Rho GTPases: biochemistry and biology

Lineage-specific expansions provide genomic complexity among sea urchin GTPases

In every organism, GTP-binding proteins control many aspects of cell signaling. Here, we examine in silico several GTPase families from the Strongylocentrotus purpuratus genome: the monomeric Ras superfamily, the heterotrimeric G proteins, the dynamin superfamily, the SRP/SR family, and the "protein biosynthesis" translational GTPases. Identified were 174 GTPases, of which over 90% are expressed in the embryo as shown by tiling array and expressed sequence tag data. Phylogenomic comparisons restricted to Drosophila, Ciona, and humans (protostomes, urochordates, and vertebrates, respectively) revealed both common and unique elements in the expected composition of these families. Galpha and dynamin families contain vertebrate expansions, consistent with whole genome duplications, whereas SRP/SR and translational GTPases are highly conserved. Unexpectedly, Ras superfamily analyses revealed several large (5+) lineage-specific expansions in the sea urchin. For Rho, Rab, Arf, and Ras subfamilies, comparing total human gene numbers to the number of sea urchin genes with vertebrate orthologs suggests reduced genomic complexity in the sea urchin. However, gene duplications in the sea urchin increase overall numbers such that total sea urchin gene numbers approximate vertebrate gene numbers for each monomeric GTPase family. These findings suggest that lineage-specific expansions may be an important component of genomic evolution in signal transduction.

Induction of transient macroapertures in endothelial cells through RhoA inhibition by Staphylococcus aureus factors

The GTPase RhoA is a major regulator of the assembly of actin stress fibers and the contractility of the actomyosin cytoskeleton. The epidermal cell differentiation inhibitor (EDIN) and EDIN-like ADP-ribosyltransferases of Staphylococcus aureus catalyze the inactivation of RhoA, producing actin cable disruption. We report that purified recombinant EDIN and EDIN-producing S. aureus provoke large transcellular tunnels in endothelial cells that we have named macroapertures (MAs). These structures open transiently, followed by the appearance of actin-containing membrane waves extending over the aperture. Disruption of actin cables, either directly or indirectly, through rhoA RNAi knockdown also triggers the formation of MAs. Intoxication of endothelial monolayers by EDIN produces a loss of barrier function and provides direct access of the endothelium basement membrane to S. aureus.

Repulsion or adhesion: receptors make the call

Repulsive signaling plays a prominent role in regulating cell-cell interactions and is fundamental to multiple developmental processes. A proper balance between repulsion from and adhesion to other cells or the extracellular matrix is also important. Semaphorin-Plexin and ephrin-Eph ligand-receptor pairs compose two major repulsive signaling systems. Recent advances have elucidated mechanisms by which Semaphorin-Plexin and ephrin-Eph signaling control repulsion versus adhesion. Semaphorins act through a complex signaling pathway to inhibit integrin-mediated adhesion, allowing cell repulsion. Ephrin-Eph interactions can directly mediate cell adhesion and several mechanisms control whether ephrin-Eph binding and signaling induces repulsion or adhesion.

A RHOse by any other name: a comparative analysis of animal and plant Rho GTPases

Rho GTPases are molecular switches that act as key regulators of a many cellular processes, including cell movement, morphogenesis, host defense, cell division and gene expression. Rho GTPases are found in all eukaryotic kingdoms. Plants lack clear homologs to conventional Rho GTPases found in yeast and animals; instead, they have over time developed a unique subfamily, ROPs, also known as RAC. The origin of ROP-like proteins appears to precede the appearance of land plants. This review aims to discuss the evolution of ROP/RAC and to compare plant ROP and animal Rho GTPases, focusing on similarities and differences in regulation of the GTPases and their downstream effectors.

TRIF-GEFH1-RhoB pathway is involved in MHCII expression on dendritic cells that is critical for CD4 T-cell activation

Dendritic cells (DC) play a central role in immune responses by presenting antigenic peptides to CD4+ T cells through MHCII molecules. Here, we demonstrate a TRIF-GEFH1-RhoB pathway is involved in MHCII surface expression on DC. We show the TRIF (TIR domain-containing adapter inducing IFNbeta)- but not the myeloid differentiation factor 88 (MyD88)-dependent pathway of lipopolysaccharide (LPS)-signaling in DC is crucial for the MHCII surface expression, followed by CD4+ T-cell activation. LPS increased the activity of RhoB, but not of RhoA, Cdc42, or Rac1/2 in a manor dependent on LPS-TRIF- but not LPS-Myd88-signaling. RhoB colocalized with MHCII+ lysosomes in DC. A dominant-negative (DN) form of RhoB (DN-RhoB) or RhoB's RNAi in DC inhibited the LPS-induced MHCII surface expression. Moreover, we found GEFH1 associated with RhoB, and DN-GEFH1 or GEFH1's RNAi suppressed the LPS-mediated RhoB activation and MHCII surface expression. DN-RhoB attenuated the DC's CD4+ T-cell stimulatory activity. Thus, our results provide a molecular mechanism relating how the MHCII surface expression is regulated during the maturation stage of DC. The activation of GEFH1-RhoB through the TRIF-dependent pathway of LPS in DC might be a critical target for controlling the activation of CD4+ T cells.

Drosophila RhoGEF4 encodes a novel RhoA-specific guanine exchange factor that is highly expressed in the embryonic central nervous system

Rho family small GTPases act as molecular switches that regulate neuronal morphogenesis, including axon growth and guidance, dendritic spine formation, and synapse formation. These proteins are positively regulated by guanine nucleotide exchange factors (GEFs) of the Dbl family. This study describes the identification and characterization of Drosophila RhoGEF4 (DRhoGEF4), a novel Dbl family protein that is specifically expressed in the central nervous system during Drosophila embryogenesis. The predicted amino acid sequence of DRhoGEF4 contains a Dbl homology (DH) domain and an adjacent C-terminal pleckstrin homology (PH) domain, which are most closely related to those of mammalian frabins. In this study, the DH-PH motif is shown to enhance the dissociation of GDP from either RhoA or Rac1 but not from Cdc42 in vitro. In addition, p21-binding domain pull-down assays demonstrate that DRhoGEF4 activates RhoA, but neither Rac1 nor Cdc42 in HEK293 cells. Finally, overexpression of DRhoGEF4 is able to induce assembly of stress fibers in cultured NIH3T3 cells. Taken together, these findings suggest that DRhoGEF4 may participate in cytoskeleton-related cellular events by specifically activating RhoA in neuronal morphogenesis.

The epithelial cell transforming sequence 2, a guanine nucleotide exchange factor for Rho GTPases, is repressed by p53 via protein methyltransferases and is required for G1-S transition

The epithelial cell transforming sequence 2 (ECT2), a member of the Dbl family of guanine nucleotide exchange factor for Rho GTPases, is required for cytokinesis. The tumor suppressor p53 plays a crucial role in coordinating cellular processes, such as cell cycle arrest and apoptosis, in response to stress signals. Here, we showed that ECT2 is negatively regulated by wild-type p53 but not tumor-derived mutant p53 or other p53 family members. In addition, ECT2 is down-regulated in multiple cell lines by DNA damage agents and Nutlin-3, an MDM2 antagonist, in a p53-dependent manner. We also showed that the activity of the ECT2 promoter is repressed by wild-type p53, and to a lesser extent, by p21. In addition, the second activation domain in p53 is necessary for the efficient repression of ECT2. Importantly, we found that the ECT2 gene is bound by p53 in vivo in response to DNA damage and Nutlin-3 treatment. Furthermore, we provided evidence that inhibition of protein methyltransferases, especially arginine methyltransferases, relieve the repression of ECT2 induced by DNA damage or Nutlin-3 in a p53-dependent manner. Finally, we generated multiple cell lines in which ECT2 is inducibly knocked down and found that ECT2 knockdown triggers cell cycle arrest in G1. Taken together, we uncovered a novel function for ECT2 and provided a novel mechanism by which p53 represses gene expression via protein methyltransferases.

The Rho-GTPase cdc42 regulates neural progenitor fate at the apical surface

Stem cell persistence into adulthood requires self-renewal from early developmental stages. In the developing mouse brain, only apical progenitors located at the ventricle are self-renewing, whereas basal progenitors gradually deplete. However, nothing is known about the mechanisms regulating the fundamental difference between these progenitors. Here we show that the conditional deletion of the small Rho-GTPase cdc42 at different stages of neurogenesis in mouse telencephalon results in an immediate increase in basal mitoses. Whereas cdc42-deficient progenitors have normal cell cycle length, orientation of cell division and basement membrane contact, the apical location of the Par complex and adherens junctions are gradually lost, leading to an increasing failure of apically directed interkinetic nuclear migration. These cells then undergo mitoses at basal positions and acquire the fate of basal progenitors. Thus, cdc42 has a crucial role at the apical pole of progenitors, thereby regulating the position of mitoses and cell fate.

Listeria monocytogenes induced Rac1-dependent signal transduction in endothelial cells

Infection of endothelial cells by Listeria monocytogenes is an essential step in the pathogenesis of listeriosis. Small GTPases of the Rho family act as molecular switches in signal transduction. We tested the hypothesis that Rho GTPases contribute to the regulation of cytokine expression following L. monocytogenes infection. L. monocytogenes induced release of distinct CC and CXC, as well as Th1 and Th2 cytokines and growth factors by endothelial cells and activated RhoA and Rac1. Inhibition of Rac1 by inhibitor Nsc23766 reduced cytokine expression, and slightly yet significantly the uptake of bacteria. Blocking of Rho proteins by Clostridium difficile toxin B-10463 (TcdB) reduced Listeria-dependent cytokine expression, whereas activating Rho proteins by Escherichia coli CNF1 increased it. We analyzed regulation of IL-8 expression in more detail: Listeria-induced IL-8 release was reduced by inhibition of RhoA, Rac1 and Cdc42 (TcdB) or Rac1 while blocking of RhoA/B/C by Clostridium limosum C3 fusion toxin (C3FT) or Rho kinase by Y27632 reduced cytokine expression only slightly. Activation of RhoA, Rac1 and Cdc42 (CNF1), but not of RhoA alone (CNF(Y)), enhanced Listeria-dependent IL-8 release significantly. Furthermore, inhibition of RhoA, Rac1 and Cdc42 (TcdB) and Rac1 (Nsc23766), but not of RhoA (C3FT) reduced Listeria-related recruitment of NF-kappaB/p65 and RNA polymerase II to the il8 promoter, as well as acetylation of histone H4 and Ser10/Lys14-phosphorylation/acetylation of histone H3 at the il8 gene promoter in HUVEC. In conclusion, Rac1 contributed to L. monocytogenes-induced cytokine expression by human endothelial cells.

Regulated synthesis and functions of laminin 5 in polarized madin-darby canine kidney epithelial cells

Renal tubular epithelial cells synthesize laminin (LN)5 during regeneration of the epithelium after ischemic injury. LN5 is a truncated laminin isoform of particular importance in the epidermis, but it is also constitutively expressed in a number of other epithelia. To investigate the role of LN5 in morphogenesis of a simple renal epithelium, we examined the synthesis and function of LN5 in the spreading, proliferation, wound-edge migration, and apical-basal polarization of Madin-Darby canine kidney (MDCK) cells. MDCK cells synthesize LN5 only when subconfluent, and they degrade the existing LN5 matrix when confluent. Through the use of small-interfering RNA to knockdown the LN5 alpha3 subunit, we were able to demonstrate that LN5 is necessary for cell proliferation and efficient wound-edge migration, but not apical-basal polarization. Surprisingly, suppression of LN5 production caused cells to spread much more extensively than normal on uncoated surfaces, and exogenous keratinocyte LN5 was unable to rescue this phenotype. MDCK cells also synthesized laminin alpha5, a component of LN10, that independent studies suggest may form an assembled basal lamina important for polarization. Overall, our findings indicate that LN5 is likely to play an important role in regulating cell spreading, migration, and proliferation during reconstitution of a continuous epithelium.

New players in TLR-mediated innate immunity: PI3K and small Rho GTPases

Toll-like receptors (TLRs) play a crucial role in the innate immune system as a first line of defense against pathogens. TLR activation in phagocytes produces pro-inflammatory cytokines and chemokines that contribute directly to elimination of infectious agents and activation of adaptive immune responses. However, a sustained inflammatory response can result in tissue damage and generalized sepsis. This review summarizes the complex and sometimes conflicting links of TLR signaling with two important regulators of immune cells functions: phosphoinositide 3-kinases (PI3Ks) and small GTPases of the Rho family. A unified model of hierarchical organization of these signaling participants is still premature, given that the tools for delineating how control of TLRmediated pathways is achieved are just emerging. Critical progress in our understanding of spatial-temporal propagation of TLR signaling will certainly be provided in the near future by pharmacological targeting of PI3Ks using recently characterized, second-generation PI3K inhibitors in combination with gene-targeting strategies for PI3K subunits and Rho GTPases targeted to the murine myeloid compartment.

Cooperative cell transformation by Myc/Mil(Raf) involves induction of AP-1 and activation of genes implicated in cell motility and metastasis

Avian fibroblasts transformed simultaneously by the v-myc and v-mil(raf) oncogenes of acute leukemia and carcinoma virus MH2 contain elevated levels of c-Fos and c-Jun, major components of the transcription factor complex AP-1. To define specific transcriptional targets in these cells, subtractive hybridization techniques were employed leading to the identification of strongly upregulated genes including OPN (osteopontin), 126MRP, and rac2. OPN is a cytokine and cell attachment protein which has been implicated in human tumor progression and metastasis, the calcium binding 126MRP protein is related to the human S100 protein family involved in invasive cell growth, and the Rac2 protein belongs to the Rho family of small GTPases regulating actin reorganization and cell migration. Promoter analysis indicated that OPN activation is mediated by a non-consensus AP-1 binding site located close to the transcription start site. Electrophoretic mobility shift assays, chromatin immunoprecipitation and transcriptional reporter gene analyses showed that c-Fos and c-Jun bind specifically to this site and that c-Fos efficiently transactivates the OPN promoter. High-level expression of OPN, 126MRP, or Rac2 proteins from a retroviral vector led to partial cell transformation, documented by morphological changes and anchorage-independent growth. The specific activation in v-myc/v-mil(raf)-transformed cells of target genes with intrinsic oncogenic potential may provide an explanation for the longstanding observation that concomitant expression of these oncogenes leads to strongly enhanced oncogenicity in vivo and in vitro compared to cell transformation by v-myc or v-mil(raf) alone.

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.

Dictyostelium RacH Regulates Endocytic Vesicular Trafficking and is Required for Localization of Vacuolin

Dictyostelium RacH localizes predominantly to membranes of the nuclear envelope, endoplasmic reticulum and Golgi apparatus. To investigate the role of this protein, we generated knockout and overexpressor strains. RacH-deficient cells displayed 50% reduced fluid-phase uptake and a moderate exocytosis defect, but phagocytosis was unaffected. Detailed examination of the endocytic pathway revealed defective acidification of early endosomes and reduced secretion of acid phosphatase in the presence of sucrose. The distribution of the post-lysosomal marker vacuolin was altered, with a high proportion of cells showing a diffuse vesicular pattern in contrast to the wild-type strain, where few intensely stained vacuoles predominate. Cytokinesis, cell motility, chemotaxis and development appeared largely unaffected. In a cell-free system, RacH stimulates actin polymerization, suggesting that this protein is involved in actin-based trafficking of vesicular compartments. We also investigated the determinants of subcellular localization of RacH by expression of green-fluorescent-protein-tagged chimeras in which the C-terminus of RacH and the plasma-membrane-targeted RacG were exchanged, the insert region was deleted or the net positive charge of the hypervariable region was increased. We show that several regions of the molecule, not only the hypervariable region, determine targeting of RacH. Overexpression of mistargeted RacH mutants did not recapitulate the phenotypes of a strain overexpressing nonmutated RacH, indicating that the function of this protein is in great part related to its subcellular localization.

Actin-based forces driving embryonic morphogenesis in Caenorhabditis elegans

Morphogenesis is the process by which multicellular organisms transform themselves from a ball of cells into an organized animal. Certain virtues of Caenorhabditis elegans make it an excellent model system for the study of this process: it is genetically tractable, develops as a transparent embryo with small cell-numbers, and yet still contains all the major tissues typical of animals. Furthermore, certain morphogenetic events are also amenable to study by direct manipulation of the cells involved. Given these advantages, it has been possible to use C. elegans to investigate the different ways in which the actin cytoskeleton drives the cellular rearrangements underlying morphogenesis, through regulated polymerization or actomyosin contraction. Recent insights from this system have determined the involvement in morphogenesis of key proteins, including the actin-regulating WASP and Ena proteins, potential guidance molecules such as the Eph and Robo receptors, and the cell-cell signaling proteins of the Wnt pathway.

Nothing but skin and bone

Skin and bone - what comes to mind at hearing this phrase? While certainly a metaphor for disease, it also defines two very different tissues, one a flexible and contiguous outer covering, the other a morphologically diverse hard tissue distributed at over 200 sites in the body. As the accompanying series of Reviews highlights, these tissues are indeed diverse, but there are also surprising similarities. Skin is the interface between the internal organs and the environment, and as such plays a crucial role in the body's defense mechanism. The skin and its many appendages are responsible for functions as diverse as epidermal barrier and defense, immune surveillance, UV protection, thermoregulation, sweating, lubrication, pigmentation, the sensations of pain and touch, and, importantly, the protection of various stem cell niches in the skin. Bone serves a number of purposes: it provides protection for vital organs, a lever for locomotion, a reservoir for calcium, and the site of adult hematopoiesis. The tissue is composed of osteoblasts, osteoclasts, and their individual precursors plus a complex mixture of mesenchymal, myeloid, and lymphoid cells in the marrow space. Finally, the endothelial microenvironment provides nutrition and is a conduit for the influx and emigration of cells that impact bone biology in several important ways. This Review series guides the reader through these various facets of 2 diverse, yet interdependent, tissues.

CNF1-induced ubiquitylation and proteasome destruction of activated RhoA is impaired in Smurf1-/- cells

Ubiquitylation of RhoA has emerged as an important aspect of both the virulence of Escherichia coli producing cytotoxic necrotizing factor (CNF) 1 toxin and the establishment of the polarity of eukaryotic cells. Owing to the molecular activity of CNF1, we have investigated the relationship between permanent activation of RhoA catalyzed by CNF1 and subsequent ubiquitylation of RhoA by Smurf1. Using Smurf1-deficient cells and by RNA interference (RNAi)-mediated Smurf1 knockdown, we demonstrate that Smurf1 is a rate-limiting and specific factor of the ubiquitin-mediated proteasomal degradation of activated RhoA. We further show that the cancer cell lines HEp-2, human embryonic kidney 293 and Vero are specifically deficient in ubiquitylation of either activated Rac, Cdc42, or Rho, respectively. In contrast, CNF1 produced the cellular depletion of all three isoforms of Rho proteins in the primary human cell types we have tested. We demonstrate that ectopic expression of Smurf1 in Vero cells, deficient for RhoA ubiquitylation, restores ubiquitylation of the activated forms of RhoA. We conclude here that Smurf1 ubiquitylates activated RhoA and that, in contrast to human primary cell types, some cancer cell lines have a lower ubiquitylation capacity of specific Rho proteins. Thus, both CNF1 and transforming growth factor-beta trigger activated RhoA ubiquitylation through Smurf1 ubiquitin-ligase.

Proteomic identification and functional characterization of a novel ARF6 GTPase-activating protein, ACAP4

ARF6 GTPase is a conserved regulator of membrane trafficking and actin-based cytoskeleton dynamics at the leading edge of migrating cells. A key determinant of ARF6 function is the lifetime of the GTP-bound active state, which is orchestrated by GTPase-activating protein (GAP) and GTP-GDP exchanging factor. However, very little is known about the molecular mechanisms underlying ARF6-mediated cell migration. To systematically analyze proteins that regulate ARF6 activity during cell migration, we performed a proteomic analysis of proteins selectively bound to active ARF6 using mass spectrometry and identified a novel ARF6-specific GAP, ACAP4. ACAP4 encodes 903 amino acids and contains two coiled coils, one pleckstrin homology domain, one GAP motif, and two ankyrin repeats. Our biochemical characterization demonstrated that ACAP4 has a phosphatidylinositol 4,5-bisphosphate-dependent GAP activity specific for ARF6. The co-localization of ACAP4 with ARF6 occurred in ruffling membranes formed upon AIF(4) and epidermal growth factor stimulation. ACAP4 overexpression limited the recruitment of ARF6 to the membrane ruffles in the absence of epidermal growth factor stimulation. Expression of GTP hydrolysis-resistant ARF6(Q67L) resulted in accumulations of ACAP4 and ARF6 in the cytoplasmic membrane, suggesting that GTP hydrolysis is required for the ARF6-dependent membrane remodeling. Significantly the depletion of ACAP4 by small interfering RNA or inhibition of ARF6 GTP hydrolysis by overexpressing GAP-deficient ACAP4 suppressed ARF6-dependent cell migration in wound healing, demonstrating the importance of ACAP4 in cell migration. Thus, our study sheds new light on the biological function of ARF6-mediated cell migration.

Cingulin regulates claudin-2 expression and cell proliferation through the small GTPase RhoA

In mouse embryoid bodies, mutation of the tight junction protein cingulin results in changes in gene expression. Here, we studied the function of cingulin using a gene silencing approach in Madin-Darby canine kidney (MDCK) cells. Cingulin-depleted cells show higher protein and mRNA levels of claudin-2 and ZO-3, increased RhoA activity, activation of G1/S phase transition, and increased cell density. The effects of cingulin depletion on claudin-2 expression, cell proliferation, and density are reversed by coexpression of either a dominant-negative form of RhoA (RhoAN19) or the Rho-inhibiting enzyme C3 transferase. However, the increase in ZO-3 protein and mRNA levels is not reversed by inhibition of either RhoA, p38, extracellular signal-regulated kinase (ERK), or c-Jun NH2-terminal kinase (JNK), suggesting that cingulin modulates ZO-3 expression by a different mechanism. JNK is implicated in the regulation of claudin-2 levels independently of cingulin depletion and RhoA activity, indicating distinct roles of RhoA- and JNK-dependent pathways in the control of claudin-2 expression. Finally, cingulin depletion does not significantly alter the barrier function of monolayers and the overall molecular organization of tight junctions. These results provide novel insights about the mechanisms of cingulin function and the signaling pathways controlling claudin-2 expression in MDCK cells.

Actin in transcription and transcription regulation

Recent research has provided convincing evidence that actin plays several important roles in gene transcription. First, actin can bind transcription factors and determine their subcellular localization. Second, actin is a component of chromatin remodeling complexes involved in transcriptional activation. Third, actin binds directly to the RNA polymerases I, II and III, and is required for their full transcriptional activity. Fourth, actin associates with nascent mRNPs and participates in the recruitment of histone modifiers to transcribed genes. We do not know yet whether these functions are general, or restricted to certain subsets of genes.

Rac1 and Cdc42 have different roles in Candida albicans development

We investigated the role of the highly conserved G protein Rac1 in the opportunistic pathogen Candida albicans. We identified and disrupted RAC1 and show here that, in contrast to CDC42, it is not necessary for viability or serum-induced hyphal growth but is essential for filamentous growth when cells are embedded in a matrix. Rac1 is localized to the plasma membrane, yet its distribution is more homogenous than that of Cdc42, with no enrichment at the tips of either buds or hyphae. In addition, fluorescence recovery after photobleaching results indicate that Rac1 and Cdc42 have different dynamics at the membrane. Furthermore, overexpression of Rac1 does not complement Cdc42 function, and conversely, overexpression of Cdc42 does not complement Rac1 function. Thus, Rac1 and Cdc42, although highly similar to one another, have different roles in C. albicans development.

Osmotic stress and the cytoskeleton: the R(h)ole of Rho GTPases

Hyperosmotic stress initiates a variety of compensatory and adaptive responses, which either serve to restore near-normal volume or remodel and reinforce the cell structure to withstand the physical challenge. The latter response is brought about by the reorganization of the cytoskeleton; however, the underlying mechanisms are not well understood. Recent research has provided major breakthroughs in our knowledge about the link between message and structure, i.e. between signalling and cytoskeletal remodelling, predominantly in the context of cell migration. The major components of this progress are the in-depth characterization of Rho family small GTPases, master regulators of the cytoskeleton, and the discovery of the actin-related protein 2/3 complex, a signalling-sensitive structural element of the actin polymerization machinery. The primary aim of this review is to find the place of these novel and crucial players in osmotically induced (volume-dependent) remodelling of the cytoskeleton. We aim to address three questions: (1) What are the major structural changes in the cytoskeleton under hyperosmotic conditions? (2) Are the Rho family small GTPases (Rho, Rac and Cdc42) regulated by osmotic stress, and if so, by what mechanisms? (3) Are Rho GTPases involved, as mediators, in major adaptive responses, including cytoskeleton rearrangement, changes in ion transport and genetic reprogramming? Our answers will show how fragmentary our current knowledge is in these areas. Therefore, this overview has been written with the hardly disguised intention that it might foster further research in this field by highlighting some intriguing questions.

LRRK2 in Parkinson's disease: protein domains and functional insights

Parkinson's disease (PD) is the most common motor neurodegenerative disease. Mutations in the gene encoding leucine-rich repeat kinase 2 (LRRK2) have been linked recently with autosomal-dominant parkinsonism that is clinically indistinguishable from typical, idiopathic, late-onset PD. Thus, the protein LRRK2 has emerged as a promising therapeutic target for treatment of PD. LRRK2 is extraordinarily large and complex, with multiple enzymatic and protein-interaction domains, each of which is targeted by pathogenic mutations in familial PD. This review places the PD-associated mutations of LRRK2 in a structural and functional framework, with the ultimate aim of deciphering the molecular basis of LRRK2-associated pathogenesis. This, in turn, should advance our understanding and treatment of familial and idiopathic PD.

Stress fibers are generated by two distinct actin assembly mechanisms in motile cells

Stress fibers play a central role in adhesion, motility, and morphogenesis of eukaryotic cells, but the mechanism of how these and other contractile actomyosin structures are generated is not known. By analyzing stress fiber assembly pathways using live cell microscopy, we revealed that these structures are generated by two distinct mechanisms. Dorsal stress fibers, which are connected to the substrate via a focal adhesion at one end, are assembled through formin (mDia1/DRF1)–driven actin polymerization at focal adhesions. In contrast, transverse arcs, which are not directly anchored to substrate, are generated by endwise annealing of myosin bundles and Arp2/3-nucleated actin bundles at the lamella. Remarkably, dorsal stress fibers and transverse arcs can be converted to ventral stress fibers anchored to focal adhesions at both ends. Fluorescence recovery after photobleaching analysis revealed that actin filament cross-linking in stress fibers is highly dynamic, suggesting that the rapid association–dissociation kinetics of cross-linkers may be essential for the formation and contractility of stress fibers. Based on these data, we propose a general model for assembly and maintenance of contractile actin structures in cells.

Functional proteomics identifies protein-tyrosine phosphatase 1B as a target of RhoA signaling

Rho GTPases are signal transduction effectors that control cell motility, cell attachment, and cell shape by the control of actin polymerization and tyrosine phosphorylation. To identify cellular targets regulated by Rho GTPases, we screened global protein responses to Rac1, Cdc42, and RhoA activation by two-dimensional gel electrophoresis and mass spectrometry. A total of 22 targets were identified of which 19 had never been previously linked to Rho GTPase pathways, providing novel insight into pathway function. One novel target of RhoA was protein-tyrosine phosphatase 1B (PTP1B), which catalyzes dephosphorylation of key signaling molecules in response to activation of diverse pathways. Subsequent analysis demonstrated that RhoA enhances post-translational modification of PTP1B, inactivates phosphotyrosine phosphatase activity, and up-regulates tyrosine phosphorylation of p130Cas, a key mediator of focal adhesion turnover and cell migration. Thus, protein profiling reveals a novel role for PTP1B as a mediator of RhoA-dependent phosphorylation of p130Cas.

Nox1-dependent reactive oxygen generation is regulated by Rac1

Rac1 has been implicated in the generation of reactive oxygen species (ROS) in several cell types, but the enzymatic origin of the ROS has not been proven. The present studies demonstrate that Nox1, a homolog of the phagocyte NADPH-oxidase component gp91(phox), is activated by Rac1. When Nox1 is co-expressed along with its regulatory subunits NOXO1 and NOXA1, significant ROS generation is seen. Herein, co-expression of constitutively active Rac1(G12V), but not wild-type Rac1, resulted in marked further stimulation of activity. Decreased Rac1 expression using small interfering RNA reduced Nox1-dependent ROS. CDC42(G12V) failed to increase activity, and small interfering RNA directed against CDC42 failed to decrease activity, pointing to specificity for Rac. TPR domain mutants of NOXA1 that interfere with Rac1 binding were ineffective in supporting Nox1-dependent ROS generation. Immunoprecipitation experiments demonstrated a complex containing Rac1(G12V), NOXO1, NOXA1, and Nox1. CDC42(G12V) could not substitute for Rac1(G12V) in such a complex. Nox1 formed a complex with Rac1(G12V) that was independent of NOXA1 and NOXO1, consistent with direct binding of Rac1(G12V) to Nox1. Rac1(G12V) interaction with NOXA1 was enhanced by Nox1 and NOXO1, suggesting cooperative binding. A model is presented comparing activation by regulatory subunits of Nox1 versus gp91(phox) (Nox2) in which Rac1 activation provides a major trigger that acutely activates Nox1-dependent ROS generation.

Synaptopodin orchestrates actin organization and cell motility via regulation of RhoA signalling

The Rho family of small GTPases (RhoA, Rac1 and Cdc42) controls signal-transduction pathways that influence many aspects of cell behaviour, including cytoskeletal dynamics. At the leading edge, Rac1 and Cdc42 promote cell motility through the formation of lamellipodia and filopodia, respectively. On the contrary, RhoA promotes the formation of contractile actin-myosin-containing stress fibres in the cell body and at the rear. Here, we identify synaptopodin, an actin-associated protein, as a novel regulator of RhoA signalling and cell migration in kidney podocytes. We show that synaptopodin induces stress fibres by competitive blocking of Smurf1-mediated ubiquitination of RhoA, thereby preventing the targeting of RhoA for proteasomal degradation. Gene silencing of synaptopodin in kidney podocytes causes the loss of stress fibres and the formation of aberrant non-polarized filopodia and impairment of cell migration. Together, these data show that synaptopodin is essential for the integrity of the podocyte actin cytoskeleton and for the regulation of podocyte cell migration.

Estrogen receptor alpha interacts with Galpha13 to drive actin remodeling and endothelial cell migration via the RhoA/Rho kinase/moesin pathway

Sex steroids control cell movement and tissue organization; however, little is known of the involved mechanisms. This report describes the ongoing dynamic regulation by estrogen of the actin cytoskeleton and cell movement in human vascular endothelial cells that depends on rapid activation of the actin-regulatory protein moesin. Moesin activation is triggered by the interaction of the C-terminal portion of cell membrane estrogen receptor alpha with the G protein Galpha(13), leading to activation of the small GTPase RhoA and of the downstream effector Rho-associated kinase. The resulting phosphorylation of moesin on Thr(558) is the means of moesin's binding to actin and the remodeling of the actin cytoskeleton. This cascade of events ensues within minutes of estradiol administration and results in changes in cell morphology and to the development of specialized cell membrane structures such as ruffles and pseudopodia that are necessary for cell movement. These findings expand our knowledge of the basis of estrogen's effects on human cells, including the regulation of actin assembly, cell movement and migration. They highlight novel pathways of signal transduction of estrogen receptor alpha through nontranscriptional mechanisms. Furthermore, exposure of this estrogen receptor-dependent, nongenomic action of estrogen on human vascular endothelial cells is especially relevant to the present interest in the role of estrogen in cardiovascular protection.

Csk-deficient boundary cells are eliminated from normal Drosophila epithelia by exclusion, migration, and apoptosis

The construction and maintenance of normal epithelia relies on local signals that guide cells into their proper niches and remove unwanted cells. Failure to execute this process properly may result in aberrant development or diseases, including cancer and associated metastasis. Here, we show that local environment influences the behavior of dCsk-deficient cells. Broad loss of dCsk led to enlarged and mispatterned tissues due to overproliferation, a block in apoptosis, and decreased cadherin-mediated adhesion. Loss of dCsk in discrete patches led to a different outcome: epithelial exclusion, invasive migration, and apoptotic death. These latter phenotypes required sharp differences in dCsk activity between neighbors; dE-cadherin, P120-catenin, Rho1, JNK, and MMP2 mediated this signal. Together, our data demonstrate how the cellular microenvironment plays a central role in determining the outcome of altered dCsk activity, and reveal a role for P120-catenin in a mechanism that protects epithelial integrity by removing abnormal cells.

Glycogen synthase kinase-3 acts upstream of ADP-ribosylation factor 6 and Rac1 to regulate epithelial cell migration

Cell sheet movement during epithelial wound closure is a complex process involving collective cell migration. We have found that glycogen synthase kinase-3 (GSK-3) activity is required for membrane protrusion and crawling of cells at the wound edge and those behind it in wounded Madin-Darby canine kidney (MDCK) epithelial cell monolayers. RNA interference-based silencing of GSK-3alpha and GSK-3beta expression also results in slowed cell sheet migration, with the effect being more pronounced with knockdown of GSK-3beta. Both GSK-3alpha and GSK-3beta are in activated states during the most active phase of cell migration. In addition to having a positive control or permissive, rather than negative, function in MDCK cell migration, GSK-3 appears to act upstream of the small GTPases ADP-ribosylation factor 6 (ARF6) and Rac1. Expression of constitutively active ARF6 restores a protrusive, migratory phenotype in cells treated with GSK-3 inhibitors. It does not, however, restore to normal levels the directional polarization of cells behind the wound edge toward the wound area, implying the existence of a separate ARF6-independent branch of the GSK-3 pathway that regulates proper wound-directed polarization of these cells. Finally, inhibition of GSK-3 also strongly reduces activation of Rac1 and cell scatter in response to hepatocyte growth factor/scatter factor, which triggers dispersal and migration of cells in monolayer culture as fibroblast-like individual cells, a mode of epithelial cell motility distinct from the collective migration of wound closure.

Mechanism of Akt1 inhibition of breast cancer cell invasion reveals a protumorigenic role for TSC2

Akt1 is frequently up-regulated in human tumors and has been shown to accelerate cell proliferation and to suppress programmed cell death; consequently, inhibition of the activity of Akt1 has been seen as an attractive target for therapeutic intervention. Paradoxically, hyperactivation of the Akt1 oncogene can also prevent the invasive behavior that underlies progression to metastasis. Here we show that overexpression of activated myr-Akt1 in human breast cancer cells phosphorylates and thereby targets the tumor suppressor tuberous sclerosis complex 2 (TSC2) for degradation, leading to reduced Rho-GTPase activity, decreased actin stress fibers and focal adhesions, and reduced motility and invasion. Overexpression of TSC2 rescues the migration phenotype of myr-Akt1-expressing tumor cells, and high levels of TSC2 in breast cancer patients correlate with increased metastasis and reduced survival. These data indicate that the functional properties of genes designated as oncogenes or tumor suppressor genes depend on the context of the cell type and the tissues studied, and suggest the need for caution in designing therapies targeting the function of individual genes in epithelial tissues.

Cortical control of plant microtubules

The cortical microtubule array of plant cells appears in early G(1) and remodels during the progression of the cell cycle and differentiation, and in response to various stimuli. Recent studies suggest that cortical microtubules are mostly formed on pre-existing microtubules and, after detachment from the initial nucleation sites, actively interact with each other to attain distinct distribution patterns. The plus end of growing microtubules is thought to accumulate protein complexes that regulate both microtubule dynamics and interactions with cortical targets. The ROP family of small GTPases and the mitogen-activated protein kinase pathways have emerged as key players that mediate the cortical control of plant microtubules.

Turning ON the switch--RhoGEFs in plants

Plant ROPs (or RACs) are Ras-related, small GTP-binding proteins that function as molecular switches in numerous signaling cascades. ROPs (RACs) cycle between a GDP-bound, inactive state and a GTP-bound, active state. Activation requires guanine-nucleotide exchange factors (GEFs), which were not identified in plants until recently. Alfred Wittinghofer and co-workers' description of a group of plant-specific RhoGEFs opens the way for the elucidation of ROP signaling cascades from membrane receptors to responses.

Rgf1p is a specific Rho1-GEF that coordinates cell polarization with cell wall biogenesis in fission yeast

Rho1p regulates cell integrity by controlling the actin cytoskeleton and cell wall synthesis. We have identified a new GEF, designated Rgf1p, which specifically regulates Rho1p during polarized growth. The phenotype of rgf1 null cells was very similar to that seen after depletion of Rho1p, 30% of cells being lysed. In addition, rgf1(+) deletion caused hypersensitivity to the antifungal drug Caspofungin and defects in the establishment of bipolar growth. rho1(+), but none of the other GTPases of the Rho-family, suppressed the rgf1Delta phenotypes. Moreover, deletion of rgf1(+) suppressed the severe growth defect in rga1(+) null mutants (a Rho1-GAP, negative regulator). Rgf1p and Rho1p coimmunoprecipitated and overexpression of rgf1(+) specifically increased the GTP-bound Rho1p; it caused changes in cell morphology, and a large increase in beta(1,3)-glucan synthase activity. These effects were similar to those elicited when the hyperactive rho1-G15V allele was expressed. A genetic relationship was observed between Rgf1p, Bgs4p (beta[1,3]-glucan synthase), and Pck1p (protein kinase C [PKC] homologue); Bgs4p and Pck1p suppressed the hypersensitivity to Caspofungin in rgf1Delta mutants. Rgf1p localized to the growing ends and the septum, where Rho1, Pck1p, and Bgs4p are known to function. Our results suggest that Rgf1p probably activates the Rho functions necessary for coordinating actin deposition with cell wall biosynthesis during bipolar growth, allowing the cells to remodel their wall without risk of rupture.

The parvins

The parvins are a family of proteins involved in linking integrins and associated proteins with intracellular pathways that regulate actin cytoskeletal dynamics and cell survival. Both alpha-parvin (PARVA) and beta-parvin (PARVB) localize to focal adhesions and function in cell adhesion, spreading, motility and survival through interactions with partners, such as integrin-linked kinase (ILK), paxillin, alpha-actinin and testicular kinase 1. A complex of PARVA with ILK and the LIM protein PINCH-1 is critical for cell survival in a variety of cells, including certain cancer cells, kidney podocytes and cardiac myocytes. While PARVA inhibits the activities of Rac1 and testicular kinase 1 and cell spreading, PARVB binds alphaPIX and alpha-actinin, and can promote cell spreading. In contrast to PARVA, PARVB inhibits ILK activity and reverses some of its oncogenic effects in cancer cells. This review focuses on the structure and function of the parvins and some possible roles in human diseases.

Oncogenic K-Ras down-regulates Rac1 and RhoA activity and enhances migration and invasion of pancreatic carcinoma cells through activation of p38

Activating mutations in the K-ras gene are genetic alterations frequently found in human carcinomas, particularly in pancreatic adenocarcinomas. Mutation of the K-ras gene is thought to be an early and important event in pancreatic tumor initiation, but the precise role of the mutant K-Ras proteins in neoplastic progression is still unknown. In the present study, we have characterized the influence of oncogenic K-Ras on the phenotype and on the signal transduction of epitheloid PANC-1 pancreatic carcinoma cells by generating PANC-1 cell clones, which stably express EGFP(enhanced green fluorescent protein)-K-Ras (V12). EGFP-K-Ras (V12)-expressing cells exhibited a more fibroblastoid cellular phenotype with irregular cell shape and disorganized cytokeratin filaments. Moreover, these cells showed a marked enhancement of their migratory and invasive properties. Stable expression of EGFP-K-Ras (V12) down-regulated the activity of Rac1 and RhoA, resulting in reduced subcortical actin filaments and stress fibers, which might contribute to the epithelial dedifferentiation. Characterization of the activity of mitogen-activated protein kinases revealed that EGFP-K-Ras (V12) enhanced the activity of p38, but did not affect the activities of the Raf/MEK/ERK cascade and JNK. While inhibition of either MEK or JNK activity had no effect on EGFP-K-Ras (V12)-induced migration, inhibition of p38 activity markedly reduced EGFP-K-Ras (V12)-induced migration. Collectively, the results suggest that oncogenic K-Ras enhances the malignant phenotype and identify the mitogen-activated protein kinase p38 as a target to inhibit oncogenic K-Ras-induced pancreatic tumor cell migration.