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Brain Cells Share Information With Virus-Like Capsules

Updated: January 12, 2018, 5:05 P.M. ET

When Jason Shepherd first saw the structures under a microscope, he thought they looked like viruses. The problem was: he wasn’t studying viruses.

Shepherd studies a gene called Arc which is active in neurons, and plays a vital role in the brain. A mouse that’s born without Arc can’t learn or form new long-term memories. If it finds some cheese in a maze, it will have completely forgotten the right route the next day. “They can’t seem to respond or adapt to changes in their environment,” says Shepherd, who works at the University of Utah, and has been studying Arc for years. “Arc is really key to transducing the information from those experiences into changes in the brain.”

Despite its importance, Arc has been a very difficult gene to study. Scientists often work out what unusual genes do by comparing them to familiar ones with similar features—but Arc is one-of-a-kind. Other mammals have their own versions of Arc, as do birds, reptiles, and amphibians. But in each animal, Arc seems utterly unique—there’s no other gene quite like it. And Shepherd learned why when his team isolated the proteins that are made by Arc, and looked at them under a powerful microscope.

He saw that these Arc proteins assemble into hollow, spherical shells that look uncannily like viruses. “When we looked at them, we thought: What are these things?” says Shepherd. They reminded him of textbook pictures of HIV, and when he showed the images to HIV experts, they confirmed his suspicions. That, to put it bluntly, was a huge surprise. “Here was a brain gene that makes something that looks like a virus,” Shepherd says.

That’s not a coincidence. The team showed that Arc descends from an ancient group of genes called gypsy retrotransposons, which exist in the genomes of various animals, but can behave like their own independent entities.* They can make new copies of themselves, and paste those…

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