Parallel processing of olfactory memories in Drosophila

One of the hallmarks of both memory and the underlying synaptic plasticity is that they each rely on short-lived and longer-lived forms. Short-lived memory is thought to rely on modification to existing proteins, whereas long-term memory requires induction of new gene expression. The most common view is that these two processes rely on signaling mechanisms within the same neurons. We recently demonstrated a dissection of the signaling requirements for short and long-lived memory into distinct sets of neurons. Using an aversive olfactory conditioning task in Drosophila, we found that cAMP signaling in different neuron cell types is sufficient to support short or long-term memory independently

Expression of pheromone binding proteins during antennal development in the gypsy moth Lymantria dispar

We have identified 2 olfactory specific proteins in the gypsy moth Lymantria dispar that are uniquely associated with the male antennae, the principal olfactory organs of this animal. These proteins were the major soluble protein components of the olfactory sensilla, present in equivalent amounts. Both proteins comigrated on SDS-PAGE, showing an apparent molecular mass of 15,000 Da but migrated separately on non-SDS-PAGE, indicating differences in net charge. N-terminal amino acid sequence analysis showed that the 2 proteins share 50% identity, indicating that they are genetically distinct homologs. Both proteins bound the L. dispar sexpheromone, associated with antisera prepared against the previously identified phermone-binding protein (PBP) of the moth Antheraea polyphemus, and shared sequence identity with the A. polyphemus PBP. These 2 proteins are therefore identified as L. dispar PBPs and are termed PBP1 and PBP2 based on their migration differences on non-SDS-PAGE. It is estimated that PBP1 and PBP2 are present in the sensilla lumen at a combined concentration of 13.4 mM. The expression of the L. dispar PBPs was examined during the 11 d development of the adult antenna. PBP1 and PBP2 were first detected by non-SDS-PAGE analysis and Coomassie blue staining 3 d before adult eclosion, on day A-3. Levels increased, reaching a plateau on day A-1 that continued into adult life. In vivo labeling studies indicated that the rate of PBP synthesis increased from A-3 to a plateau on A-2, where it remained into adult life. In vitro translations of antennal mRNAs indicated that translatable PBP mRNA was available at a very low level on day A-4, increased slightly on A-3 and dramatically on A-2, and remained at a high level into adult life. PBP mRNA represented the major translatable mRNA in the antenna during this period. It was estimated that the PBPs undergo a combined steady-state turnover of 8 x 10(7) molecules/hr/sensillum. Cursory in vivo and in vitro translation studies of antennal mRNA from A. polyphemus and Manduca sexta showed similar temporal patterns of PBP expression, suggesting that the L. dispar observations are general

MicroRNA-276a Functions in Ellipsoid Body and Mushroom Body Neurons for Naive and Conditioned Olfactory Avoidance in Drosophila

MicroRNA (miRNA)-mediated gene regulation plays a key role in brain development and function. But there are few cases in which the roles of individual miRNAs have been elucidated in behaving animals. We report a miR-276a::DopR regulatory module in Drosophila that functions in distinct circuits for naive odor responses and conditioned odor memory. Drosophila olfactory aversive memory involves convergence of the odors (conditioned stimulus) and the electric shock (unconditioned stimulus) in mushroom body (MB) neurons. Dopamine receptor DopR mediates the unconditioned stimulus inputs onto MB. Distinct dopaminergic neurons also innervate ellipsoid body (EB), where DopR function modulates arousal to external stimuli. We demonstrate that miR-276a is required in MB neurons for memory formation and in EB for naive responses to odors. Both roles of miR-276a are mediated by tuning DopR expression. The dual role of this miR-276a::DopR genetic module in these two neural circuits highlights the importance of miRNA-mediated gene regulation within distinct circuits underlying both naive behavioral responses and memory

The role of transposable elements in health and diseases of the central nervous system

First discovered in maize by Barbara McClintock in the 1940s, transposable elements (TEs) are DNA sequences that in some cases have the ability to move along chromosomes or "transpose" in the genome. This revolutionary finding was initially met with resistance by the scientific community and viewed by some as heretical. A large body of knowledge has accumulated over the last 60 years on the biology of TEs. Indeed, it is now known that TEs can generate genomic instability and reconfigure gene expression networks both in the germline and somatic cells. This review highlights recent findings on the role of TEs in health and diseases of the CNS, which were presented at the 2013 Society for Neuroscience meeting. The work of the speakers in this symposium shows that TEs are expressed and active in the brain, challenging the dogma that neuronal genomes are static and revealing that they are susceptible to somatic genomic alterations. These new findings on TE expression and function in the CNS have major implications for understanding the neuroplasticity of the brain, which could hypothetically have a role in shaping individual behavior and contribute to vulnerability to disease

Phenotypic Variation and Bistable Switching in Bacteria

Microbial research generally focuses on clonal populations. However, bacterial cells with identical genotypes frequently display different phenotypes under identical conditions. This microbial cell individuality is receiving increasing attention in the literature because of its impact on cellular differentiation, survival under selective conditions, and the interaction of pathogens with their hosts. It is becoming clear that stochasticity in gene expression in conjunction with the architecture of the gene network that underlies the cellular processes can generate phenotypic variation. An important regulatory mechanism is the so-called positive feedback, in which a system reinforces its own response, for instance by stimulating the production of an activator. Bistability is an interesting and relevant phenomenon, in which two distinct subpopulations of cells showing discrete levels of gene expression coexist in a single culture. In this chapter, we address techniques and approaches used to establish phenotypic variation, and relate three well-characterized examples of bistability to the molecular mechanisms that govern these processes, with a focus on positive feedback.

Techniques for temporal detection of neural sensitivity to external stimulation

We propose a simple measure of neural sensitivity for characterizing stimulus coding. Sensitivity is defined as the fraction of neurons that show positive responses to n stimuli out of a total of N. To determine a positive response, we propose two methods: Fisherian statistical testing and a data-driven Bayesian approach to determine the response probability of a neuron. The latter is non-parametric, data-driven, and captures a lower bound for the probability of neural responses to sensory stimulation. Both methods are compared with a standard test that assumes normal probability distributions. We applied the sensitivity estimation based on the proposed method to experimental data recorded from the mushroom body (MB) of locusts. We show that there is a broad range of sensitivity that the MB response sweeps during odor stimulation. The neurons are initially tuned to specific odors, but tend to demonstrate a generalist behavior towards the end of the stimulus period, meaning that the emphasis shifts from discrimination to feature learning

Specific requirement of NMDA receptors for long-term memory consolidation in Drosophila ellipsoid body

In humans and many other animals, memory consolidation occurs through multiple temporal phases and usually involves more than one neuroanatomical brain system. Genetic dissection of Pavlovian olfactory learning in Drosophila melanogaster has revealed multiple memory phases, but the predominant view holds that all memory phases occur in mushroom body neurons. Here, we demonstrate an acute requirement for NMDA receptors (NMDARs) outside of the mushroom body during long-term memory (LTM) consolidation. Targeted dsRNA-mediated silencing of Nmdar1 and Nmdar2 (also known as dNR1 or dNR2, respectively) in cholinergic R4m-subtype large-field neurons of the ellipsoid body specifically disrupted LTM consolidation, but not retrieval. Similar silencing of functional NMDARs in the mushroom body disrupted an earlier memory phase, leaving LTM intact. Our results clearly establish an anatomical site outside of the mushroom body involved with LTM consolidation, thus revealing both a distributed brain system subserving olfactory memory formation and the existence of a system-level memory consolidation in Drosophila

Expression and regulation of caudal in the lower cyclorrhaphan fly Megaselia

The homeobox gene caudal (cad) regulates posterior development in Drosophila. In early embryos, the cad protein (CAD) is expressed in a posterior-to-anterior concentration gradient, which contributes polarity to the developing embryo. The CAD gradient is complementary to and dependent on the anterior pattern organizer Bicoid (BCD), which represses the translation of ubiquitous maternal cad transcripts in the anterior embryo through a direct interaction with the cad 3′ untranslated region (UTR). Here, we show that early embryos of the lower cyclorrhaphan fly Megaselia express the putative cad orthologue Mab-cad throughout the posterior three quarters of the blastoderm but lack maternal transcripts. In transgenic blastoderm embryos of Drosophila, Mab-cad cis-regulatory DNA drives the expression of a reporter gene in a similar pattern, while Mab-cad 3′ UTR fails to mediate translational repression of a ubiquitously transcribed reporter. For another lower cyclorrhaphan fly (Lonchoptera) and two related outgroup taxa of Cyclorrhapha (Empis, Haematopota), we report maternal cad expression in ovarian follicles. Together, our results suggest that BCD is not required for the translational repression of Mab-cad, and that maternal cad expression was lost in the Megaselia lineage

Monomeric Bistability and the Role of Autoloops in Gene Regulation

Genetic toggle switches are widespread in gene regulatory networks (GRN). Bistability, namely the ability to choose among two different stable states, is an essential feature of switching and memory devices. Cells have many regulatory circuits able to provide bistability that endow a cell with efficient and reliable switching between different physiological modes of operation. It is often assumed that negative feedbacks with cooperative binding (i.e. the formation of dimers or multimers) are a prerequisite for bistability. Here we analyze the relation between bistability in GRN under monomeric regulation and the role of autoloops under a deterministic setting. Using a simple geometric argument, we show analytically that bistability can also emerge without multimeric regulation, provided that at least one regulatory autoloop is present