A Permissive Role of Mushroom Body α/β Core Neurons in Long-Term Memory Consolidation in Drosophila

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Memory is initially labile and gradually consolidated over time through new protein synthesis into a long-lasting stable form. Studies of odor-shock associative learning in Drosophila have established the mushroom body (MB) as a key brain structure involved in olfactory long-term memory (LTM) formation. Exactly how early neural activity encoded in thousands of MB neurons is consolidated into protein-synthesis-dependent LTM remains unclear. Here, several independent lines of evidence indicate that changes in two MB vertical lobe V3 (MB-V3) extrinsic neurons are required and contribute to an extended neural network involved in olfactory LTM: (i) inhibiting protein synthesis in MB-V3 neurons impairs LTM; (ii) MB-V3 neurons show enhanced neural activity after spaced but not massed training; (iii) MB-V3 dendrites, synapsing with hundreds of MB α/β neurons, exhibit dramatic structural plasticity after removal of olfactory inputs; (iv) neurotransmission from MB-V3 neurons is necessary for LTM retrieval; and (v) RNAi-mediated downregulation of oo18 RNA-binding protein (involved in local regulation of protein translation) in MB-V3 neurons impairs LTM. Our results suggest a model of long-term memory formation that includes a systems-level consolidation process, wherein an early, labile olfactory memory represented by neural activity in a sparse subset of MB neurons is converted into a stable LTM through protein synthesis in dendrites of MB-V3 neurons synapsed onto MB α lobes.L ong-term memory (LTM) and long-term synaptic plasticity require de novo protein synthesis, which is regulated at transcriptional and/or translational levels in a synapse-specific manner (1-3). Synapse-specific plasticity during LTM formation in some contexts may involve local regulation of protein translation by a family of RNA-binding proteins, the cytoplasmic polyadenylation element-binding proteins (CPEBs) (2). Neuronal CPEBs have two conformational states. The inactive state predominates at low levels of CPEB expression and represses translation from nascent mRNAs. The active state is achieved either via a self-perpetuating prion-like state when expression levels surpass a threshold or via Ca 2+ /calmoduline-dependent protein kinase II (CaMKII)-mediated phosphorylation, and translation is initiated by elongation of an mRNA's poly-A tail (4-6). In other species, CPEB1 has been shown to contribute to long-term facilitation or potentiation (5, 7). In Drosophila, oo18 RNA-binding protein 2 (ORB2) appears required for long-term memory formation after courtship conditioning (8, 9). Any role for ORB in fruit fly memory formation, however, remains unclear.Drosophila can learn to associate an odor (conditioned stimulus, CS) with foot-shock punishment (unconditioned stimulus, US). This odor-shock association initially is labile, lasting for only about a day after one training session. With repetitive, spaced training (ST) sessions (rest intervals between each session), a protein synthesis-dependent, LTM is formed. With repetitive, massed training (MT) ses...

Section: Resultsmentioning

confidence: 99%

Drosophila ORB protein in two mushroom body output neurons is necessary for long-term memory formation

Pai

Chen

Lin

et al. 2013

116

135

“…The procedures were described previously (71). For RU486 feeding (RU + ), flies were fed with 500 μM RU486 (mifepristone; J&K Chemicals) dissolved in control solution.…”

Section: Methodsmentioning

confidence: 99%

Inability to activate Rac1-dependent forgetting contributes to behavioral inflexibility in mutants of multiple autism-risk genes

Dong

Wang

et al. 2016

Self Cite

The etiology of autism is so complicated because it involves the effects of variants of several hundred risk genes along with the contribution of environmental factors. Therefore, it has been challenging to identify the causal paths that lead to the core autistic symptoms such as social deficit, repetitive behaviors, and behavioral inflexibility. As an alternative approach, extensive efforts have been devoted to identifying the convergence of the targets and functions of the autism-risk genes to facilitate mapping out causal paths. In this study, we used a reversal-learning task to measure behavioral flexibility in Drosophila and determined the effects of loss-of-function mutations in multiple autism-risk gene homologs in flies. Mutations of five autism-risk genes with diversified molecular functions all led to a similar phenotype of behavioral inflexibility indicated by impaired reversal-learning. These reversal-learning defects resulted from the inability to forget or rather, specifically, to activate Rac1 (Ras-related C3 botulinum toxin substrate 1)-dependent forgetting. Thus, behavior-evoked activation of Rac1-dependent forgetting has a converging function for autism-risk genes.A utism spectrum disorders are common polygenic neurodevelopmental disorders diagnosed by a cluster of core clinical syndromes characterized by impaired social interaction, communication deficits, and behavioral inflexibility (1-3). A large number of autism-risk genes have been identified through different genetic approaches (4-8). These candidate genes play highly diversified roles in synaptic organization, transmission, intracellular signaling pathways, and protein expression regulation, among others (9). How do variants of these risk genes lead to the core symptoms of autism? Are there causal paths that can determine the onset of autism? Answering these questions is difficult because the disorder is multifactorial in nature with multiple genetic variants and environmental factors contributing to the core symptoms (10-13). To face this challenge, extensive efforts have been made to identify the targeting and functional convergence of the autism-risk genes, such as those involved in translation regulation (14-16) and long-range connectivity among different brain regions (17)(18)(19). These targets and functions provide a better biological basis for elucidating the causal paths between risk genes and the disorders of autism.The aim of the this study was to determine whether Rac1 (Rasrelated C3 botulinum toxin substrate 1)-dependent forgetting is a functional converging point for autism-risk genes. This idea was inspired by two clues. First, inhibition of Rac1 activity in Drosophila leads to the failure to activate Rac1-dependent forgetting, resulting in behavioral inflexibility as assayed through a reversal-learning task (20). Behavioral inflexibility has been observed frequently in the clinical studies of autism patients (21,22), and children with autism are reported to have impaired reversal-learning, although they can learn new beh...

“…A current working model suggests that concomitant reward signals from sugar ingestion cause associative plasticity in Kenyon cells that might underlie memory formation (14-20). A single activation session of a specific cluster of dopamine neurons (PAM neurons) by sugar ingestion can induce appetitive memory that is stable over 24 h (19), underscoring the importance of sugar reward to the fly.The mushroom body (MB) is composed of the three different cell types, α/β, α′/β′, and γ, which have distinct roles in different phases of appetitive memories (11,(21)(22)(23)(24)(25). Similar to midbrain dopamine neurons in mammals (26,27), the structure and function of PAM cluster neurons are heterogeneous, and distinct dopamine neurons intersect unique segments of the MB lobes (19,(28)(29)(30)(31)(32)(33)(34).…”

mentioning

confidence: 99%

“…Recent studies have revealed aspects of the molecular and cellular mechanisms of LTM formation induced by repetitive training (8)(9)(10)(11), but how a single training induces a stable LTM is poorly understood (12).…”

mentioning

confidence: 99%

Distinct dopamine neurons mediate reward signals for short- and long-term memories

Yamagata

Ichinose

Aso

et al. 2014

228

287

Drosophila melanogaster can acquire a stable appetitive olfactory memory when the presentation of a sugar reward and an odor are paired. However, the neuronal mechanisms by which a single training induces long-term memory are poorly understood. Here we show that two distinct subsets of dopamine neurons in the fly brain signal reward for short-term (STM) and long-term memories (LTM). One subset induces memory that decays within several hours, whereas the other induces memory that gradually develops after training. They convey reward signals to spatially segregated synaptic domains of the mushroom body (MB), a potential site for convergence. Furthermore, we identified a single type of dopamine neuron that conveys the reward signal to restricted subdomains of the mushroom body lobes and induces long-term memory. Constant appetitive memory retention after a single training session thus comprises two memory components triggered by distinct dopamine neurons.dopamine | learning and memory | Drosophila | mushroom body M emory of a momentous event persists for a long time.Whereas some forms of long-term memory (LTM) require repetitive training (1-3), a highly relevant stimulus such as food or poison is sufficient to induce LTM in a single training session (4-7). Recent studies have revealed aspects of the molecular and cellular mechanisms of LTM formation induced by repetitive training (8-11), but how a single training induces a stable LTM is poorly understood (12).Appetitive olfactory learning in fruit flies is suited to address the question, as a presentation of a sugar reward paired with odor induces robust short-term memory (STM) and LTM (6, 7). Odor is represented by a sparse ensemble of the 2,000 intrinsic neurons, the Kenyon cells (13). A current working model suggests that concomitant reward signals from sugar ingestion cause associative plasticity in Kenyon cells that might underlie memory formation (14-20). A single activation session of a specific cluster of dopamine neurons (PAM neurons) by sugar ingestion can induce appetitive memory that is stable over 24 h (19), underscoring the importance of sugar reward to the fly.The mushroom body (MB) is composed of the three different cell types, α/β, α′/β′, and γ, which have distinct roles in different phases of appetitive memories (11,(21)(22)(23)(24)(25). Similar to midbrain dopamine neurons in mammals (26,27), the structure and function of PAM cluster neurons are heterogeneous, and distinct dopamine neurons intersect unique segments of the MB lobes (19,(28)(29)(30)(31)(32)(33)(34). Further circuit dissection is thus crucial to identify candidate synapses that undergo associative modulation.By activating distinct subsets of PAM neurons for reward signaling, we found that short-and long-term memories are independently formed by two complementary subsets of PAM cluster dopamine neurons. Conditioning flies with nutritious and nonnutritious sugars revealed that the two subsets could represent different reinforcing properties: sweet taste and nutritional value of sugar....