DRG1 Maintains Intestinal Epithelial Cell Junctions and Barrier Function by Regulating RAC1 Activity in Necrotizing Enterocolitis

BACKGROUND

An immature intestine is a high-risk factor for necrotizing enterocolitis (NEC), which is a serious intestinal disease in newborns. The regulation of developmentally regulated GTP-binding protein 1 (DRG1) during organ development suggests a potential role of DRG1 in the maturation process of the intestine.

AIM

To illustrate the function of DRG1 during the pathogenesis of NEC.

METHODS

DRG1 expression in the intestine was measured using immunohistochemistry and q-PCR. Immunoprecipitation coupled with mass spectrometry was used to identify the interacting proteins of DRG1. The biological functions of the potential interactors were annotated with the Database for Annotation, Visualization and Integrated Discovery. Caco2 and FHs74Int cells with stable DRG1 silencing or overexpression were used to investigate the influence of DRG1 on cell junctions and intestinal barrier permeability and to elucidate the downstream mechanism.

RESULTS

DRG1 was constitutively expressed during the intestinal maturation process but significantly decreased in the ileum in the context of NEC. Protein interaction analysis revealed that DRG1 was closely correlated with cell junctions. DRG1 deficiency destabilized the E-cadherin and occludin proteins near the cell membrane and increased the permeability of the epithelial cell monolayer, while DRG1 overexpression prevented lipopolysaccharide-induced disruption of E-cadherin and occludin expression and cell monolayer integrity. Further investigation suggested that DRG1 maintained cell junctions, especially adherens junctions, by regulating RAC1 activity, and RAC1 inhibition with NSC23766 attenuated intestinal injury and led to improved barrier integrity in experimental NEC.

CONCLUSIONS

Our findings illustrate the mechanism underlying the effect of DRG1 deficiency on epithelial cell permeability regulation and provide evidence supporting the application of RAC1 inhibitors for protection against NEC.

An RNAi screen of Rho signalling networks identifies RhoH as a regulator of Rac1 in prostate cancer cell migration

BACKGROUND

Cell migration is essential for development and tissue repair, but it also contributes to disease. Rho GTPases regulate cell migration, but a comprehensive analysis of how each Rho signalling component affects migration has not been carried out.

RESULTS

Through an RNA interference screen, and using a prostate cancer cell line, we find that approximately 25% of Rho network components alter migration. Some genes enhance migration while others decrease basal and/or hepatocyte growth factor-stimulated migration. Surprisingly, we identify RhoH as a screen hit. RhoH expression is normally restricted to haematopoietic cells, but we find it is expressed in multiple epithelial cancer cell lines. High RhoH expression in samples from prostate cancer patients correlates with earlier relapse. RhoH depletion reduces cell speed and persistence and decreases migratory polarity. Rac1 activity normally localizes to the front of migrating cells at areas of dynamic membrane movement, but in RhoH-depleted cells active Rac1 is localised around the whole cell periphery and associated with membrane regions that are not extending or retracting. RhoH interacts with Rac1 and with several p21-activated kinases (PAKs), which are Rac effectors. Similar to RhoH depletion, PAK2 depletion increases cell spread area and reduces cell migration. In addition, RhoH depletion reduces lamellipodium extension induced by PAK2 overexpression.

CONCLUSIONS

We describe a novel role for RhoH in prostate cancer cell migration. We propose that RhoH promotes cell migration by coupling Rac1 activity and PAK2 to membrane protrusion. Our results also suggest that RhoH expression levels correlate with prostate cancer progression.

Aerobic exercise regulates Rho/cofilin pathways to rescue synaptic loss in aged rats

Purpose

The role of exercise to prevent or reverse aging-induced cognitive decline has been widely reported. This neuroprotection is associated with changes in the synaptic structure plasticity. However, the mechanisms of exercise-induced synaptic plasticity in the aging brain are still unclear. Thus, the aim of the present study is to investigate the aging-related alterations of Rho-GTPase and the modulatory influences of exercise training.

Methods

Young and old rats were used in this study. Old rats were subjected to different schedules of aerobic exercise (12 m/min, 60 min/d, 3d/w or 5d/w) or kept sedentary for 12 w. After 12 w of aerobic exercise, the synapse density in the cortex and hippocampus was detected with immunofluorescent staining using synaptophysin as a marker. The total protein levels of RhoA, Rac1, Cdc42 and cofilin in the cortex and hippocampus were detected with Western Blot. The activities of RhoA, Rac1 and Cdc42 were determined using a pull down assay.

Results

We found that synapse loss occurred in aging rats. However, the change of expression and activity of RhoA, Rac1 and Cdc42 was different in the cortex and hippocampus. In the cortex, the expression and activity of Rac1 and Cdc42 was greatly increased with aging, whereas there were no changes in the expression and activity of RhoA. In the hippocampus, the expression and activity of Rac1 and Cdc42 was greatly decreased and there were no changes in the expression and activity of RhoA. As a major downstream substrate of the Rho GTPase family, the increased expression of cofilin was only observed in the cortex. High frequency exercise ameliorated all aging-related changes in the cortex and hippocampus.

Conclusions

These data suggest that aerobic exercise reverses synapse loss in the cortex and hippocampus in aging rats, which might be related to the regulation of Rho GTPases.

8-Oxoguanine DNA glycosylase-1 links DNA repair to cellular signaling via the activation of the small GTPase Rac1

8-Oxo-7,8-dihydroguanine (8-oxoG) is one of the most abundant DNA base lesions induced by reactive oxygen species (ROS). Accumulation of 8-oxoG in the mammalian genome is considered a marker of oxidative stress, to be causally linked to inflammation, and is thought to contribute to aging processes and various aging-related diseases. Unexpectedly, mice that lack 8-oxoguanine DNA glycosylase-1 (OGG1) activity and accumulate 8-oxoG in their genome have a normal phenotype and longevity; in fact, they show increased resistance to both inflammation and oxidative stress. OGG1 excises and generates free 8-oxoG base during DNA base-excision repair (BER) processes. In the present study, we report that in the presence of the 8-oxoG base, OGG1 physically interacts with guanine nucleotide-free and GDP-bound Rac1 protein. This interaction results in rapid GDP→GTP, but not GTP→GDP, exchange in vitro. Importantly, a rise in the intracellular 8-oxoG base levels increases the proportion of GTP-bound Rac1. In turn Rac1-GTP mediates an increase in ROS levels via nuclear membrane-associated NADPH oxidase type 4. These results show a novel mechanism by which OGG1 in complex with 8-oxoG is linked to redox signaling and cellular responses.

Regulation of lymphatic-blood vessel separation by endothelial Rac1

Sprouting angiogenesis and lymphatic-blood vessel segregation both involve the migration of endothelial cells, but the precise migratory molecules that govern the decision of blood vascular endothelial cells to segregate into lymphatic vasculature are unknown. Here, we deleted endothelial Rac1 in mice (Tie1-Cre(+);Rac1(fl/fl)) and revealed, unexpectedly, that whereas blood vessel morphology appeared normal, lymphatic-blood vessel separation was impaired, with corresponding edema, haemorrhage and embryonic lethality. Importantly, normal levels of Rac1 were essential for directed endothelial cell migratory responses to lymphatic-inductive signals. Our studies identify Rac1 as a crucial part of the migratory machinery required for endothelial cells to separate and form lymphatic vasculature.

Liquid chromatography electrospray ionization and matrix-assisted laser desorption ionization tandem mass spectrometry for the analysis of lipid raft proteome of monocytes

Lipid rafts are dynamic assemblies of cholesterol and glycolipid that form detergent-insoluble microdomains within membrane lipid bilayers. Because rafts can be separated by flotation on sucrose gradients, interrogation by mass spectrometry (MS) provides a valuable new insight into lipid raft function. Here we combine liquid chromatography (LC) electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) MS/MS to corroborate and extend our previous description of lipid raft proteomes derived from the monocytic cell line THP-1. Interestingly, LC-ESI and MALDI MS/MS identify largely non-overlapping, and therefore, potentially complementary protein populations. Using the combined approach, we detected 277 proteins compared to 52 proteins obtained with the original gel-based MALDI MS. We confirmed the presence of 47 of the original 52 proteins demonstrating the consistency of the lipid raft preparations. We demonstrated by immunoblotting that Rac 1 and Rac 2, two of the 52 proteins we failed to confirm, were indeed absent from the lipid raft fractions. The majority of new proteins were cytoskeletal proteins and their regulators, proteins implicated in membrane fusion and vesicular trafficking or signaling molecules. Our results therefore, confirm and extend previous evidence indicating lipid rafts of monocytic cells are specialized for cytoskeletal assembly and vesicle trafficking. Of particular interest, we detected SNAP-23, basigin, Glut-4 and pantophysin in lipid rafts. Since these proteins are implicated in both vesicular trafficking and gamete fusion, lipid rafts may play a common role in these processes. It is evident that the combination of LC-ESI and LC-MALDI MS/MS increases the proteome coverage which allows better understanding of the lipid raft function.

Right-ventricular failure is associated with increased mitochondrial complex II activity and production of reactive oxygen species

OBJECTIVE

Reactive oxygen species (ROS) have been implicated in the progression of ventricular hypertrophy to congestive heart failure. However, the source of increased oxidative stress in cardiomyocytes remains unclear.

METHODS

Here we examined NADPH oxidase and mitochondria as sources of ventricular ROS production in a rat model of right-ventricular (RV) failure (CHF) induced by pulmonary arterial hypertension (PAH).

RESULTS

Western analysis showed increased expression of the catalytic subunit gp91(phox) of NADPH oxidase as well as its activator Rac1 in RV in CHF compared to non-failing myocardium (CON). In addition, analysis of mitochondrial respiratory chain complexes showed a selective increase in the expression of Complex II subunit B. Using lucigenin chemiluminescence, tissue homogenates showed increased NADPH oxidase and Complex II-dependent ROS production in failing RV, with no increase in the left ventricle. Functional analyses of isolated RV mitochondria showed an increase in Complex II activity as well as Complex II-associated ROS production in CHF vs CON. An increase in the reduction state of the mitochondrial Coenzyme Q in failing RV, together with increased expression of hypoxia-inducible factor 1 alpha, indicated conditions in CHF that strongly favor ROS production by mitochondria. Reduced ROS-scavenging capacity was indicated by decreased mRNA levels of superoxide dismutases. Oxidative stress in failing RV was indicated by a two-fold increase in the level of phospho-p38 mitogen-activated protein kinase and by immunohistochemical evidence of extensive protein nitration.

CONCLUSIONS

These data show that the development of PAH-induced RV heart failure is associated with an increased capacity for ROS production by NADPH oxidase as well as mitochondria. The selective increase in expression and activity of mitochondrial Complex II may be particularly important for ventricular ROS production in heart failure.

Prognostic relevance of increased Rac GTPase expression in prostate carcinomas

Rac proteins of the Rho-like GTPase family, including the ubiquitous Rac1, the hematopoiesis-specific Rac2, and the least-characterized Rac3 play a major role in oncogenic transformation, tumor invasion and metastasis. However, the prognostic relevance of Rac expression in human tumors has not been investigated yet. In the present study, Rac protein expression was analyzed in benign secretory epithelium, high-grade prostatic intraepithelium neoplasia (HG-PIN), and prostate carcinomas of 60 R0-resected radical prostatectomy specimens by semiquantitative immunohistochemistry. Thus, Rac proteins were significantly strongly expressed in HG-PIN (P < 0.001) and prostate carcinomas (P < 0.001) when compared with benign secretory epithelium. Accordingly, all tumor tissues analyzed by isoform-specific real-time PCR (n = 7) exhibited significantly higher RNA expression levels of Rac (i.e. sum of Rac1 and Rac3 expression levels) than the respective benign counterparts (P = 0.018) and this appeared to result mainly from increased expression of the Rac3 isoform as verified by immunoblotting. Univariate analyses showed statistically significant associations of increased Rac protein expression in prostate cancer (P = 0.045), preoperative prostate-specific antigen levels (P = 0.044), pT stage (P = 0.002), and Gleason score (P = 0.001) with decreased disease-free survival (DFS). This prognostic effect of increased protein expression of Rac remained significant even in a multivariate analysis including all these four factors (relative risk = 3.22, 95% confidence interval = 1.04-10.00; P = 0.043). In conclusion, our data suggest that increased Rac protein expression in prostate cancer relative to the corresponding benign secretory epithelium is an independent predictor of decreased DFS and appears to result mainly from increased expression of the Rac3 isoform.

Rac1 and Rac3 have opposing functions in cell adhesion and differentiation of neuronal cells

Rac1 and Rac3 are highly homologous members of the Rho small GTPase family. Rac1 is ubiquitously expressed and regulates cell adhesion, migration and differentiation in various cell types. Rac3 is primarily expressed in brain and may therefore have a specific function in neuronal cells. We found that depletion of Rac1 by short interference RNA leads to decreased cell-matrix adhesions and cell rounding in neuronal N1E-115 cells. By contrast, depletion of Rac3 induces stronger cell adhesions and dramatically increases the outgrowth of neurite-like protrusions, suggesting opposite functions for Rac1 and Rac3 in neuronal cells. Consistent with this, overexpression of Rac1 induces cell spreading, whereas overexpression of Rac3 results in a contractile round morphology. Rac1 is mainly found at the plasma membrane, whereas Rac3 is predominantly localized in the perinuclear region. Residues 185-187, present in the variable polybasic rich region at the carboxyl terminus are responsible for the difference in phenotype induced by Rac1 and Rac3 as well as for their different intracellular localization. The Rac1-opposing function of Rac3 is not mediated by or dependent on components of the RhoA signaling pathway. It rather seems that Rac3 exerts its function through negatively affecting integrin-mediated cell-matrix adhesions. Together, our data reveal that Rac3 opposes Rac1 in the regulation of cell adhesion and differentiation of neuronal cells.

Functional analysis of the YopE GTPase-activating protein (GAP) activity of Yersinia pseudotuberculosis

YopE of Yersinia pseudotuberculosis inactivates three members of the small RhoGTPase family (RhoA, Rac1 and Cdc42) in vitro and mutation of a critical arginine abolishes both in vitro GTPase-activating protein (GAP) activity and cytotoxicity towards HeLa cells, and renders the pathogen avirulent in a mouse model. To understand the functional role of YopE, in vivo studies of the GAP activity in infected eukaryotic cells were conducted. Wild-type YopE inactivated Rac1 as early as 5 min after infection whereas RhoA was down regulated about 30 min after infection. No effect of YopE was found on the activation state of Cdc42 in Yersinia-infected cells. Single-amino-acid substitution mutants of YopE revealed two different phenotypes: (i) mutants with significantly lowered in vivo GAP activity towards RhoA and Rac1 displaying full virulence in mice, and (ii) avirulent mutants with wild-type in vivo GAP activity towards RhoA and Rac1. Our results show that Cdc42 is not an in vivo target for YopE and that YopE interacts preferentially with Rac1, and to a lesser extent with RhoA, during in vivo conditions. Surprisingly, we present results suggesting that these interactions are not a prerequisite to establish infection in mice. Finally, we show that avirulent yopE mutants translocate YopE in about sixfold higher amount compared with wild type. This raises the question whether YopE's primary function is to sense the level of translocation rather than being directly involved in downregulation of the host defence.

RhoC GTPase is required for PC-3 prostate cancer cell invasion but not motility

It is projected that in 2005, approximately 220 900 men will be newly diagnosed with carcinoma of the prostate (CaP). Men who are diagnosed with locally advanced or metastatic disease undergo androgen ablation therapy and most will relapse and progress within 18 months. Metastasis to bone is the major clinical concern during CaP progression, as it is associated with intractable pain, bone fracture and paralysis resulting from spinal cord compression. Therefore, an understanding of the key mechanisms involved in CaP cell bone metastasis is vital to development of novel treatments. The Rho GTPases are molecular switches involved in cell survival, motility and invasion. Increased expression of RhoC GTPase is linked to enhanced metastatic potential in multiple cancers; however, the role of RhoC GTPase in CaP metastasis has not been addressed. In the current study, we demonstrate that RhoC GTPase is expressed and active in PC-3 CaP cells. RhoC inhibition, either pharmacologically with C3 exotransferase or molecularly through expression of a dominant-negative RhoC, promotes IGF-I stimulated random motility but decreases in vitro invasion and experimental metastases. Inhibition of RhoC activity results in drastic morphologic changes and alterations in the expression and distribution of focal adhesion-related proteins. These data suggest that RhoC inhibition leads to activation of other GTPases involved in nondirected motility and that expression of active RhoC is required for the invasive phenotype of PC-3 cells.

Protein kinase Czeta mediates insulin-induced glucose transport through actin remodeling in L6 muscle cells

Protein kinase C (PKC) zeta has been implicated in insulin-induced glucose uptake in skeletal muscle cell, although the underlying mechanism remains unknown. In this study, we investigated the effect of PKCzeta on actin remodeling and glucose transport in differentiated rat L6 muscle cells expressing myc-tagged glucose transporter 4 (GLUT4). On insulin stimulation, PKCzeta translocated from low-density microsomes to plasma membrane accompanied by increase in GLUT4 translocation and glucose uptake. Z-scan confocal microscopy revealed a spatial colocalization of relocated PKCzeta with the small GTPase Rac-1, actin, and GLUT4 after insulin stimulation. The insulin-mediated colocalization, PKCzeta distribution, GLUT4 translocation, and glucose uptake were inhibited by wortmannin and cell-permeable PKCzeta pseudosubstrate peptide. In stable transfected cells, overexpression of PKCzeta caused an insulin-like effect on actin remodeling accompanied by a 2.1-fold increase in GLUT4 translocation and 1.7-fold increase in glucose uptake in the absence of insulin. The effects of PKCzeta overexpression were abolished by cell-permeable PKCzeta pseudosubstrate peptide, but not wortmannin. Transient transfection of constitutively active Rac-1 recruited PKCzeta to new structures resembling actin remodeling, whereas dominant negative Rac-1 prevented the insulin-mediated PKCzeta translocation. Together, these results suggest that PKCzeta mediates insulin effect on glucose transport through actin remodeling in muscle cells.

Brk activates rac1 and promotes cell migration and invasion by phosphorylating paxillin

Brk (for breast tumor kinase) is a nonreceptor tyrosine kinase containing SH3, SH2, and tyrosine kinase catalytic domains. Brk was originally identified from a human metastatic breast tumor, and its overexpression is frequently observed in breast cancer and several other cancer types. However, the molecular mechanism by which this kinase participates in tumorigenesis remains poorly characterized. In the present study, we not only identified paxillin as the binding partner and substrate of Brk but also discovered a novel signaling pathway by which Brk mediates epidermal growth factor (EGF)-induced paxillin phosphorylation. We show that EGF stimulation activates the catalytic activity of Brk, which in turn phosphorylates paxillin at Y31 and Y118. These phosphorylation events promote the activation of small GTPase Rac1 via the function of CrkII. Through this pathway, Brk is capable of promoting cell motility and invasion and functions as a mediator of EGF-induced migration and invasion. In accordance with these functional roles, Brk translocates to membrane ruffles, where it colocalizes with paxillin during cell migration. Together, our findings identify novel signaling and biological roles of Brk and indicate the first potential link between Brk and metastatic malignancy.

Cdc42, Rac1, and Rac2 display distinct patterns of activation during phagocytosis

The small G proteins Cdc42, Rac1, and Rac2 regulate the rearrangements of actin and membrane necessary for Fcgamma receptor-mediated phagocytosis by macrophages. Activated, GTP-bound Cdc42, Rac1, and Rac2 bind to the p21-binding domain (PBD) of PAK1, and this interaction provided a basis for microscopic methods to localize activation of these G proteins inside cells. Fluorescence resonance energy transfer-based stoichiometry of fluorescent chimeras of actin, PBD, Cdc42, Rac1, and Rac2 was used to quantify G protein activation relative to actin movements during phagocytosis of IgG-opsonized erythrocytes. The activation dynamics of endogenous G proteins, localized using yellow fluorescent protein-labeled PBD, was restricted to phagocytic cups, with a prominent spike of activation over an actin-poor region at the base of the cup. Refinements of fluorescence resonance energy transfer stoichiometry allowed calculation of the fractions of activated GTPases in forming phagosomes. Cdc42 activation was restricted to the leading margin of the cell, whereas Rac1 was active throughout the phagocytic cup. During phagosome closure, activation of Rac1 and Rac2 increased uniformly and transiently in the actin-poor region of phagosomal membrane. These distinct roles for Cdc42, Rac1, and Rac2 in the component activities of phagocytosis indicate mechanisms by which their differential regulation coordinates rearrangements of actin and membranes.

Morphological adjustment of senescent cells by modulating caveolin-1 status

Morphological change is one of the cardinal features of the senescent phenotype; for example, senescent human diploid cells have a flat large shape. However, the mechanisms underlying such senescence-related morphological alterations have not been well studied. To investigate this situation, we characterized the senescence-dependent changes of cellular structural determinants in terms of their levels and activities. These determinants included integrins, focal adhesion complexes, and small Rho GTPases, and special emphasis was placed on their relationships with caveolin-1 status. We observed that the expression integrin beta(1) and focal adhesion kinase (FAK) were increased and that the phosphorylations of FAK and paxillin, hallmarks of focal adhesion formation, were also increased in senescent human diploid fibroblast cells. Moreover, the Rho GTPases Rac1 and Cdc42 were found to be highly activated in senescent cells. In addition, focal adhesion complexes and Rho GTPases were up-regulated in the caveolin-rich membrane domain in the senescent cells. Activated Rac1 and Cdc42 directly interacted with caveolin-1 in senescent cells. Interestingly, caveolin-1 knock-out senescent cells, achieved by using small interfering RNA and antisense oligonucleotide, showed disrupted focal adhesion formation and actin stress fibers via the inactivation of FAK, which resulted in morphological adjustment to the young cell-like small spindle shape. Based on the results obtained, we propose that caveolin-1 plays an important role in senescence-associated morphological changes by regulating focal adhesion kinase activity and actin stress fiber formation in the senescent cells.

An activating mutant of Rac1 that fails to interact with Rho GDP-dissociation inhibitor stimulates membrane ruffling in mammalian cells

Rac1, a member of the Rho family of small GTP-binding proteins, is involved in the regulation of the actin cytoskeleton via activation of lamellipodia and membrane ruffle formation. RhoGDI (Rho-family-specific GDP-dissociation inhibitor) forms a complex with Rho proteins in the cytosol of mammalian cells. It not only regulates guanine nucleotide binding to Rho proteins, but may also function as a molecular shuttle to carry Rho proteins from an inactive cytosolic pool to the membrane for activation. These studies tested if RhoGDI is necessary for the translocation of Rac1 from the cytosol to the plasma membrane for the formation of membrane ruffles. We describe a novel mutant of Rac1, R66E (Arg66-->Glu), that fails to bind RhoGDI. This RhoGDI-binding-defective mutation is combined with a Rac1-activating mutation G12V, resulting in a double-mutant [Rac1(G12V/R66E)] that fails to interact with RhoGDI in COS-7 cells, but remains constitutively activated. This double mutant stimulates membrane ruffling to a similar extent as that observed after epidermal growth factor treatment of non-transfected cells. To confirm that Rac1 can signal ruffle formation in the absence of interaction with RhoGDI, Rac1(G12V) was overexpressed in cultured mesangial cells derived from a RhoGDI knockout mouse. Rac1-mediated membrane ruffling was indistinguishable between the RhoGDI(-/-) and RhoGDI(+/+) cell lines. In both the COS-7 and cultured mesangial cells, Rac1(G12V) and Rac1(G12V/R66E) co-localize with membrane ruffles. These findings suggest that interaction with RhoGDI is not essential in the mechanism by which Rac1 translocates to the plasma membrane to stimulate ruffle formation.

Rac2 specificity in macrophage integrin signaling: potential role for Syk kinase

Herein we report that, despite the similarity of Rac2 to Rac1 (92% amino acid identity), macrophages derived from Rac2-/- mice, which continue to express Rac1, display a marked defect in alphavbeta3/alphavbeta5 and alpha4beta1 integrin-directed migration measured on vitronectin and fibronectin fragments (FN-H296), respectively. In contrast, mouse embryo fibroblasts derived from the Rac2 knockout mice utilize Rac1 for migration via alphavbeta3/alphavbeta5 and alpha4beta1. The genetic reconstitution of bone marrow-derived macrophages (BMM) with Rac2 restores the integrin-dependent migration of Rac2-deficient macrophages on vitronectin (VN) and FN-H296. The levels of GTP-Rac2 generated upon specific integrin engagement in wild type macrophages parallels the phenotypic defect observed in Rac2-deficient macrophages; i.e. FN-H296, alpha4beta1 > VN, alphavbeta3/alphavbeta5 > FN-CH271, alpha5beta1 > intact FN. In a COS7 cell system, the expression of Syk kinase alone is sufficient to convert the alpha4beta1 migration response to Rac2 dependence. Therefore, we present the first evidence that the alpha4beta1 receptor in blood cells has evolved a Syk-Rac2 signaling axis to transmit signals required for integrin-directed migration suggesting that Syk kinase in part encodes myeloid Rac2 specificity in vivo.

Structural basis for the signaling specificity of RhoG and Rac1 GTPases

RhoG is a new GTPase that has high sequence similarity with members of the Rac subfamily (Rac1, Rac2, and Rac3), including the regions involved in effector recognition and binding. To characterize its biological properties, we have compared the activity of RhoG and Rac1 in a number of experimental systems, including the study of their subcellular localization, oncogenic potential, activation of effectors, and effect on F-actin dynamics. Our study indicates that RhoG and Rac1 share overlapping, but not identical, signal transduction pathways. In contrast to previous results, we also provide evidence that RhoG works in parallel to Rac1 rather than as a Rac1 upstream activator. Using an extensive collection of Rho/Rac1 chimeras and point mutants, we demonstrate that the different biological properties of RhoG and Rac1 can be traced to specific amino acid variations in their switch I, beta2/beta3 hairpin, alpha5 helix, and C-terminal polybasic regions. Taken collectively, our results highlight the complexity of the signal transduction pathways activated by Rho/Rac GTPases and provide insight into the structural determinants that mediate the differential engagement of biological responses by GTPases of very similar structure.

Proteomic analysis of Rac1 transgenic mice displaying dilated cardiomyopathy reveals an increase in creatine kinase M-chain protein abundance

Here, we demonstrate the application of the proteomic approach to the study of a transgenic mouse model of heart failure and provide an example of a disease-associated protein alteration that can be observed using this approach. Specifically, we applied the proteomic approach to the analysis of a mouse model of dilated cardiomyopathy in which the small GTPase, Rac1, was constitutively expressed specifically in the myocardium. We utilized the methods of two-dimensional gel electrophoresis (2-DE) for protein separation, silver-staining for protein visualization and mass spectrometry (MALDI-TOF and MS/MS) for protein spot identification. Computer-generated composite images were created which represent a normalized average of four 2-DE gel images derived from analysis of either Rac1 transgenic (n = 4) or non-transgenic (n = 4) mice. Analysis of composite images derived from NTG and Rac1 experimental groups revealed numerous statistically significant differences in mean protein spot intensities. Here, we report a statistically significant increase, of approximately 1.6-fold, in the mean protein spot intensity for creatine kinase M-chain in the composite image of Rac1 transgenic mice compared to control. This protein alteration may be consistent with an end-stage heart failure phenotype in which maximal myocardial reserve is employed to sustain survival.

[Rac subfamily expression and activity in gastrointestinal cancer cell lines]

OBJECTIVE

To investigate the significance of Rac subfamily members in the gastrointestinal carcinogenesis and progression.

METHODS

The mRNA expression of Rac1, Rac2 and Rac3 in 12 kinds of gastrointestinal cancer cell lines was examined by semi-quantitative RT-PCR. The activities of Rac1 protein in 5 kinds of gastric cancer cell lines were tested by pull-down assay.

RESULTS

Compared with the normal gastric mucosa and intestinal epithelial cell line, the mRNA expression of Rac1 and Rac3 was up-regulated in most of gastrointestinal cancer cell lines. The activities of Rac1 protein increased markedly in gastric cancer cell lines.

CONCLUSION

The increased mRNA expression of Rac1 and Rac3 in gastrointestinal cancer cell lines and the abnormal activation of Rac1 protein in gastric cancer cell lines might be correlated with the carcinogenesis of gastrointestinal cancer.

Involvement of Wiskott-Aldrich syndrome protein family verprolin-homologous protein (WAVE) and Rac1 in the phagocytosis of amyloid-beta(1-42) in rat microglia

Alzheimer's disease (AD) is characterized by the accumulation of extracellular amyloid-beta (A beta) fibrils with microglia. Recently, there has been great interest in the microglial phagocytosis of A beta, because the microglial pathway is considered to be one of the A beta clearance pathways in the brain parenchyma. However, the mechanism of microglial phagocytosis of A beta is not fully understood and, thus, was investigated in this study. At one minute after exposure to A beta(1-42) (A beta 42), A beta immunoreactivity was detected at the cell surface of microglia. After 1 h, marked immunoreactivity was observed in the cytosolic vesicles. At 12 h, delayed phagocytosis of fibrillar A beta 42 was also observed with the formation of a large phagocytic cup. The microglial cell shape rapidly changed to an ameboid form during the process of phagocytosis. Although neither neural Wiskott-Aldrich syndrome protein (N-WASP) nor WASP interacting SH3 protein (WISH) immunoreactivity was co-localized with filamentous actin (F-actin) distribution, both WASP family verprolin-homologous protein (WAVE) and Rac1 immunoreactivity was co-localized with F-actin in the lamellipodia of phogocytic microglia. These results suggest that WAVE and Rac1 participate in the phagocytosis of A beta 42 by microglia.

Distribution of Rho family GTPases in the adult rat hippocampus and cerebellum

Small GTPases are monomeric guanine nucleotide binding proteins of 20-25 kDa mass. Rho GTPases belong to the Ras superfamily of small GTPases. The small GTPases of the Rho family have been shown to participate in the organisation of the actin cytoskeleton and signal transduction pathways leading to gene transcription. Recent evidence suggests that Rho family GTPases may play an important role in synaptic communication in the brain, and particularly in synatic plasticity. In this study the distribution of RhoA, RhoB, RhoG, Cdc42, and Rac1 was investigated in hippocampal and cerebellar tissue of adult rat brain using immunohistochemical techniques. Previous studies suggest that distribution of Rho family mRNA is uniform throughout these structures. Here we provide evidence for differences in expression of these proteins between different regions of the hippocampus, and between the molecular and granular layers in the cerebellum. These differences may prove important with regard to the physiological functions of Rho family GTPases.

Effect of Rho family GTP-binding proteins on Amoeba proteus

While there is a number of studies on the effects of Rho GTPases on the actin-based cytoskeleton in higher eukaryotes, studies in protozoans are rather limited. The problem seems to be intriguing since the structure of protozoan cytoskeletons is distinct from most vertebrate cells. By blocking endogenous Rho family proteins of highly motile Amoeba proteus with C3 transferase and antibodies against human RhoA and Rac1, we tried to assess the in vivo role of these proteins in amoebae. In migrating amoebae, both proteins are concentrated in the cortical layer and seem to colocalize with filamentous actin. Endogenous Rac1, but not RhoA, is accumulated in the perinuclear cytoskeleton. Blocking Rac- or Rho-like proteins caused distinct and irreversible changes in the locomotive shape of the examined amoebae and significant inhibition of their migration. Amoebae microinjected with anti-Rac1 antibodies were contracted, shortened, and developed only few wide pseudopodia. More pronounced changes were observed in cells treated with anti-RhoA antibodies. They exhibited an atypical locomotion not leading to their effective displacement. After treatment with 50 microg of C3 transferase per ml, cells rapidly contracted and almost completely rounded up, became refractile with the granules beaten into a dense mass, detached from the surface and died. Ten times lower concentration of the enzyme caused similar changes as the inhibition of endogenous RhoA-like protein. These results indicate that Rho family-based regulation plays a key role in amoebic migration.

The expression of rho proteins decreases with human brain tumor progression: potential tumor markers

Astrocytic tumors are the most common human brain tumors. Establishment of tumor grade is a key determinant both in the choice of a therapeutic approach and in the prognosis. The diagnosis of astrocytic tumors is currently determined following histopathological analysis. The identification of molecular markers would offer a complementary tool for characterizing tumors with respect to their clinical behavior. In this study we determined the expression levels of 3 small GTP binding proteins (RhoA, RhoB and Rac1), of their inhibitor RhoGDI and of caveolin-1 in 24 human astrocytic tumors of grades I to IV. Our results demonstrated that the expression of RhoA and RhoB decreased significantly in all brain tumors studied and was inversely related with tumor of grade II to IV malignancy. The amount of caveolin-1 immunodetected was not significantly different from normal brain samples while the Rac1 expression level was diminished in astrocytic tumors of grades III and IV. Our finding that RhoA and RhoB expression levels are correlated to tumor malignancy suggests that they may serve as novel and efficient diagnostic markers for astrocytic brain tumors of histological grade II to IV and complement currently applied histopathological analysis.

Recruitment and activation of Rac1 by the formation of E-cadherin-mediated cell-cell adhesion sites

Rac1, a member of the Ρ family small GTPases, regulates E-cadherin-mediated cell-cell adhesion. However, it remains to be clarified how the localization and activation of Rac1 are regulated at sites of cell-cell contact. Here, using enhanced green fluorescence protein (EGFP)-tagged Rac1, we demonstrate that EGFP-Rac1 is colocalized with E-cadherin at sites of cell-cell contact and translocates to the cytosol during disruption of E-cadherin-mediated cell-cell adhesion by Ca(2+) chelation. Re-establishment of cell-cell adhesion by restoration of Ca(2)(+) caused EGFP-Rac1 to become relocalized, together with E-cadherin, at sites of cell-cell contact. Engagement of E-cadherin to the apical membrane by anti-E-cadherin antibody (ECCD-2) recruited EGFP-Rac1. We also investigated whether E-cadherin-mediated cell-cell adhesion induced Rac1 activation by measuring the amounts of GTP-bound Rac1 based on its specific binding to the Cdc42/Rac1 interactive binding region of p21-activated kinase. The formation of E-cadherin-mediated cell-cell adhesion induced Rac1 activation. This activation was inhibited by treatment of cells with a neutralizing antibody (DECMA-1) against E-cadherin, or with wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase). IQGAP1, an effector of Rac1, and EGFP-Rac1 behaved in a similar manner during the formation of E-cadherin-mediated cell-cell adhesion. Rac1 activation was also confirmed by measuring the amounts of coimmunoprecipitated Rac1 with IQGAP1 during the establishment of cell-cell adhesion. Taken together, these results suggest that Rac1 is recruited at sites of E-cadherin-mediated cell-cell adhesion and then activated, possibly through PI 3-kinase. http://www/biologists.com/JCS/movies/jcs2094.html

Co-localization of Rac1 and E-cadherin in human epidermal keratinocytes

The Rac1 small GTP-binding protein is known to be involved in reorganization of the actin cytoskeleton and in regulation of intracellular signal transduction. The assembly and maintenance of cadherin-based cell cell junctions in epidermal keratinocytes is thought to be dependent on activity of Rac1. In this study we have generated green fluorescent protein (GFP)-tagged wild type, dominant negative and constitutively active Rac1 expression vectors and analyzed distribution of Rac1 following microinjection of human SCC12F epidermal keratinocytes. Wild type, dominant negative and constitutively active GFP Rac1 proteins distribute to sites of cell cell adhesion and co-localize with E-cadherin and the catenins. Disruption of cadherin-based junctions by reduction in extracellular calcium concentrations, or by use of antibodies to E-cadherin, results in redistribution of Rac1 away from sites of cell cell interaction but the co-localization with E-cadherin is maintained. In addition, expression of constitutively active GFP Rac1 results in formation of membrane ruffles on the apical surface of cells and intracellular vesicles. Interestingly, co-localization of Rac1 with E-cadherin is maintained in these structures. In contrast to previously published work we find that expression of dominant negative Rac1 neither disrupts cell cell adhesion nor prevents assembly of new cadherin-based adhesion structures.

Biochemical and morphological analysis on the localization of Rac1 in neurons

The acquisition of cell type-specific morphologies is a central feature of neuronal differentiation. Many extra- and intracellular signals are known to cause the morphological changes of neuronal cells through the reconstruction of the microfilaments underneath the cell membrane. The membrane microdomain called "raft" has been paid much attention, for this domain contains many signal-transducing molecules including trimeric G proteins and cytoskeletal proteins. The raft domain is recovered in a low-density fraction after the treatment of the membrane with the non-ionic detergent such as Triton X-100 and the enrichment of cholesterol and sphingolipids is ascribed to be responsible for the detergent insolubility. In contrast to the well-known localization of trimeric G proteins in raft, the localization of small G proteins in the raft is poorly characterized. Since Rho family small G proteins (Rho, Rac, and Cdc42) regulate the microfilament system, we studied the localization of Rho family small G proteins in the raft of rat brain with western blotting. Specific localization of Rac1 was detected in the raft from 10-day-old and 8-week-old rat whole brain, and also in the raft prepared from the growth cone and synaptic plasma membrane fractions. Rho and Cdc42 were, in contrast, recovered in the Triton soluble fraction. Double immunostaining of cultured hippocampal neurons with antibodies to Rac1 and MAP-2, or Rac1 and tau, showed punctate distribution of Rac1 in axons as well as in dendrites.

The Src homology 2 domain of Vav is required for its compartmentation to the plasma membrane and activation of c-Jun NH(2)-terminal kinase 1

Vav is a hematopoietic cell-specific guanine nucleotide exchange factor (GEF) whose activation is mediated by receptor engagement. The relationship of Vav localization to its function is presently unclear. We found that Vav redistributes to the plasma membrane in response to Fcin receptor I (FcinRI) engagement. The redistribution of Vav was mediated by its Src homology 2 (SH2) domain and required Syk activity. The FcinRI and Vav were found to colocalize and were recruited to glycosphingolipid-enriched microdomains (GEMs). The scaffold protein, linker for activation of T cells (LAT), and Rac1 (a target of Vav activity) were constitutively present in GEMs. Expression of an SH2 domain-containing COOH-terminal fragment of Vav inhibited Vav phosphorylation and movement to the GEMs but had no effect on the tyrosine phosphorylation of the adaptor protein, SLP-76 (SH2 domain-containing leukocyte protein of 76 kD), and LAT. However, assembly of the multiprotein complex containing these proteins was inhibited. In addition, FcinRI-dependent activation of c-Jun NH(2)-terminal kinase 1 (JNK1) was also inhibited. Thus, Vav localization to the plasma membrane is mediated by its SH2 domain and may serve to regulate downstream effectors like JNK1.

Rab5 induces Rac-independent lamellipodia formation and cell migration

Rab5 is a regulatory GTPase of vesicle docking and fusion that is involved in receptor-mediated endocytosis and pinocytosis. Introduction of active Rab5 in cells stimulates the rate of endocytosis and vesicle fusion, resulting in the formation of large endocytic vesicles, whereas dominant negative Rab5 inhibits vesicle fusion. Here we show that introduction of active Rab5 in fibroblasts also induced reorganization of the actin cytoskeleton but not of microtubule filaments, resulting in prominent lamellipodia formation. The Rab5-induced lamellipodia formation did not require activation of PI3-K or the GTPases Ras, Rac, Cdc42, or Rho, which are all strongly implicated in cytoskeletal reorganization. Furthermore, lamellipodia formation by insulin, Ras, or Rac was not affected by expression of dominant negative Rab5. In addition, cells expressing active Rab5 displayed a dramatic stimulation of cell migration, with the lamellipodia serving as the leading edge. Both lamellipodia formation and cell migration were dependent on actin polymerization but not on microtubules. These results demonstrate that Rab5 induces lamellipodia formation and cell migration and that the Rab5-induced lamellipodia formation occurs by a novel mechanism independent of, and distinct from, PI3-K, Ras, or Rho-family GTPases. Thus, Rab5 can control not only endocytosis but also actin cytoskeleton reorganization and cell migration, which provides strong support for an intricate relationship between these processes.