(ORDO NEWS) — We borrowed the ability to form memories from viruses.
How does memory work? The further we delve into this topic, the more questions arise about how the function of memory first arose.
Scientists made a key breakthrough when they identified the Arc protein in 1995, noting that its role in neuronal plastic changes was critical to memory formation.
This protein is already a big deal, but the Arc picture just got a lot more interesting.
A team of scientists from the University of Utah, the University of Copenhagen in Denmark and the MRC Molecular Biology Laboratory in Cambridge, UK, claims that Arc took its place in the brain through a chance encounter millions of years ago.
Just as scientists believe the mitochondria in our cells originated as bacteria that engulfed the cells of our ancient ancestors, the Arc protein appears to have originated as a virus.
The researchers knew they had found something when they imaged Arc, which looks a lot like a viral capsid, an isohedral protein shell that encases the virus’s genetic material for delivery to host cells during infection.
“At the time, we didn’t know much about the molecular function or evolutionary history of Arc,” said study co-author Jason Shepard, assistant professor of neuroscience, anatomy, biochemistry, and ophthalmology at the University of Utah.
Shepard studied Arc for 15 years. “Honestly, I almost lost interest in this protein. When we saw the capsids, we knew we had found something interesting.”
The main problem that prevents neuroscientists from understanding the essence of memory is that proteins are not stored for long in the brain, although memories last almost a lifetime.
Therefore, in order for memories to be preserved, plastic changes must occur, that is, the structures of neurons must change as a result of memory consolidation.
This is where Arc comes into play. A previous study in rats showed how Arc impairs memory consolidation, suggesting that Arc plays an important role in neuronal plasticity.
But scientists never thought they would stumble upon evidence pointing to a viral origin for Arc, as the new findings suggest.
The research team needed to test this theory, so they tested to see if Arc actually acts like a virus. It turned out that the Arc capsid encapsulates its own RNA.
When they placed the Arc capsids in cultured mouse brain cells, the capsids carried their RNA into the mouse brain cells, just as they do with a viral infection.
“We started this line of research knowing that Arc was special in many ways, but when we found that Arc was able to mediate cell-to-cell transfer of RNA, we were shocked,” said study lead author Elissa Pastuzin, Ph.D. , in your statement. “No other non-viral protein known to us acts in a similar way.”
The researchers suspect this virus-mammal collaboration happened sometime between 350 and 400 million years ago, when a retrotransposon – the ancestor of modern retroviruses – got its DNA into a four-legged creature.
They also suspect that this happened more than once. If they are right, this study complicates the picture of the evolution of life as we know it.
Many mutations not only happened by chance to make us what we are today, but we actually borrowed biology from other cells and organisms to get here. A little of their history lives on in us today.
Abstract: The neural gene Arc is necessary for long-term storage of information in the mammalian brain, mediates various forms of synaptic plasticity, and is involved in neurodevelopmental disorders.
However, little is known about the molecular function of Arc and its evolutionary origins. Here we show that Arc self-assembles into virus-like capsids that encapsulate RNA.
The endogenous Arc protein is released from neurons into extracellular vesicles, which mediate the transfer of Arc mRNA to new target cells, where it can undergo active-dependent translation. Purified Arc capsids are endocytosed and able to transfer Arc mRNA into the neuronal cytoplasm.
These results indicate that Arc has similar molecular properties to retroviral Gag proteins. Evolutionary analysis indicates that Arc is descended from a vertebrate lineage of Ty3/gypsy retrotransposons, which are also the ancestors of retroviruses.
These data suggest that Gag retroelements have been repurposed during evolution to mediate cell-to-cell communication in the nervous system.
Commentary: Despite the materialistic concept of memory and “evolution”, this find is amazing. If scientists were to put aside their notions of materialistic memory storage and random evolution, science could take a big step forward in understanding how the brain works.
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