Episodic memories initially require rapid synaptic plasticity within the hippocampus for their formation and are gradually consolidated in neocortical networks for permanent storage. However, the engrams and circuits that support neocortical memory consolidation remain unknown. We found that neocortical prefrontal memory engram cells, critical for remote contextual fear memory, were rapidly generated during initial learning via inputs from both hippocampal-entorhinal cortex and basolateral amygdala. After their generation, the prefrontal engram cells, with support from hippocampal memory engram cells, became functionally mature with time. Whereas hippocampal engram cells gradually became silent with time, engram cells in the basolateral amygdala, which were necessary for fear memory, are maintained. Our data provide new insights into the functional reorganization of engrams and circuits underlying systems consolidation of memory.Memories are thought to be initially stored within the hippocampal-entorhinal cortex (HPC-EC) (recent memory) and over-time are slowly consolidated within the neocortex for permanent storage (remote memory) (1-7). Systems memory consolidation models suggest that the interaction between the HPC-EC and the neocortex during and after an experience is crucial (8)(9)(10)(11)(12). Experimentally, prolonged inhibition of hippocampal or neocortical networks during the consolidation period produces deficits in remote memory formation (13-15). However, little is known regarding specific neural circuit mechanisms underlying the formation and maturation of neocortical memories through interactions with the HPC-EC network. By employing activity-dependent cell labeling technology (16-18) combined with viral vector-based transgenic, anatomical (19, 20), and optogenetic strategies (19, 21) for We first traced entorhinal projections to frontal cortical structures (the medial prefrontal cortex (PFC), caudal anterior cingulate cortex (cACC), retrosplenial cortex (RSC)) involved in contextual fear memory, and the basolateral amygdala (BLA), with injections of the retrograde tracer cholera toxin subunit B (CTB)-Alexa555 into these regions ( fig. S1). CTB injections resulted in labeling in the medial entorhinal cortex (MEC) specifically in cells in layer Va (Fig. 1A-D, H and fig. S1A-D (Fig. 1F). Terminal inhibition during memory recall tests did not affect memory retrieval (Fig. 1G). Finally, terminal inhibition in the cACC or RSC during CFC or recall had no effect on memory throughout these periods (Fig. 1J-L and fig. S2G-I).The above results suggest that MEC-Va input into the PFC during CFC is crucial for the eventual formation of remote memory. This hypothesis was supported by several findings. First, CFC increased the number of c-Fos + cells in the PFC compared to that of homecage mice ( Fig. 1M-O), whereas context only exposure did not increase c-Fos activity in the PFC (Fig. 1O). Second, optogenetic terminal inhibition of MEC-Va projections within the PFC during CFC inhibited the observed incr...

Memory consolidation is the process by which a newly formed and unstable memory transforms into a stable long-term memory. It is unknown whether the process of memory consolidation occurs exclusively by the stabilization of memory engrams. By employing learning-dependent cell labeling, we identified an increase of synaptic strength and dendritic spine density specifically in consolidated memory engram cells. While these properties are lacking in the engram cells under protein synthesis inhibitor-induced amnesia, direct optogenetic activation of these cells results in memory retrieval, and this correlates with the retained engram cell-specific connectivity. We propose that a specific pattern of connectivity of engram cells may be crucial for memory information storage and that strengthened synapses in these cells critically contribute to the memory retrieval process. Main TextMemory consolidation is the phenomenon whereby a newly formed memory transitions from a fragile state to a stable, long-term state (1-3). The defining feature of consolidation is a finite time window that begins immediately after learning, during which a memory is susceptible to disruption such as protein synthesis inhibition (4-6), resulting in retrograde amnesia. The stabilization of synaptic potentiation is the dominant cellular model of memory consolidation (7-10) because protein synthesis inhibitors disrupt late-phase longterm potentiation of in vitro slice preparations (11-13). Although much is known about the cellular mechanisms of memory consolidation it remains unknown whether these processes occur in memory engram cells. It may be possible to characterize cellular consolidation and empirically separate mnemonic properties in retrograde amnesia by directly probing and manipulating memory engram cells in the brain. The term memory engram originally † Correspondence to: tonegawa@mit.edu. * These authors contributed equally to this work. Supplemental reference: (41)Data and Materials: pAAV-TRE-ChR2-EYFP, pAAV-TRE-ChR2-mCherry, and pAAV-TRE-mCherry were developed by X.L., in the group of S.T., at the Massachusetts Institute of Technology; therefore, a materials transfer agreement (MTA) is required to obtain these virus plasmids. HHMI Author ManuscriptHHMI Author Manuscript HHMI Author Manuscript referred to the hypothetical learned information stored in the brain, which must be reactivated for recall (14)(15). Recently, several groups demonstrated that specific hippocampal cells that are activated during memory encoding are both sufficient (16)(17)(18) and necessary (19)(20) for driving future recall of a contextual fear memory, and thus represent a component of a distributed memory engram (21). Here, we applied this engram technology to the issue of cellular consolidation and retrograde amnesia.We employed the previously established method for tagging the hippocampal dentate gyrus (DG) component of a contextual memory engram with mCherry (see Materials and Methods, fig. S1,and (16,22)). To disrupt consolidation we systemicall...

The medial temporal lobe, including the hippocampus, has been implicated in social memory. However, it remains unknown which parts of these brain regions and their circuits hold social memory. Here, we show that ventral hippocampal CA1 (vCA1) neurons of a mouse and their projections to nucleus accumbens (NAc) shell play a necessary and sufficient role in social memory. Both the proportion of activated vCA1 cells and the strength and stability of the responding cells are greater in response to a familiar mouse than to a previously unencountered mouse. Optogenetic reactivation of vCA1 neurons that respond to the familiar mouse enabled memory retrieval and the association of these neurons with unconditioned stimuli. Thus, vCA1 neurons and their NAc shell projections are a component of the storage site of social memory.

SummaryAlzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive memory decline and subsequent loss of broader cognitive functions 1 . Memory decline in early stages of Alzheimer's is mostly limited to episodic memory, for which the hippocampus (HPC) plays a crucial role 2 . However, it has been uncertain whether the observed amnesia in early stages of Alzheimer's is due to disrupted encoding and consolidation of episodic information, or an impairment in the retrieval of stored memory information. Here we show that in transgenic mouse models of early Alzheimer's, direct optogenetic activation of hippocampal memory engram cells results in memory retrieval despite the fact that these mice are amnesic in long-term memory tests when natural recall cues are utilized, revealing a retrieval, rather than a storage impairment. Prior to amyloid plaque deposition, the amnesia in these mice is age-dependent 3 -5 , which correlates with a progressive reduction of spine density of hippocampal dentate gyrus (DG) engram cells. We show that optogenetic induction of long-term potentiation (LTP) at perforant path (PP) synapses of DG engram cells restores both spine density and long-term memory. We also demonstrate that an ablation of DG engram cells containing restored spine density prevents the rescue of long-term memory. Thus, selective rescue of spine density in engram cells may lead to an effective strategy for treating memory loss in early stages of Alzheimer's disease.Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use: http://www.nature

A great deal of experimental investment is directed towards questions regarding the mechanisms of memory storage. Such studies have traditionally been restricted to investigation of the anatomical structures, physiological processes, and molecular pathways necessary for the capacity of memory storage, and have avoided the question of how individual memories are stored in the brain. Memory engram technology allows the labeling and subsequent manipulation of components of specific memory engrams in particular brain regions, and it has been established that cell ensembles labeled by this method are both sufficient and necessary for memory recall. Recent research has employed this technology to probe fundamental questions of memory consolidation, differentiating between mechanisms of memory retrieval from the true neurobiology of memory storage.