Memory is essential to human identity. Not only do they provide a mental record of our past selves, but they help us learn and make decisions. Recent research shows that components of this human capability may have evolved from viruses.
Scattered across the genomes of just about all organisms are pieces of viral DNA. To replicate, viruses inject their genetic material into the cells of their host organisms. Through millions of years of infections and evolution, bits of viral genes have embedded in the DNA of bacteria, plants and animals.
A Genetic Intruder
One such gene, called Arc, encodes a protein found in four-legged animals. Scientists have known for years that the Arc protein is important for forming and storing long term memories. They have shown that in mice without the Arc gene, memory suffers. For one, these mice forget things they learned just a day before. These mice also lack a key developmental window during which the brain rapidly learns and forms new connections. In humans, the Arc protein is likely also involved in Alzheimer's disease, schizophrenia, and other conditions.
In 2018, two groups of scientists, one from the University of Utah and another from the University of Massachusetts, revealed that the Arc protein has a virus-like structure that it uses to bring information from one neuron to another. From analyzing fly and mouse versions of Arc, these scientists figured out that it closely resembles a viral gene called gag. The protein encoded by gag transports viral genes from cell to cell during infection, similar to how the Arc protein transports information between neurons.
Arc both looks and works like viral proteins. In the animal brain, Arc plays a role in solidifying experiences into memories.
The University of Utah group's paper, published in Cell, also suggested that the virus-like protein evolved over the course of 300-400 million years as a way for neuronal cells to communicate. Since this discovery was announced, scientists have increasingly wondered what this says about how memory evolved.
Vesicle or Virus
Part of what makes this Arc protein so interesting is its role in extracellular vesicles, little membranous bubbles that contain materials for transport between cells. Arc proteins get to where they need to be via these vesicles, which only heightens the resemblance to viral structures. In a virus, a protein coating protects the virus's DNA or RNA—much like how the Arc protein encloses its messenger RNA—and this is sometimes contained in a bit cell membrane stolen from the host organism, similar to a vesicle.
As National Institute of Child Health and Human Development virologist Leonid Margolis told Quartz, "Since vesicles resemble viruses, the question of course is whether the first extracellular vesicles were primitive viruses and the viruses learned from extracellular vesicles or vice versa."
Researchers are on the hunt for more genes like Arc and an explanation of the relationship between viruses and memory. Not only will similar discoveries improve our understanding of how memory evolved, but they may also lead to developments for treating brain disorders.