Formation of microvilli and phosphorylation of ERM family proteins by CD43, a potent inhibitor for cell adhesion: cell detachment is a potential cue for ERM phosphorylation and organization of cell morphology

Interaction of SARS-CoV-2 Spike protein with ACE2 induces cortical actin modulation, including dephosphorylation of ERM proteins and reduction of cortical stiffness

Cell surface cortical actin is a regulatory target for viral infection. We aimed to investigate the effect of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection on host cell cortical stiffness, an indicator of cortical actin structure. The receptor-binding domain (RBD) of SARS-CoV-2 Spike (S) protein induced a reduction in cortical stiffness in ACE2-expressing cells. The interaction of RBD with ACE2 caused the inactivation of Ezrin/Radixin/Moesin (ERM) proteins. We further investigated the effects of the RBD of SARS-CoV-2 Omicron variants, BA.1 and BA.5. These RBDs influenced cortical stiffness depending on their affinity for ACE2. Our study provides the first evidence that the interaction of the SARS-CoV-2 S protein with ACE2 induces mechanobiological signals and attenuates the cortical actin.

Staphylococcal superantigen-like protein 3 triggers murine mast cell adhesion by binding to CD43 and augments mast cell activation

Staphylococcus aureus is a noteworthy pathogen in allergic diseases, as four staphylococcal exotoxins activate mast cells, a significant contributor to inflammation, in an IgE-independent manner. Although the adhesion of mast cells is an essential process for their immune responses, only a small number of exotoxins have been reported to affect the process. Here, we demonstrated that staphylococcal superantigen-like (SSL) 3, previously identified as a toll-like receptor 2 agonist, induced the adhesion of murine bone marrow-derived mast cells to culture substratum. SSL3-induced adhesion was mediated by fibronectin in an Arg-Gly-Asp (RGD) sequence-dependent manner, suggesting the integrins were involved in the process. Additionally, SSL3 was found to bind to an anti-adhesive surface protein CD43. SSL3 induced the adhesion of HEK293 cells expressing exogenous CD43, suggesting that CD43 is the target molecule for adhesion induced by SSL3. Evaluation of SSL3-derived mutants showed that the C-terminal region (253-326), specifically T285 and H307, are necessary to induce adhesion. SSL3 augmented the IL-13 production of mast cells in response to immunocomplex and SSL12. These findings reveal a novel function of SSL3, triggering cell adhesion and enhancing mast cell activation. This study would clarify the correlation between S. aureus and allergic diseases such as atopic dermatitis.

Alpha-mangostin dephosphorylates ERM to induce adhesion and decrease surface stiffness in KG-1 cells

Surface stiffness is a unique indicator of various cellular states and events and needs to be tightly controlled. α-Mangostin, a natural compound with numerous bioactivities, reduces the mechanical stiffness of various cells; however, the mechanism by which it affects the actin cytoskeleton remains unclear. We aimed to elucidate the mechanism underlying α-mangostin activity on the surface stiffness of leukocytes. We treated spherical non-adherent myelomonocytic KG-1 cells with α-mangostin; it clearly reduced their surface stiffness and disrupted their microvilli. The α-mangostin-induced reduction in surface stiffness was inhibited by calyculin A, a protein phosphatase inhibitor. α-Mangostin also induced KG-1 cell adhesion to a fibronectin-coated surface. In KG-1 cells, a decrease in surface stiffness and the induction of cell adhesion are largely attributed to the dephosphorylation of ezrin/radixin/moesin proteins (ERMs); α-mangostin reduced the levels of phosphorylated ERMs. It further increased protein kinase C (PKC) activity. α-Mangostin-induced KG-1 cell adhesion and cell surface softness were inhibited by the PKC inhibitor GF109203X. The results of the present study suggest that α-mangostin decreases stiffness and induces adhesion of KG-1 cells via PKC activation and ERM dephosphorylation.

Mechanical stiffness softening and cell adhesion are coordinately regulated by ERM dephosphorylation in KG-1 cells

Mechanical stiffness is closely related to cell adhesion and rounding in some cells. In leukocytes, dephosphorylation of ezrin/radixin/moesin (ERM) proteins is linked to cell adhesion events. To elucidate the relationship between surface stiffness, cell adhesion, and ERM dephosphorylation in leukocytes, we examined the relationship in the myelogenous leukemia line, KG-1, by treatment with modulation drugs. KG-1 cells have ring-shaped cortical actin with microvilli as the only F-actin cytoskeleton, and the actin structure constructs the mechanical stiffness of the cells. Phorbol 12-myristate 13-acetate and staurosporine, which induced cell adhesion to fibronectin surface and ERM dephosphorylation, caused a decrease in surface stiffness in KG-1 cells. Calyculin A, which inhibited ERM dephosphorylation and had no effect on cell adhesion, did not affect surface stiffness. To clarify whether decreasing cell surface stiffness and inducing cell adhesion are equivalent, we examined KG-1 cell adhesion by treatment with actin-attenuated cell softening reagents. Cytochalasin D clearly diminished cell adhesion, and high concentrations of Y27632 slightly induced cell adhesion. Only Y27632 slightly decreased ERM phosphorylation in KG-1 cells. Thus, decreasing cell surface stiffness and inducing cell adhesion are not equivalent, but these phenomena are coordinately regulated by ERM dephosphorylation in KG-1 cells.

Oxidative Stress Boosts the Uptake of Cerium Oxide Nanoparticles by Changing Brain Endothelium Microvilli Pattern

Vascular oxidative stress is considered a worsening factor in the progression of Alzheimer's disease (AD). Increased reactive oxygen species (ROS) levels promote the accumulation of amyloid-β peptide (Aβ), one of the main hallmarks of AD. In turn, Aβ is a potent inducer of oxidative stress. In early stages of AD, the concomitant action of oxidative stress and Aβ on brain capillary endothelial cells was observed to compromise the blood-brain barrier functionality. In this context, antioxidant compounds might provide therapeutic benefits. To this aim, we investigated the antioxidant activity of cerium oxide nanoparticles (CNP) in human cerebral microvascular endothelial cells (hCMEC/D3) exposed to Aβ oligomers. Treatment with CNP (13.9 ± 0.7 nm in diameter) restored basal ROS levels in hCMEC/D3 cells, both after acute or prolonged exposure to Aβ. Moreover, we found that the extent of CNP uptake by hCMEC/D3 was +43% higher in the presence of Aβ. Scanning electron microscopy and western blot analysis suggested that changes in microvilli structures on the cell surface, under pro-oxidant stimuli (Aβ or H₂O₂), might be involved in the enhancement of CNP uptake. This finding opens the possibility to exploit the modulation of endothelial microvilli pattern to improve the uptake of anti-oxidant particles designed to counteract ROS-mediated cerebrovascular dysfunctions.

Integrins regulate epithelial cell shape by controlling the architecture and mechanical properties of basal actomyosin networks

Forces generated by the actomyosin cytoskeleton are key contributors to many morphogenetic processes. The actomyosin cytoskeleton organises in different types of networks depending on intracellular signals and on cell-cell and cell-extracellular matrix (ECM) interactions. However, actomyosin networks are not static and transitions between them have been proposed to drive morphogenesis. Still, little is known about the mechanisms that regulate the dynamics of actomyosin networks during morphogenesis. This work uses the Drosophila follicular epithelium, real-time imaging, laser ablation and quantitative analysis to study the role of integrins on the regulation of basal actomyosin networks organisation and dynamics and the potential contribution of this role to cell shape. We find that elimination of integrins from follicle cells impairs F-actin recruitment to basal medial actomyosin stress fibers. The available F-actin redistributes to the so-called whip-like structures, present at tricellular junctions, and into a new type of actin-rich protrusions that emanate from the basal cortex and project towards the medial region. These F-actin protrusions are dynamic and changes in total protrusion area correlate with periodic cycles of basal myosin accumulation and constriction pulses of the cell membrane. Finally, we find that follicle cells lacking integrin function show increased membrane tension and reduced basal surface. Furthermore, the actin-rich protrusions are responsible for these phenotypes as their elimination in integrin mutant follicle cells rescues both tension and basal surface defects. We thus propose that the role of integrins as regulators of stress fibers plays a key role on controlling epithelial cell shape, as integrin disruption promotes reorganisation into other types of actomyosin networks, in a manner that interferes with proper expansion of epithelial basal surfaces.

Morphogenesis involves global changes in tissue architecture driven by cell shape changes. Mechanical forces generated by actomyosin networks and force transmission through adhesive complexes power these changes. The actomyosin cytoskeleton organises in different types of networks, which localise to precise regions and perform distinct roles. However, they are rarely independent and, often, reorganisation of a given structure can promote the formation of another, conversions proposed to underlie many morphogenetic processes. Nonetheless, the mechanisms controlling actomyosin network dynamics during morphogenesis remain poorly characterised. Here, using the Drosophila follicular epithelium, we show that cell-ECM interactions mediated by integrins are required for the correct distribution of actin in the different actin networks. Elimination of integrins results in redistribution of actin from stress fibers into a new type of protrusions that dynamically emanate from the cortex and extend into the stress fibers. Changes in area protrusions correlate with bursts of myosin accumulated in stress fibers and constriction pulses of the cell membrane. We also found that integrin mutant cells show increased membrane tension and reduced basal cell surface. As these defects are rescued by eliminating the F-actin protrusions, we believe these structures prevent proper basal surface growth. Thus, we propose that integrin function as regulators of stress fibers assembly and maintenance controls epithelial cell shape, as its disruption promotes reorganisation into other actomyosin networks, conversions that interfere with proper epithelial basal surface expansion.

A sticky wicket: Defining molecular functions for CD34 in hematopoietic cells

The CD34 cell surface antigen is widely expressed in tissues on cells with progenitor-like properties and on mature vascular endothelia. In adult human bone marrow, CD34 marks hematopoietic stem and progenitor cells (HSPCs) starting from the bulk of hematopoietic stem cells with long-term repopulating potential (LT-HSCs) throughout expansion and differentiation of oligopotent and unipotent progenitors. CD34 protein surface expression is typically lost as cells mature into terminal effectors. Because of this expression pattern of HSPCs, CD34 has had a central role in the evaluation or selection of donor graft tissue in HSC transplant (HSCT). Given its clinical importance, it is surprising that the biological functions of CD34 are still poorly understood. This enigma is due, in part, to CD34's context-specific role as both a pro-adhesive and anti-adhesive molecule and its potential functional redundancy with other sialomucins. Moreover, there are also critical differences in the regulation of CD34 expression on HSPCs in humans and experimental mice. In this review, we highlight some of the more well-defined functions of CD34 in HSPCs with a focus on proposed functions most relevant to HSCT biology.

Alpha-mangostin reduces mechanical stiffness of various cells

Alpha-mangostin (α-mangostin) has been identified as a naturally occurring compound with potential anticancer properties. It can induce apoptosis and inhibit the growth and metastasis of cancer cells. Moreover, α-mangostin reduces the mechanical stiffness of lung cancer cells. The objective of this study was to determine the effect of α-mangostin on the mechanical stiffness of various cells, as well as cell viability. The following cell types were examined: human fibroblast TIG-1 cells, human cancerous HeLa cells, human embryonic kidney HEK293 cells, mouse macrophage RAW 264.7 cells, and human myeloblasts KG-1 cells. Cells were treated with α-mangostin, and then examined for cell viability, actin cytoskeletal structures, and surface mechanical stiffness using atomic force microscopy. α-Mangostin demonstrated cytotoxicity against TIG-1, HeLa, HEK293, and KG-1 cells, but not against RAW 264.7 cells. The cytotoxic effect of α-mangostin varies according to cell type. On the other hand, α-mangostin reduced the mechanical stiffness of all cell types, including RAW 264.7 cells. Upon treatment with α-mangostin, F-actin was slightly reduced but the actin cytoskeletal structures were little altered in these cells. Thus, reducing mechanical stiffness of animal cells is an inherent effect of α-mangostin. Our results show that α-mangostin is a naturally occurring compound with potential to change the actin cytoskeletal micro-structures and reduce the surface stiffness of various cells.

Activation of PKC induces leukocyte adhesion by the dephosphorylation of ERM

Although circulating leukocytes are non-adherent cells, they also undergo adhesion in response to external stimuli. To elucidate this switch mechanism, we investigated PMA-induced cell adhesion in myelomonocytic KG-1 cells. PMA induced microvillius collapse, decrease of cell surface rigidity and exclusion of sialomucin from adhesion sites. All these adhesion-contributing events are linked to dephosphorylation of Ezrin/Radixin/Moesin (ERM) proteins. Indeed, PMA-treatment induced quick decrease of phosphorylated ERM proteins, while expression of Moesin-T558D, a phospho-mimetic mutant, inhibited PMA-induced cell adhesion. PMA-induced cell adhesion and ERM-dephophorylation were inhibited by PKC inhibitors or by a phosphatase inhibitor, indicating the involvement of PKC and protein phophatase in these processes. In peripheral T lymphocytes, ERM-dephosphorylation by adhesion-inducing stimuli was inhibited by a PKC inhibitor. Combined, these findings strongly suggest that external stimuli induce ERM-dephosphorylation via the activation of PKC in leukocytes and that ERM-dephosphorylation leads to leukocytes' adhesion.

Sialomucin and phosphorylated-ERM are inhibitors for cadherin-mediated aggregate formation

Cell adhesion is mediated by adhesion molecules, but also regulated by adhesion inhibitory molecules. Molecules such as leukocyte sialomucin and phosphorylated-Ezrin/Radixin/Moesin (ERM) inhibit cell-substratum adhesion. Here we show that these adhesion inhibitory molecules also inhibit aggregate formation of adherent cells in suspension culture. Expression of sialomucin, CD43 or CD34, inhibited formation of packed aggregates in HEK293T cells. Deletion mutant analysis and enzymatic cleavage indicated the significance of the extracellular sialomucin domain for this inhibition. Meanwhile, phosphorylated-ERM were decreased coincidently with aggregate formation. Combined with the inhibition of aggregate formation by the expression of phospho-mimetic Moesin mutant (Moesin-T558D), phosphorylated-ERM are inhibitors for aggregate formation. Increase of phosphorylated-ERM by CD43 and sialomucin-dependence of Moesin-T558D's inhibition indicate that sialomucin and phosphorylated-ERM collaborate to inhibit aggregate formation. Because aggregate formation of HEK293T cells is mediated by N-cadherin, sialomucin and phosphorylated-ERM inhibit cadherin-mediated cell-cell adhesion. Thus, sialomucin and phosphorylated-ERM are inhibitors for both cell-cell adhesion and cell-substratum adhesion, and regulation of these inhibitory molecules is essential for cell adhesion.

Immunohistochemical expression of P53, Ki-67, and CD34 in psoriasis and psoriasiform dermatitis

BACKGROUND

Psoriasis is the prime example of psoriasiform tissue pattern and should be differentiated from other psoriasiform dermatoses both clinically and histopathologically.

AIM

To evaluate immunohistochemical expression of P53, Ki-67, and CD34 in psoriasis and psoriasiform dermatitis for diagnostic purposes.

METHODS

An analytical cross-sectional study was performed on the paraffin blocks of 60 psoriasis and 31 psoriasiform dermatitis patients between 2014 and 2017. The selected formalin-fixed paraffin-embedded tissues from each biopsy specimen were cut into 4-micron sections. Initial sections were stained by hematoxylin and eosin staining. Primary antihuman antibodies against P53, Ki-67, and CD34 were applied. Positive control samples for biomarkers were received from former strongly positive samples of papillary endothelial hyperplasia, high grade lymphoma, and breast ductal carcinoma for CD34, Ki-67, and P53, respectively.

RESULTS

Out of 60 psoriasis patients, 56.7% were men, with the mean age of 36.8 years. From 31 psoriasiform patients, 45.2% were men, with the mean age of 37.5 years. Both groups were matched in terms of sex and age. The mean staining of three markers was more significant in psoriasiform dermatitis than psoriasis.

CONCLUSION

In spite of some other researches, the present study showed expression of P53, Ki-67, and CD34 biomarkers were significantly higher in psoriasiform dermatitis than psoriasis.

N-cadherin-mediated aggregate formation; cell detachment by Trypsin-EDTA loses N-cadherin and delays aggregate formation

Although cell aggregates/spheroids are useful tools in various fields of cell biology, the mechanism for aggregate formation is not fully resolved yet. Here I show the involvement of N-cadherin in the quick formation of packed aggregates in suspension culture. HEK293T cells detached from substratum by Trypsin alone quickly formed packed aggregates in suspension. This aggregate formation was inhibited by the down-regulation of N-cadherin. Meanwhile, aggregate formation of cells detached by Trypsin-EDTA was significantly delayed. N-cadherin was transiently lost by Trypsin-EDTA-treatment, and the re-expression of N-cadherin corresponded to delayed aggregate formation. Furthermore, packed phenotype was not observed in the absence of N-cadherin. These findings indicate that N-cadherin mediates quick formation of packed aggregates/spheroids in suspension culture.

Inhibition of cell adhesion by phosphorylated Ezrin/Radixin/Moesin

Altered phosphorylation status of the C-terminal Thr residues of Ezrin/Radixin/Moesin (ERM) is often linked to cell shape change. To determine the role of phophorylated ERM, we modified phosphorylation status of ERM and investigated changes in cell adhesion and morphology. Treatment with Calyculin-A (Cal-A), a protein phosphatase inhibitor, dramatically augmented phosphorylated ERM (phospho-ERM). Cal-A-treatment or expression of phospho-mimetic Moesin mutant (Moesin-TD) induced cell rounding in adherent cells. Moreover, reattachment of detached cells to substrate was inhibited by either treatment. Phospho-ERM, Moesin-TD and actin cytoskeleton were observed at the plasma membrane of such round cells. Augmented cell surface rigidity was also observed in both cases.

Meanwhile, non-adherent KG-1 cells were rather rich in phospho-ERM. Treatment with Staurosporine, a protein kinase inhibitor that dephosphorylates phospho-ERM, up-regulated the integrin-dependent adhesion of KG-1 cells to substrate.

These findings strongly suggest the followings: (1) Phospho-ERM inhibit cell adhesion, and therefore, dephosphorylation of ERM proteins is essential for cell adhesion. (2) Phospho-ERM induce formation and/or maintenance of spherical cell shape. (3) ERM are constitutively both phosphorylated and dephosphorylated in cultured adherent and non-adherent cells.

The Ca2+-activated Cl- channel Ano1 controls microvilli length and membrane surface area in the oocyte

Ca(2+)-activated Cl(-) channels (CaCCs) play important physiological functions in epithelia and other tissues. In frog oocytes the CaCC Ano1 regulates resting membrane potential and the block to polyspermy. Here, we show that Ano1 expression increases the oocyte surface, revealing a novel function for Ano1 in regulating cell morphology. Confocal imaging shows that Ano1 increases microvilli length, which requires ERM-protein-dependent linkage to the cytoskeleton. A dominant-negative form of the ERM protein moesin precludes the Ano1-dependent increase in membrane area. Furthermore, both full-length and the truncated dominant-negative forms of moesin co-localize with Ano1 to the microvilli, and the two proteins co-immunoprecipitate. The Ano1-moesin interaction limits Ano1 lateral membrane mobility and contributes to microvilli scaffolding, therefore stabilizing larger membrane structures. Collectively, these results reveal a newly identified role for Ano1 in shaping the plasma membrane during oogenesis, with broad implications for the regulation of microvilli in epithelia.

Distinct mechanical behavior of HEK293 cells in adherent and suspended states

The mechanical features of individual animal cells have been regarded as indicators of cell type and state. Previously, we investigated the surface mechanics of cancer and normal stromal cells in adherent and suspended states using atomic force microscopy. Cancer cells possessed specific mechanical and actin cytoskeleton features that were distinct from normal stromal cells in adherent and suspended states. In this paper, we report the unique mechanical and actin cytoskeletal features of human embryonic kidney HEK293 cells. Unlike normal stromal and cancer cells, the surface stiffness of adherent HEK293 cells was very low, but increased after cell detachment from the culture surface. Induced actin filament depolymerization revealed that the actin cytoskeleton was the underlying source of the stiffness in suspended HEK293 cells. The exclusive mechanical response of HEK293 cells to perturbation of the actin cytoskeleton resembled that of adherent cancer cells and suspended normal stromal cells. Thus, with respect to their special cell-surface mechanical features, HEK293 cells could be categorized into a new class distinct from normal stromal and cancer cells.

Regulation of cell shape and adhesion by CD34

We previously reported that expression of CD43/leukosialin induces cell rounding and microvillus formation via inhibition of cell adhesion. Here, we found that CD34, a cell surface sialomucin and marker for hematopoietic progenitor cells, also inhibited cell adhesion and induced cell rounding and microvillus formation. Forced expression of CD34-induced cell rounding, microvillus formation, and phosphorylation of ezrin/radixin/moesin (ERM) proteins in HEK293T cells, while inhibiting integrin-mediated cell re-attachment. Furthermore, CD34+ blood cells and KG-1 cells, which express endogenous CD34 on their surface, were spherical in shape, surrounded by microvilli, and non-adherent to substrata. In addition, cleavage of O-sialomucin augmented integrin-mediated cell adhesion of KG-1 cells. These results suggest the involvement of CD34 in the inhibition of integrin-mediated cell adhesion and formation of the cell surface structure. The inhibitory function of CD34 in cell adhesion may affect cell shape organization via phosphorylation of ERM proteins. Cellular structures such as the spherical shape and microvilli of CD34+ cells may also contribute to regulation of cell adhesion.

Sphingolipid regulation of ezrin, radixin, and moesin proteins family: implications for cell dynamics

A key but poorly studied domain of sphingolipid functions encompasses endocytosis, exocytosis, cellular trafficking, and cell movement. Recently, the ezrin, radixin and moesin (ERM) family of proteins emerged as novel potent targets regulated by sphingolipids. ERMs are structural proteins linking the actin cytoskeleton to the plasma membrane, also forming a scaffold for signaling pathways that are used for cell proliferation, migration and invasion, and cell division. Opposing functions of the bioactive sphingolipid ceramide and sphingosine-1-phosphate (S1P), contribute to ERM regulation. S1P robustly activates whereas ceramide potently deactivates ERM via phosphorylation/dephosphorylation, respectively. This recent dimension of cytoskeletal regulation by sphingolipids opens up new avenues to target cell dynamics, and provides further understanding of some of the unexplained biological effects mediated by sphingolipids. In addition, these studies are providing novel inroads into defining basic mechanisms of regulation and action of bioactive sphingolipids. This review describes the current understanding of sphingolipid regulation of the cytoskeleton, it also describes the biologies in which ERM proteins have been involved, and finally how these two large fields have started to converge. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Cortical rigidity of round cells in mitotic phase and suspended state

This paper describes the results of the analysis of cortical rigidity in two round cell states: mitotic round cells and detached round cells after trypsinization using atomic force microscopy (AFM). These two states are primary cell events with dynamic morphological alterations in vitro. The trypsinized detached cells were fixed on the substrate of membrane anchoring oleyl surface. Fluorescent images taken by confocal laser scanning microscopy revealed diverse cell surface protrusions and cortical actin development in the round cells under different conditions. Although the cortical actin of these cells seemed to develop similarly, cortical rigidity of the trypsinized round cells showed greater stiffness than that of mitotic round cells. The elasticity measurements by AFM may detect invisible information about the maturation or strength of F-actin structures and such measurements may indicate that the strength of the actomyosin cortex would be higher in trypsinized round cells compared to mitotic cells. The mechanical properties can help provide better insights into the characteristics of the actin cytoskeleton network in vicinity of cell surface during dynamic morphological alterations.

Radixin regulates cell migration and cell-cell adhesion through Rac1

The ERM proteins ezrin, radixin and moesin are adaptor proteins that link plasma membrane receptors to the actin cytoskeleton. Ezrin and moesin have been implicated in cell polarization and cell migration, but little is known about the involvement of radixin in these processes. Here we show that radixin is required for migration of PC3 prostate cancer cells, and that radixin, but not ezrin or moesin, depletion by RNAi increases cell spread area and cell-cell adhesion mediated by adherens junctions. Radixin depletion also alters actin organization and distribution of active phosphorylated ezrin and moesin. Similar effects were observed in MDA-MB-231 breast cancer cells. The phenotype of radixin-depleted cells is similar to that induced by constitutively active Rac1, and Rac1 is required for the radixin knockdown phenotype. Radixin depletion also increases the activity of Rac1 but not Cdc42 or RhoA. Analysis of Rac guanine nucleotide exchange factors (GEFs) suggests that radixin affects the activity of Vav GEFs. Indeed, Vav GEF depletion reverts the phenotype of radixin knockdown and reduces the effect of radixin knockdown on Rac1 activity. Our results indicate that radixin plays an important role in promoting cell migration by regulating Rac1-mediated epithelial polarity and formation of adherens junctions through Vav GEFs.