Thanks to viruses, we remember and learn

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Scientists are sure: if all viruses were removed from the surface of our globe, all life on Earth would be annihilated. As research has progressed, we have stopped viewing viruses as purely pathogenic organisms. Those that cause disease are only a small percentage.

Our guide through the fascinating world of viruses is Maciej Przybylski, habilitated doctor of medical sciences, virologist from the Chair and Department of Medical Microbiology of the Medical University of Warsaw.

1. A virion, or a viral particle, is a kind of transport capsule carrying genetic material, so when it leaves the cell in which it multiplied, it can reach another one to continue the replication process – explains Dr. Przybylski
2. As the expert explains, the record is thousands of daughter viruses replicated in one cell
3. What are giant viruses? Is it true that viruses can “travel” between continents? Is human DNA also derived from viruses? our interlocutor answers these (and many other) questions
4. More current information about the epidemic in Poland can be found on the TvoiLokony home page

Monika Zieleniewska, Medonet: All viruses are parasites that infect cells in order to multiply in them. Do they replicate in one pattern?

Dr hab. Maciej Przybylski: We can describe a common mechanism of multiplication of viruses, which is the same for all types of viruses. The virion, or viral particle, is a kind of transport capsule that carries genetic material, so when it leaves the cell in which it has multiplied, it can reach another one to continue the replication process. When this happens, it attaches to it via its surface proteins. It selects a specific receptor, i.e. a protein that is specific to a particular virus. As a result, different viruses infect different species, e.g. only humans or also other mammals, and this also determines the types of cells in the body that the virus can infect.

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The virus must hit a cell receptor with a specific three-dimensional structure corresponding to its surface protein described by a specific gene. After that, it must penetrate the cytoplasmic membrane of the cell. Once it has penetrated the cytoplasm, its genetic material, which is RNA or DNA, is released. As a general rule, for DNA viruses replication takes place in the nucleus, and for RNA viruses in the cytoplasm. However, there are exceptions, e.g. giant viruses from the DNA group replicate in the cytoplasm. Multiplication begins with the production of early genes, i.e. genes that the virus needs at the very beginning. These are genes that block certain cell functions and shift energy expenditure on virus multiplication.

When a cell is prepared so that it begins to support the replication of the virus, the genome is duplicated, that is, it produces as many copies of the DNA or RNA of the virus as possible. At the same time, the synthesis of proteins that will form its shell, called the capsid, begins. The last process is to release the virus from the cell. It may be associated, but not always, with the destruction of the cell.

How many viruses can multiply in one cell?

It depends on many factors on the part of both the virus and the cell. The record is thousands of daughter viruses replicated in one cell.

Apparently, in every milliliter of ocean water taken at random, we find 10 to the power of 8 bacteriophages. Let’s take a closer look at these viruses that were discovered at the beginning of the XNUMXth century.

Bacteriophages infect and often destroy bacteria. They were discovered when World War I was taking place in Europe, and before World War II, bacteriophage preparations were used to treat selected bacterial infections in several countries. The USSR was at the forefront of research on bacteriophages, where in the 40s bacteriophage preparations were used to treat, for example, bacterial dysentery, or dysentery. The interest of the Soviet authorities in bacteriophages translated into Poland after 1945. A research center on these viruses has been established at the Hirszfeld Institute in Wrocław. Currently, a team headed by prof. Andrzej Górski and Dr. Ryszard Międzybrodzki, who has a lot of success in the use of bacteriophages in the treatment of bacterial infections.

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The approach to bacteriophage therapy varies around the world. In Our Country or Georgia, anyone can buy phage preparations at a pharmacy. They are known to be safe, have been extensively tested, and there is not even a single set of data that would say that they can harm a person.

In Poland, the situation is different. A patient who is unsuccessful in antibiotic treatment may be assigned to an experimental therapy. A unique set of phages is prepared especially for him, it is an example of personalized medicine in the best sense of the word.

And at the beginning of the XNUMXst century, the world learned about giant viruses.

Giant viruses parasitize on amoebas, corals and other marine protozoa and invertebrates. They have quite funny names: mimi-, mama- and mumuviruses. They were officially discovered in 2002, but were known to us 10 years earlier, only then they were considered bacteria. Hence the name mimi, short for mimicking microbe, which means mimicking a bacterium. They were so large that they did not pass through virological filters (filter mesh size is 0,22 micrometers), used in laboratories to distinguish viruses from non-viruses such as bacteria. They can be seen in an optical microscope and additionally stained with standard dyes used in bacteriology.

We now know that they are not only larger but more complex than primitive bacteria. Of course, they are not able to multiply outside the host cell, but they have proteins in their genome responsible for protein synthesis and energy production, which until now were reserved for pro and eukaryotic cells, i.e. bacteria and cells containing a nucleus. They also have proteins related to nucleotide synthesis and DNA repair, and these are processes so advanced that we bind them to cellular organisms rather than viruses.

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Giant viruses create the so-called replication factories – virospheres. It is a separate region in the cytoplasm that resembles a separate cell nucleus. Scientists speculate that the giant viruses helped in the transition from prokaryotes to nuclear organisms. In addition, for example, mimivirus has its viral “satellite” called Sputnik, which infects a cell previously infected with mimivirus, and then blocks replication of the giant virus. It is, on the other hand, very small. Giant viruses are about 30 times larger than their smallest cousins.

It is said that human DNA comes in part from the viruses that once infected us. What does science say?

We started studying the human genome in earnest several years ago. We first used standard sequencing techniques (reading genetic information), and now we’re doing it much faster with next-generation sequencing. Currently, we can read sequences spanning hundreds of millions of nucleotides (letters of the genetic code) in no time. In a good laboratory, human DNA is sequenced within 24 hours.

For some time we have suspected, and now we know for sure, that the human genome includes retroviral sequences, i.e. genetic sequences of viruses from the Retroviridae family, which include, among others HIV. There are relatively many of them, this is a noticeable percentage of our genomic DNA. In the course of evolution, man came into contact with retroviruses, our evolutionary ancestors became infected. Once infected, the viral genome – initially RNA – is transcribed into DNA by an enzyme called reverse transcriptase. A DNA copy of the viral genome is made, and this DNA is incorporated into the genome of the human cell. This form of integrated viral DNA is called a provirus. In this form, the virus multiplies according to the scenario we presented earlier.

After our body has dealt with the infection, the viral DNA remains in our cells forever. And it turns out that it is inherited by us, it passes from generation to generation. With each subsequent infection with a new retrovirus species, the situation repeats itself. When we multiply this by all the species and generations that preceded modern man, it turns out that we have a fairly solid base of retroviral DNA in our genome. It contains DNA of both viruses that humans have come across relatively recently, such as HIV or HTLV, as well as ancient viral sequences, such as ERV (the so-called endogenous retroviruses). Surprisingly, some ERV sequences contain genes that our body needs, such as those related to energy production and immunity.

Scientists recently concluded that a certain retrovirus that infected primitive mammals about 100 million years ago was involved in the formation of the placenta. So without these endogenous viruses our evolution would be very different. And to make it even more interesting, if we compare the genome of mammals and the genome of these retroviruses, it turns out that they do not evolve in parallel. Thus, endogenous retroviruses provide an extra set of genes that we get completely independently.

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Are there any other similar examples?

Genes from endogenous retroviruses are involved in the process of remembering information. It is a specific type of RNA transported between neurons in the brain. It is necessary for the memorization process to run properly, and its transport takes place inside the viral pseudocapsid, made of retroviral proteins.

Imagine this: completely human nRNA transported between completely human nerve cells, but inside proteins of viral origin. The same proteins found in endogenous retroviral sequences that have been accompanying humans for a very long time. So long ago that they were part of our cells and we would not be able to learn properly without them.

A virus called HTLV, which has coexisted with humans for millennia, is used to detect prehistoric migration patterns. Research on it suggests that, for example, the inhabitants of Siberia crossed the Bering Strait. Is it true?

We have recognized sources that we use to track prehistoric migrations. It is archeology, i.e. fossil traces, these are cultural studies, e.g. ethnolinguistic ones, dealing with, among other things, the development of languages. We also have classic genetic tests that we divide into two groups: mitochondrial DNA inherited from the mother and nuclear DNA inherited from the father. Virus research is only in addition to what we already know from anthropology and genetics.

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Scientists have looked at three viruses here: HTLV retrovirus – the world’s widespread human leukemia virus; JC virus – a virus with a Janus face, because it causes infections that are common and asymptomatic, but in people with severe immunodeficiency, it can cause a serious, fatal disease called multifocal leukoencephalopathy. It is a neurological disease that destroys the nerve myelin sheaths; while the third virus is the popular herpes virus. Research on them confirmed the north-eastern migration route, i.e. the crossing of the Bering Passage by Siberian peoples. Migrations along the East Asian coast were traced with HTLV, which now also actively infects humans, and the sequences of endogenous ERV retroviruses.

In Brazil, a study was initiated that later covered Mexico and North America to compare the African and South American HTLV genotypes. It was about capturing the differences in mutations. Not surprisingly, ancient sequences of this virus have been detected that correspond to its transfer with the first nomadic peoples to the New World. It is also possible that the oceanic migration route leading directly from Africa to South America was already used then. Data is still being collected.

Interestingly, apart from this old migration route, the population of 20-30 thousand. Years ago, a common HTLV cluster was discovered, dating back to the XNUMXth – XNUMXth centuries. Then suddenly African HTLV appears in the American population. It is related to the slave trade.

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French scientists reportedly resurrected a prehistoric virus. Is it true?

It’s about a pandoravirus. It is one of the giant viruses, the cousin of mimi- and mamaviruses. Like them, it infects amoebas and is closely related to the marine environment. Based on back genetic analysis, his age was estimated at 30. years. Its shape resembles an amphora, therefore it belongs to the group of pitoviruses, from the Greek pitos – a type of amphora. The point is that Pandora received not a can from the gods, but an amphora.

To revive the virus you need to be very lucky, because the half-life of the DNA that we want to recover is quite short on this scale. The French found the pandoravirus in the frozen water brought from Chukotka. He was in the cells of prehistoric amoebas. Anyway, after frostbite, it turned out that modern amoebas are still susceptible to infection with this ancient virus.

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