The Rho GTPase family: a Racs to Wrchs story

The atypical Rho GTPase Wrch1 collaborates with the nonreceptor tyrosine kinases Pyk2 and Src in regulating cytoskeletal dynamics

The Cdc42-like GTPase Wnt responsive Cdc42 homolog 1 (Wrch1) has several atypical features; it has an N-terminal proline-rich extension that confers binding to SH3 domains, and it harbors an extremely high intrinsic nucleotide exchange activity, which overrides the normal GTPase activity. As a result, Wrch1 resides mainly in the active, GTP-loaded conformation under normal cellular conditions. We have previously shown that ectopic expression of Wrch1 in fibroblasts resulted in an altered cell morphology visible as a formation of filopodia, a loss of stress fibers, and a reduction in focal adhesions. Here, we show that Wrch1 binds to the nonreceptor tyrosine kinase Pyk2. The interaction required Wrch1 to be in a GTP conformation and also required an intact N-terminal proline-rich extension as well as an intact effector loop. Wrch1 requires Pyk2 in imposing the cytoskeletal effects, seen as the formation of filopodia, since treatment of cells with a Pyk2-specific small interfering RNA abrogated this response. Interestingly, we found that the presence and activity of Src were needed for the formation of a Wrch1-Pyk2 complex as well as for the Wrch1-induced formation of filopodia. We propose a model in which Pyk2 and Src function to coordinate the Wrch1-dependent effects on cytoskeletal dynamics.

Current knowledge of the large RhoGAP family of proteins

The Rho GTPases are implicated in almost every fundamental cellular process. They act as molecular switches that cycle between an active GTP-bound and an inactive GDP-bound state. Their slow intrinsic GTPase activity is greatly enhanced by RhoGAPs (Rho GTPase-activating proteins), thus causing their inactivation. To date, more than 70 RhoGAPs have been identified in eukaryotes, ranging from yeast to human, and based on sequence homology of their RhoGAP domain, we have grouped them into subfamilies. In the present Review, we discuss their regulation, biological functions and implication in human diseases.

START-GAP3/DLC3 is a GAP for RhoA and Cdc42 and is localized in focal adhesions regulating cell morphology

In the human genome there are three genes encoding RhoGAPs that contain the START (steroidogenic acute regulatory protein (StAR)-related lipid transfer)-domain. START-GAP3/DLC3 is a tumor suppressor gene similar to two other human START-GAPs known as DLC1 or DLC2. Although expression of START-GAP3/DLC3 inhibits the proliferation of cancer cells, its molecular function is not well understood. In this study we carried out biochemical characterization of START-GAP3/DLC3, and explored the effects of its expression on cell morphology and intracellular localization. We found that START-GAP3/DLC3 serves as a stimulator of PLCdelta1 and as a GAP for both RhoA and Cdc42 in vitro. Moreover, we found that the GAP activity is responsible for morphological changes. The intracellular localization of endogenous START-GAP3/DLC3 was explored by immunocytochemistry and was revealed in focal adhesions. These results indicate that START-GAP3/DLC3 has characteristics similar to other START-GAPs and the START-GAP family seems to share common characteristics.

The small GTPase RhoV is an essential regulator of neural crest induction in Xenopus

In vertebrates, the Rho family of GTPases is made of 20 members which regulate a variety of cellular functions, including actin cytoskeleton dynamics, cell adhesion and motility, cell growth and survival, gene transcription and membrane trafficking. To get a comprehensive view of Rho implication in physiological epithelial-mesenchymal transition, we carried out an in situ hybridization-based screen to identify Rho members expressed in Xenopus neural crest cells, in which we previously reported RhoB expression at the migrating stage. In the present study, we identify RhoV as an early expressed neural crest marker and provide evidence that its activity is essential for neural crest cell induction. RhoV mRNA is maternally expressed and accumulates shortly after gastrulation in the neural crest forming region. Using antisense morpholino injection, we show that at neurula stages, RhoV depletion impairs expression of the neural crest markers Sox9, Slug or Twist but has no effect on Snail induction. At the tailbud stage, RhoV knockdown causes a dramatic loss of cranial neural crest derived structures. All these defects are rescued by ectopic wild-type RhoV, whose overexpression on its own expands the neural crest territory. Our findings disclose an unprecedented Rho function in pathways that control neural crest cells specification.

Geldanamycin Anisimycins Activate Rho and Stimulate Rho- and ROCK-Dependent Actin Stress Fiber Formation

Heat shock protein 90 (Hsp90) is a member of the heat shock family of molecular chaperones that regulate protein conformation and activity. Hsp90 regulates multiple cell signaling pathways by controlling the abundance and activity of several important protein kinases and cell cycle-related proteins. In this report, we show that inhibition of Hsp90 by geldanamycin or its derivative, 17-allylamino-17-desmethoxygeldamycin, leads to activation of the Rho GTPase and a dramatic increase in actin stress fiber formation in human tumor cell lines. Inactivation of Rho prevents geldanamycin-induced actin reorganization. Hsp90 inactivation does not alter the appearance of filopodia or lamellipodia and tubulin architecture is not visibly perturbed. Our observations suggest that Hsp90 has an important and specific role in regulating Rho activity and Rho-dependent actin cytoskeleton remodeling.

Taking Rho GTPases to the next level: the cellular functions of atypical Rho GTPases

The Rho GTPases are influential regulators of signalling pathways that control vital cellular processes such as cytoskeletal dynamics, gene transcription, cell cycle progression and cell transformation. A vast majority of the studies involving Rho GTPases have been focused to the famous triad, Cdc42, Rac1 and RhoA, but this protein family actually harbours 20 members. Recently, the less known Rho GTPases have received increased attention. Many of the less studied Rho GTPases have structural, as well as, functional features which makes it pertinent to classify them as atypical Rho GTPases. This review article will focus on the critical aspects of the atypical Rho GTPases, RhoH, Wrch-1, Chp and RhoBTB. These proteins are involved in a broad spectre of biological processes, such as cytoskeletal dynamics, T-cell signalling and protein ubiquitinylation. We will also discuss the roles of atypical Rho GTPases as oncogenes or tumour suppressors, as well as their potential involvement in human diseases.

Autophosphorylation-dependent degradation of Pak1, triggered by the Rho-family GTPase, Chp

The Paks (p21-activated kinases) Pak1, Pak2 and Pak3 are among the most studied effectors of the Rho-family GTPases, Rac, Cdc42 (cell division cycle 42) and Chp (Cdc42 homologous protein). Pak kinases influence a variety of cellular functions, but the process of Pak down-regulation, following activation, is poorly understood. In the present study, we describe for the first time a negative-inhibitory loop generated by the small Rho-GTPases Cdc42 and Chp, resulting in Pak1 inhibition. Upon overexpression of Chp, we unexpectedly observed a T-cell migration phenotype consistent with Paks inhibition. In line with this observation, overexpression of either Chp or Cdc42 caused a marked reduction in the level of Pak1 protein in a number of different cell lines. Chp-induced degradation was accompanied by ubiquitination of Pak1, and was dependent on the proteasome. The susceptibility of Pak1 to Chp-induced degradation depended on its p21-binding domain, kinase activity and a number of Pak1 autophosphorylation sites, whereas the PIX- (Pak-interacting exchange factor) and Nck-binding sites were not required. Together, these results implicate Chp-induced kinase autophosphorylation in the degradation of Pak1. The N-terminal domain of Chp was found to be required for Chp-induced degradation, although not for Pak1 activation, suggesting that Chp provides a second function, distinct from kinase activation, to trigger Pak degradation. Collectively, our results demonstrate a novel mechanism of signal termination mediated by the Rho-family GTPases Chp and Cdc42, which results in ubiquitin-mediated degradation of one of their direct effectors, Pak1.

RhoC is essential for the metastasis of gastric cancer

Rho family members are known to regulate malignant transformation and motility of cancer cells, but the clinicopathological significance of RhoC remains unclear yet in the case of gastric cancer. In this study, we evaluated the protein expression level of RhoC in gastric cancer tissues and cell lines. Results showed that only weak staining of RhoC was detected in 3 of 33 non-tumorous cases by immunohistochemistry. The expression of RhoC was significantly higher in gastric cancer tissues (23/42, 54.8%) than in non-tumorous tissues (p < 0.01). Further analysis demonstrated that RhoC had high specificity (80.0%) in detecting gastric carcinomas with metastatic potential. RhoC was positively expressed in 18 out of 20 metastases (90.0%), even higher than that in primary gastric cancer tissues. Western blot showed that RhoC was up-regulated in five different gastric cancer cell lines but not expressed in SV40-transformed immortal gastric epithelial cell GES-1. Overexpression of RhoC GTPase in GES-1 cells could produce the motile and invasive phenotype but did not alter the monolayer growth rate. To further study the functions of RhoC, we took the powerful siRNA technology to knock down the expression of RhoC in SGC7901 cells. It was shown that down-regulation of RhoC did not affect the proliferation of SGC7901 cells. However, interference of RhoC expression could inhibit migration, invasion, and anchorage-independent growth of SGC7901 cells. In conclusion, RhoC may play a very important role in the metastasis of gastric carcinoma. Therapeutic strategies targeting RhoC and RhoC-mediated pathways may be a novel approach for treating metastasis of gastric cancer.

Molecular characterization of a novel RhoGAP, RRC-1 of the nematode Caenorhabditis elegans

The GTPase-activating proteins for Rho family GTPases (RhoGAP) transduce diverse intracellular signals by negatively regulating Rho family GTPase-mediated pathways. In this study, we have cloned and characterized a novel RhoGAP for Rac1 and Cdc42, termed RRC-1, from Caenorhabditis elegans. RRC-1 was highly homologous to mammalian p250GAP and promoted GTP hydrolysis of Rac1 and Cdc42 in cells. The rrc-1 mRNA was expressed in all life stages. Using an RRC-1::GFP fusion protein, we found that RRC-1 was localized to the coelomocytes, excretory cell, GLR cells, and uterine-seam cell in adult worms. These data contribute toward understanding the roles of Rho family GTPases in C. elegans.

Peakwide mapping on chromosome 3q13 identifies the kalirin gene as a novel candidate gene for coronary artery disease

A susceptibility locus for coronary artery disease (CAD) has been mapped to chromosome 3q13-21 in a linkage study of early-onset CAD. We completed an association-mapping study across the 1-LOD-unit-down supporting interval, using two independent white case-control data sets (CATHGEN, initial and validation) to evaluate association under the peak. Single-nucleotide polymorphisms (SNPs) evenly spaced at 100-kb intervals were screened in the initial data set (N=468). Promising SNPs (P<.1) were then examined in the validation data set (N=514). Significant findings (P<.05) in the combined initial and validation data sets were further evaluated in multiple independent data sets, including a family-based data set (N=2,954), an African American case-control data set (N=190), and an additional white control data set (N=255). The association between genotype and aortic atherosclerosis was examined in 145 human aortas. The peakwide survey found evidence of association in SNPs from multiple genes. The strongest associations were found in three SNPs from the kalirin (KALRN) gene, especially in patients with early-onset CAD (P=.00001-00028 in the combined CATHGEN data sets). In-depth investigation of the gene found that an intronic SNP, rs9289231, was associated with early-onset CAD in all white data sets examined (P<.05). In the joint analysis of all white early-onset CAD cases (N=332) and controls (N=546), rs9289231 was highly significant (P=.00008), with an odds-ratio estimate of 2.1. Furthermore, the risk allele of this SNP was associated with atherosclerosis burden (P=.03) in 145 human aortas. KALRN is a protein with many functions, including the inhibition of inducible nitric oxide synthase and guanine-exchange-factor activity. KALRN and two other associated genes identified in this study (CDGAP and MYLK) belong to the Rho GTPase-signaling pathway. Our data suggest the importance of the KALRN gene and the Rho GTPase-signaling pathway in the pathogenesis of CAD.

Evolution of the Rho family of ras-like GTPases in eukaryotes

GTPases of the Rho family are molecular switches that play important roles in converting and amplifying external signals into cellular effects. Originally demonstrated to control the dynamics of the F-actin cytoskeleton, Rho GTPases have been implicated in many basic cellular processes that influence cell proliferation, differentiation, motility, adhesion, survival, or secretion. To elucidate the evolutionary history of the Rho family, we have analyzed over 20 species covering major eukaryotic clades from unicellular organisms to mammals, including platypus and opossum, and have reconstructed the ontogeny and the chronology of emergence of the different subfamilies. Our data establish that the 20 mammalian Rho members are structured into 8 subfamilies, among which Rac is the founder of the whole family. Rho, Cdc42, RhoUV, and RhoBTB subfamilies appeared before Coelomates and RhoJQ, Cdc42 isoforms, RhoDF, and Rnd emerged in chordates. In vertebrates, gene duplications and retrotranspositions increased the size of each chordate Rho subfamily, whereas RhoH, the last subfamily, arose probably by horizontal gene transfer. Rac1b, a Rac1 isoform generated by alternative splicing, emerged in amniotes, and RhoD, only in therians. Analysis of Rho mRNA expression patterns in mouse tissues shows that recent subfamilies have tissue-specific and low-level expression that supports their implication only in narrow time windows or in differentiated metabolic functions. These findings give a comprehensive view of the evolutionary canvas of the Rho family and provide guides for future structure and evolution studies of other components of Rho signaling pathways, in particular regulators of the RhoGEF family.

A novel agent, methylophiopogonanone B, promotes Rho activation and tubulin depolymerization

Cytoskeletal reorganization, including reconstruction of actin fibers and microtubules, is essential for various biological processes, such as cell migration, proliferation and dendrite formation. We show here that methylophiopogonanone B (MOPB) induces cell morphological change via melanocyte dendrite retraction and stress fiber formation. Since members of the Rho family of small GTP-binding proteins act as master regulators of dendrite formation and actin cytoskeletal reorganization, and activated Rho promotes dendrite retraction and stress fiber formation, we studied the effects of MOPB on the small GTPases using normal human epidermal melanocytes and HeLa cells. In in vitro binding assay, MOPB significantly increased GTP-Rho, but not GTP-Rac or GTP-CDC42. Furthermore, a Rho inhibitor, a Rho kinase inhibitor and a small GTPase inhibitor each blocked MOPB-induced stress fiber formation. The effect of MOPB on actin reorganization was blocked in a Rho dominant negative mutant. These results suggest MOPB acts via the Rho signaling pathway, and it may directly or indirectly activate Rho. Quantitative Western blot analysis indicated that MOPB also induced microtubule destabilization and tubulin depolymerization. Thus, MOPB appears to induce Rho activation, resulting in actin cytoskeletal reorganization, including dendrite retraction and stress fiber formation.

Rac/Rho pathway regulates actin depolymerization induced by aminoglycoside antibiotics

Stress stimuli can lead to remodeling of the actin cytoskeleton and subsequent alteration of cell adhesion and permeation as well as cell functions and cell fate. We investigated redox-dependent Rho GTPase-linked pathways controlling the actin cytoskeleton in the inner ear of the CBA mouse, by using aminoglycoside antibiotics as a noxious stimulus that causes loss of sensory cells via the formation of reactive oxygen species. Kanamycin treatment in vivo interfered with the formation of F-actin, disturbed the arrangement of beta-actin in the stereocilia of outer hair cells, and altered the intermittent adherens junction/tight junction complexes between outer hair cells and supporting cells. The drug treatment also activated Rac1 and promoted the formation of the complex of Rac1 and p67phox while decreasing the activity of RhoA and reducing the formation of the RhoA/p140mDia complex. In inner-ear-derived cell lines, expression of mutated Rac1 changed the structural arrangement of F-actin and diminished the immunoreactivity of p140mDia. These findings suggest that actin depolymerization induced by kanamycin is mediated by Rac1 activation, followed by the formation of superoxide by NADPH oxidase. These changes will ultimately contribute to aminoglycoside-induced loss of hair cells.

Simvastatin Promotes Neurite Outgrowth in the Presence of Inhibitory Molecules Found in Central Nervous System Injury

Statins (3-hydroxy-3-methylglutaryl-CoA [HMG-CoA] reductase inhibitors) inhibit the rate-limiting step in the mevalonate pathway, conversion of HMG-CoA to mevalonate, by competitive inhibition with the enzyme HMG-CoA reductase. Statins not only lower cholesterol levels, but are also thought to exert neuroprotective and neurogenic effects that may be beneficial in treating brain and spinal cord injuries. Data presented here illustrate that simvastatin enables neurite outgrowth in the presence of growth-inhibitory molecules commonly found at central nervous system (CNS) injury sites. To assess the effect of simvastatin on neurite outgrowth in the presence of inhibitory molecules present at CNS injury sites, rat embryonic cortex explants or postnatal spinal cord explants were grown on membrane filters prepared with alternating stripes of laminin and myelin/laminin. Immunostaining indicated that myelin stripes contain myelin-associated glycoprotein (MAG), oligodendrocyte myelin glycoprotein (OMgp), and Nogo, but do not contain chondroitin sulfate proteoglycan (CSPG). When control explants were grown in the presence of alternating stripes, neurite outgrowth preferentially extended in regions containing laminin only. In contrast, neurite outgrowth from explants grown in the presence of simvastatin was significantly less selective for laminin regions and was able to extend into regions containing myelin (p < 0.01). Simvastatin-induced effects were reversed by addition of mevalonate. Isoprenyl transferase inhibitors GGTI-286 and FTI-277, inhibitors of biochemical steps subsequent to HMG-CoA conversion to mevalonate, mimicked simvastatin- induced effects. These data suggest that simvastatin counteracts myelin-associated neurite outgrowth inhibition signals via mevalonate pathway inhibition, and may be beneficial in promoting axon regeneration in brain and spinal cord injury.

Rac1 and Rac2 regulate macrophage morphology but are not essential for migration

Rac GTPases are believed to contribute to migration in leukocytes by transducing signals from cell surface receptors to the actin and microtubule cytoskeletons. Mammals have three closely related Rac isoforms, Rac1, Rac2 and Rac3, and it is widely assumed that cell migration requires the activity of these Rac GTPases. We have previously shown that Rac1-null mouse macrophages have altered cell shape and reduced membrane ruffling but normal migration speed. Here we investigate the behaviour of macrophages lacking Rac2 (Rac2(-/-)) or Rac1 and Rac2 (Rac1/2(-/-)). Rac2(-/-) macrophages have reduced F-actin levels and lack podosomes, which are integrin-based adhesion sites, and their migration speed is similar to or slightly slower than wild-type macrophages, depending on the substrate. Unexpectedly, Rac1/2(-/-) macrophages, which do not express Rac1, Rac2 or Rac3, migrate at a similar speed to wild-type macrophages on a variety of substrates and perform chemotaxis normally, although their morphology and mode of migration is altered. However, Rac1(-/-) and Rac1/2(-/-) but not Rac2(-/-) macrophages are impaired in their ability to invade through Matrigel. Together, these data show that Rac1 and Rac2 have distinct roles in regulating cell morphology, migration and invasion, but are not essential for macrophage migration or chemotaxis.

Coupling receptor tyrosine kinases to Rho GTPases--GEFs what's the link

Rho GTPases are molecular switches involved in the regulation of many cellular processes. This review summarizes work examining how stimulation of receptor tyrosine kinases (RTKs) leads to the activation of Rho guanine nucleotide exchange factors (GEFs) and their Rho GTPase substrates. The collective findings strongly suggest that RTK signaling to Rho proteins is a general signal transduction mechanism, like RTK mediated activation of phosphatidyl inositol 3-kinase, phospholipase Cgamma, and the mitogen activated protein kinase (MAPK) pathway. More than half of the 58 known human RTKs activate at least one Rho family member. Likewise, 16 Rho GEFs directly interact with and/or are phosphorylated by a RTK. The specificity of receptor tyrosine kinase/Rho GEF signaling seems to be somewhat promiscuous. There several cases where multiple RTKs activate the same Rho GEF and where a single RTK can activate multiple Rho GEFs. Expression analysis indicates that the average human tissue contains transcripts for 33 RTKs, 34 Rho GEFs, and 14 Rho GTPases with each tissue containing a unique complement of these proteins. Given the promiscuity of RTKs for Rho GEFs, Rho GEFs for Rho GTPases, and the large number of these proteins expressed in cells, a complex combinatorial network of proteins in these families may contribute to coding specific signals and cell responses from RTKs.

Function and regulation of Rnd proteins

The Rnd proteins, which form a distinct sub-group of the Rho family of small GTP-binding proteins, have been shown to regulate the organization of the actin cytoskeleton in several tissues. In the brain, they participate in neurite extension, whereas in smooth muscle, they modulate contractility. Recent evidence has shown that Rnd3 (RhoE) is also involved in the regulation of cell-cycle progression and transformation, indicating that these proteins might have other, as yet unexplored roles.

Centaureidin promotes dendrite retraction of melanocytes by activating Rho

Melanosomes synthesized within melanocytes are transferred to keratinocytes through dendrites, resulting in a constant supply of melanin to the epidermis, and this process determines skin pigmentation. During screening for inhibitors of melanosome transfer, we found a novel reagent, centaureidin, that induces significant morphological changes in normal human epidermal melanocytes and inhibits melanocyte dendrite elongation, resulting in a reduction of melanosome transfer in an in vitro melanocyte-keratinocyte co-culture system. Since members of the Rho family of small GTP-binding proteins act as master regulators of dendrite formation, and activated Rho promotes dendrite retraction, we studied the effects of centaureidin on the small GTPases. In in vitro binding assay, centaureidin activated Rho and furthermore, a Rho inhibitor (C. botulinum C3 exoenzyme), a Rho kinase inhibitor (Y27632) and a small GTPase inhibitor (Toxin B) blocked dendrite retraction induced by centaureidin. These results suggest centaureidin could act via the Rho signaling pathway, and it may directly or indirectly activate Rho. Thus, centaureidin appears to inhibit dendrite outgrowth from melanocytes by activating Rho, resulting in the inhibition of melanosome transfer from melanocytes to keratinocytes.

Evolution of developmental strategies in parasitic hymenoptera

Parasitoid wasps have evolved a wide spectrum of developmental interactions with hosts. In this review we synthesize and interpret results from the phylogenetic, ecological, physiological, and molecular literature to identify factors that have influenced the evolution of parasitoid developmental strategies. We first discuss the origins and radiation of the parasitoid lifestyle in the Hymenoptera. We then summarize how parasitoid developmental strategies are affected by ecological interactions and assess the inventory of physiological and molecular traits parasitoids use to successfully exploit hosts. Last, we discuss how certain parasitoid virulence genes have evolved and how these changes potentially affect parasitoid-host interactions. The combination of phylogenetic data with comparative and functional genomics offers new avenues for understanding the evolution of biological diversity in this group of insects.

Purification and biochemical properties of Rac1, 2, 3 and the splice variant Rac1b

Rac proteins (Rac1, 1b, 2, 3) belong to the GTP-binding proteins (or GTPases) of the Ras superfamily and thus act as molecular switches cycling between an active GTP-bound and an inactive GDP-bound form through nucleotide exchange and hydrolysis. Like most other GTPases, these proteins adopt different conformations depending on the bound nucleotide, the main differences lying in the conformation of two short and flexible loop structures designated as the switch I and switch II region. The three distinct mammalian Rac isoforms, Rac1, 2 and 3, share a very high sequence identity (up to 90%), with Rac1b being an alternative splice variant of Rac1 with a 19 amino acid insertion in vicinity to the switch II region. We have demonstrated that Rac1 and Rac3 are very closely related with respect to their biochemical properties, such as effector interaction, nucleotide binding, and hydrolysis. In contrast, Rac2 displays a slower nucleotide association and is more efficiently activated by the Rac-GEF Tiam1. Modeling and normal mode analysis corroborate the hypothesis that the altered molecular dynamics of Rac2, in particular at the switch I region, may be responsible for different biochemical properties. On the other hand, our structural and biochemical analysis of Rac1b has shown that, compared with Rac1, Rac1b has an accelerated GEF-independent GDP/GTP-exchange and an impaired GTP-hydrolysis, accounting for a self-activating GTPase. This chapter discusses the use of fluorescence spectroscopic methods, allowing real-time monitoring of the interaction of nucleotides, regulators, and effectors with the Rac proteins at submicromolar concentrations and quantification of the kinetic and equilibrium constants.

Cyclic AMP induces morphological changes of vascular smooth muscle cells by inhibiting a Rac-dependent signaling pathway

Cyclic AMP (cAMP) is a pleiotropic second messenger that regulates numerous cellular processes. In vascular smooth muscle cells (VSMCs), these include cell proliferation, migration, and contractility. Here we show that cAMP-elevating agents induce dramatic morphological changes in VSMCs, characterized by cell rounding and formation of long branching processes. The stellate morphology is associated with disassembly of actin stress fibers and lamellipodia, loss of focal adhesions, and the formation of small F-actin rings. Because of the importance of Rho family GTPases in regulating actin dynamics, we analyzed their individual roles in the cAMP phenotype. We found that pharmacological or genetic inhibition of Rac mimics cAMP effect in inducing a stellate morphology of VSMCs. Expression of activated Rac1 prevents forskolin-induced cAMP stellation, suggesting that cAMP affects cell morphology by inhibiting Rac function. Consistent with this, treatment with forskolin inhibits agonist-stimulated Rac activation in VSMCs. We further show that activated Rac1 containing the F37A effector loop substitution fails to rescue the cAMP phenotype. Our results suggest that cAMP modulates the morphology of VSMCs by inhibiting a Rac-dependent signaling pathway.

RhoA, RhoB, RhoC, Rac1, Cdc42, and Tc10 mRNA levels in spinal cord, sensory ganglia, and corticospinal tract neurons and long-lasting specific changes following spinal cord injury

Inhibition of RhoA has been shown to enhance axonal regeneration following spinal cord injury. Here we mapped mRNA expression patterns of RhoA, B, and C, Rac1, Cdc42, and Tc10 in spinal cord, sensory ganglia, and sensorimotor cortex in uninjured rats, and following spinal cord injury or sham laminectomy. In the intact spinal cord, neurons displayed high levels of Rac1, Cdc42, and Tc10 mRNA hybridization signal. GFAP-immunoreactive astrocytes expressed primarily RhoB and Rac1, while oligodendrocyte-like cells expressed RhoA, Rac1, and Cdc42. Injury caused profound, long-lasting upregulation of RhoA, Rac1, Cdc42, and Tc10 mRNA in the spinal cord, while RhoB was modestly increased and RhoC did not change. GFAP-immunoreactive reactive astrocytes exhibited a dramatic increase of RhoA mRNA expression along with increases of Rac1 and Cdc42. Injury also led to elevation of RhoA, Cdc42, and Tc10 in neurons and modest increases of RhoA, Rac1, and Tc10 in oligodendrocyte-like cells. Laminectomy caused similar, but less pronounced alterations of investigated mRNA species. In dorsal root ganglia neuronal RhoA, Rac1, Cdc42, and Tc10 mRNA levels were increased similarly by spinal cord injury and sham surgery. The CST pyramidal cells expressed Tc10 mRNA and the CST itself was Tc10-immunoreactive. Tc10-immunoreactivity disappeared distal to injury. We conclude that there are gene-specific patterns of expression of the six different Rho-GTPases in normal spinal cord and dorsal root ganglia, and that specific changes of temporal and spatial expression patterns occur in response to spinal cord injury, suggesting different roles of these GTPases in the cellular sequelae of CNS injury.

Regulation of membrane trafficking by a novel Cdc42-related protein in Caenorhabditis elegans epithelial cells

Rho GTPases are mainly known for their implication in cytoskeleton remodeling. They have also been recently shown to regulate various aspects of membrane trafficking. Here, we report the identification and the characterization of a novel Caenorhabditis elegans Cdc42-related protein, CRP-1, that shows atypical enzymatic characteristics in vitro. Expression in mouse fibroblasts revealed that, in contrast with CDC-42, CRP-1 was unable to reorganize the actin cytoskeleton and mainly localized to trans-Golgi network and recycling endosomes. This subcellular localization, as well as its expression profile restricted to a subset of epithelial-like cells in C. elegans, suggested a potential function for this protein in polarized membrane trafficking. Consistent with this hypothesis, alteration of CRP-1 expression affected the apical trafficking of CHE-14 in vulval and rectal epithelial cells and sphingolipids (C(6)-NBD-ceramide) uptake and/or trafficking in intestinal cells. However, it did not affect basolateral trafficking of myotactin in the pharynx and the targeting of IFB-2 and AJM-1, two cytosolic apical markers of intestine epithelial cells. Hence, our data demonstrate a function for CRP-1 in the regulation of membrane trafficking in a subset of cells with epithelial characteristics.

Structural features of hematopoiesis-specific RhoH/ARHH gene: high diversity of 5'-UTR in different hematopoietic lineages suggests a complex post-transcriptional regulation

The hematopoiesis-specific RhoH gene is thought to be deregulated in B-cell non-Hodgkin's lymphoma (B-NHL), by either a chromosomal translocation or mutations, which affect its 5' regulatory region. The encoded Rho protein, always GTP-bound in vivo, was hypothesized to behave as a Rac antagonist. Extensive expression analysis allowed the detection of RhoH transcripts in all hematopoietic lineages (lymphoid, erythroid, myeloid), with a high level in lymphoid cells. To initiate investigations on the molecular mechanisms that regulate RhoH gene expression, Race-PCR and primer extension were conducted in the B-cell line Raji, which allowed (i) the establishment of RhoH complex intron/exon organization and (ii) the detection of several transcription initiation sites. In addition, a high 5' end heterogeneity of RhoH mRNAs was observed, due to alternative splicing of some 5' exons and to the use of these different transcription start sites. RT-PCR analysis led to the identification of this 5' end heterogeneity in different hematopoietic lineages. Discrepancies were particularly observed between B and T cells, due to an alternative splicing of one 5' exon (1b), which might be an important element in RhoH gene regulation. Such specific features have never been described for any Rho family member gene. They provide a molecular basis to study complex mechanisms involved in the control of RhoH expression.

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.

PAK and other Rho-associated kinases--effectors with surprisingly diverse mechanisms of regulation

The Rho GTPases are a family of molecular switches that are critical regulators of signal transduction pathways in eukaryotic cells. They are known principally for their role in regulating the cytoskeleton, and do so by recruiting a variety of downstream effector proteins. Kinases form an important class of Rho effector, and part of the biological complexity brought about by switching on a single GTPase results from downstream phosphorylation cascades. Here we focus on our current understanding of the way in which different Rho-associated serine/threonine kinases, denoted PAK (p21-activated kinase), MLK (mixed-lineage kinase), ROK (Rho-kinase), MRCK (myotonin-related Cdc42-binding kinase), CRIK (citron kinase) and PKN (protein kinase novel), interact with and are regulated by their partner GTPases. All of these kinases have in common an ability to dimerize, and in most cases interact with a variety of other proteins that are important for their function. A diversity of known structures underpin the Rho GTPase-kinase interaction, but only in the case of PAK do we have a good molecular understanding of kinase regulation. The ability of Rho GTPases to co-ordinate spatial and temporal phosphorylation events explains in part their prominent role in eukaryotic cell biology.

A RhoGAP protein as a main immune suppressive factor in the Leptopilina boulardi (Hymenoptera, Figitidae)-Drosophila melanogaster interaction

To protect its eggs, the endoparasitoid wasp Leptopilina boulardi injects immune suppressive factors into Drosophila melanogaster host larvae. These factors are localized in the female long gland and reservoir. We analyzed the protein content of these tissues and found that it strongly differed between virulent and avirulent parasitoid strains. Four protein bands present in virulent long glands were eluted and their immune suppressive effect was assessed in vivo, allowing demonstrating a major effect of one of these proteins. The corresponding cDNA encodes a predicted 30 kDa subunit containing a Ras homologous GTPase Activating Protein (RhoGAP) domain, suggesting a possible involvement in the regulation of actin cytoskeleton changes. Using Western-blot experiments, we showed that this protein is abundant in virulent female long glands but is undetectable in virulent females deprived of long glands or in long glands from avirulent wasps. Its potential role in modifying the morphology and the adhesive properties of the host lamellocytes, involved in Drosophila cellular immune responses, is discussed.

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.

Critical and distinct roles of amino- and carboxyl-terminal sequences in regulation of the biological activity of the Chp atypical Rho GTPase

Chp (Cdc42 homologous protein) shares significant sequence and functional identity with the human Cdc42 small GTPase, and like Cdc42, promotes formation of filopodia and activates the p21-activated kinase serine/threonine kinase. However, unlike Cdc42, Chp contains unique amino- and carboxyl-terminal extensions. Here we determined whether Chp, like Cdc42, can promote growth transformation and evaluated the role of the amino- and carboxyl-terminal sequences in Chp function. Surprisingly, we found that a GTPase-deficient mutant of Chp exhibited low transforming activity but that deletion of the amino terminus of Chp greatly enhanced its transforming activity. Thus, the amino terminus may serve as a negative regulator of Chp function. The carboxyl terminus of Cdc42 contains a CAAX (where C is cysteine, A is aliphatic amino acid, X is terminal amino acid) tetrapeptide sequence that signals for the posttranslational modification critical for Cdc42 membrane association and biological function. Although Chp lacks aCAAXmotif, we found that Chp showed carboxyl terminus-dependent localization to the plasma membrane and to endosomes. Furthermore, an intact carboxyl terminus was required for Chp transforming activity. However, treatment with inhibitors of protein palmitoylation, but not prenylation, caused Chp to mislocalize to the cytoplasm. Thus, Chp depends on palmitoylation, rather than isoprenylation, for membrane association and function. In summary, Chp is implicated in cell transformation, and the unique amino and carboxyl termini of Chp represent atypical mechanisms of regulation of Rho GTPase function.

Reversal of the malignant phenotype of gastric cancer cells by inhibition of RhoA expression and activity

PURPOSE

The small GTPase RhoA has been implicated in the regulation of cell morphology, motility, and transformation, but the role of RhoA protein in the carcinogenesis of gastric cancer remains unclear. In the present study, we have analyzed the expression status of the RhoA protein in human gastric cancer cells and tissues and investigated the possible involvement of RhoA in regulating the malignant phenotype of gastric cancer cells.

EXPERIMENTAL DESIGN

RhoA expression was analyzed by immunohistochemistry and Western blot in gastric cancer tissues and cell lines. The RhoA-specific small interfering RNA (siRNA) vector was designed and constructed. We examined the role of RhoA in the malignant phenotype of gastric cancer cells by using siRNA knockdown and dominant-negative RhoA mutant suppression of endogenous RhoA activity.

RESULTS

RhoA was found frequently overexpressed in gastric cancer tissues and cells compared with normal tissues or gastric epithelial cells. RhoA-specific siRNA could specifically and stably reduce RhoA expression up to 90% in AGS cells. Both RhoA-specific siRNA and dominant-negative RhoA expressions could significantly inhibit the proliferation and tumorigenicity of AGS cells and enhance chemosensitivity of the cancer cells to Adriamycin and 5-fluorouracil.

CONCLUSION

RhoA may play a critical role in the carcinogenesis of gastric cancer, and the interference of RhoA expression and/or activity could provide a novel avenue in reversing the malignant phenotype of gastric cancer cells.

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.

Lipid raft proteins and their identification in T lymphocytes

This review focuses on how membrane lipid rafts have been detected and isolated, mostly from lymphocytes, and their associated proteins identified. These proteins include transmembrane antigens/receptors, GPI-anchored proteins, cytoskeletal proteins, Src-family protein kinases, G-proteins, and other proteins involved in signal transduction. To further understand the biology of lipid rafts, new methodological approaches are needed to help characterize the raft protein component, and changes that occur in this component as a result of cell perturbation. We describe the application of new proteomic approaches to the identification and quantification of raft proteins in T-lymphocytes. Similar approaches, applied to other model cell systems, will provide valuable new insights into both cellular signal transduction and lipid raft biology.

Rho-regulatory proteins in breast cancer cell motility and invasion

The importance of the Rho-GTPases in cancer progression, particularly in the area of metastasis, is becoming increasingly evident. This review will provide an overview of the role of the Rho-regulatory proteins in breast cancer metastatis.

GTPases and reactive oxygen species: switches for killing and signaling

In neutrophils and other phagocytic cells, the small GTPase Rac is an essential regulator of a multi-component NADPH oxidase that produces high levels of superoxide, which kills invading pathogens. In many other cell types, Rac and newly discovered relatives of the neutrophil burst oxidase and its subunits have been found associated with production of reactive oxygen species, implicating superoxide production in a wide range of cellular processes not related to host defense. Although the precise role played by Rac in the regulation of these novel oxidases is not known, Rac does control the cellular redox state. Through these pro-oxidant mechanisms, Rac and the novel oxidases modify gene expression, cell proliferation, adhesion and many cell-specific functions.

p68RacGAP Is a Novel GTPase-activating Protein That Interacts with Vascular Endothelial Zinc Finger-1 and Modulates Endothelial Cell Capillary Formation

The endothelium is required for maintenance of vascular integrity and homeostasis during vascular development and in adulthood. However, little is known about the coordinated interplay between transcription factors and signaling molecules that regulate endothelial cell-dependent transcriptional events. Vascular endothelial zinc finger-1 (Vezf1) is a zinc finger-containing transcription factor that is specifically expressed within the endothelium during vascular development. We have previously shown that Vezf1 potently activates transcription of the endothelin-1 promoter. We now report the identification of p68RacGAP, a novel Vezf1-interacting 68-kDa RhoGAP domain-containing protein. p68RacGAP mRNA is highly expressed in vascular endothelial cells by Northern blot analysis, and immunohistochemical staining of adult mouse tissues identified p68RacGAP in endothelial cells, vascular smooth muscle cells, and epithelial cells in vivo. Rac1 and Vezf1 both bind avidly to p68RacGAP, suggesting that p68RacGAP is not only a GTPase-activating protein for Rac1 but that p68RacGAP may also be part of the protein complex that binds to and modulates Vezf1 transcriptional activity. Functionally p68RacGAP specifically activates the GTPase activity of Rac1 in vivo but not Cdc42 or RhoA. In addition, p68RacGAP potently inhibits Vezf1/DB1-mediated transcriptional activation of the human endothelin-1 promoter and modulates endothelial cell capillary tube formation. Taken together, these data suggest that p68RacGAP is a multifunctional regulatory protein that has a Rac1-specific GTPase-activating activity, regulates transcriptional activity of the endothelin-1 promoter, and is involved in the signal transduction pathway that regulates endothelial cell capillary tube formation during angiogenesis.

Molecular Basis for Rho GTPase Signaling Specificity

There is now considerable evidence for the involvement of aberrant Rho GTPase activation in breast cancer development. Like Ras, Rho GTPases function as signaling nodes regulated by diverse extracellular stimuli. Rho GTPase activation is facilitated by multiple regulatory proteins, in particular guanine nucleotide exchange factors (GEFs) such as Dbl family proteins. Activated Rho GTPases in turn interact with and regulate a spectrum of functionally diverse downstream effectors, initiating a network of cytoplasmic and nuclear signaling cascades. Thus, Rho GTPases represent points of signaling convergence as well as relay switches that disseminate signaling divergence. In this review, we highlight issues relating to the structural basis by which Dbl family GEFs facilitate signaling convergence and Rho GTPase activation, and how Rho GTPases promote signal dissemination through downstream effectors.

RJLs: a new family of Ras-related GTP-binding proteins

The Ras superfamily of GTP binding proteins encompasses several gene families that regulate a plethora of events in the eukaryotic cell. Here we describe a novel branch of this superfamily which we have named RJLs. These are present in many unicellular organisms and also in deuterostomes but apparently missing in some intermediary phyla, suggesting an intriguing possibility of lateral gene transference between lower and higher eukaryotes. RJLs lack classical membrane targeting signals and the conserved glutamine residue that coordinates GTP hydrolysis in other proteins from the Ras superfamily. Interestingly, chordate orthologues are chimeras fused to "J" domains in their C-terminal, suggesting that these proteins recruit Hsc70 to specific sites in the cell. Expression analysis of RJLs from chordates suggests predominant expression in nervous tissues, possibly reflecting a role for RJLs in the development or maintenance of the sophisticated chordate nervous system.

Rho and Rac take center stage

Many features of cell behavior are regulated by Rho family GTPases, but the most profound effects of these proteins are on the actin cytoskeleton and it was these that first drew attention to this family of signaling proteins. Focusing on Rho and Rac, we will discuss how their effectors regulate the actin cytoskeleton. We will describe how the activity of Rho proteins is regulated downstream from growth factor receptors and cell adhesion molecules by guanine nucleotide exchange factors and GTPase activating proteins. Additionally, we will discuss how there is signaling crosstalk between family members and how various bacterial pathogens have developed strategies to manipulate Rho protein activity so as to enhance their own survival.

Comparative functional analysis of the Rac GTPases

Small GTPases of the Rho family including Rac, Rho and Cdc42 regulate different cellular processes like reorganization of the actin cytoskeleton by acting as molecular switches. The three distinct mammalian Rac proteins share very high sequence identity but how their specificity is achieved is hitherto unknown. Here we show that Rac1 and Rac3 are very closely related concerning their biochemical properties, such as effector interaction, nucleotide binding and hydrolysis. In contrast, Rac2 displays a slower nucleotide association and is more efficiently activated by the Rac-GEF Tiam1. Modeling and normal mode analysis support the idea that altered dynamics of Rac2 at the switch I region may be responsible for different biochemical properties. These results provide insight into the individual functionalities of the Rac isoforms.

A genome-wide survey of the genes for planar polarity signaling or convergent extension-related genes in Ciona intestinalis and phylogenetic comparisons of evolutionary conserved signaling components

Non-canonical Wnt signals similar to planar cell polarity (PCP) signaling in the fly control convergent extension (CE) of the dorsal mesoderm during gastrulation in vertebrates. Using the Ciona complete genome sequence and EST sequence data, we present here an initial and exhaustive search in non-vertebrate chordates, Ciona intestinalis for the family members as well as homologs or orthologs that are involved in PCP/CE signaling cascades. We clarified 7 cardinal gene families, including the MAPK, STE20 group kinase, Rho small GTPase, STAT, Glypican, Fz and Wnt gene families, as well as gene homologs or orthologs for known PCP/CE signaling components with their phylogenetic nature. As a result, we characterized 62 Ciona component genes. Among them, 59 genes were novel and functional genes which were supported by EST expressions and 15 genes belonged to PCP/CE component orthologs of other organisms or common ancestor genes. Moreover, from the phylogenetic point of view, we compared these components genome-widely with the PCP signaling components of fly and the CE signaling components of vertebrates. We then discovered not only that ascidians contain the basic ancestral signaling pathway components in chordates but also that several signaling components have not found in ascidian, indicating that ascidian CE pathway might have several gaps from vertebrate CE pathway. The present study provides an initial step for the subsequent analysis of CE in the non-vertebrate chordates, ascidians. In addition, this phylogenetic approach will help to facilitate understanding of the relationship between fly PCP signaling and the vertebrate CE pathway.

Effects of ethanol on protein kinase C alpha activity induced by association with Rho GTPases

Previous studies have shown that n-alkanols have biphasic chain length-dependent effects on protein kinase C (PKC) activity induced by association with membranes or with filamentous actin [Slater, S. J., et al. (1997) J. Biol. Chem. 272, 6167-6173; Slater, S. J., et al. (2001) Biochim. Biophys. Acta 1544, 207-216]. Recently, we showed that PKCalpha is also activated by a direct membrane lipid-independent interaction with Rho GTPases. Here, the effects of ethanol and 1-hexanol on Rho GTPase-induced activity were investigated using an in vitro assay system to provide further insight into the mechanism of the effects of n-alkanols on PKC activity. Both ethanol and 1-hexanol were found to have two competing concentration-dependent effects on the Ca(2+)- and phorbol ester- or diacylglycerol-dependent activities of PKCalpha associated with either RhoA or Cdc42, consisting of a potentiation at low alcohol levels and an attenuation of activity at higher levels. Measurements of the Ca(2+), phorbol ester, and diacylglycerol concentration-response curves for Cdc42-induced activation indicated that the activating effect corresponded to a shift in the midpoints of each of the curves to lower activator concentrations, while the attenuating effect corresponded to a decrease in the level of activity induced by maximal activator levels. The presence of ethanol enhanced the interaction of PKCalpha with Cdc42 within a concentration range corresponding to the potentiating effect, whereas the level of binding was unaffected by higher ethanol levels that were found to attenuate activity. Thus, ethanol may either enhance activation of PKCalpha by Rho GTPases by enhancing the interaction between the two proteins or attenuate the level of activity of Rho GTPase-associated PKCalpha by inhibiting the ensuing activating conformational change. The results also suggest that the effects of ethanol on Rho GTPase-induced activity may switch between an activation and inhibition depending on the concentration of Ca(2+) and other activators.

Membrane targeting by pleckstrin homology domains

Pleckstrin homology (PH) domains are small modular domains that occur once, or occasionally several times, in a large variety of signalling proteins. In a number of instances, PH domains act to target their host protein to the cytosolic face of cellular membranes through an ability to associate with phosphoinositides. In this review, we discuss recent advances in our understanding of PH domain function. In particular we describe the structural aspects of how PH domains have evolved to bind various phosphoinositides, how PH domains regulate phosphoinositide-mediated association to plasma and internals membranes, and finally raise the issue of PH domains in protein:protein interactions and the allosteric regulation of their host protein.

Slowed conduction and thin myelination of peripheral nerves associated with mutant rho Guanine-nucleotide exchange factor 10

Slowed nerve-conduction velocities (NCVs) are a biological endophenotype in the majority of the hereditary motor and sensory neuropathies (HMSN). Here, we identified a family with autosomal dominant segregation of slowed NCVs without the clinical phenotype of HMSN. Peripheral-nerve biopsy showed predominantly thinly myelinated axons. We identified a locus at 8p23 and a Thr109Ile mutation in ARHGEF10, encoding a guanine-nucleotide exchange factor (GEF) for the Rho family of GTPase proteins (RhoGTPases). Rho GEFs are implicated in neural morphogenesis and connectivity and regulate the activity of small RhoGTPases by catalyzing the exchange of bound GDP by GTP. Expression analysis of ARHGEF10, by use of its mouse orthologue Gef10, showed that it is highly expressed in the peripheral nervous system. Our data support a role for ARHGEF10 in developmental myelination of peripheral nerves.

Characterisation of a Rho homologue of Schistosoma mansoni

The development and survival of the helminth parasite, Schistosoma mansoni, is dependent on its ability to interpret signals from its environment. Currently, little is known about signal transduction in schistosomes. Rho is a member of a super-family of small GTP-binding proteins. Rho is involved in a number of cell signalling pathways with effects on actin cytoskeleton organisation, gene transcription, cell cycle progression, and membrane trafficking. We have cloned an S. mansoni protein (Rho1) that has 71-75% identity and approximately 85% similarity with human Rho A, B, and C proteins. We have optimised expression of recombinant S. mansoni Rho1 protein in Escherichia coli by co-expression with rare tRNAs. Western blot analysis results showed expression of Rho1 protein in adult worm stages especially female worms. In vitro prenylation of recombinant S. mansoni Rho1 determined that, similar to Rho from other organisms, Rho1 is geranynlgeranylated but not farnesylated. A search of the gene database indicates that Rho GTPases exist as a small family in S. mansoni including orthologues of Rho, Cdc42, and Rac. These data suggest that S. mansoni Rho1 plays a role in signalling in adult worms, especially females.

Critical role of the pleckstrin homology domain in Dbs signaling and growth regulation

Dbl family proteins act as guanine nucleotide exchange factors and positive regulators of Rho GTPase function by stimulating formation of the active, GTP-bound state. All Dbl family Rho guanine nucleotide exchange factors possess an invariant tandem domain structure consisting of a Dbl homology (DH) catalytic domain followed by a pleckstrin homology (PH) regulatory domain. We determined previously that the PH domain of Dbs was critical for the intrinsic catalytic activity of the DH domain in vitro and for Dbs transformation in vivo. In this study, we evaluated the role of phosphoinositide binding to the PH domain in regulating the DH domain function of Dbs in vitro and in vivo. We determined that mutation of basic amino acids located within the beta1-beta2 and beta3-beta4 loops of the PH domain resulted in impaired phospholipid binding in vitro, yet full guanine nucleotide exchange activity in vitro was retained for RhoA and Cdc42. Surprisingly, these mutants were compromised in their ability to activate Rho GTPases in vivo and to cause transformation of NIH 3T3 cells. However, Dbs subcellular localization was impaired by these PH domain mutations, supporting a role for phospholipid interactions in facilitating membrane association. Despite the importance of phospholipid binding for Dbs function in vivo, we found that Dbs signaling and transforming activity was not stimulated by phosphatidylinositol 3-kinase activation. We suggest that the PH domain of Dbs facilitates two distinct roles in the regulation of DH domain function, one critical for GTPase association and activation in vitro and one critical for phosphoinositide binding and GTPase interaction in vivo, that together promote Dbs association with membranes.

Regulation of lymphocyte-mediated killing by GTP-binding proteins

Exocytosis of granules containing apoptosis-inducing proteins is one mechanism of target cell killing by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Granules containing perforin and granzymes are redistributed to the area of cell contact initiated by specific interactions between surface ligands on a target cell and receptors on an effector lymphocyte. The formation of a stable conjugate between a cytotoxic lymphocyte and its potential target cell, followed by the directed delivery of granule components to the target cell are prerequisites of lymphocyte-mediated killing. Critical to understanding the development of cytotoxic function by CTLs and NK cells is the delineation of the second messenger pathways that specifically control the reorganization of the actin cytoskeleton during cell-mediated cytotoxicity. The low molecular weight guanosine 5'-triphosphate-binding proteins of the Rho family play a central role in these regulatory events controlling cytotoxic lymphocyte activation.

All roads lead to actin: the intimate relationship between TCR signaling and the cytoskeleton

Regardless of cell type, the regulation of the actin cytoskeleton is tightly linked to vital biological properties such as polarity, motility, cell-cell contact, exocytosis and proliferation. In the immune system, where rapid and efficient response to antigen-provoked stimuli is crucial, an overwhelming amount of data implicate the actin cytoskeleton and its regulators as central to immune function. Increasingly, the cytoskeleton is considered an essential amplification step in T cell receptor (TCR)-, costimulatory-, and integrin-mediated signaling. Advances in genetic manipulation and confocal imaging have led to a keener appreciation of the importance of TCR signal integration by the actin cytoskeleton. This review outlines recent advances in elucidating the regulation of T cell function through the actin cytoskeleton. We also examine intriguing parallels between the immune system and other models of cytoskeletal regulation.