2023-09-13 12:00:18
In humans and other placental mammals, gestation constitutes a unique moment during which a new organism develops, hosted and nourished by the maternal body. During this period, the placenta plays a dual role in connecting the two organisms: that of an exchange interface which allows the nutrition and respiration of the fetus as well as the elimination of its metabolic waste; and that of a protective barrier, opposing the transmission of pathogens from mother to child.
Also read our article (2014): The sex of the placenta influences the fate of the embryo
When this barrier is intact, it prevents mothers infected with herpes or AIDS viruses, for example, from transmitting them to the fetus, whose immune system is not yet developed. When it is defeated, on the other hand, the consequences for embryonic development can be disastrous. Microcephaly due to Zika virus infection during pregnancy is a tragic illustration of this.
At the placental interface, in mice as in humans, fetal cells penetrate deeply into the maternal tissues, reaching direct contact with the blood. If this intimate interaction increases the efficiency of exchanges, it also increases the risks of transmission of possible pathogens. However, an active line of defense opposes this, in the form of a continuous production of molecules specialized in communication between cells during infections: interferons, here “type III”.
Constant transcription in very large quantities
The high concentration of these interferons has the effect of implementing powerful antiviral defenses. But why are these interferons produced abundantly at this precise location, even in the absence of any infection? The team led by Hana Totary-Jain at the University of South Florida set out to understand this. In a study published in July in the journal Cell Host & Microbe, Ishani Wickramage, Jeffrey VanWye and their collaborators have demonstrated that the induction of type III interferons is maintained by a specificity of fetal placental cells: the constant transcription and in very large quantities of certain RNAs from a dedicated region of the genome, located on chromosome 19 in humans, on chromosome 2 in mice. The authors were able to specify that these RNAs belonged to a particular class of mobile elements of the genome: retrotransposons.
In most tissues of the body, these elements, whose property of “jumping” from one region to another is potentially mutagenic when activated, are kept silent. On the other hand, in an organ destined to be eliminated after a few months, such as the placenta, their mutagenic nature no longer constitutes a risk for the body. The study reveals that some of these elements are abundantly transcribed, in both directions of reading of the genome.
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