Sue-Hyun Lee

Synaptic Protein Degradation Underlies Destabilization of Retrieved Fear Memory

Reactivated memory undergoes a rebuilding process that depends on de novo protein synthesis. This suggests that retrieval is dynamic and serves to incorporate new information into preexisting memories. However, little is known about whether or not protein degradation is involved in the reorganization of retrieved memory. We found that postsynaptic proteins were degraded in the hippocampus by polyubiquitination after retrieval of contextual fear memory. Moreover, the infusion of proteasome inhibitor into the CA1 region immediately after retrieval prevented anisomycin-induced memory impairment, as well as the extinction of fear memory. This suggests that ubiquitin- and proteasome-dependent protein degradation underlies destabilization processes after fear memory retrieval. It also provides strong evidence for the existence of reorganization processes whereby preexisting memory is disrupted by protein degradation, and updated memory is reconsolidated by protein synthesis.

Goal-dependent dissociation of visual and prefrontal cortices during working memory.

To determine the specific contribution of brain regions to working memory, human participants performed two distinct tasks on the same visually presented objects. During the maintenance of visual properties, object identity could be decoded from extrastriate, but not prefrontal, cortex, whereas the opposite held for nonvisual properties. Thus, the ability to maintain information during working memory is a general and flexible cortical property, with the role of individual regions being goal-dependent.

A cellular model of memory reconsolidation involves reactivation-induced destabilization and restabilization at the sensorimotor synapse in Aplysia

The memory reconsolidation hypothesis suggests that a memory trace becomes labile after retrieval and needs to be reconsolidated before it can be stabilized. However, it is unclear from earlier studies whether the same synapses involved in encoding the memory trace are those that are destabilized and restabilized after the synaptic reactivation that accompanies memory retrieval, or whether new and different synapses are recruited. To address this issue, we studied a simple nonassociative form of memory, long-term sensitization of the gill- and siphon-withdrawal reflex in Aplysia, and its cellular analog, long-term facilitation at the sensory-to-motor neuron synapse. We found that after memory retrieval, behavioral long-term sensitization in Aplysia becomes labile via ubiquitin/proteasome-dependent protein degradation and is reconsolidated by means of de novo protein synthesis. In parallel, we found that on the cellular level, long-term facilitation at the sensory-to-motor neuron synapse that mediates long-term sensitization is also destabilized by protein degradation and is restabilized by protein synthesis after synaptic reactivation, a procedure that parallels memory retrieval or retraining evident on the behavioral level. These results provide direct evidence that the same synapses that store the long-term memory trace encoded by changes in the strength of synaptic connections critical for sensitization are disrupted and reconstructed after signal retrieval.

Nuclear Translocation of CAM-Associated Protein Activates Transcription for Long-Term Facilitation in Aplysia

Repeated pulses of serotonin (5-HT) induce long-term facilitation (LTF) of the synapses between sensory and motor neurons of the gill-withdrawal reflex in Aplysia. To explore how apCAM downregulation at the plasma membrane and CREB-mediated transcription in the nucleus, both of which are required for the formation of LTF, might relate to each other, we cloned an apCAM-associated protein (CAMAP) by yeast two-hybrid screening. We found that 5-HT signaling at the synapse activates PKA which in turn phosphorylates CAMAP to induce the dissociation of CAMAP from apCAM and the subsequent translocation of CAMAP into the nucleus of sensory neurons. In the nucleus, CAMAP acts as a transcriptional coactivator for CREB1 and is essential for the activation of ApC/EBP required for the initiation of LTF. Combined, our data suggest that CAMAP is a retrograde signaling component that translocates from activated synapses to the nucleus during synapse-specific LTF.

Multiple repressive mechanisms in the hippocampus during memory formation

Memory consolidation by gene suppression Storing a persistent memory in the brain involves dynamic gene regulation. However, our knowledge of the target genes controlled during memory formation is limited. Cho et al. used RNA sequencing and ribosome profiling to compare transcription and translational levels in the mouse hippocampus before and after memory formation. Under basal conditions, there was an unexpected translational repression of ribosomal protein-coding genes. Early after learning, specific genes were translationally repressed. Later, suppression of a group of genes resulted from the inhibition of estrogen receptor alpha signaling. Thus, suppression mechanisms in the hippocampus appear to play a major role during memory consolidation. Science, this issue p. 82 Certain genes are repressed at various time points after memory formation, and this is associated with activity-dependent plasticity. Memory stabilization after learning requires translational and transcriptional regulations in the brain, yet the temporal molecular changes that occur after learning have not been explored at the genomic scale. We used ribosome profiling and RNA sequencing to quantify the translational status and transcript levels in the mouse hippocampus after contextual fear conditioning. We revealed three types of repressive regulations: translational suppression of ribosomal protein-coding genes in the hippocampus, learning-induced early translational repression of specific genes, and late persistent suppression of a subset of genes via inhibition of estrogen receptor 1 (ESR1/ERα) signaling. In behavioral analyses, overexpressing Nrsn1, one of the newly identified genes undergoing rapid translational repression, or activating ESR1 in the hippocampus impaired memory formation. Collectively, this study unveils the yet-unappreciated importance of gene repression mechanisms for memory formation.

Identification of a serotonin receptor coupled to adenylyl cyclase involved in learning-related heterosynaptic facilitation in Aplysia

Serotonin (5-HT) plays a critical role in modulating synaptic plasticity in the marine mollusc Aplysia and in the mammalian nervous system. In Aplysia sensory neurons, 5-HT can activate several signal cascades, including PKA and PKC, presumably via distinct types of G protein-coupled receptors. However, the molecular identities of these receptors have not yet been identified. We here report the cloning and functional characterization of a 5-HT receptor that is positively coupled to adenylyl cyclase in Aplysia neurons. The cloned receptor, 5-HTapAC1, stimulates the production of cAMP in HEK293T cells and in Xenopus oocytes. Moreover, the knockdown of 5-HTapAC1 expression by RNA interference blocked 5-HT-induced cAMP production in Aplysia sensory neurons and blocked synaptic facilitation in nondepressed or partially depressed sensory-to-motor neuron synapses. These data implicate 5-HTapAC1 as a major modulator of learning related synaptic facilitation in the direct sensory to motor neuron pathway of the gill withdrawal reflex.

Synaptic Protein Degradation as a Mechanism in Memory Reorganization

An accumulating body of evidence shows that reactivated long-term memory undergoes a dynamic process called reconsolidation, in which de novo protein synthesis is required to maintain the memory. These findings open up a new dimension in the field of memory research. However, few studies have shown how once-consolidated memory becomes labile. The authors’ recent findings have demonstrated that pre-existing long-term memory becomes unstable via the ubiquitin/ proteasome-dependent protein degradation pathway and that this labile state is required for the reorganization of fear memory. Here, the authors review this finding and focus on the labile state that is critical for the reorganization of memory triggered after memory retrieval.

A Nucleolar Protein ApLLP Induces ApC/EBP Expression Required for Long-Term Synaptic Facilitation in Aplysia Neurons

In Aplysia, long-term synaptic plasticity is induced by serotonin (5-HT) or neural activity and requires gene expression. Here, we demonstrate that ApLLP, a novel nucleolus protein, is critically involved in both long-term facilitation (LTF) and behavioral sensitization. Membrane depolarization induced ApLLP expression, which activated ApC/EBP expression through a direct binding to CRE. LTF was produced by a single pulse of 5-HT 30 min after the membrane depolarization. This LTF was blocked when either ApLLP or ApC/EBP were blocked by specific antibodies. In contrast, ApLLP overexpression induced LTF in response to a single 5-HT treatment. Simultaneously, a siphon noxious stimulus (SNS) to intact Aplysia induced ApLLP and ApC/EBP expression, and single tail shock 30 min after SNS transformed short-term sensitization to long-term sensitization of siphon withdrawal reflex. These results suggest that ApLLP is an activity-dependent transcriptional activator that switches short-term facilitation to long-term facilitation.

PKA-activated ApAF-ApC/EBP heterodimer is a key downstream effector of ApCREB and is necessary and sufficient for the consolidation of long-term facilitation.

Long-term memory requires transcriptional regulation by a combination of positive and negative transcription factors. Aplysia activating factor (ApAF) is known to be a positive transcription factor that forms heterodimers with ApC/EBP and ApCREB2. How these heterodimers are regulated and how they participate in the consolidation of long-term facilitation (LTF) has not, however, been characterized. We found that the functional activation of ApAF required phosphorylation of ApAF by PKA on Ser-266. In addition, ApAF lowered the threshold of LTF by forming a heterodimer with ApCREB2. Moreover, once activated by PKA, the ApAF–ApC/EBP heterodimer transactivates enhancer response element–containing genes and can induce LTF in the absence of CRE- and CREB-mediated gene expression. Collectively, these results suggest that PKA-activated ApAF–ApC/EBP heterodimer is a core downstream effector of ApCREB in the consolidation of LTF.

Multi-Voxel Decoding and the Topography of Maintained Information During Visual Working Memory

The ability to maintain representations in the absence of external sensory stimulation, such as in working memory, is critical for guiding human behavior. Human functional brain imaging studies suggest that visual working memory can recruit a network of brain regions from visual to parietal to prefrontal cortex. In this review, we focus on the maintenance of representations during visual working memory and discuss factors determining the topography of those representations. In particular, we review recent studies employing multi-voxel pattern analysis (MVPA) that demonstrate decoding of the maintained content in visual cortex, providing support for a “sensory recruitment” model of visual working memory. However, there is some evidence that maintained content can also be decoded in areas outside of visual cortex, including parietal and frontal cortex. We suggest that the ability to maintain representations during working memory is a general property of cortex, not restricted to specific areas, and argue that it is important to consider the nature of the information that must be maintained. Such information-content is critically determined by the task and the recruitment of specific regions during visual working memory will be both task- and stimulus-dependent. Thus, the common finding of maintained information in visual, but not parietal or prefrontal, cortex may be more of a reflection of the need to maintain specific types of visual information and not of a privileged role of visual cortex in maintenance.