Mechanisms of Olfaction in Insects - ResearchSpace@Auckland ...

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General Introduction 18 1.5.2.2 Insect odorant receptor activation The novel receptor type in insects, OR83b is a 7TM domain protein that is found co- expressed with the OR in almost all ORNs. Even though OR diversity within and between insect species is high, this receptor type is highly conserved in insects such as D. melanogaster, B. mori, A. pernyi, H. virescens, Tenebrio molitor, Apis mellifera, Calliphora erythrocephala, P. xylostella, M. separate, D. indica, and E. postvittana (Vosshall et al., 1999; Hill et al., 2002; Krieger et al., 2002; Larsson et al., 2004; Melo et al., 2004; Mitsuno et al., 2008; Jordan et al., 2009). The OR/OR83b heteromeric complex is formed in the cell body and it is thought that the OR83b functions in transport of the bound OR to the dendritic membrane in the sensilla. The complex is maintained in the sensilla; suggesting a role of OR83b as a co-receptor in olfactory signalling. Supporting evidence for this could be provided by the high sequence identity of this receptor type across different insect species (64%–88%). Orthologs of OR83b in other insect species have a conserved co-expression function and may be an important factor in odorant detection by conventional ORs in insects (Larsson et al., 2004; Neuhaus et al., 2004; Nakagawa et al., 2005). The first insect ORs were identified in Drosophila through homology search of the draft Drosophila genome using novel computer programs that identify mammalian ORs called G protein-coupled receptor (GPCR) statistically by their physicochemical properties (Clyne et al., 1999; Vosshall et al., 1999). These algorithms were used to identify open reading frames of two or more transmembrane domain proteins. Both GPCRs and insect ORs have 7TM domains, however, insect ORs have been demonstrated to have an intracellular N-terminus, a topology unique to insect ORs, as shown in Figure 1.4 (Benton et al., 2006). Lundin et al. (2007) used glycosylation scanning to show the orientation and confirm the number of TM domains in Drosophila OR83b. Further evidence for insect OR topology came from epitope tagging of the termini and the predicted loop regions of Drosophila OR22a (Smart et al., 2008). This unique orientation of insect ORs suggests that these receptors would use distinct insect-specific signalling pathways.
General Introduction 19 Figure 1.4: Topologies of insect and mammalian odorant receptors in the cell membrane showing opposite orientation of the N- and C- termini in insect OR as opposed to mammalian OR, as depicted by Benton et al. (2006), Lundin et al. (2007) and Smart et al. (2008). Recent evidence suggests a novel role of OR83b as an ion channel, that when co- expressed with a non-OR83b OR, confers ligand sensitivity (response shown by light blue arrow on Figure 1.5). Sato et al. (2008) heterologously expressed a number of different insect receptors in HeLa cells, together with the co-receptor OR83b and measured calcium influx and whole-cell currents to analyse early onset of OR activation. They showed that the calcium influx response of the insect OR expressing cells to odorant stimulation is ten-fold faster than for mammalian ORs and no G proteins are involved in the signalling, therefore none were inhibited, supporting similar findings by Smart et al. (2008). The response was also independent of second messengers such a cGMP and cAMP. The ion selectivity which is a property of ion channels, was dependent on the OR subunit composition; evidence suggesting that ORs themselves act as ion channels rather than being associated with separate ion channels (Sato et al., 2008; Smart et al., 2008). A second hypothesis proposed by Wicher et al. (2008) suggests an ion channel– GPCR model (Figure 1.5). The ion channel model, similar to Sato et al. (2008) suggests OR83b forming a channel complex with conventional ORs when co- expressed in HEK293 cells to evoke odour responses upon ligand binding. The GPCR model shows that the binding of a ligand to its OR/OR83b complex leads to the
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Page 3 and 4: Abstract Abstract Olfaction or the
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The roles of Epiphyas postvittana G
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4.1 Introduction 4 Identification o
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Identification of putative odorant
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Microarray OR 454 Transcriptome OR
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5.1 Introduction 5 Concluding Discu
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Concluding Discussion 131 against t
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Concluding Discussion 133 sequencin
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Appendix B 135 Buffered TB (Sambroo
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Appendix C 137 C. Appendix C - Clus
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Appendix D 139 D. Appendix D - Solu
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Appendix E 141 I II
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Appendix E 143 V
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Appendix E 145 VII continued Figure
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References 147 Ansebo, L., Ignell,
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References 149 Butenandt, A., Beckm
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References 151 Fedurco, M., Romieu,
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References 153 Große-Wilde, E., St
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References 155 Hrdy, I., F. Kuldova
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References 157 Kaissling, K. E. (19
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References 159 Krieger, J., Große-
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References 161 Lopez-Gomez, R. and
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References 163 Miura, N., Nakagawa,
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References 165 Pell, J. K., Macaula
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References 167 Røstelien, T., Stra
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References 169 Stowers, L. and Loga
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References 171 Turner, C. T., Davy,
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References 173 Vogt, R. G., Riddifo
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References 175 Widmayer, P., Heifet
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