Heterotrimeric G proteins in C. elegans^*

Heterotrimeric G proteins, composed of α, β, and γ subunits, transduce signals from the plasma membrane through a cycle of guanine nucleotide exchange and hydrolysis. When signaling, G proteins effectively function as dimers since the signal is communicated either by the Gα subunit or the stable Gβγ complex. The classical G protein cycle is shown in Figure 1. Gα-GDP associates with Gβγ and with cytoplasmic portions of its typically seven-pass membrane receptor (also known as G protein coupled receptors; GPCRs). Upon ligand activation, the receptor acts as a guanine-nucleotide exchange factor (GEF) stimulating the exchange of GDP for GTP on the α subunit. Gα-GTP dissociates from Gβγ and both entities can interact with effectors. Hydrolysis of GTP restores Gα-GDP, which then reassociates with Gβγ and receptor to terminate signaling. In some pathways, Gα-GDP may interact with an additional GEF protein, RIC-8, which may then re-activate the α subunit (Tall et al., 2003; Reynolds et al., 2005). The rate of GTP hydrolysis can be enhanced by RGS proteins (regulator of G protein signaling; Watson et al. 1996). GEF proteins therefore act as positive regulators of G protein pathways while RGS proteins act as negative regulators (Figure 1).

G protein signaling can also be activated through a receptor-independent pathway in which the dissociation of Gα and Gβγ may be facilitated by GPR (G protein regulator) proteins, which compete with Gβγ for binding to the GDP-bound form of Gα and function as guanine nucleotide dissociation inhibitors (GDIs; Manning, 2003). Dissociation of Gα and Gβγ may also be facilitated by GEF proteins (Afshar at al., 2004). Alternatively, GEF proteins may recognize the Gα-GPR complex, and subsequently effect nucleotide exchange to generate an active Gα-GTP signaling molecule (Manning, 2003; Hess et al., 2004). Details of this pathway are discussed further under “Receptor-independent pathway” in the Gαo section.

C. elegans has 21 Gα, 2 Gβ and 2 Gγ genes (Jansen et al., 1999; Cuppen et al., 2003).

Based on sequence similarity, mammalian Gα subunits have been divided into four families: Gs, Gi/o, Gq and G12 (Neves et al., 2002). C. elegans expresses one ortholog of each of the mammalian families: GSA-1 (Gs), GOA-1 (Gi/o), EGL-30 (Gq) and GPA-12 (G12). The remaining C. elegans α subunits (GPA-1-11, GPA-13-17 and ODR-3) do not share sufficient homology to allow classification. The conserved Gα subunits, with the exception of GPA-12, are expressed broadly while 14 of the new Gα genes are expressed in subsets of chemosensory neurons (See Table 1).

Known roles for the Gα subunits in C. elegans are extensive and varied. GOA-1 and GPA-16 function redundantly and are required maternally for proper spindle positioning in the developing embryo (Gotta and Ahringer, 2001; Tsou et al., 2003). GSA-1 and EGL-30 are required for viability after hatching (Brundage et al., 1996; Korswagen et al, 1997). GSA-1, EGL-30 and GOA-1 act both pre- and post-synaptically to affect multiple behaviors and processes. GSA-1 affects locomotion and egg laying, and excess GSA-1 function triggers necrotic neuronal cell death (Korswagen et al., 1997; Berger et al., 1998; Korswagen et al., 1998). EGL-30 activates egg laying, locomotion, pharyngeal pumping, neuronal migration, spicule protraction, and may affect vulval development (Trent et al., 1983; Brundage et al., 1996; Lackner et al., 1999; Miller et al., 1999; Garcia et al., 2001; Kindt et al., 2002; Moghal et al., 2003). GOA-1 negatively regulates locomotion and egg laying, and affects fertility, neuronal migration, pharyngeal pumping, male mating, response to volatile anesthetics, and may also affect vulval development (Mendel et al., 1995; Ségalat et al., 1995; Fraser et al., 2000; Miller and Rand, 2000; Sawin et al., 2000; van Swinderen et al., 2001; Kindt et al., 2002; Keane and Avery 2003). In motor neurons, GSA-1, EGL-30 and GOA-1 act in a network to regulate acetylcholine release. The EGL-30 pathway generates the core signals for synaptic vesicle priming, while the GOA-1 pathway acts to negatively regulate EGL-30 pathway (Lackner et al., 1999; Miller et al., 1999; Nurrish et al., 1999). The GSA-1 pathway acts to modify the EGL-30 pathway, perhaps by providing positional information (Reynolds et al., 2005). Constitutive GPA-12 function affects pharyngeal pumping (van der Linden et al., 2003). Several of the C. elegans specific Gα subunits (GPAs) are involved either positively or negatively in chemosensation (e.g., Roayaie et al., 1998; Jansen et al., 1999).

Only one of the Gβ subunits, GPB-1, appears to function along with the α subunits in the classic G protein heterotrimer (van der Voorn et al., 1990). The remaining Gβ subunit, GPB-2, is thought to promote the GTP-ase activity of Gα subunits (Chase et al., 2001; Robatzek et al., 2001; van der Linden et al., 2001). Of the two Gγ subunits, GPC-2 is broadly expressed and presumably participates in most G protein pathways in C. elegans (Jansen et al., 2002). The remaining Gγ subunit, GPC-1, has a restricted role in chemosensation (Jansen et al., 2002; Hilliard et al., 2005).

The expression and function of each G protein subunit are described in detail below.

C. elegans expresses two Gβ subunits encoded by gpb-1 and gpb-2. GPB-1 shares 86% amino acid identity with mammalian Gβ subunits (van der Voorn et al., 1990), and appears to be required to mediate signaling by all Gα subunits. Inactivation of gpb-1 leads to abnormalities early in embryogenesis (Zwaal et al., 1996; see “Receptor-independent pathway” in the Go section). GPB-1 also has roles later in development that were revealed by mosaic rescue of a gpb-1 null mutant (Zwaal et al., 1996). Sterility and abnormalities in the germ line were observed in some adult mosaic animals, suggesting the gpb-1 may also play a role in germ line development. Overexpression of gpb-1 causes lethargic locomotion and delayed egg laying. These later effects are consistent with the expression pattern of gpb-1; expression is seen throughout development in nearly all somatic tissues and in the germ line (Zwaal et al., 1996). Once tissue differentiation occurs, expression is highest in neurons. In larval and adult stages, GPB-1 expression is seen in most or all neurons, the somatic gonad, vulva, and hypodermal seam cells. The intestine, pharynx, and body wall muscles appear to express GPB-1 at lower levels (Zwaal et al., 1996).

gpb-2 encodes the C. elegans ortholog of vertebrate Gβ5, a Gβ subunit of novel function (Chase et al., 2001; Robatzek et al., 2001; van der Linden et al., 2001). GPB-2 binds G protein γ-like (GGL) containing RGS proteins, and is believed to promote the GTP-ase activity of Gα subunits (Chase et al., 2001; Robatzek et al., 2001; van der Linden et al., 2001). GPB-2 is a regulator of GOA-1 and EGL-30 signaling (see “Regulators of EGL-30/G protein signaling network”).

Two genes encoding Gγ subunits have been identified in C. elegans, gpc-1 and gpc-2 (Jansen et al., 2002). GPC-1 and GPC-2 show from 22 to 79% amino acid identity to the vertebrate Gγ subunits, but are not clear orthologs of any of the human Gγ subunits (Jansen et al., 2002). GPC-1 is specifically expressed in sensory neurons while GPC-2 shows ubiquitous expression (Jansen et al., 2002).

In accordance with its more general pattern of expression, GPC-2 is required together with GPB-1, GOA-1, and GPA-16 to orient cell division axes in the C. elegans embryo (Gotta and Ahringer, 2001) (see “Receptor- independent pathway” in the Gαo section). gpc-2(RNAi) embryos exhibit defects in spindle orientation that are identical to gpb-1(RNAi) and gpb-1 mutant embryos (Gotta and Ahringer, 2001).

GPC-1 is expressed in 12 cells in the head and two cells in the tail that are identified as putative chemosensory neurons. Although GPC-1 is not essential for the detection of the water soluble attractants, NaCl, NaAc, and NH4Cl, it is essential for adaptation to these tastants (Jansen et al., 2002). In addition, gpc-1 mutants exhibit defects in the adaptation to soluble chemical repellents mediated by the ASH neurons, although they are not defective in the response to any repellents tested except for quinine (Hilliard et al., 2005). gpc-1 mutants display wild-type adaptation to volatile odorants (Jansen et al., 2002). gpc-1 overexpression may reduce locomotion and egg laying, though a null mutation in gpc-1 has no effect on those phenotypes (Jansen et al., 2002).

Many of the Gα subunits have remarkably diverse roles in C. elegans biology, and thus these genes have been identified in many genetic screens. Where not previously identified in forward genetic screens, loss-of-function (lf) mutations have been isolated for all Gα subunit genes (with the exception of gpa-17) using target-selected gene inactivation (Jansen et al., 2002). Gα subunits can be mutated to reduce their intrinsic GTP-ase activity, thus causing constitutive signaling. Such gain-of-function (gf) mutations have been isolated for gsa-1 (Schade et al., 2005) and egl-30 (Bastiani et al., 2003), and have been produced in vitro and expressed as transgenes (QL) for gsa-1, goa-1, egl-30, gpa-1, gpa-2, gpa-3, gpa-12 and odr-3 (see sections on individual Gα subunits for references). In many cases, transgenic overexpression of the wild-type Gα subunit (XS) causes effects similar to the activated transgene (although often not as severe) and reciprocal to the loss-of-function mutation. Thus, loss-of-function phenotypes have been described for each of the C. elegans Gα subunits while gain-of-function phenotypes are known for most. We discuss here the current knowledge of their functions.

GSA-1, encoded by gsa-1, shares over 60% identity with its vertebrate ortholog, Gαs. In vertebrates, Gαs is known to activate adenylyl cyclase to produce cyclic AMP, which both activates Protein Kinase A (PKA), and modulates cyclic nucleotide-gated ion channels (reviewed in Sunahara et al., 1996; Walsh and Van Patten, 1994). Vertebrate Gαs can also activate L-type voltage gated calcium channels in skeletal muscle cells and can inhibit cardiac sodium channels (reviewed in Wickman and Clapham, 1995a; Wickman and Clapham, 1995b). In C. elegans, the adenylyl cyclase ACY-1 appears to be the major effector of GSA-1 for growth and locomotion. A receptor with similarity to the vertebrate 5-HT7 family may couple to GSA-1 based on the observation that expression of SER-7b in COS-7 cells results in a dramatic increase in basal cAMP levels over untransfected cells (Hobson et al., 2003).

In C. elegans GSA-1 is required for viability; loss of GSA-1 function results in L1 arrest (Korswagen et al, 1997). GSA-1 also affects egg laying, locomotion, and necrotic neuronal cell death based on mutational analysis (Korswagen et al., 1997; Berger et al., 1998; Korswagen et al., 1998). Viable animals segregating from mosaically rescued transgenic lines show behavioral defects including sluggish locomotion and delayed egg laying (Korswagen et al., 1997). Gain-of-function mutations in gsa-1, like transgenic overexpression, result in reciprocal phenotypes (Korswagen et al, 1997; Schade et al., 2005). Constitutive activation of transgenic GSA-1 causes necrotic neuronal cell death although similar mutations in chromosomal gsa-1 cause only minimal cell death (Korswagen et al, 1997; Schade et al., 2005). GSA-1 plays an ongoing functional role in regulating movement since gsa-1(QL) transgenes are able to induce hyperactive locomotion even when expressed after adulthood (Schade et al., 2005). Gain-of-function mutation in both gsa-1 and acy-1, which encodes an adenylyl cyclase that is the major effector of GSA-1 for movement (see below), cause hypersensitivity to aldicarb, indicating that the GSA-1 pathway regulates acetycholine release by motor neurons. However, elimination of the neuronal GSA-1 pathway does not affect steady-state levels of neurotransmitter release (Reynolds et al., 2005). The implications for this are discussed further in the “Regulators of EGL-30/G protein signaling network” section. GSA-1 also affects gametogenesis, hermaphrodite genital morphogenesis, growth rate and morphogenesis of the epithelium, as determined in large-scale RNAi assays (Simmer et al., 2003).

GSA-1 is broadly expressed in neurons and muscles from the embryonic stage of development onward. It may also be expressed in intestinal and some epithelial cells (i.e. of the pharynx and vulva; Korswagen et al., 1997; Park et al., 1997). GSA-1 functions both pre- and post-synaptically in regulating movement since expression of an activated form of its effector, ACY-1, from promoters specific for either neurons or muscles causes hyperactive locomotion (Schade et al., 2005). GSA-1 pathway activity is also required in both neurons and muscles for larval growth, based on studies using heterologous promoter elements fused to acy-1 (Reynolds et al., 2005).

Two genetic studies of neurodegeneration have revealed conservation between GSA-1 signaling in C. elegans and the vertebrate Gαs signaling pathway (Korswagen et al., 1997; Berger et al., 1998). One screen employed a transgene with the constitutively active C. elegans gsa-1 gene under the control of the ubiquitously expressed heat-shock promoter (Korswagen et al., 1997) and the other employed a transgene with the constitutively active rat Gαs gene under the control of the glr-1 promoter, expressed in 17 classes of neurons, including interneurons required for locomotion (Berger et al., 1998). In both cases, constitutive activation and overproduction of either C. elegans GSA-1 or rat Gαs induced paralysis and neurodegeneration (Korswagen et al., 1997; Berger et al., 1998). Necrotic cell death was suppressed by mutations in acy-1 (sgs-1) (Korswagen et al., 1997; Berger et al., 1998). Most mutations in acy-1 fail to, or only partially suppress, the body-wall hypercontraction caused by transgenic activated Gαs, and also fail to suppress the hyperactive egg laying caused by overproduction of GSA-1 (Korswagen et al., 1997). acy-1 is expressed in the nervous system and in muscle cells (Berger et al., 1998; Korswagen et al., 1998), and null mutations affect larval viability, larval molting, life span, and pharyngeal pumping while reduction-of-function mutations affect locomotion (Moorman and Plasterk, 2002).

For growth and locomotion rate, ACY-1 appears to be the major effector of GSA-1 since null mutations in acy-1 are epistatic to gsa-1(gf) mutations (Schade et al., 2005). Downstream of ACY-1, the GSA-1 pathway likely converges with the EGL-30 pathway to control synaptic transmission from the neurons that regulate locomotion. RIC-8, a GEF protein, may regulate both GSA-1 and EGL-30 with respect to locomotion (Reynolds et al., 2005; see “Regulators of EGL-30/G protein signaling network” section for further detail). A second adenylyl cyclase gene, acy-2, was found to be essential for larval viability (Korswagen et al., 1998). The terminal phenotype of acy-2(lf) mutants resembles that of gsa-1(lf) mutants and clr-1 mutants (Korswagen et al., 1998), which phenotypically mimic worms after laser ablation of the canal-associated neurons (Kokel et al., 1998). acy-2 is expressed in the CAN cells, and some head ganglia and ventral cord neurons (Korswagen et al., 1998). These results indicate that ACY-2 may be an effector of GSA-1 in the CAN cells. Two other predicted adenylate cyclases, acy-3 and acy-4, have not been characterized genetically, nor have RNAi phenotypes been uncovered.

Gαs-induced cell death occurs via a necrotic rather than an apoptotic pathway, based on both the inability of mutations in ced-3 and ced-4 to suppress the cell death and the observation that the morphology of neuronal degeneration observed is similar to that caused by activating mutations in some ion channel genes, such as deg-1, deg-3, and mec-4. Loss-of-function mutations in unc-36, one of three calcium channel genes potentially regulated by PKA (unc-2, unc-36, and egl-19), were found to confer a slight reduction in cytotoxity in one study and slight suppression of paralysis in another (Korswagen et al., 1997; Berger et al., 1998). Cytotoxic cell death mechanisms conferred by activated Gαs and by mec-4(d), deg-1(d) and deg-3(d) alleles require ER-driven Ca²⁺ release (Xu et al., 2001), and converge upon calcium-regulated aspartyl and calpain proteases (Syntichaki et al., 2002). In all cases, degeneration is suppressed in the genetic mutants cad-1, daf-4, and unc-52, or by starvation, which all reduce aspartyl protease activity. Treatment of mec-4(d), deg-1(d), deg-3(d) and gsa-1(gf) mutant animals with Z-Val-Phe-CHO (MDL-28170), a potent calpain inhibitor, significantly reduces degeneration in these mutants (Syntichaki et al., 2002). These studies suggest that an increase in internal calcium triggers the activation of calpain proteases that subsequently engages executioner lysosomal and cytoplasmic aspartyl proteases and ultimately leads to necrotic cell death (Syntichaki et al., 2002).

egl-30 encodes the ortholog of vertebrate Gαq, and shares over 80% amino acid sequence identity with vertebrate Gαq and Gα11 (Brundage et al., 1996). EGL-30 activates diverse biological processes in C. elegans including egg laying, locomotion, pharyngeal pumping, synaptic transmission, neuronal migration, and spicule protraction in males, and may also promote vulval induction in hermaphrodites (Trent et al., 1983; Brundage et al., 1996; Lackner et al., 1999; Miller et al., 1999; Garcia et al., 2001; Kindt et al., 2002; Moghal et al., 2003). Like vertebrate Gαq family members, EGL-30 has been shown to stimulate phosphoinositide hydrolysis when expressed in COS-7 cells and can couple to the vertebrate α1-C adrenergic receptor (Brundage et al., 1996). EGL-30 has been shown to mediate the release of acetylcholine from motor neurons (Lackner et al., 1999) and is required for serotonin-induced calcium transients in vulval muscles (Shyn et al., 2003).

A complete loss-of-function mutation of egl-30 is presumably inviable (Brundage et al., 1996). Homozygotes having strong reduction-of-function mutations in egl-30 hatch, but are paralyzed with, at most, feeble contractions of body-wall, pharyngeal, and defecation muscles and arrest throughout larval development (Brundage et al. 1996). Less severe alleles cause a variety of phenotypes including sluggish movement, delayed egg laying, slow pharyngeal pumping (Trent et al., 1983; Brundage et al., 1996), and aldicarb resistance (Miller et al., 1999). Gain-of-function mutations in egl-30 or transgenic overexpression of wild type egl-30 cause hyperactive movement with exaggerated body bends, and hyperactive egg laying (Brundage et al., 1996; Bastiani et al., 2003). Overexpression of a constitutively activated form of EGL-30 results in vacuolization of cells, paralysis, and ultimately death (Bastiani et al., 2003). Both loss and gain-of-function mutations in egl-30 cause misplacement of certain migrating neural cell bodies (Kindt et al., 2002).

egl-30 is highly expressed in neurons and in pharyngeal muscle (Lackner et al., 1999; Bastiani et al., 2003). Rescuing fusions with GFP have revealed occasional expression in some other muscle types (i.e., body-wall, vulval) and in some epidermal cells (i.e., vulval; Bastiani et al., 2003). Neuronal expression of egl-30 has been shown to be sufficient for the modulation of locomotion, sensitivity to aldicarb, mediation of retrograde signaling from post-synaptic muscle cells to neurons, and for its effect on vulval development (Lackner et al., 1999; Doi and Iwasaki, 2002; Moghal et al., 2003).

In vertebrates, Gαq activates PLCβ isoforms, which hydrolyze phosphatidylinositol bisphosphate (PIP2) to inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) (for review, see Sternweis and Smrcka, 1993; Jiang et al., 1994). IP3 interacts with IP3 receptors to stimulate the release of calcium from internal stores while DAG activates PKC. A well-conserved variation of this pathway appears to be employed in C. elegans to mediate acetylcholine release from the ventral cord motor neurons. An increase in the concentration of DAG via activation of EGL-30, and subsequent activation of EGL-8 (PLCβ), is thought to promote presynaptic membrane localization of the DAG receptor protein, UNC-13, a protein that facilitates synaptic vesicle fusion at active zones (Richmond et al., 1999; Richmond et al., 2001). This model is supported by the following observations. Reduction-of-function mutations in egl-30, egl-8, and unc-13 confer reduced sensitivity to aldicarb, an inhibitor of acetylcholinesterase at synapses (Miller et al., 1996; Lackner et al., 1999; Miller et al., 1999). Loss-of-function mutations in dgk-1 (diacylglycerol kinase) can partially bypass the resistance of egl-8 mutants for aldicarb, indicating that acetylcholine release is dependent on DAG levels (Lackner et al., 1999). Both expression of a constitutively active mutation of egl-30 and exposure to phorbol esters cause hyperactive movement and hypersensitivity to aldicarb (Lackner et al., 1999; Reynolds et al., 2005). The effects of increased DAG levels, whether by egl-30(gf) mutations or phorbol ester treatment, are strongly dependent on UNC-13 activity (Lackner et al., 1999; Reynolds et al., 2005). Stimulation of acetylcholine release by phorbol esters, as measured by sensitivity to aldicarb, is blocked by a mutation that eliminates phorbol ester binding to UNC-13 (Lackner et al., 1999). An unc-13(rf) mutation is epistatic to an egl-30(gf) mutation with respect to locomotion. (Reynolds et al., 2005). Expression of an UNC-13 mutant protein that is constitutively membrane-bound restores acetylcholine release to mutants lacking EGL-8 (Lackner et al., 1999). Also, the larval arrest and paralysis of egl-30 null mutants are rescued by phorbol ester treatment (Reynolds et al., 2005). Exogenous treatment of wild-type animals with arecoline, believed to act as an agonist for receptors that are coupled to EGL-30, leads to both hypersensitivity to aldicarb and an increase in the accumulation of UNC-13 at presynaptic membranes (Lackner et al., 1999). Taken together, these data indicate that the pre-synaptic EGL-30 pathway results in synaptic vesicle priming leading to acetylcholine release. This pathway is inhibited by serotonin, via the heterotrimeric Gα protein, GOA-1 (discussed in further detail below; Nurrish et al., 1999).

A number of other processes may also be linked to egl-30-mediated release of acetylcholine. Genetic epistasis with mutations in egl-8 and unc-13 indicate that these genes act downstream of egl-30 with respect to the stimulation of pharyngeal pumping. In this case, itr-1, which is broadly expressed in the pharynx and encodes the C. elegans IP3 receptor, may also act downstream of, or in parallel to, egl-30 (Bastiani et al., 2003). Similarly, an analysis of response to emodepside, a ligand of nematode latrophilin that stimulates neuronal exocytosis and elicits pharyngeal paralysis revealed that latrophilin-dependent neurotransmitter release required egl-30, egl-8 and unc-13 (Willson et al., 2004). Mutations in genes that increase acetylcholine release, such as in unc-43, dgk-1, and egl-8, enhance ectopic axon branching induced by the amino-terminal domain of UNC-6 (Wang and Wadsworth, 2002). egl-30 gain-of-function, as well as reduction-of-function mutants, display defects in neuronal cell migration. Neuronal cell migration is also affected by mutations in egl-8 and dgk-1, although genetic epistasis analysis has not yet been performed (Kindt et al., 2002).

EGL-30 must directly activate other downstream effector molecules in addition to EGL-8, and also mediates and affects other processes in addition to acetylcholine release. The pathway for serotonin-mediated stimulation of egg laying via egl-30 has been shown to be largely independent of EGL-8, thereby specifically implicating another unidentified major effector for EGL-30 (Bastiani et al., 2003). During protraction of the male spicules, egl-30 both enhances a nicotinic pathway and may mediate a muscarinic pathway to affect spicule protraction, and ultimately regulates downstream UNC-68 ryanodine or EGL-19-containing calcium channels (Garcia et al., 2001). EGL-30 activates vulval induction in an EGL-19-dependent manner under specific growth conditions (Moghal et al., 2003). The mechanism by which EGL-30 regulates these channels is not yet known. Further analysis in C. elegans should reveal other direct effectors for EGL-30, and reveal how EGL-30 interacts with other signaling pathways.

EGL-30 may be negatively regulated by the RGS protein EAT-16, as suggested by epistatic relationships between their two genes (Hajdu-Cronin et al., 1999). eat-16(rf) mutations suppress reduction-of-function but not null mutations in egl-30, suggesting that EAT-16 may directly regulate EGL-30 in vivo. Overexpression of EAT-16 partially suppresses phenotypes due to overexpression of EGL-30, but does not suppress those caused by a GTPase deficient EGL-30 transgene (Hajdu-Cronin et al., 1999). In addition, EAT-16 can inhibit EGL-30-dependent phosphoinositide hydrolysis in cultured cells (Hajdu-Cronin et al., 1999).

ric-8 encodes a GEF expressed in neurons that affects egg laying and locomotion. RIC-8 acts genetically upstream of the EGL-30 signaling pathway (Miller and Rand, 2000). Like egl-30(rf) mutations, ric-8(rf) mutations confer aldicarb resistance, suggesting that RIC-8 may act as a positive regulator of EGL-30 for acetylcholine release (Miller et al., 1996; Miller and Rand, 2000). However, although gain-of-function mutations in egl-30 suppress the paralysis of ric-8(rf) mutants, even a strong gain-of-function mutation in egl-30 is unable to suppress a ric-8 null mutation (Reynolds et al., 2005; Schade et al., 2005). An in vitro biochemical analysis of vertebrate Ric-8 indicates that this protein can interact with, and act as a GEF for, multiple Gα subunits (Tall et al., 2003). Similarly, C. elegans RIC-8 can interact with or stimulate the GEF activity in vitro of at least two Gα subunits, GOA-1 and GPA-16 (Afshar et al., 2004; Couwenbergs et al., 2004, Hess et al., 2004). Further studies are needed to address the in vivo specificity of this GEF and its role in EGL-30 signaling.

EGL-30 and GOA-1, the C. elegans ortholog of vertebrate Gαo, confer opposite effects on a variety of behaviors in C. elegans. goa-1 null mutants are hyperactive with respect to locomotion and egg laying whereas egl-30 reduction-of-function mutants are lethargic with respect to these behaviors (Trent et al., 1983; Mendel et al., 1995; Ségalat et al., 1995; Brundage et al., 1996). goa-1 mutants are hypersensitive to aldicarb-induced paralysis and growth arrest, whereas egl-30 mutants are resistant to aldicarb (Miller et al., 1996; Lackner et al., 1999; Miller et al., 1999; Nurrish et al., 1999; van Swinderen et al., 2001).

The observation that goa-1(lf) mutants are resistant to serotonin-induced slowing of locomotion led to the proposal that serotonin and GOA-1 mediate a neuromodulatory effect on acetylcholine release from the ventral cord motorneurons. Indeed, serotonin induces resistance to aldicarb and requires goa-1 to mediate this effect (Nurrish et al., 1999). GOA-1 also negatively regulates the localization of UNC-13 (Nurrish et al., 1999). Mutations in egl-30, and mutations in genes that function in the EGL-30 pathway, are epistatic to mutations in goa-1 with respect to aldicarb sensitivity, suggesting that activation of GOA-1 by serotonin negatively regulates egl-30-mediated acetylcholine release via UNC-13 (Lackner et al., 1999; Miller et al., 1999; Nurrish et al., 1999; for further detail, see the “Pathways” subheading in the EGL-30 section). These pathways appear to intersect at the level of the RGS protein EAT-16, shown to negatively regulate EGL-30, and/or at the level of diacylglycerol kinase, DGK-1, which affects accumulation of DAG (see Figure 2). DGK-1 or