Recently researchers reported that ancient viruses have gotten on our nerves but in the best of ways. One specific retrovirus — rooted in the DNA of jawed vertebrates — helps turn on the production of a protein needed to insulate nerve fibers. Such insulation, called myelin, may have helped make quick thoughts and complex brains possible.
The team found that the retrovirus trick was so handy that it showed up many times in the evolution of vertebrates with jaws.
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Bottom of FormRetroviruses are RNA viruses that make DNA copies of themselves to embed in a host’s DNA. Rarely, these insertions can become a permanent part of who we are, being passed down from parent to children. Scientists once thought leftovers of these ancient viruses — known as jumping genes or retrotransposons — as genetic garbage, but that impression is changing, says neuroscientist Jason Shepherd, who was not involved in the study.
We’re searching more and more that these retrotransposons and retroviruses have influenced the evolution of life on the planet, says Shepherd, of the University of Utah Spencer Fox Eccles School of Medicine in Salt Lake City.
Leftovers of retroviruses were already known to have aided the evolution of the placenta, the immune system, and other important milestones in human evolution (SN: 5/16/17). Presently, they’re concerned with helping to produce myelin.
Myelin is a coating of fat and protein that encases long nerve fibers, which are known as axons. The coating works a little like the insulation around an electrical wire. Nerves sheathed in myelin can send electrical signals sooner than uninsulated nerves can.
Coated nerve fibers can also grow longer and be thinner than they would without insulation, letting animals grow bigger, says Robin Franklin, a stem cell biologist at Altos Labs-Cambridge Institute of Science in England. And, he says, thinner fibers can be packed into the nervous system more efficiently.
Franklin says, as a result of myelin, brains became more complex and vertebrates became more diverse. In early vertebrate evolution, If myelination hadn’t happened, we wouldn’t have the entire galaxy of vertebrate diversity that we see now.
Previously Franklin and colleagues went digging in collected data for proof of jumping genes, mostly those that may influence myelin production. They hit paydirt in data on the cells that cover nerves in myelin. From an ancient retrovirus, the team found high levels of RNA.
That RNA, dubbed Retro Myelin, does not have instructions for making a protein itself. Somewhat, the team found, the RNA latches on to a protein called SOX10, and together they turn on the production of myelin basic protein, which zips myelin into a tight sleeve around nerve cells.
When the researchers then used a genetic trick to lessen amounts of Retro Myelinin rat brains, zebrafish, and frogs, production of myelin basic protein dropped. That finding suggests that Retro Myelin RNA is essential for making myelin.
Other retrotransposons have shaped evolution by creating new switches in the genetic control panels of certain genes or by producing new versions of proteins that regulate the activity of genes, says Eirene Markenscoff-Papadimitriou, a developmental neuroscientist at Cornell University who was not involved in the research. However, producing RNA to influence the activity of a gene is a new trick. The finding is a very surprising and important demonstration, of a new type of developmental process being made possible by these retroviruses, she says.
Organisms with vertebrae and jaws, including fish, amphibians and mammals, have Retro Myelin, but lampreys and other jawless fish and invertebrates don’t. While that would seem to suggest that the retrovirus jumped into the common ancestor of jawed vertebrates, that doesn’t appear to have been the case.
The team found that each species they examined had its version of Retro Myelin, rather than related versions with some minor changes. That pattern recommends the retrovirus infected multiple species at different times yet resulted in the same outcome; a concept known as convergent evolution (SN: 9/6/13).
It isn’t clear why lampreys and other jawless vertebrates don’t have Retro Myelin. One option is that the viruses don’t infect lampreys, says study coauthor Tanay Ghosh, a computational biologist at Altos Labs. Or it could be that the viruses did invade lampreys but weren’t evolutionarily valuable and have been lost.
For the jawed vertebrates, Markenscoff-Papadimitriou says, having myelin basic protein and other components necessary for myelin production already in place may have been important to take advantage of Retro Myelin. We’re getting infected regularly, and some of these viruses are conferring evolutionary advantage. Scientists are seeing the success stories in which Retro Myelin probably immediately gave an advantage to the host organism and was kept, she says.
Generally, scientists are interested in genes and RNAs that produce proteins. They filter out noncoding RNAs — ones like Retro Myelin that don’t produce proteins, Markenscoff-Papadimitriou says. But, she notes, researchers are increasingly identifying that noncoding RNAs also do important jobs (SN: 4/7/19). This paper will be an inspiration to other developmental biologists to mine their data to look for the retrotransposons.
