It is the stuff with which fantasy and science fiction are made of. Remember that scene from the cult movie The Matrix (1999)? 'Jujitsu? I'm going to learn Jujitsu?' asks Neo, lying down, as the operator downloads modules of martial arts from digital tapes into the organic neural matter of Neo's brain. From kung-fu to flying a B-21 helicopter - all learning gets downloaded through a neuro-digital connection into the brain from the computer. That is an interesting version of an older fantasy, which is simply drinking a magic potion to help the brain learn a new skill automatically. If we go back in time, in the primitive age, ritual cannibalism held an axiomatic belief that all the powers, including the knowledge acquired by the victim, would become part of the one who eats the victim.
Of all these memory transfers, which one would the researchers bet on?
James V McConnell, who would now be called animal cognition scientist, was a colourful personality. Working in the psychology department of the University of Michigan, he also published his own journals. He pioneered satire in science with his Worm Runner's Digest while also publishing Journal of Biological Psychology. His work with flatworms (planarians) became a sensation in the field of learning. Those were the days of the behaviourist school when classical conditioning (the Pavlovian variety) was a rage in psychology departments. So McConnell doing a classical conditioning experiments with the flatworms - making them respond to conditioned stimulus - was not in itself unusual though it needed a lot of patience, hours of hard work and ingenious designing of the experimental setup. But planarians also have the ability for regeneration that when cut into half, they regenerate themselves into full worms. McConnell cut the worms and wanted to see if the regenerated worms - with their front part being classically conditioned still retained the memory. They did. But the real surprise came from elsewhere. The regenerated rear-part of the worm also showed signs of retaining the memory. In the words of McConnell himself:
The behaviour of our tail regenerates was, to say the least, rather unsettling. We were forced to conclude that memories were not necessarily stored just in the planarian’s brain. In fact, our data suggested that learning resulted from some biochemical change which was widespread throughout the planarian body, for subsequent informal studies showed that we got essentially the same retention whether we cut the worm in halves, thirds or fourth.James V McConnell, ‘Cannibalism and memory in flatworms’, ‘New Scientist’, 20-Feb-1964
Then came the next unsettling discovery in a more planned way. Zoologist Reeva Jacobson cut the planarians in half and the head part of the animals were given classical conditioning. Even as they learned the behaviour, the animals also regenerated. Now the animals was again cut into two pieces and now both the head and the tail parts were allowed to regenerate. What they found was the animals whose tail regenerated, also showed 'significant retention of the original learning.'
Going through the earlier work on bio-chemical basis for such memory transfers, the team discovered the work of Swedish biologist Holger Hyden, who was perhaps the first to theorise that RNA (Ribo-Nucleic Acid) had a central role to play in the biochemical basis of the memory. The RNA molecule structure would be modified by the neuronal electric impulses. This in turn resulted in changes in the protein structure, which the RNA produced. That was in 1959.
Two years later, another team from University of Rochester working on Hyden's hypothesis had further theorised that if the memory molecule was indeed RNA, then destruction of RNA (through Ribo-nuclease or RNA-ase - an enzyme that breaks RNA) should 'erase' the memory. So they did the same conditioning of planarians and cut the animals. Some of the animals were made to regenerate in pond water while others in a weak solution of RNA-ase. William Coring and his team, who conducted this study, showed that while the head regenerated in both pond water and solution retained the memory, the tail regenerated in the solution 'seemed to show relatively complete forgetting of the original learning.'
Pondering over these works McConnell and his team came to the conclusion that if memories indeed had a molecular basis, then if these molecules from one animal were to be successfully integrated into the system of another animal, then there should be a transfer of memory. But how to achieve that? They tried many techniques and failed. Then one of the early researchers, who used to train the planarians, observed that the flatworms turned cannibalistic if they were starved. So McConnell and his team starved a group of untrained flatworms and then fed them with pieces of trained flatworms. That’s when these untrained ‘cannibals’ were given their first session of training. McConnell writes:
To our great surprise (and pleasure) from their very first trails onward the cannibals showed significant evidence that they had somehow ‘ingested’ part of the training along with the trained tissue. ...Somehow, in a fashion still not clear to us, some part of the learning process seems to be transferable from one flatworm to another via ingestion.
The paper when published became sensational. It was almost like magic. However, the way McConnell took this forward, for example, by publishing the paper in his Worm Runners Digest and his appearance in sensational popular shows, which named him, McCannibal etc, did not endear him to the mainstream scientists. Further, many researchers, who tried to replicate the memory transfer experiment, failed to get results. Soon the memory transfer experiments were largely forgotten.
Meanwhile, in 1971, another neurobiologist Georges Ungar reported the discovery of another 'memory molecule' - not RNA but a small peptide chain. This peptide extracted from mice conditioned to choose light over dark, when injected into untrained mice induced in them the same behaviour, he claimed. However, the paper when published in Nature with a long commentary in 1972, received a lot of criticism and soon it was presented as an instance of 'dubious science' in an article published in The Science magazine. (Bernard Dixon, 'A Brief History of Dubious Science', Sep-1988).
This insightful passage by sociologists of science Harry Collin & Trevor Pinch, sums up how the memory transfer experiments were viewed and dismissed. Calling it 'an exemplary case of controversial science' they say:
In spite of the widespread demise of the credibility of the chemical transfer of memory, a determined upholder of the idea would find no published disproof that rests on decisive technical evidence. ... We no longer believe in memory transfer but this is because we tired of it, because more interesting problems came along, and because the principal experimenters lost their credibility. Memory transfer was never quite disproved; it just ceased to occupy the scientific imagination. The gaze of the golem turned elsewhere.
However, two decades after thus declared, the gaze of Golem would return to the memory transfer through RNA molecules. It seems to be a discovery whose time has come again.
And Now The Snails
In May 2018, a team of scientists revived the idea of memory transfer. This time they worked with sea snails (Aplysia).
Dr Priya Sethupathy's role in this series of development is quite important. A Searle scholar at Rockfeller University, her interest in neuroscience research started with her one year of work at the National Centre for Biological Sciences in Bengaluru. While working here, she began to ponder over investigating the role of micro RNAs in memory retention. In 2009, Rajasethupathy and his team of neuro-scientists at the Department of Neuroscience, Columbia University, have worked on Aplysia revealing the role of non-coding RNA 'in constraining synaptic plasticity'. In 2012, Rajasethupathy went a step further. They generated 'small RNA libraries' from the central nervous system (CNS) of the sea snails. The CNS of the snails or mollusks, is not exactly a brain, but three pairs of ganglia or groups of nerve cell bodies. In these 'libraries' they discovered 'an unexpectedly abundant expression of a 28 nucleotide sized class of piRNAs'. The pi-RNAs are the largest class of small non-coding RNA molecules expressed in animal cells. Most researchers think that the piRNAs may be involved in maternally derived epigenetic effects. In 2012, Rajasethupathy and his team concluded that their findings provided 'a small RNA-mediated gene regulatory mechanism for establishing stable long-term changes in neurons for the persistence of memory.'
In a paper published recently, neurobiologists David Glanzman, Alexis Bedecarrats and their team from the Department of Integrative Biology and Physiology at UCLA, have now claimed in a paper that the RNA extracted from a trained sea snail can induce what they call 'an epigenetic engram' in the untrained one.
The experiment was done as below.
The sea snails are given an electric shock. As a reaction they retract their protruding organ called siphons. The siphon-withdrawal reflex (SWR) is a well known reflex in these snails. With implanted platinum wires the snails were administered mild shock,s and SWR duration was noted. SWR period starts with the retraction of the siphon completely beneath lateral foot extensions called parapodia and ends with the reappearance of the siphon. A snail which has received shocks has a longer SWR than a 'naive' or an untrained snail. Now the team extracted RNA from the shock-trained snails. They made a RNA solution by dissolving it in deionised water and then injected them into the untrained snails. The control group was given RNA taken from similar untrained snails.
The ‘naive’ untrained snails, which were injected with the RNA showed significant time delay in SWR, similar to the snails that were given shock training.
What is even more important is that the team has also discovered that the RNA-based memory transfer is also related to DNA methylation, which is an epigenetic process by which gene expression is affected. Among the non-coding RNA that play an important role in epigenetics is also the piRNA we saw earlier in Dr Priya Rajasethupathy’s research. When Glanzman stopped DNA methylation in the recipient snails by injecting along with the RNA from sensitised snail, an inhibitor of the DNA methylation process called RG, the memory transfer did not happen.
There are still critical questions and the experiments need to be replicated and new experiments formulated on the basis of insights acquired. Surely, the neurobiologists are closing in on the molecular basis of memory or at least throwing light on some of the important aspects of it. And it is going to be stranger than (science) fiction. It is not just science fiction realised in labs. If proved and improved, according to Dr Glanzman, this RNA can be used 'in the not-too-distant-future ... to ameliorate the effects of Alzheimer’s disease or post-traumatic stress disorder'.
Meanwhile, looking back into the earlier decades, at the bold and insightful identification of RNA as the molecule related to memory by Holger Hyden in 1959, and those out of the box experiments by Mcconnell in 1960s, to what we have zeroed in on today, one can only say their memories and work stand honoured.
For Further Reading:
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