2009 Abstracts - Association for Chemoreception Sciences

#41 Functional evolution of chemosensory receptorsRapid evolution of two odorant-binding protein genes,Obp57d and Obp57e, in DrosophilaTakashi MatsuoTokyo Metropolitan University Tokyo, JapanOdorant-binding proteins (OBPs) are secreted molecules found ininsect chemosensory organ, where they function together withodorant and taste receptors. Two OBP genes, Obp57d andObp57e, are involved in the evolution of unique host-plantpreference in Drosophila sechellia. D. sechellia exclusivelyreproduces on the ripe fruit of Morinda citrifolia (Tahitian Noni),which contains octanoic acid that is toxic to other Drosophilaspecies. Behavioral analysis of D. melanogaster knockout strainssuggested that Obp57d and Obp57e participate in the tasteperception of octanoic acid. Comparisons of genomic sequencesat the Obp57d/e locus from 27 Drosophila species revealed thatthe OBP gene number at this locus is different between species.Phylogenetic analysis suggested that Obp57d and Obp57e aroseby gene duplication at the early stage of the melanogaster speciesgroup evolution, followed by differentiation of the ORFsequences from each other. While most species in themelanogaster species group maintain both Obp57d and Obp57e,some species have lost either gene. Expression pattern of Obp57dand Obp57e is also different between species. The two OBPs areexpressed only in the gustatory sensilla on the legs in the speciesthat have both Obp57d and Obp57e, whereas the additionalexpression in the gustatory sensilla on the mouthparts wasobserved in the other species. Behavioral analysis of variousspecies revealed that the feeding preference for octanoic acidnegatively correlates with the OBP transcripts level in themouthparts. Gene duplication and the subsequent ORFdifferentiation, as well as changes in the expression pattern, couldbe important evolutionary mechanisms by which OBP genesdevelop their functional diversity, promoting the behavioralevolution among species.#42 Functional evolution of chemosensory receptorsBimodal Function of Drosophila Odorant ReceptorsDieter Wicher, Ronny Schäfer, Marcus C. Stensmyr,Bill S. HanssonMax Planck Institute for Chemical Ecology Jena, GermanyOdorant signals are detected by binding of odor molecules toodorant receptors (ORs) that belong to the G-protein-coupledreceptor (GPCR) family. The receptors couple to G-proteins,most of which stimulate cAMP production. This opens cyclicnucleotide-gated ion channels and depolarises the olfactorysensory neuron. Insect OR proteins lack any sequence similarityto other GPCRs and show an inverted orientation in the plasmamembrane. The ORs are dimers composed of an odor-specificOR protein (e.g., Drosophila Or22a) and a ubiquitously expressedchaperone protein (as Drosophila Or83b). As G-proteins areexpressed in insect OR neurons, and olfactory perception ismodified by mutations affecting the cAMP transduction pathwaythis study was aimed at finding out the function of insect ORs.For this sake we expressed Or22a and Or83b in HEK293 cells andperformed calcium imaging and patch clamp experiments.Application of odorants produced nonselective cation currentsactivated via both an ionotropic and a metabotropic pathway,and a subsequent increase in the intracellular Ca 2+ concentration.Expression of Or83b alone provided functional ion channelsinsensitive to odorants, but directly activated by intracellularcAMP or cGMP. Furthermore, application of only 1 nM cAMPto excised inside-out patches caused a half-maximal currentresponse. However, as current production developed slowly theactivation mechanism is different from the gating known fromionotropic receptors. Insect ORs thus form ligand-gated channelsas well as complexes of odorant sensing units and cyclicnucleotide-activated nonselective cation channels.#43 Making sense of fat tasteMaking Sense of Fat TasteTimothy A. GilbertsonUtah State University Logan, UT, USAThe importance of understanding the sensory cues for dietary fathas been underscored by the recent surge in dietary-inducedobesity. This epidemic of obesity has been attributed to theincreased intake of high fat, high carbohydrate, and caloricallydensediets. While the textural properties of fats have long beenaccepted to be its most salient sensory cue, emerging data from anumber of laboratories has begun to point to their being a “tasteof fat” as well. This symposium will provide a multidisciplinaryoverview of the evidence that supports the idea that the gustatorysystem is capable of responding to the chemical cues contained indietary fat and will look at the ability of free fatty acids to actboth as a taste primer and taste modulator. The implications of thepresence of a fat detection system will be discussed. Discussants:J.-P. Montmayeur; R. Contreras.#44 Making sense of fat tasteLigand specificities to putative fat receptor candidatesCD36 and GPR120 and licking behavior correspondingto the ligands in miceShigenobu Matsumura, Takeshi Yoneda, Ai Eguchi, YasukoManabe, Satoshi Tsuzuki, Kazuo Inoue, Tohru FushikiGraduate School of Agriculture, Kyoto University Kyoto, JapanFatty foods are palatable to animals including humans.Several studies have indicated that animals recognize thepresence of fat in foods not only by the texture of the food butalso chemically in the mouth: this suggests that the chemicalperception of fat is involved in the acquisition of a strong avidityfor fat. CD36 is known as a fatty acid transporter in the muscle oradipose tissue. Previously we have reported that CD36 isexpressed in the taste bud cells of the posterior tongue. CD36-deficient mice showed lower avidity for long-chain fatty acidenrichedsolutions. In addition to CD36, we have found thatGPR120, a G-protein coupled receptor that functions as a specificunsaturated long-chain fatty acid receptor in the gastrointestinaltract, is expressed in the taste bud cells of anterior and posteriortongue. A wide variety of molecules are known to be a ligand forCD36, including saturated or unsaturated fatty acid, somelipoproteins, cholesterol, thrombospondin and so on. However,HEK293 cells overexpressing GPR120 revealed that only longchainunsaturated fatty acids are potent ligand for GPR120. Inaddition, the palatability of fatty acids for mice assessed by theshort term licking behavior is very similar to the ligand specificityfor GPR120. These results raise the possibility that GPR120Abstracts | 17