Of course, just because Arc walked and talked like a retrovirus, it doesn’t necessarily mean that’s what it once was. In order to be more certain, the team conducted a few novel experiments designed to see if Arc could “infect” cells like a real virus.
Indeed, they found that when the Arc capsids were given to brain cells of mice, the genetic material from within the capsid was transferred into the cells. A similar set of experiments in the Massachusetts-led paper on the common fruit fly found that genetic material transfer doesn’t just occur within neurons, but between neurons and muscles too.
So where did the original retrovirus for Arc come from? It’s likely that it emerged from retrotransposons, which are pieces of genetic material that can “jump” from site to site within genomes.
Credit: Jacobo Lopez, Yi-Chu Su, Hugo Vaca, University of Utah
Considered ancestral to modern retroviruses, the Utah team suspect that a retrotransposon leaped into an ancestor of ours 350-400 million years ago, which eventually morphed over time into Arc in humans. A similar process happened in flies 150 million years later.
So, Arc may be a virus-like remnant that helps control our memory – but that’s not the end of the story.
“What is the purpose of this RNA trafficking between cells, and what role does it play in memory formation?” Shepherd added. “Like most surprising observations, they end up posing more questions than answers – but we’re excited to follow this wild road!”
Either way, Arc isn't alone. "There are more than one hundred [retrovirus-like] genes in the human genome alone," the team add, but their specific roles remain an "open question."