Calmodulin Affects Sensitization of Drosophila melanogaster Odorant Receptors

Stimulus duration encoding occurs early in the moth olfactory pathway

Pheromones convey rich ethological information and guide insects' search behavior. Insects navigating in turbulent environments are tasked with the challenge of coding the temporal structure of an odor plume, obliging recognition of the onset and offset of whiffs of odor. The coding mechanisms that shape odor offset recognition remain elusive. We designed a device to deliver sharp pheromone pulses and simultaneously measured the response dynamics from pheromone-tuned olfactory receptor neurons (ORNs) in male moths and Drosophila. We show that concentration-invariant stimulus duration encoding is implemented in moth ORNs by spike frequency adaptation at two time scales. A linear-nonlinear model fully captures the underlying neural computations and offers an insight into their biophysical mechanisms. Drosophila use pheromone cis-vaccenyl acetate (cVA) only for very short distance communication and are not faced with the need to encode the statistics of the cVA plume. Their cVA-sensitive ORNs are indeed unable to encode odor-off events. Expression of moth pheromone receptors in Drosophila cVA-sensitive ORNs indicates that stimulus-offset coding is receptor independent. In moth ORNs, stimulus-offset coding breaks down for short ( < 200 ms) whiffs. This physiological constraint matches the behavioral latency of switching from the upwind surge to crosswind cast flight upon losing contact with the pheromone.

Response Plasticity of Drosophila Olfactory Sensory Neurons

In insect olfaction, sensitization refers to the amplification of a weak olfactory signal when the stimulus is repeated within a specific time window. In the vinegar fly, Drosophila melanogaster, this occurs already at the periphery, at the level of olfactory sensory neurons (OSNs) located in the antenna. In our study, we investigate whether sensitization is a widespread property in a set of seven types of OSNs, as well as the mechanisms involved. First, we characterize and compare the differences in spontaneous activity, response velocity and response dynamics, among the selected OSN types. These express different receptors with distinct tuning properties and behavioral relevance. Second, we show that sensitization is not a general property. Among our selected OSN types, it occurs in those responding to more general food odors, while OSNs involved in very specific detection of highly specific ecological cues like pheromones and warning signals show no sensitization. Moreover, we show that mitochondria play an active role in sensitization by contributing to the increase in intracellular Ca2+ upon weak receptor activation. Thus, by using a combination of single sensillum recordings (SSRs), calcium imaging and pharmacology, we widen the understanding of how the olfactory signal is processed at the periphery.

A highly conserved plant volatile odorant receptor detects a sex pheromone component of the greater wax moth, Galleria mellonella (Lepidoptera: Pyralidae)

Odorant receptors (ORs) are key specialized units for mate and host finding in moths of the Ditrysia clade, to which 98% of the lepidopteran species belong. Moth ORs have evolved to respond to long unsaturated acetates, alcohols, or aldehydes (Type I sex pheromones), falling into conserved clades of pheromone receptors (PRs). These PRs might have evolved from old lineages of non-Ditrysian moths that use plant volatile-like pheromones. However, a Ditrysian moth called the greater wax moth, Galleria mellonella (a worldwide-distributed pest of beehives), uses C9-C11 saturated aldehydes as the main sex pheromone components (i.e., nonanal and undecanal). Thus, these aldehydes represent unusual components compared with the majority of moth species that use, for instance, Type I sex pheromones. Current evidence shows a lack of consensus in the amount of ORs for G. mellonella, although consistent in that the moth does not have conserved PRs. Using genomic data, 62 OR candidates were identified, 16 being new genes. Phylogeny showed no presence of ORs in conserved PR clades. However, an OR with the highest transcript abundance, GmelOR4, appeared in a conserved plant volatile-detecting clade. Functional findings from the HEK system showed the OR as sensitive to nonanal and 2-phenylacetaldehyde, but not to undecanal. It is believed that to date GmelOR4 represents the first, but likely not unique, OR with a stable function in detecting aldehydes that help maintain the life cycle of G. mellonella around honey bee colonies.

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

Neuron-glia interaction at the receptor level affects olfactory perception in adult Drosophila

Some types of glia play an active role in neuronal signaling by modifying their activity although little is known about their role in sensory information signaling at the receptor level. In this research, we report a functional role for the glia that surround the soma of the olfactory receptor neurons (OSNs) in adult Drosophila. Specific genetic modifications have been targeted to this cell type to obtain live individuals who are tested for olfactory preference and display changes both increasing and reducing sensitivity. A closer look at the antenna by Ca²⁺ imaging shows that odor activates the OSNs, which subsequently produce an opposite and smaller effect in the glia that partially counterbalances neuronal activation. Therefore, these glia may play a dual role in preventing excessive activation of the OSNs at high odorant concentrations and tuning the chemosensory window for the individual according to the network structure in the receptor organ.

Phytopathogens Increase the Preference of Insect Vectors to Volatiles Emitted by Healthy Host Plants

Phytopathogen infections not only affect the physiology of host plants but also the preference of insect vectors; these modifications may increase the spread of infection. For this, we determined the effects of "Candidatus Liberibacter asiaticus" (CLas) infection on the preference of an insect vector (Diaphorina citri) for its uninfected or CLas-infected host (Citrus sinensis) and found that the infected vector preferred uninfected citrus, while the uninfected vector preferred infected citrus. We identified two compounds, (Z)-3 hexenyl and methyl salicylate, that were differentially abundant in the volatiles emitted by infected and uninfected citrus and two odorant-binding protein (OBP) genes differentially expressed between infected and uninfected vectors. The results of receptor-ligand binding assays indicated that CLas upregulated OBP A10 expression in the infected vector to target (Z)-3 hexenyl acetate emitted by uninfected citrus and induced citrus to emit more methyl salicylate for binding to OBP2 in the uninfected vector. Our results might be useful for the effective control of CLas infections.

Calmodulin Activity Affects the Function of the Odorant Receptor AcerOr2 in Honeybees

Bees rely on their sensitive olfactory system to perform foraging activities in the surrounding environment. This ability is associated with the existence of olfactory receptors (ORs). In this study, we identified the AcerOr2 (ortholog to the Orco) protein in Apis cerana cerana, which contains a conserved, putative calmodulin (CaM)-binding site (CBS) indicating that CaM is involved in its function. We used immunofluorescence, Western blot, and Ca2 + imaging to monitor changes in the expression and activation of the signaling pathway associated with Ca2 + and Ca2 +/CaM-dependent protein kinase II (CaMKII) in Sf9 cells heterologously expressing AcerOr2 and a CaM-binding mutant. We used the synthetic Orco agonist VUAA1 to stimulate the cells or the antagonist W7 to inhibit CaM activity. The AcerOr2 CaM-binding mutant has a point mutation in the putative CBS (K331N). When heterologously expressed in Sf9 cells, the mutant should have less CaM activity. When the cells expressing AcerOr2 were treated with W7, the Ca2 + response of AceOr2 was similar to that of the mutant stimulated by VUAA1, and the expression of the CaM, CaMKII, and p-CaMKII has similar effects. Our results suggest that CaM activity affects the function of AceOr2 in vitro and can be used to further study the interaction between the AcerOr2 and calcium/CaM signaling pathway in the pollen collection behavior of bees.

Homeostasis of Mitochondrial Ca2+ Stores Is Critical for Signal Amplification in Drosophila melanogaster Olfactory Sensory Neurons

Insects detect volatile chemosignals with olfactory sensory neurons (OSNs) that express olfactory receptors. Among them, the most sensitive receptors are the odorant receptors (ORs), which form cation channels passing Ca2+. OSNs expressing different groups of ORs show varying optimal odor concentration ranges according to environmental needs. Certain types of OSNs, usually attuned to high odor concentrations, allow for the detection of even low signals through the process of sensitization. By increasing the sensitivity of OSNs upon repetitive subthreshold odor stimulation, Drosophila melanogaster can detect even faint and turbulent odor traces during flight. While the influx of extracellular Ca2+ has been previously shown to be a cue for sensitization, our study investigates the importance of intracellular Ca2+ management. Using an open antenna preparation that allows observation and pharmacological manipulation of OSNs, we performed Ca2+ imaging to determine the role of Ca2+ storage in mitochondria. By disturbing the mitochondrial resting potential and induction of the mitochondrial permeability transition pore (mPTP), we show that effective storage of Ca2+ in the mitochondria is vital for sensitization to occur, and release of Ca2+ from the mitochondria to the cytoplasm promptly abolishes sensitization. Our study shows the importance of cellular Ca2+ management for sensitization in an effort to better understand the underlying mechanics of OSN modulation.

Targeting Insect Olfaction in vivo and in vitro Using Functional Imaging

Insects decode volatile chemical signals from its surrounding environment with the help of its olfactory system, in a fast and reliable manner for its survival. In order to accomplish this task, odorant receptors (ORs) expressed in olfactory sensory neurons (OSNs) in the fly's antenna process such odor information. In order to study such a sophisticated process, we require access to the sensory neurons to perform functional imaging. In this article, we present different preparations to monitor odor information processing in Drosophila melanogaster OSNs using functional imaging of their Ca2+ dynamics. First, we established an in vivo preparation to image specific OSN population expressing the fluorescent Ca2+ reporter GCaMP3 during OR activation with airborne odors. Next, we developed a method to extract and to embed OSNs in a silica hydrogel with OR activation by dissolved odors. The odor response dynamics under these different conditions was qualitatively similar which indicates that the reduction of complexity did not affect the concentration dependence of odor responses at OSN level.

Functional Interaction Between Drosophila Olfactory Sensory Neurons and Their Support Cells

Insects detect volatile chemicals using antennae, which house a vast variety of olfactory sensory neurons (OSNs) that innervate hair-like structures called sensilla where odor detection takes place. In addition to OSNs, the antenna also hosts various support cell types. These include the triad of trichogen, tormogen, and thecogen support cells that lie adjacent to their respective OSNs. The arrangement of OSN supporting cells occurs stereotypically for all sensilla and is widely conserved in evolution. While insect chemosensory neurons have received considerable attention, little is known about the functional significance of the cells that support them. For instance, it remains unknown whether support cells play an active role in odor detection, or only passively contribute to homeostasis, e.g., by maintaining sensillum lymph composition. To investigate the functional interaction between OSNs and support cells, we used optical and electrophysiological approaches in Drosophila. First, we characterized the distribution of various supporting cells using genetic markers. By means of an ex vivo antennal preparation and genetically-encoded Ca²⁺ and K⁺ indicators, we then studied the activation of these auxiliary cells during odor presentation in adult flies. We observed acute responses and distinct differences in Ca²⁺ and K⁺ fluxes between support cell types. Finally, we observed alterations in OSN responses upon thecogen cell ablation in mature adults. Upon inducible ablation of thecogen cells, we notice a gain in mechanical responsiveness to mechanical stimulations during single-sensillum recording, but a lack of change to the neuronal resting activity. Taken together, these results demonstrate that support cells play a more active and responsive role during odor processing than previously thought. Our observations thus reveal that support cells functionally interact with OSNs and may be important for the extraordinary ability of insect olfactory systems to dynamically and sensitively discriminate between odors in the turbulent sensory landscape of insect flight.

Understanding responses to chemical mixtures: looking forward from the past

Our goal in this article is to provide a perspective on how to understand the nature of responses to chemical mixtures. In studying responses to mixtures, researchers often identify "mixture interactions"-responses to mixtures that are not accurately predicted from the responses to the mixture's individual components. Critical in these studies is how to predict responses to mixtures and thus to identify a mixture interaction. We explore this issue with a focus on olfaction and on the first level of neural processing-olfactory sensory neurons-although we use examples from taste systems as well and we consider responses beyond sensory neurons, including behavior and psychophysics. We provide a broadly comparative perspective that includes examples from vertebrates and invertebrates, from genetic and nongenetic animal models, and from literature old and new. In the end, we attempt to recommend how to approach these problems, including possible future research directions.

Transduction and Adaptation Mechanisms in the Cilium or Microvilli of Photoreceptors and Olfactory Receptors From Insects to Humans

Sensing changes in the environment is crucial for survival. Animals from invertebrates to vertebrates use both visual and olfactory stimuli to direct survival behaviors including identification of food sources, finding mates, and predator avoidance. In primary sensory neurons there are signal transduction mechanisms that convert chemical or light signals into an electrical response through ligand binding or photoactivation of a receptor, that can be propagated to the olfactory and visual centers of the brain to create a perception of the odor and visual landscapes surrounding us. The fundamental principles of olfactory and phototransduction pathways within vertebrates are somewhat analogous. Signal transduction in both systems takes place in the ciliary sub-compartments of the sensory cells and relies upon the activation of G protein-coupled receptors (GPCRs) to close cyclic nucleotide-gated (CNG) cation channels in photoreceptors to produce a hyperpolarization of the cell, or in olfactory sensory neurons open CNG channels to produce a depolarization. However, while invertebrate phototransduction also involves GPCRs, invertebrate photoreceptors can be either ciliary and/or microvillar with hyperpolarizing and depolarizing responses to light, respectively. Moreover, olfactory transduction in invertebrates may be a mixture of metabotropic G protein and ionotropic signaling pathways. This review will highlight differences of the visual and olfactory transduction mechanisms between vertebrates and invertebrates, focusing on the implications to the gain of the transduction processes, and how they are modulated to allow detection of small changes in odor concentration and light intensity over a wide range of background stimulus levels.

Calmodulin regulates the olfactory performance in Drosophila melanogaster

Insect odorant receptors (ORs) detect volatile chemical cues with high sensitivity. These ORs operate as ligand-gated ion channels and are formed by heptahelical OrX and Orco (co-receptor) proteins. A highly conserved calmodulin (CaM) binding site (CBS) ³³⁶SAIKYWVER³⁴⁴ within the second intracellular loop of Drosophila melanogaster Orco constitutes a target for regulating OR performance. Here we asked how a point mutation K339N in this CBS affects the olfactory performance of Drosophila melanogaster. We first asked how this mutation would affect the odor responses of olfactory sensory neurons (OSNs). Using Ca²⁺ imaging in an ex-vivo antenna preparation, we activated all OR (OrX/Orco) expressing neurons using the synthetic agonist VUAA1. In a next attempt, we restricted the OR spectrum to Or22a expressing neurons (Or22a/Orco) and stimulated these OSNs with the ligand ethyl hexanoate. In both approaches, we found that flies carrying the K339N point mutation in Orco display a reduced olfactory response. We also found that the mutation abolishes the capability of OSNs to sensitize by repeated weak odor stimuli. Next, we asked whether OrcoK339N might affect the odor localization performance. Using a wind tunnel bioassay, we found that odor localization in flies carrying the OrcoK339N mutation was severely diminished.

Odor response adaptation in Drosophila-a continuous individualization process

Olfactory perception is very individualized in humans and also in Drosophila. The process that individualize olfaction is adaptation that across multiple time scales and mechanisms shape perception and olfactory-guided behaviors. Olfactory adaptation occurs both in the central nervous system and in the periphery. Central adaptation occurs at the level of the circuits that process olfactory inputs from the periphery where it can integrate inputs from other senses, metabolic states, and stress. We will here focus on the periphery and how the fast, slow, and persistent (lifelong) adaptation mechanisms in the olfactory sensory neurons individualize the Drosophila olfactory system.

Functional properties of insect olfactory receptors: ionotropic receptors and odorant receptors

The majority of insect olfactory receptors belong to two distinct protein families, the ionotropic receptors (IRs), which are related to the ionotropic glutamate receptor family, and the odorant receptors (ORs), which evolved from the gustatory receptor family. Both receptor types assemble to heteromeric ligand-gated cation channels composed of odor-specific receptor proteins and co-receptor proteins. We here present in short the current view on evolution, function, and regulation of IRs and ORs. Special attention is given on how their functional properties can meet the environmental and ecological challenges an insect has to face.

Molecular mechanisms of olfactory detection in insects: beyond receptors

Insects thrive in diverse ecological niches in large part because of their highly sophisticated olfactory systems. Over the last two decades, a major focus in the study of insect olfaction has been on the role of olfactory receptors in mediating neuronal responses to environmental chemicals. In vivo, these receptors operate in specialized structures, called sensilla, which comprise neurons and non-neuronal support cells, extracellular lymph fluid and a precisely shaped cuticle. While sensilla are inherent to odour sensing in insects, we are only just beginning to understand their construction and function. Here, we review recent work that illuminates how odour-evoked neuronal activity is impacted by sensillar morphology, lymph fluid biochemistry, accessory signalling molecules in neurons and the physiological crosstalk between sensillar cells. These advances reveal multi-layered molecular and cellular mechanisms that determine the selectivity, sensitivity and dynamic modulation of odour-evoked responses in insects.

Transcriptional variation of sensory-related genes in natural populations of Aedes albopictus

BACKGROUND

The Asian tiger mosquito, Aedes albopictus, is a highly dangerous invasive vector of numerous medically important arboviruses including dengue, chikungunya and Zika. In four decades it has spread from tropical Southeast Asia to many parts of the world in both tropical and temperate climes. The rapid invasion process of this mosquito is supported by its high ecological and genetic plasticity across different life history traits. Our aim was to investigate whether wild populations, both native and adventive, also display transcriptional genetic variability for functions that may impact their biology, behaviour and ability to transmit arboviruses, such as sensory perception.

RESULTS

Antennal transcriptome data were derived from mosquitoes from a native population from Ban Rai, Thailand and from three adventive Mediterranean populations: Athens, Greece and Arco and Trento from Italy. Clear inter-population differential transcriptional activity was observed in different gene categories related to sound perception, olfaction and viral infection. The greatest differences were detected between the native Thai and the Mediterranean populations. The two Italian populations were the most similar. Nearly one million quality filtered SNP loci were identified.

CONCLUSION

The ability to express this great inter-population transcriptional variability highlights, at the functional level, the remarkable genetic flexibility of this mosquito species. We can hypothesize that the differential expression of genes, including those involved in sensory perception, in different populations may enable Ae. albopictus to exploit different environments and hosts, thus contributing to its status as a global vector of arboviruses of public health importance. The large number of SNP loci present in these transcripts represents a useful addition to the arsenal of high-resolution molecular markers and a resource that can be used to detect selective pressure and adaptive changes that may have occurred during the colonization process.

The role of mitochondria in shaping odor responses in Drosophila melanogaster olfactory sensory neurons

Insects detect volatile chemosignals with olfactory sensory neurons (OSNs) that express olfactory receptors. Among them, the most sensitive receptors are the odorant receptors (ORs), which form cation channels passing also Ca²⁺. Here, we investigate if and how odor-induced Ca²⁺ signals in Drosophila melanogaster OSNs are controlled by intracellular Ca²⁺ stores, especially by mitochondria. Using an open antenna preparation that allows observation and pharmacological manipulation of OSNs we performed Ca²⁺ imaging to determine the role of Ca²⁺ influx and efflux pathways in OSN mitochondria. The results indicate that mitochondria participate in shaping the OR responses. The major players of this modulation are the mitochondrial Ca²⁺ uniporter and the mitochondrial permeability transition pore. Intriguingly, OR-induced Ca²⁺ signals were only mildly affected by modulating the Ca²⁺ management of the endoplasmic reticulum.

Amplification of Drosophila Olfactory Responses by a DEG/ENaC Channel

Insect olfactory receptors operate as ligand-gated ion channels that directly transduce odor stimuli into electrical signals. However, in the absence of any known intermediate transduction steps, it remains unclear whether and how these ionotropic inputs are amplified in olfactory receptor neurons (ORNs). Here, we find that amplification occurs in the Drosophila courtship-promoting ORNs through Pickpocket 25 (PPK25), a member of the degenerin/epithelial sodium channel family (DEG/ENaC). Pharmacological and genetic manipulations indicate that, in Or47b and Ir84a ORNs, PPK25 mediates Ca²⁺-dependent signal amplification via an intracellular calmodulin-binding motif. Additionally, hormonal signaling upregulates PPK25 expression to determine the degree of amplification, with striking effects on male courtship. Together, these findings advance our understanding of sensory neurobiology by identifying an amplification mechanism compatible with ionotropic signaling. Moreover, this study offers new insights into DEG/ENaC activation by highlighting a novel means of regulation that is likely conserved across species.

Double-stranded RNA targeting calmodulin reveals a potential target for pest management of Nilaparvata lugens

BACKGROUND

Calmodulin (CaM) is an essential protein in cellular activity and plays important roles in many processes in insect development. RNA interference (RNAi) has been hypothesized to be a promising method for pest control. CaM is a good candidate for RNAi target. However, the sequence and function of CaM in Nilaparvata lugens are unknown. Furthermore, the double-stranded RNA (dsRNA) target to CaM gene in pest control is still unavailable.

RESULTS

In the present study, two alternatively spliced variants of CaM transcripts, designated NlCaM1 and NlCaM2, were cloned from N. lugens. The two cDNA sequences exhibited 100% identity to each other in the open reading frame (ORF), and only differed in the 3' untranslated region (UTR). NlCaM including NlCaM1 and NlCaM2 mRNA was detectable in all developmental stages and tissues of N. lugens, with significantly increased expression in the salivary glands. Knockdown of NlCaM expression by RNAi with different dsRNAs led to an inability to molt properly, increased mortality, which ranged from 49.7 to 92.5%, impacted development of the ovaries and led to female infertility. There were no significant reductions in the transcript levels of vitellogenin and its receptor or in the total vitellogenin protein level relative to the control group. However, a significant reduction in vitellogenin protein was detected in ovaries injected with dsNlCaM. In addition, a specific dsRNA of NlCaM for control of N. lugens was designed and tested.

CONCLUSION

NlCaM plays important roles mainly in nymph development and uptake of vitellogenin by ovaries in vitellogenesis in N. lugens. dsRNA derived from the less conserved 3'-UTR of NlCaM shows great potential for RNAi-based N. lugens management. © 2018 Society of Chemical Industry.

Tuning Insect Odorant Receptors

Among the insect olfactory receptors the odorant receptors (ORs) evolved in parallel to the onset of insect flight. A special property of this receptor type is the capability to adjust sensitivity of odor detection according to previous odor contacts. This article presents a current view on regulatory processes affecting the performance of ORs and proposes a model of mechanisms contributing to OR sensitization.

Odorant receptor might be related to sperm DNA integrity in Apis cerana cerana

Olfactory receptors (ORs) are important for insects to recognize and discriminate odorants in the environment and are mainly expressed in olfactory and gustatory organs. Little is known about the potential OR functions in non-olfactory tissues. In the present study, we evaluated the possibility of odorant receptors AcerOr1 and AcerOr2 (AcerOr2 is orthologous to the co-receptor) mediating sperm DNA integrity, and the relationship between sperm DNA integrity and semen parameters in Apis cerana cerana. Based on previous findings in mammals, we speculated that the Ca²⁺/calmodulin (CaM)/CaM-dependent protein kinase II (CaMKII) signaling pathway might be involved in the regulation of sperm motility in A. cerana cerana. The results showed that both AcerOr1 and AcerOr2 are expressed in the sperms and testis, that components associated with the putative Ca²⁺/CaM/CaMKII signaling pathway are present in A. cerana cerana sperms, and that at least CaM and CaMKII are localized in the sperms and testis. The AcerOr2 agonist VUAA1 significantly improved sperm motility parameters and apoptosis of sperm cells effect DNA integrity, whereas the CaM inhibitor W7 decreased sperm motility parameters and apoptosis of sperm cells, which affects DNA integrity. We also found a positive correlation between sperm DNA integrity and semen quality. These results indirectly as well as directly suggest that OR-mediated sperm responses and the Ca²⁺/CaM/CaMKII signaling pathway might affect semen quality and might be useful in regulating insect reproduction in future.

Mass Spectrometry-Based Screening Platform Reveals Orco Interactome in Drosophila melanogaster

Animals use their odorant receptors to receive chemical information from the environment. Insect odorant receptors differ from the G protein-coupled odorant receptors in vertebrates and nematodes, and very little is known about their protein-protein interactions. Here, we introduce a mass spectrometric platform designed for the large-scale analysis of insect odorant receptor protein-protein interactions. Using this platform, we obtained the first Orco interactome from Drosophila melanogaster. From a total of 1,186 identified proteins, we narrowed the interaction candidates to 226, of which only two-thirds have been named. These candidates include the known olfactory proteins Or92a and Obp51a. Around 90% of the proteins having published names likely function inside the cell, and nearly half of these intracellular proteins are associated with the endomembrane system. In a basic loss-of-function electrophysiological screen, we found that the disruption of eight (i.e., Rab5, CG32795, Mpcp, Tom70, Vir-1, CG30427, Eaat1, and CG2781) of 28 randomly selected candidates affects olfactory responses in vivo. Thus, because this Orco interactome includes physiologically meaningful candidates, we anticipate that our platform will help guide further research on the molecular mechanisms of the insect odorant receptor family.

Access to the odor world: olfactory receptors and their role for signal transduction in insects

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.

Short-term peripheral sensitization by brief exposure to pheromone components in Spodoptera littoralis

In insects, the olfactory system displays a high degree of plasticity. In Spodoptera littoralis, pre-exposure of males to the sex pheromone has been shown to increase the sensitivity of the olfactory sensory neurons at peripheral level. In this study, we have investigated this sensitization effect by recording the electroantennographic responses of male antennae to the major sex pheromone component (Z,E)-9,11-tetradecadienyl acetate and to the minor components (Z,E)-9,12-tetradecadienyl acetate and (Z)-9-tetradecenyl acetate. Responses to the conjugated diene acetate at 1 and 10 µg and to the unconjugated ester at 10 µg at three different times (11, 22 and 33 min) after pre-exposure (T = 0 min) were significantly higher than those at T = 0, whereas no increase of sensitivity to the pheromone was elicited by any dose of the minor monoene acetate. In addition, pre-exposed antennae to sub-threshold amounts (0.1, 1 and 10 ng) of the major pheromone component also induced an increased response to the chemical at different times (5 and 15 min) after exposure. Our results revealed that pre-exposed isolated antennae display a short-term higher sensitivity at the peripheral level when compared to naive antennae. In addition, we provide evidence of a peripheral sensitization mediated not only by the major pheromone component, but also by the minor unconjugated diene acetate, and the induction of this sensitivity appears to be dependent on the pre-exposure dose and the time span between pre-exposure and subsequent recordings. Possible implications of the sensitization effect displayed by the minor component for a more effective discrimination of the pheromone bouquets of other closely related species are highlighted.

Expression differences in Aphidius ervi (Hymenoptera: Braconidae) females reared on different aphid host species

The molecular mechanisms that allow generalist parasitoids to exploit many, often very distinct hosts are practically unknown. The wasp Aphidius ervi, a generalist koinobiont parasitoid of aphids, was introduced from Europe into Chile in the late 1970s to control agriculturally important aphid species. A recent study showed significant differences in host preference and host acceptance (infectivity) depending on the host A. ervi were reared on. In contrast, no genetic differentiation between A. ervi populations parasitizing different aphid species and aphids of the same species reared on different host plants was found in Chile. Additionally, the same study did not find any fitness effects in A. ervi if offspring were reared on a different host as their mothers. Here, we determined the effect of aphid host species (Sitobion avenae versus Acyrthosiphon pisum reared on two different host plants alfalfa and pea) on the transcriptome of adult A. ervi females. We found a large number of differentially expressed genes (between host species: head: 2,765; body: 1,216; within the same aphid host species reared on different host plants: alfalfa versus pea: head 593; body 222). As expected, the transcriptomes from parasitoids reared on the same host species (pea aphid) but originating from different host plants (pea versus alfalfa) were more similar to each other than the transcriptomes of parasitoids reared on a different aphid host and host plant (head: 648 and 1,524 transcripts; body: 566 and 428 transcripts). We found several differentially expressed odorant binding proteins and olfactory receptor proteins in particular, when we compared parasitoids from different host species. Additionally, we found differentially expressed genes involved in neuronal growth and development as well as signaling pathways. These results point towards a significant rewiring of the transcriptome of A. ervi depending on aphid-plant complex where parasitoids develop, even if different biotypes of a certain aphid host species (A. pisum) are reared on the same host plant. This difference seems to persist even after the different wasp populations were reared on the same aphid host in the laboratory for more than 50 generations. This indicates that either the imprinting process is very persistent or there is enough genetic/allelic variation between A. ervi populations. The role of distinct molecular mechanisms is discussed in terms of the formation of host fidelity.

Insect odorant receptor trafficking requires calmodulin

BACKGROUND

Like most animals, insects rely on their olfactory systems for finding food and mates and in avoiding noxious chemicals and predators. Most insect olfactory neurons express an odorant-specific odorant receptor (OR) along with Orco, the olfactory co-receptor. Orco binds ORs and permits their trafficking to the dendrites of antennal olfactory sensory neurons (OSNs), where together, they are suggested to form heteromeric ligand-gated non-selective cation channels. While most amino acid residues in Orco are well conserved across insect orders, one especially well-conserved region in Orco's second intracellular loop is a putative calmodulin (CaM) binding site (CBS). In this study, we explore the relationship between Orco and CaM in vivo in the olfactory neurons of Drosophila melanogaster.

RESULTS

We first found OSN-specific knock-down of CaM at the onset of OSN development disrupts the spontaneous firing of OSNs and reduces Orco trafficking to the ciliated dendrites of OSNs without affecting their morphology. We then generated a series of Orco CBS mutant proteins and found that none of them rescue the Orco-null Orco ¹ mutant phenotype, which is characterized by an OR protein trafficking defect that blocks spontaneous and odorant-evoked OSN activity. In contrast to an identically constructed wild-type form of Orco that does rescue the Orco ¹ phenotype, all the Orco CBS mutants remain stuck in the OSN soma, preventing even the smallest odorant-evoked response. Last, we found CaM's modulation of OR trafficking is dependent on activity. Knock-down of CaM in all Orco-positive OSNs after OR expression is well established has little effect on olfactory responsiveness alone. When combined with an extended exposure to odorant, however, this late-onset CaM knock-down significantly reduces both olfactory sensitivity and the trafficking of Orco only to the ciliated dendrites of OSNs that respond to the exposed odorant.

CONCLUSIONS

In this study, we show CaM regulates OR trafficking and olfactory responses in vivo in Drosophila olfactory neurons via a well-conserved binding site on the olfactory co-receptor Orco. As CaM's modulation of Orco seems to be dependent on activity, we propose a model in which the CaM/Orco interaction allows insect OSNs to maintain appropriate dendritic levels of OR regardless of environmental odorant concentrations.

Odor-induced cAMP production in Drosophila melanogaster olfactory sensory neurons

Insect odorant receptors are seven transmembrane domain proteins that form cation channels, whose functional properties such as receptor sensitivity are subject to regulation by intracellular signaling cascades. Here, we used the cAMP fluorescent indicator Epac1-camps to investigate the occurrence of odor-induced cAMP production in olfactory sensory neurons (OSNs) of Drosophila melanogaster We show that stimulation of the receptor complex with an odor mixture or with the synthetic agonist VUAA1 induces a cAMP response. Moreover, we show that while the intracellular Ca(2+) concentration influences cAMP production, the OSN-specific receptor OrX is necessary to elicit cAMP responses in Ca(2+)-free conditions. These results provide direct evidence of a relationship between odorant receptor stimulation and cAMP production in olfactory sensory neurons in the fruit fly antenna and show that this method can be used to further investigate the role that this second messenger plays in insect olfaction.