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Cited by 221Gut microbiota and healthTryptophan and brain disordersGastrointestinal motility and disorders

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Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

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Bosi, Annalisa, Banfi, Davide, Bistoletti, Michela, Giaroni, Cristina, Baj, Andreina·2020·International Journal of Tryptophan Research
DOI 10.1177/1178646920928984PMC PMC7290275
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Abstract

The 'microbiota-gut-brain axis' plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

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Figure 1. Schematic representation of the interplay occurring between Trp metabolites and different sections of the microbiota-gut-brain axis. The gut saprophytic microflora can signal to the CNS and to the ENS via endocrine, immune, metabolic, and neuronal pathways detailed in the text. In normal conditions, the blood-brain barrier allows the access of tryptophan and kynurenines, but not of circulating 5-HT, which is synthesized from tryptophan in the brain. ACTH indicates adenocorticotrophin hormone; DRG, dorsal root ganglion; EC, enterochromaffin cells; ECC, enteroendocrine cell; IPAN, intrinsic primary afferent neurons; MP, myenteric plexus; NTS, nucleus of the solitary tract; NVG, nodose vagal ganglion; SCFA, short-chain fatty acid; SMP, submucosal plexus. Source: adapted from Baj et al.1
Figure 1. Schematic representation of the interplay occurring between Trp metabolites and different sections of the microbiota-gut-brain axis. The gut saprophytic microflora can signal to the CNS and to the ENS via endocrine, immune, metabolic, and neuronal pathways detailed in the text. In normal conditions, the blood-brain barrier allows the access of tryptophan and kynurenines, but not of circulating 5-HT, which is synthesized from tryptophan in the brain. ACTH indicates adenocorticotrophin hormone; DRG, dorsal root ganglion; EC, enterochromaffin cells; ECC, enteroendocrine cell; IPAN, intrinsic primary afferent neurons; MP, myenteric plexus; NTS, nucleus of the solitary tract; NVG, nodose vagal ganglion; SCFA, short-chain fatty acid; SMP, submucosal plexus. Source: adapted from Baj et al.1CC BY-NC · PubMed Central
Figure 2. Overview of tryptophan metabolism. Schematic representation of the major tryptophan metabolic routes yielding 5-HT, kynurenines, and indole derivatives/AhR pathways.
Figure 2. Overview of tryptophan metabolism. Schematic representation of the major tryptophan metabolic routes yielding 5-HT, kynurenines, and indole derivatives/AhR pathways.CC BY-NC · PubMed Central
Figure 3. Potential role of tryptophan metabolites in the modulation of symptoms associated with IBS and IBD. Environmental changes, stress, diet, previous infection, and dysbiosis may alter the homeostasis of the gut-brain axis underlying development of motor dysfunction, visceral pain, and CNS disorders, such as anxiety and depression in both IBS and IBD patients. Tryptophan metabolites by influencing the epithelial barrier, the immune function, and the ENS may peripherally influence symptom development, as detailed in the text. Furthermore, 5-HT and kynurenine may participate in behavioural disorders associated with both diseases. CNS indicates central nervous system; DRG, dorsal root ganglion; ENS, enteric nervous system; HPA, hypothalamic-pituitary axis; IBD, inflammatory bowel disease; IBS: irritable bowel syndrome; KYN, kynurenine; NGV, nodose vagal ganglion; NTS, nucleus of the solitary tract. Source: adapted from Baj et al.1
Figure 3. Potential role of tryptophan metabolites in the modulation of symptoms associated with IBS and IBD. Environmental changes, stress, diet, previous infection, and dysbiosis may alter the homeostasis of the gut-brain axis underlying development of motor dysfunction, visceral pain, and CNS disorders, such as anxiety and depression in both IBS and IBD patients. Tryptophan metabolites by influencing the epithelial barrier, the immune function, and the ENS may peripherally influence symptom development, as detailed in the text. Furthermore, 5-HT and kynurenine may participate in behavioural disorders associated with both diseases. CNS indicates central nervous system; DRG, dorsal root ganglion; ENS, enteric nervous system; HPA, hypothalamic-pituitary axis; IBD, inflammatory bowel disease; IBS: irritable bowel syndrome; KYN, kynurenine; NGV, nodose vagal ganglion; NTS, nucleus of the solitary tract. Source: adapted from Baj et al.1CC BY-NC · PubMed Central

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Research works printed but not linked: 6

  1. Carabotti M, 2015, Ann Gastroenterol, 28, 203
  2. Mulak A, 2004, Med Sci Monit, 10, RA55
  3. 10.2174/1570159X14666161228122 · DOI
  4. Abrams GD, 1963, Lab Invest, 12, 355
  5. Maes M, 2008, Neuro Endocrinol Lett, 29, 902
  6. 10.1038/nrgastro.2013.105 · DOI · Paper page

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Metadata

Authors
Bosi, A., Banfi, D., Bistoletti, M., Giaroni, C., & Baj, A.
Affiliations
University of Insubria (IT)
Corresponding author
Giaroni, Cristina
ORCID
Venue
International Journal of Tryptophan Research
Publisher
SAGE Publications
Published
2020-01
Volume
13
Pages
1178646920928984-1178646920928984
ISSN
1178-6469 (ISSN-L)
DOI
10.1177/1178646920928984
PMC
PMC7290275
OpenAlex
W3034589050
Cited by
221
References
360
Citations by year
2026: 38 · 2025: 35 · 2024: 31 · 2023: 40 · 2022: 41 · 2021: 30 · 2020: 4
FWCI
2.75
Type
journal article
Language
en
Open access
Licence
Locations
  • International Journal of Tryptophan Research (publishedVersion, cc-by-nc)
  • PubMed Central (publishedVersion, cc-by-nc)
  • PubMed (publishedVersion)
  • DOAJ (DOAJ: Directory of Open Access Journals) (submittedVersion, cc-by-sa)
  • IrInSubria (University of Insubria) (submittedVersion)
Topics
  • Life Sciences › Biochemistry, Genetics and Molecular Biology › Molecular Biology › Gut microbiota and health
  • Life Sciences › Neuroscience › Biological Psychiatry › Tryptophan and brain disorders
  • Health Sciences › Medicine › Gastroenterology › Gastrointestinal motility and disorders
Keywords
tryptophan metabolism, microbiota-gut-brain axis, gut microbiota, serotonin pathway, kynurenine pathway, aryl hydrocarbon receptor, irritable bowel syndrome, inflammatory bowel disease, mental illness, prebiotics, probiotics, host-microbe communication

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