Novel insights on how gut microbiota influence the circadian and stress systems
Prior research has established robust links between the circadian and stress response systems. Both rely on the same communication routes in the body: the hypothalamic-pituitary-adrenal axis (HPA-axis) and the autonomic nervous system. Also, the gut microbiota modulates HPA-axis stress-responding and baseline glucocorticoid levels, steroid hormones essential for the body´s daily functioning. However, until recently a more complete understanding of the role played by all these actors and the precise physiological network they share has remained poorly defined.
The importance of keeping to the rhythm
As an initial step, the research team performed metagenomic sequencing of mice gut microbiota, confirming previous research that indicated that microbiota populations oscillate during the day. Next, they set out to answer a clear-cut question: Is the influence of the gut microbiota on HPA-axis function time-of-day dependent? To answer this question, they used microbiota-depleted mice models to monitor the differences in plasma glucocorticoid levels across the day.
Among their discoveries was that in germ-free mice (i.e., animals completely depleted of microbiota), the sleep/wake peak characteristic of corticosterone (a glucocorticoid) in conventional mice was shifted to the dark phase. When analysing mice under antibiotic treatment, they observed that the corticosterone diurnal oscillation waves displayed higher rhythm amplitudes (for more details, please refer to Figure 1 in the manuscript). Further transcriptomic assays on brain tissues followed by the bioinformatic analysis of the detected genes led to the establishment of two fundamental facts: (1) the absence of gut microbiota profoundly alters master clock rhythmicity, and (2) microbiota exerts a major influence on the rhythmicity of stress pathways in the brain. Moreover, the same rhythmicity observed at the gene-expression level was then mimicked by later metabolomic results, which reveal the small molecule composition of a given sample. These assays determined that the individual’s microbial status alters diurnal oscillations in central glutamate metabolism. Glutamate, the most abundant neurotransmitter in the brain, is known to be affected by stress and is central in maintaining appropriate stress responsivity. Next, the team took a further step in processing the data and performed a multi-omic analysis of gene-expression and metabolomic data together. This analysis revealed that, across the day, the enrichment of small molecules that correlated with oscillations in functionally relevant genes was related to pathways important for glutamate metabolism.
On mouse behaviour and microbiota transfer
The authors also performed behavioural assays on microbiota-depleted animals at different time points, where they observed the time-of-the-day-specific alteration of social behaviours, manifesting in an enhanced anxiety-like behaviour. More importantly, when artificially blocking the synthesis of corticosterone in these mice before stress exposure, the stress-induced social impairments were absent.
Finally, crucial faecal microbiota transfer experiments, followed by metagenomic sequencing on faecal contents, revealed important differences in the bacterial composition of the samples analysed at strain and genus levels. Limosilactobacillus reuteri is highlighted as a potential circadian-sensitive strain and could be one of the microorganisms responsible for the modulation of corticosterone.
Not so fast
Among the many queries that ramify from this work´s discoveries comes a need to establish the exact nature of the putative microbial signals responsible for the observed modulation of the nervous system. An integral part of this query includes the mechanism(s) by which these signals travel from the gut to the brain to drive the regulation of the circadian and stress systems and modulate stress-sensitive brain areas. Studying the presented results in human cohorts will largely contribute to the translation value of these findings.
Why does it matter?
This work analyses the complex interplay of microbiota, glucocorticoid metabolism and brain function by integrating a multi-omics technical approach with an equally important bioinformatic analysis of the collected data and behavioural assays. This successful strategy allows the authors to determine for the first time that gut microbiota modulates both circadian and stress systems together, in a combined manner. These findings could be key to understanding microbe-host interactions in the context of immunity and metabolism.
Take home messages
1. Gut microbiota oscillates during the day and modulates both circadian and stress systems together, in a combined manner.
2. Diurnal oscillations in central glutamate metabolism are altered by microbial status.
3. reuteri is identified as a potential circadian-sensitive strain.
Guest author: Celina Galles, PhD
Reviewer: Barbara Fahmy, MS OTR, MPA
This article was written as part of a series of ‘journal club’ summaries for Scientific Writers Ltd and is based on the following publication.
Title: Gut microbiota regulates stress responsivity via the circadian system
First Author: Tofani G. S. S. et al
Journal: Cell Metabolism
Date online: 05 November 2024
Other references
1. Timmermans S, et al. A General Introduction to Glucocorticoid Biology. Immunol. 2019;10:1545.
2. Abuqwider J, et al. Limosilactobacillus reuteriin Health and Disease. 2022;10(3):522.
