Our bodies contain more bacteria than human cells, most of which live in our gut. In recent years, scientists have discovered links between the bacteria in our gut and problems such as obesity, immune disorders, and even depression. However, it is often not entirely clear whether these are causal relationships, nor how the bacteria in our gut will succeed in affecting our brains.
French researchers have now discovered a way to do this.
Our immune system is able to detect certain parts of the cell wall of intestinal bacteria, the so-called morphopeptides. The cells of our innate immune system contain special reagents, Nod2 proteins. The scientists wondered if these reagents would also show up on brain cells.
The researchers bred genetically modified mice that were unable to produce the reagent proteins. Compared to normal mice, the transgenic animals gained weight more quickly. According to the researchers, this indicates that morphopeptides play a role in the feeling of satiety.
In the second part of the experiment, the scientists fed mice slightly radioactive polypeptides. By monitoring the radioactivity in the mice’s bodies, they were able to learn how the proteins spread in the body. Within a few hours, they found the material back in the brain. “Food makes the intestinal flora thrive, and this indirectly triggers a ‘I am full’ signal to the brain,” says microbiologist Jeroen Raes (KU Leuven and Flanders Institute of Biotechnology).
The researchers found effects not only on appetite. Mice without the reagent proteins developed diabetes more often and lived shorter lives. And in female mice, the absence of the reagent raised body temperature, which led the researchers to suspect that morphopeptides also play a role in other physiological processes.
In experiments with mice, there is a good reserve. “Although it is very likely that this mechanism is also present in humans,” says Rice. “Further research will have to show exactly the effect of this. Above all, this is a very elegant experiment that reveals a new way in which bacteria in our gut communicate with the brain.”
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