The finding from the lab of David Glanzman hints at the potential for new RNA-based treatments to one day restore lost memories and, if correct, could shake up the field of memory and learning.
“It’s pretty shocking,” said Dr. Todd Sacktor, a neurologist and memory researcher at SUNY Downstate Medical Center in Brooklyn, N.Y. “The big picture is we’re working out the basic alphabet of how memories are stored for the first time.” He was not involved in the research, which was published in eNeuro, the online journal of the Society for Neuroscience.
Many scientists are expected to view the research more cautiously. The work is in snails, animals that have proven a powerful model organism for neuroscience but whose simple brains work far differently than those of humans. The experiments will need to be replicated, including in animals with more complex brains. And the results fly in the face of a massive amount of evidence supporting the deeply entrenched idea that memories are stored through changes in the strength of connections, or synapses, between neurons.
“If he’s right, this would be absolutely earth-shattering,” said Tomás Ryan, an assistant professor at Trinity College Dublin, whose lab hunts for engrams, or the physical traces of memory. “But I don’t think it’s right.”
Glanzman knows his unceremonial demotion of the synapse is not going to go over well in the field. “I expect a lot of astonishment and skepticism,” he said. “I don’t expect people are going to have a parade for me at the next Society for Neuroscience meeting.”
Even his own colleagues were dubious. “It took me a long time to convince the people in my lab to do the experiment,” he said. “They thought it was nuts.”
Glanzman’s experiments—funded by the National Institutes of Health and the National Science Foundation—involved giving mild electrical shocks to the marine snail Aplysia californica. Shocked snails learn to withdraw their delicate siphons and gills for nearly a minute as a defense when they subsequently receive a weak touch; snails that have not been shocked withdraw only briefly.
The researchers extracted RNA from the nervous systems of snails that had been shocked and injected the material into unshocked snails. RNA’s primary role is to serve as a messenger inside cells, carrying protein-making instructions from its cousin DNA. But when this RNA was injected, these naive snails withdrew their siphons for extended periods of time after a soft touch. Control snails that received injections of RNA from snails that had not received shocks did not withdraw their siphons for as long.
“It’s as if we transferred a memory,” Glanzman said.
Glanzman’s group went further, showing that Aplysia sensory neurons in Petri dishes were more excitable, as they tend to be after being shocked, if they were exposed to RNA from shocked snails. Exposure to RNA from snails that had never been shocked did not cause the cells to become more excitable.
The results, said Glanzman, suggest that memories may be stored within the nucleus of neurons, where RNA is synthesized and can act on DNA to turn genes on and off. He said he thought memory storage involved these epigenetic changes—changes in the activity of genes and not in the DNA sequences that make up those genes—that are mediated by RNA.
This view challenges the widely held notion that memories are stored by enhancing synaptic connections between neurons. Rather, Glanzman sees synaptic changes that occur during memory formation as flowing from the information that the RNA is carrying.
“This idea is radical and definitely challenges the field,” said Li-Huei Tsai, a neuroscientist who directs the Picower Institute for Learning and Memory at the Massachusetts Institute of Technology. Tsai, who recently co-authored a major review on memory formation, called Glanzman’s study “impressive and interesting” and said a number of studies support the notion that epigenetic mechanisms play some role in memory formation, which is likely a complex and multifaceted process. But she said she strongly disagreed with Glanzman’s notion that synaptic connections do not play a key role in memory storage.
Trinity College’s Ryan, like Glanzman, stands with a minority of neuroscientists—some call them rebels—who question the idea that memory is stored through synaptic strength. In 2015, Ryan was lead author of a Science paper with MIT Nobelist Susumu Tonegawa that showed memories could be retrieved even after synapse strengthening was blocked. Ryan said he is pursuing the idea that memories are stored through ensembles of neurons bound together by new synaptic connections, not by strengthening of existing connections.
Ryan knows Glanzman and trusts his work. He said he believes the data in the new paper. But he doesn’t think the behavior of the snails, or the cells, proves that RNA is transferring memories. He said he doesn’t understand how RNA, which works on a time scale of minutes to hours, could be causing memory recall that is almost instantaneous, or how RNA could connect numerous parts of the brain, like the auditory and visual systems, that are involved in more complex memories.
But Glanzman said he is convinced RNA is playing a role that eclipses the synapse. In 2014, his lab showed that memories of shocks that had been lost in snails due to a series of experimental procedures could be recovered—but the synapse patterns that were lost with the memory reformed in random ways when the memories were recovered, suggesting memories were not stored there. Glanzman’s lab and others have also shown that long-term memory formation can be blocked by preventing epigenetic changes, even when synapse formation or strengthening is not altered.
“Synapses can come and go, but the memory can still be there,” he said, saying he sees synapses as merely the “reflection of knowledge held in the nucleus.”
Glanzman has studied memory for more than three decades. He did postdoctoral work with none other than Eric Kandel—the neuroscientist who shared the 2000 Nobel prize for research on Aplysia, probing the role of the synapse in memory—and he said he has spent most of his career believing that synaptic change was the key to memory storage.
But he said a series of findings from other labs and his own in recent years have led him to start questioning the synaptic dogma. He calls himself “a recovering synaptologist.”
The skepticism over Glanzman’s research may be in part because the work harkens back to an unnerving episode in science involving an unconventional psychologist, James V. McConnell, who spent years at the University of Michigan attempting to prove that something outside the brain—a factor he called “memory RNA”—could transfer memories. In the ’50s and ’60s, McConnell trained flatworms and then fed the bodies of trained worms to untrained worms. The untrained worms then appeared to exhibit the behavior of the trained worms they’d cannibalized, suggesting that memories were somehow transferred. He also showed that trained worms that were beheaded could remember their training after they grew new heads.
Though the work was replicated by some other labs, McConnell’s work was largely ridiculed and is often described as a cautionary tale because so much time and money was spent by other labs trying, often unsuccessfully, to replicate the work. (McConnell died in 1990, five years after he’d been a target of the Unabomber Theodore Kaczynski.)
Recently, developmental biologist Michael Levin at Tufts has replicated McConnell’s experiments on headless worms under more controlled settings and thinks McConnell may have indeed been correct.
Glanzman said one of McConnell’s students, Al Jacobson, demonstrated the transfer of memories between flatworms via RNA injections, coincidentally while an assistant professor at UCLA. The work was published in Nature in 1966 but Jacobsen never received tenure, perhaps because of doubts about his findings. The experiment was, however, replicated in rats shortly afterward.
Glanzman learned about McConnell’s work—and his satirical journal “Worm Runner’s Digest”—while he was a psychology undergraduate at Indiana University but never took the results seriously. Now, while he’s still not convinced McConnell was exactly right about being able to transfer memories, he does think both McConnell and Jacobson were onto something.
Working in the memory field can be tough for those who challenge the status quo. SUNY’s Sacktor, for example, has spent more than 25 years — despite the skepticism, rejection, and outright derision of fellow scientists—chasing down a single molecule, PKMzeta, that he believes is critical to the formation of long-term memories and may be connected to the RNA mechanisms that Glanzman has uncovered.
The stakes in the field are high because memory is so key to our sense of self and many scientists feel understanding the workings of memory is something that should have been figured out by now. “It’s the last of the great 20th-century questions in biology,” Sacktor said. “Some aspect has made it difficult for neuroscientists to figure out.”
The difficulty may be due in part to the overwhelming focus on synaptic strength. Some 12,000 papers have been published on synaptic strength without providing a good explanation for how memories are stored, Ryan noted, adding that he applauds Glanzman for opening up a new path, radical as it is, to explore.
“The reality is we know so little about memory,” Ryan said. “I’m excited about any new vistas and avenues.”