Bad Virus, Good Virus

We always think that virus is a bad thing, especially nowadays in this Covid-19 global pandemic. As far as we know, it brings nothing but diseases, from common cold to HIV. But science has shown that there are always two sides in everything, including virus. It turns out virus not only has benefits to our body functions, but even our species can't survive until now if not because of virus.

Virus is a very simple organism that only consists mainly of envelope in the outside as outer layer, and genetic material in the inside covered with capsid as inner layer. The genetic material can be RNA or DNA, depends on the virus species. Virus lives as a parasite who uses its host cell to survive and replicate, and it has been around since a very long time ago, some says that it has already existed since the beginning of life itself.
Since millions of years ago, virus from Retrovirus family, whose one of its member is the famous HIV virus, had attacked our ancestors. Retrovirus attacks its host cell by a simple, yet elegant mechanism. First the ENV protein in the envelope will attach with receptor in the host cell membrane. Then the virus will go inside the cell, in there, the virus will change its RNA into DNA, and then inserts it into the host DNA. The now invaded host DNA will produce more virus and spread them into another cells, so on and so forth.

Usually this Retrovirus will only attack non-germ cells, so the virus DNA can't be passed down to the host offsprings. Germ cells are cells that are involved in reproduction process and they will pass on their DNA material to their host offsprings, i.e sperm and egg cell. But along the journey, this Retrovirus somehow managed to attack our ancestor germ cell, therefore our ancestor passed down this Retrovirus DNA to its descendants, including humans, us, and it permanently becomes part of our genome.

This process not only happened once, but many times over, so much that scientists say 8% of our genome contain this Retrovirus DNA. Most of that DNA have assimilated into our genome and don't have any effect at all. They are called "junk DNA". But a few of those DNA have mutated and hold significance impacts to our lives.

ENV gene is a gene in Retrovirus that produces ENV protein, remember? A glycoprotein in virus envelope whose role is to attach virus with host cell membrane. This gene is one of the Retrovirus DNA that passed down from our ancestor and mutated. 160 million years ago, this gene produced Syncytin 2 protein in Therians, a warm blooded, furry creature that would become ancestor of mammals. Syncytin 2 protein attached cells to one another and formed Syncytyotrophoblast, a layer of cells that separated fetus blood and maternal blood. This layer of cells facilitated nutrients and oxygen from maternal to reach embryo while protected embryo from pathogens at the same time. Thus, the first placenta was born. 25 million years ago, Syncytin 1 protein first appeared, and along with Syncytin 2 protein, they regulate the formation of Syncytyotrophoblast.

This placenta formation is an important step in mammals evolution. Now embryo can be fully developed inside the mother's womb before being born, instead of coming out as an egg that's more prone to dangerous in the outside world. Without it, our ancestors can't survive the evolution jungle, and thus, human won't exist. So we owe our existence to Retrovirus.

ARC (Activity Regulated Cytoskeleton) gene is another gene that comes from the mutation of Retrovirus DNA. This gene produces ARC mRNA, which produces ARC protein. These ARC mRNA and protein are located in dendrites, part of our neurons (nerve cells) that's functioning in signal transmission. As its name, ARC protein regulates neuron structure, which effects connection between neurons, and finally effects our ability to retain information. Thus, our capability to form memory is regulated by this ARC gene.

Not only that, ARC protein will accumulate in neuron and form capsid (like virus). This capsid will wrap ARC mRNA and other mRNA, and then shuffling them between neurons through synaptic cleft. Thus, ARC protein is capable to carry information between neurons, a task that we think can only be done by neurotransmitters before. This finding is important for researchers to find novel treatment of brain disorders.
There's another research saying that one of these Retrovirus-derived genes has role in our immune system. But there are another's who say that these genes also has roles in certain diseases such as Alzheimer's, Schizophrenia, and even cancer. (Hey, a virus gotta do what virus gotta do, right?)

In the end, nothing's completely bad, and nothing's completely good. There are always two sides in a coin. Even thing that we think is totally bad, turns out have goodness in it too.

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