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

Antagonistic PCP Signaling Pathways in the developing Drosophila eye

In Planar cell polarity (PCP), cells coordinately polarize their cytoskeletons within the plane of the epithelium in which they lie. In most insect epithelia this is indicated by the coordinated projections of the hairs secreted by the ectodermal cells. PCP of this form has been effectively studied in Drosophila, but it has proven difficult to achieve an integrated description of the roles played by the various proteins. In the insect eye, PCP is not evident as the polarization of individual cells, but as the asymmetric arrangements of the cells of the ommatidia. This different form of PCP allows different studies to be performed, and using this system we have detected the action of two antagonistic signaling pathways. Even though antagonistic, the two pathways synergize and cooperate to ensure that the correct arrangement of the cells is achieved. The cooperative use of antagonistic signaling pathways occurs in the polarization of chemotacting cells, and we discuss the possibility that a similar molecular principle may underlie PCP.

The expanding roles of Gβγ subunits in G protein-coupled receptor signaling and drug action

Gβγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gβγ" does not do justice to the fact that 5 Gβ and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gβγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gβγ subunits regulates cellular activity, and that the granularity of individual Gβ and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gβγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

Drosophila auditory organ genes and genetic hearing defects

The Drosophila auditory organ shares equivalent transduction mechanisms with vertebrate hair cells, and both are specified by atonal family genes. Using a whole-organ knockout strategy based on atonal, we have identified 274 Drosophila auditory organ genes. Only four of these genes had previously been associated with fly hearing, yet one in five of the genes that we identified has a human cognate that is implicated in hearing disorders. Mutant analysis of 42 genes shows that more than half of them contribute to auditory organ function, with phenotypes including hearing loss, auditory hypersusceptibility, and ringing ears. We not only discover ion channels and motors important for hearing, but also show that auditory stimulus processing involves chemoreceptor proteins as well as phototransducer components. Our findings demonstrate mechanosensory roles for ionotropic receptors and visual rhodopsins and indicate that different sensory modalities utilize common signaling cascades.

Protein Gq modulates termination of phototransduction and prevents retinal degeneration

Appropriate termination of the phototransduction cascade is critical for photoreceptors to achieve high temporal resolution and to prevent excessive Ca(2+)-induced cell toxicity. Using a genetic screen to identify defective photoresponse mutants in Drosophila, we isolated and identified a novel Gα(q) mutant allele, which has defects in both activation and deactivation. We revealed that G(q) modulates the termination of the light response and that metarhodopsin/G(q) interaction affects subsequent arrestin-rhodopsin (Arr2-Rh1) binding, which mediates the deactivation of metarhodopsin. We further showed that the Gα(q) mutant undergoes light-dependent retinal degeneration, which is due to the slow accumulation of stable Arr2-Rh1 complexes. Our study revealed the roles of G(q) in mediating photoresponse termination and in preventing retinal degeneration. This pathway may represent a general rapid feedback regulation of G protein-coupled receptor signaling.

Phototransduction and clock gene expression in the troglobiont beetle Ptomaphagus hirtus of Mammoth cave

Obligatory cave species exhibit dramatic trait modifications such as eye reduction, loss of pigmentation and an increase in touch receptors. As molecular studies of cave adaptation have largely concentrated on vertebrate models, it is not yet possible to probe for genetic universalities underlying cave adaptation. We have therefore begun to study the strongly cave-adapted small carrion beetle Ptomaphagus hirtus. For over 100 years, this flightless signature inhabitant of Mammoth Cave, the world's largest known cave system, has been considered blind despite the presence of residual lens structures. By deep sequencing of the adult head transcriptome, we discovered the transcripts of all core members of the phototransduction protein machinery. Combined with the absence of transcripts of select structural photoreceptor and eye pigmentation genes, these data suggest a reduced but functional visual system in P. hirtus. This conclusion was corroborated by a negative phototactic response of P. hirtus in light/dark choice tests. We further detected the expression of the complete circadian clock gene network in P. hirtus, raising the possibility of a role of light sensation in the regulation of oscillating processes. We speculate that P. hirtus is representative of a large number of animal species with highly reduced but persisting visual capacities in the twilight zone of the subterranean realm. These can now be studied on a broad comparative scale given the efficiency of transcript discovery by next-generation sequencing.

Competing activities of heterotrimeric G proteins in Drosophila wing maturation

Drosophila genome encodes six alpha-subunits of heterotrimeric G proteins. The Galphas alpha-subunit is involved in the post-eclosion wing maturation, which consists of the epithelial-mesenchymal transition and cell death, accompanied by unfolding of the pupal wing into the firm adult flight organ. Here we show that another alpha-subunit Galphao can specifically antagonize the Galphas activities by competing for the Gbeta13F/Ggamma1 subunits of the heterotrimeric Gs protein complex. Loss of Gbeta13F, Ggamma1, or Galphas, but not any other G protein subunit, results in prevention of post-eclosion cell death and failure of the wing expansion. However, cell death prevention alone is not sufficient to induce the expansion defect, suggesting that the failure of epithelial-mesenchymal transition is key to the folded wing phenotypes. Overactivation of Galphas with cholera toxin mimics expression of constitutively activated Galphas and promotes wing blistering due to precocious cell death. In contrast, co-overexpression of Gbeta13F and Ggamma1 does not produce wing blistering, revealing the passive role of the Gbetagamma in the Galphas-mediated activation of apoptosis, but hinting at the possible function of Gbetagamma in the epithelial-mesenchymal transition. Our results provide a comprehensive functional analysis of the heterotrimeric G protein proteome in the late stages of Drosophila wing development.

Division of labor: subsets of dorsal-appendage-forming cells control the shape of the entire tube

The function of an organ relies on its form, which in turn depends on the individual shapes of the cells that create it and the interactions between them. Despite remarkable progress in the field of developmental biology, how cells collaborate to make a tissue remains an unsolved mystery. To investigate the mechanisms that determine organ structure, we are studying the cells that form the dorsal appendages (DAs) of the Drosophila melanogaster eggshell. These cells consist of two differentially patterned subtypes: roof cells, which form the outward-facing roof of the lumen, and floor cells, which dive underneath the roof cells to seal off the floor of the tube. In this paper, we present three lines of evidence that reveal a further stratification of the DA-forming epithelium. Laser ablation of only a few cells in the anterior of the region causes a disproportionately severe shortening of the appendage. Genetic alteration through the twin peaks allele of tramtrack69 (ttk(twk)), a female-sterile mutation that leads to severely shortened DAs, causes no such shortening when removed from a majority of the DA-forming cells, but rather, produces short appendages only when removed from cells in the very anterior of the tube-forming tissue. Additionally we show that heterotrimeric G-protein function is required for DA morphogenesis. Like TTK69, Gbeta 13F is not required in all DA-forming follicle cells but only in the floor and leading roof cells. The different phenotypes that result from removal of Gbeta 13F from each region demonstrate a striking division of function between different DA-forming cells. Gbeta mutant floor cells are unable to control the width of the appendage while Gbeta mutant leading roof cells fail to direct the elongation of the appendage and the convergent-extension of the roof-cell population.

Role of G-proteins in odor-sensing and CO2-sensing neurons in Drosophila

A central question in insect chemoreception is whether signaling occurs via G-proteins. Two families of seven-transmembrane-domain chemoreceptors, the odor (Or) and gustatory receptor (Gr) families, have been identified in Drosophila (Clyne et al., 1999, 2000; Vosshall et al., 1999). Ors mediate odor responses, whereas two Grs, Gr21a and Gr63a, mediate CO2 response (Hallem et al., 2004; Jones et al., 2007; Kwon et al., 2007). Using single-sensillum recordings, we systematically investigate the role of Galpha proteins in vivo, initially with RNA interference constructs, competitive peptides, and constitutively active Galpha proteins. The results do not support a role for Galpha proteins in odor sensitivity. In parallel experiments, manipulations of Galpha(q), but not other Galpha proteins, affected CO2 response. Transient, conditional, and ectopic expression analyses consistently supported a role for Galpha(q) in the response of CO2-sensing neurons, but not odor-sensing neurons. Genetic mosaic analysis confirmed that odor responses are normal in the absence of Galpha(q). Ggamma30A is also required for normal CO2 response. The simplest interpretation of these results is that Galpha(q) and Ggamma30A play a role in the response of CO2-sensing neurons, but are not required for Or-mediated odor signaling.

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.

Prenylation-deficient G protein gamma subunits disrupt GPCR signaling in the zebrafish

Prenylation of G protein gamma (gamma) subunits is necessary for the membrane localization of heterotrimeric G proteins and for functional heterotrimeric G protein coupled receptor (GPCR) signaling. To evaluate GPCR signaling pathways during development, we injected zebrafish embryos with mRNAs encoding Ggamma subunits mutated so that they can no longer be prenylated. Low-level expression of these prenylation-deficient Ggamma subunits driven either ubiquitously or specifically in the primordial germ cells (PGCs) disrupts GPCR signaling and manifests as a PGC migration defect. This disruption results in a reduction of calcium accumulation in the protrusions of migrating PGCs and a failure of PGCs to directionally migrate. When co-expressed with a prenylation-deficient Ggamma, 8 of the 17 wildtype Ggamma isoforms individually confer the ability to restore calcium accumulation and directional migration. These results suggest that while the Ggamma subunits possess the ability to interact with G Beta (beta) proteins, only a subset of wildtype Ggamma proteins are stable within PGCs and can interact with key signaling components necessary for PGC migration. This in vivo study highlights the functional redundancy of these signaling components and demonstrates that prenylation-deficient Ggamma subunits are an effective tool to investigate the roles of GPCR signaling events during vertebrate development.

Heterotrimeric G proteins regulate a noncanonical function of septate junction proteins to maintain cardiac integrity in Drosophila

The gene networks regulating heart morphology and cardiac integrity are largely unknown. We previously reported a role for the heterotrimeric G protein gamma subunit 1 (Ggamma1) in mediating cardial-pericardial cell adhesion in Drosophila. Here we show G-oalpha47A and Gbeta13F cooperate with Ggamma1 to maintain cardiac integrity. Cardial-pericardial cell adhesion also relies on the septate junction (SJ) proteins Neurexin-IV (Nrx-IV), Sinuous, Coracle, and Nervana2, which together function in a common pathway with Ggamma1. Furthermore, Ggamma1 signaling is required for proper SJ protein localization, and loss of at least one SJ protein, Nrx-IV, induces cardiac lumen collapse. These results are surprising because the embryonic heart lacks SJs and suggest that SJ proteins perform noncanonical functions to maintain cardiac integrity in Drosophila. Our findings unveil the components of a previously unrecognized network of genes that couple G protein signaling with structural constituents of the heart.

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.

Excess of Gbetae over Gqalphae in vivo prevents dark, spontaneous activity of Drosophila photoreceptors

Drosophila melanogaster photoreceptor cells are capable of detecting single photons. This utmost sensitivity is critically dependent on the maintenance of an exceedingly low, dark, spontaneous activity of photoreceptor cells. However, the underlying mechanisms of this hallmark of phototransduction are not fully understood. An analysis of the Drosophila visual heterotrimeric (alphabetagamma) Gq protein revealed that wild-type Drosophila flies have about a twofold excess of Gbeta over Galpha subunits of the visual Gq protein. Studies of Gbetae mutants in which the excess of Gbeta was genetically eliminated showed dramatic dark, spontaneous activity of the photoreceptor cells, whereas concurrent genetic reduction of the Galpha subunit, which restored the excess of Gbeta, abolished this effect. These results indicate that an excess of Gbeta over Galpha is a strategy used in vivo for the suppression of spontaneous activity, thereby yielding a high signal to noise ratio, which is characteristic of the photoreceptor light response. This mechanism could be relevant to the regulation of G protein signaling in general.

G-protein gamma subunit 1 is required for sugar reception in Drosophila

Though G-proteins have been implicated in the primary step of taste signal transduction, no direct demonstration has been done in insects. We show here that a G-protein gamma subunit, Ggamma1, is required for the signal transduction of sugar taste reception in Drosophila. The Ggamma1 gene is expressed mainly in one of the gustatory receptor neurons. Behavioral responses of the flies to sucrose were reduced by the targeted suppression of neural functions of Ggamma1-expressing cells using neural modulator genes such as the modified Shaker K+ channel (EKO), the tetanus toxin light chain or the shibire (shi(ts1)) gene. RNA interference targeting to the Ggamma1 gene reduced the amount of Ggamma1 mRNA and suppressed electrophysiological response of the sugar receptor neuron. We also demonstrated that responses to sugars were lowered in Ggamma1 null mutant, Ggamma1(N159). These results are consistent with the hypothesis that Ggamma1 participates in the signal transduction of sugar taste reception.

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.

Differential functions of G protein and Baz-aPKC signaling pathways in Drosophila neuroblast asymmetric division

Drosophila melanogaster neuroblasts (NBs) undergo asymmetric divisions during which cell-fate determinants localize asymmetrically, mitotic spindles orient along the apical–basal axis, and unequal-sized daughter cells appear. We identified here the first Drosophila mutant in the Gγ1 subunit of heterotrimeric G protein, which produces Gγ1 lacking its membrane anchor site and exhibits phenotypes identical to those of Gβ13F, including abnormal spindle asymmetry and spindle orientation in NB divisions. This mutant fails to bind Gβ13F to the membrane, indicating an essential role of cortical Gγ1–Gβ13F signaling in asymmetric divisions. In Gγ1 and Gβ13F mutant NBs, Pins–Gαi, which normally localize in the apical cortex, no longer distribute asymmetrically. However, the other apical components, Bazooka–atypical PKC–Par6–Inscuteable, still remain polarized and responsible for asymmetric Miranda localization, suggesting their dominant role in localizing cell-fate determinants. Further analysis of Gβγ and other mutants indicates a predominant role of Partner of Inscuteable–Gαi in spindle orientation. We thus suggest that the two apical signaling pathways have overlapping but different roles in asymmetric NB division.

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.

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.

G beta association and effector interaction selectivities of the divergent G gamma subunit G gamma(13)

G gamma(13) is a divergent member of the G gamma subunit family considered to be a component of the gustducin G-protein heterotrimer involved in bitter and sweet taste reception in taste bud cells. G gamma(13) contains a C-terminal asparagine-proline-tryptophan (NPW) tripeptide, a hallmark of RGS protein G gamma-like (GGL) domains which dimerize exclusively with G beta(5) subunits. In this study, we investigated the functional range of G gamma(13) assembly with G beta subunits using multiple assays of G beta association and G beta gamma effector modulation. G gamma(13) was observed to associate with all five G beta subunits (G beta(1-5)) upon co-translation in vitro, as well as function with all five G beta subunits in the modulation of Kir3.1/3.4 (GIRK1/4) potassium and N-type (alpha(1B)) calcium channels. Multiple G beta/G gamma(13) pairings were also functional in cellular assays of phospholipase C (PLC) beta 2 activation and inhibition of G alpha(q)-stimulated PLC beta 1 activity. However, upon cellular co-expression of G gamma(13) with different G beta subunits, only G beta(1)/G gamma(13), G beta(3)/G gamma(13), and G beta(4)/G gamma(13) pairings were found to form stable dimers detectable by co-immunoprecipitation under high-detergent cell lysis conditions. Collectively, these data indicate that G gamma(13) forms functional G beta gamma dimers with a range of G beta subunits. Coupled with our detection of G gamma(13) mRNA in mouse and human brain and retina, these results imply that this divergent G gamma subunit can act in signal transduction pathways other than that dedicated to taste reception in sensory lingual tissue.