Interaction of p190RhoGAP with C-terminal domain of p120-catenin modulates endothelial cytoskeleton and permeability

Inflammatory lung injury is associated with endothelial cell mitochondrial fission and requires the nitration of RhoA and cytoskeletal remodeling

Higher levels of extracellular nicotinamide phosphoribosyltransferase (eNAMPT), a TLR4 agonist, are associated with poor clinical outcomes in sepsis-induced acute lung injury (ALI). Little is known regarding the mechanisms by which eNAMPT is involved in ALI. Our recent work has identified a crucial role for mitochondrial dysfunction in ALI. Thus, this study aimed to determine if eNAMPT-mediated inflammatory injury is associated with the loss of mitochondrial function. Our data show that eNAMPT disrupted mitochondrial bioenergetics. This was associated with cytoskeleton remodeling and the loss of endothelial barrier integrity. These changes were associated with enhanced mitochondrial fission and blocked when Rho-kinase (ROCK) was inhibited. The increases in mitochondrial fission were also associated with the nitration-mediated activation of the small GTPase activator of ROCK, RhoA. Blocking RhoA nitration decreased eNAMPT-mediated mitochondrial fission and endothelial barrier dysfunction. The increase in fission was linked to a RhoA-ROCK mediated increase in Drp1 (dynamin-related protein 1) at serine(S)616. Another TLR4 agonist, lipopolysaccharide (LPS), also increased mitochondrial fission in a Drp1 and RhoA-ROCK-dependent manner. To validate our findings in vivo, we challenged C57BL/6 mice with eNAMPT in the presence and absence of the Drp1 inhibitor, Mdivi-1. Mdivi-1 treatment protected against eNAMPT-induced lung inflammation, edema, and lung injury. These studies demonstrate that mitochondrial fission-dependent disruption of mitochondrial function is essential in TLR4-mediated inflammatory lung injury and identify a key role for RhoA-ROCK signaling. Reducing mitochondrial fission could be a potential therapeutic strategy to improve ARDS outcomes.

Plakophilin 4 controls the spatio-temporal activity of RhoA at adherens junctions to promote cortical actin ring formation and tissue tension

Plakophilin 4 (PKP4) is a component of cell-cell junctions that regulates intercellular adhesion and Rho-signaling during cytokinesis with an unknown function during epidermal differentiation. Here we show that keratinocytes lacking PKP4 fail to develop a cortical actin ring, preventing adherens junction maturation and generation of tissue tension. Instead, PKP4-depleted cells display increased stress fibers. PKP4-dependent RhoA localization at AJs was required to activate a RhoA-ROCK2-MLCK-MLC2 axis and organize actin into a cortical ring. AJ-associated PKP4 provided a scaffold for the Rho activator ARHGEF2 and the RhoA effectors MLCK and MLC2, facilitating the spatio-temporal activation of RhoA signaling at cell junctions to allow cortical ring formation and actomyosin contraction. In contrast, association of PKP4 with the Rho suppressor ARHGAP23 reduced ARHGAP23 binding to RhoA which prevented RhoA activation in the cytoplasm and stress fiber formation. These data identify PKP4 as an AJ component that transduces mechanical signals into cytoskeletal organization.

Tankyrase inhibition interferes with junction remodeling, induces leakiness, and disturbs YAP1/TAZ signaling in the endothelium

Tankyrase inhibitors are increasingly considered for therapeutic use in malignancies that are characterized by high intrinsic β-catenin activity. However, how tankyrase inhibition affects the endothelium after systemic application remains poorly understood. In this study, we aimed to investigate how the tankyrase inhibitor XAV939 affects endothelial cell function and the underlying mechanism involved. Endothelial cell function was analyzed using sprouting angiogenesis, endothelial cell migration, junctional dynamics, and permeability using human umbilical vein endothelial cells (HUVEC) and explanted mouse retina. Underlying signaling was studied using western blot, immunofluorescence, and qPCR in HUVEC in addition to luciferase reporter gene assays in human embryonic kidney cells. XAV939 treatment leads to altered junctional dynamics and permeability as well as impaired endothelial migration. Mechanistically, XAV939 increased stability of the angiomotin-like proteins 1 and 2, which impedes the nuclear translocation of YAP1/TAZ and consequently suppresses TEAD-mediated transcription. Intriguingly, XAV939 disrupts adherens junctions by inducing RhoA-Rho dependent kinase (ROCK)-mediated F-actin bundling, whereas disruption of F-actin bundling through the ROCK inhibitor H1152 restores endothelial cell function. Unexpectedly, this was accompanied by an increase in nuclear TAZ and TEAD-mediated transcription, suggesting differential regulation of YAP1 and TAZ by the actin cytoskeleton in endothelial cells. In conclusion, our findings elucidate the complex relationship between the actin cytoskeleton, YAP1/TAZ signaling, and endothelial cell function and how tankyrase inhibition disturbs this well-balanced signaling.

3D cell segregation geometry and dynamics are governed by tissue surface tension regulation

Tissue morphogenesis and patterning during development involve the segregation of cell types. Segregation is driven by differential tissue surface tensions generated by cell types through controlling cell-cell contact formation by regulating adhesion and actomyosin contractility-based cellular cortical tensions. We use vertebrate tissue cell types and zebrafish germ layer progenitors as in vitro models of 3-dimensional heterotypic segregation and developed a quantitative analysis of their dynamics based on 3D time-lapse microscopy. We show that general inhibition of actomyosin contractility by the Rho kinase inhibitor Y27632 delays segregation. Cell type-specific inhibition of non-muscle myosin2 activity by overexpression of myosin assembly inhibitor S100A4 reduces tissue surface tension, manifested in decreased compaction during aggregation and inverted geometry observed during segregation. The same is observed when we express a constitutively active Rho kinase isoform to ubiquitously keep actomyosin contractility high at cell-cell and cell-medium interfaces and thus overriding the interface-specific regulation of cortical tensions. Tissue surface tension regulation can become an effective tool in tissue engineering.

Critical roles of adherens junctions in diseases of the oral mucosa

The oral cavity is directly exposed to a variety of environmental stimuli and contains a diverse microbiome that continuously interacts with the oral epithelium. Therefore, establishment and maintenance of the barrier function of the oral mucosa is of paramount importance for its function and for the body's overall health. The adherens junction is a cell-cell adhesion complex that is essential for epithelial barrier function. Although a considerable body of work has associated barrier disruption with oral diseases, the molecular underpinnings of these associations have not been equally investigated. This is critical, since adherens junction components also possess significant signaling roles in the cell, in addition to their architectural ones. Here, we summarize current knowledge involving adherens junction components in oral pathologies, such as cancer and oral pathogen-related diseases, while we also discuss gaps in the knowledge and opportunities for future investigation of the relationship between adherens junctions and oral diseases.

Dephosphorylation of Y228 and Y217 and phosphorylation of Y335 in p120 catenin activate convergent extension during zebrafish gastrulation

BACKGROUND

The cadherin-associated protein p120 catenin regulates convergent extension through interactions with cadherin proteins, Cdc42, and Rac1, as we previously showed in zebrafish (Danio rerio). Phosphorylation of p120 catenin changes the nature of its activity in vitro but is virtually unexplored in embryos. We used our previously developed antisense RNA splice-site morpholino targeted to endogenous p120 catenin-δ1 to cause defects in axis elongation probing the functions of three p120 catenin tyrosine-phosphorylation sites in gastrulating zebrafish embryos.

RESULTS

The morpholino-induced defects were rescued by co-injections with mouse p120 catenin-δ1-3A mRNAs mutated at residues Y228 and Y217 to a non-phosphorylatable phenylalanine (F) or mutated at residue Y335 to a phosphomimetic glutamic acid (E). Co-injection of the complementary mutations Y228E, Y217E, or Y335F mRNAs partially rescued embryos whereas dual mutation to Y228E-Y217E blocked rescue. Immunopurification showed Y228F mutant proteins preferentially interacted with Rac1, potentially promoting cell migration. In contrast, the phosphomimetic Y228E preferentially interacted with E-cadherin increasing adhesion. Both Y228F and Y335F strongly bind VAV2.

CONCLUSIONS

p120 catenin serves dual roles during gastrulation of zebrafish. Phosphorylation and dephosphorylation of tyrosine residues Y217, Y228, and Y335 precisely balance cell adhesion and cell migration to facilitate somite compaction and axis elongation.

β-cells retain a pool of insulin-containing secretory vesicles regulated by adherens junctions and the cadherin binding protein p120 catenin

The β-cells of the islets of Langerhans are the sole producers of insulin in the human body. In response to rising glucose levels, insulin-containing vesicles inside β-cells fuse with the plasma membrane and release their cargo. However, the mechanisms regulating this process are only partly understood. Previous evidence indicated reductions in α-catenin elevate insulin release, while reductions in β-catenin decrease insulin release. α- and β-catenin contribute to cellular regulation in a range of ways but one is as members of the adherens junction complex and these contribute to the development of cell polarity in b-cells. Therefore, we investigated the effects of adherens junctions on insulin release. We show in INS-1E β-cells knockdown of either E- or N-cadherin had only small effects on insulin secretion, but simultaneous knockout of both cadherins resulted in a significant increase in basal insulin release to the same level as glucose-stimulated release. This double knockdown also significantly attenuated levels of p120 catenin, a cadherin binding partner involved in regulating cadherin turnover. Conversely, reducing p120 catenin levels with siRNA destabilized both E- and N-cadherin, and this was also associated with an increase in levels of insulin secreted from INS-1E cells. Furthermore, there were also changes in these cells consistent with higher insulin release, namely reductions in levels of F-actin and increased intracellular free Ca²⁺ levels in response to KCl-induced membrane depolarization. Taken together, these data provide evidence that adherens junctions play important roles in retaining a pool of insulin secretory vesicles within the cell and establish a role for p120 catenin in regulating this process.

DOCK4 Regulation of Rho GTPases Mediates Pulmonary Vascular Barrier Function

BACKGROUND

The vascular endothelium maintains tissue-fluid homeostasis by controlling the passage of large molecules and fluid between the blood and interstitial space. The interaction of catenins and the actin cytoskeleton with VE-cadherin (vascular endothelial cadherin) is the primary mechanism for stabilizing AJs (adherens junctions), thereby preventing lung vascular barrier disruption. Members of the Rho (Ras homology) family of GTPases and conventional GEFs (guanine exchange factors) of these GTPases have been demonstrated to play important roles in regulating endothelial permeability. Here, we evaluated the role of DOCK4 (dedicator of cytokinesis 4)-an unconventional Rho family GTPase GEF in vascular function.

METHODS

We generated mice deficient in DOCK4' used DOCK4 silencing and reconstitution approaches in human pulmonary artery endothelial cells' used assays to evaluate protein localization, endothelial cell permeability, and small GTPase activation.

RESULTS

Our data show that DOCK4-deficient mice are viable. However, these mice have hemorrhage selectively in the lung, incomplete smooth muscle cell coverage in pulmonary vessels, increased basal microvascular permeability, and impaired response to S1P (sphingosine-1-phosphate)-induced reversal of thrombin-induced permeability. Consistent with this, DOCK4 rapidly translocates to the cell periphery and associates with the detergent-insoluble fraction following S1P treatment, and its absence prevents S1P-induced Rac-1 activation and enhancement of barrier function. Moreover, DOCK4-silenced pulmonary artery endothelial cells exhibit enhanced basal permeability in vitro that is associated with enhanced Rho GTPase activation.

CONCLUSIONS

Our findings indicate that DOCK4 maintains AJs necessary for lung vascular barrier function by establishing the normal balance between RhoA (Ras homolog family member A) and Rac-1-mediated actin cytoskeleton remodeling, a previously unappreciated function for the atypical GEF family of molecules. Our studies also identify S1P as a potential upstream regulator of DOCK4 activity.

srGAP2 deactivates RhoA to control the duration of thrombin-mediated endothelial permeability

The endothelial barrier is a tightly regulated gateway in the transport of material between circulation and the tissues. Inflammatory mediators such as thrombin are able to open paracellular spaces in the endothelial monolayer to allow the extravasation of plasma proteins and leukocytes. Here we show that the protein SLIT-ROBO Rho GTPase-activating protein 2 (srGAP2) plays a critical role in regulating the extent of thrombin-mediated opening. We show that srGAP2 is not required for normal barrier function in resting endothelial cells, but that depletion of srGAP2 significantly increases the magnitude and duration of junctional opening in response to thrombin. We show that srGAP2 acts to switch off RhoA signaling after the contraction phase of thrombin-induced permeability, allowing respreading of cells and reformation of the barrier. srGAP2 is also required for effective restoration of the barrier after treatment with two other vasoactive agents that active RhoA - TNFα and angiotensin II. Taken together, we show that srGAP2 has a general function in controlling RhoA signaling in endothelial permeability, acting to limit the degree and duration of opening, by triggering the switch from endothelial cell contraction to respreading.

Effect of SARS-CoV-2 proteins on vascular permeability

Severe acute respiratory syndrome (SARS)-CoV-2 infection leads to severe disease associated with cytokine storm, vascular dysfunction, coagulation, and progressive lung damage. It affects several vital organs, seemingly through a pathological effect on endothelial cells. The SARS-CoV-2 genome encodes 29 proteins, whose contribution to the disease manifestations, and especially endothelial complications, is unknown. We cloned and expressed 26 of these proteins in human cells and characterized the endothelial response to overexpression of each, individually. Whereas most proteins induced significant changes in endothelial permeability, nsp2, nsp5_c145a (catalytic dead mutant of nsp5), and nsp7 also reduced CD31, and increased von Willebrand factor expression and IL-6, suggesting endothelial dysfunction. Using propagation-based analysis of a protein–protein interaction (PPI) network, we predicted the endothelial proteins affected by the viral proteins that potentially mediate these effects. We further applied our PPI model to identify the role of each SARS-CoV-2 protein in other tissues affected by coronavirus disease (COVID-19). While validating the PPI network model, we found that the tight junction (TJ) proteins cadherin-5, ZO-1, and β-catenin are affected by nsp2, nsp5_c145a, and nsp7 consistent with the model prediction. Overall, this work identifies the SARS-CoV-2 proteins that might be most detrimental in terms of endothelial dysfunction, thereby shedding light on vascular aspects of COVID-19.

Oxidized Phospholipids in Control of Endothelial Barrier Function: Mechanisms and Implication in Lung Injury

Earlier studies investigating the pathogenesis of chronic vascular inflammation associated with atherosclerosis described pro-inflammatory and vascular barrier disruptive effects of lipid oxidation products accumulated in the sites of vascular lesion and atherosclerotic plaque. However, accumulating evidence including studies from our group suggests potent barrier protective and anti-inflammatory properties of certain oxidized phospholipids (OxPLs) in the lung vascular endothelium. Among these OxPLs, oxidized 1-palmitoyl-2-arachdonyl-sn-glycero-3-phosphocholine (OxPAPC) causes sustained enhancement of lung endothelial cell (EC) basal barrier properties and protects against vascular permeability induced by a wide variety of agonists ranging from bacterial pathogens and their cell wall components, endotoxins, thrombin, mechanical insults, and inflammatory cytokines. On the other hand, truncated OxPLs cause acute endothelial barrier disruption and potentiate inflammation. It appears that multiple signaling mechanisms triggering cytoskeletal remodeling are involved in OxPLs-mediated regulation of EC barrier. The promising vascular barrier protective and anti-inflammatory properties exhibited by OxPAPC and its particular components that have been established in the cellular and animal models of sepsis and acute lung injury has prompted consideration of OxPAPC as a prototype therapeutic molecule. In this review, we will summarize signaling and cytoskeletal mechanisms involved in OxPLs-mediated damage, rescue, and restoration of endothelial barrier in various pathophysiological settings and discuss a future potential of OxPAPC in treating lung disorders associated with endothelial barrier dysfunction.

Interplay between actomyosin and E-cadherin dynamics regulates cell shape in the Drosophila embryonic epidermis

Precise regulation of cell shape is vital for building functional tissues. Here, we study the mechanisms that lead to the formation of highly elongated anisotropic epithelial cells in the Drosophila epidermis. We demonstrate that this cell shape is the result of two counteracting mechanisms at the cell surface that regulate the degree of elongation: actomyosin, which inhibits cell elongation downstream of RhoA (Rho1 in Drosophila) and intercellular adhesion, modulated via clathrin-mediated endocytosis of E-cadherin (encoded by shotgun in flies), which promotes cell elongation downstream of the GTPase Arf1 (Arf79F in Drosophila). We show that these two mechanisms do not act independently but are interconnected, with RhoA signalling reducing Arf1 recruitment to the plasma membrane. Additionally, cell adhesion itself regulates both mechanisms - p120-catenin, a regulator of intercellular adhesion, promotes the activity of both Arf1 and RhoA. Altogether, we uncover a complex network of interactions between cell-cell adhesion, the endocytic machinery and the actomyosin cortex, and demonstrate how this network regulates cell shape in an epithelial tissue in vivo.

p190A RhoGAP induces CDH1 expression and cooperates with E-cadherin to activate LATS kinases and suppress tumor cell growth

The ARHGAP35 gene encoding p190A RhoGAP (p190A) is significantly altered by both mutation and allelic deletion in human cancer, but the functional implications of such alterations are not known. Here, we demonstrate for the first time that p190A is a tumor suppressor using a xenograft mouse model with carcinoma cells harboring defined ARHGAP35 alterations. In vitro, restoration of p190A expression in carcinoma cells promotes contact inhibition of proliferation (CIP) through activation of LATS kinases and phosphorylation of the proto-oncogenic transcriptional co-activator YAP. In contrast, p190A forms harboring recurrent cancer mutations exhibit loss of function in modulating the Hippo pathway, inducing CIP, as well as attenuated suppression of tumor growth in mice. We determine that p190A promotes mesenchymal to epithelial transition (MET) and elicits expression of a cassette of epithelial adherens junction-associated genes in a cell density-dependent manner. This cassette includes CDH1 encoding E-cadherin, which amplifies p190A-mediated LATS activation and is necessary for CIP. Oppositely, we establish that p190A is obligatory for E-cadherin to activate LATS kinases and induce CIP. Collectively, this work defines a novel mechanism by which p190A and E-cadherin cooperate in modulating Hippo signaling to suppress tumor cell growth.

Differential Expression of a Classic Cadherin Directs Tissue-Level Contractile Asymmetry during Neural Tube Closure

Embryos control force generation at tissue boundaries, but how they do so remains poorly understood. Here we show how tissue-specific expression of the type II cadherin, Cadherin2, patterns actomyosin contractility along tissue boundaries to control zippering and neural tube closure in the basal chordate, Ciona robusta. Cadherin2 is differentially expressed and homotypically enriched in neural cells along the neural/epidermal (Ne/Epi) boundary, where RhoA and myosin are activated during zipper progression. Homotypically enriched Cadherin2 sequesters the Rho GTPase-activating protein, Gap21/23, to homotypic junctions. Gap21/23 in turn redirects RhoA/myosin activity to heterotypic Ne/Epi junctions. By activating myosin II along Ne/Epi junctions ahead of the zipper and inhibiting myosin II along newly formed Ne/Ne junctions behind the zipper, Cadherin2 promotes tissue-level contractile asymmetry to drive zipper progression. We propose that dynamic coupling of junction exchange to local changes in contractility may control fusion and separation of epithelia in many other contexts.

Vitamin D (1,25-(OH)2D3) regulates the gene expression through competing endogenous RNAs networks in high glucose-treated endothelial progenitor cells

Vitamin D (vit-D) supplementation can improve endothelial cell function in type 2 diabetes mellitus patients with vit-D insufficiency or deficiency. In the present study, we aimed to compare the expression profiles of circRNAs, lncRNAs, miRNAs, and mRNAs between 1,25-(OH)₂D₃-treated endothelial progenitor cells (EPCs) and control cells, and to further construct the 1,25-(OH)₂D₃-regulated ceRNA networks in EPCs. RNA sequencing was performed on the 1,25-(OH)₂D₃-treated EPCs and control cells derived from the bone marrow (BM). Bioinformatics analyses were performed to identify differentially expressed (DE) microRNAs (miRNAs), circular RNAs (circRNAs), mRNAs, and long non-coding RNAs (lncRNAs). Then Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were conducted to predict the function of genes. Competing endogenous RNA (ceRNA) networks were constructed with Cytoscape software. 1,25-(OH)₂D₃ application induced changes in the expression profiles of 1791 mRNAs, 2726 lncRNAs, 205 circRNAs, and 45 miRNAs in EPCs treated with high levels of glucose. These DE RNAs were associated with MMP and GTPase activities, specific signaling pathways, and components of actin, extracellular matrix, or adherens junction. DE circRNAs, which functioned independently of their linear host genes, interacted with miRNAs to serve as miRNA sponges in complex ceRNA networks. The data indicated that circRNAs and lncRNAs comprised ceRNAs to sponge effects of miRNAs on the expressions of mRNAs following 1,25-(OH)₂D₃ application in EPCs. 1,25-(OH)₂D₃ improved the function of EPCs via associated ceRNA interaction networks in diabetes patients.

SGEF forms a complex with Scribble and Dlg1 and regulates epithelial junctions and contractility

The canonical Scribble polarity complex is implicated in regulation of epithelial junctions and apical polarity. Here, we show that SGEF, a RhoG-specific GEF, forms a ternary complex with Scribble and Dlg1, two members of the Scribble complex. SGEF targets to apical junctions in a Scribble-dependent fashion and functions in the regulation of actomyosin-based contractility and barrier function at tight junctions as well as E-cadherin-mediated formation of adherens junctions. Surprisingly, SGEF does not control the establishment of polarity. However, in 3D cysts, SGEF regulates the formation of a single open lumen. Interestingly, SGEF's nucleotide exchange activity regulates the formation and maintenance of adherens junctions, and in cysts the number of lumens formed, whereas SGEF's scaffolding activity is critical for regulation of actomyosin contractility and lumen opening. We propose that SGEF plays a key role in coordinating junctional assembly and actomyosin contractility by bringing together Scribble and Dlg1 and targeting RhoG activation to cell-cell junctions.

p190RhoGAPs, the ARHGAP35- and ARHGAP5-Encoded Proteins, in Health and Disease

Small guanosine triphosphatases (GTPases) gathered in the Rat sarcoma (Ras) superfamily represent a large family of proteins involved in several key cellular mechanisms. Within the Ras superfamily, the Ras homolog (Rho) family is specialized in the regulation of actin cytoskeleton-based mechanisms. These proteins switch between an active and an inactive state, resulting in subsequent inhibiting or activating downstream signals, leading finally to regulation of actin-based processes. The On/Off status of Rho GTPases implicates two subsets of regulators: GEFs (guanine nucleotide exchange factors), which favor the active GTP (guanosine triphosphate) status of the GTPase and GAPs (GTPase activating proteins), which inhibit the GTPase by enhancing the GTP hydrolysis. In humans, the 20 identified Rho GTPases are regulated by over 70 GAP proteins suggesting a complex, but well-defined, spatio-temporal implication of these GAPs. Among the quite large number of RhoGAPs, we focus on p190RhoGAP, which is known as the main negative regulator of RhoA, but not exclusively. Two isoforms, p190A and p190B, are encoded by ARHGAP35 and ARHGAP5 genes, respectively. We describe here the function of each of these isoforms in physiological processes and sum up findings on their role in pathological conditions such as neurological disorders and cancers.

Connexin 43 Loss Triggers Cell Cycle Entry and Invasion in Non-Neoplastic Breast Epithelium: A Role for Noncanonical Wnt Signaling

(1) Background: The expression of connexin 43 (Cx43) is disrupted in breast cancer, and re-expression of this protein in human breast cancer cell lines leads to decreased proliferation and invasiveness, suggesting a tumor suppressive role. This study aims to investigate the role of Cx43 in proliferation and invasion starting from non-neoplastic breast epithelium. (2) Methods: Nontumorigenic human mammary epithelial HMT-3522 S1 cells and Cx43 shRNA-transfected counterparts were cultured under 2-dimensional (2-D) and 3-D conditions. (3) Results: Silencing Cx43 induced mislocalization of β-catenin and Scrib from apicolateral membrane domains in glandular structures or acini formed in 3-D culture, suggesting the loss of apical polarity. Cell cycle entry and proliferation were enhanced, concomitantly with c-Myc and cyclin D1 upregulation, while no detectable activation of Wnt/β-catenin signaling was observed. Motility and invasion were also triggered and were associated with altered acinar morphology and activation of ERK1/2 and Rho GTPase signaling, which acts downstream of the noncanonical Wnt pathway. The invasion of Cx43-shRNA S1 cells was observed only under permissive stiffness of the extracellular matrix (ECM). (4) Conclusion: Our results suggest that Cx43 controls proliferation and invasion in the normal mammary epithelium in part by regulating noncanonical Wnt signaling.

Signaling by Small GTPases at Cell-Cell Junctions: Protein Interactions Building Control and Networks

A number of interesting reports highlight the intricate network of signaling proteins that coordinate formation and maintenance of cell-cell contacts. We have much yet to learn about how the in vitro binding data is translated into protein association inside the cells and whether such interaction modulates the signaling properties of the protein. What emerges from recent studies is the importance to carefully consider small GTPase activation in the context of where its activation occurs, which upstream regulators are involved in the activation/inactivation cycle and the GTPase interacting partners that determine the intracellular niche and extent of signaling. Data discussed here unravel unparalleled cooperation and coordination of functions among GTPases and their regulators in supporting strong adhesion between cells.

Mind the gap: mechanisms regulating the endothelial barrier

The endothelial barrier consists of intercellular contacts localized in the cleft between endothelial cells, which is covered by the glycocalyx in a sievelike manner. Both types of barrier-forming junctions, i.e. the adherens junction (AJ) serving mechanical anchorage and mechanotransduction and the tight junction (TJ) sealing the intercellular space to limit paracellular permeability, are tethered to the actin cytoskeleton. Under resting conditions, the endothelium thereby builds a selective layer controlling the exchange of fluid and solutes with the surrounding tissue. However, in the situation of an inflammatory response such as in anaphylaxis or sepsis intercellular contacts disintegrate in post-capillary venules leading to intercellular gap formation. The resulting oedema can cause shock and multi-organ failure. Therefore, maintenance as well as coordinated opening and closure of interendothelial junctions is tightly regulated. The two principle underlying mechanisms comprise spatiotemporal activity control of the small GTPases Rac1 and RhoA and the balance of the phosphorylation state of AJ proteins. In the resting state, junctional Rac1 and RhoA activity is enhanced by junctional components, actin-binding proteins, cAMP signalling and extracellular cues such as sphingosine-1-phosphate (S1P) and angiopoietin-1 (Ang-1). In addition, phosphorylation of AJ components is prevented by junction-associated phosphatases including vascular endothelial protein tyrosine phosphatase (VE-PTP). In contrast, inflammatory mediators inhibiting cAMP/Rac1 signalling cause strong activation of RhoA and induce AJ phosphorylation finally leading to endocytosis and cleavage of VE-cadherin. This results in dissolution of TJs the outcome of which is endothelial barrier breakdown.

Small GTPases orchestrate cell-cell communication during collective cell movement

Collective cell migration is a critical mechanism involved in cell movement during various physiological and pathological processes such as angiogenesis and metastasis formation. During collective movement, cells remain functionally connected and can coordinate individual cell behaviors to ensure efficient migration. A cell-cell communication process ensures this complex coordination. Although the mechanisms regulating cell-cell communication remain unclear, recent findings indicate that it is based on acto-myosin cytoskeleton tension transmission from cell to cell through adherens junctions. As for single cell migration, small GTPases of the Rho and Rab families have been shown to be critical regulators of collective motion. Here, we discuss our current understanding on how these small GTPases are themselves regulated and how they control cell-cell communication during collective migration. Moreover, we also shed light on the key role of cell-cell communication and RhoGTPases in the physiological context of endothelial cell migration during angiogenesis.

Lipid mediators in the regulation of endothelial barriers

Lipid mediators play a critical role in the development and resolution of vascular endothelial barrier dysfunction caused by various pathologic interventions. The accumulation of excess lipids directly impairs endothelial cell (EC) barrier function that is known to contribute to the development of atherosclerosis and metabolic disorders such as obesity and diabetes as well as chronic inflammation in the vascular endothelium. Certain products of phospholipid oxidation (OxPL) such as fragmented phospholipids generated during oxidative and nitrosative stress show pro-inflammatory potential and cause endothelial barrier dysfunction. In turn, other OxPL products enhance basal EC barrier and exhibit potent barrier-protective effects in pathologic settings of acute vascular leak caused by pro-inflammatory mediators, barrier disruptive agonists and pathologic mechanical stimulation. These beneficial effects were further confirmed in rodent models of lung injury and inflammation. The bioactive oxidized lipid molecules may serve as important therapeutic prototype molecules for future treatment of acute lung injury syndromes associated with endothelial barrier dysfunction and inflammation. This review will summarize recent studies of biological effects exhibited by various groups of lipid mediators with a focus on the role of oxidized phospholipids in control of vascular endothelial barrier, agonist induced EC permeability, inflammation, and barrier recovery related to clinical settings of acute lung injury and inflammatory vascular leak.

Functional roles of p120ctn family of proteins in central neurons

The cadherin-catenin complex in central neurons is associated with a variety of cytosolic partners, collectively called catenins. The p120ctn members are a family of catenins that are distinct from the more ubiquitously expressed α- and β-catenins. It is becoming increasingly clear that the functional roles of the p120ctn family of catenins in central neurons extend well beyond their functional roles in non-neuronal cells in partnering with cadherin to regulate adhesion. In this review, we will provide an overview of the p120ctn family in neurons and their varied functional roles in central neurons. Finally, we will examine the emerging roles of this family of proteins in neurodevelopmental disorders.

Anti-Inflammatory Effects of OxPAPC Involve Endothelial Cell-Mediated Generation of LXA4

RATIONALE

Oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC) generates a group of bioactive oxidized phospholipid products with a broad range of biological activities. Barrier-enhancing and anti-inflammatory effects of OxPAPC on pulmonary endothelial cells are critical for prevention of acute lung injury caused by bacterial pathogens or excessive mechanical ventilation. Anti-inflammatory properties of OxPAPC are associated with its antagonistic effects on Toll-like receptors and suppression of RhoA GTPase signaling.

OBJECTIVE

Because OxPAPC exhibits long-lasting anti-inflammatory and lung-protective effects even after single administration in vivo, we tested the hypothesis that these effects may be mediated by additional mechanisms, such as OxPAPC-dependent production of anti-inflammatory and proresolving lipid mediator, lipoxin A4 (LXA4).

METHODS AND RESULTS

Mass spectrometry and ELISA assays detected significant accumulation of LXA4 in the lungs of OxPAPC-treated mice and in conditioned medium of OxPAPC-exposed pulmonary endothelial cells. Administration of LXA4 reproduced anti-inflammatory effect of OxPAPC against tumor necrosis factor-α in vitro and in the animal model of lipopolysaccharide-induced lung injury. The potent barrier-protective and anti-inflammatory effects of OxPAPC against tumor necrosis factor-α and lipopolysaccharide challenge were suppressed in human pulmonary endothelial cells with small interfering RNA-induced knockdown of LXA4 formyl peptide receptor-2 (FPR2/ALX) and in mFPR2^-/- (mouse formyl peptide receptor 2) mice lacking the mouse homolog of human FPR2/ALX.

CONCLUSIONS

This is the first demonstration that inflammation- and injury-associated phospholipid oxidation triggers production of anti-inflammatory and proresolution molecules, such as LXA4. This lipid mediator switch represents a novel mechanism of OxPAPC-assisted recovery of inflamed lung endothelium.

Protein Interactions at Endothelial Junctions and Signaling Mechanisms Regulating Endothelial Permeability

The monolayer of endothelial cells lining the vessel wall forms a semipermeable barrier (in all tissue except the relatively impermeable blood-brain and inner retinal barriers) that regulates tissue-fluid homeostasis, transport of nutrients, and migration of blood cells across the barrier. Permeability of the endothelial barrier is primarily regulated by a protein complex called adherens junctions. Adherens junctions are not static structures; they are continuously remodeled in response to mechanical and chemical cues in both physiological and pathological settings. Here, we discuss recent insights into the post-translational modifications of junctional proteins and signaling pathways regulating plasticity of adherens junctions and endothelial permeability. We also discuss in the context of what is already known and newly defined signaling pathways that mediate endothelial barrier leakiness (hyperpermeability) that are important in the pathogenesis of cardiovascular and lung diseases and vascular inflammation.

Rho GTPases and actomyosin: Partners in regulating epithelial cell-cell junction structure and function

Epithelial tissues are defined by polarized epithelial cells that are integrated into tissues and exhibit barrier function in order to regulate what is allowed to pass between cells. Cell-cell junctions must be stable enough to promote barrier function and tissue integrity, yet plastic enough to remodel when necessary. This remarkable ability to dynamically sense and respond to changes in cell shape and tissue tension allows cell-cell junctions to remain functional during events that disrupt epithelial homeostasis including morphogenesis, wound healing, and cell division. In order to achieve this plasticity, both tight junctions and adherens junctions are coupled to the underlying actomyosin cytoskeleton. Here, we discuss the importance of the junctional linkage to actomyosin and how a localized zone of active RhoA along with other Rho GTPases work together to orchestrate junctional actomyosin dynamics. We focus on how scaffold proteins help coordinate Rho GTPases, their upstream regulators, and their downstream effectors for efficient, localized Rho GTPase signaling output. Additionally, we highlight important roles junctional actin-binding proteins play in addition to their traditional roles in organizing actin. Together, Rho GTPases, their regulators, and effectors form compartmentalized signaling modules that regulate actomyosin structure and contractility to achieve proper cell-cell adhesion and tissue barriers.

Endothelial cell signaling and ventilator-induced lung injury: molecular mechanisms, genomic analyses, and therapeutic targets

Mechanical ventilation is a life-saving intervention in critically ill patients with respiratory failure due to acute respiratory distress syndrome (ARDS). Paradoxically, mechanical ventilation also creates excessive mechanical stress that directly augments lung injury, a syndrome known as ventilator-induced lung injury (VILI). The pathobiology of VILI and ARDS shares many inflammatory features including increases in lung vascular permeability due to loss of endothelial cell barrier integrity resulting in alveolar flooding. While there have been advances in the understanding of certain elements of VILI and ARDS pathobiology, such as defining the importance of lung inflammatory leukocyte infiltration and highly induced cytokine expression, a deep understanding of the initiating and regulatory pathways involved in these inflammatory responses remains poorly understood. Prevailing evidence indicates that loss of endothelial barrier function plays a primary role in the development of VILI and ARDS. Thus this review will focus on the latest knowledge related to 1) the key role of the endothelium in the pathogenesis of VILI; 2) the transcription factors that relay the effects of excessive mechanical stress in the endothelium; 3) the mechanical stress-induced posttranslational modifications that influence key signaling pathways involved in VILI responses in the endothelium; 4) the genetic and epigenetic regulation of key target genes in the endothelium that are involved in VILI responses; and 5) the need for novel therapeutic strategies for VILI that can preserve endothelial barrier function.

Regulation of endothelial barrier function by p120-catenin∙VE-cadherin interaction

Maintaining VE-cadherin levels by inhibiting its endocytosis through p120-catenin binding is not sufficient for forming a restrictive barrier. Instead, p120-catenin binding to VE-cadherin is required to allow tyrosine-phosphorylated VE-cadherin to contribute to barrier formation.

Endothelial p120-catenin (p120) maintains the level of vascular endothelial cadherin (VE-Cad) by inhibiting VE-Cad endocytosis. Loss of p120 results in a decrease in VE-Cad levels, leading to the formation of monolayers with decreased barrier function (as assessed by transendothelial electrical resistance [TEER]), whereas overexpression of p120 increases VE-Cad levels and promotes a more restrictive monolayer. To test whether reduced endocytosis mediated by p120 is required for VE-Cad formation of a restrictive barrier, we restored VE-Cad levels using an endocytic-defective VE-Cad mutant. This endocytic-defective mutant was unable to rescue the loss of TEER associated with p120 or VE-Cad depletion. In contrast, the endocytic-defective mutant was able to prevent sprout formation in a fibrin bead assay, suggesting that p120•VE-Cad interaction regulates barrier function and angiogenic sprouting through different mechanisms. Further investigation found that depletion of p120 increases Src activity and that loss of p120 binding results in increased VE-Cad phosphorylation. In addition, expression of a Y658F–VE-Cad mutant or an endocytic-defective Y658F–VE-Cad double mutant were both able to rescue TEER independently of p120 binding. Our results show that in addition to regulating endocytosis, p120 also allows the phosphorylated form of VE-Cad to participate in the formation of a restrictive monolayer.

Role of Cingulin in Agonist-induced Vascular Endothelial Permeability

Agonist-induced activation of Rho GTPase signaling leads to endothelial cell (EC) permeability and may culminate in pulmonary edema, a devastating complication of acute lung injury. Cingulin is an adaptor protein first discovered in epithelium and is involved in the organization of the tight junctions. This study investigated the role of cingulin in control of agonist-induced lung EC permeability via interaction with RhoA-specific activator GEF-H1. The siRNA-induced cingulin knockdown augmented thrombin-induced EC permeability monitored by measurements of transendothelial electrical resistance and endothelial cell permeability for macromolecules. Increased thrombin-induced permeability in ECs with depleted cingulin was associated with increased activation of GEF-H1 and RhoA detected in pulldown activation assays. Increased GEF-H1 association with cingulin was essential for down-regulation of thrombin-induced RhoA barrier disruptive signaling. Using cingulin-truncated mutants, we determined that GEF-H1 interaction with the rod + tail domain of cingulin was required for inactivation of GEF-H1 and endothelial cell barrier preservation. The results demonstrate the role for association of GEF-H1 with cingulin as the mechanism of RhoA pathway inactivation and rescue of EC barrier after agonist challenge.

Regulating Rho GTPases and their regulators

Rho GTPases regulate cytoskeletal and cell adhesion dynamics and thereby coordinate a wide range of cellular processes, including cell migration, cell polarity and cell cycle progression. Most Rho GTPases cycle between a GTP-bound active conformation and a GDP-bound inactive conformation to regulate their ability to activate effector proteins and to elicit cellular responses. However, it has become apparent that Rho GTPases are regulated by post-translational modifications and the formation of specific protein complexes, in addition to GTP-GDP cycling. The canonical regulators of Rho GTPases - guanine nucleotide exchange factors, GTPase-activating proteins and guanine nucleotide dissociation inhibitors - are regulated similarly, creating a complex network of interactions to determine the precise spatiotemporal activation of Rho GTPases.

Roles of the novel coiled-coil protein Rng10 in septum formation during fission yeast cytokinesis

The regulation of Rho-GAP localization is not well understood. A novel coiled-coil protein Rng10 is characterized that localizes the Rho-GAP Rga7 in fission yeast. Rng10 and Rga7 physically interact and work together to regulate the accumulation and dynamics of glucan synthases for successful septum formation during cytokinesis.

Rho GAPs are important regulators of Rho GTPases, which are involved in various steps of cytokinesis and other processes. However, regulation of Rho-GAP cellular localization and function is not fully understood. Here we report the characterization of a novel coiled-coil protein Rng10 and its relationship with the Rho-GAP Rga7 in fission yeast. Both rng10Δ and rga7Δ result in defective septum and cell lysis during cytokinesis. Rng10 and Rga7 colocalize on the plasma membrane at the cell tips during interphase and at the division site during cell division. Rng10 physically interacts with Rga7 in affinity purification and coimmunoprecipitation. Of interest, Rga7 localization is nearly abolished without Rng10. Moreover, Rng10 and Rga7 work together to regulate the accumulation and dynamics of glucan synthases for successful septum formation in cytokinesis. Our results show that cellular localization and function of the Rho-GAP Rga7 are regulated by a novel protein, Rng10, during cytokinesis in fission yeast.

Getting to the core of cadherin complex function in Caenorhabditis elegans

The classic cadherin-catenin complex (CCC) mediates cell-cell adhesion in metazoans. Although substantial insights have been gained by studying the CCC in vertebrate tissue culture, analyzing requirements for and regulation of the CCC in vertebrates remains challenging. Caenorhabditis elegans is a powerful system for connecting the molecular details of CCC function with functional requirements in a living organism. Recent data, using an “angstroms to embryos” approach, have elucidated functions for key residues, conserved across all metazoans, that mediate cadherin/β-catenin binding. Other recent work reveals a novel, potentially ancestral, role for the C. elegans p120ctn homologue in regulating polarization of blastomeres in the early embryo via Cdc42 and the partitioning-defective (PAR)/atypical protein kinase C (aPKC) complex. Finally, recent work suggests that the CCC is trafficked to the cell surface via the clathrin adaptor protein complex 1 (AP-1) in surprising ways. These studies continue to underscore the value of C. elegans as a model system for identifying conserved molecular mechanisms involving the CCC.

p120-catenin controls contractility along the vertical axis of epithelial lateral membranes

In vertebrate epithelia, p120-catenin (hereafter referred to as p120; also known as CTNND1) mediates E-cadherin stability and suppression of RhoA. Genetic ablation of p120 in various epithelial tissues typically causes striking alterations in tissue function and morphology. Although these effects could very well involve p120's activity towards Rho, ascertaining the impact of this relationship has been complicated by the fact that p120 is also required for cell-cell adhesion. Here, we have molecularly uncoupled p120's cadherin-stabilizing and RhoA-suppressing activites. Unexpectedly, removing p120's Rho-suppressing activity dramatically disrupted the integrity of the apical surface, irrespective of E-cadherin stability. The physical defect was tracked to excessive actomyosin contractility along the vertical axis of lateral membranes. Thus, we suggest that p120's distinct activities towards E-cadherin and Rho are molecularly and functionally coupled and this, in turn, enables the maintenance of cell shape in the larger context of an epithelial monolayer. Importantly, local suppression of contractility by cadherin-bound p120 appears to go beyond regulating cell shape, as loss of this activity also leads to major defects in epithelial lumenogenesis.

PDCD10 (CCM3) regulates brain endothelial barrier integrity in cerebral cavernous malformation type 3: role of CCM3-ERK1/2-cortactin cross-talk

Impairment of brain endothelial barrier integrity is critical for cerebral cavernous malformation (CCM) lesion development. The current study investigates changes in tight junction (TJ) complex organization when PDCD10 (CCM3) is mutated/depleted in human brain endothelial cells. Analysis of lesions with CCM3 mutation and brain endothelial cells transfected with CCM3 siRNA (CCM3-knockdown) showed little or no increase in TJ transmembrane and scaffolding proteins mRNA expression, but proteins levels were generally decreased. CCM3-knockdown cells had a redistribution of claudin-5 and occludin from the membrane to the cytosol with no alterations in protein turnover but with diminished protein-protein interactions with ZO-1 and ZO-1 interaction with the actin cytoskeleton. The most profound effect of CCM3 mutation/depletion was on an actin-binding protein, cortactin. CCM3 depletion caused cortactin Ser-phosphorylation, dissociation from ZO-1 and actin, redistribution to the cytosol and degradation. This affected cortical actin ring organization, TJ complex stability and consequently barrier integrity, with constant hyperpermeability to inulin. A potential link between CCM3 depletion and altered cortactin was tonic activation of MAP kinase ERK1/2. ERK1/2 inhibition increased cortactin expression and incorporation into the TJ complex and improved barrier integrity. This study highlights the potential role of CCM3 in regulating TJ complex organization and brain endothelial barrier permeability.

Phosphorylation and isoform use in p120-catenin during development and tumorigenesis

P120-catenin is essential to vertebrate development, modulating cadherin and small-GTPase functions, and growing evidence points also to roles in the nucleus. A complexity in addressing p120-catenin's functions is its many isoforms, including optional splicing events, alternative points of translational initiation, and secondary modifications. In this review, we focus upon how choices in the initiation of protein translation, or the earlier splicing of the RNA transcript, relates to primary sequences that harbor established or putative regulatory phosphorylation sites. While certain p120 phosphorylation events arise via known kinases/phosphatases and have defined outcomes, in most cases the functional consequences are still to be established. In this review, we provide examples of p120-isoforms as they relate to phosphorylation events, and thereby to isoform dependent protein-protein associations and downstream functions. We also provide a view of upstream pathways that determine p120's phosphorylation state, and that have an impact upon development and disease. Because other members of the p120 subfamily undergo similar processing and phosphorylation, as well as related catenins of the plakophilin subfamily, what is learned regarding p120 will by extension have wide relevance in vertebrates.

Concise review: Mechanotransduction via p190RhoGAP regulates a switch between cardiomyogenic and endothelial lineages in adult cardiac progenitors

Mechanical cues can have pleiotropic influence on stem cell shape, proliferation, differentiation, and morphogenesis, and are increasingly realized to play an instructive role in regeneration and maintenance of tissue structure and functions. To explore the putative effects of mechanical cues in regeneration of the cardiac tissue, we investigated therapeutically important cardiosphere-derived cells (CDCs), a heterogeneous patient- or animal-specific cell population containing c-Kit(+) multipotent stem cells. We showed that mechanical cues can instruct c-Kit(+) cell differentiation along two lineages with corresponding morphogenic changes, while also serving to amplify the initial c-Kit(+) subpopulation. In particular, mechanical cues mimicking the structure of myocardial extracellular matrix specify cardiomyogenic fate, while cues mimicking myocardium rigidity specify endothelial fates. Furthermore, we found that these cues dynamically regulate the same molecular species, p190RhoGAP, which then acts through both RhoA-dependent and independent mechanisms. Thus, differential regulation of p190RhoGAP molecule by either mechanical inputs or genetic manipulation can determine lineage type specification. Since human CDCs are already in phase II clinical trials, the potential therapeutic use of mechanical or genetic manipulation of the cell fate could enhance effectiveness of these progenitor cells in cardiac repair, and shed new light on differentiation mechanisms in cardiac and other tissues.

Rho GAPs and GEFs: controling switches in endothelial cell adhesion

Within blood vessels, endothelial cell–cell and cell–matrix adhesions are crucial to preserve barrier function, and these adhesions are tightly controlled during vascular development, angiogenesis, and transendothelial migration of inflammatory cells. Endothelial cellular signaling that occurs via the family of Rho GTPases coordinates these cell adhesion structures through cytoskeletal remodelling. In turn, Rho GTPases are regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). To understand how endothelial cells initiate changes in the activity of Rho GTPases, and thereby regulate cell adhesion, we will discuss the role of Rho GAPs and GEFs in vascular biology. Many potentially important Rho regulators have not been studied in detail in endothelial cells. We therefore will first overview which GAPs and GEFs are highly expressed in endothelium, based on comparative gene expression analysis of human endothelial cells compared with other tissue cell types. Subsequently, we discuss the relevance of Rho GAPs and GEFs for endothelial cell adhesion in vascular homeostasis and disease.

Cytoskeletal mechanisms regulating vascular endothelial barrier function in response to acute lung injury

Endothelial cells (EC) form a semi-permeable barrier between the interior space of blood vessels and the underlying tissues. In acute lung injury (ALI) the EC barrier is weakened leading to increased vascular permeability. It is widely accepted that EC barrier integrity is critically dependent upon intact cytoskeletal structure and cell junctions. Edemagenic agonists, like thrombin or endotoxin lipopolysaccharide (LPS), induced cytoskeletal rearrangement, and EC contractile responses leading to disruption of intercellular contacts and EC permeability increase. The highly clinically-relevant cytoskeletal mechanisms of EC barrier dysfunction are currently under intense investigation and will be described and discussed in the current review.

Control of vascular permeability by adhesion molecules

Vascular permeability is a vital function of the circulatory system that is regulated in large part by the limited flux of solutes, water, and cells through the endothelial cell layer. One major pathway through this barrier is via the inter-endothelial junction, which is driven by the regulation of cadherin-based adhesions. The endothelium also forms attachments with surrounding proteins and cells via 2 classes of adhesion molecules, the integrins and IgCAMs. Integrins and IgCAMs propagate activation of multiple downstream signals that potentially impact cadherin adhesion. Here we discuss the known contributions of integrin and IgCAM signaling to the regulation of cadherin adhesion stability, endothelial barrier function, and vascular permeability. Emphasis is placed on known and prospective crosstalk signaling mechanisms between integrins, the IgCAMs- ICAM-1 and PECAM-1, and inter-endothelial cadherin adhesions, as potential strategic signaling nodes for multipartite regulation of cadherin adhesion.

Rac1 functions as a reversible tension modulator to stabilize VE-cadherin trans-interaction

The role of the RhoGTPase Rac1 in stabilizing mature endothelial adherens junctions (AJs) is not well understood. In this paper, using a photoactivatable probe to control Rac1 activity at AJs, we addressed the relationship between Rac1 and the dynamics of vascular endothelial cadherin (VE-cadherin). We demonstrated that Rac1 activation reduced the rate of VE-cadherin dissociation, leading to increased density of VE-cadherin at AJs. This response was coupled to a reduction in actomyosin-dependent tension across VE-cadherin adhesion sites. We observed that inhibiting myosin II directly or through photo-release of the caged Rho kinase inhibitor also reduced the rate of VE-cadherin dissociation. Thus, Rac1 functions by stabilizing VE-cadherin trans-dimers in mature AJs by counteracting the actomyosin tension. The results suggest a new model of VE-cadherin adhesive interaction mediated by Rac1-induced reduction of mechanical tension at AJs, resulting in the stabilization of VE-cadherin adhesions.

ARVCF expression is significantly correlated with the malignant phenotype of non-small cell lung cancer

Armadillo repeat gene deleted in velo-cardio-facial syndrome (ARVCF) is a member of the p120 catenin (p120ctn) family; it contains nine central Armadillo repeats and binds to the juxtamembrane domain of E-cadherin. We used immunohistochemistry to measure ARVCF expression in 121 patients with NSCLC and western blotting to examine differences in ARVCF expression between lung cancer and adjacent normal lung tissues. We interfered with ARVCF expression in two lung cancer cell lines and measured its effects on invasion and proliferation. ARVCF expression correlated with the malignant phenotype and poor prognosis. We also observed ARVCF-dependent changes in small GTPase (mainly RhoA) activity in lung cancer cells. We confirmed that ARVCF plays an important role in the malignant phenotype.

Tropomyosin isoforms support actomyosin biogenesis to generate contractile tension at the epithelial zonula adherens

Epithelial cells generate contractile forces at their cell-cell contacts. These are concentrated at the specialized apical junction of the zonula adherens (ZA), where a ring of stabilized E-cadherin lies adjacent to prominent actomyosin bundles. Coupling of adhesion and actomyosin contractility yields tension in the junction. The biogenesis of junctional contractility requires actin assembly at the ZA as well as the recruitment of nonmuscle myosin II, but the molecular regulators of these processes are not yet fully understood. We now report a role for tropomyosins 5NM1 (Tm5NM1) and 5NM2 (Tm5NM2) in their generation. Both these tropomyosin isoforms were found at the ZA and their depletion by RNAi or pharmacological inhibition reduced both F-actin and myosin II content at the junction. Photoactivation analysis revealed that the loss of F-actin was attributable to a decrease in filament stability. These changes were accompanied by a decrease in E-cadherin content at junctions. Ultimately, both long-term depletion of Tm5NM1/2 and acute inhibition with drugs caused junctional tension to be reduced. Thus these tropomyosin isoforms are novel contributors to junctional contractility and integrity.

Mechanisms regulating endothelial permeability

The endothelial monolayer partitioning underlying tissue from blood components in the vessel wall maintains tissue fluid balance and host defense through dynamically opening intercellular junctions. Edemagenic agonists disrupt endothelial barrier function by signaling the opening of the intercellular junctions leading to the formation of protein-rich edema in the interstitial tissue, a hallmark of tissue inflammation that, if left untreated, causes fatal diseases, such as acute respiratory distress syndrome. In this review, we discuss how intercellular junctions are maintained under normal conditions and after stimulation of endothelium with edemagenic agonists. We have focused on reviewing the new concepts dealing with the alteration of adherens junctions after inflammatory stimulus.

Asef controls vascular endothelial permeability and barrier recovery in the lung

This is the first report of Asef involvement in the regulation of endothelial vascular permeability in vitro and in vivo. Asef activation in endothelial cells by hepatocyte growth factor suppressed the Rho-dependent pathway of agonist-induced endothelial permeability and promoted Rac1-dependent endothelial barrier recovery.

Increased levels of hepatocyte growth factor (HGF) in injured lungs may reflect a compensatory response to diminish acute lung injury (ALI). HGF-induced activation of Rac1 GTPase stimulates endothelial barrier protective mechanisms. This study tested the involvement of Rac-specific guanine nucleotide exchange factor Asef in HGF-induced endothelial cell (EC) cytoskeletal dynamics and barrier protection in vitro and in a two-hit model of ALI. HGF induced membrane translocation of Asef and stimulated Asef Rac1-specific nucleotide exchange activity. Expression of constitutively activated Asef mutant mimicked HGF-induced peripheral actin cytoskeleton enhancement. In contrast, siRNA-induced Asef knockdown or expression of dominant-negative Asef attenuated HGF-induced Rac1 activation evaluated by Rac-GTP pull down and FRET assay with Rac1 biosensor. Molecular inhibition of Asef attenuated HGF-induced peripheral accumulation of cortactin, formation of lamellipodia-like structures, and enhancement of VE-cadherin adherens junctions and compromised HGF-protective effect against thrombin-induced RhoA GTPase activation, Rho-dependent cytoskeleton remodeling, and EC permeability. Intravenous HGF injection attenuated lung inflammation and vascular leak in the two-hit model of ALI induced by excessive mechanical ventilation and thrombin signaling peptide TRAP6. This effect was lost in Asef^−/− mice. This study shows for the first time the role of Asef in HGF-mediated protection against endothelial hyperpermeability and lung injury.

CD148 tyrosine phosphatase promotes cadherin cell adhesion

CD148 is a transmembrane tyrosine phosphatase that is expressed at cell junctions. Recent studies have shown that CD148 associates with the cadherin/catenin complex and p120 catenin (p120) may serve as a substrate. However, the role of CD148 in cadherin cell-cell adhesion remains unknown. Therefore, here we addressed this issue using a series of stable cells and cell-based assays. Wild-type (WT) and catalytically inactive (CS) CD148 were introduced to A431D (lacking classical cadherins), A431D/E-cadherin WT (expressing wild-type E-cadherin), and A431D/E-cadherin 764AAA (expressing p120-uncoupled E-cadherin mutant) cells. The effects of CD148 in cadherin adhesion were assessed by Ca²⁺ switch and cell aggregation assays. Phosphorylation of E-cadherin/catenin complex and Rho family GTPase activities were also examined. Although CD148 introduction did not alter the expression levels and complex formation of E-cadherin, p120, and β-catenin, CD148 WT, but not CS, promoted cadherin contacts and strengthened cell-cell adhesion in A431D/E-cadherin WT cells. This effect was accompanied by an increase in Rac1, but not RhoA and Cdc42, activity and largely diminished by Rac1 inhibition. Further, we demonstrate that CD148 reduces the tyrosine phosphorylation of p120 and β-catenin; causes the dephosphorylation of Y529 suppressive tyrosine residue in Src, a well-known CD148 site, increasing Src activity and enhancing the phosphorylation of Y228 (a Src kinase site) in p120, in E-cadherin contacts. Consistent with these findings, CD148 dephosphorylated both p120 and β-catenin in vitro. The shRNA-mediated CD148 knockdown in A431 cells showed opposite effects. CD148 showed no effects in A431D and A431D/E-cadherin 764AAA cells. In aggregate, these findings provide the first evidence that CD148 promotes E-cadherin adhesion by regulating Rac1 activity concomitant with modulation of p120, β-catenin, and Src tyrosine phosphorylation. This effect requires E-cadherin and p120 association.

RhoGTPases, actomyosin signaling and regulation of the epithelial Apical Junctional Complex

Epithelial cells form regulated and selective barriers between distinct tissue compartments. The Apical Junctional Complex (AJC) consisting of the tight junction (TJ) and adherens junction (AJ) control epithelial homeostasis, paracellular permeability and barrier properties. The AJC is composed of mutliprotein complexes consisting of transmembrane proteins that affiliate with an underlying perijunctional F-actin myosin ring through cytoplasmic scaffold proteins. AJC protein associations with the apical actin-myosin cytoskeleton are tightly controlled by a number of signaling proteins including the Rho family of GTPases that orchestrate junctional biology, epithelial homeostasis and barrier function. This review highlights the vital relationship of Rho GTPases and AJCs in controlling the epithelial barrier. The pathophysiologic relationship of Rho GTPases, AJC, apical actomyosin cytoskeleton and epithelial barrier function is discussed.

IQGAP1 regulates endothelial barrier function via EB1-cortactin cross talk

Cross talk between the actin cytoskeleton and microtubules (MT) has been implicated in the amplification of agonist-induced Rho signaling, leading to increased vascular endothelial permeability. This study tested the involvement of actin-MT cross talk in the mechanisms of barrier enhancement induced by hepatocyte growth factor (HGF) and evaluated the role of the adaptor protein IQGAP1 in integrating the MT- and actin-dependent pathways of barrier enhancement. IQGAP1 knockdown by small interfering RNA attenuated the HGF-induced increase in endothelial barrier properties and abolished HGF-activated cortical actin dynamics. IQGAP1 reduction abolished HGF-induced peripheral accumulation of Rac cytoskeletal effector cortactin and cortical actin remodeling. In addition, HGF stimulated peripheral MT growth in an IQGAP1-dependent fashion. HGF also induced Rac1-dependent IQGAP1 association with the MT fraction and the formation of a protein complex containing end-binding protein 1 (EB1), IQGAP1, and cortactin. Decreasing endogenous IQGAP1 abolished HGF-induced EB1-cortactin colocalization at the cell periphery. In turn, expression of IQGAP1ΔC (IQGAP1 lacking the C-terminal domain) attenuated the cortactin association with EB1 and suppressed HGF-induced endothelial cell peripheral actin cytoskeleton enhancement. These results demonstrate for the first time the MT-actin cross talk mechanism of HGF-induced endothelial barrier enhancement and suggest that IQGAP1 functions as a hub linking HGF-induced signaling to MT and actin remodeling via EB1-IQGAP1-cortactin interactions.