Altered levels of Gq activity modulate axonal pathfinding in Drosophila

The multimodal action of G alpha q in coordinating growth and homeostasis in the Drosophila wing imaginal disc

Background

G proteins mediate cell responses to various ligands and play key roles in organ development. Dysregulation of G-proteins or Ca ²⁺ signaling impacts many human diseases and results in birth defects. However, the downstream effectors of specific G proteins in developmental regulatory networks are still poorly understood.

Methods

We employed the Gal4/UAS binary system to inhibit or overexpress Gαq in the wing disc, followed by phenotypic analysis. Immunohistochemistry and next-gen RNA sequencing identified the downstream effectors and the signaling cascades affected by the disruption of Gαq homeostasis.

Results

Here, we characterized how the G protein subunit Gαq tunes the size and shape of the wing in the larval and adult stages of development. Downregulation of Gαq in the wing disc reduced wing growth and delayed larval development. Gαq overexpression is sufficient to promote global Ca ²⁺ waves in the wing disc with a concomitant reduction in the Drosophila final wing size and a delay in pupariation. The reduced wing size phenotype is further enhanced when downregulating downstream components of the core Ca ²⁺ signaling toolkit, suggesting that downstream Ca ²⁺ signaling partially ameliorates the reduction in wing size. In contrast, Gαq -mediated pupariation delay is rescued by inhibition of IP ₃ R, a key regulator of Ca ²⁺ signaling. This suggests that Gαq regulates developmental phenotypes through both Ca ²⁺ -dependent and Ca ²⁺ -independent mechanisms. RNA seq analysis shows that disruption of Gαq homeostasis affects nuclear hormone receptors, JAK/STAT pathway, and immune response genes. Notably, disruption of Gαq homeostasis increases expression levels of Dilp8, a key regulator of growth and pupariation timing.

Conclusion

Gαq activity contributes to cell size regulation and wing metamorphosis. Disruption to Gαq homeostasis in the peripheral wing disc organ delays larval development through ecdysone signaling inhibition. Overall, Gαq signaling mediates key modules of organ size regulation and epithelial homeostasis through the dual action of Ca ²⁺ -dependent and independent mechanisms.

Septins tune lipid kinase activity and PI(4,5)P2 turnover during G-protein–coupled PLC signalling in vivo

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] hydrolysis by phospholipase C (PLC) is a conserved mechanism of signalling. Given the low abundance of PI(4,5)P2, its hydrolysis needs to be coupled to resynthesis to ensure continued PLC activity; however, the mechanism by which depletion is coupled to resynthesis remains unknown. PI(4,5)P2 synthesis is catalyzed by the phosphorylation of phosphatidylinositol 4 phosphate (PI4P) by phosphatidylinositol 4 phosphate 5 kinase (PIP5K). In Drosophila photoreceptors, photon absorption is transduced into PLC activity and during this process, PI(4,5)P2 is resynthesized by a PIP5K. However, the mechanism by which PIP5K activity is coupled to PI(4,5)P2 hydrolysis is unknown. In this study, we identify a unique isoform dPIP5KL, that is both necessary and sufficient to mediate PI(4,5)P2 synthesis during phototransduction. Depletion of PNUT, a non-redundant subunit of the septin family, enhances dPIP5KL activity in vitro and PI(4,5)P2 resynthesis in vivo; co-depletion of dPIP5KL reverses the enhanced rate of PI(4,5)P2 resynthesis in vivo. Thus, our work defines a septin-mediated mechanism through which PIP5K activity is coupled to PLC-mediated PI(4,5)P2 hydrolysis.

From spikes to intercellular waves: Tuning intercellular calcium signaling dynamics modulates organ size control

Information flow within and between cells depends significantly on calcium (Ca²⁺) signaling dynamics. However, the biophysical mechanisms that govern emergent patterns of Ca²⁺ signaling dynamics at the organ level remain elusive. Recent experimental studies in developing Drosophila wing imaginal discs demonstrate the emergence of four distinct patterns of Ca²⁺ activity: Ca²⁺ spikes, intercellular Ca²⁺ transients, tissue-level Ca²⁺ waves, and a global “fluttering” state. Here, we used a combination of computational modeling and experimental approaches to identify two different populations of cells within tissues that are connected by gap junction proteins. We term these two subpopulations “initiator cells,” defined by elevated levels of Phospholipase C (PLC) activity, and “standby cells,” which exhibit baseline activity. We found that the type and strength of hormonal stimulation and extent of gap junctional communication jointly determine the predominate class of Ca²⁺ signaling activity. Further, single-cell Ca²⁺ spikes are stimulated by insulin, while intercellular Ca²⁺ waves depend on Gαq activity. Our computational model successfully reproduces how the dynamics of Ca²⁺ transients varies during organ growth. Phenotypic analysis of perturbations to Gαq and insulin signaling support an integrated model of cytoplasmic Ca²⁺ as a dynamic reporter of overall tissue growth. Further, we show that perturbations to Ca²⁺ signaling tune the final size of organs. This work provides a platform to further study how organ size regulation emerges from the crosstalk between biochemical growth signals and heterogeneous cell signaling states.

Calcium (Ca²⁺) is a universal second messenger that regulates a myriad of cellular processes such as cell division, cell proliferation and apoptosis. Multiple patterns of Ca²⁺ signaling including single-cell spikes, multicellular Ca²⁺ transients, large-scale Ca²⁺ waves, and global “fluttering” have been observed in epithelial systems during organ development. Key molecular players and biophysical mechanisms involved in formation of these patterns during organ development are not well understood. In this work, we developed a generalized multicellular model of Ca²⁺ that captures all the key categories of Ca²⁺ activity as a function of key hormonal signals. Integration of model predictions and experiments reveals two subclasses of cell populations and demonstrates that Ca²⁺ signaling activity at the organ scale is defined by a general decrease in gap junction communication as an organ grows. Our experiments also reveal that a “goldilocks zone” of optimal Ca²⁺ activity is required to achieve optimal growth at the organ level.

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.

G-protein αq gene expression plays a role in alcohol tolerance in Drosophila melanogaster

Ethanol is a psychoactive substance causing both short- and long-term behavioural changes in humans and animal models. We have used the fruit fly Drosophila melanogaster to investigate the effect of ethanol exposure on the expression of the Gαq protein subunit. Repetitive exposure to ethanol causes a reduction in sensitivity (tolerance) to ethanol, which we have measured as the time for 50% of a set of flies to become sedated after exposure to ethanol (ST50). We demonstrate that the same treatment that induces an increase in ST50 over consecutive days (tolerance) also causes a decrease in Gαq protein subunit expression at both the messenger RNA and protein level. To identify whether there may be a causal relationship between these two outcomes, we have developed strains of flies in which Gαq messenger RNA expression is suppressed in a time- and tissue-specific manner. In these flies, the sensitivity to ethanol and the development of tolerance are altered. This work further supports the value of Drosophila as a model to dissect the molecular mechanisms of the behavioural response to alcohol and identifies G proteins as potentially important regulatory targets for alcohol use disorders.

Dopamine Receptor DAMB Signals via Gq to Mediate Forgetting in Drosophila

Prior studies have shown that aversive olfactory memory is acquired by dopamine acting on a specific receptor, dDA1, expressed by mushroom body neurons. Active forgetting is mediated by dopamine acting on another receptor, Damb, expressed by the same neurons. Surprisingly, prior studies have shown that both receptors stimulate cyclic AMP (cAMP) accumulation, presenting an enigma of how mushroom body neurons distinguish between acquisition and forgetting signals. Here, we surveyed the spectrum of G protein coupling of dDA1 and Damb, and we confirmed that both receptors can couple to Gs to stimulate cAMP synthesis. However, the Damb receptor uniquely activates Gq to mobilize Ca²⁺ signaling with greater efficiency and dopamine sensitivity. The knockdown of Gαq with RNAi in the mushroom bodies inhibits forgetting but has no effect on acquisition. Our findings identify a Damb/Gq-signaling pathway that stimulates forgetting and resolves the opposing effects of dopamine on acquisition and forgetting.

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.

Expression profiles of the Gα subunits during Xenopus tropicalis embryonic development

Heterotrimeric G protein signaling plays major roles during different cellular events. However, there is a limited understanding of the molecular mechanisms underlying G protein control during embryogenesis. G proteins are highly conserved and can be grouped into four subfamilies according to sequence homology and function. To further studies on G protein function during embryogenesis, the present analysis identified four Gα subunits representative of the different subfamilies and determined their spatiotemporal expression patterns during Xenopus tropicalis embryogenesis. Each of the Gα subunit transcripts was maternally and zygotically expressed, and, as development progressed, dynamic expression patterns were observed. In the early developmental stages, the Gα subunits were expressed in the animal hemisphere and dorsal marginal zone. While expression was observed at the somite boundaries, in vascular structures, in the eye, and in the otic vesicle during the later stages, expression was mainly found in neural tissues, such as the neural tube and, especially, in the cephalic vesicles, neural crest region, and neural crest-derived structures. Together, these results support the pleiotropism and complexity of G protein subfamily functions in different cellular events. The present study constitutes the most comprehensive description to date of the spatiotemporal expression patterns of Gα subunits during vertebrate development.

Dynamic Trk and G Protein Signalings Regulate Dopaminergic Neurodifferentiation in Human Trophoblast Stem Cells

Understanding the mechanisms in the generation of neural stem cells from pluripotent stem cells is a fundamental step towards successful management of neurodegenerative diseases in translational medicine. Albeit all-trans retinoic acid (RA) has been associated with axon outgrowth and nerve regeneration, the maintenance of differentiated neurons, the association with degenerative disease like Parkinson's disease, and its regulatory molecular mechanism from pluripotent stem cells to neural stem cells remain fragmented. We have previously reported that RA is capable of differentiation of human trophoblast stem cells to dopamine (DA) committed progenitor cells. Intracranial implantation of such neural progenitor cells into the 6-OHDA-lesioned substantia nigra pars compacta successfully regenerates dopaminergic neurons and integrity of the nigrostriatal pathway, ameliorating the behavioral deficits in the Parkinson’s disease rat model. Here, we demonstrated a dynamic molecular network in systematic analysis by addressing spatiotemporal molecular expression, intracellular protein-protein interaction and inhibition, imaging study, and genetic expression to explore the regulatory mechanisms of RA induction in the differentiation of human trophoblast stem cells to DA committed progenitor cells. We focused on the tyrosine receptor kinase (Trk), G proteins, canonical Wnt2B/β-catenin, genomic and non-genomic RA signaling transductions with Tyrosine hydroxylase (TH) gene expression as the differentiation endpoint. We found that at the early stage, integration of TrkA and G protein signalings aims for axonogenesis and morphogenesis, involving the novel RXRα/Gα_q/11 and RARβ/Gβ signaling pathways. While at the later stage, five distinct signaling pathways together with epigenetic histone modifications emerged to regulate expression of TH, a precursor of dopamine. RA induction generated DA committed progenitor cells in one day. Our results provided substantial mechanistic evidence that human trophoblast stem cell-derived neural stem cells can potentially be used for neurobiological study, drug discovery, and as an alternative source of cell-based therapy in neurodegenerative diseases like Parkinson’s disease.

Maturation of a central brain flight circuit in Drosophila requires Fz2/Ca²⁺ signaling

The final identity of a differentiated neuron is determined by multiple signaling events, including activity dependent calcium transients. Non-canonical Frizzled2 (Fz2) signaling generates calcium transients that determine neuronal polarity, neuronal migration, and synapse assembly in the developing vertebrate brain. Here, we demonstrate a requirement for Fz2/Ca²⁺ signaling in determining the final differentiated state of a set of central brain dopaminergic neurons in Drosophila, referred to as the protocerebral anterior medial (PAM) cluster. Knockdown or inhibition of Fz2/Ca²⁺ signaling during maturation of the flight circuit in pupae reduces Tyrosine Hydroxylase (TH) expression in the PAM neurons and affects maintenance of flight. Thus, we demonstrate that Fz2/Ca²⁺ transients during development serve as a pre-requisite for normal adult behavior. Our results support a neural mechanism where PAM neuron send projections to the α' and β' lobes of a higher brain centre, the mushroom body, and function in dopaminergic re-inforcement of flight.

DOI:http://dx.doi.org/10.7554/eLife.07046.001

The fruit fly Drosophila melanogaster is an aerial acrobat. These insects can suddenly change direction in less than one hundredth of a second, explaining why a moving fly can be so difficult to swat. To perform their aerial manoeuvres, the flies continually combine information from multiple senses, including vision, hearing and smell, and use these data to control the activity of the neural circuits that support flight.

These flight circuits are established during the pupal stage of fly development, during which the fly transforms from a larva into its adult form. In 2013, researchers showed that a protein called dFrizzled2 must be present in pupae for flight circuits to mature correctly. This protein forms part of a pathway that ultimately controls which specific chemicals—called neurotransmitters—are released by neurons to communicate with other cells. Agrawal and Hasan—who worked on the 2013 study—now extend their findings to investigate the role of dFrizzled2 in more detail.

The new experiments show that for the flight circuits to mature, dFrizzled2 must be active in a cluster of neurons known collectively as PAM. Specifically, dFrizzled2 is needed to make an enzyme that helps to produce a neurotransmitter called dopamine. This in turn enables the PAM neurons to communicate with a region of the fruit fly brain called the mushroom body, which it thought to play an important role in complex behaviors such as reward-based learning.

The absence of dFrizzled2 results in adult flies that rarely remain airborne for more than 20 s at a time, whereas normal flies can typically fly for over 700 s. Given that dopamine is known to signal reward, one possibility is that the dopamine signals from the PAM neurons to the mushroom body serve as a reward to encourage continuous flight. Mutant flies that lack dFrizzled2—and thus these dopamine signals—lose their motivation to fly after only a few seconds.

Overall, Agrawal and Hasan's findings suggest that the mushroom body has an important role in coordinating a fly's movements with information from it senses. Future research will be needed to determine exactly how the mushroom body performs this role.

DOI:http://dx.doi.org/10.7554/eLife.07046.002

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.

Cloning and expression analysis of G-protein Gαq subunit and Gβ1 subunit from Bemisia tabaci Gennadius (Homoptera: Aleyrodidae)

The heterotrimeric G proteins play an essential role in a wide variety of signal transduction pathways, mediating the process of chemical signals from the environment in all higher eukaryotic organisms. In this article, two G-protein subunit genes encoding Gαq and Gβ1 were cloned from Bemisia tabaci Gennadius. The full-length cDNA sequence of BtGαq consisted of 2,336 bp with an ORF of 1,062 bp encoding 353 amino acids and BtGβ1 had a full length of 1,942 bp with an ORF of 1,023 nucleotides encoding 340 amino acids. The amino acid sequences of BtGαq and BtGβ1 from B. tabaci B biotype were identical to those from the Q biotype. Phylogenetic analysis identified G protein α and β subunit families from insects based on their amino acid sequences. The expression patterns of BtGαq and BtGβ1 at different development stages and in different body regions were analyzed by real-time quantitative PCR and Western blot. The results show that BtGαq and BtGβ1 are neither developmental stage-specific nor tissue-specific. The transcript levels of BtGαq in the B biotype are similar to that in the Q biotype, the transcript levels of BtGβ1 at egg, first instar and pupae in B biotype were significantly higher than that in Q biotype. The transcript levels of BtGαq and BtGβ1 in the head were significantly higher than those in thorax and abdomen indicating that they are involved in nervous system and sensory functions.

Patterns of gene expression associated with Pten deficiency in the developing inner ear

In inner ear development, phosphatase and tensin homolog (PTEN) is necessary for neuronal maintenance, such as neuronal survival and accurate nerve innervations of hair cells. We previously reported that Pten conditional knockout (cKO) mice exhibited disorganized fasciculus with neuronal apoptosis in spiral ganglion neurons (SGNs). To better understand the genes and signaling networks related to auditory neuron maintenance, we compared the profiles of differentially expressed genes (DEGs) using microarray analysis of the inner ear in E14.5 Pten cKO and wild-type mice. We identified 46 statistically significant transcripts using significance analysis of microarrays, with the false-discovery rate set at 0%. Among the DEGs, expression levels of candidate genes and expression domains were validated by quantitative real-time RT-PCR and in situ hybridization, respectively. Ingenuity pathway analysis using DEGs identified significant signaling networks associated with apoptosis, cellular movement, and axon guidance (i.e., secreted phosphoprotein 1 (Spp1)-mediated cellular movement and regulator of G-protein signaling 4 (Rgs4)-mediated axon guidance). This result was consistent with the phenotypic defects of SGNs in Pten cKO mice (e.g., neuronal apoptosis, abnormal migration, and irregular nerve fiber patterns of SGNs). From this study, we suggest two key regulatory signaling networks mediated by Spp1 and Rgs4, which may play potential roles in neuronal differentiation of developing auditory neurons.

A genetic RNAi screen for IP₃/Ca²⁺ coupled GPCRs in Drosophila identifies the PdfR as a regulator of insect flight

Insect flight is regulated by various sensory inputs and neuromodulatory circuits which function in synchrony to control and fine-tune the final behavioral outcome. The cellular and molecular bases of flight neuromodulatory circuits are not well defined. In Drosophila melanogaster, it is known that neuronal IP₃ receptor mediated Ca²⁺ signaling and store-operated Ca²⁺ entry (SOCE) are required for air-puff stimulated adult flight. However, G-protein coupled receptors (GPCRs) that activate intracellular Ca²⁺ signaling in the context of flight are unknown in Drosophila. We performed a genetic RNAi screen to identify GPCRs that regulate flight by activating the IP₃ receptor. Among the 108 GPCRs screened, we discovered 5 IP₃/Ca²⁺ linked GPCRs that are necessary for maintenance of air-puff stimulated flight. Analysis of their temporal requirement established that while some GPCRs are required only during flight circuit development, others are required both in pupal development as well as during adult flight. Interestingly, our study identified the Pigment Dispersing Factor Receptor (PdfR) as a regulator of flight circuit development and as a modulator of acute flight. From the analysis of PdfR expressing neurons relevant for flight and its well-defined roles in other behavioral paradigms, we propose that PdfR signaling functions systemically to integrate multiple sensory inputs and modulate downstream motor behavior.

A majority of behavioral patterns in flying insects depend upon their ability to modulate flight. In Drosophila melanogaster, mutations in the IP₃ receptor gene lead to loss of voluntary flight in response to a natural stimulus like a gentle air-puff. From previous genetic and cellular studies it is known that the IP₃R in Drosophila is activated by G-protein coupled receptors (GPCRs). However, GPCRs that act upstream of the IP₃R in the context of flight are not known. Therefore, we performed a genetic RNAi screen to identify GPCRs which regulate flight. This screen was followed by a secondary suppressor screen that assessed the role of each identified GPCR in activating IP₃/Ca²⁺ signaling. We found 5 such GPCRs. Our results demonstrate that these GPCRs are required during flight circuit development and during adult flight. One flight-regulating receptor identified was the Pigment Dispersing Factor Receptor (PdfR). This receptor is known to regulate behaviors such as circadian rhythms, geotaxis and reproduction. A spatio-temporal analysis of PdfR flight function indicates that it regulates both flight circuit development and acute flight through multiple neurons. We postulate that PdfR signaling could modulate and integrate multiple behavioral inputs in Drosophila and other flying insects.

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.

The chemorepellent Slit3 promotes monocyte migration

Directional migration is an essential step for monocytes to infiltrate sites of inflammation, a process primarily regulated by chemoattractants. Slits are large matrix proteins that are secreted by endothelial cells; they were reported to inhibit the chemoattractant-induced migration of different cell types, including leukocytes. The aim of this study was to determine the effect of Slit3 on primary monocyte migration and to address the underlying mechanisms. We show that Roundabout (Robo)1, one of the Robo receptors that recognize Slit3, is the only Robo homolog expressed by CD14(+) monocytes. Interestingly, we found that stimulation with Slit3 increased the spontaneous and chemoattractant-induced migration of primary monocytes in vitro and increased the myeloid cell recruitment during peritoneal inflammation in vivo. In addition, Slit3 did not seem to act as a chemoattractant itself; it promoted directed migration triggered by chemoattractants, such as CXCL12, by inducing a chemokinetic effect. We further show that Slit3 prevented monocyte spreading and induced rounding of spread monocytes without affecting monocyte adhesion. Stimulation with Slit3 was not associated with changes in the levels of phosphorylated p38, p42/p44, or Src, known regulators of monocyte migration, but it directly acts on molecular pathways involved in basal leukocyte migration by activating RhoA. These findings show an unexpected response of monocytes to Slit3 and add insights into the possible role of Slit proteins during inflammatory cell recruitment.

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.

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.

Antagonistic regulation of neurite morphology through Gq/G11 and G12/G13

The induction of neurite retraction and growth cone collapse via G-protein-coupled receptors is involved in developmental as well as regenerative processes. The role of individual G-protein-mediated signaling processes in the regulation of neurite morphology is still incompletely understood. Using primary neurons from brains lacking Galpha(q)/Galpha(11) or Galpha(12)/Galpha(13), we show here that G(12)/G(13)-mediated signaling is absolutely required for neurite retraction and growth cone collapse induced by the blood-borne factors lysophosphatidic acid and thrombin. Interestingly, the effects of lysophosphatidic acid were mediated mainly by G(13), whereas thrombin effects required G(12). Surprisingly, lack of Galpha(q)/Galpha(11) resulted in overshooting responses to both stimuli, indicating that G(q)/G(11)-mediated signaling most likely via activation of Rac antagonizes the effects of G(12)/G(13).

Abelson tyrosine kinase and Calmodulin interact synergistically to transduce midline guidance cues in the Drosophila embryonic CNS

Calmodulin and Abelson tyrosine kinase are key signaling molecules transducing guidance cues at the Drosophila embryonic midline. A reduction in the signaling strength of either pathway alone induces ectopic midline crossing errors in a few segments. When Calmodulin and Abelson signaling levels are simultaneously reduced, the frequency of ectopic crossovers is synergistically enhanced as all segments exhibit crossing errors. But as the level of signaling is further reduced, commissures begin to fuse and large gaps form in the longitudinal connectives. Quantitative analysis suggests that the level of Abelson activity is particularly important. Like Calmodulin, Abelson interacts with son-of-sevenless to increase ectopic crossovers suggesting all three contribute to midline repulsive signaling. Axons cross the midline in almost every segment if Frazzled is co-overexpressed with the Calmodulin inhibitor, but the crossovers induced by the Calmodulin inhibitor itself do not require endogenous Frazzled. Thus, Calmodulin and Abelson tyrosine kinase are key signaling molecules working synergistically to transduce both midline attractive and repulsive cues. While they may function downstream of specific receptors, the emergence of commissural and longitudinal connective defects point to a novel convergence of Calmodulin and Abelson signaling during the regulation of actin and myosin dynamics underlying a guidance decision.

Drosophila as a genetic and cellular model for studies on axonal growth

One of the most fascinating processes during nervous system development is the establishment of stereotypic neuronal networks. An essential step in this process is the outgrowth and precise navigation (pathfinding) of axons and dendrites towards their synaptic partner cells. This phenomenon was first described more than a century ago and, over the past decades, increasing insights have been gained into the cellular and molecular mechanisms regulating neuronal growth and navigation. Progress in this area has been greatly assisted by the use of simple and genetically tractable invertebrate model systems, such as the fruit fly Drosophila melanogaster. This review is dedicated to Drosophila as a genetic and cellular model to study axonal growth and demonstrates how it can and has been used for this research. We describe the various cellular systems of Drosophila used for such studies, insights into axonal growth cones and their cytoskeletal dynamics, and summarise identified molecular signalling pathways required for growth cone navigation, with particular focus on pathfinding decisions in the ventral nerve cord of Drosophila embryos. These Drosophila-specific aspects are viewed in the general context of our current knowledge about neuronal growth.

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.

ROBO directs axon crossing of segmental boundaries by suppressing responsiveness to relocalized Netrin

Networks in the CNS consist of neural modules that are connected in a repetitive array. Whereas individual modules contain guidance information along which axons track within the unit, these guidance cues hinder axon extension across module boundaries. We investigated how axons solve this 'boundary problem' by analyzing the longitudinal connections of neuromeres in Drosophila melanogaster. The initial trajectory of the longitudinal axons is guided by Netrin, which is localized on commissural axons by its receptor, Frazzled. The Netrin cue on the commissure of the next segment can act as a barrier to longitudinal axons, inhibiting their extension and misguiding them contralaterally along the commissure. We show that, before reaching the segmental boundary, the longitudinal axons' responsiveness to Netrin presented on the commissure is suppressed by Roundabout (ROBO), through counteracting Gq signaling. The absence of suppression causes the robo phenotype: longitudinal axons project toward the midline, as if running around a roundabout (rotary).

G proteins in development

The focus of developmental biologists has expanded from the analysis of gene expression to include the analysis of cell signalling. Heterotrimeric G proteins (G proteins) mediate signalling from a superfamily of heptahelical receptors (G-protein-coupled receptors) to a smaller number of effector units that include adenylyl cyclases, phospholipase C and various ion channels. The convergence of developmental biology with cell signalling has now revealed overlaps in which G proteins mediate complex pathways in embryonic development.

The Abelson tyrosine kinase, the Trio GEF and Enabled interact with the Netrin receptor Frazzled in Drosophila

The attractive Netrin receptor Frazzled (Fra), and the signaling molecules Abelson tyrosine kinase (Abl), the guanine nucleotide-exchange factor Trio,and the Abl substrate Enabled (Ena), all regulate axon pathfinding at the Drosophila embryonic CNS midline. We detect genetic and/or physical interactions between Fra and these effector molecules that suggest that they act in concert to guide axons across the midline. Mutations in Abland trio dominantly enhance fra and Netrin mutant CNS phenotypes, and fra;Abl and fra;trio double mutants display a dramatic loss of axons in a majority of commissures. Conversely,heterozygosity for ena reduces the severity of the CNS phenotype in fra, Netrin and trio,Abl mutants. Consistent with an in vivo role for these molecules as effectors of Fra signaling, heterozygosity for Abl, trio or ena reduces the number of axons that inappropriately cross the midline in embryos expressing the chimeric Robo-Fra receptor. Fra interacts physically with Abl and Trio in GST-pulldown assays and in co-immunoprecipitation experiments. In addition, tyrosine phosphorylation of Trio and Fra is elevated in S2 cells when Abl levels are increased. Together, these data suggest that Abl, Trio, Ena and Fra are integrated into a complex signaling network that regulates axon guidance at the CNS midline.

Dynamic expression of RGS4 in the developing nervous system and regulation by the neural type-specific transcription factor Phox2b

Previous studies have shown that members of the family of regulators of G-protein signaling (RGS), including RGS4, have a discrete expression pattern in the adult brain (Gold et al., 1997). Here, we describe for RGS4 a distinct, mostly transient phase of neuronal expression, during embryonic development: transcription of RGS4 occurs in a highly dynamic manner in a small set of peripheral and central neuronal precursors. This expression pattern overlaps extensively with that of the paired-like homeodomain protein Phox2b, a determinant of neuronal identity. In embryos deficient for Phox2b, RGS4 expression is downregulated in the locus coeruleus, sympathetic ganglia, and cranial motor and sensory neurons. Moreover, Phox2b cooperates with the basic helix-loop-helix protein Mash1 to transiently switch on RGS4 after ectopic expression in the chicken spinal cord. Intriguingly, we also identify a heterotrimeric G-protein alpha-subunit, gustducin, as coexpressed with RGS4 in developing facial motor neurons, also under the control of Phox2b. Altogether, these data identify components of the heterotrimeric G-protein signaling pathway as part of the type-specific program of neuronal differentiation.