The team from Caltech (California Institute of Technology) showed that mice exposed to the gut microbial metabolite 4-ethylphenol (4EP) displayed anxiety-like behaviours. Their explanation for this change in emotional behaviour is that 4EP production induces changes in oligodendrocyte maturation and myelination.
“These findings reveal that a gut-derived molecule influences complex behaviours in mice through effects on oligodendrocyte function and myelin patterning in the brain,” wrote the scientists in the journal Nature.
What is 4EP?
4EP is a microbial metabolite (a by-product of microbes) that is produced in the gut when certain dietary components are metabolised. Initially produced by microbes in the intestines, 4EP is sulfated by the host into 4EPS (4-ethylphenol sulfate), which is then absorbed into the bloodstream and circulates throughout the body in both humans and mice.
There is some scientific evidence linking 4EP with neurobehavioural and neurodevelopment disorders. In a 2013 study, researchers showed that 4EP was present in higher levels in mice with altered neurological development, specifically, a mouse model of autism and schizophrenia. Then last year, the same Caltech researchers reported that 4EPS levels were about seven times higher in autistic children than neurotypical children.
“Poorly defined” mechanisms
However, according to the researchers, the mechanisms that mediate the gut-brain interactions remain “poorly defined”. With regard to 4EP, causation hasn’t been established between the biosynthesis of this molecule in the gut and its effect on brain function.
In this study, the Caltech scientists discovered a biosynthetic pathway for the production of 4EP. By
identifying the biosynthetic genes from the gut microbiome that convert dietary tyrosine to 4EP, they bioengineered gut bacteria to produce 4EPS in mice.
Although the current study used the amino acid tyrosine as a substrate, they showed that other dietary sources can also be metabolised by the gut microbiota into 4EP, and said that in humans they expect “diverse dietary and microbial community structures may impact circulating metabolite levels”.
Gut-derived 4EPS present in the brain
The researchers colonised separate groups of mice with strains either lacking 4EP (-) or producing 4EP (+). In the 4EP+ mice, the researchers showed that the 4EP was sulfated to 4EPS and that 4EPS had entered the brain.
“4EPS was detectable in the brains of 4EP+ mice that were treated with probenecid, which inhibits organic anion transporters that mediate efflux of small molecules through the blood-brain barrier, suggesting accumulation of 4EPS in the brain,” noted the researchers.
Next, the researchers showed that the presence of 4EPS in the brain is associated with altered activation and connectivity of specific brain regions, including several that are associated with the limbic system (the part of the brain involved in emotional and behavioural responses).
“We conclude that gut exposure to 4EP in mice results in altered functional connectivity and activity in multiple brain regions,” they said.
They also showed that 4EPS in the brain disrupts the maturation of oligodendrocytes in the brain.
“Gene expression signatures revealed altered oligodendrocyte function in the brain, and 4EPS impaired oligodendrocyte maturation in mice and decreased oligodendrocyte-neuron interactions in ex vivo brain cultures,” wrote the researchers.
Mice colonised with 4EP-producing bacteria also exhibited reduced myelination of neuronal axons. This is significant because altered myelination dynamics in the brain have been associated with behavioural outcomes in several studies.
4EPS increases anxiety-like behaviour
Next they investigated whether 4EP production in the gut is linked to certain behaviours through a series of tests. They observed that mice exposed to 4EPS displayed anxiety-like behaviours.
Finally, they sought to determine whether the 4EP-induced effects on oligodendrocytes contribute to altered behaviours. They did this by administering a drug called clemastine fumarate that promotes oligodendrocyte maturation.
“Notably, enhancing maturation of oligodendrocytes prevented behavioural changes in 4EP+ mice,” said the researchers, adding: “We observed similar improvements in anxiety-like behaviours with another myelination-inducing drug - miconazole.”
They concluded that 4EP(S) impacts anxiety-like behaviours in mice in a manner that includes effects on oligodendrocyte maturation.
Future research directions
“Future work will focus on uncovering how 4EPS leads to changes in oligodendrocyte maturation and myelination, defining brain regions that are causally affected and examining how myelination changes impact behaviour,” said the researchers.
“We propose the hypothesis that 4EP(S) represents the archetypical example of a neuroactive microbial molecule that impacts brain activity and complex behaviours in animals, conceptually similar to neurotransmitters that regulate the nervous system function,” they ventured.
Published online: doi.org/10.1038/s41586-022-04396-8
“A gut-derived metabolite alters brain activity and anxiety behaviour in mice”
Authors: Needham, B.D., Funabashi, M., Adame, M.D. et al.