Distinct neuronal circuits mediate experience-dependent, non-associative osmotactic responses in Drosophila

Choline Transporter regulates olfactory habituation via a neuronal triad of excitatory, inhibitory and mushroom body neurons

Choline is an essential component of Acetylcholine (ACh) biosynthesis pathway which requires high-affinity Choline transporter (ChT) for its uptake into the presynaptic terminals of cholinergic neurons. Previously, we had reported a predominant expression of ChT in memory processing and storing region of the Drosophila brain called mushroom bodies (MBs). It is unknown how ChT contributes to the functional principles of MB operation. Here, we demonstrate the role of ChT in Habituation, a non-associative form of learning. Odour driven habituation traces are laid down in ChT dependent manner in antennal lobes (AL), projection neurons (PNs), and MBs. We observed that reduced habituation due to knock-down of ChT in MBs causes hypersensitivity towards odour, suggesting that ChT also regulates incoming stimulus suppression. Importantly, we show for the first time that ChT is not unique to cholinergic neurons but is also required in inhibitory GABAergic neurons to drive habituation behaviour. Our results support a model in which ChT regulates both habituation and incoming stimuli through multiple circuit loci via an interplay between excitatory and inhibitory neurons. Strikingly, the lack of ChT in MBs shows characteristics similar to the major reported features of Autism spectrum disorders (ASD), including attenuated habituation, sensory hypersensitivity as well as defective GABAergic signalling. Our data establish the role of ChT in habituation and suggest that its dysfunction may contribute to neuropsychiatric disorders like ASD.

Expression of Mammalian BM88/CEND1 in Drosophila Affects Nervous System Development by Interfering with Precursor Cell Formation

We used Drosophila melanogaster as an experimental model to express mouse and pig BM88/CEND1 (cell cycle exit and neuronal differentiation 1) in order to investigate its potential functional effects on Drosophila neurogenesis. BM88/CEND1 is a neuron-specific protein whose function is implicated in triggering cells to exit from the cell cycle and differentiate towards a neuronal phenotype. Transgenic flies expressing either mouse or pig BM88/CEND1 in the nervous system had severe neuronal phenotypes with variable expressivity at various stages of embryonic development. In early embryonic stage 10, BM88/CEND1 expression led to an increase in the neural-specific antigenicity of neuroectoderm at the expense of precursor cells [neuroblasts (Nbs) and ganglion mother cells (GMCs)] including the defective formation and differentiation of the MP2 precursors, whereas at later stages (12-15), protein accumulation induced gross morphological defects primarily in the CNS accompanied by a reduction of Nb and GMC markers. Furthermore, the neuronal precursor cells of embryos expressing BM88/CEND1 failed to carry out proper cell-cycle progression as revealed by the disorganized expression patterns of specific cell-cycle markers. BM88/CEND1 accumulation in the Drosophila eye affected normal eye disc development by disrupting the ommatidia. Finally, we demonstrated that expression of BM88/CEND1 modified/reduced the levels of activated MAP kinase indicating a functional effect of BM88/CEND1 on the MAPK signaling pathway. Our findings suggest that the expression of mammalian BM88/CEND1 in Drosophila exerts specific functional effects associated with neuronal precursor cell formation during embryonic neurogenesis and proper eye disc development. This study also validates the use of Drosophila as a powerful model system in which to investigate gene function and the underlying molecular mechanisms.

Domain-Specific Cognitive Impairments in Humans and Flies With Reduced CYFIP1 Dosage

BACKGROUND

Deletions encompassing a four-gene region on chromosome 15 (BP1-BP2 at 15q11.2), seen at a population frequency of 1 in 500, are associated with increased risk for schizophrenia, epilepsy, and other common neurodevelopmental disorders. However, little is known in terms of how these common deletions impact cognition.

METHODS

We used a Web-based tool to characterize cognitive function in a novel cohort of adult carriers and their noncarrier family members. Results from 31 carrier and 38 noncarrier parents from 40 families were compared with control data from 6530 individuals who self-registered on the Lumosity platform and opted in to participate in research. We then examined aspects of sensory and cognitive function in flies harboring a mutation in Cyfip, the homologue of one of the genes within the deletion. For the fly studies, 10 or more groups of 50 individuals per genotype were included.

RESULTS

Our human studies revealed profound deficits in grammatical reasoning, arithmetic reasoning, and working memory in BP1-BP2 deletion carriers. No such deficits were observed in noncarrier spouses. Our fly studies revealed deficits in associative and nonassociative learning despite intact sensory perception.

CONCLUSIONS

Our results provide new insights into outcomes associated with BP1-BP2 deletions and call for a discussion on how to appropriately communicate these findings to unaffected carriers. Findings also highlight the utility of an online tool in characterizing cognitive function in a geographically distributed population.

Temporally specific engagement of distinct neuronal circuits regulating olfactory habituation in Drosophila

Habituation is the process that enables salience filtering, precipitating perceptual changes that alter the value of environmental stimuli. To discern the neuronal circuits underlying habituation to brief inconsequential stimuli, we developed a novel olfactory habituation paradigm, identifying two distinct phases of the response that engage distinct neuronal circuits. Responsiveness to the continuous odor stimulus is maintained initially, a phase we term habituation latency and requires Rutabaga Adenylyl-Cyclase-depended neurotransmission from GABAergic Antennal Lobe Interneurons and activation of excitatory Projection Neurons (PNs) and the Mushroom Bodies. In contrast, habituation depends on the inhibitory PNs of the middle Antenno-Cerebral Track, requires inner Antenno-Cerebral Track PN activation and defines a temporally distinct phase. Collectively, our data support the involvement of Lateral Horn excitatory and inhibitory stimulation in habituation. These results provide essential cellular substrates for future analyses of the molecular mechanisms that govern the duration and transition between these distinct temporal habituation phases.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Associative Learning of Stimuli Paired and Unpaired With Reinforcement: Evaluating Evidence From Maggots, Flies, Bees, and Rats

Finding rewards and avoiding punishments are powerful goals of behavior. To maximize reward and minimize punishment, it is beneficial to learn about the stimuli that predict their occurrence, and decades of research have provided insight into the brain processes underlying such associative reinforcement learning. In addition, it is well known in experimental psychology, yet often unacknowledged in neighboring scientific disciplines, that subjects also learn about the stimuli that predict the absence of reinforcement. Here we evaluate evidence for both these learning processes. We focus on two study cases that both provide a baseline level of behavior against which the effects of associative learning can be assessed. Firstly, we report pertinent evidence from Drosophila larvae. A re-analysis of the literature reveals that through paired presentations of an odor A and a sugar reward (A+) the animals learn that the reward can be found where the odor is, and therefore show an above-baseline preference for the odor. In contrast, through unpaired training (A/+) the animals learn that the reward can be found precisely where the odor is not, and accordingly these larvae show a below-baseline preference for it (the same is the case, with inverted signs, for learning through taste punishment). In addition, we present previously unpublished data demonstrating that also during a two-odor, differential conditioning protocol (A+/B) both these learning processes take place in larvae, i.e., learning about both the rewarded stimulus A and the non-rewarded stimulus B (again, this is likewise the case for differential conditioning with taste punishment). Secondly, after briefly discussing published evidence from adult Drosophila, honeybees, and rats, we report an unpublished data set showing that relative to baseline behavior after truly random presentations of a visual stimulus A and punishment, rats exhibit memories of opposite valence upon paired and unpaired training. Collectively, the evidence conforms to classical findings in experimental psychology and suggests that across species animals associatively learn both through paired and through unpaired presentations of stimuli with reinforcement – with opposite valence. While the brain mechanisms of unpaired learning for the most part still need to be uncovered, the immediate implication is that using unpaired procedures as a mnemonically neutral control for associative reinforcement learning may be leading analyses astray.

Interference of the complex between NCS-1 and Ric8a with phenothiazines regulates synaptic function and is an approach for fragile X syndrome

The protein complex formed by the Ca2+ sensor neuronal calcium sensor 1 (NCS-1) and the guanine exchange factor protein Ric8a coregulates synapse number and probability of neurotransmitter release, emerging as a potential therapeutic target for diseases affecting synapses, such as fragile X syndrome (FXS), the most common heritable autism disorder. Using crystallographic data and the virtual screening of a chemical library, we identified a set of heterocyclic small molecules as potential inhibitors of the NCS-1/Ric8a interaction. The aminophenothiazine FD44 interferes with NCS-1/Ric8a binding, and it restores normal synapse number and associative learning in a Drosophila FXS model. The synaptic effects elicited by FD44 feeding are consistent with the genetic manipulation of NCS-1. The crystal structure of NCS-1 bound to FD44 and the structure-function studies performed with structurally close analogs explain the FD44 specificity and the mechanism of inhibition, in which the small molecule stabilizes a mobile C-terminal helix inside a hydrophobic crevice of NCS-1 to impede Ric8a interaction. Our study shows the drugability of the NCS-1/Ric8a interface and uncovers a suitable region in NCS-1 for development of additional drugs of potential use on FXS and related synaptic disorders.

Learning and memory deficits consequent to reduction of the fragile X mental retardation protein result from metabotropic glutamate receptor-mediated inhibition of cAMP signaling in Drosophila

Loss of the RNA-binding fragile X protein [fragile X mental retardation protein (FMRP)] results in a spectrum of cognitive deficits, the fragile X syndrome (FXS), while aging individuals with decreased protein levels present with a subset of these symptoms and tremor. The broad range of behavioral deficits likely reflects the ubiquitous distribution and multiple functions of the protein. FMRP loss is expected to affect multiple neuronal proteins and intracellular signaling pathways, whose identity and interactions are essential in understanding and ameliorating FXS symptoms. We used heterozygous mutants and targeted RNA interference-mediated abrogation in Drosophila to uncover molecular pathways affected by FMRP reduction. We present evidence that FMRP loss results in excess metabotropic glutamate receptor (mGluR) activity, attributable at least in part to elevation of the protein in affected neurons. Using high-resolution behavioral, genetic, and biochemical analyses, we present evidence that excess mGluR upon FMRP attenuation is linked to the cAMP decrement reported in patients and models, and underlies olfactory associative learning and memory deficits. Furthermore, our data indicate positive transcriptional regulation of the fly fmr1 gene by cAMP, via protein kinase A, likely through the transcription factor CREB. Because the human Fmr1 gene also contains CREB binding sites, the interaction of mGluR excess and cAMP signaling defects we present suggests novel combinatorial pharmaceutical approaches to symptom amelioration upon FMRP attenuation.

Event timing in associative learning: from biochemical reaction dynamics to behavioural observations

Associative learning relies on event timing. Fruit flies for example, once trained with an odour that precedes electric shock, subsequently avoid this odour (punishment learning); if, on the other hand the odour follows the shock during training, it is approached later on (relief learning). During training, an odour-induced Ca⁺⁺ signal and a shock-induced dopaminergic signal converge in the Kenyon cells, synergistically activating a Ca⁺⁺-calmodulin-sensitive adenylate cyclase, which likely leads to the synaptic plasticity underlying the conditioned avoidance of the odour. In Aplysia, the effect of serotonin on the corresponding adenylate cyclase is bi-directionally modulated by Ca⁺⁺, depending on the relative timing of the two inputs. Using a computational approach, we quantitatively explore this biochemical property of the adenylate cyclase and show that it can generate the effect of event timing on associative learning. We overcome the shortage of behavioural data in Aplysia and biochemical data in Drosophila by combining findings from both systems.

Are mushroom bodies cerebellum-like structures?

The mushroom bodies are distinctive neuropils in the protocerebral brain segments of many protostomes. A defining feature of mushroom bodies is their intrinsic neurons, masses of cytoplasm-poor globuli cells that form a system of lobes with their densely-packed, parallel-projecting axon-like processes. In insects, the role of the mushroom bodies in olfactory processing and associative learning and memory has been studied in depth, but several lines of evidence suggest that the function of these higher brain centers cannot be restricted to these roles. The present account considers whether insight into an underlying function of mushroom bodies may be provided by cerebellum-like structures in vertebrates, which are similarly defined by the presence of masses of tiny granule cells that emit thin parallel fibers forming a dense molecular layer. In vertebrates, the shared neuroarchitecture of cerebellum-like structures has been suggested to underlie a common functional role as adaptive filters for the removal of predictable sensory elements, such as those arising from reafference, from the total sensory input. Cerebellum-like structures include the vertebrate cerebellum, the electrosensory lateral line lobe, dorsal and medial octavolateral nuclei of fish, and the dorsal cochlear nucleus of mammals. The many architectural and physiological features that the insect mushroom bodies share with cerebellum-like structures suggest that it might be fruitful to consider mushroom body function in light of a possible role as adaptive sensory filters. The present account thus presents a detailed comparison of the insect mushroom bodies with vertebrate cerebellum-like structures.

Molecular vibration-sensing component in Drosophila melanogaster olfaction

A common explanation of molecular recognition by the olfactory system posits that receptors recognize the structure or shape of the odorant molecule. We performed a rigorous test of shape recognition by replacing hydrogen with deuterium in odorants and asking whether Drosophila melanogaster can distinguish these identically shaped isotopes. We report that flies not only differentiate between isotopic odorants, but can be conditioned to selectively avoid the common or the deuterated isotope. Furthermore, flies trained to discriminate against the normal or deuterated isotopes of a compound, selectively avoid the corresponding isotope of a different odorant. Finally, flies trained to avoid a deuterated compound exhibit selective aversion to an unrelated molecule with a vibrational mode in the energy range of the carbon-deuterium stretch. These findings are inconsistent with a shape-only model for smell, and instead support the existence of a molecular vibration-sensing component to olfactory reception.

Optimizing Drosophila olfactory learning with a semi-automated training device

Drosophila olfactory aversive conditioning has served as a powerful model system with which to elucidate the molecular and neuronal mechanisms underlying memory formation. In the typical protocol, flies are exposed to a constant odor stream while receiving a pulsed electric shock in the conditioning tube of a manual apparatus. We have devised a simple, low-cost semi-automated conditioning apparatus that computationally controls the delivery of odor and shock. A semiconductor-based odor sensor is employed to monitor the change of odor concentration in the training tube. The system thus allows electric shocks to be precisely matched with odor concentration in the training tube. We found that short-term memory performance was improved with a pulsed odor flow protocol, in which odor is presented in short pulses, each paired with electric shock, rather than as a constant flow. The effect of pulsed odor flow might be ascribed to the phenomenon of 'conditioned approach', where approach toward an odor is induced when the electric shock is presented before odor pulse ends. Our data shows that the system is applicable to the study of olfactory memory formation and to the examination of conditioning parameters at a level of detail not practical with a manual apparatus.

Differential effects of Tau on the integrity and function of neurons essential for learning in Drosophila

Tauopathies are a heterogeneous group of neurodegenerative dementias involving perturbations in the levels, phosphorylation, or mutations of the microtubule-binding protein Tau. The heterogeneous pathology in humans and model organisms suggests differential susceptibility of neuronal types to wild-type (WT) and mutant Tau. WT and mutant human Tau-encoding transgenes expressed pan-neuronally in the Drosophila CNS yielded specific and differential toxicity in the embryonic neuroblasts that generate the mushroom body (MB) neurons, suggesting cell type-specific effects of Tau in the CNS. Frontotemporal dementia with parkinsonism-17-linked mutant isoforms were significantly less toxic in MB development. Tau hyperphosphorylation was essential for these MB aberrations, and we identified two novel putative phosphorylation sites, Ser(238) and Thr(245), on WT hTau essential for its toxic effects on MB integrity. Significantly, blocking putative Ser(238) and Thr(245) phosphorylation yielded animals with apparently structurally normal but profoundly dysfunctional MBs, because animals accumulating this mutant protein exhibited strongly impaired associative learning. Interestingly, the mutant protein was hyperphosphorylated at epitopes typically associated with toxicity and neurodegeneration, such as AT8, AT100, and the Par-1 targets Ser(262) and Ser(356), suggesting that these sites in the context of adult intact MBs mediate dysfunction and occupation of these sites may precede the toxicity-associated Ser(238) and Thr(245) phosphorylation. The data support the notion that phosphorylation at particular sites rather than hyperphosphorylation per se mediates toxicity or dysfunction in a cell type-specific manner.

Dimerization is essential for 14-3-3zeta stability and function in vivo

Members of the conserved 14-3-3 protein family spontaneously self-assemble as homo- and heterodimers via conserved sequences in the first four (alphaA-alphaD) of the nine helices that comprise them. Dimeric 14-3-3s bind conserved motifs in diverse protein targets involved in multiple essential cellular processes including signaling, intracellular trafficking, cell cycle regulation, and modulation of enzymatic activities. However, recent mostly in vitro evidence has emerged, suggesting functional and regulatory roles for monomeric 14-3-3s. We capitalized on the simplicity of the 14-3-3 family in Drosophila to investigate in vivo 14-3-3zeta monomer properties and functionality. We report that dimerization is essential for the stability and function of 14-3-3zeta in neurons. Moreover, we reveal the contribution of conserved amino acids in helices A and D to homo- and heterodimerization and their functional consequences on the viability of animals devoid of endogenous 14-3-3zeta. Finally, we present evidence suggesting endogenous homeostatic adjustment of the levels of the second family member in Drosophila, D14-3-3epsilon, to transgenic monomeric and dimerization-competent 14-3-3zeta.

Odour intensity learning in fruit flies

Animals' behaviour towards odours depends on both odour quality and odour intensity. While neuronal coding of odour quality is fairly well studied, how odour intensity is treated by olfactory systems is less clear. Here we study odour intensity processing at the behavioural level, using the fruit fly Drosophila melanogaster. We trained flies by pairing a MEDIUM intensity of an odour with electric shock, and then, at a following test phase, measured flies' conditioned avoidance of either this previously trained MEDIUM intensity or a LOWer or a HIGHer intensity. With respect to 3-octanol, n-amylacetate and 4-methylcyclohexanol, we found that conditioned avoidance is strongest when training and test intensities match, speaking for intensity-specific memories. With respect to a fourth odour, benzaldehyde, on the other hand, we found no such intensity specificity. These results form the basis for further studies of odour intensity processing at the behavioural, neuronal and molecular level.

A dual role for the adaptor protein DRK in Drosophila olfactory learning and memory

Participation of RAS, RAF, and mitogen-activated protein kinase (MAPK) in learning and memory has been demonstrated in a number of studies, but the molecular events requisite for cascade activation and regulation have not been explored. We demonstrate that the adapter protein DRK (downstream of receptor kinase) which is essential for signaling to RAS in developmental contexts, is preferentially distributed in the adult mushroom bodies, centers for olfactory learning and memory. We demonstrate that drk mutant heterozygotes exhibit deficits in olfactory learning and memory, apparent under limited training conditions, but are not impaired in sensory responses requisite for the association of the stimuli, or brain neuroanatomy. Furthermore, we demonstrate that the protein is required acutely within mushroom body neurons to mediate efficient learning, a process that requires RAF activation. Importantly, 90 min memory remained impaired, even after differential training yielding equivalent learning in animals with compromised DRK levels and controls and did not require RAF. Sustained MAPK activation is compromised in drk mutants and surprisingly is negatively regulated by constitutive RAF activity. The data establish a role for DRK in Drosophila behavioral neuroplasticity and suggest a dual role for the protein, first in RAF activation-dependent learning and additionally in RAF-inhibition dependent sustained MAPK activation essential for memory formation or stability.

The mushroom body of adult Drosophila characterized by GAL4 drivers

The mushroom body is required for a variety of behaviors of Drosophila melanogaster. Different types of intrinsic and extrinsic mushroom body neurons might underlie its functional diversity. There have been many GAL4 driver lines identified that prominently label the mushroom body intrinsic neurons, which are known as "Kenyon cells." Under one constant experimental condition, we analyzed and compared the the expression patterns of 25 GAL4 drivers labeling the mushroom body. As an internet resource, we established a digital catalog indexing representative confocal data of them. Further more, we counted the number of GAL4-positive Kenyon cells in each line. We found that approximately 2,000 Kenyon cells can be genetically labeled in total. Three major Kenyon cell subtypes, the gamma, alpha'/beta', and alpha/beta neurons, respectively, contribute to 33, 18, and 49% of 2,000 Kenyon cells. Taken together, this study lays groundwork for functional dissection of the mushroom body.

Circadian modulation of short-term memory in Drosophila

Endogenous biological clocks are widespread regulators of behavior and physiology, allowing for a more efficient allocation of efforts and resources over the course of a day. The extent that different processes are regulated by circadian oscillators, however, is not fully understood. We investigated the role of the circadian clock on short-term associative memory formation using a negatively reinforced olfactory-learning paradigm in Drosophila melanogaster. We found that memory formation was regulated in a circadian manner. The peak performance in short-term memory (STM) occurred during the early subjective night with a twofold performance amplitude after a single pairing of conditioned and unconditioned stimuli. This rhythm in memory is eliminated in both timeless and period mutants and is absent during constant light conditions. Circadian gating of sensory perception does not appear to underlie the rhythm in short-term memory as evidenced by the nonrhythmic shock avoidance and olfactory avoidance behaviors. Moreover, central brain oscillators appear to be responsible for the modulation as cryptochrome mutants, in which the antennal circadian oscillators are nonfunctional, demonstrate robust circadian rhythms in short-term memory. Together these data suggest that central, rather than peripheral, circadian oscillators modulate the formation of short-term associative memory and not the perception of the stimuli.

Neuronal assemblies of the Drosophila mushroom body

The mushroom body (MB) of the insect brain has important roles in odor learning and memory and in diverse other brain functions. To elucidate the anatomical basis underlying its function, we studied how the MB of Drosophila is organized by its intrinsic and extrinsic neurons. We screened for the GAL4 enhancer-trap strains that label specific subsets of these neurons and identified seven subtypes of Kenyon cells and three other intrinsic neuron types. Laminar organization of the Kenyon cell axons divides the pedunculus into at least five concentric strata. The alpha', beta', alpha, and beta lobes are each divided into three strata, whereas the gamma lobe appears more homogeneous. The outermost stratum of the alpha/beta lobes is specifically connected with a small, protruded subregion of the calyx, the accessory calyx, which does not receive direct olfactory input. As for the MB extrinsic neurons (MBENs), we found three types of antennal lobe projection neurons, among which two are novel. In addition, we resolved 17 other types of MBENs that arborize in the calyx, lobes, and pedunculus. Lobe-associated MBENs arborize in only specific areas of the lobes, being restricted along their longitudinal axes, forming two to five segmented zones in each lobe. The laminar arrangement of the Kenyon cell axons and segmented organization of the MBENs together divide the lobes into smaller synaptic units, possibly facilitating characteristic interaction between intrinsic and extrinsic neurons in each unit for different functional activities along the longitudinal lobe axes and between lobes. Structural differences between lobes are also discussed.

Paternally and maternally transmitted GAL4 transcripts contribute to UAS transgene expression in early Drosophila embryos

The GAL4/UAS binary system with its recent modifications provides a powerful tool to study gene function in Drosophila enabling control over the timing, tissue specificity, and magnitude of gene expression. GAL4 expression during early embryonic stages has been well determined for certain driver lines, but for some of the commonly used in Drosophila research it is unknown, or partially determined. By monitoring the developmental kinetics of GAL4 expression and transgene transcription, we show that particular GAL4 drivers transiently direct ectopic expression of UAS-linked transgenes at early stages of embryogenesis in a GAL4- dependent manner via a mechanism that involves parental transmission of Gal4 transcripts.

Protection from premature habituation requires functional mushroom bodies in Drosophila

Diminished responses to stimuli defined as habituation can serve as a gating mechanism for repetitive environmental cues with little predictive value and importance. We demonstrate that wild-type animals diminish their responses to electric shock stimuli with properties characteristic of short- and long-term habituation. We used spatially restricted abrogation of neurotransmission to identify brain areas involved in this behavioral response. We find that the mushroom bodies and, in particular, the alpha/beta lobes appear to guard against habituating prematurely to repetitive electric shock stimuli. In addition to protection from premature habituation, the mushroom bodies are essential for spontaneous recovery and dishabituation. These results reveal a novel modulatory role of the mushroom bodies on responses to repetitive stimuli in agreement with and complementary to their established roles in olfactory learning and memory.