Publications

Episodic memories are encoded by experience-activated neuronal ensembles that remain necessary and sufficient for recall. However, the temporal evolution of memory engrams after initial encoding is unclear. In this study, we employed computational and experimental approaches to examine how the neural composition and selectivity of engrams change with memory consolidation. Our spiking neural network model yielded testable predictions: memories transition from unselective to selective as neurons drop out of and drop into engrams; inhibitory activity during recall is essential for memory selectivity; and inhibitory synaptic plasticity during memory consolidation is critical for engrams to become selective. Using activity-dependent labeling, longitudinal calcium imaging, and a combination of optogenetic and chemogenetic manipulations in mouse dentate gyrus, we conducted contextual fear conditioning experiments that supported our model’s predictions. Our results reveal that memory engrams are dynamic and that changes in engram composition mediated by inhibitory plasticity are crucial for the emergence of memory selectivity.

Identification of synaptic partners is a fundamental task for systems neuroscience. To date, few reliable techniques exist for whole-brain labeling of downstream synaptic partners in a cell-type-dependent and monosynaptic manner. Herein, we describe a novel monosynaptic anterograde tracing system based on the deletion of the gene UL6 from the genome of a cre-dependent version of the anterograde Herpes Simplex Virus 1 strain H129. Given that this knockout blocks viral genome packaging and thus viral spread, we reasoned that co-infection of an HSV H129 ΔUL6 virus with a recombinant adeno-associated virus expressing UL6 in a cre-dependent manner would result in monosynaptic spread from target cre-expressing neuronal populations. Application of this system to five nonreciprocal neural circuits resulted in the labeling of neurons in expected projection areas. While some caveats may preclude certain applications, this system provides a reliable method to label postsynaptic partners in a brain-wide fashion.

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 thalamus engages in various functions including sensory processing, attention, decision making and memory. Classically, this diversity of function has been attributed to the nuclear organization of the thalamus, with each nucleus performing a well-defined function. Here, we highlight recent studies that used state-of-the-art expression profiling, which have revealed gene expression gradients at the single-cell level within and across thalamic nuclei. These gradients, combined with anatomical tracing and physiological analyses, point to previously unappreciated heterogeneity and redefine thalamic units of function on the basis of unique input–output connectivity patterns and gene expression. We propose that thalamic subnetworks, defined by the intersection of genetics, connectivity and computation, provide a more appropriate level of functional description; this notion is supported by behavioral phenotypes resulting from appropriately tailored perturbations. We provide several examples of thalamic subnetworks and suggest how this new perspective may both propel progress in basic neuroscience and reveal unique targets with therapeutic potential.

Parkinson’s disease causes slowness of movement, tremor and stiffness, along with mood symptoms such as depression. The brain’s parafascicular thalamus is shown to contain distinct neural circuits for locomotion, motor learning and depression-like states, and targeting these circuits alleviates some deficits in a mouse model of the disease.

Anterior cingulate cortex mediates the flexible updating of an animal’s choice responses upon rule changes in the environment. However, how the anterior cingulate cortex entrains the 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 the 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 the cingulate cortex decreases the 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 the cingulate cortex monitors behavioral errors and updates rule representations in the motor cortex, revealing a critical role for cingulate-motor circuits in adaptive choice behaviors.

In this issue of Neuron, Sun et al. (2022) identify a neuronal subpopulation in the medial paralemniscal nucleus that underlies repetitive self-grooming. Through their projections to dopamine neurons in the ventral tegmental area, this subpopulation also drives reward and alleviates anxiety.

Alterations in the structure and functional connectivity of anterior thalamic nuclei (ATN) have been linked to reduced cognition during aging. However, ATN circuits that contribute to higher cognitive functions remain understudied. We found that the anteroventral (AV) subdivision of ATN is necessary specifically during the maintenance phase of a spatial working memory task. This function engages the AV→parasubiculum (PaS)→entorhinal cortex (EC) circuit. Aged mice showed a deficit in spatial working memory, which was associated with a decrease in the excitability of AV neurons. Activation of AV neurons or the AV→PaS circuit in aged mice was sufficient to rescue their working memory performance. Furthermore, rescued aged mice showed improved behavior-induced neuronal activity in prefrontal cortex (PFC), a critical site for working memory processes. Although the direct activation of PFC neurons in aged mice also rescued their working memory performance, we found that these animals exhibited increased levels of anxiety, which was not the case for AV→PaS circuit manipulations in aged mice. These results suggest that targeting AV thalamus in aging may not only be beneficial for cognitive functions but that this approach may have fewer unintended effects compared to direct PFC manipulations.

Episodic memories are encoded by experience-activated neuronal ensembles that remain necessary and sufficient for recall. However, the temporal evolution of memory engrams after initial encoding is unclear. In this study, we employed computational and experimental approaches to examine how the neural composition and selectivity of engrams change with memory consolidation. Our spiking neural network model yielded testable predictions: memories transition from unselective to selective as neurons drop out of and drop into engrams; inhibitory activity during recall is essential for memory selectivity; and inhibitory synaptic plasticity during memory consolidation is critical for engrams to become selective. Using activity-dependent labeling, longitudinal calcium imaging, and a combination of optogenetic and chemogenetic manipulations in mouse dentate gyrus, we conducted contextual fear conditioning experiments that supported our model’s predictions. Our results reveal that memory engrams are dynamic and that changes in engram composition mediated by inhibitory plasticity are crucial for the emergence of memory selectivity.

Although bradykinesia, tremor and rigidity are the hallmark motor defects in patients with Parkinson’s disease (PD), patients also experience motor learning impairments and non-motor symptoms such as depression. The neural circuit basis for these different symptoms of PD are not well understood. Although current treatments are effective for locomotion deficits in PD therapeutic strategies targeting motor learning deficits and non-motor symptoms are lacking. Here we found that distinct parafascicular (PF) thalamic subpopulations project to caudate putamen (CPu), subthalamic nucleus (STN) and nucleus accumbens (NAc). Whereas PF→CPu and PF→STN circuits are critical for locomotion and motor learning, respectively, inhibition of the PF→NAc circuit induced a depression-like state. Whereas chemogenetically manipulating CPu-projecting PF neurons led to a long-term restoration of locomotion, optogenetic long-term potentiation (LTP) at PF→STN synapses restored motor learning behaviour in an acute mouse model of PD. Furthermore, activation of NAc-projecting PF neurons rescued depression-like phenotypes. Further, we identified nicotinic acetylcholine receptors capable of modulating PF circuits to rescue different PD phenotypes. Thus, targeting PF thalamic circuits may be an effective strategy for treating motor and non-motor deficits in PD.

Neuropsychiatric disorders are often accompanied by cognitive impairments/intellectual disability (ID). It is not clear whether there are converging mechanisms underlying these debilitating impairments. We found that many autism and schizophrenia risk genes are expressed in the anterodorsal subdivision (AD) of anterior thalamic nuclei, which has reciprocal connectivity with learning and memory structures. CRISPR-Cas9 knockdown of multiple risk genes selectively in AD thalamus led to memory deficits. While the AD is necessary for contextual memory encoding, the neighboring anteroventral subdivision (AV) regulates memory specificity. These distinct functions of AD and AV are mediated through their projections to retrosplenial cortex, using differential mechanisms. Furthermore, knockdown of autism and schizophrenia risk genes PTCHD1, YWHAG, or HERC1 from AD led to neuronal hyperexcitability, and normalization of hyperexcitability rescued memory deficits in these models. This study identifies converging cellular to circuit mechanisms underlying cognitive deficits in a subset of neuropsychiatric disease models.

The survival of an animal critically depends on its ability to identify nutrient-rich food and avoid harmful substances. Although olfactory cues and visual information contribute to these processes, the taste system is ultimately responsible for recognizing and discriminating between the different food qualities. The sensation of taste begins on the tongue, which contains specialized taste receptor cells that are sensitive to the five basic tastes (Yarmolinsky et al., 2009). In humans, taste information travels from the oral cavity to primary gustatory cortex [insular cortex (IC)] via the following three synapses: taste receptor cells to the rostral nucleus of the solitary tract (NST); NST to the gustatory thalamic nucleus [ventral posterior medial nucleus (VPM)]; and VPM to IC (Oliveira-Maia et al., 2011).

Targeting recall processes could let people who are in the disease’s early stages access what they currently can’t remember.

Neuronal ensembles that hold specific memory (memory engrams) have been identified in the hippocampus, amygdala, and cortex. It has been hypothesized that engrams for a specific memory are distributed among multiple brain regions that are functionally connected. Here, we report the hitherto most extensive engram map for contextual fear memory by characterizing activity-tagged neurons in 409 regions using SHIELD-based tissue phenotyping. The mapping was aided by a novel engram index, which identified cFos+ brain regions holding engrams with a high probability. Optogenetic manipulations confirmed previously known engrams and revealed new engrams. Many of these engram holding-regions were functionally connected to the CA1 or amygdala engrams. Simultaneous chemogenetic reactivation of multiple engrams, which mimics natural memory recall, conferred a greater level of memory recall than reactivation of a single engram ensemble. Overall, our study supports the hypothesis that a memory is stored in functionally connected engrams distributed across multiple brain regions.

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.

The invention, in part, relates to the use of optogenetic methods to increase dendritic spine density on DG memory engram cells in treatment methods for memory-impairment-associated diseases and conditions.

One of the most fascinating aspects of an animal’s brain is its ability to acquire new information from experience and retain this information over time as memory. The search for physical correlates of memory, the memory engram, has been a longstanding endeavor in modern neurobiology. Recent advances in transgenic and optogenetic tools have enabled the identification, visualization, and manipulations of natural, sensory-evoked, engram cells for a specific memory residing in specific brain regions. These studies are paving the way not only to understand memory mechanisms in unprecedented detail, but also to repair the abnormal state of mind associated with memory by engineering.

Two new protein tools translate neuronal activity into gene expression during a light-defined time window.

Memory is a central function of the brain and is essential to everyday life. Memory disorders range from those of memory transience, such as Alzheimer's disease, to those of memory persistence, such as post-traumatic stress disorder. To treat memory disorders, a thorough understanding of memory formation and retrieval is critical. To date, most research has focused on memory formation, with the neurobiological basis of memory retrieval largely ignored due to experimental limitations. Here, I present our recent advances in the study of memory retrieval using technologies to engineer the representation of a specific memory, memory engram cells, in the brain. First, using animal models of retrograde amnesia, we demonstrated that direct activation of amnesic engram cells in the hippocampus resulted in robust memory retrieval, indicating the persistence of the original memory. Subsequent experiments identified retained engram cell-specific connectivity in amnesic mice although these engram cells lacked augmented synaptic strength and dendritic spine density. We proposed that a specific pattern of connectivity of engram cells may be the crucial substrate for memory information storage and that augmented synaptic strength and spine density critically contribute to the memory retrieval process. Second, we examined memory engrams in transgenic mouse models of early Alzheimer's disease, which required the development of a novel two-virus approach. We demonstrated that optical induction of long-term potentiation at input synapses on engram cells restored both spine density and long-term memory in early Alzheimer mice, providing causal evidence for the crucial role of augmented spine density in memory retrieval. Third, using activity-dependent labeling, we found that dorsal subiculum had enhanced neuronal activity during memory retrieval as compared memory encoding. Taking advantage of a novel transgenic mouse line that permitted specific genetic access to dorsal subiculum neurons, we demonstrated that the hippocampal output circuits are functionally segregated for memory formation and memory retrieval processes. We suggested that the dorsal subiculum-containing output circuit is dedicated to meet the requirements associated with memory retrieval, such as rapid memory updating and retrieval-driven instinctive emotional responses. Together, these three related thesis projects have important implications for elucidating cellular and circuit mechanisms supporting episodic memory retrieval.

One of the most fascinating aspects of an animal's brain is its ability to acquire new information from experience and retain this information over time as memory. The search for physical correlates of memory, the memory engram, has been a long-standing question in modern neurobiology. Recent advances in transgenics and optogenetic tools have enabled the identification, visualization, and manipulation of natural, sensory-evoked, engram ensembles unique to individual brain regions, and specific learning events. We suggest that these recent studies are paving the way not only to understand memory mechanisms in unprecedented detail but also to engineer the memory-mediated state of mind and behaviors.

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 formation and retrieval of memory are 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, 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 the 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 meeting the requirements associated with recall, such as rapid memory updating and retrieval-driven instinctive fear responses.

Recent studies identified neuronal ensembles and circuits thathold specific memory information (memory engrams). Memoryengrams are retained under protein synthesis inhibition-inducedretrograde amnesia. These engram cells can be activated byoptogenetic stimulation for full-fledged recall, but not by stimulation using natural recall cues (thus, amnesia). We call this state ofengrams “silent engrams” and the cells bearing them “silent engram cells.” The retention of memory information under amnesiasuggests that the time-limited protein synthesis following learningis dispensable for memory storage, but may be necessary for effective memory retrieval processes. Here, we show that the fullfledged optogenetic recall persists at least 8 d after learning underprotein synthesis inhibition-induced amnesia. This long-term retention of memory information correlates with equally persistentretention of functional engram cell-to-engram cell connectivity.Furthermore, inactivation of the connectivity of engram cell ensembles with its downstream counterparts, but not upstreamones, prevents optogenetic memory recall. Consistent with thepreviously reported lack of retention of augmented synapticstrength and reduced spine density in silent engram cells, optogeneticmemory 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 cellsby overexpression of α-p-21–activated kinase 1, which increasesspine density in engram cells. These results indicate that memoryinformation is retained in a form of silent engram under proteinsynthesis inhibition-induced retrograde amnesia and support thehypothesis that memory is stored as the specific connectivity between engram cells.

Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by progressive memory decline and subsequent loss of broader cognitive functionsMemory decline in the early stages of AD is mostly limited to episodic memory, for which the hippocampus has a crucial rolHowever, 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-dependentwhich 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.

Memory consolidation is the process by which a newly formed and unstable memory transforms into a stable long-term memory. It is unknown whether the process of memory consolidation occurs exclusively through the stabilization of memory engrams. By using learning-dependent cell labeling, we identified an increase of synaptic strength and dendritic spine density specifically in consolidated memory engram cells. Although these properties are lacking in engram cells under protein synthesis inhibitor–induced amnesia, direct optogenetic activation of these cells results in memory retrieval, and this correlates with retained engram cell–specific connectivity. We propose that a specific pattern of connectivity of engram cells may be crucial for memory information storage and that strengthened synapses in these cells critically contribute to the memory retrieval process.

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 and the true neurobiology of memory storage.

Animals display a repertoire of different social behaviours. Appropriate behavioural responses depend on sensory input received during social interactions. In mice, social behaviour is driven by pheromones, chemical signals that encode information related to age, sex and physiological state. However, although mice show different social behaviours towards adults, juveniles, and neonates, sensory cues that enable specific recognition of juvenile mice are unknown. Here, we describe a juvenile pheromone produced by young mice before puberty, termed exocrine-gland-secreting peptide 22 (ESP22). ESP22 is secreted from the lacrimal gland and released into tears of 2- to 3-week-old mice. Upon detection, ESP22 activates high-affinity sensory neurons in the vomeronasal organ and downstream limbic neurons in the medial amygdala. Recombinant ESP22, painted on mice, exerts a powerful inhibitory effect on adult male mating behaviour, which is abolished in knockout mice lacking TRPC2, a key signaling component of the vomeronasal organ. Furthermore, knockout of TRPC2 or loss of ESP22 production results in increased sexual behaviour of adult males towards juveniles, and sexual responses towards ESP22-deficient juveniles are suppressed by ESP22 painting. Thus, we describe a pheromone of sexually immature mice that controls an innate social behaviour, a response pathway through the accessory olfactory system, and a new role for vomeronasal organ signaling in inhibiting sexual behaviour towards young. These findings provide a molecular framework for understanding how a sensory system can regulate behaviour.

Rodents use olfactory cues for species-specific behaviors. For example, mice emit odors to attract mates of the same species, but not competitors of closely related species. This implies rapid evolution of olfactory signaling, although odors and chemosensory receptors involved are unknown.

Some chemoreceptors of the trace amine-associated receptor (TAAR) family detect innately aversive odors and are proposed to activate hardwired olfactory circuits. However, the wiring of TAAR neurons, the regulatory mechanisms of Taar gene choice, and the subcellular localization of TAAR proteins remain unknown. Here, we reveal similarities between neurons expressing TAARs and odorant receptors (ORs), but also unexpected differences. Like ORs, TAARs seem to be monoallelically expressed and localized both in cilia, the site of odor detection, and in axons, where they may participate in guidance. TAAR neurons project to discrete glomeruli predominantly localized to a confined bulb region. Taar expression involves different regulatory logic than OR expression, as neurons choosing a Taar5 knockout allele frequently express a second Taar without silencing the deleted allele. Moreover, the epigenetic signature of OR gene choice is absent from Taar genes. The unique molecular and anatomical features of the TAAR neurons suggest that they constitute a distinct olfactory subsystem.

Establishment of epithelial polarity requires the reorganization of the microtubule (MT) cytoskeleton from a radial array into a network positioned along the apicobasal axis of the cell. Little is known about the mechanisms that spatially guide the remodeling of MTs during epithelial polarization. Septins are filamentous guanine triphosphatases (GTPases) that are associated with MTs, but the function of septins in MT organization and dynamics is poorly understood. In this paper, we show that in polarizing epithelia, septins guide the directionality of MT plus end movement by suppressing MT catastrophe. By enabling persistent MT growth, two spatially distinct populations of septins, perinuclear and peripheral filaments, steer the growth and capture of MT plus ends. This navigation mechanism is essential for the maintenance of perinuclear MT bundles and for the orientation of peripheral MTs as well as for the apicobasal positioning of MTs. Our results suggest that septins provide the directional guidance cues necessary for polarizing the epithelial MT network.

A biomimetic, microscale system using the mechanics of swimming bacteria has been fabricated and controlled in a low Reynolds number fluidic environment. The microswimmer consists of a polystyrene microbead conjugated to a magnetic nanoparticle via a flagellar filament using avidin-biotin linkages. The flagellar filaments were isolated from the bacterium Salmonella typhimurium. Propulsion energy was supplied by an external rotating magnetic field designed in an approximate Helmholtz configuration. Further, the finite element analysis software, COMSOL MULTIPHYSICS, was used to develop a simulation of the robotic devices within the magnetic controller. The robotic microswimmers exhibited flagellar propulsion in two-dimensional magnetic fields, which demonstrates the controllability of the biomimetically designed devices for future biomedical applications.