gut. This commentary puts in perspective the main results from this study.

Etienne-Mesmin L, Chassaing B, Gewirtz AT. Tryptophan: A gut
microbiota-derived metabolites regulating inflammation. *World J Gastrointest Pharmacol Ther* 2017; 8(1): 7-9 Available from:
URL: http://www.wjgnet.com/2150-5349/full/v8/i1/7.htm DOI:
http://dx.doi.org/10.4292/wjgpt.v8.i1.7

COMMENTARY ON HOT TOPICS

The human intestinal tract harbors a complex community including 100 trillion of microbes, referred as intestinal microbiota. This diverse microbial ecosystem provides benefits to the host, essentially through its role in energy metabolism and immunity. However, perturbations of gut microbiota (termed dysbiosis) is associated with several disorders, including inflammatory bowel disease (IBD) and metabolic syndrome (obesity-associated diseases)[1]. IBD arise as a complex interaction between host genetic factors, mucosal immune system, intestinal dysbiosis, and environmental factors among which dietary compounds being increasingly appreciated in the onset of inflammatory related disorders. Unraveling the complex crosstalk between these factors arise as a challenge for the understanding and treatment of these disorders. A study published in the June 2016 issue of Nature Medicine by Lamas et al [2] made significant progress in this area by investigating how a gene predisposing to IBD (*Card9*, encoding the caspase recruitment domain-containing protein 9) leads to a colitogenic microbiota by impairing its ability to generate tryptophan-derived metabolite.

In their study, the authors reported that the deletion of *Card9* gene, a central component of the innate anti-fungal immune response, render mice more prone to chemically-induced colitis by dextran sulfate sodium (DSS)[2]. This report strengthens previous studies conducted by others and identifying CARD9 as a gene predisposing to IBD in humans[3-5]. Lamas et al[2] also demonstrated that *Card9* knockout mice (*Card9*-/-) display alteration of immune-related signaling pathways in the colon, with a strong decrease in interleukin-22 (IL-22) production. The authors evidenced a shift in the bacterial communities and alterations in the composition of the fungal microbiota in *Card9*-/- mice. Complex inter-kingdom relationships exist in the gut microbiota, suggesting a possible role of CARD9 in shaping the bacterial and fungal communities and required to control fungi during colitis. To decipher the mechanism of such colitis susceptibility and the involvement of gut microbiota in the onset of colitis, the authors use a model of microbial transplantation to germ-free recipient animals, and showed that transfer of colitic-associated microbiota of *Card9*-/- susceptible hosts were sufficient to transferred colitis susceptibility and IL-22 cytokine production impairment in germ-free wild type (*Card9* sufficient) recipients. Those data strengthen the essential role played by the intestinal microbiota, bacteria but also fungi, in triggering intestinal inflammation following *Card9* impairment[2].

Further analysis revealed that the colitic-associated microbiota of *Card9*-/- mice is characterized by the absence of bacteria metabolizing tryptophan (an essential amino acid, whose intake is through the diet) into indoles derivatives, such as *Lactobacillus reuteri* and *Allobaculum* sp. Indoles derivatives are ligands for the aryl hydrocarbon receptor (AHR) that can drives local production of IL-22 by innate lymphoid cells and T-cells[6]. Importantly, the authors described that the treatment of *Card9*-/- susceptible animals with an AHR agonist [(i.e., 6-Formylindolo(3,2-b) carbazole named FICZ] was sufficient to restore a normal level of IL-22 production and to protect mice from DSS-induced colitis. Previous studies focusing on the amino acid tryptophan demonstrated that mice fed with a low-tryptophan diet became susceptible to chemically induced inflammation[7] and, conversely, mice or piglets fed with a tryptophan supplemented diet have a reduced inflammation and a decreased severity of DSS-induced colitis[8,9].

As a therapeutic strategy, the authors next postulated that altering the intestinal microbiota in genetically susceptible host so as to increase its ability to generate AHR ligands could protect from intestinal inflammation. Thus, the authors demonstrated that supplementation with three commensal *Lactobacillus* strains with high tryptophan-metabolic activities was sufficient to restore intestinal IL-22 production and to reverse the colitis susceptibility observed in susceptible *Card9*-/- mice. While previous studies have highlighted how diet can affect the microbiota in a detrimental way, such as the consumption of milk-fat-derived diet that lead to a bloom of pathobiont (i.e., *Bilophila wadsworthia*) and colitis in IL10-/- mice[10]; the study from Lamas et al[2] is a good example of the positive interplay between diet and the intestinal microbiota leading to the generation of microbial metabolites that play a central role in the protection against intestinal inflammation.

Finally, in their study, Lamas et al[2] further corroborated the results obtained in mice with the analysis of samples from IBD patients, and demonstrated that such patients have a reduced fecal AHR activity and fecal levels of tryptophan. The authors showed that these reductions correlate with *CARD9* polymorphism. These important findings consolidate the prominent role of dietary components and microbial-generated metabolites in mediating inflammation-related disorders. Tryptophan appears to be an important amino acid in IBD patients since they have lower levels of serum and fecal tryptophan compared to healthy subjects[2,11]. In light of the close relationship occurring between the intestinal microbiota and dietary intake, such data further highlight the need of controlling both macro- and micro-nutrients consumption in IBD patients with genetic predisposition.

In the same issue of Nature Medicine, an additional study by Rothhammer et al [12] also expand the substantial effect of tryptophan in regulating inflammation, by focusing their study on the central nervous system
(CNS), and providing evidence on the significant role
of the bidirectional communication between the gut
microbiota and the brain. The authors found that mice
fed with a tryptophan-deficient diet have exacerbated
CNS inflammation, corroborating the results from Lamas
et al [2]. These two reports support a potential probiotic
strategy, wherein tryptophan-catabolizing *Lactobacillus*
strains able to enhance AHR activity that can further
beneficially impact the immune system through IL-22
production. Further exploration of possible manipulations
of the gut microbiota through dietary modulations
by a tryptophan-enriched diet or by re-shaping the
microbiota via targeting specific populations of bacteria,
for example by favoring the tryptophan-producing
bacteria or by reducing its pro-inflammatory potential,
will provide novel insights into the development of
individual targeted approaches that can be harnessed to
prevent and/or treat IBD patients.

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P- Reviewer: Kamada N, Yu JS S- Editor: Qi Y L- Editor: A

E- Editor: Wu HL

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