Dbl family guanine nucleotide exchange factors

Association of genetic variants in the Rho guanine nucleotide exchange factor AKAP13 with familial breast cancer

The A-kinase anchor protein 13 (AKAP13, alias BRX and lbc) tethers cAMP-dependent protein kinase to its subcellular environment and catalyses Rho GTPases activity as a guanine nucleotide exchange factor. The crucial role of members of the Rho family of GTPases in carcinogenesis is well established and targeting Rho proteins with antineoplastic compounds has become a major effort in the fight against cancer. Thus, genetic alterations within the candidate cancer susceptibility gene AKAP13 would be expected to provoke a constitutive Rho signalling, thereby facilitating the development of cancer. Here, we analysed the potential impact of four polymorphic non-conservative amino acid exchanges (Arg494Trp, Lys526Gln, Asn1086Asp and Gly2461Ser) in AKAP13 on familial breast cancer. We performed a case-control study using genomic DNA of BRCA1/2 mutation-negative German female index patients from 601 unrelated families, among a subset of 356 high-risk families, and 1053 German female unrelated controls. The newfound Lys526Gln polymorphism revealed a significant association with familial breast cancer (OR = 1.58, 95% CI = 1.07-2.35) and an even stronger association with high-risk familial breast cancer (OR = 1.85, 95% CI = 1.19-2.88). Haplotype analyses were in line with genotype results displaying a similar significance as analyses of individual polymorphisms. Due to the pivotal role of AKAP13 in the Rho GTPases signalling network, this variant might affect the susceptibility to other cancers as well.

Mammalian G proteins and their cell type specific functions

Heterotrimeric G proteins are key players in transmembrane signaling by coupling a huge variety of receptors to channel proteins, enzymes, and other effector molecules. Multiple subforms of G proteins together with receptors, effectors, and various regulatory proteins represent the components of a highly versatile signal transduction system. G protein-mediated signaling is employed by virtually all cells in the mammalian organism and is centrally involved in diverse physiological functions such as perception of sensory information, modulation of synaptic transmission, hormone release and actions, regulation of cell contraction and migration, or cell growth and differentiation. In this review, some of the functions of heterotrimeric G proteins in defined cells and tissues are described.

Genetic deletion of Cdc42GAP reveals a role of Cdc42 in erythropoiesis and hematopoietic stem/progenitor cell survival, adhesion, and engraftment

Rho family GTPases are key signal transducers in cell regulation. Although a body of literature has implicated the Rho family members Rac1 and Rac2 in multiple hematopoietic-cell functions, the role of Cdc42 in hematopoiesis remains unclear. Here we have examined the hematopoietic properties and the hematopoietic stem/progenitor cell (HSP) functions of gene-targeted mice carrying null alleles of cdc42gap, a negative regulator of Cdc42. The Cdc42GAP-/- fetal liver and bone marrow cells showed a 3-fold increase in Cdc42 activity but normal Rac and RhoA activities, indicating that Cdc42GAP knockout resulted in a gain of Cdc42 activity in the hematopoietic tissues. Cdc42GAP-/- mice were anemic. The cellularity of fetal liver and bone marrow, the number and composition percentage of HSPs, and the erythroid blast-forming unit and colony-forming unit (BFU-E/CFU-E) activities were significantly reduced in the homozygous mice. The decrease in HSP number was associated with increased apoptosis of the Cdc42GAP-/- HSPs and the activation of JNK-mediated apoptotic machinery. Moreover, homozygous HSPs showed impaired cortical F-actin assembly, deficiency in adhesion and migration, and defective engraftment. These results provide evidence that Cdc42 activity is important for erythropoiesis and for multiple HSP functions, including survival, adhesion, and engraftment.

Cdc42GAP regulates c-Jun N-terminal kinase (JNK)-mediated apoptosis and cell number during mammalian perinatal growth

Rho family GTPase Cdc42 is known to regulate polarity and growth in lower eukaryotes, but its physiologic function in mammals has yet to be determined. Here we have disrupted cdc42gap, a ubiquitously expressed negative regulator of Cdc42, in mice. Cdc42GAP(-/-) embryonic fibroblasts and various organs displayed significantly elevated Cdc42 activity. The embryonic and neonatal homozygous mice were reduced in size by approximately 25-40% and suffered severe growth retardation. Major organs from Cdc42GAP(-/-) mice were proportionally smaller because of decreased cell number. Basal apoptosis was increased in Cdc42GAP(-/-) cells and tissues, and this was attributed to altered c-Jun N-terminal kinase apoptotic signals. These results reveal a role of Cdc42GAP in mammalian perinatal growth and implicate the c-Jun N-terminal kinase-mediated apoptosis machinery as a Cdc42 effector pathway in vivo.

Vav-family proteins in T-cell signalling

The Vav family proteins (Vav1, Vav2, Vav3) are cytoplasmic guanine nucleotide exchange factors (GEFs) for Rho-family GTPases. T-cell antigen receptor (TCR) signalling results in the tyrosine phosphorylation of Vav proteins and hence their activation. Results from mice deficient in one or more Vav proteins has shown that they play critical roles in T-cell development and activation. Vav1 is required for TCR-induced calcium flux, activation of the ERK MAP kinase pathway, activation of the NF-kappaB transcription factor, inside-out activation of the integrin LFA-1, TCR clustering, and polarisation of the T cell. Although many of these processes may require the GEF activity of Vav1, it is possible that Vav1 also has adaptor-like functions. Recent evidence suggests that Vav1 might also function in the nucleus, where it undergoes arginine methylation. An emerging theme is that Vav proteins may have important functions downstream of receptors other than the TCR, such as integrins and chemokine receptors.

Transforming activity of the Rho family GTPase, Wrch-1, a Wnt-regulated Cdc42 homolog, is dependent on a novel carboxyl-terminal palmitoylation motif

Wrch-1 is a Rho family GTPase that shares strong sequence and functional similarity with Cdc42. Like Cdc42, Wrch-1 can promote anchorage-independent growth transformation. We determined that activated Wrch-1 also promoted anchorage-dependent growth transformation of NIH 3T3 fibroblasts. Wrch-1 contains a distinct carboxyl-terminal extension not found in Cdc42, suggesting potential differences in subcellular location and function. Consistent with this, we found that Wrch-1 associated extensively with plasma membrane and endosomes, rather than with cytosol and perinuclear membranes like Cdc42. Like Cdc42, Wrch-1 terminates in a CAAX tetrapeptide (where C is cysteine, A is aliphatic amino acid, and X is any amino acid) motif (CCFV), suggesting that Wrch-1 may be prenylated similarly to Cdc42. Most surprisingly, unlike Cdc42, Wrch-1 did not incorporate isoprenoid moieties, and Wrch-1 membrane localization was not altered by inhibitors of protein prenylation. Instead, we showed that Wrch-1 is modified by the fatty acid palmitate, and pharmacologic inhibition of protein palmitoylation caused mislocalization of Wrch-1. Most interestingly, mutation of the second cysteine of the CCFV motif (CCFV > CSFV), but not the first, abrogated both Wrch-1 membrane localization and transformation. These results suggest that Wrch-1 membrane association, subcellular localization, and biological activity are mediated by a novel membrane-targeting mechanism distinct from that of Cdc42 and other isoprenylated Rho family GTPases.

Novel intermediate of Rac GTPase activation by guanine nucleotide exchange factor

The biochemical role of guanine nucleotide exchange factors (GEFs) in catalyzing small GTPase GDP-GTP exchange is thought to be twofold: stimulation of GDP dissociation and stabilization of a nucleotide-free GTPase intermediate. Here we report that TrioN, a Dbl family GEF, activates Rac1 by facilitating GTP binding to, as well as stimulating GDP dissociation from, Rac1. The TrioN-catalyzed GDP dissociation is dependent upon the structural nature and the concentration of free nucleotide, and nucleotide binding serves as the rate-limiting step of the GEF reaction. The TrioN-stimulated nucleotide exchange may undergo a novel two nucleotide-one G-protein intermediate involving two cryptic subsites on Rac1 induced by the GEF, with one subsite contributing to the recognition of the beta/gamma phosphates of the incoming GTP and another to the binding of the guanine base of the leaving GDP. We propose that the Rac GEF reaction may proceed by competitive displacement of bound GDP by GTP through a transient intermediate of GEF-[GTP-Rac-GDP].

Critical role for Kalirin in nerve growth factor signaling through TrkA

Kalirin is a multidomain guanine nucleotide exchange factor (GEF) that activates Rho proteins, inducing cytoskeletal rearrangement in neurons. Although much is known about the effects of Kalirin on Rho GTPases and neuronal morphology, little is known about the association of Kalirin with the receptor/signaling systems that affect neuronal morphology. Our experiments demonstrate that Kalirin binds to and colocalizes with the TrkA neurotrophin receptor in neurons. In PC12 cells, inhibition of Kalirin expression using antisense RNA decreased nerve growth factor (NGF)-induced TrkA autophosphorylation and process extension. Kalirin overexpression potentiated neurotrophin-stimulated TrkA autophosphorylation and neurite outgrowth in PC12 cells at a low concentration of NGF. Furthermore, elevated Kalirin expression resulted in catalytic activation of TrkA, as demonstrated by in vitro kinase assays and increased NGF-stimulated cellular activation of Rac, Mek, and CREB. Domain mapping demonstrated that the N-terminal Kalirin pleckstrin homology domain mediates the interaction with TrkA. The effects of Kalirin on TrkA provide a molecular basis for the requirement of Kalirin in process extension from PC12 cells and for previously observed effects on axonal extension and dendritic maintenance. The interaction of TrkA with the pleckstrin homology domain of Kalirin may be one example of a general mechanism whereby receptor/Rho GEF pairings play an important role in receptor tyrosine kinase activation and signal transduction.

Binding of GEF-H1 to the Tight Junction-Associated Adaptor Cingulin Results in Inhibition of Rho Signaling and G1/S Phase Transition

The activity of Rho GTPases is carefully timed to control epithelial proliferation and differentiation. RhoA is downregulated when epithelial cells reach confluence, resulting in inhibition of signaling pathways that stimulate proliferation. Here we show that GEF-H1/Lfc, a guanine nucleotide exchange factor for RhoA, directly interacts with cingulin, a junctional adaptor. Cingulin binding inhibits RhoA activation and signaling, suggesting that the increase in cingulin expression in confluent cells causes downregulation of RhoA by inhibiting GEF-H1/Lfc. In agreement, RNA interference of GEF-H1 or transfection of GEF-H1 binding cingulin mutants inhibit G1/S phase transition of MDCK cells, and depletion of cingulin by regulated RNA interference results in irregular monolayers and RhoA activation. These results indicate that forming epithelial tight junctions contribute to the downregulation of RhoA in epithelia by inactivating GEF-H1 in a cingulin-dependent manner, providing a molecular mechanism whereby tight junction formation is linked to inhibition of RhoA signaling.

Altered gene expression profile in mouse bladder cancers induced by hydroxybutyl(butyl)nitrosamine

A variety of genetic alterations and gene expression changes are involved in the pathogenesis of bladder tumor. To explore these changes, oligonucleotide array analysis was performed on RNA obtained from carcinogen-induced mouse bladder tumors and normal mouse bladder epithelia using Affymetrix (Santa Clara, CA) MGU74Av2 GeneChips. Analysis yielded 1164 known genes that were changed in the tumors. Certain of the upregulated genes included EGFR-Ras signaling genes, transcription factors, cell cycle-related genes, and intracellular signaling cascade genes. However, downregulated genes include mitogen-activated protein kinases, cell cycle checkpoint genes, Rab subfamily genes, Rho subfamily genes, and SH2 and SH3 domains-related genes. These genes are involved in a broad range of different pathways including control of cell proliferation, differentiation, cell cycle, signal transduction, and apoptosis. Using the pathway visualization tool GenMAPP, we found that several genes, including TbR-I, STAT1, Smad1, Smad2, Jun, NFkappaB, and so on, in the TGF-beta signaling pathway and p115 RhoGEF, RhoGDI3, MEKK4A/MEKK4B, PI3KA, and JNK in the G13 signaling pathway were differentially expressed in the tumors. In summary, we have determined the expression profiles of genes differentially expressed during mouse bladder tumorigenesis. Our results suggest that activation of the EGFR-Ras pathway, uncontrolled cell cycle, aberrant transcription factors, and G13 and TGF-beta pathways are involved, and the cross-talk between these pathways seems to play important roles in mouse bladder tumorigenesis.

Expression of Trio, a member of the Dbl family of Rho GEFs in the developing rat brain

Trio, a member of the Dbl family of guanine nucleotide exchange factors (GEFs), has a series of spectrin repeats, two GEF domains, protein interaction domains, and a putative kinase domain, potentially important in neuronal axon guidance and cell migration. Most knowledge about Trio is based on studies of Caenorhabditis elegans and Drosophila, while the function of Trio in vertebrates is unclear. The aim of these experiments was to establish the patterns of Trio expression in the postnatal rat brain. During postnatal (P) development, high levels of Trio mRNA are found in the cerebral cortex, hippocampus, thalamus, caudate/putamen, and olfactory bulb, with lower levels in the septal nucleus, nucleus accumbens, amygdala, and hypothalamus. Except for the cerebellum, Trio mRNA in major brain areas is highest at P1, decreasing gradually during development, with low but detectable levels at P30. In P14 cerebral cortex, pyramidal neurons with strongly staining soma and dendrites are observed primarily in layer 5. In hippocampus, strong staining is observed in pyramidal neurons, granule cells, and isolated interneurons. Cerebellar Purkinje neurons exhibit intense staining in the soma and in extensive dendritic arbors at P7 and P14. Levels of Trio mRNA and the intensity of Trio immunostaining in cerebellar Purkinje cells increase from P1 to P30. Consistent with the in situ hybridization pattern, Western blot analyses show that Trio levels in the hippocampus and cortex are high at P1, decreasing until P30. The data suggest that Trio plays an important role during neuronal development.

Leucine Zipper-mediated Homo-oligomerization Regulates the Rho-GEF Activity of AKAP-Lbc

AKAP-Lbc is a novel member of the A-kinase anchoring protein (AKAPs) family, which functions as a cAMP-dependent protein kinase (PKA)-targeting protein as well as a guanine nucleotide exchange factor (GEF) for RhoA. We recently demonstrated that AKAP-Lbc Rho-GEF activity is stimulated by the alpha-subunit of the heterotrimeric G protein G(12), whereas phosphorylation of AKAP-Lbc by the anchored PKA induces the recruitment of 14-3-3, which inhibits its GEF function. In the present report, using co-immunoprecipitation approaches, we demonstrated that AKAP-Lbc can form homo-oligomers inside cells. Mutagenesis studies revealed that oligomerization is mediated by two adjacent leucine zipper motifs located in the C-terminal region of the anchoring protein. Most interestingly, disruption of oligomerization resulted in a drastic increase in the ability of AKAP-Lbc to stimulate the formation of Rho-GTP in cells under basal conditions, suggesting that oligomerization maintains AKAP-Lbc in a basal-inactive state. Based on these results and on our previous findings showing that AKAP-Lbc is inactivated through the association with 14-3-3, we investigated the hypothesis that AKAP-Lbc oligomerization might be required for the regulatory action of 14-3-3. Most interestingly, we found that mutants of AKAP-Lbc impaired in their ability to undergo oligomerization were completely resistant to the inhibitory effect of PKA and 14-3-3. This suggests that 14-3-3 can negatively regulate the Rho-GEF activity of AKAP-Lbc only when the anchoring protein is in an oligomeric state. Altogether, these findings provide a novel mechanistic explanation of how oligomerization can regulate the activity of exchange factors of the Dbl family.

Regulation of the Dbl proto-oncogene by heat shock cognate protein 70 (Hsc70)

The dbl oncogene product is the defining member of a family of onco-proteins known as Dbl guanine nucleotide exchange factors (GEFs) that facilitate the activation of the small GTP-binding proteins Cdc42, Rac, and Rho. Oncogenic activation of proto-Dbl occurs through loss of the amino-terminal 497 residues, rendering the protein constitutively active. Because both onco- and proto-Dbl contain the structural elements required for GEF activity (i.e. the Dbl homology (DH) and pleckstrin homology (PH) domains), it is thought that the amino terminus of proto-Dbl somehow inhibits the biochemical activity of the protein. To better understand the molecular basis of this regulation, we set forth to identify cellular proteins that preferentially bind the proto-oncogenic form of Dbl. We identified the molecular chaperone heat shock cognate protein (Hsc70) as a binding partner that preferentially interacts with the proto-oncogenic form of Dbl. Dbl is complexed with Hsc70 in transfected cells, as well as in native mouse brain extracts. The interaction between Hsc70 and proto-Dbl is mediated by at least two regions in Dbl, the aminoterminal spectrin homology domain (residues 224-417) and the pleckstrin homology domain (residues 711-808). Overexpression of a dominant negative Hsc70 mutant leads to activation of proto-Dbl GEF activity, indicating that the chaperone negatively regulates proto-Dbl function in vivo. We propose that Hsc70 attenuates Dbl activity by maintaining an inactive conformation in which the amino terminus is "folded over" the catalytic DH-PH domain.

X-linked hypoparathyroidism region on Xq27 is evolutionarily conserved with regions on 3q26 and 13q34 and contains a novel P-type ATPase

X-linked hypoparathyroidism (HPT) has been mapped to a 988-kb region on chromosome Xq27 that contains three genes, MCF2/DBL, SOX3, and U7snRNA homologue, and a partial cDNA, AS6. We isolated the full-length AS6 cDNA, determined its genomic organization, and sought for abnormalities in HPT patients. AS6 was identified as the 3' UTR of ATP11C, a novel member of the P-type ATPases, which consists of 31 exons with alternative transcripts. The colocalization of ATP11C with SOX3 and MCF2/DBL on Xq27 mirrors that of ATP11A with SOX1 and MCF2L on 13q34 and ATP11B with SOX2 on 3q26. These colocalizations are evolutionarily conserved in mouse, and analyses indicate that SOX2 divergence likely occurred before the separation of SOX1 and SOX3. Analyses of ATP11C, MCF2, SOX3, and U7snRNA in HPT patients did not reveal mutations, implicating regulatory changes or mutation of an as yet unidentified gene in the etiology of X-linked hypoparathyroidism.

Global conformational rearrangements during the activation of the GDP/GTP exchange factor Vav3

Activation of Rho/Rac GTPases during cell signaling requires the participation of GDP/GTP exchange factors of the Dbl family. Although the structure of the catalytic core of Dbl proteins has been established recently, the molecular changes that the full-length proteins experience during normal or oncogenic conditions of stimulation are still unknown. Here, we have used single-particle electron microscopy to solve the structures of the inactive (unphosphorylated), active (phosphorylated), and constitutively active (N-terminally deleted) versions of the exchange factor Vav3. Comparison of these forms has revealed the interdomain interactions maintaining the inactive Vav3 state and the dynamic changes that the overall Vav3 structure undergoes upon tyrosine phosphorylation. We have also found that the conformations of phosphorylated Vav3 and N-terminally deleted Vav3 are distinct, indicating that the acquisition of constitutive activity by exchange factors is structurally more complex than the mere elimination of inhibitory interactions between structural domains.

Physical and functional interactions of the lysophosphatidic acid receptors with PDZ domain-containing Rho guanine nucleotide exchange factors (RhoGEFs)

Lysophosphatidic acid (LPA) is a serum-derived phospholipid that induces a variety of biological responses in various cells via heterotrimeric G protein-coupled receptors (GPCRs) including LPA1, LPA2, and LPA3. LPA-induced cytoskeletal changes are mediated by Rho family small GTPases, such as RhoA, Rac1, and Cdc42. One of these small GTPases, RhoA, may be activated via Galpha(12/13)-linked Rho-specific guanine nucleotide exchange factors (RhoGEFs) under LPA stimulation although the detailed mechanisms are poorly understood. Here, we show that the C terminus of LPA1 and LPA2 but not LPA3 interact with the PDZ domains of PDZ domain-containing RhoGEFs, PDZ-RhoGEF, and LARG, which are comprised of PDZ, RGS, Dbl homology (DH), and pleckstrin homology (PH) domains. In LPA1- and LPA2-transfected HEK293 cells, LPA-induced RhoA activation was observed although the C terminus of LPA1 and LPA2 mutants, which failed to interact with the PDZ domains, did not cause LPA-induced RhoA activation. Furthermore, overexpression of the PDZ domains of PDZ domain-containing RhoGEFs served as dominant negative mutants for LPA-induced RhoA activation. Taken together, these results indicate that formation of the LPA receptor/PDZ domain-containing RhoGEF complex plays a pivotal role in LPA-induced RhoA activation.

Direct association of Bazooka/PAR-3 with the lipid phosphatase PTEN reveals a link between the PAR/aPKC complex and phosphoinositide signaling

Cell polarity in Drosophila epithelia, oocytes and neuroblasts is controlled by the evolutionarily conserved PAR/aPKC complex, which consists of the serine-threonine protein kinase aPKC and the PDZ-domain proteins Bazooka (Baz) and PAR-6. The PAR/aPKC complex is required for the separation of apical and basolateral plasma membrane domains, for the asymmetric localization of cell fate determinants and for the proper orientation of the mitotic spindle. How the complex exerts these different functions is not known. We show that the lipid phosphatase PTEN directly binds to Baz in vitro and in vivo, and colocalizes with Baz in the apical cortex of epithelia and neuroblasts. PTEN is an important regulator of phosphoinositide turnover that antagonizes the activity of PI3-kinase. We show that Pten mutant ovaries and embryos lacking maternal and zygotic Pten function display phenotypes consistent with a function for PTEN in the organization of the actin cytoskeleton. In freshly laid eggs, the germ plasm determinants oskar mRNA and Vasa are not localized properly to the posterior cytocortex and pole cells do not form. In addition, the actin-dependent posterior movement of nuclei during early cleavage divisions does not occur and the synchrony of nuclear divisions at syncytial blastoderm stages is lost. Pten mutant embryos also show severe defects during cellularization. Our data provide evidence for a link between the PAR/aPKC complex, the actin cytoskeleton and PI3-kinase signaling mediated by PTEN.

RhoA and resistance artery remodeling

Resistance arteries are able to adapt to physiological and pathophysiological stimuli to maintain adequate perfusion according to the metabolic demand of the tissue. Although vasomotor control allows rapid adaptation of lumen diameter, vascular remodeling constitutes an active process that occurs in response to long-term alterations of hemodynamic parameters. Unfortunately, this initially adaptive process contributes to the pathology of vascular diseases. Recent studies have demonstrated the participation of Rho protein signaling pathways in several cardiovascular pathologies including hypertension, coronary artery spasm, effort angina, atherosclerosis, and restenosis. Functional analyses have further revealed that RhoA-dependent pathways are involved in excessive contraction, migration, and proliferation associated with arterial diseases. The present review focuses on the role of Rho proteins, in particular RhoA, in vascular smooth muscle cells and the involvement of Rho-dependent signaling pathways in resistance artery remodeling, more particularly in relation to hypertension.

Characterization of the Biochemical and Transforming Properties of the Neuroepithelial Transforming Protein 1

Rho family small G proteins are key regulators of cytoskeletal organization and oncogenic transformation whose activation is controlled by a family of proteins known as guanine nucleotide exchange factors (GEFs). In this work we have characterized the structural and biological determinants for cytoskeletal regulation and cell transformation by the neuroepithelioma transforming gene 1 (NET1), which is a GEF specific for RhoA, but not Cdc42 or Rac1. Previously it was shown that the biological activity and nuclear localization of NET1 is controlled by its amino terminus. Here we demonstrate that the amino terminus of NET1 does not function as cis-acting autoinhibitory domain, nor does it affect the ability of full-length NET1 to stimulate actin stress fiber formation. We also show that the nuclear localization of NET1 is controlled by two separate domains within its amino terminus, only one of which contains the previously identified NLS sequences. Importantly, we find that the ability of NET1 to stimulate actin stress fiber formation does not correlate with its transforming activity, because NET1 proteins that potently stimulate stress fiber formation do not transform cells. Furthermore, the presence of a potential PDZ binding site in the C terminus of NET1 is critical to its ability to transform cells, but is not required for enzymatic activity or for effects on the actin cytoskeleton. Thus, these data highlight a divergence between the ability of NET1 to stimulate cytoskeletal reorganization and to transform cells, and implicate the interaction with PDZ domain-containing proteins as critical to NET1-dependent transformation.

RhoH, a hematopoietic-specific Rho GTPase, regulates proliferation, survival, migration, and engraftment of hematopoietic progenitor cells

Rho guanosine triphosphatases (GT-Pases) are recognized as critical mediators of signaling pathways regulating actin assembly, migration, proliferation, and survival in hematopoietic cells. Here, we have studied a recently identified hematopoietic-specific Rho GTPase, RhoH. Unlike most members of the Rho GTPase family, RhoH is GTPase deficient and does not cycle between GTP- and guanosine diphosphate (GDP)–bound forms, suggesting that regulation of RhoH expression may be critical in its activity. We found that RhoH is expressed in murine hematopoietic progenitor cells (HPCs) and fully differentiated myeloid and lymphoid lineages. In cytokine-stimulated HPCs, knockdown of RhoH expression via RNA interference stimulates proliferation, survival, and stromal cell-derived factor-1α (SDF-1α)–induced migration in vitro. Conversely, RhoH overexpression in these cells via retrovirus-mediated gene transfer is associated with impaired activation of Rac GTPases, reduced proliferation, increased apoptosis, and defective actin polymerization and chemotaxis. In vivo, HPCs with RhoH overexpression demonstrate defective hematopoietic reconstitution capability compared with control vector-transduced cells. Our results suggest that RhoH serves as a negative regulator of both growth and actin-based function of HPCs possibly via suppression of Rac-mediated signaling.

Tech: a RhoA GEF selectively expressed in hippocampal and cortical neurons

Recent studies implicating the Rho family of small G proteins in the regulation of neuronal morphology have focused attention on identifying key components of Rho signaling pathways in neurons. To this end, we have conducted studies aimed at defining the localization and function of Tech, a Rho guanine nucleotide exchange factor (GEF) family member that is highly enriched in brain. We have found that Tech is selectively expressed in cortical and hippocampal neurons with prominent Tech immunostaining apparent in the cell bodies and dendrites of these cells. In vitro studies with prototypical members of the major Rho subfamilies, RhoA, Rac1 and Cdc42, indicate that Tech binds selectively to and activates RhoA. To assess whether Tech may be involved in the regulation of neuronal morphology, we examined the effects of Tech constructs on the morphology of cortical neurons grown in primary culture. We found that a constitutively active Tech construct, Tech 245DeltaC, decreases the number of dendritic processes present on these neurons. This reduction appears to be mediated by activation of RhoA as it is blocked by insertion of a point mutation into the DH domain of Tech which blocks its ability to activate RhoA or coexpression of a dominant negative RhoA construct. As Tech protein levels increase during post-natal development and remain at peak levels into adulthood, these results indicate that Tech regulates RhoA signaling pathways in developing and mature forebrain neurons.

PAK1 negatively regulates the activity of the Rho exchange factor NET1

Rho family small G-protein activity is controlled by guanine nucleotide exchange factors that stimulate the release of GDP, thus allowing GTP binding. Once activated, Rho proteins control cell signaling through interactions with downstream effector proteins, leading to changes in cytoskeletal organization and gene expression. The ability of Rho family members to modulate the activity of other Rho proteins is also intrinsic to these processes. In this work we show that the Rac/Cdc42hs-regulated protein kinase PAK1 down-regulates the activity of the RhoA-specific guanine nucleotide exchange factor NET1. Specifically, PAK1 phosphorylates NET1 on three sites in vitro: serines 152, 153, and 538. Replacement of serines 152 and 153 with glutamate residues down-regulates the activity of NET1 as an exchange factor in vitro and its ability to stimulate actin stress fiber formation in cells. Using a phospho-specific antibody that recognizes NET1 phosphorylated on serine 152, we show that PAK1 phosphorylates NET1 on this site in cells and that Rac1 stimulates serine 152 phosphorylation in a PAK1-dependent manner. Furthermore, coexpression of constitutively active PAK1 inhibits the ability of NET1 to stimulate actin polymerization only when serines 152 and 153 are present. These data provide a novel mechanism for the control of RhoA activity by Rac1 through the PAK-dependent phosphorylation of NET1 to reduce its activity as a guanine nucleotide exchange factor.

Cdc24 regulates nuclear shuttling and recruitment of the Ste5 scaffold to a heterotrimeric G protein in Saccharomyces cerevisiae

The Saccharomyces cerevisiae guanine nucleotide exchange factor Cdc24 regulates polarized growth by binding to Cdc42, a Rho-type GTPase that has many effectors, including Ste20 kinase, which activates multiple MAPK cascades. Here, we show that Cdc24 promotes MAPK signaling during mating through interactions with Ste5, a scaffold that must shuttle through the nucleus and bind to the beta subunit (Ste4) of a G protein for Ste20 to activate the tethered MAPK cascade. Ste5 was basally recruited to growth sites of G1 phase cells independently of Ste4. Loss of Cdc24 inhibited nuclear import and blocked basal and pheromone-induced recruitment of Ste5. Ste5 was not basally recruited and the MAPK Fus3 was not basally activated in the presence of a Cdc24 mutant (G168D) that still activates Cdc42, suggesting that Cdc24 regulates Ste5 and the associated MAPK cascade through a function that is not dependent on its guanine nucleotide exchange factor activity. Consistent with this, Cdc24 bound Ste5 and coprecipitated with Ste4 independently of Far1 and Ste5. Loss of Cdc24 decreased Ste5-Ste4 complex formation, and loss of Ste4 stimulated Cdc24-Ste5 complex formation. Collectively, these findings suggest that Cdc24 mediates site-specific localization of Ste5 to a heterotrimeric G protein and may therefore ensure localized activation of the associated MAPK cascade.

Ect2 and MgcRacGAP regulate the activation and function of Cdc42 in mitosis

Although Rho regulates cytokinesis, little was known about the functions in mitosis of Cdc42 and Rac. We recently suggested that Cdc42 works in metaphase by regulating bi-orient attachment of spindle microtubules to kinetochores. We now confirm the role of Cdc42 by RNA interference and identify the mechanisms for activation and down-regulation of Cdc42. Using a pull-down assay, we found that the level of GTP-Cdc42 elevates in metaphase, whereas the level of GTP-Rac does not change significantly in mitosis. Overexpression of dominant-negative mutants of Ect2 and MgcRacGAP, a Rho GTPase guanine nucleotide exchange factor and GTPase activating protein, respectively, or depletion of Ect2 by RNA interference suppresses this change of GTP-Cdc42 in mitosis. Depletion of Ect2 also impairs microtubule attachment to kinetochores and causes prometaphase delay and abnormal chromosomal segregation, as does depletion of Cdc42 or expression of the Ect2 and MgcRacGAP mutants. These results suggest that Ect2 and MgcRacGAP regulate the activation and function of Cdc42 in mitosis.

The guanine nucleotide exchange factor p63RhoGEF, a specific link between Gq/11-coupled receptor signaling and RhoA

The monomeric GTPase RhoA, which is a key regulator of numerous cellular processes, is activated by a variety of G protein-coupled receptors, through either G12 or G(q) family proteins. Here we report that p63RhoGEF, a recently identified RhoA-specific guanine nucleotide exchange factor, enhances the Rho-dependent gene transcription induced by agonist-stimulated G(q/11)-coupled receptors (M3-cholinoceptor, histamine H1 receptor) or GTPase-deficient mutants of G alpha(q) and G alpha11. We further demonstrate that active G alpha(q) or G alpha11, but not G alpha12 or G alpha13, strongly enhances p63RhoGEF-induced RhoA activation by direct protein-protein interaction with p63RhoGEF at its C-terminal half. Moreover, the activation of p63RhoGEF by G alpha(q/11) occurs independently of and in competition to the activation of the canonical G alpha(q/11) effector phospholipase C beta. Therefore, our results elucidate a new signaling pathway by which G alpha(q/11)-coupled receptors specifically induce Rho signaling through a direct interaction of activated G alpha(q/11) subunits with p63RhoGEF.

Cell adhesion receptors, tyrosine kinases and actin modulators: a complex three-way circuitry

The interaction of cells with surrounding matrix and neighbouring cells governs many aspects of cell behaviour. Aside from transmitting signals from the external environment, adhesion receptors also receive signals from the cell interior. Here we review the interrelationship between adhesion receptors, tyrosine kinases (both growth factor receptor and non-receptor) and modulators of the actin cytoskeletal network. Deregulation of many aspects of these signalling pathways in cancer highlights the need for a better understanding of the complexities involved.

SWAP-70 regulates c-kit-induced mast cell activation, cell-cell adhesion, and migration

SWAP-70, an unusual phosphatidylinositol-3-kinase-dependent protein that interacts with the RhoGTPase Rac, is highly expressed in mast cells. Cultured bone marrow mast cells (BMMC) from SWAP-70(-/-) mice are reduced in FcepsilonRI-triggered degranulation. This report describes the hitherto-unknown role of SWAP-70 in c-kit receptor signaling, a key proliferation and differentiation pathway in mast cells. Consistent with the role of Rac in cell motility and regulation of the actin cytoskeleton, mutant cells show abnormal actin rearrangements and are deficient in migration in vitro and in vivo. SWAP-70(-/-) BMMC are impaired in calcium flux, in proper translocation and activity of Akt kinase (required for mast cell activation and survival), and in translocation of Rac1 and Rac2 upon c-kit stimulation. Adhesion to fibronectin is reduced, but homotypic cell association induced through c-kit is strongly increased in SWAP-70(-/-) BMMC. Homotypic association requires extracellular Ca(2+) and depends on the integrin alpha(L)beta(2) (LFA-1). ERK is hyperactivated upon c-kit signaling in adherent and dispersed mutant cells. Together, we suggest that SWAP-70 is an important regulator of specific effector pathways in c-kit signaling, including mast cell activation, migration, and cell adhesion.

Vav Transformation Requires Activation of Multiple GTPases and Regulation of Gene Expression

Although Vav can act as a guanine nucleotide exchange factor for RhoA, Rac1, and Cdc42, its transforming activity has been ascribed primarily to its ability to activate Rac1. However, because activated Vav, but not Rac-specific guanine nucleotide exchange factors, exhibits very potent focus-forming transforming activity when assayed in NIH 3T3 cells, Vav transforming activity must also involve activation of Rac-independent pathways. In this study, we determined the involvement of other Rho family proteins and their signaling pathways in Vav transformation. We found that RhoA, Rac1, and Cdc42 functions are all required for Vav transforming activity. Furthermore, we determined that Vav activation of nuclear factor-κB and the Jun NH2-terminal kinase mitogen-activated protein kinase (MAPK) is necessary for full transformation by Vav, whereas p38 MAPK does not seem to play an important role. We also determined that Vav is a weak activator of Elk-1 via a Ras- and MAPK/extracellular signal-regulated kinase kinase–dependent pathway, and this activity was essential for Vav transformation. Thus, we conclude that full Vav transforming activation is mediated by the activation of multiple small GTPases and their subsequent activation of signaling pathways that regulate changes in gene expression. Because Vav is activated by the epidermal growth factor receptor and other tyrosine kinases involved in cancer development, defining the role of aberrant Vav signaling may identify activities of receptor tyrosine kinases important for human oncogenesis.

Sprouty regulates cell migration by inhibiting the activation of Rac1 GTPase

Sprouty (SPRY) protein negatively modulates fibroblast growth factor and epidermal growth factor actions. We showed that human SPRY2 inhibits cell growth and migration in response to serum and several growth factors. Using rat intestinal epithelial (IEC-6) cells, we investigated the involvement of the Rho family of GTPases, RhoA, Rac1, and cdc42 in SPRY2-mediated inhibition of cell migration and proliferation. The ability of TAT-tagged SPRY2 to inhibit proliferation and migration of IEC-6 cells transfected with constitutively active mutants of RhoA(G14V), Rac1(G12V), and cdc42 (F28L) was determined. Constitutively active RhoA(G14V), Rac1(G12V), or cdc42(F28L) did not protect cells from the anti-proliferative actions of TAT-SPRY2. The ability of TAT-hSPRY2 to inhibit migration was not altered by of RhoA(G14V) and cdc42(F28L). However, Rac1(G12V) obliterated the ability of SPRY2 to inhibit cell autonomous or serum-induced migration. Also, the activation of endogenous Rac1 was attenuated by TAT-SPRY2. Thus, SPRY2 mediates its anti-migratory actions by inhibiting Rac1 activation.

Schizosaccharomyces pombe Rgf3p is a specific Rho1 GEF that regulates cell wall beta-glucan biosynthesis through the GTPase Rho1p

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell-wall synthesis. Here, we describe the cloning and characterization of rgf3+, a member of the Rho family of guanine nucleotide exchange factors (Rho GEFs). The rgf3+ gene was cloned by complementation of a mutant (ehs2-1) hypersensitive to drugs that interfere with cell-wall biosynthesis. The rgf3+ gene was found to be essential for cell viability and depletion of Rgf3p afforded phenotypes similar to those obtained following depletion of Rho1p. However, the cell death caused by Rgf3p depletion could be rescued by the presence of 1.2 M sorbitol, whereas depletion of Rho1 was lethal under the same conditions. We show that Rgf3p is a specific Rho1-GEF. The hypersensitivity to drugs affecting the cell wall of the ehs2-1 mutant was suppressed by overexpression of rho1+ but not by any of the other GTPases of the Rho family. Rgf3p interacted with the GDP-bound form of Rho1p and promoted the GDP-GTP exchange. In addition, we show that overexpression of Rgf3p produces multiseptated cells and increases beta-1,3-glucan synthase activity and the amount of cell wall beta-1,3-glucan. Rgf3p localized to the septum and the mRNA level was regulated in a cell-cycle-dependent manner peaking during septation. Our results suggest that Rgf3p acts as a positive activator of Rho1p, probably activating the Rho functions that coordinate cell-wall biosynthesis to maintain cell integrity during septation.

The tandem BRCT domains of Ect2 are required for both negative and positive regulation of Ect2 in cytokinesis

Epithelial cell transforming protein 2 (Ect2) is a guanine nucleotide exchange factor (GEF) for Rho GTPases, molecular switches essential for the control of cytokinesis in mammalian cells. Aside from the canonical Dbl homology/pleckstrin homology cassette found in virtually all Dbl family members, Ect2 contains N-terminal tandem BRCT domains. In this study, we address the role of the Ect2 BRCT domains in the regulation of Ect2 activity and cytokinesis. First, we show that the depletion of endogenous Ect2 by small interfering RNA induces multinucleation, suggesting that Ect2 is required for cytokinesis. In addition, we provide evidence that Ect2 normally exists in an inactive conformation, which is at least partially due to an intramolecular interaction between the BRCT domains and the C-terminal domain of Ect2. This intramolecular interaction masks the catalytic domain responsible for guanine nucleotide exchange toward RhoA. Consistent with a role in regulating Ect2 GEF activity, overexpression of an N-terminal Ect2 containing the tandem BRCT domains, but not single BRCT domain or BRCT domain mutant, leads to a failure in cytokinesis. Surprisingly, although ectopically expressed wild-type Ect2 rescues the multinucleation resulting from the depletion of endogenous Ect2, expression of a BRCT mutant of Ect2 failed to restore proper cytokinesis in these cells. Taken together, the results of our study indicate that the tandem BRCT domains of Ect2 play dual roles in the regulation of Ect2. Whereas these domains negatively regulate Ect2 GEF activity in interphase cells, they are also required for the proper function of Ect2 during cytokinesis.

R-Ras controls membrane protrusion and cell migration through the spatial regulation of Rac and Rho

Although it is known that the spatial coordination of Rac and Rho activity is essential for cell migration, the molecular mechanisms regulating these GTPases during migration are unknown. We found that the expression of constitutively activated R-Ras (38V) blocked membrane protrusion and random migration. In contrast, expression of dominant negative R-Ras (41A) enhanced migrational persistence and membrane protrusion. Endogenous R-Ras is necessary for cell migration, as cells that were transfected with siRNA for R-Ras did not migrate. Expression of R-Ras (38V) decreased Rac activity and increased Rho activity around the entire cell periphery, whereas expression of dominant negative R-Ras (41A) showed the converse, suggesting that R-Ras can spatially activate Rho and inactivate Rac. Consistent with this role, endogenous R-Ras localized and was preferentially activated at the leading edge of migratory cells in response to adhesion. The effects of R-Ras on cell migration are mediated by PI3-Kinase, as an effector mutant that uncouples PI3-Kinase binding from R-Ras (38V) rescued migration. From these data, we hypothesize that R-Ras plays a key role in cell migration by locally regulating the switch from Rac to Rho activity after membrane protrusion and adhesion.

Molecular cloning, sequence and expression pattern analysis of the mouse orthologue of the leukemia-associated guanine nucleotide exchange factor

Guanine nucleotide exchange factors (GEFs) of the Dbl family activate Rho proteins and thereby participate in diverse signaling pathways involving this family of small GTPases. However, specific role of Dbl-GEFs in developmental processes, particularly cell differentiation, remains largely unexplored. Recently, a novel member of the Dbl family, leukemia-associated Rho GEF factor (LARG), has been reported to be a fusion partner with mixed-lineage leukemia (MLL) protein in the acute myeloid leukemia, suggesting potential involvement of LARG in regulation of hematopoiesis. In this study, we describe the isolation of the cDNA copy and analysis of genomic structure and expression profile of the mouse orthologue of the human LARG gene. The mouse LARG (mLARG) gene contains 42 exons. The mLARG cDNA is 10,040 bp long and contains a 4631-bp open reading frame (ORF). The predicted mLARG protein shares an overall 89% identity with its human counterpart and contains the same functional domains, namely, Dbl homology (DH), pleckstrin homology (PH), PSD-95, Dlg and ZO-1/2 (PDZ) and regulator of G protein signaling (RGS) domains, as well as two putative signals of nuclear localization. The mLARG message is expressed in all tissues studied, with comparably higher expression levels observed in brain, lung, testis, liver and heart. Using amplified cDNA samples from sorted hematopoietic cells, we showed that mLARG is highly expressed in hematopoietic stem cell fractions, as well as in immature erythroid cells. These results demonstrate high similarity between mouse and human LARG proteins and genes and provide further support to the hypothesis of the potential involvement of LARG in regulation of early stages of hematopoiesis.

P-REX2, a novel PI-3-kinase sensitive Rac exchange factor

We have identified an activator of Rac, P-REX2, that is structurally related to the exchange factor PtdIns(3,4,5)-dependent Rac exchanger (P-REX1), but exhibits distinct tissue-specific expression. P-REX2 is spliced into two RNA species, approximately 3.5 and approximately 10 kb in size. The cDNA corresponding to the smaller transcript encodes a protein that exhibits strong similarity with P-REX1 within its N-terminal domains, but differs in the C-terminal region. P-REX2 promoted increased levels of GTP-bound Rac that could be further stimulated by enhancing PI-3K activity. Thus, P-REX2 may serve as a novel link between Rac activation and the PI-3 kinase pathway.

Cdk5 and Trio modulate endocrine cell exocytosis

Hormone secretion by pituitary cells is decreased by roscovitine, an inhibitor of cyclin-dependent kinase 5 (Cdk5). Roscovitine treatment reorganizes cortical actin and ultrastructural analysis demonstrates that roscovitine limits the ability of secretory granules to approach the plasma membrane or one another. Trio, a multifunctional RhoGEF expressed in pituitary cells, interacts with peptidylglycine α-amidating monooxygenase, a secretory granule membrane protein known to affect the actin cytoskeleton. Roscovitine inhibits the ability of Trio to activate Rac, and peptides corresponding to the Cdk5 consensus sites in Trio are phosphorylated by Cdk5. Together, these data suggest that control of the cortical actin cytoskeleton, long known to modulate hormone exocytosis and subsequent endocytosis, involves Cdk5-mediated activation of Trio.

Developmentally Restricted Actin-Regulatory Molecules Control Morphogenetic Cell Movements in the Zebrafish Gastrula

Although our understanding of the regulation of cellular actin and its control during the development of invertebrates is increasing, the question as to how such actin dynamics are regulated differentially across the vertebrate embryo to effect its relatively complex morphogenetic cell movements remains poorly understood. Intercellular signaling that provides spatial and temporal cues to modulate the subcellular localization and activity of actin regulatory molecules represents one important mechanism. Here we explore whether the localized gene expression of specific actin regulatory molecules represents another developmental mechanism. We have identified a cap1 homolog and a novel guanine nucleotide exchange factor (GEF), quattro (quo), that share a restricted gene expression domain in the anterior mesendoderm of the zebrafish gastrula. Each gene is required for specific cellular behaviors during the anterior migration of this tissue; furthermore, cap1 regulates cortical actin distribution specifically in these cells. Finally, although cap1 and quo are autonomously required for the normal behaviors of these cells, they are also nonautonomously required for convergence and extension movements of posterior tissues. Our results provide direct evidence for the deployment of developmentally restricted actin-regulatory molecules in the control of morphogenetic cell movements during vertebrate development.

Activation of Rac1 by paxillin-Crk-DOCK180 signaling complex is antagonized by Rap1 in migrating NBT-II cells

Induction of epithelial cell motility is a fundamental morphogenetic event that is recapitulated during carcinoma metastasis. Random motility of NBT-II carcinoma cells on collagen critically depends on paxillin phosphorylation at Tyr-31 and Tyr-118, the binding sites for the adapter protein CrkII. Two constitutive partners of CrkII are the exchange factors DOCK180 and C3G. CrkII bound to DOCK180 formed a signaling complex with phosphorylated paxillin that was necessary for cell migration as inferred from the inhibition caused by a DOCK180-interfering mutant. DOCK180, which acts predominantly on the Rho family GTPase Rac1, restored cell locomotion in cells expressing Phe-31/118 paxillin mutants deficient in Rac1 GTP-loading, suggesting that formation of paxillin-Crk-DOCK180 signaling complex controls collagen-dependent migration mainly through Rac1 activation. In migrating cells, CrkII constitutive association with C3G was not sufficient to stimulate its GDP/GTP exchange activity toward the Ras family GTPase Rap1. However, when constitutively active RapV12 was overexpressed, it negatively regulated cell motility. Activation of the C3G/Rap1 signaling pathway resulted in down-regulation of the paxillin-Crk-DOCK180 complex and reduction of Rac1-GTP, suggesting that Rap1 activation could suppress the Rac1 signaling pathway in epithelial cells.

Tandem histone folds in the structure of the N-terminal segment of the ras activator Son of Sevenless

The Ras activator Son of Sevenless (Sos) contains a Cdc25 homology domain, responsible for nucleotide exchange, as well as Dbl/Pleckstrin homology (DH/PH) domains. We have determined the crystal structure of the N-terminal segment of human Sos1 (residues 1-191) and show that it contains two tandem histone folds. While the N-terminal domain is monomeric in solution, its structure is surprisingly similar to that of histone dimers, with both subunits of the histone "dimer" being part of the same peptide chain. One histone fold corresponds to the region of Sos that is clearly similar in sequence to histones (residues 91-191), whereas the other is formed by residues in Sos (1-90) that are unrelated in sequence to histones. Residues that form a contiguous patch on the surface of the histone domain of Sos are conserved from C. elegans to humans, suggesting a potential role for this domain in protein-protein interactions.

Role of the Sec14-like domain of Dbl family exchange factors in the regulation of Rho family GTPases in different subcellular sites

Mechanisms underlying subcellular region-specific regulation of Rho family GTPases through Dbl family guanine nucleotide exchange factors (GEFs) remain totally unknown. Here we show that the Sec14-like domain, which lies in the N-terminus of the Dbl family GEFs Dbl and Ost, directs the subcellular localization of these GEFs and also their substrate Cdc42. When coexpressed with Cdc42 in human adenocarcinoma HeLa cells, Dbl-I and Ost-I, which lack the Sec14-like domain, translocated Cdc42 to the plasma membrane, where Dbl-I or Ost-I was colocalized. In marked contrast, Dbl-II and Ost-II, which contain the Sec14-like domain, were colocalized with Cdc42 in endomembrane compartments. Furthermore, ruffle membrane formation upon epidermal growth factor treatment was mediated by Dbl-I or Ost-I, but neither Dbl-II nor Ost-II, supporting a notion that GEFs with or without the Sec14-like domain are linked to different upstream signals. By employing a novel method to detect the active GTP-bound form of Cdc42 in situ, we demonstrate that Dbl-I and Ost-I, but neither Dbl-II nor Ost-II, indeed activate colocalized Cdc42.

Inverted signaling hierarchy between RAS and RAC in T-lymphocytes

In order to generate coherent biological responses to extracellular stimuli, cells have established synergistic and antagonistic crosstalk between pathways with similar or opposing functions, respectively. Two routes cooperating in the generation of mitogenic and cytoskeletal functions are those induced by Ras and Rho/Rac GTPases. In these signaling interactions, Rho/Rac proteins have been always placed in a downstream position respect to Ras in all cell systems analysed so far. In this report, we describe that such signaling hierarchy does not apply to T-lymphocytes. Thus, we show that both Rac1 GDP/GTP exchange factors such as Vav and constitutively active versions of Rac1 can promote the effective stimulation of the Ras pathway in T-lymphocytes. The molecular link for this new type of pathway interconnectivity is RasGRP1, a diacylglycerol-dependent GDP/GTP exchange factor for Ras that translocates to the plasma membrane in a Vav- and Rac1-dependent manner. The effect of the Vav/Rac1 pathway on the Ras pathway is highly dependent on the activity of phospholipase C-gamma, the key cellular supplier of intracellular diacylglycerol. Signaling experiments suggest that this crosstalk represents a signaling strategy used by the T-cell receptor to promote robust biological responses of both the Rac/Rho and Ras pathways upon antigen engagement.

Anchoring of both PKA and 14-3-3 inhibits the Rho-GEF activity of the AKAP-Lbc signaling complex

A-kinase anchoring proteins (AKAPs) target the cAMP-regulated protein kinase (PKA) to its physiological substrates. We recently identified a novel anchoring protein, called AKAP-Lbc, which functions as a PKA-targeting protein as well as a guanine nucleotide exchange factor (GEF) for RhoA. We demonstrated that AKAP-Lbc Rho-GEF activity is stimulated by the alpha subunit of the heterotrimeric G protein G12. Here, we identified 14-3-3 as a novel regulatory protein interacting with AKAP-Lbc. Elevation of the cellular concentration of cAMP activates the PKA holoenzyme anchored to AKAP-Lbc, which phosphorylates the anchoring protein on the serine 1565. This phosphorylation event induces the recruitment of 14-3-3, which inhibits the Rho-GEF activity of AKAP-Lbc. AKAP-Lbc mutants that fail to interact with PKA or with 14-3-3 show a higher basal Rho-GEF activity as compared to the wild-type protein. This suggests that, under basal conditions, 14-3-3 maintains AKAP-Lbc in an inactive state. Therefore, while it is known that AKAP-Lbc activity can be stimulated by Galpha12, in this study we demonstrated that it is inhibited by the anchoring of both PKA and 14-3-3.

Altered Rho GTPase signaling pathways in breast cancer cells

The Rho family of GTPases have emerged as key players in regulating a diverse set of biological activities including actin organization, focal complex/adhesion assembly, cell motility, cell polarity, gene transcription and cell-cycle progression. Some Rho GTPases and their signaling components are overexpressed and/or are hyperactive in breast cancer and recent studies have shown a requirement for Rho GTPases in breast cancer cell metastasis in vivo. Herein we describe the contribution of Rho GTPase to the malignant phenotype of breast cancer cells and the role of these pathways as potential targets for breast cancer therapy. Rho GTPases promote cell-cycle progression through cyclin D1, and cyclin D1 in turn reduces cellular adhesion and promotes migration, an example of 'inside-out' signaling by cyclin D1. As cyclin D1 overexpression correlates with metastatic cancer, the 'inside-out' signaling function of cyclin D1 to promote cell migration may represent a useful new therapeutic target.

Sequential activation of phosphatidylinositol 3-kinase, beta Pix, Rac1, and Nox1 in growth factor-induced production of H2O2

The generation of reactive oxygen species (ROS) in cells stimulated with growth factors requires the activation of phosphatidylinositol 3-kinase (PI3K) and the Rac protein. We report here that the COOH-terminal region of Nox1, a protein related to gp91(phox) (Nox2) of phagocytic cells, is constitutively associated with beta Pix, a guanine nucleotide exchange factor for Rac. Both growth factor-induced ROS production and Rac1 activation were completely blocked in cells depleted of beta Pix by RNA interference. Rac1 was also shown to bind to the COOH-terminal region of Nox1 in a growth factor-dependent manner. Moreover, the depletion of Nox1 by RNA interference inhibited growth factor-induced ROS generation. These results suggest that ROS production in growth factor-stimulated cells is mediated by the sequential activation of PI3K, beta Pix, and Rac1, which then binds to Nox1 to stimulate its NADPH oxidase activity.

The C-terminal basic tail of RhoG assists the guanine nucleotide exchange factor trio in binding to phospholipids

The multidomain protein Trio regulates among others neuronal outgrowth and axonal guidance in vertebrates and invertebrates. Trio contains two Dbl-homology/pleckstrin homology (DH/PH) tandem domains that activate several RhoGTPases. Here, we present the x-ray structure of the N-terminal DH/PH, hereafter TrioN, refined to 1.7-A resolution. We show that the relative orientations of the DH and PH domains of TrioN and free Dbs are similar. However, this relative orientation is dissimilar to Dbs in the Dbs/Cdc42 structure. In vitro nucleotide exchange experiments catalyzed by TrioN show that RhoG is approximately 3x more efficiently exchanged than Rac and support the conclusion that RhoG is likely the downstream target of TrioN. Residues 54 and 69, which are not conserved between the two GTPases, are responsible for this specificity. Dot-blot assay reveals that the TrioN-PH domain does not detectably bind phosphatidylinositol 3,4-bisphosphate, PtdIns(3,4)P(2), or other phospholipids. This finding is supported by our three-dimensional structure and affinity binding experiments. Interestingly, the presence of RhoG but not Rac or a C-terminal-truncated RhoG mutant allows TrioN to bind PtdIns(3,4)P(2) with a micromolar affinity constant. We conclude the variable C-terminal basic tail of RhoG specifically assists the recruitment of the TrioN-PH domain to specific membrane-bound phospholipids. Our data suggest a role for the phosphoinositide 3-kinase, PI 3-kinase, in modulating the Trio/RhoG signaling pathway.

Phosphorylation-independent membrane relocalization of ezrin following association with Dbl in vivo

Ezrin, a widespread protein involved in cell migration, morphogenesis and cell adhesion, belongs to a large family of proteins known as ERM (ezrin, radixin, moesin). These three closely related proteins are thought to function as linkers between plasma membrane and actin cytoskeleton and their function is regulated by the small GTP-binding protein Rho. It has been previously shown that the active form of radixin can bind in vitro to Dbl, a Rho-specific guanine nucleotide exchange factor, although an in vivo interaction has not yet been demonstrated. In this paper, we attempted to investigate whether ezrin can also associate with Dbl. We show here that Dbl protein can effectively bind both in vitro and in vivo to the N-terminal region (amino acids 1-531) of a constitutively active mutant of ezrin and with the full-length molecule. We found that this binding is mediated by the Dbl pleckstrin homology domain, responsible for the proper subcellular localization of the Dbl protein. Moreover, we show that Dbl induces localization to the plasma membrane of both the active deletion mutant and the full-length ezrin proteins. Finally, we show that the relocalization of ezrin is independent of Dbl GEF activity. These results indicate that Dbl could induce translocation of ezrin to the plasma membrane through a mechanism that does not require ezrin C-terminus phosphorylation by Rho-associated kinases.

Alsin Is a Rab5 and Rac1 Guanine Nucleotide Exchange Factor

ALS2 is the gene mutated in a recessive juvenile form of amyotrophic lateral sclerosis (ALS2). ALS2 encodes a large protein termed alsin, which contains a number of predicted cell signaling and protein trafficking sequence motifs. To gain insight into the overall function of alsin and to begin to evaluate its role in motor neuron maintenance, we examined the subcellular localization of alsin and the biochemical activities associated with its individual subdomains. We found that the Vps9p domain of alsin has Rab5 guanine nucleotide exchange activity. In addition, alsin interacted specifically with and acted as a guanine nucleotide exchange factor for Rac1. Immunofluorescence and fractionation experiments in both fibroblasts and neurons revealed that alsin is a cytosolic protein, with a significant portion associated with small, punctate membrane structures. Many of these membrane structures also contained Rab5 or Rac1. Upon overexpression of full-length alsin, the overexpressed material was largely cytosolic, indicating that the association with membrane structures could be saturated. We also found that alsin was present in membrane ruffles and lamellipodia. These data suggest that alsin is involved in membrane transport events, potentially linking endocytic processes and actin cytoskeleton remodeling.

Rational design and characterization of a Rac GTPase-specific small molecule inhibitor

The signaling pathways mediated by Rho family GTPases have been implicated in many aspects of cell biology. The specificity of the pathways is achieved in part by the selective interaction between Dbl family guanine nucleotide exchange factors (GEFs) and their Rho GTPase substrates. Here, we report a first-generation small-molecule inhibitor of Rac GTPase targeting Rac activation by GEF. The chemical compound NSC23766 was identified by a structure-based virtual screening of compounds that fit into a surface groove of Rac1 known to be critical for GEF specification. In vitro it could effectively inhibit Rac1 binding and activation by the Rac-specific GEF Trio or Tiam1 in a dose-dependent manner without interfering with the closely related Cdc42 or RhoA binding or activation by their respective GEFs or with Rac1 interaction with BcrGAP or effector PAK1. In cells, it potently blocked serum or platelet-derived growth factor-induced Rac1 activation and lamellipodia formation without affecting the activity of endogenous Cdc42 or RhoA. Moreover, this compound reduced Trio or Tiam1 but not Vav, Lbc, Intersectin, or a constitutively active Rac1 mutant-stimulated cell growth and suppressed Trio, Tiam1, or Ras-induced cell transformation. When applied to human prostate cancer PC-3 cells, it was able to inhibit the proliferation, anchorage-independent growth and invasion phenotypes that require the endogenous Rac1 activity. Thus, NSC23766 constitutes a Rac-specific small-molecule inhibitor that could be useful to study the role of Rac in various cellular functions and to reverse tumor cell phenotypes associated with Rac deregulation.

Rho family GTPases cooperate with p53 deletion to promote primary mouse embryonic fibroblast cell invasion

The Rho family GTPases Rac1, RhoA and Cdc42 function as molecular switches that transduce intracellular signals regulating multiple cell functions including gene expression, adhesion, migration and invasion. p53 and its regulator p19Arf, on the other hand, are tumor suppressors that are critical in regulating cell cycle progression and apoptosis. Previously, we have demonstrated that the Rho proteins contribute to the cell proliferation, gene transcription and migration phenotypes unleashed by p19Arf or p53 deletion in primary mouse embryo fibroblasts (MEFs). To further investigate their functional interaction in the present study, we have examined the involvement of Rho signaling pathways in p53-mediated cell invasion. We found that in primary MEFs (1) p53 or p19Arf deficiency led to a marked increase in the number of focal adhesion plaques and fibronectin production, and RhoA, Rac1 and Cdc42 contribute to the p53- and p19Arf-mediated focal adhesion regulation, but not fibronectin synthesis; (2) although endogenous Rac1 activity was required for the p19Arf or p53 deficiency-induced migration phenotype, hyperactive Rho GTPases could not further enhance cell migration, rather they suppressed cell-cell adhesion of p53-/- MEFs; (3) expression of the active mutant of RhoA, Rac1 or Cdc42, but not Ras, promoted an invasion phenotype of p53-/-, not p19Arf-/-, cells; (4) although ROCK activation can partially recapitulate Rho-induced invasion phenotype, multiple pathways regulated by RhoA, in addition to ROCK, are required to fully cooperate with p53 deficiency to promote cell invasion; and (5) extracellular proteases produced by the active RhoA-transduced cells are also required for the invasion phenotype of p53-/- cells. Combined with our previous observations, these results strongly suggest that mitogenic activation of Rho family GTPases can cooperate with p53 deficiency to promote primary cell invasion as well as transformation and that multiple signaling components regulated by the Rho proteins are involved in these processes.

F-actin-dependent Translocation of the Rap1 GDP/GTP Exchange Factor RasGRP2

RasGRPs constitute a new group of diacylglycerol-dependent GDP/GTP exchange factors that activate Ras subfamily GTPases. Despite a common structure, Ras-GRPs diverge in their GTPase specificity, subcellular distribution, and downstream biological effects. The more divergent family member is RasGRP2, a Rap1-specific exchange factor with low affinity toward diacylglycerol. The regulation of RasGRP2 during signal transduction has remained elusive up to now. In this report, we show that the subcellular localization of Ras-GRP2 is highly dependent on actin dynamics. Thus, the induction of F-actin by cytoskeletal regulators such as Vav, Vav2, Dbl, and Rac1 leads to the shift of RasGRP2 from the cytosol to membrane ruffles and its co-localization with F-actin. Treatment of cells with cytoskeletal disrupting drugs abolishes this effect, leading to an abnormal localization of RasGRP2 in cytoplasmic clusters of actin. The use of Rac1 effector mutants indicates that the RasGRP2 translocation is linked exclusively to actin polymerization and is independent of other pathways such as p21-activated kinase JNK, or superoxide production. Biochemical experiments demonstrate that the translocation of RasGRP2 to membrane ruffles is mediated by the direct association of this protein with F-actin, a property contained within its 150 first amino acids. Finally, we show that the RasGRP2/F-actin interaction promotes the regionalized activation of Rap1 in juxtamembrane areas of the cell. These results reveal a novel function of the actin cytoskeleton in mediating the spatial activation of Ras subfamily GTPases through the selective recruitment of GDP/GTP exchange factors.

DNA microarray analysis of gene expression in human optic nerve head astrocytes in response to hydrostatic pressure

There is clinical and experimental evidence that elevated intraocular pressure (IOP), a mechanical stress, is involved in the pathogenesis of glaucomatous optic neuropathy. The mechanism by which astrocytes in the optic nerve head (ONH) respond to changes in IOP is under study. Gene transcription by ONH astrocytes exposed either to 60 mmHg hydrostatic pressure (HP) or control ambient pressure (CP) for 6, 24, and 48 h was compared using Affymetrix GeneChip microarrays to identify HP-responsive genes. Data were normalized across arrays within each gene. A linear regression model applied to test effect of time and HP on changes in expression level identified 596 genes affected by HP over time. Using GeneSpring analysis we selected genes whose average expression level increased or decreased more than 1.5-fold at 6, 24, or 48 h. Expression of selected genes was confirmed by real-time RT-PCR; protein levels were detected by Western blot. Among the genes highly responsive to HP were those involved in signal transduction, such as Rho nucleotide exchange factors, Ras p21 protein activator, tyrosine kinases and serine threonine kinases, and genes involved in transcriptional regulation, such as c-Fos, Egr2, and Smad3. Other genes that increased expression included ATP-binding cassettes, solute carriers, and genes associated with lipid metabolism. Among the genes that decreased expression under HP were genes encoding for dual activity phosphatases, transcription factors, and enzymes involved in protein degradation. These HP-responsive genes may be important in the establishment and maintenance of the ONH astrocyte phenotype under conditions of elevated IOP in glaucoma.