New Light on Link Between Gut Bacteria and Anxiety

Summary: Researchers discover gut microbes may influence microRNA in the prefrontal cortex and amygdala, two areas of the brain associated with depression and anxiety.
Source: BioMed Central.

Research published in the open access journal Microbiome sheds new light on how gut bacteria may influence anxiety-like behaviors. Investigating the link between gut bacteria and biological molecules called microRNAs (miRNAs) in the brain; researchers at the APC Microbiome Institute at University College Cork, which is funded by Science Foundation Ireland, found that a significant number of miRNAs were changed in the brains of microbe-free mice. These mice are reared in a germ-free bubble and typically display abnormal anxiety, deficits in sociability and cognition, and increased depressive-like behaviors.


Dr Gerard Clarke, the corresponding author said: “Gut microbes seem to influence miRNAs in the amygdala and the prefrontal cortex. This is important because these miRNAs may affect physiological processes that are fundamental to the functioning of the central nervous system and in brain regions, such as the amygdala and prefrontal cortex, which are heavily implicated in anxiety and depression.”

miRNAs are short sequences of nucleotides (the building blocks of DNA and RNA), which can act to control how genes are expressed. miRNA dysregulation or dysfunction is believed to be an underlying factor contributing to stress-related psychiatric disorders, neurodegenerative diseases and neurodevelopmental abnormalities. miRNA changes in the brain have been implicated in anxiety-like behaviors.

Dr Clarke said: “It may be possible to modulate miRNAs in the brain for the treatment of psychiatric disorders but research in this area has faced several challenges, for example, finding safe and biologically stable compounds that are able to cross the blood-brain barrier and then act at the desired location in the brain. Our study suggests that some of the hurdles that stand in the way of exploiting the therapeutic potential of miRNAs could be cleared by instead targeting the gut microbiome.”

The researchers found that levels of 103 miRNAs were different in the amygdala and 31 in the prefrontal cortex of mice reared without gut bacteria (GF mice) compared to conventional mice. Adding back the gut microbiome later in life normalized some of the changes to miRNAs in the brain.
The findings suggest that a healthy microbiome is necessary for appropriate regulation of miRNAs in these brain regions. Previous research demonstrated that manipulation of the gut microbiome affects anxiety-like behaviors but this is the first time that the gut microbiome has been linked to miRNAs in both the amygdala and prefrontal cortex, according to the authors.


The researchers used next-generation-sequencing (NGS) to find out which miRNAs were present in the amygdala and the prefrontal cortex of groups of 10-12 control mice with a normal gut microbiota, GF mice and ex-GF mice — which had been colonized with bacteria by housing them with the control mice — and adult rats whose normal microbiota had been depleted with antibiotics.

miRNAs may affect physiological processes that are fundamental to the functioning of the central nervous system and in brain regions, such as the amygdala and prefrontal cortex, which are heavily implicated in anxiety and depression. NeuroscienceNews.com image is in the public domain.

They found that depleting the microbiota of adult rats with antibiotics impacted some miRNAs in the brain in a similar way to the GF mice. This suggests that even if a healthy microbiota is present in early life, subsequent changes in adulthood can impact miRNAs in the brain relevant to anxiety-like behaviors, according to the authors.

The authors note that the exact mechanism by which the gut microbiota is able to influence the miRNAs in the brain remains unclear. Even though the study shows that effects of the microbiota on miRNAs are present in more than one species (mice and rats), further research into the possible connection between gut bacteria, miRNAs and anxiety-like behaviors is needed before the findings can be translated to a clinical setting.

Dr Clarke said: “This is early stage research but the possibility of achieving the desired impact on miRNAs in specific brain regions by targeting the gut microbiota — for example by using psychobiotics — is an appealing prospect.”

About this neuroscience research article

Source: BioMed Central
Image Source: NeuroscienceNews.com image is in the public domain.
Original Research: Full open access research for “Microbial regulation of microRNA expression in the amygdala and prefrontal cortex” by Alan E. Hoban, Roman M. Stilling, Gerard M. Moloney, Rachel D. Moloney, Fergus Shanahan, Timothy G. Dinan, John F. Cryan and Gerard Clarke in Microbiome. Published online August 25 2017 doi:10.1186/s40168-017-0321-3

BioMed Central “New Light on Link Between Gut Bacteria and Anxiety.” NeuroscienceNews. NeuroscienceNews, 25 August 2017.
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Abstract
Microbial regulation of microRNA expression in the amygdala and prefrontal cortex

Background
There is growing evidence for a role of the gut microbiome in shaping behaviour relevant to many psychiatric and neurological disorders. Preclinical studies using germ-free (GF) animals have been essential in contributing to our current understanding of the potential importance of the host microbiome for neurodevelopment and behaviour.

In particular, it has been repeatedly demonstrated that manipulation of the gut microbiome modulates anxiety-like behaviours. The neural circuits that underlie anxiety- and fear-related behaviours are complex and heavily depend on functional communication between the amygdala and prefrontal cortex (PFC).
Previously, we have shown that the transcriptional networks within the amygdala and PFC of GF mice are altered. MicroRNAs (miRNAs) act through translational repression to control gene translation and have also been implicated in anxiety-like behaviours. However, it is unknown whether these features of host post-transcriptional machinery are also recruited by the gut microbiome to exert control over CNS transcriptional networks.


Results
We conducted Illumina® next-generation sequencing (NGS) in the amygdala and PFC of conventional, GF and germ-free colonized mice (exGF). We found a large proportion of miRNAs to be dysregulated in GF animals in both brain regions (103 in the amygdala and 31 in the PFC). Additionally, colonization of GF mice normalized some of the noted alterations.

Next, we used a complementary approach to GF by manipulating the adult rat microbiome with an antibiotic cocktail to deplete the gut microbiota and found that this strategy also impacted the expression of relevant miRNAs.


Conclusion
These results suggest that the microbiome is necessary for appropriate regulation of miRNA expression in brain regions implicated in anxiety-like behaviours.
“Microbial regulation of microRNA expression in the amygdala and prefrontal cortex” by Alan E. Hoban, Roman M. Stilling, Gerard M. Moloney, Rachel D. Moloney, Fergus Shanahan, Timothy G. Dinan, John F. Cryan and Gerard Clarke in Microbiome. Published online August 25 2017 doi:10.1186/s40168-017-0321-3