Syncytin - The viral protein we owe our lives to
G'day team,
A short post today, but something I think's really cool. We'll have a chat about the virus that's integrated into our genome, and allows us successfully breed as a species!
Image source
Viral DNA in our Genome
We might think of our bodies (and the genomes that build them) as a unique evolutionary pillar, upon which our species is placed, but nothing could be further from the real truth. Our genome is a variable hodgepodge of mistakes, re-writes, and borrowed sequencing and nothing could highlight this further than it's viral component.
Everyone has heard of the human microbiome, the bacteria and fungi that live in our bodies working symbiotically to break down the sugars we can't touch and eat our waste. But not everyone is as familiar with the human virome. These viruses do not just live in our body, for our lifetimes, they live in our very genome and they live there for as long as we keep reproducing.
This all stems from how viruses work! There are many types of viruses and we won't go into details here. But large groups of viruses infect humans by gaining access to our cells, then forcing their way into the genome of that cell. Once there they use the cells own machinery to reproduce and finally destroy the cell and spread.
But sometimes things don't always go according to plan, and viruses get stuck! If this occurs in gametes (cells that are passed down generations) then we've got what we call an endogenous virus. A virus that lives in our genome. Over millennia this virus may do many things, just like the rest of our genome it's prone to mutations and these may deactivate the virus, change it's purpose or even take advantage of some of its properties.
It's one of these viruses and the protein we have hijacked that has allowed our species to thrive.
Human Endogenous Retrovirus - W
Also known simply as HERV-W is a massive retrovirus, making up about 1% of our genome. Huge when we consider that all coding DNA makes up only 4% of our genome. While most of the DNA in HERV-W is inactive, we have hijacked one of the proteins that used to compose its protective shell and put it to our own uses. The protein is known as syncytin.
Syncytin is a protein produced in the very earliest stages of embryological development, in the life-support center of a growing baby, the placenta. A placenta is composed of two layers of cells, which fuse together to allow transport of nutrients and gases from the mother's blood to the fetuses. The side that interfaces with the mother is made of cells called syncytiotrophoblasts, while the cells that interface with the fetus are called cytotrophoblasts.
Image source
Syncytin is produced in cytotrophoblasts and connects these two cell types together. Effectively anchoring the incredible delicate network that allows for mother-fetus nutrient and gas transfer.
Without this protein, the human reproductive process would be far less successful and as we've already discussed miscarriage rates recently, imagine the impact on our species if our reproductive success was even lower!
Beyond Humans
In an amazing case of convergent evolution, humans are not the only animals which have captured the syncytin protein and utilized it in placental development. In fact it's thought that this has occurred up to seven separate times, most often in rodent-like animals like guinea pigs, mice, and rabbits.
Thanks
*As always thanks for reading team and I hope everyone learned something and had as much fun reading as I had writing!
Stick around for more medical and science content!
-tfc
Resources
A large part of the information for this article was taken from the lecture 'Viruses and Viral Pathology' by Prof. Paul Young, unfortunately not publicly available.
- Retroviruses push the envelope for mammalian placentation
- Identification of an endogenous retroviral envelope gene with fusogenic activity and placenta-specific expression in the rabbit: a new "syncytin" in a third order of mammals
- Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis.
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