Understanding Taste Using <em>Drosophila melanogaster</em>

Taste is a short-range contact chemosensation required by all animals to detect nutrient rich foods and avoid consuming toxic chemicals. In insects, it is also required to select mates and appropriate oviposition sites. Organization of the fruit fly Drosophila melanogaster taste system and availability of experimental tool box, makes Drosophila gustatory system an ideal model system for studying the perception of taste and taste elicited behaviors. Like humans, fruit flies also respond to wide range of taste chemical and can differentiate between different taste categories including sweet, bitter, sour, umami and salt. This chapter will present a research progress made in the field of taste using neuroanatomical, genetic, behavioral, molecular and cellular biology techniques in the fruit fly. The compiled survey will provide an outlook of taste research done in fruit fly and its comparison with human taste behavior

Gut Feeding the Brain: <em>Drosophila</em> Gut an Animal Model for Medicine to Understand Mechanisms Mediating Food Preferences

Fruit fly, Drosophila melanogaster is a most powerful animal model for exploring fundamental biological processes and modeling molecular and cellular aspects of human diseases. It provides the flexibility and tool box with which scientists can experimentally manipulate and study behavior as well as gene expression in specific, defined population of cells in their normal tissue contexts. The utility and increasing value of a sophisticated genetic system of flies, the tool box available for studying physiological function, functional imaging, neural circuitry from gut to brain, taste receptors expression and controlling gene expression by determining the specific cells in the intestine, makes fly gut the most useful tissue for studying the regulation of feeding behavior under changing internal state. To understand the intestine and its connectivity with the brain, Drosophila has proved an ideal model organism for studying gut brain axis aspects of human metabolic diseases. Various markers and fly lines are available to characterize the expression of transgenes in the intestine. The newly generated genetic tools aim to streamline the design of experiments to target specific cells in intestine for genetic manipulations based on their type and location within physiologically specialized intestinal regions. This chapter will be useful for understanding post-ingestive sensing system that mediate food preferences and to investigate fundamental biological processes and model human diseases at the level of single cells in the fly gut. Furthermore, the utility of adult fly gut can be extended to the study of dietary and environmental factors relevant to health and disease by screening for cells and micro circuits stimulated by internal state or the consumption of various nutrients

Reduced odor responses from antennal neurons of G<SUB>q</SUB>&#945;, phospholipase C&#946;, and rdgA mutants in Drosophila support a role for a phospholipid intermediate in insect olfactory transduction

Mechanisms by which G-protein-coupled odorant receptors transduce information in insects still need elucidation. We show that mutations in the Drosophila gene for Gq&#945; (dgq) significantly reduce both the amplitude of the field potentials recorded from the whole antenna in responses to odorants as well as the frequency of evoked responses of individual sensory neurons. This requirement for Gq&#945; is for adult function and not during antennal development. Conversely, brief expression of a dominant-active form of Gq&#945; in adults leads to enhanced odor responses. To understand signaling downstream of Gq&#945; in olfactory sensory neurons, genetic interactions of dgq were tested with mutants in genes known to affect phospholipid signaling. dgq mutant phenotypes were further enhanced by mutants in a PLC&#946; (phospholipase C&#946;) gene, plc21C. Interestingly although, the olfactory phenotype of mutant alleles of diacylglycerol kinase (rdgA) was rescued by dgq mutant alleles. Our results suggest that Gq&#945;-mediated olfactory transduction in Drosophila requires a phospholipid second messenger the levels of which are regulated by a cycle of phosphatidylinositol 1,4-bisphosphate breakdown and regeneration

<em>Drosophila</em> Central Taste Circuits in Health and Obesity

When there is a perturbation in the balance between hunger and satiety, food intake gets mis-regulated leading to excessive or insufficient eating. In humans, abnormal nutrient consumption causes metabolic conditions like obesity, diabetes, and eating disorders affecting overall health. Despite this burden on society, we currently lack enough knowledge about the neuronal circuits that regulate appetite and taste perception. How specific taste neuronal circuits influence feeding behaviours is still an under explored area in neurobiology. The taste information present at the periphery must be processed by the central circuits for the final behavioural output. Identification and understanding of central neural circuitry regulating taste behaviour and its modulation by physiological changes with regard to internal state is required to understand the neural basis of taste preference. Simple invertebrate model organisms like Drosophila melanogaster can sense the same taste stimuli as mammals. Availability of powerful molecular and genetic tool kit and well characterized peripheral gustatory system with a vast array of behavioural, calcium imaging, molecular and electrophysiological approaches make Drosophila an attractive system to investigate and understand taste wiring and processing in the brain. By exploiting the gustatory system of the flies, this chapter will shed light on the current understanding of central neural taste structures that influence feeding choices. The compiled information would help us better understand how central taste neurons convey taste information to higher brain centers and guide feeding behaviours like acceptance or rejection of food to better combat disease state caused by abnormal consumption of food

Drosophila mutants in phospholipid signaling have reduced olfactory responses as adults and larvae

In this paper, we show that mutants in the gene stambhA (stmA), which encodes a putative phosphatidylinositol 4,5 bisphosphate-diacylglycerol lipase, exhibit a significant reduction in the amplitudes of odor-evoked responses recorded from the antennal surface of adult Drosophila. This lends support to previously published findings that olfactory transduction in Drosophila requires a phospholipid intermediate. Mutations in stmA also affect the olfactory behavior response of larvae. Moreover, there is a requirement for Gq&#945; and phospholipase C&#946; function in larval olfaction. The results suggest that larval olfactory transduction, like that of the adult, utilizes a phospholipid second messenger, generated by the activation of Gq&#945; and Plc&#946;21c, and modulated by the stmA gene product

Mutants in phospholipid signaling attenuate the behavioral response of adult Drosophila to trehalose

In Drosophila melanogaster, gustatory receptor genes (Grs) encode putative G-protein-coupled receptors (GPCRs) that are expressed in gustatory receptor neurons (GRNs). One of the Gr genes, Gr5a, encodes a receptor for trehalose that is expressed in a subset of GRNs. Although a role for the G protein, Gs&#945;, has been shown in Gr5a-expressing taste neurons, there is the residual responses to trehalose in Gs&#945; mutants which could suggest additional transduction mechanisms. Expression and genetic analysis of the heterotrimeric G-protein subunit, Gq, shown here suggest involvement of this G&#945; subunit in trehalose perception in Drosophila. A green fluorescent protein reporter of Gq expression is detected in gustatory neurons in the labellum, tarsal segments, and wing margins. Animals heterozygous for dgq mutations and RNA interference-mediated knockdown of dgq showed reduced responses to trehalose in the proboscis extension reflex assay and feeding behavior assay. These defects were rescued by targeted expression of the wild-type dgq&#945; transgene in the GRNs. These data together with observations from other mutants in phospholipid signaling provide insights into the mechanisms of taste transduction in Drosophila

Mutants in Drosophila TRPC channels reduce olfactory sensitivity to carbon dioxide.

BACKGROUND: Members of the canonical Transient Receptor Potential (TRPC) class of cationic channels function downstream of Gαq and PLCβ in Drosophila photoreceptors for transducing visual stimuli. Gαq has recently been implicated in olfactory sensing of carbon dioxide (CO(2)) and other odorants. Here we investigated the role of PLCβ and TRPC channels for sensing CO(2) in Drosophila. METHODOLOGY/PRINCIPAL FINDINGS: Through behavioral assays it was demonstrated that Drosophila mutants for plc21c, trp and trpl have a reduced sensitivity for CO(2). Immuno-histochemical staining for TRP, TRPL and TRPγ indicates that all three channels are expressed in Drosophila antennae including the sensory neurons that express CO(2) receptors. Electrophysiological recordings obtained from the antennae of protein null alleles of TRP (trp(343)) and TRPL (trpl(302)), showed that the sensory response to multiple concentrations of CO(2) was reduced. However, trpl(302); trp(343) double mutants still have a residual response to CO(2). Down-regulation of TRPC channels specifically in CO(2) sensing olfactory neurons reduced the response to CO(2) and this reduction was obtained even upon down-regulation of the TRPCs in adult olfactory sensory neurons. Thus the reduced response to CO(2) obtained from the antennae of TRPC RNAi strains is not due to a developmental defect. CONCLUSION: These observations show that reduction in TRPC channel function significantly reduces the sensitivity of the olfactory response to CO(2) concentrations of 5% or less in adult Drosophila. It is possible that the CO(2) receptors Gr63a and Gr21a activate the TRPC channels through Gαq and PLC21C

Odour receptors and neurons for DEET and new insect repellents.

There are major impediments to finding improved DEET alternatives because the receptors causing olfactory repellency are unknown, and new chemicals require exorbitant costs to determine safety for human use. Here we identify DEET-sensitive neurons in a pit-like structure in the Drosophila melanogaster antenna called the sacculus. They express a highly conserved receptor, Ir40a, and flies in which these neurons are silenced or Ir40a is knocked down lose avoidance to DEET. We used a computational structure-activity screen of &gt;400,000 compounds that identified &gt;100 natural compounds as candidate repellents. We tested several and found that most activate Ir40a(+) neurons and are repellents for Drosophila. These compounds are also strong repellents for mosquitoes. The candidates contain chemicals that do not dissolve plastic, are affordable and smell mildly like grapes, with three considered safe in human foods. Our findings pave the way to discover new generations of repellents that will help fight deadly insect-borne diseases worldwide

Expression of TRPC proteins in CO₂ sensing neurons located in the third antennal segment of adult <i>Drosophila</i>.

<p>TRP, TRPL and TRPγ are expressed in CO₂ responsive neurons in the adult <i>Drosophila</i> antenna. A) Frozen antennal sections (14 µm thick) from <i>Gr21aGAL4/UASH2bRFP</i> animals stained with anti-TRP, anti-TRPL and anti-TRPγ antibodies showing expression of TRP, TRPL and TRPγ respectively along the membranes of the Gr21a receptor neurons, marked by anti- RFP staining in red. The first panel shows the localization of Gr21a neurons in the antenna after staining with anti- RFP. B) Frozen antennal sections (14 µm thick) from the null mutants of <i>trpl</i> and <i>trp</i> stained with anti-TRPL and anti-TRP antibodies respectively. No expression of TRPL and TRP proteins could be observed in the respective mutant strains. mAb22C10 (anti-futch, microtubule protein) staining in red served as a neuronal marker.</p

Reduced sensitivity to CO₂ is not a developmental defect.

<p>A) RNAi lines grown at the restrictive temperature of 18°C (active GAL80) show normal electrophysiological responses to CO_2, since the CO₂ receptor neuron specific GAL4 remains inactive (absence of RNAi expression; <i>p></i>0.05). RNAi lines grown at the permissive temperature of 29°C (inactive GAL80) show reduced electrophysiological responses to CO₂ due to active GAL4 and RNAi expression (n = 10; <i>p<</i>0.0001). The RNAi heterozygotes in the absence of <i>Gr63aGAL4</i> show normal responses to CO₂ at 29°C. Error bars indicate SEM. B) Whole antennal mounts showing CO₂ sensory neurons marked using <i>UAS RedStinger</i> driven by <i>Gr21aGAL4</i> in wild type, <i>plc21C^P319/plc21C^P319</i> and <i>trpl³⁰²/trpl³⁰²</i> mutant lines. C) Quantification of CO₂ sensory neurons in the adult antennae of the same lines (n = 14; <i>p</i> value not statistically significant).</p