NMR structure and mutagenesis of the N-terminal Dbl homology domain of the nucleotide exchange factor Trio

Dbl family RhoGEFs in cancer: different roles and targeting strategies

Small Ras homologous guanosine triphosphatase (Rho GTPase) family proteins are highly associated with tumorigenesis and development. As intrinsic exchange activity regulators of Rho GTPases, Rho guanine nucleotide exchange factors (RhoGEFs) have been demonstrated to be closely involved in tumor development and received increasing attention. They mainly contain two families: the diffuse B-cell lymphoma (Dbl) family and the dedicator of cytokinesis (Dock) family. More and more emphasis has been paid to the Dbl family members for their abnormally high expression in various cancers and their correlation to poor prognosis. In this review, the common and distinctive structures of Dbl family members are discussed, and their roles in cancer are summarized with a focus on Ect2, Tiam1/2, P-Rex1/2, Vav1/2/3, Trio, KALRN, and LARG. Significantly, the strategies targeting Dbl family RhoGEFs are highlighted as novel therapeutic opportunities for cancer.

Pleckstrin Homology [PH] domain, structure, mechanism, and contribution to human disease

The pleckstrin homology [PH] domain is a structural fold found in more than 250 proteins making it the 11th most common domain in the human proteome. 25% of family members have more than one PH domain and some PH domains are split by one, or several other, protein domains although still folding to give functioning PH domains. We review mechanisms of PH domain activity, the role PH domain mutation plays in human disease including cancer, hyperproliferation, neurodegeneration, inflammation, and infection, and discuss pharmacotherapeutic approaches to regulate PH domain activity for the treatment of human disease. Almost half PH domain family members bind phosphatidylinositols [PIs] that attach the host protein to cell membranes where they interact with other membrane proteins to give signaling complexes or cytoskeleton scaffold platforms. A PH domain in its native state may fold over other protein domains thereby preventing substrate access to a catalytic site or binding with other proteins. The resulting autoinhibition can be released by PI binding to the PH domain, or by protein phosphorylation thus providing fine tuning of the cellular control of PH domain protein activity. For many years the PH domain was thought to be undruggable until high-resolution structures of human PH domains allowed structure-based design of novel inhibitors that selectively bind the PH domain. Allosteric inhibitors of the Akt1 PH domain have already been tested in cancer patients and for proteus syndrome, with several other PH domain inhibitors in preclinical development for treatment of other human diseases.

A developmental delay linked missense mutation in Kalirin-7 disrupts protein function and neuronal morphology

The Rac1 guanine exchange factor Kalirin-7 is a key regulator of dendritic spine morphology, LTP and dendritic arborization. Kalirin-7 dysfunction and genetic variation has been extensively linked to various neurodevelopmental and neurodegenerative disorders. Here we characterize a Kalirin-7 missense mutation, glu1577lys (E1577K), identified in a patient with severe developmental delay. The E1577K point mutation is located within the catalytic domain of Kalirin-7, and results in a robust reduction in Kalirin-7 Rac1 Guanosine exchange factor activity. In contrast to wild type Kalirin-7, the E1577K mutant failed to drive dendritic arborization, spine density, NMDAr targeting to, and activity within, spines. Together these results indicate that reduced Rac1-GEF activity as result of E1577K mutation impairs neuroarchitecture, connectivity and NMDAr activity, and is a likely contributor to impaired neurodevelopment in a patient with developmental delay.

The RHO Family GTPases: Mechanisms of Regulation and Signaling

Much progress has been made toward deciphering RHO GTPase functions, and many studies have convincingly demonstrated that altered signal transduction through RHO GTPases is a recurring theme in the progression of human malignancies. It seems that 20 canonical RHO GTPases are likely regulated by three GDIs, 85 GEFs, and 66 GAPs, and eventually interact with >70 downstream effectors. A recurring theme is the challenge in understanding the molecular determinants of the specificity of these four classes of interacting proteins that, irrespective of their functions, bind to common sites on the surface of RHO GTPases. Identified and structurally verified hotspots as functional determinants specific to RHO GTPase regulation by GDIs, GEFs, and GAPs as well as signaling through effectors are presented, and challenges and future perspectives are discussed.

More evidence on TRIO missense mutations in the spectrin repeat domain causing severe developmental delay and recognizable facial dysmorphism with macrocephaly

TRIO is a Dbl family guanine nucleotide exchange factor (GEF) and an important regulator of neuronal development. Most truncating and missense variants affecting the Dbl homology domain of TRIO are associated with a neurodevelopmental disorder with microcephaly (MIM617061). Recently, de novo missense variants affecting the spectrin repeat region of TRIO were associated with a novel phenotype comprising severe developmental delay and macrocephaly (MIM618825). Here, we provide more evidence on this new TRIO-associated phenotype by reporting two severely affected probands with de novo missense variants in TRIO affecting the spectrin repeat region upstream of the typically affected GEF1 domain of the protein.

Trio family proteins as regulators of cell migration and morphogenesis in development and disease - mechanisms and cellular contexts

The well-studied members of the Trio family of proteins are Trio and kalirin in vertebrates, UNC-73 in Caenorhabditis elegans and Trio in Drosophila Trio proteins are key regulators of cell morphogenesis and migration, tissue organization, and secretion and protein trafficking in many biological contexts. Recent discoveries have linked Trio and kalirin to human disease, including neurological disorders and cancer. The genes for Trio family proteins encode a series of large multidomain proteins with up to three catalytic activities and multiple scaffolding and protein-protein interaction domains. As such, Trio family proteins engage a wide array of cell surface receptors, substrates and interaction partners to coordinate changes in cytoskeletal regulatory and protein trafficking pathways. We provide a comprehensive review of the specific mechanisms by which Trio family proteins carry out their functions in cells, highlight the biological and cellular contexts in which they occur, and relate how alterations in these functions contribute to human disease.

KALRN: A central regulator of synaptic function and synaptopathies

The synaptic regulator, kalirin, plays a key role in synaptic plasticity and formation of dendritic arbors and spines. Dysregulation of the KALRN gene has been linked to various neurological disorders, including autism spectrum disorder, Alzheimer's disease, schizophrenia, addiction and intellectual disabilities. Both genetic and molecular studies highlight the importance of normal KALRN expression for healthy neurodevelopment and function. This review aims to give an in-depth analysis of the structure and molecular mechanisms of kalirin function, particularly within the brain. These data are correlated to genetic evidence of patient mutations within KALRN and animal models of Kalrn that together give insight into the manner in which this gene may be involved in neurodevelopment and the etiology of disease. The emerging links to human disease from post-mortem, genome wide association (GWAS) and exome sequencing studies are examined to highlight the disease relevance of kalirin, particularly in neurodevelopmental diseases. Finally, we will discuss efforts to pharmacologically regulate kalirin protein activity and the implications of such endeavors for the treatment of human disease. As multiple disease states arise from deregulated synapse formation and altered KALRN expression and function, therapeutics may be developed to provide control over KALRN activity and thus synapse dysregulation. As such, a detailed understanding of how kalirin regulates neuronal development, and the manner in which kalirin dysfunction promotes neurological disease, may support KALRN as a valuable therapeutic avenue for future pharmacological intervention.

Vav2 pharmaco-mimetic mice reveal the therapeutic value and caveats of the catalytic inactivation of a Rho exchange factor

The current paradigm holds that the inhibition of Rho guanosine nucleotide exchange factors (GEFs), the enzymes that stimulate Rho GTPases, can be a valuable therapeutic strategy to treat Rho-dependent tumors. However, formal validation of this idea using in vivo models is still missing. In this context, it is worth remembering that many Rho GEFs can mediate both catalysis-dependent and independent responses, thus raising the possibility that the inhibition of their catalytic activities might not be sufficient per se to block tumorigenic processes. On the other hand, the inhibition of these enzymes can trigger collateral side effects that could preclude the practical implementation of anti-GEF therapies. To address those issues, we have generated mouse models to mimic the effect of the systemic application of an inhibitor for the catalytic activity of the Rho GEF Vav2 at the organismal level. Our results indicate that lowering the catalytic activity of Vav2 below specific thresholds is sufficient to block skin tumor initiation, promotion, and progression. They also reveal that the negative side effects typically induced by the loss of Vav2 can be bypassed depending on the overall level of Vav2 inhibition achieved in vivo. These data underscore the pros and cons of anti-Rho GEF therapies for cancer treatment. They also support the idea that Vav2 could represent a viable drug target.

Interconnecting Flexibility, Structural Communication, and Function in RhoGEF Oncoproteins

Dbl family Rho guanine nucleotide exchange factors (RhoGEFs) play a central role in cell biology by catalyzing the exchange of guanosine 5'-triphosphate for guanosine 5'-diphosphate (GDP) on RhoA. Insights into the oncogenic constitutive activity of the Lbc RhoGEF were gained by analyzing the structure and dynamics of the protein in different functional states and in comparison with a close homologue, leukemia-associated RhoGEF. Higher intrinsic flexibility, less dense and extended structure network, and less stable allosteric communication pathways in Lbc, compared to the nonconstitutively active homologue, emerged as major determinants of the constitutive activity. Independent of the state, the essential dynamics of the two RhoGEFs is contributed by the last 10 amino acids of Dbl homology (DH) and the whole pleckstrin homology (PH) domains and tends to be equalized by the presence of RhoA. The catalytic activity of the RhoGEF relies on the scaffolding action of the DH domain that primarily turns the switch I (SWI) of RhoA on itself through highly conserved amino acids participating in the stability core and essential for function. Changes in the conformation of SWI and disorganization of the RhoA regions deputed to nucleotide binding are among the major RhoGEF effects leading to GDP release. Binding of RhoA reorganizes the allosteric communication on RhoGEF, strengthening the communication among the canonical RhoA binding site on DH, a secondary RhoA binding site on PH, and the binding site for heterotrimeric G proteins, suggesting dual roles for RhoA as a catalysis substrate and as a regulatory protein. The structure network-based analysis tool employed in this study proved to be useful for predicting potentially druggable regulatory sites in protein structures.

The RhoGEF protein Plekhg5 regulates apical constriction of bottle cells during gastrulation

Apical constriction regulates epithelial morphogenesis during embryonic development, but how this process is controlled is not understood completely. Here, we identify a Rho guanine nucleotide exchange factor (GEF) gene plekhg5 as an essential regulator of apical constriction of bottle cells during Xenopus gastrulation. plekhg5 is expressed in the blastopore lip and its expression is sufficient to induce ectopic bottle cells in epithelia of different germ layers in a Rho-dependent manner. This activity is not shared by arhgef3, which encodes another organizer-specific RhoGEF. Plekhg5 protein is localized in the apical cell cortex via its pleckstrin homology domain, and the GEF activity enhances its apical recruitment. Plekhg5 induces apical actomyosin accumulation and cell elongation. Knockdown of plekhg5 inhibits activin-induced bottle cell formation and endogenous blastopore lip formation in gastrulating frog embryos. Apical accumulation of actomyosin, apical constriction and bottle cell formation fail to occur in these embryos. Taken together, our data indicate that transcriptional regulation of plekhg5 expression at the blastopore lip determines bottle cell morphology via local polarized activation of Rho by Plekhg5, which stimulates apical actomyosin activity to induce apical constriction.

An autism spectrum disorder-related de novo mutation hotspot discovered in the GEF1 domain of Trio

The Rho guanine nucleotide exchange factor (RhoGEF) Trio promotes actin polymerization by directly activating the small GTPase Rac1. Recent studies suggest that autism spectrum disorder (ASD)-related behavioral phenotypes in animal models of ASD can be produced by dysregulation of Rac1’s control of actin polymerization at glutamatergic synapses. Here, in humans, we discover a large cluster of ASD-related de novo mutations in Trio’s Rac1 activating domain, GEF1. Our study reveals that these mutations produce either hypofunctional or hyperfunctional forms of Trio in rodent neurons in vitro. In accordance with pathological increases or decreases in glutamatergic neurotransmission observed in animal models of ASD, we find that these mutations result in either reduced synaptic AMPA receptor expression or enhanced glutamatergic synaptogenesis. Together, our findings implicate both excessive and reduced Trio activity and the resulting synaptic dysfunction in ASD-related pathogenesis, and point to the Trio-Rac1 pathway at glutamatergic synapses as a possible key point of convergence of many ASD-related genes.

Trio is a RhoGEF protein that promotes actin polymerization and is implicated in the regulation of glutamatergic synapses in autism spectrum disorder (ASD). Here the authors identify a large cluster of de novo mutations in the GEF1 domain of Trio in whole-exome sequencing data from individuals with ASD, and confirm that some of these mutations lead to glutamatergic dysregulation in vitro.

Membrane and Protein Interactions of the Pleckstrin Homology Domain Superfamily

The human genome encodes about 285 proteins that contain at least one annotated pleckstrin homology (PH) domain. As the first phosphoinositide binding module domain to be discovered, the PH domain recruits diverse protein architectures to cellular membranes. PH domains constitute one of the largest protein superfamilies, and have diverged to regulate many different signaling proteins and modules such as Dbl homology (DH) and Tec homology (TH) domains. The ligands of approximately 70 PH domains have been validated by binding assays and complexed structures, allowing meaningful extrapolation across the entire superfamily. Here the Membrane Optimal Docking Area (MODA) program is used at a genome-wide level to identify all membrane docking PH structures and map their lipid-binding determinants. In addition to the linear sequence motifs which are employed for phosphoinositide recognition, the three dimensional structural features that allow peripheral membrane domains to approach and insert into the bilayer are pinpointed and can be predicted ab initio. The analysis shows that conserved structural surfaces distinguish which PH domains associate with membrane from those that do not. Moreover, the results indicate that lipid-binding PH domains can be classified into different functional subgroups based on the type of membrane insertion elements they project towards the bilayer.

SLAT promotes TCR-mediated, Rap1-dependent LFA-1 activation and adhesion through interaction of its PH domain with Rap1

SLAT (also known as DEF6) promotes T cell activation and differentiation by regulating NFAT-Ca(2+) signaling. However, its role in TCR-mediated inside-out signaling, which induces integrin activation and T cell adhesion, a central process in T cell immunity and inflammation, has not been explored. Here, we show that SLAT is crucial for TCR-induced adhesion to ICAM-1 and affinity maturation of LFA-1 in CD4(+) T cells. Mechanistic studies revealed that SLAT interacts, through its PH domain, with a key component of inside-out signaling, namely the active form of the small GTPase Rap1 (which has two isoforms, Rap1A and Rap1B). This interaction has been further shown to facilitate the interdependent recruitment of Rap1 and SLAT to the T cell immunological synapse upon TCR engagement. Furthermore, a SLAT mutant lacking its PH domain drastically inhibited LFA-1 activation and CD4(+) T cell adhesion. Finally, we established that a constitutively active form of Rap1, which is present at the plasma membrane, rescues the defective LFA-1 activation and ICAM-1 adhesion in SLAT-deficient (Def6(-/-)) T cells. These findings ascribe a new function to SLAT, and identify Rap1 as a target of SLAT function in TCR-mediated inside-out signaling.

Catching Functional Modes and Structural Communication in Dbl Family Rho Guanine Nucleotide Exchange Factors

Computational approaches such as Principal Component Analysis (PCA) and Elastic Network Model-Normal Mode Analysis (ENM-NMA) are proving to be of great value in investigating relevant biological problems linked to slow motions with no demand in computer power. In this study, these approaches have been coupled to the graph theory-based Protein Structure Network (PSN) analysis to dissect functional dynamics and structural communication in the Dbl family of Rho Guanine Nucleotide Exchange Factors (RhoGEFs). They are multidomain proteins whose common structural feature is a DH-PH tandem domain deputed to the GEF activity that makes them play a central role in cell and cancer biology. While their common GEF action is accomplished by the DH domain, their regulatory mechanisms are highly variegate and depend on the PH and the additional domains as well as on interacting proteins. Major evolutionary-driven deformations as inferred from PCA concern the α6 helix of DH that dictates the orientation of the PH domain. Such deformations seem to depend on the mechanisms adopted by the GEF to prevent Rho binding, i.e. functional specialization linked to autoinhibition. In line with PCA, ENM-NMA indicates α6 and the linked PH domain as the portions of the tandem domain holding almost the totality of intrinsic and functional dynamics, with the α6/β1 junction acting as a hinge point for the collective motions of PH. In contrast, the DH domain holds a static scaffolding and hub behavior, with structural communication playing a central role in the regulatory actions by other domains/proteins. Possible allosteric communication pathways involving essentially DH were indeed found in those RhoGEFs acting as effectors of small or heterotrimeric RasGTPases. The employed methodology is suitable for deciphering structure/dynamics relationships in large sets of homologous or analogous proteins.

Trio gene is required for mouse learning ability

Trio is a guanine nucleotide exchange factor with multiple guanine nucleotide exchange factor domains. Trio regulates cytoskeleton dynamics and actin remodeling and is involved in cell migration and axonal guidance in neuronal development. The null allele of the Trio gene led to embryonic lethality, and Trio null embryos displayed aberrant organization in several regions of the brain at E18.5, including hippocampus. Nestin-Trio-/- mice, in which the Trio gene was deleted specifically in the neuronal system by the Nestin-Cre system, displayed severe phenotypes, including low survival rate, ataxia and multiple developmental defects of the cerebellum. All Nestin-Trio-/- mice died before reaching adulthood, which hinders research on Trio gene function in adult mice. Thus, we generated EMX1-Trio-/- mice by crossing Trio-floxed mice with EMX1-Cre mice in which Cre is expressed in the brain cortex and hippocampus. EMX1-Trio-/- mice can survive to adulthood. Trio gene deletion results in smaller brains, an abnormal hippocampus and disordered granule cells in the dentate gyrus (DG) and cornu ammonis (CA). Behavior tests showed that Trio deletion interfered with the hippocampal-dependent spatial learning in the mice, suggesting that Trio plays critical roles in the learning ability of adult mice. We conclude that the Trio gene regulates the neuronal development of the hippocampus and that it affects the intelligence of adult mice.

An astral simulacrum of the central spindle accounts for normal, spindle-less, and anucleate cytokinesis in echinoderm embryos

Live imaging of Ect2 and Cyk4 in echinoderm embryos shows successive recruitment to central spindle, astral microtubules, and the cleavage furrow not only in normal cells, but also between paired asters in toroidal or anucleate cells. This suggests that a common signaling ensemble underlies all functional cytokinetic furrow induction events.

Cytokinesis in animal cells depends on spindle-derived spatial cues that culminate in Rho activation, and thereby actomyosin assembly, in a narrow equatorial band. Although the nature, origin, and variety of such cues have long been obscure, one component is certainly the Rho activator Ect2. Here we describe the behavior and function of Ect2 in echinoderm embryos, showing that Ect2 migrates from spindle midzone to astral microtubules in anaphase and that Ect2 shapes the pattern of Rho activation in incipient furrows. Our key finding is that Ect2 and its binding partner Cyk4 accumulate not only at normal furrows, but also at furrows that form in the absence of associated spindle, midzone, or chromosomes. In all these cases, the cell assembles essentially the same cytokinetic signaling ensemble—opposed astral microtubules decorated with Ect2 and Cyk4. We conclude that if multiple signals contribute to furrow induction in echinoderm embryos, they likely converge on the same signaling ensemble on an analogous cytoskeletal scaffold.

Non-muscle myosin II regulates neuronal actin dynamics by interacting with guanine nucleotide exchange factors

Background

Non-muscle myosin II (NM II) regulates a wide range of cellular functions, including neuronal differentiation, which requires precise spatio-temporal activation of Rho GTPases. The molecular mechanism underlying the NM II-mediated activation of Rho GTPases is poorly understood. The present study explored the possibility that NM II regulates neuronal differentiation, particularly morphological changes in growth cones and the distal axon, through guanine nucleotide exchange factors (GEFs) of the Dbl family.

Principal Findings

NM II colocalized with GEFs, such as βPIX, kalirin and intersectin, in growth cones. Inactivation of NM II by blebbistatin (BBS) led to the increased formation of short and thick filopodial actin structures at the periphery of growth cones. In line with these observations, FRET analysis revealed enhanced Cdc42 activity in BBS-treated growth cones. BBS treatment also induced aberrant targeting of various GEFs to the distal axon where GEFs were seldom observed under physiological conditions. As a result, numerous protrusions and branches were generated on the shaft of the distal axon. The disruption of the NM II–GEF interactions by overexpression of the DH domains of βPIX or Tiam1, or by βPIX depletion with specific siRNAs inhibited growth cone formation and induced slender axons concomitant with multiple branches in cultured hippocampal neurons. Finally, stimulation with nerve growth factor induced transient dissociation of the NM II–GEF complex, which was closely correlated with the kinetics of Cdc42 and Rac1 activation.

Conclusion

Our results suggest that NM II maintains proper morphology of neuronal growth cones and the distal axon by regulating actin dynamics through the GEF–Rho GTPase signaling pathway.

Pebble/ECT2 RhoGEF negatively regulates the Wingless/Wnt signaling pathway

Wingless (Wg)/Wnt signaling is essential for patterning invertebrate and vertebrate embryos, and inappropriate Wnt activity is associated with a variety of human cancers. Despite intensive study, Wnt pathway mechanisms are not fully understood. We have discovered a new mechanism for regulating the Wnt pathway: activity of a Rho guanine nucleotide exchange factor (GEF) encoded by pebble (pbl) in Drosophila and ECT2 in humans. This RhoGEF has an essential role in cytokinesis, but also plays an unexpected, conserved role in inhibiting Wg/Wnt activity. Loss and gain of pbl function in Drosophila embryos cause pattern defects that indicate altered Wg activity. Both Pbl and ECT2 repress Wg/Wnt target gene expression in cultured Drosophila and human cells. The GEF activity is required for Wnt regulation, whereas other protein domains important for cytokinesis are not. Unlike most negative regulators of Wnt activity, Pbl/ECT2 functions downstream of Armadillo (Arm)/beta-catenin stabilization. Our results indicate GTPase regulation at a novel point in Wg/Wnt signal transduction, and provide new insight into the categorization of ECT2 as a human proto-oncogene.

The Ras superfamily G-proteins

The Ras superfamily G-proteins are monomeric proteins of approximately 21kDa that act as a molecular switch to regulate a variety of cellular processes. The structure of the Ras superfamily G-proteins, their regulators as well as posttranslational modification of these proteins leading to their membrane association have been elucidated. The Ras superfamily G-proteins interact at their effector domains with their downstream effectors via protein-protein interactions. Mutational activation or overexpression of the Ras superfamily G-proteins has been observed in a number of human cancer cases. Over the years, a variety of approaches to inhibit the Ras superfamily G-proteins have been developed. These different approaches are discussed in this volume.

Plekhg4 is a novel Dbl family guanine nucleotide exchange factor protein for rho family GTPases

Mutations in the PLEKHG4 (puratrophin-1) gene are associated with the heritable neurological disorder autosomal dominant spinocerebellar ataxia. However, the biochemical functions of this gene product have not been described. We report here that expression of Plekhg4 in the murine brain is developmentally regulated, with pronounced expression in the newborn midbrain and brainstem that wanes with age and maximal expression in the cerebellar Purkinje neurons in adulthood. We show that Plekhg4 is subject to ubiquitination and proteasomal degradation, and its steady-state expression levels are regulated by the chaperones Hsc70 and Hsp90 and by the ubiquitin ligase CHIP. On the functional level, we demonstrate that Plekhg4 functions as a bona fide guanine nucleotide exchange factor (GEF) that facilitates activation of the small GTPases Rac1, Cdc42, and RhoA. Overexpression of Plekhg4 in NIH3T3 cells induces rearrangements of the actin cytoskeleton, specifically enhanced formation of lamellopodia and fillopodia. These findings indicate that Plekhg4 is an aggregation-prone member of the Dbl family GEFs and that regulation of GTPase signaling is critical for proper cerebellar function.

The Control of semaphorin-1a-mediated reverse signaling by opposing pebble and RhoGAPp190 functions in drosophila

Transmembrane semaphorins (Semas) serve evolutionarily conserved guidance roles, and some function as both ligands and receptors. However, the molecular mechanisms underlying the transduction of these signals to the cytoskeleton remain largely unknown. We have identified two direct regulators of Rho family small GTPases, pebble (a Rho guanine nucleotide exchange factor [GEF]) and RhoGAPp190 (a GTPase activating protein [GAP]), that show robust interactions with the cytoplasmic domain of the Drosophila Sema-1a protein. Neuronal pebble and RhoGAPp190 are required to control motor axon defasciculation at specific pathway choice points and also for target recognition during Drosophila neuromuscular development. Sema-1a-mediated motor axon defasciculation is promoted by pebble and inhibited by RhoGAPp190. Genetic analyses show that opposing pebble and RhoGAPp190 functions mediate Sema-1a reverse signaling through the regulation of Rho1 activity. Therefore, pebble and RhoGAPp190 transduce transmembrane semaphorin-mediated guidance cue information that regulates the establishment of neuronal connectivity during Drosophila development.

Phosphorylation-mediated regulation of GEFs for RhoA

Spatio-temporal control of RhoA GTPase is critical for regulation of cell migration, attachment to extracellular matrix, and cell-cell adhesions. Activation of RhoA is mediated by guanine nucleotide exchange factors (GEFs), a diverse family of enzymes that are controlled by multiple signaling pathways regulating actin cytoskeleton and cell migration. GEFs can be regulated by different mechanisms. Growing evidence demonstrates that phosphorylation serves as one of the predominant signals controlling activity, interactions, and localization of RhoGEFs. It acts as a positive and a negative regulator, and allows for regulation of RhoGEFs by multiple signaling cascades. Although there are common trends in phosphorylation-mediated regulation of some RhoGEF homologs, the majority of GEFs utilize distinct mechanisms that are dictated by their unique structure and interaction networks. This diversity enables multiple signaling pathways to use different RhoGEFs for regulation of a single central-RhoA. Here, we review current examples of phosphorylation-mediated regulation of GEFs for RhoA and its role in cell migration, discuss mechanisms, and provide insights into potential future directions.

Regulation of Drosophila mesoderm migration by phosphoinositides and the PH domain of the Rho GTP exchange factor Pebble

The Drosophila RhoGEF Pebble (Pbl) is required for cytokinesis and migration of mesodermal cells. In a screen for genes that could suppress migration defects in pbl mutants we identified the phosphatidylinositol phosphate (PtdInsP) regulator pi5k59B. Genetic interaction tests with other PtdInsP regulators suggested that PtdIns(4,5)P2 levels are important for mesoderm migration when Pbl is depleted. Consistent with this, the leading front of migrating mesodermal cells was enriched for PtdIns(4,5)P2. Given that Pbl contains a Pleckstrin Homology (PH) domain, a known PtdInsP-binding motif, we examined PtdInsP-binding of Pbl and the importance of the PH domain for Pbl function. In vitro lipid blot assays showed that Pbl binds promiscuously to PtdInsPs, with binding strength associated with the degree of phosphorylation. Pbl was also able to bind lipid vesicles containing PtdIns(4,5)P2 but binding was strongly reduced upon deletion of the PH domain. Similarly, in vivo, loss of the PH domain prevented localisation of Pbl to the cell cortex and severely affected several aspects of early mesoderm development, including flattening of the invaginated tube onto the ectoderm, extension of protrusions, and dorsal migration to form a monolayer. Pbl lacking the PH domain could still localise to the cytokinetic furrow, however, and cytokinesis failure was reduced in pbl(ΔPH) mutants. Taken together, our results support a model in which interaction of the PH-domain of Pbl with PtdIns(4,5)P2 helps localise it to the plasma membrane which is important for mesoderm migration.

Pleckstrin homology (PH) like domains - versatile modules in protein-protein interaction platforms

The initial reports on pleckstrin homology (PH) domains almost 20 years ago described them as sequence feature of proteins involved in signal transduction processes. Investigated at first along the phospholipid binding properties of a small subset of PH representatives, the PH fold turned out to appear as mediator of phosphotyrosine and polyproline peptide binding to other signaling proteins. While phospholipid binding now seems rather the exception among PH-like domains, protein-protein interactions established as more and more important feature of these modules. In this review we focus on the PH superfold as a versatile protein-protein interaction platform and its three-dimensional integration in an increasing number of available multidomain structures.

Identification and evaluation of inhibitors of the EhGEF1 protein from Entamoeba histolytica

The Dbl family of guanine nucleotide exchange factors (GEFs) is made up of a vast array of members that participate in the activation of the Rho family of small GTPases. Dbl-family proteins promote the exchange of guanosine diphosphate/guanosine triphosphate (GDP/GTP) in their target molecules, resulting in the activation of a variety of signaling pathways involved in diverse cellular events, such as actin-cytoskeleton remodeling, cellular invasion, cell movement, and other functions. It has been reported that members of the Dbl family have important roles in several cellular events in Entamoeba histolytica. These include activation of the actin cytoskeleton, cytokinesis, capping, uroid formation, cellular proliferation, erythrophagocytosis, cell migration, and chemotaxis. Here, we report the identification and testing of inhibitors of the E. histolytica guanine nucleotide exchange factor 1 (EhGEF1) protein (the research compounds 2BYRF, 2BY05, 2BYT6, 2BYLX, and 2BYPD), which decreased the in vitro ability of the protein to exchange GDP/GTP at its target GTPases, EhRacG and EhRho1, by 14.9-85.2%. Interestingly, the drug 1,1'-(1,2-phenylene)-bis-(1H-pyrrole-2,5-dione), which completely inhibits the GEF activity of the Trio protein in human cells, decreases the GEF activity of the EhGEF1 protein on the EhRacG and EhRho1 GTPases by 55.7% and 3.2%, respectively. The identification and evaluation of such inhibitors opens up the possibility of obtaining a new pharmacological tool to study the function of amoeba GEF proteins, their roles in various Rho GTPase-mediated signaling pathways, and the repercussions of modulating their activities with respect to several mechanisms related to E. histolytica pathogenesis.

Essential role of the NH2-terminal region of Cdc24 guanine nucleotide exchange factor in its initial polarized localization in Saccharomyces cerevisiae

The cortical recruitment and accumulation of the small GTPase Cdc42 are crucial steps in the establishment of polarity, but this process remains obscure. Cdc24 is an upstream regulator of budding yeast Cdc42 that accelerates the exchange of GDP for GTP in Cdc42 via its Dbl homology (DH) domain. Here, we isolated five novel temperature-sensitive (ts) cdc24 mutants, the green fluorescent protein (GFP)-fused proteins of which lose their polarized localization at the nonpermissive temperature. All amino acid substitutions in the mutants were mapped to the NH2-terminal region of Cdc24, including the calponin homology (CH) domain. These Cdc24-ts mutant proteins did not interact with Bem1 at the COOH-terminal PB1 domain, suggesting a lack of exposure of the PB1 domain in the mutant proteins. The cdc24-ts mutants were also defective in polarization in the absence of Bem1. It was previously reported that a fusion protein containing Cdc24 and the p21-activated kinase (PAK)-like kinase Cla4 could bypass the requirement for Bem1 in polarity cue-independent budding (i.e., symmetry breaking). Cdc24-ts-Cla4 fusion proteins also showed ts localization at the polarity site. We propose that the NH2-terminal region unmasks the DH and PB1 domains, leading to the activation of Cdc42 and interaction with Bem1, respectively, to initiate cell polarization.

Mechanistic insights into specificity, activity, and regulatory elements of the regulator of G-protein signaling (RGS)-containing Rho-specific guanine nucleotide exchange factors (GEFs) p115, PDZ-RhoGEF (PRG), and leukemia-associated RhoGEF (LARG)

The multimodular guanine nucleotide exchange factors (GEFs) of the Dbl family mostly share a tandem Dbl homology (DH) and pleckstrin homology (PH) domain organization. The function of these and other domains in the DH-mediated regulation of the GDP/GTP exchange reaction of the Rho proteins is the subject of intensive investigations. This comparative study presents detailed kinetic data on specificity, activity, and regulation of the catalytic DH domains of four GEFs, namely p115, p190, PDZ-RhoGEF (PRG), and leukemia-associated RhoGEF (LARG). We demonstrate that (i) these GEFs are specific guanine nucleotide exchange factors for the Rho isoforms (RhoA, RhoB, and RhoC) and inactive toward other members of the Rho family, including Rac1, Cdc42, and TC10. (ii) The DH domain of LARG exhibits the highest catalytic activity reported for a Dbl protein till now with a maximal acceleration of the nucleotide exchange by 10(7)-fold, which is at least as efficient as reported for GEFs specific for Ran or the bacterial toxin SopE. (iii) A novel regulatory region at the N terminus of the DH domain is involved in its association with GDP-bound RhoA monitored by a fluorescently labeled RhoA. (iv) The tandem PH domains of p115 and PRG efficiently contribute to the DH-mediated nucleotide exchange reaction. (v) In contrast to the isolated DH or DH-PH domains, a p115 fragment encompassing both the regulator of G-protein signaling and the DH domains revealed a significantly reduced GEF activity, supporting the proposed models of an intramolecular autoinhibitory mechanism for p115-like RhoGEFs.

The minimal autoinhibited unit of the guanine nucleotide exchange factor intersectin

Intersectin-1L is a member of the Dbl homology (DH) domain guanine nucleotide exchange factors (GEF) which control Rho-family GTPase signaling. Intersectin-1L is a GEF that is specific for Cdc42. It plays an important role in endocytosis, and is regulated by several partners including the actin regulator N-WASP. Intact intersectin-1L shows low Cdc42 exchange activity, although the isolated catalytic DH domain shows high activity. This finding suggests that the molecule is autoinhibited. To investigate the mechanism of autoinhibition we have constructed a series of domain deletions. We find that the five SH3 domains of intersectin are important for autoinhibition, with the fifth domain (SH3(E)) being sufficient for the bulk of the autoinhibitory effect. This SH3 domain appears to primarily interact with the DH domain. We have determined the crystal structure of the SH3(E)-DH domain construct, which shows a domain swapped arrangement in which the SH3 from one monomer interacts with the DH domain of the other monomer. Analytical ultracentrifugation and gel filtration, however, show that under biochemical concentrations, the construct is fully monomeric. Thus we propose that the actual autoinhibited structure contains the related intramolecular SH3(E)-DH interaction. We propose a model in which this intramolecular interaction may block or distort the GTPase binding region of the DH domain.

PH-domain-driven targeting of collybistin but not Cdc42 activation is required for synaptic gephyrin clustering

Collybistin (Cb) is a brain-specific guanine nucleotide exchange factor (GEF) that is essential for the synaptic clustering of gephyrin and GABAA receptors in selected regions of the mammalian central nervous system. It has been previously proposed that Cb regulates gephyrin clustering by activating Cdc42, and thus acts as a signal transducer in a membrane activation process which labels postsynaptic membrane domains for inhibitory synapse formation. Here, we dissected the functional roles of the Dbl-homology (DH) and pleckstrin homology (PH) domains of the constitutively active splice variant Cb II by substituting conserved amino acid residues that are required for GEF activity towards Cdc42 and phosphoinositide binding, respectively. A Cb II mutant lacking any detectable GEF activity towards Cdc42 was still fully active in inducing gephyrin scaffold formation, both in transfected NIH-3T3 cells and in cultured hippocampal neurons. Furthermore, mice with a forebrain-specific inactivation of the Cdc42 gene displayed normal densities of gephyrin and GABA(A) receptor clusters in the hippocampus. In contrast, substitution of Cb II PH-domain residues essential for phosphoinositide binding abolished gephyrin recruitment to synaptic sites. Our results provide evidence that the formation of gephyrin scaffolds at inhibitory synapses requires an intact Cb II PH-domain but is Cdc42-independent.

Real-time NMR study of guanine nucleotide exchange and activation of RhoA by PDZ-RhoGEF

Small guanosine triphosphatases (GTPases) become activated when GDP is replaced by GTP at the highly conserved nucleotide binding site. This process is intrinsically very slow in most GTPases but is significantly accelerated by guanine nucleotide exchange factors (GEFs). Nucleotide exchange in small GTPases has been widely studied using spectroscopy with fluorescently tagged nucleotides. However, this method suffers from effects of the bulky fluorescent moiety covalently attached to the nucleotide. Here, we have used a newly developed real-time NMR-based assay to monitor small GTPase RhoA nucleotide exchange by probing the RhoA conformation. We compared RhoA nucleotide exchange from GDP to GTP and GTP analogues in the absence and presence of the catalytic DH-PH domain of PDZ-RhoGEF (DH-PH(PRG)). Using the non-hydrolyzable analogue guanosine-5'-O-(3-thiotriphosphate), which we found to be a reliable mimic of GTP, we obtained an intrinsic nucleotide exchange rate of 5.5 x 10(-4) min(-1). This reaction is markedly accelerated to 1179 x 10(-4) min(-1) in the presence of DH-PH(PRG) at a ratio of 1:8,000 relative to RhoA. Mutagenesis studies confirmed the importance of Arg-868 near a conserved region (CR3) of the Dbl homology (DH) domain and revealed that Glu-741 in CR1 is critical for full activity of DH-PH(PRG), together suggesting that the catalytic mechanism of PDZ-RhoGEF is similar to Tiam1. Mutation of the single RhoA (E97A) residue that contacts the pleckstrin homology (PH) domain rendered the mutant 10-fold less sensitive to the activity of DH-PH(PRG). Interestingly, this mutation does not affect RhoA activation by leukemia-associated RhoGEF (LARG), indicating that the PH domains of these two homologous GEFs may play different roles.

LARG at chromosome 11q23 has functional characteristics of a tumor suppressor in human breast and colorectal cancer

Deletion of 11q23-q24 is frequent in a diverse variety of malignancies, including breast and colorectal carcinoma, implicating the presence of a tumor suppressor gene at that chromosomal region. We examined a 6-Mb region on 11q23 by high-resolution deletion mapping, using both loss of heterozygosity analysis and customized microarray comparative genomic hybridization. LARG (leukemia-associated Rho guanine-nucleotide exchange factor) (also called ARHGEF12), identified from the analysed region, is frequently underexpressed in breast and colorectal carcinomas with a reduced expression observed in all breast cancer cell lines (n=11), in 12 of 38 (32%) primary breast cancers, 5 of 10 (50%) colorectal cell lines and in 20 of 37 (54%) primary colorectal cancers. Underexpression of the LARG transcript was significantly associated with genomic loss (P=0.00334). Hypermethylation of the LARG promoter was not detected in either breast or colorectal cancer, and treatment of four breast and four colorectal cancer cell lines with 5-aza-2'-deoxycytidine and/or trichostatin A did not result in a reactivation of LARG. Enforced expression of LARG in breast and colorectal cancer cells by stable transfection resulted in reduced cell proliferation and colony formation, as well as in a markedly slower cell migration rate in colorectal cancer cells, providing functional evidence for LARG as a candidate tumor suppressor gene.

Targeted deletion of the zebrafish obscurin A RhoGEF domain affects heart, skeletal muscle and brain development

Obscurin is a giant structural and signaling protein that participates in the assembly and structural integrity of striated myofibrils. Previous work has examined the physical interactions between obscurin and other cytoskeletal elements but its in vivo role in cell signaling, including the functions of its RhoGTPase Exchange Factor (RhoGEF) domain have not been characterized. In this study, morpholino antisense oligonucleotides were used to create an in-frame deletion of the active site of the obscurin A RhoGEF domain in order to examine its functions in zebrafish development. Cardiac myocytes in the morphant embryos lacked the intercalated disks that were present in controls by 72 and, in the more severely affected embryos, the contractile filaments were not organized into mature sarcomeres. Neural abnormalities included delay or loss of retinal lamination. Rescue of the phenotype with co-injection of mini-obscurin A expression constructs demonstrated that the observed effects were due to the loss of small GTPase activation by obscurin A. The immature phenotype of the cardiac myocytes and the retinal neuroblasts observed in the morphant embryos suggests that obscurin A-mediated small GTPase signaling promotes tissue-specific cellular differentiation. This is the first demonstration of the importance of the obscurin A-mediated RhoGEF signaling in vertebrate organogenesis and highlights the central role of obscurin A in striated muscle and neural development.

Structure and function of heterotrimeric G protein-regulated Rho guanine nucleotide exchange factors

Activation of certain classes of G protein-coupled receptors (GPCRs) can lead to alterations in the actin cytoskeleton, gene transcription, cell transformation, and other processes that are known to be regulated by Rho family small-molecular-weight GTPases. Although these responses can occur indirectly via cross-talk from canonical heterotrimeric G protein cascades, it has recently been demonstrated that Dbl family Rho guanine nucleotide exchange factors (RhoGEFs) can serve as the direct downstream effectors of heterotrimeric G proteins. Heterotrimeric Galpha(12/13), Galpha(q), and Gbetagamma subunits are each now known to directly bind and regulate RhoGEFs. Atomic structures have recently been determined for several of these RhoGEFs and their G protein complexes, providing fresh insight into the molecular mechanisms of signal transduction between GPCRs and small molecular weight G proteins. This review covers what is currently known about the structure, function, and regulation of these recently recognized effectors of heterotrimeric G proteins.

Intersectin-2L regulates caveola endocytosis secondary to Cdc42-mediated actin polymerization

Here we addressed the role of intersectin-2L (ITSN-2L), a guanine nucleotide exchange factor for the Rho GTPase Cdc42, in the mechanism of caveola endocytosis in endothelial cells (ECs). Immunoprecipitation and co-localization studies showed that ITSN-2L associates with members of the Cdc42-WASp-Arp2/3 actin polymerization pathway. Expression of Dbl homology-pleckstrin homology (DH-PH) region of ITSN-2L (DH-PH(ITSN-2L)) induced specific activation of Cdc42, resulting in formation of extensive filopodia, enhanced cortical actin, as well as a shift from G-actin to F-actin. The "catalytically dead" DH-PH domain reversed these effects and induced significant stress fiber formation, without a detectable shift in actin pools. A biotin assay for caveola internalization indicated a significant decrease in the uptake of biotinylated proteins in DH-PH(ITSN-2L)-transfected cells compared with control and 1 microM jasplakinolide-treated cells. ECs depleted of ITSN-2L by small interfering RNA, however, showed decreased Cdc42 activation and actin remodeling similar to the defective DH-PH, resulting in 62% increase in caveola-mediated uptake compared with controls. Thus, ITSN-2L, a guanine nucleotide exchange factor for Cdc42, regulates different steps of caveola endocytosis in ECs by controlling the temporal and spatial actin polymerization and remodeling sub-adjacent to the plasma membrane.

A conserved hydrophobic surface of the LARG pleckstrin homology domain is critical for RhoA activation in cells

Leukemia associated Rho guanine nucleotide exchange factor (LARG) activates RhoA in response to signals received by specific classes of cell surface receptors. The catalytic core of LARG is a Dbl homology (DH) domain whose activity is modulated by an adjacent pleckstrin homology (PH) domain. In this study, we used a transcriptional assay and confocal microscopy to examine the roles of several novel structural features of the LARG DH/PH domains, including a conserved and exposed hydrophobic patch on the PH domain that mediates protein-protein interactions in crystal structures of LARG and its close homolog PDZ-RhoGEF. Mutation of the hydrophobic patch has no effect on nucleotide exchange activity in vitro, but abolished the ability of LARG to activate RhoA and to induce stress fiber formation in cultured cells. The activity of these mutants could be rescued by fusion with exogenous membrane-targeting domains. However, because membrane recruitment by activated G alpha(13) subunits was not sufficient to rescue activity of a hydrophobic patch mutant, the LARG PH domain cannot solely contribute to membrane targeting. Instead, it seems likely that the domain is involved in regulatory interactions with other proteins near the membrane surface. We also show that the hydrophobic patch of the PH domain is likely important for the activity of all Lbc subfamily RhoGEFs.

Regulation of the Rac GTPase pathway by the multifunctional Rho GEF Pebble is essential for mesoderm migration in the Drosophila gastrula

The Drosophila guanine nucleotide exchange factor Pebble (Pbl) is essential for cytokinesis and cell migration during gastrulation. In dividing cells, Pbl promotes Rho1 activation at the cell cortex, leading to formation of the contractile actin-myosin ring. The role of Pbl in fibroblast growth factor-triggered mesoderm spreading during gastrulation is less well understood and its targets and subcellular localization are unknown. To address these issues we performed a domain-function study in the embryo. We show that Pbl is localized to the nucleus and the cell cortex in migrating mesoderm cells and found that, in addition to the PH domain, the conserved C-terminal tail of the protein is crucial for cortical localization. Moreover, we show that the Rac pathway plays an essential role during mesoderm migration. Genetic and biochemical interactions indicate that during mesoderm migration, Pbl functions by activating a Rac-dependent pathway. Furthermore, gain-of-function and rescue experiments suggest an important regulatory role of the C-terminal tail of Pbl for the selective activation of Rho1-versus Rac-dependent pathways.

Characterisation of the nucleotide exchange factor ITSN1L: evidence for a kinetic discrimination of GEF-stimulated nucleotide release from Cdc42

Cdc42, a member of the Ras superfamily of small guanine nucleotide binding proteins, plays an important role in regulating the actin cytoskeleton, intracellular trafficking, and cell polarity. Its activation is controlled by guanine nucleotide exchange factors (GEFs), which stimulate the dissociation of bound guanosine-5'-diphosphate (GDP) to allow guanosine-5'-triphosphate (GTP) binding. Here, we investigate the exchange factor activity of the Dbl-homology domain containing constructs of the adaptor protein Intersectin1L (ITSN1L), which is a specific GEF for Cdc42. A detailed kinetic characterisation comparing ITSN1L-mediated nucleotide exchange on Cdc42 in its GTP- versus GDP-bound state reveals a kinetic discrimination for GEF-stimulated dissociation of GTP: The maximum acceleration of the intrinsic mGDP [2'/3'-O-(N-methyl-anthraniloyl)-GDP] release from Cdc42 by ITSN1L is accelerated at least 68,000-fold, whereas the exchange of mGTP [2'/3'-O-(N-methyl-anthraniloyl)-GTP] is stimulated only up to 6000-fold at the same GEF concentration. The selectivity in nucleotide exchange kinetics for GDP over GTP is even more pronounced when a Cdc42 mutant, F28L, is used, which is characterised by fast intrinsic dissociation of nucleotides. We furthermore show that both GTP and Mg2+ ions are required for the interaction with effectors. We suggest a novel model for selective nucleotide exchange residing on a conformational change of Cdc42 upon binding of GTP, which enables effector binding to the Cdc42.GTP complex but, at the same time, excludes efficient modulation by the GEF. The higher exchange activity of ITSN1L towards the GDP-bound conformation of Cdc42 could represent an evolutionary adaptation of this GEF that ensures nucleotide exchange towards the formation of the signalling-active GTP-bound form of Cdc42 and avoids dissociation of the active complex.

Fission yeast rgf2p is a rho1p guanine nucleotide exchange factor required for spore wall maturation and for the maintenance of cell integrity in the absence of rgf1p

Schizosaccharomyces pombe Rho1p is essential, directly activates beta-1,3-glucan synthase, and participates in the regulation of morphogenesis. In S. pombe, Rho1p is activated by at least three guanine nucleotide exchange factors (GEFs): Rgf1p, Rgf2p, and Rgf3p. In this study we show that Rgf2p is a Rho1p GEF required for sporulation. The rgf2+ deletion did not affect forespore membrane formation and the nuclei were encapsulated properly. However, the mutant ascospores appeared dark and immature. The rgf2Delta zygotes were not able to release the ascospores spontaneously, and the germination efficiency was greatly reduced compared to wild-type (wt) spores. This phenotype resembles that of the mutants in bgs2+, which encodes a sporulation-specific glucan synthase subunit. In fact, glucan synthase activity was diminished in sporulating rgf2Delta diploids. Rgf2p also plays a role in beta-glucan biosynthesis during vegetative growth. Overexpression of rgf2+ specifically increased GTP-bound Rho1p, caused changes in cell morphology, and elicited an increase in beta-1,3-glucan synthase activity. Moreover, the simultaneous disruption of rgf1+ and rgf2+ was lethal and both Rgf1p and Rgf2p were able to partially substitute for each other. Our results suggest that Rgf1p and Rgf2p are alternative GEFs with an essential overlapping function in Rho1p activation during vegetative growth.

The Rho1p exchange factor Rgf1p signals upstream from the Pmk1 mitogen-activated protein kinase pathway in fission yeast

The Schizosaccharomyces pombe exchange factor Rgf1p specifically regulates Rho1p during polarized growth. Rgf1p activates the beta-glucan synthase (GS) complex containing the catalytic subunit Bgs4p and is involved in the activation of growth at the second end, a transition that requires actin reorganization. In this work, we investigated Rgf1p signaling and observed that Rgf1p acted upstream from the Pck2p-Pmk1p MAPK signaling pathway. We noted that Rgf1p and calcineurin play antagonistic roles in Cl(-) homeostasis; rgf1Delta cells showed the vic phenotype (viable in the presence of immunosuppressant and chlorine ion) and were unable to grow in the presence of high salt concentrations, both phenotypes being characteristic of knockouts of the MAPK components. In addition, mutations that perturb signaling through the MAPK pathway resulted in defective cell integrity (hypersensitivity to caspofungin and beta-glucanase). Rgf1p acts by positively regulating a subset of stimuli toward the Pmk1p-cell integrity pathway. After osmotic shock and cell wall damage HA-tagged Pmk1p was phosphorylated in wild-type cells but not in rgf1Delta cells. Finally, we provide evidence to show that Rgf1p regulates Pmk1p activation in a process that involves the activation of Rho1p and Pck2p, and we demonstrate that Rgf1p is unique in this signaling process, because Pmk1p activation was largely independent of the other two Rho1p-specific GEFs, Rgf2p and Rgf3p.

FGD2, a CDC42-specific exchange factor expressed by antigen-presenting cells, localizes to early endosomes and active membrane ruffles

Members of the Fgd (faciogenital dysplasia) gene family encode a group of critical guanine nucleotide exchange factors (GEFs), which, by specifically activating Cdc42, control cytoskeleton-dependent membrane rearrangements. In its first characterization, we find that FGD2 is expressed in antigen-presenting cells, including B lymphocytes, macrophages, and dendritic cells. In the B lymphocyte lineage, FGD2 levels change with developmental stage. In both mature splenic B cells and immature bone marrow B cells, FGD2 expression is suppressed upon activation through the B cell antigen receptor. FGD2 has a complex intracellular localization, with concentrations found in membrane ruffles and early endosomes. Although endosomal localization of FGD2 is dependent on a conserved FYVE domain, its C-terminal pleckstrin homology domain mediates recruitment to membrane ruffles. FGD2 overexpression promotes the activation of Cdc42 and leads to elevated JNK1 activity in a Cdc42- but not Rac1-dependent fashion. These findings are consistent with a role of FGD2 in leukocyte signaling and vesicle trafficking in cells specialized to present antigen in the immune system.

WGEF activates Rho in the Wnt-PCP pathway and controls convergent extension in Xenopus gastrulation

The Wnt-PCP (planar cell polarity, PCP) pathway regulates cell polarity and convergent extension movements during axis formation in vertebrates by activation of Rho and Rac, leading to the re-organization of the actin cytoskeleton. Rho and Rac activation require guanine nucleotide-exchange factors (GEFs), but the identity of the GEF involved in Wnt-PCP-mediated convergent extension is unknown. Here we report the identification of the weak-similarity GEF (WGEF) gene by a microarray-based screen for notochord enriched genes, and show that WGEF is involved in Wnt-regulated convergent extension. Overexpression of WGEF activated RhoA and rescued the suppression of convergent extension by dominant-negative Wnt-11, whereas depletion of WGEF led to suppression of convergent extension that could be rescued by RhoA or Rho-associated kinase activation. WGEF protein preferentially localized at the plasma membrane, and Frizzled-7 induced colocalization of Dishevelled and WGEF. WGEF protein can bind to Dishevelled and Daam-1, and deletion of the Dishevelled-binding domain generates a hyperactive from of WGEF. These results indicate that WGEF is a component of the Wnt-PCP pathway that connects Dishevelled to Rho activation.

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.

Role of Rho GTPases and Rho-GEFs in the regulation of cell shape and integrity in fission yeast

The Rho family of GTPases are highly conserved molecular switches that control some of the most fundamental processes of cell biology, including morphogenesis, vesicular transport, cell division and motility. Guanine nucleotide-exchange factors (GEFs) are directly responsible for the activation of Rho-family GTPases in response to extracellular stimuli. In fission yeast, there are seven Dbl-related GEFs and they activate six Rho-type GTPases within a particular spatio-temporal context. The failure to do so might have consequences reflected in aberrant phenotypes and in some cases lead to cell death. In this review, we briefly summarize the role of Rho GTPases and Rho-GEFs in the establishment and maintenance of cell polarity and cell integrity in Schizosaccharomyces pombe.

The DH and PH domains of Trio coordinately engage Rho GTPases for their efficient activation

Rho-family GTPases are activated by the exchange of bound GDP for GTP, a process that is catalyzed by Dbl-family guanine nucleotide exchange factors (GEFs). The catalytic unit of Dbl-family GEFs consists of a Dbl homology (DH) domain followed almost invariantly by a pleckstrin-homology (PH) domain. The majority of the catalytic interface forms between the switch regions of the GTPase and the DH domain, but full catalytic activity often requires the associated PH domain. Although PH domains are usually characterized as lipid-binding regions, they also participate in protein-protein interactions. For example, the DH-associated PH domain of Dbs must contact its cognate GTPases for efficient exchange. Similarly, the N-terminal DH/PH fragment of Trio, which catalyzes exchange on both Rac1 and RhoG, is fourfold more active in vitro than the isolated DH domain. Given continued uncertainty regarding functional roles of DH-associated PH domains, we have undertaken structural and functional analyses of the N-terminal DH/PH cassette of Trio. The crystal structure of this fragment of Trio bound to nucleotide-depleted Rac1 highlights the engagement of the PH domain with Rac1 and substitution of residues involved in this interface substantially diminishes activation of Rac1 and RhoG. Also, these mutations significantly reduce the ability of full-length Trio to induce neurite outgrowth dependent on RhoG activation in PC-12 cells. Overall, these studies substantiate a general role for DH-associated PH domains in engaging Rho GTPases directly for efficient guanine nucleotide exchange and support a parsimonious explanation for the essentially invariant linkage between DH and PH domains.

Solo/Trio8, a membrane-associated short isoform of Trio, modulates endosome dynamics and neurite elongation

With DNA microarrays, we identified a gene, termed Solo, that is downregulated in the cerebellum of Purkinje cell degeneration mutant mice. Solo is a mouse homologue of rat Trio8-one of multiple Trio isoforms recently identified in rat brain. Solo/Trio8 contains N-terminal sec14-like and spectrin-like repeat domains followed by a single guanine nucleotide exchange factor 1 (GEF1) domain, but it lacks the C-terminal GEF2, immunoglobulin-like, and kinase domains that are typical of Trio. Solo/Trio8 is predominantly expressed in Purkinje neurons of the mouse brain, and expression begins following birth and increases during Purkinje neuron maturation. We identified a novel C-terminal membrane-anchoring domain in Solo/Trio8 that is required for enhanced green fluorescent protein-Solo/Trio8 localization to early endosomes (positive for both early-endosome antigen 1 [EEA1] and Rab5) in COS-7 cells and primary cultured neurons. Solo/Trio8 overexpression in COS-7 cells augmented the EEA1-positive early-endosome pool, and this effect was abolished via mutation and inactivation of the GEF domain or deletion of the C-terminal membrane-anchoring domain. Moreover, primary cultured neurons transfected with Solo/Trio8 showed increased neurite elongation that was dependent on these domains. These results suggest that Solo/Trio8 acts as an early-endosome-specific upstream activator of Rho family GTPases for neurite elongation of developing Purkinje neurons.

Transformation by the Rho-specific guanine nucleotide exchange factor Dbs requires ROCK I-mediated phosphorylation of myosin light chain

Dbs was identified in a cDNA-based expression screen for sequences that can cause malignant growth when expressed in murine fibroblasts. In previous studies we have shown that Dbs is a Rho-specific guanine nucleotide exchange factor that can activate RhoA and/or Cdc42 in a cell-specific manner. In this current study we have used a combination of genetic and pharmacological approaches to examine the relative contributions of RhoA x PRK and RhoA x ROCK signaling to Dbs transformation. Our analysis indicates that ROCK is activated in Dbs-transformed cells and that Dbs transformation is dependent upon ROCK I activity. In contrast, there appears to be no requirement for PRK activation in Dbs transformation. Dbs transformation is also associated with increased phosphorylation of myosin light chain and stress fiber formation, both of which occur in a ROCK-dependent manner. Suppression of myosin light chain expression by small interfering RNAs impairs Dbs focus formation, thus establishing a direct link between actinomyosin contraction and Rho-specific guanine nucleotide exchange factor transformation.

Interaction of PDZRhoGEF with Microtubule-associated Protein 1 Light Chains

Rat (r) PDZRhoGEF, initially identified as a glutamate transporter EAAT4-associated protein, is a member of a novel RhoGEF subfamily. The N terminus of the protein contains a PDZ and a proline-rich domain, two motifs known to be involved in protein-protein interactions. By using the yeast two-hybrid approach, we screened for proteins that interact with the N terminus of rPDZRhoGEF. The light chain 2 of microtubule-associated protein 1 (LC2) was the only protein identified from the screen that does not contain a type I PDZ-binding motif at its extreme C terminus (-(S/T)Xphi-COOH, where phi is a hydrophobic amino acid). However, the C terminus does conform to a type II-binding motif (-phiXphi). We report here that rPDZRhoGEF interacts with LC2 via the PDZ domain, and the interaction is abolished by mutations in the carboxylate-binding loop. The specificity of the interaction was confirmed using GST fusion protein pull-down assays and coimmunoprecipitations. Expression of rPDZRhoGEF mutants that are unable to interact with proteins via the carboxylate-binding loop induced changes in cell morphology and actin organization. These mutants alter the activation of RhoGTPases, and coexpression of dominant-negative RhoGTPases prevent the morphological changes. Furthermore, in cells expressing wild type rPDZRhoGEF, drug-induced microtubule depolymerization produces changes in cell morphology that are similar to those induced by rPDZRhoGEF mutants. These results indicate that modulation of the guanine nucleotide exchange activity of rPDZRhoGEF through interaction with microtubule-associated protein light chains may coordinate microtubule integrity and the reorganization of actin cytoskeleton. This coordinated action of the actin and microtubular cytoskeletons is essential for the development and maintenance of neuronal polarity.

Cdc42p GDP/GTP cycling is necessary for efficient cell fusion during yeast mating

The highly conserved small Rho G-protein, Cdc42p plays a critical role in cell polarity and cytoskeleton organization in all eukaryotes. In the yeast Saccharomyces cerevisiae, Cdc42p is important for cell polarity establishment, septin ring assembly, and pheromone-dependent MAP-kinase signaling during the yeast mating process. In this study, we further investigated the role of Cdc42p in the mating process by screening for specific mating defective cdc42 alleles. We have identified and characterized novel mating defective cdc42 alleles that are unaffected in vegetative cell polarity. Replacement of the Cdc42p Val36 residue with Met resulted in a specific cell fusion defect. This cdc42[V36M] mutant responded to mating pheromone but was defective in cell fusion and in localization of the cell fusion protein Fus1p, similar to a previously isolated cdc24 (cdc24-m6) mutant. Overexpression of a fast cycling Cdc42p mutant suppressed the cdc24-m6 fusion defect and conversely, overexpression of Cdc24p suppressed the cdc42[V36M] fusion defect. Taken together, our results indicate that Cdc42p GDP-GTP cycling is critical for efficient cell fusion.