Olfactory physiology in the Drosophila maxillary palp requires the visual system gene rdgB

Emerging perspectives on multidomain phosphatidylinositol transfer proteins

The phosphatidylinositol transfer protein domain (PITPd) is an evolutionarily conserved protein that is able to transfer phosphatidylinositol between membranes in vitro and in vivo. However some animal genomes also include genes that encode proteins where the PITPd is found in cis with a number of additional domains and recent large scale genome sequencing efforts indicate that this type of multidomain architecture is widespread in the animal kingdom. In Drosophila photoreceptors, the multidomain phosphatidylinositol transfer protein RDGB is required to regulate phosphoinositide turnover during G-protein activated phospholipase C signalling. Recent studies in flies and mammalian cell culture models have begun to elucidate functions for the non-PITPd of RDGB and its vertebrate orthologs. We review emerging evidence on the genomics, functional and cell biological perspectives of these multi-domain PITPd containing proteins.

Measuring activity in olfactory receptor neurons in Drosophila: Focus on spike amplitude

Olfactory responses at the receptor level have been thoroughly described in Drosophila melanogaster by electrophysiological methods. Single sensilla recordings (SSRs) measure neuronal activity in intact individuals in response to odors. For sensilla that contain more than one olfactory receptor neuron (ORN), their different spontaneous spike amplitudes can distinguish each signal under resting conditions. However, activity is mainly described by spike frequency. Some reports on ORN response dynamics studied two components in the olfactory responses of ORNs: a fast component that is reflected by the spike frequency and a slow component that is observed in the LFP (local field potential, the single sensillum counterpart of the electroantennogram, EAG). However, no apparent correlation was found between the two elements. In this report, we show that odorant stimulation produces two different effects in the fast component, affecting spike frequency and spike amplitude. Spike amplitude clearly diminishes at the beginning of a response, but it recovers more slowly than spike frequency after stimulus cessation, suggesting that ORNs return to resting conditions long after they recover a normal spontaneous spike frequency. Moreover, spike amplitude recovery follows the same kinetics as the slow voltage component measured by the LFP, suggesting that both measures are connected. These results were obtained in ab2 and ab3 sensilla in response to two odors at different concentrations. Both spike amplitude and LFP kinetics depend on odorant, concentration and neuron, suggesting that like the EAG they may reflect olfactory information.

Sensing odorants and pheromones with chemosensory receptors

Olfaction is a critical sensory modality that allows living things to acquire chemical information from the external world. The olfactory system processes two major classes of stimuli: (a) general odorants, small molecules derived from food or the environment that signal the presence of food, fire, or predators, and (b) pheromones, molecules released from individuals of the same species that convey social or sexual cues. Chemosensory receptors are broadly classified, by the ligands that activate them, into odorant or pheromone receptors. Peripheral sensory neurons expressing either odorant or pheromone receptors send signals to separate odor- and pheromone-processing centers in the brain to elicit distinct behavioral and neuroendocrinological outputs. General odorants activate receptors in a combinatorial fashion, whereas pheromones activate narrowly tuned receptors that activate sexually dimorphic neural circuits in the brain. We review recent progress on chemosensory receptor structure, function, and circuitry in vertebrates and invertebrates from the point of view of the molecular biology and physiology of these sensory systems.

Controversy and consensus: noncanonical signaling mechanisms in the insect olfactory system

There is broad consensus that olfactory signaling in vertebrates and the nematode C. elegans uses canonical G-protein-coupled receptor transduction pathways. In contrast, mechanisms of insect olfactory signal transduction remain deeply controversial. Genetic disruption of G proteins and chemosensory ion channels in mice and worms leads to profound impairment in olfaction, while similar mutations in the fly show more subtle phenotypes. The literature contains contradictory claims that insect olfaction uses cAMP, cGMP, or IP3 as second messengers; that insect odorant receptors couple to G(alpha)s or G(alpha)q pathways; and that insect odorant receptors are G-protein-coupled receptors or odor-gated ion channels. Here we consider all the evidence and offer a consensus model for a noncanonical mechanism of olfactory signal transduction in insects.

A functional role for Anopheles gambiae Arrestin1 in olfactory signal transduction

Insect sensory arrestins act to desensitize visual and olfactory signal transduction pathways, as evidenced by the phenotypic effects of mutations in the genes encoding both Arr1 and Arr2 in Drosophila melanogaster. To assess whether such arrestins play similar roles in other, more medically relevant dipterans, we examined the ability of Anopheles gambiae sensory arrestin homologs AgArr1 and AgArr2 to rescue phenotypes associated with an olfactory deficit observed in D. melanogaster arrestin mutants. Of these, only AgArr1 facilitated significant phenotypic rescue of the corresponding Drosophila arr mutant olfactory phenotype, consistent with the view that functional orthology is shared by these Arr1 homologs. These results represent the first step in the functional characterization of AgArr1, which is highly expressed in olfactory appendages of An. gambiae in which it is likely to play an essential role in olfactory signal transduction. In addition to providing insight into the common elements of the peripheral olfactory system of dipterans, this work validates the importance of AgArr1 as a potential target for novel anti-malaria strategies that focus on olfactory-based behaviors in An. gambiae.

Genetic and molecular studies of olfaction in Drosophila

Drosophila melanogaster, an insect amenable to convenient molecular and genetic manipulation, has a highly sensitive olfactory system. A number of Drosophila olfactory mutants have been isolated and characterized. The smellblind mutant has a defect affecting a voltage-gated Na+ channel. The norpA mutant, defective in a phospholipase C, has a reduced response to odorants in one type of olfactory organ, providing genetic evidence for use of the inositol-1,4,5-trisphosphate signal transduction pathway in olfaction. Since the norpA gene is also required for phototransduction, this work demonstrates overlap in the molecular genetic basis of vision and olfaction. Interestingly, genetic analysis indicates that some olfactory information flows through a pathway which does not depend on norpA. Some mutants, such as ptg, acj6 and Sco, show odorant specificity, in the sense that some odorant responses are greatly reduced, whereas others are little affected, if at all. Some, but not all, mutations affect both larval and adult olfactory responses. Two tightly-linked Drosophila genes encode homologues of moth pheromone-binding proteins (PBPs). Genetic analysis may help determine whether PBPs facilitate transit of pheromones to or from olfactory receptor neurons. Information from Drosophila could be useful in designing means of controlling mosquitoes. It may also be possible to study olfactory genes, such as those encoding PBPs, from other insects by mutating them, introducing them into Drosophila and analysing their function in vivo.

Odor and pheromone detection in Drosophila melanogaster

Drosophila melanogaster has proven to be a useful model system to probe the mechanisms underlying the detection, discrimination, and perception of volatile odorants. The relatively small receptor repertoire of 62 odorant receptors makes the goal of understanding odor responses from the total receptor repertoire approachable in this system, and recent work has been directed toward this goal. In addition, new work not only sheds light but also raises more questions about the initial steps in odor perception in this system. Odorant receptor genes in Drosophila are predicted to encode seven transmembrane receptors, but surprising data suggest that these receptors may be inverted in the plasma membrane compared to classical G-protein coupled receptors. Finally, although some Drosophila odorant receptors are activated directly by odorant molecules, detection of a volatile pheromone, 11-cis vaccenyl acetate requires an extracellular adapter protein called LUSH for activation of pheromone sensitive neurons. Because pheromones are used by insects to trigger mating and other behaviors, these insights may herald new approaches to control behavior in pathogenic and agricultural pest insects.

The Inositol 1,4,5-triphosphate kinase1 gene affects olfactory reception in Drosophila melanogaster

The Inositol 1,4,5-triphosphate (IP3) route is one of the two main transduction cascades that mediate olfactory reception in Drosophila melanogaster. The activity of IP3 kinase1 reduces the levels of this substrate by phosphorylation into inositol 1,3,4,5-tetrakiphosphate (IP4). We show here that the gene is expressed in olfactory sensory organs as well as in the rest of the head. To evaluate in vivo the olfactory functional effects of up-regulating IP3K1, individuals with directed genetic changes at the reception level only were generated using the UAS/Gal4 method. In this report, we described the consequences in olfactory perception of overexpressing the IP3Kinase1 gene at eight different olfactory receptor-neuron subsets. Six out of the eight studied Gal-4/UAS-IP3K1 hybrids displayed abnormal behavioral responses to ethyl acetate, acetone, ethanol or propionaldehyde. Specific behavioral defects corresponded to the particular neuronal olfactory profile. These data confirm the role of the IP3kinase1 gene, and consequently the IP3 transduction cascade, in mediating olfactory information at the reception level.

Odorant-specific requirements for arrestin function in Drosophila olfaction

The ability to modulate olfactory sensitivity is necessary to detect chemical gradients and discriminate among a multitude of odor stimuli. Desensitization of odorant receptors has been postulated to occur when arrestins prevent the activation of downstream second messengers. A paucity of in vivo data on olfactory desensitization prompts use of Drosophila melanogaster genetics to investigate arrestins' role in regulating olfactory signaling pathways. Physiological analysis of peripheral olfactory sensitivity reveals decreased responsiveness to a host of chemically distinct odorants in flies deficient for arrestin1 (arr1), arrestin2 (arr2), or both. These phenotypes are manifest in odorant- and dose- dependent fashions. Additionally, mutants display altered adaptive properties under a prolonged exposure paradigm. Behaviorally, arr1 mutants are impaired in olfactory-based orientation towards attractive odor sources. As the olfactory deficits vary according to chemical identity and concentration, they indicate that a spectrum of arrestin activity is essential for odor processing depending upon the particular olfactory pathway involved. Arrestin mutant phenotypes are hypothesized to be a consequence of down-regulation of olfactory signaling to avoid cellular excitotoxicity. Importantly, phenotypic rescue of olfactory defects in arr1(1) mutants is achieved through transgenic expression of wild-type arr1. Taken together, these data clearly indicate that arrestins are required in a stimulus-specific manner for wild type olfactory function and add another level of complexity to peripheral odor coding mechanisms that ultimately impact olfactory behavior.

Visual arrestins in olfactory pathways of Drosophila and the malaria vector mosquito Anopheles gambiae

Arrestins are important components for desensitization of G protein-coupled receptor cascades that mediate neurotransmission as well as olfactory and visual sensory reception. We have isolated AgArr1, an arrestin-encoding cDNA from the malaria vector mosquito, Anopheles gambiae, where olfaction is critical for vectorial capacity. Analysis of AgArr1 expression revealed an overlap between chemosensory and photoreceptor neurons. Furthermore, an examination of previously identified arrestins from Drosophila melanogaster exposed similar bimodal expression, and Drosophila arrestin mutants demonstrate impaired electrophysiological responses to olfactory stimuli. Thus, we show that arrestins in Drosophila are required for normal olfactory physiology in addition to their previously described role in visual signaling. These findings suggest that individual arrestins function in both olfactory and visual pathways in Dipteran insects; these genes may prove useful in the design of control strategies that target olfactory-dependent behaviors of insect disease vectors.

Proteins of the ETS family with transcriptional repressor activity

ETS proteins form one of the largest families of signal-dependent transcriptional regulators, mediating cellular proliferation, differentiation and tumorigenesis. Most of the known ETS proteins have been shown to activate transcription. However, four ETS proteins (YAN, ERF, NET and TEL) can act as transcriptional repressors. In three cases (ERF, NET and TEL) distinct repression domains have been identified and there are indications that NET and TEL may mediate transcription via Histone Deacetylase recruitment. All four proteins appear to be regulated by MAPKs, though for YAN and ERF this regulation seems to be restricted to ERKs. YAN, ERF and TEL have been implicated in cellular proliferation although there are indications suggesting a possible involvement of YAN and TEL in differentiation as well. Other ETS-domain proteins have been shown to repress transcription in a context specific manner, and there are suggestions that the ETS DNA-binding domain may act as a transcriptional repressor. Transcriptional repression by ETS domain proteins adds an other level in the orchestrated regulation by this diverse family of transcription factors that often recognize similar if not identical binding sites on DNA and are believed to regulate critical genes in a variety of biological processes. Definitive assessment of the importance of this novel regulatory level will require the identification of ETS proteins target genes and the further analysis of transcriptional control and biological function of these proteins in defined pathways.

Olfaction in Drosophila

The fruit fly, Drosophila melanogaster, is equipped with a sophisticated olfactory sensory system that permits it to recognize and discriminate hundreds of discrete odorants. The perception of these odorants is essential for the animal to identify relevant food sources and suitable sites for egg-laying. Advances in the last year have begun to define the molecular basis of this insect's discriminatory power. The identification of a large multi-gene family of candidate Drosophila odorant receptors suggests that, as in other animals, a multitude of distinct odorants is recognized by a diversity of ligand-binding receptors. How olfactory signals are transduced and interpreted by the brain remains an important question for future analysis. The availability of genetic tools and a complete genome sequence makes Drosophila a particularly attractive organism for studying the molecular basis of olfaction.

The foraging gene affects adult but not larval olfactory-related behavior in Drosophila melanogaster

This study investigates the ability of larvae and adult rover and sitter Drosophila melanogaster to detect and migrate towards the source of a fly medium attractant using larval plate assays and an adult olfactory trap assay. Allelic variation at the foraging locus which encodes a cGMP-dependent protein kinase (PKG) did not affect larval olfactory response in the larval plate assays. In contrast, adult males of the sitter mutant for(s2) exhibited an olfactory trap response (OTR) which was significantly greater than that of males of the wild type forR strain from which for(s2) was derived and further genetic analysis showed that this was attributable to the for(s2) allele. The olfactory responses of fbrR and for(s2) flies to three odours (propionic acid, ethyl acetate and acetone) in a T-maze assay was normal indicating that they did not have general olfactory deficits. The finding that adult flies who differ in their PKG enzyme activities differ in foraging behaviours and olfactory trap responses to yeast odours suggests that PKG signalling pathways are involved in olfactory related responses to food.

Genetic dissection of sexual orientation: behavioral, cellular, and molecular approaches in Drosophila melanogaster

Insertional mutagenesis using P-element vectors yielded several independent mutations that cause male homosexuality in Drosophila melanogaster. Subsequent analyses revealed that all of these insertions were located at the same chromosomal division, 91B, where one of the inversion breakpoints responsible for the bisexual phenotype of the fruitless (fru) mutant has been mapped. In addition to the altered sexual orientation, the fru mutants displayed a range of defects in the formation of a male-specific muscle, the muscle of Lawrence. Since the male-specific formation of this muscle was dependent solely on the sex of the innervating nerve and not on the sex of the muscle itself, the primary site of action of the fru gene should be in the neural cells. satori, one of the P-insertion alleles of fru which we isolated, carried the lacZ gene of E. coli as a reporter, and beta-galactosidase expression was found in a subset of brain cells including those in the antennal lobe in the satori mutant. Targeted expression of a sex determination gene, transformer (tra), was used to produce chromosomally male flies with certain feminized glomeruli in the antennal lobe. Such sexually mosaic flies courted not only females but also males when the DM2, DA3 and DA4 glomeruli were feminized, indicating that these substructures in the antennal lobe may be involved in the determination of the sexual orientation of flies. Molecular cloning and analyses of the genomic and complementary DNAs indicated that transcription of the fru locus yields several different transcripts, one of which encodes a putative transcription regulator with a BTB domain and two zinc finger motifs. In the 5' non-coding region, three putative Transformer binding sites were identified. It appears plausible therefore that the fru gene is one of the elements in the sex determination cascade that controls sexual fates of certain neuronal cells. Improper sex determination in these neural cells may lead to altered sexual orientation and malformation of the male-specific muscle. Some implications of the results of our study on sexual orientation in other organisms will be discussed based on the Drosophila research.

Olfactory mechanisms in Drosophila melanogaster

Genetic approaches are beginning to provide valuable insights into the function of specific gene products in olfaction. Analysis of Drosophila mutants that affect olfactory responses are defining components of the olfactory signaling mechanisms. Mutations in the genes paralytic and Scutoid cause olfactory defects, as do mutations in genes encoding products that mediate visual responses. In addition, members of the family of invertebrate odorant-binding proteins have been identified in Drosophila and may play an important role in the olfactory process.

Olfaction in Drosophila: from odor to behavior

Odors elicit a variety of behavioral responses from Drosophila via a relatively simple, but sensitive, olfactory system. An increasing number of mutants have been found to be defective in olfactory function. Genetic and molecular analyses of the Drosophila olfactory system have identified some of its molecular components, and have revealed some principles of its function and organization.

Identification of a Drosophila G protein alpha subunit (dGq alpha-3) expressed in chemosensory cells and central neurons

We have identified another Drosophila GTP-binding protein (G protein) alpha subunit, dGq alpha-3. Transcripts encoding dGq alpha-3 are derived from alternative splicing of the dGq alpha locus previously shown to encode two visual-system-specific transcripts [Lee, Y.-J., Dobbs, M.B., Verardi, M.L. & Hyde, D.R. (1990) Neuron 5, 889-898]. Immunolocalization studies using dGq alpha-3 isoform-specific antibodies and LacZ fusion genes show that dGq alpha-3 is expressed in chemosensory cells of the olfactory and taste structures, including a subset of olfactory and gustatory neurons, and in cells of the central nervous system, including neurons in the lamina ganglionaris. These data are consistent with a variety of roles for dGq alpha-3, including mediating a subset of olfactory and gustatory responses in Drosophila, and supports the idea that some chemosensory responses use G protein-coupled receptors and the second messenger inositol 1,4,5-trisphosphate.

Requirement for a phospholipase C in odor response: overlap between olfaction and vision in Drosophila

A central problem in sensory system biology is the identification of the signal transduction pathways used in different sensory modalities. Genetic analysis of transduction mutants provides a means of studying in vivo the contributions of different pathways. This report shows that odorant response in one olfactory organ of Drosophila melanogaster depends on the norpA phospholipase C (EC 3.1.4.3) gene, providing evidence for use of the inositol 1,4,5-trisphosphate (IP3) signal transduction pathway. Since the norpA gene is also essential to phototransduction, this work demonstrates overlap in the genetic and molecular underpinnings of vision and olfaction. Genetic and molecular data also indicate that some olfactory information flows through a pathway which does not depend on norpA.