Myelin – The Gift From Retrovirus
In a galaxy far far away, back when creatures didn’t kill each other for political reasons when the only reason to suffer were viral infections one creature managed to do something special. While the others only inactivated the viruses, he managed to use a virus to his own benefit.
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It is no news that viral infections can sometimes add things to our genome and then we carry these reminders to our offspring. And it’s not even news that geneticists were capable of tracking one of these encounters back when vertebrates with jaws separated from those without jaws. But we will need a bit of relearning to understand the hot genetic news coming from Cambridge and Edinburgh.
What is a… retrotransposon?
Retroviruses are strange. They are covered in a phospholipid membrane and so, sometimes they build themselves into our DNA. The fact that they can cause probably the worst known ailment – AIDS – shows how capable they can be. They also have oncogenic potential. That means we owe them for a great number of cancers. But today, we care about their ability to reprogram cellular DNA.
When this happens in cells that create sperm of eggs, our offspring carries evidence of the disease. And so do their offspring, and so on. But since not the whole virus is being transferred the sequence is called a retrotransposon. This word is sometimes called a jumping gene. And it does fit. If nothing special is going on the retrotransposon stays calm. But when the organism is exposed to some kind of stress things start happening.
The retrotransposon goes wild, moves, duplicates, and appears where it previously wasn't. The organisms themselves help them. But it is a risky game. The hope is that a mutation that will help the carrier will appear. Sadly, the majority of the mutations are bad.
What is a… myelin?
In school, you might have heard that myelin is a container that surrounds the protrusions of the nerve cells called axons. They are one of the basic parts of the matter in the brain and the spinal cord. Having the neurons properly wrapped with myelin is essential to their function. Well, they do work without it but their speed is different by two orders of magnitude.
Now, anatomists and physiologists say myelin is an improvement that allowed nerves to not only function quickly but also across longer distances without the need to increase their diameter. An innovation that allowed vertebrates to increase in size and in reaction time.
Myelin seems to have appeared in vertebrates around the same time when evolution gave birth to jaws. The evidence comes in form of genes for the myelin container of nerves in the oldest known vertebrate.
So, finally, the genetic news. Molecular geneticists claim myelin-enhanced nerves were caused by an ancient retroviral infection. And that most likely it happened to one of our vertebrate ancestors and only blind luck caused the retroviral genes to get into a key position where they caused the massive production of the viral protein. So, in essence, the speed of our brains we have is only because of a viral infection.
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