The G-protein beta-subunit GPB-2 in Caenorhabditis elegans regulates the G(o)alpha-G(q)alpha signaling network through interactions with the regulator of G-protein signaling proteins EGL-10 and EAT-16

Functional Insights into Protein Kinase A (PKA) Signaling from C. elegans

Protein kinase A (PKA), which regulates a diverse set of biological functions downstream of cyclic AMP (cAMP), is a tetramer consisting of two catalytic subunits (PKA-C) and two regulatory subunits (PKA-R). When cAMP binds the PKA-R subunits, the PKA-C subunits are released and interact with downstream effectors. In Caenorhabditis elegans (C. elegans), PKA-C and PKA-R are encoded by kin-1 and kin-2, respectively. This review focuses on the contributions of work in C. elegans to our understanding of the many roles of PKA, including contractility and oocyte maturation in the reproductive system, lipid metabolism, physiology, mitochondrial function and lifespan, and a wide variety of behaviors. C. elegans provides a powerful genetic platform for understanding how this kinase can regulate an astounding variety of physiological responses.

Gα/GSA-1 works upstream of PKA/KIN-1 to regulate calcium signaling and contractility in the Caenorhabditis elegans spermatheca

Correct regulation of cell contractility is critical for the function of many biological systems. The reproductive system of the hermaphroditic nematode C. elegans contains a contractile tube of myoepithelial cells known as the spermatheca, which stores sperm and is the site of oocyte fertilization. Regulated contraction of the spermatheca pushes the embryo into the uterus. Cell contractility in the spermatheca is dependent on actin and myosin and is regulated, in part, by Ca2+ signaling through the phospholipase PLC-1, which mediates Ca2+ release from the endoplasmic reticulum. Here, we describe a novel role for GSA-1/Gαs, and protein kinase A, composed of the catalytic subunit KIN-1/PKA-C and the regulatory subunit KIN-2/PKA-R, in the regulation of Ca2+ release and contractility in the C. elegans spermatheca. Without GSA-1/Gαs or KIN-1/PKA-C, Ca2+ is not released, and oocytes become trapped in the spermatheca. Conversely, when PKA is activated through either a gain of function allele in GSA-1 (GSA-1(GF)) or by depletion of KIN-2/PKA-R, the transit times and total numbers, although not frequencies, of Ca2+ pulses are increased, and Ca2+ propagates across the spermatheca even in the absence of oocyte entry. In the spermathecal-uterine valve, loss of GSA-1/Gαs or KIN-1/PKA-C results in sustained, high levels of Ca2+ and a loss of coordination between the spermathecal bag and sp-ut valve. Additionally, we show that depleting phosphodiesterase PDE-6 levels alters contractility and Ca2+ dynamics in the spermatheca, and that the GPB-1 and GPB-2 Gβ subunits play a central role in regulating spermathecal contractility and Ca2+ signaling. This work identifies a signaling network in which Ca2+ and cAMP pathways work together to coordinate spermathecal contractions for successful ovulations.

Neurotransmitter signaling through heterotrimeric G proteins: insights from studies in C. elegans

Neurotransmitters signal via G protein coupled receptors (GPCRs) to modulate activity of neurons and muscles. C. elegans has ∼150 G protein coupled neuropeptide receptor homologs and 28 additional GPCRs for small-molecule neurotransmitters. Genetic studies in C. elegans demonstrate that neurotransmitters diffuse far from their release sites to activate GPCRs on distant cells. Individual receptor types are expressed on limited numbers of cells and thus can provide very specific regulation of an individual neural circuit and behavior. G protein coupled neurotransmitter receptors signal principally via the three types of heterotrimeric G proteins defined by the G alpha subunits Gαo, Gαq, and Gαs. Each of these G alpha proteins is found in all neurons plus some muscles. Gαo and Gαq signaling inhibit and activate neurotransmitter release, respectively. Gαs signaling, like Gαq signaling, promotes neurotransmitter release. Many details of the signaling mechanisms downstream of Gαq and Gαs have been delineated and are consistent with those of their mammalian orthologs. The details of the signaling mechanism downstream of Gαo remain a mystery. Forward genetic screens in C. elegans have identified new molecular components of neural G protein signaling mechanisms, including Regulators of G protein Signaling (RGS proteins) that inhibit signaling, a new Gαq effector (the Trio RhoGEF domain), and the RIC-8 protein that is required for neuronal Gα signaling. A model is presented in which G proteins sum up the variety of neuromodulator signals that impinge on a neuron to calculate its appropriate output level.

G Proteins and GPCRs in C. elegans Development: A Story of Mutual Infidelity

Many vital processes during C. elegans development, especially the establishment and maintenance of cell polarity in embryogenesis, are controlled by complex signaling pathways. G protein-coupled receptors (GPCRs), such as the four Frizzled family Wnt receptors, are linchpins in regulating and orchestrating several of these mechanisms. However, despite being GPCRs, which usually couple to G proteins, these receptors do not seem to activate classical heterotrimeric G protein-mediated signaling cascades. The view on signaling during embryogenesis is further complicated by the fact that heterotrimeric G proteins do play essential roles in cell polarity during embryogenesis, but their activity is modulated in a predominantly GPCR-independent manner via G protein regulators such as GEFs GAPs and GDIs. Further, the triggered downstream effectors are not typical. Only very few GPCR-dependent and G protein-mediated signaling pathways have been unambiguously defined in this context. This unusual and highly intriguing concept of separating GPCR function and G-protein activity, which is not restricted to embryogenesis in C. elegans but can also be found in other organisms, allows for essential and multi-faceted ways of regulating cellular communication and response. Although its relevance cannot be debated, its impact is still poorly discussed, and C. elegans is an ideal model to understand the underlying principles.

Genome-Wide Screen for Genes Involved in Caenorhabditis elegans Developmentally Timed Sleep

In Caenorhabditis elegans, Notch signaling regulates developmentally timed sleep during the transition from L4 larval stage to adulthood (L4/A) . To identify core sleep pathways and to find genes acting downstream of Notch signaling, we undertook the first genome-wide, classical genetic screen focused on C. elegans developmentally timed sleep. To increase screen efficiency, we first looked for mutations that suppressed inappropriate anachronistic sleep in adult hsp::osm-11 animals overexpressing the Notch coligand OSM-11 after heat shock. We retained suppressor lines that also had defects in L4/A developmentally timed sleep, without heat shock overexpression of the Notch coligand. Sixteen suppressor lines with defects in developmentally timed sleep were identified. One line carried a new allele of goa-1; loss of GOA-1 Gαo decreased C. elegans sleep. Another line carried a new allele of gpb-2, encoding a Gβ5 protein; Gβ5 proteins have not been previously implicated in sleep. In other scenarios, Gβ5 GPB-2 acts with regulators of G protein signaling (RGS proteins) EAT-16 and EGL-10 to terminate either EGL-30 Gαq signaling or GOA-1 Gαo signaling, respectively. We found that loss of Gβ5 GPB-2 or RGS EAT-16 decreased L4/A sleep. By contrast, EGL-10 loss had no impact. Instead, loss of RGS-1 and RGS-2 increased sleep. Combined, our results suggest that, in the context of L4/A sleep, GPB-2 predominantly acts with EAT-16 RGS to inhibit EGL-30 Gαq signaling. These results confirm the importance of G protein signaling in sleep and demonstrate that these core sleep pathways function genetically downstream of the Notch signaling events promoting sleep.

The C. elegans cGMP-dependent protein kinase EGL-4 regulates nociceptive behavioral sensitivity

Signaling levels within sensory neurons must be tightly regulated to allow cells to integrate information from multiple signaling inputs and to respond to new stimuli. Herein we report a new role for the cGMP-dependent protein kinase EGL-4 in the negative regulation of G protein-coupled nociceptive chemosensory signaling. C. elegans lacking EGL-4 function are hypersensitive in their behavioral response to low concentrations of the bitter tastant quinine and exhibit an elevated calcium flux in the ASH sensory neurons in response to quinine. We provide the first direct evidence for cGMP/PKG function in ASH and propose that ODR-1, GCY-27, GCY-33 and GCY-34 act in a non-cell-autonomous manner to provide cGMP for EGL-4 function in ASH. Our data suggest that activated EGL-4 dampens quinine sensitivity via phosphorylation and activation of the regulator of G protein signaling (RGS) proteins RGS-2 and RGS-3, which in turn downregulate Gα signaling and behavioral sensitivity.

Impacts of chronic low-level nicotine exposure on Caenorhabditis elegans reproduction: identification of novel gene targets

Effects and mechanisms of chronic exposure to low levels of nicotine is an area fundamentally important however less investigated. We employed the model organism Caenorhabditis elegans to investigate potential impacts of chronic (24h) and low nicotine exposure (6.17-194.5 μM) on stimulus-response, reproduction, and gene expressions. Nicotine significantly affects the organism's response to touch stimulus (p=0.031), which follows a dose-dependent pattern. Chronic nicotine exposure promotes early egg-laying events and slightly increased egg productions during the first 72 h of adulthood. The expressions of 10 (egl-10, egl-44, hlh-14, ric-3, unc-103, unc-50, unc-68, sod-1, oxi-1, and old-1) out of 18 selected genes were affected significantly. Other tested genes were cat-4, egl-19, egl-47, egl-5, lin-39, unc-43, pink-1, and age-1. Changes in gene expression were more evident at low dosages than at relatively high levels. Genes implicated in reproduction, cholinergic signaling, and stress response were regulated by nicotine, suggesting widespread physiological impacts of nicotine.

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.

Chemical dispersant potentiates crude oil impacts on growth, reproduction, and gene expression in Caenorhabditis elegans

The economic, environmental, and human health impacts of the deepwater horizon (DWH) oil spill have been of significant concern in the general public and among scientists. This study employs parallel experiments to test the effects of crude oil from the DWH oil well, chemical dispersant Corexit 9500A, and dispersant-oil mixture on growth and reproduction in the model organism Caenorhabditis elegans. Both the crude oil and the dispersant significantly inhibited the reproduction of C. elegans. Dose-dependent inhibitions of hatched larvae production were observed in worms exposed to both crude oil and dispersant. Importantly, the chemical dispersant Corexit 9500A potentiated crude oil effects; dispersant-oil mixture induced more significant effects than oil or dispersant-alone exposures. While oil-alone exposure and dispersant-alone exposure have none to moderate inhibitory effects on hatched larvae production, respectively, the mixture of dispersant and oil induced much more significant inhibition of offspring production. The production of hatched larvae was almost completely inhibited by several high concentrations of the dispersant-oil mixture. This suggests a sensitive bioassay for future investigation of oil/dispersant impacts on organisms. We also investigated the effects of crude oil/dispersant exposure at the molecular level by measuring the expressions of 31 functional genes. Results showed that the dispersant and the dispersant-oil mixture induced aberrant expressions of 12 protein-coding genes (cat-4, trxr-2, sdhb-1, lev-8, lin-39, unc-115, prdx-3, sod-1, acr-16, ric-3, unc-68, and acr-8). These 12 genes are associated with a variety of biological processes, including egg-laying, oxidative stress, muscle contraction, and neurological functions. In summary, the toxicity potentiating effect of chemical dispersant must be taken into consideration in future crude oil cleanup applications.

Structural domains required for Caenorhabditis elegans G protein-coupled receptor kinase 2 (GRK-2) function in vivo

G protein-coupled receptor kinases (GRKs) are key regulators of signal transduction that specifically phosphorylate activated G protein-coupled receptors (GPCRs) to terminate signaling. Biochemical and crystallographic studies have provided great insight into mammalian GRK2/3 interactions and structure. However, despite extensive in vitro characterization, little is known about the in vivo contribution of these described GRK structural domains and interactions to proper GRK function in signal regulation. We took advantage of the disrupted chemosensory behavior characteristic of Caenorhabditis elegans grk-2 mutants to discern the interactions required for proper in vivo Ce-GRK-2 function. Informed by mammalian crystallographic and biochemical data, we introduced amino acid substitutions into the Ce-grk-2 coding sequence that are predicted to selectively disrupt GPCR phosphorylation, Gα(q/11) binding, Gβγ binding, or phospholipid binding. Changing the most amino-terminal residues, which have been shown in mammalian systems to be required specifically for GPCR phosphorylation but not phosphorylation of alternative substrates or recruitment to activated GPCRs, eliminated the ability of Ce-GRK-2 to restore chemosensory signaling. Disrupting interaction between the predicted Ce-GRK-2 amino-terminal α-helix and kinase domain, posited to stabilize GRKs in their active ATP- and GPCR-bound conformation, also eliminated Ce-GRK-2 chemosensory function. Finally, although changing residues within the RH domain, predicted to disrupt interaction with Gα(q/11), did not affect Ce-GRK-2 chemosensory function, disruption of the predicted PH domain-mediated interactions with Gβγ and phospholipids revealed that both contribute to Ce-GRK-2 function in vivo. Combined, we have demonstrated functional roles for broadly conserved GRK2/3 structural domains in the in vivo regulation of organismal behavior.

A conserved protein interaction interface on the type 5 G protein beta subunit controls proteolytic stability and activity of R7 family regulator of G protein signaling proteins

Regulators of G protein signaling (RGS) proteins of the R7 subfamily limit signaling by neurotransmitters in the brain and by light in the retina. They form obligate complexes with the Gβ5 protein that are subject to proteolysis to control their abundance and alter signaling. The mechanisms that regulate this proteolysis, however, remain unclear. We used genetic screens to find mutations in Gβ5 that selectively destabilize one of the R7 RGS proteins in Caenorhabditis elegans. These mutations cluster at the binding interface between Gβ5 and the N terminus of R7 RGS proteins. Equivalent mutations within mammalian Gβ5 allowed the interface to still bind the N-terminal DEP domain of R7 RGS proteins, and mutant Gβ5-R7 RGS complexes initially formed in cells but were then rapidly degraded by proteolysis. Molecular dynamics simulations suggest the mutations weaken the Gβ5-DEP interface, thus promoting dynamic opening of the complex to expose determinants of proteolysis known to exist on the DEP domain. We propose that conformational rearrangements at the Gβ5-DEP interface are key to controlling the stability of R7 RGS protein complexes.

Proteins interacting with Caenorhabditis elegans Galpha subunits

To identify novel components in heterotrimeric G-protein signalling, we performed an extensive screen for proteins interacting with Caenorhabditis elegans Gα subunits. The genome of C. elegans contains homologues of each of the four mammalian classes of Gα subunits (Gs, Gi/o, Gq and G12), and 17 other Gα subunits. We tested 19 of the GGα subunits and four constitutively activated Gα subunits in a largescale yeast two-hybrid experiment. This resulted in the identification of 24 clones, representing 11 different proteins that interact with four different Gα subunits. This set includes C. elegans orthologues of known interactors of Gα subunits, such as AGS3 (LGN/PINS), CalNuc and Rap1Gap, but also novel proteins, including two members of the nuclear receptor super family and a homologue of human haspin (germ cell-specific kinase). All interactions were found to be unique for a specific Gα subunit but variable for the activation status of the Gα subunit. We used expression pattern and RNA interference analysis of the G-protein interactors in an attempt to substantiate the biological relevance of the observed interactions. Furthermore, by means of a membrane recruitment assay, we found evidence that GPA-7 and the nuclear receptor NHR-22 can interact in the animal.

Caenorhabditis elegans wsp-1 regulation of synaptic function at the neuromuscular junction

The Rho GTPase members and their effector proteins, such as the Wiskott-Aldrich syndrome protein (WASP), play critical roles in regulating actin dynamics that affect cell motility, endocytosis, cell division, and transport. It is well established that Caenorhabditis elegans wsp-1 plays an essential role in embryonic development. We were interested in the role of the C. elegans protein WSP-1 in the adult nematode. In this report, we show that a deletion mutant of wsp-1 exhibits a strong sensitivity to the neuromuscular inhibitor aldicarb. Transgenic rescue experiments demonstrated that neuronal expression of WSP-1 rescued this phenotype and that it required a functional WSP-1 Cdc42/Rac interactive binding domain. WSP-1-GFP fusion protein was found localized presynaptically, immediately adjacent to the synaptic protein RAB-3. Strong genetic interactions with wsp-1 and other genes involved in different stages of synaptic transmission were observed as the wsp-1(gm324) mutation suppresses the aldicarb resistance seen in unc-13(e51), unc-11(e47), and snt-1 (md290) mutants. These results provide genetic and pharmacological evidence that WSP-1 plays an essential role to stabilize the actin cytoskeleton at the neuronal active zone of the neuromuscular junction to restrain synaptic vesicle release.

RSBP-1 is a membrane-targeting subunit required by the Galpha(q)-specific but not the Galpha(o)-specific R7 regulator of G protein signaling in Caenorhabditis elegans

Regulator of G protein signaling (RGS) proteins inhibit G protein signaling by activating Galpha GTPase activity, but the mechanisms that regulate RGS activity are not well understood. The mammalian R7 binding protein (R7BP) can interact with all members of the R7 family of RGS proteins, and palmitoylation of R7BP can target R7 RGS proteins to the plasma membrane in cultured cells. However, whether endogenous R7 RGS proteins in neurons require R7BP or membrane localization for function remains unclear. We have identified and knocked out the only apparent R7BP homolog in Caenorhabditis elegans, RSBP-1. Genetic studies show that loss of RSBP-1 phenocopies loss of the R7 RGS protein EAT-16, but does not disrupt function of the related R7 RGS protein EGL-10. Biochemical analyses find that EAT-16 coimmunoprecipitates with RSBP-1 and is predominantly plasma membrane-associated, whereas EGL-10 does not coimmunoprecipitate with RSBP-1 and is not predominantly membrane-associated. Mutating the conserved membrane-targeting sequence in RSBP-1 disrupts both the membrane association and function of EAT-16, demonstrating that membrane targeting by RSBP-1 is essential for EAT-16 activity. Our analysis of endogenous R7 RGS proteins in C. elegans neurons reveals key differences in the functional requirements for membrane targeting between members of this protein family.

Regulators of AWC-mediated olfactory plasticity in Caenorhabditis elegans

While most sensory neurons will adapt to prolonged stimulation by down-regulating their responsiveness to the signal, it is not clear which events initiate long-lasting sensory adaptation. Likewise, we are just beginning to understand how the physiology of the adapted cell is altered. Caenorhabditis elegans is inherently attracted to specific odors that are sensed by the paired AWC olfactory sensory neurons. The attraction diminishes if the animal experiences these odors for a prolonged period of time in the absence of food. The AWC neuron responds acutely to odor-exposure by closing calcium channels. While odortaxis requires a Gα subunit protein, cGMP-gated channels, and guanylyl cyclases, adaptation to prolonged odor exposure requires nuclear entry of the cGMP-dependent protein kinase, EGL-4. We asked which candidate members of the olfactory signal transduction pathway promote nuclear entry of EGL-4 and which molecules might induce long-term adaptation downstream of EGL-4 nuclear entry. We found that initiation of long-term adaptation, as assessed by nuclear entry of EGL-4, is dependent on G-protein mediated signaling but is independent of fluxes in calcium levels. We show that long-term adaptation requires polyunsaturated fatty acids (PUFAs) that may act on the transient receptor potential (TRP) channel type V OSM-9 downstream of EGL-4 nuclear entry. We also present evidence that high diacylglycerol (DAG) levels block long-term adaptation without affecting EGL-4 nuclear entry. Our analysis provides a model for the process of long-term adaptation that occurs within the AWC neuron of C. elegans: G-protein signaling initiates long-lasting olfactory adaptation by promoting the nuclear entry of EGL-4, and once EGL-4 has entered the nucleus, processes such as PUFA activation of the TRP channel OSM-9 may dampen the output of the AWC neuron.

Caenorhabditis elegans is capable of sensing a variety of attractive volatile compounds. These odors are the worm's “best guesses” as to how to track down food. Employing calculated approximations underlies a foraging strategy that is open to failure. When C. elegans track an odor which proves unrewarding, they must modify their behavior based on this experience. They also need to prevent over-stimulating their neurons. To accomplish this, C. elegans olfactory sensory neurons adapt to odors after a sustained exposure to odor in the absence of food. Within the pair of primary odor-sensory neurons, termed the AWCs, adaptation requires the cGMP-dependent protein kinase G (PKG), EGL-4. Exposing animals to AWC–sensed odors for approximately 60 minutes results in a long-lasting (∼3 hour) adaptation that requires the nuclear translocation of EGL-4. To understand how sensory transduction and desensitization machinery converge to achieve olfactory adaptation, we asked whether odor-induced EGL-4 nuclear accumulation was affected by gene mutations that abrogate either odor sensation of or adaptation to AWC–sensed odors. We find that G-protein signaling represents the integration point where primary odor sensation and odor adaptation pathways diverge. PUFA signaling, calcium, and decreased diacylglycerol all dampen the response of the AWC neuron to odor downstream of this divergence.

Insights into RGS protein function from studies in Caenorhabditis elegans

The nematode worm, Caenorhabditis elegans, contains orthologs of most regulator of G protein signaling (RGS) protein subfamilies and all four G protein α-subunit subfamilies found in mammals. Every C. elegans RGS and Gα gene has been knocked out, and the in vivo functions and Gα targets of a number of RGS proteins have been characterized in detail. This has revealed a complex relationship between the RGS and Gα proteins, in which multiple RGS proteins can regulate the same Gα protein, either by acting redundantly or by exerting control over signaling under different circumstances or in different cells. RGS proteins that are coexpressed can also show specificity for distinct Gα targets in vivo, and the determinants of such specificity can reside outside of the RGS domain. This review will discuss how analysis in C. elegans may aid us in achieving a full understanding of the physiological functions of RGS proteins.

Prediction of protein-protein interfaces on G-protein beta subunits reveals a novel phospholipase C beta2 binding domain

Gbeta subunits from heterotrimeric G-proteins (guanine nucleotide-binding proteins) directly bind diverse proteins, including effectors and regulators, to modulate a wide array of signaling cascades. These numerous interactions constrained the evolution of the molecular surface of Gbeta. Although mammals contain five Gbeta genes comprising two classes (Gbeta1-like and Gbeta5-like), plants and fungi have a single ortholog, and organisms such as Caenorhabditis elegans and Drosophila melanogaster contain one copy from each class. A limited number of crystal structures of complexes containing Gbeta subunits and complementary biochemical data highlight specific sites within Gbetas needed for protein interactions. It is difficult to determine from these interaction sites what, if any, additional regions of the Gbeta molecular surface comprise interaction interfaces essential to Gbeta's role as a nexus in numerous signaling cascades. We used a comparative evolutionary approach to identify five known and eight previously unknown putative interfaces on the surface of Gbeta. We show that one such novel interface occurs between Gbeta and phospholipase C beta2 (PLC-beta2), a mammalian Gbeta interacting protein. Substitutions of residues within this Gbeta-PLC-beta2 interface reduce the activation of PLC-beta2 by Gbeta1, confirming that our de novo comparative evolutionary approach predicts previously unknown Gbeta-protein interfaces. Similarly, we hypothesize that the seven remaining untested novel regions contribute to putative interfaces for other Gbeta interacting proteins. Finally, this comparative evolutionary approach is suitable for application to any protein involved in a significant number of protein-protein interactions.

A network of G-protein signaling pathways control neuronal activity in C. elegans

The Caenorhabditis elegans neuromuscular junction (NMJ) is one of the best studied synapses in any organism. A variety of genetic screens have identified genes required both for the essential steps of neurotransmitter release from motorneurons as well as the signaling pathways that regulate rates of neurotransmitter release. A number of these regulatory genes encode proteins that converge to regulate neurotransmitter release. In other cases genes are known to regulate signaling at the NMJ but how they act remains unknown. Many of the proteins that regulate activity at the NMJ participate in a network of heterotrimeric G-protein signaling pathways controlling the release of synaptic vesicles and/or dense-core vesicles (DCVs). At least four heterotrimeric G-proteins (Galphaq, Galpha12, Galphao, and Galphas) act within the motorneurons to control the activity of the NMJ. The Galphaq, Galpha12, and Galphao pathways converge to control production and destruction of the lipid-bound second messenger diacylglycerol (DAG) at sites of neurotransmitter release. DAG acts via at least two effectors, MUNC13 and PKC, to control the release of both neurotransmitters and neuropeptides from motorneurons. The Galphas pathway converges with the other three heterotrimeric G-protein pathways downstream of DAG to regulate neuropeptide release. Released neurotransmitters and neuropeptides then act to control contraction of the body-wall muscles to control locomotion. The lipids and proteins involved in these networks are conserved between C. elegans and mammals. Thus, the C. elegans NMJ acts as a model synapse to understand how neuronal activity in the human brain is regulated.

The Parallel Worm Tracker: a platform for measuring average speed and drug-induced paralysis in nematodes

BACKGROUND

Caenorhabditis elegans locomotion is a simple behavior that has been widely used to dissect genetic components of behavior, synaptic transmission, and muscle function. Many of the paradigms that have been created to study C. elegans locomotion rely on qualitative experimenter observation. Here we report the implementation of an automated tracking system developed to quantify the locomotion of multiple individual worms in parallel.

METHODOLOGY/PRINCIPAL FINDINGS

Our tracking system generates a consistent measurement of locomotion that allows direct comparison of results across experiments and experimenters and provides a standard method to share data between laboratories. The tracker utilizes a video camera attached to a zoom lens and a software package implemented in MATLAB. We demonstrate several proof-of-principle applications for the tracker including measuring speed in the absence and presence of food and in the presence of serotonin. We further use the tracker to automatically quantify the time course of paralysis of worms exposed to aldicarb and levamisole and show that tracker performance compares favorably to data generated using a hand-scored metric.

CONCLUSIONS/SIGNIFICANCE

Although this is not the first automated tracking system developed to measure C. elegans locomotion, our tracking software package is freely available and provides a simple interface that includes tools for rapid data collection and analysis. By contrast with other tools, it is not dependent on a specific set of hardware. We propose that the tracker may be used for a broad range of additional worm locomotion applications including genetic and chemical screening.

Overexpression of PPK-1, the Caenorhabditis elegans Type I PIP kinase, inhibits growth cone collapse in the developing nervous system and causes axonal degeneration in adults

Growth cones are dynamic membrane structures that migrate to target tissue by rearranging their cytoskeleton in response to environmental cues. The lipid phosphatidylinositol (4,5) bisphosphate (PIP(2)) resides on the plasma membrane of all eukaryotic cells and is thought to be required for actin cytoskeleton rearrangements. Thus PIP(2) is likely to play a role during neuron development, but this has never been tested in vivo. In this study, we have characterized the PIP(2) synthesizing enzyme Type I PIP kinase (ppk-1) in Caenorhabditis elegans. PPK-1 is strongly expressed in the nervous system, and can localize to the plasma membrane. We show that PPK-1 purified from C. elegans can generate PIP(2)in vitro and that overexpression of the kinase causes an increase in PIP(2) levels in vivo. In developing neurons, PPK-1 overexpression leads to growth cones that become stalled, produce ectopic membrane projections, and branched axons. Once neurons are established, PPK-1 overexpression results in progressive membrane overgrowth and degeneration during adulthood. These data suggest that overexpression of the Type I PIP kinase inhibits growth cone collapse, and that regulation of PIP(2) levels in established neurons may be important to maintain structural integrity and prevent neuronal degeneration.

Trio's Rho-specific GEF domain is the missing Galpha q effector in C. elegans

The Galpha(q) pathway is essential for animal life and is a central pathway for driving locomotion, egg laying, and growth in Caenorhabditis elegans, where it exerts its effects through EGL-8 (phospholipase Cbeta [PLCbeta]) and at least one other effector. To find the missing effector, we performed forward genetic screens to suppress the slow growth and hyperactive behaviors of mutants with an overactive Galpha(q) pathway. Four suppressor mutations disrupted the Rho-specific guanine-nucleotide exchange factor (GEF) domain of UNC-73 (Trio). The mutations produce defects in neuronal function, but not neuronal development, that cause sluggish locomotion similar to animals lacking EGL-8 (PLCbeta). Strains containing null mutations in both EGL-8 (PLCbeta) and UNC-73 (Trio RhoGEF) have strong synthetic phenotypes that phenocopy the arrested growth and near-complete paralysis of Galpha(q)-null mutants. Using cell-based and biochemical assays, we show that activated C. elegans Galpha(q) synergizes with Trio RhoGEF to activate RhoA. Activated Galpha(q) and Trio RhoGEF appear to be part of a signaling complex, because they coimmunoprecipitate when expressed together in cells. Our results show that Trio's Rho-specific GEF domain is a major Galpha(q) effector that, together with PLCbeta, mediates the Galpha(q) signaling that drives the locomotion, egg laying, and growth of the animal.

Finding function in novel targets: C. elegans as a model organism

Despite its apparent simplicity, the nematode worm Caenorhabditis elegans has developed into an important model for biomedical research, particularly in the functional characterization of novel drug targets that have been identified using genomics technologies. The cellular complexity and the conservation of disease pathways between C. elegans and higher organisms, together with the simplicity and cost-effectiveness of cultivation, make for an effective in vivo model that is amenable to whole-organism high-throughput compound screens and large-scale target validation. This review describes how C. elegans models can be used to advance our understanding of the molecular mechanisms of drug action and disease pathogenesis.

New roles for Galpha and RGS proteins: communication continues despite pulling sisters apart

Large G protein alpha subunits and their attendant regulators of G-protein signaling (RGS) proteins control both intercellular signaling and asymmetric cell divisions by distinct pathways. The classical pathway, found throughout higher eukaryotic organisms, mediates intercellular communication via hormone binding to G-protein-coupled receptors (GPCRs). Recent studies have led to the discovery of GPCR-independent activation of Galpha subunits by the guanine nucleotide exchange factor RIC-8 in both asymmetric cell division and synaptic vesicle priming in metazoan organisms. Protein-protein interactions and protein function in each pathway are driven through the cycle of GTP binding and hydrolysis by the Galpha subunit. This review builds a conceptual framework for understanding RIC-8-mediated pathways by comparison with the mechanism of classical G-protein activation and inhibition in GPCR signaling.

MAU-8 is a Phosducin-like Protein required for G protein signaling in C. elegans

The mau-8(qm57) mutation inhibits the function of GPB-2, a heterotrimeric G protein beta subunit, and profoundly affects behavior through the Galphaq/Galphao signaling network in C. elegans. mau-8 encodes a nematode Phosducin-like Protein (PhLP), and the qm57 mutation leads to the loss of a predicted phosphorylation site in the C-terminal domain of PhLP that binds the Gbetagamma surface implicated in membrane interactions. In developing embryos, MAU-8/PhLP localizes to the cortical region, concentrates at the centrosomes of mitotic cells and remains associated with the germline blastomere. In adult animals, MAU-8/PhLP is ubiquitously expressed in somatic tissues and germline cells. MAU-8/PhLP interacts with the PAR-5/14.3.3 protein and with the Gbeta subunit GPB-1. In mau-8 mutants, the disruption of MAU-8/PhLP stabilizes the association of GPB-1 with the microtubules of centrosomes. Our results indicate that MAU-8/PhLP modulates G protein signaling, stability and subcellular location to regulate various physiological functions, and they suggest that MAU-8 might not be limited to the Galphaq/Galphao network.

Goalpha regulates olfactory adaptation by antagonizing Gqalpha-DAG signaling in Caenorhabditis elegans

The heterotrimeric G protein G(o) is abundantly expressed in the mammalian nervous system and modulates neural activities in response to various ligands. However, G(o)'s functions in living animals are less well understood. Here, we demonstrate that GOA-1 G(o)alpha has a fundamental role in olfactory adaptation in Caenorhabditis elegans. Impairment of GOA-1 G(o)alpha function and excessive activation of EGL-30 G(q)alpha cause a defect in adaptation to AWC-sensed odorants. These pathways antagonistically modulate olfactory adaptation in AWC chemosensory neurons. Wild-type animals treated with phorbol esters and double-mutant animals of diacylglycerol (DAG) kinases, dgk-3; dgk-1, also have a defect in adaptation, suggesting that elevated DAG signals disrupt normal adaptation. Constitutively active GOA-1 can suppress the adaptation defect of dgk-3; dgk-1 double mutants, whereas it fails to suppress the adaptation defect of animals with constitutively active EGL-30, implying that GOA-1 acts upstream of EGL-30 in olfactory adaptation. Our results suggest that down-regulation of EGL-30-DAG signaling by GOA-1 underlies olfactory adaptation and plasticity of chemotaxis.

Presynaptic UNC-31 (CAPS) is required to activate the G alpha(s) pathway of the Caenorhabditis elegans synaptic signaling network

C. elegans mutants lacking the dense-core vesicle priming protein UNC-31 (CAPS) share highly similar phenotypes with mutants lacking a neuronal G alpha(s) pathway, including strong paralysis despite exhibiting near normal levels of steady-state acetylcholine release as indicated by drug sensitivity assays. Our genetic analysis shows that UNC-31 and neuronal G alpha(s) are different parts of the same pathway and that the UNC-31/G alpha(s) pathway is functionally distinct from the presynaptic G alpha(q) pathway with which it interacts. UNC-31 acts upstream of G alpha(s) because mutations that activate the G alpha(s) pathway confer similar levels of strongly hyperactive, coordinated locomotion in both unc-31 null and (+) backgrounds. Using cell-specific promoters, we show that both UNC-31 and the G alpha(s) pathway function in cholinergic motor neurons to regulate locomotion rate. Using immunostaining we show that UNC-31 is often concentrated at or near active zones of cholinergic motor neuron synapses. Our data suggest that presynaptic UNC-31 activity, likely acting via dense-core vesicle exocytosis, is required to locally activate the neuronal G alpha(s) pathway near synaptic active zones.

Using microarrays to facilitate positional cloning: identification of tomosyn as an inhibitor of neurosecretion

Forward genetic screens have been used as a powerful strategy to dissect complex biological pathways in many model systems. A significant limitation of this approach has been the time-consuming and costly process of positional cloning and molecular characterization of the mutations isolated in these screens. Here, the authors describe a strategy using microarray hybridizations to facilitate positional cloning. This method relies on the fact that premature stop codons (i.e., nonsense mutations) constitute a frequent class of mutations isolated in screens and that nonsense mutant messenger RNAs are efficiently degraded by the conserved nonsense-mediated decay pathway. They validate this strategy by identifying two previously uncharacterized mutations: (1) tom-1, a mutation found in a forward genetic screen for enhanced acetylcholine secretion in Caenorhabditis elegans, and (2) an apparently spontaneous mutation in the hif-1 transcription factor gene. They further demonstrate the broad applicability of this strategy using other known mutants in C. elegans,Arabidopsis, and mouse. Characterization of tom-1 mutants suggests that TOM-1, the C. elegans ortholog of mammalian tomosyn, functions as an endogenous inhibitor of neurotransmitter secretion. These results also suggest that microarray hybridizations have the potential to significantly reduce the time and effort required for positional cloning.

Genetic screens are commonly used to figure out which genes are involved in a biological process. The first step in a genetic screen is to isolate mutant animals that are defective in the process being studied. The next step is to find which of the thousands of genes has the mutation that causes the observed defect. Positional cloning, the tried-and-true method for locating mutations, is slow and expensive. The authors propose using microarray hybridizations to speed the process. Their approach relies on the fact that a large fraction of the mutations found in screens are the results of premature stop codons, a particularly severe type of mutation. In cells, messages containing premature stop codons are rapidly destroyed by a protective pathway, called nonsense-mediated decay, thus making them directly detectable by microarray hybridization.

The authors apply this strategy retrospectively to known mutants in Caenorhabditis elegans, Arabidopsis, and mouse. They identify two uncharacterized mutations in C. elegans, including one, tom-1, found in a forward genetic screen for enhancers of neurotransmission. Interestingly, their characterization of tom-1 mutants suggests that the highly conserved protein tomosyn inhibits neurotransmission in neurons. This study shows that microarray hybridizations will help reduce the time and effort required for positional cloning.

Functional constraint and divergence in the G protein family in Caenorhabditis elegans and Caenorhabditis briggsae

Part of the challenge of the post-genomic world is to identify functional elements within the wide array of information generated by genome sequencing. Although cross-species comparisons and investigation of rates of sequence divergence are an efficient approach, the relationship between sequence divergence and functional conservation is not clear. Here, we use a comparative approach to examine questions of evolutionary rates and conserved function within the guanine nucleotide-binding protein (G protein) gene family in nematodes of the genus Caenorhabditis. In particular, we show that, in cases where the Caenorhabditis elegans ortholog shows a loss-of-function phenotype, G protein genes of C. elegans and Caenorhabditis briggsae diverge on average three times more slowly than G protein genes that do not exhibit any phenotype when mutated in C. elegans, suggesting that genes with loss of function phenotypes are subject to stronger selective constraints in relation to their function in both species. Our results also indicate that selection is as strong on G proteins involved in environmental perception as it is on those controlling other important processes. Finally, using phylogenetic footprinting, we identify a conserved non-coding motif present in multiple copies in the genomes of four species of Caenorhabditis. The presence of this motif in the same intron in the gpa-1 genes of C. elegans, C. briggsae and Caenorhabditis remanei suggests that it plays a role in the regulation of gpa-1, as well as other loci.

The GAR-3 muscarinic receptor cooperates with calcium signals to regulate muscle contraction in the Caenorhabditis elegans pharynx

Muscarinic acetylcholine receptors regulate the activity of neurons and muscle cells through G-protein-coupled cascades. Here, we identify a pathway through which the GAR-3 muscarinic receptor regulates both membrane potential and excitation-contraction coupling in the Caenorhabditis elegans pharyngeal muscle. GAR-3 signaling is enhanced in worms overexpressing gar-3 or lacking GPB-2, a G-protein beta-subunit involved in RGS-mediated inhibition of G(o)alpha- and G(q)alpha-linked pathways. High levels of signaling through GAR-3 inhibit pharyngeal muscle relaxation and impair feeding--but do not block muscle repolarization--when worms are exposed to arecoline, a muscarinic agonist. Loss of gar-3 function results in shortened action potentials and brief muscle contractions in the pharyngeal terminal bulb. High levels of calcium entry through voltage-gated channels also impair terminal bulb relaxation and sensitize worms to the toxic effects of arecoline. Mutation of gar-3 reverses this sensitivity, suggesting that GAR-3 regulates calcium influx or calcium-dependent processes. Because the effects of GAR-3 signaling on membrane depolarization and muscle contraction can be separated, we conclude that GAR-3 regulates multiple calcium-dependent processes in the C. elegans pharyngeal muscle.

Resistance to volatile anesthetics by mutations enhancing excitatory neurotransmitter release in Caenorhabditis elegans

The molecular mechanisms whereby volatile general anesthetics (VAs) disrupt behavior remain undefined. In Caenorhabditis elegans mutations in the gene unc-64, which encodes the presynaptic protein syntaxin 1A, produce large allele-specific differences in VA sensitivity. UNC-64 syntaxin normally functions to mediate fusion of neurotransmitter vesicles with the presynaptic membrane. The precise role of syntaxin in the VA mechanism is as yet unclear, but a variety of results suggests that a protein interacting with syntaxin to regulate neurotransmitter release is essential for VA action in C. elegans. To identify additional proteins that function with syntaxin to control neurotransmitter release and VA action, we screened for suppressors of the phenotypes produced by unc-64 reduction of function. Loss-of-function mutations in slo-1, which encodes a Ca(2+)-activated K+ channel, and in unc-43, which encodes CaM-kinase II, and a gain-of-function mutation in egl-30, which encodes Gqalpha, were isolated as syntaxin suppressors. The slo-1 and egl-30 mutations conferred resistance to VAs, but unc-43 mutations did not. The effects of slo-1 and egl-30 on VA sensitivity can be explained by their actions upstream or parallel to syntaxin to increase the level of excitatory neurotransmitter release. These results strengthen the link between transmitter release and VA action.

A network of stimulatory and inhibitory Galpha-subunits regulates olfaction in Caenorhabditis elegans

The two pairs of sensory neurons of C. elegans, AWA and AWC, that mediate odorant attraction, express six Galpha-subunits, suggesting that olfaction is regulated by a complex signaling network. Here, we describe the cellular localization and functions of the six olfactory Galpha-subunits: GPA-2, GPA-3, GPA-5, GPA-6, GPA-13, and ODR-3. All except GPA-6 localize to sensory cilia, suggesting a direct role in sensory transduction. GPA-2, GPA-3, GPA-5, and GPA-6 are also present in cell bodies and axons and GPA-5 specifically localizes to synaptic sites. Analysis of animals with single- to sixfold loss-of-function mutations shows that olfaction involves a balance between multiple stimulatory and inhibitory signals. ODR-3 constitutes the main stimulatory signal and is sufficient for the detection of odorants. GPA-3 forms a second stimulatory signal in the AWA and AWC neurons, also sufficient for odorant detection. In AWA, signaling is suppressed by GPA-5. In AWC, GPA-2 and GPA-13 negatively and positively regulate signaling, respectively. Finally, we show that only ODR-3 plays a role in cilia morphogenesis. Defects in this process are, however, independent of olfactory behavior. Our findings reveal the existence of a complex signaling network that controls odorant detection by C. elegans.

Convergent, RIC-8-dependent Galpha signaling pathways in the Caenorhabditis elegans synaptic signaling network

We used gain-of-function and null synaptic signaling network mutants to investigate the relationship of the G alpha(q) and G alpha(s) pathways to synaptic vesicle priming and to each other. Genetic epistasis studies using G alpha(q) gain-of-function and null mutations, along with a mutation that blocks synaptic vesicle priming and the synaptic vesicle priming stimulator phorbol ester, suggest that the G alpha(q) pathway generates the core, obligatory signals for synaptic vesicle priming. In contrast, the G alpha(s) pathway is not required for the core priming function, because steady-state levels of neurotransmitter release are not significantly altered in animals lacking a neuronal G alpha(s) pathway, even though these animals are strongly paralyzed as a result of functional (nondevelopmental) defects. However, our genetic analysis indicates that these two functionally distinct pathways converge and that they do so downstream of DAG production. Further linking the two pathways, our epistasis analysis of a ric-8 null mutant suggests that RIC-8 (a receptor-independent G alpha guanine nucleotide exchange factor) is required to maintain both the G alpha(q) vesicle priming pathway and the neuronal G alpha(s) pathway in a functional state. We propose that the neuronal G alpha(s) pathway transduces critical positional information onto the core G alpha(q) pathway to stabilize the priming of selected synapses that are optimal for locomotion.

Mutations that rescue the paralysis of Caenorhabditis elegans ric-8 (synembryn) mutants activate the G alpha(s) pathway and define a third major branch of the synaptic signaling network

To identify hypothesized missing components of the synaptic G alpha(o)-G alpha(q) signaling network, which tightly regulates neurotransmitter release, we undertook two large forward genetic screens in the model organism C. elegans and focused first on mutations that strongly rescue the paralysis of ric-8(md303) reduction-of-function mutants, previously shown to be defective in G alpha(q) pathway activation. Through high-resolution mapping followed by sequence analysis, we show that these mutations affect four genes. Two activate the G alpha(q) pathway through gain-of-function mutations in G alpha(q); however, all of the remaining mutations activate components of the G alpha(s) pathway, including G alpha(s), adenylyl cyclase, and protein kinase A. Pharmacological assays suggest that the G alpha(s) pathway-activating mutations increase steady-state neurotransmitter release, and the strongly impaired neurotransmitter release of ric-8(md303) mutants is rescued to greater than wild-type levels by the strongest G alpha(s) pathway activating mutations. Using transgene induction studies, we show that activating the G alpha(s) pathway in adult animals rapidly induces hyperactive locomotion and rapidly rescues the paralysis of the ric-8 mutant. Using cell-specific promoters we show that neuronal, but not muscle, G alpha(s) pathway activation is sufficient to rescue ric-8(md303)'s paralysis. Our results appear to link RIC-8 (synembryn) and a third major G alpha pathway, the G alpha(s) pathway, with the previously discovered G alpha(o) and G alpha(q) pathways of the synaptic signaling network.

Mechanism of extrasynaptic dopamine signaling in Caenorhabditis elegans

D1-like and D2-like dopamine receptors have synergistic and antagonistic effects on behavior. To understand the mechanisms underlying these effects, we studied dopamine signaling genetically in Caenorhabditis elegans. Knocking out a D2-like receptor, DOP-3, caused locomotion defects similar to those observed in animals lacking dopamine. Knocking out a D1-like receptor, DOP-1, reversed the defects of the DOP-3 knockout. DOP-3 and DOP-1 have their antagonistic effects on locomotion by acting in the same motor neurons, which coexpress the receptors and which are not postsynaptic to dopaminergic neurons. In a screen for mutants unable to respond to dopamine, we identified four genes that encode components of the antagonistic Galpha(o) and Galpha(q) signaling pathways, including Galpha(o) itself and two subunits of the regulator of G protein signaling (RGS) complex that inhibits Galpha(q). Our results indicate that extrasynaptic dopamine regulates C. elegans locomotion through D1- and D2-like receptors that activate the antagonistic Galpha(q) and Galpha(o) signaling pathways, respectively.

Serotonin and Go modulate functional states of neurons and muscles controlling C. elegans egg-laying behavior

From nematodes to humans, animals employ neuromodulators like serotonin to regulate behavioral patterns and states. In the nematode C. elegans, serotonin has been shown to act in a modulatory fashion to increase the rate and alter the temporal pattern of egg laying. Though many candidate effectors and regulators of serotonin have been identified in genetic studies, their effects on specific neurons and muscles in the egg-laying circuitry have been difficult to determine. Using the genetically encoded Ca(2+) indicator cameleon, we found that serotonin acts directly on the vulval muscles to increase the frequency of Ca(2+) transients. In contrast, we found that the spontaneous activity of the egg-laying motorneurons was silenced by serotonin. Mutations in G protein alpha subunit genes altered the responses of both vulval muscles and egg-laying neurons to serotonin; specifically, mutations in the G(q)alpha homolog egl-30 blocked serotonin stimulation of vulval muscle Ca(2+) transients, while mutations in the G(o)alpha homolog goa-1 prevented the silencing of motorneuron activity by serotonin. These data indicate that serotonin stimulates egg laying by directly modulating the functional state of the vulval muscles. In addition, serotonin inhibits the activity of the motorneurons that release it, providing a feedback regulatory effect that may contribute to serotonin adaptation.

Instability of GGL domain-containing RGS proteins in mice lacking the G protein beta-subunit Gbeta5

RGS (regulator of G protein signaling) proteins containing the G protein gamma-like (GGL) domain (RGS6, RGS7, RGS9, and RGS11) interact with the fifth member of the G protein beta-subunit family, Gbeta5. This interaction is necessary for the stability of both the RGS protein and for Gbeta5. Consistent with this notion, we have found that elevation of RGS9-1 mRNA levels by transgene expression does not increase RGS9-1 protein level in the retina, suggesting that Gbeta5 levels may be limiting. To examine further the interactions of Gbeta5 and the GGL domain-containing RGS proteins, we inactivated the Gbeta5 gene. We found that the levels of GGL domain-containing RGS proteins in retinas and in striatum are eliminated or reduced drastically, whereas the levels of Ggamma2 and RGS4 proteins remain normal in the absence of Gbeta5. The homozygous Gbeta5 knockout (Gbeta5-/-) mice derived from heterozygous knockout mating are runty and exhibit a high preweaning mortality rate. We concluded that complex formation between GGL domain-containing RGS proteins and the Gbeta5 protein is necessary to maintain their mutual stability in vivo. Furthermore, in the absence of Gbeta5 and all four RGS proteins that form protein complexes with Gbeta5, the animals that survive into adulthood are viable and have no gross defects in brain or retinal morphology.

Molecular evolution and quantitative variation for chemosensory behaviour in the nematode genus Caenorhabditis

Caenorhabditis elegans is a model organism in biology, yet despite the tremendous information generated from genetic, genomic and functional analyses, C. elegans has rarely been used to address questions in ecological genetics. Here, we analyse genetic variation for chemosensory behaviour, an ecologically important trait that is also genetically well characterized, at both the phenotypic and molecular levels within three species of the genus Caenorhabditis. We show that the G-protein ODR-3 plays an important role in chemosensory avoidance behaviour and identify orthologues of odr-3 in C. briggsae and C. remanei. Both quantitative genetic analysis of chemosensory behaviour and molecular population genetic analysis of odr-3 show that there is little genetic variation among a worldwide collection of isolates of the primarily selfing C. elegans, whereas there is substantially more variation within a single population of the outcrossing C. remanei. Although there are a large number of substitutions at silent sites within odr-3 among the three species, molecular evolution at the protein level is extremely conserved, suggesting that odr-3 plays an important role in cell signalling during chemosensation and/or neuronal cilia development in C. remanei and in C. briggsae as it does in C. elegans. Our results suggest that C. remanei may be a more suitable subject for ecological and evolutionary genetic studies than C. elegans.

Ggamma subunit-selective G protein beta 5 mutant defines regulators of G protein signaling protein binding requirement for nuclear localization

The signal transducing function of Gbeta(5) in brain is unknown. When studied in vitro Gbeta(5) is the only heterotrimeric Gbeta subunit known to interact with both Ggamma subunits and regulators of G protein signaling (RGS) proteins. When tested with Ggamma, Gbeta(5) interacts with other classical components of heterotrimeric G protein signaling pathways such as Galpha and phospholipase C-beta. We recently demonstrated nuclear expression of Gbeta(5) in neurons and brain (Zhang, J. H., Barr, V. A., Mo, Y., Rojkova, A. M., Liu, S., and Simonds, W. F. (2001) J. Biol. Chem. 276, 10284-10289). To gain further insight into the mechanism of Gbeta(5) nuclear localization, we generated a Gbeta(5) mutant deficient in its ability to interact with RGS7 while retaining its ability to bind Ggamma, and we compared its properties to the wild-type Gbeta(5). In HEK-293 cells co-transfection of RGS7 but not Ggamma(2) supported expression in the nuclear fraction of transfected wild-type Gbeta(5). In contrast the Ggamma-preferring Gbeta(5) mutant was not expressed in the HEK-293 cell nuclear fraction with either co-transfectant. The Ggamma-selective Gbeta(5) mutant was also excluded from the cell nucleus of transfected PC12 cells analyzed by laser confocal microscopy. These results define a requirement for RGS protein binding for Gbeta(5) nuclear expression.

Hyperactivation of the G12-mediated signaling pathway in Caenorhabditis elegans induces a developmental growth arrest via protein kinase C

The G(12) type of heterotrimeric G-proteins play an important role in development and behave as potent oncogenes in cultured cells. However, little is known about the molecular nature of the components that act in the G(12)-signaling pathway in an organism. We characterized a C. elegans Galpha subunit gene, gpa-12, which is a homolog of mammalian G(12)/G(13)alpha, and found that animals defective in gpa-12 are viable. Expression of activated GPA-12 (G(12)QL) results in a developmental growth arrest caused by a feeding behavior defect that is due to a dramatic reduction in pharyngeal pumping. To elucidate the molecular nature of the signaling pathways in which G(12) participates, we screened for suppressors of the G(12)QL phenotype. We isolated 50 suppressors that contain mutations in tpa-1, which encodes two protein kinase C isoforms, TPA-1A and TPA-1B, most similar to PKCtheta/delta. TPA-1 mediates the action of the tumor promoter PMA. Expression of G(12)QL and treatment of wild-type animals with PMA induce an identical growth arrest caused by inhibition of larval feeding, which is dependent on TPA-1A and TPA-1B function. These results suggest that TPA-1 is a downstream target of both G(12) signaling and PMA in modulating feeding and growth in C. elegans. Taken together, our findings provide a potential molecular mechanism for the transforming capability of G(12) proteins.

Differential expression of G protein alpha and beta subunit genes during development of Phytophthora infestans

A G protein alpha subunit gene (pigpa1) and a G protein beta subunit gene (pigpb1) were isolated from the oomycete Phytophthora infestans, the causal agent of potato late blight. Heterotrimeric G proteins are evolutionary conserved GTP-binding proteins that are composed of alpha,beta, and gamma subunits and participate in diverse signal transduction pathways. The deduced amino acid sequence of both pigpa1 and pigpb1, showed the typical conserved motifs present in Galpha or Gbeta proteins from other eukaryotes. Southern blot analysis revealed no additional copies of Galpha or Gbeta subunit genes in P. infestans, suggesting that pigpa1 and pigpb1 are single copy genes. By cross-hybridization homologues of gpa1 and gpb1 were detected in other Phythophthora species. Expression analyses revealed that both genes are differentially expressed during asexual development, with the highest mRNA levels in sporangia. In mycelium, no pigpa1 mRNA was detected. Western blot analysis using a polyclonal GPA1 antibody confirmed the differential expression of pigpa1. These expression patterns suggest a role for G-protein-mediated signaling during formation and germination of asexual spores of P. infestans, developmental stages representing the initial steps of the infection process.

Functional characterization of the adenylyl cyclase gene sgs-1 by analysis of a mutational spectrum in Caenorhabditis elegans

The sgs-1 (suppressor of activated Galpha(s)) gene encodes one of the four adenylyl cyclases in the nematode C. elegans and is most similar to mammalian adenylyl cyclase type IX. We isolated a complete loss-of-function mutation in sgs-1 and found it to result in animals with retarded development that arrest in variable larval stages. sgs-1 mutant animals exhibit lethargic movement and pharyngeal pumping and (while not reaching adulthood) have a mean life span that is > 50% extended compared to wild type. An extensive set of reduction-of-function mutations in sgs-1 was isolated in a screen for suppressors of a neuronal degeneration phenotype induced by the expression of a constitutively active version of the heterotrimeric Galpha(s) subunit of C. elegans. Although most of these mutations change conserved residues within the catalytic domains of sgs-1, mutations in the less-conserved transmembrane domains are also found. The sgs-1 reduction-of-function mutants are viable and have reduced locomotion rates, but do not show defects in pharyngeal pumping or life span.

A Caenorhabditis elegans pheromone antagonizes volatile anesthetic action through a go-coupled pathway

Volatile anesthetics (VAs) disrupt nervous system function by an ill-defined mechanism with no known specific antagonists. During the course of characterizing the response of the nematode C. elegans to VAs, we discovered that a C. elegans pheromone antagonizes the VA halothane. Acute exposure to pheromone rendered wild-type C. elegans resistant to clinical concentrations of halothane, increasing the EC(50) from 0.43 +/- 0.03 to 0.90 +/- 0.02. C. elegans mutants that disrupt the function of sensory neurons required for the action of the previously characterized dauer pheromone blocked pheromone-induced resistance (Pir) to halothane. Pheromone preparations from loss-of-function mutants of daf-22, a gene required for dauer pheromone production, lacked the halothane-resistance activity, suggesting that dauer and Pir pheromone are identical. However, the pathways for pheromone's effects on dauer formation and VA action were not identical. Not all mutations that alter dauer formation affected the Pir phenotype. Further, mutations in genes not known to be involved in dauer formation completely blocked Pir, including those altering signaling through the G proteins Goalpha and Gqalpha. A model in which sensory neurons transduce the pheromone activity through antagonistic Go and Gq pathways, modulating VA action against neurotransmitter release machinery, is proposed.

The G-protein gamma subunit gpc-1 of the nematode C.elegans is involved in taste adaptation

Caenorhabditis elegans has two heterotrimeric G-protein gamma subunits, gpc-1 and gpc-2. Although GPC-1 is specifically expressed in sensory neurons, it is not essential for the detection of odorants or salts. To test whether GPC-1 is involved in sensory plasticity, we developed a water soluble compound adaptation assay. The behaviour of wild-type animals in this assay confirms that prolonged exposure to salts can abolish chemo-attraction to these compounds. This process is time and concentration dependent, partly salt specific and reversible. In contrast, gpc-1 mutant animals show clear deficits in their ability to adapt to NaAc, NaCl and NH4Cl, but normal wild-type adaptation to odorants. Two other loci previously implicated in odorant adaptation, adp-1 and osm-9, are also involved in adaptation to salts. Our finding that G proteins, OSM-9 and ADP-1 are involved in taste adaptation offer the first molecular insight into this process.

An overview of C. Elegans trafficking mutants

It is almost 40 years since Sydney Brenner introduced Caenorhabditis elegans as a model genetic system. During that time mutants with defects in intracellular trafficking have been identified in a diverse range of screens for abnormalities. This should, of course, come as no surprise as it is hard to imagine any biological process in which the regulated movement of vesicles within the cells is not critical at some step. Almost all of these genes have mammalian homologs, and yet the role of many of these homologs has not been investigated. Perhaps the protein that regulates your favorite trafficking step has already been identified in C. elegans? Here I provide a brief overview of those trafficking mutants identified in C. elegans and where you can read more about them.

eat-11 encodes GPB-2, a Gbeta(5) ortholog that interacts with G(o)alpha and G(q)alpha to regulate C. elegans behavior

In C. elegans, a G(o)/G(q) signaling network regulates locomotion and egg laying [1-8]. Genetic analysis shows that activated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is suppressed by perturbations of this network, which include loss of the GOA-1 G(o)alpha, DGK-1 diacylglycerol kinase, EAT-16 G protein gamma subunit-like (GGL)-containing RGS protein, or an unidentified protein encoded by the gene eat-11 [9]. We cloned eat-11 and report that it encodes the Gbeta(5) ortholog GPB-2. Gbeta(5) binds specifically to GGL-containing RGS proteins, and the Gbeta(5)/RGS complex can promote the GTP-hydrolyzing activity of Galpha subunits [10, 11]. However, little is known about how this interaction affects G protein signaling in vivo. In addition to EAT-16, the GGL-containing RGS protein EGL-10 participates in G(o)/G(q) signaling; EGL-10 appears to act as an RGS for the GOA-1 G(o)alpha, while EAT-16 appears to act as an RGS for the EGL-30 G(q)alpha [4, 5]. We have combined behavioral, electrophysiological, and pharmacological approaches to show that GPB-2 is a central member of the G(o)/G(q) network and that GPB-2 may interact with both the EGL-10 and EAT-16 RGS proteins to mediate the opposing activities of G(o)alpha and G(q)alpha. These interactions provide a mechanism for the modulation of behavior by antagonistic G protein networks.