The mouse study joins a growing set of research that links the gut microbiome to the brain.
Published in Cell, the study found that the addition of the bacteria Lactobacillusreuteri, which is commonly found in human breast milk, increased the likelihood that previously antisocial mice would interact with each other. Lead author Shelly Buffington told us how the study could translate to the treatment of Autism Spectrum Disorder (ASD) and other neurodevelopmental disorders in humans.
ResearchGate: Can you explain your study and the significance of your findings?
Shelly Buffington: We found that a maternal high-fat diet in mice alters offspring social behavior and induces long-term changes in the offspring gut microbiome. We performed a series of co-housing experiments and fecal microbiota transplants into germ-free mice to determine if the alterations in the high-fat diet offspring microbiome were causative factors underlying their impaired social behavior. Our results suggested that there were one or more bacterial species in the regular mouse gut that are important for normal social behaviour that were missing or underrepresented in the maternal high-fat diet gut microbiome. We used whole genome shotgun sequencing to analyze the composition of the regular vs. maternal high-fat diet offspring gut microbiome. These data revealed a marked shift in microbial ecology at the species level.
The most underrepresented species in the maternal high-fat diet gut microbiome was Lactobacillus reuteri. This finding was very intriguing because Lactobacillus reuteri had previously been shown to increase the levels of the hormone oxytocin, which has been dubbed “the social hormone” as there is a lot of evidence that it plays an important role in modulating social behaviours in mammals. We then tested whether the introduction of Lactobacillus reuteri into the gut of maternal high-fat diet offspring was enough to reverse their social deficits. When we next assessed their social behaviour, we found that it was restored. We found that treatment with Lactobacillus reuteri significantly increased the number of oxytocin-producing cells in the brains of maternal high-fat diet offspring and restored, what we believe to be, social interaction-related plasticity in a key reward area of the brain.
Our results suggest that maternal diet-induced changes in the gut the microbiome can affect offspring social behaviour and that single species reconstitution of Lactobacillus reuteri can rescue these deficits. Furthermore, they add to a growing literature showing that the gut microbiome is an important player when it comes to behavior and that probiotics may hold therapeutic potential for the treatment of behavioral symptoms associated with neurodevelopmental disorders.
RG: Can you explain the method you used to reach these findings?
Buffington: We used a powerful combination of techniques including behavioural tests, 16S ribosomal RNA gene sequencing, whole genome shotgun sequencing, immunofluorescence microscopy, electrophysiology, and pharmacological approaches to study the link between maternal diet-induced changes in the offspring gut microbiome and behaviour. Our study is one of the first to combine a species-level analysis of the composition of the gut microbiome not only with behavioural assessment but also with functional analysis of plasticity in the brain.
ResearchGate: Do you know why the gut microbiome has such an impact on the brain?
Buffington: Communication between gut microbiota and the brain is complex –we don’t fully understand it yet, but we do know it’s bidirectional and multifaceted. No system works in isolation, there’s a significant amount of molecular cross-talk. One of the primary mediators between the gut and the brain is the vagus nerve, which provides two-way communication. Many, but not all, probiotics that alter the brain and behaviour in animal models depend on the integrity of the vagus nerve.
Another way gut microbiota can affect brain function is through stimulation of the immune system, altering the levels of both pro- and anti-inflammatory cytokines in the bloodstream. Dietary changes, for example, can compromise intestinal barrier integrity creating a route for bacterial products to enter the circulation and induce inflammation. Gut bacteria also generate metabolic byproducts, including short-chain fatty acids, which have been shown to modulate brain function and behaviour, as they breakdown dietary constituents. Certain species of gut bacteria even produce neurotransmitters including GABA, serotonin, dopamine, and acetylcholine as well as neurotransmitter precursors. Elucidating the mechanisms by which the gut modulates brain activity and vice versa is an exciting, active area of investigation that holds great promise for identifying novel therapeutic targets.
RG: What spurred you to look into the connection between the gut and the brain in the treatment of Autism-related antisocial behaviours?
Buffington: There has been a lot of great work published recently showing that bidirectional communication exists between the gut and the brain. This communication pathway is colloquially termed the “gut-brain axis.” Human epidemiological studies have shown that maternal obesity increases the risk of neurodevelopmental disorders in offspring. The same has been found in non-human primates.
In addition, many ASD patients co-present with gastrointestinal disorders, suggesting that they may have an imbalance in gut microbiota which contributes to their intestinal issues. When we observed abnormal social behaviour in our maternal high-fat diet mice, we hypothesized that changes in the maternal microbiome could in turn alter the offspring gut microbiome and that these alterations could underlie their behavioural deficits.
RG: What kind of behavioral improvements did you find in the mice who had a full restoration of their gut microbiome?
Buffington: We found that the maternal high-fat diet offspring that had been co-housed with mice with a normal microbiome preferred to interact with a mouse over an inanimate object. They also spent more time in contact with a stranger mouse when they were placed together in a neutral arena. Thus, they displayed normal social behaviour, for mice.
RG: Do you have any idea how this study could translate to human ASD sufferers?
Buffington: There is a lot of interest in the potential of probiotic treatments for alleviating behavioural symptoms in kids with ASD. The promise of our work lies in the finding that a single bacterial species, Lactobacillus reuteri, was able to reverse social behavioural deficits in maternal high-fat diet offspring. Not only did it restore the behavioural symptoms, but when we looked at the brains of the treated animals, we found that it also increased oxytocin levels. Several studies have suggested that oxytocin plays an important role in modulating social behaviour, not only in rodents, but in humans. Our findings suggest that Lactobacillus reuteri could prove to be useful as a novel, low-risk probiotic for treatment of behavioural symptoms associated with ASD.