Multiple Subthreshold GPCR Signals Combined by the G-Proteins Gαq and Gαs Activate the Caenorhabditis elegans Egg-Laying Muscles

Individual neurons or muscle cells express many G-protein-coupled receptors (GPCRs) for neurotransmitters and neuropeptides, yet it remains unclear how cells integrate multiple GPCR signals that all must activate the same few G-proteins. We analyzed this issue in the Caenorhabditis elegans egg-laying system, where multiple GPCRs on muscle cells promote contraction and egg laying. We genetically manipulated individual GPCRs and G-proteins specifically in these muscle cells within intact animals and then measured egg laying and muscle calcium activity. Two serotonin GPCRs on the muscle cells, Gαq-coupled SER-1 and Gαs-coupled SER-7, together promote egg laying in response to serotonin. We found that signals produced by either SER-1/Gαq or SER-7/Gαs alone have little effect, but these two subthreshold signals combine to activate egg laying. We then transgenically expressed natural or designer GPCRs in the muscle cells and found that their subthreshold signals can also combine to induce muscle activity. However, artificially inducing strong signaling through just one of these GPCRs can be sufficient to induce egg laying. Knocking down Gαq and Gαs in the egg-laying muscle cells induced egg-laying defects that were stronger than those of a SER-1/SER-7 double knockout, indicating that additional endogenous GPCRs also activate the muscle cells. These results show that in the egg-laying muscles multiple GPCRs for serotonin and other signals each produce weak effects that individually do not result in strong behavioral outcomes. However, they combine to produce sufficient levels of Gαq and Gαs signaling to promote muscle activity and egg laying.SIGNIFICANCE STATEMENT How can neurons and other cells gather multiple independent pieces of information from the soup of chemical signals in their environment and compute an appropriate response? Most cells express >20 GPCRs that each receive one signal and transmit that information through three main types of G-proteins. We analyzed how this machinery generates responses by studying the egg-laying system of C. elegans, where serotonin and multiple other signals act through GPCRs on the egg-laying muscles to promote muscle activity and egg laying. We found that individual GPCRs within an intact animal each generate effects too weak to activate egg laying. However, combined signaling from multiple GPCR types reaches a threshold capable of activating the muscle cells.

The neural G protein Gαo tagged with GFP at an internal loop is functional in C. elegans

Gα_o is the alpha subunit of the major heterotrimeric G protein in neurons and mediates signaling by every known neurotransmitter, yet the signaling mechanisms activated by Gα_o remain to be fully elucidated. Genetic analysis in Caenorhabditis elegans has shown that Gα_o signaling inhibits neuronal activity and neurotransmitter release, but studies of the molecular mechanisms underlying these effects have been limited by lack of tools to complement genetic studies with other experimental approaches. Here, we demonstrate that inserting the green fluorescent protein (GFP) into an internal loop of the Gα_o protein results in a tagged protein that is functional in vivo and that facilitates cell biological and biochemical studies of Gα_o. Transgenic expression of Gα_o-GFP rescues the defects caused by loss of endogenous Gα_o in assays of egg laying and locomotion behaviors. Defects in body morphology caused by loss of Gα_o are also rescued by Gα_o-GFP. The Gα_o-GFP protein is localized to the plasma membrane of neurons, mimicking localization of endogenous Gα_o. Using GFP as an epitope tag, Gα_o-GFP can be immunoprecipitated from C. elegans lysates to purify Gα_o protein complexes. The Gα_o-GFP transgene reported in this study enables studies involving in vivo localization and biochemical purification of Gα_o to compliment the already well-developed genetic analysis of Gα_o signaling.

Serotonin and neuropeptides are both released by the HSN command neuron to initiate Caenorhabditis elegans egg laying

Neurons typically release both a small-molecule neurotransmitter and one or more neuropeptides, but how these two types of signal from the same neuron might act together remains largely obscure. For example, serotonergic neurons in mammalian brain express the neuropeptide Substance P, but it is unclear how this co-released neuropeptide might modulate serotonin signaling. We studied this issue in C. elegans, in which all serotonergic neurons express the neuropeptide NLP-3. The serotonergic Hermaphrodite Specific Neurons (HSNs) are command motor neurons within the egg-laying circuit which have been shown to release serotonin to initiate egg-laying behavior. We found that egg-laying defects in animals lacking serotonin were far milder than in animals lacking HSNs, suggesting that HSNs must release other signal(s) in addition to serotonin to stimulate egg laying. While null mutants for nlp-3 had only mild egg-laying defects, animals lacking both serotonin and NLP-3 had severe defects, similar to those of animals lacking HSNs. Optogenetic activation of HSNs induced egg laying in wild-type animals, and in mutant animals lacking either serotonin or NLP-3, but failed to induce egg laying in animals lacking both. We recorded calcium activity in the egg-laying muscles of animals lacking either serotonin, NLP-3, or both. The single mutants, and to a greater extent the double mutant, showed muscle activity that was uncoordinated and unable to expel eggs. Specifically, the vm2 muscles cells, which are direct postsynaptic targets of the HSN, failed to contract simultaneously with other egg-laying muscle cells. Our results show that the HSN neurons use serotonin and the neuropeptide NLP-3 as partially redundant co-transmitters that together stimulate and coordinate activity of the target cells onto which they are released.

Activity of the brain results from neurons communicating with each other using chemical signals. A typical neuron releases two kinds of chemical signals: a small molecule neurotransmitter, such as serotonin, and one or more small proteins, called neuropeptides. For example, neurons in the human brain that release serotonin, a neurotransmitter thought to be involved in depression, also release the neuropeptide Substance P. Neuroscientists have typically studied the effects of neurotransmitters and neuropeptides separately, without considering how these two types of signals from the same neuron might be integrated. Here we analyzed how specific neurons in the model organism C. elegans use both serotonin and a neuropeptide together. The Hermaphrodite Specific Neurons (HSNs) activate a small group of neurons and muscles to generate egg-laying behavior. Killing the HSNs resulted in animals unable to lay eggs, but we found that eliminating either serotonin or the neuropeptide resulted in HSNs that still remained able to activate egg laying. However, eliminating both serotonin and the neuropeptide resulted in HSNs unable to activate coordinated contractions of the egg-laying muscles. Our results show that in a living animal, serotonin acts in concert with a co-released neuropeptide to carry out its functions.

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.

Activity of the C. elegans egg-laying behavior circuit is controlled by competing activation and feedback inhibition

Like many behaviors, Caenorhabditis elegans egg laying alternates between inactive and active states. To understand how the underlying neural circuit turns the behavior on and off, we optically recorded circuit activity in behaving animals while manipulating circuit function using mutations, optogenetics, and drugs. In the active state, the circuit shows rhythmic activity phased with the body bends of locomotion. The serotonergic HSN command neurons initiate the active state, but accumulation of unlaid eggs also promotes the active state independent of the HSNs. The cholinergic VC motor neurons slow locomotion during egg-laying muscle contraction and egg release. The uv1 neuroendocrine cells mechanically sense passage of eggs through the vulva and release tyramine to inhibit egg laying, in part via the LGC-55 tyramine-gated Cl^- channel on the HSNs. Our results identify discrete signals that entrain or detach the circuit from the locomotion central pattern generator to produce active and inactive states.

DOI:http://dx.doi.org/10.7554/eLife.21126.001

It has been said that if the human brain were so simple that we could understand it, we would be so simple that we couldn’t. This quote neatly captures the challenge of working out how 80 billion neurons collectively generate our thoughts and behavior. Fortunately, the nervous system is also organized into simpler units called circuits. Each consists of a relatively small number of neurons, which communicate with one another to control as little as a single behavior. These circuits should in principle be simple enough for us to understand, particularly if we study them in nervous systems less complex than our own.

Despite this, there is currently not a single circuit in any organism in which we can explain how communication between individual neurons generates behavior. Collins et al. therefore set out to characterize a simple neural circuit in one of the simplest model organisms: the egg-laying circuit of the worm C. elegans.

Using mutations, drugs and molecular genetic techniques, Collins et al. systematically altered the activity and signaling of each of the neurons within the egg-laying circuit. The experiments revealed that cells called command neurons trigger egg laying by producing signals that switch on the rest of the circuit. Once activated, the circuit is able to respond to waves of activity from a second circuit – called the central pattern generator – that also controls the worm’s movement. Finally, laying an egg activates a third set of neurons, which release a signal that returns the circuit to its inactive state.

The use of distinct signals and neurons to activate the circuit, to coordinate its ongoing activity, and to inactivate the circuit when its task is complete also applies to many other neural circuits. Now that these signals have been identified in one circuit, it should be possible to build on these findings to better understand how others work.

DOI:http://dx.doi.org/10.7554/eLife.21126.002

Evolutionary Conservation of a GPCR-Independent Mechanism of Trimeric G Protein Activation

Trimeric G protein signaling is a fundamental mechanism of cellular communication in eukaryotes. The core of this mechanism consists of activation of G proteins by the guanine-nucleotide exchange factor (GEF) activity of G protein coupled receptors. However, the duration and amplitude of G protein-mediated signaling are controlled by a complex network of accessory proteins that appeared and diversified during evolution. Among them, nonreceptor proteins with GEF activity are the least characterized. We recently found that proteins of the ccdc88 family possess a Gα-binding and activating (GBA) motif that confers GEF activity and regulates mammalian cell behavior. A sequence similarity-based search revealed that ccdc88 genes are highly conserved across metazoa but the GBA motif is absent in most invertebrates. This prompted us to investigate whether the GBA motif is present in other nonreceptor proteins in invertebrates. An unbiased bioinformatics search in Caenorhabditis elegans identified GBAS-1 (GBA and SPK domain containing-1) as a GBA motif-containing protein with homologs only in closely related worm species. We demonstrate that GBAS-1 has GEF activity for the nematode G protein GOA-1 and that the two proteins are coexpressed in many cells of living worms. Furthermore, we show that GBAS-1 can activate mammalian Gα-subunits and provide structural insights into the evolutionarily conserved determinants of the GBA–G protein interface. These results demonstrate that the GBA motif is a functional GEF module conserved among highly divergent proteins across evolution, indicating that the GBA-Gα binding mode is strongly constrained under selective pressure to mediate receptor-independent G protein activation in metazoans.

An evolutionarily conserved switch in response to GABA affects development and behavior of the locomotor circuit of Caenorhabditis elegans

The neurotransmitter gamma-aminobutyric acid (GABA) is depolarizing in the developing vertebrate brain, but in older animals switches to hyperpolarizing and becomes the major inhibitory neurotransmitter in adults. We discovered a similar developmental switch in GABA response in Caenorhabditis elegans and have genetically analyzed its mechanism and function in a well-defined circuit. Worm GABA neurons innervate body wall muscles to control locomotion. Activation of GABAA receptors with their agonist muscimol in newly hatched first larval (L1) stage animals excites muscle contraction and thus is depolarizing. At the mid-L1 stage, as the GABAergic neurons rewire onto their mature muscle targets, muscimol shifts to relaxing muscles and thus has switched to hyperpolarizing. This muscimol response switch depends on chloride transporters in the muscles analogous to those that control GABA response in mammalian neurons: the chloride accumulator sodium-potassium-chloride-cotransporter-1 (NKCC-1) is required for the early depolarizing muscimol response, while the two chloride extruders potassium-chloride-cotransporter-2 (KCC-2) and anion-bicarbonate-transporter-1 (ABTS-1) are required for the later hyperpolarizing response. Using mutations that disrupt GABA signaling, we found that neural circuit development still proceeds to completion but with an ∼6-hr delay. Using optogenetic activation of GABAergic neurons, we found that endogenous GABAA signaling in early L1 animals, although presumably depolarizing, does not cause an excitatory response. Thus a developmental depolarizing-to-hyperpolarizing shift is an ancient conserved feature of GABA signaling, but existing theories for why this shift occurs appear inadequate to explain its function upon rigorous genetic analysis of a well-defined neural circuit.

The G protein regulator AGS-3 allows C. elegans to alter behaviors in response to food deprivation

Behavioral responses to food deprivation are a fundamental aspect of nervous system function in all animals. In humans, these behavioral responses prevent dieting from being an effective remedy for obesity. Several signaling molecules in the mammalian brain act through G proteins of the Gα_i/o family to mediate responses to food restriction. The mechanisms for neural response to food deprivation may be conserved across species, allowing the power of genetic model organisms to generate insights relevant to the problem of human obesity. In a recent study, we found that C. elegans uses Gα_o signaling to mediate a number of behavioral changes that occur after food deprivation. Food deprivation causes biochemical changes in the G Protein Regulator (GPR) domain protein AGS-3 and AGS-3, together with the guanine nucleotide exchange factor RIC-8, activates Gα_o signaling to alter food-seeking behavior. These proteins are all conserved in the human brain. Thus the study of behavioral responses to food deprivation in C. elegans may reveal the details of conserved molecular mechanisms underlying neural responses to food deprivation.

LIN-12/Notch signaling instructs postsynaptic muscle arm development by regulating UNC-40/DCC and MADD-2 in Caenorhabditis elegans

The diverse cell types and the precise synaptic connectivity between them are the cardinal features of the nervous system. Little is known about how cell fate diversification is linked to synaptic target choices. Here we investigate how presynaptic neurons select one type of muscles, vm2, as a synaptic target and form synapses on its dendritic spine-like muscle arms. We found that the Notch-Delta pathway was required to distinguish target from non-target muscles. APX-1/Delta acts in surrounding cells including the non-target vm1 to activate LIN-12/Notch in the target vm2. LIN-12 functions cell-autonomously to up-regulate the expression of UNC-40/DCC and MADD-2 in vm2, which in turn function together to promote muscle arm formation and guidance. Ectopic expression of UNC-40/DCC in non-target vm1 muscle is sufficient to induce muscle arm extension from these cells. Therefore, the LIN-12/Notch signaling specifies target selection by selectively up-regulating guidance molecules and forming muscle arms in target cells.

DOI:http://dx.doi.org/10.7554/eLife.00378.001

The development of the nervous system involves the formation of complex networks of connections between diverse cell types, such as motor neurons, interneurons and pyramidal cells. However, the mechanisms by which individual cells are programmed to acquire particular identities, and how they are instructed to form connections with other specific cells, remain unclear.

In many species, the Notch signaling pathway has a role in setting up these networks. Notch is a transmembrane protein, which means that it has one component inside the cell and another outside. When a ligand binds to the extracellular part of Notch, this causes the receptor to break in two. The intracellular domain then travels to the nucleus where it can influence gene expression.

The nematode worm (C. elegans), which has two Notch receptors, is often used to study the formation of neuronal networks because each worm has only around 300 neurons, and they are connected in roughly the same way in each worm. C. elegans relies on two types of cell that are very similar to each other—type-1 and type-2 vulval muscle cells—to lay eggs, and the neurons that trigger egg-laying form synaptic connections on specialized structures called muscle arms. However, these structures are found only in type-2 vulval muscle.

To investigate the mechanisms underlying the formation of the egg-laying circuit, Li et al. screened large numbers of mutant worms to find animals that lacked muscle arms. They identified a number of such mutants, which laid fewer eggs compared to wild-type worms, and found that they all had mutations in genes that encode for proteins or ligands that are involved in the LIN-12/Notch pathway. This pathway mediates cell–cell interactions that help to specify cell fates.

Li et al. showed that type-2 vulval muscle cells develop muscle arms when their neighbors—type-1 vulval muscle cells and vulval epithelial cells—produce enough ligand to activate the LIN-12 Notch receptor on the type-2 vulval muscle cells. They also identified two of the downstream targets of LIN-12, and found that artificially expressing one of these in type-1 vulval muscle cells is sufficient to trigger the formation of muscle arms.

The work of Li et al. provides further evidence that the Notch signalling pathway, which is well known for its role in early development, also acts at later developmental stages to determine cell fate and patterns of connectivity.

DOI:http://dx.doi.org/10.7554/eLife.00378.002

Receptors and other signaling proteins required for serotonin control of locomotion in Caenorhabditis elegans

A better understanding of the molecular mechanisms of signaling by the neurotransmitter serotonin is required to assess the hypothesis that defects in serotonin signaling underlie depression in humans. Caenorhabditis elegans uses serotonin as a neurotransmitter to regulate locomotion, providing a genetic system to analyze serotonin signaling. From large-scale genetic screens we identified 36 mutants of C. elegans in which serotonin fails to have its normal effect of slowing locomotion, and we molecularly identified eight genes affected by 19 of the mutations. Two of the genes encode the serotonin-gated ion channel MOD-1 and the G-protein-coupled serotonin receptor SER-4. mod-1 is expressed in the neurons and muscles that directly control locomotion, while ser-4 is expressed in an almost entirely non-overlapping set of sensory and interneurons. The cells expressing the two receptors are largely not direct postsynaptic targets of serotonergic neurons. We analyzed animals lacking or overexpressing the receptors in various combinations using several assays for serotonin response. We found that the two receptors act in parallel to affect locomotion. Our results show that serotonin functions as an extrasynaptic signal that independently activates multiple receptors at a distance from its release sites and identify at least six additional proteins that appear to act with serotonin receptors to mediate serotonin response.

Two types of chloride transporters are required for GABA(A) receptor-mediated inhibition in C. elegans

Chloride influx through GABA-gated Cl(-) channels, the principal mechanism for inhibiting neural activity in the brain, requires a Cl(-) gradient established in part by K(+)-Cl(-) cotransporters (KCCs). We screened for Caenorhabditis elegans mutants defective for inhibitory neurotransmission and identified mutations in ABTS-1, a Na(+)-driven Cl(-)-HCO(3)(-) exchanger that extrudes chloride from cells, like KCC-2, but also alkalinizes them. While animals lacking ABTS-1 or the K(+)-Cl(-) cotransporter KCC-2 display only mild behavioural defects, animals lacking both Cl(-) extruders are paralyzed. This is apparently due to severe disruption of the cellular Cl(-) gradient such that Cl(-) flow through GABA-gated channels is reversed and excites rather than inhibits cells. Neuronal expression of both transporters is upregulated during synapse development, and ABTS-1 expression further increases in KCC-2 mutants, suggesting regulation of these transporters is coordinated to control the cellular Cl(-) gradient. Our results show that Na(+)-driven Cl(-)-HCO(3)(-) exchangers function with KCCs in generating the cellular chloride gradient and suggest a mechanism for the close tie between pH and excitability in the brain.

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.

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.

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.

The potassium chloride cotransporter KCC-2 coordinates development of inhibitory neurotransmission and synapse structure in Caenorhabditis elegans

Chloride influx through GABA-gated chloride channels, the primary mechanism by which neural activity is inhibited in the adult mammalian brain, depends on chloride gradients established by the potassium chloride cotransporter KCC2. We used a genetic screen to identify genes important for inhibition of the hermaphrodite-specific motor neurons (HSNs) that stimulate Caenorhabditis elegans egg-laying behavior and discovered mutations in a potassium chloride cotransporter, kcc-2. Functional analysis indicates that, like mammalian KCCs, C. elegans KCC-2 transports chloride, is activated by hypotonic conditions, and is inhibited by the loop diuretic furosemide. KCC-2 appears to establish chloride gradients required for the inhibitory effects of GABA-gated and serotonin-gated chloride channels on C. elegans behavior. In the absence of KCC-2, chloride gradients appear to be altered in neurons and muscles such that normally inhibitory signals become excitatory. kcc-2 is transcriptionally upregulated in the HSN neurons during synapse development. Loss of KCC-2 produces a decrease in the synaptic vesicle population within mature HSN synapses, which apparently compensates for a lack of HSN inhibition, resulting in normal egg-laying behavior. Thus, KCC-2 coordinates the development of inhibitory neurotransmission with synapse maturation to produce mature neural circuits with appropriate activity levels.

C. elegans G protein regulator RGS-3 controls sensitivity to sensory stimuli

Signal transduction through heterotrimeric G proteins is critical for sensory response across species. Regulator of G protein signaling (RGS) proteins are negative regulators of signal transduction. Herein we describe a role for C. elegans RGS-3 in the regulation of sensory behaviors. rgs-3 mutant animals fail to respond to intense sensory stimuli but respond normally to low concentrations of specific odorants. We find that loss of RGS-3 leads to aberrantly increased G protein-coupled calcium signaling but decreased synaptic output, ultimately leading to behavioral defects. Thus, rgs-3 responses are restored by decreasing G protein-coupled signal transduction, either genetically or by exogenous dopamine, by expressing a calcium-binding protein to buffer calcium levels in sensory neurons or by enhancing glutamatergic synaptic transmission from sensory neurons. Therefore, while RGS proteins generally act to downregulate signaling, loss of a specific RGS protein in sensory neurons can lead to defective responses to external stimuli.

Biogenic amine neurotransmitters in C. elegans

Four biogenic amines: octopamine, tyramine, dopamine and serotonin act in C. elegans to modulate behavior in response to changing environmental cues. These neurotransmitters act at both neurons and muscles to affect egg laying, pharyngeal pumping, locomotion and learning. A variety of experimental approaches including genetic, imaging, biochemical and pharmacological analyses have been used to identify the enzymes and cells that make and release the amines and the cells and receptors that bind them. Dopamine and serotonin act through receptors and downstream signaling mechanisms similar to those that operate in the mammalian brain suggesting that C. elegans will provide a valuable model for understanding biogenic amine signaling in the brain.

A specific subset of transient receptor potential vanilloid-type channel subunits in Caenorhabditis elegans endocrine cells function as mixed heteromers to promote neurotransmitter release

Transient receptor potential (TRP) channel subunits form homotetramers that function in sensory transduction. Heteromeric channels also form, but their physiological subunit compositions and functions are largely unknown. We found a dominant-negative mutant of the C. elegans TRPV (vanilloid-type) subunit OCR-2 that apparently incorporates into and inactivates OCR-2 homomers as well as heteromers with the TRPV subunits OCR-1 and -4, resulting in a premature egg-laying defect. This defect is reproduced by knocking out all three OCR genes, but not by any single knockout. Thus a mixture of redundant heteromeric channels prevents premature egg laying. These channels, as well as the G-protein G alpha(o), function in neuroendocrine cells to promote release of neurotransmitters that block egg laying until eggs filling the uterus deform the neuroendocrine cells. The TRPV channel OSM-9, previously suggested to be an obligate heteromeric partner of OCR-2 in sensory neurons, is expressed in the neuroendocrine cells but has no detectable role in egg laying. Our results identify a specific set of heteromeric TRPV channels that redundantly regulate neuroendocrine function and show that a subunit combination that functions in sensory neurons is also present in neuroendocrine cells but has no detectable function in these cells.

Heterotrimeric G protein signaling: Getting inside the cell

Heterotrimeric G proteins have traditionally been thought to transduce signals at the plasma membrane. In this issue, Slessareva et al. (2006) now show that a G protein alpha subunit acts at the endosome to stimulate a phosphatidylinositol 3-kinase to help yeast respond to mating pheromones.

Caenorhabditus elegans arrestin regulates neural G protein signaling and olfactory adaptation and recovery

Although regulation of G protein-coupled receptor signaling by receptor kinases and arrestins is a well established biochemical process, the physiological significance of such regulation remains poorly understood. To better understand the in vivo consequences of arrestin function, we have examined the function of the sole arrestin in Caenorhabditis elegans (ARR-1). ARR-1 is primarily expressed in the nervous system, including the HSN neuron and various chemosensory neurons involved in detecting soluble and volatile odorants. arr-1 null mutants exhibit normal chemotaxis but have significant defects in olfactory adaptation and recovery to volatile odorants. In contrast, adaptation is enhanced in animals overexpressing ARR-1. Both the adaptation and recovery defects of arr-1 mutants are rescued by transgenic expression of wild-type ARR-1, whereas expression of a C-terminally truncated ARR-1 effectively rescues only the adaptation defect. A potential mechanistic basis for these findings is revealed by in vitro studies demonstrating that wild-type ARR-1 binds proteins of the endocytic machinery and promotes receptor endocytosis, whereas C-terminally truncated ARR-1 does not. These results demonstrate that ARR-1 functions to regulate chemosensory signaling, enabling organisms to adapt to a variety of environmental cues, and provide an in vivo link between arrestin, receptor endocytosis, and temporal recovery from adaptation.

Activation of EGL-47, a Galpha(o)-coupled receptor, inhibits function of hermaphrodite-specific motor neurons to regulate Caenorhabditis elegans egg-laying behavior

Caenorhabditis elegans egg-laying behavior is inhibited by neurotransmitter signaling through the neural G-protein Galpha(o) and serves as a model for analyzing Galpha(o) signaling. Mutations that alter egg-laying frequency have identified genes encoding a number of signaling proteins that act with Galpha(o), but the receptors that activate Galpha(o) remain mostly uncharacterized. To further analyze Galpha(o) signaling, we cloned the egl-47 gene, which was identified by two dominant mutations that severely inhibit egg laying. egl-47 encodes two orphan G-protein-coupled receptor isoforms, which share all seven transmembrane domains but have different extracellular N termini. Both dominant mutations change the same alanine to valine in the sixth transmembrane domain, resulting in constitutively activated receptors. Deletion of the egl-47 gene caused no detectable egg-laying defects, suggesting that EGL-47 functions redundantly, or it inhibits egg laying under specific circumstances as yet unidentified. Using promoter::green fluorescent protein transgenes, we found that EGL-47 is expressed in a number of neurons, including the hermaphrodite-specific neurons (HSNs) that innervate the egg-laying muscles to stimulate contraction. Transgenic expression of constitutively active EGL-47 or constitutively active Galpha(o) specifically in the HSNs was sufficient to inhibit egg-laying behavior. Our results suggest that EGL-47 regulates egg laying by activating Galpha(o) in the HSN motor neurons to inhibit their activity. Because several neurotransmitters act through Galpha(o) to inhibit HSN function, it appears that loss of any one receptor, such as EGL-47, causes only mild defects. Galpha(o) apparently integrates signaling from multiple receptors in the HSNs, including EGL-47, to set the frequency of egg-laying behavior.

RGS-7 completes a receptor-independent heterotrimeric G protein cycle to asymmetrically regulate mitotic spindle positioning in C. elegans

Heterotrimeric G proteins promote microtubule forces that position mitotic spindles during asymmetric cell division in C. elegans embryos. While all previously studied G protein functions require activation by seven-transmembrane receptors, this function appears to be receptor independent. We found that mutating a regulator of G protein signaling, RGS-7, resulted in hyperasymmetric spindle movements due to decreased force on one spindle pole. RGS-7 is localized at the cell cortex, and its effects require two redundant Galphao-related G proteins and their nonreceptor activators RIC-8 and GPR-1/2. Using recombinant proteins, we found that RIC-8 stimulates GTP binding by Galphao and that the RGS domain of RGS-7 stimulates GTP hydrolysis by Galphao, demonstrating that Galphao passes through the GTP bound state during its activity cycle. While GTPase activators typically inactivate G proteins, RGS-7 instead appears to promote G protein function asymmetrically in the cell, perhaps acting as a G protein effector.

Domains, amino acid residues, and new isoforms of Caenorhabditis elegans diacylglycerol kinase 1 (DGK-1) important for terminating diacylglycerol signaling in vivo

Diacylglycerol kinases (DGKs) inhibit diacylglycerol (DAG) signaling by phosphorylating DAG. DGK-1, the Caenorhabditis elegans ortholog of human neuronal DGK, inhibits neurotransmission to control behavior. DGK-1, like DGK, has three cysteine-rich domains (CRDs), a pleckstrin homology domain, and a kinase domain. To identify DGK domains and amino acid residues critical for terminating DAG signaling in vivo, we analyzed 20 dgk-1 mutants defective in DGK-1-controlled behaviors. We found by sequencing that the mutations included nine amino acid substitutions and seven premature stop codons that impair the physiological functions of DGK-1. All nine amino acid substitutions are in the second CRD, the third CRD, or the kinase domain. Thus, these domains are important for the termination of DAG signaling by DGK-1 in vivo. Seven of the substituted amino acid residues are present in all human DGKs and likely define key residues required for the function of all DGKs. An ATP-binding site mutation expected to inactivate the kinase domain retained very little physiological function, but we found two stop codon mutants predicted to truncate DGK-1 before its kinase domain that retained significantly more function. We detected novel splice forms of dgk-1 that can reconcile this apparent conflict, as they skip exons containing the stop codons to produce DGK-1 isoforms that contain the kinase domain. Two of these isoforms lack an intact pleckstrin homology domain and yet appear to have significant function. Additional novel isoform(s) account for all of the DGK-1 function necessary for one behavior, dopamine response.

Genetic Analysis of RGS Protein Function in Caenorhabditis elegans

Caenorhabditis elegans has close homologs or orthologs of most mammalian (RGS) and G proteins, and mutants for all the RGS and G-protein genes of C. elegans have been generated. C. elegans RGS proteins can be matched to the specific Galpha proteins they regulate in vivo by comparing the defects in animals lacking or transgenically overexpressing an RGS protein with defects in a specific Galpha mutant. Transgenic expression of mutated RGS proteins or subdomains in C. elegans has also been used to carry out structure/function studies of RGS proteins. We propose that similar strategies can be used to understand the function of RGS proteins from other organisms by expressing them in C. elegans. This article describes general considerations regarding such experiments and provides detailed protocols for quantitatively measuring G-protein signaling phenotypes in C. elegans.

Genetic and cellular basis for acetylcholine inhibition of Caenorhabditis elegans egg-laying behavior

Egg-laying behavior in Caenorhabditis elegans is activated by signaling through the G-protein G(rho)q and inhibited by signaling through a second G-protein, G(rho)o. Activation of egg laying depends on the serotonergic hermaphrodite-specific neurons (HSNs), but the neurotransmitter(s) and cell(s) that signal to inhibit egg laying are not known. Mutants for G-protein signaling genes have well characterized defects in egg laying. Here we present an analysis of mutants for other genes reported to lack inhibition of egg laying. Of the nine strongest, six have morphological defects in the ventral-type C (VC) neurons, which synapse onto both the HSNs and the egg-laying muscles and are thus the third cell type comprising the egg-laying system. Laser-ablating VC neurons could also disrupt the inhibition of egg laying. The remaining three mutants (unc-4, cha-1, and unc-17) are defective for synthesis or packaging of acetylcholine in the VCs. The egg-laying defects of unc-4, cha-1, and unc-17 were rescued by VC-specific expression of the corresponding cDNAs. In addition, increasing synaptic acetylcholine by reducing acetylcholinesterase activity, with either mutations or the inhibitor aldicarb, decreased egg laying. Finally, we found that a knock-out for the HSN-expressed receptor G-protein-coupled acetylcholine receptor 2 (GAR-2) shows a partial defect in the inhibition of egg laying and fails to respond to aldicarb. Our results show that acetylcholine released from the VC neurons inhibits egg-laying behavior. This inhibition may be caused, in part, by acetylcholine signaling onto the HSN presynaptic terminals, via GAR-2, to inhibit neurotransmitter release.

An N-terminal region of Caenorhabditis elegans RGS proteins EGL-10 and EAT-16 directs inhibition of G(alpha)o versus G(alpha)q signaling

Regulator of G protein signaling (RGS) proteins contain an RGS domain that inhibits G(alpha) signaling by activating G(alpha) GTPase activity. Certain RGS proteins also contain a Ggamma-like (GGL) domain and a poorly characterized but conserved N-terminal region. We assessed the functions of these subregions in the Caenorhabditis elegans RGS proteins EGL-10 and EAT-16, which selectively inhibit GOA-1 (G(alpha)(o)) and EGL-30 (G(alpha)(q)), respectively. Using transgenes in C. elegans, we expressed EGL-10, EAT-16, their subregions, or EGL-10/EAT-16 chimeras. The chimeras showed that the GGL/RGS region of either protein can act on either GOA-1 or EGL-30 and that a key factor determining G(alpha) target selectivity is the manner in which the N-terminal and GGL/RGS regions are linked. We also found that coexpressing N-terminal and GGL/RGS fragments of EGL-10 gave full EGL-10 activity, whereas either fragment alone gave little activity. Biochemical analysis showed that coexpressing the two fragments caused both to increase in abundance and also caused the GGL/RGS fragment to move to the membrane, where the N-terminal fragment is localized. By coimmunoprecipitation, we found that the N-terminal fragment complexes with the C-terminal fragment and its associated Gbeta subunit, GPB-2. We conclude that the N-terminal region directs inhibition of G(alpha) signaling by forming a complex with the GGL/RGS region and affecting its stability, membrane localization, and G(alpha) target specificity.

Two RGS proteins that inhibit Galpha(o) and Galpha(q) signaling in C. elegans neurons require a Gbeta(5)-like subunit for function

BACKGROUND

Gbeta proteins have traditionally been thought to complex with Ggamma proteins to function as subunits of G protein heterotrimers. The divergent Gbeta(5) protein, however, can bind either Ggamma proteins or regulator of G protein signaling (RGS) proteins that contain a G gamma-like (GGL) domain. RGS proteins inhibit G protein signaling by acting as Galpha GTPase activators. While Gbeta(5) appears to bind RGS proteins in vivo, its association with Ggamma proteins in vivo has not been clearly demonstrated. It is unclear how Gbeta(5) might influence RGS activity. In C. elegans there are exactly two GGL-containing RGS proteins, EGL-10 and EAT-16, and they inhibit Galpha(o) and Galpha(q) signaling, respectively.

RESULTS

We knocked out the gene encoding the C. elegans Gbeta(5) ortholog, GPB-2, to determine its physiological roles in G protein signaling. The gpb-2 mutation reduces the functions of EGL-10 and EAT-16 to levels comparable to those found in egl-10 and eat-16 null mutants. gpb-2 knockout animals are viable, and exhibit no obvious defects beyond those that can be attributed to a reduction of EGL-10 or EAT-16 function. GPB-2 protein is nearly absent in eat-16; egl-10 double mutants, and EGL-10 protein is severely diminished in gpb-2 mutants.

CONCLUSIONS

Gbeta(5) functions in vivo complexed with GGL-containing RGS proteins. In the absence of Gbeta(5), these RGS proteins have little or no function. The formation of RGS-Gbeta(5) complexes is required for the expression or stability of both the RGS and Gbeta(5) proteins. Appropriate RGS-Gbeta(5) complexes regulate both Galpha(o) and Galpha(q) proteins in vivo.

Multiple RGS proteins alter neural G protein signaling to allow C. elegans to rapidly change behavior when fed

Regulators of G protein signaling (RGS proteins) inhibit heterotrimeric G protein signaling by activating G protein GTPase activity. Many mammalian RGS proteins are expressed in the brain and can act in vitro on the neural G protein G(o), but the biological purpose of this multiplicity of regulators is not clear. We have analyzed all 13 RGS genes in Caenorhabditis elegans and found that three of them influence the aspect of egg-laying behavior controlled by G(o) signaling. A previously studied RGS protein, EGL-10, affects egg laying under all conditions tested. The other two RGS proteins, RGS-1 and RGS-2, act as G(o) GTPase activators in vitro but, unlike EGL-10, they do not strongly affect egg laying when worms are allowed to feed constantly. However, rgs-1; rgs-2 double mutants fail to rapidly induce egg-laying behavior when refed after starvation. Thus EGL-10 sets baseline levels of signaling, while RGS-1 and RGS-2 appear to redundantly alter signaling to cause appropriate behavioral responses to food.

Multiple RGS proteins alter neural G protein signaling to allow C. elegans to rapidly change behavior when fed

Regulators of G protein signaling (RGS proteins) inhibit heterotrimeric G protein signaling by activating G protein GTPase activity. Many mammalian RGS proteins are expressed in the brain and can act in vitro on the neural G protein Go, but the biological purpose of this multiplicity of regulators is not clear. We have analyzed all 13 RGS genes in Caenorhabditis elegans and found that three of them influence the aspect of egg-laying behavior controlled by Go signaling. A previously studied RGS protein, EGL-10, affects egg laying under all conditions tested. The other two RGS proteins, RGS-1 and RGS-2, act as Go GTPase activators in vitro but, unlike EGL-10, they do not strongly affect egg laying when worms are allowed to feed constantly. However, rgs-1; rgs-2double mutants fail to rapidly induce egg-laying behavior when refed after starvation. Thus EGL-10 sets baseline levels of signaling, while RGS-1 and RGS-2 appear to redundantly alter signaling to cause appropriate behavioral responses to food.

Antagonism between G(o)alpha and G(q)alpha in Caenorhabditis elegans: the RGS protein EAT-16 is necessary for G(o)alpha signaling and regulates G(q)alpha activity

To elucidate the cellular role of the heterotrimeric G protein G(o), we have taken a molecular genetic approach in Caenorhabditis elegans. We screened for suppressors of activated GOA-1 (G(o)alpha) that do not simply decrease its expression and found mutations in only two genes, sag-1 and eat-16. Animals defective in either gene display a hyperactive phenotype similar to that of goa-1 loss-of-function mutants. Double-mutant analysis indicates that both sag-1 and eat-16 act downstream of, or parallel to, G(o)alpha and negatively regulate EGL-30 (G(q)alpha) signaling. eat-16 encodes a regulator of G protein signaling (RGS) most similar to the mammalian RGS7 and RGS9 proteins and can inhibit endogenous mammalian G(q)/G(11) in COS-7 cells. Animals defective in both sag-1 and eat-16 are inviable, but reducing function in egl-30 restores viability, indicating that the lethality of the eat-16; sag-1 double mutant is due to excessive G(q)alpha activity. Analysis of these mutations indicates that the G(o) and G(q) pathways function antagonistically in C. elegans, and that G(o)alpha negatively regulates the G(q) pathway, possibly via EAT-16 or SAG-1. We propose that a major cellular role of G(o) is to antagonize signaling by G(q).

A new family of G-protein regulators - the RGS proteins

Genetic experiments have recently been used to identify a family of 'regulator of G-protein signaling' (RGS) proteins, which downregulate signaling by heterotrimeric G proteins. The first biochemical studies of RGS proteins have shown that they accelerate the GTPase activities of G-protein alpha subunits, thus driving G proteins into their inactive GDP-bound forms. The physiological significance of the large number of different RGS proteins remains to be explored.

EGL-10 regulates G protein signaling in the C. elegans nervous system and shares a conserved domain with many mammalian proteins

The frequencies of certain periodic behaviors of the nematode C. elegans are regulated in a dose-dependent manner by the activity of the gene egl-10. These behaviors are modulated oppositely by the activity of the G protein alpha subunit gene goa-1, suggesting that egl-10 may regulate a G protein signaling pathway in a dose-dependent fashion. egl-10 encodes a protein similar to Sst2p, a negative regulator of G protein signaling in yeast. EGL-10 protein is localized in neural processes, where it may function in neurotransmitter signaling. Two previously known and 13 newly identified mammalian genes have similarity to egl-10 and SST2, and we propose that members of this family regulate many G protein signaling pathways.

DHR3: a Drosophila steroid receptor homolog

In Drosophila the steroid hormone ecdysone triggers a genetic regulatory hierarchy in which ecdysone combines with a receptor protein to form a complex that induces the transcription of a small class of "early" genes, which encode transcription factors that regulate other genes. We previously reported that one of the early genes, E75, encodes members of the steroid receptor superfamily. Using an E75 hybridization probe, we have identified two additional Drosophila genes that encode members of this superfamily. One of these is the ecdysone receptor gene, EcR, as previously reported. In this work, we examine the sequence, genomic organization, and developmental expression of the other gene, DHR3, which, like E75, encodes one of a growing number of "orphan" receptors for which ligands have not yet been identified. The structure of the DHR3 protein is strikingly similar to that of the MHR3 protein (e.g., 97% amino acid identity for the DNA binding domains), another orphan receptor encoded by an ecdysone-inducible early gene of another insect, Manduca sexta. The temporal developmental profile for DHR3 expression closely parallels that for the ecdysone titer and for the ecdysone-inducible E75 and E74 Drosophila early genes. The structural similarity to a Manduca early gene and the expression similarities to Drosophila early genes suggest that the DHR3 gene may also belong to the early gene class.

The Drosophila EcR gene encodes an ecdysone receptor, a new member of the steroid receptor superfamily

The steroid hormone ecdysone triggers coordinate changes in Drosophila tissue development that result in metamorphosis. To advance our understanding of the genetic regulatory hierarchies controlling this tissue response, we have isolated and characterized a gene, EcR, for a new steroid receptor homolog and have shown that it encodes an ecdysone receptor. First, EcR protein binds active ecdysteroids and is antigenically indistinguishable from the ecdysone-binding protein previously observed in extracts of Drosophila cell lines and tissues. Second, EcR protein binds DNA with high specificity at ecdysone response elements. Third, ecdysone-responsive cultured cells express EcR, whereas ecdysone-resistant cells derived from them are deficient in EcR. Expression of EcR in such resistant cells by transfection restores their ability to respond to the hormone. As expected, EcR is nuclear and found in all ecdysone target tissues examined. Furthermore, the EcR gene is expressed at each developmental stage marked by a pulse of ecdysone.

Visualization and quantification of transmural concentration profiles of macromolecules across the arterial wall

Transport parameters that describe a macromolecule entering the arterial wall from plasma can be obtained from concentration profiles of the labeled macromolecule entering the tissue. A new technique has been developed for measuring such concentration profiles, which offers spatial resolution superior to methods that measure profiles of radiolabeled macromolecules by serially sectioning tissue in planes parallel to the endothelium. In addition, this new method preserves cellular organization and tissue structure and permits measurement of concentration profiles underlying focal endothelial injuries or vascular lesions. The technique quantifies the concentration of a protein by measuring associated peroxidase activity. Although the present study was performed using horseradish peroxidase (HRP), the same principles can be applied to other macromolecules linked to HRP or microperoxidase. The colored reaction product of HRP was detected in transverse aortic sections using an image processing system. In the present study, profiles obtained by this new method were validated by comparison with HRP concentration profiles in rat aortas obtained by a serial slicing technique using radiolabeled HRP. We used the technique to measure high-resolution HRP concentration profiles in the intima and media of normal animals. These concentration profiles suggest that the internal elastic lamina acts as a major barrier to transport of macromolecules across the wall of the normal rat aorta. The new method should allow concentration profiles for macromolecules to be quantified in tissue surrounding vessels in the microcirculation, within the thickened intima of large vessels, and across coronary artery walls.