The precise nature of the engram, the physical substrate of memory, remains uncertain. Here, it is reported that RNA extracted from the central nervous system of Aplysia given long-term sensitization training induced sensitization when injected into untrained animals; furthermore, the RNA-induced sensitization, like training-induced sensitization, required DNA methylation. In cellular experiments, treatment with RNA extracted from trained animals was found to increase excitability in sensory neurons, but not in motor neurons, dissociated from naïve animals. Thus, the behavioral, and a subset of the cellular, modifications characteristic of a form of nonassociative long-term memory in Aplysia can be transferred by RNA. These results indicate that RNA is sufficient to generate an engram for long-term sensitization in Aplysia and are consistent with the hypothesis that RNA-induced epigenetic changes underlie memory storage in Aplysia.Here is a discussion of the paper in SciAm.
The results pretty much speak for themselves and they clearly comport very well with the GKC, even the version that garnered the greatest number of superciliously raised eyebrows when mooted (viz. that the chemical locus of memory is in our nucleic acids (RNA/DNA). The Glanzman et. al. paper proposes just this.
A major advantage of our study over earlier studies of memory transfer is that we used a
type of learning, sensitization of the defensive withdrawal reflex in Aplysia , the cellular and molecular basis of which is exceptionally well characterized (Byrne and Hawkins, 2015; Kandel, 2001; Kandel, 2012). The extensive knowledge base regarding sensitization in Aplysia enabled us to show that the RNA from sensitized donors not only produced sensitization-like behavioral change in the naïve recipients, but also caused specific electrophysiological alterations of cultured neurons that mimic those observed in sensitized animals. The cellular changes observed after exposure of cultured neurons to RNA from trained animals significantly strengthens the case for positive memory transfer in our study. Another difference between our study and earlier attempts at memory transfer via RNA is that there is now at hand a mechanism, unknown 40 years ago, whereby RNA can powerfully influence the function of neurons: epigenetic modifications (Qureshi and Mehler, 2012). In fact, the role of ncRNA-mediated epigenetic changes in neural function, particularly in learning and memory, is currently the subject of vigorous investigation (Fischer, 2014; Landry et al., 2013; Marshall and Bredy, 2016; Nestler, 2014; Smalheiser, 2014; Sweatt, 2013). Our demonstration
399 that inhibition of DNA methylation blocks the memory transfer effect (Fig. 2 ) supports the hypothesis that the behavioral and cellular effects of RNA from sensitized Aplysia in our study are mediated, in part, by DNA methylation (see also Pearce et al., 2017; Rajasethupathy et al., 2012). The discovery that RNA from trained animals can transfer the engram for long-term sensitization in Aplysia offers dramatic support for the idea that memory can be stored nonsynaptically (Gallistel and Balsam, 2014; Holliday, 1999; Queenan et al., 2017), and indicates the limitations of the synaptic plasticity model of long-term memory storage (Mayford et al., 2012; Takeuchi et al., 2014).
Two remarks: First, as the SciAm discussion makes clear, selling this idea will not be easy. Scientists are, rightfully in my opinion, a conservative lot and it takes lots of work to dislodge a well entrenched hypothesis. This is so even for views that seem to have little going for them. Gallistel (&Balsam) argued extensively that there is little good reason to buy the connectionist/associationist story that lies behind the standard cog-neuro commitment to net based cognition. Nonetheless, the idea is the guiding regulative ideal within cog-neuro and it is unlikely that it will go quietly. Or as Glanzman put it in the SciAm piece:
“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.”The reason is simple actually: if Glanzman is right, then those working in this area will need substantial retraining, as well as a big time cognitive rethink. In other words, if the GKC is on the right track, then what we think of as cog-neuro will look very different in the future than it does today. And nobody trained in earlier methods of investigation and basic concepts suffers a revolution gladly. This is why we generally measure progress in the sciences in PFTs (i.e. Plank Funereal Time).
Second, it is amazing to see just how specific the questions concerning the bio basis of memory become once one makes the shift over to the the GKC. Here are two questions that the Glanzman et. al. paper ends with. Note the detailed specificity of the chemical speculation:
Our data indicate that essential components of the engram for LTM in Aplysia can be transferred to untrained animals, or to neurons in culture, via RNA. This finding raises two questions: (1) Which specific RNA(s) mediate(s) the memory transfer?, and (2) How does the naked RNA get from the hemolymph/cell culture medium into Aplysia neurons? Regarding the first question, although we do not know the identity of the memory-bearing molecules at present, we believe it is likely that they are non-coding RNAs (ncRNAs). Note that previous results have implicated ncRNAs, notably microRNAs (miRNAs) and Piwi-interacting RNAs (piRNAs) (Fiumara et al., 2015; Rajasethupathy et al., 2012; Rajasethupathy et al., 2009), in LTM in Aplysia . Long non-coding RNAs (lncRNAs) represent other potential candidate memory transfer molecules (Mercer et al., 2008). Regarding the second question, recent evidence has revealed potential pathways for the passage of cell-free, extracellular RNA from body fluids into neurons. Thus, miRNAs, for example, have been detected in many different types of body fluids, including blood plasma; and cell-free extracellular miRNAs can become encapsulated within exosomes or attached to proteins of the Argonaut (AGO) family, thereby rendering the miRNAs resistant to degradation by extracellular nucleases (Turchinovich et al., 2013; Turchinovich et al., 2012). Moreover, miRNA-containing exosomes have been reported to pass freely through the blood-brain barrier (Ridder et al., 2014; Xu et al., 2017). And it is now appreciated that RNAs can be exchanged between cells of the body, including between neurons, via extracellular vesicles (Ashley et al., 2018; Pastuzyn et al., 2018; Smalheiser, 2007; Tkach and Théry, 2016; Valadi et al., 2007). If, as we believe, ncRNAs in the RNA extracted from sensitized animals were transferred to Aplysia neurons, perhaps via extracellular vesicles, they likely caused one or more epigenetic effects that contributed to the induction and maintenance of LTM (Fig. 2 ).
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.So, is GKC right? I bet it is. How right is it? Well, it seems that we may find out very very soon.
Oh yes, before I sign off (gloating and happy I should add), let me thank Peter and Bill and Johan and Patrick for sending me the relevant papers. Thx.
Addendum: Here's a prediction. The Glanzman paper will be taken as arguing that synaptic connections play no role in memory. Now, my completely uneducated hunch is that this strong version may well be right. However, it is not really what the Glanzman paper claims. It makes the more modest claim that the engram is at least partly located in RNA structures. It leaves open the possibility that nets and connections still play a role (though an earlier paper by him argues that it is quite unclear how they do as massive reorganization of the net seems to leave prior memories intact). So the fall back position will be that the GKC might be right in part but that a lot (most) of the heavy cog-neuro lifting will be done by neural nets. Here is a taste of that criticism from the SciAm report:
“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.Here is where the Gallistel arguments will really come into play. I believe as the urgency of answering Randy's question (how do you store a retrievable number in a connectionist net?) will increase for precisely the reasons he noted. The urgency will increase because we know how a standard computing device can do this and now that we have identified the components of a chemical computer we know how this could be done without nets. So those who think that connections are the central device will have to finally face the behavioral/computational music. There is another game in town. Let the fun begin!!
