Nat Commun︱Guan Jisong's group discovered an important indexing mechanism of the hippocampus for memory storage

Written by ︱ Wenhan Luo ︱ Sizhen Wang The hippocampal formation and cerebral cortex are the core brain regions responsible for episodic memory in mammals
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HM (Henry Gustav Molaison), a famous amnesiac in the history of science, removed the bilateral hippocampus because of the treatment of epilepsy, which led to anterograde amnesia of memory, that is, the inability to form new memories [1]
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However, there have been different hypotheses about the specific functions of the hippocampus: one hypothesis is that the hippocampus is a transit station in the process of transforming short-term memory to long-term memory, storing relevant information of memory, that is, transit theory (transition theory) [2]; Another hypothesis is that the hippocampus cannot store so much information, but only serves as an index storage brain area in the cortex to assist and regulate the memory access process before the formation of long-term memory, namely index theory [3, 4]
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And long-term memory is stored in the cerebral cortex, so how do the hippocampus and the cerebral cortex interact to form memories? How is the brain concussion, which is highly related to brain cognition, involved in this process? The answers to these questions are not yet clear
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 On March 24, 2022, the team of Guan Jisong from the School of Life Sciences, ShanghaiTech University published an article titled "Acquiring new memories in neocortex of hippocampal-lesioned mice" in Nature Communications, revealing the structure of the hippocampus.
Mechanisms of interaction between body and cerebral cortex during learning
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This study found that long-distance gamma synchronization signals in different functional areas of the cortex are regulated by the hippocampus.
It is this synchronization signal that couples the activities of memory engram cells in different brain regions and mediates the storage and retrieval process of memories in the cortex
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The researchers constructed an interesting maze behavioral paradigm, in which mice need to learn to remember the environment in the maze to navigate and ultimately obtain food rewards (Fig.
1ac)
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During the memory process, the mice's brain waves became synchronized, the study found
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The synchronous activity of multiple neurons will cause periodic oscillation of the local electric field, so brain waves can reflect the consistency of local neuron activity
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Generally speaking, the main brainwave frequencies and phases of different brain regions are different, but the researchers found that during the memory process of mice, the brainwaves of different brain regions tended to be consistent, and the synchronization improvement of the gamma frequency band was similar to theta wave.
The cycles of are highly coupled (Fig.
1dh)
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This suggests that cortical brain waves are highly involved in cognitive processes in mice
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Figure 1 Learning-induced cortical long-range gamma synchronization is linked to rhythm (Credit: Luo, et al, Nat Commun, 2022) Because the hippocampus is closely involved in the modulation of theta waves in the brain
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So the authors asked whether cortical gamma oscillations are regulated by the hippocampus during memory processing
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In loss-of-function tests, they examined behavioral performance and EEG oscillations in spatial memory tasks in mice with hippocampal disruption
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When both the dorsal and ventral parts of the hippocampus were damaged (Fig.
2 ab), the mice with hippocampal lesions took significantly longer to search for food during the learning phase of the spatial memory task, and the learning rate was much slower than that of the control group (Fig.
2 c).

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On probing trials testing memory retrieval, hippocampus-disrupted mice were less inclined to search within the food area compared with controls (Fig.
2d)
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These results suggest that these mice have strong memory deficits
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 In addition to behavioral deficits, they observed increased gamma and theta power during memory encoding and recall, but greatly reduced oscillatory power compared with normal mice (Fig.
2e)
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In addition, learning-induced cortical gamma-theta coupling was also absent in the hippocampus-disrupted mice (Fig.
2f), and the coupling of gamma-EEG synchronization was also significantly reduced (Fig.
2gi)
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Thus, the hippocampus is essential for spatial memory and for the modulation of learning-related cortical oscillations
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 The lateral entorhinal cortex (LEC) is the pathway from the hippocampus to the cerebral cortex and is also key to memory encoding
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In order to dissect the relationship between HPC (hippocampus), LEC and CTX (neocortical area), the authors performed signal delay analysis on brain waves of LEC and each brain area of ​​HPC and CTX, and found that the key theta wave is derived from HPC-LEC- CTX flow direction (Fig.
2j)
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suggesting that HPC-LEC is regulating cortical EEG
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 Further studies have shown that LECs have extensive synaptic projections to the cortex (Fig.
2k), and reverse tracing experiments in the cortex have also shown the same conclusion, if LEC neurons are artificially activated using optogenetic methods or their synapses to the cortex can be induced.
The obvious cortical gamma synchronization (Fig.
2lo), which indicates that the LEC is crucial for cortical gamma synchronization, combined with the previous theta wave coupling, the hippocampus and entorhinal cortex may induce cortical multibrain with theta wave rhythm Area synchronization promotes communication between cortical areas (Fig.
2p)
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Figure 2 The long-distance cortical gamma synchronization of HPC is regulated by the hippocampus-lateral entorhinal cortex (Source: Luo, et al, Nat Commun, 2022) Since the hippocampus-disrupted mice lack EEG synchronization and display huge memory deficits , so could artificially induced synchronization by stimulating the LEC rescue the memory deficits in these hippocampal-disrupted mice? The answer is yes
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The researchers found that inducing cortical synchronization (iSOS) by activating neuronal projections from the LEC to the cortex not only rescued the deficits of fear memory in hippocampal-damaged mice (Fig.
3 ae), but also rescued spatial memory deficits (Fig.
3 fj)
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showed that HPC-absected mice fully recovered the ability to acquire and retrieve memories after being given cortical long-range gamma synchronization during learning
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In other words, both artificial iSOS and LEC axon-mediated long-range gamma synchronization coordinate cortical units to store memories in the mouse neocortex
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   Figure 3 Artificially induced long-distance cortical synchronization (iSOS) rescues memory deficits in mice with disrupted hippocampus (Source: Luo, et al, Nat Commun, 2022) So is the synchronization signal really that important, or is it projected by the LEC Activation itself rescues memory deficits? To investigate the importance of synchronous signaling, the researchers sequentially trained two groups of mice with hippocampal disruption either with iSOS (30 Hz pulses of two synchronized lasers) or asynchronous LEC axonal activation (two lasers on different regions of the body) Asynchronous stimulation), and stimulated LEC to L2 axons at the cortical surface of the MO and VIS (or RSC) while learning two place memory tasks consecutively in two different boxes (Fig.
4 ab)
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Specifically, in the A sandbox, when group 1 mice were given iSOS and group 2 were given an asynchronous signal, group 1 mice showed a rapid reduction in the delay in retrieving food over 4 days of training
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Conversely, group 2 mice learned much slower than group 1 (Fig.
4c)
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In the memory recall test in Box A, the mice in group 1 spent significantly longer searching in the food area than mice in group 2 (Fig.
4d)
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Later, in Box B, group 2 mice were now stimulated with iSOS, and their performance improved with a marked reduction in search time (Fig.
4e)
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Conversely, group 1 mice that received asynchronous signals showed no improvement during training (Fig.
4e)
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Group 2 mice spent significantly longer in the food area than mice in group 1 in the probing test in box B of the sand table (Fig.
4f)
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Taken together, these results suggest that long-range cortical synchronization, rather than individual LEC axonal activation, is critical for the storage of spatial memory
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 Figure 4 iSOS, but not asynchronous cortical axonal activation, is key to rescue spatial memory impairment in mice with hippocampal disruption (Credit: Luo, et al, Nat Commun, 2022) Long-distance cortical gamma synchronization and cortical memory during memory retrieval How does long-distance cortical synchronization modulate memory formation and retrieval in the tightly linked activation of related neurons? We simultaneously recorded calcium activity in engram cells in the posterior compressive cortex (RSC) and local field potential (LFP) synchrony between the RSC and the VIS (visual cortex) (Fig.
5a), and tested spatial memory.
Correlation between the activity of memory-related cells during tasks and long-range synchrony
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It was found that labeled cells in RSC and VIS showed higher activity levels, especially before food-seeking (pre-exploration period), than in unfamiliar situations or living cage conditions (Fig.
5b)
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The activity of engram cells is evoked by memory retrieval, especially during free recall (when sensory information from the learned environment has not yet been presented)
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These observations were further confirmed by averaging the responses around each calcium event (Fig.
5e)
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The activity of engram cells is specific and distributed in different brain regions, and is closely related to long-range gamma synchronization, especially during memory retrieval
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  In fact, in order to successfully retrieve food, mice need to refer to an internal map
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Recent findings suggest that neurons encoding egocentric information exist in the neocortex during spatial navigation [5]
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Such egocentric coding neurons have been found in the LEC, RSC and motor cortex
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Therefore, the researchers speculate that the mice also rely on their egocentric inner map to navigate
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The authors reanalyzed behavioral data and engram cell activity to explore whether these activities encode spatial information in an egocentric manner
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The data suggest that the activity of memory imprint cells encodes information about the location of spatial objects relative to the mouse, such as a group of neurons that always fires when an object is on the left side of the mouse's body
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In this way, these neurons, distributed in different brain regions, help mice navigate and ultimately find food rewards
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Figure 5 The activation of cortical memory imprint cells is tightly coupled with long-range cortical gamma synchronization and encodes egocentric object orientation information during memory retrieval (Source: Luo, et al, Nat Commun, 2022) Finally, the researchers We dissected the role of LEC-mediated synchronization with long-range gamma in retrieval-evoked and memory-related neuronal reactivation
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It was found that inhibiting the activity of LEC neurons greatly impairs memory retrieval
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Inhibition of LEC activity by hM4D(Gi) during memory retrieval reduced mice's preference for food areas during memory trials and increased mice's delay in reaching food during retraining trials (Fig.
6ac)
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Consistent with the behavioral deficits, inhibition of LEC activity also reduced the activity frequency of memory engram cells (Fig.
6de), and the correlation between engram cells and EEG synchronization was lost (Fig.
6fh)
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By comparing the entire time series of gamma synchronization with the calcium activity of labeled neurons in RSC and VIS, they also found that LEC inhibition reduced the correlation between the activity of memory engram cells during the pre-exploration phase and long-range gamma synchronization (Fig.
6).
i)
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These data suggest that LECs are important for the reactivation of cortical memory cells during memory retrieval
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Figure 6 LEC-mediated long-range gamma synchronization is critical for cortical imprint reactivation and memory retrieval
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(Image source: Luo, et al, Nat Commun, 2022) The conclusion of the article, discussion, inspiration and prospect of the article.
In summary, this study combines electrophysiology, two-photon and genetic manipulation techniques to reveal that the hippocampal structure through the entorhinum The cortex induces extensive cortical gamma synchronization to support memory writing and retrieval
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After revealing that the engram network is responsible for the storage of episodic memory information in the cerebral cortex [6,7], this study further analyzed the coding and regulation mechanism of the engram network in the cortex: memory scattered in multiple functional areas of the cerebral cortex The engram cells integrate the complete memory information under the periodic oscillation signal, especially the EEG synchronization signal in the gamma band, forming a distributed storage and processing structure for the complete information (Fig.
6j)
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 These data confirm the speculation of human brain wave research and a number of theoretical studies, that is, brain wave synchronization may promote learning and memory.
Due to the limitations of human experiments, this speculation has not been supported by exact experimental data and has a specific mechanism before.
Very detailed data are given to support the role of brainwave synchronization in learning and memory
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For the first time, the key periodic modulation and indexing mechanism of the hippocampus in memory storage and retrieval has been suggested, an important in-depth exploration has been made to reveal the working principle of the memory engram network, and a new idea for the development of brain-like artificial intelligence technology has been provided
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Although this study largely explained the mechanism of engram cells in memory retrieval and the role of synchronization in memory formation, the principles of selection and formation of engram cells are only a little involved, and experimental methods need to be improved to further explain the engram.
Principles of cell selection and formation
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And continue to explore how to transform this application to benefit patients with memory impairment
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Link to the original text: https://doi.
org/10.
1038/s41467-022-29208-5 Professor Guan Jisong from the School of Life Science and Technology, ShanghaiTech University is the corresponding author of this paper, and postdoctoral fellows Luo Wenhan and Yun Di are the co-authors of this paper.
an author
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The research was strongly supported by Zhan Yang's research group of Shenzhen Advanced Research Institute and Xie Hong's research group of Shanghai University of Science and Technology
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The research was also assisted by Professor Hu Ji from ShanghaiTech University, Professor Lin Longnian from East China Normal University, and Professor Lu Wei from Fudan University School of Medicine
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The research was supported by the 2030 Major Research Project of the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Shanghai Science and Technology Commission Project Fund
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The first author Luo Wenhan (left), and the co-first author Yun Di (middle)
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