Like other neurodevelopmental disorders, autism has an important genetic origin: mutations in certain genes influence the severity of the disorder. But the role of these genes is not limited to our body.
Our work reveals that some of the micro-organisms that make up the intestinal flora are sensitive to mutations of these genes involved in autism. Their abundance decreases when these genes are inactivated. However, we now know that our intestinal flora plays an important role not only in digestion, but also in other functions of our body, notably neurological.
By restoring the balance of the intestinal flora of mice models for autism, we have succeeded in reducing their symptoms. This work suggests that in the future, we may be able to consider relieving patients with certain autism spectrum disorders with probiotics.
Discover the amazing story of the close dialogue that takes place between intestinal bacteria and their autistic host.Genes involved in the severity of the disorder According to the DSM-5, Autism Spectrum Disorders (ASD) cover categories of neurodevelopmental disorders characterized by persistent deficits in communication and social interaction, restricted and repetitive behaviors, interests and/or activities, hyper or hypoactivity, with or without intellectual deficits, with or without language impairment.
These disorders, which generally appear before the age of 3, are imprinted on the personality of each individual and depend as much on the person’s genetic make-up as on the environment in which he or she evolves. It is estimated that today 1 person out of 68 is concerned by these ASD. A 2014 study established a correlation between mutations in the SHANK genes (SHANK1, SHANK2 and SHANK3, which are involved in the development and functioning of neural circuits), the degree of severity of the disorder and the physical characteristics of the patients. Among the mutations affecting the three SHANK genes, patients with a mutated SHANK3 gene have a lower IQ than the other two subgroups.
The microbiota, a player in the proper functioning of our body
Recent studies have shown that certain neurodevelopmental disorders could be linked to microbiota disorders. This term refers to all non-pathogenic micro-organisms (bacteria, viruses, parasites and fungi) found on the skin, in the mouth, as well as in the respiratory, urogenital and digestive systems.
In our digestive system, the intestinal microbiota, or “intestinal flora”, alone contains up to 100,000 billion microorganisms, or about 2 kg of the total weight of the body. It is now known that it plays an important role not only in digestive and metabolic functions, but also in the immune and neurological systems. Alterations in the composition or functioning of the intestinal flora (or “dysbiosis”) are implicated in a number of pathologies ranging from cancer to Alzheimer’s or Parkinson’s disease, as well as addiction, depression, schizophrenia or autism.
This dialogue between the intestinal microbiota and the brain takes place in a direct way: the microorganisms produce compounds that act at several levels. Some of these molecules are similar to the molecules used by neurons to communicate with each other, others can activate the immune system or intervene in the regulation of hormones.
Can the genes associated with neurodevelopmental problems, such as autism, also influence the intestinal flora? In turn, can this disturbed flora aggravate pathologies?
In autistic people, a different intestinal flora. To find out, our team first analyzed the profile of the intestinal microbiota in mice carrying the Shank3 mutation and compared it to that of “normal” (control) mice. The aim was to determine which species of bacteria lived in the intestines of both categories of mice, in what proportions, etc. The composition of our microbiota depends, among other things, on the way we are born (by caesarean section or natural way), on the places where we live, on the way we eat, on our life habits… A bit like fingerprints, it is a signature that is specific to us.
The results we obtained show that the microbiota of mice carrying a mutated gene differs from that of “normal” mice both in the composition and in the diversity of the bacteria that constitute it: certain phylum, genera and species of bacteria are over or under represented.
Thus, if we consider the two major bacterial species in the human intestinal flora, Firmicutes and Bacteroidetes, the first group is under expressed while the second is over expressed, with a more marked decrease in SHANK3 mutated females compared to males.
The genera Prevotella, (oral, vaginal and intestinal bacteria, involved in obesity, inflammation of the intestine and respiratory tract…) Christensenella (intestinal bacteria, involved in obesity, cholesterol, diabetes…), Streptococcus (intestinal bacteria, involved in diabetes…), Bacteroidetes (intestinal bacteria, involved in obesity, cholesterol, diabetes…) and Bacteroidetes (intestinal bacteria, involved in obesity, diabetes…). ), Streptococcus (naturally found in mucus, oral cavities, upper respiratory tract, digestive tract), Coprococcus (bacteria of the fecal microbiota) but especially Lactobacillus (vaginal, oral, intestinal lactic acid bacteria present in mother’s milk and used in the production of fermented products) are underexpressed in the mutated mice
On the other hand, Veillonella (oral and intestinal bacteria, involved in caries formation and chronic fatigue syndrome…) and Akkermansia (intestinal bacteria, involved in obesity, diabetes, gut inflammation…) are over expressed.
We focused on the genus Lactobacillus, and in particular on the species Lactobacillus reuteri (L. reuteri). This probiotic lactic acid bacterium is marketed in pharmacies to fight against infant colic. It produces reuterin, an antibiotic molecule that inhibits the growth of potentially pathogenic bacteria such as Escherichia coli.
When gut bacteria alter brain activity
Recent studies show that L. reuteri produces and secretes GABA (or a very similar molecule), a compound used in nerve communication. This compound acts directly in the brain by binding to other molecules (called “receptors”) located on the cells.
Remember that mice with the ShanK3 mutation reproduce two of the most predominant traits of autism: compulsive repetitive behavior and avoidance of social interactions. In these mice, the number of receptors for several molecules, including GABA, differs from those in normal mice. For example, in the hippocampus (a brain area involved in memory), there are fewer GABA receptors, while in the prefrontal cortex (a region that plays an important role in emotions and mood disorders), there are more. Differences also exist for other receptors such as those for oxytocin (a hormone secreted by the pituitary gland) and glutamate (an amino acid involved in the transmission of nervous messages).
We also found that these differences are correlated with the abundance and expression of L. reuteri bacteria in the gut. These correlations are either negative or positive, but they are significant. Finally, the analysis of six cytokines (hormones of the immune system) in the plasma of our mutated and control mice allowed us to observe that the expressions of these cytokines are no longer regulated in the mutated mice and that there was a significant correlation between these cytokines and L. reuteri bacteria.
All these observations and analyses allowed us to conclude that there is a real difference between mice carrying the Shank3 mutation and control mice, not only at the cerebral level, but also at the level of the immune system.
These differences are correlated with differences in the composition of L. reuteri bacteria in the intestine, so we decided to investigate further.
L. reuteri to the rescue
Sold as a probiotic, L. reuteri has been described in recent studies as playing a role in social behavior and stress. We therefore decided to give this bacterium, cultivated by us, in gavage to four-week-old mutated mice. Two groups were set up: a control group, made up of mutated mice that had not received the bacteria, and a group made up of mutated mice that had received the bacteria. Social behavior tests were performed after the L. reuteri diet, and then we sacrificed the mice to study their different receptors and hormones again.
Autism is a human pathology, and not all autistic traits are investigated in this study using animal models. However, the results of the behavioral analyses we conducted are interesting. First of all, males and females did not respond in the same way to L. reuteri. In the former, social deficits and repetitive behaviors were reduced. In males, L. reuteri stimulated interest in unfamiliar mice. In contrast, their anxiety behaviors were only minimally affected.
Females reacted differently to L. reuteri. They were less attracted to new social interactions. In addition, their repetitive behaviors were significantly reduced, as was their anxiety, although not significantly. Molecular analyses showed that gavage with L. reuteri had a direct effect on the different brain regions and cytokine composition in the plasma of the mice.
These results suggest that the gut microbiota may have played a role during brain development. Since the Shank3 gene is also expressed in neurons in the gut, it may be involved in altering and modifying the microbiota. This would explain the differences between the gut flora of people with autism spectrum disorders due to a mutated SHANK3 gene and those without.
Continuing to decipher the interactions with our microbiota This study provides a first glimpse of the links between the host’s genetic profile and its intestinal flora. It will now be necessary to further investigate how the genome of people with autism interacts with and modifies their gut microbiota.
Moreover, these results open a new avenue to explore in the treatment of autism: that of probiotics. Despite their temporary presence in the intestine, they are active, and taking them is not without consequences.
The dialogue between the genome of autistic people and their microbiota is increasingly audible. It now remains to understand it.
We recommend to try our probiotic colon drink.
