Myelin entered the genome along with the retrovirus

(ORDO NEWS) — Myelin is an important component of the nervous system of vertebrates and humans, thanks to which their nerve impulses spread quickly and efficiently. A new study has demonstrated that the myelin gene could appear in the genome of vertebrates when infected with a retrovirus.

The myelin sheath surrounds the axons, the long extensions of nerve cells that conduct nerve impulses. The so-called glial cells, which are located next to neurons and ensure their work, participate in its formation . The design as a whole resembles a roll, in which many layers of cell membranes are wound around each other around the filling-neuron.

The emergence of myelin was an important event in the evolution of vertebrates. It is found in almost all animals of this class: fish, amphibians, reptiles, birds and mammals, including humans.

he exception is the most primitive of vertebrates, the so-called cyclostomes (for example, lampreys), which lack myelin. It is not present in a variety of invertebrates either. It follows from this that myelin arose at an early stage in the evolution of vertebrates.

The myelin sheath allows the signal to be carried out much faster, since the nerve impulse, thanks to it, seems to “jump” along the nerve fiber. This invention of evolution allowed vertebrates to acquire a more efficient and compact brain, which eventually led to the appearance of the human brain.

Neuroscientists have long been trying to figure out how and when myelin appeared in the process of evolution. A new study by scientists from the UK has convincingly shown that the emergence of myelin (in particular, its key protein Mbp) was associated with the infection of the cells of an ancient vertebrate (most likely its embryo) with a retrovirus.

In the nerve fibers outside the brain, simpler cells, called Schwann cells, form the myelin sheath. In the brain, the formation of the myelin sheath is provided by oligodendrocytes . They are a bit like octopuses, which cover several different axons with their outgrowths at once and wind around them, forming the notorious “rolls”.

It was oligodendrocytes and their development that the authors of the new article drew attention to . They used DNA microarray technology and were able to isolate an important component of the myelination process (the formation of a new “neuronal wrapper”) – RNLTR12-int .

From the sequence of RNLTR12-int it follows that it comes from the so-called endogenous retroviruses of the ERV1 family. Such viruses are called endogenous because they got into the genome a long time ago and firmly settled there, becoming its permanent and safe component.

The ERV1 family is a common retrovirus, accounting for about three percent of the entire DNA sequence. The sequences of retroviruses that turned out to be an integral part of the genome can become the so-called retrotransposons. This is one of the types of mobile elements of the genome that are able to move around it from place to place.

RNLTR12-int is a fragment of a virus that has lost its components and related functions. Initially, this sequence was involved in copying the ancient retrovirus. It should be noted that the viruses of this group, for self-copying, first “translate” their RNA into the language of DNA.

Scientists examined the genomes of various vertebrates and found many regions similar to RNLTR12-int . They were given the sonorous name “retromyelin”, which means “retrotransposons involved in the formation of myelin.”

After entering the genome, the retromyelin precursor virus could spread throughout it. Later, it was fixed in a region of the genome where it made possible the active synthesis of a characteristic protein, the main component of myelin Mbp. It was Mbp that became the basis for a whole complex of others, jointly participating in the formation of the myelin sheath.

The importance of this study is not limited to theoretical interest. Myelin is a key component of the nervous tissue and a participant in a number of normal and pathological processes occurring in it.

This is primarily about the so-called demyelinating disorders, in which the myelin sheath is destroyed, such as multiple sclerosis . Therefore, understanding the origin of myelin and the mechanisms of its functioning can be useful in the search for treatments for such disorders.

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