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Neuronal ensembles that hold specific memory (memory engrams) have been identified in the hippocampus, amygdala, or cortex. However, it has been hypothesized that engrams of a specific memory are distributed among multiple brain regions that are functionally connected, referred to as a unified engram complex. Here, we report a partial map of the engram complex for contextual fear conditioning memory by characterizing encoding activated neuronal ensembles in 247 regions using tissue phenotyping in mice. The mapping was aided by an engram index, which identified 117 cFos+ brain regions holding engrams with high probability, and brain-wide reactivation of these neuronal ensembles by recall. Optogenetic manipulation experiments revealed engram ensembles, many of which were functionally connected to hippocampal or amygdala engrams. Simultaneous chemogenetic reactivation of multiple engram ensembles conferred a greater level of memory recall than reactivation of a single engram ensemble, reflecting the natural memory recall process. Overall, our study supports the unified engram complex hypothesis for memory storage.

The attentional control of behavior is a higher-order cognitive function that operates through attention and response inhibition. The locus coeruleus (LC), the main source of norepinephrine in the brain, is considered to be involved in attentional control by modulating the neuronal activity of the prefrontal cortex (PFC). However, evidence for the causal role of LC activity in attentional control remains elusive. Here, by using behavioral and optogenetic techniques, we investigate the effect of LC neuron activation or inhibition in operant tests measuring attention and response inhibition (i.e., a measure of impulsive behavior). We show that LC neuron stimulation increases goal-directed attention and decreases impulsivity, while its suppression exacerbates distractibility and increases impulsive responding. Remarkably, we found that attention and response inhibition are under the control of two divergent projections emanating from the LC: one to the dorso-medial PFC and the other to the ventro-lateral orbitofrontal cortex, respectively. These findings are especially relevant for those pathological conditions characterized by attention deficits and elevated impulsivity.

Anterior cingulate cortex mediates the flexible updating of an animal’s choice responses upon rule changes in the environment. However, how anterior cingulate cortex entrains motor cortex to reorganize rule representations and generate required motor outputs remains unclear. Here, we demonstrate that chemogenetic silencing of the terminal projections of cingulate cortical neurons in secondary motor cortex in the rat disrupts choice performance in trials immediately following rule switches, suggesting that these inputs are necessary to update rule representations for choice decisions stored in the motor cortex. Indeed, the silencing of cingulate cortex decreases rule selectivity of secondary motor cortical neurons. Furthermore, optogenetic silencing of cingulate cortical neurons that is temporally targeted to error trials immediately after rule switches exacerbates errors in the following trials. These results suggest that cingulate cortex monitors behavioral errors and updates rule representations in motor cortex, revealing a critical role for cingulate-motor circuits in adaptive choice behaviors.

Learning from successes and failures often improves the quality of subsequent decisions. Past outcomes, however, should not influence purely perceptual decisions after task acquisition is complete since these are designed so that only sensory evidence determines the correct choice. Yet, numerous studies report that outcomes can bias perceptual decisions, causing spurious changes in choice behavior without improving accuracy. Here we show that the effects of reward on perceptual decisions are principled: past rewards bias future choices specifically when previous choice was difficult and hence decision confidence was low. We identified this phenomenon in six datasets from four laboratories, across mice, rats, and humans, and sensory modalities from olfaction and audition to vision. We show that this choice-updating strategy can be explained by reinforcement learning models incorporating statistical decision confidence into their teaching signals. Thus, reinforcement learning mechanisms are continually engaged to produce systematic adjustments of choices even in well-learned perceptual decisions in order to optimize behavior in an uncertain world.

BACKGROUND: The idea that memory is stored as enduring changes in the brain dates back at least to the time of Plato and Aristotle (circa 350 BCE), but its scientific articulation emerged in the 20th century when Richard Semon introduced the term “engram” to describe the neural substrate for storing and recalling memories. Essentially, Semon proposed that an experience activates a population of neurons that undergo persistent chemical and/or physical changes to become an engram. Subsequent reactivation of the engram by cues available at the time of the experience induces memory retrieval. After Karl Lashley failed to find the engram in a rat brain, studies attempting to localize an engram were largely abandoned. Spurred by Donald O. Hebb’s theory that augmented synaptic strength and neuronal connectivity are critical for memory formation, many researchers showed that enhanced synaptic strength was correlated with memory. Nonetheless, the causal relationship between these enduring changes in synaptic connectivity with a specific, behaviorally identifiable memory at the level of the cell ensemble (an engram) awaited further advances in experimental technologies. ADVANCES: The resurgence in research examining engrams may be linked to two complementary studies that applied intervention strategies to target individual neurons in an engram supporting a specific memory in mice. One study showed that ablating the subset of lateral amygdala neurons allocated to a putative engram disrupted subsequent memory retrieval (loss of function). The second study showed that artificially reactivating a subset of hippocampal dentate gyrus neurons that were active during a fearful experience (and, therefore, part of a putative engram) induced memory retrieval in the absence of external retrieval cues (gain of function). Subsequent findings from many labs used similar strategies to identify engrams in other brain regions supporting different types of memory. There are several recent advances in engram research. First, eligible neurons within a given brain region were shown to compete for allocation to an engram, and relative neuronal excitability determines the outcome of this competition. Excitability-based competition also guides the organization of multiple engrams in the brain and determines how these engrams interact. Second, research examining the nature of the off-line, enduring changes in engram cells (neurons that are critical components of an engram) found increased synaptic strength and spine density in these neurons as well as preferential connectivity to other downstream engram cells. Therefore, both increased intrinsic excitability and synaptic plasticity work hand in hand to form engrams, and these mechanisms are also implicated in memory consolidation and retrieval processes. Third, it is now possible to artificially manipulate memory encoding and retrieval processes to generate false memories, or even create a memory in mice without any natural sensory experience (implantation of a memory for an experience that did not occur). Fourth, “silent” engrams were discovered in amnesic mice; artificial reactivation of silent engrams induces memory retrieval, whereas natural cues cannot. Endogenous engram silencing may contribute to the change in memory over time (e.g., systems memory consolidation) or in different circumstances (e.g., fear memory extinction). These findings suggest that once formed, an engram may exist in different states (from silent to active) on the basis of their retrievability. Although initial engram studies focused on single brain regions, an emerging concept is that a given memory is supported by an engram complex, composed of functionally connected engram cell ensembles dispersed across multiple brain regions, with each ensemble supporting a component of the overall memory. OUTLOOK: The ability to identify and manipulate engram cells and brainwide engram complexes has introduced an exciting new era of memory research. The findings from many labs are beginning to define an engram as the basic unit of memory. However, many questions remain. In the short term, it is critical to characterize how information is stored in an engram, including how engram architecture affects memory quality, strength, and precision; how multiple engrams interact; how engrams change over time; and the role of engram silencing in these processes. The long-term goal of engram research is to leverage the fundamental findings from rodent engram studies to understand how information is acquired, stored, and used in humans and facilitate the treatment of human memory, or other information-processing, disorders. The development of low- to noninvasive technology may enable new human therapies based on the growing knowledge of engrams in rodents.

What happens to memories as days, weeks and years go by has long been a fundamental question in neuroscience and psychology. For decades, researchers have attempted to identify the brain regions in which memory is formed and to follow its changes across time. The theory of systems consolidation of memory (SCM) suggests that changes in circuitry and brain networks are required for the maintenance of a memory with time. Various mechanisms by which such changes may take place have been hypothesized. Recently, several studies have provided insight into the brain networks driving SCM through the characterization of memory engram cells, their biochemical and physiological changes and the circuits in which they operate. In this Review, we place these findings in the context of the field and describe how they have led to a revamped understanding of SCM in the brain.

The brain codes continuous spatial, temporal and sensory changes in daily experience. Recent studies suggest that the brain also tracks experience as segmented subdivisions (events), but the neural basis for encoding events remains unclear. Here, we designed a maze for mice, composed of four materially indistinguishable lap events, and identify hippocampal CA1 neurons whose activity are modulated not only by spatial location but also lap number. These 'event-specific rate remapping' (ESR) cells remain lap-specific even when the maze length is unpredictably altered within trials, which suggests that ESR cells treat lap events as fundamental units. The activity pattern of ESR cells is reused to represent lap events when the maze geometry is altered from square to circle, which suggests that it helps transfer knowledge between experiences. ESR activity is separately manipulable from spatial activity, and may therefore constitute an independent hippocampal code: an 'event code' dedicated to organizing experience by events as discrete and transferable units.

SummaryThe ability to extinguish conditioned fear memory is critical for adaptive control of fear response, and its impairment is a hallmark of emotional disorders like post-traumatic stress disorder (PTSD). Fear extinction is thought to take place when animals form a new memory that suppresses the original fear memory. However, little is known about the nature and the site of formation and storage of the new extinction memory. Here, we demonstrate that a fear extinction memory engram is formed and stored in a genetically distinct basolateral amygdala (BLA) neuronal population that drive reward behaviors and antagonize the BLA’s original fear neurons. The activation of the fear extinction engram neurons and natural reward-responsive neurons overlap extensively in the BLA. Furthermore, these two neuron subsets are mutually interchangeable in driving reward behaviors and fear extinction behaviors. Thus, fear extinction memory is a newly formed reward memory.

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive memory decline and subsequent loss of broader cognitive functions. Memory decline in the early stages of AD is mostly limited to episodic memory, for which the hippocampus has a crucial role. However, it has been uncertain whether the observed amnesia in the early stages of AD 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 AD, 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 used, revealing a retrieval, rather than a storage impairment. Before amyloid plaque deposition, the amnesia in these mice is age-dependent, which correlates with a progressive reduction in spine density of hippocampal dentate gyrus engram cells. We show that optogenetic induction of long-term potentiation at perforant path synapses of dentate gyrus engram cells restores both spine density and long-term memory. We also demonstrate that an ablation of dentate gyrus 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 the early stages of AD.

Animals need to optimize the efficacy of memory retrieval to adapt to environmental circumstances for survival. The recent development of memory engram labeling technology allows a precise investigation of the processes associated with the recall of a specific memory. Here, we show that engram cell excitability is transiently increased following memory reactivation. This short-term increase of engram excitability enhances the subsequent retrieval of specific memory content in response to cues and is manifest in the animal's ability to recognize contexts more precisely and more effectively. These results reveal a hitherto unknown transient enhancement of context recognition based on the plasticity of engram cell excitability. They also suggest that recall of a contextual memory is influenced by previous but recent activation of the same engram. The state of excitability of engram cells mediates differential behavioral outcomes upon memory retrieval and may be crucial for survival by promoting adaptive behavior.

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.

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., RIKEN Brain Science Institute, Howard Hughes Medical Institute, JPB Foundation

The entorhinal cortex (EC)–hippocampal (HPC) network plays an essential role for episodic memory, which preserves spatial and temporal information about the occurrence of past events. Although there has been significant progress toward understanding the neural circuits underlying the spatial dimension of episodic memory, the relevant circuits subserving the temporal dimension are just beginning to be understood. In this review, we examine the evidence concerning the role of the EC in associating events separated by time—or temporal associative learning—with emphasis on the function of persistent activity in the medial entorhinal cortex layer III (MECIII) and their direct inputs into the CA1 region of HPC. We also discuss the unique role of Island cells in the medial entorhinal cortex layer II (MECII), which is a newly discovered direct feedforward inhibitory circuit to CA1. Finally, we relate the function of these entorhinal cortical circuits to recent findings concerning hippocampal time cells, which may collectively activate in sequence to bridge temporal gaps between discontiguous events in an episode., RIKEN Brain Science Institute, Howard Hughes Medical Institute, JPB Foundation

Entorhinal-hippocampal circuits in the mammalian brain are crucial for an animal's spatial and episodic experience, but the neural basis for different spatial computations remain unknown. Medial entorhinal cortex layer II contains pyramidal island and stellate ocean cells. Here, we performed cell type-specific Ca(2+) imaging in freely exploring mice using cellular markers and a miniature head-mounted fluorescence microscope. We found that both oceans and islands contain grid cells in similar proportions, but island cell activity, including activity in a proportion of grid cells, is significantly more speed modulated than ocean cell activity. We speculate that this differential property reflects island cells' and ocean cells' contribution to different downstream functions: island cells may contribute more to spatial path integration, whereas ocean cells may facilitate contextual representation in downstream circuits.

Stress is considered a potent environmental risk factor for many behavioural abnormalities, including anxiety and mood disorders. Animal models can exhibit limited but quantifiable behavioural impairments resulting from chronic stress, including deficits in motivation, abnormal responses to behavioural challenges, and anhedonia. The hippocampus is thought to negatively regulate the stress response and to mediate various cognitive and mnemonic aspects of stress-induced impairments, although the neuronal underpinnings sufficient to support behavioural improvements are largely unknown. Here we acutely rescue stress-induced depression-related behaviours in mice by optogenetically reactivating dentate gyrus cells that were previously active during a positive experience. A brain-wide histological investigation, coupled with pharmacological and projection-specific optogenetic blockade experiments, identified glutamatergic activity in the hippocampus-amygdala-nucleus-accumbens pathway as a candidate circuit supporting the acute rescue. Finally, chronically reactivating hippocampal cells associated with a positive memory resulted in the rescue of stress-induced behavioural impairments and neurogenesis at time points beyond the light stimulation. Together, our data suggest that activating positive memories artificially is sufficient to suppress depression-like behaviours and point to dentate gyrus engram cells as potential therapeutic nodes for intervening with maladaptive behavioural states.

The memory for a new episode is formed immediately upon experience and can last up to a lifetime. It has been shown that the hippocampal network plays a fundamental role in the rapid acquisition of a memory of a one-time experience, in which the novelty component of the experience promotes the prompt formation of the memory. However, it remains unclear which neural circuits convey the novelty signal to the hippocampus for the single-trial learning. Here, we show that during encoding neuromodulatory input from locus coeruleus (LC) to CA3, but not CA1 or to the dentate gyrus, is necessary to facilitate novel contextual learning. Silencing LC activity during exposure to a novel context reduced subsequent reactivation of the engram cell ensembles in CA3 neurons and in downstream CA1 upon reexposure to the same context. Calcium imaging of the cells reactivated in both novel and familiar contexts revealed that suppression of LC inputs at the time of encoding resulted in more variable place fields in CA3 neurons. These results suggest that neuromodulatory input from LC to CA3 is crucial for the formation of a persistent memory in the hippocampus.

Recent studies identified neuronal ensembles and circuits that hold specific memory information (memory engrams). Memory engrams are retained under protein synthesis inhibition-induced retrograde amnesia. These engram cells can be activated by optogenetic stimulation for full-fledged recall, but not by stimulation using natural recall cues (thus, amnesia). We call this state of engrams "silent engrams" and the cells bearing them "silent engram cells." The retention of memory information under amnesia suggests that the time-limited protein synthesis following learning is dispensable for memory storage, but may be necessary for effective memory retrieval processes. Here, we show that the full-fledged optogenetic recall persists at least 8 d after learning under protein synthesis inhibition-induced amnesia. This long-term retention of memory information correlates with equally persistent retention of functional engram cell-to-engram cell connectivity. Furthermore, inactivation of the connectivity of engram cell ensembles with its downstream counterparts, but not upstream ones, prevents optogenetic memory recall. Consistent with the previously reported lack of retention of augmented synaptic strength and reduced spine density in silent engram cells, optogenetic memory recall under amnesia is stimulation strength-dependent, with low-power stimulation eliciting only partial recall. Finally, the silent engram cells can be converted to active engram cells by overexpression of α-p-21-activated kinase 1, which increases spine density in engram cells. These results indicate that memory information is retained in a form of silent engram under protein synthesis inhibition-induced retrograde amnesia and support the hypothesis that memory is stored as the specific connectivity between engram cells.

The formation and retrieval of a memory is thought to be accomplished by activation and reactivation, respectively, of the memory-holding cells (engram cells) by a common set of neural circuits, but this hypothesis has not been established. The medial temporal-lobe system is essential for the formation and retrieval of episodic memory for which individual hippocampal subfields and entorhinal cortex layers contribute by carrying out specific functions. One subfield whose function is poorly known is the subiculum. Here, we show that dorsal subiculum and the circuit, CA1 to dorsal subiculum to medial entorhinal cortex layer 5, play a crucial role selectively in the retrieval of episodic memories. Conversely, the direct CA1 to medial entorhinal cortex layer 5 circuit is essential specifically for memory formation. Our data suggest that the subiculum-containing detour loop is dedicated to meet the requirements associated with recall such as rapid memory updating and retrieval-driven instinctive fear responses., RIKEN Brain Science Institute, Howard Hughes Medical Institute, JPB Foundation

Basolateral amygdala (BLA) principal cells are capable of driving and antagonizing behaviors of opposing valence. BLA neurons project to the central amygdala (CeA), which also participates in negative and positive behaviors. However, the CeA has primarily been studied as the site for negative behaviors, and the causal role for CeA circuits underlying appetitive behaviors is poorly understood. Here, we identify several genetically distinct populations of CeA neurons that mediate appetitive behaviors and dissect the BLA-to-CeA circuit for appetitive behaviors. Protein phosphatase 1 regulatory subunit 1B⁺ BLA pyramidal neurons to dopamine receptor 1⁺ CeA neurons define a pathway for promoting appetitive behaviors, while R-spondin 2⁺ BLA pyramidal neurons to dopamine receptor 2⁺ CeA neurons define a pathway for suppressing appetitive behaviors. These data reveal genetically defined neural circuits in the amygdala that promote and suppress appetitive behaviors analogous to the direct and indirect pathways of the basal ganglia. Keywords: central amygdala; basolateral amygdala; direct and indirect pathways; appetitive; reward; fear; amygdala circuit; feeding; drinking; freezing, National Institutes of Health (U.S.) (Grant T32GM007287)

The authors use cell type–specific transgenic mouse lines, optogenetics and patch-clamp recordings to provide new insights into hippocampal anatomy and function. They find that dentate granule cells of the hippocampus, which were believed to not project to CA2, do indeed send functional monosynaptic inputs to CA2 pyramidal cells. CA2 innervates CA1, but, unlike CA3, projects preferentially to the deep rather than superficial sublayer of CA1. Moreover, the authors find that layer 3 of the entorhinal cortex does not project to CA2. The formation and recall of episodic memory requires precise information processing by the entorhinal-hippocampal network. For several decades, the trisynaptic circuit entorhinal cortex layer II (ECII)→dentate gyrus→CA3→CA1 and the monosynaptic circuit ECIII→CA1 have been considered the primary substrates of the network responsible for learning and memory. Circuits linked to another hippocampal region, CA2, have only recently come to light. Using highly cell type–specific transgenic mouse lines, optogenetics and patch-clamp recordings, we found that dentate gyrus cells, long believed to not project to CA2, send functional monosynaptic inputs to CA2 pyramidal cells through abundant longitudinal projections. CA2 innervated CA1 to complete an alternate trisynaptic circuit, but, unlike CA3, projected preferentially to the deep, rather than to the superficial, sublayer of CA1. Furthermore, contrary to existing knowledge, ECIII did not project to CA2. Our results allow a deeper understanding of the biology of learning and memory.

The cognitive symptoms of schizophrenia presumably result from impairments of information processing in neural circuits. We recorded neural activity in the hippocampus of freely behaving mice that had a forebrain-specific knockout of the synaptic plasticity- mediating phosphatase calcineurin and were previously shown to exhibit behavioral and cognitive abnormalities, recapitulating the symptoms of schizophrenia. Calcineurin knockout (KO) mice exhibited a 2.5-fold increase in the abundance of sharp-wave ripple (SWR) events during awake resting periods and single units in KO were overactive during SWR events. Pairwise measures of unit activity, however, revealed that the sequential reactivation of place cells during SWR events was completely abolished in KO. Since this relationship during postexperience awake rest periods has been implicated in learning, working memory, and subsequent memory consolidation, our findings provide a mechanism underlying impaired information processing that may contribute to the cognitive impairments in schizophrenia., RIKEN Brain Science Institute, Alfred P. Sloan Foundation (Fellowship), Brain & Behavior Research Foundation (NARSAD Young Investigator Award), National Institutes of Health (U.S.) (NIH grant MH78821), National Institutes of Health (U.S.) (NIH grant MH58880), National Institutes of Health (U.S.) (NIH grant MH086702)

Can You Trust Your Memory? Being highly imaginative animals, humans constantly recall past experiences. These internally generated stimuli sometimes get associated with concurrent external stimuli, which can lead to the formation of false memories. Ramirez et al. (p. 387 ; see the cover) identified a population of cells in the dentate gyrus of the mouse hippocampus that encoded a particular context and were able to generate a false memory and study its neural and behavioral interactions with true memories. Optogenetic reactivation of memory engram–bearing cells was not only sufficient for the behavioral recall of that memory, but could also serve as a conditioned stimulus for the formation of an associative memory.

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 have thus far been unknown. We found that neocortical prefrontal memory engram cells, which are critical for remote contextual fear memory, were rapidly generated during initial learning through inputs from both the hippocampal–entorhinal cortex network and the 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, were maintained. Our data provide new insights into the functional reorganization of engrams and circuits underlying systems consolidation of memory.

The basolateral amygdala (BLA) is a site of convergence of negative and positive stimuli and is critical for emotional behaviors and associations. However, the neural substrate for negative and positive behaviors and relationship between negative and positive representations in the basolateral amygdala are unknown. Here we identify two genetically distinct, spatially segregated populations of excitatory neurons in the mouse BLA that participate in valence-specific behaviors and are connected through mutual inhibition. These results identify a genetically defined neural circuit for the antagonistic control of emotional behaviors and memories., National Institutes of Health (U.S.) (Grant T32GM007287)

Patients with uterine leiomyosarcoma (LMS) typically present with vaginal bleeding, pain, and a pelvic mass, with atypical presentations of hypercalcemia and eosinophilia also being reported. Radiographic evaluation with combined positron-emission tomography/computed tomography may assist in diagnosis and surveillance in women with uterine LMS; these are commonly used with stage and tumour grade as prognostic indicators and a recently developed risk-assessment index to predict disease-specific survival. Recent studies have shown that the addition of adjuvant therapy after surgical management does not seem to improve survival, and ovarian preservation does not appear to negatively impact outcome. Experimentally, it is noteworthy that proteasome subunit beta 9 (PSMB9)/β1i-deficient mice exhibit spontaneous development of uterine LMS, with a disease prevalence of ~37% by 12 months of age. Furthermore, a recent report showed the loss of ability to induce PSMB9/β1i expression, that is up-regulated by interferon-γ (IFNγ), in human uterine LMS tissues. Here, we reviewed human uterine LMS for genetic mutations in the IFNγ signal cascade, and found serious mutations in three genes, Janus activated kinase 1 (JAK1), signal transducer and activator of transcription 1 (STAT1) and PSMB9/β1i promoter regions. Moreover, molecular experiments demonstrated differential expression of cyclin E and P27/KIP1, that regulate cell-cycle G1 arrest via PSMB9/β1i expression. The discovery of this mutational activation of a key cell-signalling pathway may provide new targets for diagnostic approaches and therapeutic intervention.

Takuma Hayashi (Shinshu University School of Medicine) discussed a novel biomarker for detecting human uterine leiomyosarcoma (LMS).

© 2016 Poo et al. The mechanism of memory remains one of the great unsolved problems of biology. Grappling with the question more than a hundred years ago, the German zoologist Richard Semon formulated the concept of the engram, lasting connections in the brain that result from simultaneous "excitations", whose precise physical nature and consequences were out of reach of the biology of his day. Neuroscientists now have the knowledge and tools to tackle this question, however, and this Forum brings together leading contemporary views on the mechanisms of memory and what the engram means today.

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.

Amnesia is a deficit of memory function that can result from trauma, stress, disease, drug use, or ageing. Though efforts are made to prevent and treat the various causes of amnesia, there remains no treatment for the symptom of memory loss itself. Because the defining feature of amnesia is an inability recall memory, any given case may be due to the possibility that the memory is damaged, or the alternative that it is present but irretrievable (Squire, 1982). Discriminating between these two scenarios would be of scientific value, because the neurobiology of memory formation is anchored in experimental amnesia. Pathological cases of amnesia that are due to retrieval deficits may in principal be treatable rather than merely preventable. Amnesia could be attributed to a retrieval deficit if the ostensible ‘lost’ memory could be evoked through brain stimulation. The challenge here is to identify exactly where in the brain a particular memory is stored., JPB Foundation, RIKEN Brain Science Institute

SummaryThe late-phase of long-term potentiation (L-LTP), the cellular correlate of long-term memory, induced at some synapses facilitates L-LTP expression at other synapses receiving stimulation too weak to induce L-LTP by itself. Using glutamate uncaging and two-photon imaging, we demonstrate that the efficacy of this facilitation decreases with increasing time between stimulations, increasing distance between stimulated spines and with the spines being on different dendritic branches. Paradoxically, stimulated spines compete for L-LTP expression if stimulated too closely together in time. Furthermore, the facilitation is temporally bidirectional but asymmetric. Additionally, L-LTP formation is itself biased toward occurring on spines within a branch. These data support the Clustered Plasticity Hypothesis, which states that such spatial and temporal limits lead to stable engram formation, preferentially at synapses clustered within dendritic branches rather than dispersed throughout the dendritic arbor. Thus, dendritic branches rather than individual synapses are the primary functional units for long-term memory storage.

Uterine tumors are the most common type of gynecologic neoplasm. Uterine leiomyosarcoma (LMS) is rare, accounting for 2% to 5% of tumors of the uterine body. Uterine LMS develops more often in the muscle tissue layer of the uterine body than in the uterine cervix. The development of gynecologic tumors is often correlated with female hormone secretion; however, the development of uterine LMS is not substantially correlated with hormonal conditions, and the risk factors are not yet known. Radiographic evaluation combined with PET/CT can be useless in the diagnosis and surveillance of uterine LMS. Importantly, a diagnostic biomarker, which distinguishes malignant LMS and benign tumor leiomyoma (LMA) is yet to be established. Accordingly, it is necessary to analyze risk factors associated with uterine LMS in order to establish a method of treatment. LMP2-deficient mice spontaneously develop uterine LMS, with a disease prevalence of ∼40% by 14 months of age. It is therefore of interest whether human uterine LMS shows a loss of LMP2 expression. We found LMP2 expression is absent in human LMS, but present in human LMA. Therefore, defective LMP2 expression may be one of the risk factors for LMS. LMP2 is potentially a diagnostic biomarker for uterine LMS, and gene therapy with LMP2-encording DNA may be a new therapeutic approach.

SUMMARY Learning and memory are fundamental brain functions affected by dietary and environmental factors. Here, we show that increasing brain magnesium using a newly developed magnesium compound (magnesium-L-threonate, MgT) leads to the enhancement of learning abilities, working memory, and short- and long-term memory in rats. The pattern completion ability was also improved in aged rats. MgT-treated rats had higher density of synaptophysin-/synaptobrevin-positive puncta in DG and CA1 subregions of hippocampus that were correlated with memory improvement. Functionally, magnesium increased the number of functional presynaptic release sites, while it reduced their release probability. The resultant synaptic reconfiguration enabled selective enhancement of synaptic transmission for burst inputs. Coupled with concurrent upregulation of NR2B-containing NMDA receptors and its downstream signaling, synaptic plasticity induced by correlated inputs was enhanced. Our findings suggest that an increase in brain magnesium enhances both short-term synaptic facilitation and long-term potentiation and improves learning and memory functions.

The acquisition of Pavlovian fear learning engages the hippocampus when the conditioned stimuli are multimodal or temporally isolated from the unconditioned stimuli. By subjecting CA3-NR1 KO mice to conditioning protocols that incorporate time-dependent components, we found that the loss of plasticity at recurrent CA3 synapses resulted in a deficits in contextual conditioning specifically when the exposure to the context was brief or when the unconditioned stimulus was signaled with a competing, predictive unimodal stimulus. Our results suggest CA3 contributes both speed and salience to contextual processing and support the theory of competition between multimodal and unimodal conditioned stimuli for associative learning.

Huntington's disease (HD), a dominantly inherited neurodegenerative disorder characterized by relatively selective degeneration of striatal neurons, is caused by an expanded polyglutamine tract of the huntingtin (htt) protein. The htt mutation reduces levels of brain-derived neurotrophic factor (BDNF) in the striatum, likely by inhibiting cortical BDNF gene expression and anterograde transport of BDNF from cortex to striatum. However, roles of the BDNF reduction in HD pathogenesis have not been established conclusively. We reasoned that increasing striatal BDNF through over-expression would slow progression of the disease if BDNF reduction plays a pivotal role in HD pathogenesis. We employed a Bdnf transgene driven by the promoter for the alpha subunit of Ca(2+)/calmodulin-dependent kinase II to over-express BDNF in the forebrain of R6/1 mice which express a fragment of mutant htt with a 116-glutamine tract. The Bdnf transgene increased BDNF levels and TrkB signaling activity in the striatum, ameliorated motor dysfunction, and reversed brain weight loss in R6/1 mice. Furthermore, it normalized DARPP-32 expression of the 32 kDa dopamine and cAMP-regulated phosphoprotein, increased the number of enkephalin-containing boutons, and reduced formation of neuronal intranuclear inclusions in the striatum of R6/1 mice. These results demonstrate crucial roles of reduced striatal BDNF in HD pathogenesis and suggest potential therapeutic values of BDNF to HD.

The hippocampus is an area of the brain involved in learning and memory. It contains parallel excitatory pathways referred to as the trisynaptic pathway (which carries information as follows: entorhinal cortex → dentate gyrus → CA3 → CA1 → entorhinal cortex) and the monosynaptic pathway (entorhinal cortex → CA1 → entorhinal cortex). We developed a generally applicable tetanus toxin–based method for transgenic mice that permits inducible and reversible inhibition of synaptic transmission and applied it to the trisynaptic pathway while preserving transmission in the monosynaptic pathway. We found that synaptic output from CA3 in the trisynaptic pathway is dispensable and the short monosynaptic pathway is sufficient for incremental spatial learning. In contrast, the full trisynaptic pathway containing CA3 is required for rapid one-trial contextual learning, for pattern completion–based memory recall, and for spatial tuning of CA1 cells.

Adult neocortical areas are characterized by marked differences in cytoarchitecture and connectivity that underlie their functional roles. The molecular determinants of these differences are largely unknown. We performed a microarray analysis to identify molecules that define the somatosensory and visual areas during the time when afferent and efferent projections are forming. We identified 122 molecules that are differentially expressed between the regions and confirmed by quantitative polymerase chain reaction 95% of the 20 genes tested. Two genes were chosen for further investigation: Bcl6 and Ten_m3. Bcl6 was highly expressed in the superficial cortical plate corresponding to developing layer IV of somatosensory cortex at postnatal day (P) 0. This had diminished by P3, but strong expression was found in layer V pyramidal cells by P7 and was maintained until adulthood. Retrograde tracing showed that Bcl6 is expressed in corticospinal neurons. Ten_m3 was expressed in a graded pattern within layer V of caudal cortex that corresponds well with visual cortex. Retrograde tracing and immunostaining showed that Ten_m3 is highly expressed along axonal tracts of projection neurons of the developing visual pathway. Overexpression demonstrated that Ten_m3 promotes homophilic adhesion and neurite outgrowth in vivo. This suggests an important role for Ten_m3 in the development of the visual pathway.

Feature-selective firing allows networks to produce representations of the external and internal environments. Despite its importance, the mechanisms generating neuronal feature selectivity are incompletely understood. In many cortical microcircuits the integration of two functionally distinct inputs occurs nonlinearly through generation of active dendritic signals that drive burst firing and robust plasticity. To examine the role of this processing in feature selectivity, we recorded CA1 pyramidal neuron membrane potential and local field potential in mice running on a linear treadmill. We found that dendritic plateau potentials were produced by an interaction between properly timed input from entorhinal cortex and hippocampal CA3. These conjunctive signals positively modulated the firing of previously established place fields and rapidly induced new place field formation to produce feature selectivity in CA1 that is a function of both entorhinal cortex and CA3 input. Such selectivity could allow mixed network level representations that support context-dependent spatial maps.

Long-term memory and its putative synaptic correlates the late phases of both long-term potentiation and long-term depression require enhanced protein synthesis. On the basis of recent data on translation-dependent synaptic plasticity and on the supralinear effect of activation of nearby synapses on action potential generation, we propose a model for the formation of long-term memory engrams at the single neuron level. In this model, which we call clustered plasticity, local translational enhancement, along with synaptic tagging and capture, facilitates the formation of long-term memory engrams through bidirectional synaptic weight changes among synapses within a dendritic branch.

Classical conditioning of the eyeblink reflex is a simple form of associative learning for motor responses. To examine the involvement of hippocampal CA3 NMDA receptors (NRs) in nonspatial associative memory, mice lacking an NR1 subunit selectively in adult CA3 pyramidal cells [CA3-NR1 knock-out (KO) mice] were subjected to eyeblink conditioning paradigms. Mice received paired presentations of an auditory conditioned stimulus (CS) and a periorbital shock unconditioned stimulus (US). With repeated presentation of the CS followed by the US, wild-type mice learned to blink in anticipation of the US before its onset. We first confirmed that wild-type mice require an intact hippocampus in the trace version of eyeblink conditioning in which the CS and US do not overlap, creating a stimulus-free time gap of 500 ms. Under the same condition, CA3-NR1 KO mice successfully acquired conditioned responses (CRs) during the 10 d acquisition sessions, whereas the extinction of CRs was impaired on the first day of extinction sessions. Importantly, CA3-NR1 KO mice were impaired in the formation of an adaptively timed CR during the first five trials in the daily acquisition sessions. The aberrantly timed CR was also observed in the extinction sessions in accordance with the impaired extinction of CRs. These results indicate that CA3-NR1 KO mice are unable to rapidly retrieve adaptive CR timing, suggesting that CA3 NRs play a crucial role in the memory of adaptive CR timing in trace conditioning.

Memory and synaptic plasticity exhibit distinct temporal phases, with long-lasting forms distinguished by their dependence on macromolecular synthesis. Prevailing models for the molecular mechanisms underlying long-lasting synaptic plasticity have largely focused on transcriptional regulation. However, a growing body of evidence now supports a crucial role for neuronal activity-dependent mRNA translation, which may occur in dendrites for a subset of neuronal mRNAs. Recent work has begun to define the signaling mechanisms coupling synaptic activation to the protein synthesis machinery. The ERK and mTOR signaling pathways have been shown to regulate the activity of the general translational machinery, while the translation of particular classes of mRNAs is additionally controlled by gene-specific mechanisms. Rapid enhancement of the synthesis of a diverse array of neuronal proteins through such mechanisms provides the components necessary for persistent forms of LTP and LTD. These findings have important implications for the synapse specificity and associativity of protein synthesis-dependent changes in synaptic strength.

Molecular and cellular mechanisms for memory consolidation in the cortex are poorly known. To study the relationships between synaptic structure and function in the cortex and consolidation of long-term memory, we have generated transgenic mice in which catalytic activity of PAK, a critical regulator of actin remodeling, is inhibited in the postnatal forebrain. Cortical neurons in these mice displayed fewer dendritic spines and an increased proportion of larger synapses compared to wild-type controls. These alterations in basal synaptic morphology correlated with enhanced mean synaptic strength and impaired bidirectional synaptic modifiability (enhanced LTP and reduced LTD) in the cortex. By contrast, spine morphology and synaptic plasticity were normal in the hippocampus of these mice. Importantly, these mice exhibited specific deficits in the consolidation phase of hippocampus-dependent memory. Thus, our results provide evidence for critical relationships between synaptic morphology and bidirectional modifiability of synaptic strength in the cortex and consolidation of long-term memory.

Enduring forms of synaptic plasticity and memory require new protein synthesis, but little is known about the underlying regulatory mechanisms. Here, we investigate the role of MAPK signaling in these processes. Conditional expression of a dominant-negative form of MEK1 in the postnatal murine forebrain inhibited ERK activation and caused selective deficits in hippocampal memory retention and the translation-dependent, transcription-independent phase of hippocampal L-LTP. In hippocampal neurons, ERK inhibition blocked neuronal activity-induced translation as well as phosphorylation of the translation factors eIF4E, 4EBP1, and ribosomal protein S6. Correspondingly, protein synthesis and translation factor phosphorylation induced in control hippocampal slices by L-LTP-generating tetanization were significantly reduced in mutant slices. Translation factor phosphorylation induced in the control hippocampus by memory formation was similarly diminished in the mutant hippocampus. These results suggest a crucial role for translational control by MAPK signaling in long-lasting forms of synaptic plasticity and memory.

Glutamate is a major excitatory neurotransmitter in the mammalian central nervous system, and the N -methyl- d -aspartate-selective glutamate receptor (NR) consisting of the NR1 subunit and an NR2 or NR3 subunit plays crucial roles in synaptic transmission, plasticity, and learning and memory. By using a knockout mouse strain, in which the NR1 gene deletion is primarily targeted to the CA1 pyramidal cells of the hippocampus, we investigated the in vivo effect of the loss of the NR1 subunit on the cellular expression and intracellular distribution of the NR2 subunits. The NR1 gene deletion had no apparent effect on the levels of NR2A or NR2B mRNA but led to severe reductions of NR2A and NR2B protein in dendrites of CA1 pyramidal cells. This reduced dendritic distribution of the NR2 subunits accompanied their robust accumulation in perikarya, where they were condensed in the lumen of the endoplasmic reticulum as electron-dense granules. These granules were also observed in CA1 pyramidal cells of the control mice but they were much fewer and contained no detectable levels of the NR2 subunit. The effect of the NR1 knockout on intracellular localization of the NR2 subunits was specific in that no such effect was observed for the GluR1 and PSD-95, two other major postsynaptic proteins. These results suggest that the NR1 subunit plays a crucial role in the release of the NR2 subunit from the endoplasmic reticulum in hippocampal pyramidal cells in vivo , and when the NR1 subunit is unavailable, the NR2 subunits are retained and aggregate into intracisternal granules.

Lesion and pharmacological intervention studies have suggested that in both human patients and animals the hippocampus plays a crucial role in the rapid acquisition and storage of information from a novel one-time experience. However, how the hippocampus plays this role is poorly known. Here, we show that mice with NMDA receptor (NR) deletion restricted to CA3 pyramidal cells in adulthood are impaired in rapidly acquiring the memory of novel hidden platform locations in a delayed matching-to-place version of the Morris water maze task but are normal when tested with previously experienced platform locations. CA1 place cells in the mutant animals had place field sizes that were significantly larger in novel environments, but normal in familiar environments relative to those of control mice. These results suggest that CA3 NRs play a crucial role in rapid hippocampal encoding of novel information for fast learning of one-time experience.

A significant fraction of the total calcium/calmodulin-dependent protein kinase II (CaMKII) activity in neurons is associated with synaptic connections and is present in nerve terminals, thus suggesting a role for CaMKII in neurotransmitter release. To determine whether CaMKII regulates neurotransmitter release, we generated and analyzed knockout mice in which the dominant α-isoform of CaMKII was specifically deleted from the presynaptic side of the CA3-CA1 hippocampal synapse. Conditional CA3 α-CaMKII knockout mice exhibited an unchanged basal probability of neurotransmitter release at CA3-CA1 synapses but showed a significant enhancement in the activity-dependent increase in probability of release during repetitive presynaptic stimulation, as was shown with the analysis of unitary synaptic currents. These data indicate that α-CaMKII serves as a negative activity-dependent regulator of neurotransmitter release at hippocampal synapses and maintains synapses in an optimal range of release probabilities necessary for normal synaptic operation.

Appropriate choice about delayed reward is fundamental to the survival of animals. Although animals tend to prefer immediate reward, delaying gratification is often advantageous. The dorsal raphe (DR) serotonergic neurons have long been implicated in the processing of delayed reward, but it has been unclear whether or when their activity causally directs choice. Here, we transiently augmented or reduced the activity of DR serotonergic neurons, while mice decided between differently delayed rewards as they performed a novel odor-guided intertemporal choice task. We found that these manipulations, precisely targeted at the decision point, were sufficient to bidirectionally influence impulsive choice. The manipulation specifically affected choices with more difficult trade-off. Similar effects were observed when we manipulated the serotonergic projections to the nucleus accumbens (NAc). We propose that DR serotonergic neurons preempt reward delays at the decision point and play a critical role in suppressing impulsive choice by regulating decision trade-off. Xu et al. optogenetically interrogate dorsal raphe serotonergic neurons in a novel odor-guided intertemporal choice task. They found that serotonergic neuronal manipulations bidirectionally regulated impulsive choice at decision point under choices involving delay-size trade-off. Keywords: serotonin; raphe; intertemporal choice; decision trade-off; impulsivity; accumbens; delay discounting

Hippocampal replays have been demonstrated to play a crucial role in memory. Chains of ripples (ripple bursts) in CA1 have been reported to co-occur with long-range place cell sequence replays during the quiet awake state, but roles of neural inputs to CA1 in ripple bursts and replays are unknown. Here we show that ripple bursts in CA1 and medial entorhinal cortex (MEC) are temporally associated. An inhibition of MECIII input to CA1 during quiet awake reduced ripple bursts in CA1 and restricted the spatial coverage of replays to a shorter distance corresponding to single ripple events. The reduction did not occur with MECIII input inhibition during slow-wave sleep. Inhibition of CA3 activity suppressed ripples and replays in CA1 regardless of behavioral state. Thus, MECIII input to CA1 is crucial for ripple bursts and long-range replays specifically in quiet awake, whereas CA3 input is essential for both, regardless of behavioral state. Yamamoto and Tonegawa aim to determine the contribution of MECIII and CA3 inputs to hippocampal ripples and replays. They found differential roles of MECIII and CA3 inputs on CA1 ripples and replays during animal's different behavioral states., RIKEN Brain Science Institute, Howard Hughes Medical Institute, JPB Foundation

Activation of the hippocampal dopamine 1-class receptors (D1R and D5R) are implicated in contextual fear conditioning (CFC). However, the specific role of the D1R versus D5R in hippocampal dependent CFC has not been investigated. Generation of D1R- and D5R-specific in situ hybridization probes showed that D1R and D5R mRNA expression was greatest in the dentate gyrus (DG) of the hippocampus. To identify the role of each receptor in CFC we generated spatially restricted KO mice that lack either the D1R or D5R in DG granule cells. DG D1R KOs displayed significant fear memory deficits, whereas DG D5R KOs did not. Furthermore, D1R KOs but not D5R KOs, exhibited generalized fear between two similar but different contexts. In the familiar home cage context, c-Fos expression was relatively low in the DG of control mice, and it increased upon exposure to a novel context. This level of c-Fos expression in the DG did not further increase when a footshock was delivered in the novel context. In DG D1R KOs, DG c-Fos levels in the home cage was higher than that of the control mice, but it did not further increase upon exposure to a novel context and remained at the same level upon a shock delivery. In contrast, the levels of DG c-Fos expression was unaffected by the deletion of DG D5R neither in the home cage nor upon a shock delivery. These results suggest that DG D1Rs, but not D5Rs, contribute to the formation of distinct contextual representations of novel environments.

How memories are formed and stored in the brain remains a fascinating question in neuroscience. Here we discuss the memory engram theory, our recent attempt to identify and manipulate memory engram cells in the brain with optogenetics, and how these methods are used to address questions such as how false memory is formed and how the valence of a memory can be changed in the brain., RIKEN Brain Science Institute, Howard Hughes Medical Institute

Calcineurin is a calcium-dependent protein phosphatase that has been implicated in various aspects of synaptic plasticity. By using conditional gene-targeting techniques, we created mice in which calcineurin activity is disrupted specifically in the adult forebrain. At hippocampal Schaffer collateral-CA1 synapses, LTD was significantly diminished, and there was a significant shift in the LTD/LTP modification threshold in mutant mice. Strikingly, although performance was normal in hippocampus-dependent reference memory tasks, including contextual fear conditioning and the Morris water maze, the mutant mice were impaired in hippocampus-dependent working and episodic-like memory tasks, including the delayed matching-to-place task and the radial maze task. Our results define a critical role for calcineurin in bidirectional synaptic plasticity and suggest a novel mechanistic distinction between working/episodic-like memory and reference memory.