Viral infections have enabled the complexity of the nervous system of vertebrates

Viral infections have enabled the complexity of the nervous system of vertebrates

Usually, we associate viruses with diseases. And often this is the case. Just as other microorganisms, such as bacteria, can live in symbiosis with us and the vast majority of them are not toxic, viruses, by their nature, usually cause us health problems. A type of virus called Retrovirus, is particularly dangerous, because copies of its genome can be introduced into our genome, and this, for example, can cause cancer. Fortunately, this genetic “loitering” is rarely passed on to offspring. But we’ve had a very long relationship with these viruses, and after thousands of years of co-evolution, the human genome ended up containing remnants of past infections in the form of viral DNA sequences mixed with our own, called retrotransposons. This is not a problem, because this DNA does not normally carry genetic information (it is part of what used to be called junk DNA) and therefore does not affect normal cell functions. But an example has been discovered that disproves this statement: the human genome contains a viral sequence that is not only “silent,” but essential for neurogenesis.

The nervous system is a network of cells responsible for controlling and coordinating the activity of an organism. They can be relatively simple, like jellyfish, which don’t even have a brain, or very complex, like ours. One of the keys to its proper functioning is that the electrical impulses that transmit information can be transmitted quickly to the part of the body that needs to receive it. In humans, as in many other species, this is done through nerves, which consist of axons, which are long projections sent by nerve cells connected to each other.

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In order for axons to communicate properly, they depend on the work of a white covering substance: myelin. Myelin is a layer of proteins and fats that acts like the plastic wrap we put around electrical wires: it insulates them from the outside and prevents signal loss. Without myelin, nerve impulses would not travel as quickly and nerves would not be as long and thin as ours. Therefore, if myelin did not exist, it would have been impossible for large animals with complex nervous systems, like us vertebrates, to have evolved.

According to an article published in the magazine cell By the group led by Dr. Robin Franklin of Altos Labs-Cambridge Institute of Science in the UK, the clade of vertebrates to which we belong (those with jaws) all possess an ancestral sequence from a retrovirus, which itself does so. They do not contain instructions for making any protein, as normal genes do, but instead have the ability to activate melanin production. They found this by analyzing genetic information for several species found in public databases, looking in particular for sequence tags that might have a viral origin.

In this way, in a group of animals, they discovered variations in a viral fragment originally known as RNLTR12-int, which they renamed RetroMyelin, which regulates a gene called SOX10. This relationship between RetroMyelin and SOX10 results in the production of more myelin around axons. They confirmed this when they blocked myelin in the laboratory in rats, frogs, and fish, and saw that the animals could not produce myelin naturally.

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Based on these data, one might believe that divine retrovirus infection that allowed the evolution of more efficient nervous systems should have occurred millions of years ago for a common ancestor of all vertebrates. But by studying the viral sequence in detail, scientists noticed that RetroMyelin had small differences and came to the conclusion that this was only possible because the retrovirus’s jump into the vertebrate genome had occurred several times during the course of evolution, and natural selection had ensured its preservation in every case. These cases. This indicates that it gave a clear advantage when it came to survival of individuals who were infected by it. It is an unusual phenomenon, called convergent evolution, that is usually seen in the biological processes of living organisms.

The condition of myelin and RNLTR12-int is not unique. It has been discovered that other retroviral residues are important for placental formation, and that a third of them are involved in the immune system. In this way, we gradually realize that in the part of the genome that we considered useless because it provides no information for making proteins, there are a series of regulatory sequences, some of them mistakenly “stolen” from viruses, which have essential functions and without which we would not have become what we are. . Who knows how many other secrets are hidden in this “dark” part of our DNA.

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