Rac 'n Rho: the music that shapes a developing embryo

An allosteric inhibitor of RhoGAP class-IX myosins suppresses the metastatic features of cancer cells

Aberrant Ras homologous (Rho) GTPase signalling is a major driver of cancer metastasis, and GTPase-activating proteins (GAPs), the negative regulators of RhoGTPases, are considered promising targets for suppressing metastasis, yet drug discovery efforts have remained elusive. Here, we report the identification and characterization of adhibin, a synthetic allosteric inhibitor of RhoGAP class-IX myosins that abrogates ATPase and motor function, suppressing RhoGTPase-mediated modes of cancer cell metastasis. In human and murine adenocarcinoma and melanoma cell models, including three-dimensional spheroid cultures, we reveal anti-migratory and anti-adhesive properties of adhibin that originate from local disturbances in RhoA/ROCK-regulated signalling, affecting actin-dynamics and actomyosin-based cell-contractility. Adhibin blocks membrane protrusion formation, disturbs remodelling of cell-matrix adhesions, affects contractile ring formation, and disrupts epithelial junction stability; processes severely impairing single/collective cell migration and cytokinesis. Combined with the non-toxic, non-pathological signatures of adhibin validated in organoids, mouse and Drosophila models, this mechanism of action provides the basis for developing anti-metastatic cancer therapies.

Filopodia-based contact stimulation of cell migration drives tissue morphogenesis

Cells migrate collectively to form tissues and organs during morphogenesis. Contact inhibition of locomotion (CIL) drives collective migration by inhibiting lamellipodial protrusions at cell-cell contacts and promoting polarization at the leading edge. Here, we report a CIL-related collective cell behavior of myotubes that lack lamellipodial protrusions, but instead use filopodia to move as a cohesive cluster in a formin-dependent manner. We perform genetic, pharmacological and mechanical perturbation analyses to reveal the essential roles of Rac2, Cdc42 and Rho1 in myotube migration. These factors differentially control protrusion dynamics and cell-matrix adhesion formation. We also show that active Rho1 GTPase localizes at retracting free edge filopodia and that Rok-dependent actomyosin contractility does not mediate a contraction of protrusions at cell-cell contacts, but likely plays an important role in the constriction of supracellular actin cables. Based on these findings, we propose that contact-dependent asymmetry of cell-matrix adhesion drives directional movement, whereas contractile actin cables contribute to the integrity of the migrating cell cluster.

Accelerated Neurite Outgrowth and Neurogenesis of PC12 Cells on an Fe-doped TiO2 Nanorod Film Triggered by Visible Light

Acceleration of neurite outgrowth and neuronal differentiation of neural cells are critical for effective neural tissue regeneration. In addition to biochemical cues, biomaterials have proven to be a valuable tool for engineering neural cellular physiological processes. However, strategies with convenient potential spatiotemporal control are still desirable. We here design a novel Fe-doped TiO₂ nanorod film using hemoglobin as the Fe source to endow it with visible-light-responsive regulated surface hydroxyl groups (-OH), which was demonstrated as the central role in mediating cell-material interactions in our previous study. The acceleration of neurite outgrowth and neuronal differentiation of PC12 cells might be attributed to the upregulated distinct terminal hydroxyl groups triggered by visible light. We also demonstrate that the actin cytoskeletal system plays a pivotal role during these processes, approved by the inhibition experiment results. This study therefore sheds light on the regulation of neurite outgrowth and neuronal differentiation of neural cells using a convenient spatiotemporal controllable strategy.

StRac1 plays an important role in potato resistance against Phytophthora infestans via regulating H2O2 production

ROP GTPases (Rho-related GTPases from plant), a unique subgroup of the Rho family in plants, is a group of key regulators of different signaling pathways controlling plant growth and development, cell polarity and differentiation, and plant response against biotic and abiotic stresses. The present study determined the potential regulatory mechanism of potato ROP GTPase (StRac1) against Phytophthora infestans (P. infestans) infection. Protein secondary structure analysis indicated that StRAC1 is a Rho GTPase. The expression level of StRac1 was variable in different tissues of potato, with the highest expression in young leaves of both Shepody and Hutou potato varieties. After challenging with P. infestans, the expression level of StRac1was higher in resistance varieties Zihuabai and Longshu 7 than in susceptible varieties Shepody and Desiree. StRAC1 fusion with GFP subcellularly localized at the plasma membrane (PM) in tobacco epidermal cells. The potato with transient or stable over-expression of CA-StRac1 (constitutively active form of StRac1)exhibited a dramatic enhancement of its resistance against P. infestans infections. The increased resistance level in transgenic potato was accompanied with elevated H2O2 levels. Importantly, silencing StRac1 via virus-induced gene silencing (VIGS) in potato resulted in higher susceptibility to P. infestans infection than in control plants. In summary, our data reveal that StRac1 regulates potato resistance against P. infestans via positively modulating the accumulation of H2O2.

Epithelial organ shape is generated by patterned actomyosin contractility and maintained by the extracellular matrix

Epithelial sheets define organ architecture during development. Here, we employed an iterative multiscale computational modeling and quantitative experimental approach to decouple direct and indirect effects of actomyosin-generated forces, nuclear positioning, extracellular matrix, and cell-cell adhesion in shaping Drosophila wing imaginal discs. Basally generated actomyosin forces generate epithelial bending of the wing disc pouch. Surprisingly, acute pharmacological inhibition of ROCK-driven actomyosin contractility does not impact the maintenance of tissue height or curved shape. Computational simulations show that ECM tautness provides only a minor contribution to modulating tissue shape. Instead, passive ECM pre-strain serves to maintain the shape independent from actomyosin contractility. These results provide general insight into how the subcellular forces are generated and maintained within individual cells to induce tissue curvature. Thus, the results suggest an important design principle of separable contributions from ECM prestrain and actomyosin tension during epithelial organogenesis and homeostasis.

The regulation and maintenance of an organ’s shape is a major outstanding problem in developmental biology. An iterative approach combining multiscale computational modelling and quantitative experimental approach was used to decouple direct and indirect roles of subcellular mechanical forces, nuclear positioning, and extracellular matrix in shaping the major axis of the wing pouch during the larval stage in fruit flies, which serves as a prototypical system for investigating epithelial morphogenesis. The research findings in this paper demonstrate that subcellular mechanical forces can effectively generate the curved tissue profile, while extracellular matrix is necessary for preserving the bent shape even in the absence of subcellular mechanical forces once the shape is generated. The developed integrated multiscale modelling environment can be readily extended to generate and test hypothesized novel mechanisms of developmental dynamics of other systems, including organoids that consist of several cellular and extracellular matrix layers.

Cdc42 controls primary mesenchyme cell morphogenesis in the sea urchin embryo

In the sea urchin embryo, gastrulation is characterized by the ingression and directed cell migration of primary mesenchyme cells (PMCs), as well as the primary invagination and convergent extension of the endomesoderm. Like all cell shape changes, individual and collective cell motility is orchestrated by Rho family GTPases and their modulation of the actomyosin cytoskeleton. And while endomesoderm specification has been intensively studied in echinoids, much less is known about the proximate regulators driving cell motility. Toward these ends, we employed anti-sense morpholinos, mutant alleles and pharmacological inhibitors to assess the role of Cdc42 during sea urchin gastrulation. While inhibition of Cdc42 expression or activity had only mild effects on PMC ingression, PMC migration, alignment and skeletogenesis were disrupted in the absence of Cdc42, as well as elongation of the archenteron. PMC migration and patterning of the larval skeleton relies on the extension of filopodia, and Cdc42 was required for filopodia in vivo as well as in cultured PMCs. Lastly, filopodial extension required both Arp2/3 and formin actin-nucleating factors, supporting models of filopodial nucleation observed in other systems. Together, these results suggest that Cdc42 plays essential roles during PMC cell motility and organogenesis.

The apical scaffold big bang binds to spectrins and regulates the growth of Drosophila melanogaster wing discs

During development, cell proliferation is regulated, ensuring that tissues reach their correct size and shape. Forest et al. show that the Drosophila melanogaster scaffold protein big bang (Bbg) controls epithelial tissue growth without affecting epithelial polarity and architecture. Bbg interacts with spectrins at the apical cortex and promotes Yki signaling and actomyosin contractility.

During development, cell numbers are tightly regulated, ensuring that tissues and organs reach their correct size and shape. Recent evidence has highlighted the intricate connections between the cytoskeleton and the regulation of the key growth control Hippo pathway. Looking for apical scaffolds regulating tissue growth, we describe that Drosophila melanogaster big bang (Bbg), a poorly characterized multi-PDZ scaffold, controls epithelial tissue growth without affecting epithelial polarity and architecture. bbg-mutant tissues are smaller, with fewer cells that are less apically constricted than normal. We show that Bbg binds to and colocalizes tightly with the β-heavy–Spectrin/Kst subunit at the apical cortex and promotes Yki activity, F-actin enrichment, and the phosphorylation of the myosin II regulatory light chain Spaghetti squash. We propose a model in which the spectrin cytoskeleton recruits Bbg to the cortex, where Bbg promotes actomyosin contractility to regulate epithelial tissue growth.

Directional Migration in Esophageal Squamous Cell Carcinoma (ESCC) is Epigenetically Regulated by SET Nuclear Oncogene, a Member of the Inhibitor of Histone Acetyltransferase Complex

Directional cell migration is of fundamental importance to a variety of biological events, including metastasis of malignant cells. Herein, we specifically investigated SET oncoprotein, a subunit of the recently identified inhibitor of acetyltransferases (INHAT) complex and identified its role in the establishment of front-rear cell polarity and directional migration in Esophageal Squamous Cell Carcinoma (ESCC). We further define the molecular circuits that govern these processes by showing that SET modulated DOCK7/RAC1 and cofilin signaling events. Moreover, a detailed analysis of the spatial distribution of RAC1 and cofilin allowed us to decipher the synergistical contributions of the two in coordinating the advancing dynamics by measuring architectures, polarities, and cytoskeletal organizations of the lamellipodia leading edges. In further investigations in vivo, we identified their unique role at multiple levels of the invasive cascade for SET cell and indicate the necessity for their functional balance to enable efficient invasion as well. Additionally, SET epigenetically repressed miR-30c expression by deacetylating histones H2B and H4 on its promoter, which was functionally important for the biological effects of SET in our cell-context. Finally, we corroborated our findings in vivo by evaluating the clinical relevance of SET signaling in the metastatic burden in mice and a large series of patients with ESCC at diagnosis, observing it's significance in predicting metastasis formation. Our findings uncovered a novel signaling network initiated by SET that epigenetically modulated ESCC properties and suggest that targeting the regulatory axis might be a promising strategy to inhibit migration and metastasis.

Mitotic-dependent phosphorylation of leukemia-associated RhoGEF (LARG) by Cdk1

Rho GTPases are integral to the regulation of actin cytoskeleton-dependent processes, including mitosis. Rho and leukemia-associated Rho guanine-nucleotide exchange factor (LARG), also known as ARHGEF12, are involved in mitosis as well as diseases such as cancer and heart disease. Since LARG has a role in mitosis and diverse signaling functions beyond mitosis, it is important to understand the regulation of the protein through modifications such as phosphorylation. Here we report that LARG undergoes a mitotic-dependent and cyclin-dependent kinase 1 (Cdk1) inhibitor-sensitive phosphorylation. Additionally, LARG is phosphorylated at the onset of mitosis and dephosphorylated as cells exit mitosis, concomitant with Cdk1 activity. Furthermore, using an in vitro kinase assay, we show that LARG can be directly phosphorylated by Cdk1. Through expression of phosphonull mutants that contain non-phosphorylatable alanine mutations at potential Cdk1 S/TP sites, we demonstrate that LARG phosphorylation occurs in both termini. Using phosphospecific antibodies, we confirm that two sites, serine 190 and serine 1176, are phosphorylated during mitosis in a Cdk1-dependent manner. In addition, these phosphospecific antibodies show phosphorylated LARG at specific mitotic locations, namely the mitotic organizing centers and flanking the midbody. Lastly, RhoA activity assays reveal that phosphonull LARG is more active in cells than phosphomimetic LARG. Our data thus identifies LARG as a phosphoregulated RhoGEF during mitosis.

AtROP1 negatively regulates potato resistance to Phytophthora infestans via NADPH oxidase-mediated accumulation of H2O2

BACKGROUND

Small GTPases are monomeric guanine nucleotide-binding proteins. In plants, ROPs regulate plant cell polarity, plant cell differentiation and development as well as biotic and abiotic stress signaling pathways.

RESULTS

We report the subcellular localization of the AtRop1 protein at the plasma membrane in tobacco epidermal cells using GFP fusions. Additionally, transient and stable expression of a dominant negative form (DN) of the Arabidopsis AtRop1 in potato led to H2O2 accumulation associated with the reduced development of Phytophthora infestans Montagne de Bary and smaller lesions on infected potato leaves. The expression of the Strboh-D gene, a NADPH oxidase homologue in potato, was analyzed by RT-PCR. Expression of this gene was maintained in DN-AtRop1 transgenic plants after infection with P. infestans. In transgenic potato lines, the transcript levels of salicylic acid (SA) and jasmonic acid (JA) marker genes (Npr1 and Lox, respectively) were analyzed. The Lox gene was induced dramatically whereas expression of Npr1, a gene up-regulated by SA, decreased slightly in DN-AtRop1 transgenic plants after infection with P. infestans.

CONCLUSIONS

In conclusion, our results indicate that DN-AtROP1 affects potato resistance to P. infestans. This is associated with increased NADPH oxidase-mediated H2O2 production and JA signaling.

Rho-guanine nucleotide exchange factors during development: Force is nothing without control

The development of multicellular organisms is associated with extensive rearrangements of tissues and cell sheets. The driving force for these rearrangements is generated mostly by the actin cytoskeleton. In order to permit the reproducible development of a specific body plan, dynamic reorganization of the actin cytoskeleton must be precisely coordinated in space and time. GTP-exchange factors that activate small GTPases of the Rho family play an important role in this process. Here we review the role of this class of cytoskeletal regulators during important developmental processes such as epithelial morphogenesis, cytokinesis, cell migration, cell polarity, neuronal growth cone extension and phagocytosis in different model systems.

In Drosophila, RhoGEF2 cooperates with activated Ras in tumorigenesis through a pathway involving Rho1-Rok-Myosin-II and JNK signalling

The Ras oncogene contributes to ≈ 30% of human cancers, but alone is not sufficient for tumorigenesis. In a Drosophila screen for oncogenes that cooperate with an activated allele of Ras (Ras(ACT)) to promote tissue overgrowth and invasion, we identified the GTP exchange factor RhoGEF2, an activator of Rho-family signalling. Here, we show that RhoGEF2 also cooperates with an activated allele of a downstream effector of Ras, Raf (Raf(GOF)). We dissect the downstream pathways through which RhoGEF2 cooperates with Ras(ACT) (and Raf(GOF)), and show that RhoGEF2 requires Rho1, but not Rac, for tumorigenesis. Furthermore, of the Rho1 effectors, we show that RhoGEF2 + Ras (Raf)-mediated tumorigenesis requires the Rho kinase (Rok)-Myosin-II pathway, but not Diaphanous, Lim kinase or protein kinase N. The Rho1-Rok-Myosin-II pathway leads to the activation of Jun kinase (JNK), in cooperation with Ras(ACT). Moreover, we show that activation of Rok or Myosin II, using constitutively active transgenes, is sufficient for cooperative tumorigenesis with Ras(ACT), and together with Ras(ACT) leads to strong activation of JNK. Our results show that Rok-Myosin-II activity is necessary and sufficient for Ras-mediated tumorigenesis. Our observation that activation of Myosin II, which regulates Filamentous actin (F-actin) contractility without affecting F-actin levels, cooperates with Ras(ACT) to promote JNK activation and tumorigenesis, suggests that increased cell contractility is a key factor in tumorigenesis. Furthermore, we show that signalling via the Tumour necrosis factor (TNF; also known as Egr)-ligand-JNK pathway is most likely the predominant pathway that activates JNK upon Rok activation. Overall, our analysis highlights the need for further analysis of the Rok-Myosin-II pathway in cooperation with Ras in human cancers.

Protein expression in the midgut of sugar-fed Aedes albopictus females

Background

Aedes albopictus is a vector for several fatal arboviruses in tropical and sub-tropical regions of the world. The midgut of the mosquito is the first barrier that pathogens must overcome to establish infection and represents one of the main immunologically active sites of the insect. Nevertheless, little is known about the proteins involved in the defense against pathogens, and even in the processing of food, and the detoxification of metabolites. The identification of proteins exclusively expressed in the midgut is the first step in understanding the complex physiology of this tissue and can provide insight into the mechanisms of pathogen-vector interaction. However, identification of the locally expressed proteins presents a challenge because the Ae. albopictus genome has not been sequenced.

Methods

In this study, two-dimensional electrophoresis (2DE) was combined with liquid chromatography in line with tandem mass spectrometry (LC-MS/MS) and data mining to identify the major proteins in the midgut of sugar-fed Ae. albopictus females.

Results

Fifty-six proteins were identified by sequence similarity to entries from the Ae. aegypti genome. In addition, two hypothetical proteins were experimentally confirmed. According to the gene ontology analysis, the identified proteins were classified into 16 clusters of biological processes. Use of the STRING database to investigate protein functional associations revealed five functional networks among the identified proteins, including a network for carbohydrate and amino acid metabolism, a group associated with ATP production and a network of proteins that interact during detoxification of toxic free radicals, among others. This analysis allowed the assignment of a potential role for proteins with unknown function based on their functional association with other characterized proteins.

Conclusion

Our findings represent the first proteome map of the Ae. albopictus midgut and denotes the first steps towards the description of a comprehensive proteome map of this vector. In addition, the data contributes to the functional annotation of Aedes spp. genomes using mass spectrometry-based proteomics data combined with complementary gene prediction methods.

Identification of novel Ras-cooperating oncogenes in Drosophila melanogaster: a RhoGEF/Rho-family/JNK pathway is a central driver of tumorigenesis

We have shown previously that mutations in the apico-basal cell polarity regulators cooperate with oncogenic Ras (Ras(ACT)) to promote tumorigenesis in Drosophila melanogaster and mammalian cells. To identify novel genes that cooperate with Ras(ACT) in tumorigenesis, we carried out a genome-wide screen for genes that when overexpressed throughout the developing Drosophila eye enhance Ras(ACT)-driven hyperplasia. Ras(ACT)-cooperating genes identified were Rac1 Rho1, RhoGEF2, pbl, rib, and east, which encode cell morphology regulators. In a clonal setting, which reveals genes conferring a competitive advantage over wild-type cells, only Rac1, an activated allele of Rho1 (Rho1(ACT)), RhoGEF2, and pbl cooperated with Ras(ACT), resulting in reduced differentiation and large invasive tumors. Expression of RhoGEF2 or Rac1 with Ras(ACT) upregulated Jun kinase (JNK) activity, and JNK upregulation was essential for cooperation. However, in the whole-tissue system, upregulation of JNK alone was not sufficient for cooperation with Ras(ACT), while in the clonal setting, JNK upregulation was sufficient for Ras(ACT)-mediated tumorigenesis. JNK upregulation was also sufficient to confer invasive growth of Ras(V12)-expressing mammalian MCF10A breast epithelial cells. Consistent with this, HER2(+) human breast cancers (where human epidermal growth factor 2 is overexpressed and Ras signaling upregulated) show a significant correlation with a signature representing JNK pathway activation. Moreover, our genetic analysis in Drosophila revealed that Rho1 and Rac are important for the cooperation of RhoGEF2 or Pbl overexpression and of mutants in polarity regulators, Dlg and aPKC, with Ras(ACT) in the whole-tissue context. Collectively our analysis reveals the importance of the RhoGEF/Rho-family/JNK pathway in cooperative tumorigenesis with Ras(ACT).

Genetic evidence for antagonism between Pak protein kinase and Rho1 small GTPase signaling in regulation of the actin cytoskeleton during Drosophila oogenesis

During Drosophila oogenesis, basally localized F-actin bundles in the follicle cells covering the egg chamber drive its elongation along the anterior-posterior axis. The basal F-actin of the follicle cell is an attractive system for the genetic analysis of the regulation of the actin cytoskeleton, and results obtained in this system are likely to be broadly applicable in understanding tissue remodeling. Mutations in a number of genes, including that encoding the p21-activated kinase Pak, have been shown to disrupt organization of the basal F-actin and in turn affect egg chamber elongation. pak mutant egg chambers have disorganized F-actin distribution and remain spherical due to a failure to elongate. In a genetic screen to identify modifiers of the pak rounded egg chamber phenotype several second chromosome deficiencies were identified as suppressors. One suppressing deficiency removes the rho1 locus, and we determined using several rho1 alleles that removal of a single copy of rho1 can suppress the pak phenotype. Reduction of any component of the Rho1-activated actomyosin contractility pathway suppresses pak oogenesis defects, suggesting that Pak counteracts Rho1 signaling. There is ectopic myosin light chain phosphorylation in pak mutant follicle cell clones in elongating egg chambers, probably due at least in part to mislocalization of RhoGEF2, an activator of the Rho1 pathway. In early egg chambers, pak mutant follicle cells have reduced levels of myosin phosphorylation and we conclude that Pak both promotes and restricts myosin light chain phosphorylation in a temporally distinct manner during oogenesis.

Macroscopic stiffening of embryonic tissues via microtubules, RhoGEF and the assembly of contractile bundles of actomyosin

During morphogenesis, forces generated by cells are coordinated and channeled by the viscoelastic properties of the embryo. Microtubules and F-actin are considered to be two of the most important structural elements within living cells accounting for both force production and mechanical stiffness. In this paper, we investigate the contribution of microtubules to the stiffness of converging and extending dorsal tissues in Xenopus laevis embryos using cell biological, biophysical and embryological techniques. Surprisingly, we discovered that depolymerizing microtubules stiffens embryonic tissues by three- to fourfold. We attribute tissue stiffening to Xlfc, a previously identified RhoGEF, which binds microtubules and regulates the actomyosin cytoskeleton. Combining drug treatments and Xlfc activation and knockdown lead us to the conclusion that mechanical properties of tissues such as viscoelasticity can be regulated through RhoGTPase pathways and rule out a direct contribution of microtubules to tissue stiffness in the frog embryo. We can rescue nocodazole-induced stiffening with drugs that reduce actomyosin contractility and can partially rescue morphogenetic defects that affect stiffened embryos. We support these conclusions with a multi-scale analysis of cytoskeletal dynamics, tissue-scale traction and measurements of tissue stiffness to separate the role of microtubules from RhoGEF activation. These findings suggest a re-evaluation of the effects of nocodazole and increased focus on the role of Rho family GTPases as regulators of the mechanical properties of cells and their mechanical interactions with surrounding tissues.

Morphogenesis of the primitive gut tube is generated by Rho/ROCK/myosin II-mediated endoderm rearrangements

During digestive organogenesis, the primitive gut tube (PGT) undergoes dramatic elongation and forms a lumen lined by a single-layer of epithelium. In Xenopus, endoderm cells in the core of the PGT rearrange during gut elongation, but the morphogenetic mechanisms controlling their reorganization are undetermined. Here, we define the dynamic changes in endoderm cell shape, polarity, and tissue architecture that underlie Xenopus gut morphogenesis. Gut endoderm cells intercalate radially, between their anterior and posterior neighbors, transforming the nearly solid endoderm core into a single layer of epithelium while concomitantly eliciting "radially convergent" extension within the gut walls. Inhibition of Rho/ROCK/Myosin II activity prevents endoderm rearrangements and consequently perturbs both gut elongation and digestive epithelial morphogenesis. Our results suggest that the cellular and molecular events driving tissue elongation in the PGT are mechanistically analogous to those that function during gastrulation, but occur within a novel cylindrical geometry to generate an epithelial-lined tube.

An ABC transporter controls export of a Drosophila germ cell attractant

Directed cell migration, which is critical for embryonic development, leukocyte trafficking, and cell metastasis, depends on chemoattraction. 3-hydroxy-3-methylglutaryl coenzyme A reductase regulates the production of an attractant for Drosophila germ cells that may itself be geranylated. Chemoattractants are commonly secreted through a classical, signal peptide-dependent pathway, but a geranyl-modified attractant would require an alternative pathway. In budding yeast, pheromones produced by a-cells are farnesylated and secreted in a signal peptide-independent manner, requiring the adenosine triphosphate-binding cassette (ABC) transporter Ste6p. Here we show that Drosophila germ cell migration uses a similar pathway, demonstrating that invertebrate germ cells, like yeast cells, are attracted to lipid-modified peptides. Components of this unconventional export pathway are highly conserved, suggesting that this pathway may control the production of similarly modified chemoattractants in organisms ranging from yeast to humans.

Mechanical control of tissue morphogenesis

Mechanical forces participate in morphogenesis from the level of individual cells to whole organism patterning. This article reviews recent research that has identified specific roles for mechanical forces in important developmental events. One well defined example is that dynein-driven cilia create fluid flow that determines left-right patterning in the early mammalian embryo. Fluid flow is also important for vasculogenesis, and evidence suggests that fluid shear stress rather than fluid transport is primarily required for remodeling the early vasculature. Contraction of the actin cytoskeleton, driven by nonmuscle myosins and regulated by the Rho family GTPases, is a recurring mechanism for controlling morphogenesis throughout development, from gastrulation to cardiogenesis. Finally, novel experimental approaches suggest critical roles for the actin cytoskeleton and the mechanical environment in determining differentiation of mesenchymal stem cells. Insights into the mechanisms linking mechanical forces to cell and tissue differentiation pathways are important for understanding many congenital diseases and for developing regenerative medicine strategies.

An image score inference system for RNAi genome-wide screening based on fuzzy mixture regression modeling

With recent advances in fluorescence microscopy imaging techniques and methods of gene knock down by RNA interference (RNAi), genome-scale high-content screening (HCS) has emerged as a powerful approach to systematically identify all parts of complex biological processes. However, a critical barrier preventing fulfillment of the success is the lack of efficient and robust methods for automating RNAi image analysis and quantitative evaluation of the gene knock down effects on huge volume of HCS data. Facing such opportunities and challenges, we have started investigation of automatic methods towards the development of a fully automatic RNAi-HCS system. Particularly important are reliable approaches to cellular phenotype classification and image-based gene function estimation. We have developed a HCS analysis platform that consists of two main components: fluorescence image analysis and image scoring. For image analysis, we used a two-step enhanced watershed method to extract cellular boundaries from HCS images. Segmented cells were classified into several predefined phenotypes based on morphological and appearance features. Using statistical characteristics of the identified phenotypes as a quantitative description of the image, a score is generated that reflects gene function. Our scoring model integrates fuzzy gene class estimation and single regression models. The final functional score of an image was derived using the weighted combination of the inference from several support vector-based regression models. We validated our phenotype classification method and scoring system on our cellular phenotype and gene database with expert ground truth labeling. We built a database of high-content, 3-channel, fluorescence microscopy images of Drosophila Kc(167) cultured cells that were treated with RNAi to perturb gene function. The proposed informatics system for microscopy image analysis is tested on this database. Both of the two main components, automated phenotype classification and image scoring system, were evaluated. The robustness and efficiency of our system were validated in quantitatively predicting the biological relevance of genes.

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

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

Identification of Rho GTPases implicated in terminal differentiation of muscle cells in ascidia

BACKGROUND INFORMATION

Members of the Rho GTPase family mediate changes in the actin cytoskeleton and are also implicated in developmental processes, including myogenesis. Nevertheless, a comprehensive analysis of these proteins during myofibrillogenesis has never been performed in any organism.

RESULTS

Using the ascidian model to identify the role of Rho GTPases on myofibrillogenesis, we show that transcripts for all Rho GTPases are detected in muscle cells of the embryo. We find that activation of RhoA, TC10 and Cdc42 (cell division cycle 42) disturbs the polarity of muscle cells, whereas that of other Rho GTPases induced cell positioning defects. Moreover, dominant negative version of five Rho GTPases, RhoA, Rac2, RCL2 (Rac- and Cdc42-like 2), TC10 and WRCH (Wnt-1 responsive Cdc42 homologue), impaired the formation of mature myofibrils.

CONCLUSIONS

Taken together, our results show that several Rho GTPase-dependent pathways are required to control the spatial localization of muscle cells in the embryo and to coordinate myofibril assembly. This stresses the importance of analysing the entire Rho family when studying a new biological process.

Regulation of Xenopus gastrulation by ErbB signaling

During Xenopus gastrulation, mesendodermal cells are internalized and display different movements. Head mesoderm migrates along the blastocoel roof, while trunk mesoderm undergoes convergent extension (C&E). Different signals are implicated in these processes. Our previous studies reveal that signals through ErbB receptor tyrosine kinases modulate Xenopus gastrulation, but the mechanisms employed are not understood. Here we report that ErbB signals control both C&E and head mesoderm migration. Inhibition of ErbB pathway blocks elongation of dorsal marginal zone explants and activin-treated animal caps without removing mesodermal gene expression. Bipolar cell shape and cell mixing in the dorsal region are impaired. Inhibition of ErbB signaling also interferes with migration of prechordal mesoderm on fibronectin. Cell-cell and cell-matrix interaction and cell spreading are reduced when ErbB signaling is blocked. Using antisense morpholino oligonucleotides, we show that ErbB4 is involved in Xenopus gastrulation morphogenesis, and it partially regulates cell movements through modulation of cell adhesion and membrane protrusions. Our results reveal for the first time that vertebrate ErbB signaling modulates gastrulation movements, thus providing a novel pathway, in addition to non-canonical Wnt and FGF signals, that controls gastrulation. We further demonstrate that regulation of cell adhesive properties and cell morphology may underlie the functions of ErbBs in gastrulation.

Global comparative transcriptome analysis identifies gene network regulating secondary xylem development in Arabidopsis thaliana

Our knowledge of the genetic control of wood formation (i.e., secondary growth) is limited. Here, we present a novel approach to unraveling the gene network regulating secondary xylem development in Arabidopsis, which incorporates complementary platforms of comparative-transcriptome analyses such as "digital northern" and "digital in situ" analysis. This approach effectively eliminated any genes that are expressed in either non-stem tissues/organs ("digital northern") or phloem and non-vascular regions ("digital in situ"), thereby identifying 52 genes that are upregulated only in the xylem cells of secondary growth tissues as "core xylem gene set". The proteins encoded by this gene set participate in signal transduction, transcriptional regulation, cell wall metabolism, and unknown functions. Five of the seven signal transduction-related genes represented in the core xylem gene set encode the essential components of ROP (Rho-related GTPase from plants) signaling cascade. Furthermore, the analysis of promoter sequences of the core xylem gene set identified a novel cis-regulatory element, ACAAAGAA. The functional significances of this gene set were verified by several independent experimental and bioinformatics methods.

Expression of RhoB in the developing Xenopus laevis embryo

Rho GTPases are signaling components that participate to the control of cell morphology, adhesion and motility through the regulation of F-actin cytoskeleton dynamics. In this paper, we report the identification of RhoB in Xenopus laevis (XRhoB) and its expression pattern during early development. Whole-mount in situ hybridization analysis indicated that XrhoB is expressed at high levels in the dorsal marginal zone early in gastrula and in the dorsal midline at later stages. At mid-neurula stages, XrhoB expression extends to the central nervous system, presomitic mesoderm and somites. Later during development, rhoB mRNA is detected in the eyes, the migrating neural crest cells as well as the dorso-lateral part of the somites.

Expression of the Rho-GEF Pbl/ECT2 is regulated by the UBE3A E3 ubiquitin ligase

We applied genetic tools available in Drosophila to identify candidate substrates of the UBE3A ubiquitin ligase, the gene responsible for Angelman syndrome (AS). Human UBE3A was expressed in Drosophila heads to identify proteins differentially regulated in UBE3A-expressing versus wild-type extracts. Using two-dimensional gel and MALDI-TOF analysis, we detected 20 proteins that were differentially regulated by over-expression of human UBE3A in Drosophila heads. One protein responsive to UBE3A was the Rho-GEF pebble (pbl). Here, we present three lines of evidence suggesting that UBE3A regulates Pbl. First, we show genetic evidence that UBE3A and the Drosophila de-ubiquitinase fat facets (faf) exert opposing effects on Pbl function. Secondly, we find that both Pbl and ECT2, the mammalian orthologue of Pbl called epithelial cell transforming sequence 2 oncogene, physically interact with their respective ubiquitin E3 ligases. Finally, we show that Ect2 expression is regulated by Ube3a in mouse neurons as the pattern of Ect2 expression is dramatically altered in the hippocampus and cerebellum of Ube3a null mice. These results suggest that an orthologous UBE3A post-translational regulatory pathway regulates neuronal outgrowth in the mammalian brain and that dysregulation of this pathway may result in neurological phenotypes including AS and possibly other autism spectrum disorders.

Migrating anterior mesoderm cells and intercalating trunk mesoderm cells have distinct responses to Rho and Rac during Xenopus gastrulation

Rho GTPases have been shown recently to be important for cell polarity and motility of the trunk mesoderm during gastrulation in Xenopus embryos. This work demonstrated that Rho and Rac have both distinct and overlapping roles in regulating cell shape, and the dynamic properties, polarity, and type of protrusive activity of these cells. Overexpression of activated or inhibitory versions of these GTPases also disrupts development of the head in Xenopus embryos. In this study, we have undertaken a detailed analysis of Rho and Rac function in migrating anterior mesendoderm cells. Scanning electron micrographs of these cells in situ revealed that their normal shingle arrangement is disrupted and both the cells and their lamellipodia are disoriented. Anterior mesendoderm explants plated on their natural blastocoel roof matrix, however, still migrated towards the animal pole, although the tendency to move in this direction is reduced compared to controls. Analysis of a number of parameters in time-lapse recordings of dissociated cells indicated that Rho and Rac also have both distinct and overlapping roles in the motility of the prospective head mesoderm; however, their effects differ to those previously seen in the trunk mesoderm. Both GTPases appear to modulate cell polarization, migration, and protrusive activity. Rho alone, however, regulates the retraction of the lagging edge of the cell. We propose that within the gastrulating Xenopus embryo, two types of mesoderm cells that undergo different motilities have distinct responses to Rho GTPases.

P190-B Rho GTPase-activating protein overexpression disrupts ductal morphogenesis and induces hyperplastic lesions in the developing mammary gland

p190-B Rho GTPase activating protein is essential for mammary gland development because p190-B deficiency prevents ductal morphogenesis. To investigate the role of p190-B during distinct stages of mammary gland development, tetracycline-regulatable p190-B-overexpressing mice were generated. Short-term induction of p190-B in the developing mammary gland results in abnormal terminal end buds (TEBs) that exhibit aberrant budding off the neck, histological anomalies, and a markedly thickened stroma. Overexpression of p190-B throughout postnatal development results in increased branching, delayed ductal elongation, and disorganization of the ductal tree. Interestingly, overexpression of p190-B during pregnancy results in hyperplastic lesions. Several cellular and molecular alterations detected within the aberrant TEBs may contribute to these phenotypes. Signaling through the IGF pathway is altered, and the myoepithelial cell layer is discontinuous at sites of aberrant budding. An increase in collagen and extensive infiltration of macrophages, which have recently been implicated in branching morphogenesis, is observed in the stroma surrounding the p190-B-overexpressing TEBs. We propose that the stromal response, disruption of the myoepithelial layer, and alterations in IGF signaling in the p190-B-overexpressing mice impact the TEB architecture, leading to disorganization and increased branching of the ductal tree. Moreover, we suggest that alterations in tissue architecture and the adjacent stroma as a consequence of p190-B overexpression during pregnancy leads to loss of growth control and the formation of hyperplasia. These data demonstrate that precise control of p190-B Rho GTPase-activating protein activity is critical for normal branching morphogenesis during mammary gland development.

Mesenchymal-to-epithelial transition during somitic segmentation: a novel approach to studying the roles of Rho family GTPases in morphogenesis

During early development in vertebrates, cells change their shapes dramatically both from epithelial to mesenchymal and also from mesenchymal to epithelial, enabling the body to form complex tissues and organs. Using somitogenesis as a novel model, Rho family GTPases have recently been shown to play essential and differential roles in individual cell behaviors in actual developing embryos. Levels of Cdc42 activity provide a binary switch wherein high Cdc42 levels allow the cells to remain mesenchymal, while low Cdc42 levels produce epithelialization. Rac1 activity needs to be precisely controlled for proper epithelialization through the bHLH transcription factor Paraxis. Somitogenesis is expected to serve as an excellent model with which one can understand how the functions of developmental genes are resolved into the morphogenetic behavior of individual cells.

Drosophila melanogaster Rac2 is necessary for a proper cellular immune response

It has been reported that during Drosophila embryonic development, and in cell culture, that the Rac GTPases are redundant. To better elucidate Rac function in Drosophila, we decided to study the role of Rac2 in larval cellular defense reactions against the parasitiod Leptopilina boulardi. Here we show a dramatic effect in the context of cellular immunity where, unlike embryonic development, Rac2 appears to have a non-redundant function. When an invading parasitoid is recognized as foreign, circulating hemocytes (blood cells) should recognize and attach to the egg chorion. After attachment the hemocytes should then spread to form a multilayered capsule surrounding the invader. In Rac2 mutants this process is disrupted. Immune surveillance cells, known as plasmatocytes, adhere to the parasitoid egg but fail to spread, and septate junctions do not assemble, possibly due to mislocalization of the Protein 4.1 homolog Coracle. Finally, larger cells known as lamellocytes attach to the capsule but also fail to spread, and there is a lack of melanization. From these results it appears that Rac2 is necessary for the larval cellular immune response.

crossveinless-c is a RhoGAP required for actin reorganisation during morphogenesis

Members of the Rho family of small GTPases are required for many of the morphogenetic processes required to shape the animal body. The activity of this family is regulated in part by a class of proteins known as RhoGTPase Activating Proteins (RhoGAPs) that catalyse the conversion of RhoGTPases to their inactive state. In our search for genes that regulate Drosophila morphogenesis, we have isolated several lethal alleles of crossveinless-c (cv-c). Molecular characterisation reveals that cv-c encodes the RhoGAP protein RhoGAP88C. During embryonic development, cv-c is expressed in tissues undergoing morphogenetic movements; phenotypic analysis of the mutants reveals defects in the morphogenesis of these tissues. Genetic interactions between cv-c and RhoGTPase mutants indicate that Rho1, Rac1 and Rac2 are substrates for Cv-c, and suggest that the substrate specificity might be regulated in a tissue-dependent manner. In the absence of cv-c activity, tubulogenesis in the renal or Malpighian tubules fails and they collapse into a cyst-like sack. Further analysis of the role of cv-c in the Malpighian tubules demonstrates that its activity is required to regulate the reorganisation of the actin cytoskeleton during the process of convergent extension. In addition, overexpression of cv-c in the developing tubules gives rise to actin-associated membrane extensions. Thus, Cv-c function is required in tissues actively undergoing morphogenesis, and we propose that its role is to regulate RhoGTPase activity to promote the coordinated organisation of the actin cytoskeleton, possibly by stabilising plasma membrane/actin cytoskeleton interactions.

DRhoGEF2 regulates actin organization and contractility in the Drosophila blastoderm embryo

Morphogenesis of the Drosophila melanogaster embryo is associated with a dynamic reorganization of the actin cytoskeleton that is mediated by small GTPases of the Rho family. Often, Rho1 controls different aspects of cytoskeletal function in parallel, requiring a complex level of regulation. We show that the guanine triphosphate (GTP) exchange factor DRhoGEF2 is apically localized in epithelial cells throughout embryogenesis. We demonstrate that DRhoGEF2, which has previously been shown to regulate cell shape changes during gastrulation, recruits Rho1 to actin rings and regulates actin distribution and actomyosin contractility during nuclear divisions, pole cell formation, and cellularization of syncytial blastoderm embryos. We propose that DRhoGEF2 activity coordinates contractile actomyosin forces throughout morphogenesis in Drosophila by regulating the association of myosin with actin to form contractile cables. Our results support the hypothesis that specific aspects of Rho1 function are regulated by specific GTP exchange factors.

All-trans retinoic acid-responsive genes identified in the human SH-SY5Y neuroblastoma cell line and their regulated expression in the nervous system of early embryos

The vitamin A metabolite, all-trans retinoic acid (atRA), is required for embryonic development. atRA binds to the nuclear retinoic acid receptors and regulates the transcription of specific target genes. In order to identify atRA-induced genes that play a role in neural development, a subtractive library was created from SH-SY5Y neuroblastoma cells, a human cell line that exhibits changes in cell adhesion and neurite outgrowth after exposure to the vitamin A acid. We report here the identification of 14 genes that are rapidly induced by atRA (retinoic acid induced in neuroblastoma or RAINB), eight of which were previously not known to be atRA responsive (BTBD11, calmin, cyclin M2, ephrin B2, HOXD10, NEDD9, RAINB6 and tenascin R). mRNA regulation by atRA was confirmed in SH-SY5Y cells by Northern blotting, and gene regulation was studied in additional human cell lines using the quantitative polymerase chain reaction. The majority of the atRA-responsive clones revealed in this screen are highly expressed in the nervous system of developing rat embryos. Further, the expression of several of these genes is perturbed in developing rat embryos exposed to excess atRA or conversely, deprived of sufficient retinoid during early development. We propose that a subset of these genes lie downstream of atRA and its receptors in the regulation of neurite outgrowth and cell adhesion in both neural and non-neural tissues within the developing embryo.

Mesenchymal-epithelial transition during somitic segmentation is regulated by differential roles of Cdc42 and Rac1

Mesenchymal-epithelial transitions (MET) are crucial for vertebrate organogenesis. The roles of Rho family GTPases in such processes during actual development remain largely unknown. By electroporating genes into chick presomitic mesenchymal cells, we demonstrate that Cdc42 and Rac1 play important and different roles in the MET that generates the vertebrate somites. Presomitic mesenchymal cells, which normally contribute to both the epithelial and mesenchymal populations of the somite, were hyperepithelialized when Cdc42 signaling was blocked. Conversely, cells taking up genes that elevate Cdc42 levels remained mesenchymal. Thus, Cdc42 activity levels appear critical for the binary decision that defines the epithelial and mesenchymal somitic compartments. Proper levels of Rac1 are necessary for somitic epithelialization, since cells with activated or inhibited Rac1 failed to undergo correct epithelialization. Furthermore, Rac1 appears to be required for Paraxis to act as an epithelialization-promoting transcription factor during somitogenesis.

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

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

A screen for modifiers of RacGAP(84C) gain-of-function in theDrosophilaeye revealed the LIM kinase Cdi/TESK1 as a downstream effector of Rac1 during spermatogenesis

In Drosophila, RotundRacGAP/RacGAP(84C) is critical to retinal organisation and spermatogenesis. We show that eye-directed expression of RacGAP(84C) or its GTPase activating protein (GAP) domain induces a dominant rough eye phenotype which we used as a starting point in a gain-of-function screen to identify new partners of RacGAP(84C). Proteins known to function in Ras, Rho and Rac signalling were identified confirming the essential role of RacGAP(84C) in crosstalk between GTPases. Other potential RacGAP(84C) partners identified by the screen are implicated in signal transduction, DNA remodelling, cytoskeletal organisation, membrane trafficking and spermatogenesis. This latter class includes the serine/threonine kinase Center divider (Cdi), which is homologous to the human LIM kinase, Testis specific kinase 1 (TESK1), involved in cytoskeleton control through Cofilin phosphorylation. Eye-directed expression of cdi strongly suppressed the phenotypes induced by either RacGAP(84C) gain-of-function or by the dominant negative form of Rac1, Rac1N17. These results are consistent with Cdi being a specific downstream target of Rac1. We showed that Rac1 and cdi are both expressed in Drosophila testis and that homozygous Rac1 mutants exhibit poor fertility that is further reduced by introducing a cdi loss-of-function mutation in trans. Thus, results from a misexpression screen in the eye led us to a putative novel Rac1-Cdi-Cofilin pathway, regulated by RacGAP(84C), coordinating Drosophila spermatogenesis.

Rho GTPases: potential candidates for anticancer therapy

Low molecular weight Rho GTPases are proteins that, in response to diverse stimuli, control key cellular processes such as cell proliferation, apoptosis, lipid metabolism, cytoarchitecture, adhesion, migration, cell polarity, and transcriptional regulation. The high incidence of overexpression of some members of the Rho family of GTPases in human tumors suggests that these proteins are important in the carcinogenic process, and therefore potential candidates for a therapeutic intervention. In recent years, the characterization of downstream effectors to Rho GTPases has increased our understanding of the general cellular effects that permit aberrant proliferation and motility of tumor cells. In addition, several transcription factors have been identified to play important roles at various levels of Rho-induced tumorigenesis. Accordingly, drugs that specifically alter Rho signaling display antineoplastic properties both at the level of tumor growth and tumor metastasis. In this review, a brief summary of the progress made in understanding the biological functions elicited by Rho GTPases that contribute to tumor biology will be made. In addition, a description of new drugs available targeted to specific elements of Rho signaling with antineoplastic or antimetastatic activity is included.

Cell and molecular biology of myoblast fusion

In organisms from Drosophila to mammals, the musculature is comprised of an elaborate array of distinct fibers that are generated by the fusion of committed myoblasts. These muscle fibers differ from each other in features that include location, pattern of innervation, site of attachment, and size. The sizes of the newly formed muscles of an embryo are controlled in large part by the number of cells that form the syncitial fiber. Over the past few decades, an extensive body of literature has described the process of myoblast fusion in vertebrates, relying primarily on the strengths of tissue culture model systems. More recently, genetic studies in Drosophila embryos have provided new insights into the process. Together, these studies define the steps necessary for myoblast differentiation, the acquisition of fusion competence, the recognition and adhesion between myoblasts, and the fusion of two lipid bilayers into one. In this review, we have attempted to combine insights from both Drosophila and vertebrate studies to trace the processes and molecules involved in myoblast fusion. Implicit in this approach is the assumption that fundamental aspects of myoblast fusion will be similar, independent of the organism in which it is occurring.

Conserved subgroups and developmental regulation in the monocot rop gene family

Rop small GTPases are plant-specific signaling proteins with roles in pollen and vegetative cell growth, abscisic acid signal transduction, stress responses, and pathogen resistance. We have characterized the rop family in the monocots maize (Zea mays) and rice (Oryza sativa). The maize genome contains at least nine expressed rops, and the fully sequenced rice genome has seven. Based on phylogenetic analyses of all available Rops, the family can be subdivided into four groups that predate the divergence of monocots and dicots; at least three have been maintained in both lineages. However, the Rop family has evolved differently in the two lineages, with each exhibiting apparent expansion in different groups. These analyses, together with genetic mapping and identification of conserved non-coding sequences, predict orthology for specific rice and maize rops. We also identified consensus protein sequence elements specific to each Rop group. A survey of ROP-mRNA expression in maize, based on multiplex reverse transcriptase-polymerase chain reaction and a massively parallel signature sequencing database, showed significant spatial and temporal overlap of the nine transcripts, with high levels of all nine in tissues in which cells are actively dividing and expanding. However, only a subset of rops was highly expressed in mature leaves and pollen. Intriguingly, the grouping of maize rops based on hierarchical clustering of expression profiles was remarkably similar to that obtained by phylogenetic analysis. We hypothesize that the Rop groups represent classes with distinct functions, which are specified by the unique protein sequence elements in each group and by their distinct expression patterns.

The polybasic region of Ras and Rho family small GTPases: a regulator of protein interactions and membrane association and a site of nuclear localization signal sequences

Many small GTPases in the Ras and Rho families have a C-terminal polybasic region (PBR) comprised of multiple lysines or arginines. The PBR controls diverse functions of these small GTPases, including their ability to associate with membranes, interact with specific proteins, and localize in subcellular compartments. Different signaling pathways mediated by Ras and Rho family members may converge when the small GTPases are directed by their PBRs to shared binding sites in specific proteins or at cell membranes. The PBR promotes the interactions of small GTPases with SmgGDS, which is a nucleocytoplasmic shuttling protein that stimulates guanine nucleotide exchange by small GTPases. The PBR of Rac1 was recently found to have a functional nuclear localization signal (NLS) sequence, which enhances the nuclear accumulation of protein complexes containing SmgGDS and Rac1. Sequence analysis demonstrates that canonical NLS sequences (K-K/R-x-K/R) are present in the PBRs of additional Ras and Rho family members, and are evolutionarily conserved across several phyla. These findings suggest that the PBR regulates the nucleocytoplasmic shuttling of some Ras and Rho family members when they are in protein complexes that are too large to diffuse through nuclear pores. These diverse functions of the PBR indicate its critical role in signaling by Ras and Rho family GTPases.

DroVav, the Drosophila melanogaster homologue of the mammalian Vav proteins, serves as a signal transducer protein in the Rac and DER pathways

Mammalian Vav signal transducer proteins couple receptor tyrosine kinase signals to the activation of the Rho/Rac GTPases, leading to cell differentiation and/or proliferation. The unique and complex structure of mammalian Vav proteins is preserved in the Drosophila melanogaster homologue, DroVav. We demonstrate that DroVav functions as a guanine-nucleotide exchange factor (GEF) for DRac. Drosophila cells overexpressing wild-type (wt) DroVav exhibited a normal morphology. However, overexpression of a truncated DroVav mutant (that functions as an oncogene when expressed in NIH3T3 cells) results in striking changes in the actin cytoskeleton, resembling those usually visible following Rac activation. Dominant-negative DRac abrogated these morphological changes, suggesting that the effect of the truncated DroVav mutant is mediated by activation of DRac. In Drosophila cells, we find that stimulation of the Drosophila EGF receptor (DER) increases tyrosine phosphorylation of DroVav, which in turn associates with tyrosine-phosphorylated DER. In addition, the following results imply that DroVav participates in downstream DER signalling, such as ERK phosphorylation: (a) overexpression of DroVav induces ERK phosphorylation; and (b) 'knockout' of DroVav by RNA interference blocks ERK phosphorylation induced by DER stimulation. Unlike mammalian Vav proteins, DroVav was not found to induce Jnk phosphorylation under the experimental circumstances tested in fly cells. These results establish the role of DroVav as a signal transducer that participates in receptor tyrosine kinase pathways and functions as a GEF for the small RhoGTPase, DRac.

Genetic Modifier Screens in Drosophila Demonstrate a Role for Rho1 Signaling in Ecdysone-Triggered Imaginal Disc Morphogenesis

Drosophila adult leg development provides an ideal model system for characterizing the molecular mechanisms of hormone-triggered morphogenesis. A pulse of the steroid hormone ecdysone at the onset of metamorphosis triggers the rapid transformation of a flat leg imaginal disc into an immature adult leg, largely through coordinated changes in cell shape. In an effort to identify links between the ecdysone signal and the cytoskeletal changes required for leg morphogenesis, we performed two large-scale genetic screens for dominant enhancers of the malformed leg phenotype associated with a mutation in the ecdysoneinducible broad early gene (br1). From a screen of >750 independent deficiency and candidate mutation stocks, we identified 17 loci on the autosomes that interact strongly with br1. In a complementary screen of ∼112,000 F1 progeny of EMS-treated br1 animals, we recovered 26 mutations that enhance the br1 leg phenotype [E(br) mutations]. Rho1, stubbloid, blistered (DSRF), and cytoplasmic Tropomyosin were identified from these screens as br1-interacting genes. Our findings suggest that ecdysone exerts its effects on leg morphogenesis through a Rho1 signaling cascade, a proposal that is supported by genetic interaction studies between the E(br) mutations and mutations in the Rho1 signaling pathway. In addition, several E(br) mutations produce unexpected defects in midembryonic morphogenetic movements. Coupled with recent evidence implicating ecdysone signaling in these embryonic morphogenetic events, our results suggest that a common ecdysone-dependent, Rho1-mediated regulatory pathway controls morphogenesis during the two major transitions in the life cycle, embryogenesis and metamorphosis.

Reciprocal interactions between neurons and glia are required for Drosophila peripheral nervous system development

A major developmental role of peripheral glia is to mediate sensory axon guidance; however, it is not known whether sensory neurons influence peripheral glial development. To determine whether glia and neurons reciprocally interact during embryonic development, we ablated each cell type by overexpressing the apoptosis gene, grim, and observed the effects on peripheral nervous system (PNS) development. When neurons are ablated, glial defects occur as a secondary effect, and vice versa. Therefore glia and neurons are codependent during embryogenesis. To further explore glial-neuronal interactions, we genetically disrupted glial migration or differentiation and observed the secondary effects on sensory neuron development. Glial migration and ensheathment of PNS axons was blocked by overexpression of activated Rho GTPase, a regulator of actin dynamics. Here, sensory axons extended to the CNS without exhibiting gross pathfinding errors. In contrast, disrupting differentiation by expression of dominant-negative Ras GTPase in glia resulted in major sensory axon pathfinding errors, similar to those seen in glial ablations. Glial overexpression of transgenic components of the epidermal growth factor receptor (EGFR) signaling pathway yielded similar sensory neuron defects and also downregulated the expression of the glial marker Neuroglian. Mutant analysis also suggested that the EGFR ligands Spitz and Vein play roles in peripheral glial development. The observations support a model in which glia express genes necessary for sensory neuron development, and these genes are potentially under the control of the EGFR/Ras signaling pathway.

A Rho GTPase controls the rate of protein synthesis in the sea urchin egg

Fertilization of the sea urchin egg triggers a Ca(2+)-dependent cortical granule exocytosis and cytoskeletal reorganization, both of which are accompanied by an accelerated protein synthesis. The signaling mechanisms leading to these events are not completely understood. The possible role of Rho GTPases in sea urchin egg activation was studied using the Clostridium botulinum C3 exotoxin, which specifically ADP-ribosylates Rho proteins and inactivates them. We observed that incubation of eggs with C3 resulted in in situ ADP-ribosylation of Rho. Following fertilization, C3-treated eggs were capable of performing cortical granule exocytosis but not the first cytokinesis. C3 caused in both unfertilized eggs and early embryos alterations in the state of actin polymerization and inhibition of the spindle formation. Moreover, C3 diminished markedly the rate of protein synthesis. These findings suggested that Rho is involved in regulating the acceleration of protein synthesis that accompanies the egg activation by sperm.

Maintenance of stem cell populations in plants

Flowering plants have the unique ability to produce new organs continuously, for hundreds of years in some species, from stem cell populations maintained at their actively growing tips. The shoot tip is called the shoot apical meristem, and it acts as a self-renewing source of undifferentiated, pluripotent stem cells whose descendents become incorporated into organ and tissue primordia and acquire different fates. Stem cell maintenance is an active process, requiring constant communication between different regions of the shoot apical meristem to coordinate loss of stem cells from the meristem through differentiation with their replacement through cell division. Stem cell research in model plant systems is facilitated by the fact that mutants with altered meristem cell identity or accumulation are viable, allowing dissection of stem cell behavior by using genetic, molecular, and biochemical methods. Such studies have determined that in the model plant Arabidopsis thaliana stem cell maintenance information flows via a signal transduction pathway that is established during embryogenesis and maintained throughout the life cycle. Signaling through this pathway results in the generation of a spatial feedback loop, involving both positive and negative interactions, that maintains stem cell homeostasis. Stem cell activity during reproductive development is terminated by a temporal feedback loop involving both stem cell maintenance genes and a phase-specific flower patterning gene. Our current investigations provide additional insights into the molecular mechanisms that regulate stem cell activity in higher plants.

Distinct functions of Rho and Rac are required for convergent extension during Xenopus gastrulation

We have undertaken the first detailed analysis of Rho GTPase function during vertebrate development by analyzing how RhoA and Rac1 control convergent extension of axial mesoderm during Xenopus gastrulation. Monitoring of a number of parameters in time-lapse recordings of mesoderm explants revealed that Rac and Rho have both distinct and overlapping roles in regulating the motility of axial mesoderm cells. The cell behaviors revealed by activated or inhibitory versions of these GTPases in native tissue were clearly distinct from those previously documented in cultured fibroblasts. The dynamic properties and polarity of protrusive activity, along with lamellipodia formation, were controlled by the two GTPases operating in a partially redundant manner, while Rho and Rac contributed separately to cell shape and filopodia formation. We propose that Rho and Rac operate in distinct signaling pathways that are integrated to control cell motility during convergent extension.