Not just bacteria
Do intestinal symbiont fungi control the host's behavior?
Not so long ago it turned out that there is a direct functional connection between such seemingly distant organs as the intestine and the brain, in which our intestinal bacteria participate. But it turns out that other symbiotic human microorganisms belonging to the kingdom of fungi can also influence the work of the intestine-immunity-brain axis.
The fact that microorganisms can control our behavior is a little scary. And by the way, such examples are not uncommon in the animal world. Thus, Entomophaga grylli mushrooms, germinating in the body of an insect such as a grasshopper, "force" it to climb higher before death and take a pose that promotes the dispersion of fungal spores. And microscopic Massospora change the sexual behavior of infected cicadas, making them sexually hyperactive.
Fortunately, in our case we are not talking about parasitic, but symbiotic fungi. As you know, fungal communities (mycobiota) are an integral part of the microbiota (microbiome) of the intestine – the totality of all the "settled" microorganisms inhabiting it.
The microbiome was formed during the joint evolution of micro- and macroorganisms, as a result of which close symbiotic relationships were formed between them. A macroorganism, such as the human body, provides its roommates with "shelter and a table", and they, in turn, increase the efficiency of digestion, participate in metabolic processes, synthesize various biologically active substances, etc. All these relationships have already been well studied in terms of intestinal bacteria, but the role of fungi in many ways it still remains a mystery.
Recently, scientists from the Weil Medical College at Cornell University (USA) identified a group of fungi living in the intestinal mucosa of not only humans, but also mice, and assessed its role in physiological processes using the example of these laboratory animals.
It turned out that fungi induce immune cells T-lymphocytes to secrete two signaling proteins – interleukins 22 and 17. The first protein works directly in the intestine. As is known, its mucous membrane, on the one hand, should ensure the absorption of nutrients and water, on the other – prevent the penetration of intestinal microorganisms into the bloodstream. It turned out that with the help of interleukin 22, fungi support the barrier functions of the intestinal mucosa, protecting it from threats such as bacterial infections. This conclusion was confirmed in an experiment on the introduction of a specific fungal community into the intestines of mice deprived of their usual microflora.
As for interleukin 17, it is known that this immune protein is pro-inflammatory and participates in many stages of the immune response. At the same time, its signaling pathway functions in completely different tissues and organs, from cartilage and intestines to the lung and brain.
As it turned out, mice whose intestines were inhabited by fungi were characterized by increased sociality. Researchers have suggested that interleukin 17 is responsible for this effect, the molecules of which penetrate into the bloodstream, and then into the brain, where they bind to their receptors. This was confirmed in the experiment: genetically engineered mice in whose brain the gene encoding the interleukin 17 receptor was "disabled" did not change their behavior in response to colonization by the fungal community.
Article by Leonardi et al. Mucosal fungi promote gut barrier function and social behavior via Type 17 immunity is published in the journal Cell.
The results of this work suggest that we still know very little about the abilities of our symbiotic intestinal microorganisms to stimulate different brain functions and thus have a diverse and often unexpected effect on the physiology and behavior of their hosts.
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