The Rap1 GTPase functions as a regulator of morphogenesis in vivo

The Rap1 small GTPase affects cell fate or survival and morphogenetic patterning during Drosophila melanogaster eye development

The Drosophila melanogaster eye has been instrumental for determining both how cells communicate with one another to determine cell fate, as well as cell morphogenesis and patterning. Here, we describe the effects of the small GTPase Rap1 on the development of multiple cell types in the D. melanogaster eye. Although Rap1 has previously been linked to RTK-Ras-MAPK signaling in eye development, we demonstrate that manipulation of Rap1 activity is modified by increase or decrease of Delta/Notch signaling during several events of cell fate specification in eye development. In addition, we demonstrate that manipulating Rap1 function either in primary pigment cells or in interommatidial cells affects cone cell contact switching, primary pigment cell enwrapment of the ommatidial cluster, and sorting of secondary and tertiary pigment cells. These data suggest that Rap1 has roles in both ommatidial cell recruitment/survival and in ommatidial morphogenesis in the pupal stage. They lay groundwork for future experiments on the role of Rap1 in these events.

Rap1 promotes epithelial integrity and cell viability in a growing tissue

Having intact epithelial tissues is critical for embryonic development and adult homeostasis. How epithelia respond to damaging insults or tissue growth while still maintaining intercellular connections and barrier integrity during development is poorly understood. The conserved small GTPase Rap1 is critical for establishing cell polarity and regulating cadherin-catenin cell junctions. Here, we identified a new role for Rap1 in maintaining epithelial integrity and tissue shape during Drosophila oogenesis. Loss of Rap1 activity disrupted the follicle cell epithelium and the shape of egg chambers during a period of major growth. Rap1 was required for proper E-Cadherin localization in the anterior epithelium and for epithelial cell survival. Both Myo-II and the adherens junction-cytoskeletal linker protein α-Catenin were required for normal egg chamber shape but did not strongly affect cell viability. Blocking the apoptotic cascade failed to rescue the cell shape defects caused by Rap1 inhibition. One consequence of increased cell death caused by Rap1 inhibition was the loss of polar cells and other follicle cells, which later in development led to fewer cells forming a migrating border cell cluster. Our results thus indicate dual roles for Rap1 in maintaining epithelia and cell survival in a growing tissue during development.

Rap1 regulates apical contractility to allow embryonic morphogenesis without tissue disruption and acts in part via Canoe-independent mechanisms

Embryonic morphogenesis is powered by dramatic changes in cell shape and arrangement driven by the cytoskeleton and its connections to adherens junctions. This requires robust linkage allowing morphogenesis without disrupting tissue integrity. The small GTPase Rap1 is a key regulator of cell adhesion, controlling both cadherin-mediated and integrin-mediated processes. We have defined multiple roles in morphogenesis for one Rap1 effector, Canoe/Afadin, which ensures robust junction-cytoskeletal linkage. We now ask what mechanisms regulate Canoe and other junction-cytoskeletal linkers during Drosophila morphogenesis, defining roles for Rap1 and one of its guanine nucleotide exchange factor (GEF) regulators, Dizzy. Rap1 uses Canoe as one effector, regulating junctional planar polarity. However, Rap1 has additional roles in junctional protein localization and balanced apical constriction-in its absence, Bazooka/Par3 localization is fragmented, and cells next to mitotic cells apically constrict and invaginate, disrupting epidermal integrity. In contrast, the GEF Dizzy has phenotypes similar to but slightly less severe than Canoe loss, suggesting that this GEF regulates Rap1 action via Canoe. Taken together, these data reveal that Rap1 is a crucial regulator of morphogenesis, likely acting in parallel via Canoe and other effectors, and that different Rap1 GEFs regulate distinct functions of Rap1.

miR-9 and miR-263 Regulate the Key Genes of the ERK Pathway in the Ovary of Mud Crab Scylla paramamosain

Mud crab Scylla paramamosain is one of the most important economic crabs in China. The molecular regulatory mechanism of ovarian development has received considerable attention in recent years. Some studies found that ERK (extracellular signal-regulated protein kinase) signaling pathway plays an important role in ovarian development and is negatively regulated by microRNAs (miRNAs). However, the study about the regulation of miRNA on the ERK pathway in crustacean's ovary remains unknown. In this study, the target genes of the ERK signaling pathway regulated by selected miRNAs identified from the ovary of mud crab in our previous research were predicted by using bioinformatics tools. The results showed that the ERK2 might be a target gene of miR-9c, miR-263a, and miR-263b; MEK2 may be a target gene of miR-263a; and Rap-1b may be a target gene of miR-9, miR-9c, and miR-263a. Results of in vitro dual-luciferase reporter assay showed that the relative luciferase activities were significantly lower in HEK293T cells co-transfected with the combination of miRNA mimics and pmir-RB-REPORTTM-target gene-3'UTR than those with the combination of mimics NC and pmir-RB-REPORTTM-target gene-3'UTR. In contrast, the relative luciferase activities were significantly higher in HEK293T cells co-transfected with miRNA inhibitor than those with inhibitor NC. To further validate in vitro results, the miRNA reagents were injected into the living female mud crabs, and the expression levels of miRNAs and target genes after the injection were analyzed by quantitative real-time PCR. The in vivo experimental results showed that miRNAs (miR-9c/miR-263a) agomir (enhancers)/antagomir (inhibitors) can enhance/decrease the expression of two miRNAs, respectively, and the expression of target genes in the ovary was declined/increased after injection of agomir/antagomir reagent. In conclusion, miR-9/miR-263 can negatively regulate the expression of the ERK pathway genes (ERK2, MEK2, and Rap-1b) in the ovary of mud crab.

Collective Migrations of Drosophila Embryonic Trunk and Caudal Mesoderm-Derived Muscle Precursor Cells

Mesoderm migration in the Drosophila embryo is a highly conserved, complex process that is required for the formation of specialized tissues and organs, including the somatic and visceral musculature. In this FlyBook chapter, we will compare and contrast the specification and migration of cells originating from the trunk and caudal mesoderm. Both cell types engage in collective migrations that enable cells to achieve new positions within developing embryos and form distinct tissues. To start, we will discuss specification and early morphogenetic movements of the presumptive mesoderm, then focus on the coordinate movements of the two subtypes trunk mesoderm and caudal visceral mesoderm, ending with a comparison of these processes including general insights gained through study.

Rottlerin acts as a therapeutic in primary open-angle glaucoma by targeting the trabecular meshwork via activation of Rap1 signaling

Primary open-angle glaucoma (POAG) is a leading cause of irreversible blindness worldwide, and elevated intraocular pressure (IOP) is a major risk factor. While IOP is mainly controlled by adjusting the outflow resistance in the trabecular meshwork (TM), drugs that act directly on the TM are rare. In this study, we discovered a novel compound and pathway that acts on the TM and decreases IOP by genomic, proteomic, and bioinformatic analyses of POAG-derived TMs and experimental validation. Overlapping differentially expressed genes of the TM between patients with POAG and normal controls from two independent gene expression profiles in public databases were analyzed and matched by using the Connectivity Map (CMap). Rottlerin was identified as a potential compound. Subsequent experiments confirmed that rottlerin reversed POAG phenotypes in vitro and that it decreased IOP and actin/extracellular matrix accumulation in vivo with no detectable ocular side effects. SwissTargetPrediction in combination with pathway analysis predicted that the effects of rottlerin may be mediated by activation of the Rap1 pathway. Finally, we confirmed that rottlerin upregulated Rap1 and the downstream PI3K/AKT pathway independent of the MAPK/ERK pathway in a dexamethasone-induced POAG cell model.

The PDZ-GEF Gef26 regulates synapse development and function via FasII and Rap1 at the Drosophila neuromuscular junction

Guanine nucleotide exchange factors (GEFs) are essential for small G proteins to activate their downstream signaling pathways, which are involved in morphogenesis, cell adhesion, and migration. Mutants of Gef26, a PDZ-GEF (PDZ domain-containing guanine nucleotide exchange factor) in Drosophila, exhibit strong defects in wings, eyes, and the reproductive and nervous systems. However, the precise roles of Gef26 in development remain unclear. In the present study, we analyzed the role of Gef26 in synaptic development and function. We identified significant decreases in bouton number and branch length at larval neuromuscular junctions (NMJs) in Gef26 mutants, and these defects were fully rescued by restoring Gef26 expression, indicating that Gef26 plays an important role in NMJ morphogenesis. In addition to the observed defects in NMJ morphology, electrophysiological analyses revealed functional defects at NMJs, and locomotor deficiency appeared in Gef26 mutant larvae. Furthermore, Gef26 regulated NMJ morphogenesis by regulating the level of synaptic Fasciclin II (FasII), a well-studied cell adhesion molecule that functions in NMJ development and remodeling. Finally, our data demonstrate that Gef26-specific small G protein Rap1 worked downstream of Gef26 to regulate the level of FasII at NMJs, possibly through a βPS integrin-mediated signaling pathway. Taken together, our findings define a novel role of Gef26 in regulating NMJ development and function.

Rap1 GTPase promotes coordinated collective cell migration in vivo

During development and in cancer, cells often move together in small to large collectives. To move as a unit, cells within collectives need to stay coupled together and coordinate their motility. How cell collectives remain interconnected and migratory, especially when moving through in vivo environments, is not well understood. The genetically tractable border cell group undergoes a highly polarized and cohesive cluster-type migration in the Drosophila ovary. Here we report that the small GTPase Rap1, through activation by PDZ-GEF, regulates border cell collective migration. We find that Rap1 maintains cell contacts within the cluster, at least in part by promoting the organized distribution of E-cadherin at specific cell-cell junctions. Rap1 also restricts migratory protrusions to the front of the border cell cluster and promotes the extension of protrusions with normal dynamics. Further, Rap1 is required in the outer migratory border cells but not in the central nonmigratory polar cells. Such cell specificity correlates well with the spatial distribution of the inhibitory Rapgap1 protein, which is higher in polar cells than in border cells. We propose that precisely regulated Rap1 activity reinforces connections between cells and polarizes the cluster, thus facilitating the coordinated collective migration of border cells.

Rap1b enhances the invasion and migration of hepatocellular carcinoma cells by up-regulating Twist 1

Rap1b was found be dysregulated in several types of cancers. Previously, we have demonstrated that Rap1b affects proliferation, migration and invasion of hepatocellular carcinoma (HCC) cells. However, the definite function of Rap1b in HCC remains unknown. Here, we reported that Rap1b was significantly up-regulated in HCC tissues compared with the non-tumoral liver tissues. Overexpression of Rap1b promoted tumor growth and migration in vitro and tumor formation in vivo. Oppositely, inhibition of Rap1b suppressed the proliferation and migration of HCC cells. Mechanism study revealed that Rap1b could up-regulate Twist 1 expression by enhancing its promoter activity. We concluded that Rap1b increased Twist 1 expression by targeting its promoter activity to induce proliferation and migration of HCC cells.

From Ras to Rap and Back, a Journey of 35 Years

Our laboratory has studied Ras and Ras-like proteins since the discovery of the Ras oncogene 35 years ago. In this review, I will give an account of what we have done in these 35 years and indicate the main papers that have guided our research. Our efforts started with the early analysis of mutant Ras in human tumors followed by deciphering of the role of Ras in signal transduction pathways. In an attempt to interfere in Ras signaling we turned to Rap proteins. These proteins are the closest relatives of Ras and were initially identified as Ras antagonists. However, our studies revealed that the Rap signaling network primarily is involved in spatiotemporal control of cell adhesion, in part through regulation of the actin cytoskeleton. More recently we returned to Ras, trying to interfere in Ras signaling by combinatorial drug testing using the organoid technology.

Rap1 acts via multiple mechanisms to position Canoe and adherens junctions and mediate apical-basal polarity establishment

Epithelial apical-basal polarity drives assembly and function of most animal tissues. Polarity initiation requires cell-cell adherens junction assembly at the apical-basolateral boundary. Defining the mechanisms underlying polarity establishment remains a key issue. Drosophila embryos provide an ideal model, as 6000 polarized cells assemble simultaneously. Current data place the actin-junctional linker Canoe (fly homolog of Afadin) at the top of the polarity hierarchy, where it directs Bazooka/Par3 and adherens junction positioning. Here we define mechanisms regulating Canoe localization/function. Spatial organization of Canoe is multifaceted, involving membrane localization, recruitment to nascent junctions and macromolecular assembly at tricellular junctions. Our data suggest apical activation of the small GTPase Rap1 regulates all three events, but support multiple modes of regulation. The Rap1GEF Dizzy (PDZ-GEF) is crucial for Canoe tricellular junction enrichment but not apical retention. The Rap1-interacting RA domains of Canoe mediate adherens junction and tricellular junction recruitment but are dispensable for membrane localization. Our data also support a role for Canoe multimerization. These multifactorial inputs shape Canoe localization, correct Bazooka and adherens junction positioning, and thus apical-basal polarity. We integrate the existing data into a new polarity establishment model.

Regulation of Rap GTPases in mammalian neurons

Small GTPases are central regulators of many cellular processes. The highly conserved Rap GTPases perform essential functions in the mammalian nervous system during development and in mature neurons. During neocortical development, Rap1 is required to regulate cadherin- and integrin-mediated adhesion. In the adult nervous system Rap1 and Rap2 regulate the maturation and plasticity of dendritic spine and synapses. Although genetic studies have revealed important roles of Rap GTPases in neurons, their regulation by guanine nucleotide exchange factors (GEFs) that activate them and GTPase activating proteins (GAPs) that inactivate them by stimulating their intrinsic GTPase activity is just beginning to be explored in vivo. Here we review how GEFs and GAPs regulate Rap GTPases in the nervous system with a focus on their in vivo function.

Unraveling molecular pathways shared by Kabuki and Kabuki-like syndromes

Kabuki syndrome (KS) is a rare genetic syndrome characterized by a typical facial gestalt, variable degrees of intellectual disability, organ malformations, postnatal growth retardation and skeletal abnormalities. So far, KMT2D or KDM6A mutation has been identified as the main cause of KS, accounting for 56%-75% and 3%-8% of cases, respectively. Patients without mutations in 1 of the 2 causative KS genes are often referred to as affected by Kabuki-like syndrome. Overall, they represent approximately 30% of KS cases, pointing toward substantial genetic heterogeneity for this condition. Here, we review all currently available literature describing KS-like phenotypes (or phenocopies) associated with genetic variants located in loci different from KMT2D and KDM6A . We also report on a new KS phenocopy harboring a 5 Mb de novo deletion in chr10p11.22-11.21. An enrichment analysis aimed at identifying functional Gene Ontology classes shared by the 2 known KS causative genes and by new candidate genes currently associated with KS-like phenotypes primarily converges upon abnormal chromatin remodeling and transcriptional dysregulation as pivotal to the pathophysiology of KS phenotypic hallmarks. The identification of mutations in genes belonging to the same functional pathways of KMT2D and KDM6A can help design molecular screenings targeted to KS-like phenotypes.

Plexins function in epithelial repair in both Drosophila and zebrafish

In most multicellular organisms, homeostasis is contingent upon maintaining epithelial integrity. When unanticipated insults breach epithelial barriers, dormant programmes of tissue repair are immediately activated. However, many of the mechanisms that repair damaged epithelia remain poorly characterized. Here we describe a role for Plexin A (PlexA), a protein with particularly well-characterized roles in axonal pathfinding, in the healing of damaged epithelia in Drosophila. Semaphorins, which are PlexA ligands, also regulate tissue repair. We show that Drosophila PlexA has GAP activity for the Rap1 GTPase, which is known to regulate the stability of adherens junctions. Our observations suggest that the inhibition of Rap1 activity by PlexA in damaged Drosophila epithelia allows epithelial remodelling, thus facilitating wound repair. We also demonstrate a role for Plexin A1, a zebrafish orthologue of Drosophila PlexA, in epithelial repair in zebrafish tail fins. Thus, plexins function in epithelial wound healing in diverse taxa.

Plexins are semaphorin receptors and are well known for their roles in neuronal pathfinding. Here the authors describe a role for Plexin A in healing damaged epithelia in Drosophila and zebrafish. In Drosophila, Plexin A inhibits the GTPase Rap1 to allow epithelial remodelling to facilitate wound repair.

Small GTPase Rap1 Is Essential for Mouse Development and Formation of Functional Vasculature

Background

Small GTPase Rap1 has been implicated in a number of basic cellular functions, including cell-cell and cell-matrix adhesion, proliferation and regulation of polarity. Evolutionarily conserved, Rap1 has been studied in model organisms: yeast, Drosophila and mice. Mouse in vivo studies implicate Rap1 in the control of multiple stem cell, leukocyte and vascular cell functions. In vitro, several Rap1 effectors and regulatory mechanisms have been proposed. In particular, Rap1 has been implicated in maintaining epithelial and endothelial cell junction integrity and linked with cerebral cavernous malformations.

Rationale

How Rap1 signaling network controls mammalian development is not clear. As a first step in addressing this question, we present phenotypes of murine total and vascular-specific Rap1a, Rap1b and double Rap1a and Rap1b (Rap1) knockout (KO) mice.

Results and Conclusions

The majority of total Rap1 KO mice die before E10.5, consistent with the critical role of Rap1 in epithelial morphogenesis. At that time point, about 50% of Tie2-double Rap1 KOs appear grossly normal and develop normal vasculature, while the remaining 50% suffer tissue degeneration and show vascular abnormalities, including hemorrhages and engorgement of perineural vessels, albeit with normal branchial arches. However, no Tie2-double Rap1 KO embryos are present at E15.5, with hemorrhages a likely cause of death. Therefore, at least one Rap1 allele is required for development prior to the formation of the vascular system; and in endothelium–for the life-supporting function of the vasculature.

The small GTPase Rap1 is a modulator of Hedgehog signaling

During development, the evolutionarily conserved Hedgehog (Hh) morphogen provides instructional cues that influence cell fate, cell affinity and tissue morphogenesis. To do so, the Hh signaling cascade must coordinate its activity with other morphogenetic signals. This can occur through engagement of or response to effectors that do not typically function as core Hh pathway components. Given the ability of small G proteins of the Ras family to impact cell survival, differentiation, growth and adhesion, we wanted to determine whether Hh and Ras signaling might intersect during development. We performed genetic modifier tests in Drosophila to examine the ability of select Ras family members to influence Hh signal output, and identified Rap1 as a positive modulator of Hh pathway activity. Our results suggest that Rap1 is activated to its GTP-bound form in response to Hh ligand, and that the GTPase exchange factor C3G likely contributes to this activation. The Rap1 effector Canoe (Cno) also impacts Hh signal output, suggesting that a C3G-Rap1-Cno axis intersects the Hh pathway during tissue morphogenesis.

Rap1 GTPase is required for mouse lens epithelial maintenance and morphogenesis

Rap1, a Ras-like small GTPase, plays a crucial role in cell-matrix adhesive interactions, cell-cell junction formation, cell polarity and migration. The role of Rap1 in vertebrate organ development and tissue architecture, however, remains elusive. We addressed this question in a mouse lens model system using a conditional gene targeting approach. While individual germline deficiency of either Rap1a or Rap1b did not cause overt defects in mouse lens, conditional double deficiency (Rap1 cKO) prior to lens placode formation led to an ocular phenotype including microphthalmia and lens opacification in embryonic mice. The embryonic Rap1 cKO mouse lens exhibited striking defects including loss of E-cadherin- and ZO-1-based cell-cell junctions, disruption of paxillin and β1-integrin-based cell adhesive interactions along with abnormalities in cell shape and apical-basal polarity of epithelium. These epithelial changes were accompanied by increased levels of α-smooth muscle actin, vimentin and N-cadherin, and expression of transcriptional suppressors of E-cadherin (Snai1, Slug and Zeb2), and a mesenchymal metabolic protein (Dihydropyrimidine dehydrogenase). Additionally, while lens differentiation was not overtly affected, increased apoptosis and dysregulated cell cycle progression were noted in epithelium and fibers in Rap1 cKO mice. Collectively these observations uncover a requirement for Rap1 in maintenance of lens epithelial phenotype and morphogenesis.

Fine-Tuning of the Actin Cytoskeleton and Cell Adhesion During Drosophila Development by the Unconventional Guanine Nucleotide Exchange Factors Myoblast City and Sponge

The evolutionarily conserved Dock proteins function as unconventional guanine nucleotide exchange factors (GEFs). Upon binding to engulfment and cell motility (ELMO) proteins, Dock-ELMO complexes activate the Rho family of small GTPases to mediate a diverse array of biological processes, including cell motility, apoptotic cell clearance, and axon guidance. Overlapping expression patterns and functional redundancy among the 11 vertebrate Dock family members, which are subdivided into four families (Dock A, B, C, and D), complicate genetic analysis. In both vertebrate and invertebrate systems, the actin dynamics regulator, Rac, is the target GTPase of the Dock-A subfamily. However, it remains unclear whether Rac or Rap1 are the in vivo downstream GTPases of the Dock-B subfamily. Drosophila melanogaster is an excellent genetic model organism for understanding Dock protein function as its genome encodes one ortholog per subfamily: Myoblast city (Mbc; Dock A) and Sponge (Spg; Dock B). Here we show that the roles of Spg and Mbc are not redundant in the Drosophila somatic muscle or the dorsal vessel. Moreover, we confirm the in vivo role of Mbc upstream of Rac and provide evidence that Spg functions in concert with Rap1, possibly to regulate aspects of cell adhesion. Together these data show that Mbc and Spg can have different downstream GTPase targets. Our findings predict that the ability to regulate downstream GTPases is dependent on cellular context and allows for the fine-tuning of actin cytoskeletal or cell adhesion events in biological processes that undergo cell morphogenesis.

The small GTPase Rap1 promotes cell movement rather than stabilizes adhesion in epithelial cells responding to insulin-like growth factor I

The Ras-related GTPase Rap1 promotes cell adhesion and migration. Although the significance of Rap1 contribution to cell migration is increasingly being recognized, little is known about the biochemical mechanisms driving this process. In the present study, we discovered a previously unidentified regulatory role of insulin-like growth factor type I (IGF-I) receptor (IGF-IR) in CRK Src homology 3 (SH3)-binding guanine-nucleotide-releasing protein (C3G)-Rap1-fascin-actin axis promoting cell movement. We demonstrate that a burst of Rap1 activity, rather than presumed hyperactivation, is imperative for the onset of cell movement. We show that while autophosphorylated IGF-IR signals to C3G to activate Rap1, subsequent IGF-IR internalization promotes gradual inactivation of Rap1 by putative Rap1 GTPase-activating protein (GAP). Additionally, IGF-IR signalling recruits active Rap1 at sites of cell motile protrusions. C3G depletion prevents IGF-I-induced fascin accumulation at actin microspikes and blocks protrusions. In the absence of IGF-IR activity, the wild-type (WT) Rap1 and the constitutively active V12Rap1 mutant remain in cell-cell contacts. Forced inactivation of Rap1 signalling by overexpressing dominant negative N17Rap1, Rap1GAP or by silencing C3G has a detrimental effect on filamentous (F)-actin and cell adhesion irrespective of IGF-IR signalling. We conclude that the basal levels of Rap1 activity holds up cell adhesion, whereas sequential regulation of C3G and GAP by IGF-IR reverses the labile Rap1 function from supporting adhesion to promoting migration.

Tumor cell migration and invasion are enhanced by depletion of Rap1 GTPase-activating protein (Rap1GAP)

The functional significance of the widespread down-regulation of Rap1 GTPase-activating protein (Rap1GAP), a negative regulator of Rap activity, in human tumors is unknown. Here we show that human colon cancer cells depleted of Rap1GAP are endowed with more aggressive migratory and invasive properties. Silencing Rap1GAP enhanced the migration of confluent and single cells. In the latter, migration distance, velocity, and directionality were increased. Enhanced migration was a consequence of increased endogenous Rap activity as silencing Rap expression selectively abolished the migration of Rap1GAP-depleted cells. ROCK-mediated cell contractility was suppressed in Rap1GAP-depleted cells, which exhibited a spindle-shaped morphology and abundant membrane protrusions. Tumor cells can switch between Rho/ROCK-mediated contractility-based migration and Rac1-mediated mesenchymal motility. Strikingly, the migration of Rap1GAP-depleted, but not control cells required Rac1 activity, suggesting that loss of Rap1GAP alters migratory mechanisms. Inhibition of Rac1 activity restored membrane blebbing and increased ROCK activity in Rap1GAP-depleted cells, suggesting that Rac1 contributes to the suppression of contractility. Collectively, these findings identify Rap1GAP as a critical regulator of aggressive tumor cell behavior and suggest that the level of Rap1GAP expression influences the migratory mechanisms that are operative in tumor cells.

Global gene expression profiling of individual human oocytes and embryos demonstrates heterogeneity in early development

Early development in humans is characterised by low and variable embryonic viability, reflected in low fecundity and high rates of miscarriage, relative to other mammals. Data from assisted reproduction programmes provides additional evidence that this is largely mediated at the level of embryonic competence and is highly heterogeneous among embryos. Understanding the basis of this heterogeneity has important implications in a number of areas including: the regulation of early human development, disorders of pregnancy, assisted reproduction programmes, the long term health of children which may be programmed in early development, and the molecular basis of pluripotency in human stem cell populations. We have therefore investigated global gene expression profiles using polyAPCR amplification and microarray technology applied to individual human oocytes and 4-cell and blastocyst stage embryos. In order to explore the basis of any variability in detail, each developmental stage is replicated in triplicate. Our data show that although transcript profiles are highly stage-specific, within each stage they are relatively variable. We describe expression of a number of gene families and pathways including apoptosis, cell cycle and amino acid metabolism, which are variably expressed and may be reflective of embryonic developmental competence. Overall, our data suggest that heterogeneity in human embryo developmental competence is reflected in global transcript profiles, and that the vast majority of existing human embryo gene expression data based on pooled oocytes and embryos need to be reinterpreted.

Distinct Rap1 activity states control the extent of epithelial invagination via α-catenin

Localized cell shape change initiates epithelial folding, while neighboring cell invagination determines the final depth of an epithelial fold. The mechanism that controls the extent of invagination remains unknown. During Drosophila gastrulation, a higher number of cells undergo invagination to form the deep posterior dorsal fold, whereas far fewer cells become incorporated into the initially very similar anterior dorsal fold. We find that a decrease in α-catenin activity causes the anterior fold to invaginate as extensively as the posterior fold. In contrast, constitutive activation of the small GTPase Rap1 restricts invagination of both dorsal folds in an α-catenin-dependent manner. Rap1 activity appears spatially modulated by Rapgap1, whose expression levels are high in the cells that flank the posterior fold but low in the anterior fold. We propose a model whereby distinct activity states of Rap1 modulate α-catenin-dependent coupling between junctions and actin to control the extent of epithelial invagination.

Segmental assembly of fibronectin matrix requires rap1b and integrin α5

BACKGROUND

During segmentation of the zebrafish embryo, inside-out signaling activates Integrin α5, which is necessary for somite border morphogenesis. The direct activator of Integrin α5 during this process is unknown. One candidate is Rap1b, a small monomeric GTPase implicated in Integrin activation in the immune system.

RESULTS

Knockdown of rap1b, or overexpression of a dominant negative rap1b, causes a mild axis elongation defect in zebrafish. However, disruption of rap1b function in integrin α5(-/-) mutants results in a strong reduction in Fibronectin (FN) matrix assembly in the paraxial mesoderm and a failure in somite border morphogenesis along the entire anterior-posterior axis. Somite patterning appears unaffected, as her1 oscillations are maintained in single and double morphants/mutants, but somite polarity is gradually lost in itgα5(-/-) ; rap1b MO embryos.

CONCLUSIONS

In itgα5(-) (/) (-) mutants, rap1b is required for proper somite border morphogenesis in zebrafish. The loss of somite borders is not a result of aberrant segmental patterning. Rather, somite boundary formation initiates but is not completed, due to the failure to assemble FN matrix along the nascent boundary. We propose a model in which Rap1b activates Integrin/Fibronectin receptors as part of an "inside-out" signaling pathway that promotes Integrin binding to FN, FN matrix assembly, and subsequent stabilization of morphological somite boundaries.

Radil controls neutrophil adhesion and motility through β2-integrin activation

Various agonists trigger β2-integrin activation in neutrophils, yet the mechanisms that regulate β2-integrin inside-out signaling remain obscure. Radil, a novel Rap downstream effector, is an important adapter in the pathway that links G protein–coupled chemoattractant receptors to adhesion complexes during neutrophil chemotaxis.

Integrin activation is required to facilitate multiple adhesion-dependent functions of neutrophils, such as chemotaxis, which is critical for inflammatory responses to injury and pathogens. However, little is known about the mechanisms that mediate integrin activation in neutrophils. We show that Radil, a novel Rap1 effector, regulates β1- and β2-integrin activation and controls neutrophil chemotaxis. On activation and chemotactic migration of neutrophils, Radil quickly translocates from the cytoplasm to the plasma membrane in a Rap1a-GTP–dependent manner. Cells overexpressing Radil show a substantial increase in cell adhesion, as well as in integrin/focal adhesion kinase (FAK) activation, and exhibit an elongated morphology, with severe tail retraction defects. This phenotype is effectively rescued by treatment with either β2-integrin inhibitory antibodies or FAK inhibitors. Conversely, knockdown of Radil causes severe inhibition of cell adhesion, β2-integrin activation, and chemotaxis. Furthermore, we found that inhibition of Rap activity by RapGAP coexpression inhibits Radil-mediated integrin and FAK activation, decreases cell adhesion, and abrogates the long-tail phenotype of Radil cells. Overall, these studies establish that Radil regulates neutrophil adhesion and motility by linking Rap1 to β2-integrin activation.

A big new job for small GTPases

Members of the Ras superfamily of small guanosine triphosphatases (GTPases) function as key nodes within signaling networks in a remarkable range of cellular processes, including cell proliferation, differentiation, growth, cell-cell adhesion and apoptosis. We recently described a novel role for the Ras-like small GTPases Rap1 and Ral in regulating cortical polarity and spindle orientation during asymmetric neuroblast division in Drosophila. The participation of these proteins in promoting cell polarization seems to be a common theme throughout evolution.

Ras-proximate-1 GTPase-activating protein and Rac2 may play pivotal roles in the initial development of myelodysplastic syndrome

Myelodysplastic syndrome (MDS) is a stem cell disease that has a characteristic morphological dysplasia. Adhesion molecules and the Wnt signaling pathway are mostly involved with the self-renewal, proliferation and differentiation of hematopoietic stem cells (HSCs) while Rho GTPases are closely correlated with the cytoskeleton and therefore cell morphology. To gain insight into the poorly understood pathophysiology of MDS, the present study focused on analyzing the gene expression profiles of these molecules with whole genomic array using CD34(+) cells from MDS patients. These profiles showed that N-cadherin, E-cadherin and c-myc binding protein tended to be downregulated, whereas β-catenin, Ras-proximate-1 GTPase-activating protein (Rap1GAP), c-myc promoter binding protein, Rac1, Rac2 and CDC42 tended to be upregulated. However, no change in the expression of genes involved in the canonical Wnt signaling pathway, with the exception of β-catenin, was observed. The array results were confirmed by real-time quantitative polymerase chain reaction (RQ-PCR) using CD34(+) cells from a cohort of patients with MDS-refractory anemia (RA) [WHO (2008) RCUD, RCMD and MDS-U] who had normal karyotypes. Only Rap1GAP and Rac2 showed higher expression levels when mononuclear cells were used from another group of patients with MDS-RA [WHO (2008) RCUD, RCMD and MDS-U] who also had normal karyotypes. We believe that the cadherin-β-catenin-c-myc signaling axis is crucial in the hematopoiesis of HSCs in the early stages of MDS. In addition, Ras-proximate-1 (Rap1), which is negatively regulated by Rap1GAP, may serve as an initiator of this axis through interplay with cadherin. This pathway is strengthened by the upregulation of Rac2, which may allow the nuclear translocation of β-catenin. The aberrant expression of Rho GTPases may also be responsible for the dysplasia characteristics observed in MDS. This study provides vital and new insights into the pathophysiology of MDS. The two small G proteins, Rap1GAP and Rac2, may act as new molecular markers for the diagnosis of MDS.

The PDZ-GEF protein Dizzy regulates the establishment of adherens junctions required for ventral furrow formation in Drosophila

The PDZ-GEF protein Dizzy (Dzy) and its downstream GTPase Rap1 have pleiotropic roles during development of the Drosophila embryo. Here, we show that maternally provided Dzy and Rap1 first function during ventral furrow formation (VFF) where they are critical to guarantee rapid apical cell constrictions. Contraction of the apical actomyosin filament system occurs independently of Dzy and Rap1, but loss of Dzy results in a delayed establishment of the apical adherens junction (AJ) belt, whereas in the absence of Rap1 only a fragmentary apical AJ belt is formed in the epithelium. The timely establishment of apical AJs appears to be essential for coupling actomyosin contractions to cell shape change and to assure completion of the ventral furrow. Immediately after VFF, the downregulation of Dzy and Rap1 is necessary to allow normal mesodermal development to continue after the epithelial-to-mesenchymal transition, as overexpression of Dzy or of constitutively active Rap1 compromises mesodermal migration and monolayer formation. We propose that Dzy and Rap1 are crucial factors regulating the dynamics of AJs during gastrulation.

The Rap1-Rgl-Ral signaling network regulates neuroblast cortical polarity and spindle orientation

The Rap1–Rgl–Ral signaling network modulates asymmetric Drosophila neuroblast division in cooperation with other intrinsic polarity cues.

A crucial first step in asymmetric cell division is to establish an axis of cell polarity along which the mitotic spindle aligns. Drosophila melanogaster neural stem cells, called neuroblasts (NBs), divide asymmetrically through intrinsic polarity cues, which regulate spindle orientation and cortical polarity. In this paper, we show that the Ras-like small guanosine triphosphatase Rap1 signals through the Ral guanine nucleotide exchange factor Rgl and the PDZ protein Canoe (Cno; AF-6/Afadin in vertebrates) to modulate the NB division axis and its apicobasal cortical polarity. Rap1 is slightly enriched at the apical pole of metaphase/anaphase NBs and was found in a complex with atypical protein kinase C and Par6 in vivo. Loss of function and gain of function of Rap1, Rgl, and Ral proteins disrupt the mitotic axis orientation, the localization of Cno and Mushroom body defect, and the localization of cell fate determinants. We propose that the Rap1–Rgl–Ral signaling network is a novel mechanism that cooperates with other intrinsic polarity cues to modulate asymmetric NB division.

Cloning and characterization of Rap GTPase from the Chinese white shrimp Fenneropenaeus chinensis

Ras-related protein Rap GTPase has been implicated in cell adhesion, cell proliferation, and cell junction formation. The first shrimp Rap cDNA (FcRap) was recently identified from the Chinese white shrimp Fenneropenaeus chinensis. The full length of FcRap is 1013 bp, with a 561 bp open reading frame that encodes a 186 amino acid protein. FcRap has a calculated molecular mass of 20.90 kDa and pI of 6.37. Phylogenetic analysis shows that FcRap and other Rap proteins are clustered into one group. Results from the quantitative real-time polymerase chain reaction show that FcRap could be detected mainly in the hemocytes, hepatopancreas, stomach, and gills, whereas a relatively lower expression level could be detected in the heart and intestines. FcRap in the hemocytes was upregulated 2h post Vibrio challenge, and it was upregulated 2h post white spot syndrome virus (WSSV) challenge, and peaked at 6h before it declined at 12h. No variation in the FcRap transcript was observed in the gills under the Vibrio challenge, but it was initially downregulated 2h post WSSV challenge, and then it was upregulated and peaked at 6h before it was eventually went down at 12h. The rFcRap protein was successfully expressed in Escherichia coli BL21DE3. The pull-down analysis showed that rFcRap protein could interact with VP28, an envelope protein of WSSV. The probable roles of Rap GTPase in shrimp innate immunity are presented for the first time.

The tumor suppressor RASSF1A is a novel effector of small G protein Rap1A

Rap1A is a small G protein implicated in a spectrum of biological processes such as cell proliferation, adhesion, differentiation, and embryogenesis. The downstream effectors through which Rap1A mediates its diverse effects are largely unknown. Here we show that Rap1A, but not the related small G proteins Rap2 or Ras, binds the tumor suppressor Ras association domain family 1A (RASSF1A) in a manner that is regulated by phosphorylation of RASSF1A. Interaction with Rap1A is shown to influence the effect of RASSF1A on microtubule behavior.

The effect of maternal subclinical hypothyroidism during pregnancy on brain development in rat offspring

BACKGROUND

Maternal thyroid hormones (THs), especially thyroxine (T(4)), are crucial to early brain development in the mammalian embryo. Epidemiological studies and case reports have shown that maternal subclinical hypothyroidism may result in significant negative effects on pregnancy and neurodevelopment of the fetus. To understand the mechanism responsible for these neurological alterations, we induced maternal subclinical hypothyroidism in pregnant rats. Behavior and several genes that are under the control of THs were evaluated in the offspring of TH-deficient rats.

METHODS

A total of 60 female rats were divided into three groups: (i) maternal subclinical hypothyroidism (total thyroidectomy with T(4) infusion), (ii) maternal hypothyroidism (total thyroidectomy without T(4) infusion), and (iii) control (sham operated). All rats were mated 10 days after the start of infusion. The infusion continued until 10 days postpartum. Pups were sacrificed at postnatal day 3 (PND 3), PND 7, and PND 21. The hippocampus was collected and tested for brain-derived neurotrophic factor (BDNF) and Rap1 protein expression by Western blotting and for BDNF and neural cell adhesion molecule mRNA expression by real-time polymerase chain reaction. On PND 41-PND 49, rat pups explored the Morris water maze. Time spent in the quadrant previously containing the platform was recorded.

RESULTS

This study found decreases in BDNF mRNA expression (on PND 3) and protein level (on PND 3 and PND 7) in hippocampi of pups from subclinical hypothyroidism dams (p < 0.05). Rap1 protein expression was higher in maternal subclinical hypothyroidism offspring than in control offspring at PND 7 and PND 21. No change was observed in neural cell adhesion molecule mRNA expression in the maternal subclinical hypothyroidism offspring. In addition, results from the Morris water maze revealed that pups from the subclinical hypothyroidism dams showed deficits in long-term memory, spending less time in the platform quadrant (p < 0.05) during testing. There was a trend toward a deficit in short-term memory (p > 0.05) in this group as well.

CONCLUSIONS

The long-term memory deficits of pups born to maternal subclinical hypothyroidism dams likely related with decreasing in BDNF mRNA expression and protein level as well as increasing in Rap1 protein expression in hippocampi.

Mesoderm migration in Drosophila is a multi-step process requiring FGF signaling and integrin activity

Migration is a complex, dynamic process that has largely been studied using qualitative or static approaches. As technology has improved, we can now take quantitative approaches towards understanding cell migration using in vivo imaging and tracking analyses. In this manner, we have established a four-step model of mesoderm migration during Drosophila gastrulation: (I) mesodermal tube formation, (II) collapse of the mesoderm, (III) dorsal migration and spreading and (IV) monolayer formation. Our data provide evidence that these steps are temporally distinct and that each might require different chemical inputs. To support this, we analyzed the role of fibroblast growth factor (FGF) signaling, in particular the function of two Drosophila FGF ligands, Pyramus and Thisbe, during mesoderm migration. We determined that FGF signaling through both ligands controls movements in the radial direction. Thisbe is required for the initial collapse of the mesoderm onto the ectoderm, whereas both Pyramus and Thisbe are required for monolayer formation. In addition, we uncovered that the GTPase Rap1 regulates radial movement of cells and localization of the beta-integrin subunit, Myospheroid, which is also required for monolayer formation. Our analyses suggest that distinct signals influence particular movements, as we found that FGF signaling is involved in controlling collapse and monolayer formation but not dorsal movement, whereas integrins are required to support monolayer formation only and not earlier movements. Our work demonstrates that complex cell migration is not necessarily a fluid process, but suggests instead that different types of movements are directed by distinct inputs in a stepwise manner.

A unique interplay between Rap1 and E-cadherin in the endocytic pathway regulates self-renewal of human embryonic stem cells

Regulatory mechanisms pertaining to the self-renewal of stem cells remain incompletely understood. Here, we show that functional interactions between small GTPase Rap1 and the adhesion molecule E-cadherin uniquely regulate the self-renewal of human embryonic stem cells (hESCs). Inhibition of Rap1 suppresses colony formation and self-renewal of hESCs, whereas overexpression of Rap1 augments hESC clonogenicity. Rap1 does not directly influence the expression of the pluripotency genes Oct4 and Nanog. Instead, it affects the endocytic recycling pathway involved in the formation and maintenance of E-cadherin-mediated cell-cell cohesion, which is essential for the colony formation and self-renewal of hESCs. Conversely, distinct from epithelial cells, disruption of E-cadherin mediated cell-cell adhesions induces lysosome delivery and degradation of Rap1. This in turn leads to a further downregulation of E-cadherin function and a subsequent reduction in hESC clonogenic capacity. These findings provide the first demonstration that the interplay between Rap1 and E-cadherin along the endocytic recycling pathway serves as a timely and efficient mechanism to regulate hESC self-renewal. Given the availability of specific activators for Rap1, this work provides a new perspective to enable better maintenance of human pluripotent stem cells.

Rap1, a mercenary among the Ras-like GTPases

The small Ras-like GTPase Rap1 is an evolutionary conserved protein that originally gained interest because of its capacity to revert the morphological phenotype of Ras-transformed fibroblasts. Rap1 is regulated by a large number of stimuli that include growth factors and cytokines, but also physical force and osmotic stress. Downstream of Rap1, a plethora of effector molecules has been proposed on the basis of biochemical studies. Here, we present an overview of genetic studies on Rap1 in various model organisms and relate the observed phenotypes to in vitro studies. The picture that emerges is one in which Rap1 is a versatile regulator of morphogenesis, by regulating diverse processes that include establishment of cellular polarity, cell-matrix interactions and cell-cell adhesion. Surprisingly, genetic experiments indicate that in the various model organisms, Rap1 uses distinct effector molecules that impinge upon the actin cytoskeleton and adhesion molecules.

Involvement of Rap-1 activation and early termination of immune synapse in CTLA-4-mediated negative signal

Cytotoxic T lymphocyte antigen 4 (CTLA-4) is a T cell co-stimulation receptor that delivers inhibitory signals upon activation. This inhibitory effect by CTLA-4 requires activation of small GTPase Rap-1. However, the precise mechanism underlying these negative signals remains unclear. Here, we show that CTLA-4-induced suppression of IL-2 production correlates with rapid destabilization of immunological synapse (IS) formation in murine normal T cell clones. Overexpression of Spa-1, a Rap-1-specific GTPase activating protein (GAP), abolished both Rap-1 activation and IL-2 suppression induced by CTLA-4. Although we failed to find any specific inhibition of activation of early signals upon CTLA-4 engagement, we found that CTLA-4 specifically up-regulates cell motility and suppresses prolonged accumulation of Talin at the contact area with antigen presenting cells upon antigen stimulation. These results suggest that Rap-1 is activated upon CTLA-4 ligation and mediates inhibitory signals through prevention of IS formation.

The Drosophila afadin homologue Canoe regulates linkage of the actin cytoskeleton to adherens junctions during apical constriction

Cadherin-based adherens junctions (AJs) mediate cell adhesion and regulate cell shape change. The nectin–afadin complex also localizes to AJs and links to the cytoskeleton. Mammalian afadin has been suggested to be essential for adhesion and polarity establishment, but its mechanism of action is unclear. In contrast, Drosophila melanogaster’s afadin homologue Canoe (Cno) has suggested roles in signal transduction during morphogenesis. We completely removed Cno from embryos, testing these hypotheses. Surprisingly, Cno is not essential for AJ assembly or for AJ maintenance in many tissues. However, morphogenesis is impaired from the start. Apical constriction of mesodermal cells initiates but is not completed. The actomyosin cytoskeleton disconnects from AJs, uncoupling actomyosin constriction and cell shape change. Cno has multiple direct interactions with AJ proteins, but is not a core part of the cadherin–catenin complex. Instead, Cno localizes to AJs by a Rap1- and actin-dependent mechanism. These data suggest that Cno regulates linkage between AJs and the actin cytoskeleton during morphogenesis.

Control of cell adhesion dynamics by Rap1 signaling

Individual cells in their particular environments adhere to the extracellular matrix (ECM) and their neighbours via integrin-containing and cadherin-containing complexes, respectively. The dynamics of these interactions regulate the formation and maintenance of complex tissues. An expanding body of evidence accentuates the role of the small Rap1 GTPase and its associated signaling network in many of these processes. In this review we will discuss more recently revealed roles of Rap1 signaling by primarily focusing on functions of the Rap1 effectors RIAM, KRIT-1/CCM1 and AF-6/Afadin in junctional regulation of the vascular system and in epithelial cells. Furthermore, we will describe novel findings on the Rap activator PDZ-GEF in the regulation of cell-cell adhesion between epithelial cells and within a stem cell niche.

Switch in FGF signalling initiates glial differentiation in the Drosophila eye

The formation of a complex nervous system requires the intricate interaction of neurons and glial cells. Glial cells generally migrate over long distances before they initiate their differentiation, which leads to wrapping and insulation of axonal processes. The molecular pathways coordinating the switch from glial migration to glial differentiation are largely unknown. Here we demonstrate that, within the Drosophila eye imaginal disc, fibroblast growth factor (FGF) signalling coordinates glial proliferation, migration and subsequent axonal wrapping. Glial differentiation in the Drosophila eye disc requires a succession from glia-glia interaction to glia-neuron interaction. The neuronal component of the fly eye develops in the peripheral nervous system within the eye-antennal imaginal disc, whereas glial cells originate from a pool of central-nervous-system-derived progenitors and migrate onto the eye imaginal disc. Initially, glial-derived Pyramus, an FGF8-like ligand, modulates glial cell number and motility. A switch to neuronally expressed Thisbe, a second FGF8-like ligand, then induces glial differentiation. This switch is accompanied by an alteration in the intracellular signalling pathway through which the FGF receptor channels information into the cell. Our findings reveal how a switch from glia-glia interactions to glia-neuron interactions can trigger formation of glial membrane around axonal trajectories. These results disclose an evolutionarily conserved control mechanism of axonal wrapping, indicating that Drosophila might serve as a model to understand glial disorders in humans.

Rap1 maintains adhesion between cells to affect Egfr signaling and planar cell polarity in Drosophila

The small GTPase Rap1 affects cell adhesion and cell motility in numerous developmental contexts. Loss of Rap1 in the Drosophila wing epithelium disrupts adherens junction localization, causing mutant cells to disperse, and dramatically alters epithelial cell shape. While the adhesive consequences of Rap1 inactivation have been well described in this system, the effects on cell signaling, cell fate specification, and tissue differentiation are not known. Here we demonstrate that Egfr-dependent cell types are lost from Rap1 mutant tissue as an indirect consequence of DE-cadherin mislocalization. Cells lacking Rap1 in the developing wing and eye are capable of responding to an Egfr signal, indicating that Rap1 is not required for Egfr/Ras/MAPK signal transduction. Instead, Rap1 regulates adhesive contacts necessary for maintenance of Egfr signaling between cells, and differentiation of wing veins and photoreceptors. Rap1 is also necessary for planar cell polarity in these tissues. Wing hair alignment and ommatidial rotation, functional readouts of planar cell polarity in the wing and eye respectively, are both affected in Rap1 mutant tissue. Finally, we show that Rap1 acts through the effector Canoe to regulate these developmental processes.

Juvenile hormone regulation of Drosophila Epac--a guanine nucleotide exchange factor

Real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) was used to characterize the effects of juvenile hormone (JH) on Epac (Exchange Protein directly Activated by Cyclic AMP; NM_001103732), a guanine nucleotide exchange factor for Rap1 in Drosophila S2 cells. JH treatment led to a rapid, dose-dependent increase in Epac relative expression ratio (RER) when compared to treatment with methyl linoleate (MLA) that lacks biological activity. The minimal level of hormone needed to elicit a response was 100 ng/ml. Time-course studies indicated a significant rise in the RER 1h after treatment. S2 cells were challenged with 20-hydroxyecdysone and a series of compounds similar in structure to JH to determine the specificity of the response. Methoprene and JH III displayed the greatest increases in RER. Late third instar (96 h) Drosophila were exposed to diet containing methoprene (500 ng/g diet); significantly higher RERs for Epac were observed 12h after exposure. JH had no effect on Epac RERs in the human cell line HEK-293.

EPAC proteins transduce diverse cellular actions of cAMP

It has now been over 10 years since efforts to completely understand the signalling actions of cAMP (3'-5'-cyclic adenosine monophosphate) led to the discovery of exchange protein directly activated by cAMP (EPAC) proteins. In the current review we will highlight important advances in the understanding of EPAC structure and function and demonstrate that EPAC proteins mediate multiple actions of cAMP in cells, revealing future targets for pharmaceutical intervention. It has been known for some time that drugs that elevate intracellular cAMP levels have proven therapeutic benefit for diseases ranging from depression to inflammation. The challenge now is to determine which of these positive actions of cAMP involve activation of EPAC-regulated signal transduction pathways. EPACs are specific guanine nucleotide exchange factors for the Ras GTPase homologues, Rap1 and Rap2, which they activate independently of the classical routes for cAMP signalling, cyclic nucleotide-gated ion channels and protein kinase A. Rather, EPAC activation is triggered by internal conformational changes induced by direct interaction with cAMP. Leading from this has been the development of EPAC-specific agonists, which has helped to delineate numerous cellular actions of cAMP that rely on subsequent activation of EPAC. These include regulation of exocytosis and the control of cell adhesion, growth, division and differentiation. Recent work also implicates EPAC in the regulation of anti-inflammatory signalling in the vascular endothelium, namely negative regulation of pro-inflammatory cytokine signalling and positive support of barrier function. Further elucidation of these important signalling mechanisms will no doubt support the development of the next generation of anti-inflammatory drugs.

Combinatorial interaction between CCM pathway genes precipitates hemorrhagic stroke

Intracranial hemorrhage (ICH) is a particularly severe form of stroke whose etiology remains poorly understood, with a highly variable appearance and onset of the disease (Felbor et al., 2006; Frizzell, 2005; Lucas et al., 2003). In humans, mutations in any one of three CCM genes causes an autosomal dominant genetic ICH disorder characterized by cerebral cavernous malformations (CCM). Recent evidence highlighting multiple interactions between the three CCM gene products and other proteins regulating endothelial junctional integrity suggests that minor deficits in these other proteins could potentially predispose to, or help to initiate, CCM, and that combinations of otherwise silent genetic deficits in both the CCM and interacting proteins might explain some of the variability in penetrance and expressivity of human ICH disorders. Here, we test this idea by combined knockdown of CCM pathway genes in zebrafish. Reducing the function of rap1b, which encodes a Ras GTPase effector protein for CCM1/Krit1, disrupts endothelial junctions in vivo and in vitro, showing it is a crucial player in the CCM pathway. Importantly, a minor reduction of Rap1b in combination with similar reductions in the products of other CCM pathway genes results in a high incidence of ICH. These findings support the idea that minor polygenic deficits in the CCM pathway can strongly synergize to initiate ICH.

Rap1a null mice have altered myeloid cell functions suggesting distinct roles for the closely related Rap1a and 1b proteins

The Ras-related GTPases Rap1a and 1b have been implicated in multiple biological events including cell adhesion, free radical production, and cancer. To gain a better understanding of Rap1 function in mammalian physiology, we deleted the Rap1a gene. Although loss of Rap1a expression did not initially affect mouse size or viability, upon backcross into C57BL/6J mice some Rap1a-/- embryos died in utero. T cell, B cell, or myeloid cell development was not disrupted in Rap1a-/- mice. However, macrophages from Rap1a null mice exhibited increased haptotaxis on fibronectin and vitronectin matrices that correlated with decreased adhesion. Chemotaxis of lymphoid and myeloid cells in response to CXCL12 or CCL21 was significantly reduced. In contrast, an increase in FcR-mediated phagocytosis was observed. Because Rap1a was previously copurified with the human neutrophil NADPH oxidase, we addressed whether GTPase loss affected superoxide production. Neutrophils from Rap1a-/- mice had reduced fMLP-stimulated superoxide production as well as a weaker initial response to phorbol ester. These results suggest that, despite 95% amino acid sequence identity, similar intracellular distribution, and broad tissue distribution, Rap1a and 1b are not functionally redundant but rather differentially regulate certain cellular events.

Defective angiogenesis, endothelial migration, proliferation, and MAPK signaling in Rap1b-deficient mice

Angiogenesis is the main mechanism of vascular remodeling during late development and, after birth, in wound healing. Perturbations of angiogenesis occur in cancer, diabetes, ischemia, and inflammation. While much progress has been made in identifying factors that control angiogenesis, the understanding of the precise molecular mechanisms involved is incomplete. Here we identify a small GTPase, Rap1b, as a positive regulator of angiogenesis. Rap1b-deficient mice had a decreased level of Matrigel plug and neonatal retinal neovascularization, and aortas isolated from Rap1b-deficient animals had a reduced microvessel sprouting response to 2 major physiological regulators of angiogenesis: vascular endothelial growth factor (VEGF) and basic fibroblasts growth factor (bFGF), indicating an intrinsic defect in endothelial cells. Proliferation of retinal endothelial cells in situ and in vitro migration of lung endothelial cells isolated from Rap1b-deficient mice were inhibited. At the molecular level, activation of 2 MAP kinases, p38 MAPK and p42/44 ERK, important regulators of endothelial migration and proliferation, was decreased in Rap1b-deficient endothelial cells in response to VEGF stimulation. These studies provide evidence that Rap1b is required for normal angiogenesis and reveal a novel role of Rap1 in regulation of proangiogenic signaling in endothelial cells.

RAP-1 and the RAL-1/exocyst pathway coordinate hypodermal cell organization in Caenorhabditis elegans

The small Ras-like GTPase Rap1 has been identified as a regulator of integrin activation and cadherin-mediated cell-cell contacts. Surprisingly, null mutants of RAP-1 in Caenorhabditis elegans are viable and fertile. In a synthetic lethal RNAi screen with C. elegans rap-1 mutants, the Ras-like GTPase ral-1 emerged as one of seven genes specifically required for viability. Depletion of exoc-8 and sec-5, encoding two putative RAL-1 effectors and members of the exocyst complex, also caused lethality of rap-1 mutants, but did not affect wild-type worms. The RAP-1 and the RAL-1/exocyst pathway appear to coordinate hypodermal cell movement and elongation during embryonic development. They mediate their effect in part through targeting the alpha-catenin homologue HMP-1 to the lateral membrane. Genetic interactions show that the RAP-1 and RAL-1/exocyst pathway also act in parallel during larval stages. Together these data provide in vivo evidence for the exocyst complex as a downstream RAL-1 effector in cell migration.

Cell surface mechanics and the control of cell shape, tissue patterns and morphogenesis

Embryonic morphogenesis requires the execution of complex mechanisms that regulate the local behaviour of groups of cells. The orchestration of such mechanisms has been mainly deciphered through the identification of conserved families of signalling pathways that spatially and temporally control cell behaviour. However, how this information is processed to control cell shape and cell dynamics is an open area of investigation. The framework that emerges from diverse disciplines such as cell biology, physics and developmental biology points to adhesion and cortical actin networks as regulators of cell surface mechanics. In this context, a range of developmental phenomena can be explained by the regulation of cell surface tension.

The Rap GTPase activator Drosophila PDZ-GEF regulates cell shape in epithelial migration and morphogenesis

Epithelial morphogenesis is characterized by an exquisite control of cell shape and position. Progression through dorsal closure in Drosophila gastrulation depends on the ability of Rap1 GTPase to signal through the adherens junctional multidomain protein Canoe. Here, we provide genetic evidence that epithelial Rap activation and Canoe effector usage are conferred by the Drosophila PDZ-GEF (dPDZ-GEF) exchange factor. We demonstrate that dPDZ-GEF/Rap/Canoe signaling modulates cell shape and apicolateral cell constriction in embryonic and wing disc epithelia. In dPDZ-GEF mutant embryos with strong dorsal closure defects, cells in the lateral ectoderm fail to properly elongate. Postembryonic dPDZ-GEF mutant cells generated in mosaic tissue display a striking extension of lateral cell perimeters in the proximity of junctional complexes, suggesting a loss of normal cell contractility. Furthermore, our data indicate that dPDZ-GEF signaling is linked to myosin II function. Both dPDZ-GEF and cno show strong genetic interactions with the myosin II-encoding gene, and myosin II distribution is severely perturbed in epithelia of both mutants. These findings provide the first insight into the molecular machinery targeted by Rap signaling to modulate epithelial plasticity. We propose that dPDZ-GEF-dependent signaling functions as a rheostat linking Rap activity to the regulation of cell shape in epithelial morphogenesis at different developmental stages.