Ecdysteroid regulation of olfactory protein expression in the developing antenna of the tobacco hawk moth, Manduca sexta

An Expanded Survey of the Moth PBP/GOBP Clade in Bombyx mori: New Insight into Expression and Functional Roles

We studied the expression profile and ontogeny (from the egg stage through the larval stages and pupal stages, to the elderly adult age) of four OBPs from the silkworm moth Bombyx mori. We first showed that male responsiveness to female sex pheromone in the silkworm moth B. mori does not depend on age variation; whereas the expression of BmorPBP1, BmorPBP2, BmorGOBP1, and BmorGOBP2 varies with age. The expression profile analysis revealed that the studied OBPs are expressed in non-olfactory tissues at different developmental stages. In addition, we tested the effect of insecticide exposure on the expression of the four OBPs studied. Exposure to a toxic macrolide insecticide endectocide molecule (abamectin) led to the modulated expression of all four genes in different tissues. The higher expression of OBPs was detected in metabolic tissues, such as the thorax, gut, and fat body. All these data strongly suggest some alternative functions for these proteins other than olfaction. Finally, we carried out ligand docking studies and reported that PBP1 and GOBP2 have the capacity of binding vitamin K1 and multiple different vitamins.

Mating precedes selective immune priming which is maintained throughout bumblebee queen diapause

Background

Understanding the mechanisms by which organisms adapt to unfavourable conditions is a fundamental question in ecology and evolutionary biology. One such mechanism is diapause, a period of dormancy typically found in nematodes, fish, crustaceans and insects. This state is a key life-history event characterised by arrested development, suppressed metabolism and increased stress tolerance and allows an organism to avoid prolonged periods of harsh and inhospitable environmental conditions. For some species, diapause is preceded by mating which can have a profound effect on female behaviour, physiology and key biological processes, including immunity. However, our understanding of how mating impacts long-term immunity and whether these effects persist throughout diapause is currently limited. To address this, we explored molecular changes in the haemolymph of the ecologically important pollinator, the buff-tailed bumblebee Bombus terrestris. B. terrestris queens mate prior to entering diapause, a non-feeding period of arrested development that can last 6–9 months. Using mass-spectrometry-based proteomics, we quantified changes in the pre-diapause queen haemolymph after mating, as well as the subsequent protein expression of mated queens during and post-diapause.

Results

Our analysis identified distinct proteome profiles associated with diapause preparation, maintenance and termination. More specifically, mating pre-diapause was followed by an increase in the abundance of antimicrobial peptides, key effectors of the immune system. Furthermore, we identified the elevated abundance of these proteins to be maintained throughout diapause. This finding was in contrast to the general reduction observed in immune proteins during diapause suggestive of selective immune priming and expression during diapause. Diapause also affected the expression of proteins involved in cuticular maintenance, olfaction, as well as proteins of unknown function, which may have roles in diapause regulation.

Conclusions

Our results provide clear molecular evidence for the consequences and benefits of mating at the immune level as it precedes the selective increased abundance of antimicrobial peptides that are sustained throughout diapause. In addition, our results provide novel insights into the molecular mechanisms by which bumblebees prepare for, survive, and recover from diapause, insights that may have implications for our general understanding of these processes in other insect groups.

Characterization and Levels of Expression of Sensory Neuron Membrane Proteins in the Adult Citrus Fruit Fly (Diptera: Tephritidae)

Sensory neuron membrane proteins (SNMPs) play an important role in insect chemoreception; however, the SNMPs for Bactrocera minax (Enderlein) (Diptera: Tephritidae), an economically important pest of citrus, remain uncharacterized. Here, we report on the molecular characterization of SNMPs (BminSNMP1 and BminSNMP2) from adult B. minax. The open-reading frames of BminSNMP1 and BminSNMP2 were 1,608 and 1,647 nucleotides, encoding proteins of 535 and 557 amino acid residues, respectively. Phylogenetic analysis showed that the two BminSNMPs belonged to two distinct subgroups, indicating the possibility of their contrasting function in insect chemoreception. Real-time PCR results showed that BminSNMP1 was expressed primarily in the antennae of males and females, where levels of expression were similar at different developmental stages of females, but lower in 1- and 5-d-old males than in 15- and 20-d-old males. In both sexes, BminSNMP2 was expressed at high levels in antennae and in nonolfactory tissues, especially in legs, where levels were higher than in other nonolfactory tissues. We found highest levels of expression of BminSNMP2 in antennae of both sexes in 30-d-old adults, while in legs of both sexes, highest levels of expression were detected in 1- and 30-d-old adults. We discuss the possible physiological functions of BminSNMPs based on our findings.

Neofunctionalization of "Juvenile Hormone Esterase Duplication" in Drosophila as an odorant-degrading enzyme towards food odorants

Odorant degrading enzymes (ODEs) are thought to be responsible, at least in part, for olfactory signal termination in the chemosensory system by rapid degradation of odorants in the vicinity of the receptors. A carboxylesterase, specifically expressed in Drosophila antennae, called "juvenile hormone esterase duplication (JHEdup)" has been previously reported to hydrolyse different fruit esters in vitro. Here we functionally characterize JHEdup in vivo. We show that the jhedup gene is highly expressed in large basiconic sensilla that have been reported to detect several food esters. An electrophysiological analysis demonstrates that ab1A olfactory neurons of jhedup mutant flies exhibit an increased response to certain food acetates. Furthermore, mutant flies show a higher sensitivity towards the same odorants in behavioural assays. A phylogenetic analysis reveals that jhedup arose as a duplication of the juvenile hormone esterase gene during the evolution of Diptera, most likely in the ancestor of Schizophora, and has been conserved in all the 12 sequenced Drosophila species. Jhedup exhibits also an olfactory-predominant expression pattern in other Drosophila species. Our results support the implication of JHEdup in the degradation of food odorants in D. melanogaster and propose a neofunctionalization of this enzyme as a bona fide ODE in Drosophilids.

Molecular identification and differential expression of sensory neuron membrane proteins in the antennae of the black cutworm moth Agrotis ipsilon

The insect sensory neuron membrane proteins (SNMPs) SNMP1 and SNMP2 are transmembrane domain-containing proteins and are homologs of the vertebrate CD36 transmembrane proteins. It has been suggested that SNMPs play a significant role in insect chemoreception. Previous studies have demonstrated that SNMP1 is expressed in the pheromone-sensitive olfactory receptor neurons (ORNs), whereas SNMP2 is expressed in the supporting cells. In this study, we identified two full-length SNMP transcripts, AipsSNMP1 and AipsSNMP2, in the black cutworm moth Agrotis ipsilon (Hufnagel). The qRT-PCR results indicated that the AipsSNMP1 and AipsSNMP2 transcripts were expressed significantly higher in the antennae than in other tissues of both sexes. The expression of AipsSNMP1 and AipsSNMP2 in the antennae from different development stages of both sexes was investigated and was shown to begin to express in the pupae stage from 3days before emergence and then increased dramatically at the day of the emergence, and the high expression levels were maintained during the following 4days after the emergence in both sexes. The mating status had no effect on the expression levels of the AipsSNMP1 and AipsSNMP2 transcripts. Consistent with previous in situ hybridization studies in other Lepidoptera insects, our immunolocalization results at protein level demonstrated that both AipsSNMP1 and AipsSNMP2 were expressed in pheromone-sensitive sensilla trichodea but with a completely different expression profile. AipsSNMP1 is more uniformed and highly expressed along the membrane of the ORN dendrites, whereas AipsSNMP2 is widely distributed at the bottom of the sensilla trichodea and highly localized in the sensillum lymph. Our studies provide further detailed evidence for the involvement and general functional role of insect SNMPs in the detection of sex pheromones and general odorant molecules.

Peripheral regulation by ecdysteroids of olfactory responsiveness in male Egyptian cotton leaf worms, Spodoptera littoralis

Physiological and behavioral plasticity allows animals to adapt to changes in external (environmental) and internal (physiological) factors. In insects, the physiological state modulates adult behavior in response to different odorant stimuli. Hormones have the potential to play a major role in the plasticity of the olfactory responses. To explore if peripheral olfactory processing could be regulated by steroid hormones, we characterized the molecular, electrophysiological, and behavioral response to changes in endogenous hormone levels in adult male Spodoptera littoralis. The expression of the receptor complex (EcR/USP) was localized by in situ hybridization in the olfactory sensilla of antennae. Injections of 20-hydroxyecdysone (20E) induced an ecdysteroid signaling pathway in antennae and increased expression of the nuclear receptors EcR, USP and E75. Diacylglycerol kinase (DGK) and CaM expression were also up-regulated by 20E. Taken together, these molecular, electrophysiological, and behavioral results suggest a hormonal regulation of the peripheral olfactory processing in S. littoralis.

Candidate chemosensory ionotropic receptors in a Lepidoptera

A new family of candidate chemosensory ionotropic receptors (IRs) related to ionotropic glutamate receptors (iGluRs) was recently discovered in Drosophila melanogaster. Through Blast analyses of an expressed sequenced tag library prepared from male antennae of the noctuid moth Spodoptera littoralis, we identified 12 unigenes encoding proteins related to D. melanogaster and Bombyx mori IRs. Their full length sequences were obtained and the analyses of their expression patterns suggest that they were exclusively expressed or clearly enriched in chemosensory organs. The deduced protein sequences were more similar to B. mori and D. melanogaster IRs than to iGluRs and showed considerable variations in the predicted ligand-binding domains; none have the three glutamate-interacting residues found in iGluRs, suggesting different binding specificities. Our data suggest that we identified members of the insect IR chemosensory receptor family in S. littoralis and we report here the first demonstration of IR expression in Lepidoptera.

An Expressed Sequence Tag collection from the male antennae of the Noctuid moth Spodoptera littoralis: a resource for olfactory and pheromone detection research

BACKGROUND

Nocturnal insects such as moths are ideal models to study the molecular bases of olfaction that they use, among examples, for the detection of mating partners and host plants. Knowing how an odour generates a neuronal signal in insect antennae is crucial for understanding the physiological bases of olfaction, and also could lead to the identification of original targets for the development of olfactory-based control strategies against herbivorous moth pests. Here, we describe an Expressed Sequence Tag (EST) project to characterize the antennal transcriptome of the noctuid pest model, Spodoptera littoralis, and to identify candidate genes involved in odour/pheromone detection.

RESULTS

By targeting cDNAs from male antennae, we biased gene discovery towards genes potentially involved in male olfaction, including pheromone reception. A total of 20760 ESTs were obtained from a normalized library and were assembled in 9033 unigenes. 6530 were annotated based on BLAST analyses and gene prediction software identified 6738 ORFs. The unigenes were compared to the Bombyx mori proteome and to ESTs derived from Lepidoptera transcriptome projects. We identified a large number of candidate genes involved in odour and pheromone detection and turnover, including 31 candidate chemosensory receptor genes, but also genes potentially involved in olfactory modulation.

CONCLUSIONS

Our project has generated a large collection of antennal transcripts from a Lepidoptera. The normalization process, allowing enrichment in low abundant genes, proved to be particularly relevant to identify chemosensory receptors in a species for which no genomic data are available. Our results also suggest that olfactory modulation can take place at the level of the antennae itself. These EST resources will be invaluable for exploring the mechanisms of olfaction and pheromone detection in S. littoralis, and for ultimately identifying original targets to fight against moth herbivorous pests.

Identification and expression pattern of the chemosensory protein gene family in the silkworm, Bombyx mori

Insect chemosensory proteins (CSPs) as well as odorant-binding proteins (OBPs) have been supposed to transport hydrophobic chemicals to receptors on sensory neurons. Compared with OBPs, CSPs are expressed more broadly in various insect tissues. We performed a genome-wide analysis of the candidate CSP gene family in the silkworm. A total of 20 candidate CSPs, including 3 gene fragments and 2 pseudogenes, were characterized based on their conserved cysteine residues and their similarity to CSPs in other insects. Some of these genes were clustered in the silkworm genome. The gene expression pattern of these candidates was investigated using RT-PCR and microarray, and the results showed that these genes were expressed primarily in mature larvae and the adult moth, suggesting silkworm CSPs may be involved in development. The majority of silkworm CSP genes are expressed broadly in tissues including the antennae, head, thorax, legs, wings, epithelium, testes, ovaries, pheromone glands, wing disks, and compound eyes.

Sensory cell proliferation within the olfactory epithelium of developing adult Manduca sexta (Lepidoptera)

BACKGROUND

Insects detect a multitude of odors using a broad array of phenotypically distinct olfactory organs referred to as olfactory sensilla. Each sensillum contains one to several sensory neurons and at least three support cells; these cells arise from mitotic activities from one or a small group of defined precursor cells. Sensilla phenotypes are defined by distinct morphologies, and specificities to specific odors; these are the consequence of developmental programs expressed by associated neurons and support cells, and by selection and expression of subpopulations of olfactory genes encoding such proteins as odor receptors, odorant binding proteins, and odor degrading enzymes.

METHODOLOGY/PRINCIPAL FINDINGS

We are investigating development of the olfactory epithelium of adult M. sexta, identifying events which might establish sensilla phenotypes. In the present study, antennal tissue was examined during the first three days of an 18 day development, a period when sensory mitotic activity was previously reported to occur. Each antenna develops as a cylinder with an outward facing sensory epithelium divided into approximately 80 repeat units or annuli. Mitotic proliferation of sensory cells initiated about 20-24 hrs after pupation (a.p.), in pre-existing zones of high density cells lining the proximal and distal borders of each annulus. These high density zones were observed as early as two hr. a.p., and expanded with mitotic activity to fill the mid-annular regions by about 72 hrs a.p. Mitotic activity initiated at a low rate, increasing dramatically after 40-48 hrs a.p.; this activity was enhanced by ecdysteroids, but did not occur in animals entering pupal diapause (which is also ecdysteroid sensitive).

CONCLUSIONS/SIGNIFICANCE

Sensory proliferation initiates in narrow zones along the proximal and distal borders of each annulus; these zones rapidly expand to fill the mid-annular regions. These zones exist prior to any mitotic activity as regions of high density cells which form either at or prior to pupation. Mitotic sensitivity to ecdysteroids may be a regulatory mechanism coordinating olfactory development with the developmental choice of diapause entry.

Developmental expression patterns of four chemosensory protein genes from the Eastern spruce budworm, Chroistoneura fumiferana

Chemosensory proteins (CSPs) are associated with insect sensory organs, including the sensillum lymph in some cases. However, they are also commonly expressed in nonsensory tissues that lack gustatory and olfactory neurones. We characterized the sex and development specific expression patterns of four CSP genes from the Eastern spruce budworm (ESB) using Northern blots. CfumAY426540.2 was detected at high levels in adult moths. Conversely, CfumAY426538 was expressed in all stages except adult moths, and was most abundant during late stages of the 6th instar. CfumAY701858 was expressed in all stages, while CfumAY426539 was detected less frequently, at specific developmental stages such as the 5th to 6th instar moult. During a natural moult, and a premature moult induced by the ecdysteroid agonist tebufenozide, CfumAY701858 and CfumAY426539 were up-regulated, while CfumAY426538 appeared to be down-regulated. Our results suggest that some members of the CSP gene family from the ESB may be involved in development, including moulting.

A comparative study of odorant binding protein genes: differential expression of the PBP1-GOBP2 gene cluster inManduca sexta(Lepidoptera) and the organization of OBP genes inDrosophila melanogaster(Diptera)

Insects discriminate odors using sensory organs called olfactory sensilla, which display a wide range of phenotypes. Sensilla express ensembles of proteins, including odorant binding proteins (OBPs), olfactory receptors (ORs) and odor degrading enzymes (ODEs); odors are thought to be transported to ORs by OBPs and subsequently degraded by ODEs. These proteins belong to multigene families. The unique combinatorial expression of specific members of each of these gene families determines, in part, the phenotype of a sensillum and what odors it can detect. Furthermore, OBPs, ORs and ODEs are expressed in different cell types, suggesting the need for cell–cell communication to coordinate their expression. This report examines the OBP gene family. In Manduca sexta, the genes encoding PBP1Msex and GOBP2Msex are sequenced, shown to be adjacent to one another, and characterized together with OBP gene structures of other lepidoptera and Drosophila melanogaster. Expression of PBP1Msex, GOBP1Msex and GOBP2Msex is characterized in adult male and female antenna and in larval antenna and maxilla. The genomic organization of 25 D. melanogaster OBPs are characterized with respect to gene locus, gene cluster, amino acid sequence similarity, exon conservation and proximity to OR loci, and their sequences are compared with 14 M. sexta OBPs. Sensilla serve as portals of important behavioral information, and genes supporting sensilla function are presumably under significant evolutionary selective pressures. This study provides a basis for studying the evolution of the OBP gene family, the regulatory mechanisms governing the coordinated expression of OBPs, ORs and ODEs, and the processes that determine specific sensillum phenotypes.

Identity and expression pattern of chemosensory proteins in Heliothis virescens (Lepidoptera, Noctuidae)

Analyzing the chemosensory organs of the moth Heliothis virescens, three proteins belonging to the family of insect chemosensory proteins (CSPs) have been cloned; they are called HvirCSP1, HvirCSP2 and HvirCSP3. The HvirCSPs show about 50% identity between each other and 30-76% identity to CSPs from other species. Overall, they are rather hydrophilic proteins but include a conserved hydrophobic motif. Tissue distribution and temporal expression pattern during the last pupal stages were assessed by Northern blots. HvirCSP mRNAs were detected in various parts of the adult body with a particular high expression level in legs. The expression of HvirCSP1 in legs started early during adult development, in parallel with the appearance of the cuticle. HvirCSP1 mRNA was detectable five days before eclosion (day E-5), increased dramatically on day E-3 and remained at high level into adult life. The tissue distribution and the time course of appearance of HvirCSPs are in agreement with a possible role in contact chemosensation.

Antennal SNMPs (sensory neuron membrane proteins) of Lepidoptera define a unique family of invertebrate CD36-like proteins

SNMP1-Apol is an antennal-specific protein of the wild silk moth Antheraea polyphemus; the protein is abundantly expressed and localized to the receptor membranes of sex-pheromone specific olfactory sensory neurons (OSNs). SNMP1-Apol is thought to function in odor detection based on its olfactory-specific expression, localization within OSNs, developmental time of expression, and apparent homology to the CD36 family of membrane-bound receptor proteins. In the current study, SNMP1-Apol homologues were identified from the moths Bombyx mori, Heliothis virescens, and Manduca sexta. These species posses antennal mRNAs encoding proteins with amino acid sequence identities ranging from 75-80%; these proteins are collectively designated SNMP1. A second M. sexta SNMP homologue, previously identified and partially sequenced [Robertson et al.: Insect Mol Biol 8:501-518, 1999] was fully sequenced and characterized. The encoded protein shares only 26-27% sequence identity with the SNMP1 proteins, and is thus designated SNMP2-Msex. The SNMP sequences were used to identify 14 and four possible homologues in Drosophila melanogaster and Caenorhabditis elegans genome databases, respectively; thus, greatly expanding CD36 family membership among the invertebrate lineages. Despite their sequence difference, SNMP1-Msex and SNMP2-Msex expression is localized to OSNs and occurs simultaneously with the onset of olfactory function. These findings suggest that SNMPs play a central role in odor detection in insects, and that the CD36 gene family is widely represented among animal phyla. The SNMPs are the only identified neuronal members of the CD36 family, and as such expand the activities of this gene family into roles influencing brain function and behavioral action.

Central processing of sex pheromone stimuli is differentially regulated by juvenile hormone in a male moth

In the male moth, Agrotis ipsilon, the neuronal basis for juvenile hormone (JH)-linked modulation of sex pheromone responsiveness was investigated following stimulation of the antenna with i) an extract of female pheromone gland, ii) the synthetic pheromone blends from A. ipsilon and a closely related species, A. segetum, and iii) single components of the A. ipsilon blend. Response characteristics of olfactory interneurons were studied in the antennal lobe (AL) at different ages and with manipulated JH levels using intracellular recording techniques. Blend-specific, generalist and component-specific neurons were identified and described according to their response pattern. The proportion of low threshold AL interneurons increased significantly with age for all stimuli tested. Changes were, however, less pronounced for the minor single components. The proportion of low threshold AL interneurons in allatectomized (JH-deprived) mature males was significantly lower for all stimuli than in intact mature males. A large proportion of low threshold AL interneurons responding to the pheromone blends, but not as pronounced for single pheromone components, could be restored/induced by injecting JH either into JH-deprived mature males or into young immature males. The specificity for the species-specific blend compared to the A. segetum blend increased with age and JH injections.

Snmp-1, a novel membrane protein of olfactory neurons of the silk moth Antheraea polyphemus with homology to the CD36 family of membrane proteins

While olfactory neurons of silk moths are well known for their exquisite sensitivity to sex pheromone odorants, molecular mechanisms underlying this sensitivity are poorly understood. In searching for proteins that might support olfactory mechanisms, we characterized the protein profile of olfactory neuron receptor membranes of the wild silk moth Antheraea polyphemus. We have purified and cloned a prominent 67-kDa protein which we have named Snmp-1 (sensory neuron membrane protein-1). Northern blot analysis suggests that Snmp-1 is uniquely expressed in antennal tissue; in situ hybridization and immunocytochemical analyses show that Snmp-1 is expressed in olfactory neurons and that the protein is localized to the cilia, dendrites, and somata but not the axons. Snmp-1 mRNA expression increases significantly 1-2 days before the end of adult development, coincident with the functional maturation of the olfactory system. Sequence analysis suggests Snmp-1 is homologous with the CD36 protein family, a phylogenetically diverse family of receptor-like membrane proteins. CD36 family proteins are characterized as having two transmembrane domains and interacting with proteinaceous ligands; Snmp-1 is the first member of this family identified in nervous tissue. These findings argue that Snmp-1 has an important role in olfaction; possible roles of Snmp-1 in odorant detection are discussed.