Memory engrams: Recalling the past and imagining the future

Abstract

In 1904, Richard Semon introduced the term "engram" to describe the neural substrate for storing memories. An experience, Semon proposed, activates a subset of cells that undergo off-line, persistent chemical and/or physical changes to become an engram. Subsequent reactivation of this engram induces memory retrieval. Although Semon's contributions were largely ignored in his lifetime, new technologies that allow researchers to image and manipulate the brain at the level of individual neurons has reinvigorated engram research. We review recent progress in studying engrams, including an evaluation of evidence for the existence of engrams, the importance of intrinsic excitability and synaptic plasticity in engrams, and the lifetime of an engram. Together, these findings are beginning to define an engram as the basic unit of memory.

Fig. 1.. Engram loss-of-function studies disrupt subsequent memory retrieval.

(A) Ablating allocated neurons. Lateral amygdala principal (excitatory) neurons were experimentally allocated to an engram (blue circles) by means of overexpression of the transcription factor CREB (122). Mice received auditory fear conditioning during which a tone (conditioned stimulus) was paired with a footshock (unconditioned stimulus). The majority of allocated neurons are active during the fear memory test (green filled circles), suggesting that allocated neurons are preferentially recruited to an engram supporting this conditioned fear memory. Specifically ablating experimentally allocated neurons (red circles) before a second memory test disrupts memory retrieval. (B) Ablating a similar number of random, nonallocated neurons does not disrupt memory retrieval. [Images: Adapted from (122)].

Fig. 2.. Gain-of-function method for engram identification and distributed engram ensembles.

(A) A c-fos–tTA transgenic mouse is injected with AAV9-TRE-ChR2-mCherry (allowing active neurons in the absence of doxycycline to express the excitatory opsin ChR2) and implanted with an optical fiber to target blue light to activate ChR2-expressing neurons in the DG. (B) Basic experimental scheme. Mice are habituated to context A with light stimulation while on doxycycline for 5 days and are then taken off doxycycline for 2 days (to open the tagging window) and exposed to contextual fear conditioning (CFC) in context B. Mice are put back on doxycycline (to close the tagging window) and tested for 5 days in context A with light stimulation. (C) Representative image showing the expression of ChR2-mCherry–positive (red) engram cells in a mouse that was taken off doxycycline for 2 days and underwent CFC training. [Image credit: X. Liu and S. Ramirez (Tonegawa lab)] (D) Mice expressing ChR2 in engram cells from CFC in context B (red) show greater freezing during test light-on epochs in context A than a control group expressing mCherry only. Error bars indicate standard error of the mean. [Graph: Adapted from Liu et al. (69)] (E) A part of the engram cell ensemble complex for contextual fear memory. It is generally thought that the engram for a specific memory is distributed in more than one brain region. For instance, for contextual fear memory, the engram cell ensemble in the entorhinal cortex layer II (EC-II) as well as hippocampal subfields [DG, CA3, CA2, CA1, and subiculum (Sub)] may represent context, whereas amygdala engram cell ensembles represent fear information. These engram cell ensembles are functionally connected to form an engram cell ensemble complex. Thus, a concept has emerged that a specific pattern of cellular connectivity within an engram cell ensemble complex serves as the substrate for a specific memory. US, unconditioned stimulus; LA, lateral nucleus of the amygdala; BLA, basolateral nucleus of the amygdala; CS, conditioned stimulus.

Fig. 3.. Neuronal allocation to an engram.

Eligible neurons compete for allocation to an engram supporting a memory, and neurons with increased relative excitability at the time of training “win” this competition for allocation. (A) Neurons that were endogenously more excitable than their neighbors at the time of training or were experimentally manipulated to become relatively more excitable (blue circles) are preferentially allocated to an engram (green filled circles). Subsequent disruption of these allocated or engram neurons disrupts memory retrieval (top right), whereas artificial reactivation of these neurons elicits memory retrieval in the absence of normal sensory retrieval cues (bottom right). (B) Neurons with relatively decreased excitability at the time of training (either endogenously or through experimental manipulation) (purple circles) are preferentially excluded from the engram (green filled circles). Subsequent disruption of nonallocated or nonengram neurons does not impact memory retrieval.

Fig. 4.. Active and silent engram cells in amnesia and during memory systems consolidation.

(A) Active engram cells have higher spine density and are activated in the conditioned context A to produce the conditioned response, freezing. Silent engram cells generated in amnesia and in a mouse designed to model early Alzheimer’s disease show lower spine density and cannot be activated in the conditioned context A to produce a conditioned response but can be activated by blue light in an unconditioned context B if they were tagged with ChR2 during encoding. (B) During memory systems consolidation, active engram cells with high spine density are formed in the hippocampus during contextual fear conditioning and for several days, the conditioned context can evoke a conditioned response. However, by two weeks (remote recall), these hippocampal engram cells demature to become silent, with reduced spine density. In the mPFC, engram cells are formed during CFC but are silent with low spine density. During the following 2 weeks, these mPFC silent engram cells acquire higher spine density and become active engram cells.

Fig. 5.. Neuronal allocation and memory linking.

Neurons with increased excitability at the time of event 1 (blue) are allocated to the engram supporting this memory (blue filled circles outlined in orange). These allocated engram neurons remain more excitable than their neighbors for several hours after event 1. If a similar event 2 (green) occurs during this time, neurons allocated to the engram supporting event 1 are more excitable and, therefore, also allocated to the engram supporting event 2 (blue and green filled circles outlined in orange). In this way, neurons are coallocated to events 1 and 2. By virtue of coallocation, these two memories become linked. After some time, neurons allocated to the engram supporting event 1 become less excitable than their neighbors (“refractory”), and if event 2 occurs in this time window, a new population of more excitable neurons wins the competition for allocation to the engram supporting event 2. This disallocation allows the two memories to be remembered separately. Circles with red dashed outlines represent less excitable neurons.