Identification and molecular characterization of the G alpha12-Rho guanine nucleotide exchange factor pathway in Caenorhabditis elegans

Divergent C-terminal motifs in Gα12 and Gα13 provide distinct mechanisms of effector binding and SRF activation

The G12/13 subfamily of heterotrimeric guanine nucleotide binding proteins comprises the α subunits Gα12 and Gα13, which transduce signals for cell growth, cytoskeletal rearrangements, and oncogenic transformation. In an increasing range of cancers, overexpressed Gα12 or Gα13 are implicated in aberrant cell proliferation and/or metastatic invasion. Although Gα12 and Gα13 bind non-redundant sets of effector proteins and participate in unique signalling pathways, the structural features responsible for functional differences between these α subunits are largely unknown. Invertebrates encode a single G12/13 homolog that participates in cytoskeletal changes yet appears to lack signalling to SRF (serum response factor), a transcriptional activator stimulated by mammalian Gα12 and Gα13 to promote growth and tumorigenesis. Our previous studies identified an evolutionarily divergent region in Gα12 for which replacement by homologous sequence from Drosophila melanogaster abolished SRF signalling, whereas the same invertebrate substitution was fully tolerated in Gα13 [Montgomery et al. (2014) Mol. Pharmacol. 85: 586]. These findings prompted our current approach of evolution-guided mutagenesis to identify fine structural features of Gα12 and Gα13 that underlie their respective SRF activation mechanisms. Our results identified two motifs flanking the α4 helix that play a key role in Gα12 signalling to SRF. We found the region encompassing these motifs to provide an interacting surface for multiple Gα12-specific target proteins that fail to bind Gα13. Adjacent to this divergent region, a highly-conserved domain was vital for SRF activation by both Gα12 and Gα13. However, dissection of this domain using invertebrate substitutions revealed different signalling mechanisms in these α subunits and identified Gα13-specific determinants of binding Rho-specific guanine nucleotide exchange factors. Furthermore, invertebrate substitutions in the C-terminal, α5 helical region were selectively disruptive to Gα12 signalling. Taken together, our results identify key structural features near the C-terminus that evolved after the divergence of Gα12 and Gα13, and should aid the development of agents to selectively manipulate signalling by individual α subunits of the G12/13 subfamily.

Gene Cascade Finder: A tool for identification of gene cascades and its application in Caenorhabditis elegans

Obtaining a comprehensive understanding of the gene regulatory networks, or gene cascades, involved in cell fate determination and cell lineage segregation in Caenorhabditis elegans is a long-standing challenge. Although RNA-sequencing (RNA-Seq) is a promising technique to resolve these questions, the bioinformatics tools to identify associated gene cascades from RNA-Seq data remain inadequate. To overcome these limitations, we developed Gene Cascade Finder (GCF) as a novel tool for building gene cascades by comparison of mutant and wild-type RNA-Seq data along with integrated information of protein-protein interactions, expression timing, and domains. Application of GCF to RNA-Seq data confirmed that SPN-4 and MEX-3 regulate the canonical Wnt pathway during embryonic development. Moreover, lin-35, hsp-3, and gpa-12 were found to be involved in MEX-1-dependent neurogenesis, and MEX-3 was found to control the gene cascade promoting neurogenesis through lin-35 and apl-1. Thus, GCF could be a useful tool for building gene cascades from RNA-Seq data.

RHO-1 and the Rho GEF RHGF-1 interact with UNC-6/Netrin signaling to regulate growth cone protrusion and microtubule organization in Caenorhabditis elegans

UNC-6/Netrin is a conserved axon guidance cue that directs growth cone migrations in the dorsal-ventral axis of C. elegans and in the vertebrate spinal cord. UNC-6/Netrin is expressed in ventral cells, and growth cones migrate ventrally toward or dorsally away from UNC-6/Netrin. Recent studies of growth cone behavior during outgrowth in vivo in C. elegans have led to a polarity/protrusion model in directed growth cone migration away from UNC-6/Netrin. In this model, UNC-6/Netrin first polarizes the growth cone via the UNC-5 receptor, leading to dorsally biased protrusion and F-actin accumulation. UNC-6/Netrin then regulates protrusion based on this polarity. The receptor UNC-40/DCC drives protrusion dorsally, away from the UNC-6/Netrin source, and the UNC-5 receptor inhibits protrusion ventrally, near the UNC-6/Netrin source, resulting in dorsal migration. UNC-5 inhibits protrusion in part by excluding microtubules from the growth cone, which are pro-protrusive. Here we report that the RHO-1/RhoA GTPase and its activator GEF RHGF-1 inhibit growth cone protrusion and MT accumulation in growth cones, similar to UNC-5. However, growth cone polarity of protrusion and F-actin were unaffected by RHO-1 and RHGF-1. Thus, RHO-1 signaling acts specifically as a negative regulator of protrusion and MT accumulation, and not polarity. Genetic interactions are consistent with RHO-1 and RHGF-1 acting with UNC-5, as well as with a parallel pathway, to regulate protrusion. The cytoskeletal interacting molecule UNC-33/CRMP was required for RHO-1 activity to inhibit MT accumulation, suggesting that UNC-33/CRMP might act downstream of RHO-1. In sum, these studies describe a new role of RHO-1 and RHGF-1 in regulation of growth cone protrusion by UNC-6/Netrin.

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.

Tick Haller's Organ, a New Paradigm for Arthropod Olfaction: How Ticks Differ from Insects

Ticks are the vector of many human and animal diseases; and host detection is critical to this process. Ticks have a unique sensory structure located exclusively on the 1st pairs of legs; the fore-tarsal Haller’s organ, not found in any other animals, presumed to function like the insect antennae in chemosensation but morphologically very different. The mechanism of tick chemoreception is unknown. Utilizing next-generation sequencing and comparative transcriptomics between the 1st and 4th legs (the latter without the Haller’s organ), we characterized 1st leg specific and putative Haller’s organ specific transcripts from adult American dog ticks, Dermacentor variabilis. The analysis suggested that the Haller’s organ is involved in olfaction, not gustation. No known odorant binding proteins like those found in insects, chemosensory lipocalins or typical insect olfactory mechanisms were identified; with the transcriptomic data only supporting a possible olfactory G-protein coupled receptor (GPCR) signal cascade unique to the Haller’s organ. Each component of the olfactory GPCR signal cascade was identified and characterized. The expression of GPCR, G_αo and β-arrestin transcripts identified exclusively in the 1st leg transcriptome, and putatively Haller’s organ specific, were examined in unfed and blood-fed adult female and male D. variabilis. Blood feeding to repletion in adult females down-regulated the expression of all three chemosensory transcripts in females but not in males; consistent with differences in post-feeding tick behavior between sexes and an expected reduced chemosensory function in females as they leave the host. Data are presented for the first time of the potential hormonal regulation of tick chemosensation; behavioral assays confirmed the role of the Haller’s organ in N,N-diethyl-meta-toluamide (DEET) repellency but showed no role for the Haller’s organ in host attachment. Further research is needed to understand the potential role of the GPCR cascade in olfaction.

A Gα12-specific Binding Domain in AKAP-Lbc and p114RhoGEF

AKAP-Lbc is a Rho-activating guanine nucleotide exchange factor (RhoGEF) important in heart development and pro-fibrotic signaling in cardiomyocytes. Heterotrimeric G proteins of the G12/13 subfamily, comprising Gα12 and Gα13, are well characterized as stimulating a specialized group of RhoGEFs through interaction with their RGS-homology (RH) domain. Despite lacking an RH domain, AKAP-Lbc is bound by Gα12 through an unknown mechanism to activate Rho signaling. We identified a Gα12-binding region near the C-terminus of AKAP-Lbc, closely homologous to a region of p114RhoGEF that we also discovered to interact with Gα12. This binding mechanism is distinct from the well-studied interface between RH-RhoGEFs and G12/13 α subunits, as demonstrated by Gα12 mutants selectively impaired in binding either this AKAP-Lbc/p114RhoGEF region or RH-RhoGEFs. AKAP-Lbc and p114RhoGEF showed high specificity for binding Gα12 in comparison to Gα13, and experiments using chimeric G12/13 α subunits mapped determinants of this selectivity to the N-terminal region of Gα12. In cultured cells expressing constitutively GDP-bound Gα12 or Gα13, the Gα12 construct was more potent in exerting a dominant-negative effect on serum-mediated signaling to p114RhoGEF, demonstrating coupling of these signaling proteins in a cellular pathway. In addition, charge-reversal of conserved residues in AKAP-Lbc and p114RhoGEF disrupted Gα12 binding for both proteins, suggesting they harbor a common structural mechanism for interaction with this α subunit. Our results provide the first evidence of p114RhoGEF as a Gα12 signaling effector, and define a novel region conserved between AKAP-Lbc and p114RhoGEF that allows Gα12 signaling input to these non-RH RhoGEFs.

Expression profiles of the Gα subunits during Xenopus tropicalis embryonic development

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

Axotomy-induced HIF-serotonin signalling axis promotes axon regeneration in C. elegans

The molecular mechanisms underlying the ability of axons to regenerate after injury remain poorly understood. Here we show that in Caenorhabditis elegans, axotomy induces ectopic expression of serotonin (5-HT) in axotomized non-serotonergic neurons via HIF-1, a hypoxia-inducible transcription factor, and that 5-HT subsequently promotes axon regeneration by autocrine signalling through the SER-7 5-HT receptor. Furthermore, we identify the rhgf-1 and rga-5 genes, encoding homologues of RhoGEF and RhoGAP, respectively, as regulators of axon regeneration. We demonstrate that one pathway initiated by SER-7 acts upstream of the C. elegans RhoA homolog RHO-1 in neuron regeneration, which functions via G12α and RHGF-1. In this pathway, RHO-1 inhibits diacylglycerol kinase, resulting in an increase in diacylglycerol. SER-7 also promotes axon regeneration by activating the cyclic AMP (cAMP) signalling pathway. Thus, HIF-1-mediated activation of 5-HT signalling promotes axon regeneration by activating both the RhoA and cAMP pathways.

RHGF-1/PDZ-RhoGEF and retrograde DLK-1 signaling drive neuronal remodeling on microtubule disassembly

Neurons remodel their connectivity in response to various insults, including microtubule disruption. How neurons sense microtubule disassembly and mount remodeling responses by altering genetic programs in the soma are not well defined. Here we show that in response to microtubule disassembly, the Caenorhabditis elegans PLM neuron remodels by retracting its synaptic branch and overextending the primary neurite. This remodeling required RHGF-1, a PDZ-Rho guanine nucleotide exchange factor (PDZ-RhoGEF) that was associated with and inhibited by microtubules. Independent of the myosin light chain activation, RHGF-1 acted through Rho-dependent kinase LET-502/ROCK and activated a conserved, retrograde DLK-1 MAPK (DLK-1/dual leucine zipper kinase) pathway, which triggered synaptic branch retraction and overgrowth of the PLM neurite in a dose-dependent manner. Our data represent a neuronal remodeling paradigm during development that reshapes the neural circuit by the coordinated removal of the dysfunctional synaptic branch compartment and compensatory extension of the primary neurite.

Rho-guanine nucleotide exchange factors during development: Force is nothing without control

The development of multicellular organisms is associated with extensive rearrangements of tissues and cell sheets. The driving force for these rearrangements is generated mostly by the actin cytoskeleton. In order to permit the reproducible development of a specific body plan, dynamic reorganization of the actin cytoskeleton must be precisely coordinated in space and time. GTP-exchange factors that activate small GTPases of the Rho family play an important role in this process. Here we review the role of this class of cytoskeletal regulators during important developmental processes such as epithelial morphogenesis, cytokinesis, cell migration, cell polarity, neuronal growth cone extension and phagocytosis in different model systems.

Gα12 structural determinants of Hsp90 interaction are necessary for serum response element-mediated transcriptional activation

The G12/13 class of heterotrimeric G proteins, comprising the α-subunits Gα12 and Gα13, regulates multiple aspects of cellular behavior, including proliferation and cytoskeletal rearrangements. Although guanine nucleotide exchange factors for the monomeric G protein Rho (RhoGEFs) are well characterized as effectors of this G protein class, a variety of other downstream targets has been reported. To identify Gα12 determinants that mediate specific protein interactions, we used a structural and evolutionary comparison between the G12/13, Gs, Gi, and Gq classes to identify "class-distinctive" residues in Gα12 and Gα13. Mutation of these residues in Gα12 to their deduced ancestral forms revealed a subset necessary for activation of serum response element (SRE)-mediated transcription, a G12/13-stimulated pathway implicated in cell proliferative signaling. Unexpectedly, this subset of Gα12 mutants showed impaired binding to heat-shock protein 90 (Hsp90) while retaining binding to RhoGEFs. Corresponding mutants of Gα13 exhibited robust SRE activation, suggesting a Gα12-specific mechanism, and inhibition of Hsp90 by geldanamycin or small interfering RNA-mediated lowering of Hsp90 levels resulted in greater downregulation of Gα12 than Gα13 signaling in SRE activation experiments. Furthermore, the Drosophila G12/13 homolog Concertina was unable to signal to SRE in mammalian cells, and Gα12:Concertina chimeras revealed Gα12-specific determinants of SRE activation within the switch regions and a C-terminal region. These findings identify Gα12 determinants of SRE activation, implicate Gα12:Hsp90 interaction in this signaling mechanism, and illuminate structural features that arose during evolution of Gα12 and Gα13 to allow bifurcated mechanisms of signaling to a common cell proliferative pathway.

Determinants at the N- and C-termini of Gα12 required for activation of Rho-mediated signaling

BACKGROUND

Heterotrimeric guanine nucleotide binding proteins of the G12/13 subfamily, which includes the α-subunits Gα12 and Gα13, stimulate the monomeric G protein RhoA through interaction with a distinct subset of Rho-specific guanine nucleotide exchange factors (RhoGEFs). The structural features that mediate interaction between Gα13 and RhoGEFs have been examined in crystallographic studies of the purified complex, whereas a Gα12:RhoGEF complex has not been reported. Several signaling responses and effector interactions appear unique to Gα12 or Gα13, despite their similarity in amino acid sequence.

METHODS

To comprehensively examine Gα12 for regions involved in RhoGEF interaction, we screened a panel of Gα12 cassette substitution mutants for binding to leukemia-associated RhoGEF (LARG) and for activation of serum response element mediated transcription.

RESULTS

We identified several cassette substitutions that disrupt Gα12 binding to LARG and the related p115RhoGEF. These Gα12 mutants also were impaired in activating serum response element mediated signaling, a Rho-dependent response. Most of these mutants matched corresponding regions of Gα13 reported to contact p115RhoGEF, but unexpectedly, several RhoGEF-uncoupling mutations were found within the N- and C-terminal regions of Gα12. Trypsin protection assays revealed several mutants in these regions as retaining conformational activation. In addition, charge substitutions near the Gα12 N-terminus selectively disrupted binding to LARG but not p115RhoGEF.

CONCLUSIONS

Several structural aspects of the Gα12:RhoGEF interface differ from the reported Gα13:RhoGEF complex, particularly determinants within the C-terminal α5 helix and structurally uncharacterized N-terminus of Gα12. Furthermore, key residues at the Gα12 N-terminus may confer selectivity for LARG as a downstream effector.

Parasite neuropeptide biology: Seeding rational drug target selection?

The rationale for identifying drug targets within helminth neuromuscular signalling systems is based on the premise that adequate nerve and muscle function is essential for many of the key behavioural determinants of helminth parasitism, including sensory perception/host location, invasion, locomotion/orientation, attachment, feeding and reproduction. This premise is validated by the tendency of current anthelmintics to act on classical neurotransmitter-gated ion channels present on helminth nerve and/or muscle, yielding therapeutic endpoints associated with paralysis and/or death. Supplementary to classical neurotransmitters, helminth nervous systems are peptide-rich and encompass associated biosynthetic and signal transduction components - putative drug targets that remain to be exploited by anthelmintic chemotherapy. At this time, no neuropeptide system-targeting lead compounds have been reported, and given that our basic knowledge of neuropeptide biology in parasitic helminths remains inadequate, the short-term prospects for such drugs remain poor. Here, we review current knowledge of neuropeptide signalling in Nematoda and Platyhelminthes, and highlight a suite of 19 protein families that yield deleterious phenotypes in helminth reverse genetics screens. We suggest that orthologues of some of these peptidergic signalling components represent appealing therapeutic targets in parasitic helminths.

Light-sensitive coupling of rhodopsin and melanopsin to G(i/o) and G(q) signal transduction in Caenorhabditis elegans

Activation of G-protein-coupled receptors (GPCRs) initiates signal transduction cascades that affect many physiological responses. The worm Caenorhabditis elegans expresses >1000 of these receptors along with their cognate heterotrimeric G proteins. Here, we report properties of 9-cis-retinal regenerated bovine opsin [(b)isoRho] and human melanopsin [(h)Mo], two light-activated, heterologously expressed GPCRs in the nervous system of C. elegans with various genetically engineered alterations. Profound transient photoactivation of G(i/o) signaling by (b)isoRho led to a sudden and transient loss of worm motility dependent on cyclic adenosine monophosphate, whereas transient photoactivation of G(q) signaling by (h)Mo enhanced worm locomotion dependent on phospholipase Cβ. These transgenic C. elegans models provide a unique way to study the consequences of G(i/o) and G(q) signaling in vivo with temporal and spatial precision and, by analogy, their relationship to human neuromotor function.

Signalling mechanisms of RhoGTPase regulation by the heterotrimeric G proteins G12 and G13

G protein-mediated signal transduction can transduce signals from a large variety of extracellular stimuli into cells and is the most widely used mechanism for cell communication at the membrane. The RhoGTPase family has been well established as key regulators of cell growth, differentiation and cell shape changes. Among G protein-mediated signal transduction, G12/13-mediated signalling is one mechanism to regulate RhoGTPase activity in response to extracellular stimuli. The alpha subunits of G12 or G13 have been shown to interact with members of the RH domain containing guanine nucleotide exchange factors for Rho (RH-RhoGEF) family of proteins to directly connect G protein-mediated signalling and RhoGTPase signalling. The G12/13-RH-RhoGEF signalling mechanism is well conserved over species and is involved in critical steps for cell physiology and disease conditions, including embryonic development, oncogenesis and cancer metastasis. In this review, we will summarize current progress on this important signalling mechanism.

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.

Regulation and physiological functions of G12/13-mediated signaling pathways

Accumulating data indicate that G12 subfamily (Galpha12/13)-mediated signaling pathways play pivotal roles in a variety of physiological processes, while aberrant regulation of this pathway has been identified in various human diseases. It has been demonstrated that Galpha12/13-mediated signals form networks with other signaling proteins at various levels, from cell surface receptors to transcription factors, to regulate cellular responses. Galpha12/13 have slow rates of nucleotide exchange and GTP hydrolysis, and specifically target RhoGEFs containing an amino-terminal RGS homology domain (RH-RhoGEFs), which uniquely function both as a GAP and an effector for Galpha12/13. In this review, we will focus on the mechanisms regulating the Galpha12/13 signaling system, particularly the Galpha12/13-RH-RhoGEF-Rho pathway, which can regulate a wide variety of cellular functions from migration to transformation.

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.

Protein crystallization in drug design: towards a rational approach

X-ray crystallography is the method of choice for the detailed characterization of stereochemistry of interactions of drug leads and potential chemotherapeutics with their protein targets. The resulting atomic models allow for rational enhancement of the lead properties and consequently for the design of high-affinity inhibitors. However, a major bottleneck of the technique is the requirement for the protein and its complexes to yield high quality single crystals. Furthermore, it is highly desirable that such crystals diffract to high resolution, preferably ≥ 1.2 Å, revealing the structures in atomic detail. Unfortunately, only a small portion of proteins readily crystallize in that fashion. New approaches are being developed to circumvent this problem. One proposed option includes rational protein surface engineering to systematically improve the crystallizability of the protein. This is accomplished by creating surface patches readily mediating weak, but specific, intermolecular interactions that take on the role of crystal contacts during nucleation and crystal growth phase.

Regulation of Rho guanine nucleotide exchange factors by G proteins

Monomeric Rho GTPases regulate cellular dynamics through remodeling of the cytoskeleton, modulation of immediate signaling pathways, and longer-term regulation of gene transcription. One family of guanine nucleotide exchange factors for Rho proteins (RhoGEFs) provides a direct pathway for regulation of RhoA by cell surface receptors coupled to heterotrimeric G proteins. Some of these RhoGEFs also contain RGS domains that can attenuate signaling by the G(12) and G(13) proteins. The regulation provided by these RhoGEFs is defined by their selective regulation by specific G proteins, phosphorylation by kinases, and potential localization with signaling partners. Evidence of their physiological importance is derived from gene knockouts in Drosophila and mice. Current understanding of the basic regulatory mechanisms of these RhoGEFs is discussed. An overview of identified interactions with other signaling proteins suggests the growing spectrum of their involvement in numerous signaling pathways.

The Galpha12-RGS RhoGEF-RhoA signalling pathway regulates neurotransmitter release in C. elegans

In Caenorhabditis elegans adults, the single Rho GTPase orthologue, RHO-1, stimulates neurotransmitter release at synapses. We show that one of the pathways acting upstream of RHO-1 in acetylcholine (ACh)-releasing motor neurons depends on Galpha12 (GPA-12), which acts via the single C. elegans RGS RhoGEF (RHGF-1). Activated GPA-12 has the same effect as activated RHO-1, inducing the accumulation of diacylglycerol and the neuromodulator UNC-13 at release sites, and increased ACh release. We showed previously that RHO-1 stimulates ACh release by two separate pathways-one that requires UNC-13 and a second that does not. We show here that a non-DAG-binding-UNC-13 mutant that partially blocks increased ACh release by activated RHO-1 completely blocks increased ACh release by activated GPA-12. Thus, the upstream GPA-12/RHGF-1 pathway stimulates only a subset of RHO-1 downstream effectors, suggesting that either the RHO-1 effectors require different levels of activated RHO-1 for activation or there are two distinct pools of RHO-1 within C. elegans neurons.

Multiple sensory G proteins in the olfactory, gustatory and nociceptive neurons modulate longevity in Caenorhabditis elegans

The life span of the nematode Caenorhabditis elegans is under control of sensory signals detected by the amphid neurons. In these neurons, C. elegans expresses at least 13 Galpha subunits and a Ggamma subunit, which are involved in the transduction and modulation of sensory signals. Here, we show that loss-of-function mutations in the Galpha subunits odr-3, gpa-1 and gpa-9, in the Ggamma subunit gpc-1 and the introduction of extra copies of the Galpha subunit gpa-11 extend the life span of C. elegans. Loss-of-function of odr-3 and extra copies of gpa-11 act synergistically and can together extend life span more than two-fold, indicating that sensory signals play an important role in regulating life span. We show that gpa-1, gpa-11, odr-3 and gpc-1 all signal via the daf-16 FOXO family transcription factor. In addition, odr-3 and gpa-11 might suppress life span extension partially independent of the insulin/IGF-1 like receptor homologue daf-2. Our results suggest that the previously unanticipated nociceptive ASH and/or ADL neurons regulate longevity. We expect that the implication of specific G proteins will eventually contribute to the identification of the sensory cues that determine the rate of aging in C. elegans.

Critical pathways in heart function: bis(2-chloroethoxy)methane-induced heart gene transcript change in F344 rats

Gene transcript changes after exposure to the heart toxin, bis(2-chloroethoxy)methane (CEM), were analyzed to elucidate mechanisms in cardiotoxicity and recovery. CEM was administered to 5-week-old male F344/N rats at 0, 200, 400, or 600 mg/kg by dermal exposure, 5 days per week, for a total of 12 doses by study day 16. Heart toxicity occurred after 2 days of dosing in all 3 regions of the heart (atrium, ventricle, interventricular septum) and was characterized by myofiber vacuolation, necrosis, mononuclear-cell infiltration, and atrial thrombosis. Ultrastructural analysis revealed that the primary site of damage was the mitochondrion. By day 5, even though dosing was continued, the toxic lesions in the heart began to resolve, and by study day 16, the heart appeared histologically normal. RNA was extracted from whole hearts after 2 or 5 days of CEM dosing. After a screen for transcript change by microarray analysis, dose-response trends for selected transcripts were analyzed by qRT-PCR. The selected transcripts code for proteins involved in energy production, control of calcium levels, and maintenance of heart function. The down-regulation of ATP subunit transcripts (Atp5j, ATP5k), which reside in the mitochondrial membranes, indicated a decrease in energy supply at day 2 and day 5. This was accompanied by down-regulation of transcripts involved in high-energy consumption processes such as membrane transport and ion channel transcripts (e.g., abc1a, kcnj12). The up-regulation of transcripts encoding for temperature regulation and calcium binding proteins (ucp1 and calb3) only at the 2 low exposure levels, suggest that these adaptive processes cannot occur in association with severe cardiotoxicity as seen in hearts at the high exposure level. Transcript expression changes occurred within 2 days of CEM exposure, and were dose-and time-dependent. The heart transcript changes suggest that CEM cardiotoxicity activates protective processes associated energy conservation and maintenance of heart function.

Regulatory network for cell shape changes during Drosophila ventral furrow formation

Rapid and sequential cell shape changes take place during the formation of the ventral furrow (VF) at the beginning of Drosophila gastrulation. At the cellular level, this morphogenetic event demands close coordination of the proteins involved in actin cytoskeletal reorganization. In order to construct a regulatory network that describes these cell shape changes, we have used published genetic and molecular data for 18 genes encoding transcriptional regulators and signaling pathway components. Based on the dynamic behavior of this network we explored the hypothesis that the combination of three recognizable phenotypes describing wild type or mutant cell types, during VF invagination, correspond to different activation states of a specific set of these gene products, which are point attractors of the regulatory network. From our results, we recognize missing components in the regulatory network and suggest alternative pathways in the regulation of cell shape changes during VF formation.

G proteins in development

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

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.

Receptors coupled to heterotrimeric G proteins of the G12 family

Much regarding the engagement of the G(12) family of heterotrimeric G proteins (G(12) and G(13)) by agonist-activated receptors remains unclear. For example, the identity of receptors that couple unequivocally to G(12) and G(13) and how signals are allocated among these and other G proteins remain open questions. Part of the problem in understanding signaling through G(12) and G(13) is that the activation of these G proteins is rarely demonstrated directly and is instead presumed usually from far removed downstream events. Furthermore, receptors that couple to G(12) and G(13) invariably couple to additional G proteins, and thus few events can be linked unambiguously to one G protein or another. In this article, we document receptors that reportedly couple to G(12), G(13) or both G(12) and G(13), evaluate the methodology used to understand the coupling of these receptors, and discuss the ability of these receptors to couple also to G(q).

RNA interference in Caenorhabditis elegans

RNA interference (RNAi) was first discovered in the nematode Caenorhabditis elegans (Fire et al., 1998; Guo and Kemphues, 1995). The completion of the C. elegans genome in 1998 coupled with the advent of RNAi techniques to knock down gene function ushered in a new age in the field of functional genomics. There are four methods for double-stranded RNA (dsRNA) delivery in C. elegans: (1) injection of dsRNA into any region of the animal (Fire et al., 1998), (2) feeding with bacteria producing dsRNA (Timmons et al., 2001), (3) soaking in dsRNA (Tabara et al., 1998), and (4) in vivo production of dsRNA from transgenic promoters (Tavernarakis et al., 2000). In this chapter, we discuss the molecular genetic mechanisms, techniques, and applications of RNAi in C. elegans.

Post-transcriptional gene silencing in neurons

The techniques evolving from the rapidly developing field of small RNAs promise accessible approaches to dissecting cellular and molecular mechanisms of higher brain function. Here, a current overview of the technology is presented, along with an outline of how these approaches might help neuroscientists to more rapidly uncover the cellular and molecular bases of behavior.

A Rho GTPase Signaling Pathway Is Used Reiteratively in Epithelial Folding and Potentially Selects the Outcome of Rho Activation

A single Rho GTPase family member is capable of initiating several different processes, including cell cycle regulation, cytokinesis, cell migration, and transcriptional regulation . It is not clear, however, how the Rho protein selects which of these processes to initiate. Guanine nucleotide exchange factors (GEFs), proteins that activate Rho GTPases, could be important in making this selection. We show here that in vivo, DRhoGEF2, a GEF that is ubiquitously expressed and specific for Rho1, is reiteratively required for epithelial folding and invagination, but not for other processes regulated by Rho. The limitation of DRhoGEF2 function supports the hypothesis that the GEF selects the outcome of Rho activation. DRhoGEF2 exerts its effects in gastrulation through the regulation of Myosin II to orchestrate coordinated apical cell constriction. Apical myosin localization is also regulated by Concertina (Cta), a Galpha(12/13) family member that is thought to activate DRhoGEF2 and is itself activated by a putative ligand, Folded gastrulation (Fog). Fog and Cta also play a role in the morphogenetic events requiring DRhoGEF2, suggesting the existence of a conserved signaling pathway in which Fog, Cta, and DRhoGEF2 locally activate Myosin for epithelial invagination and folding.