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Memory Engram Cells Have Come of Age

2015; Cell Press; Volume: 87; Issue: 5 Linguagem: Inglês

10.1016/j.neuron.2015.08.002

1097-4199

Susumu Tonegawa, Xu Liu, Steve Ramirez, Roger L. Redondo,

Neuroscience and Neural Engineering

The idea that memory is stored in the brain as physical alterations goes back at least as far as Plato, but further conceptualization of this idea had to wait until the 20th century when two guiding theories were presented: the “engram theory” of Richard Semon and Donald Hebb’s “synaptic plasticity theory.” While a large number of studies have been conducted since, each supporting some aspect of each of these theories, until recently integrative evidence for the existence of engram cells and circuits as defined by the theories was lacking. In the past few years, the combination of transgenics, optogenetics, and other technologies has allowed neuroscientists to begin identifying memory engram cells by detecting specific populations of cells activated during specific learning epochs and by engineering them not only to evoke recall of the original memory, but also to alter the content of the memory. The idea that memory is stored in the brain as physical alterations goes back at least as far as Plato, but further conceptualization of this idea had to wait until the 20th century when two guiding theories were presented: the “engram theory” of Richard Semon and Donald Hebb’s “synaptic plasticity theory.” While a large number of studies have been conducted since, each supporting some aspect of each of these theories, until recently integrative evidence for the existence of engram cells and circuits as defined by the theories was lacking. In the past few years, the combination of transgenics, optogenetics, and other technologies has allowed neuroscientists to begin identifying memory engram cells by detecting specific populations of cells activated during specific learning epochs and by engineering them not only to evoke recall of the original memory, but also to alter the content of the memory. Does the brain store memories? This seemingly obvious theme in contemporary neuroscience was actually hotly debated by leading scholars of learning and memory as recently as a century ago. For some, it was obvious that memory is represented in the brain (that is, physically), but others argued that it is stored in the mind (that is, psychically) (Bergson, 1911Bergson H. Matter and memory. Swan Sonnenschein, London1911Google Scholar, McDougall, 1911McDougall W. Body and mind: a history and a defense of animism. Methuen & Co., Ltd, London1911Google Scholar). In this paper, we will review the recent advances demonstrating that memory is indeed held in specific populations of neurons, referred to as memory engram cells, and their associated circuits. We will then sketch out a new perspective in the neuroscience of learning and memory, including potential applications for the development of therapeutic methods for brain disorders. In the first decade of the 20th century, Richard Semon, a German scientist who wrote two books on this subject (Semon, 1904Semon R. Die Mneme als erhaltendes Prinzip im Wechsel des organischen Geschehens. Wilhelm Engelmann, Leipzig1904Google Scholar, Semon, 1909Semon R. Die nmemischen Empfindungen. Wilhelm Engelmann, Leipzig1909Google Scholar), advocated the physical theory of human memory. Unfortunately, Semon’s contributions were almost completely ignored by mainstream psychologists concerned with the human memory process until the late 1970s and early 1980s, when Daniel Schacter, James Eich, and Endel Tulving revived Semon’s theory (Schacter, 1982Schacter D.L. Stranger behind the engram: theories of memory and the psychology of science. L. Erlbaum Associates, 1982Google Scholar, Schacter et al., 1978Schacter D.L. Eich J.E. Tulvlng E. Richard Semon’ s Theory of Memory.J. Verbal Learn. Verbal Behav. 1978; 17: 721-743Crossref Scopus (116) Google Scholar). Semon coined the term “engram,” which he defined as “…the enduring though primarily latent modification in the irritable substance produced by a stimulus (from an experience)…” (Semon, 1904Semon R. Die Mneme als erhaltendes Prinzip im Wechsel des organischen Geschehens. Wilhelm Engelmann, Leipzig1904Google Scholar). “Engram” is roughly equivalent to “memory trace,” the term used by some contemporary neuroscientists. Semon’s memory engram theory was built on two fundamental postulates termed the “Law of Engraphy” and the “Law of Ecphory” for memory storage and memory retrieval, respectively. The Law of Engraphy posits that “All simultaneous excitations (derived from experience)…form a connected simultaneous complex of excitations which, as such acts engraphically, that is to say leaves behind it a connected and to that extent, separate unified engram-complex,” (Semon, 1923Semon R. Mnemic Philosophy. Allen & Unwin, 1923Google Scholar). The Law of Ecphory on the other hand posits that “The partial return of an energetic situation which has fixed itself engraphically acts in an ecphoric sense upon a simultaneous engram complex,” (Semon, 1923Semon R. Mnemic Philosophy. Allen & Unwin, 1923Google Scholar). Thus, Semon’s view of retrieval is reintegrative. Only part of the total situation (i.e., stimuli) at the time of storage needs be present at the time of recall in order for retrieval of the original event in its entirety to occur (Schacter et al., 1978Schacter D.L. Eich J.E. Tulvlng E. Richard Semon’ s Theory of Memory.J. Verbal Learn. Verbal Behav. 1978; 17: 721-743Crossref Scopus (116) Google Scholar). Semon’s concept about memory retrieval is evidence for his amazing insightfulness, because it is nothing but the process of “pattern completion” theorized (Marr, 1970Marr D. A theory for cerebral neocortex.Proc. R. Soc. Lond. B Biol. Sci. 1970; 176: 161-234Crossref PubMed Google Scholar) and experimentally demonstrated many decades later (Leutgeb et al., 2004Leutgeb S. Leutgeb J.K. Treves A. Moser M.-B. Moser E.I. Distinct ensemble codes in hippocampal areas CA3 and CA1.Science. 2004; 305: 1295-1298Crossref PubMed Scopus (552) Google Scholar, Nakazawa et al., 2003Nakazawa K. Sun L.D. Quirk M.C. Rondi-Reig L. Wilson M.A. Tonegawa S. Hippocampal CA3 NMDA receptors are crucial for memory acquisition of one-time experience.Neuron. 2003; 38: 305-315Abstract Full Text Full Text PDF PubMed Scopus (391) Google Scholar). Semon’s conceptualizations of the memory process were novel at his time and were remarkably predictive of our contemporary state of memory research. However, he did not elaborate on the biological basis for the “simultaneous excitations” and “a connected unified engram complex.” This is not surprising considering that the theory was put forward nearly a century before the rapid development of molecular, cellular, and genetic biology, and sophisticated imaging and electrophysiological technologies for the analysis of the nervous system. Incorporating our current knowledge about neurons, synaptic connections, and neuronal circuits into Semon’s memory engram theory, we propose usage of the terms engram, engram cells, and other associated terminologies in these contemporary contexts as follows:•“Engram” refers to the enduring physical and/or chemical changes that were elicited by learning and underlie the newly formed memory associations.•“Engram cells” are a population of neurons that are activated by learning, have enduring cellular changes as a consequence of learning, and whose reactivation by a part of the original stimuli delivered during learning results in memory recall. Note that this goes beyond a correlational definition of the term.•“Engram cell pathway” is a set of engram cells for a given memory connected by specific neuronal circuits. It’s important to note here that these connections don’t necessarily have to be direct.•An “engram component” is the content of an engram stored in an individual engram cell population within the engram cell pathway.•“Engram complex” refers to the whole engram for a given memory that is stored in a set of engram cell populations connected by an engram cell pathway. The last three terms were introduced because the latest studies on engram cell populations have indicated that an engram of a given memory is not necessarily located in a single anatomical location, but is distributed in multiple locations connected in a pattern specific to the given memory, forming an “engram cell pathway.” The term “engram component” denotes not necessarily the specific physiological content of the engram held by a given population of engram cells, but rather the type of represented mnemonic information. Decades after the English translation of Semon’s original book was published (Semon, 1921Semon R.W. The Mneme. G. Allen & Unwin Limited, 1921Google Scholar), the American psychologist Karl Lashley pioneered a systematic hunt for engram cells in the rodent brain by introducing lesions of varying sizes into different sites of the cerebral cortex and attempting to find associations of each of these lesions with the ability of the animals to solve a maze task. The results showed that the behavioral impairments were due to lesions introduced throughout the brain and that the severity of the impairments was proportional to the size of the lesions wherever the lesions were introduced. Based on these results, Lashley concluded that the putative memory engram cells are not localized in the cerebral cortex, leading him to formulate the Mass Action Principle (Lashley, 1950Lashley K. In search of the engram.Symp. Soc. Exp. Biol. 1950; 4: 454-482Google Scholar). As discussed below, Lashley’s notion that engram cells for a specific memory are spread broadly and indiscriminately throughout the brain has not been supported by subsequent studies for at least several types of memory, including episodic memory. It is conjectured that Lashley’s failure in identifying localized engram cells is because the maze tasks he used were too complex and required multiple regions of the cerebral cortex, and/or the primary sites of the storage of this type of memory may be in subcortical regions. Lashley’s extreme view was wrong, but as will be discussed later in this article, a certain type of memory (e.g., contextual fear memory) could be distributed over limited, but multiple, brain regions (e.g., hippocampus and amygdala, etc.). Years later, Canadian neurosurgeons Wilder Penfield and Theodore Rasmussen (Penfield and Rasmussen, 1950Penfield W. Rasmussen T. The cerebral cortex of man: a clinical study of localization of function. Macmillan, New York1950Google Scholar) serendipitously obtained the first tantalizing hint that episodic memories may be localized in specific brain regions. As a pre-surgery procedure, Penfield applied small jolts of electricity to the brain to reveal which regions were centers for causing seizures. Remarkably, when stimulating parts of the lateral temporal cortex, approximately 8% of his patients reported vivid recall of random episodic memories (Penfield and Rasmussen, 1950Penfield W. Rasmussen T. The cerebral cortex of man: a clinical study of localization of function. Macmillan, New York1950Google Scholar): one patient exclaimed, “Yes, Doctor, yes, Doctor! Now I hear people laughing - my friends in South Africa … Yes, they are my two cousins, Bessie and Ann Wheliaw.” Another patient reported, “I had a dream. I had a book under my arm. I was talking to a man. The man was trying to reassure me not to worry about the book.” This study had the first glance at what geneticists call “gain-of-function” or “sufficiency” evidence for the notion that the lateral temporal lobe (LTL) region harbors a biological locus for episodic memory. This work was complemented by a study conducted several years later by the American neurosurgeon William Scoville and Canadian neuropsychologist Brenda Milner (Scoville and Milner, 1957Scoville W.B. Milner B. Loss of recent memory after bilateral hippocampal lesions.J. Neurol. Neurosurg. Psychiatry. 1957; 20: 11-21Crossref PubMed Scopus (4885) Google Scholar) that provided “loss-of-function” or “necessity” evidence. To treat the epileptic seizures of a young man (Henry Molaison [H.M.], who suffered seizures caused by a bicycle accident), Scoville resected a large portion of the medial temporal lobes from both hemispheres, including the hippocampus and the adjacent brain areas. As a consequence of this surgery, H.M. lost his ability to form new episodic memories (anterograde amnesia) as well as the ability to recall memories of episodes and events that occurred to him within a year prior to his surgery (graded retrograde amnesia). H.M.’s other types of memory, such as motor memory, were largely unaffected, indicating that episodic memories may be specifically processed in the MTL and, in particular, in the hippocampus. These pioneering studies led to a notion that at least some types of memory, in this case episodic memory, may be stored in a localized brain region. In the meantime, memory has been classified into multiple types—declarative or explicit memory and non-declarative or implicit memory. Both explicit and implicit memories are further classified into subtypes, each of which is supported by one or more specific brain areas or systems (Squire, 2004Squire L.R. Memory systems of the brain: a brief history and current perspective.Neurobiol. Learn. Mem. 2004; 82: 171-177Crossref PubMed Scopus (1311) Google Scholar). Numerous efforts have been made during the past 30 years to identify the sites where each of these types of memory is located by using lesion, physiological, or fMRI imaging methods combined with behavioral paradigms. Some of these efforts led to the identification of brain regions or brain systems that are crucial for their respective type of memory. Indeed, many of these studies advanced the field toward a better understanding of memory mechanisms (e.g., Olds et al., 1972Olds J. Disterhoft J.F. Segal M. Kornblith C.L. Hirsh R. Learning centers of rat brain mapped by measuring latencies of conditioned unit responses.J. Neurophysiol. 1972; 35: 202-219PubMed Google Scholar, Fuster and Jervey, 1981Fuster J.M. Jervey J.P. Inferotemporal neurons distinguish and retain behaviorally relevant features of visual stimuli.Science. 1981; 212: 952-955Crossref PubMed Scopus (293) Google Scholar, Miyashita, 1988Miyashita Y. Neuronal correlate of visual associative long-term memory in the primate temporal cortex.Nature. 1988; 335: 817-820Crossref PubMed Scopus (501) Google Scholar) but could not identify a specific subpopulation(s) of neurons in these brain regions or systems that would satisfy all the criteria for engram cells as defined in our proposal of a contemporary definition of engram cells mentioned above (see Observational Studies). Meeting these criteria has required a combinatorial use of new technologies, like those harnessing immediate early genes (IEGs), transgenics, optogenetics, pharmacogenetics, in vitro and in vivo physiology of single cells, and behavioral paradigms. This has recently been accomplished, but thus far mainly for hippocampus- and/or amygdala-dependent classical conditioning memories. Thus, in this review, most (but not all) of our discussion will concern this type of memory. Readers are referred to other reviews for the discussion of earlier efforts to identify brain regions or systems that play important roles in various forms of memory (Horn, 2004Horn G. Pathways of the past: the imprint of memory.Nat. Rev. Neurosci. 2004; 5: 108-120Crossref PubMed Scopus (217) Google Scholar, Horn et al., 2001Horn G. Nicol A.U. Brown M.W. Tracking memory’s trace.Proc. Natl. Acad. Sci. USA. 2001; 98: 5282-5287Crossref PubMed Scopus (56) Google Scholar, Martin and Morris, 2002Martin S.J. Morris R.G.M. New life in an old idea: the synaptic plasticity and memory hypothesis revisited.Hippocampus. 2002; 12: 609-636Crossref PubMed Scopus (356) Google Scholar, Christian and Thompson, 2003Christian K.M. Thompson R.F. Neural substrates of eyeblink conditioning: acquisition and retention.Learn. Mem. 2003; 10: 427-455Crossref PubMed Scopus (504) Google Scholar, Weinberger, 2004Weinberger N.M. Specific long-term memory traces in primary auditory cortex.Nat. Rev. Neurosci. 2004; 5: 279-290Crossref PubMed Scopus (445) Google Scholar). The general criteria for the inclusion of a study in this review article is whether it implicated a specific subpopulation of neurons within a specific brain region in a particular memory as monitored by behavioral experiments. To demonstrate that specific populations of neurons qualify as cells harboring a component of the engram complex, multiple conditions must be met according to our proposed definition. One must demonstrate that these cells are activated by learning, that they undergo enduring physical or chemical changes, and that their reactivation results in recall of the originally formed memory. To design and conduct an experiment that will satisfy all the criteria of the definition at once seemed daunting. Thus, given the limited technologies available at the time of each study, the search for memory engrams and engram cells has advanced until recently with a limited goal in mind—namely to satisfy some, but not all, of the criteria. The search for memory engrams conducted to date can be divided into the following types: observational, loss-of-function, and gain-of-function experiments (Gerber et al., 2004Gerber B. Tanimoto H. Heisenberg M. An engram found? Evaluating the evidence from fruit flies.Curr. Opin. Neurobiol. 2004; 14: 737-744Crossref PubMed Scopus (150) Google Scholar, Martin and Morris, 2002Martin S.J. Morris R.G.M. New life in an old idea: the synaptic plasticity and memory hypothesis revisited.Hippocampus. 2002; 12: 609-636Crossref PubMed Scopus (356) Google Scholar). Observational studies demonstrate correlations between certain activities of a studied cell population and the behavioral expression of a specific memory; loss-of-function studies show that a certain population of neurons is necessary for the behavioral expression of a specific memory; and gain-of-function studies indicate the activation of a certain population of neurons is sufficient for the behavioral expression of the memory (Figure 1, Table 1). Among the three types of evidence, evidence obtained by observational studies is usually non-causal and therefore weaker. Loss-of-function evidence is stronger because it reveals a specific cell population necessary for the expression of the memory, and gain-of-function evidence is the strongest because it demonstrates that activation of a specific cell population is sufficient to elicit the expression of memory. However, all of these types of evidence must be supplemented by a demonstration of learning-induced enduring changes in the putative engram cells.Table 1Three Lines of Evidence for Memory Engram Cell PopulationsEvidenceTechnologyBrain Area(s)Reference(s)Gain of functionc-Fos-tTa/TetO-ChR2/ArchTHPC, BLALiu et al., 2012Liu X. Ramirez S. Pang P.T. Puryear C.B. Govindarajan A. Deisseroth K. Tonegawa S. Optogenetic stimulation of a hippocampal engram activates fear memory recall.Nature. 2012; 484: 381-385Crossref PubMed Scopus (5) Google Scholar, Ramirez et al., 2013Ramirez S. Liu X. Lin P.-A. Suh J. Pignatelli M. Redondo R.L. Ryan T.J. Tonegawa S. Creating a false memory in the hippocampus.Science. 2013; 341: 387-391Crossref PubMed Scopus (545) Google Scholar, Redondo et al., 2014Redondo R.L. Kim J. Arons A.L. Ramirez S. Liu X. Tonegawa S. 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Inferotemporal neurons distinguish and retain behaviorally relevant features of visual stimuli.Science. 1981; 212: 952-955Crossref PubMed Scopus (293) Google Scholarin vivo electrophysiologyinferotemporal cortexMiyashita, 1988Miyashita Y. Neuronal correlate of visual associative long-term memory in the primate temporal cortex.Nature. 1988; 335: 817-820Crossref PubMed Scopus (501) Google ScholarRepresentative studies on memory engram cell populations categorized by types of supporting evidence (observational, loss of function, and gain of function), with methods used, brain areas involved, and publication listed. Open table in a new tab Representative studies on memory engram cell populations categorized by types of supporting evidence (observational, loss of function, and gain of function), with methods used, brain areas involved, and publication listed. Many observational studies have implicated selected populations of neurons in specific memories across species, although none of these cells could entirely satisfy the proposed definition of engram cells. Among early studies, and across multiple modalities, notable and pioneering examples but which still belong to this category include Olds et al., 1972Olds J. Disterhoft J.F. Segal M. Kornblith C.L. Hirsh R. Learning centers of rat brain mapped by measuring latencies of conditioned unit responses.J. Neurophysiol. 1972; 35: 202-219PubMed Google Scholar, Fuster and Jervey, 1981Fuster J.M. Jervey J.P. Inferotemporal neurons distinguish and retain behaviorally relevant features of visual stimuli.Science. 1981; 212: 952-955Crossref PubMed Scopus (293) Google Scholar, and Miyashita, 1988Miyashita Y. Neuronal correlate of visual associative long-term memory in the primate temporal cortex.Nature. 1988; 335: 817-820Crossref PubMed Scopus (501) Google Scholar. For instance, Olds et al., 1972Olds J. Disterhoft J.F. Segal M. Kornblith C.L. Hirsh R. Learning centers of rat brain mapped by measuring latencies of conditioned unit responses.J. Neurophysiol. 1972; 35: 202-219PubMed Google Scholar recorded electrical activity from multiple cortical and subcortical areas and found a variety of response latencies to auditory conditioned stimuli. The authors subsequently proposed that a subset of the brain areas analyzed (i.e., those in which response latencies matched or were shorter than responses in the inferior colliculus) indeed contained cells that comprised a “learning center” and were thus putative sites involved in processing a mnemonic record. A decade later, Fuster and Jervey, 1981Fuster J.M. Jervey J.P. Inferotemporal neurons distinguish and retain behaviorally relevant features of visual stimuli.Science. 1981; 212: 952-955Crossref PubMed Scopus (293) Google Scholar recorded single-cell activity from the inferotemporal (IT) cortex of monkeys performing a visual delayed matching-to-sample task. Many cells responded differentially to the colors of the stimuli, and notably, many cells also responded differentially to color depending on whether or not attention mechanisms were engaged, thus demonstrating their behaviorally relevant role. Fittingly, the authors demonstrated correlations of these neuronal activities to the encoding, retention, and retrieval of visual information. Then, in 1988, Miyashita revealed a neuronal correlate of visual long-term memory by studying how the anterior ventral temporal cortex represented stimulus-stimulus associations. By training monkeys to perform a visual memory task and simultaneously recording from over 200 neurons, Miyashita found that single neurons could respond conjointly to temporally related, albeit geometrically dissimilar, stimuli (i.e., these neurons displayed stimulus