1
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Franco FP, Xu P, Harris BJ, Yarov-Yarovoy V, Leal WS. Single amino acid residue mediates reciprocal specificity in two mosquito odorant receptors. eLife 2022; 11:82922. [PMID: 36511779 PMCID: PMC9799979 DOI: 10.7554/elife.82922] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2022] [Accepted: 12/12/2022] [Indexed: 12/15/2022] Open
Abstract
The southern house mosquito, Culex quinquefasciatus, utilizes two odorant receptors, CquiOR10 and CquiOR2, narrowly tuned to oviposition attractants and well conserved among mosquito species. They detect skatole and indole, respectively, with reciprocal specificity. We swapped the transmembrane (TM) domains of CquiOR10 and CquiOR2 and identified TM2 as a specificity determinant. With additional mutations, we showed that CquiOR10A73L behaved like CquiOR2. Conversely, CquiOR2L74A recapitulated CquiOR10 specificity. Next, we generated structural models of CquiOR10 and CquiOR10A73L using RoseTTAFold and AlphaFold and docked skatole and indole using RosettaLigand. These modeling studies suggested space-filling constraints around A73. Consistent with this hypothesis, CquiOR10 mutants with a bulkier residue (Ile, Val) were insensitive to skatole and indole, whereas CquiOR10A73G retained the specificity to skatole and showed a more robust response than the wildtype receptor CquiOR10. On the other hand, Leu to Gly mutation of the indole receptor CquiOR2 reverted the specificity to skatole. Lastly, CquiOR10A73L, CquiOR2, and CquiOR2L74I were insensitive to 3-ethylindole, whereas CquiOR2L74A and CquiOR2L74G gained activity. Additionally, CquiOR10A73G gave more robust responses to 3-ethylindole than CquiOR10. Thus, we suggest the specificity of these receptors is mediated by a single amino acid substitution, leading to finely tuned volumetric space to accommodate specific oviposition attractants.
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Affiliation(s)
- Flavia P Franco
- Department of Molecular and Cellular Biology, University of California, DavisDavisUnited States
| | - Pingxi Xu
- Department of Molecular and Cellular Biology, University of California, DavisDavisUnited States
| | - Brandon J Harris
- Department of Physiology and Membrane Biology, University of California, DavisDavisUnited States
| | - Vladimir Yarov-Yarovoy
- Department of Physiology and Membrane Biology, University of California, DavisDavisUnited States,Department of Anesthesiology and Pain Medicine, University of California, DavisDavisUnited States
| | - Walter S Leal
- Department of Molecular and Cellular Biology, University of California, DavisDavisUnited States
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2
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Cao S, Liu Y, Wang B, Wang G. A single point mutation causes one-way alteration of pheromone receptor function in two Heliothis species. iScience 2021; 24:102981. [PMID: 34485863 PMCID: PMC8403742 DOI: 10.1016/j.isci.2021.102981] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2021] [Revised: 07/12/2021] [Accepted: 08/11/2021] [Indexed: 11/04/2022] Open
Abstract
The sex pheromone processing system of moths has been a major focus of research on olfaction and speciation, as it is highly specific and closely related to reproductive isolation. The two noctuid moths Heliothis virescens and Heliothis subflexa have been used as a model for deciphering the mechanisms underlying differentiation in pheromone communication, but no information exist regarding the functions of the pheromone receptors (PRs) of H. subflexa. Here, we functionally characterized all candidate PRs of H. subflexa, and found that only the response profile of OR6 differed between the two species. Through domain swapping and site-directed mutation followed by functional characterization, we identified a critical amino acid in OR6 caused a one-way alteration in specificity. This result suggests HsubOR6 evolved from an ancestral OR6 gene with a HvirOR6-like function and implies that the evolutionary direction of the receptor specificity was from the H. virescens-like pattern to H. subflexa-like pattern.
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Affiliation(s)
- Song Cao
- State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, China
- Shenzhen Branch, Guangdong Laboratory for Lingnan Modern Agriculture, Genome Analysis Laboratory of the Ministry of Agriculture and Rural Affairs, Agricultural Genomics Institute at Shenzhen, Chinese Academy of Agricultural Sciences, Shenzhen 518120, China
| | - Yang Liu
- State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, China
| | - Bing Wang
- State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, China
| | - Guirong Wang
- State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, China
- Shenzhen Branch, Guangdong Laboratory for Lingnan Modern Agriculture, Genome Analysis Laboratory of the Ministry of Agriculture and Rural Affairs, Agricultural Genomics Institute at Shenzhen, Chinese Academy of Agricultural Sciences, Shenzhen 518120, China
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3
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Fleischer J, Pregitzer P, Breer H, Krieger J. Access to the odor world: olfactory receptors and their role for signal transduction in insects. Cell Mol Life Sci 2018; 75:485-508. [PMID: 28828501 PMCID: PMC11105692 DOI: 10.1007/s00018-017-2627-5] [Citation(s) in RCA: 163] [Impact Index Per Article: 27.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2017] [Revised: 08/09/2017] [Accepted: 08/14/2017] [Indexed: 12/26/2022]
Abstract
The sense of smell enables insects to recognize and discriminate a broad range of volatile chemicals in their environment originating from prey, host plants and conspecifics. These olfactory cues are received by olfactory sensory neurons (OSNs) that relay information about food sources, oviposition sites and mates to the brain and thus elicit distinct odor-evoked behaviors. Research over the last decades has greatly advanced our knowledge concerning the molecular basis underlying the reception of odorous compounds and the mechanisms of signal transduction in OSNs. The emerging picture clearly indicates that OSNs of insects recognize odorants and pheromones by means of ligand-binding membrane proteins encoded by large and diverse families of receptor genes. In contrast, the mechanisms of the chemo-electrical transduction process are not fully understood; the present status suggests a contribution of ionotropic as well as metabotropic mechanisms. In this review, we will summarize current knowledge on the peripheral mechanisms of odor sensing in insects focusing on olfactory receptors and their specific role in the recognition and transduction of odorant and pheromone signals by OSNs.
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Affiliation(s)
- Joerg Fleischer
- Department of Animal Physiology, Institute of Biology/Zoology, Martin Luther University Halle-Wittenberg, 06120, Halle (Saale), Germany
| | - Pablo Pregitzer
- Institute of Physiology, University of Hohenheim, Stuttgart, Germany
| | - Heinz Breer
- Institute of Physiology, University of Hohenheim, Stuttgart, Germany
| | - Jürgen Krieger
- Department of Animal Physiology, Institute of Biology/Zoology, Martin Luther University Halle-Wittenberg, 06120, Halle (Saale), Germany.
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4
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Yang K, Huang LQ, Ning C, Wang CZ. Two single-point mutations shift the ligand selectivity of a pheromone receptor between two closely related moth species. eLife 2017; 6:29100. [PMID: 29063835 PMCID: PMC5673308 DOI: 10.7554/elife.29100] [Citation(s) in RCA: 44] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2017] [Accepted: 10/22/2017] [Indexed: 11/13/2022] Open
Abstract
Male moths possess highly sensitive and selective olfactory systems that detect sex pheromones produced by their females. Pheromone receptors (PRs) play a key role in this process. The PR HassOr14b is found to be tuned to (Z)-9-hexadecenal, the major sex-pheromone component, in Helicoverpa assulta. HassOr14b is co-localized with HassOr6 or HassOr16 in two olfactory sensory neurons within the same sensilla. As HarmOr14b, the ortholog of HassOr14b in the closely related species Helicoverpa armigera, is tuned to another chemical (Z)-9-tetradecenal, we study the amino acid residues that determine their ligand selectivity. Two amino acids located in the transmembrane domains F232I and T355I together determine the functional difference between the two orthologs. We conclude that species-specific changes in the tuning specificity of the PRs in the two Helicoverpa moth species could be achieved with just a few amino acid substitutions, which provides new insights into the evolution of closely related moth species.
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Affiliation(s)
- Ke Yang
- State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing, China.,College of Life Sciences, University of Chinese Academy of Sciences, Beijing, China
| | - Ling-Qiao Huang
- State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing, China
| | - Chao Ning
- State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing, China.,College of Life Sciences, University of Chinese Academy of Sciences, Beijing, China
| | - Chen-Zhu Wang
- State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing, China.,College of Life Sciences, University of Chinese Academy of Sciences, Beijing, China
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5
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Paoli M, Münch D, Haase A, Skoulakis E, Turin L, Galizia CG. Minute Impurities Contribute Significantly to Olfactory Receptor Ligand Studies: Tales from Testing the Vibration Theory. eNeuro 2017; 4:ENEURO.0070-17.2017. [PMID: 28670618 PMCID: PMC5490255 DOI: 10.1523/eneuro.0070-17.2017] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2017] [Revised: 05/18/2017] [Accepted: 05/19/2017] [Indexed: 11/27/2022] Open
Abstract
Several studies have attempted to test the vibrational hypothesis of odorant receptor activation in behavioral and physiological studies using deuterated compounds as odorants. The results have been mixed. Here, we attempted to test how deuterated compounds activate odorant receptors using calcium imaging of the fruit fly antennal lobe. We found specific activation of one area of the antennal lobe corresponding to inputs from a specific receptor. However, upon more detailed analysis, we discovered that an impurity of 0.0006% ethyl acetate in a chemical sample of benzaldehyde-d5 was entirely responsible for a sizable odorant-evoked response in Drosophila melanogaster olfactory receptor cells expressing dOr42b. Without gas chromatographic purification within the experimental setup, this impurity would have created a difference in the responses of deuterated and nondeuterated benzaldehyde, suggesting that dOr42b be a vibration sensitive receptor, which we show here not to be the case. Our results point to a broad problem in the literature on use of non-GC-pure compounds to test receptor selectivity, and we suggest how the limitations can be overcome in future studies.
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Affiliation(s)
- M. Paoli
- Neurobiology, University of Konstanz, Konstanz, 78457, Germany
- Department of Physics and Center for Mind/Brain Sciences, University of Trento, Povo, TN 38123, Italy
| | - D. Münch
- Neurobiology, University of Konstanz, Konstanz, 78457, Germany
| | - A. Haase
- Department of Physics and Center for Mind/Brain Sciences, University of Trento, Povo, TN 38123, Italy
| | - E. Skoulakis
- Division of Neuroscience, Biomedical Sciences Research Centre Alexander Fleming, Vari 16672, Greece
| | - L. Turin
- Division of Neuroscience, Biomedical Sciences Research Centre Alexander Fleming, Vari 16672, Greece
| | - C. G. Galizia
- Neurobiology, University of Konstanz, Konstanz, 78457, Germany
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6
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Carraher C, Dalziel J, Jordan MD, Christie DL, Newcomb RD, Kralicek AV. Towards an understanding of the structural basis for insect olfaction by odorant receptors. INSECT BIOCHEMISTRY AND MOLECULAR BIOLOGY 2015; 66:31-41. [PMID: 26416146 DOI: 10.1016/j.ibmb.2015.09.010] [Citation(s) in RCA: 43] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2015] [Revised: 09/16/2015] [Accepted: 09/16/2015] [Indexed: 06/05/2023]
Abstract
Insects have co-opted a unique family of seven transmembrane proteins for odour sensing. Odorant receptors are believed to have evolved from gustatory receptors somewhere at the base of the Hexapoda and have expanded substantially to become the dominant class of odour recognition elements within the Insecta. These odorant receptors comprise an obligate co-receptor, Orco, and one of a family of highly divergent odorant "tuning" receptors. The two subunits are thought to come together at some as-yet unknown stoichiometry to form a functional complex that is capable of both ionotropic and metabotropic signalling. While there are still no 3D structures for these proteins, site-directed mutagenesis, resonance energy transfer, and structural modelling efforts, all mainly on Drosophila odorant receptors, are beginning to inform hypotheses of their structures and how such complexes function in odour detection. Some of the loops, especially the second extracellular loop that has been suggested to form a lid over the binding pocket, and the extracellular regions of some transmembrane helices, especially the third and to a less extent the sixth and seventh, have been implicated in ligand recognition in tuning receptors. The possible interaction between Orco and tuning receptor subunits through the final intracellular loop and the adjacent transmembrane helices is thought to be important for transducing ligand binding into receptor activation. Potential phosphorylation sites and a calmodulin binding site in the second intracellular loop of Orco are also thought to be involved in regulating channel gating. A number of new methods have recently been developed to express and purify insect odorant receptor subunits in recombinant expression systems. These approaches are enabling high throughput screening of receptors for agonists and antagonists in cell-based formats, as well as producing protein for the application of biophysical methods to resolve the 3D structure of the subunits and their complexes.
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Affiliation(s)
- Colm Carraher
- The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand
| | - Julie Dalziel
- Food Nutrition & Health Team, Food & Bio-based Products Group, AgResearch Private Bag 11008, Palmerston North 4442, New Zealand
| | - Melissa D Jordan
- The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand
| | - David L Christie
- School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
| | - Richard D Newcomb
- The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand; School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand
| | - Andrew V Kralicek
- The New Zealand Institute for Plant & Food Research Limited, Private Bag 92169, Auckland 1142, New Zealand.
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7
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de Fouchier A, Sun X, Monsempes C, Mirabeau O, Jacquin-Joly E, Montagné N. Evolution of two receptors detecting the same pheromone compound in crop pest moths of the genus Spodoptera. Front Ecol Evol 2015. [DOI: 10.3389/fevo.2015.00095] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
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8
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Hughes DT, Wang G, Zwiebel LJ, Luetje CW. A determinant of odorant specificity is located at the extracellular loop 2-transmembrane domain 4 interface of an Anopheles gambiae odorant receptor subunit. Chem Senses 2014; 39:761-9. [PMID: 25270378 DOI: 10.1093/chemse/bju048] [Citation(s) in RCA: 36] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
To explore the structural basis for odorant specificity in odorant receptors of the human malaria vector mosquito, Anopheles gambiae, odorant-binding subunits (Agam\Ors) expressed in Xenopus oocytes in combination with Agam\Orco (coreceptor subunit) were assayed by 2-electrode voltage clamp against 25 structurally related odorants. Agam\Or13 and Agam\Or15 display 82% amino acid identity and had similar, but somewhat distinct odorant response profiles. The ratio of acetophenone to 4-methylphenol responses was used in a mutation-based analysis of Agam\Or15, interchanging 37 disparate residues between Agam\Or15 and Agam\Or13. Eleven mutations caused significant changes in odorant responsiveness. Mutation of alanine 195 resulted in the largest shift in response ratio from Agam\Or15 toward Agam\Or13. Concentration-response analysis for a series of mutations of residue 195 revealed a large effect on acetophenone sensitivity, with EC50 values varying by >1800-fold and correlating with residue side chain length. Similar results were obtained for propiophenone and benzaldehyde. But, for other odorants, such as 4-methylphenol, 4-methylbenzaldehyde, and 4-methylpropiophenone, the effect of mutation was much smaller (EC50 values varied by ≤16-fold). These results show that alanine 195, putatively located at the second extracellular loop/fourth transmembrane domain interface, plays a critical role in determining the odorant response specificity of Agam\Or15.
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Affiliation(s)
- David T Hughes
- Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, FL 33101, USA and Present address: University of Colorado School of Medicine, 13001 E. 17th Place, Aurora, CO 80045, USA
| | - Guirong Wang
- Department of Biological Sciences, Vanderbilt University, Nashville, TN 37232, USA Present address: State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100101, China
| | - Laurence J Zwiebel
- Department of Biological Sciences, Vanderbilt University, Nashville, TN 37232, USA
| | - Charles W Luetje
- Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, FL 33101, USA and
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9
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Boyle SM, McInally S, Ray A. Expanding the olfactory code by in silico decoding of odor-receptor chemical space. eLife 2013; 2:e01120. [PMID: 24137542 PMCID: PMC3787389 DOI: 10.7554/elife.01120] [Citation(s) in RCA: 46] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2013] [Accepted: 08/26/2013] [Indexed: 01/30/2023] Open
Abstract
Coding of information in the peripheral olfactory system depends on two fundamental factors: interaction of individual odors with subsets of the odorant receptor repertoire and mode of signaling that an individual receptor-odor interaction elicits, activation or inhibition. We develop a cheminformatics pipeline that predicts receptor–odorant interactions from a large collection of chemical structures (>240,000) for receptors that have been tested to a smaller panel of odorants (∼100). Using a computational approach, we first identify shared structural features from known ligands of individual receptors. We then use these features to screen in silico new candidate ligands from >240,000 potential volatiles for several Odorant receptors (Ors) in the Drosophila antenna. Functional experiments from 9 Ors support a high success rate (∼71%) for the screen, resulting in identification of numerous new activators and inhibitors. Such computational prediction of receptor–odor interactions has the potential to enable systems level analysis of olfactory receptor repertoires in organisms. DOI:http://dx.doi.org/10.7554/eLife.01120.001 Although our sense of smell is regarded as inferior to that of many other species, we can nevertheless distinguish between roughly 10,000 different odors. These are made up of molecules called odorants, each of which activates a specific subset of odorant receptors in the nose. However, much of what we know about this process has come from studying the fruit fly, Drosophila, which detects odors using receptors located mainly on its antennae. The number of potential odorants in nature is vast, and only a tiny fraction of the interactions between odorants and receptors can be physically tested. To address this challenge, Boyle et al. have used a computational approach to study in depth the interactions between a subset of 24 odorant receptors in Drosophila antennae and 109 odorants. After developing a method to identify structural features shared by the odorants that activate each receptor, Boyle et al. used this information to perform a computational (in silico) screen of more than 240,000 different odorant-like volatile compounds. For each receptor, they compiled a list of the 500 odorants predicted to interact most strongly with it. They then tested their predictions for a subset of the receptors by performing experiments in living flies, and found that roughly 71% of predicted compounds did indeed activate or inhibit their receptors, compared to only 10% of a control sample. In addition to providing new insights into the nature of the interactions between odorants and their receptors, the computational screen devised by Boyle et al. could aid the development of novel insect repellents, or compounds that mask the odors used by disease-causing insects to identify their hosts. It could also be used in the future to develop novel flavors and fragrances. DOI:http://dx.doi.org/10.7554/eLife.01120.002
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Affiliation(s)
- Sean Michael Boyle
- Genetics, Genomics, and Bioinformatics Program , University of California, Riverside , Riverside , United States
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10
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The role of the coreceptor Orco in insect olfactory transduction. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2013; 199:897-909. [PMID: 23824225 DOI: 10.1007/s00359-013-0837-3] [Citation(s) in RCA: 93] [Impact Index Per Article: 8.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/02/2013] [Revised: 06/19/2013] [Accepted: 06/21/2013] [Indexed: 10/26/2022]
Abstract
Insects sense odorants with specialized odorant receptors (ORs). Each antennal olfactory receptor neuron expresses one OR with an odorant binding site together with a conserved coreceptor called Orco which does not bind odorants. Orco is necessary for localization of ORs to dendritic membranes and, thus, is essential for odorant detection. It forms a spontaneously opening cation channel, activated via phosphorylation by protein kinase C. Thereafter, Orco is also activated via cyclic adenosine monophosphate (cAMP). Orco forms homo-as well as heteromers with ORs with unknown stoichiometry. Contradictory publications suggest different mechanisms of olfactory transduction. On the one hand, evidence accumulates for the employment of more than one G protein-coupled olfactory transduction cascade in different insects. On the other hand, results from other studies suggest that the OR-Orco complex functions as an odorant-gated cation channel mediating ionotropic signal transduction. This review analyzes conflicting hypotheses concerning the role of Orco in insect olfactory transduction. In conclusion, in situ studies in hawkmoths falsify the hypothesis that Orco underlies odorant-induced ionotropic signal transduction in all insect species. Instead, Orco forms a metabotropically gated, slow cation channel which controls odorant response threshold and kinetics of the sensory neuron.
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11
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Xu P, Leal WS. Probing insect odorant receptors with their cognate ligands: insights into structural features. Biochem Biophys Res Commun 2013; 435:477-82. [PMID: 23673297 DOI: 10.1016/j.bbrc.2013.05.015] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2013] [Accepted: 05/03/2013] [Indexed: 11/28/2022]
Abstract
Odorant receptors (ORs) are essential for insect survival in the environment and thus are ideal molecular targets for the design of insect-inspired modern green chemicals to control populations of agricultural pests and insects of medical importance. Although insect ORs are known for more than a decade, their structural biology is still in its infancy. Here, we unravel the first structural features of ORs from the malaria mosquito, the Southern house mosquito and the silkworm moth. The second extracellular loops (ECL-2s) of their predicted structures are much longer than ECL-1s and ECL-3s. The 27 amino-acid-residue-long of the ECL-2s in mosquito and the 43 amino-acid-residue-long ECL2s in moth ORs are well-conserved. About one-third of the residues are identical, including 3-4 Pro residues. Thorough examination of well-conserved residues in these structures, by point mutation and functional assay with the Xenopus oocyte recording system, strongly suggest that these "loops" include three β-turns and some degree of folding. In the Southern house mosquito three Pro residues in ECL-2 are essential for full activation of the receptor, which is finely tuned to the oviposition attractant 3-methylindole. Additionally, the "corner residues" of prolines, including Gly, Tyr, and Leu are functionally important thus suggesting that turns are stabilized not only by backbone hydrogen bonds, but also by side-chain interactions. Examination of ECL-2s from a distant taxonomical group suggests these ECL-2 loops might be functionally important in all insect ORs. Two of the four Pro residues in the predicted ECL-2 of the bombykol receptor in the silkworm moth, BmorOR1, are essential for function. Experimental evidence indicates that these loops may not be specificity determinants, but they may form a cover to the yet-to-be-identified membrane embedded binding cavities of insect ORs.
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Affiliation(s)
- Pingxi Xu
- Honorary Maeda-Duffey Laboratory, University of California, Davis, CA 95616, USA
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12
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13
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Abstract
The Drosophila larva possesses just 21 unique and identifiable pairs of olfactory sensory neurons (OSNs), enabling investigation of the contribution of individual OSN classes to the peripheral olfactory code. We combined electrophysiological and computational modeling to explore the nature of the peripheral olfactory code in situ. We recorded firing responses of 19/21 OSNs to a panel of 19 odors. This was achieved by creating larvae expressing just one functioning class of odorant receptor, and hence OSN. Odor response profiles of each OSN class were highly specific and unique. However many OSN-odor pairs yielded variable responses, some of which were statistically indistinguishable from background activity. We used these electrophysiological data, incorporating both responses and spontaneous firing activity, to develop a Bayesian decoding model of olfactory processing. The model was able to accurately predict odor identity from raw OSN responses; prediction accuracy ranged from 12%–77% (mean for all odors 45.2%) but was always significantly above chance (5.6%). However, there was no correlation between prediction accuracy for a given odor and the strength of responses of wild-type larvae to the same odor in a behavioral assay. We also used the model to predict the ability of the code to discriminate between pairs of odors. Some of these predictions were supported in a behavioral discrimination (masking) assay but others were not. We conclude that our model of the peripheral code represents basic features of odor detection and discrimination, yielding insights into the information available to higher processing structures in the brain.
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14
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Nichols AS, Chen S, Luetje CW. Subunit contributions to insect olfactory receptor function: channel block and odorant recognition. Chem Senses 2011; 36:781-90. [PMID: 21677030 DOI: 10.1093/chemse/bjr053] [Citation(s) in RCA: 65] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
Insect olfactory receptors are heteromeric ligand-gated ion channels composed of at least one common subunit (Orco) and at least one subunit that confers odorant specificity. Little is known about how individual subunits contribute to the structure and function of the olfactory receptor complex. We expressed insect olfactory receptors in Xenopus oocytes to investigate 2 functional features, ion channel block and odorant recognition. The sensitivity of Drosophila olfactory receptors to inhibition by ruthenium red, a cation channel blocker, varied widely when different specificity subunits were present, suggesting that the specificity subunits contribute to the structure of the ion pore. Olfactory receptors formed by Dmel\Or35a and Orco subunits from several different species displayed highly similar odorant response profiles, suggesting that the Orco subunit does not contribute to the structure of the odorant-binding site. We further explored odorant recognition by conducting a detailed examination of the odorant specificity Dmel\Or67a + Dmel\Orco, a receptor that responds to aromatic structures. This screen identified agonists, partial agonists, and an antagonist of Dmel\Or67a + Dmel\Orco. Our findings favor specific subunit arrangements within the olfactory receptor complex and provide a preliminary odorophore for an olfactory receptor, offering a useful foundation for future exploration of insect olfactory receptor structure.
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Affiliation(s)
- Andrew S Nichols
- Department of Molecular and Cellular Pharmacology, University of Miami Miller School of Medicine, FL 33101, USA
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15
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Franco MI, Turin L, Mershin A, Skoulakis EMC. Molecular vibration-sensing component in Drosophila melanogaster olfaction. Proc Natl Acad Sci U S A 2011; 108:3797-802. [PMID: 21321219 PMCID: PMC3048096 DOI: 10.1073/pnas.1012293108] [Citation(s) in RCA: 139] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
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.
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Affiliation(s)
- Maria Isabel Franco
- Institute of Cellular and Developmental Biology, Biomedical Sciences Research Centre “Alexander Fleming,” Vari 16672, Greece; and
| | - Luca Turin
- Institute of Cellular and Developmental Biology, Biomedical Sciences Research Centre “Alexander Fleming,” Vari 16672, Greece; and
- Center for Biomedical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
| | - Andreas Mershin
- Center for Biomedical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139
| | - Efthimios M. C. Skoulakis
- Institute of Cellular and Developmental Biology, Biomedical Sciences Research Centre “Alexander Fleming,” Vari 16672, Greece; and
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Evolving olfactory systems on the fly. Trends Genet 2010; 26:307-16. [PMID: 20537755 DOI: 10.1016/j.tig.2010.04.004] [Citation(s) in RCA: 79] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2010] [Revised: 04/20/2010] [Accepted: 04/22/2010] [Indexed: 12/20/2022]
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Functional and molecular evolution of olfactory neurons and receptors for aliphatic esters across the Drosophila genus. J Comp Physiol A Neuroethol Sens Neural Behav Physiol 2009; 196:97-109. [PMID: 20033746 DOI: 10.1007/s00359-009-0496-6] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2009] [Revised: 12/02/2009] [Accepted: 12/07/2009] [Indexed: 02/01/2023]
Abstract
Insect olfactory receptor (Or) genes are large, rapidly evolving gene families of considerable interest for evolutionary studies. They determine the responses of sensory neurons which mediate critical behaviours and ecological adaptations. We investigated the evolution across the genus Drosophila of a subfamily of Or genes largely responsible for the perception of ecologically relevant aliphatic esters; products of yeast fermentation and fruits. Odour responses were recorded from eight classes of olfactory receptor neurons known to express this Or subfamily in D. melanogaster and from homologous sensilla in seven other species. Despite the fact that these species have diverged over an estimated 40 million years, we find that odour specificity is largely maintained in seven of the eight species. In contrast, we observe extensive changes in most neurons of the outgroup species D. virilis, and in two neurons across the entire genus. Some neurons show small shifts in specificity, whilst some dramatic changes correlate with gene duplication or loss. An olfactory receptor neuron response similarity tree did not match an Or sequence similarity tree, but by aligning Or proteins of likely functional equivalence we identify residues that may be relevant for odour specificity. This will inform future structure-function studies of Drosophila Ors.
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