Active Protection: Learning-Activated Raf/MAPK Activity Protects Labile Memory from Rac1-Independent Forgetting

Biological aging of two innate behaviors of Drosophila melanogaster: Escape climbing versus courtship learning and memory

Motor and cognitive aging can severely affect life quality of elderly people and burden health care systems. In search for diagnostic behavioral biomarkers, it has been suggested that walking speed can predict forms of cognitive decline, but in humans, it remains challenging to separate the effects of biological aging and lifestyle. We examined a possible association of motor and cognitive decline in Drosophila, a genetic model organism of healthy aging. Long term courtship memory is present in young male flies but absent already during mid life (4-8 weeks). By contrast, courtship learning index and short term memory (STM) are surprisingly robust and remain stable through mid (4-8 weeks) and healthy late life (>8 weeks), until courtship performance collapses suddenly at ~4.5 days prior to death. By contrast, climbing speed declines gradually during late life (>8 weeks). The collapse of courtship performance and short term memory close to the end of life occur later and progress with a different time course than the gradual late life decline in climbing speed. Thus, during healthy aging in male Drosophila, climbing and courtship motor behaviors decline differentially. Moreover, cognitive and motor performances decline at different time courses. Differential behavioral decline during aging may indicate different underlying causes, or alternatively, a common cause but different thresholds for defects in different behaviors.

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

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

Active forgetting and neuropsychiatric diseases

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

An antagonism between Spinophilin and Syd-1 operates upstream of memory-promoting presynaptic long-term plasticity

We still face fundamental gaps in understanding how molecular plastic changes of synapses intersect with circuit operation to define behavioral states. Here, we show that an antagonism between two conserved regulatory proteins, Spinophilin (Spn) and Syd-1, controls presynaptic long-term plasticity and the maintenance of olfactory memories in Drosophila. While Spn mutants could not trigger nanoscopic active zone remodeling under homeostatic challenge and failed to stably potentiate neurotransmitter release, concomitant reduction of Syd-1 rescued all these deficits. The Spn/Syd-1 antagonism converged on active zone close F-actin, and genetic or acute pharmacological depolymerization of F-actin rescued the Spn deficits by allowing access to synaptic vesicle release sites. Within the intrinsic mushroom body neurons, the Spn/Syd-1 antagonism specifically controlled olfactory memory stabilization but not initial learning. Thus, this evolutionarily conserved protein complex controls behaviorally relevant presynaptic long-term plasticity, also observed in the mammalian brain but still enigmatic concerning its molecular mechanisms and behavioral relevance.

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

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

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

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

Spontaneous recovery of reward memory through active forgetting of extinction memory

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

Genetic dissection of mutual interference between two consecutive learning tasks in Drosophila

Animals can continuously learn different tasks to adapt to changing environments and, therefore, have strategies to effectively cope with inter-task interference, including both proactive interference (Pro-I) and retroactive interference (Retro-I). Many biological mechanisms are known to contribute to learning, memory, and forgetting for a single task, however, mechanisms involved only when learning sequential different tasks are relatively poorly understood. Here, we dissect the respective molecular mechanisms of Pro-I and Retro-I between two consecutive associative learning tasks in Drosophila. Pro-I is more sensitive to an inter-task interval (ITI) than Retro-I. They occur together at short ITI (<20 min), while only Retro-I remains significant at ITI beyond 20 min. Acutely overexpressing Corkscrew (CSW), an evolutionarily conserved protein tyrosine phosphatase SHP2, in mushroom body (MB) neurons reduces Pro-I, whereas acute knockdown of CSW exacerbates Pro-I. Such function of CSW is further found to rely on the γ subset of MB neurons and the downstream Raf/MAPK pathway. In contrast, manipulating CSW does not affect Retro-I as well as a single learning task. Interestingly, manipulation of Rac1, a molecule that regulates Retro-I, does not affect Pro-I. Thus, our findings suggest that learning different tasks consecutively triggers distinct molecular mechanisms to tune proactive and retroactive interference.

Small G protein RAC-2 regulates forgetting via the JNK-1 signalling pathway in Caenorhabditis elegans

Although forgetting was once regarded as a passive decline in memory and an occasional source of embarrassment, recent research suggests that it is an active biological process of removing outdated or irrelevant memories via activation of specific genes and signal transduction pathways. Rho family G proteins are known to have a role in synaptic plasticity mediated by the actin cytoskeleton. However, the current study reveals that another Rho guanosine triphosphate enzyme (GTPase), RAC-2, facilitates the occurrence of forgetting in Caenorhabditis elegans independent of actin dynamics. Functioning downstream of RAC-2 in the same signalling pathway, JNK-1 and its phosphorylated protein are required to positively regulate forgetting. The pan-neuronal rescue of RAC-2 or JNK-1, instead of AWC neuron-specific expression, reverses the delayed forgetting caused by the rac-2 mutation, which indicates that the involvement of RAC-2/JNK-1 in more than AWCs must be required. In summary, our work elucidates the action of the Rho GTPase RAC-2 and downstream JNK-1 as a potential novel pathway in forgetting in C. elegans.

Transient active zone remodeling in the Drosophila mushroom body supports memory

Elucidating how the distinct components of synaptic plasticity dynamically orchestrate the distinct stages of memory acquisition and maintenance within neuronal networks remains a major challenge. Specifically, plasticity processes tuning the functional and also structural state of presynaptic active zone (AZ) release sites are widely observed in vertebrates and invertebrates, but their behavioral relevance remains mostly unclear. We here provide evidence that a transient upregulation of presynaptic AZ release site proteins supports aversive olfactory mid-term memory in the Drosophila mushroom body (MB). Upon paired aversive olfactory conditioning, AZ protein levels (ELKS-family BRP/(m)unc13-family release factor Unc13A) increased for a few hours with MB-lobe-specific dynamics. Kenyon cell (KC, intrinsic MB neurons)-specific knockdown (KD) of BRP did not affect aversive olfactory short-term memory (STM) but strongly suppressed aversive mid-term memory (MTM). Different proteins crucial for the transport of AZ biosynthetic precursors (transport adaptor Aplip1/Jip-1; kinesin motor IMAC/Unc104; small GTPase Arl8) were also specifically required for the formation of aversive olfactory MTM. Consistent with the merely transitory increase of AZ proteins, BRP KD did not interfere with the formation of aversive olfactory long-term memory (LTM; i.e., 1 day). Our data suggest that the remodeling of presynaptic AZ refines the MB circuitry after paired aversive conditioning, over a time window of a few hours, to display aversive olfactory memories.

Contributions of extracellular-signal regulated kinase 1/2 activity to the memory trace

The ability to learn from experience and consequently adapt our behavior is one of the most fundamental capacities enabled by complex and plastic nervous systems. Next to cellular and systems-level changes, learning and memory formation crucially depends on molecular signaling mechanisms. In particular, the extracellular-signal regulated kinase 1/2 (ERK), historically studied in the context of tumor growth and proliferation, has been shown to affect synaptic transmission, regulation of neuronal gene expression and protein synthesis leading to structural synaptic changes. However, to what extent the effects of ERK are specifically related to memory formation and stabilization, or merely the result of general neuronal activation, remains unknown. Here, we review the signals leading to ERK activation in the nervous system, the subcellular ERK targets associated with learning-related plasticity, and how neurons with activated ERK signaling may contribute to the formation of the memory trace.

A conserved domain of Drosophila RNA-binding protein Pumilio interacts with multiple CCR4-NOT deadenylase complex subunits to repress target mRNAs

Pumilio is a sequence-specific RNA-binding protein that controls development, stem cell fate, and neurological functions in Drosophila. Pumilio represses protein expression by destabilizing target mRNAs in a manner dependent on the CCR4-NOT deadenylase complex. Three unique repression domains in the N-terminal region of Pumilio were previously shown to recruit CCR4-NOT, but how they do so was not well understood. In this study, we identified the motifs that are necessary and sufficient for the activity of the third repression domain of Pumilio, designated RD3, which is present in all isoforms and has conserved regulatory function. We identified multiple conserved regions of RD3 that are important for repression activity in cell-based reporter gene assays. Using yeast two hybrid assays, we show that RD3 contacts specific regions of the Not1, Not2, and Not3 subunits of the CCR4-NOT complex. Our results indicate that RD3 makes multivalent interactions with CCR4-NOT mediated by conserved short linear interaction motifs. Specifically, two phenylalanine residues in RD3 make crucial contacts with Not1 that are essential for its repression activity. Using reporter gene assays, we also identify three new target mRNAs that are repressed by Pumilio and show that RD3 contributes to their regulation. Together, these results provide important insights into the mechanism by which Pumilio recruits CCR4-NOT to regulate the expression of target mRNAs.

Age-related memory vulnerability to interfering stimuli is caused by gradual loss of MAPK-dependent protection in Drosophila

Age‐related memory impairment (AMI) is a common phenomenon across species. Vulnerability to interfering stimuli has been proposed to be an important cause of AMI. However, the molecular mechanisms underlying this vulnerability‐related AMI remain unknown. Here we show that learning‐activated MAPK signals are gradually lost with age, leading to vulnerability‐related AMI in Drosophila. Young flies (2‐ or 3‐day‐old) exhibited a significant increase in phosphorylated MAPK levels within 15 min after learning, whereas aged flies (25‐day‐old) did not. Compared to 3‐day‐old flies, significant 1 h memory impairments were observed in 15‐, 20‐, and 30‐day‐old flies, but not in 10‐day‐old flies. However, with post‐learning interfering stimuli such as cooling or electric stimuli, 10‐day‐old flies had worse memory performance at 1 h than 3‐day‐old flies, showing a premature AMI phenomenon. Increasing learning‐activated MAPK signals through acute transgene expression in mushroom body (MB) neurons restored physiological trace of 1 h memory in a pair of MB output neurons in aged flies. Decreasing such signals in young flies mimicked the impairment of 1 h memory trace in aged flies. Restoring learning‐activated MAPK signals in MB neurons in aged flies significantly suppressed AMI even with interfering stimuli. Thus, our data suggest that age‐related loss of learning‐activated neuronal MAPK signals causes memory vulnerability to interfering stimuli, thereby leading to AMI.

The signaling pathway of levamisole-sensitive-acetylcholine receptors involved in short-term forgetting of Caenorhabditis elegans

In contrast to the popular opinion that forgetting is only the opposite of learning and memory, active forgetting explains the intrinsic instability of a labile memory that lasts for hours and has its own signal transduction pathways. However, the detailed mechanisms underlying forgetting are still lacking, though the investigations available in this field offer the first insights into their regulation. To identify the alternative signaling pathways that control the process of forgetting, we used the short-term forgetting model of Caenorhabditis elegans and discovered the involvement of lev-10, a scaffolded transmembrane protein of L-AChR, by screening the candidate genes that potentially functioned in synaptic plasticity. The LEV-9/LEV-10/L-AChR functional complex was confirmed to participate in forgetting occurrence. Furthermore, EGL-9 functioned upstream of LEV-10 and negatively regulated the latter during forgetting. Meanwhile, EGL-9 was also the target of miR-51, and hence the mutation of miR-51 similarly affected the function of L-AChR and delayed the short-term forgetting. Our findings have identified an integrated signaling pathway responsible for active forgetting, which provides the new experimental evidence on the cholinergic forgetting signal.

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

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

Dopamine, sleep, and neuronal excitability modulate amyloid-β-mediated forgetting in Drosophila

Alzheimer disease (AD) is one of the main causes of age-related dementia and neurodegeneration. However, the onset of the disease and the mechanisms causing cognitive defects are not well understood. Aggregation of amyloidogenic peptides is a pathological hallmark of AD and is assumed to be a central component of the molecular disease pathways. Pan-neuronal expression of Aβ42Arctic peptides in Drosophila melanogaster results in learning and memory defects. Surprisingly, targeted expression to the mushroom bodies, a center for olfactory memories in the fly brain, does not interfere with learning but accelerates forgetting. We show here that reducing neuronal excitability either by feeding Levetiracetam or silencing of neurons in the involved circuitry ameliorates the phenotype. Furthermore, inhibition of the Rac-regulated forgetting pathway could rescue the Aβ42Arctic-mediated accelerated forgetting phenotype. Similar effects are achieved by increasing sleep, a critical regulator of neuronal homeostasis. Our results provide a functional framework connecting forgetting signaling and sleep, which are critical for regulating neuronal excitability and homeostasis and are therefore a promising mechanism to modulate forgetting caused by toxic Aβ peptides.

Genistein enhances expression of extracellular regulated kinases (ERK) 1/2, and learning and memory of mouse

Genistein (GEN) is a well known phytoestrogen. It acts through estrogen receptor (ER) and performs plethora of functions in the brain. ERK1/2 is an activated kinase which involves in neuron differentiation, adult neurogenesis and several brain functions including learning and memory. However, GEN dependent expression of ERK1/2 and its effect in learning and memory of mice are unknown. In this study, Swiss albino male mice of 25weeks weighing 30 g were used for the experiments. Mice were placed in two groups- control (C) and genistein treated (GEN). Treated group received GEN dissolved in sesame oil (1 mg/kg/day) whereas the control group received sesame oil only. To study the effects of GEN on learning and memory, open-field (OF) test and novel object recognition (NOR) test were performed. Moreover, immunoblotting (IB) was performed to check the expression of ERK1/2 in the mouse brain of both groups. In the OF test, no significant change was observed in motor activity and anxiety in GEN treated mice as compared to control. Moreover, NOR test suggested that entry towards the dissimilar object was higher in case of GEN treated mice as compared to control. These findings suggest higher learning and memory of GEN treated mice than of control. IB showed that the expression of ERK1/2 was significantly high in GEN treated mouse brain as compared to control. Such study may be helpful to understand GEN mediated learning and memory involving ERK1/2.

Rac1 Impairs Forgetting-Induced Cellular Plasticity in Mushroom Body Output Neurons

Active memory forgetting is a well-regulated biological process thought to be adaptive and to allow proper cognitive functions. Recent efforts have elucidated several molecular players involved in the regulation of olfactory forgetting in Drosophila, including the small G protein Rac1, the dopamine receptor DAMB, and the scaffold protein Scribble. Similarly, we recently reported that dopaminergic neurons mediate both learning- and forgetting-induced plasticity in the mushroom body output neuron MBON-γ2α′1. Two open questions remain: how does forgetting affect plasticity in other, highly plastic, mushroom body compartments and how do genes that regulate forgetting affect this cellular plasticity? Here, we show that forgetting reverses short-term synaptic depression induced by aversive conditioning in the highly plastic mushroom body output neuron MBON-γ1pedc>α/β. In addition, our results indicate that genetic tampering with normal forgetting by inhibition of small G protein Rac1 impairs restoration of depressed odor responses to learned odor by intrinsic forgetting through time passing and forgetting induced acutely by shock stimulation after conditioning or reversal learning. These data further indicate that some forms of forgetting truly erase physiological changes generated by memory encoding.

Dopamine Receptor Dop1R2 Stabilizes Appetitive Olfactory Memory through the Raf/MAPK Pathway in Drosophila

In Drosophila, dopamine signaling to the mushroom body intrinsic neurons, Kenyon cells (KCs), is critical to stabilize olfactory memory. Little is known about the downstream intracellular molecular signaling underlying memory stabilization. Here we address this question in the context of sugar-rewarded olfactory long-term memory (LTM). We show that associative training increases the phosphorylation of MAPK in KCs, via Dop1R2 signaling. Consistently, the attenuation of Dop1R2, Raf, or MAPK expression in KCs selectively impairs LTM, but not short-term memory. Moreover, we show that the LTM deficit caused by the knockdown of Dop1R2 can be rescued by expressing active Raf in KCs. Thus, the Dop1R2/Raf/MAPK pathway is a pivotal downstream effector of dopamine signaling for stabilizing appetitive olfactory memory.SIGNIFICANCE STATEMENT Dopaminergic input to the Kenyon cells (KCs) is pivotal to stabilize memory in Drosophila This process is mediated by dopamine receptors like Dop1R2. Nevertheless, little is known for its underlying molecular mechanism. Here we show that the Raf/MAPK pathway is specifically engaged in appetitive long-term memory in KCs. With combined biochemical and behavioral experiments, we reveal that activation of the Raf/MAPK pathway is regulated through Dop1R2, shedding light on how dopamine modulates intracellular signaling for memory stabilization.

Ras acts as a molecular switch between two forms of consolidated memory in Drosophila

Long-lasting, consolidated memories require not only positive biological processes that facilitate long-term memories (LTM) but also the suppression of inhibitory processes that prevent them. The mushroom body neurons (MBn) in Drosophila melanogaster store protein synthesis-dependent LTM (PSD-LTM) as well as protein synthesis-independent, anesthesia-resistant memory (ARM). The formation of ARM inhibits PSD-LTM but the underlying molecular processes that mediate this interaction remain unknown. Here, we demonstrate that the Ras→Raf→rho kinase (ROCK) pathway in MBn suppresses ARM consolidation, allowing the formation of PSD-LTM. Our initial results revealed that the effects of Ras on memory are due to postacquisition processes. Ras knockdown enhanced memory expression but had no effect on acquisition. Additionally, increasing Ras activity optogenetically after, but not before, acquisition impaired memory performance. The elevated memory produced by Ras knockdown is a result of increased ARM. While Ras knockdown enhanced the consolidation of ARM, it eliminated PSD-LTM. We found that these effects are mediated by the downstream kinase Raf. Similar to Ras, knockdown of Raf enhanced ARM consolidation and impaired PSD-LTM. Surprisingly, knockdown of the canonical downstream extracellular signal-regulated kinase did not reproduce the phenotypes observed with Ras and Raf knockdown. Rather, Ras/Raf inhibition of ROCK was found to be responsible for suppressing ARM. Constitutively active ROCK enhanced ARM and impaired PSD-LTM, while decreasing ROCK activity rescued the enhanced ARM produced by Ras knockdown. We conclude that MBn Ras/Raf inhibition of ROCK suppresses the consolidation of ARM, which permits the formation of PSD-LTM.

Interplay between α2-chimaerin and Rac1 activity determines dynamic maintenance of long-term memory

Memory consolidation theory suggests that once memory formation has been completed, memory is maintained at a stable strength and is incapable of further enhancement. However, the current study reveals that even long after formation, contextual fear memory could be further enhanced. Such unexpected enhancement is possible because memory is dynamically maintained at an intermediate level that allows for bidirectional regulation. Here we find that both Rac1 activation and expression of α2-chimaerin are stimulated by single-trial contextual fear conditioning. Such sustained Rac1 activity mediates reversible forgetting, and α2-chimaerin acts as a memory molecule that reverses forgetting to sustain memory through inhibition of Rac1 activity during the maintenance stage. Therefore, the balance between activated Rac1 and expressed α2-chimaerin defines dynamic long-term memory maintenance. Our findings demonstrate that consolidated memory maintains capacity for bidirectional regulation.

Genetic dissection of active forgetting in labile and consolidated memories in Drosophila

Different memory components are forgotten through distinct molecular mechanisms. In Drosophila, the activation of 2 Rho GTPases (Rac1 and Cdc42), respectively, underlies the forgetting of an early labile memory (anesthesia-sensitive memory, ASM) and a form of consolidated memory (anesthesia-resistant memory, ARM). Here, we dissected the molecular mechanisms that tie Rac1 and Cdc42 to the different types of memory forgetting. We found that 2 WASP family proteins, SCAR/WAVE and WASp, act downstream of Rac1 and Cdc42 separately to regulate ASM and ARM forgetting in mushroom body neurons. Arp2/3 complex, which organizes branched actin polymerization, is a canonical downstream effector of WASP family proteins. However, we found that Arp2/3 complex is required in Cdc42/WASp-mediated ARM forgetting but not in Rac1/SCAR-mediated ASM forgetting. Instead, we identified that Rac1/SCAR may function with formin Diaphanous (Dia), a nucleator that facilitates linear actin polymerization, in ASM forgetting. The present study, complementing the previously identified Rac1/cofilin pathway that regulates actin depolymerization, suggests that Rho GTPases regulate forgetting by recruiting both actin polymerization and depolymerization pathways. Moreover, Rac1 and Cdc42 may regulate different types of memory forgetting by tapping into different actin polymerization mechanisms.

Recent Advances in the Genetic Dissection of Neural Circuits in Drosophila

Nervous systems endow animals with cognition and behavior. To understand how nervous systems control behavior, neural circuits mediating distinct functions need to be identified and characterized. With superior genetic manipulability, Drosophila is a model organism at the leading edge of neural circuit analysis. We briefly introduce the state-of-the-art genetic tools that permit precise labeling of neurons and their interconnectivity and investigating what is happening in the brain of a behaving animal and manipulating neurons to determine how behaviors are affected. Brain-wide wiring diagrams, created by light and electron microscopy, bring neural circuit analysis to a new level and scale. Studies enabled by these tools advances our understanding of the nervous system in relation to cognition and behavior.

ERK1/2: A Key Cellular Component for the Formation, Retrieval, Reconsolidation and Persistence of Memory

Extracellular regulated kinase 1/2 (ERK1/2) has been strongly implicated in several cellular processes. In the brain ERK1/2 activity has been primarily involved in long-term memory (LTM) formation and expression. Here, we review earlier evidence and describe recent developments of ERK1/2 signaling in memory processing focusing the attention on the role of ERK1/2 in memory retrieval and reconsolidation, and in the maintenance of the memory trace including mechanisms involving the protection of labile memories. In addition, relearning requires ERK1/2 activity in selected brain regions. Its involvement in distinct memory stages points at ERK1/2 as a core element in memory processing and as one likely target to treat memory impairments associated with neurological disorders.