Hippocampal Activation of Rac1 Regulates the Forgetting of Object Recognition Memory

Not the same as it ever was: A review of memory modification, updating, and distortion in humans and rodents

Memory is a reconstructive and continuous process that enables existing information to be modified in response to a changing environment. Being able to dynamically update outdated memories is critical to an organism's survival. Memory modifications have been extensively studied in both rodents and humans, and prior work has revealed many regional, cellular, neurotransmitter, and subcellular molecular mechanisms underlying this process. However, these diverse bodies of literature have not yet been fully integrated into a comprehensive cross-species review. Integrating the finding across rodent and human work is important for furthering our understanding of memory modifications and the underlying neural mechanisms that support memory modification in both species. Here, we discuss advances in our understanding of adaptive and maladaptive memory modifications in terms of both underlying mechanisms (regional, cellular, and molecular) and behavioral outcomes. By emphasizing findings from both humans and rodents, the two major model systems in which memory modifications have been studied, we are able to highlight converging mechanisms and point to open questions in the field. Specifically, we discuss the major findings from several memory paradigms including declarative, aversive and procedural memory designs and highlight paradigms and models that have been readily translated between rodent and human models. Ultimately, this review identifies key parallels underlying memory updating across species, paradigms, tasks, and models.

Inhibition of the Integrated Stress Response Prevents Natural Forgetting and Corrects Accelerated Forgetting Associated with Epilepsy

The neural mechanisms underlying the natural and maladaptive forgetting of established memory remain largely unknown. Brain disease states might hijack the physiological forgetting mechanisms, resulting in maladaptive forgetting such as accelerated forgetting that contributes to cognitive decline in various neurologic conditions including epilepsy. Based on the key role of the integrated stress response (ISR) in memory storage and maintenance, we determined whether the ISR underpins natural and accelerated forgetting. Here, based on the object location recognition (OLR) and novel object recognition (NOR) paradigms in mice, we found that the ISR was activated while an established memory was naturally forgotten, which was denoted by increased levels of phosphorylated eukaryotic translation initiation factor 2α (eIF2α) and activating transcription factor 4 (ATF4), and reduced general protein synthesis. Multiple administrations of ISRIB, a small molecule ISR inhibitor, during the memory retention interval attenuated the ISR activation, and prevented the natural forgetting of established OLR and NOR memories. At the same time, a single injection of ISRIB has no effect on natural forgetting and memory retrieval. Moreover, administration of pentylenetetrazole (PTZ), an inducer of epileptic seizures, during the memory retention interval provoked the ISR activation and accelerated forgetting, which was corrected by ISRIB treatment. Together, our findings suggest that the ISR is critically involved in natural forgetting and accelerated forgetting associated with epilepsy, and pharmacological inhibition of the ISR may emerge as a novel intervention strategy for accelerated forgetting in patients with epilepsy.

Rac1 in parvalbumin neurons of the medial prefrontal cortex governs rapid forgetting of social memory

Social memory can undergo rapid forgetting at first according to the Ebbinghaus forgetting curve, for which the underlying mechanism remains entirely unknown. Here, we reported that rapid forgetting of social memory did not occur as indicated by social preference on stranger 2 (S2) over stranger 1 (S1) mouse, tested shortly after social interaction with S1. However, rapid forgetting of both social and object memories occurred as indicated by no social or object preference, respectively, when the constitutive active (CA) variant of Rac1 was knocked-in parvalbumin (PV) but not somatostatin (SST) neurons of the brain. Furthermore, rapid forgetting of only social memory occurred if this CA variant was knocked-in PV but not SST neurons of the medial prefrontal cortex (mPFC). By contrast, rapid forgetting of social memory was prevented by the dominant negative (DN) variant of Rac1 knocked-in PV neurons of the mPFC. Moreover, fiber photometry revealed that PV but not SST neurons of the mPFC generated dual calcium peaks to delineate each social interaction event. Thus, PV-specific Rac1 activity of the mPFC is both necessary and sufficient for controlling social behavior via rapid forgetting of social memory, providing a novel understanding of social behaviors under health and disease conditions.

Neurobiological mechanisms of forgetting across timescales

Every species in the animal kingdom that learns, also forgets. Despite this balance between learning and forgetting, most neuroscientific explorations of memory have focused on how learning occurs, with recent studies identifying engrams as putative biological substrates for memory. Here we review an emerging literature that, in contrast, explores how our brains forget. These studies reveal that forgetting engages a broad collection of mechanisms that function to reduce engram accessibility. However, changes in accessibility emerge on vastly different timescales. At short timescales, forgetting is modulated by fluctuations in brain states that alter engram accessibility in a moment-to-moment fashion. In the intermediate- and long-term, forgetting depends, in part, on mechanisms that rewire engrams, rendering them gradually harder to access. Viewed this way, forgetting encompasses a family of plasticity mechanisms that modulate engram accessibility, perhaps in order to prioritize those memories that are most timely or relevant to the situation at hand.

Ezrin-mediated astrocyte-synapse signaling regulates cognitive function via astrocyte morphological changes in fine processes in male mice

Astrocytes, which actively participate in cognitive processes, have a complex spongiform morphology, highlighted by extensive ramified fine processes that closely enwrap the pre- and post-synaptic compartments, forming tripartite synapses. However, the role of astrocyte morphology in cognitive processes remains incompletely understood and even controversial. The actin-binding protein Ezrin is highly expressed in astrocytes and is a key structural determinant of astrocyte morphology. Here, we found that Ezrin expression and astrocyte fine process volume in the hippocampus of male mice increased after learning but decreased after lipopolysaccharide injection and in a mouse model of postoperative cognitive dysfunction, both of which involved models with impaired cognitive function. Additionally, astrocytic Ezrin knock-out led to significantly decreased astrocytic fine process volumes, decreased astrocyte-neuron proximity, and induced anxiety-like behaviors and cognitive dysfunction. Astrocytic Ezrin deficiency in the hippocampus was achieved by using a microRNA silencing technique delivered by adeno-associated viruses. Down-regulation of Ezrin in hippocampal astrocytes led to disrupted astrocyte-synapse interactions and impaired synaptic functions, including synaptic transmission and synaptic plasticity, which could be rescued by exogenous administration of D-serine. Remarkably, decreased Ezrin expression and reduced astrocyte fine processes volumes were also observed in aged mice with decreased cognitive function. Moreover, overexpression of astrocytic Ezrin increased astrocyte fine process volumes and improved cognitive function in aged mice. Overall, our results indicate Ezrin-mediated astrocyte fine processes integrity shapes astrocyte-synapse signaling contributing to cognitive function.

Natural forgetting reversibly modulates engram expression

Memories are stored as ensembles of engram neurons and their successful recall involves the reactivation of these cellular networks. However, significant gaps remain in connecting these cell ensembles with the process of forgetting. Here, we utilized a mouse model of object memory and investigated the conditions in which a memory could be preserved, retrieved, or forgotten. Direct modulation of engram activity via optogenetic stimulation or inhibition either facilitated or prevented the recall of an object memory. In addition, through behavioral and pharmacological interventions, we successfully prevented or accelerated forgetting of an object memory. Finally, we showed that these results can be explained by a computational model in which engrams that are subjectively less relevant for adaptive behavior are more likely to be forgotten. Together, these findings suggest that forgetting may be an adaptive form of engram plasticity which allows engrams to switch from an accessible state to an inaccessible state.

Hippocampal dipeptidyl peptidase 9 bidirectionally regulates memory associated with synaptic plasticity

INTRODUCTION

Subtypes of the dipeptidyl peptidase (DPP) family, such as DPP4, are reportedly associated with memory impairment. DPP9 is widely distributed in cells throughout the body, including the brain. However, whether DPP9 regulates memory has not yet been elucidated.

OBJECTIVES

This study aimed to elucidate the role of DPP9 in memory, as well as the underlying molecular mechanism.

METHODS

We performed immunofluorescence on mouse brains to explore the distribution of DPP9 in different brain regions and used AAV vectors to construct knockdown and overexpression models. The effects of changing DPP9 expression on memory were demonstrated through behavioral experiments. Finally, we used electrophysiology, proteomics and affinity purification mass spectrometry (AP-MS) to study the molecular mechanism by which DPP9 affects memory.

RESULTS

Here, we report that DPP9, which is found almost exclusively in neurons, is expressed and has enzyme activity in many brain regions, especially in the hippocampus. Hippocampal DPP9 expression increases after fear memory formation. Fear memory was impaired by DPP9 knockdown and enhanced by DPP9 protein overexpression in the hippocampus. According to subsequent hippocampal proteomics, multiple pathways, including the peptidase pathway, which can be bidirectionally regulated by DPP9. DPP9 directly interacts with its enzymatic substrate neuropeptide Y (NPY) in neurons. Hippocampal long-term potentiation (LTP) is also bidirectionally regulated by DPP9. Moreover, inhibiting DPP enzyme activity impaired both LTP and memory. In addition, AP-MS revealed that DPP9-interacting proteins are involved in the functions of dendritic spines and axons. By combining AP-MS and proteomics, DPP9 was shown to play a role in regulating actin functions.

CONCLUSION

Taken together, our findings reveal that DPP9 affects the CNS not only through enzymatic activity but also through protein-protein interactions. This study provides new insights into the molecular mechanisms of memory and DPP family functions.

Active forgetting and neuropsychiatric diseases

Recent and pioneering animal research has revealed the brain utilizes a variety of molecular, cellular, and network-level mechanisms used to forget memories in a process referred to as "active forgetting". Active forgetting increases behavioral flexibility and removes irrelevant information. Individuals with impaired active forgetting mechanisms can experience intrusive memories, distressing thoughts, and unwanted impulses that occur in neuropsychiatric diseases. The current evidence indicates that active forgetting mechanisms degrade, or mask, molecular and cellular memory traces created in synaptic connections of "engram cells" that are specific for a given memory. Combined molecular genetic/behavioral studies using Drosophila have uncovered a complex system of cellular active-forgetting pathways within engram cells that is regulated by dopamine neurons and involves dopamine-nitric oxide co-transmission and reception, endoplasmic reticulum Ca2+ signaling, and cytoskeletal remodeling machinery regulated by small GTPases. Some of these molecular cellular mechanisms have already been found to be conserved in mammals. Interestingly, some pathways independently regulate forgetting of distinct memory types and temporal phases, suggesting a multi-layering organization of forgetting systems. In mammals, active forgetting also involves modulation of memory trace synaptic strength by altering AMPA receptor trafficking. Furthermore, active-forgetting employs network level mechanisms wherein non-engram neurons, newly born-engram neurons, and glial cells regulate engram synapses in a state and experience dependent manner. Remarkably, there is evidence for potential coordination between the network and cellular level forgetting mechanisms. Finally, subjects with several neuropsychiatric diseases have been tested and shown to be impaired in active forgetting. Insights obtained from research on active forgetting in animal models will continue to enrich our understanding of the brain dysfunctions that occur in neuropsychiatric diseases.

DCC, a potential target for controlling fear memory extinction and hippocampal LTP in male mice receiving single prolonged stress

Post-traumatic stress disorder (PTSD) is a severe stress-dependent psychiatric disorder characterized by impairment of fear memory extinction; however, biological markers to determine impaired fear memory extinction in PTSD remain unclear. In male mice with PTSD-like behaviors elicited by single prolonged stress (SPS), 19 differentially expressed proteins in the hippocampus were identified compared with controls. Among them, a biological macromolecular protein named deleted in colorectal cancer (DCC) was highly upregulated. Specific overexpression of DCC in the hippocampus induced similar impairment of long-term potentiation (LTP) and fear memory extinction as observed in SPS mice. The impairment of fear memory extinction in SPS mice was improved by inhibiting the function of hippocampal DCC using a neutralizing antibody. Mechanistic studies have shown that knocking down or inhibiting μ-calpain in hippocampal neurons increased DCC expression and induced impairment of fear memory extinction. Additionally, SPS-triggered impairment of hippocampal LTP and fear memory extinction could be rescued through activation of the Rac1-Pak1 signaling pathway. Our study provides evidence that calpain-mediated regulation of DCC controls hippocampal LTP and fear memory extinction in SPS mice, which likely through activation of the Rac1-Pak1 signaling pathway.

Presynaptic Rac1 in the hippocampus selectively regulates working memory

One of the most extensively studied members of the Ras superfamily of small GTPases, Rac1 is an intracellular signal transducer that remodels actin and phosphorylation signaling networks. Previous studies have shown that Rac1-mediated signaling is associated with hippocampal-dependent working memory and longer-term forms of learning and memory and that Rac1 can modulate forms of both pre- and postsynaptic plasticity. How these different cognitive functions and forms of plasticity mediated by Rac1 are linked, however, is unclear. Here, we show that spatial working memory in mice is selectively impaired following the expression of a genetically encoded Rac1 inhibitor at presynaptic terminals, while longer-term cognitive processes are affected by Rac1 inhibition at postsynaptic sites. To investigate the regulatory mechanisms of this presynaptic process, we leveraged new advances in mass spectrometry to identify the proteomic and post-translational landscape of presynaptic Rac1 signaling. We identified serine/threonine kinases and phosphorylated cytoskeletal signaling and synaptic vesicle proteins enriched with active Rac1. The phosphorylated sites in these proteins are at positions likely to have regulatory effects on synaptic vesicles. Consistent with this, we also report changes in the distribution and morphology of synaptic vesicles and in postsynaptic ultrastructure following presynaptic Rac1 inhibition. Overall, this study reveals a previously unrecognized presynaptic role of Rac1 signaling in cognitive processes and provides insights into its potential regulatory mechanisms.

Loss of the polarity protein Par3 promotes dendritic spine neoteny and enhances learning and memory

The Par3 polarity protein is critical for subcellular compartmentalization in different developmental processes. Variants of PARD3, encoding PAR3, are associated with intelligence and neurodevelopmental disorders. However, the role of Par3 in glutamatergic synapse formation and cognitive functions in vivo remains unknown. Here, we show that forebrain-specific Par3 conditional knockout leads to increased long, thin dendritic spines in vivo. In addition, we observed a decrease in the amplitude of miniature excitatory postsynaptic currents. Surprisingly, loss of Par3 enhances hippocampal-dependent spatial learning and memory and repetitive behavior. Phosphoproteomic analysis revealed proteins regulating cytoskeletal dynamics are significantly dysregulated downstream of Par3. Mechanistically, we found Par3 deletion causes increased Rac1 activation and dysregulated microtubule dynamics through CAMSAP2. Together, our data reveal an unexpected role for Par3 as a molecular gatekeeper in regulating the pool of immature dendritic spines, a rate-limiting step of learning and memory, through modulating Rac1 activation and microtubule dynamics in vivo.

Destabilization of fear memory by Rac1-driven engram-microglia communication in hippocampus

Rac1 is a key regulator of the cytoskeleton and neuronal plasticity, and is known to play a critical role in psychological and cognitive brain disorders. To elucidate the engram specific Rac1 signaling in fear memory, a doxycycline (Dox)-dependent robust activity marking (RAM) system was used to label dorsal dentate gyrus (DG) engram cells in mice during contextual fear conditioning. Rac1 mRNA and protein levels in DG engram cells were peaked at 24 h (day 1) after fear conditioning and were more abundant in the fear engram cells than in the non-engram cells. Optogenetic activation of Rac1 in a temporal manner in DG engram cells before memory retrieval decreased the freezing level in the fear context. Optogenetic activation of Rac1 increased autophagy protein 7 (ATG7) expression in the DG engram cells and activated DG microglia. Microglia-specific transcriptomics and fluorescence in situ hybridization revealed that overexpression of ATG7 in the fear engram cells upregulated the mRNA of Toll-like receptor TLR2/4 in DG microglia. Knockdown of microglial TLR2/4 rescued fear memory destabilization induced by ATG7 overexpression or Rac1 activation in DG engram cells. These results indicate that Rac1-driven communications between engram cells and microglia contributes to contextual fear memory destabilization, and is mediated by ATG7 and TLR2/4, and suggest a novel mechanistic framework for the cytoskeletal regulator in fear memory interference.

Genetics and Molecular Biology of Memory Suppression

The brain is designed not only with molecules and cellular processes that help to form memories but also with molecules and cellular processes that suppress the formation and retention of memory. The latter processes are critical for an efficient memory management system, given the vast amount of information that each person experiences in their daily activities and that most of this information becomes irrelevant with time. Thus, efficiency dictates that the brain should have processes for selecting the most critical information for storage and suppressing the irrelevant or forgetting it later should it escape the initial filters. Such memory suppressor molecules and processes are revealed by genetic or pharmacologic insults that lead to enhanced memory expression. We review here the predominant memory suppressor molecules and processes that have recently been discovered. They are diverse, as expected, because the brain is complex and employs many different strategies and mechanisms to form memories. They include the gene-repressive actions of small noncoding RNAs, repressors of protein synthesis, cAMP-mediated gene expression pathways, inter- and intracellular signaling pathways for normal forgetting, and others. A deep understanding of memory suppressor molecules and processes is necessary to fully comprehend how the brain forms, stabilizes, and retrieves memories and to reveal how brain disorders disrupt memory.

Evidence of Active-Forgetting Mechanisms? Blocking Arachidonic Acid Release May Slow Forgetting of Sensitization in Aplysia

Long-term sensitization in Aplysia is accompanied by a persistent up-regulation of mRNA encoding the peptide neurotransmitter Phe-Met-Arg-Phe-amide (FMRFa), a neuromodulator that opposes the expression of sensitization through activation of the arachidonic acid second-messenger pathway. We completed a preregistered test of the hypothesis that FMRFa plays a critical role in the forgetting of sensitization. Aplysia received long-term sensitization training and were then given whole-body injections of vehicle (N = 27), FMRFa (N = 26), or 4-bromophenacylbromide (4-BPB; N = 31), a phospholipase inhibitor that prevents the release of arachidonic acid. FMRFa produced no changes in forgetting. 4-BPB decreased forgetting measured 6 d after training [d s = 0.55 95% CI(0.01, 1.09)], though the estimated effect size is uncertain. Our results provide preliminary evidence that forgetting of sensitization may be a regulated, active process in Aplysia, but could also indicate a role for arachidonic acid in stabilizing the induction of sensitization.

Presynaptic Rac1 in the hippocampus selectively regulates working memory

One of the most extensively studied members of the Ras superfamily of small GTPases, Rac1 is an intracellular signal transducer that remodels actin and phosphorylation signaling networks. Previous studies have shown that Rac1-mediated signaling is associated with hippocampal-dependent working memory and longer-term forms of learning and memory and that Rac1 can modulate forms of both pre- and postsynaptic plasticity. How these different cognitive functions and forms of plasticity mediated by Rac1 are linked, however, is unclear. Here, we show that spatial working memory is selectively impaired following the expression of a genetically encoded Rac1-inhibitor at presynaptic terminals, while longer-term cognitive processes are affected by Rac1 inhibition at postsynaptic sites. To investigate the regulatory mechanisms of this presynaptic process, we leveraged new advances in mass spectrometry to identify the proteomic and post-translational landscape of presynaptic Rac1 signaling. We identified serine/threonine kinases and phosphorylated cytoskeletal signaling and synaptic vesicle proteins enriched with active Rac1. The phosphorylated sites in these proteins are at positions likely to have regulatory effects on synaptic vesicles. Consistent with this, we also report changes in the distribution and morphology of synaptic vesicles and in postsynaptic ultrastructure following presynaptic Rac1 inhibition. Overall, this study reveals a previously unrecognized presynaptic role of Rac1 signaling in cognitive processes and provides insights into its potential regulatory mechanisms.

Spatially Resolved Transcriptomic Signatures of Hippocampal Subregions and Arc-Expressing Ensembles in Active Place Avoidance Memory

The rodent hippocampus is a spatially organized neuronal network that supports the formation of spatial and episodic memories. We conducted bulk RNA sequencing and spatial transcriptomics experiments to measure gene expression changes in the dorsal hippocampus following the recall of active place avoidance (APA) memory. Through bulk RNA sequencing, we examined the gene expression changes following memory recall across the functionally distinct subregions of the dorsal hippocampus. We found that recall induced differentially expressed genes (DEGs) in the CA1 and CA3 hippocampal subregions were enriched with genes involved in synaptic transmission and synaptic plasticity, while DEGs in the dentate gyrus (DG) were enriched with genes involved in energy balance and ribosomal function. Through spatial transcriptomics, we examined gene expression changes following memory recall across an array of spots encompassing putative memory-associated neuronal ensembles marked by the expression of the IEGs Arc, Egr1, and c-Jun. Within samples from both trained and untrained mice, the subpopulations of spatial transcriptomic spots marked by these IEGs were transcriptomically and spatially distinct from one another. DEGs detected between Arc+ and Arc- spots exclusively in the trained mouse were enriched in several memory-related gene ontology terms, including "regulation of synaptic plasticity" and "memory." Our results suggest that APA memory recall is supported by regionalized transcriptomic profiles separating the CA1 and CA3 from the DG, transcriptionally and spatially distinct IEG expressing spatial transcriptomic spots, and biological processes related to synaptic plasticity as a defining the difference between Arc+ and Arc- spatial transcriptomic spots.

Time-dependent inhibition of Rac1 in the VTA enhances long-term aversive memory: implications in active forgetting mechanisms

The fate of memories depends mainly on two opposing forces: the mechanisms required for the storage and maintenance of memory and the mechanisms underlying forgetting, being the latter much less understood. Here, we show the effect of inhibiting the small Rho GTPase Rac1 on the fate of inhibitory avoidance memory in male rats. The immediate post-training micro-infusion of the specific Rac1 inhibitor NSC23766 (150 ng/0.5 µl/ side) into the ventral tegmental area (VTA) enhanced long-term memory at 1, 7, and 14 days after a single training. Additionally, an opposed effect occurred when the inhibitor was infused at 12 h after training while no effect was observed immediately after testing animals at 1 day. Control experiments ruled out the possibility that post-training memory enhancement was due to facilitation of memory formation since no effect was found when animals were tested at 1 h after acquisition and no memory enhancement was observed after the formation of a weak memory. Immediate post-training micro-infusion of Rac1 inhibitor into the dorsal hippocampus, or the amygdala did not affect memory. Our findings support the idea of a Rac1-dependent time-specific active forgetting mechanism in the VTA controlling the strength of a long-term aversive memory.

Learning and memory using Drosophila melanogaster: a focus on advances made in the fifth decade of research

In the last decade, researchers using Drosophila melanogaster have made extraordinary progress in uncovering the mysteries underlying learning and memory. This progress has been propelled by the amazing toolkit available that affords combined behavioral, molecular, electrophysiological, and systems neuroscience approaches. The arduous reconstruction of electron microscopic images resulted in a first-generation connectome of the adult and larval brain, revealing complex structural interconnections between memory-related neurons. This serves as substrate for future investigations on these connections and for building complete circuits from sensory cue detection to changes in motor behavior. Mushroom body output neurons (MBOn) were discovered, which individually forward information from discrete and non-overlapping compartments of the axons of mushroom body neurons (MBn). These neurons mirror the previously discovered tiling of mushroom body axons by inputs from dopamine neurons and have led to a model that ascribes the valence of the learning event, either appetitive or aversive, to the activity of different populations of dopamine neurons and the balance of MBOn activity in promoting avoidance or approach behavior. Studies of the calyx, which houses the MBn dendrites, have revealed a beautiful microglomeruluar organization and structural changes of synapses that occur with long-term memory (LTM) formation. Larval learning has advanced, positioning it to possibly lead in producing new conceptual insights due to its markedly simpler structure over the adult brain. Advances were made in how cAMP response element-binding protein interacts with protein kinases and other transcription factors to promote the formation of LTM. New insights were made on Orb2, a prion-like protein that forms oligomers to enhance synaptic protein synthesis required for LTM formation. Finally, Drosophila research has pioneered our understanding of the mechanisms that mediate permanent and transient active forgetting, an important function of the brain along with acquisition, consolidation, and retrieval. This was catalyzed partly by the identification of memory suppressor genes-genes whose normal function is to limit memory formation.

Engram neurons: Encoding, consolidation, retrieval, and forgetting of memory

Tremendous strides have been made in our understanding of the neurobiological substrates of memory - the so-called memory "engram". Here, we integrate recent progress in the engram field to illustrate how engram neurons transform across the "lifespan" of a memory - from initial memory encoding, to consolidation and retrieval, and ultimately to forgetting. To do so, we first describe how cell-intrinsic properties shape the initial emergence of the engram at memory encoding. Second, we highlight how these encoding neurons preferentially participate in synaptic- and systems-level consolidation of memory. Third, we describe how these changes during encoding and consolidation guide neural reactivation during retrieval, and facilitate memory recall. Fourth, we describe neurobiological mechanisms of forgetting, and how these mechanisms can counteract engram properties established during memory encoding, consolidation, and retrieval. Motivated by recent experimental results across these four sections, we conclude by proposing some conceptual extensions to the traditional view of the engram, including broadening the view of cell-type participation within engrams and across memory stages. In collection, our review synthesizes general principles of the engram across memory stages, and describes future avenues to further understand the dynamic engram.

Roles of Rac1-Dependent Intrinsic Forgetting in Memory-Related Brain Disorders: Demon or Angel

Animals are required to handle daily massive amounts of information in an ever-changing environment, and the resulting memories and experiences determine their survival and development, which is critical for adaptive evolution. However, intrinsic forgetting, which actively deletes irrelevant information, is equally important for memory acquisition and consolidation. Recently, it has been shown that Rac1 activity plays a key role in intrinsic forgetting, maintaining the balance of the brain's memory management system in a controlled manner. In addition, dysfunctions of Rac1-dependent intrinsic forgetting may contribute to memory deficits in neurological and neurodegenerative diseases. Here, these new findings will provide insights into the neurobiology of memory and forgetting, pathological mechanisms and potential therapies for brain disorders that alter intrinsic forgetting mechanisms.

Spontaneous recovery of reward memory through active forgetting of extinction memory

Learned behavior can be suppressed by the extinction procedure. Such extinguished memory often returns spontaneously over time, making it difficult to treat diseases such as addiction. However, the biological mechanisms underlying such spontaneous recovery remain unclear. Here, we report that the extinguished reward memory in Drosophila recovers spontaneously because extinction training forms an aversive memory that can be actively forgotten via the Rac1/Dia pathway. Manipulating Rac1 activity does not affect sugar-reward memory and its immediate extinction effect but bidirectionally regulates spontaneous recovery-the decay process of extinction. Experiments using thermogenetic inhibition and functional imaging support that such extinction appears to be coded as an aversive experience. Genetic and pharmacological inhibition of formin Dia, a downstream effector of Rac1, specifically prevents spontaneous recovery after extinction in both behavioral performance and corresponding physiological traces. Together, our data suggest that spontaneous recovery is caused by active forgetting of the opposing extinction memory.

JADE2 Is Essential for Hippocampal Synaptic Plasticity and Cognitive Functions in Mice

BACKGROUND

Impairment of synaptic plasticity is closely correlated with a range of pathological conditions, such as cognitive deficits. However, how synaptic efficacy is regulated remains incompletely understood. Here, we report that the epigenetic factor JADE2 was indispensable for the maintenance of hippocampal synaptic plasticity and cognitive functions in mice.

METHODS

We used the Morris water maze and the fear conditioning test to examine learning-related behaviors. In addition, Western blotting, viral-mediated JADE2 manipulations, RNA sequencing, and electrophysiological recordings were used to address our questions.

RESULTS

JADE2 expression is increased upon enhanced neuronal activity in vitro and in vivo. Knockdown or genetic deletion of Jade2 in hippocampal CA1 results in impaired structural and functional synaptic plasticity, leading to memory impairment, whereas overexpression of JADE2 in CA1 neurons facilitates hippocampal-dependent learning and memory. Mechanistically, our data show that JADE2 modulates synaptic functions mainly by transcriptional activation of cytoskeletal regulator Rac1, and this activity is dependent on its interaction with histone acetyltransferase HBO1. Finally, we demonstrate that restoring RAC1 expression in Jade2 knockout mice could rescue the deficits in synaptic plasticity and learning-related behaviors.

CONCLUSIONS

Our findings reveal that JADE2 plays a critical role in regulating synaptic plasticity and memory formation, suggesting that activity-dependent epigenetic regulation is an important molecular mechanism in controlling synaptic plasticity.

Motivational salience drives habitual gazes during value memory retention and facilitates relearning of forgotten value

A habitual gaze is critical to efficiently identify and exploit valuable objects. However, it is unclear what salience components drive the habitual gaze choice. Here, we trained subjects to assign positive, neutral, and negative values to objects and found that motivational salience guided habitual gaze choices over 30 days of memory retention. The habitual preference for negatively valued objects emerged during memory retention. This habitual choice was not explained by a general model with salience components driven by physical features of objects and the rank of learned values. Instead, this is better explained by a model that contains an additional component driven by motivational salience. In a simulated value-forgotten condition, these motivational salience-based habitual choices facilitated re-learning. Our data indicate that after long-term retention, habitual gaze results from increased motivational salience, potentially facilitating the re-learning of forgotten values.

Phosphorylation of MSI-1 is implicated in the regulation of associative memory in Caenorhabditis elegans

The Musashi family of RNA-binding proteins controls several biological processes including stem cell maintenance, cell division and neural function. Previously, we demonstrated that the C. elegans Musashi ortholog, msi-1, regulates forgetting via translational repression of the Arp2/3 actin-branching complex. However, the mechanisms controlling MSI-1 activity during the regulation of forgetting are currently unknown. Here we investigated the effects of protein phosphorylation on MSI-1 activity. We showed that MSI-1 function is likely controlled by alterations of its activity rather than its expression levels. Furthermore, we found that MSI-1 is phosphorylated and using mass spectrometry we identified MSI-1 phosphorylation at three residues (T18, S19 and S34). CRISPR-based manipulations of MSI-1 phosphorylation sites revealed that phosphorylation is necessary for MSI-1 function in both short- and long-term aversive olfactory associative memory. Thus, our study provides insight into the mechanisms regulating memory-related MSI-1 activity and may facilitate the development of novel therapeutic approaches.

Understanding neural circuits and molecular mechanisms underlying learning and memory are the major challenges of neuroscience. It is a generally accepted model that a learning event causes modification of synapses; strengthening some within a circuit and weakening others (termed “synaptic plasticity”). A plastic nervous system requires not only the ability to acquire and store but also to forget new inputs. While learning and memory is widely investigated, clear-cut evidence for mechanisms involved in forgetting is still sparse. Previously, we demonstrated the role of the protein Musashi (MSI-1) in the active regulation of forgetting in the nematode C. elegans. Here we investigated the role of protein modification (phosphorylation) as a possible regulatory mechanism of the MSI-1 protein activity. We found that MSI-1 protein is modified at different positions and all of these modifications at the protein level contribute to the correct activity of the protein leading to active forgetting of short and long-term memories.

Two distinct ways to form long-term object recognition memory during sleep and wakefulness

Memory consolidation is promoted by sleep. However, there is also evidence for consolidation into long-term memory during wakefulness via processes that preferentially affect nonhippocampal representations. We compared, in rats, the effects of 2-h postencoding periods of sleep and wakefulness on the formation of long-term memory for objects and their associated environmental contexts. We employed a novel-object recognition (NOR) task, using object exploration and exploratory rearing as behavioral indicators of these memories. Remote recall testing (after 1 wk) confirmed significant long-term NOR memory under both conditions, with NOR memory after sleep predicted by the occurrence of EEG spindle-slow oscillation coupling. Rats in the sleep group decreased their exploratory rearing at recall testing, revealing successful recall of the environmental context. By contrast, rats that stayed awake after encoding showed equally high levels of rearing upon remote testing as during encoding, indicating that context memory was lost. Disruption of hippocampal function during the postencoding interval (by muscimol administration) suppressed long-term NOR memory together with context memory formation when animals slept, but enhanced NOR memory when they were awake during this interval. Testing remote recall in a context different from that during encoding impaired NOR memory in the sleep condition, while exploratory rearing was increased. By contrast, NOR memory in the wake rats was preserved and actually superior to that after sleep. Our findings indicate two distinct modes of long-term memory formation: Sleep consolidation is hippocampus dependent and implicates event-context binding, whereas wake consolidation is impaired by hippocampal activation and strengthens context-independent representations.

Inhibition of Rac1 in ventral hippocampal excitatory neurons improves social recognition memory and synaptic plasticity

Rac1 is critically involved in the regulation of the actin cytoskeleton, neuronal structure, synaptic plasticity, and memory. Rac1 overactivation is reported in human patients and animal models of Alzheimer’s disease (AD) and contributes to their spatial memory deficits, but whether Rac1 dysregulation is also important in other forms of memory deficits is unknown. In addition, the cell types and synaptic mechanisms involved remain unclear. In this study, we used local injections of AAV virus containing a dominant-negative (DN) Rac1 under the control of CaMKIIα promoter and found that the reduction of Rac1 hyperactivity in ventral hippocampal excitatory neurons improves social recognition memory in APP/PS1 mice. Expression of DN Rac1 also improves long-term potentiation, a key synaptic mechanism for memory formation. Our results suggest that overactivation of Rac1 in hippocampal excitatory neurons contributes to social memory deficits in APP/PS1 mice and that manipulating Rac1 activity may provide a potential therapeutic strategy to treat social deficits in AD.

Transient neuroinflammation following surgery contributes to long-lasting cognitive decline in elderly rats via dysfunction of synaptic NMDA receptor

Background

Perioperative neurocognitive disorders (PNDs) are considered the most common postoperative complication in geriatric patients. However, its pathogenesis is not fully understood. Surgery-triggered neuroinflammation is a major contributor to the development of PNDs. Neuroinflammation can influence N-methyl-D-aspartate receptor (NMDAR) expression or function which is closely associated with cognition. We, therefore, hypothesized that the persistent changes in NMDAR expression or function induced by transient neuroinflammation after surgery were involved in the development of PNDs.

Methods

Eighteen-month-old male Sprague–Dawley rats were subjected to abdominal surgery with sevoflurane anesthesia to establish the PNDs animal model. Then, we determined the transient neuroinflammation by detecting the protein levels of proinflammatory cytokines and microglia activation using ELISA, western blot, immunohistochemistry, and microglial morphological analysis from postoperative days 1–20. Persistent changes in NMDAR expression were determined by detecting the protein levels of NMDAR subunits from postoperative days 1–59. Subsequently, the dysfunction of synaptic NMDAR was evaluated by detecting the structural plasticity of dendritic spine using Golgi staining. Pull-down assay and western blot were used to detect the protein levels of Rac1-GTP, phosphor-cofilin, and Arp3, which contribute to the regulation of the structural plasticity of dendritic spine. Finally, glycyrrhizin, an anti-inflammatory agent, was administered to further explore the role of synaptic NMDAR dysfunction induced by transient neuroinflammation in the neuropathogenesis of PNDs.

Results

We showed that transient neuroinflammation induced by surgery caused sustained downregulation of synaptic NR2A and NR2B subunits in the dorsal hippocampus and led to a selective long-term spatial memory deficit. Meanwhile, the detrimental effect of neuroinflammation on the function of synaptic NMDARs was shown by the impaired structural plasticity of dendritic spines and decreased activity of the Rac1 signaling pathways during learning. Furthermore, anti-inflammatory treatment reversed the downregulation and hypofunction of synaptic NR2A and NR2B and subsequently rescued the long-term spatial memory deficit.

Conclusions

Our results identify sustained synaptic NR2A and NR2B downregulation and hypofunction induced by transient neuroinflammation following surgery as important contributors to the development of PNDs in elderly rats.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02528-5.

Time of Day-Dependent Alteration of Hippocampal Rac1 Activation Regulates Contextual Fear Memory in Rats

Fear memory in species varies according to the time of the day. Although the underlying molecular mechanisms have been extensively explored, they remain largely unknown. Here, we report that hippocampal Rac1 activity undergoes a time of day-dependent alteration both in nocturnal rats and diurnal tree shrews and that training at the lower hippocampal Rac1 activation period during the night leads to better contextual fear memory in rats. Furthermore, day and night reversion by 24 h darkness/24 h light housing inverses the external clock time of hippocampal Rac1 activation, but the better contextual fear memory still coincides with the lower Rac1 activation in rats during the night. Interestingly, exogenous melatonin treatment promotes hippocampal Rac1 activity and impairs better contextual fear memory acquired at the lower Rac1 activation period during the night, and Rac1-specific inhibitor NSC23766 compromises the effect of melatonin. These results suggest that the time of day-dependent alteration of hippocampal Rac1 activation regulates contextual fear memory in rats by forgetting.

Hippocampal Estrogen Signaling Mediates Sex Differences in Retroactive Interference

Despite being a crucial physiological function of the brain, the mechanisms underlying forgetting are still poorly understood. Estrogens play a critical role in different brain functions, including memory. However, the effects of sex hormones on forgetting vulnerabilitymediated by retroactive interference (RI), a phenomenon in which newly acquired information interferes with the retrieval of already stored information, are still poorly understood. The aim of our study was to characterize the sex differences in interference-mediated forgetting and identify the underlying molecular mechanisms. We found that adult male C57bl/6 mice showed a higher susceptibility to RI-dependent memory loss than females. The preference index (PI) in the NOR paradigm was 52.7 ± 5.9% in males and 62.3 ± 13.0% in females. The resistance to RI in female mice was mediated by estrogen signaling involving estrogen receptor α activation in the dorsal hippocampus. Accordingly, following RI, females showed higher phosphorylation levels (+30%) of extracellular signal-regulated kinase1/2 (ERK1/2) in the hippocampus. Pharmacological inhibition of ERK1/2 made female mice prone to RI. The PI was 70.6 ± 11.0% in vehicle-injected mice and 47.4 ± 10.8% following PD98059 administration. Collectively, our data suggest that hippocampal estrogen α receptor-ERK1/2 signaling is critically involved in a pattern separation mechanism that inhibits object-related RI in female mice.

Social experiences switch states of memory engrams through regulating hippocampal Rac1 activity

In pathological or artificial conditions, memory can be formed as silenced engrams that are unavailable for retrieval by presenting conditioned stimuli but can be artificially switched into the latent state so that natural recall is allowed. However, it remains unclear whether such different states of engrams bear any physiological significance and can be switched through physiological mechanisms. Here, we show that an acute social reward experience switches the silent memory engram into the latent state. Conversely, an acute social stress causes transient forgetting via turning a latent memory engram into a silent state. Such emotion-driven bidirectional switching between latent and silent states of engrams is mediated through regulation of Rac1 activity–dependent reversible forgetting in the hippocampus, as stress-activated Rac1 suppresses retrieval, while reward recovers silenced memory under amnesia by inhibiting Rac1. Thus, data presented reveal hippocampal Rac1 activity as the basis for emotion-mediated switching between latent and silent engrams to achieve emotion-driven behavioral flexibility.

Forgetting as a form of adaptive engram cell plasticity

One leading hypothesis suggests that memories are stored in ensembles of neurons (or 'engram cells') and that successful recall involves reactivation of these ensembles. A logical extension of this idea is that forgetting occurs when engram cells cannot be reactivated. Forms of 'natural forgetting' vary considerably in terms of their underlying mechanisms, time course and reversibility. However, we suggest that all forms of forgetting involve circuit remodelling that switches engram cells from an accessible state (where they can be reactivated by natural recall cues) to an inaccessible state (where they cannot). In many cases, forgetting rates are modulated by environmental conditions and we therefore propose that forgetting is a form of neuroplasticity that alters engram cell accessibility in a manner that is sensitive to mismatches between expectations and the environment. Moreover, we hypothesize that disease states associated with forgetting may hijack natural forgetting mechanisms, resulting in reduced engram cell accessibility and memory loss.

Rac-maninoff and Rho-vel: The symphony of Rho-GTPase signaling at excitatory synapses

Synaptic connections between neurons are essential for every facet of human cognition and are thus regulated with extreme precision. Rho-family GTPases, molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state, comprise a critical feature of synaptic regulation. Rho-GTPases are exquisitely controlled by an extensive suite of activators (GEFs) and inhibitors (GAPs and GDIs) and interact with many different signalling pathways to fulfill their roles in orchestrating the development, maintenance, and plasticity of excitatory synapses of the central nervous system. Among the mechanisms that control Rho-GTPase activity and signalling are cell surface receptors, GEF/GAP complexes that tightly regulate single Rho-GTPase dynamics, GEF/GAP and GEF/GEF functional complexes that coordinate multiple Rho-family GTPase activities, effector positive feedback loops, and mutual antagonism of opposing Rho-GTPase pathways. These complex regulatory mechanisms are employed by the cells of the nervous system in almost every step of development, and prominently figure into the processes of synaptic plasticity that underlie learning and memory. Finally, misregulation of Rho-GTPases plays critical roles in responses to neuronal injury, such as traumatic brain injury and neuropathic pain, and in neurodevelopmental and neurodegenerative disorders, including intellectual disability, autism spectrum disorder, schizophrenia, and Alzheimer's Disease. Thus, decoding the mechanisms of Rho-GTPase regulation and function at excitatory synapses has great potential for combatting many of the biggest current challenges in mental health.

Arhgap22 Disruption Leads to RAC1 Hyperactivity Affecting Hippocampal Glutamatergic Synapses and Cognition in Mice

Rho GTPases are a class of G-proteins involved in several aspects of cellular biology, including the regulation of actin cytoskeleton. The most studied members of this family are RHOA and RAC1 that act in concert to regulate actin dynamics. Recently, Rho GTPases gained much attention as synaptic regulators in the mammalian central nervous system (CNS). In this context, ARHGAP22 protein has been previously shown to specifically inhibit RAC1 activity thus standing as critical cytoskeleton regulator in cancer cell models; however, whether this function is maintained in neurons in the CNS is unknown. Here, we generated a knockout animal model for arhgap22 and provided evidence of its role in the hippocampus. Specifically, we found that ARHGAP22 absence leads to RAC1 hyperactivity and to an increase in dendritic spine density with defects in synaptic structure, molecular composition, and plasticity. Furthermore, arhgap22 silencing causes impairment in cognition and a reduction in anxiety-like behavior in mice. We also found that inhibiting RAC1 restored synaptic plasticity in ARHGAP22 KO mice. All together, these results shed light on the specific role of ARHGAP22 in hippocampal excitatory synapse formation and function as well as in learning and memory behaviors.

Memory suppressor genes: Modulating acquisition, consolidation, and forgetting

The brain has a remarkable but underappreciated capacity to limit memory formation and expression. The term "memory suppressor gene" was coined in 1998 as an attempt to explain emerging reports that some genes appeared to limit memory. At that time, only a handful of memory suppressor genes were known, and they were understood to work by limiting cAMP-dependent consolidation. In the intervening decades, almost 100 memory suppressor genes with diverse functions have been discovered that affect not only consolidation but also acquisition and forgetting. Here we highlight the surprising extent to which biological limits are placed on memory formation through reviewing the literature on memory suppressor genes. In this review, we present memory suppressors within the framework of their actions on different memory operations: acquisition, consolidation, and forgetting. This is followed by a discussion of the reasons why there may be a biological need to limit memory formation.

Bidirectional role of dopamine in learning and memory-active forgetting

Dopaminergic neurons projecting from the Substantia Nigra to the Striatum play a critical role in motor functions while dopaminergic neurons originating in the Ventral Tegmental Area (VTA) and projecting to the Nucleus Accumbens, Hippocampus and other cortical structures regulate rewarding learning. While VTA mainly consists of dopaminergic neurons, excitatory (glutamate) and inhibitory (GABA) VTA-neurons have also been described: these neurons may also modulate and contribute to shape the final dopaminergic response, which is critical for memory formation. However, given the large amount of information that is handled daily by our brain, it is essential that irrelevant information be deleted. Recently, apart from the well-established role of dopamine (DA) in learning, it has been shown that DA plays a critical role in the intrinsic active forgetting mechanisms that control storage information, contributing to the deletion of a consolidated memory. These new insights may be instrumental to identify therapies for those disorders that involve memory alterations.

Rho Signaling in Synaptic Plasticity, Memory, and Brain Disorders

Memory impairments are associated with many brain disorders such as autism, Alzheimer's disease, and depression. Forming memories involves modifications of synaptic transmission and spine morphology. The Rho family small GTPases are key regulators of synaptic plasticity by affecting various downstream molecules to remodel the actin cytoskeleton. In this paper, we will review recent studies on the roles of Rho proteins in the regulation of hippocampal long-term potentiation (LTP) and long-term depression (LTD), the most extensively studied forms of synaptic plasticity widely regarded as cellular mechanisms for learning and memory. We will also discuss the involvement of Rho signaling in spine morphology, the structural basis of synaptic plasticity and memory formation. Finally, we will review the association between brain disorders and abnormalities of Rho function. It is expected that studying Rho signaling at the synapse will contribute to the understanding of how memory is formed and disrupted in diseases.

Activation of Rac1 Has an Opposing Effect on Induction and Maintenance of Long-Term Potentiation in Hippocampus by Acting on Different Kinases

Rac1 is a small GTPase of the Rho family. A previous study showed that the activation of Rac1 had an opposing effect on induction and maintenance of long-term potentiation (LTP) in the hippocampus. However, the molecular mechanism underlying this opposing effect remains to be addressed. In the present work, we find that the activation of Rac1 during the induction of LTP leads to an activation of PKCι/λ by phosphatidylinositol-3-kinase (PI3K), whereas the activation of Rac1 during the maintenance of LTP leads to the inhibition of PKMζ by LIM_kinase (LIMK) in the hippocampus. This result suggests that during different stages of LTP, the activation of Rac1 can modulate different signaling pathways, which leads to an opposing effect on the induction and maintenance of LTP in the hippocampus.

Neurogenesis-dependent transformation of hippocampal engrams

In humans and other mammals, memories of events are encoded by neuronal ensembles (or engrams) in the hippocampus. The mnemonic information stored in these engrams can then be used to guide future behavior, including prediction- and decision-making in dynamic environments. While some hippocampal engrams may be persistently stored, others are modified over time, suggesting that the represented memories may also be transformed. How might hippocampal engrams be modified through time? Adult hippocampal neurogenesis represents one process that continuously rewires hippocampal circuitry, presumably including stored hippocampal engrams. At intermediate stages, we propose that neurogenesis-mediated rewiring of hippocampal engram circuitry induces forgetting of specific stimulus attributes, and this less precise engram allows for generalization. At more advanced stages, we propose that neurogenesis-mediated rewiring of hippocampal engram circuitry leads to silencing of hippocampal engrams, rendering them no longer accessible by natural retrieval cues.

Rac1 and Akt Exhibit Distinct Roles in Mediating Aβ-Induced Memory Damage and Learning Impairment

Accumulated beta-amyloid (Aβ) in the brain is the hallmark of Alzheimer's disease (AD). Despite Aβ accumulation is known to trigger cellular dysfunctions and learning and memory damage, the detailed molecular mechanism remains elusive. Recent studies have shown that the onset of memory impairment and learning damage in the AD animal is different, suggesting that the underlying mechanism of the development of memory impairment and learning damage may not be the same. In the current study, with the use of Aβ42 transgenic flies as models, we found that Aβ induces memory damage and learning impairment via differential molecular signaling pathways. In early stage, Aβ activates both Ras and PI3K to regulate Rac1 activity, which affects mostly on memory performance. In later stage, PI3K-Akt is strongly activated by Aβ, which leads to learning damage. Moreover, reduced Akt, but not Rac1, activity promotes cell viability in the Aβ42 transgenic flies, indicating that Akt and Rac1 exhibit differential roles in Aβ regulating toxicity. Taken together, different molecular and cellular mechanisms are involved in Aβ-induced learning damage and memory decline; thus, caution should be taken during the development of therapeutic intervention in the future.

Disruption of a RAC1-centred network is associated with Alzheimer's disease pathology and causes age-dependent neurodegeneration

The molecular biological mechanisms of Alzheimer’s disease (AD) involve disease-associated crosstalk through many genes and include a loss of normal as well as a gain of abnormal interactions among genes. A protein domain network (PDN) is a collection of physical bindings that occur between protein domains, and the states of the PDNs in patients with AD are likely to be perturbed compared to those in normal healthy individuals. To identify PDN changes that cause neurodegeneration, we analysed the PDNs that occur among genes co-expressed in each of three brain regions at each stage of AD. Our analysis revealed that the PDNs collapsed with the progression of AD stage and identified five hub genes, including Rac1, as key players in PDN collapse. Using publicly available as well as our own gene expression data, we confirmed that the mRNA expression level of the RAC1 gene was downregulated in the entorhinal cortex (EC) of AD brains. To test the causality of these changes in neurodegeneration, we utilized Drosophila as a genetic model and found that modest knockdown of Rac1 in neurons was sufficient to cause age-dependent behavioural deficits and neurodegeneration. Finally, we identified a microRNA, hsa-miR-101-3p, as a potential regulator of RAC1 in AD brains. As the Braak neurofibrillary tangle (NFT) stage progressed, the expression levels of hsa-miR-101-3p were increased specifically in the EC. Furthermore, overexpression of hsa-miR-101-3p in the human neuronal cell line SH-SY5Y caused RAC1 downregulation. These results highlight the utility of our integrated network approach for identifying causal changes leading to neurodegeneration in AD.

The neural circuitry of social homeostasis: Consequences of acute versus chronic social isolation

Social homeostasis is the ability of individuals to detect the quantity and quality of social contact, compare it to an established set-point in a command center, and adjust the effort expended to seek the optimal social contact expressed via an effector system. Social contact becomes a positive or negative valence stimulus when it is deficient or in excess, respectively. Chronic deficits lead to set-point adaptations such that reintroduction to the previous optimum is experienced as a surplus. Here, we build upon previous models for social homeostasis to include adaptations to lasting changes in environmental conditions, such as with chronic isolation.

Understanding the dynamic and destiny of memories

Memory formation enables the retention of life experiences overtime. Based on previously acquired information, organisms can anticipate future events and adjust their behaviors to maximize survival. However, in an ever-changing environment, a memory needs to be malleable to maintain its relevance. In fact, substantial evidence suggests that a consolidated memory can become labile and susceptible to modifications after being reactivated, a process termed reconsolidation. When an extinction process takes place, a memory can also be temporarily inhibited by a second memory that carries information with opposite meaning. In addition, a memory can fade and lose its significance in a process known as forgetting. Thus, following retrieval, new life experiences can be integrated with the original memory trace to maintain its predictive value. In this review, we explore the determining factors that regulate the fate of a memory after its reactivation. We focus on three post-retrieval memory destinies (reconsolidation, extinction, and forgetting) and discuss recent rodent studies investigating the biological functions and neural mechanisms underlying each of these processes.

The Rac-GEF Tiam1 Promotes Dendrite and Synapse Stabilization of Dentate Granule Cells and Restricts Hippocampal-Dependent Memory Functions

The dentate gyrus (DG) controls information flow into the hippocampus and is critical for learning, memory, pattern separation, and spatial coding, while DG dysfunction is associated with neuropsychiatric disorders. Despite its importance, the molecular mechanisms regulating DG neural circuit assembly and function remain unclear. Here, we identify the Rac-GEF Tiam1 as an important regulator of DG development and associated memory processes. In the hippocampus, Tiam1 is predominantly expressed in the DG throughout life. Global deletion of Tiam1 in male mice results in DG granule cells with simplified dendritic arbors, reduced dendritic spine density, and diminished excitatory synaptic transmission. Notably, DG granule cell dendrites and synapses develop normally in Tiam1 KO mice, resembling WT mice at postnatal day 21 (P21), but fail to stabilize, leading to dendrite and synapse loss by P42. These results indicate that Tiam1 promotes DG granule cell dendrite and synapse stabilization late in development. Tiam1 loss also increases the survival, but not the production, of adult-born DG granule cells, possibly because of greater circuit integration as a result of decreased competition with mature granule cells for synaptic inputs. Strikingly, both male and female mice lacking Tiam1 exhibit enhanced contextual fear memory and context discrimination. Together, these results suggest that Tiam1 is a key regulator of DG granule cell stabilization and function within hippocampal circuits. Moreover, based on the enhanced memory phenotype of Tiam1 KO mice, Tiam1 may be a potential target for the treatment of disorders involving memory impairments.SIGNIFICANCE STATEMENT The dentate gyrus (DG) is important for learning, memory, pattern separation, and spatial navigation, and its dysfunction is associated with neuropsychiatric disorders. However, the molecular mechanisms controlling DG formation and function remain elusive. By characterizing mice lacking the Rac-GEF Tiam1, we demonstrate that Tiam1 promotes the stabilization of DG granule cell dendritic arbors, spines, and synapses, whereas it restricts the survival of adult-born DG granule cells, which compete with mature granule cells for synaptic integration. Notably, mice lacking Tiam1 also exhibit enhanced contextual fear memory and context discrimination. These findings establish Tiam1 as an essential regulator of DG granule cell development, and identify it as a possible therapeutic target for memory enhancement.

Clmp Regulates AMPA and Kainate Receptor Responses in the Neonatal Hippocampal CA3 and Kainate Seizure Susceptibility in Mice

Synaptic adhesion molecules regulate synapse development through trans-synaptic adhesion and assembly of diverse synaptic proteins. Many synaptic adhesion molecules positively regulate synapse development; some, however, exert negative regulation, although such cases are relatively rare. In addition, synaptic adhesion molecules regulate the amplitude of post-synaptic receptor responses, but whether adhesion molecules can regulate the kinetic properties of post-synaptic receptors remains unclear. Here we report that Clmp, a homophilic adhesion molecule of the Ig domain superfamily that is abundantly expressed in the brain, reaches peak expression at a neonatal stage (week 1) and associates with subunits of AMPA receptors (AMPARs) and kainate receptors (KARs). Clmp deletion in mice increased the frequency and amplitude of AMPAR-mediated miniature excitatory post-synaptic currents (mEPSCs) and the frequency, amplitude, and decay time constant of KAR-mediated mEPSCs in hippocampal CA3 neurons. Clmp deletion had minimal impacts on evoked excitatory synaptic currents at mossy fiber-CA3 synapses but increased extrasynaptic KAR, but not AMPAR, currents, suggesting that Clmp distinctly inhibits AMPAR and KAR responses. Behaviorally, Clmp deletion enhanced novel object recognition and susceptibility to kainate-induced seizures, without affecting contextual or auditory cued fear conditioning or pattern completion-based contextual fear conditioning. These results suggest that Clmp negatively regulates hippocampal excitatory synapse development and AMPAR and KAR responses in the neonatal hippocampal CA3 as well as object recognition and kainate seizure susceptibility in mice.

Intravenous infusion of bone marrow mononuclear cells promotes functional recovery and improves impaired cognitive function via inhibition of Rho guanine nucleotide triphosphatases and inflammatory signals in a model of chronic epilepsy

Temporal lobe epilepsy is the most common form of intractable epilepsy in adults. More than 30% of individuals with epilepsy have persistent seizures and have drug-resistant epilepsy. Based on our previous findings, treatment with bone marrow mononuclear cells (BMMC) could interfere with early and chronic phase epilepsy in rats and in clinical settings. In this pilocarpine-induced epilepsy model, animals were randomly assigned to two groups: control (Con) and epileptic pre-treatment (Ep-pre-t). The latter had status epilepticus (SE) induced through pilocarpine intraperitoneal injection. Later, seizure frequency was assessed using a video-monitoring system. Ep-pre-t was further divided into epileptic treated with saline (Ep-Veh) and epileptic treated with BMMC (Ep-BMMC) after an intravenous treatment with BMMC was done on day 22 after SE. Analysis of neurobehavioral parameters revealed that Ep-BMMC had significantly lower frequency of spontaneous recurrent seizures (SRS) in comparison to Ep-pre-t and Ep-Veh groups. Hippocampus-dependent spatial and non-spatial learning and memory were markedly impaired in epileptic rats, a deficit that was robustly recovered by treatment with BMMC. Moreover, long-term potentiation-induced synaptic remodeling present in epileptic rats was restored by BMMC. In addition, BMMC was able to reduce abnormal mossy fiber sprouting in the dentate gyrus. Molecular analysis in hippocampal tissue revealed that BMMC treatment down-regulates the release of inflammatory cytokine tumor necrosis factor-α (TNF-α) and Allograft inflammatory factor-1 (AIF-1) as well as the Rho subfamily of small GTPases [Ras homolog gene family member A (RhoA) and Ras-related C3 botulinum toxin substrate 1 (Rac)]. Collectively, delayed BMMC treatment showed positive effects when intravenously infused into chronic epileptic rats.

Experimental effects of acute exercise on forgetting

Objective

Prior research has evaluated the effects of acute exercise on episodic memory function. These studies have, on occasion, demonstrated that acute exercise may enhance both short- and long-term memory. It is uncertain as to whether the acute exercise improvements in long-term memory are a result of acute exercise attenuating declines in long-term memory, or rather, are driven by the enhancement effects of acute exercise on short-term memory. The present empirical study evaluates whether the decline from short- to long-term is influenced by acute exercise. This relationship is plausible as exercise has been shown to activate neurophysiological pathways (e.g., RAC1) that are involved in the mechanisms of forgetting.

Methods

To evaluate the effects of acute exercise on forgetting, we used data from 12 of our laboratory's prior experiments (N = 538). Across these 12 experiments, acute exercise ranged from 10 to 15 mins in duration (moderate-to-vigorous intensity). Episodic memory was assessed from word-list or paragraph-based assessments. Short-term memory was assessed immediately after encoding, with long-term memory assessed approximately 20-min later. Forgetting was calculated as the difference in short- and long-term memory performance.

Results

Acute exercise (vs. seated control) was not associated with an attenuated forgetting effect (d = 0.10; 95% CI: -0.04, 0.25, P = 0.17). We observed no evidence of a significant moderation effect (Q = 6.16, df = 17, P = 0.17, I2 = 0.00) for any of the evaluated parameters, including study design, exercise intensity and delay period.

Conclusion

Across our 12 experimental studies, acute exercise was not associated with an attenuated forgetting effect. We discuss these implications for future research that evaluates the effects of acute exercise on long-term memory function.

Memory reacquisition deficit: Chicks fail to relearn pharmacologically disrupted associative response

It is generally assumed that if memory is disrupted by pharmacological inhibitors during its consolidation, it can be later acquired afresh. In our experiments, we trained day-old chicks in a one-trial passive avoidance task and interfered with memory formation using protein synthesis inhibitor anisomycin or NMDA receptor antagonist MK-801. Second training was then given to amnestic animals with either the same conditioning stimulus (retraining) or a new one (novel training). Retraining with the same stimulus failed to produce efficient memory at all the examined between-training and training-to-test intervals, while a new conditioned stimulus was learned successfully. We suggest that this memory reacquisition deficit may result from the failure of associative memory co-allocation mechanisms.

The Role of Rac GTPase in Dendritic Spine Morphogenesis and Memory

The ability to form memories in the brain is needed for daily functions, and its impairment is associated with human mental disorders. Evidence indicates that long-term memory (LTM)-related processes such as its consolidation, extinction and forgetting involve changes of synaptic efficacy produced by alterations in neural transmission and morphology. Modulation of the morphology and number of dendritic spines has been proposed to contribute to changes in neuronal transmission mediating such LTM-related processes. Rac GTPase activity is regulated by synaptic activation and it can affect spine morphology by controlling actin-regulatory proteins. Recent evidence shows that changes in Rac GTPase activity affect memory consolidation, extinction, erasure and forgetting and can affect spine morphology in brain areas that mediate these behaviors. Altered Rac GTPase activity is associated with abnormal spine morphology and brain disorders. By affecting Rac GTPase activity we can further understand the roles of spine morphogenesis in memory. Moreover, manipulation of Rac GTPase activity may serve as a therapeutic tool for the treatment of memory-related brain diseases.