dgq: a drosophila gene encoding a visual system-specific G alpha molecule

The Ntan1 gene is expressed in perineural glia and neurons of adult Drosophila

The Drosophila Ntan1 gene encodes an N-terminal asparagine amidohydrolase that we show is highly conserved throughout evolution. Protein isoforms share more than 72% of similarity with their human counterparts. At the cellular level, this gene regulates the type of glial cell growth in Drosophila larvae by its different expression levels. The Drosophila Ntan1 gene has 4 transcripts that encode 2 protein isoforms. Here we describe that although this gene is expressed at all developmental stages and adult organs tested (eye, antennae and brain) there are some transcript-dependent specificities. Therefore, both quantitative and qualitative cues could account for gene function. However, widespread developmental stage and organ-dependent expression could be masking cell-specific constraints that can be explored in Drosophila by using Gal4 drivers. We report a new genetic driver within this gene, Mz317-Gal4, that recapitulates the Ntan1 gene expression pattern in adults. It shows specific expression for perineural glia in the olfactory organs but mixed expression with some neurons in the adult brain. Memory and social behavior disturbances in mice and cancer and schizophrenia in humans have been linked to the Ntan1 gene. Therefore, these new tools in Drosophila may contribute to our understanding of the cellular basis of these alterations.

Gαq splice variants mediate phototransduction, rhodopsin synthesis, and retinal integrity in Drosophila

Heterotrimeric G proteins mediate a variety of signaling processes by coupling G protein-coupled receptors to intracellular effector molecules. In Drosophila, the Gαq gene encodes several Gαq splice variants, with the Gαq1 isoform protein playing a major role in fly phototransduction. However, Gαq1 null mutant flies still exhibit a residual light response, indicating that other Gαq splice variants or additional Gq α subunits are involved in phototransduction. Here, we isolated a mutant fly with no detectable light responses, decreased rhodopsin (Rh) levels, and rapid retinal degeneration. Using electrophysiological and genetic studies, biochemical assays, immunoblotting, real-time RT-PCR, and EM analysis, we found that mutations in the Gαq gene disrupt light responses and demonstrate that the Gαq3 isoform protein is responsible for the residual light response in Gαq1 null mutants. Moreover, we report that Gαq3 mediates rhodopsin synthesis. Depletion of all Gαq splice variants led to rapid light-dependent retinal degeneration, due to the formation stable Rh1-arrestin 2 (Arr2) complexes. Our findings clarify essential roles of several different Gαq splice variants in phototransduction and retinal integrity in Drosophila and reveal that Gαq3 functions in rhodopsin synthesis.

Calmodulin Enhances Cryptochrome Binding to INAD in Drosophila Photoreceptors

Light is the main environmental stimulus that synchronizes the endogenous timekeeping systems in most terrestrial organisms. Drosophila cryptochrome (dCRY) is a light-responsive flavoprotein that detects changes in light intensity and wavelength around dawn and dusk. We have previously shown that dCRY acts through Inactivation No Afterpotential D (INAD) in a light-dependent manner on the Signalplex, a multiprotein complex that includes visual-signaling molecules, suggesting a role for dCRY in fly vision. Here, we predict and demonstrate a novel Ca²⁺-dependent interaction between dCRY and calmodulin (CaM). Through yeast two hybrid, coimmunoprecipitation (Co-IP), nuclear magnetic resonance (NMR) and calorimetric analyses we were able to identify and characterize a CaM binding motif in the dCRY C-terminus. Similarly, we also detailed the CaM binding site of the scaffold protein INAD and demonstrated that CaM bridges dCRY and INAD to form a ternary complex in vivo. Our results suggest a process whereby a rapid dCRY light response stimulates an interaction with INAD, which can be further consolidated by a novel mechanism regulated by CaM.

A Single Residue Mutation in the Gαq Subunit of the G Protein Complex Causes Blindness in Drosophila

Heterotrimeric G proteins play central roles in many signaling pathways, including the phototransduction cascade in animals. However, the degree of involvement of the G protein subunit Gα_q is not clear since animals with previously reported strong loss-of-function mutations remain responsive to light stimuli. We recovered a new allele of Gα_q in Drosophila that abolishes light response in a conventional electroretinogram assay, and reduces sensitivity in whole-cell recordings of dissociated cells by at least five orders of magnitude. In addition, mutant eyes demonstrate a rapid rate of degeneration in the presence of light. Our new allele is likely the strongest hypomorph described to date. Interestingly, the mutant protein is produced in the eyes but carries a single amino acid change of a conserved hydrophobic residue that has been assigned to the interface of interaction between Gα_q and its downstream effector, PLC. Our study has thus uncovered possibly the first point mutation that specifically affects this interaction in vivo.

Functional genomics identifies regulators of the phototransduction machinery in the Drosophila larval eye and adult ocelli

Sensory perception of light is mediated by specialized Photoreceptor neurons (PRs) in the eye. During development all PRs are genetically determined to express a specific Rhodopsin (Rh) gene and genes mediating a functional phototransduction pathway. While the genetic and molecular mechanisms of PR development is well described in the adult compound eye, it remains unclear how the expression of Rhodopsins and the phototransduction cascade is regulated in other visual organs in Drosophila, such as the larval eye and adult ocelli. Using transcriptome analysis of larval PR-subtypes and ocellar PRs we identify and study new regulators required during PR differentiation or necessary for the expression of specific signaling molecules of the functional phototransduction pathway. We found that the transcription factor Krüppel (Kr) is enriched in the larval eye and controls PR differentiation by promoting Rh5 and Rh6 expression. We also identified Camta, Lola, Dve and Hazy as key genes acting during ocellar PR differentiation. Further we show that these transcriptional regulators control gene expression of the phototransduction cascade in both larval eye and adult ocelli. Our results show that PR cell type-specific transcriptome profiling is a powerful tool to identify key transcriptional regulators involved during several aspects of PR development and differentiation. Our findings greatly contribute to the understanding of how combinatorial action of key transcriptional regulators control PR development and the regulation of a functional phototransduction pathway in both larval eye and adult ocelli.

CULD is required for rhodopsin and TRPL channel endocytic trafficking and survival of photoreceptor cells

Endocytosis of G-protein-coupled receptors (GPCRs) and associated channels contributes to desensitization and adaptation of a variety of signaling cascades. In Drosophila melanogaster, the main light-sensing rhodopsin (Rh1; encoded by ninaE) and the downstream ion channel, transient receptor potential like (TRPL), are endocytosed in response to light, but the mechanism is unclear. By using an RNA-Sequencing (RNA-Seq) approach, we discovered a protein we named CULD, a photoreceptor-cell enriched CUB- and LDLa-domain transmembrane protein, that is required for endocytic trafficking of Rh1 and TRPL. CULD localized to endocytic Rh1-positive or TRPL-positive vesicles. Mutations in culd resulted in the accumulation of Rh1 and TRPL within endocytic vesicles, and disrupted the regular turnover of endocytic Rh1 and TRPL. In addition, loss of CULD induced light- and age-dependent retinal degeneration, and reduced levels of Rh1, but not of TRPL, suppressed retinal degeneration in culd-null mutant flies. Our data demonstrate that CULD plays an important role in the endocytic turnover of Rh1 and TRPL, and suggest that CULD-dependent rhodopsin endocytic trafficking is required for maintaining photoreceptor integrity.

Molecular Cloning and Characterization of G Alpha Proteins from the Western Tarnished Plant Bug, Lygus hesperus

The Gα subunits of heterotrimeric G proteins play critical roles in the activation of diverse signal transduction cascades. However, the role of these genes in chemosensation remains to be fully elucidated. To initiate a comprehensive survey of signal transduction genes, we used homology-based cloning methods and transcriptome data mining to identity Gα subunits in the western tarnished plant bug (Lygus hesperus Knight). Among the nine sequences identified were single variants of the Gαi, Gαo, Gαs, and Gα12 subfamilies and five alternative splice variants of the Gαq subfamily. Sequence alignment and phylogenetic analyses of the putative L. hesperus Gα subunits support initial classifications and are consistent with established evolutionary relationships. End-point PCR-based profiling of the transcripts indicated head specific expression for LhGαq4, and largely ubiquitous expression, albeit at varying levels, for the other LhGα transcripts. All subfamilies were amplified from L. hesperus chemosensory tissues, suggesting potential roles in olfaction and/or gustation. Immunohistochemical staining of cultured insect cells transiently expressing recombinant His-tagged LhGαi, LhGαs, and LhGαq1 revealed plasma membrane targeting, suggesting the respective sequences encode functional G protein subunits.

Dynamin- and Rab5-dependent endocytosis is required to prevent Drosophila photoreceptor degeneration

In Drosophila photoreceptors, Rhodopsin 1 (ninaE, Rh1) is required for proper morphogenesis and maintenance of the apical light-gathering organelle, the rhabdomere. It has been proposed that Rh1, coupled to the Rho GTPases Rac1 and Cdc42, promotes the morphogenesis of a sub-rhabdomeric F-actin meshwork or rhabdomere terminal web (RTW). The RTW provides mechanical support to the apical microvilli and is likely to guide Rab11-dependent delivery of Rh1-rich membrane to the rhabdomere from the trans Golgi network. However, the nature and function of the molecular pathway involved in RTW morphogenesis remains incomplete. Here, we show that Rh1 function in promoting RTW morphogenesis is light-independent and is conserved throughout evolution. This Rh1 function does not require G(q)α(e), which is required for phototransduction. Finally, we show that interfering with Dynamin- and Rab5-dependent endocytosis leads to a phenotype that is undistinguishable from that of the ninaE-null mutant. Importantly, the corresponding endocytic activity is essential to prevent early onset of rhabdomere degeneration. In conclusion, we propose that Rh1 function in promoting RTW morphogenesis is not only needed to sustain apical membrane delivery but is also required for proper rhabdomeric membrane endocytosis and turnover.

Cloning and expression analysis of a G-protein α subunit--Gαo in the rice water weevil Lissorhoptrus oryzophilus Kuschel

The open reading frame (ORF) encoding a novel G protein α subunit, Lo Gα(o), was cloned from the parthenogenetic rice water weevil, Lissorhoptrus oryzophilus Kuschel (Coleoptera: Curculionidae). The Lo Gα(o) ORF encodes a protein of 354 amino acid residues. The deduced protein sequence shares high homology with Gα(o) from other species. The expression patterns of Lo Gα(o) in various adult tissues were indicated by real-time quantitative PCR and Western blot. The results showed that Lo Gα(o) mRNA was expressed at similar levels in tissues except relative high levels in the antennae of adult, and Lo Gα(o) protein of an apparent molecular mass of about 40 kDa was expressed in various tissues of the adult. Immunocytochemical localization showed that Lo Gα(o) was mainly expressed in the dendrites of the trichoid sensilla in the antenna of the weevil. The tissue and cellular localization of Lo Gα(o) suggests that Lo Gα(o) may take a part in signal transduction of olfactory/gustatory.

The history of the Drosophila TRP channel: the birth of a new channel superfamily

Transient receptor potential (TRP) channels are polymodal cellular sensors involved in a wide variety of cellular processes, mainly by changing membrane voltage and increasing cellular Ca(2+). This review outlines in detail the history of the founding member of the TRP family, the Drosophila TRP channel. The field began with a spontaneous mutation in the trp gene that led to a blind mutant during prolonged intense light. It was this mutant that allowed for the discovery of the first TRP channels. A combination of electrophysiological, biochemical, Ca(2+) measurements, and genetic studies in flies and in other invertebrates pointed to TRP as a novel phosphoinositide-regulated and Ca(2+)-permeable channel. The cloning and sequencing of the trp gene provided its molecular identity. These seminal findings led to the isolation of the first mammalian homologues of the Drosophila TRP channels. We now know that TRP channel proteins are conserved through evolution and are found in most organisms, tissues, and cell-types. The TRP channel superfamily is classified into seven related subfamilies: TRPC, TRPM, TRPV, TRPA, TRPP, TRPML, and TRPN. A great deal is known today about participation of TRP channels in many biological processes, including initiation of pain, thermoregulation, salivary fluid secretion, inflammation, cardiovascular regulation, smooth muscle tone, pressure regulation, Ca(2+) and Mg(2+) homeostasis, and lysosomal function. The native Drosophila photoreceptor cells, where the founding member of the TRP channels superfamily was found, is still a useful preparation to study basic features of this remarkable channel.

Mutants in phospholipid signaling attenuate the behavioral response of adult Drosophila to trehalose

In Drosophila melanogaster, gustatory receptor genes (Grs) encode putative G-protein-coupled receptors (GPCRs) that are expressed in gustatory receptor neurons (GRNs). One of the Gr genes, Gr5a, encodes a receptor for trehalose that is expressed in a subset of GRNs. Although a role for the G protein, Gsα, has been shown in Gr5a-expressing taste neurons, there is the residual responses to trehalose in Gsα mutants which could suggest additional transduction mechanisms. Expression and genetic analysis of the heterotrimeric G-protein subunit, Gq, shown here suggest involvement of this Gα subunit in trehalose perception in Drosophila. A green fluorescent protein reporter of Gq expression is detected in gustatory neurons in the labellum, tarsal segments, and wing margins. Animals heterozygous for dgq mutations and RNA interference-mediated knockdown of dgq showed reduced responses to trehalose in the proboscis extension reflex assay and feeding behavior assay. These defects were rescued by targeted expression of the wild-type dgqα transgene in the GRNs. These data together with observations from other mutants in phospholipid signaling provide insights into the mechanisms of taste transduction in Drosophila.

Expression analysis of the 3 G-protein subunits, Galpha, Gbeta, and Ggamma, in the olfactory receptor organs of adult Drosophila melanogaster

In many species, olfactory transduction is triggered by odorant molecules that interact with olfactory receptors coupled to heterotrimeric G-proteins. The role of G-protein-linked transduction in the olfaction of Drosophila is currently under study. Here, we supply a thorough description of the expression in the olfactory receptor organs (antennae and maxillary palps) of all known Drosophila melanogaster genes that encode for G-proteins. Using RT-polymerase chain reaction, we analyzed 6 Galpha (G(s), G(i), G(q), G(o), G(f), and concertina), 3 Gbeta (G(beta5), G(beta13F), and G(beta76C)), and 2 Ggamma genes (G(gamma1) and G(gamma30A)). We found that all Galpha protein-encoding genes showed expression in both olfactory organs, but G(f) mRNA was not detected in palps. Moreover, all the Gbeta and Ggamma genes are expressed in antennae and palps, except for G(beta76C). To gain insight into the hypothesis of different G-protein subunits mediating differential signaling in olfactory receptor neurons (ORNs), we performed immunohistochemical studies to observe the expression of several Galpha and Gbeta proteins. We found that Gs, Gi, Gq, and G(beta13F) subunits displayed generalized expression in the antennal tissue, including ORNs support cells and glial cells. Finally, complete coexpression was found between Gi and Gq, which are mediators of the cyclic adenosine monophosphate and IP3 transduction cascades, respectively.

Drosophila photoreceptors and signaling mechanisms

Fly eyes have been a useful biological system in which fundamental principles of sensory signaling have been elucidated. The physiological optics of the fly compound eye, which was discovered in the Musca, Calliphora and Drosophila flies, has been widely exploited in pioneering genetic and developmental studies. The detailed photochemical cycle of bistable photopigments has been elucidated in Drosophila using the genetic approach. Studies of Drosophila phototransduction using the genetic approach have led to the discovery of novel proteins crucial to many biological processes. A notable example is the discovery of the inactivation no afterpotential D scaffold protein, which binds the light-activated channel, its activator the phospholipase C and it regulator protein kinase C. An additional protein discovered in the Drosophila eye is the light-activated channel transient receptor potential (TRP), the founding member of the diverse and widely spread TRP channel superfamily. The fly eye has thus played a major role in the molecular identification of processes and proteins with prime importance.

Reduced odor responses from antennal neurons of G(q)alpha, phospholipase Cbeta, and rdgA mutants in Drosophila support a role for a phospholipid intermediate in insect olfactory transduction

Mechanisms by which G-protein-coupled odorant receptors transduce information in insects still need elucidation. We show that mutations in the Drosophila gene for G(q)alpha (dgq) significantly reduce both the amplitude of the field potentials recorded from the whole antenna in responses to odorants as well as the frequency of evoked responses of individual sensory neurons. This requirement for G(q)alpha is for adult function and not during antennal development. Conversely, brief expression of a dominant-active form of G(q)alpha in adults leads to enhanced odor responses. To understand signaling downstream of G(q)alpha in olfactory sensory neurons, genetic interactions of dgq were tested with mutants in genes known to affect phospholipid signaling. dgq mutant phenotypes were further enhanced by mutants in a PLCbeta (phospholipase Cbeta) gene, plc21C. Interestingly although, the olfactory phenotype of mutant alleles of diacylglycerol kinase (rdgA) was rescued by dgq mutant alleles. Our results suggest that G(q)alpha-mediated olfactory transduction in Drosophila requires a phospholipid second messenger the levels of which are regulated by a cycle of phosphatidylinositol 1,4-bisphosphate breakdown and regeneration.

The RGS gene loco is essential for male reproductive system differentiation in Drosophila melanogaster

BACKGROUND

The loco gene encodes several different isoforms of a regulator of G-protein signalling. These different isoforms of LOCO are part of a pathway enabling cells to respond to external signals. LOCO is known to be required at various developmental stages including neuroblast division, glial cell formation and oogenesis. Less is known about LOCO and its involvement in male development therefore to gain further insight into the role of LOCO in development we carried out a genetic screen and analysed males with reduced fertility.

RESULTS

We identified a number of lethal loco mutants and four semi-lethal lines, which generate males with reduced fertility. We have identified a fifth loco transcript and show that it is differentially expressed in developing pupae. We have characterised the expression pattern of all loco transcripts during pupal development in the adult testes, both in wild type and loco mutant strains. In addition we also show that there are various G-protein alpha subunits expressed in the testis all of which may be potential binding partners of LOCO.

CONCLUSION

We propose that the male sterility in the new loco mutants result from a failure of accurate morphogenesis of the adult reproductive system during metamorphosis, we propose that this is due to a loss of expression of loco c3. Thus, we conclude that specific isoforms of loco are required for the differentiation of the male gonad and genital disc.

Role of protein phosphatase 2A in regulating the visual signaling in Drosophila

Drosophila visual signaling, a G-protein-coupled phospholipase Cbeta (PLCbeta)-mediated mechanism, is regulated by eye-protein kinase C (PKC) that promotes light adaptation and fast deactivation, most likely via phosphorylation of inactivation no afterpotential D (INAD) and TRP (transient receptor potential). To reveal the critical phosphatases that dephosphorylate INAD, we used several biochemical analyses and identified protein phosphatase 2A (PP2A) as a candidate. Importantly, the catalytic subunit of PP2A, microtubule star (MTS), is copurified with INAD, and an elevated phosphorylation of INAD by eye-PKC was observed in three mts heterozygotes. To explore whether PP2A (MTS) regulates dephosphorylation of INAD by counteracting eye-PKC [INAC (inactivation no afterpotential C] in vivo, we performed ERG recordings. We discovered that inaC(P209) was semidominant, because inaC(P209) heterozygotes displayed abnormal light adaptation and slow deactivation. Interestingly, the deactivation defect of inaC(P209) heterozygotes was rescued by the mts(XE2258) heterozygous background. In contrast, mts(XE2258) failed to modify the severe deactivation of norpA(P16), indicating that MTS does not modulate NORPA (no receptor potential A) (PLCbeta). Together, our results strongly indicate that dephosphorylation of INAD is catalyzed by PP2A, and a reduction of PP2A can compensate for a partial loss of function in eye-PKC, restoring the fast deactivation kinetics in vivo. We thus propose that the fast deactivation of the visual response is modulated in part by the phosphorylation of INAD.

Phototransduction and retinal degeneration in Drosophila

Drosophila visual transduction is the fastest known G-protein-coupled signaling cascade and has therefore served as a genetically tractable animal model for characterizing rapid responses to sensory stimulation. Mutations in over 30 genes have been identified, which affect activation, adaptation, or termination of the photoresponse. Based on analyses of these genes, a model for phototransduction has emerged, which involves phosphoinoside signaling and culminates with opening of the TRP and TRPL cation channels. Many of the proteins that function in phototransduction are coupled to the PDZ containing scaffold protein INAD and form a supramolecular signaling complex, the signalplex. Arrestin, TRPL, and G alpha(q) undergo dynamic light-dependent trafficking, and these movements function in long-term adaptation. Other proteins play important roles either in the formation or maturation of rhodopsin, or in regeneration of phosphatidylinositol 4,5-bisphosphate (PIP2), which is required for the photoresponse. Mutation of nearly any gene that functions in the photoresponse results in retinal degeneration. The underlying bases of photoreceptor cell death are diverse and involve mechanisms such as excessive endocytosis of rhodopsin due to stable rhodopsin/arrestin complexes and abnormally low or high levels of Ca2+. Drosophila visual transduction appears to have particular relevance to the cascade in the intrinsically photosensitive retinal ganglion cells in mammals, as the photoresponse in these latter cells appears to operate through a remarkably similar mechanism.

Drosophila retinophilin contains MORN repeats and is conserved in humans

The function of conserved novel human genes can be efficiently addressed in genetic model organisms. From a collection of genes expressed in the Drosophila visual system, cDNAs expressed in vertebrates were identified and one similar to a novel human gene was chosen for further investigation. The results reported here characterize the Drosophila retinophilin gene and demonstrate that a similar gene is expressed in the human retina. The Drosophila and human retinophilin sequences are 50% identical, and they share an additional 16% conserved substitutions. Examination of the cDNA and genomic sequence indicates that it corresponds to the gene CG10233 of the annotated genome and predicts a 22.7 kDa protein. Polyclonal antibodies generated to a predicted retinophilin peptide recognize an antigen in Drosophila photoreceptor cells. The retinophilins encode 4 copies of a repeat associated with a Membrane Occupation and Recognition Nexus (MORN) function first discovered in junctophilins, which may interact with the plasma membrane. These results therefore show that Drosophila retinophilin is expressed in fly photoreceptor cells, demonstrate that a conserved human gene is expressed in human retina, and suggest that a mutational analysis of the Drosophila gene would be valuable.

Galpha encoding gene family of the malaria vector mosquito Anopheles gambiae: expression analysis and immunolocalization of AGalphaq and AGalphao in female antennae

To initiate a comprehensive investigation of chemosensory signal transduction downstream of odorant receptors, we identified and characterized the complete set of genes that encode G-protein alpha subunits in the genome of the malaria vector mosquito An. gambiae. Data are provided on the tissue-specific expression patterns of 10 corresponding aga-transcripts in adult mosquitoes and pre-imago developmental stages. Specific immunoreactivity in chemosensory hairs of female antennae provides evidence in support of the participation of a subset of AGalphaq isoforms in olfactory signal transduction in this mosquito. In contrast, AGalphao is localized along the flagellar axon bundle but is absent from chemosensory sensilla, which suggests that this G-protein alpha subunit does not participate in olfactory signal transduction.

Compensation of inositol 1,4,5-trisphosphate receptor function by altering sarco-endoplasmic reticulum calcium ATPase activity in the Drosophila flight circuit

Ionic Ca2+ functions as a second messenger to control several intracellular processes. It also influences intercellular communication. The release of Ca2+ from intracellular stores through the inositol 1,4,5-trisphosphate receptor (InsP3R) occurs in both excitable and nonexcitable cells. In Drosophila, InsP3R activity is required in aminergic interneurons during pupal development for normal flight behavior. By altering intracellular Ca2+ and InsP3 levels through genetic means, we now show that signaling through the InsP3R is required at multiple steps for generating the neural circuit required in air puff-stimulated Drosophila flight. Decreased Ca2+ release in aminergic neurons during development of the flight circuit can be compensated by reducing Ca2+ uptake from the cytosol to intracellular stores. However, this mode of increasing intracellular Ca2+ is insufficient for maintenance of flight patterns over time periods necessary for normal flight. Our study suggests that processes such as maintenance of wing posture and formation of the flight circuit require InsP3 receptor function at a slow timescale and can thus be modulated by altering levels of cytosolic Ca2+ and InsP3. In contrast, maintenance of flight patterns probably requires fast modulation of Ca2+ levels, in which the intrinsic properties of the InsP3R play a pivotal role.

Optical imaging and control of genetically designated neurons in functioning circuits

Proteins with engineered sensitivities to light are infiltrating the biological mechanisms by which neurons generate and detect electrochemical signals. Encoded in DNA and active only in genetically specified target cells, these proteins provide selective optical interfaces for observing and controlling signaling by defined groups of neurons in functioning circuits, in vitro and in vivo. Light-emitting sensors of neuronal activity (reporting calcium increase, neurotransmitter release, or membrane depolarization) have begun to reveal how information is represented by neuronal assemblies, and how these representations are transformed during the computations that inform behavior. Light-driven actuators control the electrical activities of central neurons in freely moving animals and establish causal connections between the activation of specific neurons and the expression of particular behaviors. Anchored within mathematical systems and control theory, the combination of finely resolved optical field sensing and finely resolved optical field actuation will open new dimensions for the analysis of the connectivity, dynamics, and plasticity of neuronal circuits, and perhaps even for replacing lost--or designing novel--functionalities.

Expression and localization of three G protein alpha subunits, Go, Gq, and Gs, in adult antennae of the silkmoth (Bombyx mori)

In insect olfactory receptor neurons, rapid and transient increases in inositol triphosphate (IP3) and Ca2+ are detected upon stimulation with pheromone or nonpheromonal odorants. This suggests that heterotrimeric guanine nucleotide binding proteins (G proteins) may transduce some odorant responses in insects. We obtained cDNA clones encoding three classes of G protein alpha subunits, Bm Go, Bm Gq, and Bm Gs, from the antennae of the adult male silkmoth (Bombyx mori). RT-PCR experiments showed that the mRNA of these G protein alpha subunits was also present in the various tissues of adult and larval insects. We used immunocytochemistry to localize these G protein alpha subunits in adult male and female antennae. In the adult male antennae, anti-Go antiserum stained the nerve bundles. In contrast, anti-Gq and anti-Gs antisera stained the inner and outer dendritic segments of the putative olfactory receptor neuron. The localizations of Bm Go, Bm Gq, and Bm Gs in the female antennae were the same as in the male antennae. The localizations of Bm Gq and Bm Gs suggest that each subunit mediates a subset of the odorant response.

Diversity of G proteins in Lepidopteran cell lines: partial sequences of six G protein alpha subunits

The aim of this work was to sample the diversity of G protein alpha subunits in lepidopteran insect cell lines. Here we report the amplification by degenerate PCR of partial sequences representing six G protein alpha subunits from three different lepidopteran insect cell lines. Sequence comparisons with known G protein alpha subunits indicate that the Sf9, Ld and High Five cell lines each contain (at least) one Galpha(q)-like and one Galpha(i)-like Galpha subunit. All six PCR products are unique at the nucleotide level, but the translation products of the three Galpha q-like partial clones (Sf9-Galpha 1, Ld-Galpha 1, and Hi5-Galpha 1) are identical, as are the translation products of the three Galpha i-like partial clones (Sf9-Galpha 2, Ld-Galpha 2, and Hi5-Galpha 2). Both the Galpha(q)-like and Galpha(i)-like translation products are identical to known Galpha subunits from other Lepidoptera, are highly similar (88-98%) to Galpha subunits from other invertebrates including mosquitoes, fruit flies, lobsters, crabs, and snails, and are also highly similar (88-90%) to known mammalian Galpha subunits. Identification of G protein alpha subunits in lepidopteran cell lines will assist in host cell line selection when insect cell lines are used for the pharmacological analysis of human GPCRs.

Targeted mutagenesis of the farnesylation site of Drosophila Ggammae disrupts membrane association of the G protein betagamma complex and affects the light sensitivity of the visual system

Activation of phototransduction in the compound eye of Drosophila is mediated by a heterotrimeric G protein that couples to the effector enzyme phospholipase Cbeta. The gamma subunit of this G protein (Ggammae) as well as gamma subunits of vertebrate transducins contain a carboxyl-terminal CAAX motif (C, cysteine; A, aliphatic amino acid; X, any amino acid) with a consensus sequence for protein farnesylation. To examine the function of Ggammae farnesylation, we mutated the farnesylation site and overexpressed the mutated Ggammae in Drosophila. Mass spectrometry of overexpressed Ggammae subunits revealed that nonmutated Ggammae is modified by farnesylation, whereas the mutated Ggammae is not farnesylated. In the transgenic flies, mutated Ggammae forms a dimeric complex with Gbetae, with the consequence that the fraction of non-membrane-bound Gbetagamma is increased. Thus, farnesylation of Ggammae facilitates the membrane attachment of the Gbetagamma complex. We also expressed human Ggammarod in Drosophila photoreceptors. Despite similarities in the primary structure between the transducin gamma subunit and Drosophila Ggammae, we observed no interaction of human Ggammarod with Drosophila Gbetae. This finding indicates that human Ggammarod and Drosophila Ggammae provide different interfaces for the interaction with Gbeta subunits. Electroretinogram recordings revealed a significant loss of light sensitivity in eyes of transgenic flies that express mutated Ggammae. This loss in light sensitivity reveals that post-translational farnesylation is a critical step for the formation of membrane-associated Galphabetagamma required for transmitting light activation from rhodopsin to phospholipase Cbeta.

Novel dominant rhodopsin mutation triggers two mechanisms of retinal degeneration and photoreceptor desensitization

A variety of rod opsin mutations result in autosomal dominant retinitis pigmentosa and congenital night blindness in humans. One subset of these mutations encodes constitutively active forms of the rod opsin protein. Some of these dominant rod opsin mutant proteins, which desensitize transgenic Xenopus rods, provide an animal model for congenital night blindness. In a genetic screen to identify retinal degeneration mutants in Drosophila, we identified a dominant mutation in the ninaE gene (NinaE(pp100)) that encodes the rhodopsin that is expressed in photoreceptors R1-R6. Deep pseudopupil analysis and histology showed that the degeneration was attributable to a light-independent apoptosis. Whole-cell recordings revealed that the NinaE(pp100) mutant photoreceptor cells were strongly desensitized, which partially masked their constitutive activity. This desensitization primarily resulted from both the persistent binding of arrestin (ARR2) to the NINAE(pp100) mutant opsin and the constitutive activity of the phototransduction cascade. Whereas mutations in several Drosophila genes other than ninaE were shown to induce photoreceptor cell apoptosis by stabilizing a rhodopsin-arrestin complex, NinaE(pp100) represented the first rhodopsin mutation that stabilized this protein complex. Additionally, the NinaE(pp100) mutation led to elevated levels of G(q)alpha in the cytosol, which mediated a novel retinal degeneration pathway. Eliminating both G(q)alpha and arrestin completely rescued the NinaE(pp100)-dependent photoreceptor cell death, which indicated that the degeneration is entirely dependent on both G(q)alpha and arrestin. Such a combination of multiple pathological pathways resulting from a single mutation may underlie several dominant retinal diseases in humans.

Altered levels of Gq activity modulate axonal pathfinding in Drosophila

A majority of neurons that form the ventral nerve cord send out long axons that cross the midline through anterior or posterior commissures. A smaller fraction extend longitudinally and never cross the midline. The decision to cross the midline is governed by a balance of attractive and repulsive signals. We have explored the role of a G-protein, Galphaq, in altering this balance in Drosophila. A splice variant of Galphaq, dgqalpha3, is expressed in early axonal growth cones, which go to form the commissures in the Drosophila embryonic CNS. Misexpression of a gain-of-function transgene of dgqalpha3 (AcGq3) leads to ectopic midline crossing. Analysis of the AcGq3 phenotype in roundabout and frazzled mutants shows that AcGq3 function is antagonistic to Robo signaling and requires Frazzled to promote ectopic midline crossing. Our results show for the first time that a heterotrimeric G-protein can affect the balance of attractive versus repulsive cues in the growth cone and that it can function as a component of signaling pathways that regulate axonal pathfinding.

Expression pattern in the antennae of a newly isolated lepidopteran Gq protein alpha subunit cDNA

From the antennae of the moth Mamestra brassicae, we have identified a lepidopteran G protein alpha subunit belonging to the Gq family, through immunological detection in crude antennal extract and antennal primary cell cultures, followed by molecular cloning. The complete cDNA sequence (1540 bp) contains an open reading frame encoding a protein of 353 amino acids. This deduced sequence possesses all of the characteristics of the Gq family and shares a very high degree of amino-acid sequence identity with vertebrate (80% with mouse or human Gqalpha) and invertebrate subunits (varying between 60 and 87% for Gqalpha from organisms as diverse as sponge and Drosophila). The expression pattern of the Gq subunit in adult antennae was associated with the olfactory sensilla suggesting a specific role in olfaction. These data provide molecular evidence for a component of the phosphoinositide signaling pathway in moth antennae: this G protein alpha subunit may be involved in the olfaction transduction process through interaction with G-protein-coupled receptors, stimulating the phospholipase C mediated second messenger pathway.

Novel genomic cDNA hybrids produce effective RNA interference in adult Drosophila

Drosophila melanogaster has been a premier genetic model system for nearly 100 years, yet lacks a simple method to disrupt gene expression. Here, we show genomic cDNA fusions predicted to form double-stranded RNA (dsRNA) following splicing, effectively silencing expression of target genes in adult transgenic animals. We targeted three Drosophila genes: lush, white, and dGq(alpha). In each case, target gene expression is dramatically reduced, and the white RNAi phenotype is indistinguishable from a deletion mutant. This technique efficiently targets genes expressed in neurons, a tissue refractory to RNAi in C. elegans. These results demonstrate a simple strategy to knock out gene function in specific cells in living adult Drosophila that can be applied to define the biological function of hundreds of orphan genes and open reading frames.

Selective photostimulation of genetically chARGed neurons

To permit direct functional analyses of neural circuits, we have developed a method for stimulating groups of genetically designated neurons optically. Coexpression of the Drosophila photoreceptor genes encoding arrestin-2, rhodopsin (formed by liganding opsin with retinal), and the alpha subunit of the cognate heterotrimeric G protein--an explosive combination we term "chARGe"--sensitizes generalist vertebrate neurons to light. Illumination of a mixed population of neurons elicits action potentials selectively and cell-autonomously in its genetically chARGed members. In contrast to bath-applied photostimulants or caged neurotransmitters, which act indiscriminately throughout the illuminated volume, chARGe localizes the responsiveness to light. Distributed activity may thus be fed directly into a circumscribed population of neurons in intact tissue, irrespective of the spatial arrangement of its elements.

Drosophila Amphiphysin is a post-synaptic protein required for normal locomotion but not endocytosis

Clathrin-mediated endocytosis is required to recycle synaptic vesicles for fast and efficient neurotransmission. Amphiphysins are thought to be multiprotein adaptors that may contribute to this process by bringing together many of the proteins required for endocytosis. Their in vivo function, however, has yet to be determined. Here, we show that the Drosophila genome encodes a single amphiphysin gene that is broadly expressed during development. We also show that, unlike its vertebrate counterparts, Drosophila Amphiphysin is enriched postsynaptically at the larval neuromuscular junction. To determine the role of Drosophila Amphiphysin, we also generated null mutants which are viable but give rise to larvae and adults with pronounced locomotory defects. Surprisingly, the locomotory defects cannot be accounted for by alterations in the morphology or physiology of the neuromuscular junction. Moreover, using stimulus protocols designed to test endocytosis under moderate and extreme vesicle cycling, we could not detect any defect in the neuromuscular junction of the amphiphysin mutant. Taken together, our findings suggest that Amphiphysin is not required for viability, nor is it absolutely required for clathrin-mediated endocytosis. However, Drosophila Amphiphysin function is required in both larvae and adults for normal locomotion.

A molecular pathway for light-dependent photoreceptor apoptosis in Drosophila

Light-induced photoreceptor apoptosis occurs in many forms of inherited retinal degeneration resulting in blindness in both vertebrates and invertebrates. Though mutations in several photoreceptor signaling proteins have been implicated in triggering this process, the molecular events relating light activation of rhodopsin to photoreceptor death are yet unclear. Here, we uncover a pathway by which activation of rhodopsin in Drosophila mediates apoptosis through a G protein-independent mechanism. This process involves the formation of membrane complexes of phosphorylated, activated rhodopsin and its inhibitory protein arrestin, and subsequent clathrin-dependent endocytosis of these complexes into a cytoplasmic compartment. Together, these data define the proapoptotic molecules in Drosophila photoreceptors and indicate a novel signaling pathway for light-activated rhodopsin molecules in control of photoreceptor viability.

Dissociation of G-protein alpha from rhabdomeric membranes decreases its interaction with rhodopsin and increases its degradation by calpain

Photoactivation of invertebrate rhodopsin activates a GTP-binding protein, Gq, which in turn activates a phospholipase C (PLC) enzyme. Gqalpha is a membrane-associated protein that is progressively released from the membrane by washing with buffers containing increasing concentrations of beta-mercaptoethanol (beta-ME). Isolated, soluble Gqalpha showed a decreased ability to be activated by rhodopsin but was more active in stimulating PLC when compared with the membrane-associated form of Gqalpha. The calcium-activated protease, calpain, selectively cleaved the soluble but not the membrane-bound form of Gqalpha. Calpain cleaved a small peptide from the amino-terminus of Gqalpha reducing the ability of the G-protein to bind GTP. The uncoupling of Gqalpha from rhodopsin and subsequent calcium-dependent proteolysis to further inactivate the G-protein may therefore be a regulatory mechanism of light adaptation in rhabdomeric photoreceptors.

Phospholipase C and termination of G-protein-mediated signalling in vivo

In Drosophila photoreceptors, phospholipase C (PLC) and other signalling components form multiprotein structures through the PDZ scaffold protein INAD. Association between PLC and INAD is important for termination of responses to light; the underlying mechanism is, however, unclear. Here we report that the maintenance of large amounts of PLC in the signalling membranes by association with INAD facilitates response termination, and show that PLC functions as a GTPase-activating protein (GAP). The inactivation of the G protein by its target, the PLC, is crucial for reliable production of single-photon responses and for the high temporal and intensity resolution of the response to light.

Functional expression of a locust visual pigment in transgenic Drosophila melanogaster

The cDNA encoding a visual pigment of the locust Schistocerca gregaria has been inserted into the germline of the ninaE mutant of Drosophila melanogaster by P-element-mediated transformation. Functional expression has been documented by recording light-regulated electroretinograms in transgenic flies. The spectral properties of the expressed visual pigment were determined with detergent-solubilized material, prepared from the eyecups of the transgenic D. melanogaster. The recombinant locust pigment, as well as the genuine pigment of the fruitfly (Rh1) that served as a control for transformation/expression, showed photoreversibility between the pigment and metapigment forms. The absorptions of the difference spectra identify the locust visual pigment as a short wavelength-absorbing, blue-light-sensitive photoreceptor. The absorption maxima are similar to those recorded on living locust animals. These results show that, although locust visual pigments contain 11-cis retinal as chromophore, the expressed protein is able to adopt 3-hydroxyretinal that is provided by the transgenic fruitflies. The electrophysiological recordings reveal that the locust visual pigment is able to induce phototransduction in the fruitfly. The reported results have two important consequences: On the one hand, the binding site of the locust opsin is apparently able to interact with the 3-hydroxyretinal from Drosophila in a way that the biological signal generated by the photoisomerization of the chromophore can be used by the protein to adopt a physiologically active conformation. On the other hand, despite the relatively large phylogenetic distance between both insect species, the extent of conservation between the protein domains thought to be involved in G-protein activation is striking.

The visual G protein of fly photoreceptors interacts with the PDZ domain assembled INAD signaling complex via direct binding of activated Galpha(q) to phospholipase cbeta

Visual transduction in the compound eye of flies is a well-established model system for the study of G protein-coupled transduction pathways. Pivotal components of this signaling pathway, including the principal light-activated Ca(2+) channel transient receptor potential, an eye-specific protein kinase C, and the norpA-encoded phospholipase Cbeta, are assembled into a supramolecular signaling complex by the modular PDZ domain protein INAD. We have used immunoprecipitation assays to study the interaction of the heterotrimeric visual G protein with this INAD signaling complex. Light-activated Galpha(q)- guanosine 5'-O-(thiotriphosphate) and AlF(4)(-)-activated Galpha(q), but not Gbetagamma, form a stable complex with the INAD signaling complex. This interaction requires the presence of norpA-encoded phospholipase Cbeta, indicating that phospholipase Cbeta is the target of activated Galpha(q). Our data establish that the INAD signaling complex is a light-activated target of the phototransduction pathway, with Galpha(q) forming a molecular on-off switch that shuttles the visual signal from activated rhodopsin to INAD-linked phospholipase Cbeta.

A novel Ggamma isolated from Drosophila constitutes a visual G protein gamma subunit of the fly compound eye

Visual transduction in the compound eye of flies is a well established model system for the study of G protein-coupled transduction pathways. To characterize key components of the phototransduction cascade we performed substractive hybridization screening. We cloned the cDNA coding for the visual Ggamma (Ggamma(e)) subunit from Drosophila which had so far eluded identification at the molecular level. Northern blot analysis revealed the presence of a major, 1.4-kilobase(kb) Ggamma(e) transcript and two minor transcripts of 1.8 and 6 kb in size. The major 1.4-kb mRNA is expressed preferentially in the eye. The spatial expression pattern determined for Ggamma(e) as well as co-immunoprecipitation experiments demonstrated that Ggamma(e) dimerizes with Gbeta(e) to form the heterodimeric Gbetagamma subunit which functions in visual transduction in the Drosophila compound eye. Ggamma(e) shares common characteristics with the visual Ggamma subunits of human rod and cone photoreceptors although different classes of Galpha subunits are employed in vertebrate and invertebrate phototransduction. By the molecular cloning and characterization of the visual gamma subunit of Drosophila one of the few missing links in the well studied Drosophila phototransduction cascade has been characterized to complete our knowledge about the Drosophila visual transduction pathway.

The emerging role of mass spectrometry in molecular biosciences: studies of protein phosphorylation in fly eyes as an example

Modern mass spectrometry (MS) streamlined with two-dimensional gel electrophoresis, in-gel digestion and HPLC-interfaced electrospray ionization quadrupole MS or matrix-assisted laser desorption ionization time-of-flight MS enables us to analyse proteins at a minuscule scale. We present here two examples of MS applications in which (1) we identified the in vivo phosphorylation site of Drosophila arrestin, phosrestin I (PRI), and (2) we revealed the identity of an 80 kDa phosphoprotein (80K) in Drosophila eyes to be the InaD gene product, a member of the PDZ domain proteins. Available evidence suggests that PRI quenches the activation of rhodopsin and that the InaD protein adjusts photoreceptor responsiveness by assembling/disassembling components involved in photoreceptor transduction in flies. PRI undergoes a reversible phosphorylation at a single site, and 80K at multiple sites. The phosphorylation states of PRI and 80K depend on the intensity and/or duration of light stimuli. From these results we postulate that these proteins function as a molecular switch adjusting the signalling cascade through phosphorylation. The combination of two-dimensional gel electrophoresis with MS will be a powerful tool for detailed investigation of such complex switching processes. The techniques described here can be applied also to other complex signalling systems.

Visual transduction in Drosophila

The Drosophila phototransduction cascade has emerged as an attractive paradigm for understanding the molecular mechanisms underlying visual transduction, as well as other G protein-coupled signaling cascades that are activated and terminated with great rapidity. A large collection of mutants affecting the fly visual cascade have been isolated, and the nature and function of many of the affected gene products have been identified. Virtually all of the proteins, including those that were initially classified as novel, are highly related to vertebrate homologs. Recently, it has become apparent that most of the proteins central to Drosophila phototransduction are coupled into a supramolecular signaling complex, signalplex, through association with a PDZ-containing scaffold protein. The characterization of this complex has led to a re-evaluation of the mechanisms underlying the activation and deactivation of the phototransduction cascade.

The ordered visual transduction complex of the squid photoreceptor membrane

The study of visual transduction has given invaluable insight into the mechanisms of signal transduction by heptahelical receptors that act via guanine nucleotide binding proteins (G-proteins). However, the cyclic-GMP second messenger system seen in vertebrate photoreceptor cells is not widely used in other cell types. In contrast, the retina of higher invertebrates, such as squid, offers an equally accessible transduction system, which uses the widespread second messenger chemistry of an increase in cytosolic calcium caused by the production of inositol-(1,4,5)-trisphosphate (InsP3) by the enzyme phospholipase C, and which may be a model for store-operated calcium influx. In this article, we highlight some key aspects of invertebrate visual transduction as elucidated from the combination of biochemical techniques applied to cephalopods, genetic techniques applied to flies, and electrophysiology applied to the horseshoe crab. We discuss the importance and applicability of ideas drawn from these model systems to the understanding of some general processes in signal transduction, such as the integration of the cytoskeleton into the signal transduction process and the possible modes of regulation of store-operated calcium influx.

A G protein alpha subunit from Cochliobolus heterostrophus involved in mating and appressorium formation

A Galpha subunit-encoding gene (CGA1) was cloned from Cochliobolus heterostrophus, a heterothallic foliar pathogen of corn. The deduced amino acid sequence showed similarity to Galpha proteins from other filamentous fungi and suggested that CGA1 is a member of the Galphai class. cga1 mutants had reduced ability to form appressoria on glass surfaces and on corn leaves; mutants nevertheless caused lesions on corn plants like those of wild type. cga1 mutants were female sterile; sexual development was completely abolished when the mutant allele was homozygous in a cross. Ascospores produced in crosses heterozygous at Cga1 were all wild type. The signal transduction pathway represented by CGA1 appears to be involved in developmental pathways leading to either appressorium formation or mating; in sexual development CGA1 is required for both fertility and ascospore viability.

dRGS7 encodes a Drosophila homolog of EGL-10 and vertebrate RGS7

We identified a Drosophila gene encoding a homolog of the regulator of G-protein signaling (RGS) protein family. This gene (dRGS7) is expressed in neurons of the embryo and adult fly and is predicted to encode a 428-amino acid protein with >55% overall amino acid sequence identity with the vertebrate protein RGS7 and the C. elegans EGL-10. The dRGS7 protein is 50% conserved in the C-terminal RGS domain with RGS7 and EGL-10 but, remarkably, displays much greater conservation with the N-terminal regions of these proteins. This finding implies a conserved function for these homologs from divergent species involving domains outside the RGS domain. The dRGS7 protein also has a domain of similarity with Dishevelled and pleckstrin, raising the possibility that these proteins interact with common signaling components.

Distribution of Na+,K(+)-ATPase in photoreceptor cells of insects

Light stimulation of insect photoreceptors causes opening of cation channels and an inward current that is partially carried by Na+ ions. There is also an efflux of K+ ions upon photostimulation. Na+ and K+ gradients across the photoreceptor membrane are reestablished by the activity of the enzyme Na+,K(+)-ATPase. About two-thirds of the total amount of ATP consumed in response to a light stimulus is attributed to the activity of this ion pump, demonstrating the importance of this enzyme for photoreceptor function. Insect photoreceptor cells are polarized epithelial cells; their plasma membrane is organized into two domains having a distinct morphology, molecular composition, and function. The visual pigment rhodopsin and the molecular components of the transduction machinery are localized in the rhabdomere, an array of densely packed microvilli, whereas Na+,K(+)-ATPase resides in the nonrhabdomeric membrane. Comparative immunolocalization studies on compound eyes of diverse insect species have demonstrated subtle variations in the distribution patterns of Na+,K(+)-ATPase. These may be accounted for by differences in the mechanisms responsible for Na+,K(+)-ATPase positioning.