Induction of a dominant negative CREB transgene specifically blocks long-term memory in Drosophila

TORC2: a novel target for treating age-associated memory impairment

Memory decline is one of the greatest health threats of the twenty-first century. Because of the widespread increase in life expectancy, 20 percent of the global population will be over 60 in 2050 and the problems caused by age-related memory loss will be dramatically aggravated. However, the molecular mechanisms underlying this inevitable process are not well understood. Here we show that the activity of the recently discovered mechanistic target of rapamycin (mTOR) complex 2 (mTORC2) declines with age in the brain of both fruit flies and rodents and that the loss of mTORC2-mediated actin polymerization contributes to age-associated memory loss. Intriguingly, treatment with a small molecule that activates mTORC2 (A-443654) reverses long-term memory (LTM) deficits in both aged mice and flies. In addition, we found that pharmacologically boosting either mTORC2 or actin polymerization enhances LTM. In contrast to the current approaches to enhance memory that have primarily targeted the regulation of gene expression (epigenetic, transcriptional, and translational), our data points to a novel, evolutionarily conserved mechanism for restoring memory that is dependent on structural plasticity. These insights into the molecular basis of age-related memory loss may hold promise for new treatments for cognitive disorders.

Plasticity-related genes in brain development and amygdala-dependent learning

Learning about motivationally important stimuli involves plasticity in the amygdala, a temporal lobe structure. Amygdala-dependent learning involves a growing number of plasticity-related signaling pathways also implicated in brain development, suggesting that learning-related signaling in juveniles may simultaneously influence development. Here, we review the pleiotropic functions in nervous system development and amygdala-dependent learning of a signaling pathway that includes brain-derived neurotrophic factor (BDNF), extracellular signaling-related kinases (ERKs) and cyclic AMP-response element binding protein (CREB). Using these canonical, plasticity-related genes as an example, we discuss the intersection of learning-related and developmental plasticity in the immature amygdala, when aversive and appetitive learning may influence the developmental trajectory of amygdala function. We propose that learning-dependent activation of BDNF, ERK and CREB signaling in the immature amygdala exaggerates and accelerates neural development, promoting amygdala excitability and environmental sensitivity later in life.

NF-κB transcription factor role in consolidation and reconsolidation of persistent memories

Transcriptional regulation is an important molecular process required for long-term neural plasticity and long-term memory (LTM) formation. Thus, one main interest in molecular neuroscience in the last decades has been the identification of transcription factors that are involved in memory processes. Among them, the nuclear factor κB (NF-κB) family of transcription factors has gained interest due to a significant body of evidence that supports a key role of these proteins in synaptic plasticity and memory. In recent years, the interest was particularly reinforced because NF-κB was characterized as an important regulator of synaptogenesis. This function may be explained by its participation in synapse to nucleus communication, as well as a possible local role at the synapse. This review provides an overview of experimental work obtained in the last years, showing the essential role of this transcription factor in memory processes in different learning tasks in mammals. We focus the review on the consolidation and reconsolidation memory phases as well as on the regulation of immediate-early and late genes by epigenetic mechanisms that determine enduring forms of memories.

A new perspective on the role of the CREB family of transcription factors in memory consolidation via adult hippocampal neurogenesis

Adult neurogenesis is the process by which new neurons are generated in the brains of adults. Since its discovery 50 years ago, adult neurogenesis has been widely studied in the mammalian brain and has provided a new perspective on the pathophysiology of many psychiatric and neurodegenerative disorders, some of which affect memory. In this regard, adult hippocampal neurogenesis (AHN), which occurs in the subgranular zone (SGZ) of the dentate gyrus (DG), has been suggested to play a role in the formation and consolidation of new memories. This process involves many transcription factors, of which cyclic AMP (cAMP)-responsive element-binding protein (CREB) is a well-documented one. In the developing brain, CREB regulates crucial cell stages (e.g., proliferation, differentiation, and survival), and in the adult brain, it participates in neuronal plasticity, learning, and memory. In addition, new evidence supports the hypothesis that CREB may also participate in learning and memory through its involvement in AHN. This review examines the CREB family of transcription factors, including the different members and known signaling pathways. It highlights the role of CREB as a modulator of AHN, which could underlie its function in memory consolidation mechanisms.

Nodulisporiviridins A-H, Bioactive Viridins from Nodulisporium sp

Eight new viridins, nodulisporiviridins A-H (1-8), were isolated from the extract of an endolichenic fungal strain Nodulisporium sp. (No. 65-17-2-1) that was fermented with potato-dextrose broth. The structures were determined using spectroscopic and X-ray crystallographic analysis. Nodulisporiviridins A-D (1-4) are unique viridins with an opened ring A. The Aβ42 aggregation inhibitory activities of 1-8 were evaluated using a thioflavin T (ThT) assay with epigallocatechin gallate (EGCG) as the positive control (EGCG IC50 of 0.5 μM). Nodulisporiviridin G (7) displayed potent inhibitory activity with an IC50 value of 1.2 μM, and the preliminary trend of activity of these viridins as Aβ42 aggregation inhibitors was proposed. The short-term memory assay on an Aβ transgenic drosophila model of Alzheimer's disease showed that all eight compounds improved the short-term memory capacity, with potencies close to that of the positive control (memantine).

From Learning to Memory: What Flies Can Tell Us about Intellectual Disability Treatment

Intellectual disability (ID), previously known as mental retardation, affects 3% of the population and remains without pharmacological treatment. ID is characterized by impaired general mental abilities associated with defects in adaptive function in which onset occurs before 18 years of age. Genetic factors are increasing and being recognized as the causes of severe ID due to increased use of genome-wide screening tools. Unfortunately drug discovery for treatment of ID has not followed the same pace as gene discovery, leaving clinicians, patients, and families without the ability to ameliorate symptoms. Despite this, several model organisms have proven valuable in developing and screening candidate drugs. One such model organism is the fruit fly Drosophila. First, we review the current understanding of memory in human and its model in Drosophila. Second, we describe key signaling pathways involved in ID and memory such as the cyclic adenosine 3',5'-monophosphate (cAMP)-cAMP response element binding protein (CREB) pathway, the regulation of protein synthesis, the role of receptors and anchoring proteins, the role of neuronal proliferation, and finally the role of neurotransmitters. Third, we characterize the types of memory defects found in patients with ID. Finally, we discuss how important insights gained from Drosophila learning and memory could be translated in clinical research to lead to better treatment development.

CBP-Dependent memory consolidation in the prefrontal cortex supports object-location learning

Recognition of an object's location in space is supported by hippocampus-dependent recollection. Converging evidence strongly suggests that the interplay between the prefrontal cortex and hippocampus is critical for spatial memory. Lesion, pharmacological, and genetic studies have been successful in dissecting the role of plasticity in the hippocampal circuit in a variety of neural processes relevant to spatial memory, including memory for the location of objects. However, prefrontal mechanisms underlying spatial memory are less well understood. Here, we show that an acute hypofunction of the cyclic-AMP regulatory element binding protein (CREB) Binding Protein (CBP) histone acetyltransferase (HAT) in the medial prefrontal cortex (mPFC) results in delay-dependent disruption of object-location memory. These data suggest that mechanisms involving CBP HAT-mediated lysine acetylation of nuclear proteins support selectively long-term encoding in the mPFC circuits. Evidence from the object-location task suggests that long-term memory encoding within the mPFC complements hippocampus-dependent spatial memory mechanisms and may be critical for broader network integration of information necessary for an assessment of subtle spatial differences to guide appropriate behavioral response during retrieval of spatial memories.

Two independent mushroom body output circuits retrieve the six discrete components of Drosophila aversive memory

Understanding how the various memory components are encoded and how they interact to guide behavior requires knowledge of the underlying neural circuits. Currently, aversive olfactory memory in Drosophila is behaviorally subdivided into four discrete phases. Among these, short- and long-term memories rely, respectively, on the γ and α/β Kenyon cells (KCs), two distinct subsets of the ∼2,000 neurons in the mushroom body (MB). Whereas V2 efferent neurons retrieve memory from α/β KCs, the neurons that retrieve short-term memory are unknown. We identified a specific pair of MB efferent neurons, named M6, that retrieve memory from γ KCs. Moreover, our network analysis revealed that six discrete memory phases actually exist, three of which have been conflated in the past. At each time point, two distinct memory components separately recruit either V2 or M6 output pathways. Memory retrieval thus features a dramatic convergence from KCs to MB efferent neurons.

Insulin signaling is acutely required for long-term memory in Drosophila

Memory formation has been shown recently to be dependent on energy status in Drosophila. A well-established energy sensor is the insulin signaling (InS) pathway. Previous studies in various animal models including human have revealed the role of insulin levels in short-term memory but its role in long-term memory remains less clear. We therefore investigated genetically the spatial and temporal role of InS using the olfactory learning and long-term memory model in Drosophila. We found that InS is involved in both learning and memory. InS in the mushroom body is required for learning and long-term memory whereas long-term memory specifically is impaired after InS signaling disruption in the ellipsoid body, where it regulates the level of p70s6k, a downstream target of InS and a marker of protein synthesis. Finally, we show also that InS is acutely required for long-term memory formation in adult flies.

Distinct roles of multiple isoforms of CaMKII in signaling to the nucleus

Long-lasting synaptic changes following information acquisition are critical steps for memory. In this process, long-term potentiation (LTP) is widely considered as one of the major cellular mechanisms modifying synaptic strength. It can be classified into early phase LTP (E-LTP) and late phase LTP (L-LTP) based on its duration. Using genetically modified mice, investigators have recognized the critical role of CaMKII in E-LTP and memory. However, its function in L-LTP, which is strongly dependent on gene transcription and protein synthesis, is still unclear. In this review, we discuss how different isoforms of CaMKII are coordinated to regulate gene expression in an activity-dependent manner, and thus contribute to L-LTP and memory. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

How deeply does your mutant sleep? Probing arousal to better understand sleep defects in Drosophila

The fruitfly, Drosophila melanogaster, has become a critical model system for investigating sleep functions. Most studies use duration of inactivity to measure sleep. However, a defining criterion for sleep is decreased behavioral responsiveness to stimuli. Here we introduce the Drosophila ARousal Tracking system (DART), an integrated platform for efficiently tracking and probing arousal levels in animals. This video-based platform delivers positional and locomotion data, behavioral responsiveness to stimuli, sleep intensity measures, and homeostatic regulation effects – all in one combined system. We show how insight into dynamically changing arousal thresholds is crucial for any sleep study in flies. We first find that arousal probing uncovers different sleep intensity profiles among related genetic background strains previously assumed to have equivalent sleep patterns. We then show how sleep duration and sleep intensity can be uncoupled, with distinct manipulations of dopamine function producing opposite effects on sleep duration but similar sleep intensity defects. We conclude by providing a multi-dimensional assessment of combined arousal and locomotion metrics in the mutant and background strains. Our approach opens the door for deeper insights into mechanisms of sleep regulation and provides a new method for investigating the role of different genetic manipulations in controlling sleep and arousal.

Mushroom body output neurons encode valence and guide memory-based action selection in Drosophila

Animals discriminate stimuli, learn their predictive value and use this knowledge to modify their behavior. In Drosophila, the mushroom body (MB) plays a key role in these processes. Sensory stimuli are sparsely represented by ∼2000 Kenyon cells, which converge onto 34 output neurons (MBONs) of 21 types. We studied the role of MBONs in several associative learning tasks and in sleep regulation, revealing the extent to which information flow is segregated into distinct channels and suggesting possible roles for the multi-layered MBON network. We also show that optogenetic activation of MBONs can, depending on cell type, induce repulsion or attraction in flies. The behavioral effects of MBON perturbation are combinatorial, suggesting that the MBON ensemble collectively represents valence. We propose that local, stimulus-specific dopaminergic modulation selectively alters the balance within the MBON network for those stimuli. Our results suggest that valence encoded by the MBON ensemble biases memory-based action selection.

Rapid-onset antidepressant efficacy of glutamatergic system modulators: the neural plasticity hypothesis of depression

Depression is a devastating psychiatric disorder widely attributed to deficient monoaminergic signaling in the central nervous system. However, most clinical antidepressants enhance monoaminergic neurotransmission with little delay but require 4-8 weeks to reach therapeutic efficacy, a paradox suggesting that the monoaminergic hypothesis of depression is an oversimplification. In contrast to the antidepressants targeting the monoaminergic system, a single dose of the N-methyl-D-aspartate receptor (NMDAR) antagonist ketamine produces rapid (within 2 h) and sustained (over 7 days) antidepressant efficacy in treatment-resistant patients. Glutamatergic transmission mediated by NMDARs is critical for experience-dependent synaptic plasticity and learning, processes that can be modified indirectly by the monoaminergic system. To better understand the mechanisms of action of the new antidepressants like ketamine, we review and compare the monoaminergic and glutamatergic antidepressants, with emphasis on neural plasticity. The pathogenesis of depression may involve maladaptive neural plasticity in glutamatergic circuits that may serve as a new class of targets to produce rapid antidepressant effects.

The regulation of transcription in memory consolidation

De novo transcription of DNA is a fundamental requirement for the formation of long-term memory. It is required during both consolidation and reconsolidation, the posttraining and postreactivation phases that change the state of the memory from a fragile into a stable and long-lasting form. Transcription generates both mRNAs that are translated into proteins, which are necessary for the growth of new synaptic connections, as well as noncoding RNA transcripts that have regulatory or effector roles in gene expression. The result is a cascade of events that ultimately leads to structural changes in the neurons that mediate long-term memory storage. The de novo transcription, critical for synaptic plasticity and memory formation, is orchestrated by chromatin and epigenetic modifications. The complexity of transcription regulation, its temporal progression, and the effectors produced all contribute to the flexibility and persistence of long-term memory formation. In this article, we provide an overview of the mechanisms contributing to this transcriptional regulation underlying long-term memory formation.

Canadian Association of Neurosciences Review: Learning at a Snail's Pace

While learning and memory are related, they are distinct processes each with different forms of expression and underlying molecular mechanisms. An invertebrate model system, Lymnaea stagnalis, is used to study memory formation of a non-declarative memory. We have done so because: 1) We have discovered the neural circuit that mediates an interesting and tractable behaviour; 2) This behaviour can be operantly conditioned and intermediate-term and long-term memory can be demonstrated; and 3) It is possible to demonstrate that a single neuron in the model system is a necessary site of memory formation. This article reviews how Lymnaea has been used in the study of behavioural and molecular mechanisms underlying consolidation, reconsolidation, extinction and forgetting.

Inducible forebrain-specific ablation of the transcription factor Creb during adulthood induces anxiety but no spatial/contextual learning deficits

The cyclic AMP (cAMP)-response element binding protein (CREB) is an activity-dependent transcription factor playing a role in synaptic plasticity, learning and memory, and emotional behavior. However, the impact of Creb ablation on rodent behavior is vague as e.g., memory performance of different Creb mutant mice depends on the specific type of mutation per se but additionally on the background and learning protocol differences. Here we present the first targeted ablation of CREB induced during adulthood selectively in principal forebrain neurons in a pure background strain of C57BL/6 mice. All hippocampal principal neurons exhibited lack of CREB expression. Mutant mice showed a severe anxiety phenotype in the openfield and novel object exploration test as well as in the Dark-Light Box Test, but unaltered hippocampus-dependent long-term memory in the Morris water maze and in context dependent fear conditioning. On the molecular level, CREB ablation led to CREM up regulation in the hippocampus and frontal cortex which may at least in part compensate for the loss of CREB. BDNF, a postulated CREB target gene, was down regulated in the frontal lobe but not in the hippocampus; neurogenesis remained unaltered. Our data indicate that in the adult mouse forebrain the late onset of CREB ablation can, in case of memory functionality, be compensated for and is not essential for memory consolidation and retrieval during adulthood. In contrast, the presence of CREB protein during adulthood seems to be pivotal for the regulation of emotional behavior.

The role of progesterone in memory: an overview of three decades

Memory comprises acquisition, consolidation and retrieval of information. Many substances can influence these different phases. It is well demonstrated that sex hormones, mainly estrogen, impact cognitive function. More recently, progesterone has also been documented as playing an important role in cognition, since it influences brain regions involved in memory. Currently, many women are under hormone treatment, which contain progesterone to decrease the risk of development of endometrial cancer. This affords the opportunity to study the real effects of this hormonal replacement on cognition. There are many contradictory results regarding the role of progesterone in memory. Therefore, the aim of this review was to synthesize these studies using the new perspective of the influence of hormone replacement on cognition in women.

Spatio-temporal in vivo recording of dCREB2 dynamics in Drosophila long-term memory processing

CREB (cAMP response element-binding protein) is an evolutionarily conserved transcription factor, playing key roles in synaptic plasticity, intrinsic excitability and long-term memory (LTM) formation. The Drosophila homologue of mammalian CREB, dCREB2, is also important for LTM. However, the spatio-temporal nature of dCREB2 activity during memory consolidation is poorly understood. Using an in vivo reporter system, we examined dCREB2 activity continuously in specific brain regions during LTM processing. Two brain regions that have been shown to be important for Drosophila LTM are the ellipsoid body (EB) and the mushroom body (MB). We found that dCREB2 reporter activity is persistently elevated in EB R2/R4m neurons, but not neighboring R3/R4d neurons, following LTM-inducing training. In multiple subsets of MB neurons, dCREB2 reporter activity is suppressed immediately following LTM-specific training, and elevated during late windows. In addition, we observed heterogeneous responses across different subsets of neurons in MB αβ lobe during LTM processing. All of these changes suggest that dCREB2 functions in both the EB and MB for LTM formation, and that this activity contributes to the process of systems consolidation.

Reinstatement of long-term memory following erasure of its behavioral and synaptic expression in Aplysia

Long-term memory (LTM) is believed to be stored in the brain as changes in synaptic connections. Here, we show that LTM storage and synaptic change can be dissociated. Cocultures of Aplysia sensory and motor neurons were trained with spaced pulses of serotonin, which induces long-term facilitation. Serotonin (5HT) triggered growth of new presynaptic varicosities, a synaptic mechanism of long-term sensitization. Following 5HT training, two antimnemonic treatments-reconsolidation blockade and inhibition of PKM--caused the number of presynaptic varicosities to revert to the original, pretraining value. Surprisingly, the final synaptic structure was not achieved by targeted retraction of the 5HT-induced varicosities but, rather, by an apparently arbitrary retraction of both 5HT-induced and original synapses. In addition, we find evidence that the LTM for sensitization persists covertly after its apparent elimination by the same antimnemonic treatments that erase learning-related synaptic growth. These results challenge the idea that stable synapses store long-term memories.

Functional neuroanatomy of Drosophila olfactory memory formation

New approaches, techniques and tools invented over the last decade and a half have revolutionized the functional dissection of neural circuitry underlying Drosophila learning. The new methodologies have been used aggressively by researchers attempting to answer three critical questions about olfactory memories formed with appetitive and aversive reinforcers: (1) Which neurons within the olfactory nervous system mediate the acquisition of memory? (2) What is the complete neural circuitry extending from the site(s) of acquisition to the site(s) controlling memory expression? (3) How is information processed across this circuit to consolidate early-forming, disruptable memories to stable, late memories? Much progress has been made and a few strong conclusions have emerged: (1) Acquisition occurs at multiple sites within the olfactory nervous system but is mediated predominantly by the γ mushroom body neurons. (2) The expression of long-term memory is completely dependent on the synaptic output of α/β mushroom body neurons. (3) Consolidation occurs, in part, through circuit interactions between mushroom body and dorsal paired medial neurons. Despite this progress, a complete and unified model that details the pathway from acquisition to memory expression remains elusive.

Towards a better understanding of cognitive behaviors regulated by gene expression downstream of activity-dependent transcription factors

In the field of molecular and cellular neuroscience, it is not a trivial task to see the forest for the trees, where numerous, and seemingly independent, molecules often work in concert to control critical steps of synaptic plasticity and signalling. Here, we will first summarize our current knowledge on essential activity-dependent transcription factors (TFs) such as CREB, MEF2, Npas4 and SRF, then examine how various transcription cofactors (TcoFs) also contribute to defining the transcriptional outputs during learning and memory. This review finally attempts a provisory synthesis that sheds new light on some of the emerging principles of neuronal circuit dynamics driven by activity-regulated gene transcription to help better understand the intricate relationship between activity-dependent gene expression and cognitive behavior.

Prefrontal consolidation supports the attainment of fear memory accuracy

The neural mechanisms underlying the attainment of fear memory accuracy for appropriate discriminative responses to aversive and nonaversive stimuli are unclear. Considerable evidence indicates that coactivator of transcription and histone acetyltransferase cAMP response element binding protein (CREB) binding protein (CBP) is critically required for normal neural function. CBP hypofunction leads to severe psychopathological symptoms in human and cognitive abnormalities in genetic mutant mice with severity dependent on the neural locus and developmental time of the gene inactivation. Here, we showed that an acute hypofunction of CBP in the medial prefrontal cortex (mPFC) results in a disruption of fear memory accuracy in mice. In addition, interruption of CREB function in the mPFC also leads to a deficit in auditory discrimination of fearful stimuli. While mice with deficient CBP/CREB signaling in the mPFC maintain normal responses to aversive stimuli, they exhibit abnormal responses to similar but nonrelevant stimuli when compared to control animals. These data indicate that improvement of fear memory accuracy involves mPFC-dependent suppression of fear responses to nonrelevant stimuli. Evidence from a context discriminatory task and a newly developed task that depends on the ability to distinguish discrete auditory cues indicated that CBP-dependent neural signaling within the mPFC circuitry is an important component of the mechanism for disambiguating the meaning of fear signals with two opposing values: aversive and nonaversive.

Convergent lines of evidence support CAMKK2 as a schizophrenia susceptibility gene

Genes that are differentially expressed between schizophrenia patients and healthy controls may have key roles in the pathogenesis of schizophrenia. We analyzed two large-scale genome-wide expression studies, which examined changes in gene expression in schizophrenia patients and their matched controls. We found calcium/calmodulin (CAM)-dependent protein kinase kinase 2 (CAMKK2) is significantly downregulated in individuals with schizophrenia in both studies. To seek the potential genetic variants that may regulate the expression of CAMKK2, we investigated the association between single-nucleotide polymorphisms (SNPs) within CAMKK2 and the expression level of CAMKK2. We found one SNP, rs1063843, which is located in intron 17 of CAMKK2, is strongly associated with the expression level of CAMKK2 in human brains (P=1.1 × 10(-6)) and lymphoblastoid cell lines (the lowest P=8.4 × 10(-6)). We further investigated the association between rs1063843 and schizophrenia in multiple independent populations (a total of 130 623 subjects) and found rs1063843 is significantly associated with schizophrenia (P=5.17 × 10(-5)). Interestingly, we found the T allele of rs1063843, which is associated with lower expression level of CAMKK2, has a higher frequency in individuals with schizophrenia in all of the tested samples, suggesting rs1063843 may be a causal variant. We also found that rs1063843 is associated with cognitive function and personality in humans. In addition, protein-protein interaction (PPI) analysis revealed that CAMKK2 participates in a highly interconnected PPI network formed by top schizophrenia genes, which further supports the potential role of CAMKK2 in the pathogenesis of schizophrenia. Taken together, these converging lines of evidence strongly suggest that CAMKK2 may have pivotal roles in schizophrenia susceptibility.

Genes and signaling pathways involved in memory enhancement in mutant mice

Mutant mice have been used successfully as a tool for investigating the mechanisms of memory at multiple levels, from genes to behavior. In most cases, manipulating a gene expressed in the brain impairs cognitive functions such as memory and their underlying cellular mechanisms, including synaptic plasticity. However, a remarkable number of mutations have been shown to enhance memory in mice. Understanding how to improve a system provides valuable insights into how the system works under normal conditions, because this involves understanding what the crucial components are. Therefore, more can be learned about the basic mechanisms of memory by studying mutant mice with enhanced memory. This review will summarize the genes and signaling pathways that are altered in the mutants with enhanced memory, as well as their roles in synaptic plasticity. Finally, I will discuss how knowledge of memory-enhancing mechanisms could be used to develop treatments for cognitive disorders associated with impaired plasticity.

Transcription control pathways decode patterned synaptic inputs into diverse mRNA expression profiles

Synaptic plasticity requires transcription and translation to establish long-term changes that form the basis for long term memory. Diverse stimuli, such as synaptic activity and growth factors, trigger synthesis of mRNA to regulate changes at the synapse. The palette of possible mRNAs is vast, and a key question is how the cell selects which mRNAs to synthesize. To address this molecular decision-making, we have developed a biochemically detailed model of synaptic-activity triggered mRNA synthesis. We find that there are distinct time-courses and amplitudes of different branches of the mRNA regulatory signaling pathways, which carry out pattern-selective combinatorial decoding of stimulus patterns into distinct mRNA subtypes. Distinct, simultaneously arriving input patterns that impinge on the transcriptional control network interact nonlinearly to generate novel mRNA combinations. Our model combines major regulatory pathways and their interactions connecting synaptic input to mRNA synthesis. We parameterized and validated the model by incorporating data from multiple published experiments. The model replicates outcomes of knockout experiments. We suggest that the pattern-selectivity mechanisms analyzed in this model may act in many cell types to confer the capability to decode temporal patterns into combinatorial mRNA expression.

Shocking revelations and saccharin sweetness in the study of Drosophila olfactory memory

It is now almost forty years since the first description of learning in the fruit fly Drosophila melanogaster. Various incarnations of the classic mutagenesis approach envisaged in the early days have provided around one hundred learning defective mutant fly strains. Recent technological advances permit temporal control of neural function in the behaving fly. These approaches have radically changed experiments in the field and have provided a neural circuit perspective of memory formation, consolidation and retrieval. Combining neural perturbations with more classical mutant intervention allows investigators to interrogate the molecular and cellular processes of memory within the defined neural circuits. Here, we summarize some of the progress made in the last ten years that indicates a remarkable conservation of the neural mechanisms of memory formation between flies and mammals. We emphasize that considering an ethologically-relevant viewpoint might provide additional experimental power in studies of Drosophila memory.

Efficacy of lovastatin on learning and memory deficits caused by chronic intermittent hypoxia-hypercapnia: through regulation of NR2B-containing NMDA receptor-ERK pathway

BACKGROUND

Chronic intermittent hypoxia-hypercapnia (CIHH) exposure leads to learnning and memory deficits in rats. Overactivation of N-methyl-D-aspartate receptors(NMDARs) can lead to the death of neurons through a process termed excitotoxicity, which is involved in CIHH-induced cognitive deficits. Excessively activated NR2B (GluN2B)-containing NMDARs was reported as the main cause of excitotoxicity. The ERK1/2 (extracellular signal-regulated kinase 1/2) signaling cascade acts as a key component in NMDARs-dependent neuronal plasticity and survival. Ca2+/calmodulin-dependent protein kinase II (CaMKII), synapse-associated protein 102 (SAP102) and Ras GTPase-activating protein (SynGAP) have been shown to be involved in the regulation of NMDAR-ERK signalling cascade. Recent studies revealed statins (the HMG-CoA reductase inhibitor) have effect on the expression of NMDARs. The present study intends to explore the potential effect of lovastatin on CIHH-induced cognitive deficits and the NR2B-ERK signaling pathway.

METHODS AND FINDINGS

Eighty male Sprague Dawley rats were randomly divided into five groups. Except for those in the control group, the rats were exposed to chronic intermittent hypoxia-hypercapnia (CIHH) (9 ∼ 11%O2, 5.5 ∼ 6.5%CO2) for 4 weeks. After lovastatin administration, the rats performed better in the Morris water maze test. Electron microscopy showed alleviated hippocampal neuronal synaptic damage. Further observation suggested that either lovastatin or ifenprodil (a selective NR2B antagonist) administration similarly downregulated NR2B subunit expression leading to a suppression of CaMKII/SAP102/SynGAP signaling cascade, which in turn enhanced the phosphorylation of ERK1/2. The phosphorylated ERK1/2 induced signaling cascade involving cAMP-response element-binding protein (CREB) phosphorylation and brain-derived neurotrophic factor (BDNF) activation, which is responsible for neuroprotection.

CONCLUSIONS

These findings suggest that the ameliorative cognitive deficits caused by lovastatin are due to the downregulation of excessive NR2B expression accompanied by increased expression of ERK signaling cascade. The effect of NR2B in upregulating pERK1/2 maybe due, at least in part, to inactivation of CaMKII/SAP102/SynGAP signaling cascade.

Maged1 co-interacting with CREB through a hexapeptide repeat domain regulates learning and memory in mice

Maged1 is a member of the type II melanoma antigen (MAGE) family of proteins, which is highly conserved in the brain between mouse and human. Recently, Maged1 has been reported to be involved in depression and impaired sexual behavior. However, the role of Maged1 in learning and memory remains unknown. The aim of the present study was therefore to investigate whether Maged1 deficiency can impair learning and memory formation. By behavioral tests and electrophysiological recording, we observed that 5-6-month-old Maged1 knockout mice displayed the reduced basal synaptic transmission, pronounced hippocampal dysfunction, impaired spatial learning, and a deficit in long-term potentiation induction. Data from immunohistochemical and Western blot showed the reduced dendritic spine density and the number of synapses in the hippocampus of the Maged1 knockout mice, and Maged1 deficiency prevented the interaction of Maged1 with cAMP response element-binding protein (CREB). Furthermore, by chromatin immunoprecipitation and luciferase assay, we observed the downregulated activity of CREB and the suppressed CREB-dependent transcription after deficiency of Maged1, which lead to the decreased levels of brain-derived neurotrophic factor. Taken together, our results provide the evidence that Maged1 is involved in synaptic transmission and hippocampus-dependent learning and memory formation.

Time of day influences memory formation and dCREB2 proteins in Drosophila

Many biological phenomena oscillate under the control of the circadian system, exhibiting peaks and troughs of activity across the day/night cycle. In most animal models, memory formation also exhibits this property, but the underlying neuronal and molecular mechanisms remain unclear. The dCREB2 transcription factor shows circadian regulated oscillations in its activity, and has been shown to be important for both circadian biology and memory formation. We show that the time-of-day (TOD) of behavioral training affects Drosophila memory formation. dCREB2 exhibits complex changes in protein levels across the daytime and nighttime, and these changes in protein abundance are likely to contribute to oscillations in dCREB2 activity and TOD effects on memory formation.

Memory consolidation in aging and MCI after 1 week

OBJECTIVE

To assess consolidation in amnestic mild cognitive impairment (aMCI), controlling for differences in initial learning and using a protracted delay period for recall.

METHOD

15 individuals with aMCI were compared with 15 healthy older adult controls on a story learning task. Subjects were trained to criteria to equalize initial learning across subjects. Recall was tested at both the 30-min typically used delay and a 1-week delay used to target consolidation.

RESULTS

Using repeated measures ANOVAs adjusted for age, we found group × time point interactions across the entire task between the final trial and 30-min delay, and again between the 30-min and 1-week delay periods, with aMCI having greater declines in recall as compared with controls. Significant group main effects were also found, with aMCI recalling less than controls.

CONCLUSION

Consolidation was impaired in aMCI as compared with controls. Our findings indicate that aMCI-related performance typically measured at 30 min underestimates aMCI-associated memory deficits. This is the first study to isolate consolidation by controlling for initial learning differences and using a protracted delay period to target consolidation in an aMCI sample.

Active forgetting of olfactory memories in Drosophila

Failure to remember, or forgetting, is a phenomenon familiar to everyone and despite more than a century of scientific inquiry, why we forget what we once knew remains unclear. If the brain marshals significant resources to form and store memories, why is it that these memories become lost? In the last century, psychological studies have divided forgetting into decay theory, in which memory simply dissipates with time, and interference theory, in which additional learning or mental activity hinders memory by reducing its stability or retrieval (for review, Dewar et al., 2007; Wixted, 2004). Importantly, these psychological models of forgetting posit that forgetting is a passive property of the brain and thus a failure of the brain to retain memories. However, recent neuroscience research on olfactory memory in Drosophila has offered evidence for an alternative conclusion that forgetting is an "active" process, with specific, biologically regulated mechanisms that remove existing memories (Berry et al., 2012; Shuai et al., 2010). Similar to the bidirectional regulation of cell number by mitosis and apoptosis, protein concentration by translation and lysosomal or proteomal degradation, and protein phosphate modification by kinases and phosphatases, biologically regulated memory formation and removal would be yet another example in biological systems where distinct and separate pathways regulate the creation and destruction of biological substrates.

AKAPS act in a two-step mechanism of memory acquisition

Defining the molecular and neuronal basis of associative memories is based upon behavioral preparations that yield high performance due to selection of salient stimuli, strong reinforcement, and repeated conditioning trials. One of those preparations is the Drosophila aversive olfactory conditioning procedure where animals initiate multiple memory components after experience of a single cycle training procedure. Here, we explored the analysis of acquisition dynamics as a means to define memory components and revealed strong correlations between particular chronologies of shock impact and number experienced during the associative training situation and subsequent performance of conditioned avoidance. Analyzing acquisition dynamics in Drosophila memory mutants revealed that rutabaga (rut)-dependent cAMP signals couple in a divergent fashion for support of different memory components. In case of anesthesia-sensitive memory (ASM) we identified a characteristic two-step mechanism that links rut-AC1 to A-kinase anchoring proteins (AKAP)-sequestered protein kinase A at the level of Kenyon cells, a recognized center of olfactory learning within the fly brain. We propose that integration of rut-derived cAMP signals at level of AKAPs might serve as counting register that accounts for the two-step mechanism of ASM acquisition.

CREB regulates spine density of lateral amygdala neurons: implications for memory allocation

Neurons may compete against one another for integration into a memory trace. Specifically, neurons in the lateral nucleus of the amygdala with relatively higher levels of cAMP Responsive Element Binding Protein (CREB) seem to be preferentially allocated to a fear memory trace, while neurons with relatively decreased CREB function seem to be excluded from a fear memory trace. CREB is a ubiquitous transcription factor that modulates many diverse cellular processes, raising the question as to which of these CREB-mediated processes underlie memory allocation. CREB is implicated in modulating dendritic spine number and morphology. As dendritic spines are intimately involved in memory formation, we investigated whether manipulations of CREB function alter spine number or morphology of neurons at the time of fear conditioning. We used viral vectors to manipulate CREB function in the lateral amygdala (LA) principal neurons in mice maintained in their homecages. At the time that fear conditioning normally occurs, we observed that neurons with high levels of CREB had more dendritic spines, while neurons with low CREB function had relatively fewer spines compared to control neurons. These results suggest that the modulation of spine density provides a potential mechanism for preferential allocation of a subset of neurons to the memory trace.

RNA transport and long-term memory storage

Several studies have shown that synthesis of new proteins at the synapse is a prerequisite for the storage of long-term memories. Relatively little is known about the availability of distinct mRNA populations for translation at specific synapses, the process that determines mRNA localization, and the temporal designations of localized mRNA translation during memory storage. Techniques such as synaptosome preparation and microdissection of distal neuronal processes of cultured neurons and dendritic layers in brain slices are general approaches used to identify localized RNAs. Exploration of the association of RNA-binding proteins to the axonal transport machinery has led to the development of a strategy to identify RNAs that are transported from the cell body to synapses by molecular motor kinesin. In this article, RNA localization at the synapse, as well as its mechanisms and significance in understanding long-term memory storage, are discussed.

Evolution of the techniques used in studying associative olfactory learning and memory in adult Drosophila in vivo: a historical and technical perspective

Drosophila melanogaster behavioral mutants have been isolated in which the ability to form associative olfactory memories has been disrupted primarily by altering cyclic adenosine monophosphate signal transduction. Unfortunately, the small size of the fruit fly and its neurons has made the application of neurobiological techniques typically used to investigate the physiology underlying these behaviors daunting. However, the realization that adult fruit flies could tolerate a window in the head capsule allowing access to the central structures thought to be involved plus the development of genetically expressed reporters of neuronal function has allowed a meteoric expansion of this field over the last decade. This review attempts to summarize the evolution of the techniques involved from the first use of a window to access these brain areas thought to be involved in associative olfactory learning and memory, the mushroom bodies and antennal lobes, to the current refinements which allow both high-resolution multiphoton imaging and patch clamping of identified neurons while applying the stimuli used in the behavioral protocols. This area of research now appears poised to reveal some very exciting mechanisms underlying behavior.

Notch-inducible hyperphosphorylated CREB and its ultradian oscillation in long-term memory formation

Notch is a cell surface receptor that is known to regulate developmental processes by establishing physical contact between neighboring cells. Many recent studies show that it also plays an important role in the formation of long-term memory (LTM) in adults, implying that memory formation requires regulation at the level of cell-cell contacts among brain cells. Neither the target of Notch activity in LTM formation nor the underlying mechanism of regulation is known. We report here results of our studies in adult Drosophila melanogaster showing that Notch regulates dCrebB-17A, the CREB protein. CREB is a transcriptional factor that is pivotal for intrinsic and synaptic plasticity involved in LTM formation. Notch in conjunction with PKC activity upregulates the level of a hyperphosphorylated form of CREB (hyper-PO4 CREB) and triggers its ultradian oscillation, both of which are linked to LTM formation. One of the sites that is phosphorylated in hyper-PO4 CREB is serine 231, which is the functional equivalent of mammalian CREB serine 133, the phosphorylation of which is an important regulator of CREB functions. Our data suggest the model that Notch and PKC activities generate a cyclical accumulation of cytoplasmic hyper-PO4 CREB that is a precursor for generating the nuclear CREB isoforms. Cyclical accumulation of CREB might be important for repetitive aspects of LTM formation, such as memory consolidation. Because Notch, PKC, and CREB have been implicated in many neurodegenerative diseases (e.g., Alzheimer's disease), our data might also shed some light on memory loss and dementia.

Pattern and predictability in memory formation: from molecular mechanisms to clinical relevance

Most long-term memories are formed as a consequence of multiple experiences. The temporal spacing of these experiences is of considerable importance: experiences distributed over time (spaced training) are more easily encoded and remembered than either closely spaced experiences, or a single prolonged experience (massed training). In this article, we first review findings from studies in animal model systems that examine the cellular and molecular properties of the neurons and circuits in the brain that underlie training pattern sensitivity during long-term memory (LTM) formation. We next focus on recent findings which have begun to elucidate the mechanisms that support inter-trial interactions during the induction of LTM. Finally, we consider the implications of these findings for developing therapeutic strategies to address questions of direct clinical relevance.

Nuclear gating of a Drosophila dCREB2 activator is involved in memory formation

The transcription factor CREB is an important regulator of many adaptive processes in neurons, including sleep, cellular homeostasis, and memory formation. The Drosophila dCREB2 family includes multiple protein isoforms generated from a single gene. Overexpression of an activator or blocker isoform has been shown to enhance or block memory formation, but the molecular mechanisms underlying these phenomena remain unclear. In the present study, we generate isoform-specific antibodies and new transgenic flies to track and manipulate the activity of different dCREB2 isoforms during memory formation. We find that nuclear accumulation of a dCREB2 activator-related species, p35+, is dynamically regulated during memory formation. Furthermore, various dCREB2 genetic manipulations that enhance or block memory formation correspondingly increase or decrease p35+ levels in the nucleus. Finally, we show that overexpression of S6K can enhance memory formation and increase p35+ nuclear abundance. Taken together, these results suggest that regulation of dCREB2 localization may be a key molecular convergence point in the coordinated host of events that lead to memory formation.

Growth factor signaling and memory formation: temporal and spatial integration of a molecular network

Growth factor (GF) signaling is critically important for developmental plasticity. It also plays a crucial role in adult plasticity, such as that required for memory formation. Although different GFs interact with receptors containing distinct types of kinase domains, they typically signal through converging intracellular cascades (e.g., Ras–MEK–MAPK) to mediate overlapping functional endpoints. Several GFs have been implicated in memory formation, but due to a high level of convergent signaling, the unique contributions of individual GFs as well as the interactions between GF signaling cascades during the induction of memory is not well known. In this review, we highlight the unique roles of specific GFs in dendritic plasticity, and discuss the spatial and temporal profiles of different GFs during memory formation. Collectively, the data suggest that the roles of GF signaling in long-lasting behavioral and structural plasticity may be best viewed as interactive components in a complex molecular network.

F3/Contactin promotes hippocampal neurogenesis, synaptic plasticity, and memory in adult mice

F3/contactin, a cell-adhesion molecule belonging to the immunoglobulin supergene family, is involved in several aspects of neural development including synapse building, maintenance and functioning. Here, we examine F3/contactin function in adult hippocampal neurogenesis, synaptic plasticity, and memory, using as a model TAG/F3 transgenic mice, where F3/contactin overexpression was induced under control of regulatory sequences from the human TAG-1 (TAX-1) gene. Transgenic mice aged 5 (M5) and 12 (M12) months exhibited an increase in hippocampal size, which correlated with positive effects on precursor proliferation and NeuN expression, these data suggesting a possible role for F3/contactin in promoting adult hippocampal neurogenesis. On the functional level, TAG/F3 mice exhibited increased CA1 long-term potentiation and improved spatial and object recognition memory, notably at 12 months of age. Interestingly, these mice showed an increased expression of the phosphorylated transcription factor CREB, which may represent the main molecular correlate of the observed morphological and functional effects. Altogether, these findings indicate for the first time that F3/contactin plays a role in promoting adult hippocampal neurogenesis and that this effect correlates with improved synaptic function and memory.